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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 Transport Layer 3-1 Transport services and protocols provide logical communication between app processes running on different hosts transport protocols run in end systems more than one transport protocol available to apps Internet: TCP and UDP application transport network data link physical application transport network data link physical Transport Layer 3-2 Process socket Port 9876 UDP Process socket Port 9999 UDP 2: Application Layer 3 Process welcome socket Port 6789 socket Process socket Port 9999 Port 6789 TCP TCP Host S Host C 2: Application Layer 4 Transport vs. network layer network layer: delivers packets between hosts transport layer: delivers packets between processes relies on & enhances network layer services Transport Layer 3-5 Internet transport-layer protocols reliable, in-order delivery (TCP) connection setup congestion control flow control 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 physicalnetwork network data link physical data link physical network data link physical application transport network data link physical 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 reliable data transfer flow control connection management 3.6 Principles of congestion control 3.7 TCP congestion control Transport Layer 3-7 Multiplexing/demultiplexing Multiplexing at send host: gathering data from multiple sockets, enveloping data with header (later used for demultiplexing) Demultiplexing at rcv host: delivering received segments to correct socket = socket application transport network link = process P3 P1 P1 application transport network P2 P4 application transport network link link physical host 1 physical host 2 physical host 3 Transport Layer 3-8 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 UDP: host uses port numbers to direct segment to appropriate socket TCP: 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 Transport Layer 3-9 Connectionless demultiplexing When host receives UDP segment: checks destination port number in segment directs UDP segment to socket with that port number Transport Layer 3-10 Connection-oriented demux TCP socket (connection) identified by 4-tuple: source IP address source port number dest IP address dest port number receiving host 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 Transport Layer 3-11 Connection-oriented demux (cont) P1 P4 P5 P2 P6 P1P3 SP: 5775 DP: 80 S-IP: B D-IP:C SP: 9157 client IP: A DP: 80 S-IP: A D-IP:C SP: 9157 server IP: C DP: 80 S-IP: B D-IP:C Client IP:B Transport Layer 3-12 Connection-oriented demux: Threaded Web Server P1 P2 P4 P1P3 SP: 5775 DP: 80 S-IP: B D-IP:C SP: 9157 client IP: A DP: 80 S-IP: A D-IP:C SP: 9157 server IP: C DP: 80 S-IP: B D-IP:C Client IP:B Transport Layer 3-13 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 Transport Layer 3-14 UDP: User Datagram Protocol [RFC 768] “no frills,” “bare bones” Internet transport protocol “best effort” service, UDP segments may be: lost delivered out of order to app connectionless: no handshaking between UDP sender, receiver each UDP segment handled independently of others Why is there a UDP? no connection establishment (which can add delay) simple: no connection state at sender, receiver small segment header no congestion control: UDP can blast away as fast as desired Transport Layer 3-15 UDP: more often used for streaming multimedia apps loss tolerant rate sensitive 32 bits Length, in source port # bytes of UDP length segment, including reliable transfer over UDP: header add reliability at application layer application-specific error recovery! dest port # checksum Application data (message) UDP segment format Transport Layer 3-16 UDP checksum Goal: detect “errors” (e.g., flipped bits) in transmitted segment Sender: Receiver: treat segment contents compute checksum of as sequence of 16-bit integers checksum: addition (1’s complement sum) of segment contents sender puts checksum value into UDP checksum field received segment check if computed checksum equals checksum field value: NO - error detected YES - no error detected. But maybe errors nonetheless? More later …. Transport Layer 3-17 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 1 wraparound 1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1 sum 1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 0 checksum 