TOC: Transport Protocols

Transcription

1 TOC: Transport Protocols Why? Overview UDP TCP Summary TOC Transport

2 Why? IP provides a weak, but efficient service model (best-effort) Packets can be delayed, dropped, reordered, duplicated Packets have limited size (why?) IP packets are addressed to a host How to decide which application gets which packets? How should hosts send into the network? Too fast is bad; too slow is not efficient TOC Transport Why?

3 Overview Basic Features Illustration Ports UDP TCP Headers TOC Transport Overview

4 Basic Features Can provide more reliability, in order delivery, at most once delivery Supports messages of arbitrary length Provide a way to decide which packets go to which applications (multiplexing/demultiplexing) Govern when hosts should send data TOC Transport Overview Basic Features

5 Illustration Application HTTP RA DNS ports p1 p2 p1 p2 p3 p1 p2 Transport IP A B C [A B p1 p2 ] UDP: Not reliable TCP: Ordered, reliable, well-paced TOC Transport Overview Illustration

6 Ports Need to decide which application gets which packets Solution: map each socket to a port Client must know server s port Separate 16-bit port address space for UDP and TCP (src IP, src port, dst IP, dst port) uniquely identifies TCP connection Well known ports (0-1023): everyone agrees which services run on these ports e.g., ssh:22, on UNIX, must be root to gain access to these ports (why?) ephemeral ports(most ): given to clients e.g. chatclient gets one of these TOC Transport Overview Ports

7 UDP User Datagram Protocol minimalist transport protocol same best-effort service model as IP messages of up to 64KB provides multiplexing/demultiplexing to IP does not provide congestion control advantage over TCP: does not increase end-to-end delay over IP application example: video/audio streaming TOC Transport Overview UDP

8 TCP Transmission Control Protocol reliable, in-order, and at most once delivery messages can be of arbitrary length provides multiplexing/demultiplexing to IP provides congestion control and avoidance increases end-to-end delay over IP e.g., file transfer, chat TOC Transport Overview TCP

9 Headers IP header used for IP routing, fragmentation, error detection (we study that when we explore IP) UDP header used for multiplexing/demultiplexing, error detection TCP header used for multiplexing/demultiplexing, flow and congestion control Sender Receiver data Application Application data TCP/UDP data TCP UDP TCP UDP TCP/UDP data IP TCP/UDP data IP IP IP TCP/UDP data TOC Transport Overview Headers

10 UDP Service: Send datagram from (IPa, Port 1) to (IPb, Port 2) Service is unreliable, but error detection possible Header: Source port Destination port UDP length UDP checksum Payload (variable) UDP length is UDP packet length (including UDP header and payload, but not IP header) Optional UDP checksum is over UDP packet Why have UDP checksum in addition to IP checksum? Why not have just the UDP checksum? Why is the UDP checksum optional? TOC Transport UDP

11 TCP Service Steps 3-Way Handshake State Diagram: 1 State Diagram: 2 Header Sliding Window Protocol TOC Transport TCP

12 Service Start a connection Reliable byte stream delivery from (IPa, TCP Port 1) to (IPb, TCP Port 2) Indication if connection fails: Reset Terminate connection TOC Transport TCP Service

13 Steps SYN n; ACK k+1 ACK k+n+1 SYN k DATA k+1; ACK n+1 3-way handshake data exchange FIN ACK FIN FIN FIN ACK ½ close ½ close TOC Transport TCP Steps

14 3WH Description Rationale TOC Transport TCP 3WH

15 Description Goal: agree on a set of parameters: the start sequence number for each side Starting sequence numbers are random. Client (initiator) Active connect() Open SYN, SeqNum = x SYN and ACK, SeqNum = y and Ack = x + 1 Server listen() accept() Passive Open ACK, Ack = y + 1 allocate buffer space TOC Transport TCP 3WH - Description

16 Rationale Three-way handshake adds 1 RTT delay Why? congestion control: SYN (40 byte) acts as cheap probe Protects against delayed packets from other connection (would confuse receiver) TOC Transport TCP 3WH - Rationale

17 State Diagram 1 Closed A SYN B SYN sent Established Data + ACK SYN + ACK ACK FIN Wait-1 FIN FIN.ack Timed Wait Closed FIN Wait-2 (1) FIN FIN.ack Listen SYN received Established Close Wait Last Ack Closed (1): A waits in case B retransmits FIN and A must ack again TOC Transport TCP State Diagram 1

