Network Working Group Richard Price, Siemens/Roke Manor
INTERNET-DRAFT Jonathan Rosenberg, dynamicsoft
Expires: July 2002 Carsten Bormann, TZI/Uni Bremen
H. Hannu, Ericsson
Z. Liu, Nokia
28 January, 2002
Universal Decompressor Virtual Machine (UDVM)
<draft-ietf-rohc-sigcomp-udvm-00.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of [RFC-2026].
Internet-Drafts are working documents of the Internet Engineering
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This document is a submission to the IETF ROHC WG. Comments should be
directed to the mailing list of ROHC, rohc@cdt.luth.se.
Abstract
This draft defines a "Universal Decompressor Virtual Machine"
optimized for the task of running decompression algorithms. The UDVM
can be configured to understand the output of many well-known
compressors such as [DEFLATE].
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Revision history
Changes from <draft-ietf-rohc-sigcomp-algorithm-00.txt>
State creation mechanism modified to use MD5 hash for improved
security
Support added for streaming compressed data over TCP
Memory format modified to allow compilation of UDVM code
Additional instructions added for bit manipulation etc.
Feedback mechanism added for bidirectional UDVM operation
Table of contents
1. Introduction.................................................3
2. Terminology..................................................3
3. Description of the UDVM architecture.........................5
3.1. UDVM architecture..........................................5
3.2. Requirements on application................................7
3.3. Requirements on transport mechanism........................9
3.4. Requirements on compressor.................................10
3.5. Application-defined parameters.............................11
4. Overview of the UDVM.........................................14
4.1. UDVM memory allocation.....................................14
4.2. Well-known variables.......................................15
4.3. Instruction parameters.....................................15
4.4. Byte copying...............................................16
5. Decompressing a compressed message...........................17
5.1. Invoking the UDVM..........................................17
5.2. Successful decompression...................................19
5.3. Decompression failure......................................20
6. UDVM instruction set.........................................21
6.1. Bit manipulation instructions..............................22
6.2. Arithmetic instructions....................................23
6.3. Memory management instructions.............................23
6.4. Program flow instructions..................................25
6.5. I/O instructions...........................................27
7. Feedback information.........................................31
7.1. UDVM version...............................................33
7.2. Memory size and CPU cycles.................................33
7.3. State identifiers..........................................34
8. Security considerations......................................34
9. Acknowledgements.............................................36
10. References...................................................36
11. Authors' addresses...........................................36
Appendix A. Mnemonic language...................................38
Appendix B. Example application-defined parameters..............40
Appendix C. Example decompression algorithms....................42
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1. Introduction
This draft defines a "Universal Decompressor Virtual Machine" (UDVM).
The UDVM is a virtual machine much like the Java Virtual Machine but
with a key difference: it is designed solely for the purpose of
running decompression algorithms.
The motivation for creating the UDVM is to provide unlimited
flexibility when choosing how to compress a given item of data.
Rather than picking one of a small number of pre-negotiated
compression algorithms, the implementer has the freedom to select an
algorithm of their choice. The compressed data is then combined with
a set of UDVM instructions that allow the original data to be
extracted, and the result is outputted as UDVM bytecode.
Since the UDVM is optimized specifically for running decompression
algorithms, the code size of a typical algorithm is small (often sub
100 bytes). Moreover the UDVM approach does not add significant extra
processing or memory requirements compared to running a fixed pre-
programmed decompression algorithm.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC-2119].
Virtual machine
A machine architecture designed to be implemented in software
(although silicon implementations are of course possible).
Universal Decompressor Virtual Machine (UDVM)
The virtual machine described in this draft. The UDVM is designed
specifically for the task of running decompression algorithms.
Bytecode
Machine code that can be executed by a virtual machine. UDVM
bytecode is a combination of UDVM instructions and compressed data.
Application
Entity which invokes the UDVM. The application is also responsible
for supplying the compressed data to the UDVM and making use of the
uncompressed data.
Transport mechanism
Mechanism for passing data between two instances of an application.
The UDVM is designed to work in conjunction with a wide range of
transport mechanisms including TCP, UDP and [SCTP].
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Message-oriented transport mechanism
A transport mechanism that carries data as a set of distinct,
bounded messages.
Stream-oriented transport mechanism
A transport mechanism that carries data as a continuous stream
with no message boundaries. In this case, the UDVM reserves a
specific character to delimit messages in the compressed stream.
Compressor
Entity which converts application data into compressed data that
can be reconstructed by the UDVM.
Application-defined parameters
Parameters that must be agreed upon by the application invoking the
compressor and the application invoking the UDVM. Depending on the
application these parameters might be fixed a-priori or negotiated.
Per-message compression
Compression that does not reference data from previous messages.
The UDVM can decompress a message of this type using only the
application-defined parameters and the data in the message itself.
Dynamic compression
Compression relative to messages sent prior to the current
compressed message. The UDVM stores and retrieves this data using
the secure state reference mechanism.
State
Information which is saved by the UDVM and retrieved for the
decompression of subsequent messages. For security reasons, state
can only be saved with the permission of the application and can
only be retrieved using an [MD5] hash of the state.
State identifier
A 16-byte value used to access an item of stored state information.
(for security it is the first n bytes of an [MD5] hash of the state
to be accessed). The minimum acceptable value of n is fixed for
security purposes, but implementers can choose higher values of n.
CPU cycles
A measure of the amount of "CPU power" required to execute a UDVM
instruction (the simplest UDVM instructions require a single CPU
cycle). An upper limit is placed on the number of cycles that can
be used to decompress each bit in a compressed message.
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3. Description of the UDVM architecture
This chapter describes the overall UDVM architecture including the
interfaces between the UDVM and its environment. The requirements on
the entities external to the UDVM are also given.
In the architecture the UDVM is considered to provide a decompression
service for a certain application. The application invokes the UDVM,
and is responsible for supplying compressed data to the UDVM and
making use of the corresponding uncompressed data.
In general the UDVM can offer a decompression service to a wide range
of applications. The principal motivation for developing the UDVM has
been the compression of application-layer protocols, in particular
text-based signaling protocols such as [SIP]. The UDVM architecture
is designed to operate securely and to provide a high compression
ratio for this case.
Note however that the UDVM can be used in any situation provided that
the requirements detailed in this chapter are satisfied by the
application and the transport mechanism.
The following sections describe the overall UDVM architecture and the
requirements on entities external to the UDVM such as the transport
mechanism and the compressor.
3.1. UDVM architecture
The UDVM architecture includes the following basic entities, each of
which is defined in subsequent sections of the document:
UDVM
Application (including state)
Transport mechanism
Compressor
Two variants of the architecture are available, depending on whether
the transport mechanism offers unidirectional or bidirectional data
transport. The unidirectional architecture can be considered to be a
special case of the bidirectional version.
Note that the UDVM itself does not need to know which architecture
has been chosen, because its operation is identical for both cases.
If bidirectional data transport is unavailable or undesirable for any
other reason, then the UDVM architecture is illustrated in Figure 1.
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+--------------------+ +--------------------+
| Compressor | | UDVM |
+--------------------+ +--------------------+
^ Appl. ^ | ^ | Appl. ^
| data | | | v data |
+----|-------------|-+ +-|-------------|----+
| | | | | | [X] |
| | Appl. 1 v | Compressed data | | Appl. 2 | |
| v +---------->----------+ v |
| +-------+ | | +-------+ |
| | State | | Data transport | | State | |
| +-------+ | | +-------+ |
| | | |
| | | |
| | | |
+--------------------+ +--------------------+
Figure 1: UDVM architecture for unidirectional data transport
In the unidirectional case the UDVM has two 2-way interfaces to the
application. The first interface passes compressed data from the
application to the UDVM, and provides the corresponding uncompressed
data in return. The second interface allows the UDVM to request the
creation of state (information that may improve the compression ratio
of subsequent messages), and to access previously stored state.
Note that both of these interfaces can be provided as extensions to
an existing application (e.g. a SIP client) or as a "shim" layer
between a compression-unaware client and the UDVM. In the latter
case, the term "application" refers to the combined client and shim
layer.
The [X] symbol denotes that the application has a veto over the
corresponding interface. In this case the application has veto over
the state interface and can refuse state creation requests if it
considers them to be inappropriate. See Section 3.2.2 for further
details.
Note that although the UDVM architecture only shows one compression
entity, it is possible for the UDVM to decompress messages from
multiple compressors at different physical locations in a network.
The UDVM architecture is designed to prevent data from one compressor
interfering with data from a different compressor. A consequence of
this design choice is that it is difficult for a malicious user to
disrupt UDVM operation by inserting false compressed messages on the
transport mechanism.
If the transport mechanism exchanges data in both directions then the
architecture of Figure 2 can also be used. In this case, two
instances of the application communicate using a bidirectional
transport mechanism. Both instances of the application invoke a
compressor to compress their data, and a UDVM to retrieve the
uncompressed data sent by the remote application.
