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RFC 4413 - TCP/IP Field Behavior

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Network Working Group                                            M. West
Request for Comments: 4413                                     S. McCann
Category: Informational                      Siemens/Roke Manor Research
                                                              March 2006

                         TCP/IP Field Behavior

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).


   This memo describes TCP/IP field behavior in the context of header
   compression.  Header compression is possible because most header
   fields do not vary randomly from packet to packet.  Many of the
   fields exhibit static behavior or change in a more or less
   predictable way.  When a header compression scheme is designed, it is
   of fundamental importance to understand the behavior of the fields in
   detail.  An example of this analysis can be seen in RFC 3095.  This
   memo performs a similar role for the compression of TCP/IP headers.

Table of Contents

   1. Introduction ....................................................3
   2. General classification ..........................................4
      2.1. IP Header Fields ...........................................5
         2.1.1. IPv6 Header Fields ....................................5
         2.1.2. IPv4 Header Fields ....................................7
      2.2. TCP Header Fields .........................................10
      2.3. Summary for IP/TCP ........................................11
   3. Classification of Replicable Header Fields .....................11
      3.1. IPv4 Header (Inner and/or Outer) ..........................12
      3.2. IPv6 Header (inner and/or outer) ..........................14
      3.3. TCP Header ................................................14
      3.4. TCP Options ...............................................15
      3.5. Summary of Replication ....................................16
   4. Analysis of Change Patterns of Header Fields ...................16
      4.1. IP Header .................................................19
         4.1.1. IP Traffic-Class / Type-Of-Service (TOS) .............19
         4.1.2. ECN Flags ............................................19
         4.1.3. IP Identification ....................................20
         4.1.4. Don't Fragment (DF) flag .............................22
         4.1.5. IP Hop-Limit / Time-To-Live (TTL) ....................22
      4.2. TCP Header ................................................23
         4.2.1. Sequence Number ......................................23
         4.2.2. Acknowledgement Number ...............................24
         4.2.3. Reserved .............................................25
         4.2.4. Flags ................................................25
         4.2.5. Checksum .............................................26
         4.2.6. Window ...............................................26
         4.2.7. Urgent Pointer .......................................27
      4.3. Options ...................................................27
         4.3.1. Options Overview .....................................28
         4.3.2. Option Field Behavior ................................29
   5. Other Observations .............................................36
      5.1. Implicit Acknowledgements .................................36
      5.2. Shared Data ...............................................36
      5.3. TCP Header Overhead .......................................37
      5.4. Field Independence and Packet Behavior ....................37
      5.5. Short-Lived Flows .........................................37
      5.6. Master Sequence Number ....................................38
      5.7. Size Constraint for TCP Options ...........................38
   6. Security Considerations ........................................39
   7. Acknowledgements ...............................................39
   8. References .....................................................40
      8.1. Normative References ......................................40
      8.2. Informative References ....................................41

1.  Introduction

   This document describes the format of the TCP/IP header and the
   header field behavior, i.e., how fields vary within a TCP flow.  The
   description is presented in the context of header compression.

   Since the IP header does exhibit slightly different behavior from
   that previously presented in RFC 3095 [31] for UDP and RTP, it is
   also included in this document.

   This document borrows much of the classification text from RFC 3095
   [31], rather than inserting many references to that document.

   According to the format presented in RFC 3095 [31], TCP/IP header
   fields are classified and analyzed in two steps.  First, we have a
   general classification in Section 2, where the fields are classified
   on the basis of stable knowledge and assumptions.  This general
   classification does not take into account the change characteristics
   of changing fields, as those will vary more or less depending on the
   implementation and on the application used.  Section 3 considers how
   field values can be used to optimize short-lived flows.  A more
   detailed analysis of the change characteristics is then done in
   Section 4.  Finally, Section 5 summarizes with conclusions about how
   the various header fields should be handled by the header compression
   scheme to optimize compression.

   A general question raised by this analysis is: what 'baseline'
   definition of all possible TCP/IP implementations is to be
   considered?  This review is based on an analysis of currently
   deployed TCP implementations supporting mechanisms standardised by
   the IETF.

   The general requirement for transparency is also interesting.  A
   number of recent proposals for extensions to TCP use some of the
   previously 'reserved' bits in the TCP packet header.  Therefore, a
   'reserved' bit cannot be taken to have a guaranteed zero value; it
   may change.  Ideally, this should be accommodated by the compression

   A number of reserved bits are available for future expansion.  A
   treatment of field behavior cannot predict the future use of such
   bits, but we expect that they will be used at some point.  Given
   this, a compression scheme can optimise for the current situation but
   should be capable of supporting any arbitrary usage of the reserved
   bits.  However, it is impossible to optimise for usage patterns that
   have yet to be defined.

2.  General classification

   The following definitions (and some text) are copied from RFC 3095
   [31], Appendix A.  Differences of IP field behavior between RFC 3095
   [31] (i.e., IP/UDP/RTP behavior for audio and video applications) and
   this document have been identified.

   For the following, we define "session" as a TCP packet stream, being
   a series of packets with the same IP addresses and port numbers.  A
   packet flow is defined by certain fields (see STATIC-DEF, below) and
   may be considered a subset of a session.  See [31] for a fuller
   discussion of separation of sessions into streams of packets for
   header compression.

   At a general level, the header fields are separated into 5 classes:


         These fields contain values that can be inferred from other
         values (for example, the size of the frame carrying the packet)
         and thus do not have to be handled at all by the compression

   o  STATIC

         These fields are expected to be constant throughout the
         lifetime of the packet stream.  Static information must in some
         way be communicated once.


         STATIC fields whose values define a packet stream.  They are in
         general handled as STATIC.


         These STATIC fields are expected to have well-known values and
         therefore do not need to be communicated at all.


         These fields are expected to vary randomly within a limited
         value set or range or in some other manner.

   In this section, each of the IP and TCP header fields is assigned to
   one of these classes.  For all fields except those classified as
   CHANGING, the motives for the classification are also stated.  In
   section 4, CHANGING fields are further examined and classified on the
   basis of their expected change behavior.

2.1.  IP Header Fields

2.1.1.  IPv6 Header Fields

          |        Field        | Size (bits) |      Class     |
          | Version             |      4      |     STATIC     |
          | DSCP*               |      6      |   ALTERNATING  |
          | ECT flag*           |      1      |    CHANGING    |
          | CE  flag*           |      1      |    CHANGING    |
          | Flow Label          |     20      |   STATIC-DEF   |
          | Payload Length      |     16      |    INFERRED    |
          | Next Header         |      8      |     STATIC     |
          | Hop Limit           |      8      |    CHANGING    |
          | Source Address      |    128      |   STATIC-DEF   |
          | Destination Address |    128      |   STATIC-DEF   |
               * Differs from RFC 3095 [31].  (The DSCP, ECT,
                 and CE flags were amalgamated into the Traffic
                 Class octet in RFC 3095).

                          Figure 1.  IPv6 Header Fields

   o  Version

         The version field states which IP version is used.  Packets
         with different values in this field must be handled by
         different IP stacks.  All packets of a packet stream must
         therefore be of the same IP version.  Accordingly, the field is
         classified as STATIC.

   o  Flow Label

         This field may be used to identify packets belonging to a
         specific packet stream.  If the field is not used, its value
         should be zero.  Otherwise, all packets belonging to the same
         stream must have the same value in this field, it being one of
         the fields that define the stream.  The field is therefore
         classified as STATIC-DEF.

   o  Payload Length

         Information about packet length (and, consequently, payload
         length) is expected to be provided by the link layer.  The
         field is therefore classified as INFERRED.

   o  Next Header

         This field will usually have the same value in all packets of a
         packet stream.  It encodes the type of the subsequent header.
         Only when extension headers are sometimes absent will the field
         change its value during the lifetime of the stream.  The field
         is therefore classified as STATIC.  The classification of
         STATIC is inherited from RFC 3095 [31].  However, note that the
         next header field is actually determined by the type of the
         following header.  Thus, it might be more appropriate to view
         this as an inference, although this depends upon the specific
         implementation of the compression scheme.

   o  Source and Destination Addresses

         These fields are part of the definition of a stream and
         therefore must be constant for all packets in the stream.  The
         fields are therefore classified as STATIC-DEF.

         This might be considered as a slightly simplistic view.  In
         this document, the IP addresses are associated with the
         transport layer connection and assumed to be part of the
         definition of a flow.  More complex flow-separation could, of
         course, be considered (see also RFC 3095 [31] for more
         discussion of this issue).  Where tunneling is being performed,
         the use of the IP addresses in outer tunnel headers is also
         assumed to be STATIC-DEF.

