header compression schemes. center for teleinfrastructure 2 different header compression schemes ...

Header Compression Schemes

2Center for TeleInFrastructure

Different Header Compression schemes

Compressed TCP – Van Jacobsen RFC 1144only for TCP/IP for wired networks

Perkins improvement for of CTCP

IPHConly for IP protocolno feedback


General Structure of Header Compressors

Two entities: compressor and decompressorCompressor sends initial baseBase is used by compressor and decompressorCompressor removes redundancyDecompressor adds removed informationProposed solution differ in a possible feedback channel

N N

Base

Compressor Decompressor

Base

N*


CTCP (Van Jacobsen)

TCP/IP header compressionUsing delta compressionDesigned for wired networksNot robust against error-prone linksBase update with each new incoming packet


Loosing synchronization

Synchronization loss = decompressor’s copy of the base is different from the compressor’s copy

Synchronization is lost any time a packet is dropped Detection: using detection of TCP retransmissions. All

retransmissions are sent uncompressed


Performance of VJ scheme in case of random errors

0

0,5

1

1,5

2

2,5

3

3,5

4

344 K 328 K 550 K

VJ, random erros

no VJ, random errors

VJ, no errors

no VJ, no errors

When synchronization is lost, the decompressor starts to toss packets base update more often than needed

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199

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Kby

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s Throughput of bulk data transfersFile sizes of 344K, 328K, and 550K


Perkins – Refinement of CTCP

Perkins & Mutka – improvement of CTCP in case of noise presence Differentials are sent against a base that changes infrequently packet

loss does not cause endpoints to loose synchronization All packets refer to the first packet of the frame the same mechanisms can be used to detect loss of synchronization


Perkins – Refinement of CTCP

Base refresh (sending uncompressed header) – to combat overflow problems

Robustness introduced by periodically repetition of full base information each N packets

N packets define a frame

Larger overhead Less compression due to higher delta values Additionally, 1 byte of CID (connection identifier) is transmitted


Performance of Perkins scheme

No errors

2,8

2,9

3

3,1

3,2

3,3

3,4

3,5

3,6

3,7

344 K 328 K 550 K

kbyt

es/s Perkins, no errors

VJ, noerrorsno VJ, no errors

Random errors

0

0,5

1

1,5

2

2,5

3

344 K 328 K 550 K

kbyt

es/s

Perkins, randomerrorsVJ, random errors

no VJ, randomerrors

S. J. Perkins and M. W. Mutka, Dependency Removal for Transport Protocol Header Compression over Noisy Channels. 1997.

Throughput of bulk data transfersFile sizes of 344K, 328K, and 550K


IP Header Compression (IPHC)

Provides extensions to VJSupport UDP, IPv6, Additional TCP features

Uses delta encoding


TWICE algorithm

TCP header compression reduces throughput over lossy linksBandwidth is wasted when unharmed segments

are retransmitted after a timeout

Possible solutions:Perkins algorithmTWICE algorithm


TWICE algorithm

Decompressor can detect loss of synchronization by using TCP checksum

Motivation: totally lossless HC is not possible, make an educated guess

If inconsistency is due to a single lost segment + lost segment increments the compression state in the same way

Apply TWICE the delta of a current segment


Compressed RTP (CRTP)

Compressed RTP (RFC 2508) Compresses 40 byte header to 4 or 2 bytesFirst-order changes

Expected changes in the fields that can be predicted, no transmission of differences needed

Second-order changesChanges that have to be compressed

Enhanced Compressed RTP (RFC 3545)Refinement of CRTP in presence of packet loss,

reordering and long delaysLocal retransmissions and repeated context updates are

used


Robust Checksum-based Compression (ROCCO)

Refinement of CRTPIncludes checksum over uncompressed header

facilitation of local recovery of the synchronizationTargeted to cellular usage


Robust Header Compression

RTP/UDP/IPUDP/IPIPuncompressed


ROHC Modes


States of Compressor and Decompressor


Unidirectional


Optimistic


Reliable


Decompressor

header compression schemes. center for teleinfrastructure 2 different header compression schemes ...

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