Network Working Group                                    Krister Svanbro, Ericsson Svanbro
INTERNET-DRAFT                                                    Sweden                                                  Ericsson
Expires: May June 2002                                      November                                     December 19, 2001

    Lower Layer Guidelines for Robust RTP/UDP/IP Header Compression
          <draft-ietf-rohc-rtp-lower-layer-guidelines-02.txt>
          <draft-ietf-rohc-rtp-lower-layer-guidelines-03.txt>

Status of this memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
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   This document is a submission of the IETF ROHC WG. Comments should be
   directed to its mailing list, rohc@cdt.luth.se.

Abstract

   This document describes lower layer guidelines for robust header
   compression (ROHC) and the requirements ROHC put on lower layers. The
   purpose of this document is to support the incorporation of robust
   header compression algorithms, as specified in the ROHC working
   group, into different systems such as those specified by 3GPP, 3GPP2,
   ETSI, etc. The document covers only lower layer guidelines for
   compression of RTP/UDP/IP and UDP/IP headers as specified in
   [RFC3095]. Both general guidelines and guidelines specific for
   cellular systems are treated.

Table of Contents

   1.  Introduction..................................................2
   2.  General guidelines............................................3
         2.1.  Error detection.......................................3
         2.2.  Inferred header field information.....................3
         2.3.  Handling of header size variation.....................4
         2.4.  Negotiation of header compression parameters..........5
         2.5.  Demultiplexing of flows onto logical channels.........5
         2.6.  Packet type identification............................5
         2.7.  Packet duplication....................................6
         2.8.  Packet reordering.....................................6
         2.9.  Feedback packets......................................6
   3.  Cellular system specific guidelines...........................6
         3.1.  Handover procedures...................................7
         3.2.  Unequal error detection...............................8 detection (UED).........................8
         3.3.  Unequal error protection..............................8 protection (UEP)........................8
   4.  IANA considerations...........................................9
   5.  Security considerations.......................................9
   6.  Author's addresses............................................9
   7.  References....................................................9

1.  Introduction

   Almost all header compression algorithms [RFC1144, RFC2507, RFC2508]
   rely on some functionality from underlying link layer. Headers
   (compressed or not) are expected to be delivered without any residual
   bit errors, IP length fields are inferred from link layer length
   fields and packet type identification may be separated from the
   header compression scheme and instead done at underlying link layer.
   [RFC2509], for example, elaborates on how to incorporate IP header
   compression [RFC2507] in PPP [RFC1661].

   It is important to be aware of such assumptions on required
   functionality from underlying layers when incorporating a header
   compression scheme into a system. The functionality required by a
   specific header compression scheme from lower layers may also be
   needed if incorporation of a header compression scheme is to be
   prepared without knowing the exact details of the final scheme.

   This document describes lower layer guidelines for robust RTP/UDP/IP
   header compression [RFC3095] as specified by the ROHC working group.
   [RFC3095] will from this point be referenced to as ROHC. These
   guidelines should simplify incorporation of the robust header
   compression algorithms into cellular system systems like those standardized
   by 3GPP, 3GPP2, ETSI, etc, and also into specific link layer
   protocols such as PPP. The document should also enable preparation of
   this incorporation without requiring detailed knowledge about the
   final header compression scheme. Relevant standardization groups
   standardizing link layers should, aided by this document, include
   required functionality in "their" link layers to support robust
   header compression.

   Hence, this document clarify clarifies the requirements ROHC put on lower
   layers, while the requirements on ROHC may be found in [RFC3096].

2.  General guidelines

2.1.  Error detection

   All current header compression schemes [RFC1144, RFC2507, RFC2508]
   rely on lower layers to detect errors in (compressed) headers. This
   is usually done with link layer checksums covering at least the
   compressed header. However, any error detecting mechanism may fail to
   detect some bit errors, which are usually called residual bit errors.

   As for non-compressed IP packets, lower layers must provide similar
   error detection, at least for ROHC headers. ROHC has been designed
   not to increase the residual bit error rate (for reasonable residual
   error rates) compared to the case when no header compression is used.
   Headers passed up to the header decompressor should, however, have a
   residual bit error probability close to zero.

   A ROHC decompressor might make use of packets with erroneous headers,
   even if they must be discarded. It is therefore recommended that also
   such invalid packets are passed up to the decompressor instead of
   being discarded by lower layers, but the packet must then be
   accompanied with an error indication.

