Transport Layer

1. To learn about transport layer protocols in the Internet: TCP: connection-oriented protocol Reliability protocol UDP: connectionless protocol Transport Layer Transport Layer

2. provide logical communication between app processes running on different hosts transport protocols run in end systems send side: breaks app messages into segments, passes to network layer rcv side: reassembles segments into messages, passes to app layer more than one transport protocol available to apps Internet: TCP and UDP application transport network data link physical application transport network data link physical logical end-end transport Transport services and protocols Transport Layer

3. reliable, in-order delivery (TCP) connection oriented Unreliable delivery: UDP Not connection oriented application transport network data link physical application transport network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical logical end-end transport Internet transport-layer protocols Transport Layer

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

5. stop and wait What is reliability? Sender sends one packet, then waits for receiver response Transport Layer

6. Stop & Wait protocol Transport Layer

7. Practical scenario: underlying channel can lose packets (data or ACKs) because of many reasons What if data packet gets lost? What if Ack packet gets lost? World is not ideal Transport Layer

8. Practical scenario: underlying channel can lose packets (data or ACKs) because of many reasons What if data packet gets lost? What if Ack packet gets lost? Approach: sender waits “reasonable” amount of time for ACK retransmits if no ACK received in this time World is not ideal Transport Layer

9. Stop & wait protocol with loss Transport Layer

10. Practical scenario: underlying channel can lose packets (data or ACKs) because of many reasons What if Ack packet just delayed (not lost) Approach: sender waits “reasonable” amount of time for ACK retransmits if no ACK received in this time 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 New constraint: Stop & wait protocol with delay Transport Layer

11. Stop & wait protocol with delay Transport Layer

12. Performance of stop-and-wait sender receiver first packet bit transmitted, t = 0 last packet bit transmitted, t = L / R first packet bit arrives RTT last packet bit arrives, send ACK ACK arrives, send next packet, t = RTT + L / R Transport Layer

13. Pipelining: sender allows multiple, “in-flight”, yet-to-be-acknowledged pkts range of sequence numbers must be increased buffering at sender and/or receiver Pipelined protocols Transport Layer

14. Pipelining: increased utilization sender receiver first packet bit transmitted, t = 0 last bit transmitted, t = L / R first packet bit arrives RTT last packet bit arrives, send ACK last bit of 2nd packet arrives, send ACK last bit of 3rd packet arrives, send ACK ACK arrives, send next packet, t = RTT + L / R Increase utilization by a factor of 3! • Two generic forms of pipelined protocols: go-Back-N, selective repeat Transport Layer

15. Go-back-N: overview sender: up to N unACKed pkts in pipeline receiver: only sends cumulative ACKs doesn’t ACK pkt if there’s a gap sender: has timer for oldest unACKed pkt if timer expires: retransmit all unACKed packets Selective Repeat: overview sender: up to N unACKed packets in pipeline receiver: ACKs individual pkts sender: maintains timer for each unACKed pkt if timer expires: retransmit only unACKed packet Pipelining Protocols Transport Layer

16. GBN inaction Transport Layer

17. receiver individually acknowledges all correctly received pkts buffers pkts, as needed, for eventual in-order delivery to upper layer sender only resends pkts for which ACK not received sender timer for each unACKed pkt sender window N consecutive seq #’s again limits seq #s of sent, unACKed pkts Selective Repeat Transport Layer

18. Selective repeat in action Transport Layer

19. 32 bits source port # dest port # sequence number acknowledgement number head len not used Receive window U A P R S F checksum Urg data pointer Options (variable length) application data (variable length) TCP segment structure URG: urgent data (generally not used) counting by bytes of data (not segments!) ACK: ACK # valid PSH: push data now (generally not used) # bytes rcvr willing to accept RST, SYN, FIN: connection estab (setup, teardown commands) Internet checksum (as in UDP) Transport Layer

20. 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 implementer time TCP seq. #’s and ACKs Host B Host A User types ‘C’ Seq=42, ACK=79, data = ‘C’ host ACKs receipt of ‘C’, echoes back ‘C’ Seq=79, ACK=43, data = ‘C’ host ACKs receipt of echoed ‘C’ Seq=43, ACK=80 simple telnet scenario Transport Layer

21. Q: how to set TCP timeout value? longer than RTT but RTT varies too short: premature timeout unnecessary retransmissions 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 TCP Round Trip Time and Timeout Transport Layer

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

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

24. time-out period often relatively long: long delay before resending lost packet 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 for that segment If sender receives 3 ACKs for same data, it assumes that segment after ACKed data was lost: fast retransmit:resend segment before timer expires Fast Retransmit Transport Layer

25. Host A Host B seq # x1 seq # x2 seq # x3 ACK x1 X seq # x4 seq # x5 ACK x1 ACK x1 ACK x1 triple duplicate ACKs resend seq X2 timeout time Transport Layer

26. TCP segment structure reliable data transfer flow control congestion control outline Transport Layer

27. receive side of TCP connection has a receive buffer: speed-matching service: matching send rate to receiving application’s drain rate flow control sender won’t overflow receiver’s buffer by transmitting too much, too fast (currently) unused buffer space application process IP datagrams TCP data (in buffer) TCP Flow Control • app process may be slow at reading from buffer Transport Layer

28. (suppose TCP receiver discards out-of-order segments) unused buffer space: = rwnd = RcvBuffer-[LastByteRcvd - LastByteRead] receiver: advertises unused buffer space by including rwnd value in segment header sender: limits # of unACKed bytes to rwnd guarantees receiver’s buffer doesn’t overflow (currently) unused buffer space application process IP datagrams TCP data (in buffer) rwnd RcvBuffer TCP Flow control: how it works Transport Layer

29. Next: Principles of Congestion Control Transport Layer