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Re: [Question #232430]: Question about IR/IR-DYN remainder header?

To: rohc@xxxxxxxxxxxxxxxxxxx
From: Didier Barvaux <question232430@xxxxxxxxxxxxxxxxxxxxx>
Date: Mon, 15 Jul 2013 15:06:17 -0000
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Question #232430 on rohc changed:
https://answers.launchpad.net/rohc/+question/232430

    Status: Open => Answered

Didier Barvaux proposed the following answer:
Hello,

The code_ir_remainder() callback is used by compression profiles that
need to append some bytes at the end of the IR and IR-DYN packets. The
IP/UDP, IP/UDP-Lite and IP-only profiles make use of it to append an
additional 2-byte SN field.

The 2-byte SN field is described for the UDP profile in RFC 3095 §5.11.1 (https://tools.ietf.org/html/rfc3095#section-5.11.1):
   For ROHC UDP, the dynamic part of a UDP packet is different from
   section 5.7.7.5: a two-octet field containing the UDP SN is added
   after the Checksum field.  This affects the format of dynamic chains
   in IR and IR-DYN packets.

In source code, the c_ip_code_ir_remainder() function in the
src/comp/c_ip.c file implements the code_ir_remainder() callback and
appends the 2-byte SN field.

Regards,
Didier

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