Transport Layer 3-1 Chapter 3: Transport Layer Our goals: r understand principles behind transport layer services: m Multiplexing/demultip lexing m reliable data transfer m flow control m congestion control r learn about transport layer protocols in the Internet: m UDP: connectionless transport m TCP: connection-oriented transport m TCP congestion control

Transport Layer 3-2 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 m segment structure m reliable data transfer m flow control m connection management r 3.6 Principles of congestion control r 3.7 TCP congestion control

Transport Layer 3-3 Transport services and protocols r provide logical communication between app processes running on different hosts r transport protocols run in end systems m send side: breaks app messages into segments, passes to network layer m rcv side: reassembles segments into messages, passes to app layer r more than one transport protocol available to apps m Internet: TCP and UDP application transport network data link physical application transport network data link physical logical end-end transport

Transport Layer 3-4 Transport vs. network layer r network layer: logical communication between hosts r transport layer: logical communication between processes m relies on, enhances, network layer services A B C D Sport:4625 Dport: 80 Sport:8050 Dport: 25

Transport Layer 3-5 Internet transport-layer protocols r reliable, in-order delivery (TCP) m congestion control m flow control m connection setup r unreliable, unordered delivery: UDP r services not available: m delay guarantees m bandwidth guarantees 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 application transport network data link physical logical end-end transport

Transport Layer 3-6 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 m segment structure m reliable data transfer m flow control m connection management r 3.6 Principles of congestion control r 3.7 TCP congestion control

Transport Layer 3-7 Multiplexing/demultiplexing process socket use header info to deliver received segments to correct socket demultiplexing at receiver: handle data from multiple sockets, add transport header (later used for demultiplexing) multiplexing at sender: transport application physical link network P2P1 transport application physical link network P4 transport application physical link network P3

Transport Layer 3-8 How demultiplexing works r host receives IP datagrams m each datagram has source IP address, destination IP address m each datagram carries transport-layer segment m each segment has source, destination port number r host uses IP addresses & port numbers to direct segment to appropriate socket source port #dest port # 32 bits application data (message) other header fields TCP/UDP segment format

Transport Layer 3-9 Connectionless demultiplexing (UDP) r Create a socket binding to a port number r UDP socket identified by two-tuple: ( dest IP address, dest port number) r When host receives UDP segment: m checks destination port number in segment m directs UDP segment to socket with that port number r IP datagrams with different source IP/port can be directed to same socket

Transport Layer 3-10 Connectionless demux (cont) Client IP:B P2 client IP: A P1 P3 server IP: C Port: 6428 SP: 6428 DP: 9157 SP: 9157 DP: 6428 SP: 6428 DP: 5775 SP: 5775 DP: 6428 Socket tuple: (dest IP address, dest port number) Two clients’ traffic can be mixed together at server

Transport Layer 3-11 Connection-oriented demux (TCP) r TCP socket identified by 4- tuple: m source IP address m source port number m dest IP address m dest port number r recv host uses all four values to direct segment to appropriate socket m Two connections cannot mixed together at the receiver host r Server host may support many simultaneous TCP sockets: m each socket identified by its own 4-tuple r Web servers have different sockets for each connecting client m Remember the fork() and new socket generated by accept()

Transport Layer 3-12 Connection-oriented demux: example transport application physical link network P3 transport application physical link P4 transport application physical link network P2 source IP,port: A,9157 dest IP, port: B,80 source IP,port: B,80 dest IP,port: A,9157 host: IP address A host: IP address C network P6 P5 P3 source IP,port: C,5775 dest IP,port: B,80 source IP,port: C,9157 dest IP,port: B,80 three segments, all destined to IP address: B, dest port: 80 are demultiplexed to different sockets server: IP address B

Transport Layer 3-13 Connection-oriented demux: example transport application physical link network P3 transport application physical link transport application physical link network P2 source IP,port: A,9157 dest IP, port: B,80 source IP,port: B,80 dest IP,port: A,9157 host: IP address A host: IP address C server: IP address B network P3 source IP,port: C,5775 dest IP,port: B,80 source IP,port: C,9157 dest IP,port: B,80 P4 threaded server

Transport Layer 3-14 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 m segment structure m reliable data transfer m flow control m connection management r 3.6 Principles of congestion control r 3.7 TCP congestion control

