Chapter 3: Transport Layer

Introduction to Computer Networking Guy Leduc Chapter 3 Transport Layer Computer Networking: A Top Down Approach, 7 th edition. Jim Kurose, Keith Ross Addison-Wesley, April 2016 From Computer Networking, by Kurose&Ross 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 Internet transport layer protocols: UDP: connectionless transport TCP: connection-oriented reliable transport TCP congestion control From Computer Networking, by Kurose&Ross Transport Layer 3-2 1

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 flow control connection management 3.6 Principles of congestion control 3.7 TCP congestion control From Computer Networking, by Kurose&Ross 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 into 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 application transport network data link physical application transport network data link physical From Computer Networking, by Kurose&Ross Transport Layer 3-4 2

Transport vs. network layer network layer: logical communication between hosts transport layer: logical communication between processes relies on, enhances, network layer services Household analogy: (see example in reference book) 12 kids sending letters to 12 kids hosts = houses processes = kids app messages = letters in envelopes transport protocol = Ann and Bill who demux to inhouse siblings network-layer protocol = postal service From Computer Networking, by Kurose&Ross Transport Layer 3-5 Internet transport-layer protocols reliable, in-order delivery (TCP) congestion control flow control connection setup unreliable, unordered delivery: UDP no-frills extension of best-effort IP services not available: delay guarantees 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 network data link physical application transport network data link physical From Computer Networking, by Kurose&Ross Transport Layer 3-6 3

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 flow control connection management 3.6 Principles of congestion control 3.7 TCP congestion control From Computer Networking, by Kurose&Ross Transport Layer 3-7 Multiplexing/demultiplexing multiplexing at sender: handle data from multiple sockets, add transport header (later used for demultiplexing) demultiplexing at receiver: use header info to deliver received segments to correct socket application application P3 transport network link P1 P2 transport network link physical application P4 transport network link socket process physical physical From Computer Networking, by Kurose&Ross Transport Layer 3-8 4

How demultiplexing works host receives IP datagrams each datagram has source IP address, destination IP address each datagram carries one transport-layer segment each segment has source, destination port number host uses IP addresses & port numbers to direct segment to appropriate socket 32 bits source port # dest port # other header fields application data (message) TCP/UDP segment format From Computer Networking, by Kurose&Ross Transport Layer 3-9 Connectionless demultiplexing recall: created socket has host-local port #: DatagramSocket mysocket1 = new DatagramSocket(12534); recall: when creating datagram to send into UDP socket, must specify! destination IP address! destination port # when UDP receives segment from below: checks destination port # in segment directs UDP segment to socket with that port # IP datagrams with same dest. port #, but different source IP addresses and/or source port numbers will be directed to same socket at dest From Computer Networking, by Kurose&Ross Transport Layer 3-10 5

Connectionless demux: example DatagramSocket mysocket2 = new DatagramSocket (9157); application P3 transport network link physical DatagramSocket serversocket = new DatagramSocket (6428); application P1 transport network link physical DatagramSocket mysocket1 = new DatagramSocket (5775); application P4 transport network link physical source port: 6428 dest port: 9157 source port: 6428 dest port: 5775 source port: 9157 (set by transport) dest port: 6428 (set by P3) source port: 5775 dest port: 6428 From Computer Networking, by Kurose&Ross Transport Layer 3-11 Connection-oriented demux TCP socket identified by 4-tuple: source IP address source port number dest IP address dest port number Demux: receiver uses all four values to direct segment to appropriate socket Server host may support many simultaneous TCP sockets: each socket identified by its own 4-tuple Web servers have different sockets for each connecting client non-persistent HTTP will have different socket for each request From Computer Networking, by Kurose&Ross Transport Layer 3-12 6

Connection-oriented demux: example application P1 transport network link physical P4 application P5 transport network link physical P6 server: IP address B application P2 P3 transport network link physical host: IP address A source IP,port: B,80 dest IP,port: A,9157 source IP,port: C,5775 dest IP,port: B,80 host: IP address C source IP,port: A,9157 dest IP,port: B,80 source IP,port: C,9157 three segments, all destined for IP address: B and dest IP,port: B,80 dest port: 80, are demultiplexed to different sockets From Computer Networking, by Kurose&Ross Transport Layer 3-13 Connection-oriented demux: example threaded server application P1 transport network link physical application P4 transport network link physical server: IP address B application P2 P3 transport network link physical host: IP address A source IP,port: B,80 dest IP,port: A,9157 source IP,port: C,5775 dest IP,port: B,80 host: IP address C source IP,port: A,9157 dest IP,port: B,80 source IP,port: C,9157 dest IP,port: B,80 From Computer Networking, by Kurose&Ross Transport Layer 3-14 7

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 flow control connection management 3.6 Principles of congestion control 3.7 TCP congestion control From Computer Networking, by Kurose&Ross Transport Layer 3-15 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 " UDP use:! multimedia apps (loss tolerant, rate sensitive)! DNS! SNMP " reliable transfer over UDP:! add reliability at application layer! application-specific error recovery! From Computer Networking, by Kurose&Ross Transport Layer 3-16 8

UDP: segment header source port # dest port # length 32 bits application data (payload) checksum UDP segment format length, in bytes of UDP segment, including header why is there a UDP? no connection establishment (which can add delay) simple: no connection state at sender, receiver small header size no congestion control: UDP can blast away as fast as desired From Computer Networking, by Kurose&Ross Transport Layer 3-17 UDP checksum Goal: detect errors (e.g., flipped bits) in transmitted segment Sender: treat segment contents, including header fields, as sequence of 16-bit integers checksum: addition (one 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 nonetheless? More later. From Computer Networking, by Kurose&Ross Transport Layer 3-18 9

Internet checksum: example 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 1 wraparound sum checksum 1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 1 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 Note: when adding numbers, a carryout from the most significant bit needs to be added to the result From Computer Networking, by Kurose&Ross Transport Layer 3-19 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 flow control connection management 3.6 Principles of congestion control 3.7 TCP congestion control From Computer Networking, by Kurose&Ross Transport Layer 3-20 10

