Network Working Group R. Price
Request for Comments: 3320 Siemens/Roke Manor
Category: Standards Track C. Bormann
TZI/Uni Bremen
J. Christoffersson
H. Hannu
EriCsson
Z. Liu
Nokia
J. Rosenberg
dynamicsoft
January 2003
Signaling Compression (SigComp)
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document defines Signaling Compression (SigComp), a solution for
compressing messages generated by application protocols sUCh as the
Session Initiation Protocol (SIP) (RFC3261) and the Real Time
Streaming Protocol (RTSP) (RFC2326). The architecture and
prerequisites of SigComp are outlined, along with the format of the
SigComp message.
Decompression functionality for SigComp is provided by a Universal
Decompressor Virtual Machine (UDVM) optimized for the task of running
decompression algorithms. The UDVM can be configured to understand
the output of many well-known compressors such as DEFLATE (RFC-1951).
Table of Contents
1. Introduction...................................................2
2. Terminology....................................................3
3. SigComp architecture...........................................5
4. SigComp dispatchers...........................................15
5. SigComp compressor............................................18
6. SigComp state handler.........................................20
7. SigComp message format........................................23
8. Overview of the UDVM..........................................28
9. UDVM instruction set..........................................37
10. Security Considerations.......................................56
11. IANA Considerations...........................................58
12. Acknowledgements..............................................59
13. References....................................................59
14. Authors' Addresses............................................60
15. Full Copyright Statement......................................62
1. Introduction
Many application protocols used for multimedia communications are
text-based and engineered for bandwidth rich links. As a result the
messages have not been optimized in terms of size. For example,
typical SIP messages range from a few hundred bytes up to two
thousand bytes or more [RFC3261].
With the planned usage of these protocols in wireless handsets as
part of 2.5G and 3G cellular networks, the large message size is
problematic. With low-rate IP connectivity the transmission delays
are significant. Taking into account retransmissions, and the
multiplicity of messages that are required in some flows, call setup
and feature invocation are adversely affected. SigComp provides a
means to eliminate this problem by offering robust, lossless
compression of application messages.
This document outlines the architecture and prerequisites of the
SigComp solution, the format of the SigComp message and the Universal
Decompressor Virtual Machine (UDVM) that provides decompression
functionality.
SigComp is offered to applications as a layer between the application
and an underlying transport. The service provided is that of the
underlying transport plus compression. SigComp supports a wide range
of transports including TCP, UDP and SCTP [RFC-2960].
2. Terminology
The key Words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, RFC2119
[RFC-2119].
Application
Entity that invokes SigComp and performs the following tasks:
1. Supplying application messages to the compressor dispatcher
2. Receiving decompressed messages from the decompressor
dispatcher
3. Determining the compartment identifier for a decompressed
message.
Bytecode
Machine code that can be executed by a virtual machine.
Compressor
Entity that encodes application messages using a certain
compression algorithm, and keeps track of state that can be used
for compression. The compressor is responsible for ensuring that
the messages it generates can be decompressed by the remote UDVM.
Compressor Dispatcher
Entity that receives application messages, invokes a compressor,
and forwards the resulting SigComp compressed messages to a remote
endpoint.
UDVM Cycles
A measure of the amount of "CPU power" required to execute a UDVM
instruction (the simplest UDVM instructions require a single UDVM
cycle). An upper limit is placed on the number of UDVM cycles
that can be used to decompress each bit in a SigComp message.
Decompressor Dispatcher
Entity that receives SigComp messages, invokes a UDVM, and
forwards the resulting decompressed messages to the application.
Endpoint
One instance of an application, a SigComp layer, and a transport
layer for sending and/or receiving SigComp messages.
Message-based Transport
A transport that carries data as a set of bounded messages.
Compartment
An application-specific grouping of messages that relate to a peer
endpoint. Depending on the signaling protocol, this grouping may
relate to application concepts such as "session", "dialog",
"connection", or "association". The application allocates state
memory on a per-compartment basis, and determines when a
compartment should be created or closed.
Compartment Identifier
An identifier (in a locally chosen format) that uniquely
references a compartment.
SigComp
The overall compression solution, comprising the compressor, UDVM,
dispatchers and state handler.
SigComp Message
A message sent from the compressor dispatcher to the decompressor
dispatcher. In case of a message-based transport such as UDP, a
SigComp message corresponds to exactly one datagram. For a
stream-based transport such as TCP, the SigComp messages are
separated by reserved delimiters.
Stream-based transport
A transport that carries data as a continuous stream with no
message boundaries.
Transport
Mechanism for passing data between two endpoints. SigComp is
capable of sending messages over a wide range of transports
including TCP, UDP and SCTP [RFC-2960].
Universal Decompressor Virtual Machine (UDVM)
The machine architecture described in this document. The UDVM is
used to decompress SigComp messages.
State
Data saved for retrieval by later SigComp messages.
State Handler
Entity responsible for Accessing and storing state information
once permission is granted by the application.
State Identifier
Reference used to access a previously created item of state.
3. SigComp Architecture
In the SigComp architecture, compression and decompression is
performed at two communicating endpoints. The layout of a single
endpoint is illustrated in Figure 1:
+-------------------------------------------------------------------+
Local application
+-------------------------------------------------------------------+
^
Application message & Decompressed Compartment
compartment identifier message identifier
+-- -- -- -- -- -- -- ---- -- -- -- -- -- -- ------ -- -- -- -- -+
v v
+------------------------+ +----------------------+
+-- Compressor Decompressor <-+
dispatcher dispatcher
+------------------------+ +----------------------+
^ ^ ^
v
+--------------+ +---------------+
+-------+ v
Compressor 1 <----->State 1 +--------------+
+-------+
+--------------+ Decompressor
State handler <-->
+--------------+ (UDVM)
+-------+
+-> Compressor 2 <----->State 2 +--------------+
+-------+
+--------------+ +---------------+ SigComp layer
+- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- ---+
SigComp SigComp
message message
v
+-------------------------------------------------------------------+
Transport layer
+-------------------------------------------------------------------+
Figure 1: High-level architectural overview of one SigComp endpoint
Note that SigComp is offered to applications as a layer between the
application and the underlying transport, and so Figure 1 is an
endpoint when viewed from a transport layer perspective. From the
perspective of multi-hop application layer protocols however, SigComp
is applied on a per-hop basis.
The SigComp layer is further decomposed into the following entities:
1. Compressor dispatcher - the interface from the application. The
application supplies the compressor dispatcher with an application
message and a compartment identifier (see Section 3.1 for further
details). The compressor dispatcher invokes a particular
compressor, which returns a SigComp message to be forwarded to the
remote endpoint.
2. Decompressor dispatcher - the interface towards the application.
The decompressor dispatcher receives a SigComp message and invokes
an instance of the Universal Decompressor Virtual Machine (UDVM).
It then forwards the resulting decompressed message to the
application, which may return a compartment identifier if it
wishes to allow state to be saved for the message.
3. One or more compressors - the entities that convert application
messages into SigComp messages. Distinct compressors are invoked
on a per-compartment basis, using the compartment identifiers
supplied by the application. A compressor receives an application
message from the compressor dispatcher, compresses the message,
and returns a SigComp message to the compressor dispatcher. Each
compressor chooses a certain algorithm to encode the data (e.g.,
DEFLATE).
4. UDVM - the entity that decompresses SigComp messages. Note that
since SigComp can run over an unsecured transport layer, a
separate instance of the UDVM is invoked on a per-message basis.
However, during the decompression process the UDVM may invoke the
state handler to access existing state or create new state.
5. State handler - the entity that can store and retrieve state.
State is information that is stored between SigComp messages,
avoiding the need to upload the data on a per-message basis. For
security purposes it is only possible to create new state with the
permission of the application. State creation and retrieval are
further described in Chapter 6.
When compressing a bidirectional application protocol the choice to
use SigComp can be made independently in both directions, and
compression in one direction does not necessarily imply compression
in the reverse direction. Moreover, even when two communicating
endpoints send SigComp messages in both directions, there is no need
to use the same compression algorithm in each direction.
Note that a SigComp endpoint can decompress messages from multiple
remote endpoints at different locations in a network, as the
architecture is designed to prevent SigComp messages from one
endpoint interfering with messages from a different endpoint. A
consequence of this design choice is that it is difficult for a
malicious user to disrupt SigComp operation by inserting false
compressed messages on the transport layer.
3.1. Requirements on the Application
From an application perspective the SigComp layer appears as a new
transport, with similar behavior to the original transport used to
carry uncompressed data (for example SigComp/UDP behaves similarly to
native UDP).
Mechanisms for discovering whether an endpoint supports SigComp are
beyond the scope of this document.
All SigComp messages contain a prefix (the five most-significant bits
of the first byte are set to one) that does not occur in UTF-8
encoded text messages [RFC-2279], so for applications which use this
encoding (or ASCII encoding) it is possible to multiplex uncompressed
application messages and SigComp messages on the same port.
Applications can still reserve a new port specifically for SigComp
however (e.g., as part of the discovery mechanism).
If a particular endpoint wishes to be stateful then it needs to
partition its decompressed messages into "compartments" under which
state can be saved. SigComp relies on the application to provide
this partition. So for stateful endpoints a new interface is
required to the application in order to leverage the authentication
mechanisms used by the application itself.
When the application receives a decompressed message it maps the
message to a certain compartment and supplies the compartment
identifier to SigComp. Each compartment is allocated a separate
compressor and a certain amount of memory to store state information,
so the application must assign distinct compartments to distinct
remote endpoints. However it is possible for a local endpoint to
establish several compartments that relate to the same remote
endpoint (this should be avoided where possible as it may waste
memory and reduce the overall compression ratio, but it does not
cause messages to be incorrectly decompressed). In this case,
reliable stateful operation is possible only if the decompressor does
not lump several messages into one compartment when the compressor
eXPected them to be assigned different compartments.
The exact format of the compartment identifier is unimportant
provided that different identifiers are given to different
compartments.
Applications that wish to communicate using SigComp in a stateful
fashion should use an authentication mechanism to securely map
decompressed messages to compartment identifiers. They should also
agree on any limits to the lifetime of a compartment, to avoid the
case where an endpoint accesses state information that has already
been deleted.
3.2. SigComp feedback mechanism
If a signaling protocol sends SigComp messages in both directions and
there is a one-to-one relationship between the compartments
established by the applications on both ends ("peer compartments"),
the two endpoints can cooperate more closely. In this case, it is
possible to send feedback information that monitors the behavior of
an endpoint and helps to improve the overall compression ratio.
SigComp performs feedback on a request/response basis, so a
compressor makes a feedback request and receives some feedback data
in return. The procedure for requesting and returning feedback in
SigComp is illustrated in Figure 2:
+---------------------+ +---------------------+
+-----------------+ +-----------------+
--> Compressor ------------------------> UDVM <->
sending to B SigComp message 2
+-----------------+ requesting feedback +-----------------+
^ 1,9 3
v
+-----------------+ +-----------------+
State State
handler handler
+-----------------+ +-----------------+
^ 8 4
v
+-----------------+ +-----------------+
UDVM Compressor
<-> <------------------------ sending to A <--
6 +-----------------+ SigComp message +-----------------+
7 returning feedback 5
Endpoint A Endpoint B
+---------------------+ +---------------------+
Figure 2: Steps involved in the transmission of feedback data
The dispatchers, the application and the transport layer are omitted
from the diagram for clarity. Note that the decompressed messages
pass via the decompressor dispatcher to the application; moreover the
SigComp messages transmitted from the compressor to the remote UDVM
are sent via first the compressor dispatcher, followed by the
transport layer and finally the decompressor dispatcher.
