Библиографическая справка (Apple Talk)
В начале 1980 гг. Apple Computer готовилась к выпуску компьютера Macintosh. Инженеры компании знали, что в скором времени сети станут насущной необходимостью, а не просто интересной новинкой. Они хотели также добиться того, чтобы базирующаяся на компьютерах Macintosh сеть была бесшовным расширением интерфейса пользователя Macintosh, совершившим подлинную революцию в этой области. Имея в виду эти два фактора, Apple решила встроить сетевой интерфейс в каждый Macintosh и интегрировать этот интерфейс в окружение настольной вычислительной машины. Новая сетевая архитектура Apple получила название Apple Talk.
Хотя Apple Talk является патентованной сетью, Apple опубликовала характеристики Apple Talk, пытаясь поощрить разработку при участии третьей стороны. В настоящее время большое число компаний успешно сбывают на рынке базирующиеся на Apple Talk изделия; в их числе Novell, Inc. и Мicrosoft Corparation.
Оригинальную реализацию Apple Talk, разработанную для локальных рабочих групп, в настоящее время обычно называют Apple Talk Phase I. Однако после установки свыше 1.5 мил. компьютеров Macintosh в течение первых пяти лет существования этого изделия, Apple обнаружила, что некоторые крупные корпорации превышают встроенные возможности Apple Talk Phase I, поэтому протокол был модернизирован. Расширенные протоколы стали известнны под названием Apple Talk Phase II. Oни расширили возможности маршрутизации Apple Talk, обеспечив их успешное применение в более крупных сетях. <
Доступ к среде (Apple Talk)
Apple разработала AppleTalk таким образом, чтобы он был независимым от канального уровня. Другими словами, теоретически он может работать в дополнение к любой реализации канального уровня. Apple обеспечивает различные реализации канального уровня, включая Ethernet, Token Ring, FDDI и LocalTalk. Apple ссылается на AppleTalk, работающий в Ethernet, как нa EtherTalk, в Тоkеn Ring-кaк на TokenTalk и в FDDI-как на FDDITalk.
LocalTalk - это запатентованная компанией Apple система доступа к носителю. Он базируется на конкуренции на получение доступа, топологии об'единения с помощью шины и передаче сигналов базовой полосы (baseband signaling) и работает на носителе, представляющим собой экранированную витую пару, со скоростью 230.4 Kb/сек. Физическим интерфейсом является RS-422; это сбалансированный интерфейс для передачи электрических сигналов, поддерживаемый интерфейсом RS-449. Сегменты LocalTalk могут переноситься на расстояния до 300 метров и обеспечивать до 32 узлов.
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Назначения адреса протокола
Для обеспечения минимальных затрат, связанных с работой администратора сети, aдреса узлов AppleTalk назначаются динамично. Когда Macintosh, прогоняющий AppleTalk, начинает работать, он выбирает какой-нибудь адрес протокола (сетевого уровня) и проверяет его, чтобы убедиться, что этот адрес используется в данный момент. Если это не так, то этот новый узел успешно присваивает себе какой-нибудь адрес. Если этот адрес используется в данный момент, то узел с конфликтным адресом отправляет сообщение, указывающее на наличие проблемы, а новый узел выбирает другой адрес и повторяет этот процесс. На рисунке представлен процесс выбора адреса AppleTalk.
Фактические механизмы выбора адреса AppleTalk зависят от носителя. Для установления связи адресов AppleTalk с конкретными адресами носителя используется протокoл разрешения адреса AppleTalk (AARP). AARP также устанавливает связи между адресами других протоколов и аппаратными адресами. Если пакет протоколов AppleTalk или любого другой пакет протоколов должен отправить пакет данных в другой сетевой узел, то адрес протокола передается в AARP. AARP сначала проверяет адресный кэш, чтобы определить, является ли уже установленной связь между адресом этого протокола и аппаратным адресом. Если это так, то эта связь передается в запрашивающий пакет протоколов. Если это не так, то AARP инициирует широковещательное или многопунктовое сообщение, запрашивающее об аппаратном адресе данного протокольного адреса. Если широковещательное сообщение доходит до узла с этим протокольным адресом, то этот узел в ответном сообщении указывает свой аппаратный адрес. Эта информация передается в запрашивающий пакет протоколов, который использует этот аппаратный адрес для связи с этим узлом. <
Протокол поддепжки маршрутной таблицы (RTMP)
Протокол, который организует и поддерживает маршрутные таблицы AppleTalk, называется Протоколом поддержки маршрутной таблицы (RTMP). Маршрутные таблицы RTMP содержат данные о каждой сети, до которой может дойти дейтаграмма. В эти данные входит порт роутера, который ведет к сети пункта назначения, ID узла следующего роутера, который принимает данный пакет, расстояние до сети назначения, выраженное числом пересылок, и текущее состояние этих данных (хорошее, подозрительное или плохое). Периодический обмен маршрутными таблицами позволяет роутерам об'единенных сетей гарантировать обеспечение непротиворечивой текущей информацией. На рисунке представлен образец таблицы RTMP и соответствующая архитектура сети.
Протокол привязки по именам AppleTalk (Name Binding Protocol - NBP) устанавливает связь имен AppleTalk (которые выражаются как об'екты, видимые для сети - network-visible entities, или NVE) с адресами. NVE является адресуемой сетью AppleTalk услугой, такой как гнездо. NVE ассоциируются с более, чем одним именем об'ектов и перечнем атрибутов. Имена об'ектов представляют собой последовательность символов, например такую: printer@net1, в то время как перечень атрибутов определяет характеристики NVE.
Связь между NVE с присвоенными именами и сетевыми адресами устанавливается через процесс привязки имени. Привязка имени может быть произведена в момент запуска узла или динамично, непосредственно перед первым использованием. NBP управляет процессом привязки имени, в который входят регистрация имени, подтверждение имени, стирание имени и поиск имени.
Зоны позволяют проводить поиск имени в группе логически связанных узлов. Чтобы произвести поиск имен в пределах какой-нибудь зоны, отправляется запрос о поиске в местный роутер, который рассылает широковещательный запрос во все сети, которые имеют узлы, принадлежащие заданной зоне. Протокол информации зоны (Zone Information Protocol - ZIP) координирует эти действия.
ZIP поддерживает соответствие номер сети/номер зоны в информационных таблицах зоны (zone information tables-ZIT).
Протокол сетевого уровня DDP (Apple Talk)
Основным протоколом сетевого уровня AppleTalk является протокол DDP. DDP обеспечивает обслуживание без установления соединения между сетевыми гнездами. Гнезда могут назначаться либо статистически, либо динамически. Адреса AppleTalk, назначаемые DDP, состоят из 2 компонентов: 16-битового номера сети (network number) и 8-битового номера узла (node number). Эти два компонента обычно записываются в виде десятичных номеров, разделенных точкой (например, 10.1 означает сеть 10, узел 1). Если номер сети и номер узла дополнены 8-битовым гнездом (socket), обозначающим какой-нибудь особый процесс, то это означает, что в сети задан какой-нибудь уникальный процесс.
AppleTalk Phase II делает различие между нерасширенными (nоnextended) и расширенными (extended) сетями. В нерасширенных сетях, таких как LocalTalk, номер каждого узла AppleTalk уникален. Нерасширенные сети были единственным типом сети, определенным в AppleTalk Phase I. В расширенных сетях, таких как EtherTalk и TokenTalk, уникальной является комбинация номер каждой сети/номер узла.
Зоны определяются управляющим сети AppleTalk в процессе конфигурации роутера. Каждый узел AppleTalk принадлежит к отдельной конкретной зоне. Расширенные сети могут иметь несколько зон, которые ассоциируются с ними. Узлы в расширенных сетях могут принадлежать к любой отдельной зоне, которая ассоциируется с этой расширенной сетью.
Формат протокольного блока приведен в таблице.
| Протокол сетевого уровня DDP | |||||||||||||||||
| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | ||
| 0 | Время жизни | ст. Длина дейтаграммы мл. | для | ||||||||||||||
| 2 | Контрольная сумма | длин- | |||||||||||||||
| 4 | Номер сети назначения | ного | |||||||||||||||
| 6 | Номер сети источника | заго- | |||||||||||||||
| 8 | Номер узла назначения | Номер узла источника | ловка | ||||||||||||||
| 10 | Номер порта назначения | Номер порта источника | |||||||||||||||
| 12 | Тип протокола | Данные | |||||||||||||||
| 14 | Данные . . . |
Назначение полей.
Поле “тип протокола” определяет тип следующего протокола. Используются следующие значения: 1-RTMP, 2-NBP, 3-ATP, 4-AEP, 5-RTMP, 6-ZIP, 7-ADSP.
Протокол транспортного уровня ATP
| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | |
| 0 | Код функции | XO | EOM | STS | TREQ | Битовая карта транзакции или последовательный номер | ||||||||||
| 2 | Номер транзакции | |||||||||||||||
| 4
6 |
Пользовательские данные | |||||||||||||||
| 8 | Данные |
Назначение полей.
“Код функции”: 01 - запрос, 10 - ответ, 11 - освобождение запроса;
STS - состояние передачи посылки;
EOM- конец сообщения;
XO - признак “точно один” - запрет повторного запроса транзакции.
Транзакция может содержать не более 8 пакетов. В запросе указывается битовая карта, в которой битом помечаются требуемые части транзакции.
Протоколы архитектуры Apple Talk
Apple Talk была разработана как система распределенной сети клиент- сервер. Другими словами, пользователи совместно пользуются сетевыми ресурсами (такими, как файлы и принтеры). Компьютеры, обеспечивающие эти ресурсы, называются служебными устройствами (servers); компьютеры, использующие сетевые ресурсы служебных устройств, называются клиентами (clients). Взаимодействие со служебными устройствами в значительной степени является прозрачным для пользователя, т.к. сам компьютер определяет местоположение запрашиваемого материала и обращается к нему без получения дальнейшей информации от пользователя. В дополнение к простоте использования, распределенные системы также имеют экономические преимущества по сравнению с системами, где все равны, т.к.важные материалы могут быть помещены в нескольких, а не во многих местоположениях.
Apple Talk относительно хорошо согласуется с эталонной моделью OSI. рисунке "Apple Talk и эталонная модель OSI" представлены протоколы Apple Talk, смежные с теми уровнями OSI, с которыми у них установлено соответствие. Этот рисунок отличается от других изображений связи пакета протоколов Apple Talk с моделью OSI тем, что на нем NBP, ZIP и RTMP размещены на Уровне 3, а АЕР-на Уровне 7. По мнению Cisco, NBP, ZIP и RТМP по своим функциональным возможностям стоят в ряду ближе к Уровню 3 модели OSI, хотя они и пользуются услугами DDP, другого протокола Уровня 3. Аналогично, Cisco полагает, что АРЕ следует включить в перечень протоколов прикладного уровня, т.к. он обычно используется для обеспечения функциональных возможностей прикладного уровня. В частности, АЕР помогает определить возможность отдаленных узлов принимать следующие соединения.
Краткое описание протоколов приведено в таблице, а схему взаимодействия протоколов можно найти здесь.
| №
п/п |
Обозна- чение | Название протокола (анг.) | Назначение протокола |
| 1 | AURP | Apple Talk Update-based Routing Protocol | Протокол маршрутизации |
| 2 | RTMP | Routing Table Maintenance Protocol | Протокол обмена маршрутной
информацией |
| 3 | AEP | Apple Talk Echo Protocol | Протокол проверки доступности объектов |
| 4 | ATP | Apple Talk Transaction Protocol | Транспортный протокол, ориентированный на соединение |
| 5 | NBR | Name Binding Protocol | Протокол привязки имен к сетевым адресам |
| 6 | DDP | Datagram Delivery Protocol | Протокол сетевого уровня |
| 7 | Apple
LAP |
Link Access Protocol | |
| 8 | AARP | Apple Talk Adress Resolution Protocol | Протокол привязки адреса сетевой карты к сетевому адресу |
| 9 | AFP | Apple Talk Filing Protocol | |
| 10 | ASP | Apple Talk Sessin Protocol | Протокол сеансового уровня |
| 11 | Postscript | Протокол доступа к принтерам | |
| 12 | PAP | Printer Access Protocol | Протокол доступа к принтерам |
| 13 | ADSP | Apple Talk Data Stream Protocol | Протокол потока данных |
table border="0" cellpadding="0" cellspacing="0" width="100%">
Сетевой уровень (Apple Talk)
В данном разделе описываются концепции, принятые для сетевого уровня AppleTalk, и протоколы для этого уровня. В нем рассматриваются назначение адреса протокола, сетевые об'екты и протоколы AppleTalk, которые обеспечивают функциональные возможности Уровня 3 эталонной модели OSI.
 | Назначения адреса протокол |
| Сетевые объекты |
| Протокол доставки дейтаграмм (DDP) |
| Протокол поддепжки маршрутной таблицы (RTMP) |
Сетевые объекты (Apple Talk)
AppleTalk идентифицирует несколько сетевых об'ектов. Самым простым является узел (node), который является просто любым устройством, соединенным с сетью AppleTalk. Наиболее распространенными узлами являются компьютеры Macintosh и лазерные принтеры, однако многие другие компьютеры также способны осуществлять связь AppleTalk, в том числе компьютеры IBM PC, Digital Equipment Corparation VAX и различные АРМ. Следующим об'ектом, определяемым AppleTalk, является сеть. Сеть AppleTalk представляет собой просто отдельный логический кабель. Хотя этот логический кабель часто является отдельным физическим кабелем, некоторые вычислительные центры используют мосты для об'единения нескольких физических кабелей. И наконец, зона (zone) АppleTalk является логической группой из нескольких сетей (возможно находящихся далеко друг от друга). Объекты AppleTalk изображены на рисунке.
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Транспортный уровень (AppleTalk)
Транспортный уровень AppleTalk реализуется двумя основными протоколами AppleTalk: AppleTalk Transaction Protocol (ATP) (Протокол транзакций AppleTalk) и AppleTalk Data Stream Protocol (ADSP) (Протокол потока данных АppleTalk). АТР является транзакционно-ориентированным, в то время как ADSP является ориентированным по потоку данных.
Уровень прикладного взаимодействия
Уровень прикладного взаимодействия обеспечивает доступ пользователей ИВС к ресурсам коллективного пользования. При этом для большинства архитектур обеспечивается возможность использования коллективных ресурсов других архитектур.
Функционирование данного уровня построено по принципу клиент-сервер. В соответствии с этой технологией пользователь ИВС для доступа к ресурсу использует имеющийся у него объект-клиент (обычно программа), а ресурс коллективного пользования выступает в качестве объекта-сервера. Клиент формирует запросы на выполнение определенных услуг, а сервер обеспечивает его выполнение и посылает ответ. Типы серверов, клиентов и соответствующих им протоколов взаимодействия определяются типами предоставляемых услуг прикладного уровня. Данные услуги можно разделить на две большие группы: локальные и сетевые. Локальные услуги предоставляются каждым сетевым объектом независимо друг от друга. Выделенные ресурсы коллективного пользования в соответствии с локальной услугой, как правило, не взаимодействуют друг с другом (например, услуги доступа к сетевым дискам, сетевым принтерам и др.). Сетевые услуги реализуются с использованием совокупности серверов, взаимодействующих друг с другом. Предоставление сетевых услуг обеспечивается сетевыми службами, наибольшее распространение из которых получили электронная почта, служба обмена новостями, служба обмена гипертекстовыми документами (WWW - World Wide Web) и др. <
ETHER TYPES
ETHER TYPES
Many of the networks of all classes are Ethernets (10Mb) or
Experimental Ethernets (3Mb). These systems use a message "type"
field in much the same way the ARPANET uses the "link" field.
The following list of EtherTypes is contributed unverified information
from various sources.