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+--------------------+ +--------------------+
| Compressor 1 | | UDVM 2 |
+--------------------+ +--------------------+
^ ^ Appl. ^ | ^ | Appl. ^ |
| | data | | | v data | v
+-|--|-------------|-+ +-|-------------|--|-+
| | | | | Compressed data | | [X][X]|
| | | Appl. 1 v | plus acks | | Appl. 2 | | |
| | v +---------->----------+ v | |
| A +-------+ | | +-------+ A |
| c | State | | Data transport | | State | c |
| k +-------+ | | +-------+ k |
| s ^ +----------<----------+ ^ s |
| | | | | Compressed data | ^ | | |
|[X][X] | | plus acks | | | | |
+-|--|-------------|-+ +-|-------------|--|-+
^ | Appl. ^ | | | Appl. | |
| v data | v | v data v v
+--------------------+ +--------------------+
| UDVM 1 | | Compressor 2 |
+--------------------+ +--------------------+
Figure 2: UDVM architecture for bidirectional data transport
For a bidirectional transport mechanism an additional interface is
provided from the UDVM, via the application, to the compressor on the
reverse transport channel. This interface can be used to send
feedback information from an application to the remote compressor.
The path taken by feedback data between Application 2 and Compressor
1 is as follows:
Appl. 2 --> Compressor 2 --> UDVM 1 --> Appl. 1 --> Compressor 1
This feedback information monitors the behavior of the UDVM,
including whether data has been successfully decompressed, the amount
of available memory etc. The compressor can make use of this
information to improve the overall compression ratio.
Note that it is an implementation decision whether to use the
feedback channel or not, and compressors must operate successfully
even if no feedback information is received.
3.2. Requirements on application
The application is the entity responsible for invoking the UDVM,
supplying the UDVM with compressed data, and making use of the
corresponding uncompressed data.
Note that in order to use the UDVM decompression service an
application (e.g. a SIP client) will require interfaces to the UDVM.
These interfaces can be provided by extending the original client, or
by providing a "shim" layer between a compression-unaware client and
the UDVM.
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In addition, two instances of an application MUST agree on how to
invoke the UDVM decompression service, and MUST fix or negotiate a
common set of application-defined parameters (e.g.
maximum_compressed_size) as per Section 3.5.
Certain application-defined parameters can be modified on the fly
using the state creation mechanism and the feedback mechanism. The
application SHOULD additionally provide some external means of
resetting or renegotiating these parameters (possibly by terminating
the decompression service offered by the UDVM).
The UDVM has a total of three interfaces to the environment: a two-
way interface for exchanging compressed and uncompressed data, a two-
way interface for storing and receiving state, and a one-way
interface for forwarding feedback data. To protect against the
malicious establishment of false state or false feedback data all of
the UDVM interfaces pass through the application, and requests for
state creation and feedback can be rejected if they are not
accompanied by a valid uncompressed message.
Each of the three interfaces is described in greater detail below:
3.2.1. Compressed and uncompressed data
The first interface supplies compressed data to the UDVM and
retrieves the corresponding uncompressed data. Note that when the
UDVM is invoked it does not receive any compressed data by default,
but instead requests new data explicitly using a specific
instruction. This means that the first part of a message can be
decompressed without waiting for the entire message to arrive, which
is especially useful over a stream-oriented transport such as TCP.
Uncompressed data is also passed to the application using a specific
instruction. It is an application decision whether to make use of the
data immediately or to buffer and wait for a complete message to be
successfully decompressed.
3.2.2. Storing and retrieving state
To provide security against the malicious insertion of false
compressed data, the contents of the UDVM memory are reinitialized
after each compressed message. This ensures that damaged compressed
messages do not prevent the successful decompression of subsequent
valid messages.
Note however that the overall compression ratio is often
significantly higher if messages can be compressed relative to the
information stored in previous messages. For this reason it is
possible for the UDVM to create "state" information for access when a
later message is being decompressed.
Both the creation and access of state are designed to be secure
against malicious tampering with the compressed data. State can only
be created when a complete message has been successfully
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decompressed, and the application can veto a state creation request
based on the contents of the decompressed message. This is especially
useful if the application has an authentication mechanism that can be
applied to determine whether the uncompressed data is legitimate.
Furthermore, the UDVM can only access previously created state
information by providing an [MD5] hash of the state to be accessed.
The advantage of using a secure hash to access state information is
that it is very difficult to guess the correct hash value without
complete knowledge of the state being accessed.
Also note that state is not deleted when it is accessed. So even if a
malicious user manages to access state information, subsequent
messages compressed relative to this state can still be successfully
decompressed. Instead, the application is responsible for deleting
state information once it determines that the state will no longer be
needed.
3.2.3. Feedback information
The final interface is only used when the transport mechanism is
bidirectional. It provides feedback information from the UDVM to the
compressor on the reverse channel, and can be used to improve the
overall compression ratio.
Note that the feedback information is forwarded via the application.
Just as for the state interface above, the application can veto
feedback information if it considers the corresponding decompressed
message to be invalid.
If the transport mechanism only provides one-way data transport then
the feedback interface is considered to be null: any feedback
information sent across the interface is simply discarded by the
application.
3.3. Requirements on transport mechanism
The transport mechanism is the entity that passes data between two
instances of an application. Since the motivation for developing the
UDVM has been the compression of signaling protocols such as [SIP],
the UDVM is designed to operate successfully over both stream-
oriented protocols such as TCP and message-oriented protocols such as
UDP.
Note that the UDVM is not given direct access to the underlying
transport mechanism; instead the compressed data is considered to
first pass through the application. It is an application decision
whether to pass all data from the transport mechanism directly to the
UDVM or whether to mix compressed and uncompressed data (e.g. by
restricting compressed data to a certain port).
If the transport mechanism is message-oriented then the UDVM converts
each compressed message into a corresponding uncompressed message. It
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is not possible for one compressed message to reconstruct multiple
uncompressed messages.
If the transport mechanism is stream-oriented then the UDVM simply
converts a stream of compressed data into a stream of uncompressed
data. However, when running over a stream-oriented transport such as
TCP, applications often insert their own internal message delimiters
into the data stream. As the message is compressed, it will not be
possible to detect these delimiters in the compressed data stream.
Therefore the UDVM provides a similar character that can be inserted
into the compressed data stream to delimit messages (see Section 3.4
for further details).
No assumption is made about the reliability of the transport
mechanism. The UDVM can operate successfully over unreliable
transport mechanisms such as UDP as well as reliable transport
mechanisms such as TCP.
No assumption is made about the security of the transport mechanism.
It may be possible for a malicious user to insert or modify data on
the path between the compressor and the UDVM. In this case, the
design goal of the UDVM is to avoid presenting additional security
risks compared to simply transporting the application data
uncompressed.
3.4. Requirements on compressor
An important feature of the UDVM is that it can decompress data
generated by arbitrary compression algorithms. In particular this
means that it is not necessary to standardize a compression algorithm
for use with the UDVM; instead the choice can be left to the
implementer.
The overall requirement placed on the compressor is that of
transparency, i.e. the compressor MUST NOT send instructions which
cause the UDVM to incorrectly decompress a given message.
The following more specific requirements are also placed on the
compressor (they can be considered particular instances of the
transparency requirement):
* Since feedback information is purely optional, the compressor
MUST be able to operate successfully even if it receives no
feedback data.
* It is RECOMMENDED that the compressor supply a CRC over the
uncompressed message to ensure that successful decompression has
occurred. A UDVM instruction is provided to verify this CRC.
* If the transport mechanism is message-oriented then the
compressor MUST preserve the boundaries between messages.
* If the transport mechanism is stream-oriented but the
application defines its own internal message boundaries, then
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the compressor SHOULD preserve the boundaries between messages
by using the "end-of-message" character 0xFFFF reserved in UDVM
bytecode.
The reason for preserving the message boundaries over a stream-
oriented transport is that damage to one compressed message does not
affect the decompression of subsequent messages. Moreover, the
application typically vetoes state creation and feedback requests on
a per-message basis.
Note that the UDVM also reserves the character 0xFF00 over a stream-
oriented transport mechanism, and replaces every instance of 0xFF00
with 0xFF before decompressing the data. This ensures that arbitrary
compression algorithms can be used over a stream-oriented transport,
provided that every instance of 0xFF in the compressed data stream is
identified and replaced with 0xFF00. This "byte-stuffing" scheme
prevents the compression algorithm from inserting a message delimiter
into the data stream where one is not required.
3.4.1. Types of compression algorithm
Any of the following classes of compression algorithm may be useful
depending on the type of application:
* Generic compressor (for example [DEFLATE] or a similar
algorithm).
* Protocol-aware compressor offering excellent performance for
one particular type of data (for example the text messages
generated by [SIP]).
* Hybrid compressor with similar performance to [DEFLATE] for
generic data and superior performance for certain types of data.
Provided that the uncompressed data can be reconstructed at the UDVM
using the available memory and CPU cycles, implementers have freedom
to use a compression algorithm of their choice.
