TCP/IP Model: The Host-to-Host Layer Protocol

TCP/IP Model: The Host-to-Host Layer Protocol

Same as OSI model’s 4^th layer, Host-to-Host Layer protocols shields the upper layer application from the complexity of network.

Two protocols used here are:

          ·         TCP: Trasmission Control Protocol

          ·         UDP: User Datagram Protocol

Transmission Control Protocol (TCP):

Same as transport layer of OSI, TCP takes data stream from the upper layers and breaks them into segments. It numbers and sequences each segments so that the Destination’s TCP can put the segments back into the order the application intended.

Before transmitting, it contacts the receiving TCP stack, creating virtual circuit, and also agrees on the amout of information or data that has to be transferred. Data transfer is reliable with TCP, as it uses acknowledgement and windowing. TCP is connection-oriented.

TCP is a full-duplex, connection-oriented, reliable and accurate protocol and error checking.

User Datagram Protocol (UDP):

UDP is a scaled down economy model of TCP, and it is also known as thin protocol. It does not take much bandwith on the network. It is not reliable but it is faster than TCP.

Some situations where UDP can be used are in SNMP, NFS.

Note: it is a fatal error if UDP is used for VoIP (Voice Over Internet Protocol)

UDP is connectionless, it does not create a virtual circuit before transferring.

TCP v/s UDP

TCP	UDP
Sequenced	Unsequenced
Reliable	Unreliable
Connection-Oriented	Connectionless
Virtual Circuit is created before data transfer	Virtual circuit not created
Acknowledgments	No acknowledgments
Windowing, Flow Control	No Windowing or Flow Control

Port Numbers:

TCP and UDP uses port numbers to communicate with upper layers because they are what keep tracks of different conversations crossing the network simultaneously .

Originating Source port numbers are assigned dynamically by the source host and will be some numbers starting at 1024. 1023 and below are well known port numbers discussed in RFC 3232 (www.iana.org)

Some common protocols with their port numbers:

TCP	Port number
Telnet	23
SMTP	25
HTTP	80
HTTPS	443
FTP	21
DNS	53

UDP	Port Number
SNMP	161
TFTP	69
DNS	53
DHCP/BootP	67 and 68

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