Assignments:
decimal Hex decimal octal
000 0000-05DC - - IEEE802.3 Length Field [XEROX]
257 0101-01FF - - Experimental [XEROX]
512 0200 512 1000 XEROX PUP (see 0A00) [8,XEROX]
513 0201 - - PUP Addr Trans (see 0A01)[XEROX]
0400 Nixdorf [XEROX]
1536 0600 1536 3000 XEROX NS IDP [133,XEROX]
0660 DLOG [XEROX]
0661 DLOG [XEROX]
2048 0800 513 1001 Internet IP (IPv4) [105,JBP]
2049 0801 - - X.75 Internet [XEROX]
2050 0802 - - NBS Internet [XEROX]
2051 0803 - - ECMA Internet [XEROX]
2052 0804 - - Chaosnet [XEROX]
2053 0805 - - X.25 Level 3 [XEROX]
2054 0806 - - ARP [88,JBP]
2055 0807 - - XNS Compatability [XEROX]
2076 081C - - Symbolics Private [DCP1]
2184 0888-088A - - Xyplex [XEROX]
2304 0900 - - Ungermann-Bass net debugr[XEROX]
2560 0A00 - - Xerox IEEE802.3 PUP [XEROX]
2561 0A01 - - PUP Addr Trans [XEROX]
2989 0BAD - - Banyan Systems [XEROX]
4096 1000 - - Berkeley Trailer nego [XEROX]
4097 1001-100F - - Berkeley Trailer encap/IP[XEROX]
5632 1600 - - Valid Systems [XEROX]
16962 4242 - - PCS Basic Block Protocol [XEROX]
21000 5208 - - BBN Simnet [XEROX]
24576 6000 - - DEC Unassigned (Exp.) [XEROX]
24577 6001 - - DEC MOP Dump/Load [XEROX]
24578 6002 - - DEC MOP Remote Console [XEROX]
24579 6003 - - DEC DECNET Phase IV Route[XEROX]
24580 6004 - - DEC LAT [XEROX]
24581 6005 - - DEC Diagnostic Protocol [XEROX]
24582 6006 - - DEC Customer Protocol [XEROX]
24583 6007 - - DEC LAVC, SCA [XEROX]
24584 6008-6009 - - DEC Unassigned [XEROX]
24586 6010-6014 - - 3Com Corporation [XEROX]
28672 7000 - - Ungermann-Bass download [XEROX]
28674 7002 - - Ungermann-Bass dia/loop [XEROX]
28704 7020-7029 - - LRT [XEROX]
28720 7030 - - Proteon [XEROX]
28724 7034 - - Cabletron [XEROX]
32771 8003 - - Cronus VLN [131,DT15]
32772 8004 - - Cronus Direct [131,DT15]
32773 8005 - - HP Probe [XEROX]
32774 8006 - - Nestar [XEROX]
32776 8008 - - AT&T [XEROX]
32784 8010 - - Excelan [XEROX]
32787 8013 - - SGI diagnostics [AXC]
32788 8014 - - SGI network games [AXC]
32789 8015 - - SGI reserved [AXC]
32790 8016 - - SGI bounce server [AXC]
32793 8019 - - Apollo Computers [XEROX]
32815 802E - - Tymshare [XEROX]
32816 802F - - Tigan, Inc. [XEROX]
32821 8035 - - Reverse ARP [48,JXM]
32822 8036 - - Aeonic Systems [XEROX]
32824 8038 - - DEC LANBridge [XEROX]
32825 8039-803C - - DEC Unassigned [XEROX]
32829 803D - - DEC Ethernet Encryption [XEROX]
32830 803E - - DEC Unassigned [XEROX]
32831 803F - - DEC LAN Traffic Monitor [XEROX]
32832 8040-8042 - - DEC Unassigned [XEROX]
32836 8044 - - Planning Research Corp. [XEROX]
32838 8046 - - AT&T [XEROX]
32839 8047 - - AT&T [XEROX]
32841 8049 - - ExperData [XEROX]
32859 805B - - Stanford V Kernel exp. [XEROX]
32860 805C - - Stanford V Kernel prod. [XEROX]
32861 805D - - Evans & Sutherland [XEROX]
32864 8060 - - Little Machines [XEROX]
32866 8062 - - Counterpoint Computers [XEROX]
32869 8065 - - Univ. of Mass. @ Amherst [XEROX]
32870 8066 - - Univ. of Mass. @ Amherst [XEROX]
32871 8067 - - Veeco Integrated Auto. [XEROX]
32872 8068 - - General Dynamics [XEROX]
32873 8069 - - AT&T [XEROX]
32874 806A - - Autophon [XEROX]
32876 806C - - ComDesign [XEROX]
32877 806D - - Computgraphic Corp. [XEROX]
32878 806E-8077 - - Landmark Graphics Corp. [XEROX]
32890 807A - - Matra [XEROX]
32891 807B - - Dansk Data Elektronik [XEROX]
32892 807C - - Merit Internodal [HWB]
32893 807D-807F - - Vitalink Communications [XEROX]
32896 8080 - - Vitalink TransLAN III [XEROX]
32897 8081-8083 - - Counterpoint Computers [XEROX]
32923 809B - - Appletalk [XEROX]
32924 809C-809E - - Datability [XEROX]
32927 809F - - Spider Systems Ltd. [XEROX]
32931 80A3 - - Nixdorf Computers [XEROX]
32932 80A4-80B3 - - Siemens Gammasonics Inc. [XEROX]
32960 80C0-80C3 - - DCA Data Exchange Cluster[XEROX]
80C4 Banyan Systems [XEROX]
80C5 Banyan Systems [XEROX]
32966 80C6 - - Pacer Software [XEROX]
32967 80C7 - - Applitek Corporation [XEROX]
32968 80C8-80CC - - Intergraph Corporation [XEROX]
32973 80CD-80CE - - Harris Corporation [XEROX]
32975 80CF-80D2 - - Taylor Instrument [XEROX]
32979 80D3-80D4 - - Rosemount Corporation [XEROX]
32981 80D5 - - IBM SNA Service on Ether [XEROX]
32989 80DD - - Varian Associates [XEROX]
32990 80DE-80DF - - Integrated Solutions TRFS[XEROX]
32992 80E0-80E3 - - Allen-Bradley [XEROX]
32996 80E4-80F0 - - Datability [XEROX]
33010 80F2 - - Retix [XEROX]
33011 80F3 - - AppleTalk AARP (Kinetics)[XEROX]
33012 80F4-80F5 - - Kinetics [XEROX]
33015 80F7 - - Apollo Computer [XEROX]
33023 80FF-8103 - - Wellfleet Communications [XEROX]
33031 8107-8109 - - Symbolics Private [XEROX]
33072 8130 - - Hayes Microcomputers [XEROX]
33073 8131 - - VG Laboratory Systems [XEROX]
8132-8136 Bridge Communications [XEROX]
33079 8137-8138 - - Novell, Inc. [XEROX]
33081 8139-813D - - KTI [XEROX]
8148 Logicraft [XEROX]
8149 Network Computing Devices[XEROX]
814A Alpha Micro [XEROX]
33100 814C - - SNMP [JKR1]
814D BIIN [XEROX]
814E BIIN [XEROX]
814F Technically Elite Concept[XEROX]
8150 Rational Corp [XEROX]
8151-8153 Qualcomm [XEROX]
815C-815E Computer Protocol Pty Ltd[XEROX]
8164-8166 Charles River Data System[XEROX]
817D-818C Protocol Engines [XEROX]
818D Motorola Computer [XEROX]
819A-81A3 Qualcomm [XEROX]
81A4 ARAI Bunkichi [XEROX]
81A5-81AE RAD Network Devices [XEROX]
81B7-81B9 Xyplex [XEROX]
81CC-81D5 Apricot Computers [XEROX]
81D6-81DD Artisoft [XEROX]
81E6-81EF Polygon [XEROX]
81F0-81F2 Comsat Labs [XEROX]
81F3-81F5 SAIC [XEROX]
81F6-81F8 VG Analytical [XEROX]
8203-8205 Quantum Software [XEROX]
8221-8222 Ascom Banking Systems [XEROX]
823E-8240 Advanced Encryption Syste[XEROX]
827F-8282 Athena Programming [XEROX]
8263-826A Charles River Data System[XEROX]
829A- 829B Inst Ind Info Tech [XEROX]
829C-82AB Taurus Controls [XEROX]
82AC-8693 Walker Richer & Quinn [XEROX]
8694-869D Idea Courier [XEROX]
869E-86A1 Computer Network Tech [XEROX]
86A3-86AC Gateway Communications [XEROX]
86DB SECTRA [XEROX]
86DE Delta Controls [XEROX]
34543 86DF - - ATOMIC [JBP]
86E0-86EF Landis & Gyr Powers [XEROX]
8700-8710 Motorola [XEROX]
8A96-8A97 Invisible Software [XEROX]
36864 9000 - - Loopback [XEROX]
36865 9001 - - 3Com(Bridge) XNS Sys Mgmt[XEROX]
36866 9002 - - 3Com(Bridge) TCP-IP Sys [XEROX]
36867 9003 - - 3Com(Bridge) loop detect [XEROX]
65280 FF00 - - BBN VITAL-LanBridge cache[XEROX]
FF00-FF0F ISC Bunker Ramo [XEROX]
If you need an Ether Type, contact:
Xerox Systems Institute
3400 Hillview Ave.
PO BOX 10034
Palo Alto, CA 94303
Phone: 415-813-7164
Contact: Fonda Lix Pallone
IP_SLDPRT
Decimal Keyword Protocol References
------- ------- -------- ----------
0 Reserved [JBP]
1 ICMP Internet Control Message [RFC792,JBP]
2 IGMP Internet Group Management [RFC1112,JBP]
3 GGP Gateway-to-Gateway [RFC823,MB]
4 IP IP in IP (encasulation) [JBP]
5 ST Stream [RFC1190,IEN119,JWF]
6 TCP Transmission Control [RFC793,JBP]
7 UCL UCL [PK]
8 EGP Exterior Gateway Protocol [RFC888,DLM1]
9 IGP any private interior gateway [JBP]
10 BBN-RCC-MON BBN RCC Monitoring [SGC]
11 NVP-II Network Voice Protocol [RFC741,SC3]
12 PUP PUP [PUP,XEROX]
13 ARGUS ARGUS [RWS4]
14 EMCON EMCON [BN7]
15 XNET Cross Net Debugger [IEN158,JFH2]
16 CHAOS Chaos [NC3]
17 UDP User Datagram [RFC768,JBP]
18 MUX Multiplexing [IEN90,JBP]
19 DCN-MEAS DCN Measurement Subsystems [DLM1]
20 HMP Host Monitoring [RFC869,RH6]
21 PRM Packet Radio Measurement [ZSU]
22 XNS-IDP XEROX NS IDP [ETHERNET,XEROX]
23 TRUNK-1 Trunk-1 [BWB6]
24 TRUNK-2 Trunk-2 [BWB6]
25 LEAF-1 Leaf-1 [BWB6]
26 LEAF-2 Leaf-2 [BWB6]
27 RDP Reliable Data Protocol [RFC908,RH6]
28 IRTP Internet Reliable Transaction [RFC938,TXM]
29 ISO-TP4 ISO Transport Protocol Class 4 [RFC905,RC77]
30 NETBLT Bulk Data Transfer Protocol [RFC969,DDC1]
31 MFE-NSP MFE Network Services Protocol [MFENET,BCH2]
32 MERIT-INP MERIT Internodal Protocol [HWB]
33 SEP Sequential Exchange Protocol [JC120]
34 3PC Third Party Connect Protocol [SAF3]
35 IDPR Inter-Domain Policy Routing Protocol [MXS1]
36 XTP XTP [GXC]
37 DDP Datagram Delivery Protocol [WXC]
38 IDPR-CMTP IDPR Control Message Transport Proto [MXS1]
39 TP++ TP++ Transport Protocol [DXF]
40 IL IL Transport Protocol [DXP2]
41 SIP Simple Internet Protocol [SXD]
42 SDRP Source Demand Routing Protocol [DXE1]
43 SIP-SR SIP Source Route [SXD]
44 SIP-FRAG SIP Fragment [SXD]
45 IDRP Inter-Domain Routing Protocol [Sue Hares]
46 RSVP Reservation Protocol [Bob Braden]
47 GRE General Routing Encapsulation [Tony Li]
48 MHRP Mobile Host Routing Protocol[David Johnson]
49 BNA BNA [Gary Salamon]
50 SIPP- ESP SIPP Encap Security Payload [Steve Deering]
51 SIPP-AH SIPP Authentication Header [Steve Deering]
52 I-NLSP Integrated Net Layer Security TUBA [GLENN]
53 SWIPE IP with Encryption [JI6]
54 NHRP NBMA Next Hop Resolution Protocol
55-60 Unassigned [JBP]
61 any host internal protocol [JBP]
62 CFTP CFTP [CFTP,HCF2]
63 any local network [JBP]
64 SAT-EXPAK SATNET and Backroom EXPAK [SHB]
65 KRYPTOLAN Kryptolan [PXL1]
66 RVD MIT Remote Virtual Disk Protocol [MBG]
67 IPPC Internet Pluribus Packet Core [SHB]
68 any distributed file system [JBP]
69 SAT-MON SATNET Monitoring [SHB]
70 VISA VISA Protocol [GXT1]
71 IPCV Internet Packet Core Utility [SHB]
72 CPNX Computer Protocol Network Executive [DXM2]
73 CPHB Computer Protocol Heart Beat [DXM2]
74 WSN Wang Span Network [VXD]
75 PVP Packet Video Protocol [SC3]
76 BR-SAT-MON Backroom SATNET Monitoring [SHB]
77 SUN-ND SUN ND PROTOCOL-Temporary [WM3]
78 WB-MON WIDEBAND Monitoring [SHB]
79 WB-EXPAK WIDEBAND EXPAK [SHB]
80 ISO-IP ISO Internet Protocol [MTR]
81 VMTP VMTP [DRC3]
82 SECURE-VMTP SECURE-VMTP [DRC3]
83 VINES VINES [BXH]
84 TTP TTP [JXS]
85 NSFNET-IGP NSFNET-IGP [HWB]
86 DGP Dissimilar Gateway Protocol [DGP,ML109]
87 TCF TCF [GAL5]
88 IGRP IGRP [CISCO,GXS]
89 OSPFIGP OSPFIGP [RFC1583,JTM4]
90 Sprite-RPC Sprite RPC Protocol [SPRITE,BXW]
91 LARP Locus Address Resolution Protocol [BXH]
92 MTP Multicast Transport Protocol [SXA]
93 AX.25 AX.25 Frames [BK29]
94 IPIP IP-within-IP Encapsulation Protocol [JI6]
95 MICP Mobile Internetworking Control Pro. [JI6]
96 SCC-SP Semaphore Communications Sec. Pro. [HXH]
97 ETHERIP Ethernet-within-IP Encapsulation [RXH1]
98 ENCAP Encapsulation Header [RFC1241,RXB3]
99 any private encryption scheme [JBP]
100 GMTP GMTP [RXB5]
101-254 Unassigned [JBP]
255 Reserved [JBP]
REFERENCES
[CFTP] Forsdick, H., "CFTP", Network Message, Bolt Beranek and
Newman, January 1982.
[CISCO] Cisco Systems, "Gateway Server Reference Manual", Manual
Revision B, January 10, 1988.
[DDN] Feinler, E., Editor, "DDN Protocol Handbook", Network
Information Center, SRI International, December 1985.
[DGP] M/A-COM Government Systems, "Dissimilar Gateway Protocol
Specification, Draft Version", Contract no. CS901145,
November 16, 1987.
[ETHERNET] "The Ethernet, A Local Area Network: Data Link Layer and
Physical Layer Specification", AA-K759B-TK, Digital
Equipment Corporation, Maynard, MA. Also as: "The
Ethernet - A Local Area Network", Version 1.0, Digital
Equipment Corporation, Intel Corporation, Xerox
Corporation, September 1980. And: "The Ethernet, A Local
Area Network: Data Link Layer and Physical Layer
Specifications", Digital, Intel and Xerox, November 1982.
And: XEROX, "The Ethernet, A Local Area Network: Data Link
Layer and Physical Layer Specification", X3T51/80-50,
Xerox Corporation, Stamford, CT., October 1980.
[IEN90] Cohen, D. and J. Postel, "Multiplexing Protocol", IEN 90,
USC/Information Sciences Institute, May 1979.
[IEN119] Forgie, J., "ST - A Proposed Internet Stream Protocol",
IEN 119, MIT Lincoln Laboratory, September 1979.
[IEN158] Haverty, J., "XNET Formats for Internet Protocol Version 4",
IEN 158, October 1980.
[MFENET] Shuttleworth, B., "A Documentary of MFENet, a National
Computer Network", UCRL-52317, Lawrence Livermore Labs,
Livermore, California, June 1977.
[PUP] Boggs, D., J. Shoch, E. Taft, and R. Metcalfe, "PUP: An
Internetwork Architecture", XEROX Palo Alto Research Center,
CSL-79-10, July 1979; also in IEEE Transactions on
Communication, Volume COM-28, Number 4, April 1980.
