Network Working Group A. Surtees

Network Working Group A. Surtees
Request for Comments: 4896 M. West
Updates: 3320, 3321, 3485 Siemens/Roke Manor Research
Category: Standards Track A.B. Roach
Estacado Systems
June 2007

Signaling Compression (SigComp) Corrections and Clarifications

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 IETF Trust (2007).

Abstract

This document describes common misinterpretations and some
ambiguities in the Signaling Compression Protocol (SigComp), and
offers guidance to developers to resolve any resultant problems.
SigComp defines a scheme for compressing messages generated by
application protocols such as the Session Initiation Protocol (SIP).
This document updates the following RFCs: RFC 3320, RFC 3321, and RFC
3485.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Decompression Memory Size . . . . . . . . . . . . . . . . . . 3
2.1. Bytecode within Decompression Memory Size . . . . . . . . 3
2.2. Default Decompression Memory Size . . . . . . . . . . . . 4
3. UDVM Instructions . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Data Input Instructions . . . . . . . . . . . . . . . . . 5
3.2. MULTILOAD . . . . . . . . . . . . . . . . . . . . . . . . 5
3.3. STATE-FREE . . . . . . . . . . . . . . . . . . . . . . . . 6
3.4. Using the Stack . . . . . . . . . . . . . . . . . . . . . 6
4. Byte Copying Rules . . . . . . . . . . . . . . . . . . . . . . 7
4.1. Instructions That Use Byte Copying Rules . . . . . . . . . 9
5. State Retention Priority . . . . . . . . . . . . . . . . . . . 9
5.1. Priority Values . . . . . . . . . . . . . . . . . . . . . 9
5.2. Multiple State Retention Priorities . . . . . . . . . . . 10
5.3. Retention Priority 65535 (or -1) . . . . . . . . . . . . . 10
6. Duplicate State . . . . . . . . . . . . . . . . . . . . . . . 14
7. State Identifier Clashes . . . . . . . . . . . . . . . . . . . 14
8. Message Misordering . . . . . . . . . . . . . . . . . . . . . 15
9. Requested Feedback . . . . . . . . . . . . . . . . . . . . . . 15
9.1. Feedback When SMS Is Zero . . . . . . . . . . . . . . . . 15
9.2. Updating Feedback Requests . . . . . . . . . . . . . . . . 16
10. Advertising Resources . . . . . . . . . . . . . . . . . . . . 16
10.1. The I-bit and Local State Items . . . . . . . . . . . . . 16
10.2. Dynamic Update of Resources . . . . . . . . . . . . . . . 17
10.3. Advertisement of Locally Available State Items . . . . . . 17
10.3.1. Basic SigComp . . . . . . . . . . . . . . . . . . . . 18
10.3.2. Dictionaries . . . . . . . . . . . . . . . . . . . . 18
10.3.3. SigComp Extended Mechanisms . . . . . . . . . . . . . 19
11. Uncompressed Bytecode . . . . . . . . . . . . . . . . . . . . 19
12. RFC 3485 SIP/SDP Static Dictionary . . . . . . . . . . . . . . 20
13. Security Considerations . . . . . . . . . . . . . . . . . . . 21
14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
15. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 22
16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
16.1. Normative References . . . . . . . . . . . . . . . . . . . 23
16.2. Informative References . . . . . . . . . . . . . . . . . . 23
Appendix A. Dummy Application Protocol (DAP) . . . . . . . . . . 24
A.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . 24
A.2. Processing a DAP Message . . . . . . . . . . . . . . . . . 24
A.3. DAP Message Format in ABNF . . . . . . . . . . . . . . . . 26
A.4. An Example of a DAP Message . . . . . . . . . . . . . . . 26

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1. Introduction

SigComp [1] defines the Universal Decompressor Virtual Machine (UDVM)
for decompressing messages sent by a compliant compressor. SigComp
further describes mechanisms to deal with state handling, message
structure, and other details. While the behavior of the decompressor
is specified in great detail, the behavior of the compressor is left
as a choice for the implementer. During implementation and
interoperability tests, some areas of SigComp that need clarification
have been identified. The sections that follow enumerate the problem
areas identified in the specification, and attempt to provide
clarification.

Note that, as this document refers to sections in several other
documents, the following notation is applied:

"in Section 3.4" refers to Section 3.4 of this document
"in RFC 3320-Section 3.4" refers to Section 3.4 of RFC 3320 [1]

1.1. 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 [5].

2. Decompression Memory Size

2.1. Bytecode within Decompression Memory Size

SigComp [1] states that the default Decompression Memory Size (DMS)
is 2K. The UDVM memory size is defined in RFC 3320-Section 7 to be
(DMS - n), where n is the size of the SigComp message, for messages
transported over UDP and (DMS / 2) for those transported over TCP.
This means that when the message contains the bytecode (as it will
for at least the first message) there will actually be two copies of
the bytecode within the decompressor memory (see Figure 1). The
presence of the second copy of bytecode in decompressor memory is
correct in this case.

