ECE544: Communication Networks-II, Spring Transport Layer Protocols Sumathi Gopal March 31 st 2006

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1 ECE544: Communication Networks-II, Spring 2006 Transport Layer Protocos Sumathi Gopa March 31 st 2006

2 Lecture Outine Introduction to end-to-end protocos UDP RTP TCP Programming detais 2

3 End-To-End Protocos Enabe communication between 2 or more processes (which may be on different hosts in different networks) The Transport Layer is the owest Layer in the network stack that is an end-to-end protoco 3

4 Transport Layer Protocos Connectioness protocos considered here Basic Function: Enabe process-to-process communication via virtua process-hooks caed ports. 4-Tupe Connection Identifier: < SrcPort, SrcIPAddr, DestPort, DestIPAddr > A transport protoco may provide severa features in addition. 4

5 Most popuar transport protocos User Datagram Protoco (UDP): Provides the process identification functionaity via ports Option to check messages for correctness with CRC check Transmission Contro Protoco (TCP): Ensures reiabe deivery of packets between source and destination processes Ensures in-order deivery of packets to destination process Other options Rea Time Protoco (RTP): Serves rea-time mutimedia appications Header contains sequence number, timestamp, marker bit etc Runs over UDP 5

6 User Datagram Protoco (UDP) Appication Data bytes Socket ca UDP UDP Header Data bytes UDP Header Src Port Dest Port Checksum Length 2 bytes 2 bytes 2 bytes 2 bytes Header Fieds: Src Port: Unique identification number assigned to the source process by the kerne in source node. Dest Port: The Unique Identification number assigned to the destination process by the kerne in the destination node. Checksum: Fied on source side. Checked on receiver side to ensure message correctness. Cacuated over <Data, UDP hdr, portion of IP hdr> Length: Tota number of bytes in (UDP header + data bytes) 6

7 Exampe of an appication using UDP My appication caed Network Performance Monitor (NPM) needs to measure the pattern of packet osses in a network. Appication needs sequence numbers and timestamps in each packet UDP does not provide this faciity; So NPM adds its own header to each packet NPM NPM Header Seq Num Send Timestamp 4 bytes 8 bytes Data Bytes UDP UDP Header NPM Bytes 7

8 Appication requirements Like NPM, most appications need much more from a transport protoco than the basic functionaity Mutimedia appications require tracking of packet oss, deay and jitter. Most other appications such as HTTP, Database Management, FTP etc, require reiabe data transport TCP, UDP and RTP satisfy needs of the most common appications Appications requiring other functionaity usuay use UDP for transport protoco, and impement additiona features as part of the appication 8

9 Introduction to TCP The TCP/IP protoco suite has enabed computers of a sizes, from different vendors, different OSs, to communicate with each other. Forms the basis for the wordwide Internet that spans the gobe. First proposed by Vinton Cerf and Robert Kahn, 1974 (They were awarded the ACM Turing award 2004) Reiaby deivers data between two processes Assumes unreiabe, non-sequenced deivery Divides data passed to it from appication process into appropriate sized chunks for the network ayer beow Acknowedges received packets Sets timeouts to ensure other end acknowedges packets sent Appication can ignore detais of reiabiity 9

10 A top-eve view of TCP operation Appication Appication Write Bytes Read Bytes TCP Send Buffer TCP Receive Buffer Segment Segment Segment Transmit Segments 4-Tupe Connection Identifier: < SrcPort, SrcIPAddr, DestPort, DestIPAddr > 10

11 TCP Header Format Appication Byte Stream TCP segment TCP Data Data Data Data TCP Header Ateast 20 bytes Fags: SYN FIN RESET PUSH URG ACK Source port Destination port Sequence number Acknowedgement Hdr en 0 Fags Advertised window Checksum Urgent pointer Options (variabe) Data 11

12 Summary of TCP s Operation Sequence A Operations are sender driven; TCP protoco competey impemented at the ends Start: Connection Estabishment by a Three-Way Handshake agorithm Consensus on Initia Sequence Number (ISN) Data Transfer: An enhanced siding-window protoco is the core of TCP operation Operate in sow-start and congestion-avoidance modes Receiver acknowedges successfu reception of every segment Sender continuousy estimates round-trip time and maintains severa dependent timers to ensure reiabiity of data transfer Finish: Connection tear-down by a Three-Way Handshake agorithm Both sides independenty cose their haf of the connection 12

13 A simpe Fie Transfer Appication Receiver process waits for connection and data from sender Sender process requests receiver for a connection Once the two processes are connected, the sender process transfers the fie and coses. Receiver shoud be started first Receiver port Id shoud be known to the sender 13

14 Programming Viewpoint Data Sending Appication TCP Send bytestream to connected Socket of type SOCK_STREAM User Space Handes Appication detais Kerne Space Handes communication detais Sender appication process ony needs to provide a bytestream to the kerne Kernes on sending and receiving hosts operate TCP processes Receiver appication process ony needs to read received bytes from the assigned TCP buffers 14