2.2.  Inferred header field information

   Some fields of the RTP/UDP/IP headers may be classified as inferred,
   that is their values are to be inferred from other values or from an
   underlying link layer. A ROHC decompressor require requires that at least the
   following information can be inferred from any underlying link layer:

   Packet Length (IPv4) / Payload Length (IPv6)

     The received packet (with compressed header) length.

   Length (UDP)

     This field is redundant with the Packet Length (IPv4) or the
     Payload Length (IPv6) field.

   In summary, all these fields relate to the length of the packet the
   compressed header is included in. These fields may thus be inferred
   by the decompressor if one packet length value is signaled from the
   link layer to the decompressor on a per packet basis. This packet
   length value should be the length of the received packet including
   the (compressed) header.

2.3.  Handling of header size variations

   It is desirable for many cellular link layer technologies that bit
   rate variations and thus packet size variations are minimized.
   However, there will always be some variation in compressed header
   sizes since there is a trade-off between header size variations and
   compression efficiency, and also due to events in the header flow and
   on the channel. Variations in header sizes cause variations in packet
   sizes depending on variations of payload size. The following will
   only treat header size variations caused by ROHC and not packet size
   variations due to variations of payload size.

   The link layer must in some manner support varying header sizes from
   40 bytes (full RTP/UDP/IPv4 header) or 60 bytes (full RTP/UDP/IPv6)
   down to 1 byte for the minimal compressed header. It is likely that
   the small compressed headers dominate the flow of headers, and that
   the largest headers are sent rarely, e.g. only a few times in the
   initialization phase of the header compression scheme.

   Header size variations and thus packet size variations depend on
   numerous factors. Unpredictable changes in the RTP, UDP or IP headers
   may cause compressed headers to momentarily increase in size as well
   as size, and
   header sizes may depend on packet loss rate at lower layers. Header
   size distributions depend also on the mode ROHC operate operates in. However,
   for e.g. a voice application, carried by RTP/UDP/IPv4, with a
   constant speech frame size and silence suppression, the following
   basic header size changes may be considered as typical:

   In the very beginning of the speech session, the ROHC scheme is
   initialized by sending full headers called IR/DYN. These are the
   largest headers and headers, with sizes depending on basically on the IP-version.
   For IPv4 the size is approximately 40 bytes, and for IPv6
   approximately 60 bytes. The IR/DYN headers are used typically during
   one round trip time, possible interleaved with compressed headers.
   After that, usually only compressed headers are sent. Compressed
   headers may vary in size from 1 byte up to several bytes. The
   smallest compressed headers is are used when there is no unpredictable
   changes in header fields, typically during a talk spurt. In the
   beginning of a talk spurt, compressed header sizes may increase with by
   one or a few bytes momentarily. Apart from increases due to new talk
   spurts, compressed headers may increase in size momentarily due to
   unpredictable changes in header fields.

   ROHC provides some means to limit the amount of produced header
   sizes. In some cases a larger header than needed may be used to limit
   the number of header sizes used. Padding octets may also be used to
   fill up to a desired size. Chapter 6.3 Implementation parameters (Implementation parameters) in
   [RFC3095] provide provides optional implementation parameters that make it
   possible to mandate how a ROHC implementation should operate, for
   instance to mandate how many header sizes that may be used.

2.4.  Negotiation of header compression parameters

   ROHC have has some parameters that needs need to be configured in an initial
   setup phase. Which header compression profiles that are allowed may have
   to be determined and also what kind of context identification (CID)
   mechanism to use.

   The lower layers supporting ROHC should thus include mechanisms for
   negotiation of header compression parameters such as, as CID usage and
   header compression profile support. In certain environments, it might
   also be desirable to have mechanisms for re-negotiation of these
   parameters.

   For unidirectional links, this configuration might have to be
   performed out-of-band or a priori, and similar methods could of
   course also be used for bi-directional links if direct negotiation is
   not possible.

2.5.  Demultiplexing of flows onto logical channels

   In some cellular technologies flows are demultiplexed onto radio
   bearers suitable to the particular flows, i.e., onto logically
   separated channels. For instance, real-time flows such as voice and
   video may be carried on logically separated bearers. It is
   recommended that this kind of demultiplexing is done in the lower
   layers supporting robust header compression. By doing so, the need
   for context identification in the header compression scheme is
   reduced. If there is a one to one mapping between flow and logical
   channel, there is no need at all for context identification at the
   header compression level.

