1Transport Layer...Transport Protocols and MPLS #9Chapter 8 & Section 10.52Transport Layer...Outline Goals: Understand principles behind transport layer services Multiplexing/demultiplexing (Ports/Sockets) Examples of Transport Protocols UDP TCP– reliable data transfer– flow control– congestion control Note there are other transport layer protocols MPLS (Section 10.5)3Transport Layer...Transport services and protocols Provide logical communicationbetween 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, UDP and othersapplicationtransportnetworkdata linkphysicalapplicationtransportnetworkdata linkphysicalModified from: Computer Networking: A Top Down Approach 4thedition. Jim Kurose, Keith RossAddison-Wesley, July 2007.4Transport Layer...How multiplexing works 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 socketsource port # dest port #32 bitsapplicationdata (message)other header fieldsTCP/UDP segment formatFrom: Computer Networking: A Top Down Approach 4thedition. Jim Kurose, Keith RossAddison-Wesley, July 2007.5Transport Layer...Ports and Sockets Port address + IP address = socket address Ports are 16 bits Both sender and receive have socket addresses A connection is identified by a pair of sockets The port address is internal to the host (indicates application) A socket address is unique in the internet Once an application creates a socket and TCP connection then a write is used to send to the network and a read used to receive from the network. 6Transport Layer...Ports and Sockets Well Known Ports are those from 0 through 1023. The Registered Ports are those from 1024 through 49151 The Dynamic and/or Private Ports are those from 49152 through 65535 There are some common port numbers Example: – File data transfer (21)– TELNET (23)– Simple Mail Transfer Protocol (SMTP)(25)– Remote Procedure Call [RPC] (111)– Web servers listens on port 80 http://www.iana.org/assignments/port-numbers7Transport Layer...Transport Layer: UDP UDP Connectionless No congestion control  No acknowledgments Packets may be– lost– delivered out of order to app No handshaking between UDP sender, receiver Each UDP segment handled independently of othersFrom: “Computer Networks, 3rd Edition, A.S. Tanenbaum. Prentice Hall, 19968Transport Layer...UDP Use Cases Streaming multimedia apps loss tolerant rate sensitive DNS Simple Network Management Protocol(SNMP) Reliable transfer over UDP: add reliability at application layer application-specific error recovery!Modified from: Computer Networking: A Top Down Approach 4thedition. Jim Kurose, Keith RossAddison-Wesley, July 2007.9Transport Layer...Transport Layer: TCP TCP provides for assured delivery of PDU’s TCP Services Connection oriented (end-to-end)– Need call processing– Information on the status of each connection is available Reliable data transfer– Uses acknowledgments – Uses sequence numbers10Transport Layer...TCP HeaderFrom: “Computer Networks, 3rd Edition, A.S. Tanenbaum. Prentice Hall, 199611Transport Layer...TCP Header Source/Destination identify local end points Window size used to dynamically control source rate into the network Checksum, checks the header and data12Transport Layer...TCP Stream-oriented TCP collect user bytes and forms segments to be passed on to the IP layer Sequence number based on byte counts Push Upper layer protocol send Push message to TCP to force it to send all the bytes collected in a segment Resequencing IP may deliver information out of order, TCP must put it back together13Transport Layer...TCP Inclusive Acknowledgment Acknowledgment number, acknowledges all received bytes prior to the one specified Flow control Window size is in bytes Transmit N-bytes and the must wait for acknowledgment Window size is dynamic, i.e., it changes based on “knowledge” of network congestion14Transport Layer...TCP Multiplexing Allows multiple sessions within one host to be transmitted over an IP path (ports/sockets) Full duplex Security and precedence Graceful close All traffic in flow is acknowledged before the session is ended.15Transport Layer...From: “Computer Networks, 3rd Edition, A.S. Tanenbaum. Prentice Hall, 1996TCP SessionProcessingTCP Connection Management: Finite State Machine16Transport Layer...Host A Host BFigure 8.22From: Communications Networks, Garcia and Widaja, McGraw Hill, 2000TCP Connection Setup: Three-way Handshake17Transport Layer...tcpdump http connection set up Output columns are Time SourceIP.SourcePort > DestIP.DestPort Flags ... 11:13:38.524046 x.x.x.x.3600 > 64.233.167.104.80: S 2021815674:2021815674(0) win 64240 <mss 1460,nop,nop,sackOK> (DF) First packet is from client host x.x.x.x. Client host is using 3600 as a source port. Destination host is 64.233.167.104 on port 80 (that’s Google's webserver). The packet with flag S is a TCP SYN packet, means in words "i'd like to open a TCP connection with you“ Client host will have a temporarily opened port (3600) in order to receive data back from the server. 11:13:38.558668 64.233.167.104.80 > x.x.x.x.3600: S 3132749891:3132749891(0) ack 2021815675 win 8190 <mss 1460> Second packet is sent from Google webserver. This packet comes from 64.233.167.104 source port 80, and contains SYN/ACK TCP flags sent to client port 3600, means "ok you may open a connection with me". 11:13:38.559105 x.x.x.x.3600 > 64.233.167.104.80: . ack 1 win 64240 (DF) Third packet is the client host sending a last ACK packet, which means "ok we are now connected". Source and dest ports must stay the same here.18Transport Layer...From: “Computer Networks, 3rd Edition, A.S. Tanenbaum. Prentice Hall, 1996TCP Window Management19Transport Layer...Silly Window Syndrome Situation:  Transmitter sends large amount of data Receiver buffer depleted slowly, so buffer fills Every