path: root/doc/rfc/rfc3320.txt

Network Working Group                                           R. Price
Request for Comments: 3320                            Siemens/Roke Manor
Category: Standards Track                                     C. Bormann
                                                          TZI/Uni Bremen
                                                      J. Christoffersson
                                                                H. Hannu
                                                                Ericsson
                                                                  Z. Liu
                                                                   Nokia
                                                            J. Rosenberg
                                                             dynamicsoft
                                                            January 2003


                    Signaling Compression (SigComp)

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 (2003).  All Rights Reserved.

Abstract

   This document defines Signaling Compression (SigComp), a solution for
   compressing messages generated by application protocols such as the
   Session Initiation Protocol (SIP) (RFC 3261) and the Real Time
   Streaming Protocol (RTSP) (RFC 2326).  The architecture and
   prerequisites of SigComp are outlined, along with the format of the
   SigComp message.

   Decompression functionality for SigComp is provided by a Universal
   Decompressor Virtual Machine (UDVM) optimized for the task of running
   decompression algorithms.  The UDVM can be configured to understand
   the output of many well-known compressors such as DEFLATE (RFC-1951).










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Table of Contents

   1.  Introduction...................................................2
   2.  Terminology....................................................3
   3.  SigComp architecture...........................................5
   4.  SigComp dispatchers...........................................15
   5.  SigComp compressor............................................18
   6.  SigComp state handler.........................................20
   7.  SigComp message format........................................23
   8.  Overview of the UDVM..........................................28
   9.  UDVM instruction set..........................................37
   10. Security Considerations.......................................56
   11. IANA Considerations...........................................58
   12. Acknowledgements..............................................59
   13. References....................................................59
   14. Authors' Addresses............................................60
   15. Full Copyright Statement......................................62

1.  Introduction

   Many application protocols used for multimedia communications are
   text-based and engineered for bandwidth rich links.  As a result the
   messages have not been optimized in terms of size.  For example,
   typical SIP messages range from a few hundred bytes up to two
   thousand bytes or more [RFC3261].

   With the planned usage of these protocols in wireless handsets as
   part of 2.5G and 3G cellular networks, the large message size is
   problematic.  With low-rate IP connectivity the transmission delays
   are significant.  Taking into account retransmissions, and the
   multiplicity of messages that are required in some flows, call setup
   and feature invocation are adversely affected.  SigComp provides a
   means to eliminate this problem by offering robust, lossless
   compression of application messages.

   This document outlines the architecture and prerequisites of the
   SigComp solution, the format of the SigComp message and the Universal
   Decompressor Virtual Machine (UDVM) that provides decompression
   functionality.

   SigComp is offered to applications as a layer between the application
   and an underlying transport.  The service provided is that of the
   underlying transport plus compression.  SigComp supports a wide range
   of transports including TCP, UDP and SCTP [RFC-2960].







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2.  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 BCP 14, RFC 2119
   [RFC-2119].

   Application

      Entity that invokes SigComp and performs the following tasks:

      1. Supplying application messages to the compressor dispatcher
      2. Receiving decompressed messages from the decompressor
         dispatcher
      3. Determining the compartment identifier for a decompressed
         message.

   Bytecode

      Machine code that can be executed by a virtual machine.

   Compressor

      Entity that encodes application messages using a certain
      compression algorithm, and keeps track of state that can be used
      for compression.  The compressor is responsible for ensuring that
      the messages it generates can be decompressed by the remote UDVM.

   Compressor Dispatcher

      Entity that receives application messages, invokes a compressor,
      and forwards the resulting SigComp compressed messages to a remote
      endpoint.

   UDVM Cycles

      A measure of the amount of "CPU power" required to execute a UDVM
      instruction (the simplest UDVM instructions require a single UDVM
      cycle).  An upper limit is placed on the number of UDVM cycles
      that can be used to decompress each bit in a SigComp message.

   Decompressor Dispatcher

      Entity that receives SigComp messages, invokes a UDVM, and
      forwards the resulting decompressed messages to the application.






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   Endpoint

      One instance of an application, a SigComp layer, and a transport
      layer for sending and/or receiving SigComp messages.

   Message-based Transport

      A transport that carries data as a set of bounded messages.

   Compartment

      An application-specific grouping of messages that relate to a peer
      endpoint.  Depending on the signaling protocol, this grouping may
      relate to application concepts such as "session", "dialog",
      "connection", or "association".  The application allocates state
      memory on a per-compartment basis, and determines when a
      compartment should be created or closed.

   Compartment Identifier

      An identifier (in a locally chosen format) that uniquely
      references a compartment.

   SigComp

      The overall compression solution, comprising the compressor, UDVM,
      dispatchers and state handler.

   SigComp Message

      A message sent from the compressor dispatcher to the decompressor
      dispatcher.  In case of a message-based transport such as UDP, a
      SigComp message corresponds to exactly one datagram.  For a
      stream-based transport such as TCP, the SigComp messages are
      separated by reserved delimiters.

   Stream-based transport

      A transport that carries data as a continuous stream with no
      message boundaries.

   Transport

      Mechanism for passing data between two endpoints.  SigComp is
      capable of sending messages over a wide range of transports
      including TCP, UDP and SCTP [RFC-2960].





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   Universal Decompressor Virtual Machine (UDVM)

      The machine architecture described in this document.  The UDVM is
      used to decompress SigComp messages.

   State

      Data saved for retrieval by later SigComp messages.

   State Handler

      Entity responsible for accessing and storing state information
      once permission is granted by the application.

   State Identifier

      Reference used to access a previously created item of state.

3.  SigComp Architecture

   In the SigComp architecture, compression and decompression is
   performed at two communicating endpoints.  The layout of a single
   endpoint is illustrated in Figure 1:




























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   +-------------------------------------------------------------------+
   |                                                                   |
   |                         Local application                         |
   |                                                                   |
   +-------------------------------------------------------------------+
                           |                       ^  |
     Application message & |          Decompressed |  | Compartment
    compartment identifier |               message |  | identifier
                           |                       |  |
   +-- -- -- -- -- -- -- --|-- -- -- -- -- -- -- --|--|-- -- -- -- -- -+
                           v                       |  v
   |    +------------------------+         +----------------------+    |
        |                        |         |                      |
   | +--|       Compressor       |         |     Decompressor     |<-+ |
     |  |       dispatcher       |         |      dispatcher      |  |
   | |  |                        |         |                      |  | |
     |  +------------------------+         +----------------------+  |
   | |  ^    ^                                             ^         | |
     |  |    |                                             |         |
   | |  |    v                                             |         | |
     |  |  +--------------+   +---------------+            |         |
   | |  |  |              |   |   +-------+   |            v         | |
     |  |  | Compressor 1 |<----->|State 1|   |    +--------------+  |
   | |  |  |              |   |   +-------+   |    |              |  | |
     |  |  +--------------+   |               |    | Decompressor |  |
   | |  |                     | State handler |<-->|              |  | |
     |  |  +--------------+   |               |    |    (UDVM)    |  |
   | |  |  |              |   |   +-------+   |    |              |  | |
     |  +->| Compressor 2 |<----->|State 2|   |    +--------------+  |
   | |     |              |   |   +-------+   |                      | |
     |     +--------------+   +---------------+      SigComp layer   |
   | |                                                               | |
   +-| -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --|-+
     |                                                               |
     | SigComp                                               SigComp |
     | message                                               message |
     v                                                               |
   +-------------------------------------------------------------------+
   |                                                                   |
   |                          Transport layer                          |
   |                                                                   |
   +-------------------------------------------------------------------+

    Figure 1: High-level architectural overview of one SigComp endpoint







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RFC 3320            Signaling Compression (SigComp)         January 2003


   Note that SigComp is offered to applications as a layer between the
   application and the underlying transport, and so Figure 1 is an
   endpoint when viewed from a transport layer perspective.  From the
   perspective of multi-hop application layer protocols however, SigComp
   is applied on a per-hop basis.

   The SigComp layer is further decomposed into the following entities:

   1. Compressor dispatcher - the interface from the application.  The
      application supplies the compressor dispatcher with an application
      message and a compartment identifier (see Section 3.1 for further
      details).  The compressor dispatcher invokes a particular
      compressor, which returns a SigComp message to be forwarded to the
      remote endpoint.

   2. Decompressor dispatcher - the interface towards the application.
      The decompressor dispatcher receives a SigComp message and invokes
      an instance of the Universal Decompressor Virtual Machine (UDVM).
      It then forwards the resulting decompressed message to the
      application, which may return a compartment identifier if it
      wishes to allow state to be saved for the message.

   3. One or more compressors - the entities that convert application
      messages into SigComp messages.  Distinct compressors are invoked
      on a per-compartment basis, using the compartment identifiers
      supplied by the application.  A compressor receives an application
      message from the compressor dispatcher, compresses the message,
      and returns a SigComp message to the compressor dispatcher.  Each
      compressor chooses a certain algorithm to encode the data (e.g.,
      DEFLATE).

   4. UDVM - the entity that decompresses SigComp messages. Note that
      since SigComp can run over an unsecured transport layer, a
      separate instance of the UDVM is invoked on a per-message basis.
      However, during the decompression process the UDVM may invoke the
      state handler to access existing state or create new state.

   5. State handler - the entity that can store and retrieve state.
      State is information that is stored between SigComp messages,
      avoiding the need to upload the data on a per-message basis.  For
      security purposes it is only possible to create new state with the
      permission of the application.  State creation and retrieval are
      further described in Chapter 6.








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RFC 3320            Signaling Compression (SigComp)         January 2003


   When compressing a bidirectional application protocol the choice to
   use SigComp can be made independently in both directions, and
   compression in one direction does not necessarily imply compression
   in the reverse direction.  Moreover, even when two communicating
   endpoints send SigComp messages in both directions, there is no need
   to use the same compression algorithm in each direction.

   Note that a SigComp endpoint can decompress messages from multiple
   remote endpoints at different locations in a network, as the
   architecture is designed to prevent SigComp messages from one
   endpoint interfering with messages from a different endpoint.  A
   consequence of this design choice is that it is difficult for a
   malicious user to disrupt SigComp operation by inserting false
   compressed messages on the transport layer.

3.1.  Requirements on the Application

   From an application perspective the SigComp layer appears as a new
   transport, with similar behavior to the original transport used to
   carry uncompressed data (for example SigComp/UDP behaves similarly to
   native UDP).

   Mechanisms for discovering whether an endpoint supports SigComp are
   beyond the scope of this document.

   All SigComp messages contain a prefix (the five most-significant bits
   of the first byte are set to one) that does not occur in UTF-8
   encoded text messages [RFC-2279], so for applications which use this
   encoding (or ASCII encoding) it is possible to multiplex uncompressed
   application messages and SigComp messages on the same port.
   Applications can still reserve a new port specifically for SigComp
   however (e.g., as part of the discovery mechanism).

   If a particular endpoint wishes to be stateful then it needs to
   partition its decompressed messages into "compartments" under which
   state can be saved.  SigComp relies on the application to provide
   this partition.  So for stateful endpoints a new interface is
   required to the application in order to leverage the authentication
   mechanisms used by the application itself.

   When the application receives a decompressed message it maps the
   message to a certain compartment and supplies the compartment
   identifier to SigComp.  Each compartment is allocated a separate
   compressor and a certain amount of memory to store state information,
   so the application must assign distinct compartments to distinct
   remote endpoints.  However it is possible for a local endpoint to
   establish several compartments that relate to the same remote
   endpoint (this should be avoided where possible as it may waste



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   memory and reduce the overall compression ratio, but it does not
   cause messages to be incorrectly decompressed).  In this case,
   reliable stateful operation is possible only if the decompressor does
   not lump several messages into one compartment when the compressor
   expected them to be assigned different compartments.

   The exact format of the compartment identifier is unimportant
   provided that different identifiers are given to different
   compartments.

   Applications that wish to communicate using SigComp in a stateful
   fashion should use an authentication mechanism to securely map
   decompressed messages to compartment identifiers.  They should also
   agree on any limits to the lifetime of a compartment, to avoid the
   case where an endpoint accesses state information that has already
   been deleted.

