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RFC 4896 - Signaling Compression (SigComp) Corrections and Clari


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Network Working Group                                         A. Surtees
Request for Comments: 4896                                       M. West
Updates: 3320, 3321, 3485                    Siemens/Roke Manor Research
Category: Standards Track                                     A.B. Roach
                                                        Estacado Systems
                                                               June 2007

     Signaling Compression (SigComp) Corrections and Clarifications

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   This document describes common misinterpretations and some
   ambiguities in the Signaling Compression Protocol (SigComp), and
   offers guidance to developers to resolve any resultant problems.
   SigComp defines a scheme for compressing messages generated by
   application protocols such as the Session Initiation Protocol (SIP).
   This document updates the following RFCs: RFC 3320, RFC 3321, and RFC
   3485.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Decompression Memory Size  . . . . . . . . . . . . . . . . . .  3
     2.1.  Bytecode within Decompression Memory Size  . . . . . . . .  3
     2.2.  Default Decompression Memory Size  . . . . . . . . . . . .  4
   3.  UDVM Instructions  . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Data Input Instructions  . . . . . . . . . . . . . . . . .  5
     3.2.  MULTILOAD  . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.3.  STATE-FREE . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.4.  Using the Stack  . . . . . . . . . . . . . . . . . . . . .  6
   4.  Byte Copying Rules . . . . . . . . . . . . . . . . . . . . . .  7
     4.1.  Instructions That Use Byte Copying Rules . . . . . . . . .  9
   5.  State Retention Priority . . . . . . . . . . . . . . . . . . .  9
     5.1.  Priority Values  . . . . . . . . . . . . . . . . . . . . .  9
     5.2.  Multiple State Retention Priorities  . . . . . . . . . . . 10
     5.3.  Retention Priority 65535 (or -1) . . . . . . . . . . . . . 10
   6.  Duplicate State  . . . . . . . . . . . . . . . . . . . . . . . 14
   7.  State Identifier Clashes . . . . . . . . . . . . . . . . . . . 14
   8.  Message Misordering  . . . . . . . . . . . . . . . . . . . . . 15
   9.  Requested Feedback . . . . . . . . . . . . . . . . . . . . . . 15
     9.1.  Feedback When SMS Is Zero  . . . . . . . . . . . . . . . . 15
     9.2.  Updating Feedback Requests . . . . . . . . . . . . . . . . 16
   10. Advertising Resources  . . . . . . . . . . . . . . . . . . . . 16
     10.1. The I-bit and Local State Items  . . . . . . . . . . . . . 16
     10.2. Dynamic Update of Resources  . . . . . . . . . . . . . . . 17
     10.3. Advertisement of Locally Available State Items . . . . . . 17
       10.3.1.  Basic SigComp . . . . . . . . . . . . . . . . . . . . 18
       10.3.2.  Dictionaries  . . . . . . . . . . . . . . . . . . . . 18
       10.3.3.  SigComp Extended Mechanisms . . . . . . . . . . . . . 19
   11. Uncompressed Bytecode  . . . . . . . . . . . . . . . . . . . . 19
   12. RFC 3485 SIP/SDP Static Dictionary . . . . . . . . . . . . . . 20
   13. Security Considerations  . . . . . . . . . . . . . . . . . . . 21
   14. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 22
   15. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 22
   16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
     16.1. Normative References . . . . . . . . . . . . . . . . . . . 23
     16.2. Informative References . . . . . . . . . . . . . . . . . . 23
   Appendix A.  Dummy Application Protocol (DAP)  . . . . . . . . . . 24
     A.1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . 24
     A.2.  Processing a DAP Message . . . . . . . . . . . . . . . . . 24
     A.3.  DAP Message Format in ABNF . . . . . . . . . . . . . . . . 26
     A.4.  An Example of a DAP Message  . . . . . . . . . . . . . . . 26

1.  Introduction

   SigComp [1] defines the Universal Decompressor Virtual Machine (UDVM)
   for decompressing messages sent by a compliant compressor.  SigComp
   further describes mechanisms to deal with state handling, message
   structure, and other details.  While the behavior of the decompressor
   is specified in great detail, the behavior of the compressor is left
   as a choice for the implementer.  During implementation and
   interoperability tests, some areas of SigComp that need clarification
   have been identified.  The sections that follow enumerate the problem
   areas identified in the specification, and attempt to provide
   clarification.

   Note that, as this document refers to sections in several other
   documents, the following notation is applied:

      "in Section 3.4" refers to Section 3.4 of this document
      "in RFC 3320-Section 3.4" refers to Section 3.4 of RFC 3320 [1]

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [5].

2.  Decompression Memory Size

2.1.  Bytecode within Decompression Memory Size

   SigComp [1] states that the default Decompression Memory Size (DMS)
   is 2K.  The UDVM memory size is defined in RFC 3320-Section 7 to be
   (DMS - n), where n is the size of the SigComp message, for messages
   transported over UDP and (DMS / 2) for those transported over TCP.
   This means that when the message contains the bytecode (as it will
   for at least the first message) there will actually be two copies of
   the bytecode within the decompressor memory (see Figure 1).  The
   presence of the second copy of bytecode in decompressor memory is
   correct in this case.

    |<----------------------------DMS--------------------------------->|
    |<-----SigComp message---->|<------------UDVM memory size--------->|
    +-+----------+-------------+-----+----------+----------------------+
    | | bytecode |  comp msg   |     | bytecode | circular buffer      |
    +-+----------+-------------+-----+----------+----------------------+
     ^                            ^
     |                            |
    SigComp header          Low bytes of UDVM

            Figure 1: Bytecode and UDVM memory size within DMS

2.2.  Default Decompression Memory Size

   For many implementations, the length of decompression bytecode sent
   is in the range of three to four hundred bytes.  Because SigComp
   specifies a default DMS of 2K, the described scheme seriously
   restricts the size of the circular buffer, and of the compressed
   message itself.  In some cases, this set of circumstances has a
   damaging effect on the compression ratio; for others, it makes it
   completely impossible to send certain messages compressed.