Note that when using an "off-the-shelf" compression algorithm,
bytecode for the corresponding decompressor will need to be made
available at the UDVM. In general the decompressor bytecode is placed
at the front of the first compressed message, unless the application
offers the ability to download UDVM bytecode offline (in which case
the UDVM memory will be initialized already containing a copy of the
decompression algorithm).
3.5. Application-defined parameters
When an application invokes an instance of the UDVM, a number of
parameters are provided by the application to control the UDVM memory
size, maximum number of CPU cycles etc. The application invoking the
UDVM and the application invoking the compressor MUST initially agree
on a common set of values for these parameters.
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Note that if the transport mechanism is bidirectional then the
application invoking the UDVM can use the reverse channel to indicate
that additional memory or CPU cycles are available (compared to the
values initially agreed by the application invoking the compressor).
The compressor can then make use of these extra resources to improve
the compression ratio.
The feedback mechanism can also advertise that an upgraded version of
the UDVM is available (e.g. offering additional UDVM instructions),
provided that the upgraded version is backwards compatible with the
basic version described in this document. See Chapter 7 for further
details.
Each parameter is described in greater detail below; example values
for the parameters are listed in Appendix B.
UDVM_version
The UDVM_version parameter specifies the level of functionality
available at the UDVM. The basic version of the UDVM (Version 0)
is defined in this document.
maximum_compressed_size
The maximum_compressed_size parameter limits the size of one
compressed message. Decompression failure occurs if a message
larger than the specified value is provided.
maximum_uncompressed_size
The maximum_uncompressed_size parameter limits the size of one
uncompressed message. Decompression failure occurs if a message
larger than the specified value is provided.
minimum_hash_size
The minimum_hash_size parameter specifies the minimum size of the
state identifier that can be used to reference state. This value
needs to be sufficiently large to prevent malicious users from
guessing a state identifier by brute force.
overall_memory_size
The overall_memory_size parameter specifies the total number of
bytes in the UDVM memory.
working_memory_start
The working_memory_start parameter specifies the start of the UDVM
memory area that can be modified. Memory addresses below this
value are considered read-only by the UDVM.
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working_memory_end
The working_memory_end parameter specifies the end of the UDVM
memory area that can be modified. Memory addresses above this
value are considered read-only by the UDVM.
cycles_per_bit
The cycles_per_bit parameter specifies the number of "CPU cycles"
that can be used to decompress a single bit of data. One CPU cycle
typically corresponds to a single UDVM instruction, although some
of the high-level instructions may require additional cycles.
cycles_per_message
The cycles_per_message parameter specifies the number of additional
CPU cycles made available at the start of a compressed message.
These cycles can be useful when decompressing algorithms that
download additional data on a per-message basis, for example a new
set of Huffman codes as with [DEFLATE].
The total number of "CPU cycles" available for each compressed
message is specified by the following formula:
total_cycles = message_size * cycles_per_bit + cycles_per_message
first_instruction
The first_instruction parameter specifies the memory address of the
first instruction to be executed when the UDVM is initialized.
Initial memory contents
For each new compressed message the UDVM memory is reinitialized
with contents defined by the application. For example, the
application may be able to download UDVM bytecode for a
decompression algorithm before the first compressed message
arrives. In this case, for each new compressed message the UDVM
memory is initialized already containing a copy of the
decompression algorithm.
Initial state
As well as deciding the initial contents of the UDVM memory, the
application can also store useful information in the form of state.
This predefined state will typically contain optional data that can
be used to improve the overall compression ratio, for example a
well-known decompression algorithm or a dictionary of commonly used
[SIP] phrases. Note that unlike state created on the fly by the
UDVM, there is no need for the application-defined state to use an
[MD5] hash as the state identifier.
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4. Overview of the UDVM
This chapter describes some basic features of the UDVM, including the
memory allocation, well-known variables and instruction parameters.
4.1. UDVM memory allocation
The memory available to the UDVM is partitioned into a number of
sections, providing space for program code, variables and
miscellaneous data:
<----- working_memory_size ------>
| Fixed values | Variables | Miscellaneous data | Program code |
+--------------+-----------+--------------------+--------------+
<--------------------- overall_memory_size -------------------->
Figure 3: Memory allocation in the UDVM
Recall that the amount of memory available to the UDVM is defined by
the application-specific parameters overall_memory_size,
working_memory_start and working_memory_end. Note that all of these
parameters are initialized by the application, but can be
renegotiated on the fly using the feedback mechanism of Chapter 7.
The memory area from Address (working_memory_start) to Address
(working_memory_end) inclusive can be used to store arbitrary data
(variables, program code, Huffman codes etc.). UDVM instructions are
allowed to read from or write to any address in this memory area.
The first part of this memory area is typically used to store a
number of 2-byte variables. UDVM instructions can reference these
variables using a special instruction parameter as described in
Section 4.3.
The memory area from Address 0 to Address (working_memory_start - 1)
and from Address (working_memory_end + 1) to Address
(overall_memory_size - 1) inclusive is write-protected, so UDVM
instructions can read from this memory area but cannot write to it.
This memory area is intended for storing UDVM bytecode that can be
compiled.
Any attempt to read memory addresses beyond the overall memory size
or to write to memory addresses outside the working memory area MUST
cause a decompression failure (see Section 5.3).
The first part of the write-protected UDVM memory is intended for
storing variables whose values no longer need to be modified. The
second part of the write-protected memory is intended for storing
program code including UDVM instructions and their associated
parameters. Note that if an instruction references a variable that
has been write-protected, the compiled version of the instruction
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will typically run faster than if the referenced variable lies in the
working memory area.
4.2. Well-known variables
The first few variables in the UDVM memory have special tasks, for
example specifying the location of the stack used by the CALL and
RETURN instructions. Each of these well-known variables is a 2-byte
integer.
The following list gives the name of each well-known variable and the
memory address at which the variable can be found:
Name: Starting memory address:
byte_copy_left 0
byte_copy_right 2
stack_location 4
The MSBs of each variable are always stored before the LSBs. So, for
example, the MSBs of stack_location are stored at Address 4 whilst
the LSBs are stored at Address 5.
The use of each well-known variable is described in the following
sections of the draft.
4.3. Instruction parameters
Each of the UDVM instructions is followed by 0 or more bytes
containing the parameters required by the instruction.
To reduce the code size of a typical UDVM program, each parameter for
a UDVM instruction is compressed using variable-length encoding. The
aim is to store more common parameter values using fewer bits than
rarely occurring values.
Three different types of parameter are available: the literal, the
reference and the multitype. The parameter types that follow each
UDVM instruction are specified in Chapter 6.
The UDVM bytecode for each parameter type is illustrated in Figure 4
to Figure 6, together with the integer values represented by the
bytecode.
Note that the MSBs in the bytecode are illustrated as preceding the
LSBs. Also, any string of bits marked with k consecutive "n"s is to
be interpreted as an integer N from 0 to 2^k - 1 inclusive (with the
MSBs of n illustrated as preceding the LSBs).
The decoded integer value of the bytecode can be interpreted in two
ways. In some cases it is taken to be the actual value of the
parameter. In other cases it is taken to be a memory address at which
the 2-byte parameter value can be found (MSBs found at the specified
address, LSBs found at the following address). The latter case is
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denoted by memory[X] where X is the address and memory[X] is the 2-
byte value starting at Address X.
The simplest parameter type is the literal (#), which encodes a
constant integer from 0 to 65535 inclusive. A literal parameter may
require between 1 and 3 bytes depending on its value.
Bytecode: Parameter value: Range:
0nnnnnnn N 0 - 127
10nnnnnn nnnnnnnn N 0 - 16383
11000000 nnnnnnnn nnnnnnnn N 0 - 65535
Figure 4: Bytecode for a literal (#) parameter
The second parameter type is the reference ($), which is always used
to access a 2-byte value located elsewhere in the UDVM memory. The
bytecode for a reference parameter is decoded to be a constant
integer from 0 to 65535 inclusive, which is interpreted as the memory
address containing the actual value of the parameter.
Bytecode: Parameter value: Range:
0nnnnnnn memory[2 * N] 0 - 254
10nnnnnn nnnnnnnn memory[2 * N] 0 - 32766
11000000 nnnnnnnn nnnnnnnn memory[N] 0 - 65535
Figure 5: Bytecode for a reference ($) parameter
The third kind of parameter is the multitype (%), which can be used
to encode both actual values and memory addresses. The multitype
parameter also offers efficient encoding for small integer values
(both positive and negative) and for powers of 2.
Bytecode: Parameter value: Range:
00nnnnnn N 0 - 63
01nnnnnn memory[2 * N] 0 - 126
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 - 8191
10000000 nnnnnnnn nnnnnnnn N 0 - 65535
10000001 nnnnnnnn nnnnnnnn memory[N] 0 - 65535
Figure 6: Bytecode for a multitype (%) parameter
4.4. 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.
