ICOM : Computer Networks Chapter 6: The Transport Layer. By Dr Yi Qian Department of Electronic and Computer Engineering Fall 2006 UPRM

Transcription

1 ICOM : Computer Networks Chapter 6: The Transport Layer By Dr Yi Qian Department of Electronic and Computer Engineering Fall 2006

2 Outline The transport service Elements of transport protocols A simple transport protocol The Internet transport protocol: UDP The Internet transport protocol: TCP Performance issues 2

3 The Transport Service Services Provided to the Upper Layers Transport Service Primitives Berkeley Sockets An Example of Socket Programming: An Internet File Server 3

4 Services Provided to the Upper Layers The network, transport, and application layers. 4

5 Transport Service Primitives The primitives for a simple transport service. 5

6 Frame Format The nesting of TPDUs, packets, and frames. 6

7 A State Diagram A state diagram for a simple connection management scheme. Transitions labeled in italics are caused by packet arrivals. The solid lines show the client's state sequence. The dashed lines show the server's state sequence. 7

8 Berkeley Sockets The socket primitives for TCP. 8

9 An Internet File Server: Client Program 9

10 An Internet File Server: Server Program 10

11 Elements of Transport Protocols Addressing Connection Establishment Connection Release Flow Control and Buffering Multiplexing Crash Recovery 11

12 Transport Protocol (a) Environment of the data link layer. (b) Environment of the transport layer. Point-to-point End-to-end Packet Storage 12

13 Addressing TSAPs, NSAPs and transport connections TSAP: transport service access point NSAP: network service access point 13

14 Addressing Fixed assignment Dynamic assignment Process server Name server 14

15 Process Server Example: How a user process in host 1 establishes a connection with a time-of-day server in host 2. 15

16 Name Server Client send the request to the name server Name server reply the TSAP address for the required service Client terminate the connection with the name server Client create the connection to the server that provide the required service ** In the server side, new service must register itself with the name server 16

17 Connection Establishment Key Problem Delayed duplicates may exist Entity may crash Solution Loss memory Connection ID Packet life time Restrict subnet design Hop counter Time stamp 17

18 Three-way Handshake Three protocol scenarios for establishing a connection using a three-way handshake. CR denotes CONNECTION REQUEST. (a) Normal operation, (b) Old CONNECTION REQUEST appearing out of nowhere. (c) Duplicate CONNECTION REQUEST and duplicate ACK. 18

19 Connection Release Asymmetric Symmetric 19

20 Connection Release Abrupt disconnection with loss of data. 20

21 The Two-army Problem 21

22 Protocol Scenarios (a) Normal case of a three-way handshake. (b) final ACK lost. 22

23 Protocol Scenarios (c) Response lost. (d) Response lost and subsequent DRs lost. 6-14, c,d 23

24 Flow Control and Buffering Flow control Fast transmitter and low receiver Difference to the flow control in DLL Buffering The transport layer may handle multiple connections Share buffering The transport layer shall consider the capacity of the network Source buffering If the network service is not reliable If the network service is reliable but the destination transport entity may drop some TPDU 24

25 Buffer Management (a)chained fixed-size buffers. (b)chained variable-sized buffers. (c)one large circular buffer per connection. 25

26 Dynamic Buffer (Window) Management The arrows show the direction of transmission. An ellipsis ( ) indicates a lost TPDU. 26

27 Dynamic Buffer (Window) Management Potential problem Loss of control packet Solution Control packets shall be sent out frequently 27

28 Multiplexing (a) Upward multiplexing. (b) Downward multiplexing. 28

29 Crash Recovery Different combinations of client and server strategy. 29

30 A Simple Transport Protocol Service Primitives Transport Entity Finite State Machine 30

31 Service Primitives connum = LISTEN(local) connum = CONNECT(local, remote) status = SEND(connum, buffer, bytes) status = RECEIVE(connum, buffer, bytes) status = DISCONNECT(connum) 31

32 The Transport Entity Suppose that the network layer can provide connection-oriented service The following network layer packets will be used 32

33 States Each connection is in one of seven states: Idle Connection not established yet. Waiting CONNECT has been executed, CALL REQUEST sent. Queued A CALL REQUEST has arrived; no LISTEN yet. Established The connection has been established. Sending The user is waiting for permission to send a packet. Receiving A RECEIVE has been done. DISCONNECTING a DISCONNECT has been done locally. 33

34 Finite State Machine The example protocol as a finite state machine. Each entry has an optional predicate, an optional action, and the new state. The tilde indicates that no major action is taken. An overbar above a predicate indicate the negation of the predicate. Blank entries correspond to impossible or invalid events. 34

35 Finite State Machine The example protocol in graphical form. Transitions that leave the connection state unchanged have been omitted for simplicity. 35

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44 User Datagram Protocol Overview Remote Procedure Call (RPC) The Real-Time Transport Protocol 44