   To address this problem, those mandating the use of SigComp need to
   also provide further specification for their application that
   mandates the use of an appropriately sized DMS.  Sizing of such a DMS
   should take into account (1) the size of bytecode for algorithms
   likely to be employed in compressing the application messages, (2)
   the size of any buffers or structures necessary to execute such
   algorithms, (3) the size of application messages, and (4) the average
   entropy present within a single application message.

   For example, assume a typical compression algorithm requiring
   approximately 400 bytes of bytecode, plus about 2432 bytes of data
   structures.  The required UDVM memory size is 400 + 2432 = 2832.  For
   a TCP-based protocol, this means the DMS must be at least 5664 (2832
   * 2) bytes, which is rounded up to 8k.  For a UDP-based protocol, one
   must take into account the size of the SigComp messages themselves.
   Assuming a text-based protocol with sufficient average entropy to
   compress a single message by 50% (without any previous message
   history), and messages that are not expected to exceed 8192 bytes in
   size, the protocol message itself will add 4096 bytes to the SigComp
   message size (on top of the 400 bytes of bytecode plus a 3-byte
   header), or 4096 + 400 + 3 = 4499.  To calculate the DMS, one must
   add this to the required UDVM memory size: 2832 + 4499 = 6531, which
   is again rounded up to 8k of DMS.

3.  UDVM Instructions

3.1.  Data Input Instructions

   When inputting data from the compressed message, the INPUT-BYTES (RFC
   3320-Section 9.4.2) and INPUT-BITS (RFC 3320-Section 9.4.3)
   instructions both have the paragraph:

   "If the instruction requests data that lies beyond the end of the
   SigComp message, no data is returned.  Instead the UDVM moves program
   execution to the address specified by the address operand."

   The intent is that if n bytes/bits are requested, but only m are left
   in the message (where m < n), then the decompression dispatcher MUST
   NOT return any bytes/bits to the UDVM, and the m bytes/bits that are
   there MUST remain in the message unchanged.

   For example, if the remaining bytes of a message are: 0x01 0x02 0x03
   and the UDVM encounters an INPUT-BYTES (6, a, b) instruction.  Then
   the decompressor dispatcher returns no bytes and jumps to the
   instruction specified by b.  This contains an INPUT-BYTES (2, c, d)
   instruction so the decompressor dispatcher successfully returns the
   bytes 0x01 and 0x02.

   In the case where an INPUT-BYTES instruction follows an INPUT-BITS
   instruction that has left a partial byte in the message, the partial
   byte should still be thrown away even if there are not enough bytes
   to input.

   INPUT-BYTES (0, a, b) can be used to flush out a partial byte.

3.2.  MULTILOAD

   In order to make step-by-step implementation simpler, the MULTILOAD
   instruction is explicitly not allowed to write into any memory
   positions occupied by the MULTILOAD opcode or any of its parameters.
   Additionally, if there is any indirection of parameters, the
   indirection MUST be done at execution time.

   Any implementation technique other than a step-by-step implementation
   (e.g., decode all operands then execute, which is the model of all
   other instructions) MUST yield the same result as a step-by-step
   implementation would.

   For example:

   at (64)

   :location_a                     pad (2)
   :location_b                     pad (2)
   :location_c                     pad (2)
   pad (30)
   :udvm_memory_size               pad (2)
   :circular_buffer                pad (2)

   align (64)

   MULTILOAD (location_a, 3, circular_buffer,
                   udvm_memory_size, $location_a)

   The step-by-step implementation would: write the address of
   circular_buffer into location_a (memory address 64); write the
   address of udvm_memory_size into location_a + 2 (memory address 66);
   write the value stored in location_a (accessed using indirection -
   that is now the address of circular_buffer) into location_a + 4
   (memory address 68).  Therefore, at the end of the execution by a
   correct implementation, location_c will contain the address of
   circular_buffer.

3.3.  STATE-FREE

   The STATE-FREE instruction does not check the minimum_access_length.
   This is correct because the state cannot be freed until the
   application has authenticated the message.  The lack of checking does
   not pose a security risk because if the sender has enough information
   to create authenticated messages, then sending messages that save
   state can push previous state out of storage anyway.

   The STATE-FREE instruction can only free state in the compartment
   that corresponds to the message being decompressed.  Attempting to
   free state that is either from another compartment, or that is not
   associated with any compartment, has no effect.

3.4.  Using the Stack

   The instructions PUSH, POP, CALL, and RETURN make use of a stack that
   is set up using the well-known memory address stack_location to
   define where in memory the stack is located.  Use of the stack is
   defined in RFC 3320-Section 8.3, which states: '"Pushing" a value on
   the stack is an abbreviation for copying the value to

   stack[stack_fill] and then increasing stack_fill by 1.' and
   'stack_fill is an abbreviation for the 2-byte word at stack_location
   and stack_location + 1'.

   In the very rare case that the value of stack_fill is 0xFFFF when a
   value is pushed onto the stack, then the original stack_fill value
   MUST be increased by 1 to 0x0000 and written back to stack_location
   and stack_location + 1 (which will overwrite the value that has been
   pushed onto the stack).