1 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1 Transport Layer 3-18 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 Transport Layer 3-19 Principles of Reliable data transfer important in app., transport, link layers top-10 list of important networking topics! characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt) Transport Layer 3-20 Principles of Reliable data transfer important in app., transport, link layers top-10 list of important networking topics! characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt) Transport Layer 3-21 Principles of Reliable data transfer important in app., transport, link layers top-10 list of important networking topics! characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt) Transport Layer 3-22 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! The book uses finite state machines (FSM) to specify sender, receiver Transport Layer 3-23 Rdt1.0: reliable transfer over a reliable channel underlying channel perfectly reliable no bit errors no loss of packets no duplicates first-in-first-out Sender Send pkt Receiver Rcv pkt Send pkt Rcv pkt Send pkt Rcv pkt Transport Layer 3-24 Rdt2.0: channel with bit errors 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 receiver feedback: control msgs (ACK,NAK) from receiver to sender Transport Layer 3-25 rdt2.0 in action Sender Send pkt Rcv ACK Send pkt Rcv ACK Send pkt Receiver Rcv pkt Send ACK Rcv pkt Send ACK Rcv pkt Send ACK Rcv ACK Without error Sender Receiver Send pkt Rcv pkt Send ACK Rcv ACK Send pkt x Rcv NAK Re-send pkt Rcv pkt Dct errors Send NAK Rcv pkt Send ACK Rcv ACK With errors Transport Layer 3-26 rdt2.0 has a fatal flaw! What happens if ACK/NAK corrupted? sender doesn’t know what happened at receiver! Remedy: sender retransmits current pkt if ACK/NAK garbled problem: may introduce duplicates 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 stop and wait Sender sends one packet, then waits for receiver response Transport Layer 3-27 rdt2.1 in action Sender Send pkt0 Rcv ACK0 Send pkt1 Rcv ACK1 Send pkt0 Receiver Rcv pkt0 Send ACK Rcv pkt1 Send ACK Rcv pkt0 Send ACK Rcv ACK0 Without error Sender Receiver Send pkt0 Rcv pkt0 Send ACK Rcv ACK Send pkt1 x Rcv NAK Re-send pkt1 Rcv garbled ACK/NAK Re-send pkt1 x Rcv garbled pkt Send NAK Rcv pkt1 Send ACK Rcv pkt1 Discard pkt1 With errors Transport Layer 3-28 rdt2.1: discussion Sender: seq # added to pkt two seq. #’s (0,1) will suffice. must check if received ACK/NAK corrupted 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 Transport Layer 3-29 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 if pkt (or ACK) just delayed (not lost): retransmission will be duplicate, but use of seq. #’s already handles this receiver must specify seq # of pkt being ACKed requires countdown timer Transport Layer 3-30 rdt3.0 in action Transport Layer 3-31 rdt3.0 in action Transport Layer 3-32 Performance of rdt3.0 rdt3.0 works, but performance stinks sender receive RTT Transport Layer 3-33 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 Transport Layer 3-34 Pipelining: increased utilization sender receiver first packet bit transmitted, last bit transmitted RTT first packet bit arrives last packet bit arrives, send ACK last bit of 2nd packet arrives, send ACK last bit of 3rd packet arrives, send ACK ACK arrives, send next packet Transport Layer 3-35 Pipelining Protocols Go-back-N: overview sender: up to N unACKed pkts in pipeline receiver: only sends cumulative ACKs doesn’t ACK pkt if there’s a gap sender: has timer for oldest unACKed pkt Selective Repeat: overview sender: up to N unACKed packets in pipeline receiver: ACKs individual pkts sender: maintains timer for each unACKed pkt if timer expires: retransmit only unACKed packet if timer expires: retransmit all unACKed packets Transport Layer 3-36 Go-Back-N Sender: k-bit seq # in pkt header “window” of up to N, consecutive unACKed pkts allowed ACK(n): ACKs all pkts up to, including seq # n - “cumulative ACK” may receive duplicate ACKs (see receiver) timer for the first pkt in window timeout(n): retransmit pkt n and all higher seq # pkts in window Transport Layer 3-37 GBN: receiver ACK-only: always send ACK for correctly-received pkt with highest in-order seq # may generate duplicate ACKs need only remember expectedseqnum out-of-order pkt: discard (don’t buffer) -> no receiver buffering! Re-ACK pkt with highest