Improved Compression-Latency Improved Compression-Latency Trade-Off viaTrade-Off via

Delayed-Dictionary CompressionDelayed-Dictionary Compression

Raanan RefuaRaanan RefuaTel Aviv UniversityTel Aviv University

Under the supervision of Prof. Yossi MatiasUnder the supervision of Prof. Yossi Matias

Improved CompressionImproved Compression--Latency Latency TradeTrade--OffOff

Improving compression ratio for a given Improving compression ratio for a given latency constraintlatency constraint

Improving latency for a given compression Improving latency for a given compression ratioratio

End-to-End CompressionEnd-to-End Compression

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X

Yr BA

EncoderX X

Decoder

Lenc LdecLtx

(EncodingLatency)

(TransmissionLatency)

(DecodingLatency)

User Latency

Net 1 Net 2Y

PublicNetwork

PublicNetwork

Network ProcessorNetwork Processor

Network ProcessorNetwork Processor

The LAN-to-LANThe LAN-to-LAN Framework Framework

PublicNetwork

PublicNetwork

IP Tunnel 1

IP Tunnel 2

Network Processor

Network ProcessorNetwork Processor

IP-TunnelIP-Tunnel

PublicNetwork

PublicNetwork

Network ProcessorNetwork Processor

VPN over IPSecVPN over IPSec

Packet Compression Algorithms TypesPacket Compression Algorithms Types

Stateless compression (Packet By Packet comp.)Stateless compression (Packet By Packet comp.) Streaming compression (Continuous comp.)Streaming compression (Continuous comp.)

– dependency among packetsdependency among packets

““Offline” compressionOffline” compression– compress & sendcompress & send

||

|)(|)(A BA, BA, BA,

F

FArrrr Fofflinestreamingstateless

offlineLstreamingLstatelessL BA,BA,BA,

Compression Ratios vs Mode Compression Ratios vs Mode and Packet Sizesand Packet Sizes

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fp stateless ratio (|frag|=1500)fp stateless ratio (|frag|=125)fp streaming compression ratio (|frag|=1500)fp streaming compression ratio (|frag|=125)

Small Packets Improvement

Potential ImprovementPotential Improvement

φ(Bstateless,Bstreaming)

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Streaming Compression Streaming Compression ImplementationImplementation

X Y X

Encoder

Packets

Dictionary

Rx1

Rx2Tx2

Tx1

[ACKsData]

Encoder

Decoder

Packets

Dictionary

Rx1

Rx2Tx2

Tx1Decoder

ACKs ACKs ACKs

Latency in Streaming CompressionLatency in Streaming Compression

Large Decoding Latency due toLarge Decoding Latency due to Packet dropsPacket drops

– dominating latencydominating latency Packet reordering due toPacket reordering due to

– retransmissionsretransmissions– different pathsdifferent paths

X Y X

Encoder Decoder

Packets

Dictionary

Packets

Dictionary

Decoding Latency in StreamingDecoding Latency in Streaming

1, 2, 3, 4, 5, 1, 2, 3, 4, 5, 77, , 88, , 99, , 1010, , 66

Packet Packet 66 is dropped and retransmitted is dropped and retransmitted– LLdec dec for packet 7 = 4for packet 7 = 4

Packets Packets 7…107…10 cannot be decoded until cannot be decoded until 66 is is received.received.– The phrases of packets The phrases of packets 7…107…10 cannot be added cannot be added

to the dictionary until packet to the dictionary until packet 66 is received. is received.

Actual Decoding Latency of StreamingActual Decoding Latency of Streaming

Average LAverage Ldecdec for each of 269 Planet-Lab nodes for each of 269 Planet-Lab nodes– 26012 packets per node26012 packets per node

Total Average LTotal Average Ldecdec = 62 packets = 62 packets LLdecdec=0 for Stateless=0 for Stateless

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Decoding LatencyAverages

Total Average

Improved Compression & Latency?Improved Compression & Latency?

We would like We would like

– Compression ratio of Compression ratio of Streaming Streaming

– Latency of StatelessLatency of Stateless

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Streaming DecodingLatency Averages

Streaming TotalAverage

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fp stateless ratio (|frag|=125)fp streaming compression ratio (|frag|=125)

Delayed-Dictionary Compression Delayed-Dictionary Compression (DDC)(DDC)

Contains a sub-method called Basic-DDC Contains a sub-method called Basic-DDC (BDDC)(BDDC)

Streaming with Streaming with enforced delayenforced delay in the creation of in the creation of the dictionarythe dictionary

Dictionary does not use the most recent historyDictionary does not use the most recent history– Compression ratio is close to StreamingCompression ratio is close to Streaming– Decoding Latency is close to StatelessDecoding Latency is close to Stateless

