TCP Overview

End to end issues

Segment format

Connection establishment

TCP sliding window

Stream control Transmission Protocol

Simple demultiplexor

TCP Congestion Control Determines the network capacity Adjust the number of packets that can have safely in transit Acks to pace the transmission of packets TCP is self clocking Avoids congestion Maxwindow=MIN(CongestionWindow,AdvertisedWindow) EffectiveWindow=MaxWindow-(LastByteSent-LastByteAcked)

Caused By the shortage of buffer space. slow links. slow processors Possible solutions End-to-end versus link-by-link control Rate-Based versus Credit-Based control The rate-based traffic-flow technique constantly Integrated congestion control

Principles of Congestion Control informally: “too many sources sending too much data too fast for network to handle” different from flow control! manifestations: lost packets (buffer overflow at routers) long delays (queueing in router buffers) a top-10 problem!

Scenario 1: Queuing Delays unlimited shared output link buffers Host A lin : original data Host B lout two senders, two receivers one router, infinite buffers no retransmission large delays when congested maximum achievable throughput

Scenario 2: Retransmits one router, finite buffers sender retransmission of lost packet Host A lout lin : original data l'in : original data, plus retransmitted data Host B finite shared output link buffers

Scenario 3: Congestion Near Receiver four senders multihop paths timeout/retransmit l in Q: what happens as and increase ? l in Host A lout lin : original data l'in : original data, plus retransmitted data finite shared output link buffers Host B

Approaches towards congestion control Two broad approaches towards congestion control: End-end congestion control: no explicit feedback from network congestion inferred from end-system observed loss, delay approach taken by TCP Network-assisted congestion control: routers provide feedback to end systems single bit indicating congestion (SNA, DECbit, TCP/IP ECN, ATM) explicit rate sender should send at

TCP Congestion Control end-end control (no network assistance) sender limits transmission: LastByteSent-LastByteAcked  CongWin Roughly, CongWin is dynamic, function of perceived network congestion How does sender perceive congestion? loss event = timeout or 3 duplicate acks TCP sender reduces rate (CongWin) after loss event three mechanisms: AIMD slow start conservative after timeout events rate = CongWin RTT Bytes/sec

TCP AIMD multiplicative decrease: cut CongWin in half after loss event additive increase: increase CongWin by 1 MSS every RTT in the absence of loss events: probing Long-lived TCP connection

TCP Slow Start When connection begins, increase rate exponentially fast until first loss event When connection begins, CongWin = 1 MSS Example: MSS = 500 bytes & RTT = 200 msec initial rate = 20 kbps available bandwidth may be >> MSS/RTT desirable to quickly ramp up to respectable rate

TCP Slow Start (more) When connection begins, increase rate exponentially until first loss event: double CongWin every RTT done by incrementing CongWin for every ACK received Summary: initial rate is slow but ramps up exponentially fast Host A Host B one segment RTT two segments four segments time

Refinement (more) Implementation: Variable Threshold Q: When should the exponential increase switch to linear? A: When CongWin gets to 1/2 of its value before timeout. Implementation: Variable Threshold At loss event, Threshold is set to 1/2 of CongWin just before loss event

TCP sender congestion control Event State TCP Sender Action Commentary ACK receipt for previously unacked data Slow Start (SS) CongWin = CongWin + MSS, If (CongWin > Threshold) set state to “Congestion Avoidance” Resulting in a doubling of CongWin every RTT Congestion Avoidance (CA) CongWin = CongWin+MSS * (MSS/CongWin) Additive increase, resulting in increase of CongWin by 1 MSS every RTT Loss event detected by triple duplicate ACK SS or CA Threshold = CongWin/2, CongWin = Threshold, Set state to “Congestion Avoidance” Fast recovery, implementing multiplicative decrease. CongWin will not drop below 1 MSS. Timeout CongWin = 1 MSS, Set state to “Slow Start” Enter slow start Duplicate ACK Increment duplicate ACK count for segment being acked CongWin and Threshold not changed

Congestion Avoidance Mechanisms Helps to avoid congestion Additional functionality into the router to assist in anticipation of congestion to  control  congestion  once  it  happens to  repeatedly  increase  load  in  an  effort  to  find  the  point  at  which  congestion  occurs,  and  then  back  off

Mechanisms router-centric: DECbit and RED Gateways host-centric:  TCP  Vegas

DECbit

DECbit Add binary congestion bit to each packet header Router monitors  average  queue  length  over  last  busy+idle  cycle set  congestion  bit  if  average  queue  length  greater  than  1  when  packet  arrives attempts  to  balance  throughput  against  delay

DECbit End Hosts destination echos bit back to source source  records  how  many  packets  resulted  in  set  bit if  less  than  50%  of  last  window's  worth  had  bit  set,  then  increase  CongestionWindow  by  1  packet if  50%  or  more  of  last  window's  worth  had  bit  set,  then  decrease  CongestionWindow  by  0.875  times

UDP—User Datagram Protocol An unreliable, connectionless transport layer protocol UDP format. See picture Two additional functions beyond IP: Demultiplexing: deliver to different upper layer entities such as DNS, RTP, SNMP based on the destination port # in the header. i.e., UDP can support multiple applications in the same end systems. (Optionally) check the integrity of entire UDP. (recall IP only checks the integrity of IP header.) If source does not want to compute checksum, fill checksum with all 0s. If compute checksum and the checksum happens to be 0s, then fill all 1s. UDP checksum computation is similar to IP checksum, with two more: Add extra 0s to entire datagram if not multiple of 16 bits. Add pseudoheader to the beginning of datagram. UDP pseudoheader

Back to UDP—User Datagram Protocol UDP datagram 0 16 31 Source Port Destination Port UDP Length UDP Checksum Data Back to UDP—User Datagram Protocol Figure 8.16

Back to UDP—User Datagram Protocol UDP pseudoheader 0 8 16 31 Source IP Address Destination IP Address 0 0 0 0 0 0 0 0 Protocol = 17 UDP Length 1.Pseudoheader is to ensure that the datagram has indeed reached the correct destination host and port. 2. The padding of 0s and pseudoheader is only for the computation of checksum and not be transmitted. Figure 8.17

Thank u