A Negative Acknowledgement Mechanism for Signalling Compression

Versions: 00 01                                                         
Network Working Group                                        A. B. Roach
Internet-Draft                                               dynamicsoft
Expires: August 30, 2002                                      March 2002

    A Negative Acknowledgement Mechanism for Signalling Compression

Status of this Memo

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

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Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.


   This document describes a mechanism that allows Signalling
   Compression (SigComp) implementations to report precise error
   information upon receipt of a message which cannot be decompressed.
   This negative feedback can be used by the recipient to make fine-
   grained adjustments to the compressed message before retransmitting
   it, allowing for rapid and efficient recovery from error situations.

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Table of Contents

   1.    Introduction . . . . . . . . . . . . . . . . . . . . . . . .  3
   1.1   The Problem  . . . . . . . . . . . . . . . . . . . . . . . .  3
   1.1.1 Compartment Disposal . . . . . . . . . . . . . . . . . . . .  3
   1.1.2 Client Restart . . . . . . . . . . . . . . . . . . . . . . .  3
   1.1.3 Server Failover  . . . . . . . . . . . . . . . . . . . . . .  4
   1.2   The Solution . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.    Node Behavior  . . . . . . . . . . . . . . . . . . . . . . .  5
   2.1   Normal SigComp Message Transmission  . . . . . . . . . . . .  5
   2.2   Receiving a "Bad" SigComp Message  . . . . . . . . . . . . .  5
   2.3   Receiving a SigComp NACK . . . . . . . . . . . . . . . . . .  6
   2.3.1 Unreliable Transport . . . . . . . . . . . . . . . . . . . .  6
   2.3.2 Reliable Transport . . . . . . . . . . . . . . . . . . . . .  6
   2.4   Detecting Support for NACK . . . . . . . . . . . . . . . . .  7
   3.    Message Format . . . . . . . . . . . . . . . . . . . . . . .  8
   3.1   Message Fields . . . . . . . . . . . . . . . . . . . . . . .  8
   3.2   Reason Codes . . . . . . . . . . . . . . . . . . . . . . . . 10
   4.    Security Considerations  . . . . . . . . . . . . . . . . . . 14
   4.1   Reflector Attacks  . . . . . . . . . . . . . . . . . . . . . 14
   4.2   NACK Spoofing  . . . . . . . . . . . . . . . . . . . . . . . 14
         Normative References . . . . . . . . . . . . . . . . . . . . 15
         Non-Normative References . . . . . . . . . . . . . . . . . . 16
         Author's Address . . . . . . . . . . . . . . . . . . . . . . 16
   A.    Comments and Feedback  . . . . . . . . . . . . . . . . . . . 17
   B.    Changes  . . . . . . . . . . . . . . . . . . . . . . . . . . 18
         Full Copyright Statement . . . . . . . . . . . . . . . . . . 19

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1. Introduction

   Signalling Compression (see reference [1]), often called "SigComp",
   defines a protocol for transportation of compressed messages between
   two network elements.  One of the key features of SigComp is the
   ability of the sending node to request that the receiving node store
   state objects for later retrieval.

1.1 The Problem

   While the "SigComp - Extended Operations" document (reference [2])
   defines a mechanism that allows for confirmation of state creation,
   operational experience with the SigComp protocol has demonstrated
   that there are still several circumstances in which a sender's view
   of the shared state differs from the reciever's view.  A non-
   exhaustive list of the circumstances in which such failures may occur
   are detailed below.

1.1.1 Compartment Disposal

   In SigComp, stored states are associated with compartments.
   Conceptually, the compartments represent one instance of a remote
   application.  These compartments are used to limit the amount of
   state that each remote application is allowed to store.  Compartments
   are created upon receipt of a valid SigComp message from a remote
   application.  In the current protocol, applcations are expected to
   signal when they are finished with a compartment so that it can be
   deleted (by using the S-bit in requested feedback data).

   Unfortunately, expecting the applications to be well-behaved is not
   sufficient to prevent state from piling up.  Unexpected client
   failures, reboots, and loss of connectivity can cause compartments to
   become "stuck" and never removed.  To prevent this situation, it
   becomes necessary to implement a scheme by which compartments that
   appear disused may eventually be discarded.