   The total size of the fields in each class is as follows:

                      | Class        | Size (octets)|
                      | INFERRED     |      2       |
                      | STATIC       |      1.5     |
                      | STATIC-DEF   |     34.5     |
                      | STATIC-KNOWN |      0       |
                      | CHANGING     |      2       |

                           Figure 2: Field sizes

2.1.2.  IPv4 Header Fields

           | Field               | Size (bits) |      Class     |
           | Version             |      4      |      STATIC    |
           | Header Length       |      4      |   STATIC-KNOWN |
           | DSCP*               |      6      |   ALTERNATING  |
           | ECT flag*           |      1      |     CHANGING   |
           | CE  flag*           |      1      |     CHANGING   |
           | Packet Length       |     16      |     INFERRED   |
           | Identification      |     16      |     CHANGING   |
           | Reserved flag*      |      1      |     CHANGING   |
           | Don't Fragment flag*|      1      |     CHANGING   |
           | More Fragments flag |      1      |   STATIC-KNOWN |
           | Fragment Offset     |     13      |   STATIC-KNOWN |
           | Time To Live        |      8      |     CHANGING   |
           | Protocol            |      8      |      STATIC    |
           | Header Checksum     |     16      |     INFERRED   |
           | Source Address      |     32      |    STATIC-DEF  |
           | Destination Address |     32      |    STATIC-DEF  |
                 * Differs from RFC 3095 [31].  (The DSCP, ECT
                   and CE flags were amalgamated into the TOS
                   octet in RFC 3095; the DF flag behavior is
                   considered later; the reserved field is
                   discussed below).

                       Figure 3.  IPv4 Header Fields

   o  Version

         The version field states which IP version is used.  Packets
         with different values in this field must be handled by
         different IP stacks.  All packets of a packet stream must
         therefore be of the same IP version.  Accordingly, the field is
         classified as STATIC.

   o  Header Length

         As long as no options are present in the IP header, the header
         length is constant and well known.  If there are options, the
         fields would be STATIC, but it is assumed here that there are
         no options.  The field is therefore classified as STATIC-KNOWN.

   o  Packet Length

         Information about packet length is expected to be provided by
         the link layer.  The field is therefore classified as INFERRED.

   o  Flags

         The Reserved flag must be set to zero, as defined in RFC 791
         [1].  In RFC 3095 [31] the field is therefore classified as
         STATIC-KNOWN.  However, it is expected that reserved fields may
         be used at some future point.  It is undesirable to select an
         encoding that would preclude the use of a compression profile
         for a future change in the use of reserved fields.  For this
         reason, the alternative encoding of CHANGING is used.  (A
         compression profile can, of course, still optimise for the
         current situation, where the field value is known to be 0).

         The More Fragments (MF) flag is expected to be zero because
         fragmentation is, ideally, not expected.  However, it is also
         understood that some scenarios (for example, some tunnelling
         architectures) do cause fragmentation.  In general, though,
         fragmentation is not expected to be common in the Internet due
         to a combination of initial MSS negotiation and subsequent use
         of path-MTU discovery.  RFC 3095 [31] points out that, for RTP,
         only the first fragment will contain the transport layer
         protocol header; subsequent fragments would have to be
         compressed with a different profile.  This is also obviously
         the case for TCP.  If fragmentation were to occur, the first
         fragment, by definition, would be relatively large, minimizing
         the header overhead.  Subsequent fragments would be compressed
         with another profile.  It is therefore considered undesirable
         to optimise for fragmentation in performing header compression.
         The More Fragments flag is therefore classified as STATIC-

   o  Fragment Offset

         Under the assumption that no fragmentation occurs, the fragment
         offset is always zero.  The field is therefore classified as
         STATIC-KNOWN.  Even if fragmentation were to be further
         considered, only the first fragment would contain the TCP
         header, and the fragment offset of this packet would still be

   o  Protocol

         This field will usually have the same value in all packets of a
         packet stream.  It encodes the type of the subsequent header.

         Only where the sequence of headers changes (e.g., an extension
         header is inserted or deleted or a tunnel header is added or
         removed) will the field change its value.  The field is
         therefore classified as STATIC.  Whether such a change would
         cause the sequence of packets to be treated as a new flow (for
         header compression) is an issue for profile design.  ROHC
         profiles must be able to cope with extension headers and
         tunnelling, but the choice of strategy is outside the scope of
         this document.

   o  Header Checksum

         The header checksum protects individual hops from processing a
         corrupted header.  When almost all IP header information is
         compressed away, there is no point in having this additional
         checksum.  Instead, it can be regenerated at the decompressor
         side.  The field is therefore classified as INFERRED.

         Note that the TCP checksum does not protect the whole TCP/IP
         header, but only the TCP pseudo-header (and the payload).
         Compare this with ROHC [31], which uses a CRC to verify the
         uncompressed header.  Given the need to validate the complete
         TCP/IP header, the cost of computing the TCP checksum over the
         entire payload, and known weaknesses in the TCP checksum [37],
         an additional check is necessary.  Therefore, it is highly
         desirable that some additional checksum (such as a CRC) will be
         used to validate correct decompression.

   o  Source and Destination Addresses

         These fields are part of the definition of a stream and must
         thus be constant for all packets in the stream.  The fields are
         therefore classified as STATIC-DEF.

   The total size of the fields in each class is as follows:

                      | Class        | Size (octets)|
                      | INFERRED     |      4       |
                      | STATIC*      |      1.5     |
                      | STATIC-DEF   |      8       |
                      | STATIC-KNOWN*|      2.25    |
                      | CHANGING*    |      4.25    |
                         * Differs from RFC 3095 [31]

                          Figure 4.  Field sizes

2.2.  TCP Header Fields

          | Field               | Size (bits) |      Class     |
          | Source Port         |     16      |    STATIC-DEF  |
          | Destination Port    |     16      |    STATIC-DEF  |
          | Sequence Number     |     32      |     CHANGING   |
          | Acknowledgement Num |     32      |     CHANGING   |
          | Data Offset         |      4      |     INFERRED   |
          | Reserved            |      4      |     CHANGING   |
          | CWR flag            |      1      |     CHANGING   |
          | ECE flag            |      1      |     CHANGING   |
          | URG flag            |      1      |     CHANGING   |
          | ACK flag            |      1      |     CHANGING   |
          | PSH flag            |      1      |     CHANGING   |
          | RST flag            |      1      |     CHANGING   |
          | SYN flag            |      1      |     CHANGING   |
          | FIN flag            |      1      |     CHANGING   |
          | Window              |     16      |     CHANGING   |
          | Checksum            |     16      |     CHANGING   |
          | Urgent Pointer      |     16      |     CHANGING   |
          | Options             |   0(-352)   |     CHANGING   |

                        Figure 5: TCP header fields

   o  Source and Destination ports

      These fields are part of the definition of a stream and must thus
      be constant for all packets in the stream.  The fields are
      therefore classified as STATIC-DEF.

   o  Data Offset

      The number of 4 octet words in the TCP header, indicating the
      start of the data.  It is always a multiple of 4 octets.  It can
      be re-constructed from the length of any options, and thus it is
      not necessary to carry this explicitly.  The field is therefore
      classified as INFERRED.

2.3.  Summary for IP/TCP

   Summarizing this for IP/TCP, one obtains the following:

          | Class \ IP ver | IPv6 (octets)  | IPv4 (octets)  |
          | INFERRED       |   2 + 4 bits   |   4 + 4 bits   |
          | STATIC         |   1 + 4 bits   |   1 + 4 bits   |
          | STATIC-DEF     |  38 + 4 bits   |      12        |
          | STATIC-KNOWN   |       -        |   2 + 2 bits   |
          | CHANGING       |  17 + 4 bits   |  19 + 6 bits   |
          | Totals         |     60         |     40         |
          (Excludes options, which are all classified as CHANGING).

                      Figure 6.  Overall field sizes

3.  Classification of Replicable Header Fields

   Where multiple flows either overlap in time or occur sequentially
   within a short space of time, there can be a great deal of similarity
   in header field values.  Such commonality of field values is
   reflected in the compression context.  Thus, it should be possible to
   utilise commonality between fields across different flows to improve
   the compression ratio.  In order to do this, it is important to
   understand the 'replicable' characteristics of the various header

   The key concept is that of 'replication': an existing context is used
   as a baseline and replicated to initialise a new context.  Those
   fields that are the same are then automatically initialised in the
   new context.  Those that have changed will be updated or overwritten
   with values from the initialisation packet that triggered the
   replication.  This section considers the commonality between fields
   in different flows.

   Note, however, that replication is based on contexts (rather than on
   just field values), so compressor-created fields that are part of the
   context may also be included.  These, of course, are dependent upon
   the nature of the compression protocol (ROHC profile) being applied.

   A brief analysis of the relationship of TCP/IP fields among
   'replicable' packet streams follows.

      'N/A': The field need not be considered in the replication
            process, as it is inferred or known 'a priori' (and,
            therefore, does not appear in the context).

      'No': The field cannot be replicated since its change pattern
            between two packet flows is uncorrelated.

      'Yes': The field may be replicated.  This does not guarantee that
            the field value will be the same across two candidate
            streams, only that it might be possible to exploit
            replication to increase the compression ratio.  Specific
            encoding methods can be used to improve the compression

3.1.  IPv4 Header (Inner and/or Outer)

          | Field                 | Class         | Replicable |
          | Version               | STATIC        | N/A        |
          | Header Length         | STATIC-KNOWN  | N/A        |
          | DSCP                  | ALTERNATING   | No  (1)    |
          | ECT flag              | CHANGING      | No  (2)    |
          | CE flag               | CHANGING      | No  (2)    |
          | Packet Length         | INFERRED      | N/A        |
          | Identification        | CHANGING      | Yes (3)    |
          | Reserved flag         | CHANGING      | No  (4)    |
          | Don't Fragment flag   | CHANGING      | Yes (5)    |
          | More Fragments flag   | STATIC-KNOWN  | N/A        |
          | Fragment Offset       | STATIC-KNOWN  | N/A        |
          | Time To Live          | CHANGING      | Yes        |
          | Protocol              | STATIC        | N/A        |
          | Header Checksum       | INFERRED      | N/A        |
          | Source Address        | STATIC-DEF    | Yes        |
          | Destination Address   | STATIC-DEF    | Yes        |

                           Figure 7: IPv4 header

   (1) The DSCP is marked according to the application's requirements.
       If it can be assumed that replicable connections belong to the
       same diffserv class, then it is likely that the DSCP will be
       replicable.  The DSCP can be set not only by the sender but by
       any packet marker.  Thus, a flow may have a number of DSCP values
       at different points in the network.  However, header compression

       operates on a point-to-point link and so would expect to see a
       relatively stable value.  If re-marking is being done based on
       the state of a meter, then the value may change mid-flow.
       Overall, though, we expect supporting replication of the DSCP to
       be useful for header compression.

