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Network Working Group                                          S. CasnerRequest for Comments: 2508                                 Cisco SystemsCategory: Standards Track                                    V. Jacobson                                                           Cisco Systems                                                           February 1999Compressing IP/UDP/RTP Headers for Low-Speed Serial LinksStatus 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   This document describes a method for compressing the headers of   IP/UDP/RTP datagrams to reduce overhead on low-speed serial links.   In many cases, all three headers can be compressed to 2-4 bytes.   Comments are solicited and should be addressed to the working group   mailing list rem-conf@es.net and/or the author(s).   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 inRFC 2119.1.  Introduction   Since the Real-time Transport Protocol was published as an RFC [1],   there has been growing interest in using RTP as one step to achieve   interoperability among different implementations of network   audio/video applications.  However, there is also concern that the   12-byte RTP header is too large an overhead for 20-byte payloads when   operating over low speed lines such as dial-up modems at 14.4 or 28.8   kb/s.  (Some existing applications operating in this environment use   an application-specific protocol with a header of a few bytes that   has reduced functionality relative to RTP.)Casner & Jacobson           Standards Track                     [Page 1]

RFC 2508             Compressing IP/UDP/RTP Headers        February 1999   Header size may be reduced through compression techniques as has been   done with great success for TCP [2].  In this case, compression might   be applied to the RTP header alone, on an end-to-end basis, or to the   combination of IP, UDP and RTP headers on a link-by-link basis.   Compressing the 40 bytes of combined headers together provides   substantially more gain than compressing 12 bytes of RTP header alone   because the resulting size is approximately the same (2-4 bytes) in   either case.  Compressing on a link-by-link basis also provides   better performance because the delay and loss rate are lower.   Therefore, the method defined here is for combined compression of IP,   UDP and RTP headers on a link-by-link basis.   This document defines a compression scheme that may be used with   IPv4, IPv6 or packets encapsulated with more than one IP header,   though the initial focus is on IPv4.  The IP/UDP/RTP compression   defined here is intended to fit within the more general compression   framework specified in [3] for use with both IPv6 and IPv4.  That   framework defines TCP and non-TCP as two classes of transport above   IP.  This specification creates IP/UDP/RTP as a third class extracted   from the non-TCP class.2.  Assumptions and Tradeoffs   The goal of this compression scheme is to reduce the IP/UDP/RTP   headers to two bytes for most packets in the case where no UDP   checksums are being sent, or four bytes with checksums.  It is   motivated primarily by the specific problem of sending audio and   video over 14.4 and 28.8 dialup modems.  These links tend to provide   full-duplex communication, so the protocol takes advantage of that   fact, though the protocol may also be used with reduced performance   on simplex links.  This compression scheme performs best on local   links with low round-trip-time.   This specification does not address segmentation and preemption of   large packets to reduce the delay across the slow link experienced by   small real-time packets, except to identify inSection 4 some   interactions between segmentation and compression that may occur.   Segmentation schemes may be defined separately and used in   conjunction with the compression defined here.   It should be noted that implementation simplicity is an important   factor to consider in evaluating a compression scheme.   Communications servers may need to support compression over perhaps   as many as 100 dial-up modem lines using a single processor.   Therefore, it may be appropriate to make some simplifications in the   design at the expense of generality, or to produce a flexible design   that is general but can be subsetted for simplicity.  Higher   compression gain might be achieved by communicating more complexCasner & Jacobson           Standards Track                     [Page 2]

RFC 2508             Compressing IP/UDP/RTP Headers        February 1999   models for the changing header fields from the compressor to the   decompressor, but that complexity is deemed unnecessary.  The next   sections discuss some of the tradeoffs listed here.2.1.  Simplex vs. Full Duplex   In the absence of other constraints, a compression scheme that worked   over simplex links would be preferred over one that did not.   However, operation over a simplex link requires periodic refreshes   with an uncompressed packet header to restore compression state in   case of error.  If an explicit error signal can be returned instead,   the delay to recovery may be shortened substantially.  The overhead   in the no-error case is also reduced.  To gain these performance   improvements, this specification includes an explicit error   indication sent on the reverse path.   On a simplex link, it would be possible to use a periodic refresh   instead.  Whenever the decompressor detected an error in a particular   packet stream, it would simply discard all packets in that stream   until an uncompressed header was received for that stream, and then   resume decompression.  The penalty would be the potentially large   number of packets discarded.  The periodic refresh method described   in Section 3.3 of [3] applies to IP/UDP/RTP compression on simplex   links or links with high delay as well as to other non-TCP packet   streams.2.2.  Segmentation and Layering   Delay induced by the time required to send a large packet over the   slow link is not a problem for one-way audio, for example, because   the receiver can adapt to the variance in delay.  However, for   interactive conversations, minimizing the end-to-end delay is   critical.  Segmentation of large, non-real-time packets to allow   small real-time packets to be transmitted between segments can reduce   the delay.   This specification deals only with compression and assumes   segmentation, if included, will be handled as a separate layer.  It   would be inappropriate to integrate segmentation and compression in   such a way that the compression could not be used by itself in   situations where segmentation was deemed unnecessary or impractical.   Similarly, one would like to avoid any requirements for a reservation   protocol.  The compression scheme can be applied locally on the two   ends of a link independent of any other mechanisms except for the   requirements that the link layer provide some packet type codes, a   packet length indication, and good error detection.Casner & Jacobson           Standards Track                     [Page 3]