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) From Computer Networking, by Kurose&Ross Transport Layer 3-21 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) From Computer Networking, by Kurose&Ross Transport Layer 3-22 11

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) From Computer Networking, by Kurose&Ross Transport Layer 3-23 Reliable data transfer: getting started rdt_send(): called from above, (e.g., by app.). Passed data to deliver to receiver upper layer deliver_data(): called by rdt to deliver data to upper send side receive side udt_send(): called by rdt, to transfer packet over unreliable channel to receiver rdt_rcv(): called when packet arrives on rcv-side of channel From Computer Networking, by Kurose&Ross Transport Layer 3-24 12

Reliable data transfer: getting started We ll: 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: when in this state next state uniquely determined by next event state 1 event causing state transition actions taken on state transition event actions state 2 From Computer Networking, by Kurose&Ross Transport Layer 3-25 Rdt1.0: reliable transfer over a reliable channel assume: 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 rdt_send(data) packet = make_pkt(data) udt_send(packet) Wait for call from below rdt_rcv(packet) data = extract (packet) deliver_data(data) sender receiver From Computer Networking, by Kurose&Ross Transport Layer 3-26 13

rdt2.0: channel with bit errors assume: underlying channel may flip bits in packet checksum to detect bit errors the question: how to recover from errors? How do humans recover from errors during conversation? From Computer Networking, by Kurose&Ross Transport Layer 3-27 rdt2.0: channel with bit errors assume: 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 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 feedback: control msgs (ACK,NAK) from receiver to sender pkt A ACK pkt B NAK pkt B ACK OK Error OK From Computer Networking, by Kurose&Ross Transport Layer 3-28 14

rdt2.0: FSM specification rdt_send(data) sndpkt = make_pkt(data, checksum) udt_send(sndpkt) Wait for call from above rdt_rcv(rcvpkt) && isack(rcvpkt) sender Wait for ACK or NAK Λ rdt_rcv(rcvpkt) && isnak(rcvpkt) udt_send(sndpkt) receiver rdt_rcv(rcvpkt) && corrupt(rcvpkt) udt_send(nak) Wait for call from below rdt_rcv(rcvpkt) && not corrupt(rcvpkt) data = extract(rcvpkt) deliver_data(data) udt_send(ack) From Computer Networking, by Kurose&Ross Transport Layer 3-29 rdt2.0: operation with no errors rdt_send(data) snkpkt = make_pkt(data, checksum) udt_send(sndpkt) Wait for call from above Wait for ACK or NAK rdt_rcv(rcvpkt) && isnak(rcvpkt) udt_send(sndpkt) rdt_rcv(rcvpkt) && corrupt(rcvpkt) udt_send(nak) rdt_rcv(rcvpkt) && isack(rcvpkt) Λ Wait for call from below rdt_rcv(rcvpkt) && not corrupt(rcvpkt) extract(rcvpkt,data) deliver_data(data) udt_send(ack) From Computer Networking, by Kurose&Ross Transport Layer 3-30 15

rdt2.0: error scenario rdt_send(data) snkpkt = make_pkt(data, checksum) udt_send(sndpkt) Wait for call from above Wait for ACK or NAK rdt_rcv(rcvpkt) && isack(rcvpkt) Λ rdt_rcv(rcvpkt) && isnak(rcvpkt) udt_send(sndpkt) rdt_rcv(rcvpkt) && corrupt(rcvpkt) udt_send(nak) Wait for call from below rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) extract(rcvpkt,data) deliver_data(data) udt_send(ack) From Computer Networking, by Kurose&Ross Transport Layer 3-31 rdt2.0 has a fatal flaw! What happens if ACK/NAK corrupted? garbled ACK/NAK detected by checksum too garbled ACK/NAK discarded but sender doesn t know what happened at receiver! can t just retransmit: possible duplicate Handling duplicates: sender retransmits current pkt if ACK/NAK garbled sender adds sequence number to each pkt receiver discards (doesn t deliver up) duplicate pkt pkt garbled ACK same pkt OK Duplicate! stop and wait Sender sends one packet, then waits for receiver response From Computer Networking, by Kurose&Ross Transport Layer 3-32 16

rdt2.1: sender, handles garbled ACK/NAKs rdt_rcv(rcvpkt) && not corrupt(rcvpkt) && isack(rcvpkt) Λ rdt_rcv(rcvpkt) && ( corrupt(rcvpkt) isnak(rcvpkt) ) udt_send(sndpkt) rdt_send(data) sndpkt = make_pkt(0, data, checksum) udt_send(sndpkt) Wait for call 0 from above Wait for ACK or NAK 1 rdt_send(data) Wait for ACK or NAK 0 Wait for call 1 from above rdt_rcv(rcvpkt) && ( corrupt(rcvpkt) isnak(rcvpkt) ) udt_send(sndpkt) rdt_rcv(rcvpkt) && not corrupt(rcvpkt) && isack(rcvpkt) sndpkt = make_pkt(1, data, checksum) udt_send(sndpkt) Λ From Computer Networking, by Kurose&Ross Transport Layer 3-33 rdt2.1: receiver, handles garbled ACK/NAKs rdt_rcv(rcvpkt) && corrupt(rcvpkt) sndpkt = make_pkt(nak, chksum) udt_send(sndpkt) rdt_rcv(rcvpkt) && not corrupt(rcvpkt) && has_seq1(rcvpkt) sndpkt = make_pkt(ack, chksum) udt_send(sndpkt) rdt_rcv(rcvpkt) && not corrupt(rcvpkt) && has_seq0(rcvpkt) data = extract(rcvpkt) deliver_data(data) sndpkt = make_pkt(ack, chksum) udt_send(sndpkt) Wait for 0 from below Wait for 1 from below rdt_rcv(rcvpkt) && not corrupt(rcvpkt) && has_seq1(rcvpkt) data = extract(rcvpkt) deliver_data(data) sndpkt = make_pkt(ack, chksum) udt_send(sndpkt) rdt_rcv(rcvpkt) && corrupt(rcvpkt) sndpkt = make_pkt(nak, chksum) udt_send(sndpkt) rdt_rcv(rcvpkt) && not corrupt(rcvpkt) && has_seq0(rcvpkt) sndpkt = make_pkt(ack, chksum) udt_send(sndpkt) Note: receiver sends ACK in this case. Why? From Computer Networking, by Kurose&Ross Transport Layer 3-34 17