      The new value pushed onto the stack has, in theory, been written
      to stack [0xFFFF] = stack_location.  Stack_fill would then be
      increased by 1; however, the value at stack_location and
      stack_location + 1 has just been updated.  To maintain the
      integrity of the stack with regard to over and underflow,
      stack_fill cannot be re-read at this point, and the pushed value
      is overwritten.

4.  Byte Copying Rules

   RFC 3320-Section 8.4 states that "The string of bytes is copied in
   ascending order of memory address, respecting the bounds set by
   byte_copy_left and byte_copy_right."  This is misleading in that it
   is perfectly legitimate to copy bytes outside of the bounds set by
   byte_copy_left and byte_copy_right.  Byte_copy_left and
   byte_copy_right provide the ability to maintain a circular buffer as
   follows:

   For moving to the right

   if current_byte == ((byte_copy_right - 1) mod 2 ^ 16):
       next_byte = byte_copy_left
   else:
       next_byte = (current_byte + 1) mod 2 ^ 16

   which is equivalent to the algorithm given in RFC 3320-Section 8.4.

   For moving to the left

   if current_byte == byte_copy_left:
       previous_byte = (byte_copy_right - 1) mod 2 ^ 16
   else:
       previous_byte = (current_byte - 1) mod 2 ^ 16

   Moving to the left is only used for COPY_OFFSET.

   Consequently, copying could begin to the left of byte_copy_left and
   continue across it (and jump back to it according to the given

   algorithm if necessary) and could begin at or to the right of
   byte_copy_right (though care must be taken to prevent decompression
   failure due to writing to / reading from beyond the UDVM memory).

   For further clarity: consider the UDVM memory laid out as follows,
   with byte_copy_left and byte_copy_right in the locations indicated by
   "BCL" and "BCR", respectively:

   +----------------------------------------+
   |                                        |
   +----------^------------^----------------+
             BCL          BCR

   If an opcode read or wrote bytes starting to the left of
   byte_copy_left, it would do so in the following order:

   +----------------------------------------+
   |       abcdefghijkl                     |
   +----------^------------^----------------+
             BCL          BCR

   If the opcode continues to read or write until it reaches
   byte_copy_right, it would then wrap around to byte_copy_left and
   continue (letters after the wrap are capitalized for clarity):

   +----------------------------------------+
   |       abcQRSTUVjklmnop                 |
   +----------^------------^----------------+
             BCL          BCR

   Similarly, writing to the right of byte_copy_right is a perfectly
   valid operation for opcodes that honor byte copying rules:

   +----------------------------------------+
   |                          abcdefg       |
   +----------^------------^----------------+
             BCL          BCR

   A final, somewhat odd relic of the foregoing rules occurs when
   byte_copy_right is actually less than byte_copy_left.  In this case,
   reads and writes will skip the memory between the pointers:

   +----------------------------------------+
   |     abcde             fghijkl          |
   +----------^------------^----------------+
             BCR          BCL

4.1.  Instructions That Use Byte Copying Rules

   This document amends the list of instructions that obey byte copying
   rules in RFC 3320-Section 8.4 to include STATE-CREATE and CRC.

   RFC 3320-Section 8.4 specifies the byte copying rules and includes a
   list of the instructions that obey them.  STATE-CREATE is not in this
   list but END-MESSAGE is.  This caused confusion due to the fact that
   neither instruction actually does any byte copying; rather, both
   instructions give information to the state-handler to create state.
   Logically, both instructions should have the same information about
   byte copying.

   When state is created by the state-handler (whether from an END-
   MESSAGE or a STATE-CREATE instruction), the byte copying rules of RFC
   3320-Section 8.4 apply.

   Note that, if the contents of the UDVM changes between the occurrence
   of the STATE-CREATE instruction and the state being created, the
   bytes that are stored are those in the buffer at the time of creation
   (i.e., when the message has been decompressed and authenticated).

   CRC is not mentioned in RFC 3320-Section 8.4 in the list of
   instructions that obey byte copying rules, but its description in RFC
   3320-Section 9.3.5 states that these rules are to be obeyed.  When
   reading data over which to perform the CRC check, byte copying rules
   apply as specified in RFC 3320-Section 8.4.

   When the partial identifier for a STATE-FREE instruction is read,
   (during the execution of END-MESSAGE) byte copying rules as per RFC
   3320-Section 8.4 apply.

   Given that reading the buffer for creating and freeing state within
   the END-MESSAGE instruction obeys byte copying rules, there may be
   some confusion as to whether reading feedback items should also obey
   byte copying rules.  Byte copying rules do not apply for reading
   feedback items.

5.  State Retention Priority

5.1.  Priority Values

   For state_retention_priority, 65535 < 0 < 1 < ... < 65534.  This is
   slightly counter intuitive, but is correct.

5.2.  Multiple State Retention Priorities

   There may be confusion when the same piece of state is created at two
   different retention priorities.  The following clarifies this:

      The retention priority MUST be associated with the compartment and
      not with the piece of state.  For example, if endpoint A creates a
      piece of state with retention priority 1 and endpoint B creates
      exactly the same state with retention priority 2, there should be
      one copy (assuming the model of state management suggested in
      SigComp [1]) of the actual state, but each compartment should keep
      a record of this piece of state with its own priority.  (If this
      does not happen then the state could be kept for longer than A
      anticipated or less time than B anticipated, depending on which
      priority is used.  This could cause Decompression Failure to
      occur.)

      If the same piece of state is created within a compartment with a
      different priority, then one copy of it should be stored with the
      new priority and it MUST count only once against SMS.  That is,
      the state creation updates the priority rather than creates a new
      piece of state.