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In general, the string of bytes is copied in ascending order of
memory address. So if a byte is copied from/to Address n then the
next byte is copied from/to Address n + 1. As usual, if a byte is
read from an address beyond the overall memory size or is written to
an address outside the working memory area then decompression failure
occurs.
Note however that if a byte is copied from/to the memory address
specified in byte_copy_right, the byte copy operation continues by
copying the next byte from/to the memory address specified in
byte_copy_left. This is useful for setting up a "circular buffer"
within the UDVM memory.
Note that the string of bytes is copied on a purely byte-by-byte
basis. In particular, some of the later bytes to be copied may
themselves have been written into the UDVM memory by the byte copying
operation currently being performed.
Equally, it is possible for a byte copying operation to overwrite the
instruction that called the byte copy. If this occurs then the byte
copying operation MUST be completed as if the original instruction
were still in place in the UDVM memory (this also applies if
byte_copy_left or byte_copy_right are overwritten).
5. Decompressing a compressed message
This chapter lists the steps involved in the decompression of a
single compressed message.
5.1. Invoking the UDVM
Whenever the application receives a message to be decompressed, it
invokes a new instance of the UDVM. The overall_memory_size and
initial contents of the UDVM memory are initialized using the
corresponding application-defined parameters. The following steps are
then taken:
1.) The number of remaining CPU cycles is set equal to the
application-defined parameter cycles_per_message.
Notes:
The amount of compressed data available to the UDVM is exactly one
compressed message. If the transport mechanism is stream-oriented
then the UDVM uses the reserved byte string 0xFFFF to delimit the
compressed messages: the UDVM takes the data between a pair of
neighboring reserved byte strings to be a single compressed message.
The reserved byte string itself is not considered to be part of the
compressed message.
For a stream-oriented transport, the UDVM parses the compressed data
stream for instances of 0xFF and takes the following actions:
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Occurs in data stream: Action:
0xFFFF Delimit compressed message
0xFF00 Replace with 0xFF
0xFF01 - 0xFFFE Decompression failure
The reserved character 0xFF00 is useful for byte stuffing (if a
compression algorithm generates compressed data containing the
character 0xFF then it should be replaced by the character 0xFF00 to
avoid accidentally inserting a message delimiter into the compressed
data stream).
The compressed data is not provided to the UDVM by default. Instead,
the UDVM requests compressed data using the INPUT instructions
(useful when running over a stream-oriented transport since there is
no need to wait for the entire compressed message before
decompression can begin). Note that in particular, this means that
the application MUST define the initial contents of the UDVM memory
to contain at least one INPUT instruction. See Appendix B for an
example of how the application might initialize the UDVM memory.
The application MUST NOT make more than one compressed message
available to a given instance of the UDVM. In particular, the
application MUST NOT concatenate two messages to form a single
compressed message. This is because compressed messages are typically
padded with trailing zero bits so that they are a whole number of
bytes long. Concatenating two messages would cause these padding bits
to be incorrectly interpreted as compressed data.
2.) Next, the instructions contained within the UDVM memory are
executed beginning at the address specified in first_instruction.
Notes:
The instructions are executed consecutively unless otherwise
indicated (for example when the UDVM encounters a JUMP instruction).
If the next instruction to be executed lies outside the available
memory then decompression failure occurs (see Section 5.3).
3.) Each time an instruction is executed the number of available
CPU cycles is decreased by the amount specified in Chapter 6.
Additionally, if the UDVM requests n bits of compressed data (using
one of the INPUT instructions) then the number of available CPU
cycles is increased by n * cycles_per_bit.
Notes:
This means that the total number of CPU cycles available for
processing a compressed message is given by the formula:
total_cycles = cycles_per_message + message_size * cycles_per_bit
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The reason that this total is not allocated to the UDVM when it is
invoked is that the UDVM can begin to decompress a message that has
only been partially received. So the total message size may not be
known when the UDVM is initialized.
4.) The UDVM stops executing instructions when it encounters an
END-MESSAGE instruction or if decompression failure occurs.
Notes:
The UDVM passes uncompressed data to the application using the OUTPUT
instruction. The OUTPUT instruction can be used to output a partially
decompressed message; it is an application decision whether to use
the data immediately or whether to buffer and wait until the entire
message has been decompressed.
The UDVM passes state creation and feedback requests to the
application using the END-MESSAGE instruction. This means that it is
only possible to make a state creation and a feedback request once
the message has been decompressed, which is necessary since the
application typically checks the validity of these requests based on
the contents of the decompressed message.
5.2. Successful decompression
The END-MESSAGE instruction indicates that the compressed message has
been successfully decompressed and passed to the application. Note
that the actual uncompressed message is outputted beforehand using
the OUTPUT instruction; this allows the UDVM to output each part of
the message to the application as soon as it has been decompressed.
The END-MESSAGE instruction provides two additional pieces of
information to the application: the state creation request and the
feedback data. The state creation request mechanism is discussed
below; feedback information is discussed separately in Chapter 7.
The UDVM may optionally save part of its memory for retrieval by
later messages. However to prevent malicious storage of a large
amount of unnecessary state information, the application MUST give
permission before any state can be created. The application typically
makes a decision on whether state can be created based on the
contents of the decompressed message, particularly if the message
contains authentication data that can verify whether or not the
sender is legitimate.
The END-MESSAGE instruction requests the creation of state using the
parameters state start and state length, which together denote a byte
string state_value. Provided that the application gives permission,
state_value is byte copied from the UDVM memory (obeying the rules of
Section 4.4) and stored together with a 16-byte state identifier that
can be used to access the state by a later compressed message.
To provide security against malicious access, the identifier for any
item of state created by the UDVM is derived from the [MD5] hash of
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the state_value to be stored. The state identifier is constructed by
taking the 16-byte [MD5] hash and replacing all but the first
hash_length most significant bytes with zeroes. Note that if
hash_length is 16 then the unmodified [MD5] hash is the state
identifier. Decompression failure occurs if hash_length is less than
the application-defined parameter minimum_hash_size or greater than
16.
Each item of state stores the following information (accessed by the
state_identifier):
state_identifier
state start
state length
state_value
state_instruction
Note that state_start, state_length and state_instruction are all
parameters from the END-MESSAGE instruction, whereas state_identifier
and state_value are created as specified above.
If a state creation request is made with a state identifier that has
been used by existing state, then the request fails automatically.
This state can subsequently be accessed by using the STATE-REFERENCE
and STATE-EXECUTE instructions (by providing the correct state
identifier).
5.3. Decompression failure
If a compressed message given to the UDVM is corrupted (either
accidentally or maliciously) then the UDVM may terminate with a
decompression failure.
Reasons for decompression failure include the following:
* A compressed or uncompressed message exceeds the maximum size
defined by the application.
* The UDVM exceeds the available CPU cycles for decompressing a
message.
* The UDVM attempts to read a memory address beyond the overall
memory size, or to write into a memory address outside the
working memory area.
* An unknown instruction type is encountered.
* An unknown parameter type is encountered.
* An instruction is encountered that cannot be processed
successfully by the UDVM (for example a RETURN instruction when
no CALL instruction has previously been encountered).
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* The UDVM attempts to access non-existent state.
* A manual decompression failure is triggered using the
DECOMPRESSION-FAILURE instruction.
If a decompression failure occurs when decompressing a message then
the UDVM informs the application and takes no further action. It is
the responsibility of the application to decide how to cope with the
decompression failure. In general an application SHOULD discard the
compressed message and any decompressed data that has been outputted.
6. UDVM instruction set
The UDVM currently understands 28 instructions, chosen to support the
widest possible range of compression algorithms with the minimum
possible overhead.
Figure 7 lists the different instructions and the bytecode values
used to store the instructions at the UDVM. The cost of each
instruction in CPU cycles is also given:
Instruction: Bytecode value: Cost in CPU cycles:
DECOMPRESSION-FAILURE 0 1
AND 1 1
OR 2 1
NOT 3 1
ADD 4 1
SUBTRACT 5 1
MULTIPLY 6 1
DIVIDE 7 1
LOAD 8 1
MULTILOAD 9 1 + n
WORKING-MEMORY 10 1
COPY 11 1 + length
COPY-LITERAL 12 1 + length
COPY-OFFSET 13 1 + length + offset
JUMP 14 1
COMPARE 15 1
CALL 16 1
RETURN 17 1
SWITCH 18 1 + n
CRC 19 1 + length
END-MESSAGE 20 1 + state length
OUTPUT 21 1 + output_length
NBO 22 1
INPUT-BYTECODE 23 1 + length
INPUT-FIXED 24 1
INPUT-HUFFMAN 25 1 + n
STATE-REFERENCE 26 1 + state_length
STATE-EXECUTE 27 1 + state length
Figure 7: UDVM instructions and corresponding bytecode values
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Each UDVM instruction costs a minimum of 1 CPU cycle. Certain high-
level instructions may cost additional cycles depending on the value
of one of the instruction parameters.