   (2) It is not possible for the ECN bits to be replicated (note that
       use of the ECN nonce scheme [19] is anticipated).  However, it
       seems likely that all TCP flows between ECN-capable hosts will
       use ECN, the use (or not) of ECN for flows between the same end-
       points might be considered replicable.  See also note (4).

   (3) The replicable context for this field includes the IP-ID, NBO,
       and RND flags (as described in ROHC RTP).  This highlights that
       the replication is of the context, rather than just the header
       field values and, as such, needs to be considered based on the
       exact nature of compression applied to each field.

   (4) Since the possible future behavior of the 'Reserved Flag' cannot
       be predicted, it is not considered as replicable.  However, it
       might be expected that the behavior of the reserved flag between
       the same end-points will be similar.  In this case, any selection
       of packet formats (for example) based on this behavior might
       carry across to the new flow.  In the case of packet formats,
       this can probably be considered as a compressor-local decision.

   (5) In theory, the DF bit may be replicable.  However, this is not
       guaranteed and, in practice, it is unlikely to be useful to do
       this.  From the perspective of header compression, having to
       indicate whether or not a 1-bit flag should be replicated or
       specified explicitly is likely to require more bits than simply
       conveying the value of the flag.  We do not rule out DF

3.2.  IPv6 Header (inner and/or outer)

          | Field                 | Class         | Replicable |
          | Version               | STATIC        | N/A        |
          | Traffic Class         | CHANGING      | Yes (1)    |
          | ECT flag              | CHANGING      | No  (2)    |
          | CE flag               | CHANGING      | No  (2)    |
          | Flow Label            | STATIC-DEF    | N/A        |
          | Payload Length        | INFERRED      | N/A        |
          | Next Header           | STATIC        | N/A        |
          | Hop Limit             | CHANGING      | Yes        |
          | Source Address        | STATIC-DEF    | Yes        |
          | Destination Address   | STATIC-DEF    | Yes        |
            (1) See comment about DSCP field for IPv4, above.
            (2) See comment about ECT and CE flags for IPv4, above.

                          Figure 8.  IPv6 Header

3.3.  TCP Header

          | Field                 | Class         | Replicable |
          | Source Port           | STATIC-DEF    |  Yes (1)   |
          | Destination Port      | STATIC-DEF    |  Yes (1)   |
          | Sequence Number       | CHANGING      |  No  (2)   |
          | Acknowledgement Number| CHANGING      |  No        |
          | Data Offset           | INFERRED      |  N/A       |
          | Reserved Bits         | CHANGING      |  No  (3)   |
          | Flags                 |               |            |
          |         CWR           | CHANGING      |  No  (4)   |
          |         ECE           | CHANGING      |  No  (4)   |
          |         URG           | CHANGING      |  No        |
          |         ACK           | CHANGING      |  No        |
          |         PSH           | CHANGING      |  No        |
          |         RST           | CHANGING      |  No        |
          |         SYN           | CHANGING      |  No        |
          |         FIN           | CHANGING      |  No        |
          | Window                | CHANGING      |  Yes       |
          | Checksum              | CHANGING      |  No        |
          | Urgent Pointer        | CHANGING      |  Yes (5)   |

                           Figure 9: TCP Header

   (1) On the server side, the port number is likely to be a well-known
       value.  On the client side, the port number is generally selected
       by the stack automatically.  Whether the port number is
       replicable depends upon how the stack chooses the port number.
       Whilst most implementations use a simple scheme that sequentially
       picks the next available port number, it may not be desirable to
       rely on this behavior.

   (2) With the recommendation (and expected deployment) of TCP Initial
       Sequence Number randomization, defined in RFC 1948 [10], it will
       be impossible to share the sequence number.  Thus, this field
       will not be regarded as replicable.

   (3) See comment (4) for the IPv4 header, above.

   (4) See comment (2) on ECN flags for the IPv4 header, above.

   (5) The urgent pointer is very rarely used.  This means that, in
       practice, the field may be considered replicable.

3.4.  TCP Options

          | Option                    | SYN-only (1) | Replicable |
          | End of Option List        | No           | No   (2)   |
          | No-Operation              | No           | No   (2)   |
          | Maximum Segment Size      | Yes          | Yes        |
          | Window Scale              | Yes          | Yes        |
          | SACK-Permitted            | Yes          | Yes        |
          | SACK                      | No           | No         |
          | Timestamp                 | No           | No         |

                             Figure 10.  TCP Options

   (1) This indicates whether the option only appears in SYN packets.
       Options that are not 'SYN-only' may appear in any packet.  Many
       TCP options are used only in SYN packets.  Some options, such as
       MSS, Window Scale, and SACK-Permitted, will tend to have the same
       value among replicable packet streams.

       Thus, to support context sharing, the compressor should maintain
       such TCP options in the context (even though they only appear in
       the SYN segment).

   (2) Since these options have fixed values, they could be regarded as
       replicable.  However, the only interesting thing to convey about

       these options is their presence.  If it is known that such an
       option exists, its value is defined.

3.5.  Summary of Replication

   From the above analysis, it can be seen that there are reasonable
   grounds for exploiting redundancy between flows as well as between
   packets within a flow.  Simply consider the advantage of being able
   to elide the source and destination addresses for a repeated
   connection between two IPv6 endpoints.  There will also be a cost (in
   terms of complexity and robustness) for replicating contexts, and
   this must be considered when one decides what constitutes an
   appropriate solution.

   Finally, note that the use of replication requires that the
   compressor have a suitable degree of confidence that the source data
   is present and correct at the decompressor.  This may place some
   restrictions on which of the 'changing' fields, in particular, can be
   utilised during replication.

4.  Analysis of Change Patterns of Header Fields

   To design suitable mechanisms for efficient compression of all header
   fields, their change patterns must be analyzed.  For this reason, an
   extended classification is done based on the general classification
   in 2, considering the fields that were labeled CHANGING in that

   The CHANGING fields are separated into five different subclasses:

   o  STATIC

      These are fields that were classified as CHANGING on a general
      basis, but that are classified as STATIC here due to certain
      additional assumptions.


      These fields are STATIC most of the time.  However, occasionally
      the value changes but reverts to its original value after a known
      number of packets.


      These are fields that change their values occasionally and then
      keep their new values.


      These fields alternate between a small number of different values.


      These, finally, are the fields for which no useful change pattern
      can be identified.

   To further expand the classification possibilities without increasing
   complexity, the classification can be done either according to the
   values of the field and/or according to the values of the deltas for
   the field.

   When the classification is done, other details are also stated
   regarding possible additional knowledge about the field values and/or
   field deltas, according to the classification.  For fields classified
   as STATIC or SEMISTATIC, the value of the field could be not only
   STATIC but also well-KNOWN a priori (two states for SEMISTATIC
   fields).  For fields with non-irregular change behavior, it could be
   known that changes are usually within a LIMITED range compared to the
   maximal change for the field.  For other fields, the values are
   completely UNKNOWN.

   Figure 11 classifies all the CHANGING fields on the basis of their
   expected change patterns. (4) refers to IPv4 fields and (6) refers to

   | Field                  | Value/Delta |    Class    |  Knowledge  |
   | DSCP(4) / Tr.Class(6)  | Value       | ALTERNATING |   UNKNOWN   |
   | IP ECT flag(4)         | Value       |      RC     |   UNKNOWN   |
   | IP CE flag(4)          | Value       |      RC     |   UNKNOWN   |
   |             Sequential | Delta       |    STATIC   |    KNOWN    |
   |             -----------+-------------+-------------+-------------+
   | IP Id(4)     Seq. jump | Delta       |      RC     |   LIMITED   |
   |             -----------+-------------+-------------+-------------+
   |                 Random | Value       |  IRREGULAR  |   UNKNOWN   |
   | IP DF flag(4)          | Value       |      RC     |   UNKNOWN   |
   | IP TTL(4) / Hop Lim(6) | Value       | ALTERNATING |   LIMITED   |
   | TCP Sequence Number    | Delta       |  IRREGULAR  |   LIMITED   |
   | TCP Acknowledgement Num| Delta       |  IRREGULAR  |   LIMITED   |
   | TCP Reserved           | Value       |      RC     |   UNKNOWN   |
   | TCP flags              |             |             |             |
   |     ECN flags          | Value       |  IRREGULAR  |   UNKNOWN   |
   |     CWR flag           | Value       |  IRREGULAR  |   UNKNOWN   |
   |     ECE flag           | Value       |  IRREGULAR  |   UNKNOWN   |
   |     URG flag           | Value       |  IRREGULAR  |   UNKNOWN   |
   |     ACK flag           | Value       |  SEMISTATIC |    KNOWN    |
   |     PSH flag           | Value       |  IRREGULAR  |   UNKNOWN   |
   |     RST flag           | Value       |  IRREGULAR  |   UNKNOWN   |
   |     SYN flag           | Value       |  SEMISTATIC |    KNOWN    |
   |     FIN flag           | Value       |  SEMISTATIC |    KNOWN    |
   | TCP Window             | Value       | ALTERNATING |    KNOWN    |
   | TCP Checksum           | Value       |  IRREGULAR  |   UNKNOWN   |
   | TCP Urgent Pointer     | Value       |  IRREGULAR  |    KNOWN    |
   | TCP Options            | Value       |  IRREGULAR  |   UNKNOWN   |

               Figure 11.  Classification of CHANGING Fields

   The following subsections discuss the various header fields in
   detail.  Note that Table 1 and the discussion below do not consider
   changes caused by loss or reordering before the compression point.

