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CSS432 End-to-End Protocols Textbook Ch5.1 – 5.2

CSS432 End-to-End Protocols Textbook Ch5.1 – 5.2. Professor: Munehiro Fukuda. M5, M5, M3. IP Protocols. Underlying best-effort network drop messages re-orders messages delivers duplicate copies of a given message limits messages to some finite size

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CSS432 End-to-End Protocols Textbook Ch5.1 – 5.2

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  1. CSS432 End-to-End ProtocolsTextbook Ch5.1 – 5.2 Professor: Munehiro Fukuda CSS432: End-to-End Protocols

  2. M5, M5, M3 IP Protocols • Underlying best-effort network • drop messages • re-orders messages • delivers duplicate copies of a given message • limits messages to some finite size • delivers messages after an arbitrarily long delay Request a retransmission of M3 and M5 M5, and M3 M5 M2, M1, M4 M5, M4, M3, M2, M1 M5, M2, M1 M4 M4, M3 M3 CSS432: End-to-End Protocols

  3. 0 16 31 SrcPort DstPort Checksum Length Data Simple Demultiplexor (UDP) • Unreliable and unordered datagram service • Adds multiplexing • No flow control • Endpoints identified by ports • servers have well-known ports • see /etc/services on Unix • Header format • Optional checksum • psuedo header + UDP header + data CSS432: End-to-End Protocols

  4. int sd; sd = socket(AF_INET, SOCK_DGRAM, 0); socket() socket() socket() struct sockaddr_in server; struct hostent *hp, *gethostbyname( ); Server.sin_family = AF_INET; hp = gethostbyname( ); bcopy( hp->h_addr, &( server.sin_addr.s_addr ), sizeof( hp->h_length) ); server.sin_port = htons( 12345 ); struct sockaddr_in server; server.sin_family =AF_INET; server.sin_addr.s_addr = htonl( INADDR_ANY) server.sin_port = htons( 12345 ); bind( sd, (struct sockaddr *)&server, sizeof( server ) ); 13579 13579 bind() bind() sendto( sd, buf, sizeof( buf ), 0, (struct sockaddr *)&server, sizeof( server ) ); Port:12345 Port: 13579 recv( sd, buf, sizeof( buf ), 0 ); recv() recv() sendto() UDP: Simple Demultiplexer Packets demultiplexed UDP M5, M4, M3, M2, M1 CSS432: End-to-End Protocols

  5. End-to-End Protocols • Common end-to-end services • guarantee message delivery • deliver messages in FIFO order • deliver at most one copy of each message • support arbitrarily large messages • support synchronization • allow the receiver to flow control the sender • support multiple application processes on each host P1 P2 P3 P4 M5, M4, M3, M2, M1 Network m5, m4, m3, m2, m1 CSS432: End-to-End Protocols

  6. Application process Application process W rite Read bytes bytes … … TCP TCP Send buffer Receive buffer … Segment Segment Segment T ransmit segments TCP Overview • Full duplex • Flow control: keep sender from overrunning receiver • Congestion control: keep sender from overrunning network • Connection-oriented • Byte-stream • app writes bytes • TCP sends segments • app reads bytes CSS432: End-to-End Protocols

  7. socket() socket() socket() bind() listen() connect() connect() accept() write() write() buf2, buf1 buf2, buf1 read() read() Sockets (Code Example) int sd, newSd; sd = socket(AF_INET, SOCK_STREAM, 0); int sd = socket(AF_INET, SOCK_STREAM, 0); sockaddr_in server; bzero( (char *)&server, sizeof( server ) ); server.sin_family =AF_INET; server.sin_addr.s_addr = htonl( INADDR_ANY ) server.sin_port = htons( 12345 ); bind( sd, (sockaddr *)&server, sizeof( server ) ); struct hostent *host = gethostbyname( arg[0] ); sockaddr_in server; bzero( (char *)&server, sizeof( server ) ); server.sin_family = AF_INET; server.s_addr = inet_addr( inet_ntoa( *(struct in_addr*)*host->h_addr_list ) ); server.sin_port = htons( 12345 ); listen( sd, 5 ); connect( sd, (sockaddr *)&server, sizeof( server ) ); sockaddr_in client; socklen_t len=sizeof(client); while( true ) { newSd = accept(sd, (sockaddr *)&client, &len); write( sd, buf1, sizeof( buf ) ); write( sd, buf2, sizeof( buf ) ); if ( fork( ) == 0 ) { close( sd ); read( newSd, buf1, sizeof( buf1 ) ); read( newSd, buf2, sizeof( buf2 ) ); } close( newSd ); } close( newsd); exit( 0 ); CSS432: End-to-End Protocols