On-line Dictionary Compression On-line Dictionary Compression Algorithms ModelAlgorithms Model

Encoder - a model for incremental dictionary compression algorithm C. Themodel enables complete separation between the dictionary parser and the outputparser.

dictionary parserPd

dictionaryD

output parserPo

Input: T Output: C(T)

On-line Dictionary Compression On-line Dictionary Compression Algorithms ModelAlgorithms Model

dictionary parserPd

dictionaryD

output decoder

Compressed text: C(T)Original text: T

Decoder - a model for incremental dictionary decompression C’. The modelenables complete separation between the dictionary parser and the output decoder.

On-line Dictionary Compression On-line Dictionary Compression Algorithms Model GeneralizationAlgorithms Model Generalization

dictionary parserPd

dictionaryD(Δ)

output parserPo

Input: T Output: C(T)

D at point t is a function of text up to point t-Δ

On-line Dictionary Compression On-line Dictionary Compression Algorithms Model GeneralizationAlgorithms Model Generalization

dictionary parserPd

dictionaryD(Δ)

output decoder

Compressed text: C(T) Original text: T

Lempel-Ziv 77 with DelayLempel-Ziv 77 with Delay

ABABCAADAA... ABCBBBAC…

Coding positionPointer

Characters

|Window|=W Lookahead BufferDelay (Δ)

LZ77 with Delay in Terms of PacketsLZ77 with Delay in Terms of Packets

ABC…

Coding positionPointer

|Window|=W CurrentlyEncodedPacket

Delay (Δ)

ABC…

Basic Delayed-Dictionary Compression Basic Delayed-Dictionary Compression (BDDC) in Terms of Packets(BDDC) in Terms of Packets

Encoder Decoder

1.…………………1000…………1100..………………

Received by decoder Δ=99

Currently decoded packet

P1100=f(P1…P1000)P1101=f(P1…P1001)…

1100..……1000..………1

Encoding

Decoding

1100=f(1…1000)

BDDC ImplementationBDDC Implementation

Header Information:Header Information:

S.N. of theS.N. of the current packetcurrent packet

S.N. of theS.N. of the last packet last packet

used used for encodingfor encoding

Packet is Packet is

CompressedCompressed

Y/NY/N

(a bit)(a bit)

BDDC Implementation (cont.)BDDC Implementation (cont.)

X Y X

Encoder

Packets

Dictionary

Rx1

Rx2Tx2

Tx1

[ACKsData]

Encoder

Decoder

Packets

Dictionary

Rx1

Rx2Tx2

Tx1Decoder

ACKs ACKs ACKs

ΔQ

Decoding Latency in BDDC (Decoding Latency in BDDC (ΔΔ=5)=5)

1, 2, 3, 4, 5, 1, 2, 3, 4, 5, 77, , 88, , 99, , 1010, , 66

Packet Packet 66 dropped and retransmitted dropped and retransmitted Now we can decode packets Now we can decode packets 7…107…10

– No decoding latencyNo decoding latency– The phrases of packets The phrases of packets 7…107…10 still can not be still can not be

added to the dictionary until packet added to the dictionary until packet 66 is received is received

Effect of Encoding Delay on Effect of Encoding Delay on Compression QualityCompression Quality

Δ

r

Encoding Delay Vs. Encoding Delay Vs. Compression QualityCompression Quality

ABABABADAA... ABCDBBBAC…

Coding positionPointer

Characters

|Window|=W Lookahead Buffer

ABABCDAA...

Without delay

Encoding Delay Vs. Encoding Delay Vs. Compression Quality (cont.)Compression Quality (cont.)

ABABBADAA... ABCDBBBAC…

Coding positionPointer

Characters

|Window|=W Lookahead Buffer

ABABCDAA...

Delay (Δ)

With delay

DDC vs Stateless vs StreamingDDC vs Stateless vs StreamingLarge PacketsLarge Packets

|header|=20 bytes, |payload|=1500 bytesConcatenation of 18 Calgary-Corpus files

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Dictionary Delay Δ (packets)

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DDC-min

DDC vs Stateless vs StreamingDDC vs Stateless vs StreamingSmall PacketsSmall Packets

|header|=20 bytes, |payload|=125 bytesConcatenation of 18 Calgary-Corpus files

0.500.550.600.650.700.750.800.85

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Effect of Encoding Delay on Effect of Encoding Delay on Decoding LatencyDecoding Latency

Δ

Ldec

Effect of Encoding delay on Effect of Encoding delay on Decoding LatencyDecoding Latency