RFC 2508             Compressing IP/UDP/RTP Headers        February 1999   Conversely, separately compressing the IP/UDP and RTP layers loses   too much of the compression gain that is possible by treating them   together.  Crossing these protocol layer boundaries is appropriate   because the same function is being applied across all layers.3.  The Compression Algorithm   The compression algorithm defined in this document draws heavily upon   the design of TCP/IP header compression as described inRFC 1144 [2].   Readers are referred to that RFC for more information on the   underlying motivations and general principles of header compression.3.1.  The basic idea   In TCP header compression, the first factor-of-two reduction in data   rate comes from the observation that half of the bytes in the IP and   TCP headers remain constant over the life of the connection.  After   sending the uncompressed header once, these fields may be elided from   the compressed headers that follow.  The remaining compression comes   from differential coding on the changing fields to reduce their size,   and from eliminating the changing fields entirely for common cases by   calculating the changes from the length of the packet.  This length   is indicated by the link-level protocol.   For RTP header compression, some of the same techniques may be   applied.  However, the big gain comes from the observation that   although several fields change in every packet, the difference from   packet to packet is often constant and therefore the second-order   difference is zero.  By maintaining both the uncompressed header and   the first-order differences in the session state shared between the   compressor and decompressor, all that must be communicated is an   indication that the second-order difference was zero.  In that case,   the decompressor can reconstruct the original header without any loss   of information simply by adding the first-order differences to the   saved uncompressed header as each compressed packet is received.   Just as TCP/IP header compression maintains shared state for multiple   simultaneous TCP connections, this IP/UDP/RTP compression SHOULD   maintain state for multiple session contexts.  A session context is   defined by the combination of the IP source and destination   addresses, the UDP source and destination ports, and the RTP SSRC   field.  A compressor implementation might use a hash function on   these fields to index a table of stored session contexts.  The   compressed packet carries a small integer, called the session context   identifier or CID, to indicate in which session context that packet   should be interpreted.  The decompressor can use the CID to index its   table of stored session contexts directly.Casner & Jacobson           Standards Track                     [Page 4]

RFC 2508             Compressing IP/UDP/RTP Headers        February 1999   Because the RTP compression is lossless, it may be applied to any UDP   traffic that benefits from it.  Most likely, the only packets that   will benefit are RTP packets, but it is acceptable to use heuristics   to determine whether or not the packet is an RTP packet because no   harm is done if the heuristic gives the wrong answer.  This does   require executing the compression algorithm for all UDP packets, or   at least those with even port numbers (seesection 3.4).   Most compressor implementations will need to maintain a "negative   cache" of packet streams that have failed to compress as RTP packets   for some number of attempts in order to avoid further attempts.   Failing to compress means that some fields in the potential RTP   header that are expected to remain constant most of the time, such as   the payload type field, keep changing.  Even if the other such fields   remain constant, a packet stream with a constantly changing SSRC   field SHOULD be entered in the negative cache to avoid consuming all   of the available session contexts.  The negative cache is indexed by   the source and destination IP address and UDP port pairs but not the   RTP SSRC field since the latter may be changing.  When RTP   compression fails, the IP and UDP headers MAY still be compressed.   Fragmented IP Packets that are not initial fragments and packets that   are not long enough to contain a complete UDP header MUST NOT be sent   as FULL_HEADER packets.  Furthermore, packets that do not   additionally contain at least 12 bytes of UDP data MUST NOT be used   to establish RTP context.  If such a packet is sent as a FULL_HEADER   packet, it MAY be followed by COMPRESSED_UDP packets but MUST NOT be   followed by COMPRESSED_RTP packets.3.2.  Header Compression for RTP Data Packets   In the IPv4 header, only the total length, packet ID, and header   check-sum fields will normally change.  The total length is redundant   with the length provided by the link layer, and since this   compression scheme must depend upon the link layer to provide good   error detection (e.g., PPP's CRC [4]), the header checksum may also   be elided.  This leaves only the packet ID, which, assuming no IP   fragmentation, would not need to be communicated.  However, in order   to maintain lossless compression, changes in the packet ID will be   transmitted.  The packet ID usually increments by one or a small   number for each packet.  (Some systems increment the ID with the   bytes swapped, which results in slightly less compression.)  In the   IPv6 base header, there is no packet ID nor header checksum and only   the payload length field changes.Casner & Jacobson           Standards Track                     [Page 5]

RFC 2508             Compressing IP/UDP/RTP Headers        February 1999   In the UDP header, the length field is redundant with the IP total   length field and the length indicated by the link layer.  The UDP   check-sum field will be a constant zero if the source elects not to   generate UDP checksums.  Otherwise, the checksum must be communicated   intact in order to preserve the lossless compression.  Maintaining   end-to-end error detection for applications that require it is an   important principle.   In the RTP header, the SSRC identifier is constant in a given context   since that is part of what identifies the particular context.  For   most packets, only the sequence number and the timestamp will change   from packet to packet.  If packets are not lost or misordered   upstream from the compressor, the sequence number will increment by   one for each packet.  For audio packets of constant duration, the   timestamp will increment by the number of sample periods conveyed in   each packet.  For video, the timestamp will change on the first   packet of each frame, but then stay constant for any additional   packets in the frame.  If each video frame occupies only one packet,   but the video frames are generated at a constant rate, then again the   change in the timestamp from frame to frame is constant.  Note that   in each of these cases the second-order difference of the sequence   number and timestamp fields is zero, so the next packet header can be   constructed from the previous packet header by adding the first-order   differences for these fields that are stored in the session context   along with the previous uncompressed header.  When the second-order   difference is not zero, the magnitude of the change is usually much   smaller than the full number of bits in the field, so the size can be   reduced by encoding the new first-order difference and transmitting   it rather than the absolute value.   The M bit will be set on the first packet of an audio talkspurt and   the last packet of a video frame.  If it were treated as a constant   field such that each change required sending the full RTP header,   this would reduce the compression significantly.  Therefore, one bit   in the compressed header will carry the M bit explicitly.   If the packets are flowing through an RTP mixer, most commonly for   audio, then the CSRC list and CC count will also change.  However,   the CSRC list will typically remain constant during a talkspurt or   longer, so it need be sent only when it changes.3.3.  The protocol   The compression protocol must maintain a collection of shared   information in a consistent state between the compressor and   decompressor.  There is a separate session context for each   IP/UDP/RTP packet stream, as defined by a particular combination of   the IP source and destination addresses, UDP source and destinationCasner & Jacobson           Standards Track                     [Page 6]

