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Network Working Group                                         A.B. Roach
Request for Comments: 4077                              Estacado Systems
Category: Standards Track                                       May 2005

     A Negative Acknowledgement Mechanism for Signaling Compression

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 Internet Society (2005).


   This document describes a mechanism that allows Signaling 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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RFC 4077                      SigComp NACK                      May 2005

Table of Contents

   1. Introduction ....................................................2
      1.1. The Problem ................................................2
           1.1.1. Compartment Disposal ................................3
           1.1.2. Client Restart ......................................3
           1.1.3. Server Failover .....................................3
      1.2. The Solution ...............................................4
   2. Node Behavior ...................................................4
      2.1. Normal SigComp Message Transmission ........................4
      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 ..................................................7
      3.1. Message Fields .............................................8
      3.2. Reason Codes ...............................................9
   4. Security Considerations ........................................13
      4.1. Reflector Attacks .........................................13
      4.2. NACK Spoofing .............................................13
   5. IANA Considerations ............................................14
   6. Acknowledgements ...............................................14
   7. References .....................................................14
      7.1. Normative References ......................................14
      7.2. Informative References ....................................14

1.  Introduction

   Signaling Compression [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 [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 receiver's view.  A non-exhaustive list
   detailing the circumstances in which such failures may occur is

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RFC 4077                      SigComp NACK                      May 2005

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, applications 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 signaling 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 for 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 endpoints.  Mobile
   terminals are generally not guaranteed to be available for extended
   durations of time.  Node restarts (due to, for example, a battery
   running out) will induce situations in which the network-based server
   believes 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

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RFC 4077                      SigComp NACK                      May 2005

   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 that 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), these messages won't be sent often; in others (pure
   client/server deployments), they won't ever be sent.

   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 occurred.  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.

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 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.

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RFC 4077                      SigComp NACK                      May 2005

   In other words, if someone hands the SHA-1 hash back to the
   compressor, it needs to be able to find the compartment with which it
   was working when it sent the message with that hash.  This only
   requires that the compressor knows with which compartment it is
   working when it sends a message (which is always the case), and that
   the SHA-1 hash, when stored, points to that compartment in some way.

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 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 on which the application would typically receive
   messages.  To accommodate implementations that use connect() or
   similar constructs, the NACK will be sent from the IP address and
   port to which the uninterpretable message was sent.  From a practical
   perspective, this is probably easiest to determine by binding
   listening sockets to a specific interface; however, other mechanisms
   may also be employed.

   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: (1) the contents of the
   message, (2) an indication of why the message cannot be decoded, and
   (3) the IP address and port from which the message originated.  Note
   that none of these contains any indication of where the remote
   application is listening for messages, if it differs from the sending

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RFC 4077                      SigComp NACK                      May 2005

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 retrieve).

   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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RFC 4077                      SigComp NACK                      May 2005

   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
   occurred.  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 reset and re-establish the connection, and
   re-initiate the failed transaction.  The SigComp layer should also
   use the information contained in the NACK to make appropriate
   adjustments to the compressor associated with the proper compartment
   (similar to the adjustments made for unreliable transport).  Thus, if
   the compartment is not reset by resetting the TCP connection, the
   next message will take advantage of the adjustments.

2.4.  Detecting Support for NACK

   Detection of support for the NACK mechanism may be beneficial in
   certain circumstances.  For example, with the current definition of
   SigComp, acknowledgment of state receipt is required before a sender
   can reference such state.  When 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, any implementation that will
   send NACK messages upon decompression failure will indicate a SigComp
   version number of 0x02 in its Universal Decompressor Virtual Machine
   (UDVM).  The bytecodes sent to such an endpoint can check the version
   number, and send appropriate indication back to their compressor as
   requested feedback.  Except for the NACK mechanism described in this
   document, implementations advertising a version of 0x02 behave
   exactly like those advertising a version number of 0x01.

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

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RFC 4077                      SigComp NACK                      May 2005

   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

   The format of the NACK message and the use of the fields within it
   are shown in Figure 1.

