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TCP : Transmission Control Protocol ( Stevens TCP/ IP Illustrated Volume 1)

TCP : Transmission Control Protocol ( Stevens TCP/ IP Illustrated Volume 1). Chapter 17 and 18:. TCP is connection oriented Unit of information passed by TCP to IP is a segment A segment is retransmitted if an acknowledgement is not received within ticks of a timer

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TCP : Transmission Control Protocol ( Stevens TCP/ IP Illustrated Volume 1)

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  1. TCP : Transmission Control Protocol( Stevens TCP/ IP Illustrated Volume 1) Chapter 17 and 18: • TCP is connection oriented • Unit of information passed by TCP to IP is a segment • A segment is retransmitted if an acknowledgement is not received within • ticks of a timer • TCP upon receipt of data sends an ACK. Normally does not send this • ACK immediately • TCP reorders out of order data • TCP uses flow control

  2. TCP Connection Establishment And Termination • [ Fig 18.3] Use “tcpdump” to obtain data for this • Clint sends a SYN with initial sequence number • Notation 1415531521:1415531521 (0) Number of Bytes sent Starting Sequence Number Starting Sequence Number Plus Number of Bytes Sent • Server responds with SYN • Client ACK the SYN from server • This is 3 way handshake. It takes four segments to terminate and is called “ Half Close”

  3. TCP Header : Bit 0 4 10 31 ( 32 Bits) 15 SOURCE PORT DESTINATION PORT SEQUENCE NUMBER 20 BYTES ACKNOWLEDGEMENT NUMBER HEADER LENGTH RST FIN URG PSH SYN ACK UNUSED WINDOW CHECKSUM URGENT POINTER OPTIONS + PADDING

  4. Maximum segment size (MSS) is largest amount of data each TCP sender is • willing to receive • Advertised by each end and can be different in different directions. It is not “negotiated.” • If no MSS sent default 536 is used • For ethernet 1460 bytes is the MSS. • [ Fig 18.12 and 18.13] State Diagram Of TCP • When TCP active close occurs and sends final Ack, connection stays in Time-Wait state • for twice maximum segment lifetime (MSS) value. • 1. In case must resend final ACK • 2. In case attempt to reuse IP address and port again while old segments may still • be in network. • Reset segment RST is sent when error occurs.

  5. TCP Interactive Flow(TCP/IP Vol. 1, Stevens Chapter 19) • - Look at flow of data for rlogin connection which is a TCP application. • - Each keystroke generates a data packet. • [ Fig 19.1] • - Modified TCP dump output for session with five characters date\n (connection • establishment not shown in Fig 19.2). • [ Fig 19.2 ] • [ Fig 19.3 ] Time Line For Fig 19.2, much easier to follow • - ACK can be sent along with next data going in same direction called ACK piggyback. • - ACK usually wait up to 200 ms delay to see if must travel alone or get to piggyback. • Notation in Fig 19.2/19.3 0:1(1) means sent (1) data byte with sequence number 0. • The next starting sequence number will be 1.

  6. To prevent lots of very small data segments in a network: • Nagle Algorithm (For the sending TCP algorithm) • The sending TCP sends the (small) data even if it is only one byte. • After sending the first data segment, sending TCP accumulates data until receiver sends an ACK or until data accumulated is an MSS. At this point sender can send data. • Repeat step two for remainder of transmission. • This algorithm causes a fast application program on a slow network to accumulate and send larger (mss) segments. A slow application program on a fast network will send less than mss. • There are situations like x window mouse where small mouse movements need to immediately be sent, in that case would not want to use Nagle since want to sendsmall size data immediately. • Fig [19.5-19.8]

  7. TCP Bulk Data Flow Chapter 20 Normal Data Flow Example Transfer of 8192 bytes of data from srv4 to bsdi [Fig 20.1] Warning: This example appears to not use slow start (discussed later). - Sender transmits 3 data segments (4-6) - Next segment (7) acknowledges the first two data segments only. This is because of the following: - When TCP processes segment 4 the connection is marked to generate a delayed ACK - When segment 5 arrives TCP has two outstanding segments and immediately acknowledges. * TCP Immediately ACK's two outstanding segments. - Next segment (8) ACK's the third data segment due to ACK acknowledgement timer reaching a “ 200 ms Interval”. - Window of only 3072 advertised since 1024 bytes of data still in TCP receive buffer - In TCP ACK's are cumulative. They acknowledge receipt of up through ACK sequence number minus one. Note in Fig. 20.1 that the FIN segment has no data and is 8193:8193(0). The ACK for this is an ACK8194 which is one more than the last data. This is always true. A FIN uses up one ACK number.

  8. Another Normal Data Flow Example: [Fig 20.2] Same as before but data sent a bit different. Warning: This example appears to not use slow start (discussed later). Fast Sender, Slow Receiver Example: [Fig 20.3] Warning: This example appears to not use slow start (discussed later). - Sender transmits four back-to-back data segments (4-7) to fill receivers window. - Receiver sends ACK but with advertised window 0. Application has not yet read data - Another ACK called a window update is sent later announcing some room at the Inn. Warning: Fig 20.3 does not follow the ack no more than every other segment rule.

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