Network Working Group
Request for Comments: 2686
Category: Standards Track
C. Bormann
Universitaet Bremen TZI
September 1999

The Multi-Class Extension to Multi-Link PPP

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 © 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 place.

This document proposes the fragment-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 provide a small number of extensions to add functionality and reduce the overhead.

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 fragment-oriented solution for the real-time encapsulation format part of the architecture, i.e. for the queues-of-fragments type sender [1]. 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. The PPP extensions defined in this document allow a sender to fragment the packets of various priorities into multiple classes of fragments, allowing high-priority packets to be sent between fragments of lower priorities.

A companion document based on these extensions [5] defines a suspend/resume-oriented solution for those cases where the best possible delay is required and the senders are of type 1 [1].

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 main design goal for the components of an architecture that addresses real-time multimedia flows over low-bitrate links is that of minimizing the end-to-end delay. More specifically, the worst case delay (after removing possible outliers, which are equivalent to packet losses from an application point of view) is what determines the playout points selected by the applications and thus the delay actually perceived by the user.

In addition, every attempt should obviously be undertaken to maximize the bandwidth actually available to media data; overheads must be minimized.

The solution should not place unnecessary burdens on the non-real- time flows. In particular, the usual MTU should be available to these flows.

The most general approach would provide the ability to suspend any packet (real-time or not) for a more urgent real-time packet, up to an infinite number of levels of nesting. On the other hand, it is likely that there would rarely be a requirement for a real-time packet to suspend another real-time packet that is not at least about twice as long. Typically, the largest packet size to be expected on a PPP link is the default MTU of 1500 bytes. The smallest high- priority packets are likely to have on the order of 22 bytes (compressed RTP/G.723.1 packets). In the 1:72 range of packet sizes to be expected, this translates to a maximum requirement of about eight levels of suspension (including one level where long real-time packets suspend long non-real-time packets). On 28.8kbit/s modems, there seems to be a practical requirement for at least two levels of suspension (i.e., audio suspends any longer packet including video, video suspends other very long packets).

On an architectural level, there are several additional requirements for the fragmentation scheme:

   a)   The scheme must be predictable enough that admission control can
        make decisions based on its characteristics.  As is argued in
        [1], this will often only be the case when additional hints
        about the characteristics of the flow itself are available
        (application hints).
   b)   The scheme must be robust against errors, at least with the same
        level of error detection as PPP.
   c)   The scheme must in general cooperate nicely with PPP.  In
        particular, it should be as compatible to existing PPP standards
        as possible.  On a link that (based on PPP negotiation) makes
        use of the scheme, it should always be possible to fall back to
        standard LCP (PPP Link Control Protocol [6, 7]) without
   d)   The scheme must work well with existing chips and router
        systems.  (See [1] for a more extensive discussion of
        implementation models.)  For synchronous links this means using
        HDLC framing; with much existing hardware, it is also hard to
        switch off the HDLC per-frame CRC.  For asynchronous links,
        there is much more freedom in design; on the other hand, a
        design that treats them much different from synchronous links
        would lose a number of desirable properties of PPP.
   e)   The scheme must be future proof.  In particular, the emergence
        of V.80 based modems may significantly change the way PPP is
        used with modems.

This document does not address additional requirements that may be relevant in conjunction with Frame Relay; however, there seems to be little problem in applying the principles of this document to "PPP in Frame Relay" [3].

3. Using PPP Multilink as-is

Transmitting only part of a packet to allow higher-priority traffic to intervene and resuming its transmission later on is a kind of fragmentation. The existing PPP Multilink Protocol (MP, [2]) provides for sequence numbering and begin/end bits, allowing packets to be split into fragments (Figure 1).

