Subject: [rohc] How to determine the structure of static chain of IR packet

From: shilong zhao <[email-masked]>
Date: Mon, 26 Apr 2004 07:15:17 +0000 (GMT)

hi, Dear all ROHC experts:
    
    There is a basic question when I will implement RFC3095.

In RFC3095

5.7.7 RTP IR and IR-DYN packets 
The subheaders which are compressible are split into a STATIC part and a DYNAMIC part. These parts are defined in sections 5.7.7.3 through 5.7.7.7. 

The structure of a chain of subheaders is determined by each header having a Next Header, or Protocol, field. This field identifies the type of the following header. Each Static part below that is followed by another Static part contains the Next Header/Protocol field and allows parsing of the Static chain; the Dynamic chain, if present, is structured analogously.

For RTP/UDP/IPv4.
we know that we can know what is the next static chain from IPV4 static part:

   +---+---+---+---+---+---+---+---+
   |  Version = 4  |       0       |
   +---+---+---+---+---+---+---+---+
   |           Protocol            |
   +---+---+---+---+---+---+---+---+
   /        Source Address         /   4 octets
   +---+---+---+---+---+---+---+---+
   /      Destination Address      /   4 octets
   +---+---+---+---+---+---+---+---+

through protocol field,  we know the next static should be UDP static part.

However,  How we know what is next static part(It should be RTP static part)  from UDP static part?

UDP Static part as following: 

      +---+---+---+---+---+---+---+---+
      /          Source Port          /   2 octets
      +---+---+---+---+---+---+---+---+
      /       Destination Port        /   2 octets
      +---+---+---+---+---+---+---+---+

Who can point out for me.
Thankx in advance.

Shilong  Zhao

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