Univ. of TehranIntroduction to Computer Network1 An Introduction Computer Networks An Introduction to Computer Networks University of Tehran Dept. of EE and Computer Engineering By: Dr. Nasser Yazdani Transport Layer Lecture 13: Transport Layer

Univ. of TehranIntroduction to Computer Network2 Outline Connection Establishment/Termination Sliding Window Revisited Flow Control Adaptive Timeout

Univ. of TehranIntroduction to Computer Network3 End-to-End Protocols Underlying best-effort network drop messages re-orders messages delivers duplicate copies of a given message limits messages to some finite size delivers messages after an arbitrarily long delay Common end-to-end services guarantee message delivery deliver messages in the same order they are sent deliver at most one copy of each message support arbitrarily large messages support synchronization allow the receiver to flow control the sender support multiple application processes on each host

Univ. of TehranIntroduction to Computer Network4 Transport Layer Function(s) Multiplexing/demultiplexing between network application processes. Others? Error Detection within a segment Reliability Flow Control. Congestion control. Connection Management. Difference between Error detection and reliability? Difference between flow control and congestion control?

Univ. of TehranIntroduction to Computer Network5 Simple Demultiplexor (UDP) Unreliable and unordered datagram service Adds multiplexing No flow control Endpoints identified by ports servers have well-known ports see /etc/services on Unix Header format Optional checksum pseudo header + UDP header + data Pseudo header= Protocol num. +source and dest. IP address SrcPortDstPort ChecksumLength Data 01631

Univ. of TehranIntroduction to Computer Network6 Using UDP Non-standard protocols can be implemented on top of UDP. » Non-standard = non-TCP in practice » use the port addressing provided by UDP » implement their own reliability, flow control, ordering, congestion control  Examples: » remote procedure calls » multimedia » distributed computing communication libraries » look at some examples later

Univ. of TehranIntroduction to Computer Network7 TCP Overview Connection-oriented Byte-stream app writes bytes TCP sends segments app reads bytes Application process Write bytes TCP Send buffer Segment Transmit segments Application process Read bytes TCP Receive buffer … …… Full duplex Flow control: keep sender from overrunning receiver Congestion control: keep sender from overrunning network

Univ. of TehranIntroduction to Computer Network8 TCP Characteristics Connection-oriented reliable byte-stream protocol. » Used for file transfers, telnet, web access, …. Two way connections. » control information for one direction piggy-backed on data flow in other direction » header fields fall in three classes: general, forward flow, opposite flow Protocol has evolved over time and will continue to do so. » Nearly impossible to change the header » Uses options to add information to the header » Change processing at endpoints » Backward compatibility is what makes it TCP

Univ. of TehranIntroduction to Computer Network9 Data Link Versus Transport Potentially connects many different hosts (logical connection) need explicit connection establishment and termination Potentially different RTT need adaptive timeout mechanism Potentially reordering packets. (How far?), Maximum segment life time. Currently 120 sec. Potentially long delay in network need to be prepared for arrival of very old packets Delay X bandwidth? (window size?) Buffer size

Univ. of TehranIntroduction to Computer Network10 Data Link Versus Transport Data Link Versus Transport (cont) Potentially different capacity at destination need to accommodate different node capacity Potentially different network capacity need to be prepared for network congestion Differences with X.25? X.25 uses sliding window in hop by hop bases. It can not guarantee overall reliable transmission. Errors in the intermediate nodes.

Univ. of TehranIntroduction to Computer Network11 Data delivery in TCP TCP is byte oriented, however transmit data in segments (messages). How TCP knows the time for delivery? Maximum Segment Size (MSS) Pushed by the application. Like telnet. Timer

Univ. of TehranIntroduction to Computer Network12 Segment Format Options (variable) Data Checksum SrcPortDstPort HdrLen 0Flags UrgPtr AdvertisedWindow SequenceNum Acknowledgment

Univ. of TehranIntroduction to Computer Network13 Segment Format (cont) Each connection identified with 4-tuple: ( SrcPort, SrcIPAddr, DsrPort, DstIPAddr) Sliding window + flow control acknowledgment, SequenceNum, AdvertisedWinow Checksum pseudo header + TCP header + data Sender Data(SequenceNum) Acknowledgment + AdvertisedWindow Receiver

Univ. of TehranIntroduction to Computer Network14 Segment Format (cont) Flags SYN, FIN, RESET, PUSH, URG, ACK SYN and FIN for establish and tear down connection. ACK indicates the Ack field is valid. URG shows some part of data is urgent.(Up to UrgPtr). PUSH shows the sender had push operation. RESET shows confusion in receiver.

Univ. of TehranIntroduction to Computer Network15 Connection Establishment and Termination Active participant (client) Passive participant (server) SYN, SequenceNum = x SYN + ACK, SequenceNum = y, ACK, Acknowledgment = y + 1 Acknowledgment = x + 1 Client does active connection. Server must do passive connection first. Closing is symmetric, both side must tear down the connection. Three way handshaking. Connection parameter are exchanged first. Acknowledgment shows the next expected sequence number.