The steps for requesting and returning feedback data are described in
more detail below:
1. The compressor that sends messages to Endpoint B piggybacks a
feedback request onto a SigComp message.
2. When the application receives the decompressed message, it may
return the compartment identifier for the message.
3. The UDVM in Endpoint B forwards the requested feedback data to the
state handler.
4. If the UDVM can supply a valid compartment identifier, then the
state handler forwards the feedback data to the appropriate
compressor (namely the compressor sending to Endpoint A).
5. The compressor returns the requested feedback data to Endpoint A
piggybacked onto a SigComp message.
6. When the application receives the decompressed message, it may
return the compartment identifier for the message.
7. The UDVM in Endpoint A forwards the returned feedback data to the
state handler.
8. If the UDVM can supply a valid compartment identifier, then the
state handler forwards the feedback data to the appropriate
compressor (namely the compressor sending to Endpoint B).
9. The compressor makes use of the returned feedback data.
The detailed role played by each entity in the transmission of
feedback data is explained in subsequent chapters.
3.3. SigComp Parameters
An advantage of using a virtual machine for decompression is that
almost all of the implementation flexibility lies in the SigComp
compressors. When receiving SigComp messages an endpoint generally
behaves in a predictable manner.
Note however that endpoints implementing SigComp will typically have
a wide range of capabilities, each offering a different amount of
working memory, processing power etc. In order to support this wide
variation in endpoint capabilities, the following parameters are
provided to modify SigComp behavior when receiving SigComp messages:
decompression_memory_size
state_memory_size
cycles_per_bit
SigComp_version
locally available state (a set containing 0 or more state items)
Each parameter has a minimum value that MUST be offered by all
receiving SigComp endpoints. Moreover, endpoints MAY offer
additional resources if available; these resources can be advertised
to remote endpoints using the SigComp feedback mechanism.
Particular applications may also agree a-priori to offer additional
resources as mandatory (e.g., SigComp for SIP offers a dictionary of
common SIP phrases as a mandatory state item).
Each of the SigComp parameters is described in greater detail below.
3.3.1. Memory Size and UDVM Cycles
The decompression_memory_size parameter specifies the amount of
memory available to decompress one SigComp message. (Note that the
term "amount of memory" is used on a conceptual level in order to
specify decompressor behavior and allow resource planning on the side
of the compressor -- an implementation could require additional,
bounded amounts of actual memory resources or could even organize its
memory in a completely different way as long as this does not cause
decompression failures where the conceptual model would not.) A
portion of this memory is used to buffer a SigComp message before it
is decompressed; the remainder is given to the UDVM. Note that the
memory is allocated on a per-message basis and can be reclaimed after
the message has been decompressed. All endpoints implementing
SigComp MUST offer a decompression_memory_size of at least 2048
bytes.
The state_memory_size parameter specifies the number of bytes offered
to a particular compartment for the creation of state. This
parameter is set to 0 if the endpoint is stateless.
Unlike the other SigComp parameters, the state_memory_size is offered
on a per-compartment basis and may vary for different compartments.
The memory for a compartment is reclaimed when the application
determines that the compartment is no longer required.
The cycles_per_bit parameter specifies the number of "UDVM cycles"
available to decompress each bit in a SigComp message. Executing a
UDVM instruction requires a certain number of UDVM cycles; a complete
list of UDVM instructions and their cost in UDVM cycles can be found
in Chapter 9. An endpoint MUST offer a minimum of 16 cycles_per_bit.
Each of the three parameter values MUST be chosen from the limited
set given below, so that the parameters can be efficiently encoded
for transmission using the SigComp feedback mechanism.
The cycles_per_bit parameter is encoded using 2 bits, whilst the
decompression_memory_size and state_memory_size are both encoded
using 3 bits. The bit encodings and their corresponding values are
as follows:
Encoding: cycles_per_bit: Encoding: state_memory_size (bytes):
00 16 000 0
01 32 001 2048
10 64 010 4096
11 128 011 8192
100 16384
101 32768
110 65536
111 131072
The decompression_memory_size is encoded in the same manner as the
state_memory_size, except that the bit pattern 000 cannot be used (as
an endpoint cannot offer a decompression_memory_size of 0 bytes).
3.3.2. SigComp Version
The SigComp_version parameter specifies whether only the basic
version of SigComp is available, or whether an upgraded version is
available offering additional instructions etc. Within the UDVM, it
is available as a 2-byte value, generated by zero-extending the 1-
byte SigComp_version parameter (i.e., the first byte of the 2-byte
value is always zero).
The basic version of SigComp is Version 0x01, which is the version
described in this document.
To ensure backwards compatibility, if a SigComp message is
successfully decompressed by Version 0x01 of SigComp then it will be
successfully decompressed on upgraded versions. Similarly, if the
message triggers a manual decompression failure (see Section 8.7),
then it will also continue to do so.
However, messages that cause an unexpected decompression failure on
Version 0x01 of SigComp may be successfully decompressed by upgraded
versions.
The simplest way to upgrade SigComp in a backwards-compatible manner
is to add additional UDVM instructions, as this will not affect the
decompression of SigComp messages compatible with Version 0x01.
Reserved addresses in the UDVM memory (Useful Values, see Section
7.2) may also be assigned values in future versions of SigComp.
3.3.3. Locally Available State Items
A SigComp state item is an item of data that is retained between
SigComp messages. State items can be retrieved and loaded into the
UDVM memory as part of the decompression process, often significantly
improving the compression ratio as the same information does not have
to be uploaded on a per-message basis.
Each endpoint maintains a set of state items where every item is
composed of the following information:
Name: Type of data:
state_identifier 20-byte value
state_length 2-byte value
state_address 2-byte value
state_instruction 2-byte value
minimum_access_length 2-byte value from 6 to 20 inclusive
state_value String of state_length consecutive bytes
State items are typically created at an endpoint upon successful
decompression of a SigComp message. The remote compressor sending
the message makes a state creation request by invoking the
appropriate UDVM instruction, and the state is saved once permission
is granted by the application.
However, an endpoint MAY also wish to offer a set of locally
available state items that have not been uploaded as part of a
SigComp message. For example it might offer well-known decompression
algorithms, dictionaries of common phrases used in a specific
signaling protocol, etc.
Since these state items are established locally without input from a
remote endpoint, they are most useful if publicly documented so that
a wide collection of remote endpoints can determine the data
contained in each state item and how it may be used. Further
Internet Documents and RFCs may be published to describe particular
locally available state items.
Although there are no locally available state items that are
mandatory for every SigComp endpoint, certain state items can be made
mandatory in a specific environment (e.g., the dictionary of common
phrases for a specific signaling protocol could be made mandatory for
that signaling protocol's usage of SigComp). Also, remote endpoints
can indicate their interest in receiving a list of some of the state
items available locally at an endpoint using the SigComp feedback
mechanism.
It is a matter of local decision for an endpoint what items of
locally available state it advertises; this decision has no influence
on interoperability, but may increase or decrease the efficiency of
the compression achievable between the endpoints.
4. SigComp Dispatchers
This chapter defines the behavior of the compressor and decompressor
dispatcher. The function of these entities is to provide an
interface between SigComp and its environment, minimizing the effort
needed to integrate SigComp into an existing protocol stack.
4.1. Compressor Dispatcher
The compressor dispatcher receives messages from the application and
passes the compressed version of each message to the transport layer.
Note that SigComp invokes compressors on a per-compartment basis, so
when the application provides a message to be compressed it must also
provide a compartment identifier. The compressor dispatcher forwards
the application message to the correct compressor based on the
compartment identifier (invoking a new compressor if a new
compartment identifier is encountered). The compressor returns a
SigComp message that can be passed to the transport layer.
Additionally, the application should indicate to the compressor
dispatcher when it wishes to close a particular compartment, so that
the resources taken by the corresponding compressor can be reclaimed.
4.2. Decompressor Dispatcher
The decompressor dispatcher receives messages from the transport
layer and passes the decompressed version of each message to the
application.
To ensure that SigComp can run over an unsecured transport layer, the
decompressor dispatcher invokes a new instance of the UDVM for each
new SigComp message. Resources for the UDVM are released as soon as
the message has been decompressed.
The dispatcher MUST NOT make more than one SigComp message available
to a given instance of the UDVM. In particular, the dispatcher MUST
NOT concatenate two SigComp messages to form a single message.
4.2.1. Decompressor Dispatcher Strategies
Once the UDVM has been invoked it is initialized using the SigComp
message of Chapter 7. The message is then decompressed by the UDVM,
returned to the decompressor dispatcher, and passed on to the
receiving application. Note that the UDVM has no awareness of
whether the underlying transport is message-based or stream-based,
and so it always outputs decompressed data as a stream. It is the
responsibility of the dispatcher to provide the decompressed message
to the application in the expected form (i.e., as a stream or as a
distinct, bounded message). The dispatcher knows that the end of a
decompressed message has been reached when the UDVM instruction END-
MESSAGE is invoked (see Section 9.4.9).
For a stream-based transport, two strategies are therefore possible
for the decompressor dispatcher:
1) The dispatcher collects a complete SigComp message and then
invokes the UDVM. The advantage is that, even in implementations
that have multiple incoming compressed streams, only one instance
of the UDVM is ever required.
2) The dispatcher collects the SigComp header (see Section 7) and
invokes the UDVM; the UDVM stays active while the rest of the
message arrives. The advantage is that there is no need to buffer
up the rest of the message; the message can be decompressed as it
arrives, and any decompressed output can be relayed to the
application immediately.
In general, which of the strategies is used is an implementation
choice.
However, the compressor may want to take advantage of strategy 2 by
expecting that some of the application message is passed on to the
application before the SigComp message is terminated, e.g., by
keeping the UDVM active while expecting the application to
continuously receive decompressed output. This approach ("continuous
mode") invalidates some assumptions of the SigComp security model and
can only be used if the transport itself can provide the required
protection against denial of service attacks. Also, since only
strategy 2 works in this approach, the use of continuous mode
requires previous agreement between the two endpoints.
4.2.2. Record Marking
For a stream-based transport, the dispatcher delimits messages by
parsing the compressed data stream for instances of 0xFF and taking
the following actions:
Occurs in data stream: Action:
0xFF 00 one 0xFF byte in the data stream
0xFF 01 same, but the next byte is quoted (could
be another 0xFF)
: :
0xFF 7F same, but the next 127 bytes are quoted
0xFF 80 to 0xFF FE (reserved for future standardization)
0xFF FF end of SigComp message
The combinations 0xFF01 to 0xFF7F are useful to limit the worst case
expansion of the record marking scheme: the 1 (0xFF01) to 127
(0xFF7F) bytes following the byte combination are copied literally by
the decompressor without taking any special action on 0xFF. (Note
that 0xFF00 is just a special case of this, where zero following
bytes are copied literally.)
In UDVM version 0x01, any occurrence of the combinations 0xFF80 to
0xFFFE that are not protected by quoting causes decompression
failure; the decompressor SHOULD close the stream-based transport in
this case.
