Network Working Group Richard Price, Siemens/Roke Manor
INTERNET-DRAFT Hans Hannu, Ericsson
Expires: August 2002 Carsten Bormann, TZI/Uni Bremen
Jan Christoffersson, Ericsson
Zhigang Liu, Nokia
Jonathan Rosenberg, dynamicsoft
February 14, 2002
Signaling Compression (SigComp)
<draft-ietf-rohc-sigcomp-04.txt>
Status of this memo
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all provisions of Section 10 of RFC2026.
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Abstract
This document defines SigComp, a solution for compressing messages
generated by text-based protocols such as SIP [SIP] and RTSP [RTSP].
The architecture and pre-requisites of SigComp are outlined, along
with the format of the SigComp message.
Decompression functionality for the SigComp solution is provided by a
"Universal Decompressor Virtual Machine" 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].
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Table of contents
1. Introduction..................................................2
2. Terminology...................................................3
3. SigComp Architecture..........................................5
4. SigComp message flow..........................................11
5. SigComp compressor............................................15
6. State handling and capability announcement....................18
7. Overview of the UDVM..........................................22
8. Decompressing a SigComp message...............................26
9. UDVM instruction set..........................................30
10. Security considerations.......................................41
11. IANA considerations...........................................43
12. Acknowledgements..............................................43
13. AuthorsÆ addresses............................................44
14. References....................................................45
Appendix A. Mnemonic language.....................................46
Appendix B. Example application-defined parameters................48
Appendix C. Example decompression algorithms......................49
Appendix D. Document history......................................51
1. Introduction
The Session Initiation Protocol (SIP) [SIP], along with many other
application protocols used for multimedia communications such as RTSP
[RTSP], is a textual protocol engineered for bandwidth rich links. As
a result, the SIP messages have not been optimized in terms of size.
Typical SIP messages are from a few hundred bytes to as high as 2000.
To date, this has not been a significant problem.
With the planned usage of these protocols in wireless handsets as
part of 2.5G and 3G cellular networks, the large size of these
messages is problematic. With low-rate IP connectivity, store-and-
forward delays are significant. Taking into account retransmits, and
the multiplicity of messages that are required in some flows, call
setup and feature invocation are adversely affected. Therefore, we
believe there is merit in reducing these message sizes.
This document outlines the architecture and pre-requisites of the
SigComp solution including the capability announcement and UDVM
algorithm upload, along with the format of the SigComp message.
SigComp is typically offered to applications as a "shim" layer
between the application and the transport. The service provided is
that of the underlying transport plus compression.
This document focuses on the signaling scenario where an endpoint
sends and receives data to/from an outbound/inbound proxy. However,
SigComp is designed to run over both connectionless and connection-
oriented transports and hence may be applicable to other scenarios
with multiple endpoints compressing and decompressing data.
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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 [RFC-2119].
SigComp
The overall solution for signaling compression, comprising the
compressor, decompressor, dispatchers and state handler.
Application
For the purpose of this document, an application is a text-based
protocol software that:
a) sends application data to the compressor dispatcher
b) receives data from the decompressor dispatcher
c) decides whether state information may be saved by SigComp
Transport
Mechanism for passing data between two instances of an application.
SigComp is capable of sending messages over a wide range of
transports including TCP, UDP and [SCTP].
Message-based transport
A transport that carries data as a set of distinct, bounded
messages.
Stream-based transport
A transport that carries data as a continuous stream with no
message boundaries. In this case, SigComp reserves the character
0xFFFF to delimit messages in the compressed stream.
Application-defined parameters
Parameters that must be agreed upon by the applications invoking
SigComp. Depending on the situation these parameters might be fixed
a-priori or negotiated.
Application message
An uncompressed message, as provided from or to the application,
which is to be compressed by the compressor. When delivered
from the decompressor the data has passed through the decompression
process and is referred to as decompressed data or a decompressed
message.
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SigComp message
May contain a compressed application message in the form of UDVM
bytecode. In case of a message- based transport, such as UDP, a
SigComp message corresponds to exactly one (UDP) datagram. For a
stream-based transport, such as TCP, each SigComp message is
separated by a 0xFFFF delimiter.
Compressor
The compressor invokes the encoder, and keeps track of states that
can be used for compression. It is responsible for supplying UDVM
bytecode to the remote decompressor in order for compressed
data to be decompressed.
Encoder
Encodes data according to a (compression) algorithm into UDVM
bytecode. The encoded data can be decoded by a UDVM.
Compressor dispatcher
A layer that receives uncompressed application messages, invokes a
compressor, and forwards the SigComp messages to a remote SigComp
layer.
Decompressor
The decompressor is responsible for converting a SigComp message
into uncompressed data. Decompression functionality is provided by
the UDVM.
Decompressor dispatcher
A layer that receives SigComp messages, invokes a decompressor, and
forwards the decompressed application messages to an application.
Virtual machine
A machine architecture designed to be implemented in software
(although silicon implementations are of course possible).
Universal Decompressor Virtual Machine (UDVM)
The virtual machine described in this document. The UDVM is used
for decompression of SigComp messages.
Bytecode
Machine code that can be executed by a virtual machine. UDVM
bytecode is a combination of UDVM instructions and compressed data.
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Per-message compression
Compression that does not reference data from previous messages.
SigComp can decompress a message of this type using only the
application-defined parameters and the data in the message itself.
Dynamic compression
Compression relative to messages sent prior to the current
compressed message. SigComp stores and retrieves this data using
the state handler.
State
Data saved for retrieval by later SigComp messages. The data
typically reflects the contents of the UDVM memory after
decompressing a message, but state can also be saved by the
compressor or by the application.
State identifier
Reference used to access an item of state previously saved by the
compressor, the decompressor or the application.
CPU cycles
A measure of the amount of "CPU power" required to execute a UDVM
instruction (the simplest UDVM instructions require a single CPU
cycle). An upper limit is placed on the number of cycles that can
be used to decompress each bit in a compressed message.
3. SigComp Architecture
In the SigComp architecture compression and decompression is
performed at two communicating endpoints. Figure 1 shows the layout
of a communicating endpoint that implements a SigComp layer. The
figure does not mandate any particular implementation, but is shown
to the reader for the sake of clarity.
SigComp is typically offered to applications as a "shim" layer
between the application and the transport. Note however that for
certain applications the compressed SigComp message may be passed
back to the application itself for additional processing before
transmission. For example, the application may wish to apply
encryption to the compressed message before handing it to the
transport.
The SigComp layer is common for several text based protocol
applications (identified as Application 1 and Application 2 in Figure
1). These applications are not part of the SigComp layer.
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The SigComp layer is further decomposed in the following components:
- A compressor dispatcher: this is the interface from the
applications. Application messages are received by the compressor
dispatcher, and based on the application requirements, the
compressor dispatcher invokes a particular compressor to achieve
the desired compression ratio using the allocated processing and
memory resources. The compressor returns a SigComp message that is
forwarded to the remote SigComp peer.
- A decompressor dispatcher: this is the interface towards the
applications. A SigComp message is received by the decompressor
dispatcher and an instance of the UDVM is invoked. Once the
dispatcher has received the (decompressed) application data it
determines the target application and forward the message to it.
- One or more compressors: the compressors each contain an algorithm
to perform the compression. A compressor receives an (uncompressed)
application message from the compressor dispatcher, compresses the
message, and returns a SigComp message to the compressor
dispatcher. During the compression process, the compressor may
invoke the state handler to restore a previous state or save a new
one. The compressor is responsible for providing the remote
decompressor with suitable UDVM bytecode to reconstruct the
original application message. Within the compressor, the entity
which runs the actual compression algorithm (minus state management
issues) is known as the "encoder".
- One or more decompressors: the decompressors contain the needed
UDVM to perform the decompression. The decompressor receives a
SigComp message from the decompressor dispatcher, decompress the
message, and returns the (decompressed) application message to the
decompressor dispatcher. During the decompression process, the
decompressor may invoke the state handler to restore a previous
state or save a new one.
- State handler: this entity contains enough logic to store and
retrieve states. A state is data that is stored between SigComp
messages: this data can be saved either by a compressor, a
decompressor or an application. The saved state may be used for
(de)compression between a compressor and its peer decompressor.
The state handler is also responsible for asking the application to
grant permission for states to be saved by the state handler. State
parameters and retrieval of states are further described in Chapter
6.
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+-----------------+ +-----------------+
| | | |
| Application 1 |<-+ +->| Application 2 |
| | | | | |
| | | | | |
+-----------------+ | | +-----------------+
| ^ | | | ^
| | Application msgs. | |
| +-------------------------+ |
+-- -- -- -- --| | | - -- -- -- -- -- -- -- |-- -- -- -- --+
| | | +-----------------------+ | |
v v | | | |
| +--------------+ | | +--------------+ |
SigComp | | | | | | SigComp
message | Compressor | | | | Decompressor | message
<-------| dispatcher | | | | dispatcher |<-------
| | | | | | | |
+--------------+ | | +--------------+
| ^ ^ | | ^ ^ |
| | | | | |
| | | | | | | |
| | | | | |
| v | | | v | |
+--------------+ v | +--------------+
| | Compressor 1 | +---------+ |Decompressor 1| |
| |<-->| State |<-->| |
| | (Encoder) | | Handler | | (UDVM) | |
| | +---------+ | |
| +--------------+ | +--------------+ |
| | |
| v | v |
+--------------+ v +--------------+
| | Compressor 2 | +---------+ |Decompressor 2| |
| |<-->| State |<-->| |
| | (Encoder) | | Handler | | (UDVM) | |
| | +---------+ | |
| +--------------+ +--------------+ |
| SigComp layer |
+-- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --+
Figure 1: High-level architectural overview of one SigComp peer.