Transport Layer 3-15 UDP: User Datagram Protocol [RFC 768] r “no frills,” “bare bones” Internet transport protocol r “best effort” service, UDP segments may be: m lost m delivered out of order to app r connectionless: m no handshaking between UDP sender, receiver m each UDP segment handled independently of others Why is there a UDP? r no connection establishment (which can add delay) r simple: no connection state at sender, receiver r small segment header r no congestion control: UDP can blast away as fast as desired m UDP worm (Slammer)

Transport Layer 3-16 UDP-based Worm: Slammer r Worm code flow: m Exploit code (buffer overflow) m Generate random target IP address x: m Sendto() worm code to x on udp port 1434 r Fast spreading worm code (Jan. 2003) m Single UDP packet: 376 bytes m Average scan rate: 4000 scans/sec m Infect 90% in 10 minutes m ~ 100,000 infected in an hour r Bandwidth-limited worm m Severely congested Internet m Stopped ATM, Flight checking, … r TCP-based worm is much slower m TCP connection setup Connect() is a blocking call m Multiple threads for spreading

Transport Layer 3-17 UDP: more r often used for streaming multimedia apps m loss tolerant m rate sensitive r other UDP uses m DNS m SNMP r reliable transfer over UDP: add reliability at application layer m 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

Transport Layer 3-18 UDP checksum Sender: r treat segment contents as sequence of 16-bit integers r checksum: 1’s complement of addition of segment contents r sender puts checksum value into UDP checksum field Receiver: r Add all received 16-bit segments, including checksum r check if result is : m NO - error detected m YES - no error detected. But maybe errors nonetheless? More later …. Goal: detect “errors” (e.g., flipped bits) in transmitted segment

Transport Layer 3-19 Internet Checksum Example r Note m When adding numbers, a carryout from the most significant bit needs to be added to the result r Example: add two 16-bit integers wraparound sum checksum

Internet Checksum Example 2 r Suppose a 6-bytes packet content is m 0xABCC, 0x960B, 0x5A3D What is the checksum for this packet? 0x is a hexadecimal representation that each symbol (0-9, A-F) represents 4 bits binary within the value of For more details see: Normal summation: 0xABCC+0x960B+0x5A3D = 0x19C14 Wrap up carry-out value: 0x9C14 + 0x1 = 0x9C15 So the checksum is: 0xFFFF – 0x9C15 = 0x63EA Transport Layer 3-20

Transport Layer 3-21 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 m segment structure m reliable data transfer m flow control m connection management r 3.6 Principles of congestion control r 3.7 TCP congestion control

Transport Layer 3-22 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) Network layer u

Transport Layer 3-23 Reliable data transfer: getting started send side receive side rdt_send(): called from above, (e.g., by app.). Passed data to deliver to receiver upper layer udt_send(): called by rdt, to transfer packet over unreliable channel to receiver udt_rcv(): called when packet arrives on rcv-side of channel deliver_data(): called by rdt to deliver data to upper u

Transport Layer 3-24 Reliable data transfer: getting started We’ll: r incrementally develop sender, receiver sides of reliable data transfer protocol (rdt) r consider only unidirectional data transfer m but control info will flow on both directions! r use finite state machines (FSM) to specify sender, receiver state 1 state 2 event causing state transition actions taken on state transition state: when in this “state” next state uniquely determined by next event event actions

Transport Layer 3-25 Rdt1.0: reliable transfer over a reliable channel r Assumption: underlying channel perfectly reliable m no bit errors m no loss of packets r separate FSMs for sender, receiver: m sender sends data into underlying channel m receiver read data from underlying channel Wait for call from above packet = make_pkt(data) udt_send(packet) rdt_send(data) extract (packet,data) deliver_data(data) Wait for call from below udt_rcv(packet) sender receiver Only need to chop bit-stream data into packets and send Modern Internet packet has Maximum Transition Unit (MTU) of 1500 Bytes (Ethernet)

Transport Layer 3-26 Rdt2.0: channel with bit errors r Assumption #1: underlying channel may flip bits in packet m checksum to detect bit errors r Assumption # 2: no packet will be lost r the question: how to recover from errors: m acknowledgements (ACKs): receiver explicitly tells sender that pkt received OK m negative acknowledgements (NAKs): receiver explicitly tells sender that pkt had errors m sender retransmits pkt on receipt of NAK  new mechanisms in rdt2.0 (beyond rdt1.0 ): m Error detection (checksum) m Receiver feedback: control msgs (ACK,NAK) rcvr->sender m Sender retransmit if NAK

