Subject: [AVT] Evaluation of header compression algorithm ECRTP (Master's Thesis )

From: "Lars-Erik Jonsson (LU/EAB)" <[email-masked]>
Date: Thu, 27 Jan 2005 10:04:49 +0100

Header Compression folks in AVT and ROHC,

The following master thesis from Luleå University presents an evaluation of eCRTP, with special attention to the operation in scenarios with potential reordering between compressor and decompressor. The paper does not only cover the performance and capabilities of eCRTP, but looks at these aspects of CRTP and ROHC RTP as well.

The paper can be found at:
http://epubl.luth.se/1402-1617/2004/286/LTU-EX-04286-SE.pdf

Rgds,
/Lars-Erik

Abstract
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This master's thesis presents an evaluation of Enhanced Compressed RealTime Protocol (ECRTP). ECRTP an extension to CRTP, is a header compression scheme for real time traffic. ECRTP exploits the hop-by-hop redundancy in a stream of IP/UDP/RTP packets, were virtually the same header is sent over and over again. The header and additional information is saved in a context at the compressor and decompressor. If the context is synchronized, the compressor can send only the differences since the last header and decompressor will be able to reconstruct the header. 

ECRTP was developed to overcome the problems of CRTP. CRTP does not perform well over links with long round trip time that lose and reorder packets. ECRTP extends CRTP by repeating context updates and by sending absolute values along with delta values when encoding monotonically increasing header fields to increase robustness. It inserts a header checksum when UDP checksum is missing, to improve error recovery and fail checks for the compression. 

The evaluation of ECRTP consists of two parts, simulations and a theoretical investigation. With the results of the simulations and the theory in mind, weaknesses and strength are pointed out and suggestions of improvement will be presented. 

A possible error in the establishment of the repetition value in the decompressor was found. The error can be avoided by sending the repetition value in the compressed header at the beginning of a session. 

The investigation shows that ECRTP handles packet loss and large round trip times well. ECRTP can be configured to handle packet reordering by exclusive use of absolute encoding for monotonically increasing header fields, by using most commonly changing fields that is not protected by the checksum as context-defining fields. The checksum-protected RTP fields should, if changed between two consecutive headers, be included in every packet and not compressed on the assumption of in-order delivery. All the suggested changes will decrease the compression efficiency. 

ECRTP is simulated together with two other schemes; CRTP and ROHC. It is shown that Robust header compression (ROHC) handles reordering better than expected. Especially the least significant bit encoding shows promising results and the handling of CSRC list. But also the handling of the checksum-protected RTP fields show that ROHC with some modifications could possibly be made to handle compression during reordering more efficiently than ECRTP. 

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