Note that it is possible to decompress messages from multiple
compressors at different physical locations in a network. The
architecture is designed to prevent data from one compressor
interfering with data from a different compressor. A consequence of
this design choice is that it is difficult for a malicious user to
disrupt decompressor operation by inserting false compressed messages
on the transport.
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The decompressors in Figure 1 should be viewed as containers for
UDVMs; the actual decompressor functionality is handled by invoking
an instance of the UDVM.
Figure 2 gives a more detailed view of a UDVM, including all of the
interfaces between the UDVM and its environment.
+----------------+ +----------------+
| | Request compressed data | |
| |-------------------------------->| |
| |<--------------------------------| |
| | Provide compressed data | |
| | | Dispatcher |
| | | |
| | Output uncompressed data | |
| |-------------------------------->| |
| | | |
| | +----------------+
| UDVM |
| | +----------------+
| | Request state information | |
| |-------------------------------->| |
| |<--------------------------------| |
| | Provide state information | |
| | | State |
| | | Handler |
| | Make state creation request | |
| |-------------------------------->| |
| | Forward capability announcement | |
| | | |
+----------------+ +----------------+
Figure 2: Interfaces between the UDVM and its environment
Note that for simplicity, the UDVM indicates when it requires
additional compressed data or state information using an explicit
instruction. It then pauses and waits for the information to be
supplied before continuing with the next instruction. This prevents
the arrival of more data from interfering with the operation of the
UDVM (e.g. by accidentally overwriting UDVM memory that is currently
in use).
3.1. Requirements on application
From an application perspective the SigComp layer typically 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).
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If the application wishes to mix SigComp messages with other types of
data (e.g. uncompressed data) on the same transport then the
transport must distinguish between the two types of data. For UDP and
TCP this means that a new port will need to be reserved for
compressed data. For example [SIP] uses port 5060 for TCP and port
5061 for TLS/TCP, so it could similarly reserve another port for
SigComp/TCP.
In the interests of security, a new interface is required to the
signaling application in order to leverage the authentication
functions built into the application itself. For each decompressed
message that is accompanied by a state creation request, the state
handler needs to find out whether the application considers the
message to be legitimate. If the decompressed message is considered
to be invalid then the state handler cannot create the requested
state information. This interface is marked on the architecture of
Figure 1.
3.2. Application-defined parameters
When an application invokes SigComp, a number of parameters are
provided by the application to control the maximum size of compressed
messages, the UDVM memory size etc. The two instances of the
application that wish to communicate MUST initially agree on a common
set of values for these parameters.
Note that if a reverse channel is available then SigComp can perform
an internal "capability announcement" to indicate that additional
memory or CPU cycles are available. This means that it is generally
sufficient to set fixed values for each application-defined parameter
(there is no need to provide an external, application-specific
negotiation mechanism).
Each application-defined parameter is described below. Appendix B
discusses how each of the parameters affects SigComp operation in
greater detail, and recommends default values for the parameters.
UDVM_version
The UDVM_version parameter specifies the level of functionality
available at the UDVM. The basic version of the UDVM (Version 0)
is defined in this document.
minimum_compression_ratio
The minimum_compression_ratio parameter prevents the generation of
excessively large SigComp messages. For an n byte uncompressed
message, the corresponding SigComp message must be no larger than
(n / minimum_compression_ratio) rounded down to the nearest byte.
Note that this parameter can be less than 1, (in which case a
certain amount of message expansion is allowed) or 0 (in which case
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no minimum_compression_ratio needs to be met). Any value other than
0 bans the creation of standalone SigComp messages (i.e. messages
that do not contain a compressed application message).
maximum_compressed_size
The maximum_compressed_size parameter limits the size of one
compressed message. SigComp rejects any message larger than the
specified value.
maximum_uncompressed_size
The maximum_uncompressed_size parameter limits the size of one
uncompressed message. SigComp rejects any message larger than the
specified value.
minimum_hash_size
The minimum_hash_size parameter specifies the minimum size of the
state identifier when creating new state information. This value
needs to be sufficiently large to prevent malicious users from
guessing a state identifier by brute force.
overall_memory_size
The overall_memory_size parameter specifies the total number of
bytes in the UDVM memory.
working_memory_start
The working_memory_start parameter specifies the start of the UDVM
memory area that can be modified. Memory addresses below this
value are considered read-only by the UDVM.
working_memory_end
The working_memory_end parameter specifies the end of the UDVM
memory area that can be modified. Memory addresses above this
value are considered read-only by the UDVM.
cycles_per_bit
The cycles_per_bit parameter specifies the number of "CPU cycles"
that can be used to decompress a single bit of data. One CPU cycle
typically corresponds to a single UDVM instruction, although some
of the high-level instructions may require additional cycles.
cycles_per_message
The cycles_per_message parameter specifies the number of additional
CPU cycles made available at the start of a compressed message.
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These cycles can be useful when decompressing algorithms that
upload additional data on a per-message basis, for example a new
set of Huffman codes as with [DEFLATE].
The total number of "CPU cycles" available for each compressed
message is specified by the following formula:
total_cycles = message_size * cycles_per_bit + cycles_per_message
first_instruction
The first_instruction parameter specifies the memory address of the
first instruction to be executed when the UDVM is initialized.
Initial memory contents
When the UDVM is invoked its memory is reset to contents defined by
the application. This code is executed for every SigComp message
(so typically it performs a simple task such as extracting the
first n bytes from the message and interpreting them as a state
identifier).
Initial state
As well as deciding the initial contents of the UDVM memory, the
application can also store useful information in the form of state.
This predefined state is used to offer a range of well-known
decompression algorithms to the compressor, which can choose to
avoid uploading bytecode for a new algorithm if it supports one of
the well-known algorithms. Each item of initial state can be made
mandatory for every instance of the application, or it can be made
optional (in which case support for the relevant state will need to
be advertised before the state can be used).
4. SigComp message flow
This chapter describes the SigComp message flow, including the
initialization, capability announcement and exchange of compressed
messages.
In the architecture of Figure 1, this chapter describes the operation
of the compressor and decompressor dispatcher.
4.1. Message exchange
The local SigComp layer may send compressed data to a remote SigComp
layer, and the local SigComp layer may also receive compressed data.
However, compression in one direction does not necessarily imply
compression in the reverse direction. Furthermore, even in the case
that there are two unidirectional compressed flows between two
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SigComp layer, there is no need to use the same compression algorithm
at both compressors.
4.1.1. Operation for each compressor-decompressor pair
An endpoint that wants to send compressed data to a remote party must
initialize a SigComp layer at the local party prior to its use, so
that the decompressor dispatcher in the remote endpoint assigns a
decompressor to be used and the UDVM is loaded with the compression
algorithm. The process is described in Figure 3.
+--------------+ +--------------+
| | | |
| Endpoint A | | Endpoint B |
| | | |
+--------------+ +--------------+
| |
| |
| SigComp Discovery |
|<------------------------------->|
| |
| SigComp Request |
|-------------------------------->|
| |
| |
| Capabilities Announcement |
|<--------------------------------|
| |
| |
| UDVM Upload |
|-------------------------------->|
| |
| Compressed Messages |
|- - - - - - - - - - - - - - - >|
Figure 3: Compressor-decompressor pair operation
The SigComp discovery mechanism itself is outside the scope of this
specification.
The following three paragraphs specify a message flow for discovering
the capabilities of Endpoint B and (if necessary) uploading a new
decompression algorithm to this endpoint. Note that if an
application-defined default algorithm is available at all endpoints
then Endpoint A can immediately begin to compress messages and the
following stages may be skipped.
Endpoint A may send a SigComp Request message to Endpoint B. The
SigComp Request message is a request to initialize a SigComp layer
for the application at Endpoint B and to know B's capabilities, i.e.
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the parameters in Section 3.2. If Endpoint A uses a UDVM
decompression algorithm which only requires the default application-
defined parameters, then this step may be omitted.
Endpoint B SHOULD answer the SigComp Request message with a
Capabilities Announcement message, which includes the SigComp
parameters that constrain the operation of the UDVM.
Once Endpoint A has received the Capabilities Announcement message,
it chooses a suitable compression algorithm that B is able to
decompress and sends a message containing the UDVM decompression
algorithm (unless Endpoint B already has the algorithm available).
At this point, Endpoint B contains enough information to start
decompressing messages received from the application at Endpoint A.
4.1.2. Bi-directional initialization
In scenarios where both endpoints decide to compress data in each of
the directions, a double initialization process must be done prior to
start with the normal operation.