  8. Data Link Versus Transport • Potentially connects many different hosts • need explicit connection establishment and termination • Potentially different RTT (Round Trip Time) • need adaptive timeout mechanism • Potentially long delay in network • need to be prepared for arrival of very old packets • need to set MSL (Maximum Segment Lifetime) • Potentially different capacity at destination • need to accommodate different node capacity • Potentially different network capacity • need to be prepared for network congestion CSS432: End-to-End Protocols

  9. Data (SequenceNum) Sender Receiver Acknowledgment + AdvertisedWindow Segment Format • Each connection identified with 4-tuple: • (SrcPort, SrcIPAddr, DsrPort, DstIPAddr) • Sliding window + flow control • Acknowledgment, SequenceNum, AdvertisedWinow • Flags • SYN, FIN, RESET, PUSH, URG, ACK • SYN: Establishing a conneciton • FIN: terminating a connection • RESET: Confused and Terminating • PUSH: Section 5.2.7 • URG: Sending urgent data • ACK: Validating acknowledgment field • SequenceNum is incremented in all cases other than ACK. CSS432: End-to-End Protocols

  10. Active participant Passive participant (client) (server) SYN, SequenceNum = x , y 1 + SYN + ACK, SequenceNum = x Acknowledgment = ACK, Acknowledgment = y + 1 Connection Establishment and Termination • Tree-Way Handshake • Client • Initiate a connection to a server with seq=x • Set a timer and retransmit the request upon an expiration • Server • Acknowledge the client request with ack=++x • Initiate a reverse connection with seq=y • Set a timer and retransmit the request upon an expiration • Client • Acknowledge the server request with ack=++y • X and y are chosen at random • Protect against two incarnations of the same connection using the same sequence number. CSS432: End-to-End Protocols

  11. CLOSED Active open /SYN Passive open Close Close LISTEN SYN/SYN + ACK Send/ SYN SYN/SYN + ACK SYN_RCVD SYN_SENT ACK SYN + ACK/ACK Close /FIN ESTABLISHED Close /FIN FIN/ACK FIN_WAIT_1 CLOSE_WAIT FIN/ACK ACK Close /FIN ACK + FIN/ACK FIN_WAIT_2 CLOSING LAST_ACK Timeout after two ACK ACK segment lifetimes (2 * MSL) FIN/ACK TIME_WAIT CLOSED State Transition Diagram • Open • Active open • A client • connect( ) • Passive open • A server • listen( ) • Can change active • Close • Active close • A client or a server • First close( ) • Both side can be active • Passive close • A client or server • close( ) in response to the first close( ) CSS432: End-to-End Protocols

  12. State Transition Diagram • In what condition can the state transit from FIN_WAIT_1 to TIME_WAIT? • What is the purpose of the TIME_WAIT state? CSS432: End-to-End Protocols

  13. Timing Chart Client Server ( connect( ) ) SYN_SENT LISTEN ( listen( ) ) SYN seq=x SYN_RCVD Establishment SYN seq=y, ACK=x + 1 ESTABLISHED ACK=y + 1 ESTABLISHED ( write( ) ) seq=x+1 ACK=y + 1 ( read( ) ) Data Transfer ACK x + 2 ( close( ) ) FIN_WAIT_1 FIN seq=x+2 ACK=y + 1 CLOSE_WAIT ACK x + 3 Termination FIN seq = y + 1 FIN_WAIT_2 LAST_ACK( close( ) ) TIME_WAIT ACK=y + 2 Peek such a flow with tcpdump in assignment 3. CSS432: End-to-End Protocols

  14. Sending application Receiving application TCP TCP LastByteWritten LastByteRead LastByteAcked LastByteSent NextByteExpected LastByteRcvd Sliding Window Revisited • Sending side • LastByteAcked ≤ LastByteSent • LastByteSent ≤ LastByteWritten • buffer bytes between [LastByteAcked, LastByteWritten] • Receiving side • LastByteRead < NextByteExpected • NextByteExpected ≤ LastByteRcvd+1 • buffer bytes between [NextByteRead, LastByteRcvd] CSS432: End-to-End Protocols