4.3. Returning a Compartment Identifier
Upon receiving a decompressed message the application may supply the
dispatcher with a compartment identifier. Supplying this identifier
grants permission for the following:
1. Items of state accompanying the decompressed message can be saved
using the state memory reserved for the specified compartment.
2. The feedback data accompanying the decompressed message can be
trusted sufficiently that it can be used when sending SigComp
messages that relate to the compressor's equivalent for the
compartment.
The dispatcher passes the compartment identifier to the UDVM, where
it is used as per the END-MESSAGE instruction (see Section 9.4.9).
The application uses a suitable authentication mechanism to determine
whether the decompressed message belongs to a legitimate compartment
or not. If the application fails to authenticate the message with
sufficient confidence to allow state to be saved or feedback data to
be trusted, it supplies a "no valid compartment" error to the
dispatcher and the UDVM is terminated without creating any state or
forwarding any feedback data.
5. SigComp Compressor
An important feature of SigComp is that decompression functionality
is provided by a Universal Decompressor Virtual Machine (UDVM). This
means that the compressor can choose any algorithm to generate
compressed SigComp messages, and then upload bytecode for the
corresponding decompression algorithm to the UDVM as part of the
SigComp message.
To help with the implementation and testing of a SigComp endpoint,
further Internet Documents and RFCs may be published to describe
particular compression algorithms.
The overall requirement placed on the compressor is that of
transparency, i.e., the compressor MUST NOT send bytecode which
causes the UDVM to incorrectly decompress a given SigComp message.
The following more specific requirements are also placed on the
compressor (they can be considered particular instances of the
transparency requirement):
1. For robustness, it is recommended that the compressor supply some
form of integrity check (not necessarily of cryptographic
strength) over the application message to ensure that successful
decompression has occurred. A UDVM instruction is provided for
CRC verification; also, another instruction can be used to compute
a SHA-1 cryptographic hash.
2. The compressor MUST ensure that the message can be decompressed
using the resources available at the remote endpoint.
3. If the transport is message-based, then the compressor MUST map
each application message to exactly one SigComp message.
4. If the transport is stream-based but the application defines its
own internal message boundaries, then the compressor SHOULD map
each application message to exactly one SigComp message.
Message boundaries should be preserved over a stream-based transport
so that accidental or malicious damage to one SigComp message does
not affect the decompression of subsequent messages.
Additionally, if the state handler passes some requested feedback to
the compressor, then it SHOULD be returned in the next SigComp
message generated by the compressor (unless the state handler passes
some newer requested feedback before the older feedback has been
sent, in which case the older feedback is deleted).
If present, the requested feedback item SHOULD be copied unmodified
into the returned_feedback_item field provided in the SigComp
message. Note that there is no need to transmit any requested
feedback item more than once.
The compressor SHOULD also upload the local SigComp parameters to the
remote endpoint, unless the endpoint has indicated that it does not
wish to receive these parameters or the compressor determines that
the parameters have already successfully arrived (see Section 5.1 for
details of how this can be achieved). The SigComp parameters are
uploaded to the UDVM memory at the remote endpoint as described in
Section 9.4.9.
5.1. Ensuring Successful Decompression
A compressor MUST be certain that all of the data needed to
decompress a SigComp message is available at the receiving endpoint.
One way to ensure this is to send all of the needed information in
every SigComp message (including bytecode to decompress the message).
However, the compression ratio for this method will be relatively
low.
To oBTain the best overall compression ratio the compressor needs to
request the creation of new state items at the remote endpoint. The
information saved in these state items can then be accessed by later
SigComp messages, avoiding the need to upload the data on a per-
message basis.
Before the compressor can access saved state however, it must ensure
that the SigComp message carrying the state creation request arrived
successfully at the receiving endpoint. For a reliable transport
(e.g., TCP or SCTP) this is guaranteed. For an unreliable transport
however, the compressor must provide a suitable mechanism itself (see
[RFC-3321] for further details).
The compressor must also ensure that the state item it wishes to
access has not been rejected due to a lack of state memory. This can
be accomplished by checking the state_memory_size parameter using the
SigComp feedback mechanism (see Section 9.4.9 for further details).
5.2. Compression Failure
The compressor SHOULD make every effort to successfully compress an
application message, but in certain cases this might not be possible
(particularly if resources are scarce at the receiving endpoint). In
this case a "compression failure" is called.
If a compression failure occurs then the compressor informs the
dispatcher and takes no further action. The dispatcher MUST report
this failure to the application so that it can try other methods to
deliver the message.
6. State Handling and Feedback
This chapter defines the behavior of the SigComp state handler. The
function of the state handler is to retain information between
received SigComp messages; it is the only SigComp entity that is
capable of this function, and so it is of particular importance from
a security perspective.
6.1. Creating and Accessing State
To provide security against the malicious insertion or modification
of SigComp messages, a separate instance of the UDVM is invoked to
decompress each message. This ensures that damaged SigComp messages
do not prevent the successful decompression of subsequent valid
messages.
Note, however, that the overall compression ratio is often
significantly higher if messages can be compressed relative to the
information contained in previous messages. For this reason, it is
possible to create state items for access when a later message is
being decompressed. Both the creation and access of state are
designed to be secure against malicious tampering with the compressed
data. The UDVM can only create a state item when a complete message
has been successfully decompressed and the application has returned a
compartment identifier under which the state can be saved.
State access cannot be protected by relying on the application alone,
since the authentication mechanism may require information from the
decompressed message (which of course is not available until after
the state has been accessed). Instead, SigComp protects state access
by creating a state identifier that is a hash over the item of state
to be retrieved. This state_identifier must be supplied to retrieve
an item of state from the state handler.
Also note that state is not deleted when it is accessed. So even if
a malicious sender manages to access some state information,
subsequent messages compressed relative to this state can still be
successfully decompressed.
Each state item contains a state_identifier that is used to access
the state. One state identifier can be supplied in the SigComp
message header to initialize the UDVM (see Chapter 7); additional
state items can be retrieved using the STATE-ACCESS instruction. The
UDVM can also request the creation of a new state item by using the
STATE-CREATE and END-MESSAGE instructions (see Chapter 9 for further
details).
6.2. Memory Management
The state handler manages state memory on a per-compartment basis.
Each compartment can store state up to a certain state_memory_size
(where the application may assign different values for the
state_memory_size parameter to different compartments).
As well as storing the state items themselves, the state handler
maintains a list of the state items created by a particular
compartment and ensures that no compartment exceeds its allocated
state_memory_size. For the purpose of calculation, each state item
is considered to cost (state_length + 64) bytes.
Each instance of the UDVM can pass up to four state creation requests
to the state handler, as well as up to four state free requests (the
latter are requests to free the memory taken by a state item in a
certain compartment). When the state handler receives a state
creation request from the UDVM it takes the following steps:
1. The state handler MUST reject all state creation requests that are
not accompanied by a valid compartment identifier, or if the
compartment is allocated 0 bytes of state memory. Note that if a
state creation request fails due to lack of state memory then it
does not mean that the corresponding SigComp message is damaged;
compressors will often make state creation requests in the first
SigComp message of a compartment, before they have discovered the
state_memory_size using the SigComp feedback mechanism.
2. If the state creation request needs more state memory than the
total state_memory_size for the compartment, the state handler
deletes all but the first (state_memory_size - 64) bytes from the
state_value. It sets the state_length to (state_memory_size -
64), and recalculates the state_identifier as defined in Section
9.4.9.
3. If the state creation request contains a state_identifier that
already exists then the state handler checks whether the requested
state item is identical to the established state item and counts
the state creation request as successful if this is the case. If
not then the state creation request is unsuccessful (although the
probability that this will occur is vanishingly small).
4. If the state creation request exceeds the state memory allocated
to the compartment, sufficient items of state created by the same
compartment are freed until enough memory is available to
accommodate the new state. When a state item is freed, it is
removed from the list of states created by the compartment and the
memory cost of the state item no longer counts towards the total
cost for the compartment. Note, however, that identical state
items may be created by several different compartments, so a state
item must not be physically deleted unless the state handler
determines that it is no longer required by any compartment.
5. The order in which the existing state items are freed is
determined by the state_retention_priority, which is set when the
state items are created. The state_retention_priority of 65535 is
reserved for locally available states; these states must always be
freed first. Apart from this special case, states with the lowest
state_retention_priority are always freed first. In the event of
a tie, then the state item created first in the compartment is
also the first to be freed.
The state_retention_priority is always stored on a per-compartment
basis as part of the list of state items created by each compartment.
In particular, the same state item might have several priority values
if it has been created by several different compartments.
Note that locally available state items (as described in Section
3.3.3) need not be mapped to any particular compartment. However, if
they are created on a per-compartment basis, then they must not
interfere with the state created at the request of the remote
endpoint. The special state_retention_priority of 65535 is reserved
for locally available state items to ensure that this is the case.
The UDVM may also explicitly request the state handler to free a
specific state item in a compartment. In this case, the state
handler deletes the state item from the list of state items created
by the compartment (as before the state item itself must not be
physically deleted unless the state handler determines that it is not
longer required by any compartment).
The application should indicate to the state handler when it wishes
to close a particular compartment, so that the resources taken by the
corresponding state can be reclaimed.
6.3. Feedback Data
The SigComp feedback mechanism allows feedback data to be received by
a UDVM and forwarded via the state handler to the correct compressor.
Since this feedback data is retained between SigComp messages, it is
considered to be part of the overall state and can only be forwarded
if accompanied by a valid compartment identifier. If this is the
case, then the state handler forwards the feedback data to the
compressor responsible for sending messages that pertain to the peer
compartment of the specified compartment.
7. SigComp Message Format
This chapter describes the format of the SigComp message and how the
message is used to initialize the UDVM memory.
Note that the SigComp message is not copied into the UDVM memory as
soon as it arrives; instead, the UDVM indicates when it requires
compressed data using a specific instruction. It then pauses and
waits for the information to be supplied before executing the next
instruction. This means that the UDVM can begin to decompress a
SigComp message before the entire message has been received.
A consequence of the above behavior is that when the UDVM is invoked,
the size of the UDVM memory depends on whether the transport used to
provide the SigComp message is stream-based or message-based. If the
transport is message-based then sufficient memory must be available
to buffer the entire SigComp message before it is passed to the UDVM.
So if the message is n bytes long, then the UDVM memory size is set
to (decompression_memory_size - n), up to a maximum of 65536 bytes.
If the transport is stream-based however, then a fixed-size input
buffer is required to accommodate the stream, independently of the
size of each SigComp message. So, for simplicity, the UDVM memory
size is set to (decompression_memory_size / 2).
As a separate instance of the UDVM is invoked on a per-message basis,
each SigComp message must explicitly indicate its chosen
decompression algorithm as well as any additional information that is
needed to decompress the message (e.g., one or more previously
received messages, a dictionary of common SIP phrases etc.). This
information can either be uploaded as part of the SigComp message or
retrieved from an item of state.
A SigComp message takes one of two forms depending on whether it
accesses a state item at the receiving endpoint. The two variants of
a SigComp message are given in Figure 3. (The T-bit controls the
format of the returned feedback item and is defined in Section 7.1.)