Transport Layer 3-27 rdt2.0: FSM specification Wait for call from above snkpkt = make_pkt(data, checksum) udt_send(sndpkt) extract(rcvpkt,data) deliver_data(data) udt_send(ACK) udt_rcv(rcvpkt) && notcorrupt(rcvpkt) udt_rcv(rcvpkt) && isACK(rcvpkt) udt_send(sndpkt) udt_rcv(rcvpkt) && isNAK(rcvpkt) udt_send(NAK) udt_rcv(rcvpkt) && corrupt(rcvpkt) Wait for ACK or NAK Wait for call from below sender receiver rdt_send(data)   means no action

Transport Layer 3-28 rdt2.0: operation with no errors Wait for call from above snkpkt = make_pkt(data, checksum) udt_send(sndpkt) extract(rcvpkt,data) deliver_data(data) udt_send(ACK) rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) rdt_rcv(rcvpkt) && isACK(rcvpkt) udt_send(sndpkt) rdt_rcv(rcvpkt) && isNAK(rcvpkt) udt_send(NAK) rdt_rcv(rcvpkt) && corrupt(rcvpkt) Wait for ACK or NAK Wait for call from below rdt_send(data) 

Transport Layer 3-29 rdt2.0: error scenario Wait for call from above snkpkt = make_pkt(data, checksum) udt_send(sndpkt) extract(rcvpkt,data) deliver_data(data) udt_send(ACK) udt_rcv(rcvpkt) && notcorrupt(rcvpkt) udt_rcv(rcvpkt) && isACK(rcvpkt) udt_send(sndpkt) rdt_rcv(rcvpkt) && isNAK(rcvpkt) udt_send(NAK) rdt_rcv(rcvpkt) && corrupt(rcvpkt) Wait for ACK or NAK Wait for call from below rdt_send(data) 

Transport Layer 3-30 rdt2.0 has a fatal flaw! What happens if ACK/NAK corrupted? r sender doesn’t know what happened at receiver! m Time-out and retransmit 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 3-31 rdt2.1: sender, handles garbled ACK/NAKs Wait for call 0 from above sndpkt = make_pkt(0, data, checksum) udt_send(sndpkt) rdt_send(data) Wait for ACK or NAK 0 udt_send(sndpkt) udt_rcv(rcvpkt) && ( corrupt(rcvpkt) || isNAK(rcvpkt) ) sndpkt = make_pkt(1, data, checksum) udt_send(sndpkt) rdt_send(data) udt_rcv(rcvpkt) && notcorrupt(rcvpkt) && isACK(rcvpkt) udt_send(sndpkt) udt_rcv(rcvpkt) && ( corrupt(rcvpkt) || isNAK(rcvpkt) ) udt_rcv(rcvpkt) && notcorrupt(rcvpkt) && isACK(rcvpkt) Wait for call 1 from above Wait for ACK or NAK 1  

Transport Layer 3-32 extract(rcvpkt,data) deliver_data(data) sndpkt = make_pkt(ACK, chksum) udt_send(sndpkt) rdt2.1: receiver, handles garbled ACK/NAKs Wait for 0 from below sndpkt = make_pkt(NAK, chksum) udt_send(sndpkt) udt_rcv(rcvpkt) && not corrupt(rcvpkt) && has_seq0(rcvpkt) udt_rcv(rcvpkt) && notcorrupt(rcvpkt) && has_seq1(rcvpkt) extract(rcvpkt,data) deliver_data(data) sndpkt = make_pkt(ACK, chksum) udt_send(sndpkt) Wait for 1 from below udt_rcv(rcvpkt) && notcorrupt(rcvpkt) && has_seq0(rcvpkt) udt_rcv(rcvpkt) && (corrupt(rcvpkt) sndpkt = make_pkt(ACK, chksum) udt_send(sndpkt) udt_rcv(rcvpkt) && not corrupt(rcvpkt) && has_seq1(rcvpkt) udt_rcv(rcvpkt) && (corrupt(rcvpkt) sndpkt = make_pkt(ACK, chksum) udt_send(sndpkt) sndpkt = make_pkt(NAK, chksum) udt_send(sndpkt) Why ACK for wrong sequence packet?

Transport Layer 3-33 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 m state must “remember” whether “current” pkt has 0 or 1 seq. # Receiver: r must check if received packet is duplicate m 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