                      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          |
                    |  OPCODE of failed instruction |
                    |   PC of failed instruction    |
                    |                               |
                    |                               |
                    : 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 3.2.

   o  "OPCODE of failed instruction" is a one-byte field that includes
      the opcode to which the PC was pointing when the failure occurred.
      If failure occurred before the UDVM began executing any code, this
      field is set to 0.

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RFC 4077                      SigComp NACK                      May 2005

   o  "PC of failed instruction" is a two-byte field containing the
      value of the program counter when failure occurred (i.e., the
      memory address of the failed UDVM instruction).  The field is
      encoded with the most significant byte of the PC first (i.e., in
      network or big endian order).  If failure occurred before the UDVM
      began executing any code, this field is set to 0.

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

   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 recipient to encode future messages so as to not use
   the indicated state.

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

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RFC 4077                      SigComp NACK                      May 2005

       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_INPUT_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
       INVALID_STATE_PROBE         20
       ID_NOT_UNIQUE               21  State ID (6 - 20 bytes)
       STATE_TOO_SHORT             23  State ID (6 - 20 bytes)
       INTERNAL_ERROR              24
       FRAMING_ERROR               25

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

                    Figure 2: SigComp NACK Reason Codes

        A state that was referenced cannot be found.  The state may have
        been referenced by the UDVM executing a STATE-ACCESS
        instruction; it also may have been referenced by the "partial
        state identifier" field in a SigComp message.  The "details"
        field contains the state identifier for the state that could not
        be found.  This is also the proper error to return in the case
        that a unique state item was matched but fewer bytes of state ID
        were sent than required by the minimum_access_length.

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RFC 4077                      SigComp NACK                      May 2005

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

        The DECOMPRESSION-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 13 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

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        An INPUT-BITS or INPUT-HUFFMAN instruction was encountered that
        attempted to input more than 16 bits.

        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

        When attempting to retrieve state, the state_length operand is
        set to 0 but the state_begin operand is non-zero.

        A partial state identifier that was used to access state matched
        more than one state item.  Note that this error might be
        returned as the result of executing a STATE-ACCESS instruction
        or attempting to locate a unique piece of state as identified by
        the "partial state identifier" in a SigComp message.  The
        "details" field contains the partial state identifier that was

        A MULTILOAD instruction attempted to overwrite itself.

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        A STATE-ACCESS instruction has attempted to copy more bytes from
        a state item than the state item actually contains.  The
        "details" field contains the partial state identifier that was
        requested.  Implementors are cautioned to return only the
        partial state identifier that was requested; if the NACK
        contains any state identifier in addition to what was requested,
        attackers may be able to use that additional information to
        access the state.

        The UDVM encountered an unexpected condition that prevented it
        from decompressing the message.

        The UDVM encountered a framing error (unquoted 0xFF 80 .. 0xFF
        FE in an input stream.)  This error is applicable only to
        messages received on a stream transport.  In the case of a
        framing error, a SHA-1 hash for a unique message cannot be
        determined.  Consequently, when a FRAMING_ERROR NACK is sent,
        the "SHA-1 Hash of failed message" field should be set to all

4.  Security Considerations

4.1.  Reflector Attacks

   Because SigComp NACK messages are by necessity sent in response to
   other messages, 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.
   Further, 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 messages 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.

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

5.  IANA Considerations

   This document defines a new value for the IANA registered attribute

   Value (in hex): 02

   Description: SigComp version 2 (NACK support)

   Reference: [RFC4077]

6.  Acknowledgements

   Thanks to Carsten Bormann, Zhigang Liu, Pekka Pessi, and Robert Sugar
   for their comments and suggestions.  Special thanks to Abigail
   Surtees and Richard Price for several very detailed reviews and

7.  References

7.1.  Normative References

   [1]  Price, R., Bormann, 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.-C., Liu,
        Z., and R. Price, "Signaling Compression (SigComp) - Extended
        Operations", RFC 3321, January 2003.

7.2.  Informative 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.

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RFC 4077                      SigComp NACK                      May 2005

Author's Address

   Adam Roach
   Estacado Systems
   17210 Campbell Road
   Suite 250
   Dallas, TX 75252

   EMail: adam@estacado.net

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RFC 4077                      SigComp NACK                      May 2005

Full Copyright Statement

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