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+---+
1 1 1 1 1 T len 1 1 1 1 1 T 0
+---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+---+
: returned feedback item : : returned feedback item :
+---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+---+
code_len
: partial state identifier : +---+---+---+---+---+---+---+---+
code_len destination
+---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+---+
: remaining SigComp message : : uploaded UDVM bytecode :
+---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+---+
: remaining SigComp message :
+---+---+---+---+---+---+---+---+
Figure 3: Format of a SigComp message
Decompression failure occurs if the SigComp message is too short to
contain the expected fields (see Section 8.7 for further details).
The fields except for the "remaining SigComp message" are referred to
as the "SigComp header" (note that this may include the uploaded UDVM
bytecode).
7.1. Returned feedback item
For both variants of the SigComp message, the T-bit is set to 1
whenever the SigComp message contains a returned feedback item. The
format of the returned feedback item is illustrated in Figure 4.
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+---+
0 returned_feedback_field 1 returned_feedback_length
+---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+---+
: returned_feedback_field :
+---+---+---+---+---+---+---+---+
Figure 4: Format of returned feedback item
Note that the returned feedback length specifies the size of the
returned feedback field (from 0 to 127 bytes). So the total size of
the returned feedback item lies between 1 and 128 bytes.
The returned feedback item is not copied to the UDVM memory; instead,
it is buffered until the UDVM has successfully decompressed the
SigComp message. It is then forwarded to the state handler with the
rest of the feedback data (see Section 9.4.9 for further details).
7.2. Accessing Stored State
The len field of the SigComp message determines which fields follow
the returned feedback item. If the len field is non-zero, then the
SigComp message contains a state identifier to access a state item at
the receiving endpoint. All state items include a 20-byte state
identifier as per Section 3.3.3, but it is possible to transmit as
few as 6 bytes from the identifier if the sender believes that this
is sufficient to match a unique state item at the receiving endpoint.
The len field encodes the number of transmitted bytes as follows:
Encoding: Length of partial state identifier
01 6 bytes
10 9 bytes
11 12 bytes
The partial state identifier is passed to the state handler, which
compares it with the most significant bytes of the state_identifier
in every currently stored state item. Decompression failure occurs
if no state item is matched or if more than one state item is
matched.
Decompression failure also occurs if exactly one state item is
matched but the state item contains a minimum_access_length greater
than the length of the partial state identifier. This prevents
especially sensitive state items from being accessed maliciously by
brute force guessing of the state_identifier.
If a state item is successfully accessed then the state_value byte
string is copied into the UDVM memory beginning at state_address.
The first 32 bytes of UDVM memory are then initialized to special
values as illustrated in Figure 5.
0 7 8 15
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
UDVM_memory_size 0 - 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
cycles_per_bit 2 - 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
SigComp_version 4 - 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
partial_state_ID_length 6 - 7
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
state_length 8 - 9
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: reserved : 10 - 31
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: Initializing Useful Values in UDVM memory
The first five 2-byte words are initialized to contain some values
that might be useful to the UDVM bytecode (Useful Values). Note that
these values are for information only and can be overwritten when
executing the UDVM bytecode without any effect on the endpoint. The
MSBs of each 2-byte word are stored preceding the LSBs.
Addresses 0 to 5 indicate the resources available to the receiving
endpoint. The UDVM memory size is expressed in bytes modulo 2^16, so
in particular, it is set to 0 if the UDVM memory size is 65536 bytes.
The cycles_per_bit is expressed as a 2-byte integer taking the value
16, 32, 64 or 128. The SigComp_version is expressed as a 2-byte
value as per Section 3.3.2.
Addresses 6 to 9 are initialized to the length of the partial state
identifier, followed by the state_length from the retrieved state
item. Both are expressed as 2-byte values.
Addresses 10 to 31 are reserved and are initialized to 0 for Version
0x01 of SigComp. Future versions of SigComp can use these locations
for additional Useful Values, so a decompressor MUST NOT rely on
these values being zero.
Any remaining addresses in the UDVM memory that have not yet been
initialized MUST be set to 0.
The UDVM then begins executing instructions at the memory address
contained in state_instruction (which is part of the retrieved item
of state). Note that the remaining SigComp message is held by the
decompressor dispatcher until requested by the UDVM.
(Note that the Useful Values are only set at UDVM startup; there is
no special significance to this memory area afterwards. This means
that the UDVM bytecode is free to use these locations for any other
purpose a memory location might be used for; it just has to be aware
they are not necessarily initialized to zero.)
7.3. Uploading UDVM bytecode
If the len field is set to 0 then the bytecode needed to decompress
the SigComp message is supplied as part of the message itself. The
12-bit code_len field specifies the size of the uploaded UDVM
bytecode (from 0 to 4095 bytes inclusive); eight most significant
bits are in the first byte, followed by the four least significant
bits in the most significant bits in the second byte. The remaining
bits in the second byte are interpreted as a 4-bit destination field
that specifies the starting memory address to which the bytecode is
copied. The destination field is encoded as follows:
Encoding: Destination address:
0000 reserved
0001 2 * 64 = 128
0010 3 * 64 = 196
0011 4 * 64 = 256
: :
1111 16 * 64 = 1024
Note that the encoding 0000 is reserved for future SigComp versions,
and causes a decompression failure in Version 0x01.
The UDVM memory is initialized as per Figure 5, except that addresses
6 to 9 inclusive are set to 0 because no state item has been
accessed. The UDVM then begins executing instructions at the memory
address specified by the destination field. As above, the remaining
SigComp message is held by the decompressor dispatcher until needed
by the UDVM.
8. Overview of the UDVM
Decompression functionality for SigComp is provided by a Universal
Decompressor Virtual Machine (UDVM). The UDVM is a virtual machine
much like the Java Virtual Machine but with a key difference: it is
designed solely for the purpose of running decompression algorithms.
The motivation for creating the UDVM is to provide flexibility when
choosing how to compress a given application message. Rather than
picking one of a small number of pre-negotiated algorithms, the
compressor implementer has the freedom to select an algorithm of
their choice. The compressed data is then combined with a set of
UDVM instructions that allow the original data to be extracted, and
the result is outputted as a SigComp message. Since the UDVM is
optimized specifically for running decompression algorithms, the code
size of a typical algorithm is small (often sub 100 bytes).
Moreover, the UDVM approach does not add significant extra processing
or memory requirements compared to running a fixed preprogrammed
decompression algorithm.
Figure 6 gives a detailed view of the interfaces between the UDVM and
its environment.
+----------------+ +----------------+
Request compressed data
-------------------------------->
<--------------------------------
Provide compressed data
Output decompressed data Decompressor
--------------------------------> dispatcher
Indicate end of message
-------------------------------->
<--------------------------------
UDVM Provide compartment identifier
+----------------+
+----------------+
Request state information
-------------------------------->
<--------------------------------
Provide state information State
handler
Make state creation request
-------------------------------->
Forward feedback information
+----------------+ +----------------+
Figure 6: Interfaces between the UDVM and its environment
Note that once the UDVM has been initialized, additional compressed
data and state information are only provided at the request of a
specific UDVM instruction.
This chapter describes the basic features of the UDVM including the
UDVM registers and the format of UDVM bytecode.
8.1. UDVM Registers
The UDVM registers are 2-byte words in the UDVM memory that have
special tasks, for example specifying the location of the stack used
by the CALL and RETURN instructions.
The UDVM registers are illustrated in Figure 7.
0 7 8 15
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
byte_copy_left 64 - 65
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
byte_copy_right 66 - 67
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
input_bit_order 68 - 69
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
stack_location 70 - 71
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: Memory addresses of the UDVM registers
The MSBs of each register are always stored before the LSBs. So, for
example, the MSBs of byte_copy_left are stored at Address 64 whilst
the LSBs are stored at Address 65.
The use of each UDVM register is defined in the following sections.
(Note that the UDVM registers start at Address 64, that is 32 bytes
after the area reserved for Useful Values. The intention is that the
gap, i.e., the area between Address 32 and Address 63, will often be
used as scratch-pad memory that is guaranteed to be zero at UDVM
startup and is efficiently addressable in operand types reference ($)
and multitype (%).)
8.2. Requesting Additional Compressed Data
The decompressor dispatcher stores the compressed data from the
SigComp message before it is requested by the UDVM via one of the
INPUT instructions. When the UDVM bytecode is first executed, the
dispatcher contains the remaining SigComp message after the header
has been used to initialize the UDVM as per Chapter 7.
Note that the INPUT-BITS and INPUT-HUFFMAN instructions retrieve a
stream of individual compressed bits from the dispatcher. To provide
bitwise compatibility with various well-known compression algorithms,
the input_bit_order register can modify the order in which individual
bits are passed within a byte.
The input_bit_order register contains the following three flags:
0 7 8 15
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
reserved FHP 68 - 69
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The P-bit controls the order in which bits are passed from the
dispatcher to the INPUT instructions. If set to 0, it indicates that
the bits within an individual byte are passed to the INPUT
instructions in MSB to LSB order. If it is set to 1, the bits are
passed in LSB to MSB order.
Note that the input_bit_order register cannot change the order in
which the bytes themselves are passed to the INPUT instructions
(bytes are always passed in the same order as they occur in the
SigComp message).
The following diagram illustrates the order in which bits are passed
to the INPUT instructions for both cases:
MSB LSB MSB LSB MSB LSB MSB LSB
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 1 2 3 4 5 6 78 9 ... 7 6 5 4 3 2 1 0 ... 9 8
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Byte 0 Byte 1 Byte 0 Byte 1
P = 0 P = 1
Note that after one or more INPUT instructions the dispatcher may
hold a fraction of a byte (what used to be the LSBs if P = 0, or, the
MSBs, if P = 1). If an INPUT instruction is encountered and the P-
bit has changed since the last INPUT instruction, any fraction of a
byte still held by the dispatcher MUST be discarded (even if the
INPUT instruction requests zero bits). The first bit passed to the
INPUT instruction is taken from the subsequent byte.
When an INPUT instruction requests n bits of compressed data, it
interprets the received bits as an integer between 0 and 2^n - 1.
The F-bit and the H-bit specify whether the bits in these integers
are considered to arrive in MSB to LSB order (bit set to 0) or in LSB
to MSB order (bit set to 1).
If the F-bit is set to 0, the INPUT-BITS instruction interprets the
received bits as arriving MSBs first, and if it is set to 1, it
interprets the bits as arriving LSBs first. The H-bit performs the
same function for the INPUT-HUFFMAN instruction. Note that it is
possible to set these two bits to different values in order to use
different bit orders for the two instructions (certain algorithms
actually require this, e.g., DEFLATE [RFC-1951]). (Note that there
are no special considerations for changing the F- or H-bit between
INPUT instructions, unlike the discard rule for the P-bit described
above.)
Decompression failure occurs if an INPUT-BITS or an INPUT-HUFFMAN
instruction is encountered and the input_bit_order register does not
lie between 0 and 7 inclusive.
8.3. UDVM Stack
Certain UDVM instructions make use of a stack of 2-byte words stored
at the memory address specified by the 2-byte word stack_location.
