DOC PREVIEW
USC CSCI 551 - 15b_snoop-6up

This preview shows page 1 out of 2 pages.

Save
View full document
View full document
Premium Document
Do you want full access? Go Premium and unlock all 2 pages.
Access to all documents
Download any document
Ad free experience
Premium Document
Do you want full access? Go Premium and unlock all 2 pages.
Access to all documents
Download any document
Ad free experience

Unformatted text preview:

1 Computer Communications - CSCI 551 Copyright © William C. ChengCS551Handoff Performance inCellular Networks[Balakrishnan95b]Bill Chenghttp://merlot.usc.edu/cs551-f12packet loss (corruption)Deals with TCP in mobile environments2Key Ideas Computer Communications - CSCI 551 Copyright © William C. Chenghandoff (changing from one base station to another)base stations cache TCP segments and quickly retransmitSnoopcache TCP segments at nearby base-stations to allowrapid handoff HandoffTCP’s reaction: reduce sending rateTCP assumes loss implies congestion3Problem: TCP Loss Handling in Wireless Computer Communications - CSCI 551 Copyright © William C. Chengdesired reaction: retransmit lost packets quicklyWireless adds losses due to corruption, collision, handofflet base-station help outApproach:alternative is to do link-level reliabilityfrom BS, use one TCP connection to FH and another to MHSplit-connection TCP:4Alternatives Computer Communications - CSCI 551 Copyright © William C. Chengbut requires changes to FH, BS, MHgood idea: done with ECNMake TCP distinguish congestion vs. other kinds of lossbut done after this work and not widely deployed eventodaygood idea, but must be careful to avoid interactionsbetween link-layer and TCP (works if on differenttimescale)Link-layer retransmissionrequires changes to FH and MHwhat does an ACK mean now?Solution should not require modifications to fixed hostsIncremental deployment5Constraints Computer Communications - CSCI 551 Copyright © William C. ChengIf possible, avoid modifying mobile hostsACK of a packet means it’s at the receiver, not the basestationPreserve TCP end-to-end semantics6Snoop Overview Computer Communications - CSCI 551 Copyright © William C. ChengBS adds SACK support(even if FH doesn’t support it)Base Station (BS) snoopspassing traffic (data/acks);quickly retx’s dataFixed Host (FH)sends data to MHno change to FH codeMobile Host (MH) receives data, sends ACKs as usualFH-to-MH:FHBSMHDataACKsMH-to-FH:FHBSMHACKsData2. Forward packetAdd to cache and pass onPacket in sequence7FH-to-MH Snoop Data Processing Computer Communications - CSCI 551 Copyright © William C. ChengGreater than last acked:pass onOut of sequence, cachedLost or delayed out-of-orderOut of sequence, not cachedElse: generate ACK to fixedhost (may be caused by alost ACK)Pass on, and keepinformationIn-sequence?New pkt?YesYesPacket arrives from FHNoNoCommon case1.2.Cache packetForward tomobileSender Retransmission1.2.Forward packetReset localretransmissioncounterCongestion loss1. Mark ascongestion lossShould not be commonPass on to FHsuppress other dupacksNew ACK8Snoop ACK Processing Computer Communications - CSCI 551 Copyright © William C. ChengClean up cacheIf data not in cache, orsender retransmit, pass on to FH (not in flowchart)Duplicate ACKIf in cache, respondimmediatelyDup ack?New ack?NoYesAck arrives from MHNoYesLater dup acksfor lost packetDiscardCommon case1.2.Free buffersUpdate RTTestimateFirst one?NoYesNext packet lostRetransmit lostpacket with highprioritySpurious ackDiscard3. PropagateACK to senderextend mobile IP to multicast packets to several FA’s (basestations, BSes)General approach:9Handoff Support Computer Communications - CSCI 551 Copyright © William C. ChengMH informs BS when it’s changingBSes are pre-loaded w/data, can run snoop and quicklyrepair lossesModify snoop module to generate SACKsWhat about mobile-to-fixed communication?10Other Issues Computer Communications - CSCI 551 Copyright © William C. ChengOpen area of researchTCP over ad-hoc networks?fast-retransmit in TCP11 Computer Communications - CSCI 551 Copyright © William C. Cheng Discussiondeployable solution for wireless performance enhancementImpactDoes this violate the end-to-end argument?layer-4 caching? (i.e., caching HTTP without the end pointsknowing it)Other examples?minimal changes to improve performanceNice aspects of Snoopsoft-state designpreserves TCP


View Full Document

USC CSCI 551 - 15b_snoop-6up

Download 15b_snoop-6up
Our administrator received your request to download this document. We will send you the file to your email shortly.
Loading Unlocking...
Login

Join to view 15b_snoop-6up and access 3M+ class-specific study document.

or
We will never post anything without your permission.
Don't have an account?
Sign Up

Join to view 15b_snoop-6up 2 2 and access 3M+ class-specific study document.

or

By creating an account you agree to our Privacy Policy and Terms Of Use

Already a member?