Chapter 3 Transport Layer slides are modified from J. Kurose & K. Ross CPE 400 / 600 Computer Communication Networks Lecture 10

Transport Layer 2 Chapter 3 outline r 3.1 Transport-layer services r 3.2 Multiplexing and demultiplexing r 3.3 Connectionless transport: UDP r 3.4 Principles of reliable data transfer r 3.5 Connection-oriented transport: TCP r 3.6 Principles of congestion control r 3.7 TCP congestion control

Transport Layer 3 Principles of Reliable data transfer r important in app., transport, link layers r top-10 list of important networking topics! r characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt)

Transport Layer 4 Rdt1.0: reliable transfer over a reliable channel r underlying channel perfectly reliable  no bit errors  no loss of packets r separate FSMs for sender, receiver:  sender sends data into underlying channel  receiver read data from underlying channel Wait for call from above packet = make_pkt(data) udt_send(packet) rdt_send(data) sender extract (packet,data) deliver_data(data) Wait for call from below rdt_rcv(packet) receiver

Transport Layer 5 Rdt2.0: channel with bit errors r underlying channel may flip bits in packet  checksum to detect bit errors r the question: how to recover from errors:  acknowledgements (ACKs): receiver explicitly tells sender that pkt received OK  negative acknowledgements (NAKs): receiver explicitly tells sender that pkt had errors  sender retransmits pkt on receipt of NAK  new mechanisms in rdt2.0 (beyond rdt1.0 ):  error detection  receiver feedback: control msgs (ACK,NAK) rcvr->sender

Transport Layer 6 rdt2.0 has a fatal flaw! What happens if ACK/NAK corrupted? r sender doesn’t know what happened at receiver! r can’t just retransmit: possible duplicate Handling duplicates: r sender retransmits current pkt if ACK/NAK garbled r sender adds sequence number to each pkt r receiver discards (doesn’t deliver up) duplicate pkt Sender sends one packet, then waits for receiver response stop and wait

Transport Layer 7 rdt2.1: discussion Sender: r seq # added to pkt r two seq. #’s (0,1) will suffice. Why? r must check if received ACK/NAK corrupted r twice as many states  state must “remember” whether “current” pkt has 0 or 1 seq. # Receiver: r must check if received packet is duplicate  state indicates whether 0 or 1 is expected pkt seq # r note: receiver can not know if its last ACK/NAK received OK at sender

Transport Layer 8 rdt2.2: a NAK-free protocol r same functionality as rdt2.1, using ACKs only r instead of NAK, receiver sends ACK for last pkt received OK  receiver must explicitly include seq # of pkt being ACKed r duplicate ACK at sender results in same action as NAK: retransmit current pkt

Transport Layer 9 rdt3.0: channels with errors and loss New assumption: underlying channel can also lose packets (data or ACKs)  checksum, seq. #, ACKs, retransmissions will be of help, but not enough Approach: sender waits “reasonable” amount of time for ACK r retransmits if no ACK received in this time r if pkt (or ACK) just delayed (not lost):  retransmission will be duplicate, but use of seq. #’s already handles this  receiver must specify seq # of pkt being ACKed r requires countdown timer

Transport Layer 10 Performance of rdt3.0 r rdt3.0 works, but performance stinks r ex: 1 Gbps link, 15 ms prop. delay, 8000 bit packet: m U sender : utilization – fraction of time sender busy sending m 1KB pkt every 30 msec -> 33kB/sec thruput over 1 Gbps link m network protocol limits use of physical resources!

