TCP in Heterogeneous Network Md. Ehtesamul Haque # P

TCP (Transmission Control Protocol) – Most widely used transport layer protocol. – Connection-oriented. – Full duplex. – Flow control. – Congestion control. Different versions of TCP –TCP Tahoe –TCP Reno, NewReno –TCP Vegas etc… Basic Terms

Heterogeneous Network A network where connected systems have different –Operating systems –Protocols –Access technologies –Links We focus on heterogeneous network with different links –wired and wireless links

Outline TCP Congestion control Wireless Link characteristics TCP Peach [2001] TCP Westwood [2002] TCP Hybla [2004] Future Direction

TCP Congestion Control

Slow Start The source starts with Congestion window, cwnd = 1. Every time an ACK arrives, cwnd is Doubled. Slow start runs until –any packet loss or –cwnd reaches some threshold, ssthreshold. Packet loss is detected either by Retransmission timeout or Three duplicate acks.

Congestion window Slow start Congestion avoidance Congestion happens Threshold Round Trip Time, RTT Congestion control in TCP Tahoe

Fast Retransmit Only applied if loss is recognized by –Duplicate acks. Retransmit the lost packet. Better in non-congestion errors. Time out leads to slow start.

Fast Recovery Added by TCP Reno After a fast-retransmit –cwnd = cwnd / 2 (vs. 1 in Tahoe) – threshold = cwnd After a timeout –threshold = cwnd –cwnd = 1 –Do slow start –Same as Tahoe

Wireless Network Packet loss in wireless networks due to… –Bit errors –Handoffs –Possibly congestion, but not often TCP congestion avoidance can be triggered by non-congestion loss –May reduce throughput

Wireless Network (Cont.) Bursts of errors –More than one packet lost in TCP Delay is often very high –TCP’s timeout mechanisms do not work well Links may be asymmetric –ACKs in the slow bandwidth

Different approaches Link-Layer Protocols –Hide losses by making link appear to be more reliable Split-connection protocols –Intermediaries scoop the state of a TCP connection –Violates end-to-end semantics of TCP End-to-end protocols –No intermediaries to scoop the state of a TCP connection –Preserves the E2E semantics of TCP connection

TCP Peach [2001]

Basic concept Avoid slow start Probe network by dummy packet Dummy packets –Small –Low priority Initially high transmission rate

Components Sudden Start Congestion Avoidance Fast Retransmit Rapid Recovery Over Transmit

Flow of TCP Peach

Sudden Start Sudden Start substitues Slow Start. Initially, cwnd=1 1 data segment + (rwnd-1) dummy segments transmitted in 1 RTT After one round trip time, the sender enters the Congestion Avoidance phase. Cwnd= min {rwnd,r}, r is the maximum with available bandwidth.

Rapid Recovery Set cwnd = C 0 / 2. Recover by sending C 0 dummy packets in next RTT. At t 0 cwnd is halved. Send C 0 dummy packets in [t 0,t 0 +RTT]. Continue data packet transmission. t0t0 t 0 + RTT t 0 + 2*RTT

TCP Westwood [2002]

Basic concept Monitor the ack reception rate to estimate the bandwidth. set the cwnd and ssthresh based on the estimated bandwidth. Bandwidth, b k = d k /Δt k, –d k = data acked –Δt k = time difference between two ack. Estimated bandwidth, –BE k+1 = a k BE k + (1-a k ) (b k +b k-1 )/2

Basic concept (cont.) Threshold = (BE * RTT min ) / seg_size cwnd = 1, slow start cwnd = threshold, recovery

TCP Hybla cwnd size independent of RTT. Support SACK for multiple loss. Westwood like bandwidth estimation.

Future Direction Correctly differentiate between –Congestion loss and –Non-congestion loss Solve the problem of asymmetry in bandwidth Develop a unified TCP

Questions?