The double initialization process is comprised of two of the above
initialization processes, one in each direction, as described in
Section 4.1.1.
4.2. SigComp message format
The basic SigComp message consists of a block of UDVM bytecode, the
first n bytes of which are interpreted as a state identifier that
accesses some previously stored state information.
This state information comprises the decompression algorithm that
will be used to decompress the remainder of the SigComp message, as
well as any needed additional information (e.g. one or more
previously received messages if dynamic compression is in use).
A decompressor dispatcher MUST be able to separate two SigComp
messages; in the case of UDP a SigComp message corresponds exactly to
one UDP datagram. For TCP each 0xFFFF delimiter is followed by a new
SigComp message.
The format of the basic SigComp message is given in Figure 4:
[Editors' Note: Specific SigComp messages such as the SigComp
Request, the Capabilities Announcement and the UDVM Upload may need
to be defined. A state identifier could be reserved for each specific
type of message, just as state identifiers are reserved for each
well-known algorithm.]
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0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| |
: state_identifier (n-bytes) :
| |
+---+---+---+---+---+---+---+---+
| |
: Remaining UDVM bytecode :
| |
+---+---+---+---+---+---+---+---+
Figure 4: Basic SigComp message
Note that n is the application-defined parameter minimum_hash_size,
an example value for which is given in Appendix B.
Note also that the state information is loaded into the UDVM memory
and executed as defined by the following piece of UDVM bytecode:
reserve state_identifier (n)
INPUT-BYTECODE (n, state_identifier, fail)
STATE-EXECUTE (state_identifier, n)
:fail
DECOMPRESSION-FAILURE
If the UDVM memory is initialized containing the above bytecode then
the state identifier will automatically be extracted from the SigComp
message and the corresponding state information will be accessed and
executed.
4.3. Interfaces to and from the dispatcher
Once the remote party has initialized the SigComp layer at the local
party, the decompressor dispatcher is ready to receive compressed
messages from a particular remote party, decompress those messages,
and pass them onto the application.
The application provides the compressor dispatcher with messages to
be compressed. The encoder in the compressor compresses messages in
such a way that the remote decompressor with the UDVM can decompress
the data correctly (providing that the compressed data is not lost or
damaged during transport).
When a message is to be compressed, the compressor selects the state
to use. The identifier of the used state MUST be sent along with the
compressed message to the remote decompressor. The SigComp message is
then passed on to underlying layers for transport to the remote
decompressor.
[Editor's Note: State identifiers will need to be reserved for well-
known decompression algorithms, and an additional state identifier
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will be needed to indicate that the algorithm is being uploaded as
part of the compressed message.]
Upon the arrival of a SigComp message the decompressor dispatcher
invokes the decompressor that loads the UDVM with the indicated
state. The message is then decompressed by the UDVM, returned to the
decompressor dispatcher, and possibly passed on to the receiving
application.
Note that when the UDVM is invoked it does not receive any compressed
data by default, but instead requests new data explicitly using a
specific instruction. Therefore, the dispatcher is responsible for
buffering each SigComp message and passing the data to the UDVM when
it is requested.
Uncompressed data is also outputted by the UDVM using a specific
instruction. Depending on the particular application, the dispatcher
decides whether to forward a partially decompressed message
immediately to the application, or to buffer and wait for a complete
message to be successfully decompressed.
For a stream-based transport, the dispatcher delimits messages by
parsing the compressed data stream for instances of 0xFF and taking
the following actions:
Occurs in data stream: Action:
0xFFFF Delimit compressed message
0xFF00 Replace with 0xFF
0xFF01 - 0xFFFE Decompression failure
The reserved character 0xFF00 is useful for byte stuffing (if a
compression algorithm generates compressed data containing the
character 0xFF then it should be replaced by the character 0xFF00 to
avoid accidentally inserting a message delimiter into the compressed
data stream).
5. SigComp compressor
An important feature of SigComp is that if two endpoints cannot agree
on a common algorithm with which to send and receive data, it is
possible for the compressor to upload bytecode for its own choice of
algorithm to the decompressor. In particular this means that it is
not necessary to force all compressors to use the same default
algorithm; instead each implementer has the freedom to pick one of
the predefined algorithms or to upload their own if needed.
The overall requirement placed on the compressor is that of
transparency, i.e. the compressor MUST NOT send bytecode which cause
the UDVM to incorrectly decompress a given message.
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The following more specific requirements are also placed on the
compressor (they can be considered particular instances of the
transparency requirement):
* It is RECOMMENDED that the compressor supply a CRC over the
uncompressed message to ensure that successful decompression has
occurred. A UDVM instruction is provided to verify this CRC.
* If the transport is message-based then the compressor MUST
preserve the boundaries between messages.
* If the transport is stream-based but the application defines its
own internal message boundaries, then the compressor SHOULD
preserve the boundaries between messages by using the "end-of-
message" character 0xFFFF reserved by SigComp.
* The compressor MUST achieve the minimum_compression_ratio and
MUST ensure that the message can be decompressed using no more
than the resources available at the remote decompressor.
The reason for preserving the message boundaries over a stream-based
transport is that damage to one compressed message does not affect
the decompression of subsequent messages. Moreover, the application
typically vetoes state creation requests on a per-message basis.
Note that SigComp also reserves the character 0xFF00 over a stream-
based transport, and replaces every instance of 0xFF00 with 0xFF
before decompressing the data. This ensures that arbitrary
compression algorithms can be used over a stream-based transport,
provided that every instance of 0xFF in the compressed data stream is
identified and replaced with 0xFF00. This "byte-stuffing" scheme
prevents the compression algorithm from inserting a message delimiter
into the data stream where one is not required.
5.1. Types of compression algorithm
Any of the following classes of compression algorithm may be useful
for particular applications:
* Generic compressor (for example [DEFLATE] or a similar
algorithm).
* Protocol-aware compressor offering excellent performance for
one particular type of data (for example the text messages
generated by [SIP]).
* Hybrid compressor with similar performance to [DEFLATE] for
generic data and superior performance for certain types of data.
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Provided that the uncompressed data can be reconstructed at the UDVM
using the available memory and CPU cycles, implementers have freedom
to use a compression algorithm of their choice.
5.2. Supplying bytecode to the UDVM
A compressor MUST be certain that compressed data can be decompressed
before the data is to be sent, i.e. the UDVM instructions for
decompression MUST be available at the peer decompressor. Several
options exist for ensuring that this bytecode is available:
1. Each SigComp message sent from the compressor contains the
necessary UDVM instructions for decompression.
2. By setting up a reliable connection, such as a TCP connection,
between a compressor and its peer decompressor the UDVM
instructions can be transferred and saved as state.
3. If there are predefined UDVM codes for well-known algorithms, a
compressor only needs to send the state identifier of that UDVM
decompression algorithm code to its peer decompressor. The
decompressor can then populate the UDVM locally.
In order to save delay for "time-critical" sessions, the UDVM
instructions should be uploaded prior to any initiation of "time-
critical" sessions.
5.3. 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 a high minimum_compression_ratio has been set by the
application). In this case a "compression failure" is called.
Reasons for compression failure include the following:
* A compressed or uncompressed message exceeds the maximum size
defined by the application.
* The minimum_compression_ratio cannot be achieved for a certain
message.
* Insufficient resources are available at the compressor or at the
remote decompressor.
If a compression failure occurs when compressing a message then the
compressor informs the dispatcher and takes no further action. The
dispatcher can then report this failure to the application.
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6. State handling and capability announcement
This chapter defines the behavior of the SigComp state handler. The
function of the state handler is to retain information between
successive 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. Storing and retrieving state
To provide security against the malicious insertion of false
compressed data, the UDVM memory is reset after each compressed
message. This ensures that damaged compressed 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 stored in previous messages. For this reason it is
possible to create "state" information 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. State can only
be created when a complete message has been successfully
decompressed, and the state handler MUST veto a state creation
request if instructed by the application based on the contents of the
decompressed message. This is especially useful if the application
has an authentication mechanism that can be applied to determine
whether the decompressed data is legitimate.
Furthermore, a compressor can only access previously created state
information by providing an [MD5] hash of the state to be accessed.
The advantage of using a secure hash to access state information is
that it is very difficult to guess the correct hash value without
complete knowledge of the state being accessed.
Also note that state is not deleted when it is accessed. So even if a
malicious user manages to access state information, subsequent
messages compressed relative to this state can still be successfully
decompressed. Instead, the state handler is responsible for deleting
state information once it determines that the state will no longer be
needed.
Each item of state stores the following information:
Name: Type of data:
state_identifier 16-byte value
state start 2-byte value
state_instruction 2-byte value
state length 2-byte value
state_value String of bytes
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The state_identifier must be supplied to retrieve an item of state
from the state handler. State can be accessed using the UDVM
instructions STATE-REFERENCE and STATE-EXECUTE, and can be created
using the END-MESSAGE instruction.
The state_value is a byte string that contains the actual value that
is copied from/to the UDVM memory. The state_length specifies the
number of bytes contained within state_value, and state_start gives
the UDVM memory address from/to which the state_value is copied.