4.1.  IP Header

4.1.1.  IP Traffic-Class / Type-Of-Service (TOS)

   The Traffic-Class (IPv6) or Type-Of-Service/DSCP (IPv4) field might
   be expected to change during the lifetime of a packet stream.  This
   analysis considers several RFCs that describe modifications to the
   original RFC 791 [1].

   The TOS byte was initially described in RFC 791 [1] as 3 bits of
   precedence followed by 3 bits of TOS and 2 reserved bits (defined to
   be zero).  RFC 1122 [21] extended this to specify 5 bits of TOS,
   although the meanings of the additional 2 bits were not defined.  RFC
   1349 [23] defined the 4th bit of TOS as 'minimize monetary cost'.
   The next significant change was in RFC 2474 [14] (obsoleting RFC 1349
   [23]).  RFC 2474 reworked the TOS octet as 6 bits of DSCP (DiffServ
   Code Point) plus 2 unused bits.  Most recently, RFC 2780 [30]
   identified the 2 reserved bits in the TOS or traffic class octet for
   experimental use with ECN.

   It is therefore proposed that the TOS (or traffic class) octet be
   classified as 6 bits for the DSCP and 2 additional bits.  These 2
   bits may be expected to be zero or to contain ECN data.  From a
   future-proofing perspective, it is preferable to assume the use of
   ECN, especially with respect to TCP.

   It is also considered important that the profile work with legacy
   stacks, since these will be in existence for some considerable time
   to come.  For simplicity, this will be considered as 6 bits of TOS
   information and 2 bits of ECN data, so the fields are always
   considered to be structured the same way.

   The DSCP (as for TOS in ROHC RTP) is not expected to change
   frequently (although it could change mid-flow, for example, as a
   result of a route change).

4.1.2.  ECN Flags

   Initially, we describe the ECN flags as specified in RFC 2481 [15]
   and RFC 3168 [18].  Subsequently, a suggested update is described
   that would alter the behavior of the flags.

   In RFC 2481 [15] there are 2 separate flags, the ECT (ECN Capable
   Transport) flag and the CE (Congestion Experienced) flag.  The ECT

   flag, if negotiated by the TCP stack, will be '1' for all data
   packets and '0' for all 'pure acknowledgement' packets.  This means
   that the behavior of the ECT flag is linked to behavior in the TCP
   stack.  Whether this can be exploited for compression is not clear.

   The CE flag is only used if ECT is set to '1'.  It is set to '0' by
   the sender and can be set to '1' by an ECN-capable router in the
   network to indicate congestion.  Thus the CE flag is expected to be
   randomly set to '1' with a probability dependent on the congestion
   state of the network and the position of the compressor in the path.
   Therefore, a compressor located close to the receiver in a congested
   network will see the CE bit set frequently, but a compressor located
   close to a sender will rarely, if ever, see the CE bit set to '1'.

   A recent experimental proposal [19] suggests an alternative view of
   these 2 bits.  This considers the two bits together to have 4
   possible codepoints.  Meanings are then assigned to the codepoints:

      00 Not ECN capable
      01 ECN capable, no congestion (known as ECT(0))
      10 ECN capable, no congestion (known as ECT(1))
      11 Congestion experienced

   The use of 2 codepoints for signaling ECT allows the sender to detect
   when a receiver is not reliably echoing congestion information.

   For the purposes of compression, this update means that ECT(0) and
   ECT(1) are equally likely (for an ECN capable flow) and that '11'
   will be seen relatively rarely.  The probability of seeing a
   congestion indication is discussed above in the description of the CE

   It is suggested that, for the purposes of compression, ECN with
   nonces be assumed as the baseline, although the compression scheme
   must be able to compress the original ECN scheme transparently.

4.1.3.  IP Identification

   The Identification field (IP ID) of the IPv4 header identifies which
   fragments constitute a datagram, when fragmented datagrams are
   reassembled.  The IPv4 specification does not specify exactly how
   this field is to be assigned values, only that each packet should get
   an IP ID that is unique for the source-destination pair and protocol
   for the time during which the datagram (or any of its fragments)
   could be alive in the network.  This means that assignment of IP ID
   values can be done in various ways, which we have separated into
   three classes:

   o  Sequential jump

      This is the most common assignment policy in today's IP stacks.  A
      single IP ID counter is used for all packet streams.  When the
      sender is running more than one packet stream simultaneously, the
      IP ID can increase by more than one between packets in a stream.
      The IP ID values will be much more predictable and will require
      fewer bits to transfer than random values, and the packet-to-
      packet increment (determined by the number of active outgoing
      packet streams and sending frequencies) will usually be limited.

   o  Random

      Some IP stacks assign IP ID values by using a pseudo-random number
      generator.  There is thus no correlation between the ID values of
      subsequent datagrams.  Therefore, there is no way to predict the
      IP ID value for the next datagram.  For header compression
      purposes, this means that the IP ID field needs to be sent
      uncompressed with each datagram, resulting in two extra octets of
      header.  IP stacks in cellular terminals that need optimum header
      compression efficiency should not use this IP ID assignment

   o  Sequential

      This assignment policy keeps a separate counter for each outgoing
      packet stream, and thus the IP ID value will increment by one for
      each packet in the stream, except at wrap around.  Therefore, the
      delta value of the field is constant and well known a priori.
      This assignment policy is the most desirable for header
      compression purposes.  However, its usage is not as common as it
      perhaps should be.

      In order to avoid violating RFC 791 [1], packets sharing the same
      IP address pair and IP protocol number cannot use the same IP ID
      values.  Therefore, implementations of sequential policies must
      make the ID number spaces disjoint for packet streams of the same
      IP protocol going between the same pair of nodes.  This can be
      done in a number of ways, all of which introduce occasional jumps
      and thus make the policy less than perfectly sequential.  For
      header compression purposes, less frequent jumps are preferred.

   Note that the ID is an IPv4 mechanism and is therefore not a problem
   for IPv6.  For IPv4, the ID could be handled in three different ways.
   First, we have the inefficient but reliable solution where the ID
   field is sent as-is in all packets, increasing the compressed headers
   by two octets.  This is the best way to handle the ID field if the
   sender uses random assignment of the ID field.  Second, there can be

   solutions with more flexible mechanisms that require fewer bits for
   the ID handling as long as sequential jump assignment is used.  Such
   solutions will probably require even more bits if random assignment
   is used by the sender.  Knowledge about the sender's assignment
   policy could therefore be useful when choosing between the two
   solutions above.  Finally, even for IPv4, header compression could be
   designed without any additional information for the ID field included
   in compressed headers.  To use such schemes, it must be known which
   assignment policy for the ID field is being used by the sender.  That
   might not be possible to know, which implies that the applicability
   of such solutions is very uncertain.  However, designers of IPv4
   stacks for cellular terminals should use an assignment policy close
   to sequential.

   With regard to TCP compression, the behavior of the IP ID field is
   essentially the same.  However, in RFC 3095 [31], the IP ID is
   generally inferred from the RTP Sequence Number.  There is no obvious
   candidate in the TCP case for a field to offer this 'master sequence
   number' role.

   Clearly, from a busy server, the observed behavior may well be quite
   erratic.  This is a case where the ability to share the IP
   compression context between a number of flows (between the same end-
   points) could offer potential benefits.  However, this would only
   have any real impact where there is a large number of flows between
   one machine and the server.  If context sharing is being considered,
   then it is preferable to share the IP part of the context.

4.1.4.  Don't Fragment (DF) flag

   Path-MTU discovery (RFC 1191 for IPv4 [6] and RFC 1981 for IPv6 [11])
   is widely deployed for TCP, in contrast to little current use for UDP
   packet streams.  This employs the DF flag value of '1' to detect the
   need for fragmentation in the end-to-end path and to probe the
   minimum MTU along the network path.  End hosts using this technique
   may be expected to send all packets with DF set to '1', although a
   host may end PMTU discovery by clearing the DF bit to '0'.  Thus, for
   compression, we expect the field value to be stable.

4.1.5.  IP Hop-Limit / Time-To-Live (TTL)

   The Hop-Limit (IPv6) or Time-To-Live (IPv4) field is expected to be
   constant during the lifetime of a packet stream or to alternate
   between a limited number of values due to route changes.

4.2.  TCP Header

   Any discussion of compressability of TCP fields borrows heavily from
   RFC 1144 [22].  However, the premise of how the compression is
   performed is slightly different, and the protocol has evolved
   slightly in the intervening time.

4.2.1.  Sequence Number

   Understanding the sequence and acknowledgement number behavior is
   essential for a TCP compression scheme.