rdt2.1: discussion Sender: seq # added to pkt two seq. # s (0,1) will suffice. Why? must check if received ACK/NAK corrupted twice as many states state must remember whether current pkt has seq. # of 0 or 1 Receiver: must check if received packet is duplicate state indicates whether 0 or 1 is expected pkt seq # From Computer Networking, by Kurose&Ross Transport Layer 3-35 rdt3.0: channels with errors and loss New assumption: underlying channel can also lose packets (data or ACKs) checksum, seq. #, ACKs, retransmissions will be of help but not enough Approach: sender waits reasonable amount of time for ACK retransmits if no ACK received in this time requires countdown timer Timeout pkt(0) pkt(0) ACK X Timeout X pkt(0) ACK pkt(0) ACK Discard but ACK! No duplicate From Computer Networking, by Kurose&Ross Transport Layer 3-36 18

rdt3.1: actually no need for NAKs! Up to now: Timer for packet loss NAK for packet errors Simpler: Timer for both packet loss and errors! NAK would improve recovery time, but it s not our concern here! rdt3.0: With ACKs & NAKs rdt3.1: With ACKs only pkt(0) ACK pkt(1) NAK pkt(1) ACK OK Error OK Timeout pkt(0) ACK pkt(1) pkt(1) ACK OK Error OK From Computer Networking, by Kurose&Ross Transport Layer 3-37 Flaw in rdt3.1: Delayed packets or ACKs If pkt (or ACK) just delayed (not lost): retransmission will be duplicate, but use of seq. # s already handles this Timeout A B B ACKed but lost! C C ACKed but lost! pkt(0) pkt(0) pkt(0) pkt(1) ACK ACK ACK X A received Discard (no dupl) Resend ACK B not recovered Discard again, Still expects #1! C not recovered However, race conditions are possible between the received ACK and the retransmitted packet! Note: such race conditions are only possible over full-duplex channels From Computer Networking, by Kurose&Ross Transport Layer 3-38 19

rdt3.2: adding seq # in ACKs receiver must specify seq # of pkt being ACKed Timeout B not ACKed A B Timeout pkt(0) pkt(0) ACK(0) ACK(0) pkt(1) X pkt(1) ACK(1) A received Discard (no dupl) Resend ACK B recovered From Computer Networking, by Kurose&Ross Transport Layer 3-39 rdt3.2 sender rdt_rcv(rcvpkt) Λ Wait for call 0from above rdt_rcv(rcvpkt) && not corrupt(rcvpkt) && isack(rcvpkt,1) stop_timer timeout udt_send(sndpkt) start_timer rdt_rcv(rcvpkt) && ( corrupt(rcvpkt) isack(rcvpkt,0) ) Λ rdt_send(data) sndpkt = make_pkt(0, data, chksum) udt_send(sndpkt) start_timer Wait for ACK1 rdt_send(data) Wait for ACK0 Wait for call 1 from above sndpkt = make_pkt(1, data, chksum) udt_send(sndpkt) start_timer rdt_rcv(rcvpkt) && ( corrupt(rcvpkt) isack(rcvpkt,1) ) Λ timeout udt_send(sndpkt) start_timer rdt_rcv(rcvpkt) && not corrupt(rcvpkt) && isack(rcvpkt,0) stop_timer rdt_rcv(rcvpkt) Λ From Computer Networking, by Kurose&Ross Transport Layer 3-40 20

rdt3.2 in action: Alternating-Bit Protocol (1969) sender send pkt0 rcv ack0 send pkt1 rcv ack1 send pkt0 pkt0 ack0 pkt1 ack1 pkt0 ack0 (a) no loss receiver rcv pkt0 send ack0 rcv pkt1 send ack1 rcv pkt0 send ack0 sender send pkt0 rcv ack0 send pkt1 timeout resend pkt1 rcv ack1 send pkt0 pkt0 ack0 pkt1 X loss pkt1 ack1 pkt0 ack0 receiver rcv pkt0 send ack0 rcv pkt1 send ack1 rcv pkt0 send ack0 (b) packet loss From Computer Networking, by Kurose&Ross Transport Layer 3-41 rdt3.2 in action sender send pkt0 rcv ack0 send pkt1 timeout resend pkt1 rcv ack1 send pkt0 pkt0 ack0 pkt1 ack1 X loss pkt1 ack1 pkt0 ack0 receiver rcv pkt0 send ack0 rcv pkt1 send ack1 rcv pkt1 (detect duplicate) send ack1 rcv pkt0 send ack0 sender send pkt0 rcv ack0 send pkt1 timeout resend pkt1 rcv ack1 send pkt0 rcv ack1 send pkt0 pkt0 ack0 pkt1 ack1 pkt1 pkt0 ack1 ack0 receiver rcv pkt0 send ack0 rcv pkt1 send ack1 rcv pkt1 (detect duplicate) send ack1 rcv pkt0 send ack0 (c) ACK loss (d) premature timeout/ delayed ACK From Computer Networking, by Kurose&Ross Transport Layer 3-42 21

rdt3.2 still incorrect if pkt or ACK reordering is possible pkt(0) Can be arbitrarily small Timeout on pkt(0) pkt(0) ACK(0) pkt reordering pkt(1) ACK(1) pkt(0) ACKed but lost! ACK(0) X pkt(0) Duplicate! Considered as new pkt(0)! Solution: Choose timeout so large that when a pkt is retransmitted the sender is sure that the previous copy of this pkt and its ACK have disappeared from the network. Better solution: Use a much larger seq# space (see later). From Computer Networking, by Kurose&Ross Transport Layer 3-43 Performance of rdt3.2: stop-and-wait operation first packet bit transmitted, t = 0 last packet bit transmitted, t = L / R sender receiver RTT first packet bit arrives last packet bit arrives, send ACK ACK arrives, send next packet, t = RTT + L / R U sender : utilization = fraction of time sender busy sending U sender = L R RTT + L R = 1 1+ R RTT L R. RTT/2 = Bandwidth-delay product = in-flight bits From Computer Networking, by Kurose&Ross Transport Layer 3-44 22