Finally, state_instruction specifies the memory address of the next
UDVM instruction to execute when state is accessed.
The kind of information which is included in the state_value is up to
a particular compressor and the uploaded instructions in the remote
UDVM. However a compressor MUST not use a state that is not known to
be established at the remote decompressor.
6.2. Guidelines for saving and deleting states
[Editors' Note: Do we need something more?]
A decompressor SHOULD NOT delete a state before it is confident
enough that the state is not used by a peer compressor any more.
6.3. Capability announcement
The capability announcement information is used to modify the value
of certain application-defined parameters. Since these parameter
values are saved between SigComp messages, they are considered to be
part of the overall state and hence are supplied from the UDVM to the
state handler.
If the state handler rejects a state creation request then the
accompanying capability announcements MUST be rejected also.
If the unidirectional version of SigComp is running then the
capability announcement information is automatically discarded by the
state handler.
The following block of parameters is passed to the state handler
using the appropriate UDVM instruction (currently the END-MESSAGE
instruction):
[Editors' Note: The capability announcement block is yet to be
finalized. More items may be added in future.]
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Total length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| UDVM_version |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| overall_memory_size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cycles_per_bit |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cycles_per_message |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Requested feedback length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
: Requested feedback :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Returned feedback length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
: Returned feedback :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: Capability announcement block
Note that the three 2-byte length fields specify the lengths of the
entire capability announcement block, the requested feedback data and
the returned feedback data respectively. As usual, MSBs are stored
preceding LSBs.
The remaining items of data are explained in greater detail below:
6.3.1. UDVM version
The first 2 bytes of the capability announcement block specify
whether only the basic version of the UDVM is available, or whether
an upgraded version of the UDVM is available offering additional
instructions etc.
The basic version of the UDVM is Version 0, which is the version
described in this document. Upgraded versions MUST be backwards-
compatible with the basic version in the following sense:
* If some UDVM bytecode reaches the END-MESSAGE or DECOMPRESSION-
FAILURE instructions when running on Version 0 of the UDVM, then
the upgraded version MUST run the bytecode in an identical
manner.
This condition ensures that all bytecode that is valid for Version 0
of the UDVM will continue to be valid for upgraded versions of the
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UDVM. However, bytecode that is invalid on Version 0 of the UDVM
(i.e. bytecode that produces a decompression failure that is not
manually triggered) may become valid on upgraded versions.
The simplest way to upgrade the UDVM in a backwards-compatible manner
is to add additional UDVM instructions, as this will not affect the
operation of existing UDVM bytecode.
6.3.2. Memory size and CPU cycles
The next 6 bytes of data specify new values for the application-
defined parameters overall_memory_size, cycles_per_bit and
cycles_per_message.
Note that this data can only be used to increase the amount of
resources available at the remote UDVM. If the data specifies a
parameter value that is smaller than the value already possessed by
the state handler, the parameter keeps its original value (i.e. the
capability announcement data for this parameter is simply ignored).
In particular, only allowing the parameter values to increase means
that the announcement mechanism is robust against message loss or
reordering.
The parameters can only be restored to their original values if reset
or renegotiated by the application.
6.3.3. Requested feedback
The requested feedback data is provided to the UDVM by the remote
compressor. By providing this data, the remote compressor is
requesting that the data be returned to the compressor via a reverse
channel (assuming that one is present).
The compressor in control of the reverse channel SHOULD return this
data by uploading it into the returned feedback data block at the
remote UDVM. The data will then be passed back to the remote
compressor as explained below.
6.3.4. Returned feedback
The returned feedback data is an item of feedback data that has
successfully returned from the remote entity. This data is passed to
the local compressor (assuming that permission is granted by the
application), which can make use of it as it wishes.
Note that a compressor MUST only populate the returned feedback data
with the bit-exact contents of a requested feedback data block
previously provided to it.
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7. 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 unlimited
flexibility when choosing how to compress a given item of data.
Rather than picking one of a small number of pre-negotiated
compression algorithms, the 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 UDVM bytecode.
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 pre-
programmed decompression algorithm.
This chapter describes some basic features of the UDVM, including the
memory allocation, well-known variables and instruction parameters.
7.1. UDVM memory allocation
The memory available to the UDVM is partitioned into a number of
sections, providing space for program code, variables and
miscellaneous data:
<----- working_memory_size ------>
| Fixed values | Variables | Miscellaneous data | Program code |
+--------------+-----------+--------------------+--------------+
<--------------------- overall_memory_size -------------------->
Figure 6: Memory allocation in the UDVM
Recall that the amount of memory available to the UDVM is specified
by the application-defined parameters overall_memory_size,
working_memory_start and working_memory_end. Note that all of these
parameters are initialized by the application, but can be
renegotiated on the fly using the capabilities announcement
mechanism.
The memory area from Address (working_memory_start) to Address
(working_memory_end) inclusive can be used to store arbitrary data
(variables, program code, Huffman codes etc.). UDVM instructions are
allowed to read from or write to any address in this memory area.
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The first part of this memory area is typically used to store a
number of 2-byte variables. UDVM instructions can reference these
variables using a special instruction parameter as described in
Section 7.3.
The memory area from Address 0 to Address (working_memory_start - 1)
and from Address (working_memory_end + 1) to Address
(overall_memory_size - 1) inclusive is write-protected, so UDVM
instructions can read from this memory area but cannot write to it.
This memory area is intended for storing UDVM bytecode that can be
compiled.
Any attempt to read memory addresses beyond the overall memory size
or to write to memory addresses outside the working memory area MUST
cause a decompression failure (see Section 8.3).
The first part of the write-protected UDVM memory is intended for
storing variables whose values no longer need to be modified. The
second part of the write-protected memory is intended for storing
program code including UDVM instructions and their associated
parameters. Note that if an instruction references a variable that
has been write-protected, the compiled version of the instruction
will typically run faster than if the referenced variable lies in the
working memory area.
7.2. Well-known variables
The first few variables in the UDVM memory have special tasks, for
example specifying the location of the stack used by the CALL and
RETURN instructions. Each of these well-known variables is a 2-byte
integer.
The following list gives the name of each well-known variable and the
memory address at which the variable can be found:
Name: Starting memory address:
byte_copy_left 0
byte_copy_right 2
stack_location 4
The MSBs of each variable are always stored before the LSBs. So, for
example, the MSBs of stack_location are stored at Address 4 whilst
the LSBs are stored at Address 5.
The use of each well-known variable is described in the following
sections of the document.
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7.3. Instruction parameters
Each of the UDVM instructions is followed by 0 or more bytes
containing the parameters required by the instruction.
To reduce the code size of a typical UDVM program, each parameter for
a UDVM instruction is compressed using variable-length encoding. The
aim is to store more common parameter values using fewer bits than
rarely occurring values.
Three different types of parameter are available: the literal, the
reference and the multitype. The parameter types that follow each
UDVM instruction are specified in Chapter 9.
The UDVM bytecode for each parameter type is illustrated in Figure 7
to Figure 9, 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
parameter. In other cases it is taken to be a memory address at which
the 2-byte parameter value can be found (MSBs found at the specified
address, LSBs found at the following address). The latter case is
denoted by memory[X] where X is the address and memory[X] is the 2-
byte value starting at Address X.
The simplest parameter type is the literal (#), which encodes a
constant integer from 0 to 65535 inclusive. A literal parameter may
require between 1 and 3 bytes depending on its value.
Bytecode: Parameter value: Range:
0nnnnnnn N 0 - 127
10nnnnnn nnnnnnnn N 0 - 16383
11000000 nnnnnnnn nnnnnnnn N 0 - 65535
Figure 7: Bytecode for a literal (#) parameter
The second parameter type is the reference ($), which is always used
to access a 2-byte value located elsewhere in the UDVM memory. The
bytecode for a reference parameter is decoded to be a constant
integer from 0 to 65535 inclusive, which is interpreted as the memory
address containing the actual value of the parameter.
Note that reference parameters can always take values from 0 to 65535
inclusive, as they reference 2-byte values.
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Bytecode: Parameter value: Range:
0nnnnnnn memory[2 * N] 0 - 65535
10nnnnnn nnnnnnnn memory[2 * N] 0 - 65535
11000000 nnnnnnnn nnnnnnnn memory[N] 0 - 65535
Figure 8: Bytecode for a reference ($) parameter
The third kind of parameter is the multitype (%), which can be used
to encode both actual values and memory addresses. The multitype
parameter also offers efficient encoding for small integer values
(both positive and negative) and for powers of 2.
Bytecode: Parameter 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 9: Bytecode for a multitype (%) parameter
7.4. 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.
In general, the string of bytes is copied in ascending order of
memory address. So if a byte is copied from/to Address n then the
next byte is copied from/to Address n + 1. As usual, if a byte is
read from an address beyond the overall memory size or is written to
an address outside the working memory area then decompression failure
occurs.
Note however that if a byte is copied from/to the memory address
specified in byte_copy_right, the byte copy operation continues by
copying the next byte from/to the memory address specified in
byte_copy_left. This is useful for setting up a "circular buffer"
within the UDVM memory.
Note that the string of bytes is copied on a purely byte-by-byte
basis. 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.