3.2.  SigComp feedback mechanism

   If a signaling protocol sends SigComp messages in both directions and
   there is a one-to-one relationship between the compartments
   established by the applications on both ends ("peer compartments"),
   the two endpoints can cooperate more closely.  In this case, it is
   possible to send feedback information that monitors the behavior of
   an endpoint and helps to improve the overall compression ratio.
   SigComp performs feedback on a request/response basis, so a
   compressor makes a feedback request and receives some feedback data
   in return.  The procedure for requesting and returning feedback in
   SigComp is illustrated in Figure 2:






















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    +---------------------+                     +---------------------+
    | +-----------------+ |                     | +-----------------+ |
   -->|   Compressor    |------------------------>|      UDVM       |<->
    | |  sending to B   | |   SigComp message   | |                 | |2
    | +-----------------+ | requesting feedback | +-----------------+ |
    |          ^     1,9  |                     |  3       |          |
    |          |          |                     |          v          |
    | +-----------------+ |                     | +-----------------+ |
    | |      State      | |                     | |      State      | |
    | |     handler     | |                     | |     handler     | |
    | +-----------------+ |                     | +-----------------+ |
    |          ^       8  |                     |  4       |          |
    |          |          |                     |          v          |
    | +-----------------+ |                     | +-----------------+ |
    | |      UDVM       | |                     | |   Compressor    | |
   <->|                 |<------------------------|  sending to A   |<--
   6| +-----------------+ |   SigComp message   | +-----------------+ |
    |                  7  | returning feedback  |  5                  |
    |     Endpoint A      |                     |     Endpoint B      |
    +---------------------+                     +---------------------+

       Figure 2: Steps involved in the transmission of feedback data

   The dispatchers, the application and the transport layer are omitted
   from the diagram for clarity.  Note that the decompressed messages
   pass via the decompressor dispatcher to the application; moreover the
   SigComp messages transmitted from the compressor to the remote UDVM
   are sent via first the compressor dispatcher, followed by the
   transport layer and finally the decompressor dispatcher.

   The steps for requesting and returning feedback data are described in
   more detail below:

   1. The compressor that sends messages to Endpoint B piggybacks a
      feedback request onto a SigComp message.

   2. When the application receives the decompressed message, it may
      return the compartment identifier for the message.

   3. The UDVM in Endpoint B forwards the requested feedback data to the
      state handler.

   4. If the UDVM can supply a valid compartment identifier, then the
      state handler forwards the feedback data to the appropriate
      compressor (namely the compressor sending to Endpoint A).

   5. The compressor returns the requested feedback data to Endpoint A
      piggybacked onto a SigComp message.



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RFC 3320            Signaling Compression (SigComp)         January 2003


   6. When the application receives the decompressed message, it may
      return the compartment identifier for the message.

   7. The UDVM in Endpoint A forwards the returned feedback data to the
      state handler.

   8. If the UDVM can supply a valid compartment identifier, then the
      state handler forwards the feedback data to the appropriate
      compressor (namely the compressor sending to Endpoint B).

   9. The compressor makes use of the returned feedback data.

   The detailed role played by each entity in the transmission of
   feedback data is explained in subsequent chapters.

3.3.  SigComp Parameters

   An advantage of using a virtual machine for decompression is that
   almost all of the implementation flexibility lies in the SigComp
   compressors.  When receiving SigComp messages an endpoint generally
   behaves in a predictable manner.

   Note however that endpoints implementing SigComp will typically have
   a wide range of capabilities, each offering a different amount of
   working memory, processing power etc.  In order to support this wide
   variation in endpoint capabilities, the following parameters are
   provided to modify SigComp behavior when receiving SigComp messages:

   decompression_memory_size
   state_memory_size
   cycles_per_bit
   SigComp_version
   locally available state (a set containing 0 or more state items)

   Each parameter has a minimum value that MUST be offered by all
   receiving SigComp endpoints.  Moreover, endpoints MAY offer
   additional resources if available; these resources can be advertised
   to remote endpoints using the SigComp feedback mechanism.

   Particular applications may also agree a-priori to offer additional
   resources as mandatory (e.g., SigComp for SIP offers a dictionary of
   common SIP phrases as a mandatory state item).

   Each of the SigComp parameters is described in greater detail below.







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RFC 3320            Signaling Compression (SigComp)         January 2003


3.3.1.  Memory Size and UDVM Cycles

   The decompression_memory_size parameter specifies the amount of
   memory available to decompress one SigComp message.  (Note that the
   term "amount of memory" is used on a conceptual level in order to
   specify decompressor behavior and allow resource planning on the side
   of the compressor -- an implementation could require additional,
   bounded amounts of actual memory resources or could even organize its
   memory in a completely different way as long as this does not cause
   decompression failures where the conceptual model would not.)  A
   portion of this memory is used to buffer a SigComp message before it
   is decompressed; the remainder is given to the UDVM.  Note that the
   memory is allocated on a per-message basis and can be reclaimed after
   the message has been decompressed.  All endpoints implementing
   SigComp MUST offer a decompression_memory_size of at least 2048
   bytes.

   The state_memory_size parameter specifies the number of bytes offered
   to a particular compartment for the creation of state.  This
   parameter is set to 0 if the endpoint is stateless.

   Unlike the other SigComp parameters, the state_memory_size is offered
   on a per-compartment basis and may vary for different compartments.
   The memory for a compartment is reclaimed when the application
   determines that the compartment is no longer required.

   The cycles_per_bit parameter specifies the number of "UDVM cycles"
   available to decompress each bit in a SigComp message.  Executing a
   UDVM instruction requires a certain number of UDVM cycles; a complete
   list of UDVM instructions and their cost in UDVM cycles can be found
   in Chapter 9.  An endpoint MUST offer a minimum of 16 cycles_per_bit.

   Each of the three parameter values MUST be chosen from the limited
   set given below, so that the parameters can be efficiently encoded
   for transmission using the SigComp feedback mechanism.

   The cycles_per_bit parameter is encoded using 2 bits, whilst the
   decompression_memory_size and state_memory_size are both encoded
   using 3 bits.  The bit encodings and their corresponding values are
   as follows:











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   Encoding:   cycles_per_bit:   Encoding:   state_memory_size (bytes):

   00          16                000         0
   01          32                001         2048
   10          64                010         4096
   11          128               011         8192
                                 100         16384
                                 101         32768
                                 110         65536
                                 111         131072

   The decompression_memory_size is encoded in the same manner as the
   state_memory_size, except that the bit pattern 000 cannot be used (as
   an endpoint cannot offer a decompression_memory_size of 0 bytes).

3.3.2.  SigComp Version

   The SigComp_version parameter specifies whether only the basic
   version of SigComp is available, or whether an upgraded version is
   available offering additional instructions etc.  Within the UDVM, it
   is available as a 2-byte value, generated by zero-extending the 1-
   byte SigComp_version parameter (i.e., the first byte of the 2-byte
   value is always zero).

   The basic version of SigComp is Version 0x01, which is the version
   described in this document.

   To ensure backwards compatibility, if a SigComp message is
   successfully decompressed by Version 0x01 of SigComp then it will be
   successfully decompressed on upgraded versions.  Similarly, if the
   message triggers a manual decompression failure (see Section 8.7),
   then it will also continue to do so.

   However, messages that cause an unexpected decompression failure on
   Version 0x01 of SigComp may be successfully decompressed by upgraded
   versions.

   The simplest way to upgrade SigComp in a backwards-compatible manner
   is to add additional UDVM instructions, as this will not affect the
   decompression of SigComp messages compatible with Version 0x01.
   Reserved addresses in the UDVM memory (Useful Values, see Section
   7.2) may also be assigned values in future versions of SigComp.









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3.3.3.  Locally Available State Items

   A SigComp state item is an item of data that is retained between
   SigComp messages.  State items can be retrieved and loaded into the
   UDVM memory as part of the decompression process, often significantly
   improving the compression ratio as the same information does not have
   to be uploaded on a per-message basis.

   Each endpoint maintains a set of state items where every item is
   composed of the following information:

   Name:                      Type of data:

   state_identifier           20-byte value
   state_length               2-byte value
   state_address              2-byte value
   state_instruction          2-byte value
   minimum_access_length      2-byte value from 6 to 20 inclusive
   state_value                String of state_length consecutive bytes

   State items are typically created at an endpoint upon successful
   decompression of a SigComp message.  The remote compressor sending
   the message makes a state creation request by invoking the
   appropriate UDVM instruction, and the state is saved once permission
   is granted by the application.

   However, an endpoint MAY also wish to offer a set of locally
   available state items that have not been uploaded as part of a
   SigComp message.  For example it might offer well-known decompression
   algorithms, dictionaries of common phrases used in a specific
   signaling protocol, etc.

   Since these state items are established locally without input from a
   remote endpoint, they are most useful if publicly documented so that
   a wide collection of remote endpoints can determine the data
   contained in each state item and how it may be used.  Further
   Internet Documents and RFCs may be published to describe particular
   locally available state items.

   Although there are no locally available state items that are
   mandatory for every SigComp endpoint, certain state items can be made
   mandatory in a specific environment (e.g., the dictionary of common
   phrases for a specific signaling protocol could be made mandatory for
   that signaling protocol's usage of SigComp).  Also, remote endpoints
   can indicate their interest in receiving a list of some of the state
   items available locally at an endpoint using the SigComp feedback
   mechanism.




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RFC 3320            Signaling Compression (SigComp)         January 2003


   It is a matter of local decision for an endpoint what items of
   locally available state it advertises; this decision has no influence
   on interoperability, but may increase or decrease the efficiency of
   the compression achievable between the endpoints.

4.  SigComp Dispatchers

   This chapter defines the behavior of the compressor and decompressor
   dispatcher.  The function of these entities is to provide an
   interface between SigComp and its environment, minimizing the effort
   needed to integrate SigComp into an existing protocol stack.

4.1.  Compressor Dispatcher

   The compressor dispatcher receives messages from the application and
   passes the compressed version of each message to the transport layer.

   Note that SigComp invokes compressors on a per-compartment basis, so
   when the application provides a message to be compressed it must also
   provide a compartment identifier.  The compressor dispatcher forwards
   the application message to the correct compressor based on the
   compartment identifier (invoking a new compressor if a new
   compartment identifier is encountered).  The compressor returns a
   SigComp message that can be passed to the transport layer.

   Additionally, the application should indicate to the compressor
   dispatcher when it wishes to close a particular compartment, so that
   the resources taken by the corresponding compressor can be reclaimed.

4.2.  Decompressor Dispatcher

   The decompressor dispatcher receives messages from the transport
   layer and passes the decompressed version of each message to the
   application.

   To ensure that SigComp can run over an unsecured transport layer, the
   decompressor dispatcher invokes a new instance of the UDVM for each
   new SigComp message.  Resources for the UDVM are released as soon as
   the message has been decompressed.

   The dispatcher MUST NOT make more than one SigComp message available
   to a given instance of the UDVM.  In particular, the dispatcher MUST
   NOT concatenate two SigComp messages to form a single message.








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4.2.1.  Decompressor Dispatcher Strategies

   Once the UDVM has been invoked it is initialized using the SigComp
   message of Chapter 7.  The message is then decompressed by the UDVM,
   returned to the decompressor dispatcher, and passed on to the
   receiving application.  Note that the UDVM has no awareness of
   whether the underlying transport is message-based or stream-based,
   and so it always outputs decompressed data as a stream.  It is the
   responsibility of the dispatcher to provide the decompressed message
   to the application in the expected form (i.e., as a stream or as a
   distinct, bounded message).  The dispatcher knows that the end of a
   decompressed message has been reached when the UDVM instruction END-
   MESSAGE is invoked (see Section 9.4.9).

   For a stream-based transport, two strategies are therefore possible
   for the decompressor dispatcher:

   1) The dispatcher collects a complete SigComp message and then
      invokes the UDVM.  The advantage is that, even in implementations
      that have multiple incoming compressed streams, only one instance
      of the UDVM is ever required.

   2) The dispatcher collects the SigComp header (see Section 7) and
      invokes the UDVM; the UDVM stays active while the rest of the
      message arrives.  The advantage is that there is no need to buffer
      up the rest of the message; the message can be decompressed as it
      arrives, and any decompressed output can be relayed to the
      application immediately.

   In general, which of the strategies is used is an implementation
   choice.

   However, the compressor may want to take advantage of strategy 2 by
   expecting that some of the application message is passed on to the
   application before the SigComp message is terminated, e.g., by
   keeping the UDVM active while expecting the application to
   continuously receive decompressed output.  This approach ("continuous
   mode") invalidates some assumptions of the SigComp security model and
   can only be used if the transport itself can provide the required
   protection against denial of service attacks.  Also, since only
   strategy 2 works in this approach, the use of continuous mode
   requires previous agreement between the two endpoints.