5.3.  Retention Priority 65535 (or -1)

   There is potentially a problem with storing multiple pieces of state
   with the minimum retention priority (65535) as defined in SigComp
   [1].  This can be shown by considering the following examples that
   are of shared mode, which is documented in SigComp Extended [2].  The
   key thing about state with retention priority 65535 is that it can be
   created by an endpoint in the decompressor compartment without the
   knowledge of the remote compressor (which controls state creation in
   the decompressor compartment).

   Example 1:

       [SMn state is shared mode state (priority 65535),
        BC is bytecode state (priority 1),
        BFn is buffer state (priority 0)]

       Endpoint A                  Endpoint B
       [decomp cpt]                [comp cpt]

       [SM1]
       ------------------------------->
                                   [SM1]

       [SM1, SM2]
       --------------------X (message lost)

                                   [SM1, BC, BF1]
       <------------ref SM1------------
       [SM2, BC, BF1]
                                   endpoint B still believes SM1
                                   is at endpoint A

                                   [BC, BF1, BF2]
       <------------ref SM1------------

       decompression failure at A
       because SM1 has already been deleted

   Example 2:

       Endpoint A                  Endpoint B
       [decomp cpt]                [comp cpt]

       [SM1]
       ------------------------------->
                                   [SM1]

                                   [SM1, BC, BF1]
       (message lost)X------ref SM1-----

       [SM1, SM2]
       ------------------------------->
                                   endpoint B does not create SM2
                                   because there is no space
                                   [SM1, BC, BF1]

                                   [SM1, BC, BF1, BF2]
       <------------ref SM1------------
       [SM2, BC, BF2]
                                   endpoint B still believes SM1
                                   is at endpoint A

                                   [BC, BF1, BF2, BF3]
       <------------ref SM1------------

       decompression failure at A
       because SM1 has already been deleted

                Figure 2: Retention priority 65535 examples

   Once there is more than one piece of minimum priority state created
   in a decompressor compartment, the corresponding compressor cannot be
   certain about which pieces of state are present in that
   (decompressor) compartment.  If there is only one piece of state,
   then no such ambiguity exists.

   The problem is a consequence of the different rules for the creation
   of minimum priority state.  In particular, the creation of the second
   piece of state without the knowledge of the compressor could mean
   that the first piece is pushed out earlier than the compressor
   expects (despite the fact that the state processing rules from
   SigComp [1] are being implemented correctly).

   SigComp [1] also states that a compressor MUST be certain that all of
   the data needed to decompress a SigComp message is available at the
   receiving endpoint.  Thus, it SHOULD NOT reference any state unless
   it can be sure that the state exists.  The fact that the compressor
   at B has no way of knowing how much state has been created at A can
   lead to a loss of synchronization between the endpoints, which is not
   acceptable.

   One observation is that it is always safe to reference a piece of
   minimum priority state following receipt of the advertisement of the
   state.

   If it is known that both endpoints are running SigComp version 2, as
   defined in NACK [3], then an endpoint MAY assume that the likelihood
   of a loss of synchronization is very small, and rely on the NACK
   mechanism for recovery.

   However, for a compressor to try and avoid causing the generation of
   NACKs, it has to be able to make some assumptions about the behavior
   of the peer compressor.  Also, if one of the endpoints does not
   support NACK, then some other solution is needed.

   Consequently, where NACK is not supported or for NACK averse
   compressors, the recommendation is that only one piece of minimum
   priority state SHOULD be present in a compartment at any one time.
   If both endpoints support NACK [3], then this recommendation MAY be
   relaxed, but implementers need to think carefully about the
   consequences of creating multiple pieces of minimum priority state.
   In either case, if the behavior of the application restricts the
   message flow, this fact could be exploited to allow safe creation of
   multiple minimum priority states; however, care must still be taken.

   Note that if a compressor wishes the remote endpoint to be able to
   create a new piece of minimum priority state, it can use the STATE-
   FREE instruction to remove the existing piece of state.

6.  Duplicate State

   If a piece of state is created in a compartment in which it already
   exists, the time of its creation SHOULD be updated as if it had just
   been created, irrespective of whether or not there is a new state
   retention priority.

7.  State Identifier Clashes

   RFC 3320-Section 6.2 states that when creating a piece of state, the
   full 20-byte hash should be checked to see whether or not another
   piece of state with this identifier exists.  If it does, and the
   state item is not identical, then the new creation MUST fail.  It is
   stated that the probability of this occurring is vanishingly small
   (and so it is, see below).

   However, when state is accessed, only the first n bytes of the state
   identifier are used, where n could be as low as 6.  At this point, if
   there are two pieces of state with the same first n bytes of state
   identifier, the STATE-ACCESS instruction will cause decompression
   failure.  The compressor referencing the state will not expect this
   failure mode because the state creation succeeded without a clash.
   At a server endpoint where there could be thousands or millions of
   pieces of state, how likely is this to actually happen?

   Consider the birthday paradox (where there only have to be 23 people
   in a room to have a greater than 50% chance that two of them will
   have the same birthday (Birthday [8])).

   The naive calculation using factorials gives:

                      N!
   Pd(N,s) = 1 - -------------
                 (N - s)! N^s

   where N is the number of possible values and s is the sample size.

