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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.


 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
 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

Bormann Standards Track [Page 1] RFC 2687 PPP in Real-time Oriented HDLC-like Framing September 1999

 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",
 document are to be interpreted as described in RFC 2119 [8].

2. Requirements

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

Bormann Standards Track [Page 2] RFC 2687 PPP in Real-time Oriented HDLC-like Framing September 1999

 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
 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

Bormann Standards Track [Page 3] RFC 2687 PPP in Real-time Oriented HDLC-like Framing September 1999

 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:
  1. If the class field is 0 to 6, the class number is 0 to 6,
  1. if the class field is 7 and the sequence field is 0 to 7, the

class number is 7 to 14.

Bormann Standards Track [Page 4] RFC 2687 PPP in Real-time Oriented HDLC-like Framing September 1999

 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

Bormann Standards Track [Page 5] RFC 2687 PPP in Real-time Oriented HDLC-like Framing September 1999

 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
 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
 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

Bormann Standards Track [Page 6] RFC 2687 PPP in Real-time Oriented HDLC-like Framing September 1999

 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
 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
 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

Bormann Standards Track [Page 7] RFC 2687 PPP in Real-time Oriented HDLC-like Framing September 1999

 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
          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.

Bormann Standards Track [Page 8] RFC 2687 PPP in Real-time Oriented HDLC-like Framing September 1999

               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

Bormann Standards Track [Page 9] RFC 2687 PPP in Real-time Oriented HDLC-like Framing September 1999

 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]:
  1. Code = 11: basic and extended compact real-time fragment format

with classes, in FSE-encoded HDLC frame

  1. Code = 15: basic compact real-time fragment format with classes,

in FSE-encoded HDLC frame

Bormann Standards Track [Page 10] RFC 2687 PPP in Real-time Oriented HDLC-like Framing September 1999

 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

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
 [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.

Bormann Standards Track [Page 11] RFC 2687 PPP in Real-time Oriented HDLC-like Framing September 1999

 [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


 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.

Bormann Standards Track [Page 12] RFC 2687 PPP in Real-time Oriented HDLC-like Framing September 1999

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 Internet Society.

Bormann Standards Track [Page 13]

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