Chapter 22 Transport Layer

Chapter 22 Transport Layer 22.1 Duties

22.2 Connection 22.3 The OSI Transport Protocol

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Transport Layer(cont’d) 

Transport layer acts as a liaison between the upper-layer protocols and the lower-layer protocols.

 To make this separation possible, the transport layer is  

independent of the physical network. Examples: TCP, UDP, …

An internetwork

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22.1 Duties of the Transport Layer 

Transport layer concept

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Duties of the Transport Layer(cont’d) 

Transport layer compared with data link layer

 The services of the transport layer are similar to those of the data link layer.

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Duties of the Transport Layer(cont’d) 

The services provided by transport layer protocols:

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End-to-End Delivery 

The network layer treats each packet as an independent entry, even those belonging to a single message.



The transport layer oversees the end-to-end (source-todestination) delivery of an entire message.

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Addressing 

Levels of Service Access Points

 Data link level protocols need to know which two computers  

within a network are communicating. • MAC address Network level protocols need to know which two computers within an internet are communicating. • IP address Transport level protocols need to know which upper-layer protocols are communicating. • Port

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Addressing (cont’d) 

In transport layer, service access points identifies upper-layer services (applications).

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Reliable Delivery 

Aspects of reliable delivery

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Reliable Delivery(cont’d) 

Error Control

 Mechanisms for error control are based on error detection and  

retransmission. Error detections are performed using algorithms implemented in software, such as checksum. We already have error handling at the data link layer, why do we need it at the transport layer?

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Reliable Delivery(cont’d) 

Sequence Control Segmentation

Reassembly

 What is important is that segments are properly reassembled at the destination.

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Reliable Delivery(cont’d) 

Segmentation and Reassembly

 Transport layer adds a sequence number at each segment.  This number indicates the order for reassembly.  Each segment carries a field that indicates whether it is the final segment or middle segment of a transmission. 

Concatenation and Separation

 When the size of the data unit belonging to a single session is



so small that several units can fit together into a single datagram. A sequence number at each unit allows correct separation at the destination.

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Reliable Delivery(cont’d) 

Loss Control

 Sequence numbers allow the receiver’s transport layer protocol to identify any missing segments and request redelivery.

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Reliable Delivery(cont’d) 

Duplication Control

 Sequence numbers allow the receiver to identify and discard duplicate segments.

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Flow Control 

Flow control at this layer is performed end-to-end rather than across a single link.



A sliding window is used to make data transmission more efficient as well as to control the flow of data so that the receiver does not become overwhelmed.



Some points about sliding windows at the transport layer:

 The sender does not have to send a full window’s worth of data.  An acknowledgment can expand the size of the window based  

on the sequence number of the acknowledged data segment. The size of the window can be increased or decreased by the receiver. The receiver can send an acknowledgment at anytime.

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Flow Control(cont’d) 

Sliding window

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Flow Control(cont’d) 

Sliding windows used at the transport layer are usually byte oriented rather than frame oriented.

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Multiplexing 

Upward Multiplexing  The transport layer can send several transmissions bound for the same destination along the same path.  It is useful when the underlying networks have high throughput.



Downward Multiplexing  It is useful when the underlying networks have low or slow capacity(e.g., X.25’s three bit sequence code).

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22.2 Connection 

End-to-end delivery can be accomplished in either of two modes:  Connection-oriented transmission has three stages: • connection establishment, • data transfer, • connection termination.

 Connectionless transmission

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Connection(cont’d) 

Connection establishment(three-way handshaking)

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Connection(cont’d) 

Connection termination(three-way handshaking)

 Termination at TCP: four-way handshaking Data Communications and Networking

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22.3 The OSI Transport Protocol 

To avoid redundant services, the OSI model defines five types of transport classes:

    

TP0: Simple class TP1: Basic error recovery class TP2: Multiplexing class TP3: Error recovery and multiplexing class TP4: Error detection and recovery class

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The OSI Transport Protocol(cont’d) 

Which class is used depends on the type of service required by the upper layers.

 TP0 and TP2 are used with perfect network layers.

• In the perfect network layer, the number of packets that are lost or damaged is almost zero.

 TP1 and TP3 are used with residual-error network layers.

• In the residual-error network layer, some percentage of errors are never corrected.

 TP4 is used with unreliable network layers.

• TP4 provides fully reliable, full-duplex, connection-oriented services similar to TCP.

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The OSI Transport Protocol(cont’d) 

TPDU(Transport Protocol Data Unit)

 Length : Total number of bytes (excluding the length field itself)



in the TPDU. Fixed Parameters :

• Code : CR, CC, DR, DC, DT(data), ED, AK, EA, RJ, ER • Source and destination reference • Sequence number • Credit allocation : It enables a receiver to tell the sender how many more data units may be sent.

 Variable(Optional) Parameters :

• These parameters are used mostly for management.

 Data

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The OSI Transport Protocol(cont’d) 

Connection-Oriented and Connectionless Service

 The OSI model supports both COTS and CLTS.  Connection-oriented model is more commonly used.

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