in-order seq # Transport Layer 3-38 GBN in action Transport Layer 3-39 Selective Repeat receiver individually acknowledges all correctly received pkts buffers pkts, as needed, for eventual in-order delivery to upper layer sender only resends pkts for which ACK not received sender timer for each unACKed pkt sender window N consecutive seq #’s again limits seq #s of sent, unACKed pkts Transport Layer 3-40 Selective repeat: sender, receiver windows Transport Layer 3-41 Selective repeat sender data from above : receiver pkt n in [rcvbase, rcvbase+N-1] if next available seq # in send ACK(n) timeout(n): in-order: deliver (also window, send pkt resend pkt n, restart timer ACK(n) in [sendbase,sendbase+N1]: mark pkt n as received if n smallest unACKed pkt, advance window base to next unACKed seq # out-of-order: buffer deliver buffered, in-order pkts), advance window to next not-yet-received pkt pkt n in [rcvbase-N,rcvbase-1] ACK(n) otherwise: ignore Transport Layer 3-42 Selective repeat in action Transport Layer 3-43 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) Q: what relationship between seq # size and window size? Transport Layer 3-44 Two-way communication Two-way communication, say, between A and B Run two GBN’s (or SR’s) In one of them, A is sender, B receiver In the other, B is sender, A receiver A and B each send both data packets and ACKs Piggybacking Transport Layer 3-45 Example Sender Receiver Sender Receiver pkt0 ACK0 ACK4 pkt5 pkt1 ACK1 pkt2 ACK2 ACK5 pkt6 ACK6 pkt7 ACK7 Transport Layer 3-46 Piggybacking Sender Receiver Send pkt0,4 Send pkt5,0 Send pkt1,5 Send pkt6,1 Send pkt2,6 Send pkt7,2 Send ACK7 ACK(n): ACKs all pkts up to seq # n Transport Layer 3-47 Alternative ACK(n) in GBN Sender Receiver Sender Receiver Send pkt0 Send pkt0 Send ACK1 Send ACK0 Send pkt1 Send pkt1 Send ACK2 Send ACK1 Send pkt2 Send pkt2 Send ACK2 ACK(n): ACKs all pkts up to seq # n Send ACK3 ACK(n): ACKs all pkts up to seq # n-1 Transport Layer 3-48 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 Transport Layer 3-49 TCP: Overview point-to-point: one sender, one receiver reliable, in-order byte steam: no “message boundaries” pipelined: TCP congestion and flow control set window size send & receive buffers socket door application writes data application reads data TCP send buffer TCP receive buffer RFCs: 793, 1122, 1323, 2018, 2581 full duplex data: bi-directional data flow in same connection MSS: maximum segment size connection-oriented: handshaking (exchange of control msgs) init’s sender, receiver state before data exchange flow controlled: sender will not socket door overwhelm receiver segment Transport Layer 3-50 TCP segment structure 32 bits 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) source port # dest port # sequence number acknowledgement number head not UA P R S F len used checksum Receive window Urg data pointer Options (variable length) counting by bytes of data (not segments!) # bytes rcvr willing to accept application data (variable length) Transport Layer 3-51 TCP seq. #’s and ACKs Seq. #’s: byte stream “number” of first byte in segment’s data ACKs: seq # of next byte expected from other side cumulative ACK Q: how receiver handles out-of-order segments A: TCP spec doesn’t say, - up to implementer Host A User types ‘C’ Host B host ACKs receipt of ‘C’, echoes back ‘C’ host ACKs receipt of echoed ‘C’ simple telnet scenario time Transport Layer 3-52 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 Transport Layer 3-53 TCP reliable data transfer TCP creates rdt service on top of IP’s unreliable service pipelined segments cumulative ACKs TCP uses single retransmission timer retransmissions are triggered by: timeout events duplicate ACKs initially consider simplified TCP sender: ignore duplicate ACKs ignore flow control, congestion control Transport Layer 3-54 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 acknowledges previously unACKed segments update what is known to be ACKed start timer if there are outstanding segments TimeoutInterval = EstimatedRTT + 4*DevRTT Transport Layer 3-55 NextSeqNum = InitialSeqNum SendBase = InitialSeqNum loop (forever) { switch(event) event: data received from application above create TCP segment with sequence number NextSeqNum if (timer currently not running) start timer pass segment to IP NextSeqNum = NextSeqNum + length(data) event: timer timeout retransmit