  15. Flow Control Receiving application Sending application TCP TCP LastByteRead LastByteWritten y LastByteSent NextByteExpected LastByteRcvd LastByteAcked LastByteRcvd – LastByteRead ≤ MaxRcvbuffer LastByteSent – LastByteAcked ≤ AdvertisedWindow AdvertisedWindow = MaxRcvBuffer – (NextByteExpected – NextByteRead) EffectiveWindow = AdvertisedWindow – (LastByteSent – LastByteAcked) • Send ACK with an advertise window • in response to arriving data segments • as long as all the preceding bytes have • also arrived and • until the advertised window reaches 0. • (ACK returned at the first time when it reaches 0) LastByteWritten – LastByteAcked ≤ MaxsendBuffer block sender if (LastByteWritten - LastByteAcked) + y > MaxSenderBuffer CSS432: End-to-End Protocols

  16. y y Flow Control with A Slower Receiver Receiving application Sending application Read slow. TCP TCP LastByteRead LastByteWritten LastByteSent LastByteRcvd LastByteAcked NextByteExpected LastByteRcvd – LastByteRead ≤ MaxRcvbuffer LastByteSent – LastByteAcked ≤ AdvertisedWindow 0 0 < 0 AdvertisedWindow = MaxRcvBuffer – (NextByteExpected – LastByteRead) EffectiveWindow = AdvertisedWindow – (LastByteSent – LastByteAcked) 0 No more send, no more ack, thus it stays In the same value LastByteWritten – LastByteAcked ≤ MaxsendBuffer block sender since (LastByteWritten - LastByteAcked) + y > MaxSenderBuffer CSS432: End-to-End Protocols

  17. Flow Control • The sender won’t send any more data. • The receiver won’t initiate to send any advertised window. • Then, how can the sender find out when the receiver can receive more data? CSS432: End-to-End Protocols

  18. Protection Against Wrap Around • 32-bit SequenceNum • MSL (Maximum Segment Lifetime) = 120sec. • SequenceNum should not be wrapped around within 120 seconds. Bandwidth Time Until Wrap Around T1 (1.5 Mbps) 6.4 hours Ethernet (10 Mbps) 57 minutes T3 (45 Mbps) 13 minutes FDDI (100 Mbps) 6 minutes STS-3 (155 Mbps) 4 minutes STS-12 (622 Mbps) 55 seconds STS-24 (1.2 Gbps) 28 seconds CSS432: End-to-End Protocols

  19. Keeping the Pipe Full • 16-bit AdvertisedWindow = 64KB • Assuming that RTT = 100msec, the advertised window’s capacity is at least RTT x Network bandwidth Bandwidth RTT(100msec) x Bandwidth Product T1 (1.5 Mbps) 18KB Ethernet (10 Mbps) 122KB T3 (45 Mbps) 549KB FDDI (100 Mbps) 1.2MB STS-3 (155 Mbps) 1.8MB STS-12 (622 Mbps) 7.4MB STS-24 (1.2 Gbps) 14.8MB CSS432: End-to-End Protocols

  20. Segment Transmission A segment is transmitted out: • When a segment to send reaches Maximum segment size (MMS) = Maximum Transfer Unit (MTU) • When a TCP receives a push operation that flushes the unsent data (Peek with tcpdump in programming assignment 3) • When a timer expires • A trigger by a timer may cause the silly window syndrome. CSS432: End-to-End Protocols

  21. Silly Window Syndrome small MMS 2 Sender MMS 1 Receiver • If a sender aggressively takes advantage of any available window, • The receiver empties every window regardless of its size and thus small windows will never disappear. • The problem occurs only when either the sender transmits a small segment or the receiver opens the window a small amount • The receiver can delay ACKs to make a larger window • How long does it wait? • The sender should make a decision • Nagle’s Algorithm (Programming assignment 3) Ad Window Ad Window CSS432: End-to-End Protocols