4.2.2.  Record Marking

   For a stream-based transport, the dispatcher delimits messages by
   parsing the compressed data stream for instances of 0xFF and taking
   the following actions:



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   Occurs in data stream:     Action:

   0xFF 00                    one 0xFF byte in the data stream
   0xFF 01                    same, but the next byte is quoted (could
                              be another 0xFF)
      :                                           :
   0xFF 7F                    same, but the next 127 bytes are quoted
   0xFF 80 to 0xFF FE         (reserved for future standardization)
   0xFF FF                    end of SigComp message

   The combinations 0xFF01 to 0xFF7F are useful to limit the worst case
   expansion of the record marking scheme:  the 1 (0xFF01) to 127
   (0xFF7F) bytes following the byte combination are copied literally by
   the decompressor without taking any special action on 0xFF.  (Note
   that 0xFF00 is just a special case of this, where zero following
   bytes are copied literally.)

   In UDVM version 0x01, any occurrence of the combinations 0xFF80 to
   0xFFFE that are not protected by quoting causes decompression
   failure; the decompressor SHOULD close the stream-based transport in
   this case.

4.3.  Returning a Compartment Identifier

   Upon receiving a decompressed message the application may supply the
   dispatcher with a compartment identifier.  Supplying this identifier
   grants permission for the following:

   1. Items of state accompanying the decompressed message can be saved
      using the state memory reserved for the specified compartment.

   2. The feedback data accompanying the decompressed message can be
      trusted sufficiently that it can be used when sending SigComp
      messages that relate to the compressor's equivalent for the
      compartment.

   The dispatcher passes the compartment identifier to the UDVM, where
   it is used as per the END-MESSAGE instruction (see Section 9.4.9).

   The application uses a suitable authentication mechanism to determine
   whether the decompressed message belongs to a legitimate compartment
   or not.  If the application fails to authenticate the message with
   sufficient confidence to allow state to be saved or feedback data to
   be trusted, it supplies a "no valid compartment" error to the
   dispatcher and the UDVM is terminated without creating any state or
   forwarding any feedback data.





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5.  SigComp Compressor

   An important feature of SigComp is that decompression functionality
   is provided by a Universal Decompressor Virtual Machine (UDVM).  This
   means that the compressor can choose any algorithm to generate
   compressed SigComp messages, and then upload bytecode for the
   corresponding decompression algorithm to the UDVM as part of the
   SigComp message.

   To help with the implementation and testing of a SigComp endpoint,
   further Internet Documents and RFCs may be published to describe
   particular compression algorithms.

   The overall requirement placed on the compressor is that of
   transparency, i.e., the compressor MUST NOT send bytecode which
   causes the UDVM to incorrectly decompress a given SigComp message.

   The following more specific requirements are also placed on the
   compressor (they can be considered particular instances of the
   transparency requirement):

   1. For robustness, it is recommended that the compressor supply some
      form of integrity check (not necessarily of cryptographic
      strength) over the application message to ensure that successful
      decompression has occurred.  A UDVM instruction is provided for
      CRC verification; also, another instruction can be used to compute
      a SHA-1 cryptographic hash.

   2. The compressor MUST ensure that the message can be decompressed
      using the resources available at the remote endpoint.

   3. If the transport is message-based, then the compressor MUST map
      each application message to exactly one SigComp message.

   4. If the transport is stream-based but the application defines its
      own internal message boundaries, then the compressor SHOULD map
      each application message to exactly one SigComp message.

   Message boundaries should be preserved over a stream-based transport
   so that accidental or malicious damage to one SigComp message does
   not affect the decompression of subsequent messages.

   Additionally, if the state handler passes some requested feedback to
   the compressor, then it SHOULD be returned in the next SigComp
   message generated by the compressor (unless the state handler passes
   some newer requested feedback before the older feedback has been
   sent, in which case the older feedback is deleted).




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   If present, the requested feedback item SHOULD be copied unmodified
   into the returned_feedback_item field provided in the SigComp
   message.  Note that there is no need to transmit any requested
   feedback item more than once.

   The compressor SHOULD also upload the local SigComp parameters to the
   remote endpoint, unless the endpoint has indicated that it does not
   wish to receive these parameters or the compressor determines that
   the parameters have already successfully arrived (see Section 5.1 for
   details of how this can be achieved).  The SigComp parameters are
   uploaded to the UDVM memory at the remote endpoint as described in
   Section 9.4.9.

5.1.  Ensuring Successful Decompression

   A compressor MUST be certain that all of the data needed to
   decompress a SigComp message is available at the receiving endpoint.
   One way to ensure this is to send all of the needed information in
   every SigComp message (including bytecode to decompress the message).
   However, the compression ratio for this method will be relatively
   low.

   To obtain the best overall compression ratio the compressor needs to
   request the creation of new state items at the remote endpoint.  The
   information saved in these state items can then be accessed by later
   SigComp messages, avoiding the need to upload the data on a per-
   message basis.

   Before the compressor can access saved state however, it must ensure
   that the SigComp message carrying the state creation request arrived
   successfully at the receiving endpoint.  For a reliable transport
   (e.g., TCP or SCTP) this is guaranteed.  For an unreliable transport
   however, the compressor must provide a suitable mechanism itself (see
   [RFC-3321] for further details).

   The compressor must also ensure that the state item it wishes to
   access has not been rejected due to a lack of state memory.  This can
   be accomplished by checking the state_memory_size parameter using the
   SigComp feedback mechanism (see Section 9.4.9 for further details).

5.2.  Compression Failure

   The compressor SHOULD make every effort to successfully compress an
   application message, but in certain cases this might not be possible
   (particularly if resources are scarce at the receiving endpoint). In
   this case a "compression failure" is called.





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   If a compression failure occurs then the compressor informs the
   dispatcher and takes no further action.  The dispatcher MUST report
   this failure to the application so that it can try other methods to
   deliver the message.

6.  State Handling and Feedback

   This chapter defines the behavior of the SigComp state handler.  The
   function of the state handler is to retain information between
   received SigComp messages; it is the only SigComp entity that is
   capable of this function, and so it is of particular importance from
   a security perspective.

6.1.  Creating and Accessing State

   To provide security against the malicious insertion or modification
   of SigComp messages, a separate instance of the UDVM is invoked to
   decompress each message.  This ensures that damaged SigComp messages
   do not prevent the successful decompression of subsequent valid
   messages.

   Note, however, that the overall compression ratio is often
   significantly higher if messages can be compressed relative to the
   information contained in previous messages.  For this reason, it is
   possible to create state items for access when a later message is
   being decompressed.  Both the creation and access of state are
   designed to be secure against malicious tampering with the compressed
   data.  The UDVM can only create a state item when a complete message
   has been successfully decompressed and the application has returned a
   compartment identifier under which the state can be saved.

   State access cannot be protected by relying on the application alone,
   since the authentication mechanism may require information from the
   decompressed message (which of course is not available until after
   the state has been accessed).  Instead, SigComp protects state access
   by creating a state identifier that is a hash over the item of state
   to be retrieved.  This state_identifier must be supplied to retrieve
   an item of state from the state handler.

   Also note that state is not deleted when it is accessed.  So even if
   a malicious sender manages to access some state information,
   subsequent messages compressed relative to this state can still be
   successfully decompressed.

   Each state item contains a state_identifier that is used to access
   the state.  One state identifier can be supplied in the SigComp
   message header to initialize the UDVM (see Chapter 7); additional
   state items can be retrieved using the STATE-ACCESS instruction.  The



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   UDVM can also request the creation of a new state item by using the
   STATE-CREATE and END-MESSAGE instructions (see Chapter 9 for further
   details).

6.2.  Memory Management

   The state handler manages state memory on a per-compartment basis.
   Each compartment can store state up to a certain state_memory_size
   (where the application may assign different values for the
   state_memory_size parameter to different compartments).

   As well as storing the state items themselves, the state handler
   maintains a list of the state items created by a particular
   compartment and ensures that no compartment exceeds its allocated
   state_memory_size.  For the purpose of calculation, each state item
   is considered to cost (state_length + 64) bytes.

   Each instance of the UDVM can pass up to four state creation requests
   to the state handler, as well as up to four state free requests (the
   latter are requests to free the memory taken by a state item in a
   certain compartment).  When the state handler receives a state
   creation request from the UDVM it takes the following steps:

   1. The state handler MUST reject all state creation requests that are
      not accompanied by a valid compartment identifier, or if the
      compartment is allocated 0 bytes of state memory. Note that if a
      state creation request fails due to lack of state memory then it
      does not mean that the corresponding SigComp message is damaged;
      compressors will often make state creation requests in the first
      SigComp message of a compartment, before they have discovered the
      state_memory_size using the SigComp feedback mechanism.

   2. If the state creation request needs more state memory than the
      total state_memory_size for the compartment, the state handler
      deletes all but the first (state_memory_size - 64) bytes from the
      state_value.  It sets the state_length to (state_memory_size -
      64), and recalculates the state_identifier as defined in Section
      9.4.9.

   3. If the state creation request contains a state_identifier that
      already exists then the state handler checks whether the requested
      state item is identical to the established state item and counts
      the state creation request as successful if this is the case.  If
      not then the state creation request is unsuccessful (although the
      probability that this will occur is vanishingly small).






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   4. If the state creation request exceeds the state memory allocated
      to the compartment, sufficient items of state created by the same
      compartment are freed until enough memory is available to
      accommodate the new state.  When a state item is freed, it is
      removed from the list of states created by the compartment and the
      memory cost of the state item no longer counts towards the total
      cost for the compartment.  Note, however, that identical state
      items may be created by several different compartments, so a state
      item must not be physically deleted unless the state handler
      determines that it is no longer required by any compartment.

   5. The order in which the existing state items are freed is
      determined by the state_retention_priority, which is set when the
      state items are created.  The state_retention_priority of 65535 is
      reserved for locally available states; these states must always be
      freed first.  Apart from this special case, states with the lowest
      state_retention_priority are always freed first.  In the event of
      a tie, then the state item created first in the compartment is
      also the first to be freed.

   The state_retention_priority is always stored on a per-compartment
   basis as part of the list of state items created by each compartment.
   In particular, the same state item might have several priority values
   if it has been created by several different compartments.

   Note that locally available state items (as described in Section
   3.3.3) need not be mapped to any particular compartment.  However, if
   they are created on a per-compartment basis, then they must not
   interfere with the state created at the request of the remote
   endpoint.  The special state_retention_priority of 65535 is reserved
   for locally available state items to ensure that this is the case.

   The UDVM may also explicitly request the state handler to free a
   specific state item in a compartment.  In this case, the state
   handler deletes the state item from the list of state items created
   by the compartment (as before the state item itself must not be
   physically deleted unless the state handler determines that it is not
   longer required by any compartment).

   The application should indicate to the state handler when it wishes
   to close a particular compartment, so that the resources taken by the
   corresponding state can be reclaimed.









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6.3.  Feedback Data

   The SigComp feedback mechanism allows feedback data to be received by
   a UDVM and forwarded via the state handler to the correct compressor.

   Since this feedback data is retained between SigComp messages, it is
   considered to be part of the overall state and can only be forwarded
   if accompanied by a valid compartment identifier.  If this is the
   case, then the state handler forwards the feedback data to the
   compressor responsible for sending messages that pertain to the peer
   compartment of the specified compartment.

7.  SigComp Message Format

   This chapter describes the format of the SigComp message and how the
   message is used to initialize the UDVM memory.

   Note that the SigComp message is not copied into the UDVM memory as
   soon as it arrives; instead, the UDVM indicates when it requires
   compressed data using a specific instruction.  It then pauses and
   waits for the information to be supplied before executing the next
   instruction.  This means that the UDVM can begin to decompress a
   SigComp message before the entire message has been received.

   A consequence of the above behavior is that when the UDVM is invoked,
   the size of the UDVM memory depends on whether the transport used to
   provide the SigComp message is stream-based or message-based.  If the
   transport is message-based then sufficient memory must be available
   to buffer the entire SigComp message before it is passed to the UDVM.
   So if the message is n bytes long, then the UDVM memory size is set
   to (decompression_memory_size - n), up to a maximum of 65536 bytes.

   If the transport is stream-based however, then a fixed-size input
   buffer is required to accommodate the stream, independently of the
   size of each SigComp message. So, for simplicity, the UDVM memory
   size is set to (decompression_memory_size / 2).