Univ. of TehranIntroduction to Computer Network16 State Transition Diagram CLOSED LISTEN SYN_RCVDSYN_SENT ESTABLISHED CLOSE_WAIT LAST_ACKCLOSING TIME_WAIT FIN_WAIT_2 FIN_WAIT_1 Passive openClose Send/SYN SYN/SYN + ACK SYN + ACK/ACK SYN/SYN + ACK ACK Close/FIN FIN/ACKClose/FIN FIN/ACK ACK + FIN/ACK Timeout after two segment lifetimes FIN/ACK ACK Close/FIN Close CLOSED Active open/SYN

Univ. of TehranIntroduction to Computer Network17 State Transition (cont) Sliding window is in ESTABLISHED. All connections start with CLOSED. Arcs are tagged with event/action. Triggering a transition. Arriving a segment from a peer. Invoking by an application.

Univ. of TehranIntroduction to Computer Network18 Sliding Window Revisited Sending side LastByteAcked < = LastByteSent LastByteSent < = LastByteWritten buffer bytes between LastByteAcked and LastByteWritten Sending application LastByteWritten TCP LastByteSentLastByteAcked Receiving application LastByteRead TCP LastByteRcvdNextByteExpected Receiving side LastByteRead < NextByteExpected NextByteExpected < = LastByteRcvd +1 buffer bytes between NextByteRead and LastByteRcvd

Univ. of TehranIntroduction to Computer Network19 Flow Control Send buffer size: MaxSendBuffer Receive buffer size: MaxRcvBuffer Receiving side LastByteRcvd - LastByteRead < = MaxRcvBuffer AdvertisedWindow = MaxRcvBuffer - ( NextByteExpected - NextByteRead ) Sending side LastByteSent - LastByteAcked < = AdvertisedWindow EffectiveWindow = AdvertisedWindow - ( LastByteSent - LastByteAcked ) LastByteWritten - LastByteAcked < = MaxSendBuffer block sender if ( LastByteWritten - LastByteAcked ) + y > MaxSenderBuffer Always send ACK in response to arriving data segment Persist when AdvertisedWindow = 0

Univ. of TehranIntroduction to Computer Network20 Adaptive Retransmission (Original Algorithm) Measure SampleRTT for each segment/ ACK pair Compute weighted average of RTT EstRTT =  x EstRTT +  x SampleRTT where  +  = 1   between 0.8 and 0.9   between 0.1 and 0.2 Set timeout based on EstRTT TimeOut = 2 x EstRTT

Univ. of TehranIntroduction to Computer Network21 Karn/Partridge Algorithm Do not sample RTT when retransmitting Double timeout after each retransmission Sender Receiver Original transmission ACK SampleRTT Retransmission SenderReceiver Original transmission ACK SampleRTT Retransmission

Univ. of TehranIntroduction to Computer Network22 Jacobson/ Karels Algorithm New Calculations for average RTT Diff = SampleRTT - EstRTT EstRTT = EstRTT + (  x Diff) Dev = Dev +  ( |Diff| - Dev) where  is a factor between 0 and 1 Consider variance when setting timeout value TimeOut =  x EstRTT +  x Dev where  = 1 and  = 4 Notes algorithm only as good as granularity of clock (500ms on Unix) accurate timeout mechanism important to congestion control (later)

Univ. of TehranIntroduction to Computer Network23 TCP Extensions Implemented using TCP options Timestamp Protection from sequence number wraparound Large windows SACK- Selected Ack

Univ. of TehranIntroduction to Computer Network24 Timestamp Extension Used to improve timeout mechanism by more accurate measurement of RTT When sending a packet, insert current timestamp into option 4 bytes for seconds, 4 bytes for microseconds Receiver echoes timestamp in ACK Actually will echo whatever is in timestamp Removes retransmission ambiguity Can get RTT sample on any packet

Univ. of TehranIntroduction to Computer Network25 Protection From Wraparound Wraparound time vs. Link speed 1.5Mbps: 6.4 hours 10Mbps: 57 minutes 45Mbps: 13 minutes 100Mbps: 6 minutes 622Mbps: 55 seconds  < MSL! 1.2Gbps: 28 seconds Use timestamp to distinguish sequence number wraparound

Univ. of TehranIntroduction to Computer Network26 Large Windows Delay-bandwidth product for 100ms delay 1.5Mbps: 18KB 10Mbps: 122KB > max 16bit window 45Mbps: 549KB 100Mbps: 1.2MB 622Mbps: 7.4MB 1.2Gbps: 14.8MB Scaling factor on advertised window Specifies how many bits window must be shifted to the left Scaling factor exchanged during connection setup

Univ. of TehranIntroduction to Computer Network27 Selected Ack If a segment was lost, what we should do First Option: Probably the other following packets has been lost. Then, do not wait, send all of them- Go back N Fast recovery Unnecessary retransmission. Just send that packet Slow recovery Retransmit all lost packets in one window. Then, Selected Ack.

Univ. of TehranIntroduction to Computer Network28 Maximum Segment Size (MSS) Exchanged at connection setup Typically pick MTU of local link What all does this effect? Efficiency Congestion control Retransmission Path MTU discovery Why should MTU match MSS?

Univ. of TehranIntroduction to Computer Network29 Problems with TCP Silly Window Syndrome: advertising small window sizes. Solution1: do not send small segments and advertise a small window. push Question? What we should do with push by application? Solution2:Delays ACKs or combine them: Question? Receiver does not know how long is safe to postpone ACK Solution3:Introduce timer, i.e, every 100 ms. Good for interactive applications like Telnet.

Univ. of TehranIntroduction to Computer Network30 Nagles’s Algorithm Self-clocking In sending any data If data & the window >= MSS send a full segment. else if there is unACKed data in flight buffer the new data until an ACK arrive else send all the new data now