Performance of rdt3.2 Was OK over local low-speed networks, but Example: 1 Gbps link, 15 ms end-to-end prop. delay, 1KB packet: T transmit = L (packet length in bits) R (transmission rate, bps) = 8 10 3 bits 10 9 bps = 8 µsec U sender = L / R RTT + L / R =.008 30.008 = 0.00027 microsec 1 KByte packet every 30 msec -> 266.7 kbps throughput over 1 Gbps link network protocol limits use of physical resources! From Computer Networking, by Kurose&Ross Transport Layer 3-45 Pipelined protocols Pipelining: sender allows multiple, in-flight, yet-tobe-acknowledged pkts range of sequence numbers must be increased buffering at sender and/or receiver Two generic forms of pipelined protocols: go-back-n, selective repeat From Computer Networking, by Kurose&Ross Transport Layer 3-46 23

Pipelining: increased utilization first packet bit transmitted, t = 0 last bit transmitted, t = L / R sender receiver RTT ACK arrives, send next packet, t = RTT + L / R first packet bit arrives last packet bit arrives, send ACK last bit of 2 nd packet arrives, send ACK last bit of 3 rd packet arrives, send ACK 3-packet pipelining increases utilization by a factor of 3! U sender = 3 * L / R RTT + L / R =.024 30.008 = 0.0008 microsecon From Computer Networking, by Kurose&Ross Transport Layer 3-47 Pipelined protocols: overview Go-back-N: Sender can have up to N unacked packets in pipeline Receiver only sends cumulative ACKs doesn t ACK pkt if there s a gap Sender has timer for oldest unacked pkt when timer expires, retransmit all unacked packets Selective Repeat: Sender can have up to N unacked packets in pipeline Receiver sends individual ACKs for each packet Sender maintains timer for each unacked packet when timer expires, retransmit only that unacked packet From Computer Networking, by Kurose&Ross Transport Layer 3-48 24

Go-Back-N (GBN): sender k-bit seq # in pkt header window of up to N, consecutive unacked packets allowed ACK(n): ACKs all packets up to, including seq # n - cumulative ACK may receive duplicate ACKs (see receiver) single timer: conceptually for the oldest in-flight packet (i.e., sent but unacked packet) Timeout(n): retransmit packet n and all higher seq # packets in window From Computer Networking, by Kurose&Ross Transport Layer 3-49 GBN: sender extended FSM Λ base=1 nextseqnum=1 rdt_rcv(rcvpkt) && corrupt(rcvpkt) Λ 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) Wait rdt_rcv(rcvpkt) && not corrupt(rcvpkt) timeout start_timer udt_send(sndpkt[base]) udt_send(sndpkt[base+1]) udt_send(sndpkt[nextseqnum-1]) base = getacknum(rcvpkt)+1 If (base == nextseqnum) stop_timer else From Computer Networking, by Kurose&Ross start_timer Transport Layer 3-50 25

GBN: receiver extended FSM Λ default expectedseqnum=1 sndpkt = make_pkt(0,ack,chksum) udt_send(sndpkt) Wait rdt_rcv(rcvpkt) && not corrupt(rcvpkt) && hasseqnum(rcvpkt,expectedseqnum) data = extract(rcvpkt) deliver_data(data) sndpkt = make_pkt(expectedseqnum,ack,chksum) udt_send(sndpkt) expectedseqnum++ ACK-only: always send ACK for correctly-received packet with highest in-order seq # need only remember expectedseqnum out-of-order packet: discard (don t buffer) -> no receiver buffering! re-ack packet with highest in-order seq # will generate duplicate ACKs From Computer Networking, by Kurose&Ross Transport Layer 3-51 GBN in action sender window (N=4) 0 1 2 3 4 5 6 7 8 0 1 2 3 4 5 6 7 8 0 1 2 3 4 5 6 7 8 0 1 2 3 4 5 6 7 8 0 1 2 3 4 5 6 7 8 0 1 2 3 4 5 6 7 8 0 1 2 3 4 5 6 7 8 0 1 2 3 4 5 6 7 8 0 1 2 3 4 5 6 7 8 0 1 2 3 4 5 6 7 8 sender send pkt0 send pkt1 send pkt2 send pkt3 (wait) rcv ack0, send pkt4 rcv ack1, send pkt5 ignore duplicate ACK pkt 2 timeout send pkt2 send pkt3 send pkt4 send pkt5 X loss receiver receive pkt0, send ack0 receive pkt1, send ack1 receive pkt3, discard, (re)send ack1 receive pkt4, discard, (re)send ack1 receive pkt5, discard, (re)send ack1 rcv pkt2, deliver, send ack2 rcv pkt3, deliver, send ack3 rcv pkt4, deliver, send ack4 rcv pkt5, deliver, send ack5 From Computer Networking, by Kurose&Ross Transport Layer 3-52 26

Maximum window size with GBN If seq# size = K ( The modulo ) 0 1 2 3 4 5 6 7 0 1 2 Q: What is the maximum window size N? Suppose K = 8, N =8 No pkt loss ACK2 0 1 2 3 4 5 6 7 0 1 2 With pkt loss: For sender: same view For receiver: different view! X X XX X X X ACK2 W=[3..2] ACK2 W=[3..2] Discard ACK2 3 3 From Computer Networking, by Kurose&Ross Transport Layer 3-53 GBN: maximum window size Constraint: window size (N) seq# size (K) - 1 0 1 2 3 4 5 6 7 0 1 W=[3..1] X X XX X X ACK2 Discard ACK2 resend 3 resend 4 From Computer Networking, by Kurose&Ross Transport Layer 3-54 27

