Network Working Group                                         W. Simpson
Request for Comments: 1618                                    Daydreamer
Category: Standards Track                                       May 1994

                             PPP over ISDN

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.


   The Point-to-Point Protocol (PPP) [1] provides a standard method for
   transporting multi-protocol datagrams over point-to-point links.
   This document describes the use of PPP over Integrated Services
   Digital Network (ISDN) switched circuits.

   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 mailing list.


   This specification is intended for those implementations which desire
   to use the PPP encapsulation over ISDN point-to-point links.  PPP is
   not designed for multi-point or multi-access environments.

   "It is clear that there is never likely to be a single, monolithic,
   worldwide ISDN." [3] The goal of this document is to describe a few
   common implementations, chosen from the current wide variety of
   alternatives, in an effort to promote interoperability.

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RFC 1618                     PPP over ISDN                      May 1994

                           Table of Contents

     1.     Introduction ..........................................    1

     2.     Physical Layer Requirements ...........................    1

     3.     Framing ...............................................    3

     4.     Out-of-Band signaling .................................    4

     5.     Configuration Details .................................    5

     SECURITY CONSIDERATIONS ......................................    5

     REFERENCES ...................................................    5

     ACKNOWLEDGEMENTS .............................................    6

     CHAIR'S ADDRESS ..............................................    6

     AUTHOR'S ADDRESS .............................................    6

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RFC 1618                     PPP over ISDN                      May 1994

1.  Introduction

   PPP was designed as a standard method of communicating over point-
   to-point links.  Initial deployment has been over short local lines,
   leased lines, and plain-old-telephone-service (POTS) using modems.
   As new packet services and higher speed lines are introduced, PPP is
   easily deployed in these environments as well.

   This specification is primarily concerned with the use of the PPP
   encapsulation over ISDN links.  Since the ISDN B-channel is by
   definition a point-to-point circuit, PPP is well suited to use over
   these links.

   The ISDN Primary Rate Interface (PRI) may support many concurrent B-
   channel links.  The PPP LCP and NCP mechanisms are particularly
   useful in this situation in reducing or eliminating hand
   configuration, and facilitating ease of communication between diverse

   The ISDN D-channel can also be used for sending PPP packets when
   suitably framed, but is limited in bandwidth and often restricts
   communication links to a local switch.

   The terminology of ISDN can be confusing.  Here is a simple graphical
   representation of the points used in subsequent descriptions:

                   +-------+     +-------+     +-------+
               R   |       |  S  |       |  T  |       |   U
               +---+  TA   +--+--+  NT2  +--+--+  NT1  +---+
                   |       |     |       |     |       |
                   +-------+     +-------+     +-------+

   These elements are frequently combined into a single device.

2.  Physical Layer Requirements

   PPP treats ISDN channels as bit or octet oriented synchronous links.
   These links MUST be full-duplex, but MAY be either dedicated or

   Interface Format

      PPP presents an octet interface to the physical layer.  There is
      no provision for sub-octets to be supplied or accepted.  The octet
      stream is applied primarily at the R or T reference points.

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RFC 1618                     PPP over ISDN                      May 1994

   Transmission Rate

      PPP does not impose any restrictions regarding transmission rate,
      other than that of the particular ISDN channel interface.

   Control Signals

      PPP does not require the use of control signals.  When available,
      using such signals can allow greater functionality and
      performance.  Implications are discussed in [2].

      Control signals MAY be required by some of the framing techniques
      described, and is outside the scope of this specification.


      The definition of various encodings and scrambling is the
      responsibility of the DTE/DCE equipment in use, and is outside the
      scope of this specification.

      While PPP will operate without regard to the underlying
      representation of the bit stream, lack of standards for
      transmission will hinder interoperability as surely as lack of
      data link standards.  The D-channel LAPD interface requires NRZ
      encoding at the T reference point.  Therefore, as a default, it is
      recommended that NRZ be used over the B-channel interface at the T
      reference point.  This will allow frames to be easily exchanged
      between the B and D channels.

      When configuration of the encoding is allowed, NRZI is recommended
      as an alternative in order to ensure a minimum ones density where
      required over the clear B-channel, with caveats regarding FCS [2].

      Historically, some implementations have used Inverted NRZ (merely
      switching the sense of mark and space), in order to ensure a
      minimum ones density with bit-synchronous HDLC.  The use of
      Inverted NRZ is deprecated.

      Automatic Detection

         Implementations which desire to interoperate with multiple
         encodings MAY choose to detect those encodings automatically.
         Automatic encoding detection is particularly important for
         Primary Rate Interfaces, to avoid extensive pre-configuration.
         Only simple encodings are currently distinguished.

         The only reliable method of detection available is to switch
         modes between the supported encodings.  Transmission of the LCP

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RFC 1618                     PPP over ISDN                      May 1994

         Configure-Request SHOULD be tried twice for each mode before
         switching in rotation.  This ensures that sufficient time is
         available for a response to arrive from the peer.

         Max-Configure MUST be set such that the cumulative attempts
         result in no more than 59 seconds of time before disconnect.
         It is preferable that the usual limit of 30 seconds be

      Prior Configuration

         By prior configuration, PPP MAY also be used with other
         encodings.  Because of difficulty distinguishing them, it is
         not recommended that these encodings be automatically detected.