   However, due to dealing with large numbers, an approximation is
   needed:

   Pd(N,s) = 1 - e^( LnFact(N) - LnFact(N-s) - s Ln(N) )

   where LnFact (x) is the log of x!, which can be approximated by:

   LnFact(x) ~ (x + 1/2) Ln(x) - x + Ln(2*Pi)/2 +

                1       1         1           1
               --- - ------- + -------- - --------
               12x   360 x^3   1260 x^5   1680 x^7

   which using N = 2^48 [6 octet partial state identifier] gives:

   s = 1 000 000: Pd (N,s) = 0.018%
   s = 10 000 000: Pd (N,s) = 16.28%
   s = 100 000 000: Pd (N,s) = 100.00%

   so when implementing, thought should be given as to whether or not 6
   octets of state identifier is enough to ensure that state access will
   be successful (particularly at a server).

   The likelihood of a clash when using the full 20 octets of state
   identifier, does indeed have a vanishingly small probability:
   using N = 2^160 [full 20 octet state identifier] gives:

   s = 1 000 000: Pd (N,s) = 3.42E-35%
   s = 10 000 000: Pd (N,s) = 3.42E-33%
   s = 100 000 000: Pd (N,s) = 3.42E-31%

   Consequently, care must be taken when deciding how many octets of
   state identifier to use to access state at the server.

8.  Message Misordering

   SigComp [1] makes only one reference to the possibility of misordered
   messages.  However, the statement that the 'compressor MUST ensure
   that the message can be decompressed using the resources available at
   the remote endpoint' puts the onus on the compressor to take account
   of the possibility of misordering occurring.

   Whether misordering can occur and whether that would have an impact
   depends on the compartment definition and the transport protocol in
   use.  Therefore, it is up to the implementer of the compressor to
   take these factors into account.

9.  Requested Feedback

9.1.  Feedback When SMS Is Zero

   If an endpoint receives a request for feedback, then it SHOULD return
   the feedback even if its SMS is zero.  The storage overhead of the
   requested feedback is NOT part of the SMS.

9.2.  Updating Feedback Requests

   When an endpoint receives a valid message it updates the requested
   feedback data for that compartment.  RFC 3320-Section 5 states that
   there is no need to transmit any requested feedback item more than
   once.  However, there are cases where it would be beneficial for the
   feedback to be sent more than once (e.g., a retransmitted 200 OK SIP
   message [9] to an INVITE SIP message implies that the original 200
   OK, and the feedback it carried, might not have reached the remote
   endpoint).  Therefore, an endpoint SHOULD transmit feedback
   repeatedly until it receives another valid message that updates the
   feedback.

   RFC 3320-Section 9.4.9 states that when requested_feedback_location
   equals zero, no feedback request is made.  However, there is no
   indication of whether this means that the existing feedback data is
   left untouched or if this means that the existing feedback data
   SHOULD be overwritten to be 'no feedback data'.  If
   requested_feedback_location equals zero, the existing feedback data
   SHOULD be left untouched and returned in any subsequent messages as
   before.

   RFC 3320-Section 9.4.9 also makes no statement about what happens to
   existing feedback data when requested_feedback_location does not
   equal zero but the Q flag indicating the presence/absence of a
   requested_feedback_item is zero.  In this case, the existing feedback
   data SHOULD be overwritten to be 'no feedback data'.

10.  Advertising Resources

10.1.  The I-bit and Local State Items

   The I-bit in requested feedback is a mechanism by which a compressor
   can tell a remote endpoint that it is not going to access any local
   state items.  By doing so, it gives the remote endpoint the option of
   not advertising them in subsequent messages.  Setting the I-bit does
   not obligate the remote endpoint to cease sending advertisements.

   The remote endpoint SHOULD still advertise its parameters such as DMS
   and state memory size (SMS).  (This is particularly important; if the
   sender of the first message sets the I-bit, it will still want the
   advertisement of parameters from the receiver.  If it doesn't receive
   these, it has to assume the default parameters which will affect
   compression efficiency.)

   The endpoint receiving an I-bit of 1 can reclaim the memory used to
   store the locally available state items.  However, this has NO impact

   on any state that has been created by the sender using END-MESSAGE or
   STATE-CREATE instructions.

10.2.  Dynamic Update of Resources

   Decompressor resources such as SMS and DMS can be dynamically updated
   at the compressor by use of the SMS and DMS bits in returned
   parameters feedback (see RFC 3320-Section 9.4.9).  Changing resources
   dynamically (apart from initial advertisements for each compartment)
   is not expected to happen very often.

   If additional resources are advertised to a compressor, then it is up
   to the implementation at the compressor whether or not to make use of
   these resources.  For example, if the decompressor advertises 8k SMS
   but the compressor only has 4k SMS, then the compressor MAY choose
   not to use the extra 4k (e.g., in order to monitor state saved at the
   decompressor).  In this case, there is no synchronization problem.
   The compressor MUST NOT use more than the most recently advertised
   resources.  Note that the compressor SMS is unofficial (it enables
   the compressor to monitor decompressor state) and is separate from
   the SMS advertised by the decompressor.

   Reducing the resources has potential synchronization issues and so
   SHOULD NOT be done unless absolutely necessary.  If this is the case
   then the memory MUST NOT be reclaimed until the remote endpoint has
   acknowledged the message sent with the advertisement.  If state is to
   be deleted to accommodate a reduction in SMS then both endpoints MUST
   delete it according to the state retention priority (see RFC 3320-
   Section 6.2).  The compressor MUST NOT then use more than the amount
   of resources most recently advertised.

10.3.  Advertisement of Locally Available State Items

   RFC 3320-Section 3.3.3 defines locally available state items to be
   the pieces of state that an endpoint has available but that have not
   been uploaded by the SigComp message.  The examples given are
   dictionaries and well known pieces of bytecode; and the advertisement
   mechanism discussed in RFC 3320-Section 9.4.9 provides a way for the
   endpoint to advertise the pieces of locally available state that it
   has.

