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Chapter 6 -2 The Transport Layer

Chapter 6 -2 The Transport Layer. TCP, UDP and Performance Issues. The Internet Transport Protocols: UDP. Introduction to UDP Remote Procedure Call The Real-Time Transport Protocol. UDP (User Data Protocol). Provides a way to send IP datagrams without establishing a connection

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Chapter 6 -2 The Transport Layer

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  1. Chapter 6-2The Transport Layer TCP, UDP and Performance Issues Computer Networks II

  2. The Internet Transport Protocols: UDP • Introduction to UDP • Remote Procedure Call • The Real-Time Transport Protocol Computer Networks II

  3. UDP (User Data Protocol) • Provides a way to send IP datagrams without establishing a connection • A UDP segment has 8-byte header followed by data • UDP length includes header and data • UDP checksum includes the same format pseudoheader as in TCP Computer Networks II

  4. Remote Procedure Call • Allows programs to call procedures located on remote hosts • Looks as much possible as local call • Two library procedures called client stub and server stub hide the fact that call is remote • The client program makes a local call to client stub to handle the RPC • Information is transported in parameters • For this purpose, client stub packs parameters into a message (parameter marshalling) Computer Networks II

  5. Remote Procedure Call Steps in making a remote procedure call. The stubs are shaded. Computer Networks II

  6. Problems with RPC • Passing pointers is impossible • Array size must be specified and fixed • It may not be possible to deduce the types of the parameters • Global variables can not be used • RPC commonly uses UDP Computer Networks II

  7. Real Time Transport Protocol (RTP) • Used to transport multimedia streams • Can multiplex different streams such as the audio and video into a single UDP socket • Each packet is given a unique increasing number • No flow control, no error control, no acks,no retransmission mechanism • Contains a payload type field to specify encoding that allows changing quality of the multimedia data when available bandwidth drops • Contains timestamps that are used to synchronize streams. However, these timestamps are used to compute the relative time from the start rather than absolute time • Has a sister protocol, Real Time Control Protocol (RTCP) that carries delay, jitter, bandwidth and congestion information for feedback purposes Computer Networks II

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

  9. The Real-Time Transport Protocol (2) The RTP header. Computer Networks II

  10. 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 Transmission Policy • TCP Congestion Control • TCP Timer Management • Wireless TCP and UDP • Transactional TCP Computer Networks II

  11. TCP Basic Functionality • Provides reliable end-to-end byte stream over an unreliable internetwork • Transport entity • A piece of software that is either • A user process or • Part of kernel • Accepts user data streams from local processes • Breaks them into pieces not exceeding 64K bytes • Send each piece as an IP datagram • At the receiver, it reconstructs original byte streams Computer Networks II

  12. The TCP Service Model • The sender and receiver create end points called sockets • Each socket has • IP address • Port number (16 bit and local) • A socket may be used for multiple connections at the same time • Some port numbers are reserved • FTP: 21 • Telnet: 23 Computer Networks II

  13. The TCP Service Model Some assigned ports. Port Protocol Use 21 FTP File transfer 23 Remote login Telnet E-mail 25 SMTP 69 Trivial File Transfer Protocol TFTP Finger Lookup info about a user 79 80 World Wide Web HTTP POP-3 110 Remote e-mail access USENET news 119 NNTP Computer Networks II

  14. The TCP Service Model (2) (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. • A TCP connection is a byte stream, not a message stream • Message boundaries are not preserved end to end Computer Networks II

  15. The TCP Service Model (cont’d) • When an application passes data to TCP, TCP may send it immediately or buffer it at its discretion • To force data out, applications can use PUSH flag, which tells TCP not to delay transmission • At the receiver side, the same effect can be obtained by using URGENT flag • However, at the receiver side, on the contrary of PUSH flag, URGENT flag causes an interrupt to the application (provides signaling) Computer Networks II

