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Re: [Question #195843]: What is needed for RoHC with SIP?

 

Question #195843 on rohc changed:
https://answers.launchpad.net/rohc/+question/195843

Didier Barvaux posted a new comment:
Cédric,

> Using ROHC for that purpose implies that your 2 end points are able to communicate using
> ROHC packets, that is not > straighforward (use of IP tunneling is not really useful if your
> objective is to reduce overhead) . In addition, the main part of the overhead is due to the
> rich text part (plain text XML) and not the header part.

You're right for the modifications of the 2 end points and for the
overhead in the signalling packets (SIP). However, for data packets
(RTP), the overhead is still the header part. For those packets, ROHC
may be interesting even if IP tunneling is required.

Gain for ROHC without tunneling:
 - non-compressed IPv4/UDP/RTP packet = 20-byte IPv4 header + 8-byte UDP header
    + 12-byte RTP header + 40-byte audio data = 80 bytes
 - ROHC-compressed IPv4/UDP/RTP packet = 1-byte ROHC header + 40-byte audio
    data = 41 bytes
 = 49% gain

Gain for ROHC with IPv4 tunneling:
 - non-compressed IPv4/UDP/RTP packet = 20-byte IPv4 header + 8-byte UDP header
    + 12-byte RTP header + 40-byte audio data = 80 bytes
 - ROHC-compressed IPv4/UDP/RTP packet in IPv4 tunnel = 20-byte IPv4 header for
    tunnel + 1-byte ROHC header + 40-byte audio data = 61 bytes
 = 24% gain

A 24% gain is still interesting, but the IPv4 tunnel degrades the
compression gain.

We may group several ROHC packets together (packing) to reduce the overhead
introduced by the IPv4 tunnel:
 - 4 non-compressed IPv4/UDP/RTP packet = 4 * (20-byte IPv4 header + 8-byte UDP header
    + 12-byte RTP header + 40-byte audio data) = 320 bytes
 - 4 ROHC-compressed IPv4/UDP/RTP packet in IPv4 tunnel = 20-byte IPv4 header for
    tunnel + 4 * (1-byte for packing + 1-byte ROHC header + 40-byte audio data) = 188 bytes
 = 41% gain

A 41% gain is definitely  interesting :)


Regards,
Didier

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