   As a separate instance of the UDVM is invoked on a per-message basis,
   each SigComp message must explicitly indicate its chosen
   decompression algorithm as well as any additional information that is
   needed to decompress the message (e.g., one or more previously
   received messages, a dictionary of common SIP phrases etc.).  This
   information can either be uploaded as part of the SigComp message or
   retrieved from an item of state.







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   A SigComp message takes one of two forms depending on whether it
   accesses a state item at the receiving endpoint.  The two variants of
   a SigComp message are given in Figure 3.  (The T-bit controls the
   format of the returned feedback item and is defined in Section 7.1.)

     0   1   2   3   4   5   6   7       0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+   +---+---+---+---+---+---+---+---+
   | 1   1   1   1   1 | T |  len  |   | 1   1   1   1   1 | T |   0   |
   +---+---+---+---+---+---+---+---+   +---+---+---+---+---+---+---+---+
   |                               |   |                               |
   :    returned feedback item     :   :    returned feedback item     :
   |                               |   |                               |
   +---+---+---+---+---+---+---+---+   +---+---+---+---+---+---+---+---+
   |                               |   |           code_len            |
   :   partial state identifier    :   +---+---+---+---+---+---+---+---+
   |                               |   |   code_len    |  destination  |
   +---+---+---+---+---+---+---+---+   +---+---+---+---+---+---+---+---+
   |                               |   |                               |
   :   remaining SigComp message   :   :    uploaded UDVM bytecode     :
   |                               |   |                               |
   +---+---+---+---+---+---+---+---+   +---+---+---+---+---+---+---+---+
                                       |                               |
                                       :   remaining SigComp message   :
                                       |                               |
                                       +---+---+---+---+---+---+---+---+

                   Figure 3: Format of a SigComp message

   Decompression failure occurs if the SigComp message is too short to
   contain the expected fields (see Section 8.7 for further details).

   The fields except for the "remaining SigComp message" are referred to
   as the "SigComp header" (note that this may include the uploaded UDVM
   bytecode).

7.1.  Returned feedback item

   For both variants of the SigComp message, the T-bit is set to 1
   whenever the SigComp message contains a returned feedback item.  The
   format of the returned feedback item is illustrated in Figure 4.











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     0   1   2   3   4   5   6   7       0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+   +---+---+---+---+---+---+---+---+
   | 0 |  returned_feedback_field  |   | 1 | returned_feedback_length  |
   +---+---+---+---+---+---+---+---+   +---+---+---+---+---+---+---+---+
                                       |                               |
                                       :    returned_feedback_field    :
                                       |                               |
                                       +---+---+---+---+---+---+---+---+

                Figure 4: Format of returned feedback item

   Note that the returned feedback length specifies the size of the
   returned feedback field (from 0 to 127 bytes).  So the total size of
   the returned feedback item lies between 1 and 128 bytes.

   The returned feedback item is not copied to the UDVM memory; instead,
   it is buffered until the UDVM has successfully decompressed the
   SigComp message.  It is then forwarded to the state handler with the
   rest of the feedback data (see Section 9.4.9 for further details).

7.2.  Accessing Stored State

   The len field of the SigComp message determines which fields follow
   the returned feedback item.  If the len field is non-zero, then the
   SigComp message contains a state identifier to access a state item at
   the receiving endpoint.  All state items include a 20-byte state
   identifier as per Section 3.3.3, but it is possible to transmit as
   few as 6 bytes from the identifier if the sender believes that this
   is sufficient to match a unique state item at the receiving endpoint.

   The len field encodes the number of transmitted bytes as follows:

   Encoding:   Length of partial state identifier

   01          6 bytes
   10          9 bytes
   11          12 bytes

   The partial state identifier is passed to the state handler, which
   compares it with the most significant bytes of the state_identifier
   in every currently stored state item.  Decompression failure occurs
   if no state item is matched or if more than one state item is
   matched.








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   Decompression failure also occurs if exactly one state item is
   matched but the state item contains a minimum_access_length greater
   than the length of the partial state identifier.  This prevents
   especially sensitive state items from being accessed maliciously by
   brute force guessing of the state_identifier.

   If a state item is successfully accessed then the state_value byte
   string is copied into the UDVM memory beginning at state_address.

   The first 32 bytes of UDVM memory are then initialized to special
   values as illustrated in Figure 5.

                      0             7 8            15
                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     |       UDVM_memory_size        |  0 - 1
                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     |        cycles_per_bit         |  2 - 3
                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     |        SigComp_version        |  4 - 5
                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     |    partial_state_ID_length    |  6 - 7
                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     |         state_length          |  8 - 9
                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     |                               |
                     :           reserved            :  10 - 31
                     |                               |
                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 5: Initializing Useful Values in UDVM memory

   The first five 2-byte words are initialized to contain some values
   that might be useful to the UDVM bytecode (Useful Values).  Note that
   these values are for information only and can be overwritten when
   executing the UDVM bytecode without any effect on the endpoint.  The
   MSBs of each 2-byte word are stored preceding the LSBs.

   Addresses 0 to 5 indicate the resources available to the receiving
   endpoint.  The UDVM memory size is expressed in bytes modulo 2^16, so
   in particular, it is set to 0 if the UDVM memory size is 65536 bytes.
   The cycles_per_bit is expressed as a 2-byte integer taking the value
   16, 32, 64 or 128.  The SigComp_version is expressed as a 2-byte
   value as per Section 3.3.2.

   Addresses 6 to 9 are initialized to the length of the partial state
   identifier, followed by the state_length from the retrieved state
   item.  Both are expressed as 2-byte values.




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   Addresses 10 to 31 are reserved and are initialized to 0 for Version
   0x01 of SigComp.  Future versions of SigComp can use these locations
   for additional Useful Values, so a decompressor MUST NOT rely on
   these values being zero.

   Any remaining addresses in the UDVM memory that have not yet been
   initialized MUST be set to 0.

   The UDVM then begins executing instructions at the memory address
   contained in state_instruction (which is part of the retrieved item
   of state).  Note that the remaining SigComp message is held by the
   decompressor dispatcher until requested by the UDVM.

   (Note that the Useful Values are only set at UDVM startup; there is
   no special significance to this memory area afterwards.  This means
   that the UDVM bytecode is free to use these locations for any other
   purpose a memory location might be used for; it just has to be aware
   they are not necessarily initialized to zero.)

7.3.  Uploading UDVM bytecode

   If the len field is set to 0 then the bytecode needed to decompress
   the SigComp message is supplied as part of the message itself.  The
   12-bit code_len field specifies the size of the uploaded UDVM
   bytecode (from 0 to 4095 bytes inclusive); eight most significant
   bits are in the first byte, followed by the four least significant
   bits in the most significant bits in the second byte.  The remaining
   bits in the second byte are interpreted as a 4-bit destination field
   that specifies the starting memory address to which the bytecode is
   copied.  The destination field is encoded as follows:

                     Encoding:   Destination address:

                     0000        reserved
                     0001        2  *  64  =  128
                     0010        3  *  64  =  196
                     0011        4  *  64  =  256
                       :                :
                     1111        16 *  64  =  1024

   Note that the encoding 0000 is reserved for future SigComp versions,
   and causes a decompression failure in Version 0x01.









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   The UDVM memory is initialized as per Figure 5, except that addresses
   6 to 9 inclusive are set to 0 because no state item has been
   accessed.  The UDVM then begins executing instructions at the memory
   address specified by the destination field.  As above, the remaining
   SigComp message is held by the decompressor dispatcher until needed
   by the UDVM.

8.  Overview of the UDVM

   Decompression functionality for SigComp is provided by a Universal
   Decompressor Virtual Machine (UDVM).  The UDVM is a virtual machine
   much like the Java Virtual Machine but with a key difference:  it is
   designed solely for the purpose of running decompression algorithms.

   The motivation for creating the UDVM is to provide flexibility when
   choosing how to compress a given application message.  Rather than
   picking one of a small number of pre-negotiated algorithms, the
   compressor implementer has the freedom to select an algorithm of
   their choice.  The compressed data is then combined with a set of
   UDVM instructions that allow the original data to be extracted, and
   the result is outputted as a SigComp message.  Since the UDVM is
   optimized specifically for running decompression algorithms, the code
   size of a typical algorithm is small (often sub 100 bytes).
   Moreover, the UDVM approach does not add significant extra processing
   or memory requirements compared to running a fixed preprogrammed
   decompression algorithm.

   Figure 6 gives a detailed view of the interfaces between the UDVM and
   its environment.






















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   +----------------+                                 +----------------+
   |                |     Request compressed data     |                |
   |                |-------------------------------->|                |
   |                |<--------------------------------|                |
   |                |     Provide compressed data     |                |
   |                |                                 |                |
   |                |    Output decompressed data     |  Decompressor  |
   |                |-------------------------------->|   dispatcher   |
   |                |                                 |                |
   |                |     Indicate end of message     |                |
   |                |-------------------------------->|                |
   |                |<--------------------------------|                |
   |      UDVM      | Provide compartment identifier  |                |
   |                |                                 +----------------+
   |                |
   |                |                                 +----------------+
   |                |    Request state information    |                |
   |                |-------------------------------->|                |
   |                |<--------------------------------|                |
   |                |    Provide state information    |     State      |
   |                |                                 |    handler     |
   |                |   Make state creation request   |                |
   |                |-------------------------------->|                |
   |                |  Forward feedback information   |                |
   +----------------+                                 +----------------+

         Figure 6: Interfaces between the UDVM and its environment

   Note that once the UDVM has been initialized, additional compressed
   data and state information are only provided at the request of a
   specific UDVM instruction.

   This chapter describes the basic features of the UDVM including the
   UDVM registers and the format of UDVM bytecode.

8.1.  UDVM Registers

   The UDVM registers are 2-byte words in the UDVM memory that have
   special tasks, for example specifying the location of the stack used
   by the CALL and RETURN instructions.

   The UDVM registers are illustrated in Figure 7.









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                      0             7 8            15
                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     |        byte_copy_left         |  64 - 65
                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     |        byte_copy_right        |  66 - 67
                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     |        input_bit_order        |  68 - 69
                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     |        stack_location         |  70 - 71
                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Figure 7: Memory addresses of the UDVM registers

   The MSBs of each register are always stored before the LSBs.  So, for
   example, the MSBs of byte_copy_left are stored at Address 64 whilst
   the LSBs are stored at Address 65.

   The use of each UDVM register is defined in the following sections.

   (Note that the UDVM registers start at Address 64, that is 32 bytes
   after the area reserved for Useful Values.  The intention is that the
   gap, i.e., the area between Address 32 and Address 63, will often be
   used as scratch-pad memory that is guaranteed to be zero at UDVM
   startup and is efficiently addressable in operand types reference ($)
   and multitype (%).)

8.2.  Requesting Additional Compressed Data

   The decompressor dispatcher stores the compressed data from the
   SigComp message before it is requested by the UDVM via one of the
   INPUT instructions.  When the UDVM bytecode is first executed, the
   dispatcher contains the remaining SigComp message after the header
   has been used to initialize the UDVM as per Chapter 7.

   Note that the INPUT-BITS and INPUT-HUFFMAN instructions retrieve a
   stream of individual compressed bits from the dispatcher.  To provide
   bitwise compatibility with various well-known compression algorithms,
   the input_bit_order register can modify the order in which individual
   bits are passed within a byte.

   The input_bit_order register contains the following three flags:

                      0             7 8            15
                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     |         reserved        |F|H|P|  68 - 69
                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+





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   The P-bit controls the order in which bits are passed from the
   dispatcher to the INPUT instructions.  If set to 0, it indicates that
   the bits within an individual byte are passed to the INPUT
   instructions in MSB to LSB order.  If it is set to 1, the bits are
   passed in LSB to MSB order.

   Note that the input_bit_order register cannot change the order in
   which the bytes themselves are passed to the INPUT instructions
   (bytes are always passed in the same order as they occur in the
   SigComp message).

   The following diagram illustrates the order in which bits are passed
   to the INPUT instructions for both cases:

    MSB         LSB MSB         LSB     MSB         LSB MSB         LSB

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 1 2 3 4 5 6 7|8 9 ...        |   |7 6 5 4 3 2 1 0|        ... 9 8|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

        Byte 0           Byte 1              Byte 0          Byte 1

                 P = 0                               P = 1

   Note that after one or more INPUT instructions the dispatcher may
   hold a fraction of a byte (what used to be the LSBs if P = 0, or, the
   MSBs, if P = 1).  If an INPUT instruction is encountered and the P-
   bit has changed since the last INPUT instruction, any fraction of a
   byte still held by the dispatcher MUST be discarded (even if the
   INPUT instruction requests zero bits).  The first bit passed to the
   INPUT instruction is taken from the subsequent byte.