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|<----------------------------DMS--------------------------------->|
|<-----SigComp message---->|<------------UDVM memory size--------->|
+-+----------+-------------+-----+----------+----------------------+
| | bytecode | comp msg | | bytecode | circular buffer |
+-+----------+-------------+-----+----------+----------------------+
^ ^
| |
SigComp header Low bytes of UDVM

Figure 1: Bytecode and UDVM memory size within DMS

2.2. Default Decompression Memory Size

For many implementations, the length of decompression bytecode sent
is in the range of three to four hundred bytes. Because SigComp
specifies a default DMS of 2K, the described scheme seriously
restricts the size of the circular buffer, and of the compressed
message itself. In some cases, this set of circumstances has a
damaging effect on the compression ratio; for others, it makes it
completely impossible to send certain messages compressed.

To address this problem, those mandating the use of SigComp need to
also provide further specification for their application that
mandates the use of an appropriately sized DMS. Sizing of such a DMS
should take into account (1) the size of bytecode for algorithms
likely to be employed in compressing the application messages, (2)
the size of any buffers or structures necessary to execute such
algorithms, (3) the size of application messages, and (4) the average
entropy present within a single application message.

For example, assume a typical compression algorithm requiring
approximately 400 bytes of bytecode, plus about 2432 bytes of data
structures. The required UDVM memory size is 400 + 2432 = 2832. For
a TCP-based protocol, this means the DMS must be at least 5664 (2832
* 2) bytes, which is rounded up to 8k. For a UDP-based protocol, one
must take into account the size of the SigComp messages themselves.
Assuming a text-based protocol with sufficient average entropy to
compress a single message by 50% (without any previous message
history), and messages that are not expected to exceed 8192 bytes in
size, the protocol message itself will add 4096 bytes to the SigComp
message size (on top of the 400 bytes of bytecode plus a 3-byte
header), or 4096 + 400 + 3 = 4499. To calculate the DMS, one must
add this to the required UDVM memory size: 2832 + 4499 = 6531, which
is again rounded up to 8k of DMS.

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3. UDVM Instructions

3.1. Data Input Instructions

When inputting data from the compressed message, the INPUT-BYTES (RFC
3320-Section 9.4.2) and INPUT-BITS (RFC 3320-Section 9.4.3)
instructions both have the paragraph:

"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."

The intent is that if n bytes/bits are requested, but only m are left
in the message (where m < n), then the decompression dispatcher MUST
NOT return any bytes/bits to the UDVM, and the m bytes/bits that are
there MUST remain in the message unchanged.

For example, if the remaining bytes of a message are: 0x01 0x02 0x03
and the UDVM encounters an INPUT-BYTES (6, a, b) instruction. Then
the decompressor dispatcher returns no bytes and jumps to the
instruction specified by b. This contains an INPUT-BYTES (2, c, d)
instruction so the decompressor dispatcher successfully returns the
bytes 0x01 and 0x02.

In the case where an INPUT-BYTES instruction follows an INPUT-BITS
instruction that has left a partial byte in the message, the partial
byte should still be thrown away even if there are not enough bytes
to input.

INPUT-BYTES (0, a, b) can be used to flush out a partial byte.

3.2. MULTILOAD

In order to make step-by-step implementation simpler, the MULTILOAD
instruction is explicitly not allowed to write into any memory
positions occupied by the MULTILOAD opcode or any of its parameters.
Additionally, if there is any indirection of parameters, the
indirection MUST be done at execution time.

Any implementation technique other than a step-by-step implementation
(e.g., decode all operands then execute, which is the model of all
other instructions) MUST yield the same result as a step-by-step
implementation would.

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For example:

at (64)

:location_a pad (2)
:location_b pad (2)
:location_c pad (2)
pad (30)
:udvm_memory_size pad (2)
:circular_buffer pad (2)

align (64)

MULTILOAD (location_a, 3, circular_buffer,
udvm_memory_size, $location_a)

The step-by-step implementation would: write the address of
circular_buffer into location_a (memory address 64); write the
address of udvm_memory_size into location_a + 2 (memory address 66);
write the value stored in location_a (accessed using indirection -
that is now the address of circular_buffer) into location_a + 4
(memory address 68). Therefore, at the end of the execution by a
correct implementation, location_c will contain the address of
circular_buffer.

3.3. STATE-FREE

The STATE-FREE instruction does not check the minimum_access_length.
This is correct because the state cannot be freed until the
application has authenticated the message. The lack of checking does
not pose a security risk because if the sender has enough information
to create authenticated messages, then sending messages that save
state can push previous state out of storage anyway.

The STATE-FREE instruction can only free state in the compartment
that corresponds to the message being decompressed. Attempting to
free state that is either from another compartment, or that is not
associated with any compartment, has no effect.

3.4. Using the Stack

The instructions PUSH, POP, CALL, and RETURN make use of a stack that
is set up using the well-known memory address stack_location to
define where in memory the stack is located. Use of the stack is
defined in RFC 3320-Section 8.3, which states: '"Pushing" a value on
the stack is an abbreviation for copying the value to

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stack[stack_fill] and then increasing stack_fill by 1.' and
'stack_fill is an abbreviation for the 2-byte word at stack_location
and stack_location + 1'.

In the very rare case that the value of stack_fill is 0xFFFF when a
value is pushed onto the stack, then the original stack_fill value
MUST be increased by 1 to 0x0000 and written back to stack_location
and stack_location + 1 (which will overwrite the value that has been
pushed onto the stack).

