EE 122 Transport Protocols UDP and TCP Computer Science Division Department of Electrical Engineering and Computer Science University of California Berkeley Berkeley CA 94720 1776 October 5 2004 Katz Stoica F04 Today s Lecture Application Transport Network IP Link Physical Katz Stoica F04 2 Outline Motivation Transport layer TCP UDP Katz Stoica F04 3 Motivation 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 packets into the network Too fast may overwhelm the network Too slow is not efficient Katz Stoica F04 4 Outline Motivation Transport layer TCP UDP Katz Stoica F04 5 Transport Layer Provide a way to decide which packets go to which applications multiplexing demultiplexing Can Provide reliability in order delivery at most once delivery Support messages of arbitrary length Govern when hosts should send data can implement congestion and flow control Katz Stoica F04 6 Congestion vs Flow Control Flow Control avoid overflowing the receiver Congestion Control avoid congesting the network What is network congestion Katz Stoica F04 7 Transport Layer cont d ports HTTP RA DNS Application p1 p2 p1 p2 p3 p1 p2 Transport IP A B C A B p1 p2 UDP Not reliable TCP Ordered reliable well paced Katz Stoica F04 8 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 http 80 On UNIX must be root to gain access to these ports why Ephemeral ports most 1024 65535 given to clients e g chat clients p2p networks Katz Stoica F04 9 Headers IP IP header used for IP routing fragmentation error detection UDP header used for multiplexing demultiplexing error detection TCP header used for multiplexing demultiplexing flow and congestion control Receiver data Sender Application Application TCP UDP data TCP UDP TCP UDP TCP UDP data IP IP data TCP UDP data TCP UDP data Katz Stoica F04 10 IP Outline Motivation Transport Layer UDP TCP Katz Stoica F04 11 UDP User Unreliable Data Protocol Minimalist transport protocol Same best effort service model as IP Messages up to 64KB Provides multiplexing demultiplexing to IP Does not provide flow and congestion control Application examples video audio streaming Katz Stoica F04 12 UDP Service Header Service Send datagram from IPa Port1 to IPb Port2 Service is unreliable but error detection possible Header 0 16 Source port UDP length 31 Destination port 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 Katz Stoica F04 13 Outline Motivation Transport Layer UDP TCP Katz Stoica F04 14 TCP Transport Control Protocol Reliable in order and at most once delivery Stream oriented messages can be of arbitrary length Provides multiplexing demultiplexing to IP Provides congestion control and avoidance Application examples file transfer chat Katz Stoica F04 15 TCP Service 1 2 Open connection 3 way handshaking Reliable byte stream transfer from IPa TCP Port1 to IPb TCP Port2 3 Indication if connection fails Reset Close tear down connection Katz Stoica F04 16 Open Connection 3 Way Handshaking Goal agree on a set of parameters the start sequence number for each side Starting sequence numbers are random Server Client initiator Active connect Open SYN Seq Num x k x 1 c A d n a y qNum e S K C A SYN and ACK Ack y 1 listen accept Passive Open allocate buffer space Katz Stoica F04 17 3 Way Handshaking cont d 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 Katz Stoica F04 18 Close Connection Two Army Problem Goal both sides agree to close the connection Two army problem Two blue armies need to simultaneously attack the white army to win otherwise they will be defeated The blue army can communicate only across the area controlled by the white army which can intercept the messengers What is the solution Katz Stoica F04 19 Close Connection 4 ways tear down connection Host 1 Host 2 FIN close FIN ACK FIN FIN ACK timeout Avoid reincarnation Can retransmit FIN ACK if it is lost close closed Katz Stoica F04 20 Reliable Transfer Retransmit missing packets Numbering of packets and ACKs Do this efficiently Keep transmitting whenever possible Detect missing ACKs and retransmit quickly Two schemes Stop Wait Sliding Window Go back n and Selective Repeat Katz Stoica F04 21 Stop Wait Send wait for ack If timeout retransmit else repeat TRANS DATA Receiver Sender RTT ACK Time Inefficient if TRANS RTT Katz Stoica F04 22 Sliding Window window set of adjacent sequence numbers The size of the set is the window size Assume window size is n 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 Katz Stoica F04 23 Go Back n GBN Transmit up to n unacknowledged packets If timeout for ACK k retransmit k k 1 Katz Stoica F04 24 GBN Example w o Errors Sender Window 1 1 2 1 2 3 2 3 4 3 4 5 4 5 6 1 2 3 4 5 6 Window size 3 packets Receiver Window Time Sender Receiver Katz Stoica F04 25 GBN Example with Errors Timeout Packet 4 1 2 3 4 5 6 4 5 6 Sender Window size 3 packets 5 5 6 4 is missing Receiver Katz Stoica F04 26 Selective Repeat SR Sender transmit up to n unacknowledged packets assume packet k is lost Receiver indicate packet k is missing Sender retransmit packet k Katz Stoica F04 27 SR Example with Errors 1 1 2 1 2 3 2 3 4 3 4 5 4 5 6 1 2 3 Window size 3 packets 4 5 6 4 5 6 4 Nack 4 Time 7 7 Sender Receiver Katz Stoica F04 28 Observations With sliding windows it is possible to fully utilize a link provided the window size is large enough Throughput is n RTT Stop Wait is like n 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 Katz Stoica F04 29 TCP Flow Control Each byte has a sequence number Initial sequence
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