not-yet-acknowledged segment with smallest sequence number start timer event: ACK received, with ACK field value of y if (y > SendBase) { SendBase = y if (there are currently not-yet-acknowledged segments) start timer } } /* end of loop forever */ TCP sender (simplified) Comment: • SendBase-1: last cumulatively ACKed byte Example: • SendBase-1 = 71; y= 73, so the rcvr wants 73+ ; y > SendBase, so that new data is ACKed Transport Layer 3-56 TCP: retransmission scenarios Host A X loss Sendbase = 100 SendBase = 120 SendBase = 100 time SendBase = 120 lost ACK scenario Host B Seq=92 timeout Host B Seq=92 timeout timeout Host A time premature timeout Transport Layer 3-57 TCP retransmission scenarios (more) timeout Host A Host B X loss SendBase = 120 time Cumulative ACK scenario Transport Layer 3-58 TCP ACK generation [RFC 1122, RFC 2581] Event at Receiver TCP Receiver action Arrival of in-order segment with expected seq #. All data up to expected seq # already ACKed Delayed ACK. Wait up to 500ms for next segment. If no next segment, send ACK Arrival of in-order segment with expected seq #. One other segment has ACK pending Immediately send single cumulative ACK, ACKing both in-order segments Arrival of out-of-order segment higher-than-expect seq. # . Gap detected Immediately send (duplicate) ACK, indicating seq. # of next expected byte Arrival of segment that partially or completely fills gap Immediate send ACK, provided that segment starts at lower end of gap Transport Layer 3-59 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 back-toback if segment is lost, there will likely be many duplicate ACKs for that segment If sender receives 3 duplicate ACKs (i.e., a total of 4 ACKs) for same data, it assumes that segment after ACKed data was lost: fast retransmit: resend segment before timer expires Transport Layer 3-60 Host A seq # x1 seq # x2 seq # x3 seq # x4 seq # x5 Host B X ACK x1 ACK x1 ACK x1 ACK x1 timeout triple duplicate ACKs time Transport Layer 3-61 Fast retransmit algorithm: event: ACK received, with ACK field value of y if (y > SendBase) { SendBase = y if (there are currently not-yet-acknowledged segments) start timer } else /* y < SendBase */ { increment count of dup ACKs received for y if (count of dup ACKs received for y = 3) { resend segment with sequence number y } a duplicate ACK for already ACKed segment fast retransmit Transport Layer 3-62 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 Transport Layer 3-63 TCP Flow Control receive side of TCP connection has a receive buffer: (currently) TCP data application IP unused buffer (in buffer) process datagrams space flow control sender won’t overflow receiver’s buffer by transmitting too much, too fast speed-matching service: matching send rate to receiving application’s drain rate app process may be slow at reading from buffer Transport Layer 3-64 TCP Flow control: how it works (currently) TCP data application IP unused buffer (in buffer) process datagrams space rwnd RcvBuffer (suppose TCP receiver discards out-of-order segments) unused buffer space: receiver: advertises unused buffer space by including rwnd value in segment header sender: limits # of unACKed bytes to rwnd guarantees receiver’s buffer doesn’t overflow = rwnd = RcvBuffer-[LastByteRcvd LastByteRead] Transport Layer 3-65 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 Transport Layer 3-66 Process welcome socket Port 6789 socket Process socket Port 9999 Port 6789 TCP TCP Host S Host C 2: Application Layer 67 TCP Connection Management Recall: TCP sender, receiver establish “connection” before exchanging data segments initialize TCP variables: seq. #s buffers, flow control info (e.g. RcvWindow) client: connection initiator Socket clientSocket = new Socket("hostname","port number"); server: contacted by client Socket connectionSocket = welcomeSocket.accept(); Three way handshake: Step 1: client host sends TCP SYN segment to server specifies initial seq # no data Step 2: server host receives SYN, replies with SYNACK segment server allocates buffers specifies server initial seq. # Step 3: client receives SYNACK, replies with ACK segment, which may contain data Transport Layer 3-68 Three-way handshak client server • x, y randomly chosen. Why? •The third segment may carry data Transport Layer 3-69 TCP Connection Management (cont.) Closing a connection: client closes socket: clientSocket.close(); client close