Transport Layer

Transport Layer

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The transport layer of both the OSI and TCP/IP models is very important. At this layer the data being prepared to be sent over the internet is broken into pieces called segments.Overview 

The PDU or protocol data unit at this layer is called a segment. Their are two main protocols that function at this layer TCP and UDP. TCP or transmission control protocol is a very reliable and connection oriented protocol. TCP is characterized as being reliable because of the fact that it will only send data once a three way handshake has first been established, it uses sequence numbers to track all segments and it also uses system of syns and acks (acknowledgments), and it will not send new data until an acknowledgement has been received for data already sent. If the acknowledgement is not received it will resend data. UDP or user datagram protocol on the other hand is not reliable, it is a best effort delivery system, a connectionless protocol, that does not require an established connection with another computer before sending data. UDP's advantage is the fact that its header fields or control information is a lot smaller than TCP's so there is a lot less to process and as a result it is a faster  but less reliable protocol.

TCP	UDP
segments- sequence numbers,acknowledgements, many header fields, lots of overhead	datagrams- no sequence numbering, few header fields,little overhead = fast
reliability -due to sequence numbering, and resending of data if no acknowledgement is received	unreliable - sends all data regardless of whether or not it was received
connection oriented - Three way handshake receiving computer prior to sending data	connectionless -  no handshake to establish connection
source and destination port numbers in the header	source and destination port numbers in the header
flow control - dynamically change the windows size to not overwhelm the receiver with data	no flow control

Here is a short list of some of the most useful port numbers. You should memorize these ports.

Port Number	Protocol
80	HTTP
23	Telnet
20,21	FTP
22	SSH
25	SMTP
53	DNS
110	POP

Well Known Ports	0 - 1023
Registered Ports	1024 - 49151
Dynamic Ports	49152 - 65535