Transport Layer 11 rdt3.0: stop-and-wait operation first packet bit transmitted, t = 0 senderreceiver RTT last bit transmitted, t = L / R first packet bit arrives Last bit arrives, send ACK ACK arrives, send next packet, t = RTT + L / R

Transport Layer 12 Pipelined protocols Pipelining: sender allows multiple, “in-flight”, yet-to-be- acknowledged pkts  range of sequence numbers must be increased  buffering at sender and/or receiver r Two generic forms of pipelined protocols: go-Back-N, selective repeat

Transport Layer 13 Pipelining: increased utilization First bit transmitted, t = 0 senderreceiver RTT last bit transmitted, t = L / R first packet bit arrives last packet bit arrives, send ACK ACK arrives, send next packet, t = RTT + L / R last bit of 2 nd pckt arrives, send ACK last bit of 3 rd pckt arrives, send ACK Increase utilization by a factor of 3!

Transport Layer 14 Pipelining Protocols Go-back-N: big picture: r Sender can have up to N unacked packets in pipeline r Rcvr only sends cumulative acks  Doesn’t ack packet if there’s a gap r Sender has timer for oldest unacked packet  If timer expires, retransmit all unacked packets Selective Repeat: big pic r Sender can have up to N unacked packets in pipeline r Rcvr acks individual packets r Sender maintains timer for each unacked packet  When timer expires, retransmit only unack packet

Transport Layer 15 Go-Back-N Sender: r k-bit seq # in pkt header r “window” of up to N, consecutive unack’ed pkts allowed r ACK(n): ACKs all pkts up to, including seq # n - “cumulative ACK” m may receive duplicate ACKs (see receiver) r timer for each in-flight pkt r timeout(n): retransmit pkt n and all higher seq # pkts in window

Transport Layer 16 Selective repeat: sender, receiver windows

Transport Layer 17 Selective repeat: dilemma Example: r seq #’s: 0, 1, 2, 3 r window size=3 r receiver sees no difference in two scenarios! r incorrectly passes duplicate data as new in (a) Q: what relationship between seq # size and window size?

Transport Layer 18 Lecture 10 outline r 3.4 Principles of reliable data transfer  rdt (1.0, 2.0, 2.1, 2.2, 3.0)  Pipelined Protocols  Go-Back-N (GBN)  Selective Repeat (SR) r 3.5 Connection-oriented transport: TCP  segment structure  reliable data transfer  flow control  connection management

Transport Layer 19 TCP: Overview RFCs: 793, 1122, 1323, 2018, 2581 r point-to-point:  one sender, one receiver r reliable, in-order byte steam:  no “message boundaries” r pipelined:  TCP congestion and flow control set window size r send & receive buffers r full duplex data:  bi-directional data flow in same connection  MSS: maximum segment size r connection-oriented:  handshaking (exchange of control msgs) init’s sender, receiver state before data exchange r flow controlled:  sender will not overwhelm receiver

Transport Layer 20 TCP segment structure source port # dest port # 32 bits application data (variable length) sequence number acknowledgement number Receive window Urg data pnter checksum F SR PAU head len not used Options (variable length) URG: urgent data (generally not used) ACK: ACK # valid PSH: push data now (generally not used) RST, SYN, FIN: connection estab (setup, teardown) # bytes rcvr willing to accept counting by bytes of data (not segments!) Internet checksum (as in UDP)

Transport Layer 21 TCP seq. #’s and ACKs Seq. #’s:  byte stream “number” of first byte in segment’s data ACKs:  seq # of next byte expected from other side  cumulative ACK Q: how receiver handles out-of-order segments  A: TCP spec doesn’t say, - up to implementor Host A Host B Seq=42, ACK=79, data = ‘C’ Seq=79, ACK=43, data = ‘C’ Seq=43, ACK=80 User types ‘C’ host ACKs receipt of echoed ‘C’ host ACKs receipt of ‘C’, echoes back ‘C’ time simple telnet scenario

Transport Layer 22 TCP Round Trip Time and Timeout Q: how to set TCP timeout value? r longer than RTT  but RTT varies r too short: premature timeout  unnecessary retransmissions r too long: slow reaction to segment loss Q: how to estimate RTT?  SampleRTT : measured time from segment transmission until ACK receipt  ignore retransmissions  SampleRTT will vary, want estimated RTT “smoother”  average several recent measurements, not just current SampleRTT