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 seq # s again limits seq #s of sent, unacked packets From Computer Networking, by Kurose&Ross Transport Layer 3-55 Selective repeat: sender, receiver windows From Computer Networking, by Kurose&Ross Transport Layer 3-56 28

Selective repeat sender data from above : if next available seq # in window, send packet timeout(n): resend packet n, restart timer (n) ACK(n) in [sendbase,sendbase+n-1]: mark packet n as received if n is smallest unacked packet, advance window base to next unacked seq # receiver pkt n in [rcvbase, rcvbase+n-1] send ACK(n) out-of-order: buffer in-order: deliver (also deliver buffered, in-order pkts), advance window to next not-yet-received pkt pkt n in [rcvbase-n, rcvbase-1] ACK(n) otherwise: ignore From Computer Networking, by Kurose&Ross Transport Layer 3-57 Selective repeat in action sender window (N=4) 0 1 2 3 4 5 6 7 8 0 1 2 3 4 5 6 7 8 0 1 2 3 4 5 6 7 8 0 1 2 3 4 5 6 7 8 0 1 2 3 4 5 6 7 8 0 1 2 3 4 5 6 7 8 0 1 2 3 4 5 6 7 8 0 1 2 3 4 5 6 7 8 0 1 2 3 4 5 6 7 8 0 1 2 3 4 5 6 7 8 sender send pkt0 send pkt1 send pkt2 send pkt3 (wait) rcv ack0, send pkt4 rcv ack1, send pkt5 record ack3 arrived pkt 2 timeout send pkt2 record ack4 arrived record ack5 arrived X loss receiver receive pkt0, send ack0, deliver pkt0 receive pkt1, send ack1 deliver pkt1 receive pkt3, buffer, send ack3 receive pkt4, buffer, send ack4 receive pkt5, buffer, send ack5 rcv pkt2; deliver pkt2, pkt3, pkt4, pkt5; send ack2 Q: what happens when ack2 arrives? From Computer Networking, by Kurose&Ross Transport Layer 3-58 29

Selective repeat: dilemma example: seq # s: 0, 1, 2, 3 window size=3 " receiver sees no difference in two scenarios! " duplicate data accepted as new in (b) Q: what relationship between seq # size and window size to avoid problem in (b)? sender window (after receipt) 0 1 2 3 0 1 2 0 1 2 3 0 1 2 0 1 2 3 0 1 2 0 1 2 3 0 1 2 pkt0 0 1 2 3 0 1 2 pkt1 0 1 2 3 0 1 2 pkt2 X X timeout retransmit pkt0 X 0 1 2 3 0 1 2 pkt0 (b) oops! receiver window (after receipt) 0 1 2 3 0 1 2 0 1 2 3 0 1 2 0 1 2 3 0 1 2 will accept packet with seq number 0 From Computer Networking, by Kurose&Ross Transport Layer 3-59 pkt0 pkt1 pkt2 0 1 2 3 0 1 2 pkt3 X 0 1 2 3 0 1 2 pkt0 (a) no problem 0 1 2 3 0 1 2 0 1 2 3 0 1 2 0 1 2 3 0 1 2 will accept packet with seq number 0 receiver can t see sender side. receiver behavior identical in both cases! something s (very) wrong! Maximum window size with selective repeat If seq# size = K ( The modulo ) Q: What is the maximum window size N? A: N K/2 Suppose K = 4, N =3 K = 4, but N =2 Retransmission of received pkts fall outside rec window Sender 0 1 2 3 0 1 2 3 0 1 2 3 0 1 2 3 Receiver 0 1 2 3 0 1 2 3 0 1 2 3 0 1 2 3 (a) (b) (c) (d) (a) Initial situation with a window of size N=3. (b) After 3 frames have been sent and received but not ACK ed. The upper side of the rec window falls in the sending window (c) Initial situation with a window of size N=2. (d) After 2 frames sent and received but not ACK ed. The upper side of the rec window does not fall in the sending window From Computer Networking, by Kurose&Ross Transport Layer 3-60 30

Maximum window size: general principle Notations: seq# space = [0..K-1], i.e. modulo K Maximum sender window size = Ns Maximum receiver window size = Nr Requirement: Ns + Nr K Particular cases: GBN: Nr = 1, Ns K - 1 Selective repeat: Ns = Nr K/2 Alternating-bit: K = 2, Ns = Nr = 1 Special case of both GBN and Selective Repeat From Computer Networking, by Kurose&Ross Transport Layer 3-61 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 flow control connection management 3.6 Principles of congestion control 3.7 TCP congestion control From Computer Networking, by Kurose&Ross Transport Layer 3-62 31

TCP: Overview RFCs: 793, 1122, 1323, 2018, 2581 point-to-point: one sender, one receiver reliable, in-order byte stream: no message boundaries pipelined: TCP congestion and flow control set window size full duplex data: bi-directional data flow in same connection MSS: maximum segment size connection-oriented: handshaking (exchange of control msgs) inits sender, receiver state before data exchange flow controlled: sender will not overwhelm receiver From Computer Networking, by Kurose&Ross Transport Layer 3-63 TCP segment structure URG: urgent data (generally not used) ACK: ACK # valid PSH: push data now (generally not used) RST, SYN, FIN: connection estab (setup, teardown commands) Internet checksum (as in UDP) 32 bits source port # dest port # head len sequence number acknowledgement number not used checksum U A P R S F Receive window Urg data pnter Options (variable length) application data (variable length) counting by bytes of data (not segments!) # bytes rcvr willing to accept From Computer Networking, by Kurose&Ross Transport Layer 3-64 32

