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Chapter 3: roadmap Transport-layer services Multiplexing and demultiplexing Connectionless transport: UDP Principles of reliable data transfer Connection-oriented transport: TCP• segment structure• reliable data transfer• flow control• connection management Principles of congestion control TCP congestion controlTransport Layer: 3-1

TCP: overview RFCs: 793,1122, 2018, 5681, 7323 cumulative ACKs pipelining:• TCP congestion and flow controlset window size connection-oriented:• handshaking (exchange of controlmessages) initializes sender,receiver state before data exchange flow controlled:• sender will not overwhelm receiver point-to-point:• one sender, one receiver reliable, in-order bytestream:• no “message boundaries" full duplex data:• bi-directional data flow insame connection• MSS: maximum segment sizeTransport Layer: 3-2

TCP segment structuresource port # dest port #32 bitsnotused receive window flow control: # bytesreceiver willing to acceptsequence numbersegment seq #: countingbytes of data into bytestream(not segments!)applicationdata(variable length)data sent byapplication intoTCP socketAacknowledgement numberACK: seq # of next expectedbyte; A bit: this is an ACKoptions (variablelength)TCP optionsheadlenlength (of TCP header)checksumInternet checksumRST, SYN, FIN: connectionmanagementFSRUrg data pointerPUC EC, E: congestion notificationTransport Layer: 3-3RST: reset the connectionSyn: Synchronize the sequence numberFIN: Finish the connection

4The 4-bit header length field:•This specifies the length of the TCP header in 32-bit words•The TCP header can be of variable length due to the TCP options field•Typically, the options field is empty, so that the length of the typical TCP header is 20bytes- The flag field: contains 6 bits•The ACK bit is used to indicate that the value carried in the acknowledgment field isvalid, that is, the segment contains an acknowledgment for a segment that has beensuccessfully received•The RST, SYN, and FIN bits are used for connection setup and teardown•Other flags are typically not used

5Sequence Numbers and ACK Numbers- As discussed earlier for RDT protocols, the sequence number and ACK number arerequired for ensuring reliability- But, TCP interprets the sequence and ACK numbers differently from our earlystudy of RDT protocols- In RDT protocols discussed earlier, the sequence and ACK numbers areinterpreted to be the index of the data packet and the index of the data packetbeing acknowledged, respectively- But, TCP views the application data as an unstructured but ordered stream ofbytes rather than as a sequence of data packets- TCP’s use of sequence numbers reflects this view•That is the TCP sequence numbers are over the stream of transmitted bytes and notover the series of transmitted segments

6Thus the sequence number for a segment is therefore the byte-stream number of thefirst byte in the segment;for example•Suppose that the data stream consists of a file consisting of 500,000 bytes, that the MSS is1,000 bytes, and that the first byte of the data stream is numbered 0•Then, TCP constructs 500 segments out of the data stream•The first segment gets assigned sequence number 0, the second segment gets assignedsequence number 1,000, the third segment gets assigned sequence number 2,000, and soon•Each sequence number is inserted in the sequence number field in the header of theappropriate TCP segment

7TCP is full-duplex, so that Host A may be receiving data from Host B while it sends data to Host B (as partof the same TCP connection).Host B to Host AEach of the segments that arrive from Host B has a sequence number for the data flowing from B to A.The acknowledgment number that Host A puts in its segment is the sequence number of the next byteHost A is expecting from Host B.Suppose that Host A has received all bytes numbered 0 through 535 from B and suppose that it is aboutto send a segment to Host B. Host A is waiting for byte 536 and all the subsequent bytes in Host B’s datastream. So Host A puts 536 in the acknowledgment number field of the segment it sends to B..

9Host B to Host Asuppose that Host A has received one segment from Host B containing bytes 0 through 535 and anothersegment containing bytes 900 through 1,000.For some reason Host A has not yet received bytes 536 through 899.In this example, Host A is still waiting for byte 536 (and beyond) in order to re-create B’s data stream.Thus, A’s next segment to B will contain 536 in the acknowledgment number field.Because TCP only acknowledges bytes up to the first missing byte in the stream, TCP is said to providecumulative acknowledgments.

