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RFC 2687 - PPP in a Real-time Oriented HDLC-like Framing


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Network Working Group                                         C. Bormann
Request for Comments: 2687                       Universitaet Bremen TZI
Category: Standards Track                                 September 1999

             PPP in a Real-time Oriented HDLC-like Framing

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.

Copyright Notice

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

Abstract

   A companion document describes an architecture for providing
   integrated services over low-bitrate links, such as modem lines, ISDN
   B-channels, and sub-T1 links [1].  The main components of the
   architecture are: a real-time encapsulation format for asynchronous
   and synchronous low-bitrate links, a header compression architecture
   optimized for real-time flows, elements of negotiation protocols used
   between routers (or between hosts and routers), and announcement
   protocols used by applications to allow this negotiation to take
   place.

   This document proposes the suspend/resume-oriented solution for the
   real-time encapsulation format part of the architecture.  The general
   approach is to start from the PPP Multilink fragmentation protocol
   [2] and its multi-class extension [5] and add suspend/resume in a way
   that is as compatible to existing hard- and firmware as possible.

1.  Introduction

   As an extension to the "best-effort" services the Internet is well-
   known for, additional types of services ("integrated services") that
   support the transport of real-time multimedia information are being
   developed for, and deployed in the Internet.

   The present document defines the suspend/resume-oriented solution for
   the real-time encapsulation format part of the architecture.  As
   described in more detail in the architecture document, a real-time
   encapsulation format is required as, e.g., a 1500 byte packet on a

   28.8 kbit/s modem link makes this link unavailable for the
   transmission of real-time information for about 400 ms.  This adds a
   worst-case delay that causes real-time applications to operate with
   round-trip delays on the order of at least a second -- unacceptable
   for real-time conversation.

   A true suspend/resume-oriented approach can only be implemented on a
   type-1 sender [1], but provides the best possible delay performance
   to this type of senders.  The format defined in this document may
   also be of interest to certain type-2-senders that want to exploit
   the better bit-efficiency of this format as compared to [5].  The
   format was designed so that it can be implemented by both type-1 and
   type-2 receivers.

1.1.  Specification Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [8].

2.  Requirements

   The requirements for this document are similar to those listed in
   [5].

   A suspend/resume-oriented solution can provide better worst-case
   latency than the pre-fragmenting-oriented solution defined in [5].
   Also, as this solution requires a new encapsulation scheme, there is
   an opportunity to provide a slightly more efficient format.

   Predictability, robustness, and cooperation with PPP and existing
   hard- and firmware installations are as important with suspend/resume
   as with pre-fragmenting.  A good suspend/resume solution achieves
   good performance even with type-2 receivers [1] and is able to work
   with PPP hardware such as async-to-sync converters.

   Finally, a partial non-requirement: While the format defined in this
   draft is based on the PPP multilink protocol ([2], also abbreviated
   as MP), operation over multiple links is in many cases not required.

3.  General Approach

   As in [5], the general approach is to start out from PPP multilink
   and add multiple classes to obtain multiple levels of suspension.
   However, in contrast to [5], more significant changes are required to
   be able to suspend the transmission of a packet at any point and
   inject a higher priority packet.

   The applicability of the multilink header for suspend/resume type
   implementations is limited, as the "end" bit is in the multilink
   header, which is the wrong place for suspend/resume operation.  To
   make a big packet suspendable, it must be sent with the "end" bit
   off, and (unless the packet was suspended a small number of bytes
   before its end) an empty fragment has to be sent afterwards to
   "close" the packet.  The minimum overhead for sending a suspendable
   packet thus is twice the multilink header size (six bytes, including
   a compressed multilink protocol field) plus one PPP framing (three
   bytes).  Each suspension costs another six bytes (not counting the
   overhead of the framing for the intervening packet).

   Also, the existing multi-link header is relatively large; as the
   frequency of small high-priority packets increases, the overhead
   becomes significant.

   The general approach of this document is to start from PPP Multilink
   with classes and provide a number of extensions to add functionality
   and reduce the overhead of using PPP Multilink for real-time
   transmission.

   This document introduces two new features:

   1)   A compact fragment format and header, and

   2)   a real-time frame format.