Step 1: client sends TCP close FIN, replies with ACK. Closes connection, sends FIN. timed wait FIN control segment to server Step 2: server receives server closed Transport Layer 3-70 TCP Connection Management (cont.) Step 3: client receives FIN, replies with ACK. client server closing Enters “timed wait” will respond with ACK to received FINs closing Step 4: server, receives Note: with small modification, can handle simultaneous FINs. timed wait ACK. Connection closed. closed closed Transport Layer 3-71 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 Transport Layer 3-72 Principles of Congestion Control Congestion: 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 (queueing in router buffers) a top-10 problem! Transport Layer 3-73 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 explicit rate sender should send at Transport Layer 3-74 TCP congestion control: goal: TCP sender should transmit as fast as possible, but without congesting network Q: how to find rate just below congestion level decentralized: each TCP sender sets its own rate, based on implicit feedback: ACK: segment received (a good thing!), network not congested, so increase sending rate lost segment: assume loss due to congested network, so decrease sending rate Transport Layer 3-75 TCP congestion control: bandwidth probing “probing for bandwidth”: increase transmission rate on receipt of ACK, until eventually loss occurs, then decrease transmission rate continue to increase on ACK, decrease on loss (since available bandwidth is changing, depending on other connections in network) sending rate ACKs being received, so increase rate X X loss, so decrease rate X X TCP’s “sawtooth” behavior X time Q: how fast to increase/decrease? details to follow Transport Layer 3-76 TCP Congestion Control: overview three “phases” slow start congestion avoidance fast recovery important variables: cwnd ssthresh (slow start threshold) send window size = min(cwnd,rwnd) cwnd is dynamic, function of perceived network congestion cwnd is ajusted by the congestion control algorithm Transport Layer 3-77 TCP congestion control FSM: overview cwnd > ssthresh slow start loss: timeout congestion avoidance loss: timeout loss: timeout loss: 3dupACK new ACK loss: 3dupACK fast recovery Transport Layer 3-78 TCP Slowstart Host A • init cwnd = 1 MSS • increase cwnd by 1MSS for each segment ACKed until loss event or when threshold reached RTT Slowstart algorithm Host B cwnd doubles per RTT (starts at a slow rate but increases fast) loss event: timeout or three duplicate ACKs time 2: Transport Layer 4 79 TCP Congestion Avoidance when cwnd > ssthresh grow cwnd linearly increase cwnd by 1 MSS per RTT implementation: increase cwnd by MSS/(cwnd /MSS) for each ACK received ssthresh: one half of cwnd when last loss event occurred 2: Transport Layer 4 80 Segment loss event ssthresh = cwnd/2 timeout: cut cwnd to 1 enter slowstart 3 duplicate ACKs (Tahoe): same as timeout 3 duplicate ACKs (Reno): cwnd = cwnd/2 + 3 enter fast recovery TCP congestion control FSM: details duplicate ACK dupACKcount++ L cwnd = 1 MSS ssthresh = 64 KB dupACKcount = 0 slow start timeout ssthresh = cwnd/2 cwnd = 1 MSS dupACKcount = 0 retransmit missing segment dupACKcount == 3 ssthresh= cwnd/2 cwnd = ssthresh + 3 retransmit missing segment new ACK cwnd = cwnd+MSS dupACKcount = 0 transmit new segment(s),as allowed cwnd > ssthresh L timeout ssthresh = cwnd/2 cwnd = 1 MSS dupACKcount = 0 retransmit missing segment timeout ssthresh = cwnd/2 cwnd = 1 dupACKcount = 0 retransmit missing segment new ACK cwnd = cwnd + MSS (MSS/cwnd) dupACKcount = 0 transmit new segment(s),as allowed . congestion avoidance duplicate ACK dupACKcount++ New ACK cwnd = ssthresh dupACKcount = 0 dupACKcount == 3 ssthresh= cwnd/2 cwnd = ssthresh + 3 retransmit missing segment fast recovery duplicate ACK cwnd = cwnd + MSS transmit new segment(s), as allowed Transport Layer 3-82 cwnd window size (in segments) Popular “flavors” of TCP TCP Reno ssthresh ssthresh TCP Tahoe Transmission round Transport Layer 3-83 Summary: TCP Congestion Control when cwnd < ssthresh, sender in slow-start phase, window grows exponentially. when cwnd > ssthresh, sender is in congestion- avoidance phase, window grows linearly. when triple duplicate ACK occurs, cut cwnd in half (cwnd/2 + 3) when timeout occurs, cwnd set to 1 MSS. ssthresh = cwnd/2 on loss event Transport Layer 3-84