! errors caused by signal attenuation, noise.!! receiver detects presence of errors:!

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1 Daa Link Layer! The Daa Link layer can be furher subdivided ino:!.! Logical Link Conrol (LLC): error and flow conrol!.! Media Access Conrol (MAC): framing and media access! differen link proocols may provide differen services, e.g., Eherne doesn provide reliable delivery (error recovery)! MAC opics:!! framing and MAC address assignmen!! LAN forwarding!! IP o MAC address resoluion!! IP o MAC: Address esoluion Proocol (AP)!! MAC o IP: everse AP (AP), BOOTsrap Proocol (BOOTP), Dynamic Hos Configuraion Proocol (DHCP)!! media access conrol! applicaion ranspor nework LLC MAC physical Link Layer ervices! Half-duplex and full-duplex!! wih half duplex, nodes a boh ends of link can ransmi, bu no a same ime! Framing, link access:!! encapsulae daagram ino frame, adding header, railer!! channel access if shared medium!! MAC addresses used in frame headers o idenify source, des!! differen from IP address Error Deecion:!! errors caused by signal aenuaion, noise.!! receiver deecs presence of errors:!! signals sender for reransmission or drops frame! Link Layer ervices! Error Correcion:!! receiver idenifies and correcs bi error(s) wihou resoring o reransmission! Flow Conrol:!! pacing beween adjacen sending and receiving nodes! eliable delivery beween adjacen nodes!! seldom used on low bi error link (fiber, some wised pair)!! wireless links: high error raes!! Q: why boh link-level and end-end reliabiliy?! Flow Conrol! Wha is flow conrol?! Why do you need flow conrol?! Flow conrol proocols a daa link layer (single hop):!! ON/OFF!! op & Wai Proocol (WP)!! liding Window Proocol! imilar issues and mechanisms apply a he ranspor layer!

2 ON/OFF! Algorihm:! sender receiver! sends sream of daa!! sends OFF, sops ransmission!! sends ON, resumes ransmission! " propagaion Works OK if! is small, oherwise sender can overrun receiver (Why?)! op and Wai (&W) Proocol! Afer each, sender mus wai for acknowledgmen () before sending he nex! Time ender + eceiver +! op & Wai Performance! Disadvanages:!! slow!! mus wai for even if no overrun!! max ransmission bandwidh /r! Performance ok if! is small, else inefficien! Example :!! link bandwih (µ ) = Mbps, wih size (L) = Kbis, " ransmission ime is L/µ = ms!! if r (! ) = 9 ms, we can send 00 s/sec!! he hroughpu (T g ) is 00 Kbps (0% of capaciy)! op & Wai Performance! firs packe bi ransmied, = 0 las packe bi ransmied, = L / µ Example :! sender TT (! ) arrives, send nex! packe, = TT + L / µ receiver firs packe bi arrives las packe bi arrives, send! link bandwih (µ ) = Gbps, wih size (L) = 8 Kbis, " ransmission ime is L/µ = 8 µs!! sender uilizaion (U s ), fracion of ime sender is sending:! L / µ U s =! + L / µ = 8 0 /0 9 = "

3 liding Window: " Pipelined Flow Conrol! Pipelining: sender allows muliple, in-fligh, yeo-be-acknowledged s!!range of sequence numbers mus be increased!!buffering a sender and/or receiver! liding Window! end w number of s before waiing for an (can have w ousanding, i.e., uned, s)! evens! On receiving an, slide window (over daa) by " (&W is sliding window wih w = )! Throughpu of he sliding window proocol (T w ):! T w = T g *w send window size w limied by buffer size a receiver (w ):! T w = T g *MIN(w,w )! Pipelining: Increased Uilizaion! firs packe bi ransmied, = 0 las bi ransmied, = L / µ Example :! TT (! ) arrives, send nex! packe, = TT + L / µ sender receiver firs packe bi arrives las bi of s arrives, send las bi of nd arrives, send las bi of rd arrives, send! link bandwih (µ) = Gbps, wih size (L) = 8 Kbis, " ransmission ime is L/µ = 8 µs, window size (w) =!! sender uilizaion (U s ), fracion of ime sender is sending:! U s = w * L / µ! + L / µ = * 8 0 /0 9 = "6 increase uilizaion! by a facor of liding Window: Max Window ize! Wha is he opimal window size?! i.e., wha s he maximum number of s one can have ousanding (o fill he pipe )?! Le µ be link bandwidh, pipe size = r * µ =! *µ" (commonly called he bandwidh- produc)! Normally you don wan o, and can, fill he pipe compleely!