4.  The Compact Fragment Format

   This section describes an optional multilink fragment format that is
   more optimized towards single-link operation and frequent suspension
   (type 1 senders)/a small fragment size (type 2 senders), with
   optional support for multiple links.

   When operating over a single link, the Multilink sequence number is
   used only for loss detection.  Even a 12-bit sequence number clearly
   is larger than required for this application on most kinds of links.
   We therefore define the following compact multilink header format
   option with a three-bit sequence number.

   As, with a compact header, there is little need for sending packets
   outside the multilink, we can provide an additional compression
   mechanism for this format: the MP protocol identifier is not sent
   with the compact fragment header.  This obviously requires prior
   negotiation (similar to the way address and control field compression
   are negotiated), as well as a method for avoiding the bit combination

   0xFF (the first octet in an LCP frame before any LCP options have
   been negotiated), as the start of a new LCP negotiation could
   otherwise not be reliably detected.

                  Figure 1:  Compact Fragment Format

                    0   1   2   3   4   5   6   7
                  +---+---+---+---+---+---+---+---+
                  | R |  sequence |   class   | 1 |
                  +---+---+---+---+---+---+---+---+
                  |            data               |
                  :                               :
                  +---+---+---+---+---+---+---+---+

   Having the least significant bit always be 1 helps with HDLC chips
   that operate specially on least significant bits in HDLC addresses.
   (Initial bytes with the least significant bit set to zero are used
   for the extended compact fragment format, see next section.)

   The R bit is the inverted equivalent of the B bit in the other
   multilink fragment formats, i.e. R = 1 means that this fragment
   resumes a packet previous fragments of which have been sent already.

   The following trick avoids the case of a header byte of 0xFF (which
   would mean R=1, sequence=7, and class=7): If the class field is set
   to 7, the R bit MUST never be set to one.  I.e., class 7 frames by
   design cannot be suspended/resumed.  (This is also the reason the
   sense of the B bit is inverted to an R bit in the compact fragment
   format -- class 7 would be useless otherwise, as a new packet could
   never be begun.)

   As the sequence number is not particularly useful with the class
   field set to 7, it is used to distinguish eight more classes -- for
   some minor additional complexity, the applicability of prefix elision
   is significantly increased by providing more classes with possibly
   different elided prefixes.

   For purposes of prefix elision, the actual class number of a fragment
   is computed as follows:

   -  If the class field is 0 to 6, the class number is 0 to 6,

   -  if the class field is 7 and the sequence field is 0 to 7, the
      class number is 7 to 14.

   As a result of this scheme, the classes 0 to 6 can be used for
   suspendable packets, and classes 7 to 14 (where the class field is 7
   and the R bit must always be off) can be used for non-suspendable
   high-priority classes, e.g., eight highly compressed voice streams.

5.  The Extended Compact Fragment Format

   For operation over multiple links, a three-bit sequence number will
   rarely be sufficient.  Therefore, we define an optional extended
   compact fragment format.  The option, when negotiated, allows both
   the basic compact fragment format and the extended compact fragment
   format to be used; each fragment indicates which format it is in.

               Figure 1:  Extended Compact Fragment Format

                     0   1   2   3   4   5   6   7
                   +---+---+---+---+---+---+---+---+
                   | R |  seq LSB  |   class   | 0 |
                   +---+---+---+---+---+---+---+---+
                   |      sequence -- MSB      | 1 |
                   +---+---+---+---+---+---+---+---+
                   |            data               |
                   :                               :
                   +---+---+---+---+---+---+---+---+

   In the extended compact fragment format, the sequence number is
   composed of three least significant bits from the first octet of the
   fragment header and seven most significant bits from the second
   octet.  (Again, the least significant bit of the second octet is
   always set to one for compatibility with certain HDLC chips.)

   For prefix elision purposes, fragments with a class field of 7 can
   use the basic format to indicate classes 7 to 14 and the extended
   format to indicate classes 7 to 1030.  Different classes may use
   different formats concurrently without problems.  (This allows some
   classes to be spread over a multi-link and other classes to be
   confined to a single link with greater efficiency.)  For class fields
   0 to 6, i.e. suspendable classes, one of the two compact fragment
   formats SHOULD be used consistently within each class.

