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TCP: Transmission Control Protocol Part I : Protocol basics Surasak Sanguanpong nguan@ku.ac.th

TCP: Transmission Control Protocol Part I : Protocol basics Surasak Sanguanpong nguan@ku.ac.th http://www.cpe.ku.ac.th/~nguan Last updated: July 30, 1999. Agenda. Services provided by TCP TCP format How TCP reliability is achieved Sliding window TCP Connection TCP State.

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TCP: Transmission Control Protocol Part I : Protocol basics Surasak Sanguanpong nguan@ku.ac.th

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  1. TCP: Transmission Control Protocol Part I : Protocol basics Surasak Sanguanpong nguan@ku.ac.th http://www.cpe.ku.ac.th/~nguan Last updated: July 30, 1999

  2. Agenda • Services provided by TCP • TCP format • How TCP reliability is achieved • Sliding window • TCP Connection • TCP State

  3. TCP encapsulation • with Ethernet frame segment Ethernet hdr IP header TCP header data

  4. TCP & UDP Services • TCP : Transmission Control Protocol • RFC 793 • connection-oriented service • full duplex • reliable service by adding more overhead to manage acknowledgment, flow control, timer

  5. TCP: Transmission Control Protocol • TCP performs typical transport layer functions: • passed data to relevant application-level services • mux and demux data from applications to and from IP layer • error recovery • flow control data stream (avoid buffer overflow)

  6. TCP properties • byte stream with full duplex transferring • adaptive to LAN/WAN • congestion avoidance and control

  7. TCP data stream • TCP provides a full duplex service that simultaneous manages two streams of data • stream of octets passed between sender/receiver application receive send application send receive

  8. Applications 1 2 3 4 ( ) ( ) ( ) ( ) Transport Network access Ports • Port - a 16 bit address allocated for the most common application layer services • UDP and TCP use port addressing to deliver info to applications • Servers are known by ports number • FTP 20, TELNET 23, SMTP 25, HTTP 80 • Port numbers are generally allocated by • 0 --not used • 1-255 --Reserved ports for well-known services • 256-1023 --Other reserved ports • 1024-65535 --user-defined server ports • Unix store general used ports in /etc/services

  9. Sockets • socket : a pair of the IP address and the port number IP address port number <158.108.33.3, 3000> IP address is unique to a node, the port is unique on a node the socket gives a unique identification of an application layer services

  10. Socket address • A connection is identified by the socket address at its to ends • client socket: 158.108.33.3,3000; 158.108.2.71,21 • server socket: 158.108.2.71,21; 158.108.33.3,3000; IP : 158.108.2.71 IP : 158.108.33.3 connection port : 21 port : 3000 server client

  11. Socket multiple connection • server’s unique socket address can be accessed simultaneously by clients client server IP : 158.108.33.2 IP : 158.108.2.71 port : 3000 port : 21 connection client IP : 158.108.33.3 port : 3120

  12. Transmission Control Protocol • TCP passed block of data to IP, consisting of the TCP header and application layer data, called segment • adding reliability in TCP is achieved by • Error detection and correction (due to segments corrupted) • Flow control (prevent a transmitter overrunning a receiver owing a resource limitations) • Resequencing (IP can deliver datagrams in any order) • Removing duplicate segments (due to error-recovery mechanisms used by TCP)

  13. How TCP handles reliability • Using sequence numbers to identify data • positive acknowledgments of data received in the correct sequence • retransmission of segments which have not been acknowledged within a (variable) time limit • Let’s see these mechanisms in TCP header

  14. TCP header 0 15 16 31 source port:16 destination port:16 sequence number:32 acknowledgment number:32 data offs:4 resv:6 flag:6 window size:16 checksum :16 urgent pointer:16 options and padding

  15. TCP header details (I) • source, destination port:16,16 - identify applications at ends of the connection • sequence:32 - indicates 1st data octet in this segment • acknowledgment:32 - next expected sequence number, valid only when the ACK bit (reside in flag) is set

  16. TCP header details (II) • data offset:4 - 32 bit words offset tells the receiver where user data begins • reserved:6 -not used • flag:6 • URG : validity of urgent pointer field • ACK : validity of acknowledge field • PSH : push request (pass segment to appl layer immediately) • RST : reset the connection • SYN : initial synchronization • FIN : sender at end of byte stream

  17. TCP header details (III) • window:16 - advertise amount of buffer space this node has allocated • checksum:16 - 16 bits 1’s complement of pseudo header, TCP header and data • urgent pointer:16 - byte position of data that should be processed first • options - variable length option e.g. MSS (max segment size) tells destination node

  18. Sliding window principle • send and wait for acknowledgment • no ACK within a certain time, retransmit the packet • use for flow control : • prevent sender from overloading receiver with data, e.g. high-performance server to slow PC • congestion inside network, e.g. router performance, slow link speed • How to provide flow control? • set the appropriate size of sliding window size

  19. Sliding window flow control • Receiver “ advertises” it’s windows size in acknowledgments • Sender will adjusts its allowed to send pointer as receiver’s advertisement no! no! I can load only 200 kg.

