Lecture 3: The Transport Layer: UDP and TCP

Transcription

1 Lecture 3: The Transport Layer: UDP and TCP Prof. Shervin Shirmohammadi SITE, University of Ottawa Prof. Shervin Shirmohammadi CEG

2 The Transport Layer Provides efficient and robust end-to-end service to upper layers. Not all services need to be provided! Uses services provided by the network layer. End-to-end delivery of entire message. Connection Prof. Shervin Shirmohammadi CEG

3 End-to-End Delivery Transport layer Networking layer Services are done from an end-to-end perspective Error control, flow control, Connectionless or connection-oriented Prof. Shervin Shirmohammadi CEG

4 Addressing Service Access Point (SAP), also referred to as Ports at the transport layer. Prof. Shervin Shirmohammadi CEG

5 Reliable Delivery Note that it is not compulsory for a transport layer protocol to provide such services. The idea is that if any of these services are to be provided in an end-to-end sense, then the transport layer is where you would implement them. Prof. Shervin Shirmohammadi CEG

6 Error Control Error checking is done for the Transport Layer PDU (T-PDU). Even though T-PDU is put into the N-PDU as payload, the networking layer may or may not provide error control for it. What does IP do? error checked at networking error checked at networking error checked at networking Prof. Shervin Shirmohammadi CEG

7 Sequence Control Prof. Shervin Shirmohammadi CEG

8 Loss Control Prof. Shervin Shirmohammadi CEG

9 Duplication Control What can cause duplication? Prof. Shervin Shirmohammadi CEG

10 Flow Control Controls the flow of information, again from an end-to-end perspective. Factors affecting this are the end host, but also the network. Prof. Shervin Shirmohammadi CEG

11 Multiplexing and Demultiplexing Data Link Networking Transport Prof. Shervin Shirmohammadi CEG

12 App1 App2 App3 Multiplexing data data data TCP UDP SPort=125 SPort=123 SPort=250 App 3 data App 1 data App 2 data Protocol=TCP Protocol=TCP Protocol=UDP IP TCP data TCP data UDP data to data Link layer Prof. Shervin Shirmohammadi CEG Protocol=TCP TCP data

13 App1 App2 App3 Demultiplexing data data data TCP UDP DPort=1090 DPort=560 DPort=440 App 3 data App 1 data App 2 data Protocol=TCP Protocol=UDP Protocol=TCP Protocol=TCP IP TCP data UDP data TCP data TCP data From data Link layer Prof. Shervin Shirmohammadi CEG

14 Connection Obviously required only if we re talking about a connection-oriented transport protocol. Otherwise just send the packet and hope for the best. Need two protocols really: connection establishment connection release Prof. Shervin Shirmohammadi CEG

15 User Datagram Protocol (UDP) Defined in RFC 768. Connectionless service Multiplexing/Demultiplexing Error Detection (not really error control) Detection only, no recovery; and even the detection is optional No flow control No congestion control What does it do then? Used by SNMP, DNS, RTP, Multicasting. Prof. Shervin Shirmohammadi CEG

16 Multiplexing/Demultiplexing Uses Port numbers Also used for addressing Prof. Shervin Shirmohammadi CEG

17 ICANN Ranges Internet Corporation for Assigned Names and Numbers 13: daytime 53: name server 69: TFTP 111: RPC 161 SNMP Prof. Shervin Shirmohammadi CEG

18 UDP Packet Format There is also a pseudo-header that is created and computed. The pseudo-header does not really exist as a header! Prof. Shervin Shirmohammadi CEG

19 Pseudoheader This is done to detect wrong delivery of the packet by the Networking layer. Prof. Shervin Shirmohammadi CEG

20 Checksum One s complement of the one s complement sum modulo Prof. Shervin Shirmohammadi CEG

21 Multicasting The ability to send a packet to more than one destination. (send once, receive at multiple destinations). Uses the IP multicasting (class D) addresses. Interested parties must join a multicast group. Network infrastructure (routers, bridges, ) must support IP Multicast for this to happen Mostly not supported on the Internet. Prof. Shervin Shirmohammadi CEG

22 Transmission Control Protocol (TCP) Prof. Shervin Shirmohammadi CEG

23 TCP - Transmission Control Protocol Provides reliable end-to-end byte stream over unreliable IP subnet. Designed to dynamically adapt to properties of subnet (bandwidth, delay, loss) and for robust handling of failures. Initial definition in standard RFC 793. Updates in RFC 1122, RFC 1323, RFC 2001 TCP accepts user data from local processes, splits into pieces no larger than 64K bytes (usually about 1500 bytes in practice), and sends each piece as a separate IP datagram. When IP datagrams arrive at the destination, TCP reconstructs the original byte stream. Prof. Shervin Shirmohammadi CEG

24 The Service TCP Provides Connection-oriented Connection release Point-to-point communication Addressing, Multiplexing and Demultiplexing Complete reliability Full-duplex communication Reliable connection startup and shutdown 3-way handshake Flow Control Congestion Control Prof. Shervin Shirmohammadi CEG

