Subject: RE: [rohc] List Compression

From: [email-masked]
Date: Wed, 2 Jul 2003 16:25:47 -0500

Vishal,

> Considering a scenario where we have an IP stream - IP/AH/UDP/RTP.
> (snip)
> In static list the next header field in IP header will point to UDP ,
> And therefore there is no way to determine which extension header is
> present
> in "Generic extension header list".

See following text from Section 5.8.4.1 of RFC 3095 (I know we could
have done a better job to make the document more readable...)

   All communicated uncompressed extension header items indicate their
   own type in their Next Header field.

> Another question is considering the scenario where we have multiple IP
> headers - eg. IP/IP/IP/AH/UDP/RTP then how these headers will be
> organised/arranged  in compressed header ? One way could be we can use the
> "Generic extension header list" to do so
> OR  is there some other way to do so ??
> For example in case of IR it could be -
> static chain - (First IP header static info )  +  (Second IP header static
> info ) + UDP static info + RTP static info
> dynamic chain - (First IP header dynamic info ) + (Second dynamic IP
> header dynamic info ) + UDP dynamic info + RTP dynamic info

Do you mean two or three IP headers (you showed three on top and then
two in the chains at bottom)?

Assuming you meant 3 IP headers, here are the rules/answers:
1) "IPv6 base headers and IPv4 headers cannot be part of an extension
header chain" (section 5.8). So, whether to use "generic extension header
list" depends on IP version.
2) For IR packet, you can certainly "chain" the IP headers as in your
example. However, for compressed packets, ROHC does not provide
efficient handling other than the first two IP headers.

ROHC is pretty flexible and efficient for most common cases. But it is
not designed to handle (not even mentioning efficiency) arbitrary header
structures. I hope the above is a rare case which can be avoided in
practice.

BR,
Zhigang

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