CSC 600 Internetworking with TCP/IP

1. CSC 600Internetworking with TCP/IP Unit 3: Transport Layer (Ch. 13, 12) Dr. Cheer-Sun Yang Spring 2001

2. Introduction • Transmission Control Protocol provides connection-oriented reliable transport services. • User Datagram Protocol (UDP) provides connectionless unreliable transport services.

3. TCP & UDP • Transmission Control Protocol • Connection oriented • RFC 793 • User Datagram Protocol (UDP) • Connectionless • RFC 768

5. Reliable vs. Unreliable • Reliable transport service handles error recovery at the transport level. • Unreliable transport service does not provide error recovery at at the transport level.

6. Connection-oriented vs.Connection-less • Connection-oriented service must establish connection between the source and the destination first. • Connection-less service does not establish connection first. It simply does store-and-forward.

7. Properties of the Reliable Delivery Service • Stream orientation - ordered delivery • Virtual circuit connection – connection establishment is must prior to segment delivery • Buffered transfer – data buffering is needed • Unstructured stream – TCP segments may not be as big as a record in a payroll application. • Full duplex connection – Connections provided by the TCP/IP stream service allow concurrent transfer in both direction.

8. Properties of the Reliable Delivery Service • TCP provides reliable transport service using sliding window protocol as defined in the Data Link Layer Protocol.

9. TCP is a communication protocol, not a piece of software. Transmission Control Protocol

10. TCP vs. the Implementation • TCP is the communication protocol. • TCP is implemented by many venders in software as part of the Operating System. • The difference between a protocol and the software that implements it is analogous to the difference between the definition of a programming language and a compiler.

11. What does TCP Specify? • Data segment format • Timing • Meanings of header fields • Functions of TCP – also referred to as services provided by TCP

12. What does TCP not specify? • The user interface is not specified. • The underlying communication system can be a dialup telephone line, a local area network, a high speed fiber optical network, or a lower speed long haul network.

13. TCP Services • Reliable communication between pairs of processes • Across variety of reliable and unreliable networks and internets • Two labeling facilities • Data stream push • TCP user can require transmission of all data up to push flag • Receiver will deliver in same manner • Avoids waiting for full buffers • Urgent data signal • Indicates urgent data is upcoming in stream • User decides how to handle it

15. TCP Header

17. Items Passed to IP • TCP passes some parameters down to IP • Precedence • Normal delay/low delay • Normal throughput/high throughput • Normal reliability/high reliability • Security

18. TCP Header Field • Port Number • source and destination port numbers (why source port number?) • why not IP addresses? • Identifies an application • Together with IP address to form an end point

19. TCP Header Field • Sequence Number • 32 bits long • the range of sequence number is 0 <= seq <= 2 32 -1 • Each sequence number identifies the byte in the stream of data from the sending TCP to the receiving TCP where the first byte of data is located in the segment • Initial Sequence Number (ISN) of a connection is set during connection management 1 200 201 400 401 600 segment 1 segment 2 segment 3 (seq = 1) (seq = 201) (seq = 401)

20. TCP Header Field • Acknowledgement Nubmer • Acknowledgements are piggybacked if there is a segment ready to be sent from the receiver to the sender • The acknowledgement segment consists of the next sequence number expected

21. TCP Header Field • Header Length • Why is this needed ?

22. TCP Header Field

23. TCP Header Field • Flags • URG - if the URG =1, the following bytes contain an urgent message: seq <= urgent message <= seq + urgent pointer • ACK: acknowledgement number is valid • PSH: • notification from sender to receiver to force the TCP on the receiver side to pass all data received to the application layer • Normally sent by the sender when the sender’s buffer is empty so the sender does not wait for more data • RST: Reset the connection • SYN: synchronization request for the sequence number • FIN: Finish flag

24. TCP Header Field • Options: • End of options: 1 byte • NOP: 1 byte • Maximum segment size: 4 bytes • Window scale factor: 3 bytes • increases the TCP window size from 16 bits to 32 bits • 1-byte shift count is between 0 and 14 • used in the connection establishment for window size negotiation • Timestamp: 10 bytes • sender places a timestamp in a segment • receiver places an echo reply • this allows the sender to calculate the Round-Trip Time per window