  16. The TCP Protocol • Every byte on a TCP connection has its own 32-bit sequence number to be used for acks and sliding window sequencing • Window mechanism has its own 32-bit header • Data is exchanged in form of segments • A segment consists of a 20-byte header and zero or more bytes • A segment must fit into a IP payload (65635 bytes) or Maximum Transfer Unit, MTU of the underlying network (further segmentation is a major problem) • Sliding window acks next expected sequence number Computer Networks II

  17. The TCP Segment Header TCP Header. Computer Networks II

  18. The TCP Segment Header (2) The pseudoheader included in the TCP checksum. Computer Networks II

  19. The TCP Segment Header (cont’d) • Every segment begins with a fixed format 20-byte header • Segments without any data are legal • Ack messages or other control messages • Ack: Next byte expected • Options • Specifies beginning of data • Urgent pointer specifies offset of urgent data when URG flag is set • ACK bit indicates that the ack field is valid Computer Networks II

  20. The TCP Segment Header (cont’d) • PSH flag indicates pushed data • RST flag indicates problem with connection • SYN flag indicates connection request or connection accepted together with ACK flag • FIN flag is used to release a connection • Window Size field indicates how many bytes can be accepted starting at the byte acknowledged. • Checksum is a control field on header, data and pseudo header that indicates portions of IP header Computer Networks II

  21. The TCP segment Header (cont’d) • Options • A host may specify the maximum TCP payload it is willing to accept • If not specified, the default is 536 bytes • Window scale option • In general 16 bit window size is not sufficient for current systems • This option scales up the window size • Selective Reject instead of go back n can be indicated via options field Computer Networks II

  22. TCP Connection Management • Server waits for an incoming connection by executing the LISTEN and ACCEPT primitives • Client executes CONNECT primitive • IP address, port number and options specified • Destination checks to see if a process has done a LISTEN on the indicated port • If not,request is rejected • If there is a process which has done a LISTEN, it is asked if it would accept the connection • If process rejects, destination sends a segment with RST set. If process accepts, request is acked Computer Networks II

  23. TCP Connection Establishment (a) TCP connection establishment in the normal case. (b) Call collision. 6-31 Computer Networks II

  24. TCP Connection Management (cont’d) • If simultaneous connection request happens only one connection is established • TCP connections are full duplex • However, they are considered as two simplex connections and each of them is released independently • To release, a node sends a segment with FIN bit set and the other side acks the release • Same procedure is repeated for the other simplex connection. Computer Networks II

  25. TCP Connection Management (cont’d) • To avoid two-army problem, timers are used in release process • That is, if ack for release does not arrive on time, timer goes off and client releases the connection • After release, both sides wait for maximum packet lifetime to erase tables Computer Networks II

  26. TCP Connection Management Modeling The states used in the TCP connection management finite state machine. Computer Networks II

  27. TCP Connection Management Modeling (2) 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. Computer Networks II

  28. TCP Transmission Policy • Window management is not directly tied to acks as in data link protocols • Exclusive buffer messages manage the transmission • If no buffer at receiver, then no transmission by sender except • Urgent data may be sent • One byte segment can be sent to ask receiver to renounce its buffer status (to prevent deadlock) Computer Networks II

  29. TCP Transmission Policy Window management in TCP. Computer Networks II

  30. TCP Transmission Policy (cont’d) • Sender and receiver are not forced to transmit or receive as soon as they receive data from the application. This improves performance as follows: • If one byte messages are sent (like in TELNET) then use NAGLE’s algorithm • Send first byte and keep the rest until ack comes back • As ack comes in send the rest and keep the further incoming bytes until ack is received Computer Networks II

  31. TCP Transmission Policy (2) • Silly Window Syndrome: Receive bytes one by one and send window messaging accordingly Computer Networks II

  32. TCP Transmission Policy (cont’d) • Clark’s Solution to Silly Window Syndrome • Prevent receiver from sending one byte updates and make it wait until decent amount of space available before it sends buffer messages • Sender may also pospone sending messages • Nagle’s Algorithm and Clark’s solution are complementary and they can be used at the same time Computer Networks II