   At the simplest level, the behavior of the sequence number can be
   described relatively easily.  However, there are a number of
   complicating factors that also need to be considered.

   For transferring in-sequence data packets, the sequence number will
   increment for each packet by between 0 and an upper limit defined by
   the MSS (Maximum Segment Size) and, if it is being used, by Path-MTU

   There are common MSS values, but these can be quite variable and
   unpredictable for any given flow.  Given this variability and the
   range of window sizes, it is hard (compared with the RTP case, for
   example) to select a 'one size fits all' encoding for the sequence
   number.  (The same argument applies equally to the acknowledgement

   Note that the increment of the sequence number in a packet is the
   size of the data payload of that packet (including the SYN and FIN
   flags).  This is, of course, exactly the relationship that RFC 1144
   [22] exploits to compress the sequence number in the most efficient
   case.  This technique may not be directly applicable to a robust
   solution, but it may be a useful relationship to consider.

   However, at any point on the path (i.e., wherever a compressor might
   be deployed), the sequence number can be anywhere within a range
   defined by the TCP window.  This is a combination of a number of
   values (buffer space at the sender; advertised buffer size at the
   receiver; and TCP congestion control algorithms).  Missing packets or
   retransmissions can cause the TCP sequence number to fluctuate within
   the limits of this window.

   It is desirable to be able to predict the sequence number with some
   regularity.  However, this also appears to be difficult to do.  For
   example, during bulk data transfer, the sequence number will tend to
   go up by 1 MSS per packet (assuming no packet loss).  Higher layer
   values have been seen to have an impact as well, where sequence

   number behavior has been observed with an 8 kbyte repeating pattern
   -- 5 segments of 1460 bytes followed by 1 segment of 892 bytes.  The
   implementation of TCP and the management of buffers within a protocol
   stack can affect the behavior of the sequence number.

   It may be possible to track the TCP window by the compressor,
   allowing it to bound the size of these jumps.

   For interactive flows (for example, telnet), the sequence number will
   change by small, irregular amounts.  In this case, the Nagle
   algorithm [3] commonly applies, coalescing small packets where
   possible in order to reduce the basic header overhead.  This may also
   mean that predictable changes in the sequence number are less likely
   to occur.  The Nagle algorithm is an optimisation and is not required
   to be used (applications can disable its use).  However, it is turned
   on by default in all common TCP implementations.

   Note also that the SYN and FIN flags (which have to be acknowledged)
   each consume 1 byte of sequence space.

4.2.2.  Acknowledgement Number

   Much of the information about the sequence number applies equally to
   the acknowledgement number.  However, there are some important

   For bulk data transfers, there will tend to be 1 acknowledgement for
   every 2 data segments.  The algorithm is specified in RFC 2581 [16].
   An ACK need not always be sent immediately on receipt of a data
   segment, but it must be sent within 500ms and should be generated for
   at least every second full-size segment (MSS) of received data.  It
   may be seen from this that the delta for the acknowledgement number
   is roughly twice that of the sequence number.  This is not always the
   case, and the discussion about sequence number irregularity should be

   As an aside, a common implementation bug is 'stretch ACKs' [33]
   (acknowledgements may be generated less frequently than every two
   full-size data segments).  This pattern can also occur following loss
   on the return path.

   Since the acknowledgement number is cumulative, dropped packets in
   the forward path will result in the acknowledgement number remaining
   constant for a time in the reverse direction.  Retransmission of a
   dropped segment can then cause a substantial jump in the
   acknowledgement number.  These jumps in acknowledgement number are
   bounded by the TCP window, just as for the jumps in sequence number.

   In the acknowledgement case, information about the advertised
   received window gives a bound to the size of any ACK jump.

4.2.3.  Reserved

   This field is reserved, and it therefore might be expected to be
   zero.  This can no longer be assumed, due to future-proofing.  It is
   only a matter of time before a suggestion for using the flag is made.

4.2.4.  Flags

   o  ECN-E (Explicit Congestion Notification)

      '1' to echo CE bit in IP header.  It will be set in several
      consecutive headers (until 'acknowledged' by CWR).  If ECN nonces
      are used, then there will be a 'nonce-sum' (NS) bit in the flags,
      as well.  Again, transparency of the reserved bits is crucial for
      future-proofing this compression scheme.  From an
      efficiency/compression standpoint, the NS bit will either be
      unused (always '0') or randomly changing.  The nonce sum is the
      1-bit sum of the ECT codepoints, as described in [19].

   o  CWR (Congestion Window Reduced)

      '1' to signal congestion window reduced on ECN.  It will generally
      be set in individual packets.  The flag will be set once per loss
      event.  Thus, the probability of its being set is proportional to
      the degree of congestion in the network, but it is less likely to
      be set than the CE flag.

   o  ECE (Echo Congestion Experience)

      If 'congestion experienced' is signaled in a received IP header,
      this is echoed through the ECE bit in segments sent by the
      receiver until acknowledged by seeing the CWR bit set.  Clearly,
      in periods of high congestion and/or long RTT, this flag will
      frequently be set to '1'.

      During connection open (SYN and SYN/ACK packets), the ECN bits
      have special meaning:

      * CWR and ECN-E are both set with SYN to indicate desire to use

      * CWR only is set in SYN-ACK, to agree to ECN.

        (The difference in bit-patterns for the negotiation is such that
        it will work with broken stacks that reflect the value of
        reserved bits).

   o  URG (Urgent Flag)

      '1' to indicate urgent data (which is unlikely with any flag other
      than ACK).

   o  ACK (Acknowledgement)

      '1' for all except the initial 'SYN' packet.

   o  PSH (Push Function Field)

      Generally accepted to be randomly '0' or '1'.  However, it may be
      biased more to one value than the other (this is largely caused by
      the implementation of the stack).

   o  RST (Reset Connection)

      '1' to reset a connection (unlikely with any flag other than ACK).

   o  SYN (Synchronize Sequence Number)

      '1' for the SYN/SYN-ACK, only at the start of a connection.

   o  FIN (End of Data: FINished)

      '1' to indicate 'no more data' (unlikely with any flag other than

4.2.5.  Checksum

   Carried as the end-to-end check for the TCP data.  See RFC 1144 [22]
   for a discussion of why this should be carried.  A header compression
   scheme should not rely upon the TCP checksum for robustness, though,
   and should apply appropriate error-detection mechanisms of its own.
   The TCP checksum has to be considered to be randomly changing.

4.2.6.  Window

   This may oscillate randomly between 0 and the receiver's window limit
   (for the connection).

   In practice, the window will either not change or alternate between a
   relatively small number of values.  Particularly when the window is
   closing (its value is getting smaller), the change in window is
   likely to be related to the segment size, but it is not clear that
   this necessarily offers any compression advantage.  When the window
   is opening, the effect of 'Silly-Window Syndrome' avoidance should be
   remembered.  This prevents the window from opening by small amounts
   that would encourage the sender to clock out small segments.

   When thinking about what fields might change in a sequence of TCP
   segments, one should note that the receiver can generate 'window
   update' segments in which only the window advertisement changes.

4.2.7.  Urgent Pointer

   From a compression point of view, the Urgent Pointer is interesting
   because it offers an example where 'semantically identical'
   compression is not the same as 'bitwise identical'.  This is because
   the value of the Urgent Pointer is only valid if the URG flag is set.

   However, the TCP checksum must be passed transparently, in order to
   maintain its end-to-end integrity checking property.  Since the TCP
   checksum includes the Urgent Pointer in its coverage, this enforces
   bitwise transparency of the Urgent Pointer.  Thus, the issue of
   'semantic' vs. 'bitwise' identity is presented as a note: the Urgent
   Pointer must be compressed in a way that preserves its value.

   If the URG flag is set, then the Urgent Pointer indicates the end of
   the urgent data and thus can point anywhere in the window.  It may be
   set (and changing) over several segments.  Note that urgent data is
   rarely used, since it is not a particularly clean way of managing
   out-of-band data.

4.3.  Options

   Options occupy space at the end of the TCP header.  All options are
   included in the checksum.  An option may begin on any byte boundary.
   The TCP header must be padded with zeros to make the header length a
   multiple of 32 bits.

   Optional header fields are identified by an option kind field.
   Options 0 and 1 are exactly one octet, which is their kind field.
   All other options have their one-octet kind field, followed by a
   one-octet length field, followed by length-2 octets of option data.