The stack contains the following words:
Name: Starting memory address:
stack_fill stack_location
stack[0] stack_location + 2
stack[1] stack_location + 4
stack[2] stack_location + 6
: :
The notation stack_location is an abbreviation for the contents of
the stack_location register, i.e., the 2-byte word at locations 70
and 71. The notation stack_fill is an abbreviation for the 2-byte
word at stack_location and stack_location+1. Similarly, the notation
stack[n] is an abbreviation for the 2-byte word at
stack_location+2*n+2 and stack_location+2*n+3. (As always, the
arithmetic is modulo 2^16.)
The stack is used by the CALL, RETURN, PUSH and POP instructions.
"Pushing" a value on the stack is an abbreviation for copying the
value to stack[stack_fill] and then increasing stack_fill by 1. CALL
and PUSH push values on the stack.
"Popping" a value from the stack is an abbreviation for decreasing
stack_fill by 1, and then using the value stored in
stack[stack_fill]. Decompression failure occurs if stack_fill is
zero at the commencement of a popping operation. POP and RETURN pop
values from the stack.
For both of these abstract operations, the UDVM first takes note of
the current value of stack_location and uses this value for both
sub-operations (accessing the stack and manipulating stack_fill),
i.e., overwriting stack_location in the course of the operation is
inconsequential for the operation.
8.4. Byte copying
A number of UDVM instructions require a string of bytes to be copied
to and from areas of the UDVM memory. This section defines how the
byte copying operation should be performed.
The string of bytes is copied in ascending order of memory address,
respecting the bounds set by byte_copy_left and byte_copy_right.
More precisely, if a byte is copied from/to Address m then the next
byte is copied from/to Address n where n is calculated as follows:
Set k := m + 1 (modulo 2^16)
If k = byte_copy_right then set n := byte_copy_left, else set n := k
Decompression failure occurs if a byte is copied from/to an address
beyond the UDVM memory.
Note that the string of bytes is copied one byte at a time. In
particular, some of the later bytes to be copied may themselves have
been written into the UDVM memory by the byte copying operation
currently being performed.
Equally, it is possible for a byte copying operation to overwrite the
instruction that invoked the byte copy. If this occurs, then the
byte copying operation MUST be completed as if the original
instruction were still in place in the UDVM memory (this also applies
if byte_copy_left or byte_copy_right are overwritten).
Byte copying is used by the following UDVM instructions:
SHA-1, COPY, COPY-LITERAL, COPY-OFFSET, MEMSET, INPUT-BYTES, STATE-
ACCESS, OUTPUT, END-MESSAGE
8.5. Instruction operands and UDVM bytecode
Each of the UDVM instructions in a piece of UDVM bytecode is
represented by a single byte, followed by 0 or more bytes containing
the operands required by the instruction.
During instruction execution, conceptually the UDVM first fetches the
first byte of the instruction, determines the number and types of
operands required for this instruction, and then decodes all the
operands in sequence before starting to act on the instruction.
(Note that the UDVM instructions have been designed in such a way
that this sequence remains conceptual in those cases where it would
result in an unreasonable burden on the implementation.)
To reduce the size of typical UDVM bytecode, each operand for a UDVM
instruction is compressed using variable-length encoding. The aim is
to store more common operand values using fewer bytes than rarely
occurring values.
Four different types of operand are available: the literal, the
reference, the multitype and the address. Chapter 9 gives a complete
list of UDVM instructions and the operand types that follow each
instruction.
The UDVM bytecode for each operand type is illustrated in Figure 8 to
Figure 10, together with the integer values represented by the
bytecode.
Note that the MSBs in the bytecode are illustrated as preceding the
LSBs. Also, any string of bits marked with k consecutive "n"s is to
be interpreted as an integer N from 0 to 2^k - 1 inclusive (with the
MSBs of n illustrated as preceding the LSBs).
The decoded integer value of the bytecode can be interpreted in two
ways. In some cases it is taken to be the actual value of the
operand. In other cases it is taken to be a memory address at which
the 2-byte operand value can be found (MSBs found at the specified
address, LSBs found at the following address). The latter cases are
denoted by memory[X] where X is the address and memory[X] is the 2-
byte value starting at Address X.
The simplest operand type is the literal (#), which encodes a
constant integer from 0 to 65535 inclusive. A literal operand may
require between 1 and 3 bytes depending on its value.
Bytecode: Operand value: Range:
0nnnnnnn N 0 - 127
10nnnnnn nnnnnnnn N 0 - 16383
11000000 nnnnnnnn nnnnnnnn N 0 - 65535
Figure 8: Bytecode for a literal (#) operand
The second operand type is the reference ($), which is always used to
access a 2-byte value located elsewhere in the UDVM memory. The
bytecode for a reference operand is decoded to be a constant integer
from 0 to 65535 inclusive, which is interpreted as the memory address
containing the actual value of the operand.
Bytecode: Operand value: Range:
0nnnnnnn memory[2 * N] 0 - 65535
10nnnnnn nnnnnnnn memory[2 * N] 0 - 65535
11000000 nnnnnnnn nnnnnnnn memory[N] 0 - 65535
Figure 9: Bytecode for a reference ($) operand
Note that the range of a reference operand is always 0 - 65535
independently of how many bits are used to encode the reference,
because the operand always references a 2-byte value in the memory.
The third kind of operand is the multitype (%), which can be used to
encode both actual values and memory addresses. The multitype
operand also offers efficient encoding for small integer values (both
positive and negative) and for powers of 2.
Bytecode: Operand value: Range:
00nnnnnn N 0 - 63
01nnnnnn memory[2 * N] 0 - 65535
1000011n 2 ^ (N + 6) 64 , 128
10001nnn 2 ^ (N + 8) 256 , ... , 32768
111nnnnn N + 65504 65504 - 65535
1001nnnn nnnnnnnn N + 61440 61440 - 65535
101nnnnn nnnnnnnn N 0 - 8191
110nnnnn nnnnnnnn memory[N] 0 - 65535
10000000 nnnnnnnn nnnnnnnn N 0 - 65535
10000001 nnnnnnnn nnnnnnnn memory[N] 0 - 65535
Figure 10: Bytecode for a multitype (%) operand
The fourth operand type is the address (@). This operand is decoded
as a multitype operand followed by a further step: the memory address
of the UDVM instruction containing the address operand is added to
obtain the correct operand value. So if the operand value from
Figure 10 is D then the actual operand value of an address is
calculated as follows:
operand_value = (memory_address_of_instruction + D) modulo 2^16
Address operands are always used in instructions that control program
flow, because they ensure that the UDVM bytecode is position-
independent code (i.e., it will run independently of where it is
placed in the UDVM memory).
8.6. UDVM Cycles
Once the UDVM has been invoked it executes the instructions contained
in its memory consecutively unless otherwise indicated (for example
when the UDVM encounters a JUMP instruction). If the next
instruction to be executed lies outside the available memory then
decompression failure occurs (see Section 8.7).
To ensure that a SigComp message cannot consume excessive processing
resources, SigComp limits the number of "UDVM cycles" allocated to
each message. The number of available UDVM cycles is initialized to
1000 plus the number of bits in the SigComp header (as described in
Section 7); this sum is then multiplied by cycles_per_bit. Each time
an instruction is executed the number of available UDVM cycles is
decreased by the amount specified in Chapter 9. Additionally, if the
UDVM successfully requests n bits of compressed data using one of the
INPUT instructions then the number of available UDVM cycles is
increased by n * cycles_per_bit once the instruction has been
executed.
This means that the maximum number of UDVM cycles available for
processing an n-byte SigComp message is given by the formula:
maximum_UDVM_cycles = (8 * n + 1000) * cycles_per_bit
The reason that this total is not allocated to the UDVM when it is
invoked is that the UDVM can begin to decompress a message that has
only been partially received. So the total message size may not be
known when the UDVM is initialized.
Note that the number of UDVM cycles MUST NOT be increased if a
request for additional compressed data fails.
The UDVM stops executing instructions when it encounters an END-
MESSAGE instruction or if decompression failure occurs (see Section
8.7 for further details).
8.7. Decompression Failure
If a compressed message given to the UDVM is corrupted (either
accidentally or maliciously), then the UDVM may terminate with a
decompression failure.
Reasons for decompression failure include the following:
1. A SigComp message contains an invalid header as per Chapter 7.
2. A SigComp message is larger than the decompression_memory_size.
3. An instruction costs more than the number of remaining UDVM
cycles.
4. The UDVM attempts to read from or write to a memory address beyond
its memory size.
5. An unknown instruction is encountered.
6. An unknown operand is encountered.
7. An instruction is encountered that cannot be processed
successfully by the UDVM (for example a RETURN instruction when no
CALL instruction has previously been encountered).
8. A request to access some state information fails.
9. A manual decompression failure is triggered using the
DECOMPRESSION-FAILURE instruction.
If a decompression failure occurs when decompressing a message then
the UDVM informs the dispatcher and takes no further action. It is
the responsibility of the dispatcher to decide how to cope with the
decompression failure. In general a dispatcher SHOULD discard the
compressed message (or the compressed stream if the transport is
stream-based) and any decompressed data that has been outputted but
not yet passed to the application.
9. UDVM Instruction Set
The UDVM currently understands 36 instructions, chosen to support the
widest possible range of compression algorithms with the minimum
possible overhead.
Figure 11 lists the different instructions and the bytecode values
used to encode the instructions. The cost of each instruction in
UDVM cycles is also given:
Instruction: Bytecode value: Cost in UDVM cycles:
DECOMPRESSION-FAILURE 0 1
AND 1 1
OR 2 1
NOT 3 1
LSHIFT 4 1
RSHIFT 5 1
ADD 6 1
SUBTRACT 7 1
MULTIPLY 8 1
DIVIDE 9 1
REMAINDER 10 1
SORT-ASCENDING 11 1 + k * (ceiling(log2(k)) + n)
SORT-DESCENDING 12 1 + k * (ceiling(log2(k)) + n)
SHA-1 13 1 + length
LOAD 14 1
MULTILOAD 15 1 + n
PUSH 16 1
POP 17 1
COPY 18 1 + length
COPY-LITERAL 19 1 + length
COPY-OFFSET 20 1 + length
MEMSET 21 1 + length
JUMP 22 1
COMPARE 23 1
CALL 24 1
RETURN 25 1
SWITCH 26 1 + n
CRC 27 1 + length
INPUT-BYTES 28 1 + length
INPUT-BITS 29 1
INPUT-HUFFMAN 30 1 + n
STATE-ACCESS 31 1 + state_length
STATE-CREATE 32 1 + state_length
STATE-FREE 33 1
OUTPUT 34 1 + output_length
END-MESSAGE 35 1 + state_length
Figure 11: UDVM instructions and corresponding bytecode values
Each UDVM instruction costs a minimum of 1 UDVM cycle. Certain
instructions may cost additional cycles depending on the values of
the instruction operands. Named variables in the cost expressions
refer to the values of the instruction operands with these names.
Note that for the SORT instructions, the formula ceiling(log2(k))
calculates the smallest value i such that k <= 2^i.
The UDVM instruction set offers a mix of low-level and high-level
instructions. The high-level instructions can all be emulated using
combinations of low-level instructions, but given a choice it is
generally preferable to use a single instruction rather than a large
number of general-purpose instructions. The resulting bytecode will
be more compact (leading to a higher overall compression ratio) and
decompression will typically be faster because the implementation of
the high-level instructions can be more easily optimized.