The only exception when calculating the number of CPU cycles is that
the STATE-EXECUTE instruction takes (1 + state_length) cycles even
though it does not have a state_length parameter; instead the value
of state length is provided by the application as part of the state
being accessed.
All instructions are stored as a single byte to indicate the
instruction type, followed by 0 or more bytes containing the
parameters required by the instruction. The instruction specifies
which of the three parameter types of Section 4.3 is used in each
case. For example, the ADD instruction is followed by two parameters
as shown below:
ADD ($parameter_1, %parameter_2)
When converted into bytecode the number of bytes required by the ADD
instruction depends on the size of each parameter value, and whether
the second (multitype) parameter contains the parameter value itself
or a memory address where the actual value of the parameter can be
found.
The instruction set available for the UDVM offers a mix of low-level
and high-level instructions. The high-level instructions can all be
emulated using the low-level instructions provided, but given a
choice it is generally preferable to use a single instruction rather
than a large number of general-purpose instructions. The resulting
bytecode will be more compact (leading to a higher overall
compression ratio) and decompression will typically be faster because
the implementation of the compression-specific instructions can be
optimized for the UDVM.
Each instruction is explained in more detail below:
6.1. Bit manipulation instructions
The AND, OR and NOT instructions provide simple bit manipulation on
2-byte words.
AND ($parameter_1, %parameter_2)
OR ($parameter_1, %parameter_2)
NOT ($parameter_1)
After the operation is complete, the value of the first parameter is
overwritten with the result. Note that since this parameter is a
reference, the memory address specified by the parameter is always
overwritten and not the parameter itself.
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6.2. Arithmetic instructions
The ADD, SUBTRACT, MULTIPLY and DIVIDE instructions perform
arithmetic on 2-byte words.
ADD ($parameter_1, %parameter_2)
SUBTRACT ($parameter_1, %parameter_2)
MULTIPLY ($parameter_1, %parameter_2)
DIVIDE ($parameter_1, %parameter_2)
After the operation is complete, the first parameter is overwritten
with the result.
Note that in all cases the arithmetic operation is performed modulo
2^16. So for example, subtracting 1 from 0 gives the result 65535.
For the SUBTRACT instruction the second parameter is subtracted from
the first. Similarly, for the DIVIDE instruction the first parameter
is divided by the second parameter. Note that if the second parameter
does not divide exactly into the first parameter then the remainder
is ignored.
6.3. Memory management instructions
The following instructions are used to manipulate the UDVM memory.
Bytes can be copied from one area of memory to another, and areas of
memory can be write-protected to make it easier for UDVM code to be
compiled.
6.3.1. LOAD
The LOAD instruction sets a 2-byte variable to a certain specified
value. The format of a LOAD instruction is as follows:
LOAD (%address, %value)
The first parameter specifies the starting address of the 2-byte
variable, whilst the second parameter specifies the value to be
loaded into this variable. As usual, MSBs are stored before LSBs in
the UDVM memory.
6.3.2. MULTILOAD
The MULTILOAD instruction sets a contiguous block of 2-byte variables
to specified values.
MULTILOAD (%address, #n, %value_0, ..., %value_n-1)
The first parameter specifies the starting address of the contiguous
variables, whilst the parameters value_0 through to value_n-1 specify
the values to load into these variables (in the same order as they
appear in the instruction).
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6.3.3. WORKING-MEMORY
The WORKING-MEMORY instruction is used to prevent part of the UDVM
memory from being modified. This can be very useful when offering
UDVM code for compilation.
WORKING-MEMORY (%memory_start, %memory_end)
The parameters memory_start and memory_end specify the new working
memory area for the UDVM. These parameters replace the application-
defined parameters working_memory_start and working_memory_end, but
only while the current message is being decompressed. When a new
instance of the UDVM is invoked the working memory area is set by the
original application-defined parameters.
If memory_end < memory_start, or if the parameters reference a memory
address beyond the overall UDVM memory size, then decompression
failure occurs.
After the WORKING-MEMORY instruction has been encountered, the only
way to write into UDVM memory within the protected region is to
cancel the protection using another WORKING-MEMORY instruction (or to
invoke a new instance of the UDVM).
6.3.4. COPY
The COPY instruction is used to copy a string of bytes from one part
of the UDVM memory to another.
COPY (%position, %length, %destination)
The position parameter specifies the memory address of the first byte
in the string to be copied, and the length parameter specifies the
number of bytes to be copied.
The destination parameter gives the address to which the first byte
in the string will be copied.
Note that byte copying is performed as per the rules of Section 4.4.
6.3.5. COPY-LITERAL
A modified version of the COPY instruction is given below:
COPY-LITERAL (%position, %length, $destination)
The COPY-LITERAL instruction behaves as a COPY instruction except
that after copying, the destination parameter is replaced with the
memory address immediately following the address to which the final
byte was copied. If the final byte was copied to the memory address
specified in byte_copy_right, the destination parameter is set to the
memory address specified in byte_copy_left.
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6.3.6. COPY-OFFSET
A further version of the COPY-LITERAL instruction is given below:
COPY-OFFSET (%offset, %length, $destination)
The COPY-OFFSET instruction behaves as a COPY-LITERAL instruction
except that an offset parameter is given instead of a position
parameter.
To derive a suitable position parameter, starting at the memory
address specified by destination, the UDVM counts backwards a total
of offset memory addresses. If the memory address specified in
byte_copy_left is reached, the next memory address is taken to be
byte_copy_right.
The COPY-OFFSET instruction then behaves as a COPY-LITERAL
instruction, taking the position parameter to be the last memory
address reached in the above step.
6.4. Program flow instructions
The following instructions alter the flow of UDVM code. Each
instruction jumps to one of a number of memory addresses based on a
certain specified criterion. Note that all of the instructions give
the memory addresses in the form of deltas relative to the memory
address of the instruction. The actual memory address is calculated
as follows:
memory_address = (memory_address_of_instruction + delta) modulo 2^16
Note that certain I/O instructions (see Section 6.5) can also alter
program flow.
6.4.1. JUMP
The JUMP instruction moves program execution to the specified memory
address.
JUMP (%delta)
Note that if the address (specified as a delta from the address of
the JUMP instruction) lies beyond the overall UDVM memory size then
decompression failure occurs.
6.4.2. COMPARE
The COMPARE instruction compares two parameters and then jumps to one
of three specified memory addresses depending on the result.
COMPARE (%parameter_1, %parameter_2, %delta_1, %delta_2, %delta_3)
If parameter_1 < parameter_2 then the UDVM continues instruction
execution at the (relative) memory address specified by delta 1. If
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parameter_1 = parameter_2 then it jumps to the address specified by
delta_2. If parameter_1 > parameter_2 then it jumps to the address
specified by delta_3.
6.4.3. CALL and RETURN
The CALL and RETURN instructions provide support for compression
algorithms with a nested structure.
CALL (%delta)
RETURN
The CALL and RETURN instructions make use of a stack of 2-byte
variables stored at the memory address specified by the well-known
variable stack_location. The stack contains the following variables:
Name: Starting memory address:
stack_free stack_location
stack[0] stack_location + 2
stack[1] stack_location + 4
stack[2] stack_location + 6
: :
The MSBs of these variables are stored before the LSBs in the UDVM
memory.
When the UDVM reaches a CALL instruction, it finds the memory address
of the instruction immediately following the CALL instruction and
copies this 2-byte value into stack[stack_free] ready for later
retrieval. It then increases stack_free by 1 and continues
instruction execution at the (relative) memory address specified by
the parameter.
When the UDVM reaches a RETURN instruction it decreases stack_free by
1, and then continues instruction execution at the byte position
stored in stack[stack_free].
If the variable stack_free is ever increased beyond 65535 or
decreased below 0 then a bad compressed message has been received and
decompression failure occurs (see Section 5.3).
Decompression failure also occurs if one of the above instructions is
encountered and the value of stack_location is smaller than 6 (this
prevents the stack from overwriting the well-known variables).
6.4.4. SWITCH
The SWITCH instruction performs a conditional jump based on the value
of one of its parameters.
SWITCH (#n, %j, %delta_0, %delta_1, ... , %delta_n-1)
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When a SWITCH instruction is encountered the UDVM reads the value of
j. It then continues instruction execution at the (relative) address
specified by delta j.
If j specifies a value of n or more, a bad compressed message has
been received and decompression failure occurs.
6.4.5. CRC
The CRC instruction verifies a string of bytes using a 2-byte CRC.
CRC (%value, %position, %length, %delta)
The actual CRC calculation is performed using the generator
polynomial x^16 + x^12 + x^5 + 1, which coincides with the 2-byte
Frame Check Sequence (FCS) of [RFC-1662].
The position and length parameters define the string of bytes over
which the CRC is evaluated. Byte copying rules are enforced as per
Section 4.4.
Important note: Since a CRC calculation is always performed over a
bitstream, for interoperability it is necessary to define the order
in which bits are supplied within each individual byte. In this case
the MSBs of the byte MUST be supplied to the CRC calculation before
the LSBs.