   While the preceding facts make such a practice necessary, discarding
   compartments without explicit signalling can have the unfortunate
   side effect that active compartments are sometimes discarded.  This
   leads to a different view of state between the server and the client.

1.1.2 Client Restart

   The prime motivation behind SigComp was compression of messages to be
   sent over a radio interface.  Consequently, most deployments of
   SigComp will involve a mobile unit as one of the the endpoints.  Such
   units are not generally highly available.  Node restarts (due to e.g.
   a battery running out) will induce situations in which the network-

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   based server beleives that the client contains several states that
   are no longer actually available.

1.1.3 Server Failover

   Many applications for which SigComp will be used (e.g., SIP [3]) use
   DNS SRV records for server lookup.  One of the important features of
   DNS SRV records is the ability to specify multiple servers from which
   clients will select at random, with probabilities determined by the
   q-value weighting.  The reason for defining this behavior for SRV
   records is to allow load distribution through a set of equivalent
   servers, and to permit clients to continue to function even if the
   server with which they are communicating fails.  When using protocols
   that use SRV for such distribution, the traffic to a failed server is
   typically sent by the client to an equivalent server that can serve
   the same purpose.  From an application perspective, this new server
   often appears to be the same endpoint as the failed server, and will
   consequently resolve to the same compartment.

   Although SigComp state can be replicated amongst such a cluster of
   servers, maintaining integrity of such states requires a two-phase
   commit process, which adds a great deal of complexity to the server,
   and can degrade performance significantly.

1.2 The Solution

   Although SigComp allows returned SigComp parameters to signal that
   all states have been lost (by setting "state_memory_size" to 0 for
   one message in the reverse direction), such an approach provides an
   incomplete solution to the problem.  In addition to wiping out an
   entire compartment when only one state is corrupt or missing, this
   approach suffers from the unfortunate behavior that it requires a
   message in the reverse direction that the remote application will
   authorize.  Unless a lower-layer security mechanism is employed (e.g.
   TLS), this would typically mean that a compressed application-level
   message in the reverse direction must be sent before recovery can
   occur.  In many cases, such as SIP-based mobile terminals, such
   messages may be seldom; in others (pure client/server deployments),
   they won't ever happen.

   The proposed solution to this problem is a simple Negative
   Acknowledgement (NACK) mechanism which allows the recipient to
   communicate to the sender that a failure has occured.  This NACK
   contains a reason code that communicates the nature of the failure.
   For certain types of failures, the NACK will also contain additional
   details that might be useful in recovering from the failure.

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2. Node Behavior

   The following sections detail the behavior of nodes sending and
   receiving SigComp NACKs.  The actual format and values are described
   in section Section 3.

2.1 Normal SigComp Message Transmission

   Although normal in all other respects, SigComp implementations that
   use the NACK mechanism need to calculate and store a SHA-1 hash for
   each SigComp message that they send.  This must be stored in such a
   way that, given the SHA-1 hash, the implementation is able to locate
   the compartment with which the sent message was associated.  Further,
   when a reliable transport is being used, the implementation must be
   able to retrieve the plain-text version of the original message.

2.2 Receiving a "Bad" SigComp Message

   When a received SigComp message causes a decompression failure, the
   recipient forms and sends a SigComp NACK message.  This NACK message
   contains a SHA-1 hash of the received SigComp message that could not
   be decompressed.  It also contains the exact reason decompression
   failed, as well as any additional details that might assist the NACK
   recipient to correct any problems.  See section Section 3 for more
   information about formatting the NACK message and its fields.

   For a connection-oriented transport, such as TCP, the NACK message is
   sent back to the originator of the failed message over that same

   For a stream-based transport, such as TCP, the standard SigComp
   delimiter of 0xFFFF is used to terminate the NACK message.

   For a connectionless transport, such as UDP, the NACK message is sent
   back to the originator of the failed message at the port and IP
   address from which the message was sent.  Note that this may or may
   not be the same port to which the appliation would typically receive

      The behavior specified above is strictly necessary for any
      generally useful form of a NACK mechanism.  In the most general
      case, when an implementation receives a message that it cannot
      decompress, it has exactly three useful pieces of information: the
      contents of the message, an indication of why the message cannot
      be decoded, and the contents of the compressed message.  Note that
      none of these contain any indication of where the remote
      applicaition is listening for messages, if it differs from the
      sending port.