[SPRITE] Welch, B., "The Sprite Remote Procedure Call System",
Technical Report, UCB/Computer Science Dept., 86/302,
University of California at Berkeley, June 1986.
[RFC741] Cohen, D., "Specifications for the Network Voice Protocol",
RFC 741, ISI/RR 7539, USC/Information Sciences Institute,
March 1976.
[RFC768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
USC/Information Sciences Institute, August 1980.
[RFC791] Postel, J., "Internet Protocol - DARPA Internet Program
Protocol Specification", STD 5, RFC 791, DARPA, September
1981.
[RFC792] Postel, J., "Internet Control Message Protocol - DARPA
Internet Program Protocol Specification", STD 5, RFC 792,
USC/Information Sciences Institute, September 1981.
[RFC793] Postel, J., "Transmission Control Protocol - DARPA
Internet Program Protocol Specification", STD 7, RFC 793,
USC/Information Sciences Institute, September 1981.
[RFC823] Hinden, R., and A. Sheltzer, "The DARPA Internet Gateway",
RFC 823, BBN, September 1982.
[RFC869] Hinden, R., "A Host Monitoring Protocol", RFC 869,
Bolt Beranek and Newman, December 1983.
[RFC888] Seamonson, L., and E. Rosen, "STUB" Exterior Gateway
Protocol", RFC 888, BBN Communications Corporation,
January 1984.
[RFC905] International Standards Organization, "ISO Transport Protocol
Specification - ISO DP 8073", RFC 905, April 1984.
[RFC908] Velten, D., R. Hinden, and J. Sax, "Reliable Data Protocol",
RFC 908, BBN Communications Corporation, July 1984.
[RFC938] Miller, T., "Internet Reliable Transaction Protocol", RFC 938,
ACC, February 1985.
[RFC969] Clark, D., M. Lambert, and L. Zhang, "NETBLT: A Bulk Data
Transfer Protocol", RFC 969, MIT Laboratory for Computer
Science, December 1985.
[RFC1112] Deering, S., "Host Extensions for IP Multicasting",
STD 5, RFC 1112, Stanford University, August 1989.
[RFC1190] Topolcic, C., Editor, "Experimental Internet Stream
Protocol, Version 2 (ST-II)", RFC 1190, CIP Working Group,
October 1990.
[RFC1241] Woodburn, W., and D. Mills, " A Scheme for an Internet
Encapsulation Protocol: Version 1", RFC 1241, SAIC,
University of Delaware, July 1991.
[RFC1583] Moy, J., "The OSPF Specification", RFC 1583, Proteon,
March 1994.
PEOPLE
[BCH2] Barry Howard
[BK29] Brian Kantor
[BN7]
[BWB6] Barry Boehm
[BXH] Brian Horn <---none--->
[BXW] Bruce Willins <---none--->
[DDC1] David Clark
[DLM1] David Mills
[DRC3] Dave Cheriton
[DXE1] Deborah Estrin
[DXF] Dirk Fromhein
[DXM2] David Mittnacht <---none--->
[DXP2] Dave Presotto
[David Johnson]
[GAL5] Guillermo A. Loyola
[GLENN] K. Robert Glenn
[GXC] Greg Chesson
[GXS] Guenther Schreiner
[GXT1] Gene Tsudik
[HCF2] Harry Forsdick
[HWB] Hans-Werner Braun
[HXH] Howard Hart
[JBP] Jon Postel
[JC120]
[JFH2] Jack Haverty
[JI6] John Ioannidis
[JTM4] John Moy
[JWF] Jim Forgie
[JXS] Jim Stevens
[KATZ] Dave Katz
[MB] Mike Brescia
[MBG] Michael Greenwald
[ML109] Mike Little
[MTR] Marshall T. Rose
[MXS1] Martha Steenstrup
[NC3] J. Noel Chiappa
[PK] Peter Kirstein
[PXL1] Paul Liu <---none--->
[RH6] Robert Hinden
[RTB3] Bob Braden
[RC77]
[RWS4] Robert W. Scheifler
[RXB3] Robert Woodburn
[RXH1] Russ Housley
[SAF3] Stuart A. Friedberg
[SC3] Steve Casner
[SGC] Steve Chipman
[SHB] Steven Blumenthal
[ Sue Hares] Sue Hares
[SXA] Susie Armstrong
[SXD] Steve Deering
[Tony Li] Tony Li
[TXM] Trudy Miller
[VXD] Victor Dafoulas <---none--->
[WM3] William Melohn
[WXC] Wesley Craig
[ZSU] Zaw-Sing Su
[]
URL = ftp://ftp.isi.edu/in-notes/iana/assignments/protocol-numbers
Request for Comments: 1065 K.
Network Working Group M. Rose
Request for Comments: 1065 K. McCloghrie
TWG
August 1988
Structure and Identification of Management Information
for TCP/IP-based internets
Table of Contents
1. Status of this Memo ............................................. 1
2. Introduction .................................................... 2
3. Structure and Identification of Management Information........... 4
3.1 Names .......................................................... 4
3.1.1 DIRECTORY .................................................... 5
3.1.2 MGMT ......................................................... 6
3.1.3 EXPERIMENTAL ................................................. 6
3.1.4 PRIVATE ...................................................... 7
3.2 Syntax ......................................................... 7
3.2.1 Primitive Types .............................................. 7
3.2.1.1 Guidelines for Enumerated INTEGERs ......................... 7
3.2.2 Constructor Types ............................................ 8
3.2.3 Defined Types ................................................ 8
3.2.3.1 NetworkAddress ............................................. 8
3.2.3.2 IpAddress .................................................. 8
3.2.3.3 Counter .................................................... 8
3.2.3.4 Gauge ...................................................... 9
3.2.3.5 TimeTicks .................................................. 9
3.2.3.6 Opaque ..................................................... 9
3.3 Encodings ...................................................... 9
4. Managed Objects ................................................. 10
4.1 Guidelines for Object Names .................................... 10
4.2 Object Types and Instances ..................................... 10
4.3 Macros for Managed Objects ..................................... 14
5. Extensions to the MIB ........................................... 16
6.
Definitions ..................................................... 17
7. Acknowledgements ................................................ 20
8. References ...................................................... 21
1. Status of this Memo
This memo provides the common definitions for the structure and
identification of management information for TCP/IP-based internets.
In particular, together with its companion memos which describe the
initial management information base along with the initial network
management protocol, these documents provide a simple, workable
Rose & McCloghrie [Page 1]
RFC 1065 SMI August 1988
architecture and system for managing TCP/IP-based internets and in
particular, the Internet.
This memo specifies a draft standard for the Internet community.
TCP/IP implementations in the Internet which are network manageable
are expected to adopt and implement this specification.
Distribution of this memo is unlimited.
2. Introduction
This memo describes the common structures and identification scheme
for the definition of management information used in managing
TCP/IP-based internets. Included are descriptions of an object
information model for network management along with a set of generic
types used to describe management information. Formal descriptions
of the structure are given using Abstract Syntax Notation One (ASN.1)
[1].
This memo is largely concerned with organizational concerns and
administrative policy: it neither specifies the objects which are
managed, nor the protocols used to manage those objects. These
concerns are addressed by two companion memos: one describing the
Management Information Base (MIB) [2], and the other describing the
Simple Network Management Protocol (SNMP) [3].
This memo is based in part on the work of the Internet Engineering
Task Force, particularly the working note titled "Structure and
Identification of Management Information for the Internet" [4]. This
memo uses a skeletal structure derived from that note, but differs in
one very significant way: that note focuses entirely on the use of
OSI-style network management. As such, it is not suitable for use in
the short-term for which a non-OSI protocol, the SNMP, has been
designated as the standard.
This memo attempts to achieve two goals: simplicity and
extensibility. Both are motivated by a common concern: although the
management of TCP/IP-based internets has been a topic of study for
some time, the authors do not feel that the depth and breadth of such
understanding is complete. More bluntly, we feel that previous
experiences, while giving the community insight, are hardly
conclusive. By fostering a simple SMI, the minimal number of
constraints are imposed on future potential approaches; further, by
fostering an extensible SMI, the maximal number of potential
approaches are available for experimentation.
It is believed that this memo and its two companions comply with the
guidelines set forth in RFC 1052, "IAB Recommendations for the
Rose & McCloghrie [Page 2]
RFC 1065 SMI August 1988
Development of Internet Network Management Standards" [5]. In
particular, we feel that this memo, along with the memo describing
the initial management information base, provide a solid basis for
network management of the Internet.
Rose & McCloghrie [Page 3]
RFC 1065 SMI August 1988
3. Structure and Identification of Management Information
Managed objects are accessed via a virtual information store, termed
the Management Information Base or MIB. Objects in the MIB are
defined using Abstract Syntax Notation One (ASN.1) [1].
Each type of object (termed an object type) has a name, a syntax, and
an encoding. The name is represented uniquely as an OBJECT
IDENTIFIER. An OBJECT IDENTIFIER is an administratively assigned
name. The administrative policies used for assigning names are
discussed later in this memo.
The syntax for an object type defines the abstract data structure
corresponding to that object type.
For example, the structure of a
given object type might be an INTEGER or OCTET STRING. Although in
general, we should permit any ASN.1 construct to be available for use
in defining the syntax of an object type, this memo purposely
restricts the ASN.1 constructs which may be used. These restrictions
are made solely for the sake of simplicity.
The encoding of an object type is simply how instances of that object
type are represented using the object's type syntax. Implicitly tied
to the notion of an object's syntax and encoding is how the object is
represented when being transmitted on the network. This memo
specifies the use of the basic encoding rules of ASN.1 [6].
It is beyond the scope of this memo to define either the initial MIB
used for network management or the network management protocol. As
mentioned earlier, these tasks are left to the companion memos. This
memo attempts to minimize the restrictions placed upon its companions
so as to maximize generality. However, in some cases, restrictions
have been made (e.g., the syntax which may be used when defining
object types in the MIB) in order to encourage a particular style of
management. Future editions of this memo may remove these
restrictions.
3.1. Names
Names are used to identify managed objects. This memo specifies
names which are hierarchical in nature. The OBJECT IDENTIFIER
concept is used to model this notion. An OBJECT IDENTIFIER can be
used for purposes other than naming managed object types; for
example, each international standard has an OBJECT IDENTIFIER
assigned to it for the purposes of identification. In short, OBJECT
IDENTIFIERs are a means for identifying some object, regardless of
the semantics associated with the object (e.g., a network object, a
standards document, etc.)
Rose & McCloghrie [Page 4]
RFC 1065 SMI August 1988
An OBJECT IDENTIFIER is a sequence of integers which traverse a
global tree. The tree consists of a root connected to a number of
labeled nodes via edges.
Each node may, in turn, have children of
its own which are labeled. In this case, we may term the node a
subtree. This process may continue to an arbitrary level of depth.
Central to the notion of the OBJECT IDENTIFIER is the understanding
that administrative control of the meanings assigned to the nodes may
be delegated as one traverses the tree. A label is a pairing of a
brief textual description and an integer.
The root node itself is unlabeled, but has at least three children
directly under it: one node is administered by the International
Standards Organization, with label iso(1); another is administrated
by the International Telegraph and Telephone Consultative Committee,
with label ccitt(2); and the third is jointly administered by the ISO
and the CCITT, joint-iso-ccitt(3).
Under the iso(1) node, the ISO has designated one subtree for use by
other (inter)national organizations, org(3). Of the children nodes
present, two have been assigned to the U.S. National Bureau of
Standards. One of these subtrees has been transferred by the NBS to
the U.S. Department of Defense, dod(6).
As of this writing, the DoD has not indicated how it will manage its
subtree of OBJECT IDENTIFIERs. This memo assumes that DoD will
allocate a node to the Internet community, to be administered by the
Internet Activities Board (IAB) as follows:
internet OBJECT IDENTIFIER ::= { iso org(3) dod(6) 1 }
That is, the Internet subtree of OBJECT IDENTIFIERs starts with the
prefix:
1.3.6.1.
This memo, as an RFC approved by the IAB, now specifies the policy
under which this subtree of OBJECT IDENTIFIERs is administered.
Initially, four nodes are present:
directory OBJECT IDENTIFIER ::= { internet 1 }
mgmt OBJECT IDENTIFIER ::= { internet 2 }
experimental OBJECT IDENTIFIER ::= { internet 3 }
private OBJECT IDENTIFIER ::= { internet 4 }
3.1.1. DIRECTORY
The directory(1) subtree is reserved for use with a future memo that
discusses how the OSI Directory may be used in the Internet.
Rose & McCloghrie [Page 5]
RFC 1065 SMI August 1988
3.1.2. MGMT
The mgmt(2) subtree is used to identify objects which are defined in
IAB-approved documents. Administration of the mgmt(2) subtree is
delegated by the IAB to the Assigned Numbers authority for the
Internet. As RFCs which define new versions of the Internet-standard
Management Information Base are approved, they are assigned an OBJECT
IDENTIFIER by the Assigned Numbers authority for identifying the
objects defined by that memo.
For example, the RFC which defines the initial Internet standard MIB
would be assigned management document number 1. This RFC would use
the OBJECT IDENTIFIER
{ mgmt 1 }
or
1.3.6.1.2.1
in defining the Internet-standard MIB.
The generation of new versions of the Internet-standard MIB is a
rigorous process. Section 5 of this memo describes the rules used
when a new version is defined.
3.1.3. EXPERIMENTAL
The experimental(3) subtree is used to identify objects used in
Internet experiments. Administration of the experimental(3) subtree
is delegated by the IAB to the Assigned Numbers authority of the
Internet.
For example, an experimenter might received number 17, and would have
available the OBJECT IDENTIFIER
{ experimental 17 }
or
1.3.6.1.3.17
for use.
As a part of the assignment process, the Assigned Numbers authority
may make requirements as to how that subtree is used.
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RFC 1065 SMI August 1988
3.1.4. PRIVATE
The private(4) subtree is used to identify objects defined
unilaterally. Administration of the private(4) subtree is delegated
by the IAB to the Assigned Numbers authority for the Internet.
Initially, this subtree has at least one child:
enterprises OBJECT IDENTIFIER ::= { private 1 }
The enterprises(1) subtree is used, among other things, to permit
parties providing networking subsystems to register models of their
products.
Upon receiving a subtree, the enterprise may, for example, define new
MIB objects in this subtree.
In addition, it is strongly recommended
that the enterprise will also register its networking subsystems
under this subtree, in order to provide an unambiguous identification
mechanism for use in management protocols. For example, if the
"Flintstones, Inc." enterprise produced networking subsystems, then
they could request a node under the enterprises subtree from the
Assigned Numbers authority. Such a node might be numbered:
1.3.6.1.4.1.42
The "Flintstones, Inc." enterprise might then register their "Fred
Router" under the name of:
1.3.6.1.4.1.42.1.1
3.2. Syntax
Syntax is used to define the structure corresponding to object types.
ASN.1 constructs are used to define this structure, although the full
generality of ASN.1 is not permitted.
The ASN.1 type ObjectSyntax defines the different syntaxes which may
be used in defining an object type.
3.2.1. Primitive Types
Only the ASN.1 primitive types INTEGER, OCTET STRING, OBJECT
IDENTIFIER, and NULL are permitted. These are sometimes referred to
as non-aggregate types.
3.2.1.1. Guidelines for Enumerated INTEGERs
If an enumerated INTEGER is listed as an object type, then a named-
number having the value 0 shall not be present in the list of
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RFC 1065 SMI August 1988
enumerations. Use of this value is prohibited.
3.2.2. Constructor Types
The ASN.1 constructor type SEQUENCE is permitted, providing that it
is used to generate either lists or tables.
For lists, the syntax takes the form:
SEQUENCE { , ..., }
where each resolves to one of the ASN.1 primitive types listed
above. Further, these ASN.1 types are always present (the DEFAULT
and OPTIONAL clauses do not appear in the SEQUENCE definition).
For tables, the syntax takes the form:
SEQUENCE OF
where resolves to a list constructor.
Lists and tables are sometimes referred to as aggregate types.
3.2.3. Defined Types
In addition, new application-wide types may be defined, so long as
they resolve into an IMPLICITly defined ASN.1 primitive type, list,
table, or some other application-wide type. Initially, few
application-wide types are defined. Future memos will no doubt
define others once a consensus is reached.
3.2.3.1. NetworkAddress
This CHOICE represents an address from one of possibly several
protocol families. Currently, only one protocol family, the Internet
family, is present in this CHOICE.