TCP seq. numbers, ACKs sequence numbers: byte stream number of first byte in segment s data acknowledgements: seq # of next byte expected from other side cumulative ACK Q: how receiver handles outof-order segments A: TCP spec doesn t say, - outgoing segment from sender source port # dest port # sequence number acknowledgement number rwnd checksum sent ACKed urg pointer window size N sender sequence number space sent, notyet ACKed ( inflight ) incoming segment to sender up to implementor source port # dest port # sequence number acknowledgement number A rwnd checksum usable but not yet sent urg pointer not usable From Computer Networking, by Kurose&Ross Transport Layer 3-65 TCP seq. numbers, ACKs Host A Host B User types C host ACKs receipt of echoed C Seq=42, ACK=79, data = C Seq=79, ACK=43, data = C Seq=43, ACK=80 host ACKs receipt of C, echoes back C simple telnet scenario From Computer Networking, by Kurose&Ross Transport Layer 3-66 33

TCP Round Trip Time and Timeout Q: how to set TCP timeout value? longer than RTT but RTT varies too short: premature timeout unnecessary retransmissions too long: slow reaction to segment loss Over a network 0.3 T 0.3 T 1 T 2 Over a link 0.2 Prob. density 0.1 0.2 Prob. density 0.1 0 0 1 2 3 4 5 RTT (msec) From Computer Networking, by Kurose&Ross Transport Layer 3-67 From Computer Networks, by Tanenbaum Prentice Hall 0 0 10 20 30 40 50 RTT (msec) TCP Round Trip Time and Timeout Q: how to estimate RTT? SampleRTT: measured time from segment transmission until ACK receipt ignore retransmissions SampleRTT will vary, want estimated RTT smoother average several recent measurements, not just current SampleRTT EstimatedRTT = (1-α)*EstimatedRTT + α*samplertt Exponential weighted moving average Influence of past sample decreases exponentially fast Typical value: α = 0.125 From Computer Networking, by Kurose&Ross Transport Layer 3-68 34

Example RTT estimation: RTT: gaia.cs.umass.edu to fantasia.eurecom.fr 350 RTT: gaia.cs.umass.edu to fantasia.eurecom.fr 300 RTT (milliseconds) RTT (milliseconds) 250 200 150 100 1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106 time (seconnds) time (seconds) SampleRTT Estimated RTT samplertt EstimatedRTT From Computer Networking, by Kurose&Ross Transport Layer 3-69 TCP Round Trip Time, timeout Timeout interval: EstimatedRTT plus safety margin large variation in EstimatedRTT -> larger safety margin Estimate SampleRTT deviation from EstimatedRTT: DevRTT = (1-β)*DevRTT + β* SampleRTT-EstimatedRTT (typically, β = 0.25) TimeoutInterval = EstimatedRTT + 4*DevRTT estimated RTT safety margin From Computer Networking, by Kurose&Ross Transport Layer 3-70 35

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 flow control connection management 3.6 Principles of congestion control 3.7 TCP congestion control From Computer Networking, by Kurose&Ross Transport Layer 3-71 TCP reliable data transfer TCP creates rdt service on top of IP s unreliable service pipelined segments cumulative acks single retransmission timer Retransmissions triggered by: timeout events duplicate acks Let s initially consider simplified TCP sender: ignore duplicate acks ignore flow control, congestion control From Computer Networking, by Kurose&Ross Transport Layer 3-72 36

TCP sender events: data rcvd from app: create segment with seq # seq # is byte-stream number of first data byte in segment start timer if not already running think of timer as for oldest unacked segment expiration interval: TimeOutInterval timeout: retransmit segment that caused timeout restart timer Ack rcvd: if ack acknowledges previously unacked segments update what is known to be acked start timer if there are still unacked segments From Computer Networking, by Kurose&Ross Transport Layer 3-73 TCP sender (simplified) Λ NextSeqNum = InitialSeqNum SendBase = InitialSeqNum wait for event ACK received, with ACK field value y data received from application above create segment, seq. #: NextSeqNum pass segment to IP (i.e., send ) NextSeqNum = NextSeqNum + length(data) if (timer currently not running) start timer timeout retransmit not-yet-acked segment with smallest seq. # start timer if (y > SendBase) { SendBase = y /* SendBase 1: last cumulatively ACKed byte */ if (there are currently not-yet-acked segments) start timer else stop timer } From Computer Networking, by Kurose&Ross Transport Layer 3-74 37

TCP: retransmission scenarios Host A Host B Host A Host B Seq=92, 8 bytes of data SendBase=92 Seq=92, 8 bytes of data timeout X ACK=100 timeout Seq=100, 20 bytes of data ACK=100 ACK=120 Seq=92, 8 bytes of data ACK=100 lost ACK scenario SendBase=100 SendBase=120 SendBase=120 Seq=92, 8 bytes of data ACK=120 premature timeout From Computer Networking, by Kurose&Ross Transport Layer 3-75 TCP: retransmission scenarios Host A Host B Seq=92, 8 bytes of data Seq=100, 20 bytes of data timeout X ACK=100 ACK=120 Seq=120, 15 bytes of data cumulative ACK From Computer Networking, by Kurose&Ross Transport Layer 3-76 38

TCP ACK generation [RFC 1122, RFC 2581] Event at Receiver Arrival of in-order segment with expected seq #. All data up to expected seq # already ACKed Arrival of in-order segment with expected seq #. One other segment has ACK pending Arrival of out-of-order segment higher-than-expect seq. #. Gap detected Arrival of segment that partially or completely fills gap TCP Receiver action Delayed ACK. Wait up to 500ms for next segment. If no next segment, send ACK Immediately send single cumulative ACK, ACKing both in-order segments Immediately send duplicate ACK, indicating seq. # of next expected byte (if SACK option, also send selective ACK) Immediately send ACK, provided that segment starts at lower end of gap From Computer Networking, by Kurose&Ross Transport Layer 3-77 TCP fast retransmit time-out period often relatively long: long delay before resending lost packet detect lost segments via duplicate ACKs sender often sends many segments backto-back if segment is lost, there will likely be many duplicate ACKs TCP fast retransmit if sender receives 4 ACKs for same data ( triple duplicate ACKs ), resend unacked segment with smallest seq #! likely that unacked segment lost, so don t wait for timeout From Computer Networking, by Kurose&Ross Transport Layer 3-78 39

