1. ICT 6621 : Advanced NetworkingKhaled Mahbub, IICT, BUET, 2008 Lecture 7 TCP/IP Transport Layer (1)

2. ICT 6621 : Advanced NetworkingKhaled Mahbub, IICT, BUET, 2008 Outline (Transport Layer) Principles behind transport layer services: –multiplexing/demultiplexing –principles of reliable data transfer learn about transport layer protocols in the Internet: –UDP: connectionless transport –TCP: connection-oriented transport segment structure reliable data transfer flow control connection management

3. ICT 6621 : Advanced NetworkingKhaled Mahbub, IICT, BUET, 2008 Transport Services and Protocols provide logical communication between application processes running on different hosts transport protocols run in end systems –sender side: breaks application messages into segments, passes to network layer –receiver side: reassembles segments into messages, passes to application layer more than one transport protocol available to applications, e.g. –Internet: TCP and UDP

4. ICT 6621 : Advanced NetworkingKhaled Mahbub, IICT, BUET, 2008 Transport vs. Network Layer network layer: logical communication between hosts transport layer: logical communication between processes –relies on, enhances, network layer services Household analogy: 5 kids from house A sending letters to 5 kids in house B In house A, Ali collects letters from each kid and wrap them in single envelope. Ali also distributes received mails in house A. Babu does same task in house B. Postal service is used to send mails –processes = kids –app messages = letters from each kid –hosts = houses –transport protocol = Ali and Babu –network-layer protocol = postal service

5. ICT 6621 : Advanced NetworkingKhaled Mahbub, IICT, BUET, 2008 Internet Transport Layer protocols User Datagram Protocol (UDP) –unreliable unordered data transfer between sending and receiving process –does not provide: connection setup, reliability, flow control, congestion control, timing, or bandwidth guarantee Transmission Control Protocol (TCP) –reliable, in-order delivery –reliable transport: acknowledgement, retransmissions –flow control: sender won’t overwhelm receiver –congestion control: for benefit of the Internet –Does not provide: timing, minimum bandwidth guarantees

6. ICT 6621 : Advanced NetworkingKhaled Mahbub, IICT, BUET, 2008 Outline (Transport Layer) Principles behind transport layer services: –multiplexing/demultiplexing –principles of reliable data transfer learn about transport layer protocols in the Internet: –UDP: connectionless transport –TCP: connection-oriented transport segment structure reliable data transfer flow control connection management

7. ICT 6621 : Advanced NetworkingKhaled Mahbub, IICT, BUET, 2008 Multiplexing/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 = process= socket delivering received segments to correct application layers processes. Demultiplexing at rcv host: gathering data from multiple application layer process, enveloping data with header (later used for demultiplexing) Multiplexing at send host:

8. ICT 6621 : Advanced NetworkingKhaled Mahbub, IICT, BUET, 2008 How Demultiplexing Works host receives IP datagrams –each datagram has source IP address, destination IP address –each datagram carries 1 transport-layer segment –each segment has source, destination port number –The port number is a 16-bit number, ranging from 0 to 65535. Port numbers ranging from 0 - 1023 are called well- known port numbers and are used by well-known application protocols such as HTTP, FTP, Telnet etc. 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

9. ICT 6621 : Advanced NetworkingKhaled Mahbub, IICT, BUET, 2008 Demultiplexing Client IP:B P2 client IP: A P1 P3 server IP: C SP: 6428 DP: 9157 D-IP:A SP: 9157 DP: 6428 D-IP:C SP: 6428 DP: 5775 D-IP:B SP: 5775 DP: 6428 D-IP:C SP provides “return address”

10. ICT 6621 : Advanced NetworkingKhaled Mahbub, IICT, BUET, 2008 Outline (Transport Layer) Principles behind transport layer services: –multiplexing/demultiplexing –principles of reliable data transfer learn about transport layer protocols in the Internet: –UDP: connectionless transport –TCP: connection-oriented transport segment structure reliable data transfer flow control connection management

11. ICT 6621 : Advanced NetworkingKhaled Mahbub, IICT, BUET, 2008 UDP: User Datagram 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 segment header –no congestion control: UDP can blast away as fast as desired often used for streaming multimedia applications –loss tolerant –rate sensitive other UDP uses –DNS –SNMP reliable transfer over UDP: add reliability at application layer –application-specific error recovery!