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2.3 Receiving a SigComp NACK

   The first action taken upon receipt of a NACK is an attempt to find
   the message to which the NACK corresponds.  This search is performed
   using the 20-byte SHA-1 hash contained in the NACK.  Once the
   matching message is located, further operations are performed based
   on the compartment that was associated with the sent message.

   Further behavior of a node upon receiving a SigComp NACK depends on
   whether a reliable or unreliable transport is being used.

2.3.1 Unreliable Transport

   When SigComp is used over an unreliable transport, the application
   has no reasonable expectation that the transport layer will deliver
   any particular message.  It then becomes the application layer's
   responsibility to ensure that data is retransmitted as necessary.  In
   these circumstances, the NACK mechanism relies on such behavior to
   ensure delivery of the message, and never performs retransmissions on
   the application's behalf.

   When a NACK is received for a message sent over an unreliable
   transport, the NACK recipient uses the contained information to make
   appropriate adjustments to the compressor associated with the proper
   compartment.  The exact nature of these adjustments are specific to
   the compression scheme being used, and will vary from implementation
   to implementation.  The only requirement on these adjustments is that
   they must have the effect of compensating for the error that has been
   indicated (e.g.  by removing the state that the remote node indicates
   it cannot retreive).

   In particular, when an unreliable transport is used, the original
   message must not be retransmitted by the SigComp layer upon receipt
   of a NACK.  Instead, the next application initiated transmission of a
   message will take advantage of the adjustments made as a result of
   processing the NACK.

2.3.2 Reliable Transport

   When a reliable transport is employed, the application makes a basic
   assumption that any message passed down the stack will be
   retransmitted as necessary to ensure that the remote node receives it
   unless a failure is indicated by the transport layer.  Because
   SigComp acts as a shim between the transport-layer and the
   application, it becomes the responsibility of the SigComp
   implementation to ensure that any failure to transmit a message is
   communicated to the application.

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   When a NACK is received for a message sent over a reliable transport,
   the SigComp layer must indicate to the application that an error has
   occured.  In general, the application should react in the same way as
   it does for any other transport layer error, such as a TCP connection
   reset.  For most applications, this reaction will initially be an
   attempt to reestablish the connection, and re-initiate the failed

2.4 Detecting Support for NACK

   Detection of support for the NACK mechanism may be beneficial in some
   certain circumstances.  For example, with the current definition of
   SigComp, acknowlegement of state receipt is required before a sender
   can reference such state.  In cases in which multiple messages are
   sent before a response is received, the need to wait for such
   responses can cause significant decreases in message compression
   efficiency.  If it is known that the receiver supports the NACK
   mechanism, the sender can instead optimistically assume that the
   state created by a sent message has been created, and is allowed to
   be referenced; if such an assumption turns out to be false (due to,
   for example, packet loss or packet reordering), the sender can
   recover upon receipt of a NACK.

   In order to facilitate such detection, implementations that will send
   NACK messages upon decompression failure MUST set the least
   significant bit of memory position 11 to "1" when initializing their
   UDVMs.  The bytecodes sent to such an endpoint can check whether this
   bit is set, and send appropriate indication back to their compressor
   as requested feedback.  The other bits of bytes 10 and 11 are
   reserved for future extensions and MUST be ignored for the purpose of
   detection of NACK support.

      Open Issue: Is the above behavior the best way to detect support?
      Even without this additional behavior, it is trivial to probe for
      NACK support by sending a message intentionally designed to fail
      (e.g.  message format 1 with random data for the state
      identifier), and check whether a NACK is received in response.
      The downside to such probing, of course, is that doing so adds
      another round-trip of messages when communication is initiated.
      Further, if a response to the probe is not received on an
      unreliable transport, the endpoint performing the probe has no
      clear way to determine whether the absence of a response is due to
      lack of support by the remote endpoint, or due to packet loss.
      Consequently, such a probe mechanism would require repeated
      retransmissions if no response is received.

