Notes - The Transport Service

The following are notes from Computer Networks written by Tanenbaum 5th edition.

• Introduction to transport service and its service to application layer

Services Provided to the Upper Layers

• provide efficient, reliable, and cost-effective data transmission to users
• software/hardware within layer is called transport entity
• located in the os kernel, or library package in network apps, separate user process or network interface card
• links application or session layer to network layer
• two types of transport service
• both have 3 phases
• establishment
• data transfer
• release
• Addressing and flow control similar to network layer
• difference between network and transport layer despite similarities?
• transport code is all run on users machines
• network runs on routers
• users have little control over network layer
• if packets lost or mangled, transport layer corrects for errors and asks for retransmit
• if connection lost, transport layer sets up new link
• makes transport service more reliable than network
• programmers can write code that works on a variety of networks without dealing with reliability of network
• bottom 4 layers
• transport service provider
• upper layers
• transport service user

Transport Service Primitives

• many programs require transport entities, but few require network services/entities because those are hidden
• five primitives
• LISTEN
• block until process tries to connect
• CONNECT
• attempts to establish a connection
• SEND
• sends data/information
• RECEIVE
• blocks until data packet arrives
• DISCONNECT
• releases connection
• segments
• messages sent from transport entity to transport entity
• TCP or UDP will use this term, older protocols may call segments TPDU(Transport Protocol Data Unit)
• segments exchanged by transport layer are contained in 
• packets exchanged by network layer
• packets contained in frames exchanged by data link layer
• when frame arrives
• data link layer processes frame header, if match
• passes contents up to network, if packet header match
• pass up to transport layer
• Client-Server
• CONNECT call causes connection request segment sent to server
• on arrivale transport checks to see if server blocked
• if it is unblocks server and sends connection accepted segment back to client
• when connection accepted arrives client unblocked
• data can be now be sent and receive with SEND RECEIVE
• In transport even one direction data exchange is more complicated than network layer, every data packet gets ack eventually
• control segments also gets ack, managed by transport entities
• transport entities don't need to worry about timer/retransmit
• When connection no longer needed DISCONNECT primitive
• asymmetric disconnect either can issue disconnect
• symmetric variant each side closed separately

Berkeley Sockets

• socket primitives used for TCP

• Server side
• The SOCKET primitive creates new endpoint, allocates space for it within transport entity
• returns ordinary file descriptor, kind of like OPEN on a file
• sockets don't have network addresses
• BIND primitive assign sockets network address
• remote clients can now connect
• LISTEN primitive allocates space to queue to incoming calls for several clients to try to connect
• not blocking call
• ACCEPT primitive returns file descriptor for read and write
• creates a new socket with same properties as original and returns file descriptor for it
• server can now fork process or thread
• Client Side
• SOCKET primitive to be created, but BIND not necessary
• CONNECT primitive blocks the caller and starts connection
• both sides can now use SEND and RECEIVE
• Connection release is symmetric
• CLOSE connection is released
• socket API is often used with TCP to create a reliable byte stream
• DCCP(Datagram Congestion Controlled Protocol)
• version of UDP with congestion control Kohler et al 2006
• sockets probably not final word, structure for congestion control suboptimal because applied separately to each stream, and one stream per object
• SCTP(Stream Control Transmission Protocol)
• SST(Structured Stream Transport)

An Example of Socket Programming: An Internet File Server

Client

Server

• Server code
• includes standard headers
• defines SERVER_PORT
• any number will work between 1024 and 65535, 1023 below is reserved
• local server code begins
• call to memset sets data structure to 0
• htonl and htons convert values to standard format so code runs correctly on both little endian machines and big endian machines
• server creates sockets and checks for errors
• setsockopt is needed to allow port to be reused so run indefinitely
• now ip address is bound to socket, check to see if bind succeed
• then call to listen to server's willingness to accept incoming call, and tell system ot hold up to QUEUE_SIZE
• server enters main loop which never leaves
• call to accept locks, if accept call succeeds 
• returns socket descriptor that can read and write to pipes, unidirectional
• sockets bidirectional
• after connection established, the server reads file name
• if name not available, block to wait for file name
•  when file name received open file and alternately, read blocks and write them to socket until file copied, then waits for next file
• Client Code
• invoked, only works if server is already running
• if call success file transfer to internet written to f after which client exits
• starts with right number of arguments
• socket created and initialized
• client attempts to establish TCP connection using CONNECT
• procedure fatal prints error and exits, since compile separately can't share code of fatal

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