12. ICT 6621 : Advanced NetworkingKhaled Mahbub, IICT, BUET, 2008 UDP Segment Format source port # dest port # 32 bits Application data (message) UDP segment format length checksum Length, in bytes of UDP segment, including header identifies receiving process (for multiplexing demultiplexing) For UDP header and UDP data (optional)

13. ICT 6621 : Advanced NetworkingKhaled Mahbub, IICT, BUET, 2008 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. Does not guarantee error free. Goal: detect “errors” (e.g., flipped bits) in transmitted segment

14. ICT 6621 : Advanced NetworkingKhaled Mahbub, IICT, BUET, 2008 Outline (Transport Layer) Principles behind transport layer services: –multiplexing/demultiplexing –principles of reliable data transfer learn about transport layer protocols in the Internet: –UDP: connectionless transport –TCP: connection-oriented transport segment structure reliable data transfer flow control connection management

15. ICT 6621 : Advanced NetworkingKhaled Mahbub, IICT, BUET, 2008 Principles of Reliable Data Transfer important in application, transport, link layers top-10 list of important networking topics! characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt)

16. ICT 6621 : Advanced NetworkingKhaled Mahbub, IICT, BUET, 2008 Reliable Data Transfer 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 rdt_rcv(): called when packet arrives on rcv-side of channel deliver_data(): called by rdt to deliver data to upper

17. ICT 6621 : Advanced NetworkingKhaled Mahbub, IICT, BUET, 2008 Reliable Data Transfer We will: incrementally develop sender, receiver sides 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 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

18. ICT 6621 : Advanced NetworkingKhaled Mahbub, IICT, BUET, 2008 Rdt1.0: Reliable Transfer Over a Reliable Channel underlying channel perfectly reliable –no bit errors –no loss of packets separate FSMs for sender, receiver: –sender sends data into underlying channel –receiver reads 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 rdt_rcv(packet) sender receiver

19. ICT 6621 : Advanced NetworkingKhaled Mahbub, IICT, BUET, 2008 Rdt2.0: Channel with Bit Errors all transmitted packets are received in the order in which they were sent. underlying channel may flip bits in packet –checksum to detect bit errors the question: how to recover from errors: –acknowledgements (ACKs): receiver explicitly tells sender that packet received OK –negative acknowledgements (NAKs): receiver explicitly tells sender that packet had errors –sender retransmits packet on receipt of NAK new mechanisms in rdt2.0 (beyond rdt1.0 ): –error detection –receiver feedback: control msgs (ACK,NAK) receiver->sender

20. ICT 6621 : Advanced NetworkingKhaled Mahbub, IICT, BUET, 2008 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) udt_send(NAK) rdt_rcv(rcvpkt) && corrupt(rcvpkt) Wait for ACK or NAK Wait for call from below rdt_send(data)  Sender sends one packet, then waits for receiver response stop and wait

21. ICT 6621 : Advanced NetworkingKhaled Mahbub, IICT, BUET, 2008 Rdt2.0 Flaws What happens if ACK/NAK corrupted? sender doesn’t know what happened at receiver! –add enough checksum bits to allow the sender to not only detect, but recover from, bit errors. –resend the current data packet when it receives a garbled ACK or NAK packet. possible duplicate Handling duplicates: sender adds sequence number to each packet sender retransmits current packet if ACK/NAK garbled receiver discards (doesn’t deliver up) duplicate packet

22. ICT 6621 : Advanced NetworkingKhaled Mahbub, IICT, BUET, 2008 Rdt2.1: Mechanism Sender: sequence number added to packet two sequence numbers (0,1) will suffice. must check if received ACK/NAK corrupted twice as many states –state must “remember” whether “current” packet has 0 or 1 sequence number. Receiver: must check if received packet is duplicate –state indicates whether 0 or 1 is expected packet sequence number note: receiver can not know if its last ACK/NAK received OK at sender

23. ICT 6621 : Advanced NetworkingKhaled Mahbub, IICT, BUET, 2008 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) 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(sndpkt) rdt_rcv(rcvpkt) && ( corrupt(rcvpkt) || isNAK(rcvpkt) ) rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && isACK(rcvpkt) Wait for call 1 from above Wait for ACK or NAK 1  