   When an INPUT instruction requests n bits of compressed data, it
   interprets the received bits as an integer between 0 and 2^n - 1.
   The F-bit and the H-bit specify whether the bits in these integers
   are considered to arrive in MSB to LSB order (bit set to 0) or in LSB
   to MSB order (bit set to 1).

   If the F-bit is set to 0, the INPUT-BITS instruction interprets the
   received bits as arriving MSBs first, and if it is set to 1, it
   interprets the bits as arriving LSBs first.  The H-bit performs the
   same function for the INPUT-HUFFMAN instruction.  Note that it is
   possible to set these two bits to different values in order to use
   different bit orders for the two instructions (certain algorithms
   actually require this, e.g., DEFLATE [RFC-1951]).  (Note that there
   are no special considerations for changing the F- or H-bit between
   INPUT instructions, unlike the discard rule for the P-bit described
   above.)



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   Decompression failure occurs if an INPUT-BITS or an INPUT-HUFFMAN
   instruction is encountered and the input_bit_order register does not
   lie between 0 and 7 inclusive.

8.3.  UDVM Stack

   Certain UDVM instructions make use of a stack of 2-byte words stored
   at the memory address specified by the 2-byte word stack_location.
   The stack contains the following words:

               Name:                 Starting memory address:

               stack_fill            stack_location
               stack[0]              stack_location + 2
               stack[1]              stack_location + 4
               stack[2]              stack_location + 6
                  :                       :

   The notation stack_location is an abbreviation for the contents of
   the stack_location register, i.e., the 2-byte word at locations 70
   and 71.  The notation stack_fill is an abbreviation for the 2-byte
   word at stack_location and stack_location+1.  Similarly, the notation
   stack[n] is an abbreviation for the 2-byte word at
   stack_location+2*n+2 and stack_location+2*n+3.  (As always, the
   arithmetic is modulo 2^16.)

   The stack is used by the CALL, RETURN, PUSH and POP instructions.

   "Pushing" a value on the stack is an abbreviation for copying the
   value to stack[stack_fill] and then increasing stack_fill by 1.  CALL
   and PUSH push values on the stack.

   "Popping" a value from the stack is an abbreviation for decreasing
   stack_fill by 1, and then using the value stored in
   stack[stack_fill].  Decompression failure occurs if stack_fill is
   zero at the commencement of a popping operation.  POP and RETURN pop
   values from the stack.

   For both of these abstract operations, the UDVM first takes note of
   the current value of stack_location and uses this value for both
   sub-operations (accessing the stack and manipulating stack_fill),
   i.e., overwriting stack_location in the course of the operation is
   inconsequential for the operation.








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8.4.  Byte copying

   A number of UDVM instructions require a string of bytes to be copied
   to and from areas of the UDVM memory.  This section defines how the
   byte copying operation should be performed.

   The string of bytes is copied in ascending order of memory address,
   respecting the bounds set by byte_copy_left and byte_copy_right.
   More precisely, if a byte is copied from/to Address m then the next
   byte is copied from/to Address n where n is calculated as follows:

   Set k := m + 1 (modulo 2^16)
   If k = byte_copy_right then set n := byte_copy_left, else set n := k

   Decompression failure occurs if a byte is copied from/to an address
   beyond the UDVM memory.

   Note that the string of bytes is copied one byte at a time.  In
   particular, some of the later bytes to be copied may themselves have
   been written into the UDVM memory by the byte copying operation
   currently being performed.

   Equally, it is possible for a byte copying operation to overwrite the
   instruction that invoked the byte copy.  If this occurs, then the
   byte copying operation MUST be completed as if the original
   instruction were still in place in the UDVM memory (this also applies
   if byte_copy_left or byte_copy_right are overwritten).

   Byte copying is used by the following UDVM instructions:

   SHA-1, COPY, COPY-LITERAL, COPY-OFFSET, MEMSET, INPUT-BYTES, STATE-
   ACCESS, OUTPUT, END-MESSAGE

8.5.  Instruction operands and UDVM bytecode

   Each of the UDVM instructions in a piece of UDVM bytecode is
   represented by a single byte, followed by 0 or more bytes containing
   the operands required by the instruction.

   During instruction execution, conceptually the UDVM first fetches the
   first byte of the instruction, determines the number and types of
   operands required for this instruction, and then decodes all the
   operands in sequence before starting to act on the instruction.
   (Note that the UDVM instructions have been designed in such a way
   that this sequence remains conceptual in those cases where it would
   result in an unreasonable burden on the implementation.)





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   To reduce the size of typical UDVM bytecode, each operand for a UDVM
   instruction is compressed using variable-length encoding.  The aim is
   to store more common operand values using fewer bytes than rarely
   occurring values.

   Four different types of operand are available: the literal, the
   reference, the multitype and the address.  Chapter 9 gives a complete
   list of UDVM instructions and the operand types that follow each
   instruction.

   The UDVM bytecode for each operand type is illustrated in Figure 8 to
   Figure 10, together with the integer values represented by the
   bytecode.

   Note that the MSBs in the bytecode are illustrated as preceding the
   LSBs.  Also, any string of bits marked with k consecutive "n"s is to
   be interpreted as an integer N from 0 to 2^k - 1 inclusive (with the
   MSBs of n illustrated as preceding the LSBs).

   The decoded integer value of the bytecode can be interpreted in two
   ways.  In some cases it is taken to be the actual value of the
   operand.  In other cases it is taken to be a memory address at which
   the 2-byte operand value can be found (MSBs found at the specified
   address, LSBs found at the following address).  The latter cases are
   denoted by memory[X] where X is the address and memory[X] is the 2-
   byte value starting at Address X.

   The simplest operand type is the literal (#), which encodes a
   constant integer from 0 to 65535 inclusive.  A literal operand may
   require between 1 and 3 bytes depending on its value.

   Bytecode:                       Operand value:      Range:

   0nnnnnnn                        N                   0 - 127
   10nnnnnn nnnnnnnn               N                   0 - 16383
   11000000 nnnnnnnn nnnnnnnn      N                   0 - 65535

               Figure 8: Bytecode for a literal (#) operand

   The second operand type is the reference ($), which is always used to
   access a 2-byte value located elsewhere in the UDVM memory.  The
   bytecode for a reference operand is decoded to be a constant integer
   from 0 to 65535 inclusive, which is interpreted as the memory address
   containing the actual value of the operand.







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   Bytecode:                       Operand value:      Range:

   0nnnnnnn                        memory[2 * N]       0 - 65535
   10nnnnnn nnnnnnnn               memory[2 * N]       0 - 65535
   11000000 nnnnnnnn nnnnnnnn      memory[N]           0 - 65535

              Figure 9: Bytecode for a reference ($) operand

   Note that the range of a reference operand is always 0 - 65535
   independently of how many bits are used to encode the reference,
   because the operand always references a 2-byte value in the memory.

   The third kind of operand is the multitype (%), which can be used to
   encode both actual values and memory addresses.  The multitype
   operand also offers efficient encoding for small integer values (both
   positive and negative) and for powers of 2.

   Bytecode:                       Operand value:      Range:

   00nnnnnn                        N                   0 - 63
   01nnnnnn                        memory[2 * N]       0 - 65535
   1000011n                        2 ^ (N + 6)        64 , 128
   10001nnn                        2 ^ (N + 8)    256 , ... , 32768
   111nnnnn                        N + 65504       65504 - 65535
   1001nnnn nnnnnnnn               N + 61440       61440 - 65535
   101nnnnn nnnnnnnn               N                   0 - 8191
   110nnnnn nnnnnnnn               memory[N]           0 - 65535
   10000000 nnnnnnnn nnnnnnnn      N                   0 - 65535
   10000001 nnnnnnnn nnnnnnnn      memory[N]           0 - 65535

              Figure 10: Bytecode for a multitype (%) operand

   The fourth operand type is the address (@).  This operand is decoded
   as a multitype operand followed by a further step: the memory address
   of the UDVM instruction containing the address operand is added to
   obtain the correct operand value.  So if the operand value from
   Figure 10 is D then the actual operand value of an address is
   calculated as follows:

   operand_value = (memory_address_of_instruction + D) modulo 2^16

   Address operands are always used in instructions that control program
   flow, because they ensure that the UDVM bytecode is position-
   independent code (i.e., it will run independently of where it is
   placed in the UDVM memory).






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8.6.  UDVM Cycles

   Once the UDVM has been invoked it executes the instructions contained
   in its memory consecutively unless otherwise indicated (for example
   when the UDVM encounters a JUMP instruction).  If the next
   instruction to be executed lies outside the available memory then
   decompression failure occurs (see Section 8.7).

   To ensure that a SigComp message cannot consume excessive processing
   resources, SigComp limits the number of "UDVM cycles" allocated to
   each message.  The number of available UDVM cycles is initialized to
   1000 plus the number of bits in the SigComp header (as described in
   Section 7); this sum is then multiplied by cycles_per_bit.  Each time
   an instruction is executed the number of available UDVM cycles is
   decreased by the amount specified in Chapter 9.  Additionally, if the
   UDVM successfully requests n bits of compressed data using one of the
   INPUT instructions then the number of available UDVM cycles is
   increased by n * cycles_per_bit once the instruction has been
   executed.

   This means that the maximum number of UDVM cycles available for
   processing an n-byte SigComp message is given by the formula:

           maximum_UDVM_cycles = (8 * n + 1000) * cycles_per_bit

   The reason that this total is not allocated to the UDVM when it is
   invoked is that the UDVM can begin to decompress a message that has
   only been partially received.  So the total message size may not be
   known when the UDVM is initialized.

   Note that the number of UDVM cycles MUST NOT be increased if a
   request for additional compressed data fails.

   The UDVM stops executing instructions when it encounters an END-
   MESSAGE instruction or if decompression failure occurs (see Section
   8.7 for further details).

8.7.  Decompression Failure

   If a compressed message given to the UDVM is corrupted (either
   accidentally or maliciously), then the UDVM may terminate with a
   decompression failure.









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   Reasons for decompression failure include the following:

   1. A SigComp message contains an invalid header as per Chapter 7.

   2. A SigComp message is larger than the decompression_memory_size.

   3. An instruction costs more than the number of remaining UDVM
      cycles.

   4. The UDVM attempts to read from or write to a memory address beyond
      its memory size.

   5. An unknown instruction is encountered.

   6. An unknown operand is encountered.

   7. An instruction is encountered that cannot be processed
      successfully by the UDVM (for example a RETURN instruction when no
      CALL instruction has previously been encountered).

   8. A request to access some state information fails.

   9. A manual decompression failure is triggered using the
      DECOMPRESSION-FAILURE instruction.

   If a decompression failure occurs when decompressing a message then
   the UDVM informs the dispatcher and takes no further action.  It is
   the responsibility of the dispatcher to decide how to cope with the
   decompression failure.  In general a dispatcher SHOULD discard the
   compressed message (or the compressed stream if the transport is
   stream-based) and any decompressed data that has been outputted but
   not yet passed to the application.

9.  UDVM Instruction Set

   The UDVM currently understands 36 instructions, chosen to support the
   widest possible range of compression algorithms with the minimum
   possible overhead.

   Figure 11 lists the different instructions and the bytecode values
   used to encode the instructions.  The cost of each instruction in
   UDVM cycles is also given:









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   Instruction:       Bytecode value:   Cost in UDVM cycles:

   DECOMPRESSION-FAILURE     0          1
   AND                       1          1
   OR                        2          1
   NOT                       3          1
   LSHIFT                    4          1
   RSHIFT                    5          1
   ADD                       6          1
   SUBTRACT                  7          1
   MULTIPLY                  8          1
   DIVIDE                    9          1
   REMAINDER                 10         1
   SORT-ASCENDING            11         1 + k * (ceiling(log2(k)) + n)
   SORT-DESCENDING           12         1 + k * (ceiling(log2(k)) + n)
   SHA-1                     13         1 + length
   LOAD                      14         1
   MULTILOAD                 15         1 + n
   PUSH                      16         1
   POP                       17         1
   COPY                      18         1 + length
   COPY-LITERAL              19         1 + length
   COPY-OFFSET               20         1 + length
   MEMSET                    21         1 + length
   JUMP                      22         1
   COMPARE                   23         1
   CALL                      24         1
   RETURN                    25         1
   SWITCH                    26         1 + n
   CRC                       27         1 + length
   INPUT-BYTES               28         1 + length
   INPUT-BITS                29         1
   INPUT-HUFFMAN             30         1 + n
   STATE-ACCESS              31         1 + state_length
   STATE-CREATE              32         1 + state_length
   STATE-FREE                33         1
   OUTPUT                    34         1 + output_length
   END-MESSAGE               35         1 + state_length

      Figure 11: UDVM instructions and corresponding bytecode values

   Each UDVM instruction costs a minimum of 1 UDVM cycle.  Certain
   instructions may cost additional cycles depending on the values of
   the instruction operands.  Named variables in the cost expressions
   refer to the values of the instruction operands with these names.