TCP fast retransmit Host A Host B Seq=92, 8 bytes of data Seq=100, 20 bytes of data X fast retransmit after sender receipt of triple duplicate ACK timeout ACK=100 ACK=100 ACK=100 ACK=100 Seq=100, 20 bytes of data From Computer Networking, by Kurose&Ross Transport Layer 3-79 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 flow control connection management 3.6 Principles of congestion control 3.7 TCP congestion control From Computer Networking, by Kurose&Ross Transport Layer 3-80 40

TCP flow control application may remove data from TCP socket buffers. slower than TCP receiver is delivering (sender is sending) application process TCP socket receiver buffers TCP code application OS flow control receiver controls sender, so sender won t overflow receiver s buffer by transmitting too much, too fast from sender IP code receiver protocol stack From Computer Networking, by Kurose&Ross Transport Layer 3-81 TCP flow control receiver advertises free buffer space by including rwnd value in TCP header of receiver-to-sender segments RcvBuffer size set via socket options (typical default is 4096 bytes) many operating systems autoadjust RcvBuffer sender limits amount of unacked ( in-flight ) data to receiver s rwnd value guarantees receive buffer will not overflow to application process RcvBuffer buffered data rwnd free buffer space TCP segment payloads receiver-side buffering From Computer Networking, by Kurose&Ross Transport Layer 3-82 41

Nagle algorithm When data come in from socket one byte at the time send first byte and buffer all the rest until the outstanding byte is acknowledged sends other segments as one per RTT Useful for Telnet Otherwise: 41 bytes segments containing 1 byte of data 40 bytes of TCP/IP header overhead From Computer Networking, by Kurose&Ross Transport Layer 3-83 Silly window syndrome Still a problem with applications reading one byte at a time Receiver s buffer is full Application reads 1 byte Room for one more byte Header Header 1 Byte Window update segment sent New byte arrives Receiver s buffer is full Clark's solution: receiver sends a window update only if the buffer is half empty, or if a full segment can be received From Computer Networking, by Kurose&Ross Transport Layer 3-84 From Computer Networks, by Tanenbaum Prentice Hall 42

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 flow control connection management 3.6 Principles of congestion control 3.7 TCP congestion control From Computer Networking, by Kurose&Ross Transport Layer 3-85 Connection Management before exchanging data, sender/receiver handshake : agree to establish connection (each knowing the other willing to establish connection) agree on connection parameters application connection state: ESTAB connection variables: seq # client-to-server server-to-client rcvbuffer size at server,client network application connection state: ESTAB connection Variables: seq # client-to-server server-to-client rcvbuffer size at server,client network Socket clientsocket = newsocket("hostname","port number"); Socket connectionsocket = welcomesocket.accept(); From Computer Networking, by Kurose&Ross Transport Layer 3-86 43

Agreeing to establish a connection 2-way handshake: Let s talk ESTAB OK ESTAB Q: will 2-way handshake always work in network? variable delays retransmitted messages (e.g. req_conn(x)) due to message loss message reordering can t see other side choose x ESTAB req_conn(x) acc_conn(x) ESTAB From Computer Networking, by Kurose&Ross Transport Layer 3-87 Agreeing to establish a connection 2-way handshake failure scenarios: choose x retransmit req_conn(x) req_conn(x) acc_conn(x) ESTAB choose x retransmit req_conn(x) req_conn(x) acc_conn(x) ESTAB ESTAB client terminates req_conn(x) connection x completes server forgets x ESTAB retransmit data(x+1) client terminates data(x+1) connection x completes req_conn(x) accept data(x+1) server forgets x half open connection! (no client!) ESTAB data(x+1) ESTAB accept data(x+1) From Computer Networking, by Kurose&Ross Transport Layer 3-88 44

TCP 3-way handshake client state LISTEN SYNSENT ESTAB choose init seq num, x send TCP SYN msg without data; start timer for SYN retransmission received SYNACK(x+1) indicates server is live; send ACK for SYNACK; this segment may contain client-to-server data with Seq=x+1 SYNbit=1, Seq=x SYNbit=1, Seq=y ACKbit=1; ACKnum=x+1 ACKbit=1, ACKnum=y+1 choose init seq num, y send TCP SYNACK msg, acking SYN received ACK(y+1) indicates client is live; allocate buffer server state LISTEN SYN RCVD ESTAB If server port not open, TCP server sends back a RST segment From Computer Networking, by Kurose&Ross Transport Layer 3-89 TCP 3-way handshake: FSM closed Socket connectionsocket = welcomesocket.accept(); SYN(x) SYNACK(seq=y,ACKnum=x+1) create new socket for communication back to client Λ listen Socket clientsocket = newsocket("hostname","port number"); SYN(seq=x) SYN rcvd SYN sent ACK(ACKnum=y+1) Λ ESTAB SYNACK(seq=y,ACKnum=x+1) ACK(ACKnum=y+1) From Computer Networking, by Kurose&Ross Transport Layer 3-90 45

TCP: closing a connection client, server each close their sending side of connection send TCP segment with FIN bit = 1 symmetric/graceful release respond to received FIN with ACK on receiving FIN, ACK can be combined with own FIN simultaneous FIN exchanges can be handled From Computer Networking, by Kurose&Ross Transport Layer 3-91 TCP: closing a connection client state server state ESTAB ESTAB clientsocket.close() FIN_WAIT_1 FIN_WAIT_2 can no longer send but can receive data wait for server close FINbit=1, seq=x ACKbit=1; ACKnum=x+1 can still send data CLOSE_WAIT TIMED_WAIT timed wait for 2*max segment lifetime; will respond with ACK to received FINs FINbit=1, seq=y ACKbit=1; ACKnum=y+1 can no longer send data LAST_ACK CLOSED CLOSED FINs have seq#: can recover loss of last data bytes From Computer Networking, by Kurose&Ross Transport Layer 3-92 46

