CMPE 257 Wireless and Mobile Networking Spring 2005 E2E Protocols point to point CMPE 257 Spring 2005 1 Announcements CMPE 257 Spring 2005 2 Today E2E protocols cont d CMPE 257 Spring 2005 3 Recap ing CMPE 257 Spring 2005 4 Cross Layer Approaches Link layer error recovery Link layer retransmission TCP awareness TCP unawareness Split connection CMPE 257 Spring 2005 5 Link Layer Mechanisms Error Correction Example Forward Error Correction FEC Lin83 can be used to correct limited number of errors Correctable errors hidden from the TCP sender FEC incurs overhead even when errors do not occur Adaptive FEC schemes can reduce the overhead by choosing appropriate FEC dynamically CMPE 257 Spring 2005 6 Link Layer Mechanisms Link Level Retransmissions Link level retransmission schemes retransmit a packet at the link layer if errors are detected Retransmission overhead incurred only if errors occur CMPE 257 Spring 2005 7 Link Layer Mechanisms May combine both FEC and retransmissions Use FEC to correct small number of errors Use link level retransmission when FEC capability is exceeded CMPE 257 Spring 2005 8 Link Level Retransmissions TCP connection Link layer state application application application transport transport transport network network link link link physical physical physical rxmt network wireless CMPE 257 Spring 2005 9 Link Level Retransmissions Issues How many times to retransmit at the link level before giving up Finite bound semi reliable link layer No bound reliable link layer What triggers link level retransmissions Link layer timeout mechanism Link level acks negative acks dupacks CMPE 257 Spring 2005 10 Link Level Retransmissions Issues How much time is required to trigger link layer retransmission Small fraction of end to end TCP RTT Multiple of end to end TCP RTT Should link layer deliver packets as they arrive or deliver them in order Link layer may need to buffer packets and reorder if necessary so as to deliver packets in order CMPE 257 Spring 2005 11 Link Layer Schemes Summary When is a reliable link layer beneficial to TCP performance If it provides almost in order delivery and TCP retransmission timeout large enough to tolerate additional delays due to link level retransmits CMPE 257 Spring 2005 12 Cross Layer Approaches Link layer error recovery Link layer retransmission TCP awareness TCP unawareness Split connection CMPE 257 Spring 2005 13 TCP Aware Link Layer CMPE 257 Spring 2005 14 Snoop Protocol Balakrishnan95 Retains local recovery of Split Connection approach Link level retransmissions Differs from split connection schemes End to end semantics retained Soft state at base station CMPE 257 Spring 2005 15 Snoop Protocol Per TCP connection stat TCP connection application application application transport transport transport network network link link link physical physical physical FH BS rxmt wireless CMPE 257 Spring 2005 network MH 16 Snoop Protocol Buffers data packets at base station Data sent by FH not yet ack d by MH Allow link layer retransmission When dupacks received by BS from MH or local timeout retransmit on wireless link if packet in buffer Prevents fast retransmit by TCP sender at FH by suppressing dupacks at BS CMPE 257 Spring 2005 17 Snoop Example 35 36 TCP state maintained at link layer 37 38 40 39 38 FH 37 BS 34 MH 36 Example assumes delayed ack every other packet ack d CMPE 257 Spring 2005 18 Snoop Example 35 39 36 37 38 41 40 39 34 38 36 CMPE 257 Spring 2005 19 Snoop Example 37 40 38 39 42 41 40 36 39 36 dupack Duplicate acks are not delayed CMPE 257 Spring 2005 20 Snoop Example 37 40 38 41 39 43 42 41 36 40 36 36 Duplicate acks CMPE 257 Spring 2005 21 Snoop Example 44 37 40 38 41 39 42 43 FH 37 41 BS Dupack triggers retransmission of packet 37 from base station BS needs to be TCP aware to be able to interpret TCP headers Discard dupack MH 36 36 36 CMPE 257 Spring 2005 22 Snoop Example 45 37 40 38 41 39 42 44 43 42 37 36 36 36 36 CMPE 257 Spring 2005 23 Snoop Example 46 37 40 43 38 41 44 39 42 45 43 42 36 TCP sender does not fast retransmit 41 36 36 36 CMPE 257 Spring 2005 24 Snoop Example 47 37 40 43 38 41 44 39 42 45 46 44 43 41 TCP sender does not fast retransmit 36 36 36 36 CMPE 257 Spring 2005 25 Snoop Example 42 45 43 46 44 48 47 45 FH 44 BS 41 MH 43 36 36 36 36 CMPE 257 Spring 2005 26 bits sec Performance 2000000 1600000 1200000 800000 400000 0 base TCP Snoop no error 256K 128K 64K 32K 16K 1 error rate in bytes 2 Mbps Wireless link CMPE 257 Spring 2005 27 Snoop Protocol Advantages Snoop prevents fast retransmit from sender despite transmission errors and out oforder delivery on the wireless link If wireless link delay bandwidth product less than 4 packets simple TCP unaware link level retransmission scheme can suffice Since delay bandwidth product is small retransmission scheme can deliver lost packet without causing MH to send 3 dupacks CMPE 257 Spring 2005 28 Snoop Protocol Advantages Higher throughput can be achieved Local recovery from wireless losses Fast retransmit not triggered at sender despite out of order link layer delivery End to end semantics retained Soft state at base station Loss of the soft state affects performance but not correctness CMPE 257 Spring 2005 29 Snoop Protocol Disadvantages Link layer at base station needs to be TCP aware Not useful if TCP headers are encrypted IPsec Cannot be used if TCP data and TCP ACKs traverse different paths CMPE 257 Spring 2005 30 Delayed Dupacks Approach TCP unaware approximation of TCP aware link layer Attempts to imitate Snoop without making BS TCP aware Snoop implements two features at BS Link layer retransmission Dupack handling reduced interference between TCP and link layer retransmissions drop dupacks CMPE 257 Spring 2005 31 Delayed Dupacks Implements same two features at BS link layer retransmission at MH reducing interference between TCP and link layer retransmissions by delaying dupacks CMPE 257 Spring 2005 32 Delayed Dupacks Protocols TCP receiver delays dupacks third and subsequent for interval D when out of order packets received Dupack delay intended to give link level retransmit time to succeed Benefit Delayed dupacks can result in recovery from a transmission loss without triggering a response from the TCP sender Disadvantage Recovery from congestion losses delayed CMPE 257 Spring 2005 33 Delayed Dupacks Protocols Delayed dupacks released after interval D if missing packet not received Link layer maintains state to allow retransmission CMPE 257 Spring 2005 34