  22. Nagle’s Algorithm When the application produces data to send { if both the available data and the window ≥ MSS send a full segment // no problem, send it right now else if there is unAcked data in transit // a full empty segment returns soon buffer the new data until an ACK arrives else // the receiver won’t reply. Can’t avoid the silly window syndrome send all the new data now } • Ack works as a timer to fire a new segment transmission. • The algorithm may not respond to interactive or real-time applications • TCP_NODELAY Option: Transmit data as soon as possible • setsockopt(sockfd, SOL_TCP, TCP_NODELAY, &intFlag, sizeof(intFlag)) CSS432: End-to-End Protocols

  23. How to Estimate RTT • Timeout is necessary • To retransmit a lost packet • To detect congestions and invoke AIMD. • RTT estimation algorithms • Original Algorithm • Karn/Partridge Algorithm • Jacobson/Karels Algorithm CSS432: End-to-End Protocols

  24. Original Algorithm • Measure SampleRTT for each segment/ ACK pair • Compute weighted average of RTT • EstRTT = ax EstRTT + b x SampleRTT • where a+b = 1 • a between 0.8 and 0.9 • b between 0.1 and 0.2 • Set timeout based on EstRTT • TimeOut=2 x EstRTT • Why double? EstRTT cannot respond to deviated SampleRTT quickly. CSS432: End-to-End Protocols

  25. Karn/Partridge Algorithm Sender Receiver Sender Receiver • Do not sample RTT when retransmitting • Can’t figure out which transmission the latest ACK corresponds to. • Double timeout after each retransmission • Congestion causes this retransmission. • Modestly retransmit segments. Original transmission Original transmission TT TT ACK Retransmission SampleR SampleR Retransmission ACK CSS432: End-to-End Protocols

  26. Jacobson/ Karels Algorithm • Original Algorithm • EstRTT = ax EstRTT + b x SampleRTT 0.8 and 0.9 0.1 and 0.2 • TimeOut = EstRTT * 2 • New Algorithm that takes into a consideration if the variation among smapleRTTs are large. • EstRTT = EstRTT + d(SampleRTT – EstRTT) 0.125Diff • Dev = Dev + d(|SampleRTT – EstRTT| – Dev) Diff • TimeOut = m x EstRTT + f x Dev 14 CSS432: End-to-End Protocols

  27. Jacobson/ Karels Algorithm EstimatedRTT = 8 EstRTT Deviation = 8 Dev SampleRTT = (SampleRTT – 8 EstRTT) / 8 = SampleRTT – EstRTT) diff SampleRTT –= (EstimatedRTT >> 3) EstimatedRTT += SampleRTT; If (SampleRTT < 0) SampleRTT = – SampelRTT; SampleRTT –= (Deviation >> 3); Deviation += SampleRTT; TimeOut = (EstimatedRTT >> 3) + (Deviation >> 1) • Notes • Algorithm does not work accurately with a large granularity of clock (500ms on Unix) • Accurate timeout mechanism important to congestion control 8 EstRTT = 8EstRTT + SampleRTT = 8EstRTT + (SampleRTT – EstRTT) EstRTT = EstRTT + 1/8 (SampleRTT – EstRTT) diff |SampleRTT – EstRTT| – 8Dev/8 |diff| 8 Dev = 8 Dev + |SampleRTT – EsRTT| - Dev Dev = Dev + 1/8( |SampleRTT – EstRTT| - Dev ) TimeOut = 8 EstRTT / 8 + 8 Dev / 2 TimeOut = EstRTT + 4 Dev CSS432: End-to-End Protocols

  28. TCP Extensions • RTT Measurement • Store a 32-bit timestamp in outgoing segments’ header option • Receive an ack with the original timestamp • Sample RTT by subtracting the timestamp from the current timer • Resolving the quick wrap-around of sequence number • The 32-bit timestamp and the 32-bit sequence number gives a 64-bit sequence space • Extending an advertised window • Scale up the advertised window • How many bytes to send → How many 16-byte units to send CSS432: End-to-End Protocols

  29. Reviews • UDP • TCP: three-way handshake and state transition • Sliding window and flow control • Segment transmission: silly window syndrome and Nagle’s algorithm • Adaptive retransmission: original, Karn/Partridge, and Jacobson/Karels • Exercises in Chapter 5 • Ex. 5, 14, 22, and 39 (TCP state transition) • Ex. 9(a) (Sliding window) • Ex. 20 (Nagle’s algorithm) CSS432: End-to-End Protocols

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