1 15-441 Computer Networking Lecture 15 – Transport Protocols Copyright ©, 2007-10 Carnegie Mellon University 7 Outline • Transport introduction • Error recovery & flow control 8 Transport Protocols • Lowest level end-to-end protocol. • Header generated by sender is interpreted only by the destination • Routers view transport header as part of the payload 7 6 5 7 6 5 Transport IP Datalink Physical Transport IP Datalink Physical IP router 2 2 1 1 9 Functionality Split • Network provides best-effort delivery • End-systems implement many functions • Reliability • In-order delivery • Demultiplexing • Message boundaries • Connection abstraction • Congestion control • …2 10 Transport Protocols • UDP provides just integrity and demux • TCP adds… • Connection-oriented • Reliable • Ordered • Point-to-point • Byte-stream • Full duplex • Flow and congestion controlled 11 UDP: User Datagram Protocol [RFC 768] • “No frills,” “bare bones” Internet transport protocol • “Best effort” service, UDP segments may be: • Lost • Delivered out of order to app • Connectionless: • No handshaking between UDP sender, receiver • Each UDP segment handled independently of others Why is there a UDP? • No connection establishment (which can add delay) • Simple: no connection state at sender, receiver • Small header • No congestion control: UDP can blast away as fast as desired 12 UDP, cont. • Often used for streaming multimedia apps • Loss tolerant • Rate sensitive • Other UDP uses (why?): • DNS, SNMP • Reliable transfer over UDP • Must be at application layer • Application-specific error recovery Source port # Dest port # 32 bits Application data (message) UDP segment format Length Checksum Length, in bytes of UDP segment, including header 13 UDP Checksum Sender: • Treat segment contents as sequence of 16-bit integers • Checksum: addition (1’s complement sum) of segment contents • Sender puts checksum value into UDP checksum field Receiver: • Compute checksum of received segment • Check if computed checksum equals checksum field value: • NO - error detected • YES - no error detected But maybe errors nonethless? Goal: detect “errors” (e.g., flipped bits) in transmitted segment – optional use!3 14 High-Level TCP Characteristics • Protocol implemented entirely at the ends • Fate sharing • Protocol has evolved over time and will continue to do so • Nearly impossible to change the header • Use options to add information to the header • Change processing at endpoints • Backward compatibility is what makes it TCP 15 TCP Header Source port Destination port Sequence number Acknowledgement Advertised window HdrLen Flags 0 Checksum Urgent pointer Options (variable) Data Flags: SYN FIN RESET PUSH URG ACK 16 Evolution of TCP 1975 1980 1985 1990 1982 TCP & IP RFC 793 & 791 1974 TCP described by Vint Cerf and Bob Kahn In IEEE Trans Comm 1983 BSD Unix 4.2 supports TCP/IP 1984 Nagel’s algorithm to reduce overhead of small packets; predicts congestion collapse 1987 Karn’s algorithm to better estimate round-trip time 1986 Congestion collapse observed 1988 Van Jacobson’s algorithms congestion avoidance and congestion control (most implemented in 4.3BSD Tahoe) 1990 4.3BSD Reno fast retransmit delayed ACK’s 1975 Three-way handshake Raymond Tomlinson In SIGCOMM 75 17 TCP Through the 1990s 1993 1994 1996 1994 ECN (Floyd) Explicit Congestion Notification 1993 TCP Vegas (Brakmo et al) delay-based congestion avoidance 1994 T/TCP (Braden) Transaction TCP 1996 SACK TCP (Floyd et al) Selective Acknowledgement 1996 Hoe NewReno startup and loss recovery 1996 FACK TCP (Mathis et al) extension to SACK4 18 Outline • Transport introduction • Error recovery & flow control 19 Stop and Wait Time Packet ACK Timeout • ARQ • Receiver sends acknowledgement (ACK) when it receives packet • Sender waits for ACK and timeouts if it does not arrive within some time period • Simplest ARQ protocol • Send a packet, stop and wait until ACK arrives Sender Receiver 20 Recovering from Error Packet ACK Timeout Packet ACK Timeout Packet Timeout Packet ACK Timeout Time Packet ACK Timeout Packet ACK Timeout ACK lost Packet lost Early timeout DUPLICATE PACKETS!!! 21 • How to recognize a duplicate • Performance • Can only send one packet per round trip Problems with Stop and Wait5 22 How to Recognize Resends? • Use sequence numbers • both packets and acks • Sequence # in packet is finite How big should it be? • For stop and wait? • One bit – won’t send seq #1 until received ACK for seq #0 Pkt 0 ACK 0 Pkt 0 ACK 1 Pkt 1 ACK 0 23 How to Keep the Pipe Full? • Send multiple packets without waiting for first to be acked • Number of pkts in flight = window • Reliable, unordered delivery • Several parallel stop & waits • Send new packet after each ack • Sender keeps list of unack’ed packets; resends after timeout • Receiver same as stop & wait • How large a window is needed? • Suppose 10Mbps link, 4ms delay, 500byte pkts • 1? 10? 20? • Round trip delay * bandwidth = capacity of pipe 24 Sliding Window • Reliable, ordered delivery • Receiver has to hold onto a packet until all prior packets have arrived • Why might this be difficult for just parallel stop & wait? • Sender must prevent buffer overflow at receiver • Circular buffer at sender and receiver • Packets in transit ≤ buffer size • Advance when sender and receiver agree packets at beginning have been received 25 Receiver Sender Sender/Receiver State … … Sent & Acked Sent Not Acked OK to Send Not Usable … … Max acceptable Receiver window Max ACK received Next seqnum Received & Acked Acceptable Packet Not Usable Sender window Next expected6 26 Sequence Numbers • How large do sequence numbers need to be? • Must be able to detect wrap-around • Depends on sender/receiver window size • E.g. • Max seq = 7, send win=recv win=7 • If pkts 0..6 are sent succesfully and all acks lost • Receiver expects 7,0..5, sender retransmits old 0..6!!! • Max sequence must be ≥ send window +
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