1………………m…………………………..i1=m+Δ

Received bythe decoder

Δ1=Δ

Currently decoded packet

1………………m……………i2=m+Δ/2

Δ2=Δ/2

1………………m……i3=m+Δ/4

Δ3=Δ/4

Δ1 > Δ2 > Δ3

Ldec1 < Ldec2 < Ldec3

The Compression-Latency Trade-offThe Compression-Latency Trade-off

Ldec

Δ

r

BDDC ConflictBDDC Conflict

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Ldec(packets)

DDCTraffic

CompressionRatio

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Dictionary Delay Δ (packets)

DDC Traffic Compression Ratio

BDDC Av Ldec (packets)

Trade-off between the Decoding-Trade-off between the Decoding-Latency and the Compression RatioLatency and the Compression Ratio

Compression Ratio

Ldec

Stateless (Δ=∞)

Streaming (Δ=0)DDC (Δ)

Confirmed-Dictionary Compression (ΔConf)

DDC algorithmsDDC algorithms

BDDC - Fixed Delay AlgorithmBDDC - Fixed Delay Algorithm BDDC - Adaptive Delay AlgorithmBDDC - Adaptive Delay Algorithm Confirmed-Dictionary Compression (CDC)Confirmed-Dictionary Compression (CDC) DDC-minDDC-min DDCDDC

DDC Algorithms (cont.)DDC Algorithms (cont.)

Fixed Delay AlgorithmFixed Delay Algorithm ∆ ∆ is constantis constant ∆ ∆ is proportional to the network is proportional to the network

propagation delay (requires calibration)propagation delay (requires calibration)

DDC Algorithms (cont.)DDC Algorithms (cont.)

Adaptive Delay AlgorithmAdaptive Delay Algorithm Maintains a Maintains a delay delay parameter, depending onparameter, depending on

– decoding successdecoding success– changes in changes in pp for packet drops for packet drops

DDC Algorithms (cont.)DDC Algorithms (cont.)Confirmed-Dictionary Compression (CDC) AlgorithmConfirmed-Dictionary Compression (CDC) Algorithm The Encoder registers the ACKsThe Encoder registers the ACKs max max ΔΔ s.t. s.t.

ACK received for all packets prior to previous ACK received for all packets prior to previous ΔΔ packets packets

Ensures immediate decodingEnsures immediate decoding Tradeoff in the compression ratioTradeoff in the compression ratio Does not required a retransmissionDoes not required a retransmission

1…………………x……x……x……x……x…..

All packets receivedby the decoder

max Δ

Currently decoded packetACKs List

DDC Algorithms (cont.)DDC Algorithms (cont.)CDC Variant – good for lossy trafficCDC Variant – good for lossy traffic Choose a large enough Choose a large enough ΔΔ If Packet If Packet ii was not ACKed was not ACKed

– Throw it from the queueThrow it from the queue

Does not required a retransmissionDoes not required a retransmission

DDC vs Stateless, StreamingDDC vs Stateless, Streaming

DDC vs StatelessDDC vs Stateless Saves much larger amounts of BandwidthSaves much larger amounts of Bandwidth

– Especially for small packetsEspecially for small packets

DDC vs StreamingDDC vs Streaming Smaller latencySmaller latency DDC good for latency sensitive applicationsDDC good for latency sensitive applications

– Packet drops allowed (up to Packet drops allowed (up to ΔΔ) for decoding) for decoding– Packet reordering allowedPacket reordering allowed

DDC-minDDC-min

Compress in parallel with Stateless and Compress in parallel with Stateless and BDDCBDDC

Transmit according to Transmit according to

min {Stateless,BDDC,Uncompressed}min {Stateless,BDDC,Uncompressed} Use a bit in the header to sign the Use a bit in the header to sign the

compression methodcompression method

DDC = BDDC DDC = BDDC Stateless Stateless

DDBDDC BDDC i i == DD (1…i-(1…i-ΔΔ-1)-1)

DDDDC DDC i i = D= DBDDC BDDC ii D DStateless Stateless ii

DBDDC i = D(1…i-Δ-1) DBDDC i+1

DDDCi =

DBDDC i DStateless i

DDDC i+1DDDC i-1

Rollback

DDC ImplementationDDC Implementation

DDBDDCBDDC i i DDStatelessStateless i i

DDDDC DDC ii

Last Phrase Number=j First Phrase Number=j+1

Dictionary Insertion:if (phrase xDBDDC i) & (phrase xDStateless i)

add x to DStateless i

BDDC DDCBDDC

Graph Based DictionariesGraph Based Dictionaries

Non Compact TriesNon Compact Tries

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A new phrase

Eager DecodingEager Decoding

Each packet contains data about the Each packet contains data about the historyhistory