24. ICT 6621 : Advanced NetworkingKhaled Mahbub, IICT, BUET, 2008 Rdt2.1: Receiver Handles Garbled ACK/NAKs Wait for 0 from below sndpkt = make_pkt(NAK, chksum) udt_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) Wait for 1 from below rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && has_seq0(rcvpkt) extract(rcvpkt,data) deliver_data(data) sndpkt = make_pkt(ACK, chksum) udt_send(sndpkt) rdt_rcv(rcvpkt) && (corrupt(rcvpkt) 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)

25. ICT 6621 : Advanced NetworkingKhaled Mahbub, IICT, BUET, 2008 Rdt2.2: a NAK-free Protocol same functionality as rdt2.1, using ACKs only instead of NAK, receiver sends ACK for last packet received OK –receiver must explicitly include sequence number of packet being ACKed duplicate ACK at sender results in same action as NAK: retransmit current packet

26. ICT 6621 : Advanced NetworkingKhaled Mahbub, IICT, BUET, 2008 Rdt2.2: Sender Receiver FSM Wait 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) || isACK(rcvpkt,1) ) rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && isACK(rcvpkt,0) Wait for ACK 0 sender FSM fragment Wait for 0 from below rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && has_seq1(rcvpkt) extract(rcvpkt,data) deliver_data(data) sndpkt = make_pkt(ACK1, chksum) udt_send(sndpkt) rdt_rcv(rcvpkt) && (corrupt(rcvpkt) || has_seq1(rcvpkt)) udt_send(sndpkt) receiver FSM fragment 

27. ICT 6621 : Advanced NetworkingKhaled Mahbub, IICT, BUET, 2008 Rdt3.0: Channels with Errors and Loss New assumption: underlying channel can also lose packets (data or ACKs) how to detect packet loss and what to do when this occurs. –use of checksum, sequence numbers, ACK packets, and retransmissions Approach: sender waits “reasonable” amount of time for ACK retransmits if no ACK received in this time if packet (or ACK) just delayed (not lost): –retransmission will be duplicate, but use of sequence numbers already handles this –receiver must specify sequence number of packet being ACKed requires countdown timer

28. ICT 6621 : Advanced NetworkingKhaled Mahbub, IICT, BUET, 2008 Rdt3.0 sender sndpkt = make_pkt(0, data, checksum) udt_send(sndpkt) start_timer rdt_send(data) Wait for ACK0 rdt_rcv(rcvpkt) && ( corrupt(rcvpkt) || isACK(rcvpkt,1) ) Wait for call 1 from above sndpkt = make_pkt(1, data, checksum) udt_send(sndpkt) start_timer rdt_send(data) rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && isACK(rcvpkt,0) rdt_rcv(rcvpkt) && ( corrupt(rcvpkt) || isACK(rcvpkt,0) ) rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && isACK(rcvpkt,1) stop_timer udt_send(sndpkt) start_timer timeout udt_send(sndpkt) start_timer timeout rdt_rcv(rcvpkt) Wait for call 0from above Wait for ACK1  rdt_rcv(rcvpkt)   

29. ICT 6621 : Advanced NetworkingKhaled Mahbub, IICT, BUET, 2008 Rdt3.0 in Action

30. ICT 6621 : Advanced NetworkingKhaled Mahbub, IICT, BUET, 2008 Rdt3.0 in Action

31. ICT 6621 : Advanced NetworkingKhaled Mahbub, IICT, BUET, 2008 Performance of Rdt3.0 rdt3.0 works, but performance stinks example: 1 Gbps link, 15 ms e-e prop. delay, 1KB packet: T transmit = 8kb/pkt 10**9 b/sec = 8 microsec –U sender : utilization – fraction of time sender busy sending –1KB packet every 30 msec -> 33kB/sec throughput over 1 Gbps link –network protocol limits use of physical resources! L (packet length in bits) R (transmission rate, bps) =

32. ICT 6621 : Advanced NetworkingKhaled Mahbub, IICT, BUET, 2008 Rdt3.0: stop-and-wait operation first packet bit transmitted, t = 0 senderreceiver RTT last packet bit transmitted, t = L / R first packet bit arrives last packet bit arrives, send ACK ACK arrives, send next packet, t = RTT + L / R

33. ICT 6621 : Advanced NetworkingKhaled Mahbub, IICT, BUET, 2008 Pipelined Protocols Pipelining: sender allows multiple, “in-flight”, yet-to-be-acknowledged packets –range of sequence numbers must be increased –buffering at sender and/or receiver Two generic forms of pipelined protocols: go- Back-N, selective repeat