3.2.3.2. IpAddress
This application-wide type represents a 32-bit internet address. It
is represented as an OCTET STRING of length 4, in network byte-order.
When this ASN.1 type is encoded using the ASN.1 basic encoding rules,
only the primitive encoding form shall be used.
3.2.3.3. Counter
This application-wide type represents a non-negative integer which
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RFC 1065 SMI August 1988
monotonically increases until it reaches a maximum value, when it
wraps around and starts increasing again from zero. This memo
specifies a maximum value of 2^32-1 (4294967295 decimal) for
counters.
3.2.3.4. Gauge
This application-wide type represents a non-negative integer, which
may increase or decrease, but which latches at a maximum value. This
memo specifies a maximum value of 2^32-1 (4294967295 decimal) for
gauges.
3.2.3.5. TimeTicks
This application-wide type represents a non-negative integer which
counts the time in hundredths of a second since some epoch. When
object types are defined in the MIB which use this ASN.1 type, the
description of the object type identifies the reference epoch.
3.2.3.6. Opaque
This application-wide type supports the capability to pass arbitrary
ASN.1 syntax. A value is encoded using the ASN.1 basic rules into a
string of octets. This, in turn, is encoded as an OCTET STRING, in
effect "double-wrapping" the original ASN.1 value.
Note that a conforming implementation need only be able to accept and
recognize opaquely-encoded data. It need not be able to unwrap the
data and then interpret its contents.
Further note that by use of the ASN.1 EXTERNAL type, encodings other
than ASN.1 may be used in opaquely-encoded data.
3.3. Encodings
Once an instance of an object type has been identified, its value may
be transmitted by applying the basic encoding rules of ASN.1 to the
syntax for the object type.
Rose & McCloghrie [Page 9]
RFC 1065 SMI August 1988
4. Managed Objects
Although it is not the purpose of this memo to define objects in the
MIB, this memo specifies a format to be used by other memos which
define these objects.
An object type definition consists of five fields:
OBJECT:
-------
A textual name, termed the OBJECT DESCRIPTOR, for the object type,
along with its corresponding OBJECT IDENTIFIER.
Syntax:
The abstract syntax for the object type. This must resolve to an
instance of the ASN.1 type ObjectSyntax (defined below).
Definition:
A textual description of the semantics of the object type.
Implementations should ensure that their instance of the object
fulfills this definition since this MIB is intended for use in
multi-vendor environments. As such it is vital that objects have
consistent meaning across all machines.
Access:
One of read-only, read-write, write-only, or not-accessible.
Status:
One of mandatory, optional, or obsolete.
Future memos may also specify other fields for the objects which they
define.
4.1. Guidelines for Object Names
No object type in the Internet-Standard MIB shall use a sub-
identifier of 0 in its name. This value is reserved for use with
future extensions.
Each OBJECT DESCRIPTOR corresponding to an object type in the
internet-standard MIB shall be a unique, but mnemonic, printable
string. This promotes a common language for humans to use when
discussing the MIB and also facilitates simple table mappings for
user interfaces.
4.2. Object Types and Instances
An object type is a definition of a kind of managed object; it is
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RFC 1065 SMI August 1988
declarative in nature. In contrast, an object instance is an
instantiation of an object type which has been bound to a value. For
example, the notion of an entry in a routing table might be defined
in the MIB. Such a notion corresponds to an object type; individual
entries in a particular routing table which exist at some time are
object instances of that object type.
A collection of object types is defined in the MIB. Each such
subject type is uniquely named by its OBJECT IDENTIFIER and also has
a textual name, which is its OBJECT DESCRIPTOR. The means whereby
object instances are referenced is not defined in the MIB. Reference
to object instances is achieved by a protocol-specific mechanism: it
is the responsibility of each management protocol adhering to the SMI
to define this mechanism.
An object type may be defined in the MIB such that an instance of
that object type represents an aggregation of information also
represented by instances of some number of "subordinate" object
types. For example, suppose the following object types are defined
in the MIB:
OBJECT:
-------
atIndex { atEntry 1 }
Syntax:
INTEGER
Definition:
The interface number for the physical address.
Access:
read-write.
Status:
mandatory.
OBJECT:
-------
atPhysAddress { atEntry 2 }
Syntax:
OCTET STRING
Definition:
The media-dependent physical address.
Rose & McCloghrie [Page 11]
RFC 1065 SMI August 1988
Access:
read-write.
Status:
mandatory.
OBJECT:
-------
atNetAddress { atEntry 3 }
Syntax:
NetworkAddress
Definition:
The network address corresponding to the media-dependent physical
address.
Access:
read-write.
Status:
mandatory.
Then, a fourth object type might also be defined in the MIB:
OBJECT:
-------
atEntry { atTable 1 }
Syntax:
AtEntry ::= SEQUENCE {
atIndex
INTEGER,
atPhysAddress
OCTET STRING,
atNetAddress
NetworkAddress
}
Definition:
An entry in the address translation table.
Access:
read-write.
Rose & McCloghrie [Page 12]
RFC 1065 SMI August 1988
Status:
mandatory.
Each instance of this object type comprises information represented
by instances of the former three object types. An object type
defined in this way is called a list.
Similarly, tables can be formed by aggregations of a list type. For
example, a fifth object type might also be defined in the MIB:
OBJECT:
------
atTable { at 1 }
Syntax:
SEQUENCE OF AtEntry
Definition:
The address translation table.
Access:
read-write.
Status:
mandatory.
such that each instance of the atTable object comprises information
represented by the set of atEntry object types that collectively
constitute a given atTable object instance, that is, a given address
translation table.
Consider how one might refer to a simple object within a table.
Continuing with the previous example, one might name the object type
{ atPhysAddress }
and specify, using a protocol-specific mechanism, the object instance
{ atNetAddress } = { internet "10.0.0.52" }
This pairing of object type and object instance would refer to all
instances of atPhysAddress which are part of any entry in some
address translation table for which the associated atNetAddress value
is { internet "10.0.0.52" }.
To continue with this example, consider how one might refer to an
aggregate object (list) within a table. Naming the object type
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RFC 1065 SMI August 1988
{ atEntry }
and specifying, using a protocol-specific mechanism, the object
instance
{ atNetAddress } = { internet "10.0.0.52" }
refers to all instances of entries in the table for which the
associated atNetAddress value is { internet "10.0.0.52" }.
Each management protocol must provide a mechanism for accessing
simple (non-aggregate) object types. Each management protocol
specifies whether or not it supports access to aggregate object
types. Further, the protocol must specify which instances are
"returned" when an object type/ instance pairing refers to more than
one instance of a type.
To afford support for a variety of management protocols, all
information by which instances of a given object type may be usefully
distinguished, one from another, is represented by instances of
object types defined in the MIB.
4.3. Macros for Managed Objects
In order to facilitate the use of tools for processing the definition
of the MIB, the OBJECT-TYPE macro may be used. This macro permits
the key aspects of an object type to be represented in a formal way.
OBJECT-TYPE MACRO ::=
BEGIN
TYPE NOTATION ::= "SYNTAX" type (TYPE ObjectSyntax)
"ACCESS" Access
"STATUS" Status
VALUE NOTATION ::= value (VALUE ObjectName)
Access ::= "read-only"
| "read-write"
| "write-only"
| "not-accessible"
Status ::= "mandatory"
| "optional"
| "obsolete"
END
Given the object types defined earlier, we might imagine the
following definitions being present in the MIB:
atIndex OBJECT-TYPE
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RFC 1065 SMI August 1988
SYNTAX INTEGER
ACCESS read-write
STATUS mandatory
::= { atEntry 1 }
atPhysAddress OBJECT-TYPE
SYNTAX OCTET STRING
ACCESS read-write
STATUS mandatory
::= { atEntry 2 }
atNetAddress OBJECT-TYPE
SYNTAX NetworkAddress
ACCESS read-write
STATUS mandatory
::= { atEntry 3 }
atEntry OBJECT-TYPE
SYNTAX AtEntry
ACCESS read-write
STATUS mandatory
::= { atTable 1 }
atTable OBJECT-TYPE
SYNTAX SEQUENCE OF AtEntry
ACCESS read-write
STATUS mandatory
::= { at 1 }
AtEntry ::= SEQUENCE {
atIndex
INTEGER,
atPhysAddress
OCTET STRING,
atNetAddress
NetworkAddress
}
The first five definitions describe object types, relating, for
example, the OBJECT DESCRIPTOR atIndex to the OBJECT IDENTIFIER {
atEntry 1 }. In addition, the syntax of this object is defined
(INTEGER) along with the access permitted (read-write) and status
(mandatory). The sixth definition describes an ASN.1 type called
AtEntry.
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RFC 1065 SMI August 1988
5. Extensions to the MIB
Every Internet-standard MIB document obsoletes all previous such
documents. The portion of a name, termed the tail, following the
OBJECT IDENTIFIER
{ mgmt version-number }
used to name objects shall remain unchanged between versions. New
versions may:
(1) declare old object types obsolete (if necessary), but not
delete their names;
(2) augment the definition of an object type corresponding to a
list by appending non-aggregate object types to the object types
in the list; or,
(3) define entirely new object types.
New versions may not:
(1) change the semantics of any previously defined object without
changing the name of that object.
These rules are important because they admit easier support for
multiple versions of the Internet-standard MIB. In particular, the
semantics associated with the tail of a name remain constant
throughout different versions of the MIB. Because multiple versions
of the MIB may thus coincide in "tail-space," implementations
supporting multiple versions of the MIB can be vastly simplified.
However, as a consequence, a management agent might return an
instance corresponding to a superset of the expected object type.
Following the principle of robustness, in this exceptional case, a
manager should ignore any additional information beyond the
definition of the expected object type. However, the robustness
principle requires that one exercise care with respect to control
actions: if an instance does not have the same syntax as its expected
object type, then those control actions must fail. In both the
monitoring and control cases, the name of an object returned by an
operation must be identical to the name requested by an operation.
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RFC 1065 SMI August 1988
6. Definitions
RFC1065-SMI DEFINITIONS ::= BEGIN
EXPORTS -- EVERYTHING
internet, directory, mgmt,
experimental, private, enterprises,
OBJECT-TYPE, ObjectName, ObjectSyntax, SimpleSyntax,
ApplicationSyntax, NetworkAddress, IpAddress,
Counter, Gauge, TimeTicks, Opaque;
-- the path to the root
internet OBJECT IDENTIFIER ::= { iso org(3) dod(6) 1 }
directory OBJECT IDENTIFIER ::= { internet 1 }
mgmt OBJECT IDENTIFIER ::= { internet 2 }
experimental OBJECT IDENTIFIER ::= { internet 3 }
private OBJECT IDENTIFIER ::= { internet 4 }
enterprises OBJECT IDENTIFIER ::= { private 1 }
-- definition of object types
OBJECT-TYPE MACRO ::=
BEGIN
TYPE NOTATION ::= "SYNTAX" type (TYPE ObjectSyntax)
"ACCESS" Access
"STATUS" Status
VALUE NOTATION ::= value (VALUE ObjectName)
Access ::= "read-only"
| "read-write"
| "write-only"
| "not-accessible"
Status ::= "mandatory"
| "optional"
| "obsolete"
END
-- names of objects in the MIB
ObjectName ::=
OBJECT IDENTIFIER
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RFC 1065 SMI August 1988
-- syntax of objects in the MIB
ObjectSyntax ::=
CHOICE {
simple
SimpleSyntax,
-- note that simple SEQUENCEs are not directly
-- mentioned here to keep things simple (i.e.,
-- prevent mis-use). However, application-wide
-- types which are IMPLICITly encoded simple
-- SEQUENCEs may appear in the following CHOICE
application-wide
ApplicationSyntax
}
SimpleSyntax ::=
CHOICE {
number
INTEGER,
string
OCTET STRING,
object
OBJECT IDENTIFIER,
empty
NULL
}
ApplicationSyntax ::=
CHOICE {
address
NetworkAddress,
counter
Counter,
gauge
Gauge,
ticks
TimeTicks,
arbitrary
Opaque
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RFC 1065 SMI August 1988
-- other application-wide types, as they are
-- defined, will be added here
}
-- application-wide types
NetworkAddress ::=
CHOICE {
internet
IpAddress
}
IpAddress ::=
[APPLICATION 0] -- in network-byte order
IMPLICIT OCTET STRING (SIZE (4))
Counter ::=
[APPLICATION 1]
IMPLICIT INTEGER (0..4294967295)
Gauge ::=
[APPLICATION 2]
IMPLICIT INTEGER (0..4294967295)
TimeTicks ::=
[APPLICATION 3]
IMPLICIT INTEGER
Opaque ::=
[APPLICATION 4] -- arbitrary ASN.1 value,
IMPLICIT OCTET STRING -- "double-wrapped"
END
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RFC 1065 SMI August 1988
7. Acknowledgements
This memo was influenced by three sets of contributors:
First, Lee Labarre of the MITRE Corporation, who as author of the
NETMAN SMI [4], presented the basic roadmap for the SMI.
Second, several individuals who provided valuable comments on this
memo prior to its initial distribution:
James Davin, Proteon
Mark S. Fedor, NYSERNet
Craig Partridge, BBN Laboratories
Martin Lee Schoffstall, Rensselaer Polytechnic Institute
Wengyik Yeong, NYSERNet
Third, the IETF MIB working group:
Karl Auerbach, Epilogue Technology
K. Ramesh Babu, Excelan
Lawrence Besaw, Hewlett-Packard
Jeffrey D. Case, University of Tennessee at Knoxville
James R. Davin, Proteon
Mark S. Fedor, NYSERNet
Robb Foster, BBN
Phill Gross, The MITRE Corporation
Bent Torp Jensen, Convergent Technology
Lee Labarre, The MITRE Corporation
Dan Lynch, Advanced Computing Environments
Keith McCloghrie, The Wollongong Group
Dave Mackie, 3Com/Bridge
Craig Partridge, BBN (chair)
Jim Robertson, 3Com/Bridge
Marshall T. Rose, The Wollongong Group
Greg Satz, cisco
Martin Lee Schoffstall, Rensselaer Polytechnic Institute
Lou Steinberg, IBM
Dean Throop, Data General
Unni Warrier, Unisys
Rose & McCloghrie [Page 20]
RFC 1065 SMI August 1988
8. References
[1] Information processing systems - Open Systems Interconnection,
"Specification of Abstract Syntax Notation One (ASN.1)",
International Organization for Standardization, International
Standard 8824, December 1987.
[2] McCloghrie K., and M. Rose, "Management Information Base for
Network Management of TCP/IP-based internets", RFC 1066, TWG,
August 1988.
[3] Case, J., M.
Fedor, M. Schoffstall, and J. Davin, The Simple
Network Management Protocol", RFC 1067, University of Tennessee
at Knoxville, NYSERNet, Rensselaer Polytechnic, Proteon, August
1988.
[4] LaBarre, L., "Structure and Identification of Management
Information for the Internet", Internet Engineering Task Force
working note, Network Information Center, SRI International,
Menlo Park, California, April 1988.
[5] Cerf, V., "IAB Recommendations for the Development of Internet
Network Management Standards", RFC 1052, IAB, April 1988.
[6] Information processing systems - Open Systems Interconnection,
"Specification of Basic Encoding Rules for Abstract Notation One
(ASN.1)", International Organization for Standardization,
International Standard 8825, December 1987.
Rose & McCloghrie [Page 21]
Network Working Group W. Simpson,
Network Working Group W. Simpson, Editor
Request for Comments: 1661 Daydreamer
STD: 51 July 1994
Obsoletes: 1548
Category: Standards Track
The Point-to-Point Protocol (PPP)
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
The Point-to-Point Protocol (PPP) provides a standard method for
transporting multi-protocol datagrams over point-to-point links. PPP
is comprised of three main components:
1. A method for encapsulating multi-protocol datagrams.
2. A Link Control Protocol (LCP) for establishing, configuring,
and testing the data-link connection.
3. A family of Network Control Protocols (NCPs) for establishing
and configuring different network-layer protocols.
This document defines the PPP organization and methodology, and the
PPP encapsulation, together with an extensible option negotiation
mechanism which is able to negotiate a rich assortment of
configuration parameters and provides additional management
functions. The PPP Link Control Protocol (LCP) is described in terms
of this mechanism.