The value parameter contains the expected integer value of the 2-byte
CRC. If the calculated CRC matches the expected value then the UDVM
continues at the following instruction. Otherwise the UDVM jumps to
the (relative) memory address specified by delta.
6.5. I/O instructions
The following instructions allow the UDVM to interface with its
environment. Note that in the current UDVM architecture all of the
interfaces pass through the application (which has a veto over any
information supplied to or from the UDVM).
6.5.1. END-MESSAGE
The END-MESSAGE instruction successfully terminates the UDVM and
passes feedback and state information to the application.
END-MESSAGE (%hash_length, %state_start, %state_length,
%state_instruction, %feedback_location)
The actions taken by the UDVM upon encountering the END-MESSAGE
instruction are described in Section 5.2.
6.5.2. DECOMPRESSION-FAILURE
The DECOMPRESSION-FAILURE instruction triggers a manual decompression
failure. This is useful if the UDVM program discovers that it cannot
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successfully decompress the message (e.g. by using the CRC
instruction).
This instruction has no parameters.
6.5.3. OUTPUT
The OUTPUT instruction provides successfully decompressed data to the
application.
OUTPUT (%output_start, %output_length)
The parameters define the starting memory address and length of the
byte string to be provided to the application. Note that the OUTPUT
instruction can be used to output a partially decompressed message;
each time the instruction is encountered it appends a byte string to
the end of the data previously passed to the application via the
OUTPUT instruction.
The string of data is byte copied from the UDVM memory obeying the
rules of Section 4.4.
Decompression failure occurs if the cumulative number of bytes
provided to the application exceeds the application-defined parameter
maximum_uncompressed_size.
Since there is technically a difference between outputting a 0-byte
decompressed message, and not outputting a decompressed message at
all, the OUTPUT instruction needs to distinguish between the two
cases. Thus, if the UDVM terminates before encountering an OUTPUT
instruction it is considered not to have outputted a decompressed
message. If it encounters one or more OUTPUT instructions, each of
which provides 0 bytes of data to the application, then it is
considered to have outputted a 0-byte decompressed message.
6.5.4. NBO
The NBO instruction modifies the order in which compressed bits are
passed to the UDVM.
As the INPUT-FIXED and INPUT-HUFFMAN instructions read individual
bits from within a byte, to avoid ambiguity it is necessary to define
the order in which these bits are read. The default operation is to
read the MSBs before the LSBs, but if the NBO instruction is
encountered then the LSBs are read before the MSBs. Both cases are
illustrated below:
MSB LSB MSB LSB MSB LSB MSB LSB
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 1 2 3 4 5 6 7|8 9 ... | |7 6 5 4 3 2 1 0| ... 9 8|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Byte 0 Byte 1 Byte 0 Byte 1
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Default operation After NBO instruction
The NBO instruction can only be used before bitwise compressed data
is passed to the UDVM. Therefore, a decompression failure occurs if
it is encountered after an INPUT-FIXED or an INPUT-HUFFMAN
instruction has been used.
6.5.5. INPUT-BYTECODE
The INPUT-BYTECODE instruction requests a certain number of bytes of
compressed data from the application.
INPUT-BYTECODE (%length, %destination, %delta)
The length parameter indicates the requested number of bytes of
compressed data, and the destination parameter specifies the starting
memory address to which they should be copied. Byte copying is
performed as per the rules of Section 4.4.
If the instruction requests data that lies beyond the end of the
compressed message, no data is returned. Instead the UDVM moves
program execution to the memory address specified by the formula
(memory_address_of_INPUT-BYTECODE_instruction + delta) modulo 2^16.
The INPUT-BYTECODE instruction can only be used before bitwise
compressed data is passed to the UDVM. Therefore, a decompression
failure occurs if it is encountered after an INPUT-FIXED or an INPUT-
HUFFMAN instruction has been used.
6.5.6. INPUT-FIXED
The INPUT-FIXED instruction requests a certain number of bits of
compressed data from the application.
INPUT-FIXED (%length, %destination, %delta)
The length parameter indicates the requested number of bits. If this
parameter does not lie between 1 and 16 inclusive then a
decompression failure occurs.
The destination parameter specifies the memory address to which the
compressed data should be copied. Note that the requested bits are
interpreted as a 2-byte integer ranging from 0 to 2^length - 1. Under
default operation the MSBs of this integer are provided first, but if
an NBO instruction has been executed then the LSBs are provided
first.
If the instruction requests data that lies beyond the end of the
compressed message, no data is returned. Instead the UDVM moves
program execution to the memory address specified by the formula
(memory_address_of_INPUT-FIXED_instruction + delta) modulo 2^16.
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6.5.7. INPUT-HUFFMAN
The INPUT-HUFFMAN instruction requests a variable number of bits of
compressed data from the application. The instruction initially
requests a small number of bits and compares the result against a
certain criterion; if the criterion is not met then additional bits
are requested until the criterion is achieved.
The INPUT-HUFFMAN instruction is followed by three mandatory
parameters plus n additional sets of parameters. Every additional set
contains four parameters as shown below:
INPUT-HUFFMAN (%destination, %delta, #n, %bits_1, %lower_bound_1,
%upper_bound_1, %uncompressed_1, ... , %bits_n, %lower_bound_n,
%upper_bound_n, %uncompressed_n)
Note that if n = 0 then the INPUT-HUFFMAN instruction is ignored by
the UDVM. If bits_1 = 0 or (bits_1 + ... + bits_n) > 16 then
decompression failure occurs.
In all other cases, the behavior of the INPUT-HUFFMAN instruction is
defined below:
1.) Set j = 1.
2.) Request an additional bits_j compressed bits. Interpret the
total (bits_1 + ... + bits_j) bits of compressed data requested so
far as an integer H, with the first bit to be supplied as the MSB and
the last bit to be supplied as the LSB (note that this is always the
case, independently of whether the NBO instruction has been used).
3.) If data is requested that lies beyond the end of the compressed
message, terminate the INPUT-HUFFMAN instruction and move program
execution to the memory address specified by the formula
(memory_address_of_INPUT-HUFFMAN_instruction + delta) modulo 2^16.
4.) If (H < lower_bound_j) or (H > upper_bound_j) then set j = j +
1. Then go back to Step 2, unless j > n in which case decompression
failure occurs.
5.) Copy (H + uncompressed_j - lower_bound_j) modulo 2^16 to the
memory address specified by the destination parameter.
6.5.8. STATE-REFERENCE
The STATE-REFERENCE instruction retrieves some previously stored
state information.
STATE-REFERENCE (%id_start, %id_length, %state_start, %state_length,
%state_destination)
The id_start and id_length parameters specify the location of the
state identifier used to retrieve the state information. The state
identifier is always 16 bytes long; if id_length is less than 16 then
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the remaining least significant bytes of the identifier are padded
with zeroes.
Decompression failure occurs if id_length is greater than 16.
Decompression failure also occurs if no state information matching
the state identifier can be found.
Note that when accessing state information that has been previously
created by the UDVM, the state identifier is always taken from an
[MD5] hash of the state to be retrieved. However this is not
necessarily the case for application-defined state as per Section
3.5.
The state_start and state_length parameters define the starting byte
and number of bytes to copy from the state_value contained in the
identified item of state. If more state is requested than is actually
available then decompression failure occurs.
The state_destination parameter contains a UDVM memory address. The
requested state is byte copied to this memory address using the rules
of Section 4.4.
6.5.9. STATE-EXECUTE
The STATE-EXECUTE instruction retrieves and runs some previously
stored state information.
STATE-EXECUTE (%id_start, %id_length)
The id_start and id_length parameters function as per the STATE-
REFERENCE instruction.
STATE-EXECUTE is similar to STATE-REQUEST except that it does not
require the amount of state being requested or the proposed
destination for the state to be specified explicitly. Instead, it
simply puts the state back into the UDVM memory using the original
parameters from the END-MESSAGE instruction that created the state.
The entire state_value (all state length bytes of it) is byte copied
into the memory address specified by state start The UDVM then jumps
to the (absolute) memory address specified by state_instruction.
Note that state start, state length and state_instruction are all
stored together with state_value as part of an item of state
information.
7. Feedback information
If the transport mechanism offers bidirectional data transport then
the compression ratio can be improved by sending feedback
information. Since feedback data is optional, compressors must be
able to function correctly even if no feedback information is
provided.
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In the bidirectional UDVM architecture, suppose that Application 2
wishes to send feedback information to Compressor 1. The path taken
by the feedback information is as follows:
Appl. 2 --> Compressor 2 --> UDVM 1 --> Appl. 1 --> Compressor 1
The first hop along the path is between Application 2 and Compressor
2. If permitted by the application, Compressor 2 MAY be supplied with
some or all of the following items of data:
overall_memory_size
cycles_per_bit
cycles_per_message
id lengths and id values of successfully established state
Since the design of each compressor is left as an implementation
decision, there is no need to standardize the format in which this
data is provided to Compressor 2.