45 Overview Connectionless Standard: RFC 768 Header: 8 bytes The main value of UDP over IP: port address No flow control No error control No retransmission 45

46 The UDP Header 46

47 Remote Procedure Call Steps in making a remote procedure call. The stubs are shaded. 47

48 The Real-Time Transport Protocol (a) The position of RTP in the protocol stack. (b) Packet nesting. 48

49 RTP Header 49

50 Transmission Control Protocol Overview Connection Establishment Connection Release Connection Management Modeling Transmission Policy Congestion Control Timer Management Others 50

51 Overview Standard: RFC 793, 1122, 1323 Connection-oriented Full duplex TCP connection: Byte stream Provide reliable communication The service model The protocol The segment header 51

52 Service Model A connection between two entities ID: Socket (IP and port) Some assigned ports Port Protocol Use 21 FTP File transfer 23 Telnet Remote login 25 SMTP 69 TFTP Trivial File Transfer Protocol 79 Finger Lookup info about a user 80 HTTP World Wide Web 110 POP-3 Remote access 119 NNTP USENET news 52

53 Byte Stream (a) Four 512-byte segments sent as separate IP datagrams. (b) The 2048 bytes of data delivered to the application in a single READ CALL. 53

54 The Protocol Every byte has a 32-bit sequence number The sequence number will be used in the connection initiation and flow control Data unit: segment Header: 20 bytes plue optional parts Size of segment The payload size can be zero The total length of a segment < (for IP) The total length of a segment < MTU MTU=Maximum transfer unit, usually is 1500 bytes 54

55 The Segment Header 55

56 Flags SYN: setup connection ACK: acknowledgement # is valid FIN: release connection RST: reset connection Reject invalid attempts PSH: do not buffer the data URG: urgent pointer is valid Urgent data must be processed first 56

57 Other Fields Window size: the number of BYTES that can be sent after the ACK number The window size can be zero Urgent pointer: the offset to indicate the end of the urgent data in the byte stream Checksum: When calculate the checksum, a pseudo header is included Optional Increase window size The maximum size in the header (16-bit) might be too small Utilize selective repeat instead of go back n 57

58 The Pseudoheader 58

59 Connection Establishment Three-way handshake The service must be available at the destination side Otherwise a RST packet will be sent back 59

60 Examples (a) TCP connection establishment in the normal case (b) Call collision: only one connection is established, denoted as (x,y)

61 Connection Release Symmetric 61

62 Connection Management Modeling The states used in the TCP connection management finite state machine. 62

63 Finite State Machine TCP connection management finite state machine. The heavy solid line is the normal path for a client. The heavy dashed line is the normal path for a server. The light lines are unusual events. Each transition is labeled by the event causing it and the action resulting from it, separated by a slash. 63

64 Transmission Policy 64

65 Performance Issues In some applications, one entity may send only one byte as the payload Example: a key stroke in telnet may result in a 41 byte IP packet, with only one byte information To improve the performance, the entity can delay the ACK for up to 500 ms Nagle s algorithm Buffer at the sender side 65

66 Silly Window Syndrome 66

67 Congestion Control (a) A fast network feeding a low capacity receiver. (b) A slow network feeding a high-capacity receiver. 67

68 Control Algorithm 68

69 Timer Management (a) Probability density of ACK arrival times in the data link layer. (b) Probability density of ACK arrival times for TCP. 69

70 Wireless TCP and UDP Splitting a TCP connection into two connections. 70

71 Transitional TCP (a) RPC using normal TPC. (b) RPC using T/TCP. 71

72 Performance Issues Performance Problems in Computer Networks Network Performance Measurement System Design for Better Performance Fast TPDU Processing Protocols for Gigabit Networks 72

73 Performance Problems The state of transmitting one megabit from San Diego to Boston (a) At t = 0, (b) After 500 μsec, (c) After 20 msec, (d) after 40 msec. 73

74 Network Performance Measurement The basic loop for improving network performance Measure relevant network parameters Try to understand what is going on Change one parameter 74

75 System Design for Better Performance Rules: CPU speed is more important than network speed Reduce packet count to reduce software overhead Minimize context switches Minimize copying You can buy more bandwidth but not lower delay Avoiding congestion is better than recovering from it Avoid timeouts 75

76 System Design for Better Performance Response as a function of load. 76

77 System Design for Better Performance Four context switches to handle one packet with a user-space network manager. 77

78 Fast TPDU Processing The fast path from sender to receiver is shown with a heavy line. The processing steps on this path are shaded. 78

79 Fast TPDU Processing (a) TCP header (b) IP header In both cases, the shaded fields are taken from the prototype without change 79

80 A timing wheel. Fast TPDU Processing 80

81 Protocols for Gigabit Networks Time to transfer and acknowledge a 1-megabit file over a 4000-km line. 81