   However, SigComp [1] does not (nor was it ever intended to) fully
   define the use of locally available state items, in particular, the
   length of time for which they will be available.  The use of locally
   available state items is left for definition in other documents.
   However, this fact, coupled with the fact that SigComp does contain
   some hooks for uses of locally available state items and the fact
   that some of the definitions of such uses (in SigComp Extended [2])

   are incomplete has caused some confusion.  Therefore, this section
   clarifies the situation.

   Note that any definitions of uses of locally available state items
   MUST NOT conflict with any other uses.

10.3.1.  Basic SigComp

   SigComp provides a mechanism for an endpoint to advertise locally
   available state (RFC 3320-Section 9.4.9).  If the endpoint receiving
   the advertisement does not 'recognize' it and therefore know the
   properties of the state e.g., its length and lifetime, the compressor
   needs to consider very carefully whether or not to access the state;
   especially if NACK [3] is not available.

   SigComp provides the following hooks for use in conjunction with
   locally available state items.  Without further definition, locally
   available state SHOULD NOT be used.

   RFC 3320-Section 6.2 allows for the possibility to map locally
   available state items to a compartment and states that, if this is
   done, the state items MUST have state retention priority 65535 in
   order to not interfere with state created at the request of the
   remote compressor.  Note that Section 5.3 also recommends that only
   one such piece of state SHOULD be created per compartment.

   The I-bit in the requested_feedback_location (see RFC 3320-Section
   9.4.9) allows a compressor to indicate to the remote endpoint that it
   will not reference any of the previously advertised locally available
   state.  Depending on the implementation model for state handling at
   the remote endpoint, this could allow the remote endpoint to reclaim
   the memory being used by such state items.

10.3.2.  Dictionaries

   The most basic use of the local state advertisement is the
   advertisement of a dictionary (e.g., the dictionary specified by SIP/
   SDP Static Dictionary [4]) or a piece of bytecode.  In general, these
   pieces of state:

   o  are not mapped to compartments
   o  are local to the endpoint
   o  are available for at least the duration of the compartment
   o  do not have any impact on the compartment SMS

   However, for a given piece of state the exact lifetime needs to be
   defined e.g., in public specifications such as SigComp for SIP [7] or

   the 3GPP IMS specification [10].  Such a specification should also
   indicate whether or not advertisement of the state is needed.

10.3.3.  SigComp Extended Mechanisms

   SigComp Extended [2] defines some uses of local state advertisements
   for which additional clarification is provided here.

   Shared-mode (see RFC 3321-Section 5.2) is well-defined (when combined
   with the clarification in Section 5.3).  In particular, the states
   that are created and advertised are mapped into the compartment, have
   the minimum retention priority and persist only until they are
   deleted by the creation of new (non-minimum retention priority) state
   or use of a STATE-FREE instruction.

   The definition of endpoint initiated acknowledgments (RFC 3321-
   Section 5.1.2) requires clarification in order to ensure that the
   definition does not preclude advertisements being used to indicate
   that state will be kept beyond the lifetime of the compartment (as
   discussed in SigComp for SIP [7]).  Thus the clarification is:

      Where Endpoint A requests state creation at Endpoint B, Endpoint B
      MAY subsequently advertise the hash of the created state item to
      Endpoint A.  This conveys to Endpoint A (i) that the state has
      been successfully created within the compartment; and (ii) that
      the state will be available for at least the lifetime of the state
      as defined by the state deletion rules according to age and
      retention priority of SigComp [1].  If the state is available at
      Endpoint B after it would be deleted from the compartment
      according to [1], then the state no longer counts towards the SMS
      of the compartment.  Since there is no guarantee of such state
      being available beyond its normally defined lifetime, endpoints
      SHOULD only attempt to access the state after this time where it
      is known that NACK [3] is available.

11.  Uncompressed Bytecode

   It is possible to write bytecode that simply instructs the
   decompressor to output the entire message (effectively sending it
   uncompressed, but within a SigComp message).  This is particularly
   useful if the bytecode is well-known (so that decompressors can
   recognize and output the bytes without running a VM if they wish);
   therefore, it is documented here.

   The mnemonic code is:

   at (0)
   :udvm_memory_size         pad (2)
   :cycles_per_bit           pad (2)
   :sigcomp_version          pad (2)
   :partial_state_id_length  pad (2)
   :state_length             pad (2)
   :reserved                 pad (2)
   at (64)
   :byte_copy_left           pad (2)
   :byte_copy_right          pad (2)
   :input_bit_order          pad (2)
   :stack_location           pad (2)

   ; Simple loop
   ;       Read a byte
   ;       Output a byte
   ; Until there are no more bytes!

   at (128)
   :start
   INPUT-BYTES (1, byte_copy_left, end)
   OUTPUT (byte_copy_left, 1)
   JUMP (start)

   :end
   END-MESSAGE (0,0,0,0,0,0,0)

   which translates to give the following SigComp message:

   0xf8, 0x00, 0xa1, 0x1c, 0x01, 0x86, 0x09, 0x22, 0x86, 0x01, 0x16,
   0xf9, 0x23

12.  RFC 3485 SIP/SDP Static Dictionary

   SIP/SDP Static Dictionary [4] provides a dictionary of strings
   frequently used in SIP and SDP messages.  The format of the
   dictionary is the list of strings followed by a table of offset
   references to the strings so that a compressor can choose to
   reference the address of the string or the entry in the table.  Both
   parts of the dictionary are divided into 5 prioritized sections to
   allow compressors to choose how much of it they use (which is
   particularly useful in the case where it has to be downloaded).  If
   only part of the dictionary is used, then the corresponding sections
   of both parts (strings and offset table) are used.

