Robust Header Compression for Lossy Links

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					ROHC Draft Material

Lars-Erik Jonsson, Ericsson September 06, 2000

ROHC Chapter 5.9 This document includes an updated version of chapter 5.9 of the ROHC protocol draft. There are still open issues in ROHC that may affect this section further but I have tried to get this part in line with today's status regarding the various issues.

Bormann (ed.)

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Robust Header Compression

July 14, 2000

Header compression CRCs, coverage and polynomials

This chapter contains a description of how to calculate the different CRCs used in the packet headers defined in this document. 5.9.1. FULL HEADER packet CRC

The CRC in the FULL HEADER packet is calculated over the original IP/UDP/RTP header. Before the calculation of the CRC, the IPv4 header checksum and the UDP checksum have to be set to 0. This makes it possible to recalculate the checksums after the decompression. Calculation over the full IP/UDP/RTP headers ensures that the decompressed IP/UDP/RTP header is a correct header. The CRC polynomial to be used in FULL HEADER packets is: C(x) = 1 + x + x^2 + x^8 5.9.2. COMPRESSED packet CRCs

The header compression CRC in COMPRESSED packets is calculated over the same headers as the CRC in FULL HEADER packets but with different polynomials depending on the size of the CRC. The polynomial to be used for 3 bit CRC is: C(x) = 1 + x + x^3 The polynomial to be used for 7 bit CRC is: C(x) = ????

Bormann (ed.)

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