Table of Contents
1. Introduction .......................................... 1
1.1 Specification of Requirements ................... 2
1.2 Terminology ..................................... 3
2. PPP Encapsulation ..................................... 4
Simpson [Page i]
RFC 1661 Point-to-Point Protocol July 1994
3. PPP Link Operation .................................... 6
3.1 Overview ........................................ 6
3.2 Phase Diagram ................................... 6
3.3 Link Dead (physical-layer not ready) ............ 7
3.4 Link Establishment Phase ........................ 7
3.5 Authentication Phase ............................ 8
3.6 Network-Layer Protocol Phase .................... 8
3.7 Link Termination Phase .......................... 9
4. The Option Negotiation Automaton ...................... 11
4.1 State Transition Table .......................... 12
4.2 States .......................................... 14
4.3 Events .......................................... 16
4.4 Actions ......................................... 21
4.5 Loop Avoidance .................................. 23
4.6 Counters and Timers ............................. 24
5. LCP Packet Formats .................................... 26
5.1 Configure-Request ............................... 28
5.2 Configure-Ack ................................... 29
5.3 Configure-Nak ................................... 30
5.4 Configure-Reject ................................ 31
5.5 Terminate-Request and Terminate-Ack ............. 33
5.6 Code-Reject ..................................... 34
5.7 Protocol-Reject ................................. 35
5.8 Echo-Request and Echo-Reply ..................... 36
5.9 Discard-Request ................................. 37
6. LCP Configuration Options ............................. 39
6.1 Maximum-Receive-Unit (MRU) ...................... 41
6.2 Authentication-Protocol ......................... 42
6.3 Quality-Protocol ................................ 43
6.4 Magic-Number .................................... 45
6.5 Protocol-Field-Compression (PFC) ................ 48
6.6 Address-and-Control-Field-Compression (ACFC)
SECURITY CONSIDERATIONS ...................................... 51
REFERENCES ................................................... 51
ACKNOWLEDGEMENTS ............................................. 51
CHAIR'S ADDRESS .............................................. 52
EDITOR'S ADDRESS ............................................. 52
Simpson [Page ii]
RFC 1661 Point-to-Point Protocol July 1994
1. Introduction
The Point-to- Point Protocol is designed for simple links which
transport packets between two peers.
These links provide full-duplex
simultaneous bi- directional operation, and are assumed to deliver
packets in order. It is intended that PPP provide a common solution
for easy connection of a wide variety of hosts, bridges and routers
[1].
Encapsulation
The PPP encapsulation provides for multiplexing of different
network-layer protocols simultaneously over the same link. The
PPP encapsulation has been carefully designed to retain
compatibility with most commonly used supporting hardware.
Only 8 additional octets are necessary to form the encapsulation
when used within the default HDLC-like framing. In environments
where bandwidth is at a premium, the encapsulation and framing may
be shortened to 2 or 4 octets.
To support high speed implementations, the default encapsulation
uses only simple fields, only one of which needs to be examined
for demultiplexing. The default header and information fields
fall on 32-bit boundaries, and the trailer may be padded to an
arbitrary boundary.
Link Control Protocol
In order to be sufficiently versatile to be portable to a wide
variety of environments, PPP provides a Link Control Protocol
(LCP). The LCP is used to automatically agree upon the
encapsulation format options, handle varying limits on sizes of
packets, detect a looped-back link and other common
misconfiguration errors, and terminate the link. Other optional
facilities provided are authentication of the identity of its peer
on the link, and determination when a link is functioning properly
and when it is failing.
Network Control Protocols
Point-to-Point links tend to exacerbate many problems with the
current family of network protocols. For instance, assignment and
management of IP addresses, which is a problem even in LAN
environments, is especially difficult over circuit-switched
point-to-point links (such as dial-up modem servers). These
problems are handled by a family of Network Control Protocols
(NCPs), which each manage the specific needs required by their
Simpson [Page 1]
RFC 1661 Point-to-Point Protocol July 1994
respective network- layer protocols. These NCPs are defined in
companion documents.
Configuration
It is intended that PPP links be easy to configure. By design,
the standard defaults handle all common configurations. The
implementor can specify improvements to the default configuration,
which are automatically communicated to the peer without operator
intervention. Finally, the operator may explicitly configure
options for the link which enable the link to operate in
environments where it would otherwise be impossible.
This self-configuration is implemented through an extensible
option negotiation mechanism, wherein each end of the link
describes to the other its capabilities and requirements.
Although the option negotiation mechanism described in this
document is specified in terms of the Link Control Protocol (LCP),
the same facilities are designed to be used by other control
protocols, especially the family of NCPs.
1.1. Specification of Requirements
In this document, several words are used to signify the requirements
of the specification. These words are often capitalized.
MUST This word, or the adjective "required", means that the
definition is an absolute requirement of the specification.
MUST NOT This phrase means that the definition is an absolute
prohibition of the specification.
SHOULD This word, or the adjective "recommended", means that there
may exist valid reasons in particular circumstances to
ignore this item, but the full implications must be
understood and carefully weighed before choosing a
different course.
MAY This word, or the adjective "optional", means that this
item is one of an allowed set of alternatives. An
implementation which does not include this option MUST be
prepared to interoperate with another implementation which
does include the option.
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RFC 1661 Point-to-Point Protocol July 1994
1.2. Terminology
This document frequently uses the following terms:
datagram The unit of transmission in the network layer (such as IP).
A datagram may be encapsulated in one or more packets
passed to the data link layer.
frame The unit of transmission at the data link layer. A frame
may include a header and/or a trailer, along with some
number of units of data.
packet The basic unit of encapsulation, which is passed across the
interface between the network layer and the data link
layer. A packet is usually mapped to a frame; the
exceptions are when data link layer fragmentation is being
performed, or when multiple packets are incorporated into a
single frame.
peer The other end of the point-to-point link.
silently discard
The implementation discards the packet without further
processing. The implementation SHOULD provide the
capability of logging the error, including the contents of
the silently discarded packet, and SHOULD record the event
in a statistics counter.
Simpson [Page 3]
RFC 1661 Point-to-Point Protocol July 1994
2. PPP Encapsulation
The PPP encapsulation is used to disambiguate multiprotocol
datagrams. This encapsulation requires framing to indicate the
beginning and end of the encapsulation. Methods of providing framing
are specified in companion documents.
A summary of the PPP encapsulation is shown below. The fields are
transmitted from left to right.
+----------+-------------+---------+
| Protocol | Information | Padding |
| 8/16 bits| * | * |
+----------+-------------+---------+
Protocol Field
The Protocol field is one or two octets, and its value identifies
the datagram encapsulated in the Information field of the packet.
The field is transmitted and received most significant octet
first.
The structure of this field is consistent with the ISO 3309
extension mechanism for address fields. All Protocols MUST be
odd; the least significant bit of the least significant octet MUST
equal "1". Also, all Protocols MUST be assigned such that the
least significant bit of the most significant octet equals "0".
Frames received which don't comply with these rules MUST be
treated as having an unrecognized Protocol.
Protocol field values in the "0***" to "3***" range identify the
network-layer protocol of specific packets, and values in the
"8***" to "b***" range identify packets belonging to the
associated Network Control Protocols (NCPs), if any.
Protocol field values in the "4***" to "7***" range are used for
protocols with low volume traffic which have no associated NCP.
Protocol field values in the "c***" to "f***" range identify
packets as link-layer Control Protocols (such as LCP).
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Up-to-date values of the Protocol field are specified in the most
recent "Assigned Numbers" RFC [2]. This specification reserves
the following values:
Value (in hex) Protocol Name
0001 Padding Protocol
0003 to 001f reserved (transparency inefficient)
007d reserved (Control Escape)
00cf reserved (PPP NLPID)
00ff reserved (compression inefficient)
8001 to 801f unused
807d unused
80cf unused
80ff unused
c021 Link Control Protocol
c023 Password Authentication Protocol
c025 Link Quality Report
c223 Challenge Handshake Authentication Protocol
Developers of new protocols MUST obtain a number from the Internet
Assigned Numbers Authority (IANA), at IANA@isi.edu.
Information Field
The Information field is zero or more octets. The Information
field contains the datagram for the protocol specified in the
Protocol field.
The maximum length for the Information field, including Padding,
but not including the Protocol field, is termed the Maximum
Receive Unit (MRU), which defaults to 1500 octets. By
negotiation, consenting PPP implementations may use other values
for the MRU.
Padding
On transmission, the Information field MAY be padded with an
arbitrary number of octets up to the MRU.
It is the
responsibility of each protocol to distinguish padding octets from
real information.
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3. PPP Link Operation
3.1. Overview
In order to establish communications over a point-to-point link, each
end of the PPP link MUST first send LCP packets to configure and test
the data link. After the link has been established, the peer MAY be
authenticated.
Then, PPP MUST send NCP packets to choose and configure one or more
network-layer protocols. Once each of the chosen network-layer
protocols has been configured, datagrams from each network-layer
protocol can be sent over the link.
The link will remain configured for communications until explicit LCP
or NCP packets close the link down, or until some external event
occurs (an inactivity timer expires or network administrator
intervention).
3.2. Phase Diagram
In the process of configuring, maintaining and terminating the
point-to-point link, the PPP link goes through several distinct
phases which are specified in the following simplified state diagram:
+------+ +-----------+ +--------------+
| | UP | | OPENED | | SUCCESS/NONE
| Dead |------->| Establish |---------->| Authenticate |--+
| | | | | | |
+------+ +-----------+ +--------------+ |
^ | | |
| FAIL | FAIL | |
+<--------------+ +----------+ |
| | |
| +-----------+ | +---------+ |
| DOWN | | | CLOSING | | |
+------------| Terminate |<---+<----------| Network |<-+
| | | |
+-----------+ +---------+
Not all transitions are specified in this diagram. The following
semantics MUST be followed.
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3.3. Link Dead (physical-layer not ready)
The link necessarily begins and ends with this phase. When an
external event (such as carrier detection or network administrator
configuration) indicates that the physical-layer is ready to be used,
PPP will proceed to the Link Establishment phase.
During this phase, the LCP automaton (described later) will be in the
Initial or Starting states. The transition to the Link Establishment
phase will signal an Up event to the LCP automaton.
Implementation Note:
Typically, a link will return to this phase automatically after
the disconnection of a modem. In the case of a hard-wired link,
this phase may be extremely short -- merely long enough to detect
the presence of the device.
3.4. Link Establishment Phase
The Link Control Protocol (LCP) is used to establish the connection
through an exchange of Configure packets. This exchange is complete,
and the LCP Opened state entered, once a Configure-Ack packet
(described later) has been both sent and received.
All Configuration Options are assumed to be at default values unless
altered by the configuration exchange. See the chapter on LCP
Configuration Options for further discussion.
It is important to note that only Configuration Options which are
independent of particular network-layer protocols are configured by
LCP. Configuration of individual network-layer protocols is handled
by separate Network Control Protocols (NCPs) during the Network-Layer
Protocol phase.
Any non-LCP packets received during this phase MUST be silently
discarded.
The receipt of the LCP Configure-Request causes a return to the Link
Establishment phase from the Network-Layer Protocol phase or
Authentication phase.
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3.5. Authentication Phase
On some links it may be desirable to require a peer to authenticate
itself before allowing network-layer protocol packets to be
exchanged.
By default, authentication is not mandatory. If an implementation
desires that the peer authenticate with some specific authentication
protocol, then it MUST request the use of that authentication
protocol during Link Establishment phase.
Authentication SHOULD take place as soon as possible after link
establishment. However, link quality determination MAY occur
concurrently. An implementation MUST NOT allow the exchange of link
quality determination packets to delay authentication indefinitely.
Advancement from the Authentication phase to the Network-Layer
Protocol phase MUST NOT occur until authentication has completed. If
authentication fails, the authenticator SHOULD proceed instead to the
Link Termination phase.
Only Link Control Protocol, authentication protocol, and link quality
monitoring packets are allowed during this phase. All other packets
received during this phase MUST be silently discarded.
Implementation Notes:
An implementation SHOULD NOT fail authentication simply due to
timeout or lack of response. The authentication SHOULD allow some
method of retransmission, and proceed to the Link Termination
phase only after a number of authentication attempts has been
exceeded.
The implementation responsible for commencing Link Termination
phase is the implementation which has refused authentication to
its peer.
3.6. Network-Layer Protocol Phase
Once PPP has finished the previous phases, each network-layer
protocol (such as IP, IPX, or AppleTalk) MUST be separately
configured by the appropriate Network Control Protocol (NCP).
Each NCP MAY be Opened and Closed at any time.
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Implementation Note:
Because an implementation may initially use a significant amount
of time for link quality determination, implementations SHOULD
avoid fixed timeouts when waiting for their peers to configure a
NCP.
After a NCP has reached the Opened state, PPP will carry the
corresponding network-layer protocol packets. Any supported
network-layer protocol packets received when the corresponding NCP is
not in the Opened state MUST be silently discarded.
Implementation Note:
While LCP is in the Opened state, any protocol packet which is
unsupported by the implementation MUST be returned in a Protocol-
Reject (described later). Only protocols which are supported are
silently discarded.
During this phase, link traffic consists of any possible combination
of LCP, NCP, and network-layer protocol packets.
3.7. Link Termination Phase
PPP can terminate the link at any time. This might happen because of
the loss of carrier, authentication failure, link quality failure,
the expiration of an idle-period timer, or the administrative closing
of the link.
LCP is used to close the link through an exchange of Terminate
packets. When the link is closing, PPP informs the network-layer
protocols so that they may take appropriate action.
After the exchange of Terminate packets, the implementation SHOULD
signal the physical-layer to disconnect in order to enforce the
termination of the link, particularly in the case of an
authentication failure. The sender of the Terminate-Request SHOULD
disconnect after receiving a Terminate-Ack, or after the Restart
counter expires. The receiver of a Terminate-Request SHOULD wait for
the peer to disconnect, and MUST NOT disconnect until at least one
Restart time has passed after sending a Terminate-Ack. PPP SHOULD
proceed to the Link Dead phase.
Any non-LCP packets received during this phase MUST be silently
discarded.
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Implementation Note:
The closing of the link by LCP is sufficient. There is no need
for each NCP to send a flurry of Terminate packets. Conversely,
the fact that one NCP has Closed is not sufficient reason to cause
the termination of the PPP link, even if that NCP was the only NCP
currently in the Opened state.
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4. The Option Negotiation Automaton
The finite-state automaton is defined by events, actions and state
transitions. Events include reception of external commands such as
Open and Close, expiration of the Restart timer, and reception of
packets from a peer. Actions include the starting of the Restart
timer and transmission of packets to the peer.
Some types of packets -- Configure-Naks and Configure-Rejects, or
Code-Rejects and Protocol-Rejects, or Echo-Requests, Echo-Replies and
Discard-Requests -- are not differentiated in the automaton
descriptions. As will be described later, these packets do indeed
serve different functions. However, they always cause the same
transitions.