All instructions are encoded as a single byte to indicate the
instruction type, followed by 0 or more bytes containing the operands
required by the instruction. The instruction specifies which of the
four operand types of Section 8.5 is used in each case. For example
the ADD instruction is followed by two operands:
ADD ($operand_1, %operand_2)
When converted into bytecode the number of bytes required by the ADD
instruction depends on the value of each operand, and whether the
multitype operand contains the operand value itself or a memory
address where the actual value of the operand can be found.
Each instruction is explained in more detail below.
Whenever the description of an instruction uses the expression "and
then", the intended semantics is that the effect explained before
"and then" is completed before work on the effect explained after the
"and then" is commenced.
9.1. Mathematical Instructions
The following instructions provide a number of mathematical
operations including bit manipulation, arithmetic and sorting.
9.1.1. Bit Manipulation
The AND, OR, NOT, LSHIFT and RSHIFT instructions provide simple bit
manipulation on 2-byte words.
AND ($operand_1, %operand_2)
OR ($operand_1, %operand_2)
NOT ($operand_1)
LSHIFT ($operand_1, %operand_2)
RSHIFT ($operand_1, %operand_2)
After the operation is complete, the value of the first operand is
overwritten with the result. (Note that since this operand is a
reference, it is the 2-byte word at the memory address specified by
the operand that is overwritten.)
The precise definitions of LSHIFT and RSHIFT are given below. Note
that m and n are the 2-byte values encoded by the operands, and that
floor(x) calculates the largest integer not greater than x:
LSHIFT (m, n) := m * 2^n (modulo 2^16)
RSHIFT (m, n) := floor(m / 2^n)
9.1.2. Arithmetic
The ADD, SUBTRACT, MULTIPLY, DIVIDE and REMAINDER instructions
perform arithmetic on 2-byte words.
ADD ($operand_1, %operand_2)
SUBTRACT ($operand_1, %operand_2)
MULTIPLY ($operand_1, %operand_2)
DIVIDE ($operand_1, %operand_2)
REMAINDER ($operand_1, %operand_2)
After the operation is complete, the value of the first operand is
overwritten with the result.
The precise definition of each instruction is given below:
ADD (m, n) := m + n (modulo 2^16)
SUBTRACT (m, n) := m - n (modulo 2^16)
MULTIPLY (m, n) := m * n (modulo 2^16)
DIVIDE (m, n) := floor(m / n)
REMAINDER (m, n) := m - n * floor(m / n)
Decompression failure occurs if a DIVIDE or REMAINDER instruction
encounters an operand_2 that is zero.
9.1.3. Sorting
The SORT-ASCENDING and SORT-DESCENDING instructions sort lists of 2-
byte words.
SORT-ASCENDING (%start, %n, %k)
SORT-DESCENDING (%start, %n, %k)
The start operand specifies the starting memory address of the block
of data to be sorted.
The block of data itself is divided into n lists each containing k
2-byte words. The SORT-ASCENDING instruction applies a certain
permutation to the lists, such that the first list is sorted into
ascending order (treating each 2-byte word as an unsigned integer).
The same permutation is applied to all n lists, so lists other than
the first will not necessarily be sorted into order.
In the case that two words have the same value, the original ordering
of the list is preserved.
For example, the first list might contain a set of integers to be
sorted whilst the second list might be used to keep track of where
the integers appear in the sorted list:
Before sorting After sorting
List 1 List 2 List 1 List 2
8 1 1 2
1 2 1 3
1 3 3 4
3 4 8 1
The SORT-DESCENDING instruction behaves as above, except that the
first list is sorted into descending order.
9.1.4. SHA-1
The SHA-1 instruction calculates a 20-byte SHA-1 hash [RFC-3174] over
the specified area of UDVM memory.
SHA-1 (%position, %length, %destination)
The position and length operands specify the starting memory address
and the length of the byte string over which the SHA-1 hash is
calculated. Byte copying rules are enforced as per Section 8.4.
The destination operand gives the starting address to which the
resulting 20-byte hash will be copied. Byte copying rules are
enforced as above.
9.2. Memory Management Instructions
The following instructions are used to set up the UDVM memory, and to
copy byte strings from one memory location to another.
9.2.1. LOAD
The LOAD instruction sets a 2-byte word to a certain specified value.
The format of a LOAD instruction is as follows:
LOAD (%address, %value)
The first operand specifies the starting address of a 2-byte word,
whilst the second operand specifies the value to be loaded into this
word. As usual, MSBs are stored before LSBs in the UDVM memory.
9.2.2. MULTILOAD
The MULTILOAD instruction sets a contiguous block of 2-byte words in
the UDVM memory to specified values.
MULTILOAD (%address, #n, %value_0, ..., %value_n-1)
The first operand specifies the starting address of the contiguous
2-byte words, whilst the operands value_0 through to value_n-1
specify the values to load into these words (in the same order as
they appear in the instruction).
Decompression failure occurs if the set of 2-byte words set by the
instruction would overlap the memory locations held by the
instruction (including its operands) itself, i.e., if the instruction
would be self-modifying. (This restriction makes it simpler to
implement MULTILOAD step-by-step instead of having to decode all
operands before being able to copy data, as is implied by the
conceptual model of instruction execution.)
9.2.3. PUSH and POP
The PUSH and POP instructions read from and write to the UDVM stack
(as defined in Section 8.3).
PUSH (%value)
POP (%address)
The PUSH instruction pushes the value specified by its operand on the
stack.
The POP instruction pops a value from the stack and then copies the
value to the specified memory address. (Note that the expression
"and then" implies that the copying of the value is inconsequential
for the stack operation itself, which happens beforehand.)
See Section 8.3 for possible error conditions.
9.2.4. COPY
The COPY instruction is used to copy a string of bytes from one part
of the UDVM memory to another.
COPY (%position, %length, %destination)
The position operand specifies the memory address of the first byte
in the string to be copied, and the length operand specifies the
number of bytes to be copied.
The destination operand gives the address to which the first byte in
the string will be copied.
Byte copying is performed as per the rules of Section 8.4.
9.2.5. COPY-LITERAL
A modified version of the COPY instruction is given below:
COPY-LITERAL (%position, %length, $destination)
The COPY-LITERAL instruction behaves as a COPY instruction except
that after copying is completed, the value of the destination operand
is replaced by the address to which the next byte of data would be
copied. More precisely it is replaced by the value n, derived as per
Section 8.4 with m set to the destination address of the last byte to
be copied, if any (i.e., if the value of the length operand is zero,
the value of the destination operand is not changed).
9.2.6. COPY-OFFSET
A further version of the COPY-LITERAL instruction is given below:
COPY-OFFSET (%offset, %length, $destination)
The COPY-OFFSET instruction behaves as a COPY-LITERAL instruction
except that an offset operand is given instead of a position operand.
To derive the value of the position operand, starting at the memory
address specified by destination, the UDVM counts backwards a total
of offset memory addresses.
If the memory address specified in byte_copy_left is reached, the
next memory address is taken to be (byte_copy_right - 1) modulo 2^16.
The COPY-OFFSET instruction then behaves as a COPY-LITERAL
instruction, taking the value of the position operand to be the last
memory address reached in the above step.
9.2.7. MEMSET
The MEMSET instruction initializes an area of UDVM memory to a
specified sequence of values. The format of a MEMSET instruction is
as follows:
MEMSET (%address, %length, %start_value, %offset)
The sequence of values used by the MEMSET instruction is specified by
the following formula:
Seq[n] := (start_value + n * offset) modulo 256
The values Seq[0] to Seq[length - 1] inclusive are each interpreted
as a single byte, and then concatenated to form a byte string where
the first byte has value Seq[0], the second byte has value Seq[1] and
so on up to the last byte which has value Seq[length - 1].
The string is then byte copied into the UDVM memory beginning at the
memory address specified as an operand to the MEMSET instruction,
obeying the rules of Section 8.4. (Note that the byte string may
overwrite the MEMSET instruction or its operands; as explained in
Section 8.5, the MEMSET instruction must be executed as if the
original operands were still in place in the UDVM memory.)
9.3. Program Flow Instructions
The following instructions alter the flow of UDVM code. Each
instruction jumps to one of a number of memory addresses based on a
certain specified criterion.
Note that certain I/O instructions (see Section 9.4) can also alter
program flow.
9.3.1. JUMP
The JUMP instruction moves program execution to the specified memory
address.
JUMP (@address)
Decompression failure occurs if the value of the address operand lies
beyond the overall UDVM memory size.
9.3.2. COMPARE
The COMPARE instruction compares two operands and then jumps to one
of three specified memory addresses depending on the result.
COMPARE (%value_1, %value_2, @address_1, @address_2, @address_3)
If value_1 < value_2 then the UDVM continues instruction execution at
the memory address specified by address 1. If value_1 = value_2 then
it jumps to the address specified by address_2. If value_1 > value_2
then it jumps to the address specified by address_3.
9.3.3. CALL and RETURN
The CALL and RETURN instructions provide support for compression
algorithms with a nested structure.
CALL (@address)
RETURN
Both instructions use the UDVM stack of Section 8.3. When the UDVM
reaches a CALL instruction, it finds the memory address of the
instruction immediately following the CALL instruction and pushes
this 2-byte value on the stack, ready for later retrieval. It then
continues instruction execution at the memory address specified by
the address operand.
When the UDVM reaches a RETURN instruction it pops a value from the
stack and then continues instruction execution at the memory address
just popped.
See Section 8.3 for error conditions.
9.3.4. SWITCH
The SWITCH instruction performs a conditional jump based on the value
of one of its operands.
SWITCH (#n, %j, @address_0, @address_1, ... , @address_n-1)
When a SWITCH instruction is encountered the UDVM reads the value of
j. It then continues instruction execution at the address specified
by address j.
Decompression failure occurs if j specifies a value of n or more, or
if the address lies beyond the overall UDVM memory size.
9.3.5. CRC
The CRC instruction verifies a string of bytes using a 2-byte CRC.
CRC (%value, %position, %length, @address)
The actual CRC calculation is performed using the generator
polynomial x^16 + x^12 + x^5 + 1, which coincides with the 2-byte
Frame Check Sequence (FCS) of PPP [RFC-1662].
The position and length operands define the string of bytes over
which the CRC is evaluated. Byte copying rules are enforced as per
Section 8.4.
The CRC value is computed exactly as defined for the 16-bit FCS
calculation in [RFC-1662].
The value operand contains the expected integer value of the 2-byte
CRC. If the calculated CRC matches the expected value then the UDVM
continues instruction execution at the following instruction.
Otherwise the UDVM jumps to the memory address specified by the
address operand.
9.4. I/O instructions
The following instructions allow the UDVM to interface with its
environment. Note that in the overall SigComp architecture all of
these interfaces pass to the decompressor dispatcher or to the state
handler.
9.4.1. DECOMPRESSION-FAILURE
The DECOMPRESSION-FAILURE instruction triggers a manual decompression
failure. This is useful if the UDVM bytecode discovers that it
cannot successfully decompress the message (e.g., by using the CRC
instruction).
This instruction has no operands.
9.4.2. INPUT-BYTES
The INPUT-BYTES instruction requests a certain number of bytes of
compressed data from the decompressor dispatcher.
INPUT-BYTES (%length, %destination, @address)
The length operand indicates the requested number of bytes of
compressed data, and the destination operand specifies the starting
memory address to which they should be copied. Byte copying is
performed as per the rules of Section 8.4.