4.3.1.  Options Overview

   The IANA provides the authoritative list of TCP options.  Figure 12
   describes the current allocations at the time of publication.  Any
   new option would have a 'kind' value assigned by IANA.  The list is
   available at [20].  Where applicable, the associated RFC is also

   |Kind|Length |               Meaning              |    RFC   | Use |
   |    |octets |                                    |          |     |
   |  0 |   -   | End of Option List                 | RFC 793  |  *  |
   |  1 |   -   | No-Operation                       | RFC 793  |  *  |
   |  2 |   4   | Maximum Segment Size               | RFC 793  |  *  |
   |  3 |   3   | WSopt - Window Scale               | RFC 1323 |  *  |
   |  4 |   2   | SACK Permitted                     | RFC 2018 |  *  |
   |  5 |   N   | SACK                               | RFC 2018 |  *  |
   |  6 |   6   | Echo (obsoleted by option 8)       | RFC 1072 |     |
   |  7 |   6   | Echo Reply (obsoleted by option 8) | RFC 1072 |     |
   |  8 |  10   | TSopt - Time Stamp Option          | RFC 1323 |  *  |
   |  9 |   2   | Partial Order Connection Permitted | RFC 1693 |     |
   | 10 |   3   | Partial Order Service Profile      | RFC 1693 |     |
   | 11 |   6   | CC                                 | RFC 1644 |     |
   | 12 |   6   | CC.NEW                             | RFC 1644 |     |
   | 13 |   6   | CC.ECHO                            | RFC 1644 |     |
   | 14 |   3   | Alternate Checksum Request         | RFC 1146 |     |
   | 15 |   N   | Alternate Checksum Data            | RFC 1146 |     |
   | 16 |       | Skeeter                            |          |     |
   | 17 |       | Bubba                              |          |     |
   | 18 |   3   | Trailer Checksum Option            |          |     |
   | 19 |  18   | MD5 Signature Option               | RFC 2385 |     |
   | 20 |       | SCPS Capabilities                  |          |     |
   | 21 |       | Selective Negative Acks            |          |     |
   | 22 |       | Record Boundaries                  |          |     |
   | 23 |       | Corruption experienced             |          |     |
   | 24 |       | SNAP                               |          |     |
   | 25 |       | Unassigned (released 12/18/00)     |          |     |
   | 26 |       | TCP Compression Filter             |          |     |

                      Figure 12.  Common TCP Options

   The 'use' column is marked with '*' to indicate options that are most
   likely to be seen in TCP flows.  Also note that RFC 1072 [4] has been
   obsoleted by RFC 1323 [7], although the original bit usage is defined
   only in RFC 1072.

4.3.2.  Option Field Behavior

   Generally speaking, all option fields have been classified as
   changing.  This section describes the behavior of each option
   referenced within an RFC, listed by 'kind' indicator.

      0: End of Option List

         This option code indicates the end of the option list.  This
         might not coincide with the end of the TCP header according to
         the Data Offset field.  This is used at the end of all options,
         not at the end of each option, and it need only be used if the
         end of the options would not otherwise coincide with the end of
         the TCP header.  Defined in RFC 793 [2].

         There is no data associated with this option, so a compression
         scheme must simply be able to encode its presence.  However,
         note that since this option marks the end of the list and the
         TCP options are 4-octet aligned, there may be octets of padding
         (defined to be '0' in [2]) after this option.

      1: No-Operation

         This option code may be used between options, for example, to
         align the beginning of a subsequent option on a word boundary.
         There is no guarantee that senders will use this option, so
         receivers must be prepared to process options even if they do
         not begin on a word boundary RFC 793 [2].  There is no data
         associated with this option, so a compression scheme must
         simply be able to encode its presence.  This may be done by
         noting that the option simply maintains a certain alignment and
         that compression need only convey this alignment.  In this way,
         padding can just be removed.

      2: Maximum Segment Size

         If this option is present, then it communicates the maximum
         segment size that may be used to send a packet to this end-
         host.  This field must only be sent in the initial connection
         request (i.e., in segments with the SYN control bit set).  If
         this option is not used, any segment size is allowed RFC 793

         This option is very common.  The segment size is a 16-bit
         quantity.  Theoretically, this could take any value; however
         there are a number of values that are common.  For example,
         1460 bytes is very common for TCP/IPv4 over Ethernet (though
         with the increased prevalence of tunnels, for example, smaller

         values such as 1400 have become more popular). 536 bytes is the
         default MSS value.  This may allow for common values to be
         encoded more efficiently.

      3: Window Scale Option (WSopt)

         This option may be sent in a SYN segment by the TCP end-host
         (1) to indicate that the sending TCP end-host is prepared to
             perform both send and receive window scaling, and
         (2) to communicate a scale factor to be applied to its receive

         The scale factor is encoded logarithmically as a power of 2
         (presumably to be implemented by binary shifts).  Note that the
         window in the SYN segment itself is never scaled (RFC 1072
         [4]).  This option may be sent in an initial segment (i.e., in
         a segment with the SYN bit on and the ACK bit off).  It may
         also be sent in later segments, but only if a Window Scale
         option was received in the initial segment.  A Window Scale
         option in a segment without a SYN bit should be ignored.  The
         Window field in a SYN segment itself is never scaled (RFC 1323

         The use of window scaling does not affect the encoding of any
         other field during the lifetime of the flow.  Only the encoding
         of the window scaling option itself is important.  The window
         scale must be between 0 and 14 (inclusive).  Generally, smaller
         values would be expected (a window scale of 14 allows for a
         1Gbyte window, which is extremely large).

      4: SACK-Permitted

         This option may be sent in a SYN by a TCP that has been
         extended to receive (and presumably to process) the SACK option
         once the connection has opened RFC 2018 [12].  There is no data
         in this option all that is required is for the presence of the
         option to be encoded.

      5: SACK

         This option is to be used to convey extended acknowledgment
         information over an established connection.  Specifically, it
         is to be sent by a data receiver to inform the data transmitter
         of non-contiguous blocks of data that have been received and
         queued.  The data receiver awaits the receipt of data in later
         retransmissions to fill the gaps in sequence space between
         these blocks.  At that time, the data receiver acknowledges the
         data, normally by advancing the left window edge in the

         Acknowledgment Number field of the TCP header.  It is important
         to understand that the SACK option will not change the meaning
         of the Acknowledgment Number field, whose value will still
         specify the left window edge, i.e., one byte beyond the last
         sequence number of fully received data (RFC 2018 [12]).

         If SACK has been negotiated (through an exchange of SACK-
         Permitted options), then this option may occur when dropped
         segments are noticed by the receiver.  Because this identifies
         ranges of blocks within the receiver's window, it can be viewed
         as a base value with a number of offsets.  The base value (left
         edge of the first block) can be viewed as offset from the TCP
         acknowledgement number.  There can be up to 4 SACK blocks in a
         single option.  SACK blocks may occur in a number of segments
         (if there is more out-of-order data 'on the wire'), and this
         will typically extend the size of or add to the existing

         Alternative proposals such as DSACK RFC 2883 [17] do not
         fundamentally change the behavior of the SACK block, from the
         point of view of the information contained within it.

      6: Echo

         This option carries information that the receiving TCP may send
         back in a subsequent TCP Echo Reply option (see below).  A TCP
         may send the TCP Echo option in any segment, but only if a TCP
         Echo option was received in a SYN segment for the connection.
         When the TCP echo option is used for RTT measurement, it will
         be included in data segments, and the four information bytes
         will define the time at which the data segment was transmitted
         in any format convenient to the sender (see RFC 1072 [4]).

         The Echo option is generally not used in practice -- it is
         obsoleted by the Timestamp option.  However, for transparency
         it is desirable that a compression scheme be able to transport
         it.  (However, there is no benefit in attempting any treatment
         more sophisticated than viewing it as a generic 'option').

      7: Echo Reply

         A TCP that receives a TCP Echo option containing four
         information bytes will return these same bytes in a TCP Echo
         Reply option.  This TCP Echo Reply option must be returned in
         the next segment (e.g., an ACK segment) that is sent.  If more
         than one Echo option is received before a reply segment is
         sent, the TCP must choose only one of the options to echo,

         ignoring the others; specifically, it must choose the newest
         segment with the oldest sequence number (see RFC 1072 [4]).

         The Echo Reply option is generally not used in practice -- it
         is obsoleted by the Timestamp option.  However, for
         transparency it is desirable that a compression scheme be able
         to transport it.  (However, there is no benefit in attempting
         any more sophisticated treatment than viewing it as a generic

      8: Timestamps

         This option carries two four-byte timestamp fields.  The
         Timestamp Value field (TSval) contains the current value of the
         timestamp clock of the TCP sending the option.  The Timestamp
         Echo Reply field (TSecr) is only valid if the ACK bit is set in
         the TCP header; if it is valid, it echoes a timestamp value
         that was sent by the remote TCP in the TSval field of a
         Timestamps option.  When TSecr is not valid, its value must be
         zero.  The TSecr value will generally be from the most recent
         Timestamp option that was received; however, there are
         exceptions that are explained below.  A TCP may send the
         Timestamps option (TSopt) in an initial segment (i.e., a
         segment containing a SYN bit and no ACK bit), and it may send a
         TSopt in other segments only if it received a TSopt in the
         initial segment for the connection (see RFC 1323 [7]).
         Timestamps are quite commonly used.  If timestamp options are
         exchanged in the connection set-up phase, then they are
         expected to appear on all subsequent segments.  If this
         exchange does not happen, then they will not appear for the
         remainder of the flow.

         Because the value being carried is a timestamp, it is logical
         to expect that the entire value need not be carried.  There is
         no obvious pattern of increments that might be expected,

         An important reason for using the timestamp option is to allow
         detection of sequence space wrap-around (Protection Against
         Wrapped Sequence-number, or PAWS, see RFC 1323 [7]).  It is not
         expected that this is a serious concern on the links on which
         TCP header compression would be deployed, but it is important
         that the integrity of this option be maintained.  This issue is
         discussed in, for example, RFC 3150 [32].  However, the
         proposed Eifel algorithm [35] makes use of timestamps, so it is
         currently recommended that timestamps be used for cellular-type
         links [34].

         With regard to compression, note that the range of resolutions
         for the timestamp suggested in RFC 1323 [7] is quite wide (1ms
         to 1s per 'tick').  This (along with the perhaps wide variation
         in RTT) makes it hard to select a set of encodings that will be
         optimal in all cases.