  33. TCP Congestion Control • When congestion occurs, IP has limited effect on managing congestion • Most of the congestion control is done by TCP by cutting down the data rate • Indication of congestion • Timeouts • Packet discards • In fiber optic cable transmission errors are minimized so timeouts mainly due to congestion Computer Networks II

  34. Congestion Window • In addition to receiver’s buffer information, the sender also maintains a congestion window. • This is mainly due to the fact that even if receiver may have space for fast data transfer, network may not carry it due to congestion. • The number of bytes to be sent is the minimum of the two windows Computer Networks II

  35. TCP Congestion Control (a) A fast network feeding a low capacity receiver. (b) A slow network feeding a high-capacity receiver. Computer Networks II

  36. Slow Start Algorithm • Initialize the size of the congestion window to the maximum segment size • If no timeout occurs increase the size of the window by one segment and send again. • Repeat above n times. If no timeout, then increase the window by n segments. • Internet uses threshold in addition to the above algorithm Computer Networks II

  37. TCP Congestion Control (2) An example of the Internet congestion algorithm. Computer Networks II

  38. TCP Timer Management:Retransmission Timer • When goes off, segment is retransmitted • Setting the timer value is much more difficult than that of data-link layer, beacuse the delay is not easily predictable • TCP uses a dynamic algorithm to solve this problem • For each connection, TCP maintains a variable called RTT (round trip time) Computer Networks II

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

  40. Retransmission Timer (cont’d) • M: The time to receive ack • :Smoothing factor, usually 7/8 • RTT=RTT+(1-)M • D=D+(1-)|RTT-M| • Timeout=RTT+4D • Karn’s Algorithm • Do not update RTT on any segments that have been retransmitted (difficult to figure out for what segment the ack is) Computer Networks II

  41. TCP Timers (cont’d) • Persistence Timer • Used to prevent deadlock when buffer management messages are lost • Keepalive Timer • Check the other side if the line is idle for a long time • Timed Wait • Used to close the connection Computer Networks II

  42. Wireless TCP and UDP • In theory, transport protocol should be independent of the technology of the underlying network layer • However, if the transmission is wireless, performance of TCP degrades drastically • Wired connections • Timeouts are considered to be caused by congestion not by lost packets • Wireless Connections • Timeouts are primarily due to lost packets Computer Networks II

  43. Wireless TCP and UDP (cont’d) • Wired TCP Timeout • Slowdown to alleviate congestion • Wireless TCP Timeout • Retransmit as soon as possible • Indirect TCP • Split TCP connection into two separate connections • Sender-to-base station • Base station to mobile host Computer Networks II

  44. Wireless TCP and UDP Splitting a TCP connection into two connections. Computer Networks II

  45. Indirect TCP • Snooping Agent • Observes and caches TCP segments going out to the mobile host and acknowledges coming back from it • When an ack for a segment to mobile host is late, it retransmits the segment to mobile host without telling anything to the source • Congestion control algorithm will never start unless there is congestion on the wired part • Wireless UDP • Stations should not expect reliability Computer Networks II

  46. Translational TCP • If RPC requires long replies, then UDP is not suitable and TCP becomes the choice • However, TCP requires nine packets and this not efficient • A variant of TCP called T/TCP (Translational TCP) solves this problem • Allows transfer of data during setup Computer Networks II

  47. Transitional TCP (a) RPC using normal TCP. (b) RPC using T/TCP. Computer Networks II

  48. Performance Issues • Performance Problems in Computer Networks • Network Performance Measurement • System Design for Better Performance • Fast TPDU Processing • Protocols for Gigabit Networks Computer Networks II

  49. PERFORMANCE ISSUES:Performance Problems • Fast line-slow CPU • Broadcast storm • Power failure (RARP density when machines are up) • Lack of memory allocation • Timeouts set incorrectly • Bandwidth-delay product (BDP) • The receiver window should be at least BDP • Jitter Computer Networks II

  50. Performance Problems in Computer Networks 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. Computer Networks II

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