   However, there are some minor bugs in the dictionary.  In a number of
   places, the entry in the offset table refers to an address that is
   not in the corresponding priority section in the list of strings.
   Consequently, if the bytecode uses the offset table and limits use of
   the dictionary to priorities less than 4, then care must be taken not
   to use the following strings in the dictionary:

      'application' at 0x0334 is not at priority 2 (it's priority 4)
      'sdp' at 0x064b is not at priority 2 (it's priority 4)
      'send' at 0x089d is not at priority 2 (it's priority 3)
      'recv' at 0x0553 is not at priority 2 (it's priority 4)
      'phone' at 0x00f2 is not at priority 3 (it's priority 4)

   This document does not correct the dictionary, as any changes to the
   dictionary itself would be non-backwards-compatible, and require all
   implementations to maintain two different copies of the dictionary.
   Such a cost is far too high for a bug that is trivial to work around
   and has a negligible effect on compression ratios.  Instead, the flaw
   is pointed out to allow implementers to avoid any consequent
   problems.  Specifically, if the bytecode sent to a remote endpoint
   contains instructions that load only a sub-portion of the SIP/SDP
   dictionary, then the input stream provided to that bytecode cannot
   reference any of these five offsets in the offset table, unless the
   corresponding string portion of the dictionary has also been loaded.
   For example, if bytecode loads only the first three priorities of the
   dictionary (both string and offset table), use of the offset for
   "send" (at 0x089d) would be valid; however, use of the offset for
   "phone" (at 0x00f2) would not.

13.  Security Considerations

   This document updates SigComp [1], SigComp Extended [2], and the
   SigComp Static Dictionary [4].  The security considerations for [2]
   and [4] are the same as for [1]; therefore, this section discusses
   only how the security considerations for [1] are affected by the
   updates.

   Several security risks are discussed in [1].  These are discussed
   briefly here; however, this update does not change the security
   considerations of SigComp:

      Snooping into state of other users - this is mitigated by using at
      least 48 bits from the hash.  This update does not reduce the
      minimum and recommends use of more bits under certain
      circumstances.

      Faking state or making unauthorized changes - this is mitigated by
      the fact that the application layer has to authorize state
      manipulation.  This update does not change that mechanism.

      Use of SigComp as a tool in a Denial of Service (DoS) attack -
      this is mitigated by the fact that SigComp only generates one
      decompressed message per incoming compressed message.  That is not
      changed by this update.

      Attacking SigComp as the DoS target by filling with state - this
      is mitigated by the fact that the application layer has to
      authorize state manipulation.  This update does not change that
      mechanism.

      Attacking the UDVM by sending it looping code - this is mitigated
      by the upper limit of "UDVM cycles", which is unchanged by this
      update.

14.  IANA Considerations

   This document updates SigComp [1], but does not change the version.
   Consequently, the IANA considerations are the same as those for [1].

   This document updates SigComp Extended [2], but does not change the
   version.  Consequently, the IANA considerations are the same as those
   for [2].

   This document updates Static Dictionary [4], but does not change the
   version.  Consequently, the IANA considerations are the same as those
   for [4].

15.  Acknowledgements

   We would like to thank the following people who, largely through
   being foolish enough to be authors or implementors of SigComp, have
   provided us their confusion, suggestions, and comments:

      Richard Price
      Lajos Zaccomer
      Timo Forsman
      Tor-Erik Malen
      Jan Christoffersson
      Kwang Mien Chan
      William Kembery
      Pekka Pessi

16.  References

16.1.  Normative References

   [1]   Price, R., Borman, C., Christoffersson, J., Hannu, H., Liu, Z.,
         and J. Rosenberg, "Signaling Compression (SigComp)", RFC 3320,
         January 2003.

   [2]   Hannu, H., Christoffersson, J., Forsgren, S., Leung, K., Liu,
         Z., and R. Price, "Signaling Compression (SigComp) - Extended
         Operations", RFC 3321, January 2003.

   [3]   Roach, A., "A Negative Acknowledgement Mechanism for Signaling
         Compression)", RFC 4077, October 2004.

   [4]   Garcia-Martin, M., Borman, C., Ott, J., Price, R., and A.
         Roach, "The Session Initiation Protocol (SIP) and Session
         Description Protocol (SDP) Static Dictionary for Signaling
         Compression (SigComp)", RFC 3485, February 2003.

   [5]   Bradner, S., "Key words for use in RFCs to Indicate Requirement
         Levels", RFC 2119, March 1997.

16.2.  Informative References

   [6]   Crocker, D. and P. Overell, "Augmented BNF for Syntax
         Specifications (ABNF)", RFC 2234, November 1997.

   [7]   Borman, C., Liu, Z., Price, R., and G. Camarillo, "Applying
         Signaling Compression (SigComp) to the Session Initiation
         Protocol (SIP)", Work in Progress, November 2006.

   [8]   Ritter, T., "Estimating Population from Repetitions in
         Accumulated Random Samples", 1994,
         <http://www.ciphersbyritter.com/ARTS/BIRTHDAY.HTM>.

   [9]   Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
         Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
         Session Initiation Protocol", RFC 3261, June 2002.

   [10]  "IP Multimedia Call Control Protocol based on Session
         Initiation Protocol (SIP)", October 2006.

Appendix A.  Dummy Application Protocol (DAP)

A.1.  Introduction

   This appendix defines a simple dummy application protocol (DAP) that
   can be used for SigComp interoperability testing.  This is handy for
   SigComp implementations that are not integrated with a SIP stack.  It
   also provides some features that facilitate the testing of SigComp
   internal operations.

