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IP DATAGRAM
An IP datagram is a basic unit of
information used by the IP layer to exchange data between two
hosts. A IP datagram consists of an IP header and data. All the
datagram headers in the TCP/IP protocol suite are transmitted in
the "big endian" byte order. i.e. The most significant byte is
transmitted first. This is also called as "network byte order".
The size of the IP header is not
fixed. Depending on the IP options present, the size of the IP
header will vary. A separate field for the IP header length is
added, so that the destination system can separate the IP
datagram header from the payload.
The IP datagram identifier is just
a sequence number assigned by the transmitting host. The
algorithm for assigning value to this field is not specified by
the IP protocol. The IP datagram identifier field is used to
uniquely identify and assemble the different fragments of an IP
datagram. The IP datagram identifier field is different for each
IP datagram transmitted. The fragments of an IP datagram will
have the same identifier value.
The Type Of Service (TOS) field is
used TCP to describe the desired quality of service for an IP
datagram by upper layer protocols like TCP. This field can be
used to specify the nature and priority of a IP datagram (like
Network Control, Immediate, Critical, etc) and the criteria for
selecting a path for forwarding a datagram by a gateway.
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Time to Live (TTL)
The TTL field is used to limit the
lifetime of a IP datagram and to prevent indefinite looping of
IP datagrams. The TTL field contains a counter value set by the
source host. Each gateway that processes this datagram,
decreases the TTL value by one. When the TTL value reaches zero,
the datagram is discarded. The typical value for a TTL field is
32 or 64.
If a datagram cannot be delivered
to the destination host due to some reason, it is considered an
undeliverable datagram.
A datagram may become
undeliverable, if
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The destination host is down.
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The route for the destination
host is not found.
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A network in the route to the
destination host is down.
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The Time To Live (TTL) value of
the datagram becomes zero.
An undeliverable datagram is
discarded and an ICMP error message is sent to the source host.
A host may receive the same copy of an IP datagram twice. It is
upto the higher layer protocols to discard the duplicate copy of
the datagram.
The checksum field in the IP header covers only the IP header.
The payload data is not used for calculating this checksum
Author: Asanka
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