15 Connection Estabishment Sender Receiver SYN, Sequence Num = x SYN+ACK, Sequence Num = y Acknowedgement = x + 1 ACK, Acknowedgement = y + 1 Figure 5.6 from text book Three-Way Handshake Agorithm SYN and ACK fags in the header used Initia Sequence numbers x and y seected at random Required to avoid same number for previous incarnation on the same connection 15

16 Connection Estabishment 16

17 Connection Tear-down Sender FIN FIN-ACK FIN Receiver Data write Data ack FIN-ACK Each side coses its haf of the connection independenty 17

18 Connection Tear-down State Diagram Observe that a connection in the TIME_WAIT state can move to CLOSED state ony after waiting for 2*Max-TTL 18

19 TCP Data Transfer Operation Goa of TCP: Deiver data reiaby and in order whie maximizing end-to-end throughput (Throughput = bytes deivered/ time taken) Fow Contro: On sender side with congestion window; On receiver side with advertised window Saturate network pipe : (deay X bandwidth) bytes If more bytes sent: packets ost due to overfows If fewer bytes sent: Network resources underutiized Ensure receiver is not fooded with data Error Contro or Congestion Contro: Lost ACK for a packet => ost packet Packet oss interpreted as due to congestion => overfow of a queue in some router aong the way. 19

20 The Reiabiity Mechanism Receiver generates ACKs each time a segment is received ACKs are cumuative Sender data Ack Receiver 3 ost ack 3 ack

21 TCP s Congestion Window The Congestion Window (cwnd) is TCP s main too of operation A siding window used for both Fow contro and Error Contro Error Contro: cwnd maintains a packets not-yet acknowedged Fow contro: Size of cwnd is the burst size that can be sent at one time Higher the size of cwnd, better the net throughput Sequence numbers maintained in bytes (remember, TCP serves a byte stream!) 21

22 cwnd Operation congestion window Sent but not ACKed Bytes in TCP buffer; Not yet sent A bytes ACKed n m Sequence numbers Receive (ACK n+k) => Receiver has received a bytes upto (not inc.) n+k NEW congestion window ACKed Bytes expunge n A bytes ACKed Sent but not ACKed New Bytes sent n+k m m+k m+k+ x Not yet sent x depends on mode of operation Sequence numbers 22

23 Modes of Operation Sow-start mode: cwnd growth in this mode when cwnd size < sow-start-threshod AND No congestion has been detected cwnd increases by a segment with every incoming ACK Exponentia increase cwnd incremented by the number of ACKs received in one round-triptime congestion-avoidance mode cwnd growth in this mode in a other cases cwnd incremented by (1/cwnd)* number of bytes acked with each incoming ACK Additive increase cwnd incremented by at most one segment in each round-trip-time 23

24 Visuaization of sow-start and congestion avoidance ssthresh Assumes: ssthresh = cwnd size (segments) sow-start congestion-avoidance A segments are ACKed and there are no packet osses round-trip times Courtesy: TCP/IP Iustrated, Vo 1 by W.R.Stevens 24

25 Receiver-side fow contro Avoid fooding receiver with data Notifies sender of number of bytes it can accept in advertisedwindow fied in ACK header. Sender bytes sent = MIN(cwnd, advertisedwindow) Receiver deivers bytes in correct order to appication process by maintaining a receive window Receive buffer Acked but not deivered to user Not yet acked Gap window Sequence numbers 25

26 TCP s Error Contro Mechanism Data segments and ACKs may get ost in transit; Losses interpreted as due to network congestion (i.e. buffer overfow in an intermediate router) TCP sender sets deadines for ACK arriva using timers; Deadines a function of estimated RTT If deadines not met: cwnd scaed down Segment(s) retransmitted Accurate Round-trip Time estimation critica for efficient TCP operation Premature timeouts and retransmissions pace huge to on the net throughput 26

27 Round-trip time (RTT) Estimation Two important timers : Retransmission Timer and RTO Timer depend on accurate RTT estimation Round-trip time is variabe. Smoothed RTT estimator: R = αr + (1- α)m R: smoothed RTT M: new RTT measurement α : smoothing factor (typicay = 0.9) RTO = function of(smoothed RTT, RTT variance) (ook in the book for expression) A singe RTT estimator active at a time. Cumuative ACKs aso considered Karn s Agorithm: Retransmitted segments not considered for RTT estimation because of retransmission ambiguity probem 27

28 Timeout and Retransmission RTT Timer Expiry: Dupicate ACKs arrive, but the ACK for a specific segment does not arrive TCP interprets this as oss of a singe segment, and a transient network congestion RTT Timer expires and triggers an immediate retransmission of the segment requested in the dupicate ACKs. RTO Timer Expiry: No ACKs arrive at a before RTO timer expires TCP interprets this as heaviy congested network Exponentia Backoff triggered when no segment is transmitted and network is given time to recover from congestion After the backoff duration, the first unacknowedged segment is retransmitted subsequent resumption of data fow ony after Backoff duration Expiration of either timer sets cwnd=1, and sow-start mode 28

