Institute of Technology Sligo - Dept of Computing Chapter 12 The Transport Layer.

Institute of Technology Sligo - Dept of Computing

Chapter 12

The Transport Layer

Institute of Technology Sligo - Dept of Computing• Layer 1 allows bit streams to be created and to travel;

• Layer 2 packages those data packets into frames to be converted to bit streams and makes LAN delivery possible;

• Layer 3 packages data from upper layers in packets and makes routing and WAN delivery possible.

• But we have made no provision for assuring our data travels reliably end-to-end across the often vast network path.

• Layer 4 performs multiple functions to provide this “quality of service”:

•segmenting upper-layer application data

•establishing end-to-end operations

•sending segments from one end host to another

•ensuring data reliability

•providing flow control

Institute of Technology Sligo - Dept of Computing

Flow control and reliability of the transport layer can be compared to a student who studies only one year of a foreign language, then visits the country that uses that language. Whenever the student tries to join a conversation, he or she has to ask everyone, frequently, to repeat his/her words (reliability) and to speak slowly (flow control).

Institute of Technology Sligo - Dept of ComputingTwo particularly important Layer 4 protocols are

Transmission Control Protocol (TCP) and User Datagram Protocol (UDP).

Institute of Technology Sligo - Dept of ComputingTCP supplies a virtual circuit between end-user applications.

connection-oriented reliable divides outgoing messages into segments reassembles messages at the destination station re-sends anything not received

Following are the characteristics of UDP: connectionless unreliable transmit messages provides no software checking for segment

delivery (unreliable) does not reassemble incoming messages uses no acknowledgments provides no flow control

Institute of Technology Sligo - Dept of Computing

The TCP/IP Protocol Stack

IP is a Layer 3 protocol and TCP is a Layer 4 protocol. TCP is connection-oriented and ensures reliability but IP is connection-less with best effort attempts at delivery.

Institute of Technology Sligo - Dept of ComputingTransmission Control Protocol (TCP) is a connection-oriented Layer 4 (transport layer) protocol that provides reliable full-duplex data transmission. TCP is part of the TCP/IP protocol stack.

Institute of Technology Sligo - Dept of ComputingField definitions in the TCP segment: source port - number of the calling port destination port - number of the called port sequence number - number used to ensure correct sequencing

of the arriving data acknowledgment number - next expected TCP octet HLEN - number of 32-bit words in the header reserved - set to zero code bits - control functions (such as setup and termination of a

session) window - number of octets that the sender is willing to accept checksum - calculated checksum of the header and data fields urgent pointer - indicates the end of the urgent data option-one option - maximum TCP segment size data - upper-layer protocol data

Institute of Technology Sligo - Dept of Computing

User Datagram Protocol (UDP) is the connectionless transport protocol in the TCP/IP protocol stack. UDP is a simple protocol that exchanges datagrams, without acknowledgments or guaranteed delivery, requiring that error processing and retransmission be handled by other protocols.

Institute of Technology Sligo - Dept of Computing

UDP uses no windowing or acknowledgments; therefore, application-layer protocols provide the reliability. UDP is designed for applications that do not need to put sequences of segments together. Protocols that use UDP include:

TFTP

SNMP

DHCP

DNS (Domain Name System)

Institute of Technology Sligo - Dept of Computing

Both TCP and UDP use port (or socket) numbers to pass information to the upper layers. Port numbers are used to keep track of different conversations that cross the network at the same time. Application software developers have agreed to use the well-known port numbers that are defined in RFC1700. Port numbers below 255 are reserved for TCP and UDP public applications.

Institute of Technology Sligo - Dept of Computing

Conversations that do not involve applications with well-known port numbers are assigned port numbers that have been randomly selected from within a specific range.

These port numbers are used as source and destination addresses in the TCP segment.

Institute of Technology Sligo - Dept of Computing

Originating source port numbers are dynamically assigned by the source host. Usually, it is a number larger than 1023.

Institute of Technology Sligo - Dept of Computing

A three-way handshake/open connection sequence synchronizes a connection at both ends before the transferred data reaches the ends. The exchange of introductory sequence numbers during the connection sequence ensures that any data that is lost due to problems that may occur later can be recovered.

Institute of Technology Sligo - Dept of Computing

Window size determines the amount of data that can be transmitted at one time before receiving an acknowledgment. After a host transmits the window-sized number of bytes, it must receive an acknowledgment before it can send any more messages.

Institute of Technology Sligo - Dept of Computing

Window size refers to the number of messages that can be transmitted before receiving an acknowledgment. The larger the window size number (bytes), the greater amount of data that can be transmitted. TCP uses expectational acknowledgments, meaning that the acknowledgment number refers to the octet that is expected next. The "sliding" part of sliding window refers to the fact that the window size is negotiated dynamically during the TCP session.

Institute of Technology Sligo - Dept of Computing

Since the segments are encapsulated in packets and since packets travel connectionless paths through internetworks, sequence and acknowledgment numbers become necessary for TCP to track since two successive IP packets may in many instances NOT travel the same path and arrive at the destination host out of order.