If the instruction requests data that lies beyond the end of the
SigComp message, no data is returned. Instead the UDVM moves program
execution to the address specified by the address operand.
If the INPUT-BYTES is encountered after an INPUT-BITS or an INPUT-
HUFFMAN instruction has been used, and the dispatcher currently holds
a fraction of a byte, then the fraction MUST be discarded before any
data is passed to the UDVM. The first byte to be passed is the byte
immediately following the discarded data.
9.4.3. INPUT-BITS
The INPUT-BITS instruction requests a certain number of bits of
compressed data from the decompressor dispatcher.
INPUT-BITS (%length, %destination, @address)
The length operand indicates the requested number of bits.
Decompression failure occurs if this operand does not lie between 0
and 16 inclusive.
The destination operand specifies the memory address to which the
compressed data should be copied. Note that the requested bits are
interpreted as a 2-byte integer ranging from 0 to 2^length - 1, as
explained in Section 8.2.
If the instruction requests data that lies beyond the end of the
SigComp message, no data is returned. Instead the UDVM moves program
execution to the address specified by the address operand.
9.4.4. INPUT-HUFFMAN
The INPUT-HUFFMAN instruction requests a variable number of bits of
compressed data from the decompressor dispatcher. The instruction
initially requests a small number of bits and compares the result
against a certain criterion; if the criterion is not met, then
additional bits are requested until the criterion is achieved.
The INPUT-HUFFMAN instruction is followed by three mandatory operands
plus n additional sets of operands. Every additional set contains
four operands as shown below:
INPUT-HUFFMAN (%destination, @address, #n, %bits_1, %lower_bound_1,
%upper_bound_1, %uncompressed_1, ... , %bits_n, %lower_bound_n,
%upper_bound_n, %uncompressed_n)
Note that if n = 0 then the INPUT-HUFFMAN instruction is ignored and
program execution resumes at the following instruction.
Decompression failure occurs if (bits_1 + ... + bits_n) > 16.
In all other cases, the behavior of the INPUT-HUFFMAN instruction is
defined below:
1. Set j := 1 and set H := 0.
2. Request bits_j compressed bits. Interpret the returned bits as an
integer k from 0 to 2^bits_j - 1, as explained in Section 8.2.
3. Set H := H * 2^bits_j + k.
4. If data is requested that lies beyond the end of the SigComp
message, terminate the INPUT-HUFFMAN instruction and move program
execution to the memory address specified by the address operand.
5. If (H < lower_bound_j) or (H > upper_bound_j) then set j := j + 1.
Then go back to Step 2, unless j > n in which case decompression
failure occurs.
6. Copy (H + uncompressed_j - lower_bound_j) modulo 2^16 to the
memory address specified by the destination operand.
9.4.5. STATE-ACCESS
The STATE-ACCESS instruction retrieves some previously stored state
information.
STATE-ACCESS (%partial_identifier_start, %partial_identifier_length,
%state_begin, %state_length, %state_address, %state_instruction)
The partial_identifier_start and partial_identifier_length operands
specify the location of the partial state identifier used to retrieve
the state information. This identifier has the same function as the
partial state identifier transmitted in the SigComp message as per
Section 7.2.
Decompression failure occurs if partial_identifier_length does not
lie between 6 and 20 inclusive. Decompression failure also occurs if
no state item matching the partial state identifier can be found, if
more than one state item matches the partial identifier, or if
partial_identifier_length is less than the minimum_access_length of
the matched state item. Otherwise, a state item is returned from the
state handler.
If any of the operands state_address, state_instruction or
state_length is set to 0 then its value is taken from the returned
item of state instead.
Note that when calculating the number of UDVM cycles the STATE-ACCESS
instruction costs (1 + state_length) cycles. The value of
state_length MUST be taken from the returned item of state in the
case that the state_length operand is set to 0.
The state_begin and state_length operands define the starting byte
and number of bytes to copy from the state_value contained in the
returned item of state. Decompression failure occurs if bytes are
copied from beyond the end of the state_value. Note that
decompression failure will always occur if the state_length operand
is set to 0 but the state_begin operand is non-zero.
The state_address operand contains a UDVM memory address. The
requested portion of the state_value is byte copied to this memory
address using the rules of Section 8.4.
Program execution then resumes at the memory address specified by
state_instruction, unless this address is 0 in which case program
execution resumes at the next instruction following the STATE-ACCESS
instruction. Note that the latter case only occurs if both the
state_instruction operand and the state_instruction value from the
requested state are set to 0.
9.4.6. STATE-CREATE
The STATE-CREATE instruction requests the creation of a state item at
the receiving endpoint.
STATE-CREATE (%state_length, %state_address, %state_instruction,
%minimum_access_length, %state_retention_priority)
Note that the new state item cannot be created until a valid
compartment identifier has been returned by the application.
Consequently, when a STATE-CREATE instruction is encountered the UDVM
simply buffers the five supplied operands until the END-MESSAGE
instruction is reached. The steps taken at this point are described
in Section 9.4.9.
Decompression failure MUST occur if more than four state creation
requests are made before the END-MESSAGE instruction is encountered.
Decompression failure also occurs if the minimum_access_length does
not lie between 6 and 20 inclusive, or if the
state_retention_priority is 65535.
9.4.7. STATE-FREE
The STATE-FREE instruction informs the receiving endpoint that the
sender no longer wishes to use a particular state item.
STATE-FREE (%partial_identifier_start, %partial_identifier_length)
Note that the STATE-FREE instruction does not automatically delete a
state item, but instead reclaims the memory taken by the state item
within a certain compartment, which is generally not known before the
END-MESSAGE instruction is reached. So just as for the STATE-CREATE
instruction, when a STATE-FREE instruction is encountered the UDVM
simply buffers the two supplied operands until the END-MESSAGE
instruction is reached. The steps taken at this point are described
in Section 9.4.9.
Decompression failure MUST occur if more than four state free
requests are made before the END-MESSAGE instruction is encountered.
Decompression failure also occurs if partial_identifier_length does
not lie between 6 and 20 inclusive.
9.4.8. OUTPUT
The OUTPUT instruction provides successfully decompressed data to the
dispatcher.
OUTPUT (%output_start, %output_length)
The operands define the starting memory address and length of the
byte string to be provided to the dispatcher. Note that the OUTPUT
instruction can be used to output a partially decompressed message;
each time the instruction is encountered it provides a new byte
string that the dispatcher appends to the end of any bytes previously
passed to the dispatcher via the OUTPUT instruction.
The string of data is byte copied from the UDVM memory obeying the
rules of Section 8.4.
Decompression failure occurs if the cumulative number of bytes
provided to the dispatcher exceeds 65536 bytes.
Since there is technically a difference between outputting a 0-byte
decompressed message, and not outputting a decompressed message at
all, the OUTPUT instruction needs to distinguish between the two
cases. Thus, if the UDVM terminates before encountering an OUTPUT
instruction it is considered not to have outputted a decompressed
message. If it encounters one or more OUTPUT instructions, each of
which provides 0 bytes of data to the dispatcher, then it is
considered to have outputted a 0-byte decompressed message.
9.4.9. END-MESSAGE
The END-MESSAGE instruction successfully terminates the UDVM and
forwards the state creation and state free requests to the state
handler together with any supplied feedback data.
END-MESSAGE (%requested_feedback_location,
%returned_parameters_location, %state_length, %state_address,
%state_instruction, %minimum_access_length,
%state_retention_priority)
When the END-MESSAGE instruction is encountered, the decompressor
dispatcher indicates to the application that a complete message has
been decompressed. The application may return a compartment
identifier, which the UDVM forwards to the state handler together
with the state creation and state free requests and any supplied
feedback data.
The actual decompressed message is outputted separately using the
OUTPUT instruction; this conserves memory at the UDVM because there
is no need to buffer an entire decompressed message before it can be
passed to the dispatcher.
The END-MESSAGE instruction may pass up to four state creation
requests and up to four state free requests to the state handler.
The requests are passed to the state handler in the same order as
they are made; in particular it is possible for the state creation
requests and the state free requests to be interleaved.
The state creation requests are made by the STATE-CREATE instruction.
Note however that the END-MESSAGE can make one state creation request
itself using the supplied operands. If the specified
minimum_access_length does not lie between 6 and 20 inclusive, or if
the state_retention_priority is 65535 then the END-MESSAGE
instruction fails to make a state creation request of its own
(however decompression failure does not occur and the state creation
requests made by the STATE-CREATE instruction are still valid).
Note that there is a maximum limit of four state creation requests
per instance of the UDVM. Therefore, decompression failure occurs if
the END-MESSAGE instruction makes a state creation request and four
instances of the STATE-CREATE instruction have already been
encountered.
When creating a state item it is necessary to give the state_length,
state address, state_instruction and minimum_access_length; these are
supplied as operands in the STATE-CREATE instruction (or the END-
MESSAGE instruction). A complete item of state also requires a
state_value and a state_identifier, which are derived as follows:
The UDVM byte copies a string of state_length bytes from the UDVM
memory beginning at state_address (obeying the rules of Section 8.4).
This is the state_value.
The UDVM then calculates a 20-byte SHA-1 hash [RFC-3174] over the
byte string formed by concatenating the state_length, state_address,
state_instruction, minimum_access_length and state_value (in the
order given). This is the state_identifier.
The state_retention_priority is not part of the state item itself,
but instead determines the order in which state will be deleted when
the compartment exceeds its allocated state memory. The
state_retention_priority is supplied as an operand in the STATE-
CREATE or END-MESSAGE instruction and is passed to the state handler
as part of each state creation request.
The state free requests are made by the STATE-FREE instruction. Each
STATE-FREE instruction supplies the values partial_identifier_start
and partial_identifier_length; upon reaching the END-MESSAGE
instruction these values are used to byte copy a partial state
identifier from the UDVM memory. If no state item matching the
partial state identifier can be found or if more than one state item
in the compartment matches the partial state identifier, then the
state free request is ignored (this does not cause decompression
failure to occur). Otherwise, the state handler frees the matched
state item as specified in Section 6.2.
As well as forwarding the state creation and state free requests, the
END-MESSAGE instruction may also pass feedback data to the state
handler. Feedback data is used to inform the receiving endpoint
about the capabilities of the sending endpoint, which can help to
improve the overall compression ratio and to reduce the working
memory requirements of the endpoints.
Two types of feedback data are available: requested feedback and
returned feedback. The format of the requested feedback data is
given in Figure 12. As outlined in Section 3.2, the requested
feedback data can be used to influence the contents of the returned
feedback data in the reverse direction.
The returned feedback data is itself subdivided into a returned
feedback item and a list of returned SigComp parameters. The
returned feedback item is of sufficient importance to warrant its own
field in the SigComp header as described in Section 7.1. The
returned SigComp parameters are illustrated in Figure 13.
Note that the formats of Figure 12 and Figure 13 are only for local
presentation of the feedback data on the interface between the UDVM
and state handler. The formats do not mandate any bits on the wire;
the compressor can transmit the data in any form provided that it is
loaded into the UDVM memory at the correct addresses.
Moreover, the responsibility for ensuring that feedback data arrives
successfully over an unreliable transport lies with the sender. The
receiving endpoint always uses the last received value for each field
in the feedback data, even if the values are out of date due to
packet loss or misordering.