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Equally, it is possible for a byte copying operation to overwrite the
instruction that called 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).
8. Decompressing a SigComp message
This chapter lists the steps involved in the decompression of a
single SigComp message.
8.1. Invoking the UDVM
Whenever the dispatcher receives a message to be decompressed, it
invokes a new instance of the UDVM. The overall_memory_size and
initial contents of the UDVM memory are initialized using the
corresponding application-defined parameters. The following steps are
then taken:
1.) The number of remaining CPU cycles is set equal to the
application-defined parameter cycles_per_message.
Notes:
The amount of compressed data available to the UDVM is exactly one
compressed message. If the transport is stream-based then SigComp
uses the reserved byte string 0xFFFF to delimit the compressed
messages: the dispatcher takes the data between a pair of neighboring
reserved byte strings to be a single compressed message. The reserved
byte string itself is not considered to be part of the compressed
message.
The compressed data is not provided to the UDVM by default. Instead,
the UDVM requests compressed data using the INPUT instructions
(useful when running over a stream-based transport since there is no
need to wait for the entire compressed message before decompression
can begin). Note that in particular, this means that the application
MUST define the initial contents of the UDVM memory to contain at
least one INPUT instruction. See Section 4.2 for an example of how
the application might initialize the UDVM memory.
The dispatcher MUST NOT make more than one compressed message
available to a given instance of the UDVM. In particular, the
dispatcher MUST NOT concatenate two messages to form a single
compressed message. This is because compressed messages are typically
padded with trailing zero bits so that they are a whole number of
bytes long. Concatenating two messages would cause these padding bits
to be incorrectly interpreted as compressed data.
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2.) Next, the instructions contained within the UDVM memory are
executed beginning at the address specified in first_instruction.
Notes:
The instructions are executed 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 8.3).
3.) Each time an instruction is executed the number of available
CPU cycles is decreased by the amount specified in Chapter 9.
Additionally, if the UDVM requests n bits of compressed data (using
one of the INPUT instructions) then the number of available CPU
cycles is increased by n * cycles_per_bit.
Notes:
This means that the total number of CPU cycles available for
processing a compressed message is given by the formula:
total_cycles = cycles_per_message + message_size * 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.
4.) The UDVM stops executing instructions when it encounters an
END-MESSAGE instruction or if decompression failure occurs.
Notes:
The UDVM passes uncompressed data to the dispatcher using the OUTPUT
instruction. The OUTPUT instruction can be used to output a partially
decompressed message; it is a dispatcher decision whether to use the
data immediately or whether to buffer and wait until the entire
message has been decompressed.
The UDVM passes state creation requests to the state handler using
the END-MESSAGE instruction. This means that it is only possible to
make a state creation request once the message has been decompressed,
which is necessary since the application typically determines the
validity of these requests based on the contents of the decompressed
message.
8.2. Successful decompression
The END-MESSAGE instruction indicates that the compressed message has
been successfully decompressed and passed to the dispatcher. Note
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that the actual uncompressed message is outputted beforehand using
the OUTPUT instruction; this allows the UDVM to output each part of
the message to the dispatcher as soon as it has been decompressed.
The END-MESSAGE instruction provides two additional pieces of
information to the state handler: the state creation request and the
capability announcement block. The state creation request mechanism
is discussed below:
The UDVM may optionally save part of its memory for retrieval by
later messages. However to prevent malicious storage of a large
amount of unnecessary state information, the application itself MUST
give permission before any state can be created. The state handler
typically makes a decision on whether state can be created based on
the contents of the decompressed message, particularly if the message
contains authentication data that can verify whether or not the
sender is legitimate.
The END-MESSAGE instruction requests the creation of state using the
parameters state start and state length, which together denote a byte
string state_value. Provided that the application gives permission,
state_value is byte copied from the UDVM memory (obeying the rules of
Section 7.4) and stored together with a 16-byte state identifier that
can be used to access the state by a later compressed message.
To provide security against malicious access, the identifier for any
item of state created by the UDVM is derived from the [MD5] hash of
the state_value to be stored. The state identifier is constructed by
taking the 16-byte [MD5] hash and replacing all but the first
hash_length most significant bytes with zeroes. Note that if
hash_length is 16 then the unmodified [MD5] hash is the state
identifier. Decompression failure occurs if hash_length is less than
the application-defined parameter minimum_hash_size or greater than
16.
If a state identifier already exists (hash collision occurs), the
decompressor should check whether the requested state is identical to
the established state, and count the state creation request as
successful if this is the case.
If not then the state creation request is unsuccessful. The existing
state MUST NOT be replaced with the requested state to be saved. This
is to avoid the situation where a compressed message cannot be
decompressed because a needed item of state has been replaced
(possibly by a malicious sender).
Each item of state stores the following information (accessed by the
state_identifier):
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Name: Type of data:
state_identifier 16-byte value
state start 2-byte value
state_instruction 2-byte value
state length 2-byte value
state_value String of bytes
Note that state_start, state_length and state_instruction are all
parameters from the END-MESSAGE instruction, whereas state_identifier
and state_value are created as specified above.
This state can subsequently be accessed by using the STATE-REFERENCE
and STATE-EXECUTE instructions (by providing the correct state
identifier).
8.3. 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.
Reasons for decompression failure include the following:
* A compressed or uncompressed message exceeds the maximum size
defined by the application.
* The UDVM exceeds the available CPU cycles for decompressing a
message.
* The UDVM attempts to read a memory address beyond the overall
memory size, or to write into a memory address outside the
working memory area.
* An unknown instruction type is encountered.
* An unknown parameter type is encountered.
* 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).
* The UDVM attempts to access non-existent state.
* 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
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decompression failure. In general a dispatcher SHOULD discard the
compressed message and any decompressed data that has been outputted.
9. UDVM instruction set
The UDVM currently understands 28 instructions, chosen to support the
widest possible range of compression algorithms with the minimum
possible overhead.
Figure 10 lists the different instructions and the bytecode values
used to store the instructions at the UDVM. The cost of each
instruction in CPU cycles is also given:
Instruction: Bytecode value: Cost in CPU cycles:
DECOMPRESSION-FAILURE 0 1
AND 1 1
OR 2 1
NOT 3 1
ADD 4 1
SUBTRACT 5 1
MULTIPLY 6 1
DIVIDE 7 1
LOAD 8 1
MULTILOAD 9 1 + n
WORKING-MEMORY 10 1
COPY 11 1 + length
COPY-LITERAL 12 1 + length
COPY-OFFSET 13 1 + length + offset
JUMP 14 1
COMPARE 15 1
CALL 16 1
RETURN 17 1
SWITCH 18 1 + n
CRC 19 1 + length
END-MESSAGE 20 1 + state length
OUTPUT 21 1 + output_length
NBO 22 1
INPUT-BYTECODE 23 1 + length
INPUT-FIXED 24 1
INPUT-HUFFMAN 25 1 + n
STATE-REFERENCE 26 1 + state_length
STATE-EXECUTE 27 1 + state length
Figure 10: UDVM instructions and corresponding bytecode values
Each UDVM instruction costs a minimum of 1 CPU cycle. Certain high-
level instructions may cost additional cycles depending on the value
of one of the instruction parameters.
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The only exception when calculating the number of CPU cycles is that
the STATE-EXECUTE instruction takes (1 + state_length) cycles even
though it does not have a state_length parameter; instead the value
of state length is provided by the state handler as part of the state
being accessed.
All instructions are stored as a single byte to indicate the
instruction type, followed by 0 or more bytes containing the
parameters required by the instruction. The instruction specifies
which of the three parameter types of Section 7.3 is used in each
case. For example, the ADD instruction is followed by two parameters
as shown below:
ADD ($parameter_1, %parameter_2)
When converted into bytecode the number of bytes required by the ADD
instruction depends on the size of each parameter value, and whether
the second (multitype) parameter contains the parameter value itself
or a memory address where the actual value of the parameter can be
found.
The instruction set available for the UDVM offers a mix of low-level
and high-level instructions. The high-level instructions can all be
emulated using the low-level instructions provided, but given a
choice it is generally preferable to use a single instruction rather
than a large number of general-purpose instructions. The resulting
bytecode will be more compact (leading to a higher overall
compression ratio) and decompression will typically be faster because
the implementation of the compression-specific instructions can be
optimized for the UDVM.
Each instruction is explained in more detail below:
9.1. Bit manipulation instructions
The AND, OR and NOT instructions provide simple bit manipulation on
2-byte words.
AND ($parameter_1, %parameter_2)
OR ($parameter_1, %parameter_2)
NOT ($parameter_1)
After the operation is complete, the value of the first parameter is
overwritten with the result. Note that since this parameter is a
reference, the memory address specified by the parameter is always
overwritten and not the parameter itself.
9.2. Arithmetic instructions
The ADD, SUBTRACT, MULTIPLY and DIVIDE instructions perform
arithmetic on 2-byte words.
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ADD ($parameter_1, %parameter_2)
SUBTRACT ($parameter_1, %parameter_2)
MULTIPLY ($parameter_1, %parameter_2)
DIVIDE ($parameter_1, %parameter_2)
After the operation is complete, the first parameter is overwritten
with the result.