Transport Layer 23 TCP Round Trip Time and Timeout EstimatedRTT = (1-  )*EstimatedRTT +  *SampleRTT r Exponential weighted moving average r influence of past sample decreases exponentially fast  typical value:  = 0.125

Transport Layer 24 Example RTT estimation:

Transport Layer 25 TCP Round Trip Time and Timeout Setting the timeout  EstimtedRTT plus “safety margin”  large variation in EstimatedRTT -> larger safety margin r first estimate of how much SampleRTT deviates from EstimatedRTT: TimeoutInterval = EstimatedRTT + 4*DevRTT DevRTT = (1-  )*DevRTT +  *|SampleRTT-EstimatedRTT| (typically,  = 0.25) Then set timeout interval:

Transport Layer 26 Lecture 10 outline r 3.4 Principles of reliable data transfer  rdt (1.0, 2.0, 2.1, 2.2, 3.0)  Pipelined Protocols  Go-Back-N (GBN)  Selective Repeat (SR) r 3.5 Connection-oriented transport: TCP  segment structure  reliable data transfer  flow control  connection management

Transport Layer 27 TCP reliable data transfer r TCP creates rdt service on top of IP’s unreliable service r Pipelined segments r Cumulative acks r TCP uses single retransmission timer r Retransmissions are triggered by:  timeout events  duplicate acks r Initially consider simplified TCP sender:  ignore duplicate acks  ignore flow control, congestion control

Transport Layer 28 TCP sender events: data rcvd from app: r Create segment with seq # r seq # is byte-stream number of first data byte in segment r start timer if not already running (think of timer as for oldest unacked segment)  expiration interval: TimeOutInterval timeout: r retransmit segment that caused timeout r restart timer Ack rcvd: r If acknowledges previously unacked segments  update what is known to be acked  start timer if there are outstanding segments

Transport Layer 29 TCP sender (simplified) NextSeqNum = InitialSeqNum SendBase = InitialSeqNum loop (forever) { switch(event) event: data received from application above create TCP segment with sequence number NextSeqNum if (timer currently not running) start timer pass segment to IP NextSeqNum = NextSeqNum + length(data) event: timer timeout retransmit not-yet-acknowledged segment with smallest sequence number start timer event: ACK received, with ACK field value of y if (y > SendBase) { SendBase = y if (there are currently not-yet-acknowledged segments) start timer } } /* end of loop forever */ Comment: SendBase-1: last cumulatively ack’ed byte Example: SendBase-1 = 71; y= 73, so the rcvr wants 73+ ; y > SendBase, so that new data is acked

Transport Layer 30 TCP: retransmission scenarios Host A Seq=100, 20 bytes data ACK=100 time premature timeout Host B Seq=92, 8 bytes data ACK=120 Seq=92, 8 bytes data Seq=92 timeout ACK=120 Host A Seq=92, 8 bytes data ACK=100 loss timeout lost ACK scenario Host B X Seq=92, 8 bytes data ACK=100 time Seq=92 timeout SendBase = 100 SendBase = 120 SendBase = 120 Sendbase = 100

Transport Layer 31 TCP retransmission scenarios (more) Host A Seq=92, 8 bytes data ACK=100 loss timeout Cumulative ACK scenario Host B X Seq=100, 20 bytes data ACK=120 time SendBase = 120

Transport Layer 32 TCP ACK generation [RFC 1122, RFC 2581] Event at Receiver Arrival of in-order segment with expected seq #. All data up to expected seq # already ACKed Arrival of in-order segment with expected seq #. One other segment has ACK pending Arrival of out-of-order segment higher-than-expect seq. #. Gap detected Arrival of segment that partially or completely fills gap TCP Receiver action Delayed ACK. Wait up to 500ms for next segment. If no next segment, send ACK Immediately send single cumulative ACK, ACKing both in-order segments Immediately send duplicate ACK, indicating seq. # of next expected byte Immediate send ACK, provided that segment starts at lower end of gap