29 Timeout impact on Throughput A timeout reduces cwnd size to 1 => Just 1 segment transmitted in 1 RTT. cwnd subsequenty grows very cautiousy in sow-start mode Bad for ossy high bandwidth-deay paths A segments foowing the ost segment are aso retransmitted: even if they have been successfuy received (out of order) at the receiver A possibe buk retransmission of a arge portion, may further contribute to network congestion 29

30 Idea behavior of TCP Tahoe cwnd size ssthresh(0) ssthresh(t) 0 t t+rto Exponentia backoff At t2, t3, t4: Dupicate ACKs arrive Retransmission timer expires Singe packet retransmitted in sow-start mode at cwnd=1 Next segments sent based on cumuative ACKs received t2 t3 t4 time 30

31 Optimizations in various TCP favors Severa optimizations for better TCP throughput in the past 30 years. Most important among them: Fast Retransmit Fast Recovery Seective Acknowedgement (SACK) Deayed ACKs 31

32 Fast Retransmit Don t wait for retransmission timer to expire that causes cwnd to drop to 1 React to dupicate ACKs instead Don t know if dupicate ACKs are because of packet oss or reordering. Threshod set to 3 dupicate ACKs On receiving 3 dupicate ACKs: Requested segment retransmitted cwnd growth continues Set ssthresh = (½ * MIN(cwnd, rcvr_adv_window) ) bytes Set cwnd = (ssthresh + num-dup-acks*segment_size) bytes cwnd continues to grow with arriving dup-acks Each dupicate ACK impies that a segment has eft the network and reached the receiver New segment transmitted if cwnd size permits 32

33 Fast Recovery When ACK for retransmission received cwnd growth resumes in congestion avoidance mode with cwnd=ssthresh rather than starting in sow-start mode with cwnd=1 For an exampe of Fast Retransmit and Fast Recovery refer TCP/IP Vo 1, W.R.Stevens, section 21.8, Figures & 21.11, Page

34 TCP Reno Most popuar TCP favor; impemented in most operating systems Impements Fast Retransmit and Fast Recovery in addition to defaut TCP congestion contro and fow contro mechanisms. cwnd size network deay-bandwidth product time 34

35 Programming viewpoint Open a TCP fow SENDING PROCESS socket(sock_stream,...) KERNEL Sender side int TCPsock = returned socket id detais.set_oca_port detais.set_oca_ipaddr detais.set_dest_port detais.set_dest_ipaddr assign socket bind(tcpsock, detais,...) bound connect(tcpsock,...) Fi Buffer with bytes to send Fi Buffer with more bytes to send connect(tcpsock,...) connected socket send(bytes, Buffer) return success/faiure send(bytes, Buffer) Estabish connection with destination Kerne handes a data transfer procedures of TCP Done sending a data; indicate finish to kerne cose(s) Kerne initiates connection teardown 35

36 Programming viewpoint Open TCP Socket RECVING USER PROCESS socket(sock_stream,...) KERNEL assign socket Receiver side int rtcpsock = returned socket id rtcpsock.set_oca_port rtcpsock.set_oca_ipaddr bind(tcpsock, detais,...) bound Indicate to kerne you're expecting a connection on this socket isten(rtcpsock,..) OK Now wait for a connection request accept(rtcpsock,...) connected A connection request received Now wait and read bytes when avaiabe Save received bytes; read more data recv(rtcpsock, Buffer) return numbytes recv(rtcpsock, Buffer) Kerne maintains TCP recv process; Receive data for connected socket Store in SOCKBUF; fi Buffer and notify user process Interpret as data finished from sender recv(rtcpsock, Buffer) return -1 cose(s) Cose connection request by sender Kerne handes connection teardown 36

37 TCP is not the idea for a appications TCP optimized for wired networks Performance is poor in wireess networks Appications with stringent deay requirements do not use TCP, because of possibe unbounded deays 37

38 Rea-Time Protoco Quaity of Service (QoS) factors: Reiabiity, Deay and Jitter Because of possiby unbounded retransmissions in TCP, arge deay and jitter may ensue. Appications prefer UDP instead. RTP protoco operates over UDP, and with header containing timestamp sequence number A marker bit Packet concatenation etc RTP provides no other correction strategies ike in TCP; Appications hande a aspects themseves. RTP modues run in user-space. RTP ibraries incuded in the appication. 38

39 Summary Numerous transport protocos proposed TCP sustained because of its distributed nature and because of the TCP/IP protoco suite that enabed computer systems to connect across boundaries Ampe scope exists for new transport protocos given proiferation of heterogeneous networks and devices 39

40 Homework (from 2 nd edition of text

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