25. TCP Header Field(Options) 0 End of options NOP 1 2 4 MSS Window scale factor 3 3 S S: shift count 8 10 timestamp timestamp echo reply Timestamp

26. Transport Layer Issues • Addressing • Connection establishment • Connection termination • Flow Control • Timeout and retransmission • Congestion Control • Multiplexing • Duplication detection • Crash recovery

27. TCP Mechanisms • Connection establishment • Data transfer • Send policy • Deliver policy • Accept policy: in-order, in-window • Retransmission policy: first-only, batch, individual • Acknowledgement Policy

28. Addressing • Target user specified by: • User identification • Usually host, port • Called a socket in TCP • Port represents a particular transport service (TS) user • Transport entity identification • Generally only one per host • If more than one, then usually one of each type • Specify transport protocol (TCP, UDP) • Host address • An attached network device • In an internet, a global internet address • Network number

29. Finding Addresses • Four methods • Know address ahead of time • e.g. collection of network device stats • Well known addresses • Name server • Sending process request to well known address

30. Ports, Connections, and Endpoints • TCP uses the connection, not the protocol port, as its fundamental abstraction; connections are identified by a pair of endpoints, i.e., (18.26.0.36, 1069) and (128.10.2.3, 25). • An endpoint is a pair of integers = (host, port). • Because TCP identifies a connection by a pair of endpoints, a given TCP port number can be shared by multiple connections on the same machine.

31. Connection Establishment • Connection establishment • Three way handshake • Between pairs of ports • One port can connect to multiple destinations

33. Passive and Active Opens • A client requests for a connection – an active open request. • A server must be waiting for the request for connection – a passive open.

34. Connection Establishment • Two way handshake • A send SYN, B replies with SYN • Lost SYN handled by re-transmission • Can lead to duplicate SYNs • Ignore duplicate SYNs once connected • Lost or delayed data segments can cause connection problems • Segment from old connections • Start segment numbers fare removed from previous connection • Use SYN i • Need ACK to include i • Three Way Handshake

35. Two Way Handshake:Obsolete Data Segment

36. Two Way Handshake:Obsolete SYN Segment

37. Three WayHandshake:Examples

38. Connection Establishment

39. Three Way Handshake:State Diagram

41. Initial Sequence Number • When a new connection is being established, the SYN flag is turned on. The sequence number field contains the ISN chosen by the host for this connection. • The sequence number of the first byte of data sent by the host will be the ISN plus one because the SYN flag consumes a sequence number.

42. Connection Termination • Entity in CLOSE WAIT state sends last data segment, followed by FIN • FIN arrives before last data segment • Receiver accepts FIN • Closes connection • Loses last data segment • Associate sequence number with FIN • Receiver waits for all segments before FIN sequence number • Loss of segments and obsolete segments • Must explicitly ACK FIN

43. Data Transfer • Data transfer • Logical stream of octets • Octets numbered modulo 223 • Flow control by credit allocation of number of octets • Data buffered at transmitter and receiver

44. Send Policy • If no push or close TCP entity transmits at its own convenience • Data buffered at transmit buffer • May construct segment per data batch • May wait for certain amount of data

45. Deliver Policy • In absence of push, deliver data at own convenience • May deliver as each in order segment received • May buffer data from more than one segment

46. Accept Policy • Segments may arrive out of order • In order • Only accept segments in order • Discard out of order segments • In windows • Accept all segments within receive window

47. Not Listening • Reject with RST (Reset) • Queue request until matching open issued • Signal TS user to notify of pending request • May replace passive open with accept

48. Connection Termination • Connection termination • Graceful close • TCP users issues CLOSE primitive • Transport entity sets FIN flag on last segment sent • Abrupt termination by ABORT primitive • Entity abandons all attempts to send or receive data • RST segment transmitted

49. Termination • Either or both sides • By mutual agreement • Abrupt termination • Or graceful termination • Close wait state must accept incoming data until FIN received