| List Of Transport Layer Services |
There is a long list of services that can be optionally provided by the
transport layer. None of them are compulsory, because not all applications want
all the services available. Some can be wasted overhead, or even
counterproductive in some cases.
- Connection-oriented
- This is normally easier to deal with than connection-less models, so
where the Network layer only provides a connection-less service, often a
connection-oriented service is built on top of that in the Transport layer.
- Same Order Delivery
- The Network layer doesn't generally guarantee that packets of data will
arrive in the same order that they were sent, but often this is a desirable
feature, so the Transport layer provides it. The simplest way of doing this
is to give each packet a number, and allow the receiver to reorder the
packets.
- Reliable Data
- Packets may be lost in routers, switches, bridges and hosts due to
network congestion, when the packet queues are filled and the network nodes
have to delete packets. Packets may be lost or corrupted in for example
Ethernet due to interference and noise, since Ethernet does not retransmit
corrupt packets. Packets may be delivered in the wrong order by an
underlying network. Some transport layer protocols, for example TCP, can fix
this. By means of an error detection code, for example a checksum, the
transport protocol may check that the data is not corrupted, and verify that
by sending an ACK message to the sender. Automatic repeat request schemes
may be used to retransmit lost or corrupted data. By introducing segment
numbering in the transport layer packet headers, the packets can be sorted
in order. Of course, error free is impossible, but it is possible to
substantially reduce the numbers of undetected errors.
- Flow Control
- The amount of memory on a computer is limited, and without flow control
a larger computer might flood a computer with so much information that it
can't hold it all before dealing with it. Nowadays, this is not a big issue,
as memory is cheap while bandwidth is comparatively expensive, but in
earlier times it was more important. Flow control allows the receiver to say
"Whoa!" before it is overwhelmed. Sometimes this is already provided by the
network, but where it is not, the Transport layer may add it on.
- Congestion avoidance
- Network congestion occurs when a queue buffer of a network node is full
and starts to drop packets. Automatic repeat request may keep the network in
a congested state. This situation can be avoided by adding congestion
avoidance to the flow control, including slow-start. This keeps the
bandwidth consumption at a low level in the beginning of the transmission,
or after packet retransmission.
- Byte orientation
- Rather than dealing with things on a packet-by-packet basis, the
Transport layer may add the ability to view communication just as a stream
of bytes. This is nicer to deal with than random packet sizes, however, it
rarely matches the communication model which will normally be a sequence of
messages of user defined sizes.
- Ports
- (Part of the transport layer in the TCP/IP model, but of the session
layer in the OSI model) Ports are essentially ways to address multiple
entities in the same location. For example, the first line of a postal
address is a kind of port, and distinguishes between different occupants of
the same house. Computer applications will each listen for information on
their own ports, which is why you can use more than one network-based
application at the same time.
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