Transport Layer Protocols

Transport Layer

• Provides logical communication between application processes on different hosts
• From an application's perspective, it appears hosts are directly connected
• In reality, hosts may be on opposite sides of the world, connected via routers with different link types
• Application processes use the transport layer's logical communication to exchange messages
• Transport Layer protocol is implemented in the end systems, like computers.
• Routers that handle network traffic do not have transport Layer protocols enabled.
• On the sending side, the transport layer converts application layer messages into transport layer packets

Segments

• Transport layer breaks application messages into smaller chunks (segments)
• Transport layer adds a header to each chunk, creating transport layer segments
• The transport layer passes the segment to the network layer
• Inside the network layer the segment is encapsulated within a network layer packet (datagram)
• The datagram is sent to the final destination
• On the receiving side, the network layer extracts the transport layer segment from the datagram
• The network layer passes the segment up to the transport layer.
• The transport layer processes the received segment
• The transport layer makes the data in the segment available to the receiving application
• There is often more than one transport layer protocol available to network applications

UDP and TCP

• Internet has two protocols: TCP and UDP.
• Each provide a certain set of transport layer services to the application using it
• Internet makes two distinct transport layer protocols available to the application layer
• UDP (User Datagram Protocol) provides an unreliable, connectionless service
• TCP (Transmission Control Protocol) provides a reliable, connection-oriented service
• Application developers must select one of these two for network applications
• IP (Internet Protocol) is the Internet's network-layer protocol
• IP provides logical communication between hosts
• The IP service model is a "best effort" delivery service
• IP tries its best to deliver segments between communicating hosts but does not guarantee it

Service Guarantees

• IP does not guarantee segment delivery
• IP does not guarantee orderly delivery of segments
• IP does not guarantee data integrity in segments
• IP can be said to provide an unreliable service
• UDP and TCP fundamentally extend IP's delivery service between two end systems
• UDP and TCP extend IP's delivery service to a delivery service between two processes running on end systems
• Extending host-to-host delivery to process-to-process delivery is called transport-layer multiplexing and demultiplexing
• UDP and TCP provide integrity checking via error-detection fields in segment headers
• Process-to-process data delivery and error checking are the only two UDP services
• Like IP, UDP has an unreliable service

TCP Additional Services

• TCP offers applications reliable data transfer
• TCP employs flow control, sequence numbers, acknowledgments, and timers.
• TCP ensures data is delivered from sending to receiving process correctly and in order
• TCP converts IP's unreliable service between end systems into reliable data transport service between processes
• TCP provides congestion control
• TCP congestion control prevents a TCP connection from swamping links/routers between communicating hosts with traffic
• TCP aims to give each connection traversing a congested link an equal share of the link bandwidth
• TCP regulates the rate at which sending sides of TCP connections can send traffic into the network
• Application UDP traffic is unregulated

Relationship Between Transport and Network Layers

• The transport layer sits above the network layer in the protocol stack
• A transport-layer protocol enables logical communication between processes running on different hosts
• A network-layer protocol provides logical communication between hosts

Multiplexing and Demultiplexing

• Transport-layer multiplexing and demultiplexing extends the network layer's host-to-host delivery service
• Transport-layer multiplexing enables a process-to-process delivery service for applications running on the hosts
• At the destination Host, the transport layer receives segments from the network layer
• The transport layer delivers the data in these segments to the appropriate application process
• Suppose your computer is downloading Web pages, running one FTP session, and two Telnet sessions

Sockets

• Sockets are the doors through which data passes from the network to the process and vice versa.
• Transport layer in receiving host doesn't deliver data directly to a process but to an intermediary socket
• There can be more than one socket in the receiving host at any given time.
• Every socket has a unique identifier
• Unique identifier format depends on whether the socket is UDP or TCP
• A receiving host directs the data to a socket because each transport-layer segment has a set of fields for this
• The transport layer checks the data to identify the receiving socket and directs the segment to that socket
• This delivery of data in a transport-layer segment to the correct socket is called demultiplexing
• Taking data chunks at the source host from different sockets, encapsulating each chunk with header information, creating segments, and passing them to the network layer is called multiplexing

Transport Layer

• Transport layer in middle host must demultiplex segments arriving from the network layer below
• Directs data to either process P1 or P2 using the corresponding process's socket
• Transport layer in middle host gathers outgoing data from these sockets
• Transport layer forms the segments, and passes these segments down to the network layer
• Transport-layer multiplexing requires sockets to have unique identifiers
• Transport-layer multiplexing requires each segment to have special fields to indicate the destination socket

Special Fields

• Source port number field
• The destination port number field.
• UDP and TCP segments have other fields as well
• Each port number is a 16-bit number, ranging from 0 to 65535
• Port numbers 0-1023 are "well-known" and restricted
• "Well-known" numbers reserved for well-known application protocols
• HTTP uses port number 80
• FTP uses port number 21