2.6.  Packet type identification

   Header compression schemes like [RFC2507, RFC2508] have relied on the
   underlying link layer to identify different kinds of headers by means
   of packet type identifiers on link layers. This kind of mechanism is
   not necessarily needed for ROHC since a ROHC packet type identifier
   is included in all compressed ROHC headers. Only if ROHC packets are
   to be mixed with other packets, such as packets compressed by other
   header compression schemes, the link layer must provide a packet type
   identifier. In such cases, or if ROHC is used on top of link layers
   already providing packet type identification, one (1) packet type
   identifier must be reserved for identification of ROHC packets. Thus,
   only one ROHC packet type is needed to mix ROHC and e.g. RFC2507
   flows, or to support ROHC on links where packet type identifiers are
   already present.

2.7.  Packet duplication

   Exact duplications of one and the same packet may waste transmission
   resources and is in contradiction to compression. Even so, packet
   duplication may occur for various reasons. Packet duplication may
   also occur in different places along the path for a packet.

   ROHC can handle packet duplication before the compressor but such
   packet duplications should be avoided for optimal compression
   efficiency. For correct ROHC operation, lower layers are not allowed
   to duplicate packets on the ROHC compressor-decompressor path.

2.8.  Packet reordering

   Lower layers between compressor and decompressor is are assumed not to
   reorder packets, i.e., the decompressor must receive packets in the
   same order as the compressor sends them. ROHC handles, however,
   reordering before the compression point. That is, there is no
   assumption that the compressor will only receive packets in sequence.

2.9.  Feedback packets

   ROHC may operate in three different modes; Unidirectional mode (U-
   mode), bidirectional optimistic mode (O-mode) and bidirectional
   reliable mode (R-mode). A brief description of the modes can be found
   in chapter 4.4 of [RFC3095].

   In U-mode it is not necessary to send any feedback from the
   decompressor to the compressor. O-mode and R-mode requires however
   that feedback messages from the decompressor to the compressor may be
   sent. Feedback message messages consist of small ROHC internal packets
   without any application payload. It is possible in ROHC to piggy-back
   feedback packets onto regular packets with ROHC compressed headers
   and payload, if there is ROHC type of compression in both the forward
   and reverse direction. However, this piggy-backing may not be desired
   or possible in some cases.

   To support ROHC O-mode or R-mode operation, lower layers must provide
   transport of feedback packets from decompressor to compressor. If
   piggybacking of feedback packets is not used, lower layers must be
   able to handle feedback as small stand-alone packets. For optimal
   compression efficiency, feedback packets from the decompressor should
   be delivered as soon as possible to the compressor.

3.  Cellular system specific guidelines

   An important group of link layer technologies where robust header
   compression will be needed are future cellular systems, which may
   have a very large number of users in some years. The need for header
   compression is large in these kinds of systems to achieve spectrum
   efficiency. Hence, it is important that future cellular systems can
   efficiently incorporate the robust header compression scheme.

3.1.  Handover procedures

   One cellular specific property that may affect header compression is
   mobility and thus, handover (i.e., change of serving base station or
   radio network controller).

   The main characteristics of handovers relevant for robust header
   compression are: the length of the longest packet loss event due to
   handover (i.e. the number of consecutive packet losses); losses), and
   relocation of header compression context when necessary.

   Depending on the location of the header compressor/decompressor in
   the radio access network and the type of handover, handover may or
   may not cause disruptions or packet loss events in the (compressed)
   header flow relevant for the header compression scheme. For instance,
   if soft handover is used and if the header compressor/decompressor
   reside above the combining point for soft handover, there will be no
   extra packet losses visible to the decompressor due to handover. In
   hard handovers, where packet loss events due to handover is
   introduced, the length of the longest consecutive packet loss is most
   relevant and should thus, thus be minimized.

   To maintain efficient ROHC operation, it should be ensured that
   handover events do not cause significant long events of consecutive
   packet loss. Significant The term "significant" in this context relates to the
   kind of loss tolerable for the carried real-time application.

   If hard handovers are performed, which may cause significant long
   events of consecutive packet loss, the radio access network should
   notify the compressor when such a handover has started and completed.
   The compressor could then be implemented to take proper actions and
   prevent consequences from such long loss events.

