Technical Summary
This document describes common misinterpretations and some
ambiguities in the Signalling Compression Protocol (SigComp), and
offers guidance to developers to clarify any resultant problems.
Working Group Summary
This document has grown through concerns from implementers. The
final document has been reviewed by the WG, and there is WG
consensus that the document should now be published as an RFC.
Document Quality
There are several independent implementations of SigComp, which
are in general already implemented according to this document.
Personnel
Document Sheperd for this document is Lars-Erik Jonsson, and Magnus
Westerlund is the Responsible Area Director.
Note to RFC Editor
First page header:
NEW:
Updates: RFC 3320, RFC 3321, RFC 3485 (once approved)
Title:
OLD:
Implementer's Guide for SigComp
NEW:
Signaling Compression (SigComp) Corrections and Clarifications
Abstract:
OLD:
This document (if approved) clarifies and corrects text in the
following updated RFCs: RFC 3320, RFC 3321, RFC 3485.
NEW:
This document updates the following RFCs: RFC 3320, RFC 3321, RFC 3485.
Section 1:
OLD:
"in section 3.4" refers to section 3.4 of this document
"in section RFC-3320:3.4 refers to section 3.4 of RFC 3320 [1]
NEW:
"in section 3.4" refers to section 3.4 of this document
"in section RFC3320-3.4" refers to section 3.4 of RFC 3320 [1]
Section 2.1, First paragraph:
OLD:
SigComp [1] states that the default Decompression Memory Size (DMS)
is 2K. The UDVM memory size is defined in section RFC3320:7 to be
(DMS - sizeof (sigcomp_msg)) for messages transported over UDP and
(DMS / 2) for those transported over TCP.
NEW:
SigComp [1] states that the default Decompression Memory Size (DMS)
is 2K. The UDVM memory size is defined in section RFC3320-7 to be
(DMS - sizeof (sigcomp_msg)) for messages transported over UDP and
(DMS / 2) for those transported over TCP.
Section 12, final paragraph
OLD:
These are seen to be relatively low cost bugs as only these 5 strings
are affected and they are only affected under certain conditions.
NEW:
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 negligible effect on compression ratios. Instead, we point
out the flaw so as to allow implementers to avoid any consequent
problems. Specifically: if the bytecodes sent to a remote endpoint
contain instructions that load only a sub-portion of the SIP/SDP
dictionary, then the input stream provided to those bytecodes 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 bytecodes loaded 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.
Please replace all occurances of "Sigcomp" with "SigComp".