      9: Partial Order Connection (POC) permitted

         This option represents a simple indicator communicated between
         the two peer transport entities to establish the operation of
         the POC protocol.  See RFC 1693 [9].

         The Partial Order Connection option sees little (or no) use in
         the current Internet, so the only requirement is that the
         header compression scheme be able to encode it.

      10: POC service profile

         This option serves to communicate the information necessary to
         carry out the job of the protocol -- the type of information
         that is typically found in the header of a TCP segment.  The
         Partial Order Connection option sees little (or no) use in the
         current Internet, so the only requirement is that the header
         compression scheme be able to encode it.

      11: Connection Count (CC)

         This option is part of the implementation of TCP Accelerated
         Open (TAO) that effectively bypasses the TCP Three-Way
         Handshake (3WHS).  TAO introduces a 32-bit incarnation number,
         called a "connection count" (CC), that is carried in a TCP
         option in each segment.  A distinct CC value is assigned to
         each direction of an open connection.  The implementation
         assigns monotonically increasing CC values to successive
         connections that it opens actively or passively (see RFC 1644
         [8]).  This option sees little (or no) use in the current
         Internet, so the only requirement is that the header
         compression scheme be able to encode it.

      12: CC.NEW

         Correctness of the TAO mechanism requires that clients generate
         monotonically increasing CC values for successive connection
         initiations.  Receiving a CC.NEW causes the server to
         invalidate its cache entry and to do a 3WHS.  See RFC 1644 [8].
         This option sees little (or no) use in the current Internet, so
         the only requirement is that the header compression scheme be
         able to encode it.

      13: CC.ECHO

         When a server host sends a segment, it echoes the connection
         count from the initial in a CC.ECHO option, which is used by
         the client host to validate the segment (see RFC 1644 [8]).
         This option sees little (or no) use in the current Internet, so
         the only requirement is that the header compression scheme be
         able to encode it.

      14: Alternate Checksum Request

         This option may be sent in a SYN segment by a TCP to indicate
         that the TCP is prepared to both generate and receive checksums
         based on an alternate algorithm.  During communication, the
         alternate checksum replaces the regular TCP checksum in the
         checksum field of the TCP header.  Should the alternate
         checksum require more than 2 octets to transmit, either the
         checksum may be moved into a TCP Alternate Checksum Data Option
         and the checksum field of the TCP header be sent as zero, or
         the data may be split between the header field and the option.
         Alternate checksums are computed over the same data as the
         regular TCP checksum; see RFC 1146 [5].

         This option sees little (or no) use in the current Internet, so
         the only requirement is that the header compression scheme be
         able to encode it.  It would only occur in connection set-up
         (SYN) packets.  Even if this option were used, it would not
         affect the handling of the checksum, since this should be
         carried transparently in any case.

      15: Alternate Checksum Data

         This field is used only when the alternate checksum that is
         negotiated is longer than 16 bits.  These checksums will not
         fit in the checksum field of the TCP header and thus at least
         part of them must be put in an option.  Whether the checksum is
         split between the checksum field in the TCP header and the
         option or the entire checksum is placed in the option is
         determined on a checksum-by-checksum basis.  The length of this
         option will depend on the choice of alternate checksum
         algorithm for this connection; see RFC 1146 [5].

         If an alternative checksum was negotiated in the connection
         set-up, then this option may appear on all subsequent packets
         (if needed to carry the checksum data).  However, this option
         is in practice never seen, so the only requirement is that the
         header compression scheme be able to encode it.

      16 - 18:

         These non-RFC option types are not considered in this document.

      19: MD5 Digest

         Every segment sent on a TCP connection to be protected against
         spoofing will contain the 16-byte MD5 digest produced by
         applying the MD5 algorithm to a concatenated block of data

         Upon receiving a signed segment, the receiver must validate it
         by calculating its own digest from the same data (using its own
         key) and comparing the two digests.  A failing comparison must
         result in the segment's being dropped and must not produce any
         response back to the sender.  Logging the failure is probably

         Unlike other TCP extensions (e.g., the Window Scale option
         [7]), the absence of the option in the SYN-ACK segment must not
         cause the sender to disable its sending of signatures.  This
         negotiation is typically done to prevent some TCP
         implementations from misbehaving upon receiving options in non-
         SYN segments.  This is not a problem for this option, since the
         SYN-ACK sent during connection negotiation will not be signed
         and will thus be ignored.  The connection will never be made,
         and non-SYN segments with options will never be sent.  More
         importantly, the sending of signatures must be under the
         complete control of the application, not at the mercy of a
         remote host not understanding the option.  MD5 digest
         information should, like any cryptographically secure data, be
         incompressible.  Therefore the compression scheme must simply
         transparently carry this option, if it occurs.

      20 - 26;

         Thse non-RFC option types are not considered in this document.
         This only means that their behavior is not described in detail,
         as a compression scheme is not expected to be optimised for
         these options.  However, any unrecognised option must be
         carried by a TCP compression scheme transparently, in order to
         work efficiently in the presence of new or rare options.

   The above list covers options known at the time of writing.  Other
   options are expected to be defined.  It is important that any future
   options can be handled by a header compression scheme.  The
   processing of as-yet undefined options cannot be optimised but, at
   the very least, unknown options should be carried transparently.

   The current model for TCP options is that an option is negotiated in
   the SYN exchange and used thereafter, if the negotiation succeeds.
   This leads to some assumptions about the presence of options (being
   only on packets with the SYN flag set, or appearing on every packet,
   for example).  Where such assumptions hold true, it may be possible
   to optimise compression of options slightly.  However, it is seen as
   undesirable to be so constrained, as there is no guarantee that
   option handling and negotiation will remain the same in the future.
   Also note that a compressor may not process the SYN packets of a flow
   and cannot, therefore, be assumed to know which options have been

5.  Other Observations

5.1.  Implicit Acknowledgements

   There may be a small number of cues for 'implicit acknowledgements'
   in a TCP flow.  Even if the compressor only sees the data transfer
   direction, for example, seeing a packet without the SYN flag set
   implies that the SYN packet has been received.

   There is a clear requirement for the deployment of compression to be
   topologically independent.  This means that it is not actually
   possible to be sure that seeing a data packet at the compressor
   guarantees that the SYN packet has been correctly received by the
   decompressor (as the SYN packet may have taken an alternative path).

   However, there may be other such cues, which may be used in certain
   circumstances to improve compression efficiency.

5.2.  Shared Data

   It can be seen that there are two distinct deployments (i) where the
   forward (data) and reverse (ACK) path are both carried over a common
   link, and (ii) where the forward (data) and reverse (ACK) path are
   carried over different paths, with a specific link carrying packets
   corresponding to only one direction of communication.

   In the former case, a compressor and decompressor could be colocated.
   It may then be possible for the compressor and decompressor at each
   end of the link to exchange information.  This could lead to possible

   For example, acknowledgement numbers are generally taken from the
   sequence numbers in the opposite direction.  Since an acknowledgement
   cannot be generated for a packet that has not passed across the link,
   this offers an efficient way of encoding acknowledgements.

5.3.  TCP Header Overhead

   For a TCP bulk data-transfer, the overhead of the TCP header does not
   form a large proportion of the data packet (e.g., < 3% for a 1460
   octet packet), particularly compared to the typical RTP voice case.
   Spectral efficiency is clearly an important goal.  However,
   extracting every last bit of compression gain offers only marginal
   benefit at a considerable cost in complexity.  This trade-off, of
   efficiency and complexity, must be addressed in the design of a TCP
   compression profile.

   However, in the acknowledgement direction (i.e., for 'pure'
   acknowledgement headers), the overhead could be said to be infinite
   (since there is no data being carried).  This is why optimizations
   for the acknowledgement path may be considered useful.

   There are a number of schemes for manipulating TCP acknowledgements
   to reduce the ACK bandwidth.  Many of these are documented in [33]
   and [32].  Most of these schemes are entirely compatible with header
   compression, without requiring any particular support.  While it is
   not expected that a compression scheme will be optimised for
   experimental options, it is useful to consider these when developing
   header compression schemes, and vice versa.  A header compression
   scheme must be able to support any option (including ones as yet

5.4.  Field Independence and Packet Behavior

   It should be apparent that direct comparisons with the highly
   'packet'-based view of RTP compression are hard.  RTP header fields
   tend to change regularly per-packet, and many fields (IPv4 IP ID, RTP
   sequence number, and RTP timestamp, for example) typically change in
   a dependent manner.  However, TCP fields, such as sequence number
   tend to change more unpredictably, partly because of the influence of
   external factors (size of TCP windows, application behavior, etc.).
   Also, the field values tend to change independently.  Overall, this
   makes compression more challenging and makes it harder to select a
   set of encodings that can successfully trade off efficiency and

5.5.  Short-Lived Flows

   It is hard to see what can be done to improve performance for a
   single, unpredictable, short-lived connection.  However, there are
   commonly cases where there will be multiple TCP connections between
   the same pair of hosts.  As a particular example, consider web
   browsing (this is more the case with HTTP/1.0 [25] than with HTTP/1.1

   When a connection closes, either it is the last connection between
   that pair of hosts or it is likely that another connection will open
   within a relatively short space of time.  In this case, the IP header
   part of the context (i.e., those fields characterised in Section 2.1)
   will probably be almost identical.  Certain aspects of the TCP
   context may also be similar.