Events Actions
Up = lower layer is Up tlu = This-Layer-Up
Down = lower layer is Down tld = This-Layer-Down
Open = administrative Open tls = This-Layer-Started
Close= administrative Close tlf = This-Layer-Finished
TO+ = Timeout with counter > 0 irc = Initialize-Restart-Count
TO- = Timeout with counter expired zrc = Zero-Restart-Count
RCR+ = Receive-Configure-Request (Good) scr = Send-Configure-Request
RCR- = Receive-Configure-Request (Bad)
RCA = Receive-Configure-Ack sca = Send-Configure-Ack
RCN = Receive-Configure-Nak/Rej scn = Send-Configure-Nak/Rej
RTR = Receive-Terminate-Request str = Send-Terminate-Request
RTA = Receive-Terminate-Ack sta = Send-Terminate-Ack
RUC = Receive-Unknown-Code scj = Send-Code-Reject
RXJ+ = Receive-Code-Reject (permitted)
or Receive-Protocol-Reject
RXJ- = Receive-Code-Reject (catastrophic)
or Receive-Protocol-Reject
RXR = Receive-Echo-Request ser = Send-Echo-Reply
or Receive-Echo-Reply
or Receive-Discard-Request
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4.1. State Transition Table
The complete state transition table follows. States are indicated
horizontally, and events are read vertically. State transitions and
actions are represented in the form action/new-state. Multiple
actions are separated by commas, and may continue on succeeding lines
as space requires; multiple actions may be implemented in any
convenient order. The state may be followed by a letter, which
indicates an explanatory footnote. The dash ('-') indicates an
illegal transition.
| State
| 0 1 2 3 4 5
Events| Initial Starting Closed Stopped Closing Stopping
------+-----------------------------------------------------------
Up | 2 irc,scr/6 - - - -
Down | - - 0 tls/1 0 1
Open | tls/1 1 irc,scr/6 3r 5r 5r
Close| 0 tlf/0 2 2 4 4
|
TO+ | - - - - str/4 str/5
TO- | - - - - tlf/2 tlf/3
|
RCR+ | - - sta/2 irc,scr,sca/8 4 5
RCR- | - - sta/2 irc,scr,scn/6 4 5
RCA | - - sta/2 sta/3 4 5
RCN | - - sta/2 sta/3 4 5
|
RTR | - - sta/2 sta/3 sta/4 sta/5
RTA | - - 2 3 tlf/2 tlf/3
|
RUC | - - scj/2 scj/3 scj/4 scj/5
RXJ+ | - - 2 3 4 5
RXJ- | - - tlf/2 tlf/3 tlf/2 tlf/3
|
RXR | - - 2 3 4 5
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| State
| 6 7 8 9
Events| Req-Sent Ack-Rcvd Ack-Sent Opened
------+-----------------------------------------
Up | - - - -
Down | 1 1 1 tld/1
Open | 6 7 8 9r
Close|irc,str/4 irc,str/4 irc,str/4 tld,irc,str/4
|
TO+ | scr/6 scr/6 scr/8 -
TO- | tlf/3p tlf/3p tlf/3p -
|
RCR+ | sca/8 sca,tlu/9 sca/8 tld,scr,sca/8
RCR- | scn/6 scn/7 scn/6 tld,scr,scn/6
RCA | irc/7 scr/6x irc,tlu/9 tld,scr/6x
RCN |irc,scr/6 scr/6x irc,scr/8 tld,scr/6x
|
RTR | sta/6 sta/6 sta/6 tld,zrc,sta/5
RTA | 6 6 8 tld,scr/6
|
RUC | scj/6 scj/7 scj/8 scj/9
RXJ+ | 6 6 8 9
RXJ- | tlf/3 tlf/3 tlf/3 tld,irc,str/5
|
RXR | 6 7 8 ser/9
The states in which the Restart timer is running are identifiable by
the presence of TO events. Only the Send-Configure-Request, Send-
Terminate-Request and Zero-Restart-Count actions start or re-start
the Restart timer. The Restart timer is stopped when transitioning
from any state where the timer is running to a state where the timer
is not running.
The events and actions are defined according to a message passing
architecture, rather than a signalling architecture. If an action is
desired to control specific signals (such as DTR), additional actions
are likely to be required.
[p] Passive option; see Stopped state discussion.
[r] Restart option; see Open event discussion.
[x] Crossed connection; see RCA event discussion.
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4.2. States
Following is a more detailed description of each automaton state.
Initial
In the Initial state, the lower layer is unavailable (Down), and
no Open has occurred. The Restart timer is not running in the
Initial state.
Starting
The Starting state is the Open counterpart to the Initial state.
An administrative Open has been initiated, but the lower layer is
still unavailable (Down). The Restart timer is not running in the
Starting state.
When the lower layer becomes available (Up), a Configure-Request
is sent.
Closed
In the Closed state, the link is available (Up), but no Open has
occurred. The Restart timer is not running in the Closed state.
Upon reception of Configure-Request packets, a Terminate-Ack is
sent. Terminate-Acks are silently discarded to avoid creating a
loop.
Stopped
The Stopped state is the Open counterpart to the Closed state. It
is entered when the automaton is waiting for a Down event after
the This-Layer-Finished action, or after sending a Terminate-Ack.
The Restart timer is not running in the Stopped state.
Upon reception of Configure-Request packets, an appropriate
response is sent. Upon reception of other packets, a Terminate-
Ack is sent. Terminate-Acks are silently discarded to avoid
creating a loop.
Rationale:
The Stopped state is a junction state for link termination,
link configuration failure, and other automaton failure modes.
These potentially separate states have been combined.
There is a race condition between the Down event response (from
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the This-Layer-Finished action) and the Receive-Configure-
Request event. When a Configure-Request arrives before the
Down event, the Down event will supercede by returning the
automaton to the Starting state. This prevents attack by
repetition.
Implementation Option:
After the peer fails to respond to Configure-Requests, an
implementation MAY wait passively for the peer to send
Configure-Requests. In this case, the This-Layer-Finished
action is not used for the TO- event in states Req-Sent, Ack-
Rcvd and Ack-Sent.
This option is useful for dedicated circuits, or circuits which
have no status signals available, but SHOULD NOT be used for
switched circuits.
Closing
In the Closing state, an attempt is made to terminate the
connection. A Terminate-Request has been sent and the Restart
timer is running, but a Terminate-Ack has not yet been received.
Upon reception of a Terminate-Ack, the Closed state is entered.
Upon the expiration of the Restart timer, a new Terminate-Request
is transmitted, and the Restart timer is restarted. After the
Restart timer has expired Max-Terminate times, the Closed state is
entered.
Stopping
The Stopping state is the Open counterpart to the Closing state.
A Terminate-Request has been sent and the Restart timer is
running, but a Terminate-Ack has not yet been received.
Rationale:
The Stopping state provides a well defined opportunity to
terminate a link before allowing new traffic. After the link
has terminated, a new configuration may occur via the Stopped
or Starting states.
Request-Sent
In the Request-Sent state an attempt is made to configure the
connection. A Configure-Request has been sent and the Restart
timer is running, but a Configure-Ack has not yet been received
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nor has one been sent.
Ack-Received
In the Ack-Received state, a Configure-Request has been sent and a
Configure-Ack has been received. The Restart timer is still
running, since a Configure-Ack has not yet been sent.
Ack-Sent
In the Ack-Sent state, a Configure-Request and a Configure-Ack
have both been sent, but a Configure-Ack has not yet been
received. The Restart timer is running, since a Configure-Ack has
not yet been received.
Opened
In the Opened state, a Configure-Ack has been both sent and
received. The Restart timer is not running.
When entering the Opened state, the implementation SHOULD signal
the upper layers that it is now Up. Conversely, when leaving the
Opened state, the implementation SHOULD signal the upper layers
that it is now Down.
4.3. Events
Transitions and actions in the automaton are caused by events.
Up
This event occurs when a lower layer indicates that it is ready to
carry packets.
Typically, this event is used by a modem handling or calling
process, or by some other coupling of the PPP link to the physical
media, to signal LCP that the link is entering Link Establishment
phase.
It also can be used by LCP to signal each NCP that the link is
entering Network-Layer Protocol phase. That is, the This-Layer-Up
action from LCP triggers the Up event in the NCP.
Down
This event occurs when a lower layer indicates that it is no
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longer ready to carry packets.
Typically, this event is used by a modem handling or calling
process, or by some other coupling of the PPP link to the physical
media, to signal LCP that the link is entering Link Dead phase.
It also can be used by LCP to signal each NCP that the link is
leaving Network-Layer Protocol phase. That is, the This-Layer-
Down action from LCP triggers the Down event in the NCP.
Open
This event indicates that the link is administratively available
for traffic; that is, the network administrator (human or program)
has indicated that the link is allowed to be Opened. When this
event occurs, and the link is not in the Opened state, the
automaton attempts to send configuration packets to the peer.
If the automaton is not able to begin configuration (the lower
layer is Down, or a previous Close event has not completed), the
establishment of the link is automatically delayed.
When a Terminate-Request is received, or other events occur which
cause the link to become unavailable, the automaton will progress
to a state where the link is ready to re-open. No additional
administrative intervention is necessary.
Implementation Option:
Experience has shown that users will execute an additional Open
command when they want to renegotiate the link. This might
indicate that new values are to be negotiated.
Since this is not the meaning of the Open event, it is
suggested that when an Open user command is executed in the
Opened, Closing, Stopping, or Stopped states, the
implementation issue a Down event, immediately followed by an
Up event. Care must be taken that an intervening Down event
cannot occur from another source.
The Down followed by an Up will cause an orderly renegotiation
of the link, by progressing through the Starting to the
Request-Sent state. This will cause the renegotiation of the
link, without any harmful side effects.
Close
This event indicates that the link is not available for traffic;
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that is, the network administrator (human or program) has
indicated that the link is not allowed to be Opened. When this
event occurs, and the link is not in the Closed state, the
automaton attempts to terminate the connection. Futher attempts
to re-configure the link are denied until a new Open event occurs.
Implementation Note:
When authentication fails, the link SHOULD be terminated, to
prevent attack by repetition and denial of service to other
users. Since the link is administratively available (by
definition), this can be accomplished by simulating a Close
event to the LCP, immediately followed by an Open event. Care
must be taken that an intervening Close event cannot occur from
another source.
The Close followed by an Open will cause an orderly termination
of the link, by progressing through the Closing to the Stopping
state, and the This-Layer-Finished action can disconnect the
link. The automaton waits in the Stopped or Starting states
for the next connection attempt.
Timeout (TO+,TO-)
This event indicates the expiration of the Restart timer. The
Restart timer is used to time responses to Configure-Request and
Terminate-Request packets.
The TO+ event indicates that the Restart counter continues to be
greater than zero, which triggers the corresponding Configure-
Request or Terminate-Request packet to be retransmitted.
The TO- event indicates that the Restart counter is not greater
than zero, and no more packets need to be retransmitted.
Receive-Configure-Request (RCR+,RCR-)
This event occurs when a Configure-Request packet is received from
the peer. The Configure-Request packet indicates the desire to
open a connection and may specify Configuration Options. The
Configure-Request packet is more fully described in a later
section.
The RCR+ event indicates that the Configure-Request was
acceptable, and triggers the transmission of a corresponding
Configure-Ack.
The RCR- event indicates that the Configure-Request was
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unacceptable, and triggers the transmission of a corresponding
Configure-Nak or Configure-Reject.
Implementation Note:
These events may occur on a connection which is already in the
Opened state. The implementation MUST be prepared to
immediately renegotiate the Configuration Options.
Receive-Configure-Ack (RCA)
This event occurs when a valid Configure-Ack packet is received
from the peer. The Configure-Ack packet is a positive response to
a Configure-Request packet. An out of sequence or otherwise
invalid packet is silently discarded.
Implementation Note:
Since the correct packet has already been received before
reaching the Ack-Rcvd or Opened states, it is extremely
unlikely that another such packet will arrive. As specified,
all invalid Ack/Nak/Rej packets are silently discarded, and do
not affect the transitions of the automaton.
However, it is not impossible that a correctly formed packet
will arrive through a coincidentally-timed cross-connection.
It is more likely to be the result of an implementation error.
At the very least, this occurance SHOULD be logged.
Receive-Configure-Nak/Rej (RCN)
This event occurs when a valid Configure-Nak or Configure-Reject
packet is received from the peer. The Configure-Nak and
Configure- Reject packets are negative responses to a Configure-
Request packet. An out of sequence or otherwise invalid packet is
silently discarded.
Implementation Note:
Although the Configure-Nak and Configure-Reject cause the same
state transition in the automaton, these packets have
significantly different effects on the Configuration Options
sent in the resulting Configure-Request packet.
Receive-Terminate-Request (RTR)
This event occurs when a Terminate-Request packet is received.
The Terminate-Request packet indicates the desire of the peer to
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close the connection.
Implementation Note:
This event is not identical to the Close event (see above), and
does not override the Open commands of the local network
administrator. The implementation MUST be prepared to receive
a new Configure-Request without network administrator
intervention.
Receive-Terminate-Ack (RTA)
This event occurs when a Terminate-Ack packet is received from the
peer. The Terminate-Ack packet is usually a response to a
Terminate-Request packet. The Terminate-Ack packet may also
indicate that the peer is in Closed or Stopped states, and serves
to re-synchronize the link configuration.
Receive-Unknown-Code (RUC)
This event occurs when an un-interpretable packet is received from
the peer. A Code-Reject packet is sent in response.
Receive-Code-Reject, Receive-Protocol-Reject (RXJ+,RXJ-)
This event occurs when a Code-Reject or a Protocol-Reject packet
is received from the peer.
The RXJ+ event arises when the rejected value is acceptable, such
as a Code-Reject of an extended code, or a Protocol-Reject of a
NCP. These are within the scope of normal operation. The
implementation MUST stop sending the offending packet type.
The RXJ- event arises when the rejected value is catastrophic,
such as a Code-Reject of Configure-Request, or a Protocol-Reject
of LCP! This event communicates an unrecoverable error that
terminates the connection.
Receive-Echo-Request, Receive-Echo-Reply, Receive-Discard-Request
(RXR)
This event occurs when an Echo-Request, Echo-Reply or Discard-
Request packet is received from the peer. The Echo-Reply packet
is a response to an Echo-Request packet. There is no reply to an
Echo-Reply or Discard-Request packet.
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4.4. Actions
Actions in the automaton are caused by events and typically indicate
the transmission of packets and/or the starting or stopping of the
Restart timer.
Illegal-Event (-)
This indicates an event that cannot occur in a properly
implemented automaton. The implementation has an internal error,
which should be reported and logged. No transition is taken, and
the implementation SHOULD NOT reset or freeze.
This-Layer-Up (tlu)
This action indicates to the upper layers that the automaton is
entering the Opened state.
Typically, this action is used by the LCP to signal the Up event
to a NCP, Authentication Protocol, or Link Quality Protocol, or
MAY be used by a NCP to indicate that the link is available for
its network layer traffic.
This-Layer-Down (tld)
This action indicates to the upper layers that the automaton is
leaving the Opened state.
Typically, this action is used by the LCP to signal the Down event
to a NCP, Authentication Protocol, or Link Quality Protocol, or
MAY be used by a NCP to indicate that the link is no longer
available for its network layer traffic.
This-Layer-Started (tls)
This action indicates to the lower layers that the automaton is
entering the Starting state, and the lower layer is needed for the
link. The lower layer SHOULD respond with an Up event when the
lower layer is available.
This results of this action are highly implementation dependent.
This-Layer-Finished (tlf)
This action indicates to the lower layers that the automaton is
entering the Initial, Closed or Stopped states, and the lower
layer is no longer needed for the link.
The lower layer SHOULD
respond with a Down event when the lower layer has terminated.
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Typically, this action MAY be used by the LCP to advance to the
Link Dead phase, or MAY be used by a NCP to indicate to the LCP
that the link may terminate when there are no other NCPs open.
This results of this action are highly implementation dependent.
Initialize-Restart-Count (irc)
This action sets the Restart counter to the appropriate value
(Max-Terminate or Max-Configure). The counter is decremented for
each transmission, including the first.
Implementation Note:
In addition to setting the Restart counter, the implementation
MUST set the timeout period to the initial value when Restart
timer backoff is used.
Zero-Restart-Count (zrc)
This action sets the Restart counter to zero.
Implementation Note:
This action enables the FSA to pause before proceeding to the
desired final state, allowing traffic to be processed by the
peer. In addition to zeroing the Restart counter, the
implementation MUST set the timeout period to an appropriate
value.
Send-Configure-Request (scr)
A Configure-Request packet is transmitted. This indicates the
desire to open a connection with a specified set of Configuration
Options. The Restart timer is started when the Configure-Request
packet is transmitted, to guard against packet loss. The Restart
counter is decremented each time a Configure-Request is sent.
Send-Configure-Ack (sca)
A Configure-Ack packet is transmitted. This acknowledges the
reception of a Configure-Request packet with an acceptable set of
Configuration Options.
Send-Configure-Nak (scn)
A Configure-Nak or Configure-Reject packet is transmitted, as
appropriate. This negative response reports the reception of a
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Configure-Request packet with an unacceptable set of Configuration
Options.
Configure-Nak packets are used to refuse a Configuration Option
value, and to suggest a new, acceptable value. Configure-Reject
packets are used to refuse all negotiation about a Configuration
Option, typically because it is not recognized or implemented.
The use of Configure-Nak versus Configure-Reject is more fully
described in the chapter on LCP Packet Formats.
Send-Terminate-Request (str)
A Terminate-Request packet is transmitted. This indicates the
desire to close a connection. The Restart timer is started when
the Terminate-Request packet is transmitted, to guard against
packet loss. The Restart counter is decremented each time a
Terminate-Request is sent.
Send-Terminate-Ack (sta)
A Terminate-Ack packet is transmitted. This acknowledges the
reception of a Terminate-Request packet or otherwise serves to
synchronize the automatons.
Send-Code-Reject (scj)
A Code-Reject packet is transmitted. This indicates the reception
of an unknown type of packet.