   If the use of the extended compact fragment format has been
   negotiated, receivers MAY keep 10-bit sequence numbers for all
   classes to facilitate senders switching formats in a class.  When a
   sender starts sending basic format fragments in a class that was
   using extended format fragments, the 3-bit sequence number can be
   taken as a modulo-8 version of the 10-bit sequence number, and no
   discontinuity need result.  In the inverse case, if a 10-bit sequence
   number has been kept throughout by the receiver (and no major slips

   of the sequence number have occurred), no discontinuity will result,
   although this cannot be guaranteed in the presence of errors.
   (Discontinuity, in this context, means that a receiver has to
   resynchronize sequence numbers by discarding fragments until a
   fragment with R=0 has been seen.)

6.  Real-Time Frame Format

   This section defines how fragments with compact fragment headers are
   mapped into real-time frames.  This format has been designed to
   retain the overall HDLC based format of frames, so that existing
   synchronous HDLC chips and async to sync converters can be used on
   the link.  Note that if the design could be optimized for async only
   operation, more design alternatives would be available [4]; with the
   advent of V.80 style modems, asynchronous communications is likely to
   decrease in importance, though.

   The compact fragment format provides a compact rendition of the PPP
   multilink header with classes and a reduced sequence number space.
   However, it does not encode the E-bit of the PPP multilink header,
   which indicates whether the fragment at hand is the last fragment of
   a packet.

   For a solution where packets can be suspended at any point in time,
   the E-bit needs to be encoded near the end of each fragment.  The
   real-time frame format, to ensure maximum compatibility with type 2
   receivers, encodes the E-bit in the following way: Any normal frame
   ending also ends the current fragment with E implicitly set to one.
   This ensures that packets that are ready for delivery to the upper
   layers immediately trigger a receive interrupt even at type-2
   receivers.

   Fragments of packets that are to be suspended are terminated within
   the HDLC frame by a special "fragment suspend escape" byte (FSE).
   The overall structure of the HDLC frame does not change; the
   detection and handling of FSE bytes is done at a layer above HDLC
   framing.

   The suspend/resume format with FSE detection is an alternative to
   address/control field compression (ACFC, LCP option 8).  It does not
   apply to frames that start with 0xFF, the standard PPP-in-HDLC
   address field; these frames are handled as defined in [6] and [7].
   (This provision ensures that attempts to renegotiate LCP do not cause
   ambiguities.)

   For frames that do not start with 0xFF, suspend/resume processing
   performs a scan of every HDLC frame received.  The FCS of the HDLC
   frame is checked and stripped.  Compact fragment format headers (both
   basic and extended) are handled without further FSE processing.
   (Note that, as the FSE byte was chosen such that it never occurs in
   compact fragment format headers, this does not require any specific
   code.)

   Within the remaining bytes of the HDLC frame ("data part"), an FSE
   byte is used to indicate the end of the current fragment, with an E
   bit implicitly cleared.  All fragments up to the last FSE are
   considered suspended (E = 0); the final fragment is terminated (E =
   1), or, if it is empty, ignored (i.e., the data part of an HDLC frame
   can end in an FSE to indicate that the last fragment has E = 0).

   Each fragment begins with a normal header, so the structure of a
   frame could be:

                Figure 2:  Example frame with FSE delimiter

     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | R |  sequence |   class   | 1 |
   +---+---+---+---+---+---+---+---+
   |            data               |
   :                               :
   +---+---+---+---+---+---+---+---+
   +              FSE              + previous fragment implicitly E = 0
   +---+---+---+---+---+---+---+---+
   | R |  sequence |   class   | 1 |
   +---+---+---+---+---+---+---+---+
   |            data               |
   :                               :
   +---+---+---+---+---+---+---+---+
   |             Frame             | previous fragment implicitly E = 1
   |              CRC              |
   +---+---+---+---+---+---+---+---+

   The value chosen for FSE is 0xDE (this is a relatively unlikely byte
   to occur in today's data streams, it does not trigger octet stuffing
   and triggers bit stuffing only for 1/8 of the possible preceding
   bytes).