4 (Link Layer) eliabiliy! Frames could be corruped or loss!! underlying channel may flip bis in packe! How is corrupion deeced?! How o recover from errors?!! sender mus reransmi los/corruped s! How does sender know when and which s o reransmi?! Auomaic epea reques (AQ)! How does sender know when " and which s o reransmi?!! by he use of s and imeou! General algorihm:!! receiver acknowledges (s) receip of s!! sender reransmis s no ed by imeou!! a.k.a. PA: Posiive Acknowledgemen wih eransmission! Time eliabiliy proocols:!! Alernaing Bi Proocol (ABP)!! Go-Back-N (GBN, wih or wihou NAK)!! elecive epea Proocol (P)! ender + los rexmied eceiver +! Alernaing Bi Proocol (ABP)! ABP in Acion! &W wih un-numbered s and s causes ender confusion on reransmission:!! how o differeniae a rexmied" Time frame from he nex frame?!! wih rexmission, which frame is " + being ed?!! he original frame?!! or he rexmied frame?! ender! (wha implicaion?)! ABP: use bi o number " s and s! Time + for which? los eceiver +! how o deec duplicae? eceiver +!

5 ABP in Acion! Performance of ABP! ABP works, bu performance is bad! Example:!! link bandwidh (µ) = Gbps link!! propagaion (!) = ms!! size (L) = KB!! sender uilizaion (Us) :" L / µ U s =! + L / µ = 8 0 /0 9 = # 0 " !roughly, every 0 ms!! KBps or 6 Kbps hroughpu over a Gbps link!nework proocol limis use of physical resources Pipelined AQ Proocols! Pipelined AQ proocols:!!go-back-n!!go-back-n wih NAK!!elecive epea Proocol! Go-Back-N! ender:!! runs sliding window flow conrol:!! k-bi seq# in header!! window of up o w, consecuive uned s allowed"! keeps a reransmission imer for each in-fligh!! when he imer for i imes ou, reransmis i and all subsequen higher seq# s in window!

6 Go-Back-N! Go-Back-N wih Negaive (NAK)! eceiver:!! remembers nex expeced seq#!! s and delivers o app in-order s!! discards ou-of-order s no buffering!! s ou-of-order packes if seq# is " smaller han nex expeced number " (why?)! ender rexmission imeou (ro) Pks s eceiver ender Pks s eceiver discard hese discard eceiver:!! s and delivers in-order packes!! sends NAK for firs ou of order and discards!! s and discards subsequen ou of order packes! ender: rexmis on receiving NAK or ro! sender! discard all hese Walrand! receiver! elecive epea Proocol (P)! eceiver:!! s all correcly received s!! buffers ou of order s (up o w ), " for evenual in-order delivery o upper layer! ender:!! keeps a rexmi imer for each!! reransmis only uned s!! mus keep rack of w and ensures " ha w > (larges uned " smalles uned)! canno send! (why?)! ok o send! 6 (why?)! Walrand! implifying Assumpions! Infinie sequence# space size! uppose you have only a -bi sequence space:! ender rexmission imeou (ro) Pks s eceiver elecive Acknowledgemen: Piggy-back NAK wih., e.g. [,NAK], [,NAK]! s or h?

7 Oher Issues a Transpor Layer! Connecionless nework layer means each can:!! ake a differen pah!! experience differen congesion! Implicaions:!! non-deerminisic r!! ou of order s mus be buffered for Go-Back-N!! complicaes compuaion of w