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Network Working Group                                         W. SimpsonRequest for Comments: 1618                                    DaydreamerCategory: Standards Track                                       May 1994PPP over ISDNStatus 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) [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 ietf-ppp@merit.edu mailing list.Applicability   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.Simpson                                                         [Page i]

RFC 1618                     PPP over ISDN                      May 1994                           Table of Contents1.     Introduction ..........................................12.     Physical Layer Requirements ...........................13.     Framing ...............................................34.     Out-of-Band signaling .................................45.     Configuration Details .................................5     SECURITY CONSIDERATIONS ......................................5     REFERENCES ...................................................5     ACKNOWLEDGEMENTS .............................................6     CHAIR'S ADDRESS ..............................................6     AUTHOR'S ADDRESS .............................................6Simpson                                                         [Page ii]

RFC 1618                     PPP over ISDN                      May 19941.  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   implementations.   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   circuit-switched.   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.Simpson                                                         [Page 1]

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.   Encoding      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 LCPSimpson                                                         [Page 2]

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         observed.      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 TSimpson                                                         [Page 3]

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   used.   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.Simpson                                                         [Page 4]

RFC 1618                     PPP over ISDN                      May 19945.  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   negotiated.   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   value.   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.References   [1]   Simpson, W., Editor, "The Point-to-Point Protocol (PPP)",RFC1548, 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,         1992.   [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.Simpson                                                         [Page 5]

RFC 1618                     PPP over ISDN                      May 1994Acknowledgments   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      EMail: fbaker@acc.comAuthor'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      EMail: Bill.Simpson@um.cc.umich.edu             bsimpson@MorningStar.comSimpson                                                         [Page 6]