   Support for context replication is discussed in more detail in
   Section 3.  Overall, support for sub-context sharing or initializing
   one context from another offers useful optimizations for a sequence
   of short-lived connections.

   Note that, although TCP is connection oriented, it is hard for a
   compressor to tell whether a TCP flow has finished.  For example,
   even in the 'bi-directional' link case, seeing a FIN and the ACK of
   the FIN at the compressor/decompressor does not mean that the FIN
   cannot be retransmitted.  Thus, it may be more useful to think about
   initializing a new context from an existing one, rather than re-using
   an existing one.

   As mentioned previously in Section 4.1.3, the IP header can clearly
   be shared between any transport-layer flows between the same two
   end-points.  There may be limited scope for initialisation of a new
   TCP header from an existing one.  The port numbers are the most
   obvious starting point.

5.6.  Master Sequence Number

   As pointed out earlier, in Section 4.1.3, there is no obvious
   candidate for a 'master sequence number' in TCP.  Moreover, it is
   noted that such a master sequence number is only required to allow a
   decompressor to acknowledge packets in bi-directional mode.  It can
   also be seen that such a sequence number would not be required for
   every packet.

   While the sequence number only needs to be 'sparse', it is clear that
   there is a requirement for an explicitly added sequence number.
   There are no obvious ways to guarantee the unique identity of a
   packet other than by adding such a sequence number (sequence and
   acknowledgement numbers can both remain the same, for example).

5.7.  Size Constraint for TCP Options

   As can be seen from the above analysis, most TCP options, such as
   MSS, WSopt, or SACK-Permitted, may appear only on a SYN segment.
   Every implementation should (and we expect that most will) ignore
   unknown options on SYN segments.  TCP options will be sent on non-SYN
   segments only when an exchange of options on the SYN segments has

   indicated that both sides understand the extension.  Other TCP
   options, such as MD5 Digest or Timestamp, also tend to be sent when
   the connection is initiated (i.e., in the SYN packet).

   The total header size is also an issue.  The TCP header specifies
   where segment data starts with a 4-bit field that gives the total
   size of the header (including options) in 32-bit words.  This means
   that the total size of the header plus option must be less than or
   equal to 60 bytes.  This leaves 40 bytes for options.

6.  Security Considerations

   Since this document only describes TCP field behavior, it raises no
   direct security concerns.

   This memo is intended to be used to aid the compression of TCP/IP
   headers.  Where authentication mechanisms such as IPsec AH [24] are
   used, it is important that compression be transparent.  Where
   encryption methods such as IPsec ESP [27] are used, the TCP fields
   may not be visible, preventing compression.

7.  Acknowledgements

   Many IP and TCP RFCs (hopefully all of which have been collated
   below), together with header compression schemes from RFC 1144 [22],
   RFC 3544 [36], and RFC 3095 [31], and of course the detailed analysis
   of RTP/UDP/IP in RFC 3095, have been sources of ideas and knowledge.
   Further background information can also be found in [28] and [29].

   This document also benefited from discussion on the ROHC mailing list
   and in various corridors (virtual or otherwise) about many key
   issues; special thanks go to Qian Zhang, Carsten Bormann, and Gorry

   Qian Zhang and Hongbin Liao contributed the extensive analysis of
   shareable header fields.

   Any remaining misrepresentation or misinterpretation of information
   is entirely the fault of the authors.

8.  References

8.1.  Normative References

   [1]   Postel, J., "Internet Protocol", STD 5, RFC 791, September

   [2]   Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
         September 1981.

   [3]   Nagle, J., "Congestion control in IP/TCP internetworks", RFC
         896, January 1984.

   [4]   Jacobson, V. and R. Braden, "TCP extensions for long-delay
         paths", RFC 1072, October 1988.

   [5]   Zweig, J. and C. Partridge, "TCP alternate checksum options",
         RFC 1146, March 1990.

   [6]   Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
         November 1990.

   [7]   Jacobson, V., Braden, B., and D. Borman, "TCP Extensions for
         High Performance", RFC 1323, May 1992.

   [8]   Braden, B., "T/TCP -- TCP Extensions for Transactions
         Functional Specification", RFC 1644, July 1994.

   [9]   Connolly, T., Amer, P., and P. Conrad, "An Extension to TCP:
         Partial Order Service", RFC 1693, November 1994.

   [10]  Bellovin, S., "Defending Against Sequence Number Attacks", RFC
         1948, May 1996.

   [11]  McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery for
         IP version 6", RFC 1981, August 1996.

   [12]  Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
         Selective Acknowledgment Options", RFC 2018, October 1996.

   [13]  Heffernan, A., "Protection of BGP Sessions via the TCP MD5
         Signature Option", RFC 2385, August 1998.

   [14]  Nichols, K., Blake, S., Baker, F., and D. Black, "Definition of
         the Differentiated Services Field (DS Field) in the IPv4 and
         IPv6 Headers", RFC 2474, December 1998.

   [15]  Ramakrishnan, K. and S. Floyd, "A Proposal to add Explicit
         Congestion Notification (ECN) to IP", RFC 2481, January 1999.

   [16]  Allman, M., Paxson, V., and W. Stevens, "TCP Congestion
         Control", RFC 2581, April 1999.

   [17]  Floyd, S., Mahdavi, J., Mathis, M., and M. Podolsky, "An
         Extension to the Selective Acknowledgement (SACK) Option for
         TCP", RFC 2883, July 2000.

   [18]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition of
         Explicit Congestion Notification (ECN) to IP", RFC 3168,
         September 2001.

   [19]  Spring, N., Wetherall, D., and D. Ely, "Robust Explicit
         Congestion Notification (ECN)  Signaling with Nonces", RFC
         3540, June 2003.

8.2.  Informative References

   [20]  IANA, "IANA", IANA TCP options, February 1998,

   [21]  Braden, R., "Requirements for Internet Hosts - Communication
         Layers", STD 3, RFC 1122, October 1989.

   [22]  Jacobson, V., "Compressing TCP/IP headers for low-speed serial
         links", RFC 1144, February 1990.

   [23]  Almquist, P., "Type of Service in the Internet Protocol Suite",
         RFC 1349, July 1992.

   [24]  Kent, S. and R. Atkinson, "IP Authentication Header", RFC 2402,
         November 1998.

   [25]  Berners-Lee, T., Fielding, R., and H. Nielsen, "Hypertext
         Transfer Protocol -- HTTP/1.0", RFC 1945, May 1996.

   [27]  Kent, S. and R. Atkinson, "IP Encapsulating Security Payload
         (ESP)", RFC 2406, November 1998.

   [26]  Fielding, R., Gettys, J., Mogul, J., Nielsen, H., and T.
         Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC
         2068, January 1997.

   [28]  Degermark, M., Nordgren, B., and S. Pink, "IP Header
         Compression", RFC 2507, February 1999.

   [29]  Casner, S. and V. Jacobson, "Compressing IP/UDP/RTP Headers for
         Low-Speed Serial Links", RFC 2508, February 1999.

   [30]  Bradner, S. and V. Paxson, "IANA Allocation Guidelines For
         Values In the Internet Protocol and Related Headers", BCP 37,
         RFC 2780, March 2000.

   [31]  Bormann, C., Burmeister, C., Degermark, M., Fukushima, H.,
         Hannu, H., Jonsson, L-E., Hakenberg, R., Koren, T., Le, K.,
         Liu, Z., Martensson, A., Miyazaki, A., Svanbro, K., Wiebke, T.,
         Yoshimura, T., and H. Zheng, "RObust Header Compression (ROHC):
         Framework and four profiles: RTP, UDP, ESP, and uncompressed",
         RFC 3095, July 2001.

   [32]  Dawkins, S., Montenegro, G., Kojo, M., and V. Magret, "End-to-
         end Performance Implications of Slow Links", BCP 48, RFC 3150,
         July 2001.

   [33]  Balakrishnan, Padmanabhan, V., Fairhurst, G., and M.
         Sooriyabandara, "TCP Performance Implications of Network Path
         Asymmetry", RFC 3449, December 2002.

   [34]  Inamura, H., Montenegro, G., Ludwig, R., Gurtov, A., and F.
         Khafizov, "TCP over Second (2.5G) and Third (3G) Generation
         Wireless Networks", RFC 3481, February 2003.

   [35]  Ludwig, R. and M. Meyer, "The Eifel Detection Algorithm for
         TCP", RFC 3522, April 2003.

   [36]  Engan, M., Casner, S., Bormann, C., and T. Koren, "IP Header
         Compression over PPP", RFC 3544, July 2003.

   [37]  Karn, P., Bormann, C., Fairhurst, G., Grossman, D., Ludwig, R.,
         Mahdavi, J., Montenegro, G., Touch, J., and L. Wood, "Advice
         for Internet Subnetwork Designers", BCP 89, RFC 3819, July

Authors' Addresses

   Mark A. West
   Siemens/Roke Manor Research
   Roke Manor Research Ltd.
   Romsey, Hants  SO51 0ZN

   Phone: +44 (0)1794 833311
   EMail: mark.a.west@roke.co.uk
   URI:   http://www.roke.co.uk

   Stephen McCann
   Siemens/Roke Manor Research
   Roke Manor Research Ltd.
   Romsey, Hants  SO51 0ZN

   Phone: +44 (0)1794 833341
   EMail: stephen.mccann@roke.co.uk
   URI:   http://www.roke.co.uk

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