RFC 2508             Compressing IP/UDP/RTP Headers        February 1999   ports, and the RTP SSRC field.  The number of session contexts to be   maintained MAY be negotiated between the compressor and decompressor.   Each context is identified by an 8- or 16-bit identifier, depending   upon the number of contexts negotiated, so the maximum number is   65536.  Both uncompressed and compressed packets MUST carry the   context ID and a 4-bit sequence number used to detect packet loss   between the compressor and decompressor.  Each context has its own   separate sequence number space so that a single packet loss need only   invalidate one context.   The shared information in each context consists of the following   items:      o The full IP, UDP and RTP headers, possibly including a CSRC        list, for the last packet sent by the compressor or        reconstructed by the decompressor.      o The first-order difference for the IPv4 ID field, initialized to        1 whenever an uncompressed IP header for this context is        received and updated each time a delta IPv4 ID field is received        in a compressed packet.      o The first-order difference for the RTP timestamp field,        initialized to 0 whenever an uncompressed packet for this        context is received and updated each time a delta RTP timestamp        field is received in a compressed packet.      o The last value of the 4-bit sequence number, which is used to        detect packet loss between the compressor and decompressor.      o The current generation number for non-differential coding of UDP        packets with IPv6 (see [3]).  For IPv4, the generation number        may be set to zero if the COMPRESSED_NON_TCP packet type,        defined below, is never used.      o A context-specific delta encoding table (seesection 3.3.4) may        optionally be negotiated for each context.   In order to communicate packets in the various uncompressed and   compressed forms, this protocol depends upon the link layer being   able to provide an indication of four new packet formats in addition   to the normal IPv4 and IPv6 packet formats:      FULL_HEADER - communicates the uncompressed IP header plus any      following headers and data to establish the uncompressed header      state in the decompressor for a particular context.  The FULL-      HEADER packet also carries the 8- or 16-bit session context      identifier and the 4-bit sequence number to establishCasner & Jacobson           Standards Track                     [Page 7]

RFC 2508             Compressing IP/UDP/RTP Headers        February 1999      synchronization between the compressor and decompressor.  The      format is shown insection 3.3.1.      COMPRESSED_UDP - communicates the IP and UDP headers compressed to      6 or fewer bytes (often 2 if UDP checksums are disabled), followed      by any subsequent headers (possibly RTP) in uncompressed form,      plus data.  This packet type is used when there are differences in      the usually constant fields of the (potential) RTP header.  The      RTP header includes a potentially changed value of the SSRC field,      so this packet may redefine the session context.  The format is      shown insection 3.3.3.      COMPRESSED_RTP - indicates that the RTP header is compressed along      with the IP and UDP headers.  The size of this header may still be      just two bytes, or more if differences must be communicated.  This      packet type is used when the second-order difference (at least in      the usually constant fields) is zero.  It includes delta encodings      for those fields that have changed by other than the expected      amount to establish the first-order differences after an      uncompressed RTP header is sent and whenever they change.  The      format is shown insection 3.3.2.      CONTEXT_STATE - indicates a special packet sent from the      decompressor to the compressor to communicate a list of context      IDs for which synchronization has or may have been lost.  This      packet is only sent across the point-to-point link so it requires      no IP header.  The format is shown insection 3.3.5.   When this compression scheme is used with IPv6 as part of the general   header compression framework specified in [3], another packet type   MAY be used:      COMPRESSED_NON_TCP - communicates the compressed IP and UDP      headers as defined in [3] without differential encoding.  If it      were used for IPv4, it would require one or two bytes more than      the COMPRESSED_UDP form listed above in order to carry the IPv4 ID      field.  For IPv6, there is no ID field and this non-differential      compression is more resilient to packet loss.   Assignments of numeric codes for these packet formats in the Point-   to-Point Protocol [4] are to be made by the Internet Assigned Numbers   Authority.3.3.1.  FULL_HEADER (uncompressed) packet format   The definition of the FULL_HEADER packet given here is intended to be   the consistent with the definition given in [3].  Full details on   design choices are given there.Casner & Jacobson           Standards Track                     [Page 8]

RFC 2508             Compressing IP/UDP/RTP Headers        February 1999   The format of the FULL_HEADER packet is the same as that of the   original packet.  In the IPv4 case, this is usually an IP header,   followed by a UDP header and UDP payload that may be an RTP header   and its payload.  However, the FULL_HEADER packet may also carry IP   encapsulated packets, in which case there would be two IP headers   followed by UDP and possibly RTP.  Or in the case of IPv6, the packet   may be built of some combination of IPv6 and IPv4 headers.  Each   successive header is indicated by the type field of the previous   header, as usual.   The FULL_HEADER packet differs from the corresponding normal IPv4 or   IPv6 packet in that it must also carry the compression context ID and   the 4-bit sequence number.  In order to avoid expanding the size of   the header, these values are inserted into length fields in the IP   and UDP headers since the actual length may be inferred from the   length provided by the link layer.  Two 16-bit length fields are   needed; these are taken from the first two available headers in the   packet.  That is, for an IPv4/UDP packet, the first length field is   the total length field of the IPv4 header, and the second is the   length field of the UDP header.  For an IPv4 encapsulated packet, the   first length field would come from the total length field of the   first IP header, and the second length field would come from the   total length field of the second IP header.   As specified in Sections5.3.2 of [3], the position of the context ID   (CID) and 4-bit sequence number varies depending upon whether 8- or   16-bit context IDs have been selected, as shown in the following   diagram (16 bits wide, with the most-significant bit is to the left):           For 8-bit context ID:           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+           |0|1| Generation|      CID      |  First length field           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+           |            0          |  seq  |  Second length field           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+           For 16-bit context ID:           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+           |1|1| Generation|   0   |  seq  |  First length field           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+           |              CID              |  Second length field           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+Casner & Jacobson           Standards Track                     [Page 9]

