Chapter 7. The Transport Layer

Transcription

1 Chapter 7 The Transport Layer 1

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12 Addressing TSAPs, NSAPs and transport connections. 12

13 For rarely used processes, the initial connection protocol is used. A special process server, which acts as a proxy, listens to a set of ports waiting for a connection request. Potential users of a service begin by doing a CONNECT request, specifying the TSAP address of the service they want. If no server is waiting for them, they get a connection to the process server. Then the process server spawns the requested server which does the requested work. And the process server goes back to listen for new requests. 13

14 Addressing (2) How a user process in host 1 establishes a connection with a mail server in host 2 via a process server. 14

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27 Congestion Control Desirable bandwidth allocation Regulating the sending rate 27

28 Desirable Bandwidth Allocation (1) (a) Goodput and (b) delay as a function of offered load 28

29 Desirable Bandwidth Allocation (2) Max-min bandwidth allocation for four flows 29

30 Desirable Bandwidth Allocation (3) Changing bandwidth allocation over time 30

31 Regulating the Sending Rate (1) A fast network feeding a low-capacity receiver 31

32 Regulating the Sending Rate (2) A slow network feeding a high-capacity receiver 32

33 Regulating the Sending Rate (3) Some congestion control protocols 33

34 Regulating the Sending Rate (4) 34

35 Regulating the Sending Rate (5) User 2 s allocation User 1 s allocation Additive Increase Multiplicative Decrease (AIMD) control law. 35

36 Regulating the Sending Rate (6) 36

37 The Internet Transport Protocols: UDP Introduction to UDP Remote Procedure Call The Real-Time Transport Protocol 37

38 Introduction to UDP The UDP does not do flow control, error control, or retransmission. It provides an interface to the IP protocol with the added feature of demultiplexing multiple processes using the ports (especially useful in client-server situations, e.g. DNS) The source port is needed when a reply must be sent to the source The UDP length field includes the 8-bytes header and the data The UDP checksum is optional and stored as 0 if not computed (a true computed 0 is stored as all 1s) The UDP header. 38

39 Introduction to UDP (2) The IPv4 pseudoheader included in the UDP checksum. 39

40 Remote Procedure Call A remote call is similar to a function call in programming (start with parameters and get back a result). Which leads to arrange request/reply interaction Steps in making a remote procedure call. The stubs are shaded. 40

41 Definition: Client stub: a small library procedure in the client Server stub: a small library procedure in the server Marshaling: packing the parameters Procedure 1. The client calls the client stub 2. The client stub packs (marshals) the parameters into a message and makes a system call to send the message 3. The kernel sends the message from the client to the server 4. The kernel in the server passes the incoming packet to the server stub 5. The server stub calls the server procedure with the unmarshaled parameters The reply traces the same path in the other direction, some restrictions are needed to avoid problems 41

42 The Real-Time Transport Protocol RTP is for multimedia applications (e.g. Internet radio, telephony, video on demand ) RTP looks like an application protocol. On the other hand, it is a generic application-independent protocol that just provides transport facilities, so it also looks like a transport protocol. The basic function of RTP is to multiplex several real-time data streams onto a single stream of UDP packets (unicasting or multicasting) (a) The position of RTP in the protocol stack. (b) Packet nesting. 42

43 The Real-Time Transport Protocol (2) Ver: version, P: padded, X: extension header existing, CC: how many contributing Sources (0~15), M: an application-specific marker bit (e.g. start of a video frame) Payload type: encoding algorithm (e.g. MP3) timestamp: helping reduce jitter, source identifier: identifies the stream the packet belongs to, contributing source identifier: used when mixers are presented in the studio The RTP header. 43

44 Real-Time Transport (3) Smoothing the output stream by buffering packets 44

45 Real-Time Transport (3) High jitter 45

46 Real-Time Transport (4) Low jitter 46

47 The Internet Transport Protocols: TCP Introduction to TCP The TCP Service Model The TCP Protocol The TCP Segment Header TCP Connection Establishment TCP Connection Release TCP Connection Management Modeling TCP Sliding Window TCP Timer Management TCP Congestion Control TCP futures 47

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49 The TCP Service Model (1) Some assigned ports 49

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51 When an application passes data to TCP, TCP may send it immediately or buffer it. PUSH flag can be used by applications to tell TCP not to delay the transmission. If an interactive user (e.g. hits DEL or CTRL-C key) wants to put some control information in the data stream, the URGENT flag may be used. The URGENT flag will cause TCP to stop accumulating data and transmit everything it has for that connection immediately. 51

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53 The TCP Segment Header The TCP header. 53

54 Source and Destination port fields identify the local end points. Sequence number and Acknowledgement number specify the byte number in the data field and the next byte expected. CWR: congestion Window reduced ECE: Explicit Congestion Notification TCP header length tells how many 32-bit words in the TCP header. URG: Urgent pointer (1) is used indicate a byte offset of urgent data. ACK: (1) Ack number is valid otherwise is ignored PSH: This bit indicates PUSH data RST: Reset SYN: is used to establish connections FIN: this bit is used to release a connection (no more data) Window Size: It tells how many bytes may be sent starting at the byte acknowledged 54

55 ACK 55

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62 SRTT = α SRTT + (1 α) R where α is a smoothing factor that determines how quickly the old values are forgotten. Typically, α = 7/8. This kind of formula is an EWMA (Exponentially Weighted Moving Average) or low-pass filter that discards noise in the samples. Round-Trip Time VARiation, RTTVAR, is updated using the formula RTTVAR = β RTTVAR + (1 β ) This is an EWMA as before, and typically β = 3/4. The retransmission timeout, RTO, is set to be SRTT R RTO = SRTT + 4 RTTVAR 62

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64 TCP Congestion Control (1) Slow start from an initial congestion window of 1 segment 64

65 TCP Congestion Control (2) Additive increase from an initial congestion window of 1 segment. 65

66 TCP Congestion Control (3) Slow start followed by additive increase in TCP Tahoe. 66

67 TCP Congestion Control (4) Fast recovery and the sawtooth pattern of TCP Reno. 67

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69 Performance Issues Performance problems in computer networks Network performance measurement System design for better performance Fast TPDU processing Protocols for high-speed networks 69

70 Performance Issues Five aspects of network performance 1.Performance problems. 2.Measuring network performance. 3.Host design for fast networks. 4.Header Compression. 5.Protocols for long fat networks. 70

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76 System Design for Better Performance Rules of thumb 1. CPU speed more important than network speed 2. Reduce packet count to reduce software overhead 3. Minimize data touching 4. Minimize context switches 5. Minimize copying 6. You can buy more bandwidth but not lower delay 7. Avoiding congestion is better than recovering from it 8. Avoid timeouts 76