TSIN02 - Internetworking Literature: Lecture 4: Transport Layer Forouzan: ch 11-12 Transport layer responsibilities UDP TCP 2004 Image Coding Group, Linköpings Universitet 2 Transport layer in OSI model TSIN02 - Internetworking The physical layer transmits signals representing individual bits on a channel The link layer provides reliable link communication through framing, physical addressing, flow- error- and access control The network layer provides end to end delivery of individual packets through logical addressing and routing support 3 4
The transport layer is responsible for end to end delivery of messages. This is achieved through Service point addressing Segmentation and reassembly Connection control Flow control Error control 5 6 Port numbers are used as a process identifier. They are needed since computers can run multiple programs at the same time. IP addresses identify computers Port numbers identify processes An (IP address, port number) pair is called a socket. Messages must define source and destination sockets 7 8
Addresses and Ports TSIN02 - Internetworking 9 10 The IANA has divided the port numbers into three ranges: Well known ports (0-1023) Assigned and controlled by IANA Registered ports (1024-49151) Ports in this range can be registered with IANA to prevent duplication. Dynamic ports (49152-65535) The ephemeral ports can be used by any process. Port number Application 20 FTP 23 Telnet 25 SNMP 80 HTTP 11 12
There are two transport layer protocols in the TCP/IP stack: UDP - User Datagram Protocol Connectionless unreliable service TCP - Transmission Control Protocol Connection-oriented reliable stream service TCP UDP IGMP ICMP IP ARP RARP UDP is a connectionless, unreliable transport protocol UDP provides process to process communication limited error checking UDP does not provide acknowledgement for recieved packets segmentation and reassembly flow control Underlying networks 13 14 Why use UDP? UDP is suitable for processes that provide internal flow and error control mechanisms, eg TFTP multicasting and broadcasting management processes, eg SNMP some route updating protocols, eg RIP processes that only send short messages 15 16
UDP packets (datagrams): 8 byte header max 65507 bytes of data (=IP length IP header UDP header) The UDP checksum includes pseudoheader UDP header data from the application layer pad byte (if needed) 8 bits 8 bits Protocol All 0s (17) Source port address Source IP address... 8 bits 8 bits Destination IP address UDP total length Data UDP total length Destination port address UDP Checksum 17 18 Messages are encapsulated in UDP datagrams Usually each port is associated with one or two queues UDP handles multiplexing and demultiplexing of messages. 19 20
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TCP is a connection-oriented transport protocol TCP provides full duplex connections reliable stream service flow control error control 25 26 Sending and receiving buffers are used for storage. This makes it possible to handle differences in speed between the sending and receiving processes. Buffers are also used in flow and error control mechanisms. 27 28
29 30 TCP provides reliable service by breaking application data into suitably sized blocks called segments using an acknowledgement mechanism retransmission when errors occur providing flow control checksum control of header and data 31 32
All bytes transmitted in a connection are numbered, starting from a random position 32 bits are used for numbering The value in the header is the number of the first byte of data contained in the segment. This number is used to announce the sequence number of the next expected segment and thus acknowledging that all prior segments (with lower sequence numbers) have been received. 33 34 6 flags can be used to determine the purpose of the segment (more than one can be set) URG - urgent pointer valid (set when sender wants the receiver to read a piece of data urgently and possibly out of order) ACK - acknowledgment number valid PSH - push data, receiver should immediately pass the data to the application (buffers should be emptied!) RST - reset the connection A three-way handshake is used to establish a connection Negotiation on initial sequence numbers Initial segment number (ISN) chosen so that each active connection has its own ISN Normally initiation is made by the client SYN - synchronize sequence numbers to initiate connection FIN - terminate the connection 35 36
4 segments needed to close a connection MSS is the largest block of data TCP will send to the other side. MSS can be announced in the options field during connection establishment. Default MSS is 536 The larger the better (until fragmentation occurs) 37 38 Up to 40 bytes of optional informationcan be included in the TCP header Used to convey additional information or to align other options How much can a source send without receiving an ACK? The sliding window protocol is used in TCP offered window usable window A sliding window is used to make transmission more efficient as well as to control the flow of data so that the destination does not become overwhelmed with data. TCP s sliding windows are byte oriented. 39 40
41 42 Sender sends 1-byte segments because the application produces data slowly. Solution - Nagle's algorithm After sending one segment, wait until either an ack is received or a full window can be transmitted. 1-byte windows are announced by the receiver because the application consumes data slowly. Solution - Clark's Send acks as soon as possible, but announce zero window size until a maximum size segment fits the buffer Solution - delayed ack Wait with acknowledgments until there is room in the buffer 43 44
The source does not have to send a full window's worth of data. The size of the window can be increased or decreased by the destination The destination can send an acknowledgement at any time. 45 46 47 48
Handles bottlenecks in the network TCP assumes that the cause of a lost segment is due to congestion in the network. If the cause of the lost segment is congestion, retransmission of the segment will not help, it will make things worse. Without congestion control the network will collaps! Solution: Congestion window (CWND) slow start Additive increase of CWND Multiplicative decrease of CWND Fast retransmit and Fast recovery 49 50 Retransmission timer how long should sender wait for an ACK? Persistence timer how long should sender wait if window size = 0? Keepalive timer Prevents connections to live forever Time-Waited timer Each connection held in limbo before actually closed 51 52
TCP packet TSIN02 - Internetworking 53 54 Transport layer basics UDP - a fairly simple connectionless protocol TCP - a very complex protocol Reliability Connection management Flow control Congestion avoidance Timers 55