RFC 2508             Compressing IP/UDP/RTP Headers        February 1999   The first bit in the first length field indicates the length of the   CID.  The length of the CID MUST either be constant for all contexts   or two additional distinct packet types MUST be provided to   separately indicate COMPRESSED_UDP and COMPRESSED_RTP packet formats   with 8- and 16-bit CIDs.  The second bit in the first length field is   1 to indicate that the 4-bit sequence number is present, as is always   the case for this IP/UDP/RTP compression scheme.   The generation field is used with IPv6 for COMPRESSED_NON_TCP packets   as described in [3].  For IPv4-only implementations that do not use   COMPRESSED_NON_TCP packets, the compressor SHOULD set the generation   value to zero.  For consistent operation between IPv4 and IPv6, the   generation value is stored in the context when it is received by the   decompressor, and the most recent value is returned in the   CONTEXT_STATE packet.   When a FULL_HEADER packet is received, the complete set of headers is   stored into the context selected by the context ID.  The 4-bit   sequence number is also stored in the context, thereby   resynchronizing the decompressor to the compressor.   When COMPRESSED_NON_TCP packets are used, the 4-bit sequence number   is inserted into the "Data Field" of that packet and the D bit is set   as described in Section 6 of [3].  When a COMPRESSED_NON_TCP packet   is received, the generation number is compared to the value stored in   the context.  If they are not the same, the context is not up to date   and MUST be refreshed by a FULL_HEADER packet.  If the generation   does match, then the compressed IP and UDP header information, the   4-bit sequence number, and the (potential) RTP header are all stored   into the saved context.   The amount of memory required to store the context will vary   depending upon how many encapsulating headers are included in the   FULL_HEADER packet.  The compressor and decompressor MAY negotiate a   maximum header size.3.3.2.  COMPRESSED_RTP packet format   When the second-order difference of the RTP header from packet to   packet is zero, the decompressor can reconstruct a packet simply by   adding the stored first-order differences to the stored uncompressed   header representing the previous packet.  All that need be   communicated is the session context identifier and a small sequence   number (not related to the RTP sequence number) to maintain   synchronization and detect packet loss between the compressor and   decompressor.Casner & Jacobson           Standards Track                    [Page 10]

RFC 2508             Compressing IP/UDP/RTP Headers        February 1999   If the second-order difference of the RTP header is not zero for some   fields, the new first-order difference for just those fields is   communicated using a compact encoding.  The new first-order   difference values are added to the corresponding fields in the   uncompressed header in the decompressor's session context, and are   also stored explicitly in the context to be added to the   corresponding fields again on each subsequent packet in which the   second-order difference is zero.  Each time the first-order   difference changes, it is transmitted and stored in the context.   In practice, the only fields for which it is useful to store the   first-order difference are the IPv4 ID field and the RTP timestamp.   For the RTP sequence number field, the usual increment is 1.  If the   sequence number changes by other than 1, the difference must be   communicated but does not set the expected difference for the next   packet.  Instead, the expected first-order difference remains fixed   at 1 so that the difference need not be explicitly communicated on   the next packet assuming it is in order.   For the RTP timestamp, when a FULL_HEADER, COMPRESSED_NON_TCP or   COMPRESSED_UDP packet is sent to refresh the RTP state, the stored   first-order difference is initialized to zero.  If the timestamp is   the same on the next packet (e.g., same video frame), then the   second-order difference is zero.  Otherwise, the difference between   the timestamps of the two packets is transmitted as the new first-   order difference to be added to the timestamp in the uncompressed   header stored in the decompressor's context and also stored as the   first-order difference in that context.  Each time the first-order   difference changes on subsequent packets, that difference is again   transmitted and used to update the context.   Similarly, since the IPv4 ID field frequently increments by one, the   first-order difference for that field is initialized to one when the   state is refreshed by a FULL_HEADER packet, or when a   COMPRESSED_NON_TCP packet is sent since it carries the ID field in   uncompressed form.  Thereafter, whenever the first-order difference   changes, it is transmitted and stored in the context.   A bit mask will be used to indicate which fields have changed by   other than the expected difference.  In addition to the small link   sequence number, the list of items to be conditionally communicated   in the compressed IP/UDP/RTP header is as follows:Casner & Jacobson           Standards Track                    [Page 11]

