The Transport Layer application transport network data link physical application transport network data link physical application transport network data link physical This critical layer provides communications services directly to distributed processes running on different hosts.

The Most Frequently Used Internet Transport Protocols r Transmission Control Protocol (TCP) r User Datagram Protocol (UDP)

TCP/IP Protocol Data Units (PDUs)

TCP/IP Concepts

TCP/IP Concepts – Data Flow application transport network link physical application transport network link physical application transport network link physical application transport network link physical network link physical data

TCP Applications FTP client FTP server TCP control connection port 21 TCP data connection port 20 mail server user agent user agent user agent mail server user agent user agent mail server user agent SMTP SMTP SMTP PC running Explorer Serverrunning Apache Web Server Mac running Navigator http request http response Web browsing File transfer

The transport level addressing r Network Service Access Point (NSAP): it is between the network and transport levels r Transport Service Access Point (Transport address, TSAP): it is between the transport and the application layers r Port Number = Application Selection Address (ASA)

Port r An abstraction, which is used by the transport protocols to take the difference among each addressee in a given host r The TCP/IP protocol identify the port with a small integer r The port equivalent to the Transport Selector (TSEL) in the transport layer of the OSI model

Assigned Port or Socket Number

Some well-known port numbers

Transport services and protocols r provide logical communication between application processes running on different hosts r transport protocols run in end systems r transport vs. network layer services: m network layer: data transfer between end systems (hosts) m transport layer: data transfer between processes r Transport layer relies on and enhances network layer services application transport network data link physical application transport network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical logical end-end transport

The source and destination port in the client- server communication

Socket, the realization of the port in the programming TCP is a transport level protocol, see later

application transport network M P2 application transport network Multiplexing/demultiplexing Recall: segment - unit of data exchanged between transport layer entities m aka TPDU: transport protocol data unit receiver H t H n Demultiplexing: delivering received segments to correct app layer processes segment M application transport network P1 MMM P3 P4 segment header application-layer data

Transport level multiplexing

Multiplexing/demultiplexing: examples host A server B source port: x dest. port: 23 source port:23 dest. port: x port use: simple telnet app Web client host A Web server B Web client host C Source IP: C Dest IP: B source port: x dest. port: 80 Source IP: C Dest IP: B source port: y dest. port: 80 port use: Web server Source IP: A Dest IP: B source port: x dest. port: 80

Transmission Control Protocol (TCP) r Connection-oriented r End-to-end reliability m Reliability: positive acknowledging r It is intended for use as a highly reliable host-to- host protocol between hosts in m packet-switched computer communication networks, and m in interconnected systems of such networks r Flow control with sliding window m protocol-adaptation to: round-trip time (RTT) or available transmission velocity Protocol Layering:

TCP: Overview RFCs: 793, 1122, 1323, 2018, 2581  Point-to-point: one sender, one receiver  Reliable transport using sliding window protocol  Flow controlled and congestion controlled

User Datagram Protocol (UDP) r The UDP gives the transport mechanism with the possible smallest overhead r Connection-less r It carry out multiplexing/demultiplexing and error check, only r The delivery is not reliable r The delivery is not ordered

Relation With Other Protocols Application Layer (SMTP, FTP, HTTP, etc) Transport Layer (TCP, UDP) Internet Layer (IP, ICMP, ARP) Network Interface Layer (Ethernet, X25, SLIP, PPP) PHYSICAL NETWORK TCP/IP STACK

UDP: User Datagram Protocol [RFC 768] r “best effort” service, UDP segments may be: m lost m delivered out of order to app r connectionless: m no handshaking between UDP sender, receiver m each UDP segment handled independently of others Why is there a UDP? r no connection establishment (which can add delay) r simple: no connection state at sender, receiver r small segment header r no congestion control: UDP can blast away and send data as fast as desired

UDP: more r often used for streaming multimedia apps m loss tolerant m rate sensitive r other UDP uses: m DNS m SNMP r reliable transfer over UDP: add reliability at application layer m application-specific error recover! source port #dest port # 32 bits Application data (message) UDP segment format length checksum Length, in bytes of UDP segment, including header

UDP-based Error Check r If sum of the whole 16-bit words of the received UDP datagram equals to FFFFh, the datagram is probably error free r Calculation of the control sum: m E.g. let the next 16-bit words: m Their sum: m (1st complement: all 0 are exchanged to 1 and vice versa) m So we get the control sum: m At the receiver side the sum of the whole 16-bit word plus the control sum (possible carry bit is eliminated): =FFFFh r This error check is not very safe

Features of the Application of UDP r Small header overhead r Non-controlled sending speed r No connection establishing r No connection state r Lack of error control r Lack of congestion protection r Message carried by UDP restricted to 512 bytes, not counting the IP or UDP headers m Longer message are truncated

Services provided by Internet transport protocols TCP service: r connection-oriented: setup required between client, server r reliable transport between sending and receiving process r flow control: sender won’t overwhelm receiver r congestion control: throttle sender when network overloaded r does not provide: timing, minimum bandwidth guarantees UDP service: r unreliable data transfer between sending and receiving process r does not provide: connection setup, reliability, flow control, congestion control, timing, or bandwidth guarantee Why is there a UDP? Q: Why is there a UDP?

Application of the Transport Protocols