Note that in all cases the arithmetic operation is performed modulo
2^16. So for example, subtracting 1 from 0 gives the result 65535.
For the SUBTRACT instruction the second parameter is subtracted from
the first. Similarly, for the DIVIDE instruction the first parameter
is divided by the second parameter. Note that if the second parameter
does not divide exactly into the first parameter then the remainder
is ignored.
9.3. Memory management instructions
The following instructions are used to manipulate the UDVM memory.
Bytes can be copied from one area of memory to another, and areas of
memory can be write-protected to make it easier for UDVM code to be
compiled.
9.3.1. LOAD
The LOAD instruction sets a 2-byte variable to a certain specified
value. The format of a LOAD instruction is as follows:
LOAD (%address, %value)
The first parameter specifies the starting address of the 2-byte
variable, whilst the second parameter specifies the value to be
loaded into this variable. As usual, MSBs are stored before LSBs in
the UDVM memory.
9.3.2. MULTILOAD
The MULTILOAD instruction sets a contiguous block of 2-byte variables
to specified values.
MULTILOAD (%address, #n, %value_0, ..., %value_n-1)
The first parameter specifies the starting address of the contiguous
variables, whilst the parameters value_0 through to value_n-1 specify
the values to load into these variables (in the same order as they
appear in the instruction).
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9.3.3. WORKING-MEMORY
The WORKING-MEMORY instruction is used to prevent part of the UDVM
memory from being modified. This can be very useful when offering
UDVM code for compilation.
WORKING-MEMORY (%memory_start, %memory_end)
The parameters memory_start and memory_end specify the new working
memory area for the UDVM. These parameters replace the application-
defined parameters working_memory_start and working_memory_end, but
only while the current message is being decompressed. When a new
instance of the UDVM is invoked the working memory area is set by the
original application-defined parameters.
If memory_end < memory_start, or if the parameters reference a memory
address beyond the overall UDVM memory size, then decompression
failure occurs.
After the WORKING-MEMORY instruction has been encountered, the only
way to write into UDVM memory within the protected region is to
cancel the protection using another WORKING-MEMORY instruction (or to
invoke a new instance of the UDVM).
9.3.4. 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 parameter specifies the memory address of the first byte
in the string to be copied, and the length parameter specifies the
number of bytes to be copied.
The destination parameter gives the address to which the first byte
in the string will be copied.
Note that byte copying is performed as per the rules of Section 7.4.
9.3.5. 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, the destination parameter is replaced with the
memory address immediately following the address to which the final
byte was copied. If the final byte was copied to the memory address
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specified in byte_copy_right, the destination parameter is set to the
memory address specified in byte_copy_left.
9.3.6. 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 parameter is given instead of a position
parameter.
To derive a suitable position parameter, 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.
The COPY-OFFSET instruction then behaves as a COPY-LITERAL
instruction, taking the position parameter to be the last memory
address reached in the above step.
9.4. 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 all of the instructions give
the memory addresses in the form of deltas relative to the memory
address of the instruction. The actual memory address is calculated
as follows:
memory_address = (memory_address_of_instruction + delta) modulo 2^16
Note that certain I/O instructions (see Section 9.5) can also alter
program flow.
9.4.1. JUMP
The JUMP instruction moves program execution to the specified memory
address.
JUMP (%delta)
Note that if the address (specified as a delta from the address of
the JUMP instruction) lies beyond the overall UDVM memory size then
decompression failure occurs.
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9.4.2. COMPARE
The COMPARE instruction compares two parameters and then jumps to one
of three specified memory addresses depending on the result.
COMPARE (%parameter_1, %parameter_2, %delta_1, %delta_2, %delta_3)
If parameter_1 < parameter_2 then the UDVM continues instruction
execution at the (relative) memory address specified by delta 1. If
parameter_1 = parameter_2 then it jumps to the address specified by
delta_2. If parameter_1 > parameter_2 then it jumps to the address
specified by delta_3.
9.4.3. CALL and RETURN
The CALL and RETURN instructions provide support for compression
algorithms with a nested structure.
CALL (%delta)
RETURN
The CALL and RETURN instructions make use of a stack of 2-byte
variables stored at the memory address specified by the well-known
variable stack_location. The stack contains the following variables:
Name: Starting memory address:
stack_free stack_location
stack[0] stack_location + 2
stack[1] stack_location + 4
stack[2] stack_location + 6
: :
The MSBs of these variables are stored before the LSBs in the UDVM
memory.
When the UDVM reaches a CALL instruction, it finds the memory address
of the instruction immediately following the CALL instruction and
copies this 2-byte value into stack[stack_free] ready for later
retrieval. It then increases stack_free by 1 and continues
instruction execution at the (relative) memory address specified by
the parameter.
When the UDVM reaches a RETURN instruction it decreases stack_free by
1, and then continues instruction execution at the byte position
stored in stack[stack_free].
If the variable stack_free is ever increased beyond 65535 or
decreased below 0 then a bad compressed message has been received and
decompression failure occurs (see Section 8.3).
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Decompression failure also occurs if one of the above instructions is
encountered and the value of stack_location is smaller than 6 (this
prevents the stack from overwriting the well-known variables).
9.4.4. SWITCH
The SWITCH instruction performs a conditional jump based on the value
of one of its parameters.
SWITCH (#n, %j, %delta_0, %delta_1, ... , %delta_n-1)
When a SWITCH instruction is encountered the UDVM reads the value of
j. It then continues instruction execution at the (relative) address
specified by delta j.
If j specifies a value of n or more, a bad compressed message has
been received and decompression failure occurs.
9.4.5. CRC
The CRC instruction verifies a string of bytes using a 2-byte CRC.
CRC (%value, %position, %length, %delta)
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 [RFC-1662].
The position and length parameters define the string of bytes over
which the CRC is evaluated. Byte copying rules are enforced as per
Section 7.4.
Important note: Since a CRC calculation is always performed over a
bitstream, for interoperability it is necessary to define the order
in which bits are supplied within each individual byte. In this case
the MSBs of the byte MUST be supplied to the CRC calculation before
the LSBs.
The value parameter contains the expected integer value of the 2-byte
CRC. If the calculated CRC matches the expected value then the UDVM
continues at the following instruction. Otherwise the UDVM jumps to
the (relative) memory address specified by delta.
9.5. 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.
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9.5.1. END-MESSAGE
The END-MESSAGE instruction successfully terminates the UDVM and
passes state information to the state handler.
END-MESSAGE (%hash_length, %state_start, %state_length,
%state_instruction, %capability_announcement_location)
The actions taken by the UDVM upon encountering the END-MESSAGE
instruction are described in Section 8.2. Note also that the
capability_announcement_location parameter points to the starting
memory address of the capability announcement block of Section 6.3.
9.5.2. DECOMPRESSION-FAILURE
The DECOMPRESSION-FAILURE instruction triggers a manual decompression
failure. This is useful if the UDVM program discovers that it cannot
successfully decompress the message (e.g. by using the CRC
instruction).
This instruction has no parameters.
9.5.3. OUTPUT
The OUTPUT instruction provides successfully decompressed data to the
dispatcher.
OUTPUT (%output_start, %output_length)
The parameters 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 appends a byte string to
the end of the data 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 7.4.
Decompression failure occurs if the cumulative number of bytes
provided to the dispatcher exceeds the application-defined parameter
maximum_uncompressed_size.
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.
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9.5.4. NBO
The NBO instruction modifies the order in which compressed bits are
passed to the UDVM.
As the INPUT-FIXED and INPUT-HUFFMAN instructions read individual
bits from within a byte, to avoid ambiguity it is necessary to define
the order in which these bits are read. The default operation is to
read the MSBs before the LSBs, but if the NBO instruction is
encountered then the LSBs are read before the MSBs. Both cases are
illustrated below:
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
Default operation After NBO instruction
The NBO instruction can only be used before bitwise compressed data
is passed to the UDVM. Therefore, a decompression failure occurs if
it is encountered after an INPUT-FIXED or an INPUT-HUFFMAN
instruction has been used.
9.5.5. INPUT-BYTECODE
The INPUT-BYTECODE instruction requests a certain number of bytes of
compressed data from the dispatcher.
INPUT-BYTECODE (%length, %destination, %delta)
The length parameter indicates the requested number of bytes of
compressed data, and the destination parameter specifies the starting
memory address to which they should be copied. Byte copying is
performed as per the rules of Section 7.4.
If the instruction requests data that lies beyond the end of the
compressed message, no data is returned. Instead the UDVM moves
program execution to the memory address specified by the formula
(memory_address_of_INPUT-BYTECODE_instruction + delta) modulo 2^16.
The INPUT-BYTECODE instruction can only be used before bitwise
compressed data is passed to the UDVM. Therefore, a decompression
failure occurs if it is encountered after an INPUT-FIXED or an INPUT-
HUFFMAN instruction has been used.
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9.5.6. INPUT-FIXED
The INPUT-FIXED instruction requests a certain number of bits of
compressed data from the dispatcher.
INPUT-FIXED (%length, %destination, %delta)
The length parameter indicates the requested number of bits. If this
parameter does not lie between 1 and 16 inclusive then a
decompression failure occurs.