TCP: choosing initial seq# Consider choosing x connect connected client server SYN (seq=x) SYNACK (seq=y, ack=x+1) ACK (seq=x+1, ack=y+1) listen + accept connected Constraint: x not used recently in a former connection by this client (i.e. same port#) to send TCP segments to this server (same port#) Recently = less than packet lifetime Otherwise a still alive duplicate segment from the former connection can be confused with this SYN From Computer Networking, by Kurose&Ross Transport Layer 3-93 TCP: choosing initial seq# (2) 1. In practice If client can only reuse the same port# when a maximum packet lifetime has elapsed after the last packet has been sent, and If, when a host fails (= unknown duration), it waits at least a maximum packet lifetime before opening the first connection, Then, any initial seq# x and y are OK, they could be randomly chosen 2. In principle, we could avoid these delays by using clocks (see next slide) From Computer Networking, by Kurose&Ross Transport Layer 3-94 47

Using clocks Let T be the maximum packet lifetime in the network Each host has a "clock that never stops running, even when the station is down that need not be synchronized with other clocks The clock is merely a regular and reliable counter of say k bits such that 2 k ticks > T 2 k -1 t From Computer Networking, by Kurose&Ross Transport Layer 3-95 Using clocks (2) clock z y Numbers used by connection T Initial seq# x is the local clock value Don t enter forbidden region (neither from top, nor from bottom) Throughput is bounded! x t SYN x is sent DT x+1 is sent DT x+k is sent SYN y is sent New connection between same ports of same hosts At that time, an old duplicate DT z From Computer Networking, by Kurose&Ross Transport Layer 3-96 and a new packet DT z can be both present in the network 48

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 flow control connection management 3.6 Principles of congestion control 3.7 TCP congestion control From Computer Networking, by Kurose&Ross Transport Layer 3-97 Principles of Congestion Control Transmission rate adjustment Need flow control Transmission network Need congestion control Internal congestion Small-capacity receiver informally: too many sources sending too much data too fast for network to handle ; different from flow control! manifestations: lost packets (buffer overflow at routers) long delays (queuing in router buffers) a top-10 problem! Large-capacity receiver From Computer Networks, by Tanenbaum Prentice Hall From Computer Networking, by Kurose&Ross Transport Layer 3-98 49

Causes/costs of congestion: scenario 1 two senders, two receivers one router, infinite buffers output link capacity: R no retransmission Host B original data: λ in Host A unlimited shared output link buffers throughput: λ out R R/2 λ out delay λ in R/2 λ in R/2 maximum per-connection throughput: R/2 large delays as arrival rate, λ in, approaches capacity From Computer Networking, by Kurose&Ross Transport Layer 3-99 Causes/costs of congestion: scenario 2 one router, finite buffers sender retransmission of timed-out packet application-layer input = application-layer output: λ in = λ out goodput transport-layer input includes retransmissions : λ in λ in λ in : original data λ' in : original data, plus retransmitted data λ out Host A finite shared output Host B link buffers From Computer Networking, by Kurose&Ross Transport Layer 3-100 50

Causes/costs of congestion: scenario 2 Idealization 1: perfect knowledge = sender sends only when router buffers available R/2 λ out λ in R/2 copy λ in : original data λ' in : original data, plus retransmitted data λ out Host A free buffer space! R finite shared output Host B link buffers From Computer Networking, by Kurose&Ross Transport Layer 3-101 Causes/costs of congestion: scenario 2 Idealization 2: known loss - packets can be lost, dropped at router due to full buffers - sender only resends if packet known to be lost copy λ in : original data λ' in : original data, plus retransmitted data λ out Host A no buffer space! Host B From Computer Networking, by Kurose&Ross Transport Layer 3-102 51

Causes/costs of congestion: scenario 2 Idealization 2: known loss R/2 λ out λ in R/2 when sending at R/2, some packets are retransmissions but asymptotic goodput is still R/2 (why?) λ in : original data λ' in : original data, plus retransmitted data λ out Host A free buffer space! R Host B From Computer Networking, by Kurose&Ross Transport Layer 3-103 Causes/costs of congestion: scenario 2 Realistic: duplicates " packets can be lost, dropped at router due to full buffers " sender times out prematurely, sending two copies, both of which are delivered R/2 λ out λ in R/2 when sending at R/2, some packets are retransmissions including duplicates that are delivered! timeout copy λ in λ' in λ out Host A free buffer space! Host B From Computer Networking, by Kurose&Ross Transport Layer 3-104 52

Causes/costs of congestion: scenario 2 Realistic: duplicates " packets can be lost, dropped at router due to full buffers " sender times out prematurely, sending two copies, both of which are delivered λ out R/2 λ in R/2 when sending at R/2, some packets are retransmissions including duplicated that are delivered! costs of congestion: " more work (retransmissions) for given goodput " unneeded retransmissions: link carries multiple copies of packet! decreasing goodput From Computer Networking, by Kurose&Ross Transport Layer 3-105 Causes/costs of congestion: scenario 3 four senders multihop paths timeout/retransmit Q: what happens as λ in and λ in increase? A: as red λ in increases, all arriving blue packets at upper queue are dropped, blue throughput! 0 Host A λ in : original data λ' in : original data, plus retransmitted data finite shared output link buffers λ out Host B Host D Host C From Computer Networking, by Kurose&Ross Transport Layer 3-106 53

Causes/costs of congestion: scenario 3 C/2 λ out λ in C/2 another cost of congestion: " when packet dropped, any upstream transmission capacity used for that packet was wasted! From Computer Networking, by Kurose&Ross Transport Layer 3-107 Approaches towards congestion control Two broad approaches towards congestion control: End-end congestion control: no explicit feedback from network congestion inferred from end-system observed loss, delay approach taken by TCP Network-assisted congestion control: routers provide feedback to end-systems single bit indicating congestion (SNA, DECbit, TCP/IP ECN, ATM) explicit rate for sender to send at (ATM ABR) Not covered in this course From Computer Networking, by Kurose&Ross Transport Layer 3-108 54