Chapter 22. Transport LayerData Communications and Networking
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, …
Transport layer concept
Transport layer compared with data link layer
The services of the transport layer are similar to those of the data link layer.
Data Communications and Networking
The services provided by transport layer protocols:
Data Communications and Networking
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-to-destination) delivery of an entire message.
Data Communications and Networking
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
Data Communications and Networking
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?
Data Communications and Networking
Sequence Control
What is important is that segments are properly reassembled at the destination.
Segmentation
Reassembly
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.
Data Communications and Networking
Loss Control
Sequence numbers allow the receiver’s transport layer protocol to identify any missing segments and request redelivery.
Data Communications and Networking
Duplication Control
Sequence numbers allow the receiver to identify and discard duplicate segments.
Data Communications and Networking
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.
Data Communications and Networking
Flow Control(cont’d)
Sliding windows used at the transport layer are usually byte oriented rather than frame oriented.
Data Communications and Networking
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).
Data Communications and Networking
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
Data Communications and Networking
22.3 The OSI Transport Protocol
To avoid redundant services, the OSI model defines five types of transport classes:
TP0: Simple class
TP2: Multiplexing class
Data Communications and Networking
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.
Data Communications and Networking
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 :
Data
Connection-Oriented and Connectionless Service
Connection-oriented model is more commonly used.