Transport Protocols and MPLS 9 Chapter 8 Section 10 5 1 Transport 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 2 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 UDP and others application transport network data link physical application transport network data link physical 3 Modified from Computer Networking A Top Down Approach 4th edition Jim Kurose Keith Ross Addison Wesley July 2007 Transport 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 socket 32 bits source port dest port other header fields application data message TCP UDP segment format From Computer Networking A Top Down Approach 4th edition Jim Kurose Keith Ross Addison Wesley July 2007 4 Transport 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 5 Transport 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 numbers 6 Transport 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 others From Computer Networks 3rd Edition A S Tanenbaum Prentice Hall 1996 7 Transport 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 4th edition Jim Kurose Keith Ross Addison Wesley July 2007 8 Transport 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 numbers 9 Transport Layer TCP Header From Computer Networks 3rd Edition A S Tanenbaum Prentice Hall 1996 10 Transport 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 data 11 Transport Layer TCP Stream oriented Push TCP collect user bytes and forms segments to be passed on to the IP layer Sequence number based on byte counts 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 together 12 Transport 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 congestion 13 Transport 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 14 Transport Layer TCP Session Processing TCP Connection Management Finite State Machine From Computer Networks 3rd Edition A S Tanenbaum Prentice Hall 1996 15 Transport Layer TCP Connection Setup Three way Handshake Host A Host B Figure 8 22 From Communications Networks Garcia and Widaja McGraw Hill 2000 16 Transport 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 11 13 38 558668 64 233 167 104 80 x x x x 3600 S 3132749891 3132749891 0 ack 2021815675 win 8190 mss 1460 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 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 17 Transport Layer TCP Window Management From Computer Networks 3rd Edition A S Tanenbaum Prentice Hall 1996 18 Transport Layer Silly Window Syndrome Situation Transmitter sends large amount of data Receiver buffer depleted slowly so buffer fills Every time a few bytes read from buffer a new advertisement to transmitter is generated Sender immediately sends data fills buffer Many small inefficient segments are transmitted Solution Receiver does not advertize window until window is at least of receiver buffer or maximum segment size Transmitter refrains from sending small segments From Communication Networks Fundamentals Concepts and Key Architectures Authors A Leon Garcia and I Widjaja 19 Transport Layer Delay BW Product Advertised