Transport Layer 33 Fast Retransmit r Time-out period often relatively long:  long delay before resending lost packet r Detect lost segments via duplicate ACKs.  Sender often sends many segments back-to-back  If segment is lost, there will likely be many duplicate ACKs. r If sender receives 3 ACKs for the same data, it supposes that segment after ACKed data was lost:  fast retransmit: resend segment before timer expires

Transport Layer 34 Host A timeout Host B time X resend 2 nd segment Resending a segment after triple duplicate ACK

Transport Layer 35 event: ACK received, with ACK field value of y if (y > SendBase) { SendBase = y if (there are currently not-yet-acknowledged segments) start timer } else { increment count of dup ACKs received for y if (count of dup ACKs received for y = 3) { resend segment with sequence number y } Fast retransmit algorithm: a duplicate ACK for already ACKed segment fast retransmit

Transport Layer 36 Lecture 10 outline r 3.4 Principles of reliable data transfer  rdt (1.0, 2.0, 2.1, 2.2, 3.0)  Pipelined Protocols  Go-Back-N (GBN)  Selective Repeat (SR) r 3.5 Connection-oriented transport: TCP  segment structure  reliable data transfer  flow control  connection management

Transport Layer 37 TCP Flow Control r receive side of TCP connection has a receive buffer: r speed-matching service: matching the send rate to the receiving app’s drain rate r app process may be slow at reading from buffer sender won’t overflow receiver’s buffer by transmitting too much, too fast flow control

Transport Layer 38 TCP Flow control: how it works (Suppose TCP receiver discards out-of-order segments)  spare room in buffer = RcvWindow = RcvBuffer-[LastByteRcvd - LastByteRead]  Rcvr advertises spare room by including value of RcvWindow in segments  Sender limits unACKed data to RcvWindow  guarantees receive buffer doesn’t overflow

Transport Layer 39 Lecture 10 outline r 3.4 Principles of reliable data transfer  rdt (1.0, 2.0, 2.1, 2.2, 3.0)  Pipelined Protocols  Go-Back-N (GBN)  Selective Repeat (SR) r 3.5 Connection-oriented transport: TCP  segment structure  reliable data transfer  flow control  connection management

Transport Layer 40 TCP Connection Management Recall: TCP sender, receiver establish “connection” before exchanging data segments  initialize TCP variables: seq. #s, buffers, flow control info  client: connection initiator Socket clientSocket = new Socket("hostname","port number");  server: contacted by client Socket connectionSocket = welcomeSocket.accept(); Three way handshake: Step 1: client host sends TCP SYN segment to server  specifies initial seq #  no data Step 2: server host receives SYN, replies with SYNACK segment  server allocates buffers  specifies server initial seq. # Step 3: client receives SYNACK, replies with ACK segment, which may contain data

Transport Layer 41 TCP Connection Management (cont.) Closing a connection: client closes socket: clientSocket.close(); Step 1: client end system sends TCP FIN control segment to server Step 2: server receives FIN, replies with ACK. Closes connection, sends FIN. client FIN server ACK FIN close

Transport Layer 42 TCP Connection Management (cont.) Step 3: client receives FIN, replies with ACK.  Enters “timed wait” - will respond with ACK to received FINs Step 4: server, receives ACK. Connection closed. client FIN server ACK FIN closing closed timed wait closed

Transport Layer 43 TCP Connection Management (cont) TCP client lifecycle TCP server lifecycle

Transport Layer 44 Lecture 10 summary Connection-oriented transport: TCP r segment structure  Round Trip Time, timeout r reliable data transfer  ACK  Fast Retransmit r flow control  Receiver buffer, receiver window r connection management  Three way handshake  Connection termination