   The remaining problem is that of data transparency.  In the scheme
   described so far, an FSE is always followed by a compact fragment
   header.  In these headers, the combination of a class field set to 7

   with R=1 is reserved.  Data transparency is achieved by making the
   occurrence of an FSE byte followed by one of 0x8F, 0x9F, ... to 0xFF
   special.

            Figure 3:  Data transparency with FSE bytes present

           0   1   2   3   4   5   6   7
          +---+---+---+---+---+---+---+---+
          | R |  sequence |   class   | 1 |
          +---+---+---+---+---+---+---+---+
          |            data               |
          :                               :
          +---+---+---+---+---+---+---+---+
          +              FSE              + fragment NOT terminated
          +---+---+---+---+---+---+---+---+
          | R | S | T | U | 1 | 1 | 1 | 1 | R always is 1
          +---+---+---+---+---+---+---+---+
          |            data               | fragment continues
          :                               :

   In a combination of FSE/0xnF (where n is the first four-bit field in
   the second byte, RSTU in Figure 3), the n field gives a sequence of
   four bits indicating where in the received data stream FSE bytes,
   which cannot simply be transmitted in the data stream, are to be
   added by the receiver:

0x8F: insert one FSE, back to data
0x9F: insert one FSE, copy two data bytes, insert one FSE, back to data
0xAF: insert one FSE, copy one data byte, insert one FSE, back to data
0xBF: insert one FSE, copy one data byte, insert two FSE bytes, back
      to data
0xCF: insert two FSE bytes, back to data
0xDF: insert two FSE bytes, copy one data byte, insert one FSE, back
      to data
0xEF: insert three FSE bytes, back to data
0xFF: insert four FSE bytes, back to data

   The data bytes following the FSE/0xnF combinations and corresponding
   to the zero bits in the N field may not be FSE bytes.

   This scheme limits the worst case expansion factor by FSE processing
   to about 25 %.  Also, it is designed such that a single data stream
   can either trigger worst-case expansion by octet stuffing (or by bit
   stuffing) or worst-case FSE processing, but never both.  Figure 4
   illustrates the scheme in a few examples; FSE/0xnF pairs are written
   in lower case.

                 Figure 4:  Data transparency examples

            Data stream                     FSE-stuffed stream

            DD DE DF E0                     DD de 8f DF E0
            01 DE 02 DE 03                  01 de af 02 03
            DE DA DE DE DB                  de bf DA DB
            DE DE DE DE DE DA               de ff de 8f DA

   In summary, the real-time frame format is a HDLC-like frame delimited
   by flags and containing a final FCS as defined in [7], but without
   address and control fields, containing as data a sequence of FSE-
   stuffed fragments in compact fragment format, delimited by FSE bytes.
   As a special case, the final FSE may occur as the last byte of the
   data content (i.e. immediately before the FCS bytes) of the HDLC-like
   frame, to indicate that the last fragment in the frame is suspended
   and no final fragment is in the frame (e.g., because the desirable
   maximum size of the frame has been reached).

7.  Implementation notes

7.1.  MRU Issues

   The LCP parameter MRU defines the maximum size of the packets sent on
   the link.  Async-to-sync converters that are monitoring the LCP
   negotiations on the link may interpret the MRU value as the maximum
   HDLC frame size to be expected.

   Implementations of this specification should preferably negotiate a
   sufficiently large MRU to cover the worst-case 25 % increase in frame
   size plus the increase caused by suspended fragments.  If that is not
   possible, the HDLC frame size should be limited by monitoring the
   HDLC frame sizes and possibly suspending the current fragment by
   sending an FSE with an empty final fragment (FSE immediately followed
   by the end of the information field, i.e. by CRC bytes and a flag) to
   be able to continue in a new HDLC frame.  This strategy also helps
   minimizing the impact of lengthening the HDLC frame on the safety of
   the 16-bit FCS at the end of the HDLC frame.