The new value pushed onto the stack has, in theory, been written
to stack [0xFFFF] = stack_location. Stack_fill would then be
increased by 1; however, the value at stack_location and
stack_location + 1 has just been updated. To maintain the
integrity of the stack with regard to over and underflow,
stack_fill cannot be re-read at this point, and the pushed value
is overwritten.

4. Byte Copying Rules

RFC 3320-Section 8.4 states that "The string of bytes is copied in
ascending order of memory address, respecting the bounds set by
byte_copy_left and byte_copy_right." This is misleading in that it
is perfectly legitimate to copy bytes outside of the bounds set by
byte_copy_left and byte_copy_right. Byte_copy_left and
byte_copy_right provide the ability to maintain a circular buffer as
follows:

For moving to the right

if current_byte == ((byte_copy_right - 1) mod 2 ^ 16):
next_byte = byte_copy_left
else:
next_byte = (current_byte + 1) mod 2 ^ 16

which is equivalent to the algorithm given in RFC 3320-Section 8.4.

For moving to the left

if current_byte == byte_copy_left:
previous_byte = (byte_copy_right - 1) mod 2 ^ 16
else:
previous_byte = (current_byte - 1) mod 2 ^ 16

Moving to the left is only used for COPY_OFFSET.

Consequently, copying could begin to the left of byte_copy_left and
continue across it (and jump back to it according to the given

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algorithm if necessary) and could begin at or to the right of
byte_copy_right (though care must be taken to prevent decompression
failure due to writing to / reading from beyond the UDVM memory).

For further clarity: consider the UDVM memory laid out as follows,
with byte_copy_left and byte_copy_right in the locations indicated by
"BCL" and "BCR", respectively:

+----------------------------------------+
| |
+----------^------------^----------------+
BCL BCR

If an opcode read or wrote bytes starting to the left of
byte_copy_left, it would do so in the following order:

+----------------------------------------+
| abcdefghijkl |
+----------^------------^----------------+
BCL BCR

If the opcode continues to read or write until it reaches
byte_copy_right, it would then wrap around to byte_copy_left and
continue (letters after the wrap are capitalized for clarity):

+----------------------------------------+
| abcQRSTUVjklmnop |
+----------^------------^----------------+
BCL BCR

Similarly, writing to the right of byte_copy_right is a perfectly
valid operation for opcodes that honor byte copying rules:

+----------------------------------------+
| abcdefg |
+----------^------------^----------------+
BCL BCR

A final, somewhat odd relic of the foregoing rules occurs when
byte_copy_right is actually less than byte_copy_left. In this case,
reads and writes will skip the memory between the pointers:

+----------------------------------------+
| abcde fghijkl |
+----------^------------^----------------+
BCR BCL

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4.1. Instructions That Use Byte Copying Rules

This document amends the list of instructions that obey byte copying
rules in RFC 3320-Section 8.4 to include STATE-CREATE and CRC.

RFC 3320-Section 8.4 specifies the byte copying rules and includes a
list of the instructions that obey them. STATE-CREATE is not in this
list but END-MESSAGE is. This caused confusion due to the fact that
neither instruction actually does any byte copying; rather, both
instructions give information to the state-handler to create state.
Logically, both instructions should have the same information about
byte copying.

When state is created by the state-handler (whether from an END-
MESSAGE or a STATE-CREATE instruction), the byte copying rules of RFC
3320-Section 8.4 apply.

Note that, if the contents of the UDVM changes between the occurrence
of the STATE-CREATE instruction and the state being created, the
bytes that are stored are those in the buffer at the time of creation
(i.e., when the message has been decompressed and authenticated).

CRC is not mentioned in RFC 3320-Section 8.4 in the list of
instructions that obey byte copying rules, but its description in RFC
3320-Section 9.3.5 states that these rules are to be obeyed. When
reading data over which to perform the CRC check, byte copying rules
apply as specified in RFC 3320-Section 8.4.

When the partial identifier for a STATE-FREE instruction is read,
(during the execution of END-MESSAGE) byte copying rules as per RFC
3320-Section 8.4 apply.

Given that reading the buffer for creating and freeing state within
the END-MESSAGE instruction obeys byte copying rules, there may be
some confusion as to whether reading feedback items should also obey
byte copying rules. Byte copying rules do not apply for reading
feedback items.

5. State Retention Priority

5.1. Priority Values

For state_retention_priority, 65535 < 0 < 1 < ... < 65534. This is
slightly counter intuitive, but is correct.

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5.2. Multiple State Retention Priorities

There may be confusion when the same piece of state is created at two
different retention priorities. The following clarifies this:

The retention priority MUST be associated with the compartment and
not with the piece of state. For example, if endpoint A creates a
piece of state with retention priority 1 and endpoint B creates
exactly the same state with retention priority 2, there should be
one copy (assuming the model of state management suggested in
SigComp [1]) of the actual state, but each compartment should keep
a record of this piece of state with its own priority. (If this
does not happen then the state could be kept for longer than A
anticipated or less time than B anticipated, depending on which
priority is used. This could cause Decompression Failure to
occur.)

If the same piece of state is created within a compartment with a
different priority, then one copy of it should be stored with the
new priority and it MUST count only once against SMS. That is,
the state creation updates the priority rather than creates a new
piece of state.