10Urgent DataFor example, if the segment sequence number is 15000 and the value of the urgent pointer is 200, thefirst byte of urgent data is the byte 15000 and the last byte is the byte 15200. The rest of the bytes in thesegment (if present) are non-urgent.

TCP sequence numbers, ACKshost ACKs receiptof echoed ‘C’host ACKs receipt of‘C’,echoes back ‘C’simple telnet scenarioHost BHost AUser types‘C’Seq=42, ACK=79, data = ‘C’Seq=79, ACK=43, data = ‘C’Seq=43, ACK=80Transport Layer: 3-11Telnet (short for "telecommunications network") is aclient/server application protocol that provides access to virtualterminals of remote systems on local area networks or theInternet.

12suppose the starting sequence numbers are 42 and 79 for the client and server, respectively.Recall that the sequence number of a segment is the sequence number of the first byte in the data field. Thus,the first segment sent from the client will have sequence number 42; the first segment sent from the server willhave sequence number 79.Recall that the acknowledgment number is the sequence number of the next byte of data that the host iswaiting for. After the TCP connection is established but before any data is sent, the client is waiting for byte 79and the server is waiting for byte 42.

TCP round trip time, timeoutQ: how to set TCP timeoutvalue? longer than RTT, but RTT varies! too short: premature timeout,unnecessary retransmissions too long: slow reaction tosegment lossQ: how to estimate RTT? SampleRTT:measured timefrom segment transmission untilACK receipt• ignore retransmissions SampleRTT will vary, wantestimated RTT “smoother”• average several recentmeasurements, not just currentSampleRTTTransport Layer: 3-13the SampleRTT values will fluctuate from segment to segment dueto congestion in the routers and to the varying load on the end systems

TCP round trip time, timeoutEstimatedRTT = (1- )*EstimatedRTT + *SampleRTT exponential weighted moving average (EWMA) influence of past sample decreases exponentially fast typical value:  = 0.125RTT: gaia.cs.umass.edu to fantasia.eurecom.fr1001502002503003501 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106time (seconnds)RTT(milliseconds)SampleRTT Estimated RTTRTT(milliseconds)RTT: gaia.cs.umass.edu to fantasia.eurecom.frsampleRTTEstimatedRTTtime(seconds) Transport Layer: 3-14

TCP sequence numbers, ACKsSequence numbers:• byte stream “number” offirst byte in segment’s datasource port # dest port #sequence numberacknowledgement numberchecksumrwndurg pointeroutgoing segment from receiverAsentACKedsent, not-yet ACKed(“in-flight”)usablebut notyet sentnotusablewindow sizeNsender sequence number spacesource port # dest port #sequence numberacknowledgement numberchecksumrwndurg pointeroutgoing segment from senderAcknowledgements:• seq # of next byte expectedfrom other side• cumulative ACKTransport Layer: 3-15

TCP round trip time, timeout timeout interval: EstimatedRTT plus “safety margin”• large variation in EstimatedRTT: want a larger safety marginTimeoutInterval = EstimatedRTT + 4*DevRTTestimated RTT “safety margin”DevRTT = (1-)*DevRTT + *|SampleRTT-EstimatedRTT|(typically,  = 0.25) DevRTT: EWMA of SampleRTT deviation from EstimatedRTT:Transport Layer: 3-16

18TCP maintains a single timer and sets the timeout value asTimeoutInterval = EstimatedRTT + 4 *DevRTTThink of the timer as being associated with the oldest unacknowledged segmentAn initial TimeoutInterval value of 1 second is recommended when the connection is established; assoon as a segment is received and EstimatedRTT is updated, the TimeoutInterval is computed using theformula aboveWhen a timeout occurs, the value of TimeoutInterval is doubled to avoid a premature timeout occurringfor a subsequent segment; as soon as a segment is received and EstimatedRTT is updated, theTimeoutInterval is computed using the formula aboveSimilarly, if application layer data is received but no timer is already running (this could happen, forexample, when the application did not generate data for a sufficiently long time) then theTimeoutInterval is computed using the formula above