How Demultiplexing Works

• Host receives IP datagram
• Every datagram has a Source IP address and a destination IP address.
• Every datagram takes 1 transport-layer segment
• Every segment has source and destination port number
• The host uses IP addresses and port numbers to direct segments to the correct socket
• TCP sockets are identified by (Source-IP, Destination IP, Source Port, Destination Port)
• UDP sockets are identified by Destination IP and Destination Port
• When a UDP socket is created, the transport layer automatically assigns a port number
• The transport layer assigns a port number between 1024 to 65535 that is not already in use
• Typically the client side of the application automatically requests for a port number.
• The server side of the application is is assigned a specific port number

UDP Example

• A process in Host A with UDP port 19157 wants to send application data to a process with UDP port 46428 in Host B
• The transport layer in Host A creates a transport-layer segment comprising the application data Includes:
• source port number (19157)
• destination port number (46428)
• other values
• The transport layer passes the resulting segment to the network layer in the IP datagram
• The network layer encapsulates the segment and attempts to deliver the segment to its destination by any means
• The transport layer examines the destination port number and delivers segment by port 46428
• Host B could be running multiple processes, each with its own UDP socket and associated port number
• As UDP segments arrive, Host B "demultiplexes" the segment to the appropriate socket
• A UDP socket is fully identified by a two-tuple consisting of a destination IP address and a destination port number

TCP Sockets

• TCP sockets are identified by a four-tuple: (source IP address, source port number, destination IP address, destination port number)
• When TCP segments arrive, the host uses all four values to "demultiplex" and direct the segment to the socket

TCP Client-Server

• The TCP server application has has a welcoming socket that waits for connection requests from the TCP clients on port 12000
• The TCP client creates a socket and sends a connection establishment request segment
• A connection-establishment request is a TCP segment with destination port 12000 a/nd a connection-establishment bit set in the TCP header
• The segment also includes a source port number chosen by the client
• When the host operating system of the computer running the server process receives the incoming connection-request segment with port 12000
• It locates the server process that is waiting to accept a connection on port number 12000
• The server process then creates a new socket The transport layer at the server notes four values in the connection-request segment:
  • Source Port Number
  • IP address of the source host
  • Destination Port Number
  • IP address
• The new socket is then identified by the four numbers above
• All segments arriving whose source port, source IP address, destination port, destination IP address match
• Once these four values are a validated then then the client and server can send data to each other

Fundamentals of Reliable Data Transfer Protocol

• Significant in applications, transport layer, and link layers Techniques:
  • Error detection and correction: A packet is received but may be incorrect.
  • Loss detection and recovery: A packet is lost. Design Challenge:
  • Efficiency: Maximum utilization of bandwidth resources.
  • Utilization = max. app. data rate / available bandwidth

UDP Checksum

• UDP checksum is an error detection: To determine if bits within UDP segment have been altered
• Noise in the links or while stored in a router can lead to bit changes
• Before one application process can begin to send TCP data, the two process must have a "handshake"

TCP Connection

• Preliminary segments are sent to establish the parameters of the ensuing data transfer
• As part of TCP connection establishment, sides of the connection will initialize TCP state variables
• TCP runs only in the end systems and intermediate network elements do not maintain TCP connection state
• Intermediate routers do not maintain TCP connection state
• Intermediate routers are completely oblivious to TCP connections, they see datagrams.

TCP Connection Overview

• The "TCP Connection" service is full-duplex
• If there is a TCP connection between Process A on one host and Process B on another, application-layer data can simultaneously flow from Process A to B and vice versa
• A TCP connection is always point-to-point. (between a single sender and a single receiver)
• Data transfer from one sender to many receivers is impossible with TCP.
• The process initiating the connection is called the client process
• The process that is receiving the connection is called the server process
• A client application process must first inform the client transport layer wanting to establish a connection to a process on the server
• A Python client program does this by issuing the command: clientSocket.connect((serverName, serverPort))
• serverName is the name of the server and serverPort identifies the process on the server
• TCP in the client and server proceeds to establish a TCP connection by sending a special TCP segment

3-Way Handshake

• The client first sends a special TCP segment
• The server responds with a second special TCP segment
• Finally the client responds again with a third special segment
• The first two segments have no payload
• The third segment may carry a payload
• Because three segments are sent between the two hosts, connection-establishment is referred to as a "three-way handshake"
• Once the two hosts have an established TCP connection they can now send data to each other
• A client passes a stream of data to the socket; if the client hands the data off to TCP
• TCP then directs the data to the connection's send buffer set up during the three way handshake previously
• TCP will grab chunks of data from the send buffer and pass the data to the network layer
• The maximum amount of data that can be grabbed and placed in a segment is limited by the maximum segment size (MSS)
• MSS is the maximum amount of application-layer data in the segment, not the maximum size of the TCP segment
• TCP pairs data with headers making up TCP Segments
• The segments are then encapsulated within network layer IP datagrams
• The IP datagram is then sent into the network
• When the IP datagram is received, it is in the connections receive buffer
• Then the application reads the stream of data from the buffer
• Each side of the connection has it's own send and receive buffer