5.3. Retention Priority 65535 (or -1)

There is potentially a problem with storing multiple pieces of state
with the minimum retention priority (65535) as defined in SigComp
[1]. This can be shown by considering the following examples that
are of shared mode, which is documented in SigComp Extended [2]. The
key thing about state with retention priority 65535 is that it can be
created by an endpoint in the decompressor compartment without the
knowledge of the remote compressor (which controls state creation in
the decompressor compartment).

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Example 1:

[SMn state is shared mode state (priority 65535),
BC is bytecode state (priority 1),
BFn is buffer state (priority 0)]

Endpoint A Endpoint B
[decomp cpt] [comp cpt]

[SM1]
------------------------------->
[SM1]

[SM1, SM2]
--------------------X (message lost)

[SM1, BC, BF1]
<------------ref SM1------------
[SM2, BC, BF1]
endpoint B still believes SM1
is at endpoint A

[BC, BF1, BF2]
<------------ref SM1------------

decompression failure at A
because SM1 has already been deleted

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Example 2:

Endpoint A Endpoint B
[decomp cpt] [comp cpt]

[SM1]
------------------------------->
[SM1]

[SM1, BC, BF1]
(message lost)X------ref SM1-----

[SM1, SM2]
------------------------------->
endpoint B does not create SM2
because there is no space
[SM1, BC, BF1]

[SM1, BC, BF1, BF2]
<------------ref SM1------------
[SM2, BC, BF2]
endpoint B still believes SM1
is at endpoint A

[BC, BF1, BF2, BF3]
<------------ref SM1------------

decompression failure at A
because SM1 has already been deleted

Figure 2: Retention priority 65535 examples

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Once there is more than one piece of minimum priority state created
in a decompressor compartment, the corresponding compressor cannot be
certain about which pieces of state are present in that
(decompressor) compartment. If there is only one piece of state,
then no such ambiguity exists.

The problem is a consequence of the different rules for the creation
of minimum priority state. In particular, the creation of the second
piece of state without the knowledge of the compressor could mean
that the first piece is pushed out earlier than the compressor
expects (despite the fact that the state processing rules from
SigComp [1] are being implemented correctly).

SigComp [1] also states that a compressor MUST be certain that all of
the data needed to decompress a SigComp message is available at the
receiving endpoint. Thus, it SHOULD NOT reference any state unless
it can be sure that the state exists. The fact that the compressor
at B has no way of knowing how much state has been created at A can
lead to a loss of synchronization between the endpoints, which is not
acceptable.

One observation is that it is always safe to reference a piece of
minimum priority state following receipt of the advertisement of the
state.

If it is known that both endpoints are running SigComp version 2, as
defined in NACK [3], then an endpoint MAY assume that the likelihood
of a loss of synchronization is very small, and rely on the NACK
mechanism for recovery.

However, for a compressor to try and avoid causing the generation of
NACKs, it has to be able to make some assumptions about the behavior
of the peer compressor. Also, if one of the endpoints does not
support NACK, then some other solution is needed.

Consequently, where NACK is not supported or for NACK averse
compressors, the recommendation is that only one piece of minimum
priority state SHOULD be present in a compartment at any one time.
If both endpoints support NACK [3], then this recommendation MAY be
relaxed, but implementers need to think carefully about the
consequences of creating multiple pieces of minimum priority state.
In either case, if the behavior of the application restricts the
message flow, this fact could be exploited to allow safe creation of
multiple minimum priority states; however, care must still be taken.

Note that if a compressor wishes the remote endpoint to be able to
create a new piece of minimum priority state, it can use the STATE-
FREE instruction to remove the existing piece of state.

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6. Duplicate State

If a piece of state is created in a compartment in which it already
exists, the time of its creation SHOULD be updated as if it had just
been created, irrespective of whether or not there is a new state
retention priority.

7. State Identifier Clashes

RFC 3320-Section 6.2 states that when creating a piece of state, the
full 20-byte hash should be checked to see whether or not another
piece of state with this identifier exists. If it does, and the
state item is not identical, then the new creation MUST fail. It is
stated that the probability of this occurring is vanishingly small
(and so it is, see below).

However, when state is accessed, only the first n bytes of the state
identifier are used, where n could be as low as 6. At this point, if
there are two pieces of state with the same first n bytes of state
identifier, the STATE-ACCESS instruction will cause decompression
failure. The compressor referencing the state will not expect this
failure mode because the state creation succeeded without a clash.
At a server endpoint where there could be thousands or millions of
pieces of state, how likely is this to actually happen?

Consider the birthday paradox (where there only have to be 23 people
in a room to have a greater than 50% chance that two of them will
have the same birthday (Birthday [8])).

The naive calculation using factorials gives:

N!
Pd(N,s) = 1 - -------------
(N - s)! N^s

where N is the number of possible values and s is the sample size.