RFC 2508             Compressing IP/UDP/RTP Headers        February 1999      I = IPv4 packet ID (always 0 if no IPv4 header)      U = UDP checksum      M = RTP marker bit      S = RTP sequence number      T = RTP timestamp      L = RTP CSRC count and list   If 4 bits are needed for the link sequence number to get a reasonable   probability of loss detection, there are too few bits remaining to   assign one bit to each of these items and still fit them all into a   single byte to go along with the context ID.   It is not necessary to explicitly carry the U bit to indicate the   presence of the UDP checksum because a source will typically include   check-sums on all packets of a session or none of them.  When the   session state is initialized with an uncompressed header, if there is   a nonzero checksum present, an unencoded 16-bit checksum will be   inserted into the compressed header in all subsequent packets until   this setting is changed by sending another uncompressed packet.   Of the remaining items, the L bit for the CSRC count and list may be   the one least frequently used.  Rather than dedicating a bit in the   mask to indicate CSRC change, an unusual combination of the other   bits may be used instead.  This bit combination is denoted MSTI.  If   all four of the bits for the IP packet ID, RTP marker bit, RTP   sequence number and RTP timestamp are set, this is a special case   indicating an extended form of the compressed RTP header will follow.   That header will include an additional byte containing the real   values of the four bits plus the CC count.  The CSRC list, of length   indicated by the CC count, will be included just as it appears in the   uncompressed RTP header.   The other fields of the RTP header (version, P bit, X bit, payload   type and SSRC identifier) are assumed to remain relatively constant.   In particular, the SSRC identifier is defined to be constant for a   given context because it is one of the factors selecting the context.   If any of the other fields change, the uncompressed RTP header MUST   sent as described inSection 3.3.3.   The following diagram shows the compressed IP/UDP/RTP header with   dotted lines indicating fields that are conditionally present.  The   most significant bit is numbered 0.  Multi-byte fields are sent in   network byte order (most significant byte first).  The delta fields   are often a single byte as shown but may be two or three bytes   depending upon the delta value as explained inSection 3.3.4.Casner & Jacobson           Standards Track                    [Page 12]

RFC 2508             Compressing IP/UDP/RTP Headers        February 1999             0   1   2   3   4   5   6   7           +...............................+           :   msb of session context ID   :  (if 16-bit CID)           +-------------------------------+           |   lsb of session context ID   |           +---+---+---+---+---+---+---+---+           | M | S | T | I | link sequence |           +---+---+---+---+---+---+---+---+           :                               :           +         UDP checksum          +  (if nonzero in context)           :                               :           +...............................+           :                               :           +        "RANDOM" fields        +  (if encapsulated)           :                               :           +...............................+           : M'| S'| T'| I'|      CC       :  (if MSTI = 1111)           +...............................+           :         delta IPv4 ID         :  (if I or I' = 1)           +...............................+           :      delta RTP sequence       :  (if S or S' = 1)           +...............................+           :      delta RTP timestamp      :  (if T or T' = 1)           +...............................+           :                               :           :           CSRC list           :  (if MSTI = 1111           :                               :   and CC nonzero)           :                               :           +...............................+           :                               :           :      RTP header extension     :  (if X set in context)           :                               :           :                               :           +-------------------------------+           |                               |           |            RTP data           |           /                               /           /                               /           |                               |           +-------------------------------+           :            padding            :  (if P set in context)           +...............................+   When more than one IPv4 header is present in the context as   initialized by the FULL_HEADER packet, then the IP ID fields of   encapsulating headers MUST be sent as absolute values as described inCasner & Jacobson           Standards Track                    [Page 13]

RFC 2508             Compressing IP/UDP/RTP Headers        February 1999   [3].  These fields are identified as "RANDOM" fields.  They are   inserted into the COMPRESSED_RTP packet in the same order as they   appear in the original headers, immediately following the UDP   checksum if present or the MSTI byte if not, as shown in the diagram.   Only if an IPv4 packet immediately precedes the UDP header will the   IP ID of that header be sent differentially, i.e., potentially with   no bits if the second difference is zero, or as a delta IPv4 ID field   if not.  If there is not an IPv4 header immediately preceding the UDP   header, then the I bit MUST be 0 and no delta IPv4 ID field will be   present.3.3.3.  COMPRESSED_UDP packet format   If there is a change in any of the fields of the RTP header that are   normally constant (such as the payload type field), then an   uncompressed RTP header MUST be sent.  If the IP and UDP headers do   not also require updating, this RTP header MAY be carried in a   COMPRESSED_UDP packet rather than a FULL_HEADER packet.  The   COMPRESSED_UDP packet has the same format as the COMPRESSED_RTP   packet except that the M, S and T bits are always 0 and the   corresponding delta fields are never included:             0   1   2   3   4   5   6   7           +...............................+           :   msb of session context ID   :  (if 16-bit CID)           +-------------------------------+           |   lsb of session context ID   |           +---+---+---+---+---+---+---+---+           | 0 | 0 | 0 | I | link sequence |           +---+---+---+---+---+---+---+---+           :                               :           +         UDP checksum          +  (if nonzero in context)           :                               :           +...............................+           :                               :           +        "RANDOM" fields        +  (if encapsulated)           :                               :           +...............................+           :         delta IPv4 ID         :  (if I = 1)           +-------------------------------+           |           UDP data            |           :   (uncompressed RTP header)   :   Note that this constitutes a form of IP/UDP header compression   different from COMPRESSED_NON_TCP packet type defined in [3].  The   motivation is to allow reaching the target of two bytes when UDP   checksums are disabled, as IPv4 allows.  The protocol in [3] does not   use differential coding for UDP packets, so in the IPv4 case, twoCasner & Jacobson           Standards Track                    [Page 14]