Figure 1: Multilink Short Sequence Number Fragment Format [2]

   PPP Header:  | Address 0xff  | Control 0x03  |
                | PID(H)  0x00  | PID(L)  0x3d  |
   MP Header:   |B|E|0|0|    sequence number    |
                |    fragment data              |
                |               .               |
                |               .               |
                |               .               |
   PPP FCS:     |              FCS              |

(Note that the address, control, and most significant PID bytes are often negotiated to be compressed away.)

MP's monotonically increasing sequence numbering (contiguous numbers are needed for all fragments of a packet) does not allow suspension of the sending of a sequence of fragments of one packet in order to send another packet. It is, however, possible to send intervening packets that are not encapsulated in multilink headers; thus, MP supports two levels of priority.

The multilink-as-is approach can be built using existing standards; multilink capability is now widely deployed and only the sending side needs to be aware that they are using this for giving priority to real-time packets.

3.1. Limitations of multilink as-is

   Multilink-as-is is not the complete solution for a number of reasons.
   First, because of the single monotonically increasing serial number,
   there is only one level of suspension:  "Big" packets that are sent
   via multilink can be suspended by "small" packets sent outside of
   multilink; the latter are not fragmentable (and therefore, the
   content of one packet cannot be sent in parallel on multiple links;

if the packets are sent in rounds on multiple links, the order they are processed at the receiver may differ from the order they were sent).

A problem not solved by this specification is that the multi-link header is relatively large; as delay bounds become small (for queues-of-fragments type implementations) the overhead may become significant.

4. Extending PPP Multilink to multiple classes

The obvious approach to providing more than one level of suspension with PPP Multilink is to run Multilink multiple times over one link. Multilink as it is defined provides no way for more than one instance to be active. Fortunately, a number of bits are unused in the Multilink header: two bits in the short sequence number format (as can be seen in Figure 1), six in the long sequence number format.

This document defines (some of the) previously unused bits as a class number:

Figure 2: Short Sequence Number Fragment Format With Classes

   PPP Header:  | Address 0xff  | Control 0x03  |
                | PID(H)  0x00  | PID(L)  0x3d  |
   MP Header:   |B|E|cls|    sequence number    |
                |    fragment data              |
                |               .               |
                |               .               |
                |               .               |
   PPP FCS:     |              FCS              |

Each class runs a separate copy of the mechanism defined in [2], i.e. uses a separate sequence number space and reassembly buffer.

Similarly, for the long sequence number format:

       Figure 3:  Long Sequence Number Fragment Format With Classes
   PPP Header:  | Address 0xff  | Control 0x03  |
                | PID(H)  0x00  | PID(L)  0x3d  |
   MP Header:   |B|E| class |0|0|sequence number|
                |      sequence number (L)      |
                |        fragment data          |
                |               .               |
                |               .               |
                |               .               |
   PPP FCS:     |              FCS              |

Together with the ability to send packets without a multilink header, this provides four levels of suspension with 12-bit headers (probably sufficient for many practical applications) and sixteen levels with 24-bit headers (only four of the six free bits are used in this case -- based on the rationale given above, sixteen levels should generally be more than sufficient).

5. Prefix elision: Compressing common header bytes

For some applications, all packets of a certain class will have a common protocol identifier (or even more than one common prefix byte). In this case, the following optimization is possible: the class number can be associated with a prefix of bytes that are removed from each packet before transmission and that are implicitly prepended to the reassembled packet after reception.

Note that if only some of the packets to be transmitted at a certain level of priority have the common prefix, it may still be possible to utilize this method by allocating two class numbers and only associating one of them with the prefix. (This is the reason why four of the unused bits in the long sequence number format have been allocated to the class number instead of the three that generally should suffice.)

Prefix elision is not a replacement for header compression or data compression: it allows implementations to compress away prefixes that often are not reachable by header or data compression methods.

6. Negotiable options

The following PPP LCP options are already defined by MP:

   o    Multilink Maximum Received Reconstructed Unit
   o    Multilink Short Sequence Number Header Format
   o    Endpoint Discriminator

This document defines two new LCP options:

   o    Multilink Header Format
   o    Prefix Elision

6.1. Multilink header format option

A summary of the Multilink Header Format Option format is shown below. The fields are transmitted from left to right.