However, due to dealing with large numbers, an approximation is
needed:

Pd(N,s) = 1 - e^( LnFact(N) - LnFact(N-s) - s Ln(N) )

where LnFact (x) is the log of x!, which can be approximated by:

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LnFact(x) ~ (x + 1/2) Ln(x) - x + Ln(2*Pi)/2 +

1 1 1 1
--- - ------- + -------- - --------
12x 360 x^3 1260 x^5 1680 x^7

which using N = 2^48 [6 octet partial state identifier] gives:

s = 1 000 000: Pd (N,s) = 0.018%
s = 10 000 000: Pd (N,s) = 16.28%
s = 100 000 000: Pd (N,s) = 100.00%

so when implementing, thought should be given as to whether or not 6
octets of state identifier is enough to ensure that state access will
be successful (particularly at a server).

The likelihood of a clash when using the full 20 octets of state
identifier, does indeed have a vanishingly small probability:
using N = 2^160 [full 20 octet state identifier] gives:

s = 1 000 000: Pd (N,s) = 3.42E-35%
s = 10 000 000: Pd (N,s) = 3.42E-33%
s = 100 000 000: Pd (N,s) = 3.42E-31%

Consequently, care must be taken when deciding how many octets of
state identifier to use to access state at the server.

8. Message Misordering

SigComp [1] makes only one reference to the possibility of misordered
messages. However, the statement that the 'compressor MUST ensure
that the message can be decompressed using the resources available at
the remote endpoint' puts the onus on the compressor to take account
of the possibility of misordering occurring.

Whether misordering can occur and whether that would have an impact
depends on the compartment definition and the transport protocol in
use. Therefore, it is up to the implementer of the compressor to
take these factors into account.

9. Requested Feedback

9.1. Feedback When SMS Is Zero

If an endpoint receives a request for feedback, then it SHOULD return
the feedback even if its SMS is zero. The storage overhead of the
requested feedback is NOT part of the SMS.

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9.2. Updating Feedback Requests

When an endpoint receives a valid message it updates the requested
feedback data for that compartment. RFC 3320-Section 5 states that
there is no need to transmit any requested feedback item more than
once. However, there are cases where it would be beneficial for the
feedback to be sent more than once (e.g., a retransmitted 200 OK SIP
message [9] to an INVITE SIP message implies that the original 200
OK, and the feedback it carried, might not have reached the remote
endpoint). Therefore, an endpoint SHOULD transmit feedback
repeatedly until it receives another valid message that updates the
feedback.

RFC 3320-Section 9.4.9 states that when requested_feedback_location
equals zero, no feedback request is made. However, there is no
indication of whether this means that the existing feedback data is
left untouched or if this means that the existing feedback data
SHOULD be overwritten to be 'no feedback data'. If
requested_feedback_location equals zero, the existing feedback data
SHOULD be left untouched and returned in any subsequent messages as
before.

RFC 3320-Section 9.4.9 also makes no statement about what happens to
existing feedback data when requested_feedback_location does not
equal zero but the Q flag indicating the presence/absence of a
requested_feedback_item is zero. In this case, the existing feedback
data SHOULD be overwritten to be 'no feedback data'.

10. Advertising Resources

10.1. The I-bit and Local State Items

The I-bit in requested feedback is a mechanism by which a compressor
can tell a remote endpoint that it is not going to access any local
state items. By doing so, it gives the remote endpoint the option of
not advertising them in subsequent messages. Setting the I-bit does
not obligate the remote endpoint to cease sending advertisements.

The remote endpoint SHOULD still advertise its parameters such as DMS
and state memory size (SMS). (This is particularly important; if the
sender of the first message sets the I-bit, it will still want the
advertisement of parameters from the receiver. If it doesn't receive
these, it has to assume the default parameters which will affect
compression efficiency.)

The endpoint receiving an I-bit of 1 can reclaim the memory used to
store the locally available state items. However, this has NO impact

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on any state that has been created by the sender using END-MESSAGE or
STATE-CREATE instructions.

10.2. Dynamic Update of Resources

Decompressor resources such as SMS and DMS can be dynamically updated
at the compressor by use of the SMS and DMS bits in returned
parameters feedback (see RFC 3320-Section 9.4.9). Changing resources
dynamically (apart from initial advertisements for each compartment)
is not expected to happen very often.

If additional resources are advertised to a compressor, then it is up
to the implementation at the compressor whether or not to make use of
these resources. For example, if the decompressor advertises 8k SMS
but the compressor only has 4k SMS, then the compressor MAY choose
not to use the extra 4k (e.g., in order to monitor state saved at the
decompressor). In this case, there is no synchronization problem.
The compressor MUST NOT use more than the most recently advertised
resources. Note that the compressor SMS is unofficial (it enables
the compressor to monitor decompressor state) and is separate from
the SMS advertised by the decompressor.

Reducing the resources has potential synchronization issues and so
SHOULD NOT be done unless absolutely necessary. If this is the case
then the memory MUST NOT be reclaimed until the remote endpoint has
acknowledged the message sent with the advertisement. If state is to
be deleted to accommodate a reduction in SMS then both endpoints MUST
delete it according to the state retention priority (see RFC 3320-
Section 6.2). The compressor MUST NOT then use more than the amount
of resources most recently advertised.

10.3. Advertisement of Locally Available State Items

RFC 3320-Section 3.3.3 defines locally available state items to be
the pieces of state that an endpoint has available but that have not
been uploaded by the SigComp message. The examples given are
dictionaries and well known pieces of bytecode; and the advertisement
mechanism discussed in RFC 3320-Section 9.4.9 provides a way for the
endpoint to advertise the pieces of locally available state that it
has.