   Note that for the SORT instructions, the formula ceiling(log2(k))
   calculates the smallest value i such that k <= 2^i.



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   The UDVM instruction set offers a mix of low-level and high-level
   instructions.  The high-level instructions can all be emulated using
   combinations of low-level instructions, but given a choice it is
   generally preferable to use a single instruction rather than a large
   number of general-purpose instructions.  The resulting bytecode will
   be more compact (leading to a higher overall compression ratio) and
   decompression will typically be faster because the implementation of
   the high-level instructions can be more easily optimized.

   All instructions are encoded as a single byte to indicate the
   instruction type, followed by 0 or more bytes containing the operands
   required by the instruction.  The instruction specifies which of the
   four operand types of Section 8.5 is used in each case. For example
   the ADD instruction is followed by two operands:

   ADD ($operand_1, %operand_2)

   When converted into bytecode the number of bytes required by the ADD
   instruction depends on the value of each operand, and whether the
   multitype operand contains the operand value itself or a memory
   address where the actual value of the operand can be found.

   Each instruction is explained in more detail below.

   Whenever the description of an instruction uses the expression "and
   then", the intended semantics is that the effect explained before
   "and then" is completed before work on the effect explained after the
   "and then" is commenced.

9.1.  Mathematical Instructions

   The following instructions provide a number of mathematical
   operations including bit manipulation, arithmetic and sorting.

9.1.1.  Bit Manipulation

   The AND, OR, NOT, LSHIFT and RSHIFT instructions provide simple bit
   manipulation on 2-byte words.

   AND ($operand_1, %operand_2)
   OR ($operand_1, %operand_2)
   NOT ($operand_1)
   LSHIFT ($operand_1, %operand_2)
   RSHIFT ($operand_1, %operand_2)







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   After the operation is complete, the value of the first operand is
   overwritten with the result.  (Note that since this operand is a
   reference, it is the 2-byte word at the memory address specified by
   the operand that is overwritten.)

   The precise definitions of LSHIFT and RSHIFT are given below.  Note
   that m and n are the 2-byte values encoded by the operands, and that
   floor(x) calculates the largest integer not greater than x:

   LSHIFT (m, n) := m * 2^n (modulo 2^16)
   RSHIFT (m, n) := floor(m / 2^n)

9.1.2.  Arithmetic

   The ADD, SUBTRACT, MULTIPLY, DIVIDE and REMAINDER instructions
   perform arithmetic on 2-byte words.

   ADD ($operand_1, %operand_2)
   SUBTRACT ($operand_1, %operand_2)
   MULTIPLY ($operand_1, %operand_2)
   DIVIDE ($operand_1, %operand_2)
   REMAINDER ($operand_1, %operand_2)

   After the operation is complete, the value of the first operand is
   overwritten with the result.

   The precise definition of each instruction is given below:

   ADD (m, n)       := m + n (modulo 2^16)
   SUBTRACT (m, n)  := m - n (modulo 2^16)
   MULTIPLY (m, n)  := m * n (modulo 2^16)
   DIVIDE (m, n)    := floor(m / n)
   REMAINDER (m, n) := m - n * floor(m / n)

   Decompression failure occurs if a DIVIDE or REMAINDER instruction
   encounters an operand_2 that is zero.

9.1.3.  Sorting

   The SORT-ASCENDING and SORT-DESCENDING instructions sort lists of 2-
   byte words.

   SORT-ASCENDING (%start, %n, %k)
   SORT-DESCENDING (%start, %n, %k)

   The start operand specifies the starting memory address of the block
   of data to be sorted.




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   The block of data itself is divided into n lists each containing k
   2-byte words.  The SORT-ASCENDING instruction applies a certain
   permutation to the lists, such that the first list is sorted into
   ascending order (treating each 2-byte word as an unsigned integer).
   The same permutation is applied to all n lists, so lists other than
   the first will not necessarily be sorted into order.

   In the case that two words have the same value, the original ordering
   of the list is preserved.

   For example, the first list might contain a set of integers to be
   sorted whilst the second list might be used to keep track of where
   the integers appear in the sorted list:

            Before sorting              After sorting

         List 1        List 2        List 1        List 2

            8             1             1             2
            1             2             1             3
            1             3             3             4
            3             4             8             1

   The SORT-DESCENDING instruction behaves as above, except that the
   first list is sorted into descending order.

9.1.4.  SHA-1

   The SHA-1 instruction calculates a 20-byte SHA-1 hash [RFC-3174] over
   the specified area of UDVM memory.

   SHA-1 (%position, %length, %destination)

   The position and length operands specify the starting memory address
   and the length of the byte string over which the SHA-1 hash is
   calculated.  Byte copying rules are enforced as per Section 8.4.

   The destination operand gives the starting address to which the
   resulting 20-byte hash will be copied.  Byte copying rules are
   enforced as above.

9.2.  Memory Management Instructions

   The following instructions are used to set up the UDVM memory, and to
   copy byte strings from one memory location to another.






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9.2.1.  LOAD

   The LOAD instruction sets a 2-byte word to a certain specified value.
   The format of a LOAD instruction is as follows:

   LOAD (%address, %value)

   The first operand specifies the starting address of a 2-byte word,
   whilst the second operand specifies the value to be loaded into this
   word.  As usual, MSBs are stored before LSBs in the UDVM memory.

9.2.2.  MULTILOAD

   The MULTILOAD instruction sets a contiguous block of 2-byte words in
   the UDVM memory to specified values.

   MULTILOAD (%address, #n, %value_0, ..., %value_n-1)

   The first operand specifies the starting address of the contiguous
   2-byte words, whilst the operands value_0 through to value_n-1
   specify the values to load into these words (in the same order as
   they appear in the instruction).

   Decompression failure occurs if the set of 2-byte words set by the
   instruction would overlap the memory locations held by the
   instruction (including its operands) itself, i.e., if the instruction
   would be self-modifying.  (This restriction makes it simpler to
   implement MULTILOAD step-by-step instead of having to decode all
   operands before being able to copy data, as is implied by the
   conceptual model of instruction execution.)

9.2.3.  PUSH and POP

   The PUSH and POP instructions read from and write to the UDVM stack
   (as defined in Section 8.3).

   PUSH (%value)
   POP (%address)

   The PUSH instruction pushes the value specified by its operand on the
   stack.

   The POP instruction pops a value from the stack and then copies the
   value to the specified memory address.  (Note that the expression
   "and then" implies that the copying of the value is inconsequential
   for the stack operation itself, which happens beforehand.)

   See Section 8.3 for possible error conditions.



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9.2.4.  COPY

   The COPY instruction is used to copy a string of bytes from one part
   of the UDVM memory to another.

   COPY (%position, %length, %destination)

   The position operand specifies the memory address of the first byte
   in the string to be copied, and the length operand specifies the
   number of bytes to be copied.

   The destination operand gives the address to which the first byte in
   the string will be copied.

   Byte copying is performed as per the rules of Section 8.4.

9.2.5.  COPY-LITERAL

   A modified version of the COPY instruction is given below:

   COPY-LITERAL (%position, %length, $destination)

   The COPY-LITERAL instruction behaves as a COPY instruction except
   that after copying is completed, the value of the destination operand
   is replaced by the address to which the next byte of data would be
   copied.  More precisely it is replaced by the value n, derived as per
   Section 8.4 with m set to the destination address of the last byte to
   be copied, if any (i.e., if the value of the length operand is zero,
   the value of the destination operand is not changed).

9.2.6.  COPY-OFFSET

   A further version of the COPY-LITERAL instruction is given below:

   COPY-OFFSET (%offset, %length, $destination)

   The COPY-OFFSET instruction behaves as a COPY-LITERAL instruction
   except that an offset operand is given instead of a position operand.

   To derive the value of the position operand, starting at the memory
   address specified by destination, the UDVM counts backwards a total
   of offset memory addresses.

   If the memory address specified in byte_copy_left is reached, the
   next memory address is taken to be (byte_copy_right - 1) modulo 2^16.






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   The COPY-OFFSET instruction then behaves as a COPY-LITERAL
   instruction, taking the value of the position operand to be the last
   memory address reached in the above step.

9.2.7.  MEMSET

   The MEMSET instruction initializes an area of UDVM memory to a
   specified sequence of values. The format of a MEMSET instruction is
   as follows:

   MEMSET (%address, %length, %start_value, %offset)

   The sequence of values used by the MEMSET instruction is specified by
   the following formula:

   Seq[n] := (start_value + n * offset) modulo 256

   The values Seq[0] to Seq[length - 1] inclusive are each interpreted
   as a single byte, and then concatenated to form a byte string where
   the first byte has value Seq[0], the second byte has value Seq[1] and
   so on up to the last byte which has value Seq[length - 1].

   The string is then byte copied into the UDVM memory beginning at the
   memory address specified as an operand to the MEMSET instruction,
   obeying the rules of Section 8.4.  (Note that the byte string may
   overwrite the MEMSET instruction or its operands; as explained in
   Section 8.5, the MEMSET instruction must be executed as if the
   original operands were still in place in the UDVM memory.)

9.3.  Program Flow Instructions

   The following instructions alter the flow of UDVM code.  Each
   instruction jumps to one of a number of memory addresses based on a
   certain specified criterion.

   Note that certain I/O instructions (see Section 9.4) can also alter
   program flow.

9.3.1.  JUMP

   The JUMP instruction moves program execution to the specified memory
   address.

   JUMP (@address)

   Decompression failure occurs if the value of the address operand lies
   beyond the overall UDVM memory size.




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9.3.2.  COMPARE

   The COMPARE instruction compares two operands and then jumps to one
   of three specified memory addresses depending on the result.

   COMPARE (%value_1, %value_2, @address_1, @address_2, @address_3)

   If value_1 < value_2 then the UDVM continues instruction execution at
   the memory address specified by address 1. If value_1 = value_2 then
   it jumps to the address specified by address_2. If value_1 > value_2
   then it jumps to the address specified by address_3.

9.3.3.  CALL and RETURN

   The CALL and RETURN instructions provide support for compression
   algorithms with a nested structure.

   CALL (@address)
   RETURN

   Both instructions use the UDVM stack of Section 8.3.  When the UDVM
   reaches a CALL instruction, it finds the memory address of the
   instruction immediately following the CALL instruction and pushes
   this 2-byte value on the stack, ready for later retrieval.  It then
   continues instruction execution at the memory address specified by
   the address operand.

   When the UDVM reaches a RETURN instruction it pops a value from the
   stack and then continues instruction execution at the memory address
   just popped.

   See Section 8.3 for error conditions.

9.3.4.  SWITCH

   The SWITCH instruction performs a conditional jump based on the value
   of one of its operands.

   SWITCH (#n, %j, @address_0, @address_1, ... , @address_n-1)

   When a SWITCH instruction is encountered the UDVM reads the value of
   j. It then continues instruction execution at the address specified
   by address j.

   Decompression failure occurs if j specifies a value of n or more, or
   if the address lies beyond the overall UDVM memory size.





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9.3.5.  CRC

   The CRC instruction verifies a string of bytes using a 2-byte CRC.

   CRC (%value, %position, %length, @address)

   The actual CRC calculation is performed using the generator
   polynomial x^16 + x^12 + x^5 + 1, which coincides with the 2-byte
   Frame Check Sequence (FCS) of PPP [RFC-1662].

   The position and length operands define the string of bytes over
   which the CRC is evaluated.  Byte copying rules are enforced as per
   Section 8.4.

   The CRC value is computed exactly as defined for the 16-bit FCS
   calculation in [RFC-1662].