RFC 2508             Compressing IP/UDP/RTP Headers        February 1999   bytes of IP ID, and two bytes of UDP checksum if nonzero, would   always be transmitted in addition to two bytes of compression prefix.   For IPv6, the COMPRESSED_NON_TCP packet type MAY be used instead.3.3.4.  Encoding of differences   The delta fields in the COMPRESSED_RTP and COMPRESSED_UDP packets are   encoded with a variable-length mapping for compactness of the more   commonly-used values.  A default encoding is specified below, but it   is RECOMMENDED that implementations use a table-driven delta encoder   and decoder to allow negotiation of a table specific for each session   if appropriate, possibly even an optimal Huffman encoding.  Encodings   based on sequential interpretation of the bit stream, of which this   default table and Huffman encoding are examples, allow a reasonable   table size and may result in an execution speed faster than a non-   table-driven implementation with explicit tests for ranges of values.   The default delta encoding is specified in the following table.  This   encoding was designed to efficiently encode the small changes that   may occur in the IP ID and in RTP sequence number when packets are   lost upstream from the compressor, yet still handling most audio and   video deltas in two bytes.  The column on the left is the decimal   value to be encoded, and the column on the right is the resulting   sequence of bytes shown in hexadecimal and in the order in which they   are transmitted (network byte order).  The first and last values in   each contiguous range are shown, with ellipses in between:         Decimal  Hex          -16384  C0 00 00               :  :            -129  C0 3F 7F            -128  80 00               :  :              -1  80 7F               0  00               :  :             127  7F             128  80 80               :  :           16383  BF FF           16384  C0 40 00               :  :         4194303  FF FF FF   For positive values, a change of zero through 127 is represented   directly in one byte.  If the most significant two bits of the byte   are 10 or 11, this signals an extension to a two- or three-byteCasner & Jacobson           Standards Track                    [Page 15]

RFC 2508             Compressing IP/UDP/RTP Headers        February 1999   value, respectively.  The least significant six bits of the first   byte are combined, in decreasing order of significance, with the next   one or two bytes to form a 14- or 22-bit value.   Negative deltas may occur when packets are misordered or in the   intentionally out-of-order RTP timestamps on MPEG video [5].  These   events are less likely, so a smaller range of negative values is   encoded using otherwise redundant portions of the positive part of   the table.   A change in the RTP timestamp value less than -16384 or greater than   4194303 forces the RTP header to be sent uncompressed using a   FULL_HEADER, COMPRESSED_NON_TCP or COMPRESSED_UDP packet type.  The   IP ID and RTP sequence number fields are only 16 bits, so negative   deltas for those fields SHOULD be masked to 16 bits and then encoded   (as large positive 16-bit numbers).3.3.5.  Error Recovery   Whenever the 4-bit sequence number for a particular context   increments by other than 1, except when set by a FULL_HEADER or   COMPRESSED_NON_TCP packet, the decompressor MUST invalidate that   context and send a CONTEXT_STATE packet back to the compressor   indicating that the context has been invalidated.  All packets for   the invalid context MUST be discarded until a FULL_HEADER or   COMPRESSED_NON_TCP packet is received for that context to re-   establish consistent state (unless the "twice" algorithm is used as   described later in this section).  Since multiple compressed packets   may arrive in the interim, the decompressor SHOULD NOT retransmit the   CONTEXT_STATE packet for every compressed packet received, but   instead SHOULD limit the rate of retransmission to avoid flooding the   reverse channel.   When an error occurs on the link, the link layer will usually discard   the packet that was damaged (if any), but may provide an indication   of the error.  Some time may elapse before another packet is   delivered for the same context, and then that packet would have to be   discarded by the decompressor when it is observed to be out of   sequence, resulting in at least two packets lost.  To allow faster   recovery if the link does provide an explicit error indication, the   decompressor MAY optionally send an advisory CONTEXT_STATE packet   listing the last valid sequence number and generation number for one   or more recently active contexts (not necessarily all).  For a given   context, if the compressor has sent no compressed packet with a   higher sequence number, and if the generation number matches the   current generation, no corrective action is required.  Otherwise, the   compressor MAY choose to mark the context invalid so that the next   packet is sent in FULL_HEADER or COMPRESSED_NON_TCP mode (FULL_HEADERCasner & Jacobson           Standards Track                    [Page 16]

RFC 2508             Compressing IP/UDP/RTP Headers        February 1999   is required if the generation doesn't match).  However, note that if   the link round-trip-time is large compared to the inter-packet   spacing, there may be several packets from multiple contexts in   flight across the link, increasing the probability that the sequence   numbers will already have advanced when the CONTEXT_STATE packet is   received by the compressor.  The result could be that some contexts   are invalidated unnecessarily, causing extra bandwidth to be   consumed.   The format of the CONTEXT_STATE packet is shown in the following   diagrams.  The first byte is a type code to allow the CONTEXT_STATE   packet type to be shared by multiple compression schemes within the   general compression framework specified in [3].  The contents of the   remainder of the packet depends upon the compression scheme.  For the   IP/UDP/RTP compression scheme specified here, the remainder of the   CONTEXT_STATE packet is structured as a list of blocks to allow the   state for multiple contexts to be indicated, preceded by a one-byte   count of the number of blocks.   Two type code values are used for the IP/UDP/RTP compression scheme.   The value 1 indicates that 8-bit session context IDs are being used:             0   1   2   3   4   5   6   7           +---+---+---+---+---+---+---+---+           | 1 = IP/UDP/RTP with 8-bit CID |           +---+---+---+---+---+---+---+---+           |         context count         |           +---+---+---+---+---+---+---+---+           +---+---+---+---+---+---+---+---+           |       session context ID      |           +---+---+---+---+---+---+---+---+           | I | 0 | 0 | 0 |    sequence   |           +---+---+---+---+---+---+---+---+           | 0 | 0 |       generation      |           +---+---+---+---+---+---+---+---+                          ...           +---+---+---+---+---+---+---+---+           |       session context ID      |           +---+---+---+---+---+---+---+---+           | I | 0 | 0 | 0 |    sequence   |           +---+---+---+---+---+---+---+---+           | 0 | 0 |       generation      |           +---+---+---+---+---+---+---+---+   The value 2 indicates that 16-bit session context IDs are being used.   The session context ID is sent in network byte order (most   significant byte first):Casner & Jacobson           Standards Track                    [Page 17]