However, SigComp [1] does not (nor was it ever intended to) fully
define the use of locally available state items, in particular, the
length of time for which they will be available. The use of locally
available state items is left for definition in other documents.
However, this fact, coupled with the fact that SigComp does contain
some hooks for uses of locally available state items and the fact
that some of the definitions of such uses (in SigComp Extended [2])

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are incomplete has caused some confusion. Therefore, this section
clarifies the situation.

Note that any definitions of uses of locally available state items
MUST NOT conflict with any other uses.

10.3.1. Basic SigComp

SigComp provides a mechanism for an endpoint to advertise locally
available state (RFC 3320-Section 9.4.9). If the endpoint receiving
the advertisement does not 'recognize' it and therefore know the
properties of the state e.g., its length and lifetime, the compressor
needs to consider very carefully whether or not to access the state;
especially if NACK [3] is not available.

SigComp provides the following hooks for use in conjunction with
locally available state items. Without further definition, locally
available state SHOULD NOT be used.

RFC 3320-Section 6.2 allows for the possibility to map locally
available state items to a compartment and states that, if this is
done, the state items MUST have state retention priority 65535 in
order to not interfere with state created at the request of the
remote compressor. Note that Section 5.3 also recommends that only
one such piece of state SHOULD be created per compartment.

The I-bit in the requested_feedback_location (see RFC 3320-Section
9.4.9) allows a compressor to indicate to the remote endpoint that it
will not reference any of the previously advertised locally available
state. Depending on the implementation model for state handling at
the remote endpoint, this could allow the remote endpoint to reclaim
the memory being used by such state items.

10.3.2. Dictionaries

The most basic use of the local state advertisement is the
advertisement of a dictionary (e.g., the dictionary specified by SIP/
SDP Static Dictionary [4]) or a piece of bytecode. In general, these
pieces of state:

o are not mapped to compartments
o are local to the endpoint
o are available for at least the duration of the compartment
o do not have any impact on the compartment SMS

However, for a given piece of state the exact lifetime needs to be
defined e.g., in public specifications such as SigComp for SIP [7] or

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the 3GPP IMS specification [10]. Such a specification should also
indicate whether or not advertisement of the state is needed.

10.3.3. SigComp Extended Mechanisms

SigComp Extended [2] defines some uses of local state advertisements
for which additional clarification is provided here.

Shared-mode (see RFC 3321-Section 5.2) is well-defined (when combined
with the clarification in Section 5.3). In particular, the states
that are created and advertised are mapped into the compartment, have
the minimum retention priority and persist only until they are
deleted by the creation of new (non-minimum retention priority) state
or use of a STATE-FREE instruction.

The definition of endpoint initiated acknowledgments (RFC 3321-
Section 5.1.2) requires clarification in order to ensure that the
definition does not preclude advertisements being used to indicate
that state will be kept beyond the lifetime of the compartment (as
discussed in SigComp for SIP [7]). Thus the clarification is:

Where Endpoint A requests state creation at Endpoint B, Endpoint B
MAY subsequently advertise the hash of the created state item to
Endpoint A. This conveys to Endpoint A (i) that the state has
been successfully created within the compartment; and (ii) that
the state will be available for at least the lifetime of the state
as defined by the state deletion rules according to age and
retention priority of SigComp [1]. If the state is available at
Endpoint B after it would be deleted from the compartment
according to [1], then the state no longer counts towards the SMS
of the compartment. Since there is no guarantee of such state
being available beyond its normally defined lifetime, endpoints
SHOULD only attempt to access the state after this time where it
is known that NACK [3] is available.

11. Uncompressed Bytecode

It is possible to write bytecode that simply instructs the
decompressor to output the entire message (effectively sending it
uncompressed, but within a SigComp message). This is particularly
useful if the bytecode is well-known (so that decompressors can
recognize and output the bytes without running a VM if they wish);
therefore, it is documented here.

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The mnemonic code is:

at (0)
:udvm_memory_size pad (2)
:cycles_per_bit pad (2)
:sigcomp_version pad (2)
:partial_state_id_length pad (2)
:state_length pad (2)
:reserved pad (2)
at (64)
:byte_copy_left pad (2)
:byte_copy_right pad (2)
:input_bit_order pad (2)
:stack_location pad (2)

; Simple loop
; Read a byte
; Output a byte
; Until there are no more bytes!

at (128)
:start
INPUT-BYTES (1, byte_copy_left, end)
OUTPUT (byte_copy_left, 1)
JUMP (start)

:end
END-MESSAGE (0,0,0,0,0,0,0)

which translates to give the following SigComp message:

0xf8, 0x00, 0xa1, 0x1c, 0x01, 0x86, 0x09, 0x22, 0x86, 0x01, 0x16,
0xf9, 0x23

12. RFC 3485 SIP/SDP Static Dictionary

SIP/SDP Static Dictionary [4] provides a dictionary of strings
frequently used in SIP and SDP messages. The format of the
dictionary is the list of strings followed by a table of offset
references to the strings so that a compressor can choose to
reference the address of the string or the entry in the table. Both
parts of the dictionary are divided into 5 prioritized sections to
allow compressors to choose how much of it they use (which is
particularly useful in the case where it has to be downloaded). If
only part of the dictionary is used, then the corresponding sections
of both parts (strings and offset table) are used.

