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[Didier Barvaux <didier@xxxxxxxxxxx>]

Hello Ramya,

> Thanks,I got the output
> I could execute it succesfully using the ping application after which
> i got the output that has been attached.

Congratulation! You just compressed your first packets with the ROHC
library :)


> In the last line of the output,it gives the size of the ROHC
> Packet,feedback,compressed header size and payload size,.
> I sent a packet with size 56 to which 8 bytes of header is added..In
> the output statistics the payload
> is being printed as 64 and the ROHC size is being printed as the
> payload plus the compressed header size..
> Is this right?

According to the screenshot you sent, you compressed a 84-byte ICMPv4
packet. See the trace "[rohc_comp.c:175 rohc_compress()] size of IP
packet = 84 bytes".

Your 84-byte ICMPv4 packet is composed of a 20-byte IPv4 header, an
8-byte ICMPv4 header and 56 bytes of ICMPv4 payload. The ROHC library
compresses the IPv4 header but not the ICMPv4 header and the ICMPv4
payload.

In the case of the last packet that I see on your screenshot (this is
the packet #164), the resulting ROHC packet is 67-byte long. It is made
of a 3-byte ROHC header (ie. the compressed equivalent of the 20-byte
uncompressed IPv4 header) and a 64-byte ROHC payload (ie. the 8-byte
ICMPv4 header and the 56-byte ICMPv4 payload unchanged). See the trace
"[rohc_comp.c:362 rohc_compress()] ROHC size = 67 (feedback = 0, header
= 3, payload = 64), output buffer = 5120" in the screenshot.

ROHC compression with the last packet results in a 85% gain for header
(20 bytes -> 3 bytes) and 20% gain for header+payload (84 bytes -> 67
bytes).

I updated FAQ #639 "How to easily test the ROHC library ?"
(https://answers.launchpad.net/rohc/+faq/639) to explain the rohctunnel
traces the same way I did in this email. Let me know if the
explanations are sufficient.

Regards,
Didier Barvaux

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