34. ICT 6621 : Advanced NetworkingKhaled Mahbub, IICT, BUET, 2008 Pipelining: Increased Utilization first packet bit transmitted, t = 0 senderreceiver RTT last bit transmitted, t = L / R first packet bit arrives last packet bit arrives, send ACK ACK arrives, send next packet, t = RTT + L / R last bit of 2 nd packet arrives, send ACK last bit of 3 rd packet arrives, send ACK

35. ICT 6621 : Advanced NetworkingKhaled Mahbub, IICT, BUET, 2008 Go-Back-N Sender: k-bit sequence number in packet header “window” of up to N, consecutive unack’ed packets allowed ACK(n): ACKs all packets up to, including sequence number n - “cumulative ACK” –may deceive duplicate ACKs (see receiver) timer for each in-flight packet timeout(n): retransmit packet n and all higher sequence number packets in window

36. ICT 6621 : Advanced NetworkingKhaled Mahbub, IICT, BUET, 2008 GBN: Sender Extended FSM Wait start_timer udt_send(sndpkt[base]) udt_send(sndpkt[base+1]) … udt_send(sndpkt[nextseqnum-1]) timeout rdt_send(data) if (nextseqnum < base+N) { sndpkt[nextseqnum] = make_pkt(nextseqnum,data,chksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) start_timer nextseqnum++ } else refuse_data(data) base = getacknum(rcvpkt)+1 If (base == nextseqnum) stop_timer else start_timer rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) base=1 nextseqnum=1 rdt_rcv(rcvpkt) && corrupt(rcvpkt) 

37. ICT 6621 : Advanced NetworkingKhaled Mahbub, IICT, BUET, 2008 GBN: Receiver Extended FSM ACK-only: always send ACK for correctly-received packet with highest in-order sequence number –may generate duplicate ACKs –need only remember expectedseqnum out-of-order packet: –discard (don’t buffer) -> no receiver buffering! –Re-ACK packet with highest in-order sequence Wait udt_send(sndpkt) default rdt_rcv(rcvpkt) && notcurrupt(rcvpkt) && hasseqnum(rcvpkt,expectedseqnum) extract(rcvpkt,data) deliver_data(data) sndpkt = make_pkt(expectedseqnum,ACK,chksum) udt_send(sndpkt) expectedseqnum++ expectedseqnum=1 sndpkt = make_pkt(expectedseqnum,ACK,chksum) 

38. ICT 6621 : Advanced NetworkingKhaled Mahbub, IICT, BUET, 2008 GBN in Action

39. ICT 6621 : Advanced NetworkingKhaled Mahbub, IICT, BUET, 2008 Selective Repeat receiver individually acknowledges all correctly received packets –buffers packets, as needed, for eventual in-order delivery to upper layer sender only resends packets for which ACK not received –sender timer for each unACKed packet sender window –N consecutive sequence numbers –again limits sequence numbers of sent, unACKed packets

40. ICT 6621 : Advanced NetworkingKhaled Mahbub, IICT, BUET, 2008 Selective Repeat: Sender, Receiver Windows

41. ICT 6621 : Advanced NetworkingKhaled Mahbub, IICT, BUET, 2008 Selective Repeat data from above : if next available sequence number in window, send packet timeout(n): resend packet n, restart timer ACK(n) in [sendbase,sendbase+N]: mark packet n as received if n smallest unACKed packet, advance window base to next unACKed sequence number sender packet n in [rcvbase, rcvbase+N-1] send ACK(n) out-of-order: buffer in-order: deliver (also deliver buffered, in-order packets), advance window to next not- yet-received packet packet n in [rcvbase- N,rcvbase-1] ACK(n) otherwise: ignore receiver

42. ICT 6621 : Advanced NetworkingKhaled Mahbub, IICT, BUET, 2008 Selective Repeat in Action

43. ICT 6621 : Advanced NetworkingKhaled Mahbub, IICT, BUET, 2008 Selective Repeat: Dilemma Example: seq #’s: 0, 1, 2, 3 window size=3 receiver sees no difference in two scenarios! incorrectly passes duplicate data as new in (a) a window size that is one smaller than the size of the sequence number space won't work. the window size must be less than or equal to half the size of the sequence number space.

44. ICT 6621 : Advanced NetworkingKhaled Mahbub, IICT, BUET, 2008 Reading Material Chapter 3 – text3 (Kurose) Chapter 6 – text2 (Tanenbaum)