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However, there are some minor bugs in the dictionary. In a number of
places, the entry in the offset table refers to an address that is
not in the corresponding priority section in the list of strings.
Consequently, if the bytecode uses the offset table and limits use of
the dictionary to priorities less than 4, then care must be taken not
to use the following strings in the dictionary:

'application' at 0x0334 is not at priority 2 (it's priority 4)
'sdp' at 0x064b is not at priority 2 (it's priority 4)
'send' at 0x089d is not at priority 2 (it's priority 3)
'recv' at 0x0553 is not at priority 2 (it's priority 4)
'phone' at 0x00f2 is not at priority 3 (it's priority 4)

This document does not correct the dictionary, as any changes to the
dictionary itself would be non-backwards-compatible, and require all
implementations to maintain two different copies of the dictionary.
Such a cost is far too high for a bug that is trivial to work around
and has a negligible effect on compression ratios. Instead, the flaw
is pointed out to allow implementers to avoid any consequent
problems. Specifically, if the bytecode sent to a remote endpoint
contains instructions that load only a sub-portion of the SIP/SDP
dictionary, then the input stream provided to that bytecode cannot
reference any of these five offsets in the offset table, unless the
corresponding string portion of the dictionary has also been loaded.
For example, if bytecode loads only the first three priorities of the
dictionary (both string and offset table), use of the offset for
"send" (at 0x089d) would be valid; however, use of the offset for
"phone" (at 0x00f2) would not.

13. Security Considerations

This document updates SigComp [1], SigComp Extended [2], and the
SigComp Static Dictionary [4]. The security considerations for [2]
and [4] are the same as for [1]; therefore, this section discusses
only how the security considerations for [1] are affected by the
updates.

Several security risks are discussed in [1]. These are discussed
briefly here; however, this update does not change the security
considerations of SigComp:

Snooping into state of other users - this is mitigated by using at
least 48 bits from the hash. This update does not reduce the
minimum and recommends use of more bits under certain
circumstances.

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Faking state or making unauthorized changes - this is mitigated by
the fact that the application layer has to authorize state
manipulation. This update does not change that mechanism.

Use of SigComp as a tool in a Denial of Service (DoS) attack -
this is mitigated by the fact that SigComp only generates one
decompressed message per incoming compressed message. That is not
changed by this update.

Attacking SigComp as the DoS target by filling with state - this
is mitigated by the fact that the application layer has to
authorize state manipulation. This update does not change that
mechanism.

Attacking the UDVM by sending it looping code - this is mitigated
by the upper limit of "UDVM cycles", which is unchanged by this
update.

14. IANA Considerations

This document updates SigComp [1], but does not change the version.
Consequently, the IANA considerations are the same as those for [1].

This document updates SigComp Extended [2], but does not change the
version. Consequently, the IANA considerations are the same as those
for [2].

This document updates Static Dictionary [4], but does not change the
version. Consequently, the IANA considerations are the same as those
for [4].

15. Acknowledgements

We would like to thank the following people who, largely through
being foolish enough to be authors or implementors of SigComp, have
provided us their confusion, suggestions, and comments:

Richard Price
Lajos Zaccomer
Timo Forsman
Tor-Erik Malen
Jan Christoffersson
Kwang Mien Chan
William Kembery
Pekka Pessi

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16. References

16.1. Normative References

[1] Price, R., Borman, C., Christoffersson, J., Hannu, H., Liu, Z.,
and J. Rosenberg, "Signaling Compression (SigComp)", RFC 3320,
January 2003.

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

[3] Roach, A., "A Negative Acknowledgement Mechanism for Signaling
Compression)", RFC 4077, October 2004.

[4] Garcia-Martin, M., Borman, C., Ott, J., Price, R., and A.
Roach, "The Session Initiation Protocol (SIP) and Session
Description Protocol (SDP) Static Dictionary for Signaling
Compression (SigComp)", RFC 3485, February 2003.

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

16.2. Informative References

[6] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications (ABNF)", RFC 2234, November 1997.

[7] Borman, C., Liu, Z., Price, R., and G. Camarillo, "Applying
Signaling Compression (SigComp) to the Session Initiation
Protocol (SIP)", Work in Progress, November 2006.

[8] Ritter, T., "Estimating Population from Repetitions in
Accumulated Random Samples", 1994,
<http://www.ciphersbyritter.com/ARTS/BIRTHDAY.HTM>.

[9] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.

[10] "IP Multimedia Call Control Protocol based on Session
Initiation Protocol (SIP)", October 2006.

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Appendix A. Dummy Application Protocol (DAP)

A.1. Introduction

This appendix defines a simple dummy application protocol (DAP) that
can be used for SigComp interoperability testing. This is handy for
SigComp implementations that are not integrated with a SIP stack. It
also provides some features that facilitate the testing of SigComp
internal operations.

The message format is quite simple. Each message consists of a
8-line message-header, an empty line, and an OPTIONAL message-body.
The style resembles that of SIP and HTTP.

The exact message format is given later in augmented Backus-Naur Form
(ABNF) [6]. Here are a few notes:

Each line of message-header MUST be terminated with CRLF.

The empty line MUST be present even if the message-body is not.