The destination parameter 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. Under
default operation the MSBs of this integer are provided first, but if
an NBO instruction has been executed then the LSBs are provided
first.
If the instruction requests data that lies beyond the end of the
compressed message, no data is returned. Instead the UDVM moves
program execution to the memory address specified by the formula
(memory_address_of_INPUT-FIXED_instruction + delta) modulo 2^16.
9.5.7. INPUT-HUFFMAN
The INPUT-HUFFMAN instruction requests a variable number of bits of
compressed data from the 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
parameters plus n additional sets of parameters. Every additional set
contains four parameters as shown below:
INPUT-HUFFMAN (%destination, %delta, #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 by
the UDVM. If bits_1 = 0 or (bits_1 + ... + bits_n) > 16 then
decompression failure occurs.
In all other cases, the behavior of the INPUT-HUFFMAN instruction is
defined below:
1.) Set j = 1.
2.) Request an additional bits_j compressed bits. Interpret the
total (bits_1 + ... + bits_j) bits of compressed data requested so
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far as an integer H, with the first bit to be supplied as the MSB and
the last bit to be supplied as the LSB (note that this is always the
case, independently of whether the NBO instruction has been used).
3.) If data is requested that lies beyond the end of the compressed
message, terminate the INPUT-HUFFMAN instruction and move program
execution to the memory address specified by the formula
(memory_address_of_INPUT-HUFFMAN_instruction + delta) modulo 2^16.
4.) 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.
5.) Copy (H + uncompressed_j - lower_bound_j) modulo 2^16 to the
memory address specified by the destination parameter.
9.5.8. STATE-REFERENCE
The STATE-REFERENCE instruction retrieves some previously stored
state information.
STATE-REFERENCE (%id_start, %id_length, %state_start, %state_length,
%state_destination)
The id_start and id_length parameters specify the location of the
state identifier used to retrieve the state information. The state
identifier is always 16 bytes long; if id_length is less than 16 then
the remaining least significant bytes of the identifier are padded
with zeroes.
Decompression failure occurs if id_length is greater than 16.
Decompression failure also occurs if no state information matching
the state identifier can be found.
Note that when accessing state information that has been previously
created by the UDVM, the state identifier is always taken from an
[MD5] hash of the state to be retrieved. However this is not
necessarily the case for application-defined state as per Section
3.2.
The state_start and state_length parameters define the starting byte
and number of bytes to copy from the state_value contained in the
identified item of state. If more state is requested than is actually
available then decompression failure occurs.
The state_destination parameter contains a UDVM memory address. The
requested state is byte copied to this memory address using the rules
of Section 7.4.
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9.5.9. STATE-EXECUTE
The STATE-EXECUTE instruction retrieves and runs some previously
stored state information.
STATE-EXECUTE (%id_start, %id_length)
The id_start and id_length parameters function as per the STATE-
REFERENCE instruction.
STATE-EXECUTE is similar to STATE-REQUEST except that it does not
require the amount of state being requested or the proposed
destination for the state to be specified explicitly. Instead, it
simply puts the state_value back into the UDVM memory using the
parameters state_start and state_length contained as part of the
state information.
The entire state_value (all state length bytes of it) is byte copied
into the memory address specified by state start. The UDVM then jumps
to the (absolute) memory address specified by state_instruction.
Note that state start, state length and state_instruction are all
stored together with state_value as part of an item of state
information.
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 protocols, as it always can be circumvented by
not using compression. More specifically, the high-level security
goals can be described as:
-- do not worsen security of existing application protocol
-- do not create any new security issues
-- do not hinder deployment of application security
10.2 Security risks and mitigations
This subsection identifies the potential security risks associated
with the overall SigComp architecture, and details the proposed
solution for each risk.
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** Confidentiality risks
*** Attacking SigComp by snooping into state of other users
State can only be accessed using a state identifier, which is a
(prefix of a) cryptographic hash of the state being referenced. This
implies that the referencing packet already needs knowledge about the
state. To enforce this, a reference length of 48 bits is defined.
This also minimizes the probability of an accidental state collision.
Generally, ways to obtain knowledge about the state identifier (e.g.,
passive attacks) will also easily provide knowledge about the state
referenced, so no new vulnerability results.
The application needs to handle state identifiers with the same care
it would handle the state itself.
** Integrity risks
The SigComp approach assumes that there is appropriate integrity
protection below and/or above the SigComp layer. However, the state
establishment mechanism provides additional potential to compromise
the integrity of the messages (which, however, would most likely be
detectable at the application layer).
*** Attacking SigComp by faking state or making unauthorized changes
to state
State cannot be destroyed or changed by a malicious sender -- it can
only add new state. Faking state is only possible if the hash allows
intentional collision.
** Availability risks (avoid 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 packet is then handed to the application; the utility
as a reflection amplifier is therefore limited by the utility of the
application.
However, it must be noted that SigComp can be used to generate larger
packets as input to the application than have to be sent from the
malicious sender; this therefore can send smaller packets (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 state from a message where it would not have
installed state itself.
If this is desirable to increase the compression ratio, the effect
can be mitigated by adding 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.
*** Attacking the UDVM by faking state or making unauthorized changes
to state
(See "Integrity risks" above.)
*** Attacking the UDVM by sending it looping code
The application sets an upper limit to the number of "CPU 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 CPU cycles are offered by the UDVM.
However, this would be true for any decompressor that can receive
packets from anywhere.
11. IANA considerations
The SigComp solution currently requires two identifiers to be
assigned by IANA: the UDVM_version and the state identifier.
Upgraded versions of the UDVM will contain additional instructions to
improve the performance of the overall SigComp solution; new
UDVM_version parameters will be needed in this case.
Well-known decompression algorithms will also need to be assigned
fixed state identifiers.
12. Acknowledgements
Thanks to
Abigail Surtees (abigail.surtees@roke.co.uk)
Mark A West (mark.a.west@roke.co.uk)
Lawrence Conroy (lwc@roke.co.uk)
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Christian Schmidt (christian.schmidt@icn.siemens.de)
Max Riegel (maximilian.riegel@icn.siemens.de)
Lars-Erik Jonsson (lars-erik.jonsson@epl.ericsson.se)
Stefan Forsgren (stefan.forsgren@epl.ericsson.se)
Krister Svanbro (krister.svanbro@epl.ericsson.se)
Christopher Clanton (christopher.clanton@nokia.com)
Khiem Le (khiem.le@nokia.com)
Ka Cheong Leung (kacheong.leung@nokia.com)
for valuable input and review.
13. AuthorsÆ addresses
Richard Price Tel: +44 1794 833681
Email: richard.price@roke.co.uk
Roke Manor Research Ltd
Romsey, Hants, SO51 0ZN
United Kingdom
Hans Hannu Tel: +46 920 20 21 84
Email: hans.hannu@epl.ericsson.se
Box 920
Ericsson Erisoft AB
SE-971 28 Lulea, Sweden
Carsten Bormann Tel: +49 421 218 7024
Email: cabo@tzi.org
Universitaet Bremen TZI
Postfach 330440
D-28334 Bremen, Germany
Jan Christoffersson Tel: +46 920 20 28 40
Email: jan.christoffersson@epl.ericsson.se
Box 920
Ericsson Erisoft AB
SE-971 28 Lulea, Sweden
Zhigang Liu Tel: +1 972 894-5935
Email: zhigang.liu@nokia.com
Nokia Research Center
6000 Connection Drive
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Irving, TX 75039
USA
Jonathan Rosenberg
Email: jdrosen@dynamicsoft.com
dynamicsoft
72 Eagle Rock Avenue
First Floor
East Hanover, NJ 07936
14. References
[SIP] "SIP: Session Initiation Protocol", Handley et al,
RFC 2543, Internet Engineering Task Force, March 1999
[RTSP] "Real Time Streaming Protocol (RTSP)", H. Schulzrinne, A.
Rao and R. Lanphier, , RFC 2326, April 1998
[HTTP] "HyperText Transfer Protocol, HTTP/1.1", R. Fielding et
al.", RFC 2616, June 1999
[SIPsrv] "SIP: Locating SIP Servers", J. Rosenberg, H.
Schulzrinne, draft-ietf-sip-srv-04.txt, January 2002,
work in progress
[DEFLATE] "DEFLATE Compressed Data Format Specification version
1.3", P. Deutsch, RFC 1951, Internet Engineering Task
Force, May 1996
[SCTP] "Stream Control Transmission Protocol", Stewart et al,
RFC 2960, Internet Engineering Task Force, October 2000
[MD5] "The MD5 Message-Digest Algorithm", R. Rivest, RFC 1321,
Internet Engineering Task Force, April 1992
[RFC-1662] "PPP in HDLC-like Framing", Simpson et al, Internet
Engineering Task Force, July 1994
[RFC-2026] "The Internet Standards Process - Revision 3", Scott
Bradner, Internet Engineering Task Force, October 1996
[RFC-2119] "Key words for use in RFCs to Indicate Requirement
Levels", Scott Bradner, Internet Engineering Task Force,
March 1997
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Appendix A. Mnemonic language
Writing UDVM programs directly in bytecode would be a daunting task,
so a simple mnemonic language is provided to facilitate the creation
of new decompression algorithms. Most importantly, the language
allows the parameters of an instruction to be specified as text names
rather than as integer values.