The second hop along the path is between Compressor 2 and UDVM 1. For
this step Compressor 2 transmits the feedback information to UDVM 1
across the same transport mechanism used to carry compressed data.
Typically this feedback information is piggybacked onto existing
compressed messages (standalone feedback messages are generally
vetoed by the application due to the lack of a corresponding
decompressed message).
Note that Compressor 2 can send the feedback information compressed
in order to reduce the total number of bits transmitted. Equally,
Compressor 2 may opt not to send feedback information at all.
If Compressor 2 chooses not to send feedback information then it sets
the feedback_location parameter in the END-MESSAGE instruction to 0.
Otherwise, it copies the following block of data to the memory of
UDVM 1 and places the starting memory address of this block in the
feedback_location parameter:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| UDVM_version |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| overall_memory_size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cycles_per_bit |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cycles_per_message |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|S| n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| id_length 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
: id_value_1 :
| |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| id_length 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
: id_value_2 :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| id_length n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
: id_value_n :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Each of the items of data is explained in greater detail below:
7.1. UDVM version
The first 2 bytes of feedback data specify whether only the basic
version of the UDVM is available, or whether an upgraded version of
the UDVM is available offering additional instructions, feedback data
etc.
The basic version of the UDVM is Version 0, which is the version
described in this document. Upgraded versions MUST be backwards-
compatible with the basic version in the following sense:
* If some UDVM bytecode reaches the END-MESSAGE or DECOMPRESSION-
FAILURE instructions when running on Version 0 of the UDVM, then
the upgraded version MUST run the bytecode in an identical
manner.
This condition ensures that all bytecode that is valid for Version 0
of the UDVM will continue to be valid for upgraded versions of the
UDVM. However, bytecode that is invalid on Version 0 of the UDVM
(i.e. bytecode that produces a decompression failure that is not
manually triggered) may become valid on upgraded versions.
Examples of how to upgrade the UDVM in a backwards-compatible manner
include: adding new UDVM instructions, adding more items of feedback
data etc.
7.2. Memory size and CPU cycles
The next 6 bytes of feedback data specify new values for the
application-defined parameters overall_memory_size, cycles_per_bit
and cycles_per_message. This allows Application 2 to inform
Compressor 1 that it has additional memory or processing power
available that could be used to improve the overall compression
ratio.
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Note that the feedback data can only be used to increase the amount
of resources available for Compressor 1 to use. If the feedback data
specifies a parameter value that is smaller than the value already
possessed by Compressor 1, the parameter keeps its original value
(i.e. the feedback data for this parameter is simply ignored).
The reason for this behavior is that if UDVM 2 is initialized with
more memory than expected by Compressor 1 then no problem arises, but
if UDVM 2 is initialized with less memory that expected by Compressor
1 then decompression failure may occur. Therefore, only allowing the
parameter values to increase means that the feedback mechanism is
robust against message loss or reordering on the feedback channel.
The parameters can only be restored to their original values if reset
or renegotiated by the application.
7.3. State identifiers
The variable n specifies the number of state identifiers to be
acknowledged.
Each state identifier is usually the first few bytes from an [MD5]
hash of the state being acknowledged. When a state identifier is
placed in the feedback information of UDVM 1, it is known by
Compressor 1 that the corresponding state has been successfully
established and can be referenced in future by using a STATE-
REFERENCE or a STATE-EXECUTE instruction. The feedback information
includes the length and value of each hash to be acknowledged.
Note that the MSB of n has a special meaning; if set to 1 then it
acknowledges the state that is currently being created by UDVM_1 via
the END-MESSAGE instruction. This saves having to transmit the
id_length and id_value explicitly on the feedback channel.
8. Security considerations
The following chapter identifies the potential security risks
associated with the overall UDVM architecture, and details the
proposed solution for each risk.
** Avoid snooping into state of other users
State can only be accessed using a state identifier, which is a
(prefix of a) cryptographic hash of the state being referenced. This
implies that the referencing packet already needs knowledge about the
state. To enforce this, a minimum reference length of 48 bits is
RECOMMENDED for applications running over an unsecure transport
mechanism. This also minimises the probability of an accidental state
collision.
Generally, ways to obtain knowledge about the state identifier (e.g.,
passive attacks) will also easily provide knowledge about the state
referenced, so no new vulnerability results.
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The application needs to handle state identifiers with the same care
it would handle the state itself.
** Avoid DoS vulnerabilities
*** Use of the UDVM as a tool in a DoS attack to another target
The UDVM cannot easily be used as an amplifier in a reflection
attack, as it only generates one decompressed message per incoming
compressed message. This packet is then handed to the application;
the utility as a reflection amplifier is therefore limited by the
utility of the application.
However, it must be noted that the UDVM can be used to generate
larger packets as input to the application than have to be sent from
the malicious sender; this therefore can send smaller packets (at a
lower bandwidth) than are delivered to the application. Depending on
the reflection characteristics of the application, this can be
considered a mild form of amplification. The application MUST limit
the number of packets reflected to a potential target - even if the
UDVM is used to generate a large amount of information from a small
incoming attack packet.
*** Attacking the UDVM 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 UDVM and application is thus approximately as
vulnerable as the application itself, unless it allows the
installation of state from a message where it would not have
installed state itself.
If this is desirable to increase the compression ratio, the effect
can be mitigated by adding feedback at the application level that
indicates whether the state was actually installed - this allows a
system under attack to gracefully degrade by no longer installing
compressor state that is not matched by application state.
*** Attacking the UDVM by faking state or making unauthorized changes
to state
State cannot be destroyed or changed by a malicious sender - it can
only add new state.
*** Attacking the UDVM by sending it looping code
The application sets an upper limit to the number of "CPU cycles"
that can be used per compressed message and per input bit in the
compressed message. The damage inflicted by sending packets with
looping code is therefore limited, although this may still be
substantial if a large number of CPU cycles are offered by the UDVM.
However, this would be true for any decompressor that can receive
packets from anywhere.
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9. Acknowledgements
Individual compression algorithms such as [DEFLATE] have been
important sources of ideas and knowledge.
Thanks to
Abigail Surtees (abigail.surtees@roke.co.uk)
Mark A West (mark.a.west@roke.co.uk)
Lawrence Conroy (lwc@roke.co.uk)
Christian Schmidt (christian.schmidt@icn.siemens.de)
Max Riegel (maximilian.riegel@icn.siemens.de)
Jan Christoffersson (jan.christoffersson@epl.ericsson.se)
Stefan Forsgren (stefan.forsgren@epl.ericsson.se)
Krister Svanbro (krister.svanbro@epl.ericsson.se)
Christopher Clanton (christopher.clanton@nokia.com)
Khiem Le (khiem.le@nokia.com)
Ka Cheong Leung (kacheong.leung@nokia.com)
for valuable input and review.
10. References
[DEFLATE] "DEFLATE Compressed Data Format Specification version
1.3", P. Deutsch, RFC 1951, Internet Engineering Task
Force, May 1996
[SCTP] "Stream Control Transmission Protocol", Stewart et al,
RFC 2960, Internet Engineering Task Force, October 2000
[SIP] "SIP: Session Initiation Protocol", Handley et al,
RFC 2543, Internet Engineering Task Force, March 1999
[MD5] "The MD5 Message-Digest Algorithm", R. Rivest, RFC 1321,
Internet Engineering Task Force, April 1992
[RFC-1662] "PPP in HDLC-like Framing", Simpson et al, Internet
Engineering Task Force, July 1994
[RFC-2026] "The Internet Standards Process - Revision 3", Scott
Bradner, Internet Engineering Task Force, October 1996
[RFC-2119] "Key words for use in RFCs to Indicate Requirement
Levels", Scott Bradner, Internet Engineering Task Force,
March 1997
11. Authors' addresses
Richard Price Tel: +44 1794 833681
Email: richard.price@roke.co.uk
Roke Manor Research Ltd
Romsey, Hants, SO51 0ZN
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United Kingdom
Jonathan Rosenberg
Email: jdrosen@dynamicsoft.com
dynamicsoft
72 Eagle Rock Avenue
First Floor
East Hanover, NJ 07936
Carsten Bormann Tel: +49 421 218 7024
Email: cabo@tzi.org
Universitaet Bremen TZI
Postfach 330440
D-28334 Bremen, Germany
Hans Hannu Tel: +46 920 20 21 84
Email: hans.hannu@epl.ericsson.se
Box 920
Ericsson Erisoft AB
SE-971 28 Lulea, Sweden
Zhigang Liu Tel: +1 972 894-5935
Email: zhigang.liu@nokia.com
Nokia Research Center
6000 Connection Drive
Irving, TX 75039
USA
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Appendix A. Mnemonic language
Writing UDVM programs directly in bytecode would be a daunting task,
so a simple mnemonic language is provided to facilitate the creation
of new decompression algorithms. Most importantly, the language
allows the parameters of an instruction to be specified as text names
rather than as integer values.