Body-length is the length of the message-body, excluding the CRLF
that separates the message-body from the message-header.

All strings in the message-header are case-insensitive.

For implementation according to this appendix, the DAP-version
MUST be set to 1.

A.2. Processing a DAP Message

A message with an invalid format will be discarded by a DAP receiver

For testing purposes, a message with a valid format will be returned
to the original sender (IP address, port number) in clear text, i.e.,
without compression. This is the case even if the sender requests
this receiver to reject the message. Note that the entire DAP
message (message-header + CRLF + message-body) is returned. This
allows the sender to compare what it sent with what the receiver
decompressed.

Endpoint-ID is the global identifier of the sending endpoint. It can
be used to test the case where multiple SigComp endpoints communicate
with the same remote SigComp endpoint. For simplicity, the IPv4
address is used for this purpose.

Compartment-ID is the identifier of the *compressor* compartment that
the *sending* endpoint used to compress this message. It is assigned

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by the sender and therefore only unique per sending endpoint; i.e.,
DAP messages sent by different endpoints MAY carry the same
compartment-ID. Therefore, the receiver SHOULD use the (endpoint-ID,
compartment-ID) pair carried in a message to determine the
decompressor compartment identifier for that message. The exact
local representation of the derived compartment identifier is an
implementation choice.

To test SigComp feedback [1], peer compartments between two endpoints
are defined in DAP as those with the same compartment-ID. For
example, (endpoint-A, 1) and (endpoint-B, 1) are peer compartments.
That means, SigComp feedback for a DAP message sent from compartment
1 of endpoint-A to endpoint-B will be piggybacked on a DAP message
sent from compartment 1 of endpoint-B to endpoint-A.

A DAP receiver will follow the instruction carried in message-header
line-5 to either accept or reject a DAP message. Note: line-6 and
line-7 will be ignored if the message is rejected.

A DAP receiver will follow the instruction in line-6 to create or
close the decompressor compartment that is associated with the
received DAP message (see above).

If line-7 of a received DAP message-header carries "TRUE", the
receiver will send back a response message to the sender. This
allows the test of SigComp feedback. As mentioned above, the
response message MUST be compressed by, and sent from, the local
compressor compartment that is a peer of the remote compressor
compartment. Other than this constraint, the response message is
just a regular DAP message that can carry arbitrary message-header
and message-body. For example, the "need-response" field of the
response can also be set to TRUE, which will trigger a response to
response, and so on. Note that since each endpoint has control over
the "need-response" field of its own messages, this does not lead to
a dead loop. A sensible implementation of a DAP sender SHOULD NOT
blindly set this field to TRUE unless a response is desired. For
testing, the message-body of a response MAY contain the message-
header of the original message that triggered the response.

Message-seq can be used by a DAP sender to track each message it
sends, e.g., in case of losses. Message loss can happen either on
the path or at the receiving endpoint (i.e., due to decompression
failure). The assignment of message-seq is up to the sender. For
example, it could be either assigned per compartment or per endpoint.
This has no impact on the receiving side.

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A.3. DAP Message Format in ABNF

(Note: see (ABNF) [6] for basic rules.)

DAP-message = message-header CRLF [ message-body ]

message-body = *OCTET

message-header = line-1 line-2 line-3 line-4 line-5 line-6 line-7 line-8

line-1 = "DAP-version" ":" 1*DIGIT CRLF
line-2 = "endpoint-ID" ":" IPv4address CRLF
line-3 = "compartment-ID" ":" 1*DIGIT CRLF
line-4 = "message-seq" ":" 1*DIGIT CRLF
line-5 = "message-auth" ":" ( "ACCEPT" / "REJECT" ) CRLF
line-6 = "compartment-op" ":" ( "CREATE" / "CLOSE" / "NONE" ) CRLF
line-7 = "need-response" ":" ( "TRUE" / "FALSE" )
line-8 = "body-length" ":" 1*DIGIT CRLF

IPv4address = 1*3DIGIT "." 1*3DIGIT "." 1*3DIGIT "." 1*3DIGIT

A.4. An Example of a DAP Message

DAP-version: 1
endpoint-ID: 123.45.67.89
compartment-ID: 2
message-seq: 0
message-auth: ACCEPT
compartment-op: CREATE
need-response: TRUE
body-length: 228

This is a DAP message sent from SigComp endpoint at IP address
123.45.67.89. This is the first message sent from compartment 2.
Please accept the message, create the associated compartment, and
send back a response message.

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Authors' Addresses

Abigail Surtees
Siemens/Roke Manor Research
Roke Manor Research Ltd.
Romsey, Hants SO51 0ZN
UK

Phone: +44 (0)1794 833131
EMail: [email protected]
URI: http://www.roke.co.uk

Mark A. West
Siemens/Roke Manor Research
Roke Manor Research Ltd.
Romsey, Hants SO51 0ZN
UK

Phone: +44 (0)1794 833311
EMail: [email protected]
URI: http://www.roke.co.uk

Adam Roach
Estacado Systems
17210 Campbell Rd.
Suite 250
Dallas, TX 75252
US

Phone: sip:[email protected]
EMail: [email protected]

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Full Copyright Statement

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This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
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Surtees, et al. Standards Track [Page 28]