   The value operand contains the expected integer value of the 2-byte
   CRC.  If the calculated CRC matches the expected value then the UDVM
   continues instruction execution at the following instruction.
   Otherwise the UDVM jumps to the memory address specified by the
   address operand.

9.4.  I/O instructions

   The following instructions allow the UDVM to interface with its
   environment.  Note that in the overall SigComp architecture all of
   these interfaces pass to the decompressor dispatcher or to the state
   handler.

9.4.1.  DECOMPRESSION-FAILURE

   The DECOMPRESSION-FAILURE instruction triggers a manual decompression
   failure.  This is useful if the UDVM bytecode discovers that it
   cannot successfully decompress the message (e.g., by using the CRC
   instruction).

   This instruction has no operands.

9.4.2.  INPUT-BYTES

   The INPUT-BYTES instruction requests a certain number of bytes of
   compressed data from the decompressor dispatcher.

   INPUT-BYTES (%length, %destination, @address)






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   The length operand indicates the requested number of bytes of
   compressed data, and the destination operand specifies the starting
   memory address to which they should be copied.  Byte copying is
   performed as per the rules of Section 8.4.

   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.

   If the INPUT-BYTES is encountered after an INPUT-BITS or an INPUT-
   HUFFMAN instruction has been used, and the dispatcher currently holds
   a fraction of a byte, then the fraction MUST be discarded before any
   data is passed to the UDVM.  The first byte to be passed is the byte
   immediately following the discarded data.

9.4.3.  INPUT-BITS

   The INPUT-BITS instruction requests a certain number of bits of
   compressed data from the decompressor dispatcher.

   INPUT-BITS (%length, %destination, @address)

   The length operand indicates the requested number of bits.
   Decompression failure occurs if this operand does not lie between 0
   and 16 inclusive.

   The destination operand specifies the memory address to which the
   compressed data should be copied.  Note that the requested bits are
   interpreted as a 2-byte integer ranging from 0 to 2^length - 1, as
   explained in Section 8.2.

   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.

9.4.4.  INPUT-HUFFMAN

   The INPUT-HUFFMAN instruction requests a variable number of bits of
   compressed data from the decompressor dispatcher.  The instruction
   initially requests a small number of bits and compares the result
   against a certain criterion; if the criterion is not met, then
   additional bits are requested until the criterion is achieved.

   The INPUT-HUFFMAN instruction is followed by three mandatory operands
   plus n additional sets of operands.  Every additional set contains
   four operands as shown below:





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   INPUT-HUFFMAN (%destination, @address, #n, %bits_1, %lower_bound_1,
   %upper_bound_1, %uncompressed_1, ... , %bits_n, %lower_bound_n,
   %upper_bound_n, %uncompressed_n)

   Note that if n = 0 then the INPUT-HUFFMAN instruction is ignored and
   program execution resumes at the following instruction.
   Decompression failure occurs if (bits_1 + ... + bits_n) > 16.

   In all other cases, the behavior of the INPUT-HUFFMAN instruction is
   defined below:

   1. Set j := 1 and set H := 0.

   2. Request bits_j compressed bits.  Interpret the returned bits as an
      integer k from 0 to 2^bits_j - 1, as explained in Section 8.2.

   3. Set H := H * 2^bits_j + k.

   4. If data is requested that lies beyond the end of the SigComp
      message, terminate the INPUT-HUFFMAN instruction and move program
      execution to the memory address specified by the address operand.

   5. If (H < lower_bound_j) or (H > upper_bound_j) then set j := j + 1.
      Then go back to Step 2, unless j > n in which case decompression
      failure occurs.

   6. Copy (H + uncompressed_j - lower_bound_j) modulo 2^16 to the
      memory address specified by the destination operand.

9.4.5.  STATE-ACCESS

   The STATE-ACCESS instruction retrieves some previously stored state
   information.

   STATE-ACCESS (%partial_identifier_start, %partial_identifier_length,
   %state_begin, %state_length, %state_address, %state_instruction)

   The partial_identifier_start and partial_identifier_length operands
   specify the location of the partial state identifier used to retrieve
   the state information.  This identifier has the same function as the
   partial state identifier transmitted in the SigComp message as per
   Section 7.2.

   Decompression failure occurs if partial_identifier_length does not
   lie between 6 and 20 inclusive.  Decompression failure also occurs if
   no state item matching the partial state identifier can be found, if





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   more than one state item matches the partial identifier, or if
   partial_identifier_length is less than the minimum_access_length of
   the matched state item. Otherwise, a state item is returned from the
   state handler.

   If any of the operands state_address, state_instruction or
   state_length is set to 0 then its value is taken from the returned
   item of state instead.

   Note that when calculating the number of UDVM cycles the STATE-ACCESS
   instruction costs (1 + state_length) cycles.  The value of
   state_length MUST be taken from the returned item of state in the
   case that the state_length operand is set to 0.

   The state_begin and state_length operands define the starting byte
   and number of bytes to copy from the state_value contained in the
   returned item of state.  Decompression failure occurs if bytes are
   copied from beyond the end of the state_value.  Note that
   decompression failure will always occur if the state_length operand
   is set to 0 but the state_begin operand is non-zero.

   The state_address operand contains a UDVM memory address.  The
   requested portion of the state_value is byte copied to this memory
   address using the rules of Section 8.4.

   Program execution then resumes at the memory address specified by
   state_instruction, unless this address is 0 in which case program
   execution resumes at the next instruction following the STATE-ACCESS
   instruction.  Note that the latter case only occurs if both the
   state_instruction operand and the state_instruction value from the
   requested state are set to 0.

9.4.6.  STATE-CREATE

   The STATE-CREATE instruction requests the creation of a state item at
   the receiving endpoint.

   STATE-CREATE (%state_length, %state_address, %state_instruction,
   %minimum_access_length, %state_retention_priority)

   Note that the new state item cannot be created until a valid
   compartment identifier has been returned by the application.
   Consequently, when a STATE-CREATE instruction is encountered the UDVM
   simply buffers the five supplied operands until the END-MESSAGE
   instruction is reached.  The steps taken at this point are described
   in Section 9.4.9.





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   Decompression failure MUST occur if more than four state creation
   requests are made before the END-MESSAGE instruction is encountered.
   Decompression failure also occurs if the minimum_access_length does
   not lie between 6 and 20 inclusive, or if the
   state_retention_priority is 65535.

9.4.7.  STATE-FREE

   The STATE-FREE instruction informs the receiving endpoint that the
   sender no longer wishes to use a particular state item.

   STATE-FREE (%partial_identifier_start, %partial_identifier_length)

   Note that the STATE-FREE instruction does not automatically delete a
   state item, but instead reclaims the memory taken by the state item
   within a certain compartment, which is generally not known before the
   END-MESSAGE instruction is reached.  So just as for the STATE-CREATE
   instruction, when a STATE-FREE instruction is encountered the UDVM
   simply buffers the two supplied operands until the END-MESSAGE
   instruction is reached.  The steps taken at this point are described
   in Section 9.4.9.

   Decompression failure MUST occur if more than four state free
   requests are made before the END-MESSAGE instruction is encountered.
   Decompression failure also occurs if partial_identifier_length does
   not lie between 6 and 20 inclusive.

9.4.8.  OUTPUT

   The OUTPUT instruction provides successfully decompressed data to the
   dispatcher.

   OUTPUT (%output_start, %output_length)

   The operands define the starting memory address and length of the
   byte string to be provided to the dispatcher.  Note that the OUTPUT
   instruction can be used to output a partially decompressed message;
   each time the instruction is encountered it provides a new byte
   string that the dispatcher appends to the end of any bytes previously
   passed to the dispatcher via the OUTPUT instruction.

   The string of data is byte copied from the UDVM memory obeying the
   rules of Section 8.4.

   Decompression failure occurs if the cumulative number of bytes
   provided to the dispatcher exceeds 65536 bytes.





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   Since there is technically a difference between outputting a 0-byte
   decompressed message, and not outputting a decompressed message at
   all, the OUTPUT instruction needs to distinguish between the two
   cases.  Thus, if the UDVM terminates before encountering an OUTPUT
   instruction it is considered not to have outputted a decompressed
   message.  If it encounters one or more OUTPUT instructions, each of
   which provides 0 bytes of data to the dispatcher, then it is
   considered to have outputted a 0-byte decompressed message.

9.4.9.  END-MESSAGE

   The END-MESSAGE instruction successfully terminates the UDVM and
   forwards the state creation and state free requests to the state
   handler together with any supplied feedback data.

   END-MESSAGE (%requested_feedback_location,
   %returned_parameters_location, %state_length, %state_address,
   %state_instruction, %minimum_access_length,
   %state_retention_priority)

   When the END-MESSAGE instruction is encountered, the decompressor
   dispatcher indicates to the application that a complete message has
   been decompressed.  The application may return a compartment
   identifier, which the UDVM forwards to the state handler together
   with the state creation and state free requests and any supplied
   feedback data.

   The actual decompressed message is outputted separately using the
   OUTPUT instruction; this conserves memory at the UDVM because there
   is no need to buffer an entire decompressed message before it can be
   passed to the dispatcher.

   The END-MESSAGE instruction may pass up to four state creation
   requests and up to four state free requests to the state handler.
   The requests are passed to the state handler in the same order as
   they are made; in particular it is possible for the state creation
   requests and the state free requests to be interleaved.

   The state creation requests are made by the STATE-CREATE instruction.
   Note however that the END-MESSAGE can make one state creation request
   itself using the supplied operands. If the specified
   minimum_access_length does not lie between 6 and 20 inclusive, or if
   the state_retention_priority is 65535 then the END-MESSAGE
   instruction fails to make a state creation request of its own
   (however decompression failure does not occur and the state creation
   requests made by the STATE-CREATE instruction are still valid).





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   Note that there is a maximum limit of four state creation requests
   per instance of the UDVM.  Therefore, decompression failure occurs if
   the END-MESSAGE instruction makes a state creation request and four
   instances of the STATE-CREATE instruction have already been
   encountered.

   When creating a state item it is necessary to give the state_length,
   state address, state_instruction and minimum_access_length; these are
   supplied as operands in the STATE-CREATE instruction (or the END-
   MESSAGE instruction).  A complete item of state also requires a
   state_value and a state_identifier, which are derived as follows:

   The UDVM byte copies a string of state_length bytes from the UDVM
   memory beginning at state_address (obeying the rules of Section 8.4).
   This is the state_value.

   The UDVM then calculates a 20-byte SHA-1 hash [RFC-3174] over the
   byte string formed by concatenating the state_length, state_address,
   state_instruction, minimum_access_length and state_value (in the
   order given).  This is the state_identifier.

   The state_retention_priority is not part of the state item itself,
   but instead determines the order in which state will be deleted when
   the compartment exceeds its allocated state memory.  The
   state_retention_priority is supplied as an operand in the STATE-
   CREATE or END-MESSAGE instruction and is passed to the state handler
   as part of each state creation request.

   The state free requests are made by the STATE-FREE instruction. Each
   STATE-FREE instruction supplies the values partial_identifier_start
   and partial_identifier_length; upon reaching the END-MESSAGE
   instruction these values are used to byte copy a partial state
   identifier from the UDVM memory.  If no state item matching the
   partial state identifier can be found or if more than one state item
   in the compartment matches the partial state identifier, then the
   state free request is ignored (this does not cause decompression
   failure to occur).  Otherwise, the state handler frees the matched
   state item as specified in Section 6.2.

   As well as forwarding the state creation and state free requests, the
   END-MESSAGE instruction may also pass feedback data to the state
   handler.  Feedback data is used to inform the receiving endpoint
   about the capabilities of the sending endpoint, which can help to
   improve the overall compression ratio and to reduce the working
   memory requirements of the endpoints.






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   Two types of feedback data are available: requested feedback and
   returned feedback.  The format of the requested feedback data is
   given in Figure 12.  As outlined in Section 3.2, the requested
   feedback data can be used to influence the contents of the returned
   feedback data in the reverse direction.

   The returned feedback data is itself subdivided into a returned
   feedback item and a list of returned SigComp parameters.  The
   returned feedback item is of sufficient importance to warrant its own
   field in the SigComp header as described in Section 7.1.  The
   returned SigComp parameters are illustrated in Figure 13.

   Note that the formats of Figure 12 and Figure 13 are only for local
   presentation of the feedback data on the interface between the UDVM
   and state handler.  The formats do not mandate any bits on the wire;
   the compressor can transmit the data in any form provided that it is
   loaded into the UDVM memory at the correct addresses.