If an instruction parameter is given as a text name, it should
correspond to exactly one instance of a label, a reserved memory
address or an externally defined keyword. A label is simply a text
name preceded by a colon, for example:
:loop
JUMP (loop)
For any parameters corresponding to a label, the integer value of the
parameter is calculated by the following formula:
parameter_value = (instruction_address - label_address) modulo 2^16
Note that the "label address" is simply the memory address of the
instruction immediately following the label. In particular, the above
example can be rewritten as JUMP (0).
A reserved memory address is specified using the "reserve" keyword
followed by a text_name and (optionally) an integer value. For
example:
reserve apples
reserve pears (8)
reserve bananas
LOAD (bananas, 5)
For any parameters corresponding to a reserved memory address, the
integer value of the parameter is the next free memory address that
has not yet been reserved. Starting at this address, the specified
number of bytes of memory are then reserved (if no value is given
then a total of 2 bytes is reserved).
The first instance of a "reserve" keyword begins reserving memory at
Address 6 (to avoid overwriting the three well-known variables of
Section 4.2). So the above example can be rewritten as LOAD (16, 5).
An externally defined keyword is specified outside of the mnemonic
language. All of the application-defined parameters are considered to
be externally defined keywords and can be referenced in the mnemonic
code (useful for adapting the code based on the available memory or
CPU cycles). The following additional keywords can also be used:
Keyword: Corresponding value:
byte_copy_left 0
byte_copy_right 2
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stack_location 4
reserved_end See below
bytecode_length See below
total_length See below
The keyword reserved_end specifies the highest reserved memory
address for the entire mnemonic code (taking into account all the
occasions where memory is reserved).
The keyword bytecode_length specifies the total size of the bytecode
corresponding to the mnemonic code. Any instances of bytecode_length
are initially replaced with 3 bytes of zeroes, and then are filled in
after the remainder of the bytecode has been generated.
Similarly, the keyword total_length specifies the total amount of
memory required at the UDVM including bytecode and reserved memory
addresses.
A complete description of the mnemonic language and how it should be
translated into bytecode is given below:
Instructions: Instruction names are given in capitals. Replace
each name with the corresponding 1-byte value as
per Chapter 6.
$: When appended to the front of an instruction
parameter then the parameter is a memory address
rather than a direct value. This symbol is
mandatory for reference parameters, optional for
multitype parameters and disallowed for literals.
Integers: Instruction parameters can be given in the form of
decimal integers. They are converted into the
shortest bytecode capable of representing the
integer by the rules of Section 4.3.
Text references: Instruction parameters can also be given in the
form of lowercase names. These names should match
exactly one label, reserved memory address or
externally defined keyword as described above.
Labels: Label names are given as a colon followed by
lowercase text. They are deleted when converting
the mnemonics to bytecode.
Reserved memory: Memory addresses are reserved using the "reserve"
keyword. The line containing the reserve keyword
is deleted when converting to bytecode.
.LSB: When appended to the end of a text name, the
integer value corresponding to the name is
increased by 1. This is useful for addressing the
LSBs of a 2-byte variable.
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0b, 0d: Bytecode values can be specified directly in
binary or decimal via the appropriate prefix. The
direct bytecode continues until a character occurs
that is not an integer or whitespace.
Whitespace: All whitespace (plus brackets and commas) just
delimit the instructions. Delete.
Comments: These are indicated by a semicolon and continue
to the end of the line. Delete.
Once the mnemonic code has been converted into bytecode, it can be
executed by copying the bytecode into the UDVM memory beginning at
the first memory address that has not been reserved by an instance of
the "reserve" keyword. Program execution is assumed to begin at this
address.
Note that further to the rules outlined above, well-written mnemonic
code will also have the following properties:
* Any instance of a memory address will be specified as a text
reference rather than an integer value. This ensures that the
mnemonic code is portable.
* The mnemonic code will not write to any memory address except
those reserved by the "reserve" keyword. This ensures that the
code can be compiled.
Appendix B. Example application-defined parameters
This appendix gives some example values for each of the application-
defined parameters. These values are geared towards the compression
of a text-based protocol running over UDP or TCP, for example a
signaling protocol such as [SIP].
Note that all of the proposed values are fixed and not negotiated
between the two instances of the application invoking the compressor
and the UDVM. This is because it is possible for the application
invoking the UDVM to receive compressed messages from several
different applications, and it is difficult to determine which
message corresponds to which application. [SIP] does this using
"From:" and "To:" fields in the message itself, but these are not
visible until the message has been decompressed. It is simpler just
to fix a set of parameter for every instance of the application.
UDVM_version 0
maximum_compressed_size 65535
maximum_uncompressed_size 65535
minimum_hash_size 6
overall_memory_size 8192
working_memory_start 0
working_memory_end 8191
cycles_per_bit 20
cycles_per_message 2000
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first_instruction 26
Note that the parameters overall_memory_size, cycles_per_bit and
cycles_per_message can be increased on the fly using the feedback
mechanism of Chapter 7. This mechanism is designed to be function
correctly even when the application invoking the UDVM is sent
compressed messages from several different applications.
The initial contents of the UDVM memory also need to be defined. It
is not enough simply to initialize the memory containing all zeroes,
as the UDVM would be unable to input any compressed data. Instead,
for each new compressed message the memory should be initialized
containing a simple decompressor capable of extracting the first few
bytes of compressed data. These bytes can then be interpreted as UDVM
instructions for a more powerful decompression algorithm, a state
reference to retrieve a previously stored algorithm etc.
As an example, the following mnemonic code can be converted to
bytecode and pasted into the UDVM memory beginning at Address 26:
reserve length
reserve destination
reserve hash (16)
INPUT-BYTECODE (1, length, fail)
COMPARE (length, 16, retrieve_state, retrieve_state, new_code)
:retrieve_state
INPUT-BYTECODE ($length, hash, fail)
STATE-EXECUTE (hash, $length)
:new_code
INPUT-BYTECODE (2, destination, fail)
INPUT-BYTECODE ($length, $destination, fail)
SUBTRACT ($destination, execute_new_code)
:execute_new_code
JUMP ($destination)
:fail
DECOMPRESSION-FAILURE
The mnemonic code requests a single byte of compressed data, which is
considered to be a length from 0 to 255. Lengths from 0 to 16
inclusive are interpreted as the length of a hash value that is used
to retrieve and run bytecode previously stored as state. Lengths from
17 to 255 are interpreted as an amount of new UDVM bytecode to be
extracted from the start of the compressed data.
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Finally, the application can define initial state that is available
to the UDVM. Examples of application-defined state include common
decompression algorithms, dictionaries of common text phrases etc.
Appendix C. Example decompression algorithms
This appendix gives examples of decompression algorithms which can be
downloaded to the UDVM in the form of bytecode.
C.1. Example UDVM code for simple LZ77 decompression
The first example gives the code required to decompress data from a
very simple LZ77-based algorithm. The UDVM is instructed to interpret
a compressed message as a set of 4-byte characters, where each
character contains a 2-byte position integer followed by a 2-byte
length integer. Taken together these integers point to a previously
received text string in the UDVM memory, which is then copied to the
end of the uncompressed message.
Since the compressor can only send references to strings already
present in the UDVM memory, before the first message is decompressed
the memory must be initialized with a static dictionary containing
the 256 ASCII characters.
The algorithm write-protects the memory containing the UDVM
instructions used to decompress each character, so that they can
easily be compiled to improve the speed of decompression.
A 2-byte CRC over the uncompressed message is appended to the end of
the compressed message, to verify that correct decompression has
occurred. The algorithm also requests that the contents of the UDVM
memory be saved using the state request mechanism, so that it can be
retrieved by sending the appropriate 6-byte hash.
reserve byte_copy_left
reserve byte_copy_right
reserve uncompressed_start
reserve uncompressed_end
reserve uncompressed_length
reserve position
reserve length
reserve static_dictionary (256)
reserve circular_buffer (2048)
WORKING-MEMORY (uncompressed_start, reserved_end)
MULTILOAD (0, 7, circular_buffer, reserved_end, static_dictionary,
circular_buffer, 0, 0, 0)
:unpack_static_dictionary
; The following instructions initialize the static dictionary.
COPY-LITERAL (position.LSB, 1, $uncompressed_start)
ADD ($position, 1)
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COMPARE ($position, 256, unpack_static_dictionary, next_character, 0)
:next_character
INPUT-FIXED (16, position, fail)
INPUT-FIXED (16, length, end_of_message)
COPY-LITERAL ($position, $length, $uncompressed_end)
ADD ($uncompressed_length, $length)
JUMP (next_character)
:fail
DECOMPRESSION-FAILURE
:end_of_message
CRC ($position, $uncompressed_start, $uncompressed_length, fail)
OUTPUT ($uncompressed_start, $uncompressed_length)
END-MESSAGE (6, 0, total_length, next_character, 0)
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