Header compression schemes
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Header Compression Schemes. Different Header Compression schemes. Compressed TCP – Van Jacobsen RFC 1144 only for TCP/IP for wired networks Perkins improvement for of CTCP IPHC only for IP protocol no feedback. General Structure of Header Compressors.

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Header Compression Schemes

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Header compression schemes

Header Compression Schemes


Different header compression schemes

Different Header Compression schemes

  • Compressed TCP – Van Jacobsen RFC 1144

    • only for TCP/IP

    • for wired networks

  • Perkins

    • improvement for of CTCP

  • IPHC

    • only for IP protocol

    • no feedback


General structure of header compressors

General Structure of Header Compressors

  • Two entities: compressor and decompressor

  • Compressor sends initial base

  • Base is used by compressor and decompressor

  • Compressor removes redundancy

  • Decompressor adds removed information

  • Proposed solution differ in a possible feedback channel

Base

Base

Decompressor

Compressor

N

N

N*


Ctcp van jacobsen

CTCP (Van Jacobsen)

  • TCP/IP header compression

  • Using delta compression

  • Designed for wired networks

  • Not robust against error-prone links

  • Base update with each new incoming packet


Loosing synchronization

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

Performance of VJ scheme in case of random errors

Throughput of bulk data transfers

File sizes of 344K, 328K, and 550K

Kbytes/s

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

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


Perkins refinement of ctcp

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 ctcp1

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

Performance of Perkins scheme

Throughput of bulk data transfers

File sizes of 344K, 328K, and 550K

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


Ip header compression iphc

IP Header Compression (IPHC)

  • Provides extensions to VJ

    • Support UDP, IPv6,

    • Additional TCP features

  • Uses delta encoding


Twice algorithm

TWICE algorithm

  • TCP header compression reduces throughput over lossy links

    • Bandwidth is wasted when unharmed segments are retransmitted after a timeout

  • Possible solutions:

    • Perkins algorithm

    • TWICE algorithm


Twice algorithm1

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 (CRTP)

  • Compressed RTP (RFC 2508)

    • Compresses 40 byte header to 4 or 2 bytes

    • First-order changes

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

    • Second-order changes

      • Changes that have to be compressed

  • Enhanced Compressed RTP (RFC 3545)

    • Refinement of CRTP in presence of packet loss, reordering and long delays

    • Local retransmissions and repeated context updates are used


Robust checksum based compression rocco

Robust Checksum-based Compression (ROCCO)

  • Refinement of CRTP

  • Includes checksum over uncompressed header  facilitation of local recovery of the synchronization

  • Targeted to cellular usage


Robust header compression

Robust Header Compression

  • RTP/UDP/IP

  • UDP/IP

  • IP

  • uncompressed


Rohc modes

ROHC Modes


States of compressor and decompressor

States of Compressor and Decompressor


Unidirectional

Unidirectional


Optimistic

Optimistic


Reliable

Reliable


Decompressor

Decompressor


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