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3. Message Format

   SigComp NACK packets are syntactically valid SigComp messages which
   have been specifically designed to be safely ignored by
   implementations that do not support the NACK mechanism.

   In particular, NACK messages are formatted as the second variant of a
   SigComp message (typically used for code upload) with a "code_len"
   field of zero.  The NACK information (message identifier, reason for
   failure, and error details) is encoded in the "remaining SigComp
   message" area, typically used for input data.  Further, the
   "destination" field is used as a version identifier to indicate which
   version of NACK is being employed.

3.1 Message Fields

   Although the format of NACK messages are the same as the second
   variant of normal SigComp messages, it is useful to demonstrate the
   specific fields as they appear inside the "returned feedback item"

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             0   1   2   3   4   5   6   7
           | 1   1   1   1   1 | T |   0   |
           |                               |
           :    returned feedback item     :
           |                               |
           |         code_len = 0          |
           | code_len = 0  |  version = 1  |
           |          Reason Code          |
           |                               |
           : SHA-1 Hash of failed message  :
           |                               |
           |                               |
           :         Error Details         :
           |                               |

                 Figure 1: SigComp NACK Message Format


   o  "Reason Code" is a one-byte value that indicates the nature of the
      decompression failure.  The specific codes are given in section
      Section 3.2

   o  "SHA-1 Hash of failed message" contains the full 20-byte SHA-1
      hash of the SigComp message that could not be decompressed.  This
      information allows the NACK recipient to locate the message that
      failed to decompress so that adjustments to the correct
      compartment can be performed.  When performing this hash, the
      entire SigComp message is used, from the header byte (binary
      11111xxx) to the end of the input.  Any lower-level protocol
      headers (such as UDP or IP) and message delimiters (the 0xFFFF
      that marks message boundaries in stream protocols) are not
      included in the hash.  When used over a stream based protocol, any
      0xFFxx escape sequences are un-escaped before performing the hash

   o  "Error Details" provides any additional information that might be

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      useful in correcting the problem that caused decompression
      failure.  Its meaning is specific to the "Reason Code".  See
      section Section 3.2 for specific information on what appears in
      this field.

   o  "Code_len" is the "code_len" field from a standard SigComp
      message.  It is always set to "0" for NACK messages.

   o  "Version" gives the version of the NACK mechanism being employed.
      This document defines version 1.

3.2 Reason Codes

   Note that many of the status codes are more useful in debugging
   interoperability problems than with on-the-fly correction of errors.
   The "STATE_NOT_FOUND" error is a notable exception: it will generally
   cause the NACK receipient to encode future messages so as to not use
   the indicated state.

   Upon receiving the other status messages, an implementation would
   typically be expected to either use a different set of bytecodes or,
   if that is not an option, to send that specific message uncompressed.

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       Error                      Code Details
       -------------------------- ---- ---------------------------
       STATE_NOT_FOUND              1  State ID (6 - 20 bytes)
       CYCLES_EXHAUSTED             2  Cycles Per Bit (1 byte)
       USER_REQUESTED               3
       SEGFAULT                     4
       OUTPUT_OVERFLOW              8
       STACK_UNDERFLOW              9
       BAD_BITORDER                10
       DIV_BY_ZERO                 11
       SWITCH_VALUE_TOO_HIGH       12
       INVALID_OPERAND             14
       HUFFMAN_NO_MATCH            15
       MESSAGE_TOO_SHORT           16
       BYTECODES_TOO_LARGE         18  Memory size (2 bytes)
       INVALID_OPCODE              19
       ID_TOO_SHORT                20  State ID (6 - 19 bytes)
       ID_NOT_UNIQUE               21  State ID (6 - 20 bytes)
       STATE_TOO_SHORT             23  State ID (6 - 20 bytes)

   Only the six errors "STATE_NOT_FOUND", "CYCLES_EXHAUSTED",
   "STATE_TOO_SHORT" contain details; for all other error codes, the
   "Error Details" field has zero length.

                  Figure 2: SigComp NACK Reason Codes


        A state that was referenced (either using STATE-ACCESS
        instruction or in the actual SigComp message itself) cannot be
        found.  The "details" field contains the state identifier for
        the state that could not be found.