7.2.  Implementing octet-stuffing and FSE processing in one automaton

   The simplest way to add real-time framing to an implementation may be
   to perform HDLC processing as usual and then, on the result, to
   perform FSE processing.  A more advanced implementation may want to
   combine the two levels of escape character processing.  Note,
   however, that FSE processing needs to wait until two bytes from the
   HDLC frame are available and followed by a third to ensure that the
   bytes are not the final HDLC FCS bytes, which are not subject to FSE

   processing.  I.e., on the reception of normal data byte, look for an
   FSE in the second-to-previous byte, and, on the reception of a
   frame-end, look for an FSE as the last data byte.

8.  Negotiable options

   The following options are already defined by MP [2]:

   o    Multilink Maximum Received Reconstructed Unit

   o    Multilink Short Sequence Number Header Format

   o    Endpoint Discriminator

   The following options are already defined by MCML [5]:

   o    Multilink Header Format

   o    Prefix Elision

   This document defines two new code points for the Multilink Header
   Format option.

8.1.  Multilink header format option

   The multilink header format option is defined in [5].  A summary of
   the Multilink Header Format Option format is shown below.  The fields
   are transmitted from left to right.

           Figure 5:  Multilink header format option

     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 = 27   |  Length = 4   |     Code      | # Susp Clses  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    As defined in [5], this LCP option advises the peer that the
    implementation wishes to receive fragments with a format given by
    the code number, with the maximum number of suspendable classes (see
    below) given.  This specification defines two additional values for
    Code, in addition to those defined in [5]:

   -  Code = 11: basic and extended compact real-time fragment format
      with classes, in FSE-encoded HDLC frame

   -  Code = 15: basic compact real-time fragment format with classes,
      in FSE-encoded HDLC frame

   An implementation MUST NOT request a combination of both LCP
   Address-and-Control-Field-Compression (ACFC) and the code values 11
   or 15 for this option.

   The number of suspendable classes negotiated for the compact real-
   time fragment format only limits the use of class numbers that allow
   suspending.  As class numbers of 7 and higher do not require
   additional reassembly space, they are not subject to the class number
   limit negotiated.

9.  Security Considerations

   Operation of this protocol is believed to be no more and no less
   secure than operation of the PPP multilink protocol [2].  Operation
   with a small sequence number range increases the likelihood that
   fragments from different packets could be incorrectly reassembled
   into one packet.  While most such packets will be discarded by the
   receiver because of higher-layer checksum failures or other
   inconsistencies, there is an increase in likelihood that contents of
   packets destined for one host could be delivered to another host.
   Links that carry packets where this raises security considerations
   SHOULD use the extended sequence number range for multi-fragment
   packets.

10.  References

   [1]  Bormann, C., "Providing Integrated Services over Low-bitrate
        Links", RFC 2689, September 1999.

   [2]  Sklower, K., Lloyd, B., McGregor, G., Carr, D. and  T.
        Coradetti, "The PPP Multilink Protocol (MP)", RFC 1990, August
        1996.

   [3]  Simpson, W., "PPP in Frame Relay", RFC 1973, June 1996.

   [4]  Andrades, R. and F. Burg, "QOSPPP Framing Extensions to PPP",
        Work in Progress.

   [5]  Bormann, C., "The Multi-Class Extension to Multi-Link PPP", RFC
        2686, September 1999.

   [6]  Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", STD
        51, RFC 1661, July 1994.

   [7]  Simpson, W., Editor, "PPP in HDLC-like Framing", STD 51, RFC
        1662, July 1994.

   [8]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

11.  Author's Address

   Carsten Bormann
   Universitaet Bremen FB3 TZI
   Postfach 330440
   D-28334 Bremen, GERMANY

   Phone: +49.421.218-7024
   Fax:   +49.421.218-7000
   EMail: cabo@tzi.org

Acknowledgements

   The participants in a lunch BOF at the Montreal IETF 1996 gave useful
   input on the design tradeoffs in various environments.  Richard
   Andrades, Fred Burg, and Murali Aravamudan insisted that there should
   be a suspend/resume solution in addition to the pre-fragmenting one
   defined in [5].  The members of the ISSLL subgroup on low bitrate
   links (ISSLOW) have helped in coming up with a set of requirements
   that shaped this solution.

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