If an instruction parameter is given as a text name, it should
correspond to exactly one instance of a label, a reserved memory
address or an externally defined keyword. A label is simply a text
name preceded by a colon, for example:
:loop
JUMP (loop)
For any parameters corresponding to a label, the integer value of the
parameter is calculated by the following formula:
parameter_value = (instruction_address - label_address) modulo 2^16
Note that the "label address" is simply the memory address of the
instruction immediately following the label. In particular, the above
example can be rewritten as JUMP (0).
A reserved memory address is specified using the "reserve" keyword
followed by a text_name and (optionally) an integer value. For
example:
reserve apples
reserve pears (8)
reserve bananas
LOAD (bananas, 5)
For any parameters corresponding to a reserved memory address, the
integer value of the parameter is the next free memory address that
has not yet been reserved. Starting at this address, the specified
number of bytes of memory are then reserved (if no value is given
then a total of 2 bytes is reserved).
The first instance of a "reserve" keyword begins reserving memory at
Address 6 (to avoid overwriting the three well-known variables of
Section 7.2). So the above example can be rewritten as LOAD (16, 5).
An externally defined keyword is specified outside of the mnemonic
language. All of the application-defined parameters are considered to
be externally defined keywords and can be referenced in the mnemonic
code (useful for adapting the code based on the available memory or
CPU cycles). The following additional keywords can also be used:
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Keyword: Corresponding value:
byte_copy_left 0
byte_copy_right 2
stack_location 4
reserved_end See below
bytecode_length See below
total_length See below
The keyword reserved_end specifies the highest reserved memory
address for the entire mnemonic code (taking into account all the
occasions where memory is reserved).
The keyword bytecode_length specifies the total size of the bytecode
corresponding to the mnemonic code. Any instances of bytecode_length
are initially replaced with 3 bytes of zeroes, and then are filled in
after the remainder of the bytecode has been generated.
Similarly, the keyword total_length specifies the total amount of
memory required at the UDVM including bytecode and reserved memory
addresses.
A complete description of the mnemonic language and how it should be
translated into bytecode is given below:
Instructions: Instruction names are given in capitals. Replace
each name with the corresponding 1-byte value as
per Chapter 9.
$: When appended to the front of an instruction
parameter then the parameter is a memory address
rather than a direct value. This symbol is
mandatory for reference parameters, optional for
multitype parameters and disallowed for literals.
Integers: Instruction parameters can be given in the form of
decimal integers. They are converted into the
shortest bytecode capable of representing the
integer by the rules of Section 7.3.
Text references: Instruction parameters can also be given in the
form of lowercase names. These names should match
exactly one label, reserved memory address or
externally defined keyword as described above.
Labels: Label names are given as a colon followed by
lowercase text. They are deleted when converting
the mnemonics to bytecode.
Reserved memory: Memory addresses are reserved using the "reserve"
keyword. The line containing the reserve keyword
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is deleted when converting to bytecode.
.LSB: When appended to the end of a text name, the
integer value corresponding to the name is
increased by 1. This is useful for addressing the
LSBs of a 2-byte variable.
0b, 0d: Bytecode values can be specified directly in
binary or decimal via the appropriate prefix. The
direct bytecode continues until a character occurs
that is not an integer or whitespace.
Whitespace: All whitespace (plus brackets and commas) just
delimit the instructions. Delete.
Comments: These are indicated by a semicolon and continue
to the end of the line. Delete.
Once the mnemonic code has been converted into bytecode, it can be
executed by copying the bytecode into the UDVM memory beginning at
the first memory address that has not been reserved by an instance of
the "reserve" keyword. Program execution is assumed to begin at this
address.
Note that further to the rules outlined above, well-written mnemonic
code will also have the following properties:
* Any instance of a memory address will be specified as a text
reference rather than an integer value. This ensures that the
mnemonic code is portable.
* The mnemonic code will not write to any memory address except
those reserved by the "reserve" keyword. This ensures that the
code can be compiled.
Appendix B. Example application-defined parameters
This appendix gives some example values for each of the application-
defined parameters. These values are geared towards the compression
of a signaling protocol such as [SIP].
Note that all of the proposed values are fixed and not negotiated
between the two instances of the application invoking SigComp. This
is because it is possible for the application invoking the
decompressor to receive compressed messages from several different
applications, and it is difficult to determine which message
corresponds to which application. [SIP] does this using "From:" and
"To:" fields in the message itself, but these are not visible until
the message has been decompressed. It is simpler just to fix a set of
parameter for every instance of the application.
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UDVM_version 0
minimum_compression_ratio 0.5
maximum_compressed_size 65535
maximum_uncompressed_size 65535
minimum_hash_size 6
overall_memory_size 8192
working_memory_start 0
working_memory_end 8191
cycles_per_bit 20
cycles_per_message 2000
first_instruction 6
Note that the parameters overall_memory_size, cycles_per_bit and
cycles_per_message can be increased on the fly using the capabilities
announcement mechanism. This mechanism is designed to function
correctly even when the receiving application is sent compressed
messages from several different applications.
The initial contents of the UDVM memory also need to be defined. It
is not enough simply to initialize the memory containing all zeroes,
as the UDVM would be unable to input any compressed data. Instead,
for each new compressed message the memory should be initialized
containing a simple decompressor capable of extracting the first few
bytes of compressed data. These bytes can then be interpreted as a
state identifier to retrieve the correct decompression algorithm.
As an example, the following mnemonic code can be converted to
bytecode and pasted into the UDVM memory beginning at Address 6:
reserve state_identifier (6)
INPUT-BYTECODE (6, state_identifier, fail)
STATE-EXECUTE (state_identifier, 6)
:fail
DECOMPRESSION-FAILURE
Finally, the application can define initial state that is available
to the UDVM. Examples of application-defined state include common
decompression algorithms, dictionaries of common text phrases etc.
Appendix C. Example decompression algorithms
This appendix gives examples of decompression algorithms which can be
run on the UDVM in the form of bytecode.
C.1. Example UDVM code for simple LZ77 decompression
The first example gives the code required to decompress data from a
very simple LZ77-based algorithm. The UDVM is instructed to interpret
a compressed message as a set of 4-byte characters, where each
character contains a 2-byte position integer followed by a 2-byte
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length integer. Taken together these integers point to a previously
received text string in the UDVM memory, which is then copied to the
end of the uncompressed message.
Since the compressor can only send references to strings already
present in the UDVM memory, before the first message is decompressed
the memory must be initialized with a static dictionary containing
the 256 ASCII characters.
The algorithm write-protects the memory containing the UDVM
instructions used to decompress each character, so that they can
easily be compiled to improve the speed of decompression.
A 2-byte CRC over the uncompressed message is appended to the end of
the compressed message, to verify that correct decompression has
occurred. The algorithm also requests that the contents of the UDVM
memory be saved using the state request mechanism, so that it can be
retrieved by sending the appropriate 6-byte hash.
reserve byte_copy_left
reserve byte_copy_right
reserve uncompressed_start
reserve uncompressed_end
reserve uncompressed_length
reserve position
reserve length
reserve static_dictionary (256)
reserve circular_buffer (2048)
WORKING-MEMORY (uncompressed_start, reserved_end)
MULTILOAD (0, 7, circular_buffer, reserved_end, static_dictionary,
circular_buffer, 0, 0, 0)
:unpack_static_dictionary
; The following instructions initialize the static dictionary.
COPY-LITERAL (position.LSB, 1, $uncompressed_start)
ADD ($position, 1)
COMPARE ($position, 256, unpack_static_dictionary, next_character, 0)
:next_character
INPUT-FIXED (16, position, fail)
INPUT-FIXED (16, length, end_of_message)
COPY-LITERAL ($position, $length, $uncompressed_end)
ADD ($uncompressed_length, $length)
JUMP (next_character)
:fail
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DECOMPRESSION-FAILURE
:end_of_message
CRC ($position, $uncompressed_start, $uncompressed_length, fail)
OUTPUT ($uncompressed_start, $uncompressed_length)
END-MESSAGE (6, 0, total_length, next_character, 0)
Appendix D. Document history
- October 19, 2001, version 00
First version. The draft describes the current ideas, from people
involved in the ROHC WG, of how to perform compression of
application signaling messages.
- October 31, 2001, version 01
Second version. Additional section, 5.2.1, which describes when a
message identifier can be reused.
- November 21, 2001, version 02
Third version. Section 6 has been moved to a separate draft. The
third version describes a modular solution, providing flexibility
for implementers to decide which functions they want to integrate.
- January 28, 2002, version 03
Fourth version. SigComp version 02 is divided into this draft, a UDVM
draft and a extended operation mechanisms draft.
Compressor/decompressor (UDVM) state approach has been introduced for
security reasons.
- February 14, 2002, version 04
Fifth version. Describes the complete base SigComp solution including
the UDVM.
This Internet-Draft expires in August 2002.
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