25 Port Numbers Prof. Shervin Shirmohammadi CEG

26 Reserved TCP Port Numbers Prof. Shervin Shirmohammadi CEG

27 TCP Error Control Sources of error: Segment damaged in transit Segment fails to arrive Transmitter does not know of failure Receiver must acknowledge successful receipt Too many ACKs if sent for each segment Use cumulative acknowledgement Time out waiting for ACK triggers retransmission (retransmission timer) Prof. Shervin Shirmohammadi CEG

28 Duplication Detection If ACK lost, segment is re-transmitted Causes duplication Receiver must recognize duplicates Duplicate received prior to closing connection Receiver assumes ACK lost and ACKs duplicate Sender must not get confused with multiple ACKs Sequence number space large enough to not cycle within maximum life of segment Duplicate received after closing connection Prof. Shervin Shirmohammadi CEG

29 Flow Control: Credit Allocation Prof. Shervin Shirmohammadi CEG

30 Congestion Caused by too much traffic going through a network; more than the network can handle. Results in Routers dropping packets. Causes time-outs: equally likely from lost messages due to unreliable transmission media, as from congestion. Simply retransmitting a lost message makes congestion worst. Why? There needs to be a way to control congestion. Prof. Shervin Shirmohammadi CEG

31 TCP Congestion Control RFC 1122, Requirements for Internet hosts Retransmission timer management Estimate round trip delay by observing pattern of delay Set time to value somewhat greater than estimate Simple average Exponential average RTT Variance Estimation (Jacobson s algorithm) Prof. Shervin Shirmohammadi CEG

32 Window Management Slow start awnd = MIN[credit, cwnd] Start connection with cwnd=1 Increment cwnd (double) with ACK, to some threshold Increment cwnd by 1 with ACK after the threshold Dynamic windows sizing on congestion When a timeout occurs Set slow start threshold to half current window ssthresh=cwnd/2 Set cwnd = 1 and slow start until cwnd=ssthresh Increasing cwnd by 1 for every ACK For cwnd >=ssthresh, increase cwnd by 1 for each RTT Prof. Shervin Shirmohammadi CEG

33 TCP Slow Start Prof. Shervin Shirmohammadi CEG

34 TCP Data Transfer Connection is a stream of bytes, not messages. Four messages of 512 bytes may be sent by TCP as any combination of pieces totaling 2048 bytes: bytes bytes bytes bytes... When application sends data to TCP, it may not be sent immediately: TCP may decide to collect more data before transmission. TCP PUSH flag is used to request no transmission delay. TCP URGENT flag immediately sends all pending data, and causes receiver to send an interrupt to its user. Prof. Shervin Shirmohammadi CEG

35 TCP Header Prof. Shervin Shirmohammadi CEG

36 Control Field Prof. Shervin Shirmohammadi CEG

37 TCP Header Fields Source and destination ports: 16 bit address of local port (socket). Sequence and acknowledgment numbers: Every byte is numbered in a TCP stream. Acknowledgment number is next byte number expected. 32 bits each. Header length: Needed because options field can vary in length. Number of 32 bits words in header. URG: set to 1 if urgent pointer in use Pointer indicates offset from current sequence number at which urgent data ends. Meaning that the receiving program should be notified of its arrival as soon as possible. Prof. Shervin Shirmohammadi CEG

38 TCP Header Fields (cont d) ACK: Set to 1 to indicate acknowledgment number is valid If 0, no acknowledgment in this segment. PSH: Set to 1 to indicate pushed data. Force delivery of bytes currently in the stream without waiting for buffer to fill. RST: Set to 1 to indicate reset. Host has become confused due to crash or for other reason. Also used to reject a connection, or refuse an invalid segment. SYN: used to establish connections. SYN = 1, ACK = 0 in connection request. SYN = 1, ACK = 1 in connection acceptance. FIN: set to 1 to indicate end of user data. Used to close connection. May continue to receive data. Prof. Shervin Shirmohammadi CEG

39 TCP Pseudo-header TCP Error Checking Protocol = 6 Source address Destination address 32 bits TCP segment length Checksum field provides error detection information for TCP segment header, plus the pseudo-header shown above. Checksum computation: Set checksum field to all zeros. Pad user data with extra 0 byte, if needed, so that user data has an even number of bytes. Add all 16 bit words in 1 s complement, and take 1 s complement of the sum. When receiver performs this computation, including checksum field, result should be 0. Prof. Shervin Shirmohammadi CEG

40 Additional Timer Issues TCP persistence timer: Prevents deadlock when current window size is 0, and acknowledgment increasing window size is lost. When persistence timer expires, ask receiver for window size. TCP keep alive timer: Timer expires after long interval with no messages. On expiry, send a message to receiver asking are you still there? Connection terminated if no response. TCP close timer ensures all packets die on connection termination. Prof. Shervin Shirmohammadi CEG