RFC 2508             Compressing IP/UDP/RTP Headers        February 1999             0   1   2   3   4   5   6   7           +---+---+---+---+---+---+---+---+           | 2 = IP/UDP/RTP with 16-bit CID|           +---+---+---+---+---+---+---+---+           |         context count         |           +---+---+---+---+---+---+---+---+           +---+---+---+---+---+---+---+---+           |                               |           +       session context ID      +           |                               |           +---+---+---+---+---+---+---+---+           | I | 0 | 0 | 0 |    sequence   |           +---+---+---+---+---+---+---+---+           | 0 | 0 |       generation      |           +---+---+---+---+---+---+---+---+                          ...           +---+---+---+---+---+---+---+---+           |                               |           +       session context ID      +           |                               |           +---+---+---+---+---+---+---+---+           | I | 0 | 0 | 0 |    sequence   |           +---+---+---+---+---+---+---+---+           | 0 | 0 |       generation      |           +---+---+---+---+---+---+---+---+   The bit labeled "I" is set to one for contexts that have been marked   invalid and require a FULL_HEADER of COMPRESSED_NON_TCP packet to be   transmitted.  If the I bit is zero, the context state is advisory.   The I bit is set to zero to indicate advisory context state that MAY   be sent following a link error indication.   Since the CONTEXT_STATE packet itself may be lost, retransmission of   one or more blocks is allowed.  It is expected that retransmission   will be triggered only by receipt of another packet, but if the line   is near idle, retransmission MAY be triggered by a relatively long   timer (on the order of 1 second).   If a CONTEXT_STATE block for a given context is retransmitted, it may   cross paths with the FULL_HEADER or COMPRESSED_NON_TCP packet   intended to refresh that context.  In that case, the compressor MAY   choose to ignore the error indication.   In the case where UDP checksums are being transmitted, the   decompressor MAY attempt to use the "twice" algorithm described in   section 10.1 of [3].  In this algorithm, the delta is applied more   than once on the assumption that the delta may have been the same on   the missing packet(s) and the one subsequently received.  Success isCasner & Jacobson           Standards Track                    [Page 18]

RFC 2508             Compressing IP/UDP/RTP Headers        February 1999   indicated by a checksum match.  For the scheme defined here, the   difference in the 4- bit sequence number tells number of times the   delta must be applied.  Note, however, that there is a nontrivial   risk of an incorrect positive indication.  It may be advisable to   request a FULL_HEADER or COMPRESSED_NON_TCP packet even if the   "twice" algorithm succeeds.   Some errors may not be detected, for example if 16 packets are lost   in a row and the link level does not provide an error indication.  In   that case, the decompressor will generate packets that are not valid.   If UDP checksums are being transmitted, the receiver will probably   detect the invalid packets and discard them, but the receiver does   not have any means to signal the decompressor.  Therefore, it is   RECOMMENDED that the decompressor verify the UDP checksum   periodically, perhaps one out of 16 packets.  If an error is   detected, the decompressor would invalidate the context and signal   the compressor with a CONTEXT_STATE packet.3.4.  Compression of RTCP Control Packets   By relying on the RTP convention that data is carried on an even port   number and the corresponding RTCP packets are carried on the next   higher (odd) port number, one could tailor separate compression   schemes to be applied to RTP and RTCP packets.  For RTCP, the   compression could apply not only to the header but also the "data",   that is, the contents of the different packet types.  The numbers in   Sender Report (SR) and Receiver Report (RR) RTCP packets would not   compress well, but the text information in the Source Description   (SDES) packets could be compressed down to a bit mask indicating each   item that was present but compressed out (for timing purposes on the   SDES NOTE item and to allow the end system to measure the average   RTCP packet size for the interval calculation).   However, in the compression scheme defined here, no compression will   be done on the RTCP headers and "data" for several reasons (though   compression SHOULD still be applied to the IP and UDP headers).   Since the RTP protocol specification suggests that the RTCP packet   interval be scaled so that the aggregate RTCP bandwidth used by all   participants in a session will be no more than 5% of the session   bandwidth, there is not much to be gained from RTCP compression.   Compressing out the SDES items would require a significant increase   in the shared state that must be stored for each context ID.  And, in   order to allow compression when SDES information for several sources   was sent through an RTP "mixer", it would be necessary to maintain a   separate RTCP session context for each SSRC identifier.  In a session   with more than 255 participants, this would cause perfect thrashing   of the context cache even when only one participant was sending data.Casner & Jacobson           Standards Track                    [Page 19]