         Terminal adapters conforming to V.120 [4] can be used as a
         simple interface to workstations.  Asynchronous HDLC framing
         [2] is accepted at the R reference point.  The terminal adapter
         provides async-sync conversion.  Multiple B-channels can be
         used in parallel.  Unfortunately, V.120 has a framing mode of
         its own for rate adaptation, which is difficult to distinguish
         from Frame Relay, and which can confuse in-band frame
         detection.  V.120 is not interoperable with bit-synchronous
         links, since V.120 does not provide octet-stuffing to bit-
         stuffing conversion.  Therefore, V.120 is deprecated in favor
         of more modern standards, such as "PPP in Frame Relay".

         The "Bandwidth On Demand Interoperability Group" has defined a
         proposal called BONDING.  Multiple B-channels can be used in
         parallel.  BONDING has an initialization period of its own,
         which might conflict with the simple detection technique
         described above, and requires extensive individual
         configuration in some current implementations when multiple B-
         channels are involved.  It is recommended that the PPP Multi-
         Link Procedure be used instead of BONDING.

3.  Framing

   For B-channels, in the absence of prior configuration, the
   implementation MUST first use bit-synchronous HDLC [2], as opposed to
   other framings, for initial link establishment.  This assumes that
   circuit-switched communications are generally [host | router] to
   [host | router].

   By prior configuration, octet-synchronous HDLC [2] is recommended
   where the network termination equipment interfaces directly to the T

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RFC 1618                     PPP over ISDN                    May 1994

   reference point, and octet boundaries are available at the time of
   framing.  Such equipment is likely to be highly integrated, and the
   elimination of bit-synchronous hardware can reduce the part count,
   resulting in lower cost interfaces and simpler configuration.
   Octet-synchronous HDLC MUST be used with NRZ bit encoding.

   For D-channels, by default no data service is expected.  By prior
   configuration, "PPP in X.25" or "PPP in Frame Relay" framing MAY be

   Despite the fact that HDLC, LAPB, LAPD, and LAPF are nominally
   distinguishable, multiple methods of framing SHOULD NOT be used
   concurrently on the same ISDN channel.  There is no requirement that
   PPP recognize alternative framing techniques, or switch between
   framing techniques without specific configuration.

4.  Out-of-Band signaling

   Experience has shown that the LLC Information Element is not reliably
   transmitted end to end.  The deployment of compatible switches is too
   limited, and the subscription policies of the providers are too
   diverse.  Therefore, transmission of the LLC-IE SHOULD NOT be relied
   upon for framing or encoding determination.

   No LLC-IE values which pertain to PPP have been assigned.  Any other
   values which are received are not valid for PPP links, and can be
   ignored for PPP service.

   As an alternative administrative measure, multiple directory numbers
   can point to the same physical access facility, by binding particular
   services to each directory number.  The called party identifier has
   proven to be reliably provided by the local switch.

   When a called party identifier is used, or when a future LLC-IE value
   is assigned to PPP and the PPP value is received, if the LCP has not
   had the administrative Open event, the call MUST be rejected.
   Receivers MUST NOT accept an incoming call, only to close the circuit
   or ignore packets from the circuit.

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RFC 1618                     PPP over ISDN                      May 1994

5.  Configuration Details

   The LCP recommended sync configuration options apply to ISDN links.

   The standard LCP sync configuration defaults apply to ISDN links.

   The typical network feeding the link is likely to have a MRU of
   either 1500, or 2048 or greater.  To avoid fragmentation, the
   Maximum-Transmission-Unit (MTU) at the network layer SHOULD NOT
   exceed 1500, unless a peer MRU of 2048 or greater is specifically

   Instead of a constant value for the Restart timer, the exponential
   backoff method is recommended.  The Restart Timer SHOULD be 250
   milliseconds for the initial value, and 3 seconds for the final

   Implementations that include persistent dialing features, such as
   "demand dialing" or "redialing", SHOULD use mechanisms to limit their
   persistence.  Examples of such mechanisms include exponential
   backoff, and discarding packet queues after failure to complete link
   establishment.  In some implementations, discarding the transmit
   queue can temporarily remove the stimulus to retry the connection.

Security Considerations

   Security issues are not discussed in this memo.


   [1]   Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", RFC
         1548, Daydreamer, December 1993.

   [2]   Simpson, W., Editor, "PPP in HDLC Framing", RFC 1549,
         Daydreamer, December 1993.

   [3]   Stallings, W, "ISDN and Broadband ISDN - 2nd ed", Macmillan,

   [4]   CCITT Recommendations I.465 and V.120, "Data Terminal Equipment
         Communications over the Telephone Network with Provision for
         Statistical Multiplexing", CCITT Blue Book, Volume VIII,
         Fascicle VIII.1, 1988.

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RFC 1618                     PPP over ISDN                      May 1994


   This design was inspired by previous drafts of C. Frost, B. Gorsline,
   D. Leifer, K. Muramaki, S. Sheldon, K. Sklower, and T. Sugawara.

   Thanks to Oliver Korfmacher (NetCS) for European considerations, Dory
   Leifer (University of Michigan) for technical details and called
   party signalling, and Vernon Schryver (Silicon Graphics) regarding
   handling of link misconfiguration and timeouts.

   Special thanks to Morning Star Technologies for providing computing
   resources and network access support for writing this specification.

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


Author's Address

   Questions about this memo can also be directed to:

      William Allen Simpson
      Computer Systems Consulting Services
      1384 Fontaine
      Madison Heights, Michigan  48071


Simpson                                                         [Page 6]