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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
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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*
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CTCP (Van Jacobsen)
TCP/IP header compressionUsing delta compressionDesigned for wired networksNot robust against error-prone linksBase update with each new incoming packet
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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
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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
S. J
. Per
kins
and
M. W
. Mut
ka,
Dep
ende
ncy
Rem
oval
for
Tra
nspo
rt
Pro
toco
l Hea
der
Com
pres
sion
ove
r N
oisy
C
hann
els.
199
7.
Kby
tes/
s Throughput of bulk data transfersFile sizes of 344K, 328K, and 550K
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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
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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
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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
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IP Header Compression (IPHC)
Provides extensions to VJSupport UDP, IPv6, Additional TCP features
Uses delta encoding
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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
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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
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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
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Robust Checksum-based Compression (ROCCO)
Refinement of CRTPIncludes checksum over uncompressed header
facilitation of local recovery of the synchronizationTargeted to cellular usage
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Robust Header Compression
RTP/UDP/IPUDP/IPIPuncompressed
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ROHC Modes
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States of Compressor and Decompressor
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Unidirectional
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Optimistic
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Reliable
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Decompressor