The Decoder may be able to decode The Decoder may be able to decode received packets, without all the received packets, without all the required historyrequired history

When to Use DDC?When to Use DDC? Latency critical applicationsLatency critical applications Order is not required by application Order is not required by application

(preferable)(preferable) Packet drops are allowedPacket drops are allowed

– use retransmission for dictionary update use retransmission for dictionary update purposespurposes

Example ApplicationsExample Applications

VoIP CompressionVoIP Compression Raw Video CompressionRaw Video Compression TFTP (over UDP)TFTP (over UDP)

– Fast transfer of filesFast transfer of files– Fast transfer of memory areasFast transfer of memory areas

SMSC Encoder Decoder

PC1

PC2

PC4

PC3

TCP SS7

ICQClients

MobilePhoneUsers

User Latency

DecodingLatency

Example Applications (cont.)Example Applications (cont.) SMS Chat from a PC to a Mobile PhoneSMS Chat from a PC to a Mobile Phone

– SMS messages are transferred via the SS7 networkSMS messages are transferred via the SS7 network– SMS uses “Submit ACK”SMS uses “Submit ACK”

Instant MessagingInstant Messaging– A packet should be enough for each messageA packet should be enough for each message

Network ManagementNetwork Management– Multicast/Broadcast of control messagesMulticast/Broadcast of control messages– Deploying information to remote nodes Deploying information to remote nodes – Sensor networksSensor networks

Example Applications (cont.)Example Applications (cont.)

Experimentation on InternetExperimentation on Internet

Tx: planetlab1.tau.ac.ilRx: 269 Planet-Lab nodes

TAU

Packet Drop ProbabilityPacket Drop ProbabilityExperiment Description:Experiment Description: Concatenation of 18 Calgary-Corpus filesConcatenation of 18 Calgary-Corpus files

– 3.25MB3.25MB Transmission with UDP Transmission with UDP

– Each packet contains 125B of dataEach packet contains 125B of data– Transmission to 269 nodesTransmission to 269 nodes

Results:Results:PPdropdrop=1.27%=1.27%PPsuccesssuccess=98.73%=98.73%Var=0.002Var=0.002

Decoding Latency vs Encoding DelayDecoding Latency vs Encoding Delay

020406080

Dictionary Delay Δ (packets)

To

tal A

v L

dec (p

ackets

)

BDDC

Streaming

LLdecdec Distribution of Streaming vs. Distribution of Streaming vs. BDDC with BDDC with ΔΔ=300=300

110

1001000

10000100000

100000010000000

1 225 449 673 897

Ldec (packets)

Nu

mb

er

of P

ackets

Ldec BDDCΔ=300

LdecStreaming

A favorable trade-offA favorable trade-off

Compression Compression ratio close to ratio close to streamingstreaming

Decoding Decoding latency close to latency close to statelessstateless

020406080

Dictionary Delay Δ (packets)

To

tal A

v L

dec (p

ackets

)

BDDC

Streaming

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DDC-min

Related WorkRelated Work

Header Compression:Header Compression: J. Lilley, J. Yang, H. Balakrishnan, and S.

Seshan, “A unified header compression framework for low-bandwidth links,” ACM/IEEE International Conference on Mobile Computing and Networking, pp. 131–142, 2000.

Related Work (cont.)Related Work (cont.)

Stateless Payload Compression:Stateless Payload Compression: A. Shacham et al “IP payload compression

protocol (IPComp),” IETF, RFC 2393, 1998 R. Friend and R. Monsour, “IP payload

compression using LZS,” IETF, RFC 2395, 1998

R. Pereira, “IP payload compression using DEFLATE,” IETF, RFC 2394, 1998

Related Work (cont.)Related Work (cont.)

Payload Compression: S. Dorward and S. Quinlan, “Robust data

compression of network packets,” Bell Labs, 2000, Unpublished manuscript

Related Work (cont.)Related Work (cont.)S. Dorward and S. Quinlan - thwack

Possible ExtensionsPossible Extensions

DDC can be used on any on-line DDC can be used on any on-line compression algorithm (e.g. Deflate)compression algorithm (e.g. Deflate)

Implementation of eager decodingImplementation of eager decoding Multi-Media applicationsMulti-Media applications

– voice compressionvoice compression– download Raw images to mobile phonesdownload Raw images to mobile phones

ConclusionsConclusions

Improved Compression-Latency Trade-offImproved Compression-Latency Trade-off Delayed-Dictionary Compression AlgorithmDelayed-Dictionary Compression Algorithm Experimentation over the internet with Experimentation over the internet with

Planet-LabPlanet-Lab