Subject: [rohc] List Compression

From: "Shweta Dass" <[email-masked]>
Date: Mon, 15 Oct 2007 13:00:58 +0530

Hi,
 
Post as found at
http://www1.ietf.org/mail-archive/web/rohc/current/msg01604.html
 
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.

Referring to the above post at 
 
http://www1.ietf.org/mail-archive/web/rohc/current/msg01604.html
 
1)I had query relating to following text mentioned in the section
5.8.4.4 

   A one-octet Next Header field is inserted as the first octet of the
header.  

   The value of the Next Header field corresponds to GRE (this value is
47

   according to the Assigned Internet Protocol Number section of

   [ASSIGNED]) when the uncompressed item is to be inserted in a list,

   and to the type of the subsequent header when the uncompressed item

   is in a Generic list.

As per the above text, it is clear that when an uncompressed item is
part of a generic list (which will be typically the scenario in case of
IR and IR-DYN packets), next header field will correspond to the
subsequent header and if that is the case, then there is no way to
determine the first extension header present in "Generic extension
header list".
 
Assuming that the header items of generic list also indicate their own
types in their next header field (as stated above),does that mean the
generic lists are ordered lists??
 
2) Also, wanted to know how is the presence of extension header list in
the dynamic chain detected??
 
  Considering the above example of IP/AH/UDP/RTP,
 
For an IR packet, 
 
 static chain  : IP(Static part),UDP(static part) and RTP(static part)
 dynamic chain : IP(Dynamic part including IP Generic Extension header
list) followed by UDP checksum (UDP dynamic part) and then RTP Sequence
number (RTP dynamic part).... Now how does de-compressor detect that IP
Dynamic part contains the "Generic extension header list" part or not???

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