RFC 2508             Compressing IP/UDP/RTP Headers        February 1999   Even though RTCP is not compressed, the fraction of the total   bandwidth occupied by RTCP packets on the compressed link remains no   more than 5% in most cases, assuming that the RTCP packets are sent   as COMPRESSED_UDP packets.  Given that the uncompressed RTCP traffic   consumes no more than 5% of the total session bandwidth, then for a   typical RTCP packet length of 90 bytes, the portion of the compressed   bandwidth used by RTCP will be no more than 5% if the size of the   payload in RTP data packets is at least 108 bytes.  If the size of   the RTP data payload is smaller, the fraction will increase, but is   still less than 7% for a payload size of 37 bytes.  For large data   payloads, the compressed RTCP fraction is less than the uncompressed   RTCP fraction (for example, 4% at 1000 bytes).3.5.  Compression of non-RTP UDP Packets   As described earlier, the COMPRESSED_UDP packet MAY be used to   compress UDP packets that don't carry RTP.  Whatever data follows the   UDP header is unlikely to have some constant values in the bits that   correspond to usually constant fields in the RTP header.  In   particular, the SSRC field would likely change.  Therefore, it is   necessary to keep track of the non-RTP UDP packet streams to avoid   using up all the context slots as the "SSRC field" changes (since   that field is part of what identifies a particular RTP context).   Those streams may each be given a context, but the encoder would set   a flag in the context to indicate that the changing SSRC field should   be ignored and COMPRESSED_UDP packets should always be sent instead   of COMPRESSED_RTP packets.4.  Interaction With Segmentation   A segmentation scheme may be used in conjunction with RTP header   compression to allow small, real-time packets to interrupt large,   presumably non-real-time packets in order to reduce delay.  It is   assumed that the large packets bypass the compressor and decompressor   since the interleaving would modify the sequencing of packets at the   decompressor and cause the appearance of errors.  Header compression   should be less important for large packets since the overhead ratio   is smaller.   If some packets from an RTP session context are selected for   segmentation (perhaps based on size) and some are not, there is a   possibility of re-ordering.  This would reduce the compression   efficiency because the large packets would appear as lost packets in   the sequence space.  However, this should not cause more serious   problems because the RTP sequence numbers should be reconstructed   correctly and will allow the application to correct the ordering.Casner & Jacobson           Standards Track                    [Page 20]

RFC 2508             Compressing IP/UDP/RTP Headers        February 1999   Link errors detected by the segmentation scheme using its own   sequencing information MAY be indicated to the compressor with an   advisory CONTEXT_STATE message just as for link errors detected by   the link layer itself.   The context ID byte is placed first in the COMPRESSED_RTP header so   that this byte MAY be shared with the segmentation layer if such   sharing is feasible and has been negotiated.  Since the compressor   may assign context ID values arbitrarily, the value can be set to   match the context identifier from the segmentation layer.5.  Negotiating Compression   The use of IP/UDP/RTP compression over a particular link is a   function of the link-layer protocol.  It is expected that such   negotiation will be defined separately for PPP [4], for example.  The   following items MAY be negotiated:      o The size of the context ID.      o The maximum size of the stack of headers in the context.      o A context-specific table for decoding of delta values.6.  Acknowledgments   Several people have contributed to the design of this compression   scheme and related problems.  Scott Petrack initiated discussion of   RTP header compression in the AVT working group at Los Angeles in   March, 1996.  Carsten Bormann has developed an overall architecture   for compression in combination with traffic control across a low-   speed link, and made several specific contributions to the scheme   described here.  David Oran independently developed a note based on   similar ideas, and suggested the use of PPP Multilink protocol for   segmentation.  Mikael Degermark has contributed advice on integration   of this compression scheme with the IPv6 compression framework.Casner & Jacobson           Standards Track                    [Page 21]

RFC 2508             Compressing IP/UDP/RTP Headers        February 19997.  References:   [1] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, "RTP:       A Transport Protocol for real-time applications",RFC 1889,       January 1996.   [2] Jacobson, V., "TCP/IP Compression for Low-Speed Serial Links",RFC 1144, February 1990.   [3] Degermark, M., Nordgren, B. and S. Pink, "Header Compression for       IPv6",RFC 2507, February 1999.   [4] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51,RFC1661, July 1994.   [5] Hoffman, D., Fernando, G., Goyal, V. and M. Civanlar, "RTP       Payload Format for MPEG1/MPEG2 Video",RFC 2250, January 1998.8.  Security Considerations   Because encryption eliminates the redundancy that this compression   scheme tries to exploit, there is some inducement to forego   encryption in order to achieve operation over a low-bandwidth link.   However, for those cases where encryption of data and not headers is   satisfactory, RTP does specify an alternative encryption method in   which only the RTP payload is encrypted and the headers are left in   the clear.  That would allow compression to still be applied.   A malfunctioning or malicious compressor could cause the decompressor   to reconstitute packets that do not match the original packets but   still have valid IP, UDP and RTP headers and possibly even valid UDP   check-sums.  Such corruption may be detected with end-to-end   authentication and integrity mechanisms which will not be affected by   the compression.  Constant portions of authentication headers will be   compressed as described in [3].   No authentication is performed on the CONTEXT_STATE control packet   sent by this protocol.  An attacker with access to the link between   the decompressor and compressor could inject false CONTEXT_STATE   packets and cause compression efficiency to be reduced, probably   resulting in congestion on the link.  However, an attacker with   access to the link could also disrupt the traffic in many other ways.   A potential denial-of-service threat exists when using compression   techniques that have non-uniform receiver-end computational load.   The attacker can inject pathological datagrams into the stream which   are complex to decompress and cause the receiver to be overloaded and   degrading processing of other streams.  However, this compressionCasner & Jacobson           Standards Track                    [Page 22]

RFC 2508             Compressing IP/UDP/RTP Headers        February 1999   does not exhibit any significant non-uniformity.   A security review of this protocol found no additional security   considerations.9.  Authors' Addresses   Stephen L. Casner   Cisco Systems, Inc.   170 West Tasman Drive   San Jose, CA 95134-1706   United States   EMail: casner@cisco.com   Van Jacobson   Cisco Systems, Inc.   170 West Tasman Drive   San Jose, CA 95134-1706   United States   EMail: van@cisco.comCasner & Jacobson           Standards Track                    [Page 23]

RFC 2508             Compressing IP/UDP/RTP Headers        February 199910.  Full Copyright Statement   Copyright (C) The Internet Society (1999).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Casner & Jacobson           Standards Track                    [Page 24]