Page 155 - DCAP207_NETWORKS_DCAP406_COMPUTER_NETWORKS

Computer Networks/Networks

Notes type is normally specified by user level applications. Examples of service type include
minimum and maximum throughput, requested by applications such as the File Transfer
Protocol (FTP) and Simple Mail Transfer Protocol (SMTP).
 Total length has everything in the datagram (max. 64 KB). If it is subtracted from the IHL
field, it indicates to IP the actual length of the data field.

 Identification enables the destination host to determine which datagram a newly arrived
fragment belongs to:
 DF means Do not Fragment.

 MF is for More Fragments.
 Fragment offset indicates the source location of the current datagram. The elementary
fragment unit size is 8 bytes.

 Time to live that counts hops is expressed in seconds. A zero count indicates that the packet
is discarded. TTL is employed by IP to prevent a lost datagram from endlessly looping
around the network. IP achieves this objective by initializing the TTL field to the maximum
number of routers that the packet can traverse on the network. Every time the datagram
traverses a router, IP decrements the TTL field by 1.
 Protocol indicates the destination which transports process to give the datagram to (TCP,
UDP, or others).

 Header checksum verifies the header only. The algorithm is to add up all the
16-bit half-words as they arrive, using one’s complement arithmetic.
 Source/Destination address tells the network number and host number.

 Options provides an escape to allow subsequent versions of the protocol to have
information not present in the original design, to allow experimenters to try out new
ideas, and to avoid allocating header bits to information that is rarely needed. On its
presence, it includes optional control information. An example of optional information
includes the route record, which includes a record of every router that the datagram
traversed during its trip around the network.

10.1.1 IP Addresses

Using Internet has become common. We will now understand how Internet interprets the Internet
address. The Internet addresses are written as www.hotmail.com, for the instance we write one
more address as server.institution.domain. The address www.hotmail.com is not actual address;
it is a text version of the Internet address, which is basically a binary representation. Now we
compare www.hotmail.com, and server.institution.domain. WWW is the name of the server
owned by the institution (in this case, it is hotmail) and this server is connected to the Internet to
a domain server namely (com in this case) which maintains a database of the addresses of
different servers using the same domain com. The domain name has no geographical relevance
and two sites with same domain name may exist at two end of this world.
The above case is the simplest case. In another instance an organization may be large enough
and have several other servers for different purposes such as web server, email server, print
server, etc. Suppose we now take an example www.sun.planet.universe.in. This address has five
parts separated by three dots. If we try to understand this address, this address will indicate that
a group Planets (planet) comes under an Universe sub domain which is a part of India domain

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