Send-Echo-Reply (ser)
An Echo-Reply packet is transmitted. This acknowledges the
reception of an Echo-Request packet.
4.5. Loop Avoidance
The protocol makes a reasonable attempt at avoiding Configuration
Option negotiation loops. However, the protocol does NOT guarantee
that loops will not happen. As with any negotiation, it is possible
to configure two PPP implementations with conflicting policies that
will never converge. It is also possible to configure policies which
do converge, but which take significant time to do so. Implementors
should keep this in mind and SHOULD implement loop detection
mechanisms or higher level timeouts.
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4.6. Counters and Timers
Restart Timer
There is one special timer used by the automaton. The Restart
timer is used to time transmissions of Configure-Request and
Terminate-Request packets. Expiration of the Restart timer causes
a Timeout event, and retransmission of the corresponding
Configure-Request or Terminate-Request packet. The Restart timer
MUST be configurable, but SHOULD default to three (3) seconds.
Implementation Note:
The Restart timer SHOULD be based on the speed of the link.
The default value is designed for low speed (2,400 to 9,600
bps), high switching latency links (typical telephone lines).
Higher speed links, or links with low switching latency, SHOULD
have correspondingly faster retransmission times.
Instead of a constant value, the Restart timer MAY begin at an
initial small value and increase to the configured final value.
Each successive value less than the final value SHOULD be at
least twice the previous value. The initial value SHOULD be
large enough to account for the size of the packets, twice the
round trip time for transmission at the link speed, and at
least an additional 100 milliseconds to allow the peer to
process the packets before responding. Some circuits add
another 200 milliseconds of satellite delay. Round trip times
for modems operating at 14,400 bps have been measured in the
range of 160 to more than 600 milliseconds.
Max-Terminate
There is one required restart counter for Terminate-Requests.
Max-Terminate indicates the number of Terminate-Request packets
sent without receiving a Terminate-Ack before assuming that the
peer is unable to respond. Max-Terminate MUST be configurable,
but SHOULD default to two (2) transmissions.
Max-Configure
A similar counter is recommended for Configure-Requests. Max-
Configure indicates the number of Configure-Request packets sent
without receiving a valid Configure-Ack, Configure-Nak or
Configure-Reject before assuming that the peer is unable to
respond. Max-Configure MUST be configurable, but SHOULD default
to ten (10) transmissions.
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Max-Failure
A related counter is recommended for Configure-Nak. Max-Failure
indicates the number of Configure-Nak packets sent without sending
a Configure-Ack before assuming that configuration is not
converging. Any further Configure-Nak packets for peer requested
options are converted to Configure-Reject packets, and locally
desired options are no longer appended. Max-Failure MUST be
configurable, but SHOULD default to five (5) transmissions.
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5. LCP Packet Formats
There are three classes of LCP packets:
1. Link Configuration packets used to establish and configure a
link (Configure-Request, Configure-Ack, Configure-Nak and
Configure-Reject).
2. Link Termination packets used to terminate a link (Terminate-
Request and Terminate-Ack).
3. Link Maintenance packets used to manage and debug a link
(Code-Reject, Protocol-Reject, Echo-Request, Echo-Reply, and
Discard-Request).
In the interest of simplicity, there is no version field in the LCP
packet. A correctly functioning LCP implementation will always
respond to unknown Protocols and Codes with an easily recognizable
LCP packet, thus providing a deterministic fallback mechanism for
implementations of other versions.
Regardless of which Configuration Options are enabled, all LCP Link
Configuration, Link Termination, and Code-Reject packets (codes 1
through 7) are always sent as if no Configuration Options were
negotiated. In particular, each Configuration Option specifies a
default value. This ensures that such LCP packets are always
recognizable, even when one end of the link mistakenly believes the
link to be open.
Exactly one LCP packet is encapsulated in the PPP Information field,
where the PPP Protocol field indicates type hex c021 (Link Control
Protocol).
A summary of the Link Control Protocol packet format is shown below.
The fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data ...
+-+-+-+-+
Code
The Code field is one octet, and identifies the kind of LCP
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packet. When a packet is received with an unknown Code field, a
Code-Reject packet is transmitted.
Up-to-date values of the LCP Code field are specified in the most
recent "Assigned Numbers" RFC [2]. This document concerns the
following values:
1 Configure-Request
2 Configure-Ack
3 Configure-Nak
4 Configure-Reject
5 Terminate-Request
6 Terminate-Ack
7 Code-Reject
8 Protocol-Reject
9 Echo-Request
10 Echo-Reply
11 Discard-Request
Identifier
The Identifier field is one octet, and aids in matching requests
and replies. When a packet is received with an invalid Identifier
field, the packet is silently discarded without affecting the
automaton.
Length
The Length field is two octets, and indicates the length of the
LCP packet, including the Code, Identifier, Length and Data
fields. The Length MUST NOT exceed the MRU of the link.
Octets outside the range of the Length field are treated as
padding and are ignored on reception. When a packet is received
with an invalid Length field, the packet is silently discarded
without affecting the automaton.
Data
The Data field is zero or more octets, as indicated by the Length
field. The format of the Data field is determined by the Code
field.
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5.1. Configure-Request
Description
An implementation wishing to open a connection MUST transmit a
Configure-Request. The Options field is filled with any desired
changes to the link defaults. Configuration Options SHOULD NOT be
included with default values.
Upon reception of a Configure-Request, an appropriate reply MUST
be transmitted.
A summary of the Configure-Request packet format is shown below. The
fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+
Code
1 for Configure-Request.
Identifier
The Identifier field MUST be changed whenever the contents of the
Options field changes, and whenever a valid reply has been
received for a previous request. For retransmissions, the
Identifier MAY remain unchanged.
Options
The options field is variable in length, and contains the list of
zero or more Configuration Options that the sender desires to
negotiate. All Configuration Options are always negotiated
simultaneously. The format of Configuration Options is further
described in a later chapter.
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5.2. Configure-Ack
Description
If every Configuration Option received in a Configure-Request is
recognizable and all values are acceptable, then the
implementation MUST transmit a Configure-Ack. The acknowledged
Configuration Options MUST NOT be reordered or modified in any
way.
On reception of a Configure-Ack, the Identifier field MUST match
that of the last transmitted Configure-Request. Additionally, the
Configuration Options in a Configure-Ack MUST exactly match those
of the last transmitted Configure-Request. Invalid packets are
silently discarded.
A summary of the Configure-Ack packet format is shown below. The
fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+
Code
2 for Configure-Ack.
Identifier
The Identifier field is a copy of the Identifier field of the
Configure-Request which caused this Configure-Ack.
Options
The Options field is variable in length, and contains the list of
zero or more Configuration Options that the sender is
acknowledging. All Configuration Options are always acknowledged
simultaneously.
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5.3. Configure-Nak
Description
If every instance of the received Configuration Options is
recognizable, but some values are not acceptable, then the
implementation MUST transmit a Configure-Nak. The Options field
is filled with only the unacceptable Configuration Options from
the Configure-Request. All acceptable Configuration Options are
filtered out of the Configure-Nak, but otherwise the Configuration
Options from the Configure-Request MUST NOT be reordered.
Options which have no value fields (boolean options) MUST use the
Configure-Reject reply instead.
Each Configuration Option which is allowed only a single instance
MUST be modified to a value acceptable to the Configure-Nak
sender. The default value MAY be used, when this differs from the
requested value.
When a particular type of Configuration Option can be listed more
than once with different values, the Configure-Nak MUST include a
list of all values for that option which are acceptable to the
Configure-Nak sender. This includes acceptable values that were
present in the Configure-Request.
Finally, an implementation may be configured to request the
negotiation of a specific Configuration Option. If that option is
not listed, then that option MAY be appended to the list of Nak'd
Configuration Options, in order to prompt the peer to include that
option in its next Configure-Request packet. Any value fields for
the option MUST indicate values acceptable to the Configure-Nak
sender.
On reception of a Configure-Nak, the Identifier field MUST match
that of the last transmitted Configure-Request. Invalid packets
are silently discarded.
Reception of a valid Configure-Nak indicates that when a new
Configure-Request is sent, the Configuration Options MAY be
modified as specified in the Configure-Nak. When multiple
instances of a Configuration Option are present, the peer SHOULD
select a single value to include in its next Configure-Request
packet.
Some Configuration Options have a variable length.
Since the
Nak' d Option has been modified by the peer, the implementation
MUST be able to handle an Option length which is different from
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the original Configure-Request.
A summary of the Configure-Nak packet format is shown below. The
fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+
Code
3 for Configure-Nak.
Identifier
The Identifier field is a copy of the Identifier field of the
Configure-Request which caused this Configure-Nak.
Options
The Options field is variable in length, and contains the list of
zero or more Configuration Options that the sender is Nak'ing.
All Configuration Options are always Nak'd simultaneously.
5.4. Configure-Reject
Description
If some Configuration Options received in a Configure-Request are
not recognizable or are not acceptable for negotiation (as
configured by a network administrator), then the implementation
MUST transmit a Configure-Reject. The Options field is filled
with only the unacceptable Configuration Options from the
Configure-Request. All recognizable and negotiable Configuration
Options are filtered out of the Configure-Reject, but otherwise
the Configuration Options MUST NOT be reordered or modified in any
way.
On reception of a Configure-Reject, the Identifier field MUST
match that of the last transmitted Configure-Request.
Additionally, the Configuration Options in a Configure-Reject MUST
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be a proper subset of those in the last transmitted Configure-
Request. Invalid packets are silently discarded.
Reception of a valid Configure-Reject indicates that when a new
Configure-Request is sent, it MUST NOT include any of the
Configuration Options listed in the Configure-Reject.
A summary of the Configure-Reject packet format is shown below. The
fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+
Code
4 for Configure-Reject.
Identifier
The Identifier field is a copy of the Identifier field of the
Configure-Request which caused this Configure-Reject.
Options
The Options field is variable in length, and contains the list of
zero or more Configuration Options that the sender is rejecting.
All Configuration Options are always rejected simultaneously.
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5.5. Terminate-Request and Terminate-Ack
Description
LCP includes Terminate-Request and Terminate-Ack Codes in order to
provide a mechanism for closing a connection.
An implementation wishing to close a connection SHOULD transmit a
Terminate-Request. Terminate-Request packets SHOULD continue to
be sent until Terminate-Ack is received, the lower layer indicates
that it has gone down, or a sufficiently large number have been
transmitted such that the peer is down with reasonable certainty.
Upon reception of a Terminate-Request, a Terminate-Ack MUST be
transmitted.
Reception of an unelicited Terminate-Ack indicates that the peer
is in the Closed or Stopped states, or is otherwise in need of
re-negotiation.
A summary of the Terminate-Request and Terminate-Ack packet formats
is shown below. The fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data ...
+-+-+-+-+
Code
5 for Terminate-Request;
6 for Terminate-Ack.
Identifier
On transmission, the Identifier field MUST be changed whenever the
content of the Data field changes, and whenever a valid reply has
been received for a previous request. For retransmissions, the
Identifier MAY remain unchanged.
On reception, the Identifier field of the Terminate-Request is
copied into the Identifier field of the Terminate-Ack packet.
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Data
The Data field is zero or more octets, and contains uninterpreted
data for use by the sender. The data may consist of any binary
value. The end of the field is indicated by the Length.
5.6. Code-Reject
Description
Reception of a LCP packet with an unknown Code indicates that the
peer is operating with a different version. This MUST be reported
back to the sender of the unknown Code by transmitting a Code-
Reject.
Upon reception of the Code-Reject of a code which is fundamental
to this version of the protocol, the implementation SHOULD report
the problem and drop the connection, since it is unlikely that the
situation can be rectified automatically.
A summary of the Code-Reject packet format is shown below. The
fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Rejected-Packet ...
+-+-+-+-+-+-+-+-+
Code
7 for Code-Reject.
Identifier
The Identifier field MUST be changed for each Code-Reject sent.
Rejected-Packet
The Rejected-Packet field contains a copy of the LCP packet which
is being rejected. It begins with the Information field, and does
not include any Data Link Layer headers nor an FCS. The
Rejected-Packet MUST be truncated to comply with the peer's
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RFC 1661 Point-to-Point Protocol July 1994
established MRU.
5.7. Protocol-Reject
Description
Reception of a PPP packet with an unknown Protocol field indicates
that the peer is attempting to use a protocol which is
unsupported. This usually occurs when the peer attempts to
configure a new protocol. If the LCP automaton is in the Opened
state, then this MUST be reported back to the peer by transmitting
a Protocol-Reject.
Upon reception of a Protocol-Reject, the implementation MUST stop
sending packets of the indicated protocol at the earliest
opportunity.
Protocol-Reject packets can only be sent in the LCP Opened state.
Protocol-Reject packets received in any state other than the LCP
Opened state SHOULD be silently discarded.
A summary of the Protocol-Reject packet format is shown below. The
fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Rejected-Protocol | Rejected-Information ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code
8 for Protocol-Reject.
Identifier
The Identifier field MUST be changed for each Protocol-Reject
sent.
Rejected-Protocol
The Rejected-Protocol field is two octets, and contains the PPP
Protocol field of the packet which is being rejected.
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Rejected-Information
The Rejected-Information field contains a copy of the packet which
is being rejected. It begins with the Information field, and does
not include any Data Link Layer headers nor an FCS. The
Rejected-Information MUST be truncated to comply with the peer's
established MRU.
5.8. Echo-Request and Echo-Reply
Description
LCP includes Echo-Request and Echo-Reply Codes in order to provide
a Data Link Layer loopback mechanism for use in exercising both
directions of the link. This is useful as an aid in debugging,
link quality determination, performance testing, and for numerous
other functions.
Upon reception of an Echo- Request in the LCP Opened state, an
Echo-Reply MUST be transmitted.
Echo-Request and Echo-Reply packets MUST only be sent in the LCP
Opened state. Echo-Request and Echo-Reply packets received in any
state other than the LCP Opened state SHOULD be silently
discarded.
A summary of the Echo-Request and Echo-Reply packet formats is shown
below. The fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Magic-Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data ...
+-+-+-+-+
Code
9 for Echo-Request;
10 for Echo-Reply.
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Identifier
On transmission, the Identifier field MUST be changed whenever the
content of the Data field changes, and whenever a valid reply has
been received for a previous request. For retransmissions, the
Identifier MAY remain unchanged.
On reception, the Identifier field of the Echo-Request is copied
into the Identifier field of the Echo-Reply packet.
Magic-Number
The Magic-Number field is four octets, and aids in detecting links
which are in the looped-back condition. Until the Magic-Number
Configuration Option has been successfully negotiated, the Magic-
Number MUST be transmitted as zero. See the Magic-Number
Configuration Option for further explanation.
Data
The Data field is zero or more octets, and contains uninterpreted
data for use by the sender. The data may consist of any binary
value. The end of the field is indicated by the Length.
5.9. Discard-Request
Description
LCP includes a Discard-Request Code in order to provide a Data
Link Layer sink mechanism for use in exercising the local to
remote direction of the link. This is useful as an aid in
debugging, performance testing, and for numerous other functions.
Discard- Request packets MUST only be sent in the LCP Opened state.
On reception, the receiver MUST silently discard any Discard-
Request that it receives.
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RFC 1661 Point-to-Point Protocol July 1994
A summary of the Discard-Request packet format is shown below. The
fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Magic-Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data ...
+-+-+-+-+
Code
11 for Discard-Request.
Identifier
The Identifier field MUST be changed for each Discard-Request
sent.
Magic-Number
The Magic-Number field is four octets, and aids in detecting links
which are in the looped-back condition. Until the Magic-Number
Configuration Option has been successfully negotiated, the Magic-
Number MUST be transmitted as zero. See the Magic-Number
Configuration Option for further explanation.
Data
The Data field is zero or more octets, and contains uninterpreted
data for use by the sender. The data may consist of any binary
value. The end of the field is indicated by the Length.
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6. LCP Configuration Options
LCP Configuration Options allow negotiation of modifications to the
default characteristics of a point-to-point link. If a Configuration
Option is not included in a Configure-Request packet, the default
value for that Configuration Option is assumed.
Some Configuration Options MAY be listed more than once. The effect
of this is Configuration Option specific, and is specified by each
such Configuration Option description. (None of the Configuration
Options in this specification can be listed more than once.)
The end of the list of Configuration Options is indicated by the
Length field of the LCP packet.
Unless otherwise specified, all Configuration Options apply in a
half-duplex fashion; typically, in the receive direction of the link
from the point of view of the Configure-Request sender.
