Subject: RE: [rohc] How to determine the structure of static chain of IR p acket

From: "Lars-Erik Jonsson (LU/EAB)" <[email-masked]>
Date: Mon, 26 Apr 2004 09:37:56 +0200

By initiating compression with the RTP profile, one can implicitly know
that UDP will be followed by RTP, at least this is how the compressor
will operate if it uses the RTP profile.

One can never know that it is RTP, but one can guess. As has previously
been explained on this list, the compressor will use heuristics to find out
if it is RTP or not. It can initiate compression for all UDP flows as if they
were RTP, and then switch back to only-UDP if it turns out to be an
invalid guess. The RTP compression is transparent, independent of
actual data, so this approach will never fail.

/L-E

> -----Original Message-----
> From: [email-masked] [mailto:[email-masked]]On 
> Behalf Of shilong zhao
> Sent: den 26 april 2004 09:15
> To: [email-masked]
> Subject: [rohc] How to determine the structure of static 
> chain of IR packet
> 
> 
> 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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