If the requested_feedback_location operand is set to 0, then no
feedback request is made; otherwise, it points to the starting memory
address of the requested feedback data as shown in Figure 12.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
reserved Q S I requested_feedback_location
+---+---+---+---+---+---+---+---+
: requested feedback item : if Q = 1
+---+---+---+---+---+---+---+---+
Figure 12: Format of requested feedback data
The reserved bits may be used in future versions of SigComp, and are
set to 0 in Version 0x01. Non-zero values should be ignored by the
receiving endpoint.
The Q-bit indicates whether a requested feedback item is present or
not. The compressor can set the requested feedback item to an
arbitrary value, which will then be transmitted unmodified in the
reverse direction as a returned feedback item. See Chapter 5 for
further details of how the requested feedback item is returned.
The format of the requested feedback item is identical to the format
of the returned feedback item illustrated in Figure 4.
The compressor sets the S-bit to 1 if it does not wish (or no longer
wishes) to save state information at the receiving endpoint and also
does not wish to access state information that it has previously
saved. Consequently, if the S-bit is set to 1 then the receiving
endpoint can reclaim the state memory allocated to the remote
compressor and set the state_memory_size for the compartment to 0.
The compressor may change its mind and switch the S-bit back to 0 in
a later message. However, the receiving endpoint is under no
obligation to use the original state_memory_size for the compartment;
it may choose to allocate less memory to the compartment or possibly
none at all.
Similarly the compressor sets the I-bit to 1 if it does not wish (or
no longer wishes) to access any of the locally available state items
offered by the receiving endpoint. This can help to conserve
bandwidth because the list of locally available state items no longer
needs to be returned in the reverse direction. It may also conserve
memory at the receiving endpoint, as the state handler can delete any
locally available state items that it determines are no longer
required by any remote endpoint. Note that the compressor can set
the I-bit back to 0 in a later message, but it cannot access any
locally available state items that were previously offered by the
receiving endpoint unless they are subsequently re-announced.
If the returned_parameters_location operand is set to 0, then no
SigComp parameters are returned; otherwise, it points to the starting
memory address of the returned parameters as shown in Figure 13.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
cpb dms sms returned_parameters_location
+---+---+---+---+---+---+---+---+
SigComp_version
+---+---+---+---+---+---+---+---+
length_of_partial_state_ID_1
+---+---+---+---+---+---+---+---+
: partial_state_identifier_1 :
+---+---+---+---+---+---+---+---+
: :
+---+---+---+---+---+---+---+---+
length_of_partial_state_ID_n
+---+---+---+---+---+---+---+---+
: partial_state_identifier_n :
+---+---+---+---+---+---+---+---+
Figure 13: Format of returned SigComp parameters
The first byte encodes the SigComp parameters cycles_per_bit,
decompression_memory_size and state_memory_size as per Section 3.3.1.
The byte can be set to 0 if the three parameters are not included in
the feedback data. (This may be useful to save bits in the
compressed message if the remote endpoint is already satisfied all
necessary information has reached the endpoint receiving the
message.)
The second byte encodes the SigComp_version as per Section 3.3.2.
Similar to the first byte, the second byte can be set to 0 if the
parameter is not included in the feedback data.
The remaining bytes encode a list of partial state identifiers for
the locally available state items offered by the sending endpoint.
Each state item is encoded as a 1-byte length field, followed by a
partial state identifier containing as many bytes as indicated in the
length field. The sender can choose to send as few as 6 bytes if it
believes that this is sufficient for the receiver to determine which
state item is being offered.
The list of state identifiers is terminated by a byte in the position
where the next length field would be expected that is set to a value
below 6 or above 20. Note that upgraded SigComp versions may append
additional items of data after the final length field.
10. Security Considerations
10.1. Security Goals
The overall security goal of the SigComp architecture is to not
create risks that are in addition to those already present in the
application protocols. There is no intention for SigComp to enhance
the security of the application, as it always can be circumvented by
not using compression. More specifically, the high-level security
goals can be described as:
1. Do not worsen security of existing application protocol
2. Do not create any new security issues
3. Do not hinder deployment of application security.
10.2. Security Risks and Mitigation
This section identifies the potential security risks associated with
SigComp, and explains how each risk is minimized by the scheme.
10.2.1. Confidentiality Risks
- Attacking SigComp by snooping into state of other users:
State is accessed by supplying a state identifier, which is a
cryptographic hash of the state being referenced. This implies that
the referencing message already needs knowledge about the state. To
enforce this, a state item cannot be accessed without supplying a
minimum of 48 bits from the hash. This also minimizes the
probability of an accidental state collision. A compressor can,
using the minimum_access_length operand of the STATE-CREATE and END-
MESSAGE instructions, increase the number of bits that need to be
supplied to access the state, increasing the protection against
attacks.
Generally, ways to obtain knowledge about the state identifier (e.g.,
passive attacks) will also easily provide knowledge about the
referenced state, so no new vulnerability results.
An endpoint needs to handle state identifiers with the same care it
would handle the state itself.
10.2.2. Integrity Risks
The SigComp approach assumes that there is appropriate integrity
protection below and/or above the SigComp layer. The state creation
mechanism provides some additional potential to compromise the
integrity of the messages; however, this would most likely be
detectable at the application layer.
- Attacking SigComp by faking state or making unauthorized changes to
state:
State cannot be destroyed by a malicious sender unless it can send
messages that the application identifies as belonging to the same
compartment the state was created under; this adds additional
security risks only when the application allows the installation of
SigComp state from a message where it would not have installed state
itself.
Faking or changing state is only possible if the hash allows
intentional collision.
10.2.3. Availability Risks (Avoiding DoS Vulnerabilities)
- Use of SigComp as a tool in a DoS attack to another target:
SigComp cannot easily be used as an amplifier in a reflection attack,
as it only generates one decompressed message per incoming compressed
message. This message is then handed to the application; the utility
as a reflection amplifier is therefore limited by the utility of the
application for this purpose.
However, it must be noted that SigComp can be used to generate larger
messages as input to the application than have to be sent from the
malicious sender; this therefore can send smaller messages (at a
lower bandwidth) than are delivered to the application. Depending on
the reflection characteristics of the application, this can be
considered a mild form of amplification. The application MUST limit
the number of packets reflected to a potential target - even if
SigComp is used to generate a large amount of information from a
small incoming attack packet.
- Attacking SigComp as the DoS target by filling it with state:
Excessive state can only be installed by a malicious sender (or a set
of malicious senders) with the consent of the application. The
system consisting of SigComp and application is thus approximately as
vulnerable as the application itself, unless it allows the
installation of SigComp state from a message where it would not have
installed application state itself.
If this is desirable to increase the compression ratio, the effect
can be mitigated by making use of feedback at the application level
that indicates whether the state requested was actually installed -
this allows a system under attack to gracefully degrade by no longer
installing compressor state that is not matched by application state.
Obviously, if a stream-based transport is used, the streams
themselves constitute state that has to be handled in the same way
that the application itself would handle a stream-based transport; if
an application is not equipped for stream-based transport, it should
not allow SigComp connections on a stream-based transport. For the
alternative SigComp usage described as "continuous mode" in Section
4.2.1, an attacker could create any number of active UDVMs unless
there is some DoS protection at a lower level (e.g., by using TLS in
appropriate configurations).
- Attacking the UDVM by faking state or making unauthorized changes
to state:
This is covered in Section 10.2.2.
- Attacking the UDVM by sending it looping code:
The application sets an upper limit to the number of "UDVM cycles"
that can be used per compressed message and per input bit in the
compressed message. The damage inflicted by sending packets with
looping code is therefore limited, although this may still be
substantial if a large number of UDVM cycles are offered by the UDVM.
However, this would be true for any decompressor that can receive
packets over an unsecured transport.
11. IANA Considerations
SigComp requires a 1-byte name space, the SigComp_version, which has
been created by the IANA. Upgraded versions of SigComp must be
backwards-compatible with Version 0x01, described in this document.
Adding additional UDVM instructions and assigning values to the
reserved UDVM memory addresses are two possible upgrades for which
this is the case.
Following the policies outlined in [RFC-2434], the IANA policy for
assigning a new value for the SigComp_version shall require a
Standards Action. Values are thus assigned only for Standards Track
RFCs approved by the IESG.
12. Acknowledgements
Thanks to
Abigail Surtees
Mark A West
Lawrence Conroy
Christian Schmidt
Max Riegel
Lars-Erik Jonsson
Stefan Forsgren
Krister Svanbro
Miguel Garcia
Christopher Clanton
Khiem Le
Ka Cheong Leung
Robert Sugar
for valuable input and review.
13. References
13.1. Normative References
[RFC-1662] Simpson, W., "PPP in HDLC-like Framing", STD 51, RFC
1662, July 1994.
[RFC-2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC2119, March 1997.
[RFC-3174] Eastlake, 3rd, D. and P. Jones, "US Secure Hash Algorithm
1 (SHA1)", RFC3174, September 2001.
13.2. Informative References
[RFC-1951] Deutsch, P., "DEFLATE Compressed Data Format
Specification version 1.3", RFC1951, May 1996.
[RFC-2026] Bradner, S., "The Internet Standards Process - Revision
3", BCP 9, RFC2026, October 1996.
[RFC-2279] Yergeau, F., "UTF-8, a transformation format of ISO
10646", RFC2279, January 1998.
[RFC-2326] Schulzrinne, H., Rao, A. and R. Lanphier, "Real Time
Streaming Protocol (RTSP)", RFC2326, April 1998.
[RFC-2434] Alvestrand, H. and T. Narten, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC2434,
October 1998.
[RFC-2960] Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
Schwartzbauer, H., Taylor, T., Rytina, I., Kalla, M.,
Zhang, L. and V. Paxson, "Stream Control Transmission
Protocol", RFC2960, October 2000.
[RFC-3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M. and E.
Schooler, "SIP: Session Initiation Protocol", RFC3261,
June 2002.
[RFC-3321] Hannu, H., Christoffersson, J., Forsgren, S., Leung,
K.-C., Liu, Z. and R. Price, "Signaling Compression
(SigComp) - Extended Operations", RFC3321, January
2003.
14. Authors' Addresses
Richard Price
Roke Manor Research Ltd
Romsey, Hants, SO51 0ZN
United Kingdom
Phone: +44 1794 833681
EMail: richard.price@roke.co.uk
Carsten Bormann
Universitaet Bremen TZI
Postfach 330440
D-28334 Bremen, Germany
Phone: +49 421 218 7024
EMail: cabo@tzi.org
Jan Christoffersson
Box 920
Ericsson AB
SE-971 28 Lulea, Sweden
Phone: +46 920 20 28 40
EMail: jan.christoffersson@epl.ericsson.se
Hans Hannu
Box 920
Ericsson AB
SE-971 28 Lulea, Sweden
Phone: +46 920 20 21 84
EMail: hans.hannu@epl.ericsson.se
Zhigang Liu
Nokia Research Center
6000 Connection Drive
Irving, TX 75039
Phone: +1 972 894-5935
EMail: zhigang.c.liu@nokia.com
Jonathan Rosenberg
dynamicsoft
72 Eagle Rock Avenue
First Floor
East Hanover, NJ 07936
EMail: jdrosen@dynamicsoft.com
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