Introduction to Networks CSEN 404 Introduction to Networks Amr El Mougy Gina Maher ** Slides are attributed to J. F. Kurose

Chapter 3 outline Connection-oriented transport: TCP segment structure reliable data transfer flow control connection management Principles of congestion control Transport-layer services Multiplexing and demultiplexing Connectionless transport: UDP Principles of reliable data transfer Transport Layer

Transport services and protocols provide logical communication between app processes running on different hosts transport protocols run in end systems send side: breaks app messages into segments, passes to network layer rcv side: reassembles segments into messages, passes to app layer more than one transport protocol available to apps Internet: TCP and UDP application transport network data link physical logical end-end transport application transport network data link physical Transport Layer

Transport vs. network layer network layer: logical communication between hosts transport layer: logical communication between processes relies on, enhances, network layer services Household analogy: 12 kids sending letters to 12 kids processes = kids app messages = letters in envelopes hosts = houses transport protocol = Ann and Bill network-layer protocol = postal service Transport Layer

Internet transport-layer protocols reliable, in-order delivery (TCP) congestion control flow control connection setup unreliable, unordered delivery: UDP no-frills extension of “best- effort” IP services not available: delay guarantees bandwidth guarantees application transport network data link physical network data link physical network data link physical logical end-end transport network data link physical network data link physical network data link physical network data link physical application transport network data link physical Transport Layer

Multiplexing/demultiplexing Demultiplexing at rcv host: Multiplexing at send host: delivering received segments to correct socket gathering data from multiple sockets, enveloping data with header (later used for demultiplexing) = socket = process application application application P3 P1 P2 P4 P1 transport transport transport network network network link link link physical physical physical host 3 host 1 host 2 Transport Layer

How demultiplexing works 32 bits host receives IP datagrams each datagram has source IP address, destination IP address each datagram carries 1 transport-layer segment each segment has source, destination port number host uses IP addresses & port numbers to direct segment to appropriate socket source port # dest port # other header fields application data (message) TCP/UDP segment format Transport Layer

Choosing Port Numbers UDP TCP Source port: 26145 Dest. port: 80 Src IP: C Dest. IP: B UDP TCP Source port: 19157 Dest. port: 53 Source port: 7532 Dest. port: 80 Src IP: C Dest. IP: B Source port: 53 Dest. port: 19157 Source port: 26145 Dest. port: 80 Src IP: A Dest. IP: B Server is always listening on the well-known port number Client chooses a source port number and inserts it in the header. The destination port number is the well- known port number In the reply, the server inserts the port number received in the packet in the destination port number Server is always listening on the well-known port number Client chooses a source port number and inserts it in the header. The destination port number is the well-known port number The server creates a socket for the client that is identifiable via the combination of src IP and src port address

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

Examples of Transport-layer Protocols used by Applications Application-Layer Protocol Underlying Transport Protocol E-mail SMTP TCP Remote terminal access Telnet Web HTTP File transfer FTP Remote file server NFS Typically UDP Streaming multimedia Typically proprietary UDP or TCP Internet telephony Network management SNMP Routing protocol RIP Name translation DNS Transport Layer

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