  20. SENDER RECEIVER Sliding window: small window size window size =1 • 1 byte window size utilizes efficiency of channel in half (half-duplex transmission) • why not send many packets and get back cumulative ACK? Send 1 Receive 1 Ack 2 Send 2 Receive 2 Ack 3

  21. SENDER RECEIVER Sliding window: larger window size window size =3 • A larger window size allows more data to be transmitted pending acknowledgment • Window size specifies how many bytes the receiver is willing to accept Send 1 Send 2 Send 3 Receive 1 Receive 2 Receive 3 Receive ACK 4 Send ACK 4 Send 4 Send 5 Send 6 Receive 4 Receive 5 Receive 6 Receive ACK 4 Send ACK 4

  22. data bytes 500-599 data bytes 600-699 data bytes 700-799 I received all up to 799! ACK 800 Sequence number in segment • Data continuously sent more than segment • need not to wait for acknowledgment every segment host A host B

  23. data bytes 500-599 data bytes 600-699 data bytes 700-799 ACK 700 data bytes 700-799 Retransmit a loss segment host A host B

  24. Error recovery (I) • receiver has to send ACK with sequence number • sender reset timer when receives ACK host A host B segment #i, start timer ACK cancel timer

  25. Error recovery (II) • on time out, sender will retransmit the segment • this mechanism is used for error recovery host A host B segment #i, start timer timer expires, resend #i ACK cancel timer

  26. Sliding window buffer • sender groups its packet to be transmitted with window indication offered windows SndWnd can send ASAP sent and not ACKed sent and ACK can’t send now 1000… 1099 1100… 1199 ....999 1200… SndUna+SndWnd SndNxt SndUna

  27. Sliding window example • movement of the right and left edges of the window 1000… 1099 1100… 1199 initial ....999 1200… SndUna+SndWnd SndUna, SndNxt 1000… 1099 1100… 1199 send 100 bytes ....999 1200… SndUna+SndWnd SndUna SndNxt 1000… 1099 1100... 1199 more 100 bytes ....999 1200… SndNxt, SndUna+SndWnd SndUna ACK 100 bytes 1100… 1199 1200 1299 ....999 1000... 1099 1300… SndNxt SndUna+SndWnd SndUna

  28. TCP in actions • before data could be transferred, a connection must be opened • servers do passive open (listen) • clients do active open (connect) • when it finished, the connection is closed • TCP has general 3 phases • connection setup phase • data phase • connection close phase

  29. TCP connection establishment • TCP uses 3-way handshake to establish a connection • exchange the sequence number • ensures that both ends are ready and sync sequence number host A host B (1) send SYN (seq=x) (2) send SYN (seq =y, ack=x+1) (3) send ACK (ack=y+1) connection is setup!

  30. Transfer phase • simple example with terminal connection such as Telnet. Host echoes back each received character host A host B SEQ=92, ACK=109 DATA=“w” Host echoes back “w” SEQ=109, ACK=93 DATA=“w” SEQ=93, ACK=110 DATA=“…”

  31. TCP Connection close • use FIN flag to close connection host A host B SEQ=x, FIN ACK=x+1 SEQ=Y inform application application response with SEQ=y, FIN ACK=y+1

  32. Open/Close mechanisms • Half open - one end has closed, aborted without the knowledge of the other end • host may be crashed, power off • no detection if no data transfer • reset segment (RST bit) is sent when detected • Half close - one end of connection terminated its output, but still receiving data from the other end • Simultaneous open- both end perform an active open to each other • Simultaneous close - both end perform an active open to each other

  33. appl passive opens; send nothing LISTEN appl active opens; send SYN passive open got SYN; send SYN,ACK got RST appl closes; or time-out got SYN; send SYN, ACK simultaneous open SYN_RCVD SYN_SENT active open got ACK; send nothing got SYN, ACK; send ACK appl closes; send FIN ESTABLISHED TCP state diagram : open start CLOSED (normal) client transition (normal) server transition

  34. TCP state diagram : close back to CLOSED got FIN; send ACK ESTABLISHED CLOSE_WAIT appl close; send FIN appl closes; send FIN appl closes; send FIN simultaneous close got FIN; send ACK got ACK; send nothing FIN_WAIT_1 CLOSING LAST_ACK passive close get FIN, ACK; send: ACK got ACK; send nothing got ACK; send nothing got FIN; send ACK FIN_WAIT_2 TIME_WAIT 2MSL time-out (normal) client transition (normal) server transition active close

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