Design Philosophy
The options indicate additional capabilities or requirements of
the implementation that is requesting the option. An
implementation which does not understand any option SHOULD
interoperate with one which implements every option.
A default is specified for each option which allows the link to
correctly function without negotiation of the option, although
perhaps with less than optimal performance.
Except where explicitly specified, acknowledgement of an option
does not require the peer to take any additional action other than
the default.
It is not necessary to send the default values for the options in
a Configure-Request.
A summary of the Configuration Option format is shown below. The
fields are transmitted from left to right.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Data ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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RFC 1661 Point-to-Point Protocol July 1994
Type
The Type field is one octet, and indicates the type of
Configuration Option. Up-to-date values of the LCP Option Type
field are specified in the most recent "Assigned Numbers" RFC [2].
This document concerns the following values:
0 RESERVED
1 Maximum-Receive-Unit
3 Authentication-Protocol
4 Quality-Protocol
5 Magic-Number
7 Protocol-Field-Compression
8 Address-and-Control-Field-Compression
Length
The Length field is one octet, and indicates the length of this
Configuration Option including the Type, Length and Data fields.
If a negotiable Configuration Option is received in a Configure-
Request, but with an invalid or unrecognized Length, a Configure-
Nak SHOULD be transmitted which includes the desired Configuration
Option with an appropriate Length and Data.
Data
The Data field is zero or more octets, and contains information
specific to the Configuration Option. The format and length of
the Data field is determined by the Type and Length fields.
When the Data field is indicated by the Length to extend beyond
the end of the Information field, the entire packet is silently
discarded without affecting the automaton.
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RFC 1661 Point-to-Point Protocol July 1994
6.1. Maximum-Receive-Unit (MRU)
Description
This Configuration Option may be sent to inform the peer that the
implementation can receive larger packets, or to request that the
peer send smaller packets.
The default value is 1500 octets. If smaller packets are
requested, an implementation MUST still be able to receive the
full 1500 octet information field in case link synchronization is
lost.
Implementation Note:
This option is used to indicate an implementation capability.
The peer is not required to maximize the use of the capacity.
For example, when a MRU is indicated which is 2048 octets, the
peer is not required to send any packet with 2048 octets. The
peer need not Configure-Nak to indicate that it will only send
smaller packets, since the implementation will always require
support for at least 1500 octets.
A summary of the Maximum-Receive-Unit Configuration Option format is
shown below. The fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Maximum-Receive-Unit |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
1
Length
4
Maximum-Receive-Unit
The Maximum-Receive-Unit field is two octets, and specifies the
maximum number of octets in the Information and Padding fields.
It does not include the framing, Protocol field, FCS, nor any
transparency bits or bytes.
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6.2. Authentication-Protocol
Description
On some links it may be desirable to require a peer to
authenticate itself before allowing network-layer protocol packets
to be exchanged.
This Configuration Option provides a method to negotiate the use
of a specific protocol for authentication. By default,
authentication is not required.
An implementation MUST NOT include multiple Authentication-
Protocol Configuration Options in its Configure-Request packets.
Instead, it SHOULD attempt to configure the most desirable
protocol first. If that protocol is Configure-Nak'd, then the
implementation SHOULD attempt the next most desirable protocol in
the next Configure-Request.
The implementation sending the Configure-Request is indicating
that it expects authentication from its peer. If an
implementation sends a Configure-Ack, then it is agreeing to
authenticate with the specified protocol. An implementation
receiving a Configure-Ack SHOULD expect the peer to authenticate
with the acknowledged protocol.
There is no requirement that authentication be full-duplex or that
the same protocol be used in both directions. It is perfectly
acceptable for different protocols to be used in each direction.
This will, of course, depend on the specific protocols negotiated.
A summary of the Authentication-Protocol Configuration Option format
is shown below. The fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Authentication-Protocol |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data ...
+-+-+-+-+
Type
3
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RFC 1661 Point-to-Point Protocol July 1994
Length
>= 4
Authentication-Protocol
The Authentication-Protocol field is two octets, and indicates the
authentication protocol desired. Values for this field are always
the same as the PPP Protocol field values for that same
authentication protocol.
Up-to-date values of the Authentication-Protocol field are
specified in the most recent "Assigned Numbers" RFC [2].
Current
values are assigned as follows:
Value (in hex) Protocol
c023 Password Authentication Protocol
c223 Challenge Handshake Authentication Protocol
Data
The Data field is zero or more octets, and contains additional
data as determined by the particular protocol.
6.3. Quality-Protocol
Description
On some links it may be desirable to determine when, and how
often, the link is dropping data. This process is called link
quality monitoring.
This Configuration Option provides a method to negotiate the use
of a specific protocol for link quality monitoring. By default,
link quality monitoring is disabled.
The implementation sending the Configure-Request is indicating
that it expects to receive monitoring information from its peer.
If an implementation sends a Configure-Ack, then it is agreeing to
send the specified protocol. An implementation receiving a
Configure-Ack SHOULD expect the peer to send the acknowledged
protocol.
There is no requirement that quality monitoring be full-duplex or
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RFC 1661 Point-to-Point Protocol July 1994
that the same protocol be used in both directions. It is
perfectly acceptable for different protocols to be used in each
direction. This will, of course, depend on the specific protocols
negotiated.
A summary of the Quality-Protocol Configuration Option format is
shown below. The fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Quality-Protocol |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data ...
+-+-+-+-+
Type
4
Length
>= 4
Quality-Protocol
The Quality-Protocol field is two octets, and indicates the link
quality monitoring protocol desired. Values for this field are
always the same as the PPP Protocol field values for that same
monitoring protocol.
Up-to-date values of the Quality-Protocol field are specified in
the most recent "Assigned Numbers" RFC [2]. Current values are
assigned as follows:
Value (in hex) Protocol
c025 Link Quality Report
Data
The Data field is zero or more octets, and contains additional
data as determined by the particular protocol.
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RFC 1661 Point-to-Point Protocol July 1994
6.4. Magic-Number
Description
This Configuration Option provides a method to detect looped-back
links and other Data Link Layer anomalies. This Configuration
Option MAY be required by some other Configuration Options such as
the Quality-Protocol Configuration Option. By default, the
Magic-Number is not negotiated, and zero is inserted where a
Magic-Number might otherwise be used.
Before this Configuration Option is requested, an implementation
MUST choose its Magic-Number. It is recommended that the Magic-
Number be chosen in the most random manner possible in order to
guarantee with very high probability that an implementation will
arrive at a unique number. A good way to choose a unique random
number is to start with a unique seed. Suggested sources of
uniqueness include machine serial numbers, other network hardware
addresses, time-of-day clocks, etc. Particularly good random
number seeds are precise measurements of the inter-arrival time of
physical events such as packet reception on other connected
networks, server response time, or the typing rate of a human
user. It is also suggested that as many sources as possible be
used simultaneously.
When a Configure-Request is received with a Magic-Number
Configuration Option, the received Magic-Number is compared with
the Magic-Number of the last Configure-Request sent to the peer.
If the two Magic-Numbers are different, then the link is not
looped-back, and the Magic-Number SHOULD be acknowledged. If the
two Magic-Numbers are equal, then it is possible, but not certain,
that the link is looped-back and that this Configure-Request is
actually the one last sent. To determine this, a Configure-Nak
MUST be sent specifying a different Magic-Number value. A new
Configure- Request SHOULD NOT be sent to the peer until normal
processing would cause it to be sent (that is, until a Configure-
Nak is received or the Restart timer runs out).
Reception of a Configure-Nak with a Magic-Number different from
that of the last Configure-Nak sent to the peer proves that a link
is not looped-back, and indicates a unique Magic-Number. If the
Magic-Number is equal to the one sent in the last Configure-Nak,
the possibility of a looped-back link is increased, and a new
Magic-Number MUST be chosen. In either case, a new Configure-
Request SHOULD be sent with the new Magic-Number.
If the link is indeed looped-back, this sequence (transmit
Configure-Request, receive Configure-Request, transmit Configure-
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RFC 1661 Point-to-Point Protocol July 1994
Nak, receive Configure-Nak) will repeat over and over again. If
the link is not looped-back, this sequence might occur a few
times, but it is extremely unlikely to occur repeatedly. More
likely, the Magic-Numbers chosen at either end will quickly
diverge, terminating the sequence. The following table shows the
probability of collisions assuming that both ends of the link
select Magic-Numbers with a perfectly uniform distribution:
Number of Collisions Probability
-------------------- ---------------------
1 1/2**32 = 2.3 E-10
2 1/2**32**2 = 5.4 E-20
3 1/2**32**3 = 1.3 E-29
Good sources of uniqueness or randomness are required for this
divergence to occur. If a good source of uniqueness cannot be
found, it is recommended that this Configuration Option not be
enabled; Configure-Requests with the option SHOULD NOT be
transmitted and any Magic-Number Configuration Options which the
peer sends SHOULD be either acknowledged or rejected. In this
case, looped-back links cannot be reliably detected by the
implementation, although they may still be detectable by the peer.
If an implementation does transmit a Configure-Request with a
Magic- Number Configuration Option, then it MUST NOT respond with a
Configure-Reject when it receives a Configure-Request with a
Magic-Number Configuration Option. That is, if an implementation
desires to use Magic Numbers, then it MUST also allow its peer to
do so. If an implementation does receive a Configure-Reject in
response to a Configure-Request, it can only mean that the link is
not looped-back, and that its peer will not be using Magic-
Numbers. In this case, an implementation SHOULD act as if the
negotiation had been successful (as if it had instead received a
Configure-Ack).
The Magic-Number also may be used to detect looped-back links
during normal operation, as well as during Configuration Option
negotiation. All LCP Echo-Request, Echo-Reply, and Discard-
Request packets have a Magic-Number field. If Magic-Number has
been successfully negotiated, an implementation MUST transmit
these packets with the Magic-Number field set to its negotiated
Magic-Number.
The Magic-Number field of these packets SHOULD be inspected on
reception. All received Magic-Number fields MUST be equal to
either zero or the peer's unique Magic-Number, depending on
whether or not the peer negotiated a Magic-Number.
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Reception of a Magic-Number field equal to the negotiated local
Magic-Number indicates a looped-back link. Reception of a Magic-
Number other than the negotiated local Magic-Number, the peer's
negotiated Magic-Number, or zero if the peer didn't negotiate one,
indicates a link which has been (mis)configured for communications
with a different peer.
Procedures for recovery from either case are unspecified, and may
vary from implementation to implementation. A somewhat
pessimistic procedure is to assume a LCP Down event. A further
Open event will begin the process of re-establishing the link,
which can't complete until the looped-back condition is
terminated, and Magic-Numbers are successfully negotiated.
A more
optimistic procedure (in the case of a looped-back link) is to
begin transmitting LCP Echo-Request packets until an appropriate
Echo-Reply is received, indicating a termination of the looped-
back condition.
A summary of the Magic- Number Configuration Option format is shown
below. The fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Magic-Number
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Magic-Number (cont) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
5
Length
6
Magic-Number
The Magic-Number field is four octets, and indicates a number
which is very likely to be unique to one end of the link. A
Magic-Number of zero is illegal and MUST always be Nak'd, if it is
not Rejected outright.
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RFC 1661 Point-to-Point Protocol July 1994
6.5. Protocol-Field-Compression (PFC)
Description
This Configuration Option provides a method to negotiate the
compression of the PPP Protocol field. By default, all
implementations MUST transmit packets with two octet PPP Protocol
fields.
PPP Protocol field numbers are chosen such that some values may be
compressed into a single octet form which is clearly
distinguishable from the two octet form. This Configuration
Option is sent to inform the peer that the implementation can
receive such single octet Protocol fields.
As previously mentioned, the Protocol field uses an extension
mechanism consistent with the ISO 3309 extension mechanism for the
Address field; the Least Significant Bit (LSB) of each octet is
used to indicate extension of the Protocol field. A binary "0" as
the LSB indicates that the Protocol field continues with the
following octet. The presence of a binary "1" as the LSB marks
the last octet of the Protocol field. Notice that any number of
"0" octets may be prepended to the field, and will still indicate
the same value ( consider the two binary representations for 3,
00000011 and 00000000 00000011).
When using low speed links, it is desirable to conserve bandwidth
by sending as little redundant data as possible. The Protocol-
Field-Compression Configuration Option allows a trade-off between
implementation simplicity and bandwidth efficiency. If
successfully negotiated, the ISO 3309 extension mechanism may be
used to compress the Protocol field to one octet instead of two.
The large majority of packets are compressible since data
protocols are typically assigned with Protocol field values less
than 256.
Compressed Protocol fields MUST NOT be transmitted unless this
Configuration Option has been negotiated. When negotiated, PPP
implementations MUST accept PPP packets with either double-octet
or single-octet Protocol fields, and MUST NOT distinguish between
them.
The Protocol field is never compressed when sending any LCP
packet. This rule guarantees unambiguous recognition of LCP
packets.
When a Protocol field is compressed, the Data Link Layer FCS field
is calculated on the compressed frame, not the original
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RFC 1661 Point-to-Point Protocol July 1994
uncompressed frame.
A summary of the Protocol-Field-Compression Configuration Option
format is shown below. The fields are transmitted from left to
right.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
7
Length
2
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RFC 1661 Point-to-Point Protocol July 1994
6.6. Address-and-Control-Field-Compression (ACFC)
Description
This Configuration Option provides a method to negotiate the
compression of the Data Link Layer Address and Control fields. By
default, all implementations MUST transmit frames with Address and
Control fields appropriate to the link framing.
Since these fields usually have constant values for point-to-point
links, they are easily compressed. This Configuration Option is
sent to inform the peer that the implementation can receive
compressed Address and Control fields.
If a compressed frame is received when Address-and-Control-Field-
Compression has not been negotiated, the implementation MAY
silently discard the frame.
The Address and Control fields MUST NOT be compressed when sending
any LCP packet. This rule guarantees unambiguous recognition of
LCP packets.
When the Address and Control fields are compressed, the Data Link
Layer FCS field is calculated on the compressed frame, not the
original uncompressed frame.
A summary of the Address-and-Control-Field-Compression configuration
option format is shown below. The fields are transmitted from left
to right.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
8
Length
2
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RFC 1661 Point-to-Point Protocol July 1994
Security Considerations
Security issues are briefly discussed in sections concerning the
Authentication Phase, the Close event, and the Authentication-
Protocol Configuration Option.
References
[1] Perkins, D., "Requirements for an Internet Standard Point-to-
Point Protocol", RFC 1547, Carnegie Mellon University,
December 1993.
[2] Reynolds, J., and Postel, J., "Assigned Numbers", STD 2, RFC
1340, USC/Information Sciences Institute, July 1992.
Acknowledgements
This document is the product of the Point-to-Point Protocol Working
Group of the Internet Engineering Task Force (IETF). Comments should
be submitted to the ietf-ppp@merit.edu mailing list.
Much of the text in this document is taken from the working group
requirements [1]; and RFCs 1171 & 1172, by Drew Perkins while at
Carnegie Mellon University, and by Russ Hobby of the University of
California at Davis.
William Simpson was principally responsible for introducing
consistent terminology and philosophy, and the re-design of the phase
and negotiation state machines.
Many people spent significant time helping to develop the Point-to-
Point Protocol. The complete list of people is too numerous to list,
but the following people deserve special thanks: Rick Adams, Ken
Adelman, Fred Baker, Mike Ballard, Craig Fox, Karl Fox, Phill Gross,
Kory Hamzeh, former WG chair Russ Hobby, David Kaufman, former WG
chair Steve Knowles, Mark Lewis, former WG chair Brian Lloyd, John
LoVerso, Bill Melohn, Mike Patton, former WG chair Drew Perkins, Greg
Satz, John Shriver, Vernon Schryver, and Asher Waldfogel.
Special thanks to Morning Star Technologies for providing computing
resources and network access support for writing this specification.
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RFC 1661 Point-to-Point Protocol July 1994
Chair's Address
The working group can be contacted via the current chair:
Fred Baker
Advanced Computer Communications
315 Bollay Drive
Santa Barbara, California 93117
fbaker@acc.com
Editor's Address
Questions about this memo can also be directed to:
William Allen Simpson
Daydreamer
Computer Systems Consulting Services
1384 Fontaine
Madison Heights, Michigan 48071
Bill.Simpson@um.cc.umich.edu
bsimpson@MorningStar.com
Simpson [Page 52]