18 State Diagram 2 TOC Transport TCP State Diagram 2

19 Header Source port Destination port Sequence number Acknowledgement HdrLen Flags Advertised window Checksum Urgent pointer Options (variable) Payload (variable) Sequence number, acknowledgement, and advertised window used by sliding-window based flow control Flags: SYN, FIN establishing/terminating a TCP connection ACK set when Acknowledgement field is valid URG urgent data; Urgent Pointer says where non-urgent data starts PUSH don t wait to fill segment RESET abort connection TOC Transport TCP Header

20 Sliding Window Protocol Objectives Stop & Wait Go-Back-n TOC Transport TCP SWP

21 Objectives Retransmit missing packets Numbering of packets and ACKs Do this efficiently Keep transmitting whenever possible Detect missing ACKs and retransmit quickly TOC Transport TCP SWP Objectives

22 Stop & Wait Send; wait for ack If timeout, retransmit; else repeat TRANS Sender DATA Receiver RTT ACK Inefficient if TRANS << RTT Time TOC Transport TCP SWP S&W

23 Go-Back-n (GBN) Definition Illustration without errors Illustration with errors Sliding window rules Sliding window example Observations Round-Trip Timing The question of ACKs TOC Transport TCP SWP GBN

24 Definition Transmit up to n unacknowledged packets If timeout for ACK(k), retransmit k, k+1, TOC Transport TCP SWP GBN Definition

25 Example without errors n = 9 packets in one RTT instead of 1 Fully efficient Time TOC Transport TCP SWP GBN No Errors

26 Example with errors Timeout Packet Window size = 3 packets Time Sender Receiver TOC Transport TCP SWP GBN Errors

27 Sliding Window Rules window = collection of adjacent sequence numbers the size of the collection is the window size Let A be the last ack d packet of sender without gap; then window of sender = {A+1, A+2,, A+n} Sender can send packets in its window Let B be the last received packet without gap by receiver, then window of receiver = {B+1,, B+n} Receiver can accept out of sequence, if in window TOC Transport TCP SWP GBN Rules

28 Sliding Window Example Last ACKed (without gap) Last received (without gap) TOC Transport TCP SWP GBN SW Ex.

29 Observations With sliding windows, it is possible to fully utilize a link, provided the window size is large enough. Throughput is ~ (w/rtt); Stop & Wait is like w = 1. Sender has to buffer all unacknowledged packets, because they may require retransmission Receiver may be able to accept out-of-order packets, but only up to its buffer limits TOC Transport TCP SWP GBN Observations

30 Timing Objective Illustration Adaptation Algorithm TOC Transport TCP SWP GBN Timing

31 Objective So, the sender needs to set timers in order to know when to retransmit a packet the may have been lost How long to set the timer for? Too short: may retransmit before data or ACK has arrived, creating duplicates Too long: if a packet is lost, will take a long time to recover (inefficient) TOC Transport TCP SWP GBN Timing Objective

32 Illustrations 1 1 RTT Timer too long Timer too short TOC Transport TCP SWP GBN Timing Illustrations

33 Adaptation The amount of time the sender should wait is about the round-trip time (RTT) between the sender and receiver For link-layer networks (LANs), this value is essentially known For multi-hop WANS, rarely known Must work in both environments, so protocol should adapt to the path behavior Measure successive ack delays T(n) Set timeout = average + 4 deviations TOC Transport TCP SWP GBN Timing Adaptation

34 Algorithm Use exponential averaging: A(n) = ba(n- 1) + (1 b)t(n) D(n) = bd(n-1) + (1 b) T(n) A(n) Timeout(n) = A(n) +4D(n) Notes: 1. Measure T(n) only for original transmissions 2. Double Timeout after timeout Justification: timeout indicates likely congestion; Further retransmissions would make things worse 3. Reset Timeout = A + 4D for new packet and when receive ACK TOC Transport TCP SWP GBN Timing Algorithm

35 The question of ACKs What exactly should the receiver ACK? Some possibilities: ACK every packet, giving its sequence number use cumulative ACK, where an ACK for number n implies ACKS for all k < n use negative ACKs (NACKs), indicating which packet did not arrive use selective ACKs (SACKs), indicating those that did arrive, even if not in order TOC Transport TCP SWP GBN Timing ACKs

36 Summary UDP: Multiplex, detect errors TCP: Reliable Byte Stream 3WH; Exchange; Close Reliable transmissions: ACKs S&W not efficient Go-Back-n What to ACK? (cumulative, ) Timer Value: based on measured RTT Next: Congestion and Flow Control TOC Transport Summary