   Moreover, the responsibility for ensuring that feedback data arrives
   successfully over an unreliable transport lies with the sender.  The
   receiving endpoint always uses the last received value for each field
   in the feedback data, even if the values are out of date due to
   packet loss or misordering.

   If the requested_feedback_location operand is set to 0, then no
   feedback request is made; otherwise, it points to the starting memory
   address of the requested feedback data as shown in Figure 12.

        0   1   2   3   4   5   6   7
      +---+---+---+---+---+---+---+---+
      |     reserved      | Q | S | I |  requested_feedback_location
      +---+---+---+---+---+---+---+---+
      |                               |
      :    requested feedback item    :  if Q = 1
      |                               |
      +---+---+---+---+---+---+---+---+

               Figure 12: Format of requested feedback data

   The reserved bits may be used in future versions of SigComp, and are
   set to 0 in Version 0x01.  Non-zero values should be ignored by the
   receiving endpoint.

   The Q-bit indicates whether a requested feedback item is present or
   not.  The compressor can set the requested feedback item to an
   arbitrary value, which will then be transmitted unmodified in the
   reverse direction as a returned feedback item.  See Chapter 5 for
   further details of how the requested feedback item is returned.



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   The format of the requested feedback item is identical to the format
   of the returned feedback item illustrated in Figure 4.

   The compressor sets the S-bit to 1 if it does not wish (or no longer
   wishes) to save state information at the receiving endpoint and also
   does not wish to access state information that it has previously
   saved.  Consequently, if the S-bit is set to 1 then the receiving
   endpoint can reclaim the state memory allocated to the remote
   compressor and set the state_memory_size for the compartment to 0.

   The compressor may change its mind and switch the S-bit back to 0 in
   a later message.  However, the receiving endpoint is under no
   obligation to use the original state_memory_size for the compartment;
   it may choose to allocate less memory to the compartment or possibly
   none at all.

   Similarly the compressor sets the I-bit to 1 if it does not wish (or
   no longer wishes) to access any of the locally available state items
   offered by the receiving endpoint.  This can help to conserve
   bandwidth because the list of locally available state items no longer
   needs to be returned in the reverse direction.  It may also conserve
   memory at the receiving endpoint, as the state handler can delete any
   locally available state items that it determines are no longer
   required by any remote endpoint.  Note that the compressor can set
   the I-bit back to 0 in a later message, but it cannot access any
   locally available state items that were previously offered by the
   receiving endpoint unless they are subsequently re-announced.

   If the returned_parameters_location operand is set to 0, then no
   SigComp parameters are returned; otherwise, it points to the starting
   memory address of the returned parameters as shown in Figure 13.




















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        0   1   2   3   4   5   6   7
      +---+---+---+---+---+---+---+---+
      |  cpb  |    dms    |    sms    |  returned_parameters_location
      +---+---+---+---+---+---+---+---+
      |        SigComp_version        |
      +---+---+---+---+---+---+---+---+
      | length_of_partial_state_ID_1  |
      +---+---+---+---+---+---+---+---+
      |                               |
      :  partial_state_identifier_1   :
      |                               |
      +---+---+---+---+---+---+---+---+
              :               :
      +---+---+---+---+---+---+---+---+
      | length_of_partial_state_ID_n  |
      +---+---+---+---+---+---+---+---+
      |                               |
      :  partial_state_identifier_n   :
      |                               |
      +---+---+---+---+---+---+---+---+

             Figure 13: Format of returned SigComp parameters

   The first byte encodes the SigComp parameters cycles_per_bit,
   decompression_memory_size and state_memory_size as per Section 3.3.1.
   The byte can be set to 0 if the three parameters are not included in
   the feedback data.  (This may be useful to save bits in the
   compressed message if the remote endpoint is already satisfied all
   necessary information has reached the endpoint receiving the
   message.)

   The second byte encodes the SigComp_version as per Section 3.3.2.
   Similar to the first byte, the second byte can be set to 0 if the
   parameter is not included in the feedback data.

   The remaining bytes encode a list of partial state identifiers for
   the locally available state items offered by the sending endpoint.
   Each state item is encoded as a 1-byte length field, followed by a
   partial state identifier containing as many bytes as indicated in the
   length field.  The sender can choose to send as few as 6 bytes if it
   believes that this is sufficient for the receiver to determine which
   state item is being offered.

   The list of state identifiers is terminated by a byte in the position
   where the next length field would be expected that is set to a value
   below 6 or above 20.  Note that upgraded SigComp versions may append
   additional items of data after the final length field.




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10. Security Considerations

10.1.  Security Goals

   The overall security goal of the SigComp architecture is to not
   create risks that are in addition to those already present in the
   application protocols.  There is no intention for SigComp to enhance
   the security of the application, as it always can be circumvented by
   not using compression.  More specifically, the high-level security
   goals can be described as:

   1. Do not worsen security of existing application protocol

   2. Do not create any new security issues

   3. Do not hinder deployment of application security.

10.2.  Security Risks and Mitigation

   This section identifies the potential security risks associated with
   SigComp, and explains how each risk is minimized by the scheme.

10.2.1.  Confidentiality Risks

   - Attacking SigComp by snooping into state of other users:

   State is accessed by supplying a state identifier, which is a
   cryptographic hash of the state being referenced.  This implies that
   the referencing message already needs knowledge about the state.  To
   enforce this, a state item cannot be accessed without supplying a
   minimum of 48 bits from the hash.  This also minimizes the
   probability of an accidental state collision.  A compressor can,
   using the minimum_access_length operand of the STATE-CREATE and END-
   MESSAGE instructions, increase the number of bits that need to be
   supplied to access the state, increasing the protection against
   attacks.

   Generally, ways to obtain knowledge about the state identifier (e.g.,
   passive attacks) will also easily provide knowledge about the
   referenced state, so no new vulnerability results.

   An endpoint needs to handle state identifiers with the same care it
   would handle the state itself.








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10.2.2.  Integrity Risks

   The SigComp approach assumes that there is appropriate integrity
   protection below and/or above the SigComp layer.  The state creation
   mechanism provides some additional potential to compromise the
   integrity of the messages; however, this would most likely be
   detectable at the application layer.

   - Attacking SigComp by faking state or making unauthorized changes to
     state:

   State cannot be destroyed by a malicious sender unless it can send
   messages that the application identifies as belonging to the same
   compartment the state was created under; this adds additional
   security risks only when the application allows the installation of
   SigComp state from a message where it would not have installed state
   itself.

   Faking or changing state is only possible if the hash allows
   intentional collision.

10.2.3.  Availability Risks (Avoiding DoS Vulnerabilities)

   - Use of SigComp as a tool in a DoS attack to another target:

   SigComp cannot easily be used as an amplifier in a reflection attack,
   as it only generates one decompressed message per incoming compressed
   message.  This message is then handed to the application; the utility
   as a reflection amplifier is therefore limited by the utility of the
   application for this purpose.

   However, it must be noted that SigComp can be used to generate larger
   messages as input to the application than have to be sent from the
   malicious sender; this therefore can send smaller messages (at a
   lower bandwidth) than are delivered to the application.  Depending on
   the reflection characteristics of the application, this can be
   considered a mild form of amplification.  The application MUST limit
   the number of packets reflected to a potential target - even if
   SigComp is used to generate a large amount of information from a
   small incoming attack packet.











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   - Attacking SigComp as the DoS target by filling it with state:

   Excessive state can only be installed by a malicious sender (or a set
   of malicious senders) with the consent of the application.  The
   system consisting of SigComp and application is thus approximately as
   vulnerable as the application itself, unless it allows the
   installation of SigComp state from a message where it would not have
   installed application state itself.

   If this is desirable to increase the compression ratio, the effect
   can be mitigated by making use of feedback at the application level
   that indicates whether the state requested was actually installed -
   this allows a system under attack to gracefully degrade by no longer
   installing compressor state that is not matched by application state.

   Obviously, if a stream-based transport is used, the streams
   themselves constitute state that has to be handled in the same way
   that the application itself would handle a stream-based transport; if
   an application is not equipped for stream-based transport, it should
   not allow SigComp connections on a stream-based transport.  For the
   alternative SigComp usage described as "continuous mode" in Section
   4.2.1, an attacker could create any number of active UDVMs unless
   there is some DoS protection at a lower level (e.g., by using TLS in
   appropriate configurations).

   - Attacking the UDVM by faking state or making unauthorized changes
     to state:

   This is covered in Section 10.2.2.

   - Attacking the UDVM by sending it looping code:

   The application sets an upper limit to the number of "UDVM cycles"
   that can be used per compressed message and per input bit in the
   compressed message.  The damage inflicted by sending packets with
   looping code is therefore limited, although this may still be
   substantial if a large number of UDVM cycles are offered by the UDVM.
   However, this would be true for any decompressor that can receive
   packets over an unsecured transport.

11. IANA Considerations

   SigComp requires a 1-byte name space, the SigComp_version, which has
   been created by the IANA.  Upgraded versions of SigComp must be
   backwards-compatible with Version 0x01, described in this document.
   Adding additional UDVM instructions and assigning values to the
   reserved UDVM memory addresses are two possible upgrades for which
   this is the case.



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   Following the policies outlined in [RFC-2434], the IANA policy for
   assigning a new value for the SigComp_version shall require a
   Standards Action.  Values are thus assigned only for Standards Track
   RFCs approved by the IESG.

12. Acknowledgements

   Thanks to

      Abigail Surtees
      Mark A West
      Lawrence Conroy
      Christian Schmidt
      Max Riegel
      Lars-Erik Jonsson
      Stefan Forsgren
      Krister Svanbro
      Miguel Garcia
      Christopher Clanton
      Khiem Le
      Ka Cheong Leung
      Robert Sugar

   for valuable input and review.

13. References

13.1. Normative References

   [RFC-1662]  Simpson, W., "PPP in HDLC-like Framing", STD 51, RFC
               1662, July 1994.

   [RFC-2119]  Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC-3174]  Eastlake, 3rd, D. and P. Jones, "US Secure Hash Algorithm
               1 (SHA1)", RFC 3174, September 2001.

13.2. Informative References

   [RFC-1951]  Deutsch, P., "DEFLATE Compressed Data Format
               Specification version 1.3", RFC 1951, May 1996.

   [RFC-2026]  Bradner, S., "The Internet Standards Process - Revision
               3", BCP 9, RFC 2026, October 1996.

   [RFC-2279]  Yergeau, F., "UTF-8, a transformation format of ISO
               10646", RFC 2279, January 1998.



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   [RFC-2326]  Schulzrinne, H., Rao, A. and R. Lanphier, "Real Time
               Streaming Protocol (RTSP)", RFC 2326, April 1998.

   [RFC-2434]  Alvestrand, H. and T. Narten, "Guidelines for Writing an
               IANA Considerations Section in RFCs", BCP 26, RFC 2434,
               October 1998.

   [RFC-2960]  Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
               Schwartzbauer, H., Taylor, T., Rytina, I., Kalla, M.,
               Zhang, L. and V. Paxson, "Stream Control Transmission
               Protocol", RFC 2960, October 2000.

   [RFC-3261]  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.

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

14. Authors' Addresses

   Richard Price
   Roke Manor Research Ltd
   Romsey, Hants, SO51 0ZN
   United Kingdom

   Phone: +44 1794 833681
   EMail: richard.price@roke.co.uk


   Carsten Bormann
   Universitaet Bremen TZI
   Postfach 330440
   D-28334 Bremen, Germany

   Phone: +49 421 218 7024
   EMail: cabo@tzi.org











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   Jan Christoffersson
   Box 920
   Ericsson AB
   SE-971 28 Lulea, Sweden

   Phone: +46 920 20 28 40
   EMail: jan.christoffersson@epl.ericsson.se


   Hans Hannu
   Box 920
   Ericsson AB
   SE-971 28 Lulea, Sweden

   Phone: +46 920 20 21 84
   EMail: hans.hannu@epl.ericsson.se


   Zhigang Liu
   Nokia Research Center
   6000 Connection Drive
   Irving, TX 75039

   Phone: +1 972 894-5935
   EMail: zhigang.c.liu@nokia.com


   Jonathan Rosenberg
   dynamicsoft
   72 Eagle Rock Avenue
   First Floor
   East Hanover, NJ 07936

   EMail: jdrosen@dynamicsoft.com

















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15.  Full Copyright Statement

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
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   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.



















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