   Cellular systems supporting robust header compression may have
   internal mechanisms for transferring the header compression context
   between nodes where contexts may reside, at or before handover. If no
   such mechanism for transferring header compression context between
   nodes is available, the contexts may be resynchronized by the header
   compression scheme itself by means of a context refresh. The header
   compressor will than then perform a new header compression initialization,
   e.g. by sending full headers. This will, however, introduce an
   increase in the average header size dependent on how often a transfer
   of context is needed. To reinitialize the context in such cases, the
   lower layers must indicate to the header compressor when a handover
   has occurred, so that it knows when to refresh the context. Chapter
   6.3 "Implementation parameters" (Implementation parameters) in [RFC3095] provide provides optional
   implementation parameters that make it possible to trigger e.g. a
   complete context refresh.

3.2.  Unequal error detection

   Chapter (UED)

   Section 3.1 states that ROHC requires error detection from lower
   layers for at least the compressed header. However, some cellular
   technologies may differentiate the amount of error detection for
   different parts of a packet. For instance, it could be possible to
   have a stronger error detection for the header part of a packet, if
   the application payload part of the packet is less sensitive to
   errors, e.g. some cellular types of speech codecs.

   ROHC does not require UED from lower layers, ROHC requires only a an
   error detection mechanism that detects errors in at least the header
   part of the packet. There is thus no requirement on lower layers to
   provide separate error detection for the header and payload part of a
   packet. However, overall performance may be increased if UED is used.

   For example, if equal error detection is used in the form of one link
   layer checksum covering the entire packet including both header and
   payload part, any bit error will cause the packet to be discarded at
   the ROHC decompressor. It is not possible to distinguish between
   errors in the header and the payload part of the packet with this
   error detection mechanism and the ROHC decompressor must assume that
   the header is damaged, even if the bit error hit the payload part of
   the packet. If the header is assumed to be damaged, it is not
   possible to ensure correct decompression and that packet will thus be
   discarded. If the application is such that it tolerates some errors
   in the payload, it could have been better to deliver that packet to
   the application and let the application judge whether the payload was
   usable or not. Hence, with a an unequal error detection scheme where it
   is possible to separate detection of errors in the header and payload
   part of a packet, more packets may be delivered to applications in
   some cases for the same lower layer error rates. The final benefit
   depend
   depends of course on the cost of UED for the radio interface and
   related protocols.

3.3.  Unequal error protection (UEP)

   Some cellular technologies can provide different error probabilities
   for different parts of a packet, unequal error protection (UEP). For
   instance, the lower layers may provide a stronger error protection
   for the header part of a packet compared to the payload part of the
   packet.

   ROHC does not require UEP. UEP may be beneficial in some cases to
   reduce the error rate in ROHC headers, but only if it is possible to
   distinguish between errors in header and payload parts of a packets.
   I.e. packet,
   i.e. only if unequal error detection (UED) is used. The benefit of
   UEP depend depends of course on the cost of UEP for the radio interface and
   related protocols.

4.  IANA Considerations

   A protocol which follows these guidelines, e.g., [RFC3095], will
   require the IANA to assign various numbers. This document by itself,
   however, does not require IANA involvement.

5.  Security Considerations

   A protocol which follows these guidelines, e.g., [RFC3095], must be
   able to compress packets containing IPSEC headers according to
   [RFC3096]. There may be other sequrity aspects to consider in such
   protocols. This document by itself, however, does not add security
   risks.

6.  Author's Addresses

   Krister Svanbro               Tel:   +46 920 20 20 77
   Box 920                       Fax:   +46 920 20 20 99
   Ericsson Erisoft AB           Email: krister.svanbro@ericsson.com
   SE-971 28 Lulea, Sweden

7.  References

   [RFC3095]   Carsten Borman et al, "Robust Header Compression (ROHC)",
               RFC 3095, July 2001.

   [RFC3096]   Mikael Degermark, "Requirements for robust IP/UDP/RTP
               header compression", RFC 3096, July 2001.

   [RFC1144]   Van Jacobson, "Compressing TCP/IP Headers for Low-Speed
               Serial Links", RFC 1144, February 1990.

   [RFC2507]   Mikael Degermark, Bjorn Nordgren, Stephen Pink, "IP
               Header Compression", RFC 2507, February 1999.

   [RFC2508]   Steven Casner, Van Jacobson, "Compressing IP/UDP/RTP
               Headers for Low-Speed Serial Links", RFC 2508,
               February 1999.

   [RFC2509]   Mattias Engan, Steven Casner, Carsten Bormann, "IP Header
               Compression over PPP", RFC2509, February 1999

   [RFC1661]   Simpson, W., Ed., "The Point-To-Point Protocol (PPP)",
               STD 51, RFC 1661, July 1994.

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This Internet-Draft expires May June 19, 2002.