[Sipping] P-Headers in the static SIP/SDP compression dictionary

In preparing draft-ietf-sipping-sigcomp-sip-dictionary-03.txt, there
has been some discussion about the best way to handle P-headers.
Obviously, P-headers are private, so maybe they should not be part of
a static SIP dictionary that all SIP implementations that support
compressed signaling have to use.

On the other hand, P-headers are likely to be present in many messages
exchanged over low-bandwidth cellular links.  Not supporting them in
the static dictionary increases the size of these messages.

So what should we do for -04?  Here is a proposal for a compromise
position.  Instead of bloating the dictionary by including every
P-header in sight, we could collect a set of strings likely to turn up
in P-headers.  Here is my provisional list:

[CRLF]P-

Access-
Associated-
Called-
Charging-
Function-
Party-
Visited-
Network-

Adresses:[SP]
URI:[SP]
ID:[SP]
Info:[SP]
Vector:[SP]

This means that your typical P-header will need 3 to 4 dictionary
references instead of one if we just pasted all of them in.  This may
(depending on the compression algorithm) still be a sizable win.
The dictionary size will only increase moderately, in particular since
some of these strings are already present in the dictionary.

Comments?

WG meetings where this could turn up:
ROHC, Wed 0900-1130, Room 502
SIP, Wed 1300-1500, Room 301/302

Gruesse, Carsten

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