        Decompression of the message has taken more cycles than were
        allocated to it.  The "details" field contains a one-byte value
        that communicates the number of cycles per bit.  The cycles per

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        bit is represented as an unsigned 8-bit integer (i.e.  not

        The DECOMPRESSON-FAILURE opcode has been executed.

   4.   SEGFAULT
        An attempt to read from or write to memory that is outside of
        the UDVM's memory space has been attempted.

        More than four requests to store or delete state objects have
        been requested.

        A state id length less than 6 or greater than 20 has been

        A state priority of 65535 has been specified when attempting to
        store a state.

        The decompressed message is too large to be decoded by the
        receiving node.

        An attempt to pop a value off the UDVM stack was made with a
        stack_fill value of 0.

        An INPUT-BITS or INPUT-HUFFMAN instruction was encountered with
        the "input_bit_order" register set to an invalid value (i.e.
        one of the upper five bits is set).

   11.  DIV_BY_ZERO
        A DIVIDE or REMAINDER opcode was encountered with a divisor of

        The input to a SWITCH opcode exceeds the number of branches

        An INPUT instruction was encountered that attempted to input
        more than 16 bits.


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        An operand for an instruction could not be resolved to an
        integer value (e.g.  a literal or reference operand beginning
        with 11111111).

        The input string does not match any of the bitcodes in the
        INPUT-HUFFMAN opcode.

        When attempting to decode a SigComp message, the recipient
        determined that there were not enough bytes in the message for
        it to be valid.

        The "code location" field in the SigComp message was set to the
        invalid value of 0.

        The bytecodes that a SigComp message attempted to upload exceed
        the amount of memory available in the receiving UDVM.  The
        details field is a two-byte expression of the
        DECOMPRESSION_MEMORY_SIZE of the receiving UDVM.  This value is
        communicated most-significant-byte first.

        The UDVM attempted to identify an undefined byte value as an

        A partial state identifier that was used to access state matched
        more than one state item.

   21.  ID_TOO_SHORT
        A unique state item was matched but fewer bytes of state ID were
        sent than required by the minimum_access_length.

        A MULTILOAD instruction attempted to overwrite itself.

        A STATE-ACCESS instruction has attempted to copy more bytes from
        a state item than the state item actually contains.

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4. Security Considerations

4.1 Reflector Attacks

   Because SigComp NACK messages trigger responses, it is possible to
   trigger them by intentionally sending malformed messages to a SigComp
   implementation with a spoofed IP address.  However, because such
   actions can only generate one message for each message sent, they
   don't serve as amplifier attacks.  Futher, due to the reasonably
   small size of NACK packets, there cannot be a significant increase in
   the size of the packet generated.

   It is worth noting that nearly all deployed protocols exhibit this
   same behavior.

4.2 NACK Spoofing

   Although it is possible to forge NACK message as if they were
   generated by a different node, the damage that can be caused is
   minimal.  Reporting a loss of state will typically result in nothing
   more than the re-transmission of that state in a subsequent message.
   Other failure codes would result in the next message being sent using
   an alternate compression mechanism, or possibly uncompressed.

   Although all of the above consequences result in slightly larger
   messages, none of them have particularly catastrophic implications
   for security.

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Normative References

   [1]  Price, R., Bormann, C., Christoffersson, J., Hannu, H., Liu, Z.
        and J. Rosenberg, "Signaling Compression", RFC 3320, January

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

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Non-Normative References

   [3]  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.

Author's Address

   Adam Roach
   5100 Tennyson Pkwy
   Suite 1200
   Plano, TX  75024

   EMail: adam@dynamicsoft.com

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Appendix A. Comments and Feedback

   Editorial comments should be directed to the author at
   adam@dynamicsoft.com.  Discussion of the mechanism described in this
   document should be directed to the ROHC mailing list (rohc@ietf.org).

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Appendix B. Changes

   o  Moved NACK parameters to end of message, so that NACK messages cen
      be distinguished from standalone feedback messages

   o  Changed behavior of endpoint receiving a NACK for a message sent
      on a reliable transport.

   o  Clarified some of the motivating text relating to server failover

   o  Added mechanism for detection of NACK support

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