Figure 4:

    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  |

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.

When this option is negotiated, the accepting 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. (Note that an implementation MAY continue to send packets outside of multilink in any case.) If this option is offered on a link which is intended to join an existing bundle, a system MUST offer the same multilink header format option value previously negotiated for the bundle, or none if none was negotiated previously.

The values defined in this document for the use of this option are:

   -    Code = 2: long sequence number fragment format with classes
   -    Code = 6: short sequence number fragment format with classes

The Multilink Header Format option MUST NOT occur more than once in a Configure-Request or Configure-Ack, and, if it is present, the Short Sequence Number Header Format option ([2]) MUST NOT also be present. If no instance of this option or the Short Sequence Number Header Format option is present, but an MRRU option [2] is present, then by default, long sequence number multilink headers with class 0 only are used; this is equivalent to code equals 2 and number of suspendable classes equals 1. An instance of the Short Sequence Number Header Format Option is equivalent to an instance of this option with code equals 6 and number of suspendable classes equal to 1.

The number of suspendable classes bounds the allowable class numbers: only class numbers numerically lower than this limit can be used for suspendable classes. Implementations MAY want to negotiate a number smaller than made possible by the packet format to limit their reassembly buffer space requirements. Implementations SHOULD at least support the value 4 for the short sequence number fragment format, and the value 8 for the long sequence number fragment format, unless configured differently. Bit combinations that would indicate class numbers outside the negotiated range MAY be used for other semantics if negotiated by other means outside the scope of this document (e.g., [6]).

6.2. Prefix elision option

This LCP option advises the peer that, in each of the given classes, the implementation expects to receive only packets with a certain prefix; this prefix is not to be sent as part of the information in the fragment(s) of this class. By default, this common prefix is empty for all classes. When this option is negotiated, the accepting implementation MUST either transmit all subsequent multilink packets of each of the given classes with the given prefix removed from the start of the packet or Configure-Nak or Configure-Reject the option. If none of the formats with classes has been negotiated, class number 0 may be used to indicate a common prefix for all packets sent within multilink fragments.

Apart from the type and length octets common to all LCP options, the option contains a sequence of zero or more sequences of a single- octet class number, a single-octet length of the prefix for that class, and the octets in that prefix:

                                 Figure 5:
    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 = 26   | Option Length |    Class      | Prefix Length |
   |   Prefix...                                   |    Class      |
   | Prefix Length |   Prefix...

The Prefix Elision option MUST NOT occur more than once in a Configure-Request or Configure-Nak. If this option is offered on a link which is intended to join an existing multilink bundle, a system MUST offer the same prefix elision option value previously negotiated for the bundle, or none if none was negotiated previously.

IMPLEMENTATION NOTE: as with most PPP options that indicate capabilities of the receiver to the sender, the sense of this option is an indication from the receiver to the sender of the packets concerned. Often, only the senders will have sufficient control over their usage of classes to be able to supply useful values for this option. A receiver willing to accept prefix-elided packets SHOULD request this option with empty content; the sender then can use Configure-Nak to propose the class-to-prefix mapping desired.

7. Security Considerations

Operation of this protocol is believed to be no more and no less secure than operation of the PPP multilink protocol [2].

8. 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., "PPP in a Real-time Oriented HDLC-like Framing",
        RFC 2687, 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.

9. 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

10. Acknowledgements

David Oran suggested using PPP Multilink for real-time framing and reminded the author of his earlier attempts of making Multilink more useful for this purpose. The participants in a lunch BOF at the 1996 Montreal IETF gave useful input on the design tradeoffs in various environments. The members of the ISSLL subgroup on low bitrate links (ISSLOW) have helped reducing the large set of options that initial versions of this specification had.

11. Full Copyright Statement

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

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