Internet Info 1997 December/Internet_Info_CD-ROM_Walnut_Creek_December_1997.iso/drafts/draft_ietf_i/draft-ietf-issll-isslow-rtf-00.txt

Text File | 1997-03-26 | 23KB | 581 lines

INTERNET-DRAFT                                           Carsten Bormann
Expires: September 1997                              Universitaet Bremen
                                                              March 1997


             PPP in a real-time oriented HDLC-like framing
                   draft-ietf-issll-isslow-rtf-00.txt


Status of this memo

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
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   To learn the current status of any Internet-Draft, please check the
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   ftp.isi.edu (US West Coast).

   Distribution of this document is unlimited.

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.

   This document is a product of the IETF ISSLL working group.
   Comments are solicited and should be addressed to the two working
   groups' mailing lists at [e-mail] and ietf-
   [e-mail] and/or the author.



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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.

   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.

   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].  It can
   be implemented by both type-1 and type-2 receivers.


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 non-requirement: While the format defined in this draft is
   based on the PPP multilink protocol ([2], also abbreviated as MP), it

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   specifically does NOT address operation over multiple links.


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).

   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 the multilink, we can provide an additional compression

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   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, 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.
   Headers with the least significant bit set to zero are reserved for
   future extensions.

   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: If the
   class field is set to 7, the R bit can never be 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.

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5.  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.  Within the remaining bytes of the
   HDLC frame, 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; the final fragment is terminated,
   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:




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                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,

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   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


   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).


6.  Implementation notes


6.1.  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.


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   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 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.

6.2.  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.

7.  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 is already defined by MCML [5]:

   o    Multilink Header Format

   o    Prefix Elision

   This document defines one new code point for the Multilink Header
   Format option.

7.1.  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.






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INTERNET-DRAFTPPP in a real-time oriented HDLC-like framing   March 1997


                 Figure 5:  Multilink header format option

    0                   1                   2
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Type = TBD   |  Length = 3   |     Code      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   This option advises the peer that the implementation wishes to
   receive fragments with a format given by the code number.  When this
   option is received, an implementation MUST either transmit all
   subsequent multilink packets on all links of the bundle with the
   multilink header format given or configure-NAK or configure-Reject
   the option.  This specification defines one additional value for
   Code, in addition to those defined in [5]:

   -    This option present with code = 15: compact real-time fragment
        format with classes in FSE-encoded HDLC frame

   An implementation MUST NOT request a combination of both the Short
   Sequence Number Header Format Option and this option, or of both LCP
   Address-and-Control-Field-Compression (ACFC) and the code value 15
   for this option.


8.  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.

9.  References


   [1]  C. Bormann, Providing integrated services over low-bitrate
        links, work in progress (draft-ietf-issll-isslow-01.txt), March
        1997.

   [2]  K. Sklower, B. Lloyd, G. McGregor, D. Carr, T. Coradetti, ``The
        PPP Multilink Protocol (MP)'', RFC 1990, August 1996 (obsoletes
        RFC1717).

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

   [4]  R. Andrades, F. Burg, ``QOSPPP Framing Extensions to PPP'',
        September 20, 1996, Work in Progress (draft-andrades-framing-
        ext-00.txt).

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INTERNET-DRAFTPPP in a real-time oriented HDLC-like framing   March 1997

   [5]  C. Bormann, The Multi-Class Extension to Multi-Link PPP, work in
        progress (draft-ietf-issll-isslow-mcml-01.txt), March 1997.

   [6]  W. Simpson, Editor. ``The Point-to-Point Protocol (PPP)'', RFC
        1661, July 1994 (Obsoletes RFC1548, also STD0051).

   [7]  W. Simpson, Editor. ``PPP in HDLC-like Framing'', RFC 1662, July
        1994 (Obsoletes RFC1549, also STD0051)


10.  Addresses


10.1.  Working Group

   The ISSLL working group can be contacted via the co-chairs, Eric
   Crawley <[e-mail]> and John Wroclawski <[e-mail]>,
   or via its WG mailing list <[e-mail]>.

10.2.  Author's address


   Carsten Bormann
   Universitaet Bremen FB3 TZI
   Postfach 330440
   D-28334 Bremen, GERMANY
   [e-mail]
   phone +49.421.218-7024
   fax +49.421.218-7000

























Bormann                                                        [Page 10]