19A Simplified TCP Sender (uses only timeouts to recover fromlost segments)- There are three major events related to data transmissionand retransmission in the simplified TCP sender:•Data received from application above•Timer timeout•ACK receipt

20- Data received from above• TCP receives data from the application, encapsulates the data ina segment, and passes the segment to IP• Each segment includes a sequence number that is the byte-stream number of the first data byte in the segment• If the timer is already not running for some other segment, TCPstarts the timer when the segment is passed to IP and sets theTimeoutInterval to a value that is calculated from EstimatedRTTand DevRTT- TimeoutTCP retransmits only the oldest unacknowledged segment andthen restarts the timer

21Arrival of an ACK (i.e., a segment containing the ACK bit set)• TCP compares the ACK value y with its variable SendBase• The TCP state variable SendBase is the sequence number ofthe oldest unacknowledged byte• Thus SendBase - 1 is the sequence number of the last byte thatis known to have been received correctly and in order at thereceiver• Since TCP uses cumulative acknowledgments, y acknowledgesthe receipt of all bytes before byte number y• If y > SendBase, then the ACK is acknowledging one or morepreviously unacknowledged segments; then the senderupdates its SendBase variable; it also restarts the timer if thereare any not-yet-acknowledged segments

TCP fast retransmitHost BHost AtimeoutACK=100ACK=100ACK=100ACK=100XSeq=92, 8 bytes of dataSeq=100, 20 bytes ofdataSeq=100, 20 bytes of dataReceipt of three duplicate ACKsindicates 3 segments receivedafter a missing segment – lostsegment is likely. So retransmit!if sender receives 3 additionalACKs for same data (“tripleduplicate ACKs”), resend unACKedsegment with smallest seq # likely that unACKed segment lost,so don’t wait for timeoutTCP fast retransmitTransport Layer: 3-25

26Mid Term exam portion till this slide

27Is TCP like Go-Back-N or Selective-Repeat?• TCP’s error-recovery mechanism is probably bestcategorized as a hybrid of GBN and SR protocols• - Features like Go-Back-N• TCP acknowledgments are cumulative and correctlyreceived but out-of-order segments are not individuallyACKed by the receiver• TCP sender need only maintain the smallest sequencenumber of a transmitted but unacknowledged byte(SendBase) and the sequence number of the next byte tobe sent (NextSeqNum)

33Features like Selective-Repeat• Many TCP implementations buffer correctly received butout-of-order segments• TCP retransmits only one segment when a timeout occursor triple-duplicate ACKs are recived, unlike Go-Back-N thatretransmits all outstanding segments• A proposed modification to TCP, the so-called selectiveacknowledgment [RFC 2018], allows a TCP receiver toacknowledge out-of-order segments selectively rather thanjust cumulatively acknowledging the last correctly received,in-order segment

TCP 3-way handshakeSYNbit=1, Seq=xchoose init seq num, xsend TCP SYN msgESTABSYNbit=1, Seq=yACKbit=1; ACKnum=x+1choose init seq num, ysend TCP SYNACKmsg, acking SYNACKbit=1, ACKnum=y+1received SYNACK(x)indicates server is live;send ACK for SYNACK;this segment may containclient-to-server datareceived ACK(y)indicates client is liveSYNSENTESTABSYN RCVDClient stateLISTENServer stateLISTENclientSocket = socket(AF_INET, SOCK_STREAM)serverSocket = socket(AF_INET,SOCK_STREAM)serverSocket.bind((‘’,serverPort))serverSocket.listen(1)connectionSocket, addr = serverSocket.accept()clientSocket.connect((serverName,serverPort))Transport Layer: 3-36

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