1、Transport Layer,3-1,Chapter 3: Transport Layer,Our goals: understand principles behind transport layer services: multiplexing/demultiplexing reliable data transfer flow control congestion control,learn about transport layer protocols in the Internet: UDP: connectionless transport TCP: connection-ori
2、ented transport TCP congestion control,Transport Layer,3-2,Chapter 3 outline,3.1 Transport-layer services 3.2 Multiplexing and demultiplexing 3.3 Connectionless transport: UDP 3.4 Principles of reliable data transfer,3.5 Connection-oriented transport: TCP segment structure reliable data transfer flo
3、w control connection management 3.6 Principles of congestion control 3.7 TCP congestion control,Transport Layer,3-3,Transport services and protocols,provide logical communication between app processes running on different hosts transport protocols run in end systems send side: breaks app messages in
4、to 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,Transport Layer,3-4,Transport vs. network layer,network layer: logical communication between hosts transport layer: logical
5、communication between processes 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,reliable, in-order delivery (TCP) congestion control flow control connection setup unreliable, unordered delivery: UDP
6、services not available: delay guarantees bandwidth guarantees,Transport Layer,3-6,Chapter 3 outline,3.1 Transport-layer services 3.2 Multiplexing and demultiplexing 3.3 Connectionless transport: UDP 3.4 Principles of reliable data transfer,3.5 Connection-oriented transport: TCP segment structure rel
7、iable data transfer flow control connection management 3.6 Principles of congestion control 3.7 TCP congestion control,Transport Layer,3-7,Multiplexing/demultiplexing,= process,= socket,delivering received segments to correct socket,gathering data from multiple sockets, enveloping data with header (
8、later used for demultiplexing),application,transport,network,link,physical,P1,application,transport,network,link,physical,application,transport,network,link,physical,P2,P3,P4,P1,host 1,host 2,host 3,Transport Layer,3-8,How demultiplexing works,host receives IP datagrams each datagram has source IP a
9、ddress, destination IP address each datagram carries transport-layer segment each segment has source, destination port number 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
10、 format,Transport Layer,3-9,Connectionless demultiplexing (UDP),Create a socket binding to a port numberUDP socket identified by two-tuple: (dest IP address, dest port number),When host receives UDP segment: checks destination port number in segment directs UDP segment to socket with that port numbe
11、r IP datagrams with different source IP/port can be directed to same socket,Transport Layer,3-10,Connectionless demux (cont),Client IP:B,server IP: C Port: 6428,SP: 9157,DP: 6428,Socket tuple: (dest IP address, dest port number) Two clients traffic can be mixed together at server,Transport Layer,3-1
12、1,Connection-oriented demux (TCP),TCP socket identified by 4-tuple: source IP address source port number dest IP address dest port number recv host uses all four values to direct segment to appropriate socket Two connections cannot mixed together at the receiver host,Server host may support many sim
13、ultaneous TCP sockets: each socket identified by its own 4-tuple Web servers have different sockets for each connecting client Remember the fork() and new socket generated by accept(),Transport Layer,3-12,Connection-oriented demux (cont),Client IP:B,server IP: C,SP: 9157,DP: 80,D-IP:C,S-IP: A,D-IP:C
14、S-IP: B,D-IP:C,S-IP: B,Transport Layer,3-13,Connection-oriented demux: Threaded Web Server,Client IP:B,server IP: C Port: 80,SP: 9157,DP: 80,P4,D-IP:C,S-IP: A,D-IP:C,S-IP: B,D-IP:C,S-IP: B,Transport Layer,3-14,Chapter 3 outline,3.1 Transport-layer services 3.2 Multiplexing and demultiplexing 3.3 Co
15、nnectionless transport: UDP 3.4 Principles of reliable data transfer,3.5 Connection-oriented transport: TCP segment structure reliable data transfer flow control connection management 3.6 Principles of congestion control 3.7 TCP congestion control,Transport Layer,3-15,UDP: User Datagram Protocol RFC
16、 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 (whic
17、h can add delay) simple: no connection state at sender, receiver small segment header no congestion control: UDP can blast away as fast as desired UDP worm (Slammer),Transport Layer,3-16,UDP-based Worm: Slammer,Worm code flow: Exploit code (buffer overflow) Generate random target IP address x: Send(
18、) worm code to x on udp port 1434,Fast spreading worm code (Jan. 2003) Single UDP packet: 376 bytes Average scan rate: 4000 scans/sec Infect 90% in 10 minutes 100,000 infected in an hour,Bandwidth-limited worm Severely congested Internet Stopped ATM, Flight checking, ,TCP-based worm is much slower T
19、CP connection setup Connect() is a blocking call Multiple threads for spreading,Transport Layer,3-17,UDP: more,often used for streaming multimedia apps loss tolerant rate sensitive other UDP uses DNS SNMP reliable transfer over UDP: add reliability at application layer application-specific error rec
20、overy!,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: treat segment contents as sequence of 16-bit integers checksum: 1s complement of addition of segment cont
21、ents sender puts checksum value into UDP checksum field,Receiver: Add all received 16-bit segments, including checksum check if result is 1111 1111 1111 1111: NO - error detected YES - no error detected. But maybe errors nonetheless? More later .,Goal: detect “errors” (e.g., flipped bits) in transmi
