A Unified Header Compression Framework for Low-Bandwidth

Links

Jeremy Lilley • Jason YangHari Balakrishnan • Srinivasan

Seshan

MIT Laboratory for Computer ScienceNetworks and Mobile Systems Group

http://nms.lcs.mit.edu/

Motivation

• Low bandwidth on many wireless links• Protocol headers are often large• Header compression is attractive and

beneficial in many cases• But…

– Many, many protocols in practice- compressing them is where the real gains are– Treating each as a separate problem: tedious!

• Wanted: A general approach!

The Problem:Painful Implementation Path

• Standardization (packet format, RFCs)• Implementation hassles

– Requires detailed knowledge of kernel internals

– Needs understanding of link error characteristics

• Each protocol treated as a new problem• Result: Header-compressed protocols

lag way behind!

Solution:Unified approach to the problem

Source code implementati

on for different platforms

High-level protocol description

Protocol designer writes

Code Generation

Toolkit runs

Goal: to make header compression absurdly simple

Previous Work

• 1984 – Thinwire• 1990 – V.

Jacobson– TCP/IP

• 1996 – Degermark– UDP/TCP/IPv6

• 1999 – Casner– RTP

Contributions

• Unified header compression scheme • High-level description language• Header compression software toolkit• Error-tolerance analysis• Comparison to conventional header

compression

Compression Scheme: An Example

Source IP Address

Destination IP Address

ChecksumProtocolTTL

Length

Flags

HLenVer TOS

ID Fragment Offset

Constant Inferrable Delta Random

Unified Compression Scheme

• Goal: To send original packets with minimal amount of bandwidth

• Reference packets– Send complete header

• Compressed packets (common case)– Send only changes

Base StationMobile Host

Internet

Reference Packets

• Fields sent unchanged– Initializes decompressor state– Keeps state synchronized

• Contexts– Want to allow many packet streams

concurrently– Provide different “contexts” for them– Context ID to identify the stream

Compressed Headers

• Only delta and random fields sent• Send only non-zero delta values

– Encoded in change bit-vector

• Field ordering must be consistent

0 0 0 0 1 0 1 0 Payload

Delta flags RandomDelta values

Corresponding flagand delta values

Context ID

Specification Language

• Set of header fields– Size in bits– Compression category (CONSTANT,

INFERRED, DELTA, RANDOM)– Category-specific parameters: inference formula, encoding method…

PField sourceIP, fsize=32, ptype=CONSTANT;PField length, fsize=16, ptype=INFERRED, formula=[framelength];

Rules and Actions

• Rules can:– Bypass routines altogetherPRule SendAsIP, ruletext=[protover]!=4;

– Force a reference packet to be sentPRule SendReference,

ruletext=[curTime]<{expireTime};

• Actions can keep state, write logs {expiretime}=[curTime]

+5*[tickspersecond]

Complete Sample Description

class IPcompressor {NiceName IP_Compression_Protocol;Compressed_ID 0x0081;UnCompressed_ID 0x0083;

PField protover, fsize=4, ptype=NOCHANGE;PField hdrlen, fsize=4, ptype=NOCHANGE; PField tos, fsize=8, ptype=NOCHANGE;PField totlen, fsize=16, ptype=INFERRED, formula=[length];PField packetid, fsize=16, ptype=DELTA, encoding=VARONETHREE, negatives=DISALLOWED;PField fragments, fsize=16, ptype=NOCHANGE;PField ttl, fsize=8, ptype=NOCHANGE;PField protocol, fsize=8, ptype=NOCHANGE;PField checksum, fsize= 16, ptype=RANDOM;PField sourceIP, fsize=32, ptype=NOCHANGE;PField destIP, fsize=32, ptype=NOCHANGE;

PRule SendAsIP, ruletext=[protover]!=4; # want IP version 4}

class IPcompressor {NiceName IP_Compression_Protocol;Compressed_ID 0x0081;UnCompressed_ID 0x0083;

PField protover, fsize=4, ptype=NOCHANGE;PField hdrlen, fsize=4, ptype=NOCHANGE; PField tos, fsize=8, ptype=NOCHANGE;PField totlen, fsize=16, ptype=INFERRED, formula=[length];PField packetid, fsize=16, ptype=DELTA, encoding=VARONETHREE, negatives=DISALLOWED;PField fragments, fsize=16, ptype=NOCHANGE;PField ttl, fsize=8, ptype=NOCHANGE;PField protocol, fsize=8, ptype=NOCHANGE;PField checksum, fsize= 16, ptype=RANDOM;PField sourceIP, fsize=32, ptype=NOCHANGE;PField destIP, fsize=32, ptype=NOCHANGE;

PRule SendAsIP, ruletext=[protover]!=4; # want IP version 4}

Errors

• Errors desynchronize compressor and decompressor– State changes missing, corrupted– As errors increase, header compression can

be a losing proposition

• Need to keep synchronized, preferably with implicit semantics/in-band signaling

Base StationMobile Host

Internet

Impact of Link Errors TCP/IP over a 28.8 kbps link

0

1

2

3

4

5

6

7

1.0E-08 1.0E-07 1.0E-06 1.0E-05 1.0E-04

Bit Error Rate

Thro

ughp

ut R

atio

0-byte

1-byte

10-byte

100-byte

1460-byte

Break-even at 1.0

Errors Decrease Effectiveness

Error handling

• Subject of ongoing research, we implement basic techniques– Connection-oriented protocols: implicit

signaling– Connectionless protocols: periodic refresh

• TCP effectiveness analysis in paper• Flexible framework enables other

techniques to be incorporated

Compression Comparisons

Generated TCP/IP vs. VJ TCP/IP,based on original packet length

0

0.2

0.4

0.6

0.8

1

Bulk ACKs Interactive

Traffic Type

Com

pres

sion

Rat

io

TCP/IPSemanticsVJ TCP/IP

Compression vs. SpecificityHow much does extra information help?

0

0.2

0.4

0.6

0.8

1

Bulk ACKs Interactive

Traffic Type

Com

pres

sion

Rat

io

Naïve IP

IP Semantics

TCP/IP Semantics

Low-overhead ImplementationCPU time for compressing outgoing packet

0

5

10

15

20

25

Bulk ACKs Interactive

Traff ic Type

Mic

rose

cond

s

Naïve IP

IP Semantics

TCP/IP Semantics

VJ TCP

Further Directions

• String protocol compression– E.g., HTTP transfers

• Advanced error recovery• Handling IPSec

All <250k

<50k <10k

6.3%

9.3% 11.1% 19.3%

Fraction of transfer as http header

Conclusion

• New approach to header compression– Unified header compression scheme– High-level description language– Software toolkit:

http://nms.lcs.mit.edu/software/• Performance comparable to hand-

optimized algorithms for TCP• Eases implementation and

deployment of header-compressed protocols