   The message format is quite simple.  Each message consists of a
   8-line message-header, an empty line, and an OPTIONAL message-body.
   The style resembles that of SIP and HTTP.

   The exact message format is given later in augmented Backus-Naur Form
   (ABNF) [6].  Here are a few notes:

      Each line of message-header MUST be terminated with CRLF.

      The empty line MUST be present even if the message-body is not.

      Body-length is the length of the message-body, excluding the CRLF
      that separates the message-body from the message-header.

      All strings in the message-header are case-insensitive.

      For implementation according to this appendix, the DAP-version
      MUST be set to 1.

A.2.  Processing a DAP Message

   A message with an invalid format will be discarded by a DAP receiver

   For testing purposes, a message with a valid format will be returned
   to the original sender (IP address, port number) in clear text, i.e.,
   without compression.  This is the case even if the sender requests
   this receiver to reject the message.  Note that the entire DAP
   message (message-header + CRLF + message-body) is returned.  This
   allows the sender to compare what it sent with what the receiver
   decompressed.

   Endpoint-ID is the global identifier of the sending endpoint.  It can
   be used to test the case where multiple SigComp endpoints communicate
   with the same remote SigComp endpoint.  For simplicity, the IPv4
   address is used for this purpose.

   Compartment-ID is the identifier of the *compressor* compartment that
   the *sending* endpoint used to compress this message.  It is assigned

   by the sender and therefore only unique per sending endpoint; i.e.,
   DAP messages sent by different endpoints MAY carry the same
   compartment-ID.  Therefore, the receiver SHOULD use the (endpoint-ID,
   compartment-ID) pair carried in a message to determine the
   decompressor compartment identifier for that message.  The exact
   local representation of the derived compartment identifier is an
   implementation choice.

   To test SigComp feedback [1], peer compartments between two endpoints
   are defined in DAP as those with the same compartment-ID.  For
   example, (endpoint-A, 1) and (endpoint-B, 1) are peer compartments.
   That means, SigComp feedback for a DAP message sent from compartment
   1 of endpoint-A to endpoint-B will be piggybacked on a DAP message
   sent from compartment 1 of endpoint-B to endpoint-A.

   A DAP receiver will follow the instruction carried in message-header
   line-5 to either accept or reject a DAP message.  Note: line-6 and
   line-7 will be ignored if the message is rejected.

   A DAP receiver will follow the instruction in line-6 to create or
   close the decompressor compartment that is associated with the
   received DAP message (see above).

   If line-7 of a received DAP message-header carries "TRUE", the
   receiver will send back a response message to the sender.  This
   allows the test of SigComp feedback.  As mentioned above, the
   response message MUST be compressed by, and sent from, the local
   compressor compartment that is a peer of the remote compressor
   compartment.  Other than this constraint, the response message is
   just a regular DAP message that can carry arbitrary message-header
   and message-body.  For example, the "need-response" field of the
   response can also be set to TRUE, which will trigger a response to
   response, and so on.  Note that since each endpoint has control over
   the "need-response" field of its own messages, this does not lead to
   a dead loop.  A sensible implementation of a DAP sender SHOULD NOT
   blindly set this field to TRUE unless a response is desired.  For
   testing, the message-body of a response MAY contain the message-
   header of the original message that triggered the response.

   Message-seq can be used by a DAP sender to track each message it
   sends, e.g., in case of losses.  Message loss can happen either on
   the path or at the receiving endpoint (i.e., due to decompression
   failure).  The assignment of message-seq is up to the sender.  For
   example, it could be either assigned per compartment or per endpoint.
   This has no impact on the receiving side.

A.3.  DAP Message Format in ABNF

   (Note: see (ABNF) [6] for basic rules.)

DAP-message = message-header CRLF [ message-body ]

message-body = *OCTET

message-header = line-1 line-2 line-3 line-4 line-5 line-6 line-7 line-8

line-1 = "DAP-version" ":" 1*DIGIT CRLF
line-2 = "endpoint-ID" ":" IPv4address CRLF
line-3 = "compartment-ID" ":" 1*DIGIT CRLF
line-4 = "message-seq" ":" 1*DIGIT CRLF
line-5 = "message-auth" ":" ( "ACCEPT" / "REJECT" ) CRLF
line-6 = "compartment-op" ":" ( "CREATE" / "CLOSE" / "NONE" ) CRLF
line-7 = "need-response" ":" ( "TRUE" / "FALSE" )
line-8 = "body-length" ":" 1*DIGIT CRLF

IPv4address = 1*3DIGIT "." 1*3DIGIT "." 1*3DIGIT "." 1*3DIGIT

A.4.  An Example of a DAP Message

      DAP-version: 1
      endpoint-ID: 123.45.67.89
      compartment-ID: 2
      message-seq: 0
      message-auth: ACCEPT
      compartment-op: CREATE
      need-response: TRUE
      body-length: 228

   This is a DAP message sent from SigComp endpoint at IP address
   123.45.67.89.  This is the first message sent from compartment 2.
   Please accept the message, create the associated compartment, and
   send back a response message.

Authors' Addresses

   Abigail Surtees
   Siemens/Roke Manor Research
   Roke Manor Research Ltd.
   Romsey, Hants  SO51 0ZN
   UK

   Phone: +44 (0)1794 833131
   EMail: abigail.surtees@roke.co.uk
   URI:   http://www.roke.co.uk

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

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

   Adam Roach
   Estacado Systems
   17210 Campbell Rd.
   Suite 250
   Dallas, TX  75252
   US

   Phone: sip:adam@estacado.net
   EMail: adam@estacado.net

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