22、tted segment,Transport Layer,3-19,Internet Checksum Example,Note When adding numbers, a carryout from the most significant bit needs to be added to the result Example: add two 16-bit integers,1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 1 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 11 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 11 1 0 1
23、1 1 0 1 1 1 0 1 1 1 1 0 0 1 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1,wraparound,sum,checksum,Transport Layer,3-20,Chapter 3 outline,3.1 Transport-layer services 3.2 Multiplexing and demultiplexing 3.3 Connectionless transport: UDP 3.4 Principles of reliable data transfer,3.5 Connection-oriented transport: TC
24、P segment structure reliable data transfer flow control connection management 3.6 Principles of congestion control 3.7 TCP congestion control,Transport Layer,3-21,Principles of Reliable data transfer,important in app., transport, link layers top-10 list of important networking topics!,characteristic
25、s of unreliable channel will determine complexity of reliable data transfer protocol (rdt),Network layer,Transport Layer,3-22,Reliable data transfer: getting started,send side,receive side,Transport Layer,3-23,Reliable data transfer: getting started,Well: incrementally develop sender, receiver sides
26、 of reliable data transfer protocol (rdt) consider only unidirectional data transfer but control info will flow on both directions! use finite state machines (FSM) to specify sender, receiver,event causing state transition,actions taken on state transition,state: when in this “state” next state uniq
27、uely determined by next event,Transport Layer,3-24,Rdt1.0: reliable transfer over a reliable channel,Assumption: underlying channel perfectly reliable no bit errors no loss of packets separate FSMs for sender, receiver: sender sends data into underlying channel receiver read data from underlying cha
28、nnel,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,rdt_rcv(packet),sender,receiver,Only need to chop bit-stream data into packets and send,Modern Internet packet has Maximum Transition Unit (MTU) of
29、1500 Bytes (Ethernet),Transport Layer,3-25,Rdt2.0: channel with bit errors,Assumption #1: underlying channel may flip bits in packet checksum to detect bit errors Assumption # 2: no packet will be lost the question: how to recover from errors: acknowledgements (ACKs): receiver explicitly tells sende
30、r that pkt received OK negative acknowledgements (NAKs): receiver explicitly tells sender that pkt had errors sender retransmits pkt on receipt of NAK new mechanisms in rdt2.0 (beyond rdt1.0): Error detection (checksum) Receiver feedback: control msgs (ACK,NAK) rcvr-sender Sender retransmit if NAK,T
31、ransport Layer,3-26,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),rdt_rcv(rcvpkt) & notcorrupt(rcvpkt),rdt_rcv(rcvpkt) & isACK(rcvpkt),udt_send(sndpkt),rdt_rcv(rcvpkt) &isNAK(rcvpkt),sender,
32、receiver,rdt_send(data),L,L : means no action,Transport Layer,3-27,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),u
33、dt_send(sndpkt),rdt_rcv(rcvpkt) &isNAK(rcvpkt),Wait for call from below,rdt_send(data),L,Transport Layer,3-28,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),rdt_rcv(rcvpkt) & notcorrupt(rcvpkt),
34、rdt_rcv(rcvpkt) & isACK(rcvpkt),udt_send(sndpkt),rdt_rcv(rcvpkt) &isNAK(rcvpkt),Wait for call from below,rdt_send(data),L,Transport Layer,3-29,rdt2.0 has a fatal flaw!,What happens if ACK/NAK corrupted? sender doesnt know what happened at receiver! Time-out and retransmit cant just retransmit: possi
35、ble duplicate,Handling duplicates: sender retransmits current pkt if ACK/NAK garbled sender adds sequence number to each pkt receiver discards (doesnt deliver up) duplicate pkt,Sender sends one packet, then waits for receiver response,Transport Layer,3-30,rdt2.1: sender, handles garbled ACK/NAKs,Wai
36、t for call 0 from above,sndpkt = make_pkt(0, data, checksum) udt_send(sndpkt),rdt_send(data),udt_send(sndpkt),rdt_rcv(rcvpkt) & ( corrupt(rcvpkt) | isNAK(rcvpkt) ),sndpkt = make_pkt(1, data, checksum) udt_send(sndpkt),rdt_send(data),rdt_rcv(rcvpkt) & notcorrupt(rcvpkt) & isACK(rcvpkt),udt_send(sndpk
37、t),rdt_rcv(rcvpkt) & ( corrupt(rcvpkt) | isNAK(rcvpkt) ),rdt_rcv(rcvpkt) & notcorrupt(rcvpkt) & isACK(rcvpkt),L,L,Transport Layer,3-31,extract(rcvpkt,data) deliver_data(data) sndpkt = make_pkt(ACK, chksum) udt_send(sndpkt),rdt2.1: receiver, handles garbled ACK/NAKs,sndpkt = make_pkt(NAK, chksum) udt
38、send(sndpkt),rdt_rcv(rcvpkt) & not corrupt(rcvpkt) &has_seq0(rcvpkt),rdt_rcv(rcvpkt) & notcorrupt(rcvpkt) & has_seq1(rcvpkt),extract(rcvpkt,data) deliver_data(data) sndpkt = make_pkt(ACK, chksum) udt_send(sndpkt),rdt_rcv(rcvpkt) & notcorrupt(rcvpkt) & has_seq0(rcvpkt),rdt_rcv(rcvpkt) & (corrupt(rcv
39、pkt),sndpkt = make_pkt(ACK, chksum) udt_send(sndpkt),rdt_rcv(rcvpkt) & not corrupt(rcvpkt) &has_seq1(rcvpkt),rdt_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,3-32,rdt2
40、1: discussion,Sender: seq # added to pkt two seq. #s (0,1) will suffice. Why? must check if received ACK/NAK corrupted What if seq. # error? twice as many states state must “remember” whether “current” pkt has 0 or 1 seq. #,Receiver: must check if received packet is duplicate state indicates whether 0 or 1 is expected pkt seq # note: receiver can not know if its last ACK/NAK received OK at sender,
