Tcp header/IP Header/Authentication header

TCP Header/
IP Header/
Authentication
Header
NETWORK SECURITY
By Faizan Shaikh
T.Y.B.Sc.IT
Roll no.37

What is TCP/IP?
 TCP/IP is the basic communication language or protocol(set of
rules) of the Internet.
 It can also be used as a communications protocol in a private
network(either an intranet or an extranet).
 Basically, it can be said as the TCP works in order to divide file into
packets and send it to the workstation. The IP handels the shortest
route. So it can be said as both work together.

Source Port Number: This 2 bytes number
signifies the port number of the source
computer.
Destination Port Number: This 2 bytes number
signifies the port number of the destination.
Sequence Number: This field defines the
number assigned to the first byte of data
portion contained in each TCP segment. For
the data to be delivered correctly, each byte to
be transferred is numbered in an increasing
sequence.
Acknowledgement Number:
If the destination host receives the
sequence number correctly, it sends X+1 as the acknowledgement number back to the source.
Header Length: We know that the header length can be between 20-to-60 bytes. Hence the value of the field
can be between 5(5*4=20) and 15(15*4=60).
Reserved: This 6 byte field is reserved for future use and is currently unused.
TCP Header

Flag: This 6-byte field defines 6 different control flags,each one of them occupying one bit. Out of these 6 flags
two are most important. The SYN flag indicates that the source wants to establish a connection with the
destination. The FIN flag indicates that the sender wants to terminate the TCP connection.
Window Size: This field determines the size of the window the other party must maintain.
Checksum: This 16-bit field contains the checksum for facilitating the error detection and correction.
Urgent Pointer: This fields is used in situations where data in TCP segment is more important or urgent
than other data in the same TCP connection.

IP Header
The TCP header plus the
original message is now
passed to the IP layer. The IP
header treats this whole
package and adds its own
header to it. The format of an
IP datagram is shown.
Version: This field contains a
value 4, which indicates IPv4.
In future, this field would
contain 6 when IPv6 becomes
the standard.
Header Length(HLEN)
indicates the size of the
header in a multiple four-byte
words.
Service Type: This field defines the service parameters such as priority of the datagram and the level of
reliability desired..
Total Length: This field contains the total length of the IP datagram. Because it is two bytes long, an IP
datagram cannot be more than 65,536 bytes.

Identification: This field is used
in situations when a datagram
is fragmented. As a datagram
passes through different
networks, it might be
fragmented into smaller sub-
datagrams. In such a situation,
the sub-datagram are
sequenced using the
identification field, so that the
original datagram can be
reconstructed from them.
Flags: This field corresponds to Identification. It indicates whether a datagram can be fragmented and if it
can be fragmented, whether it is the first or the last or the middle fragment.
Fragmentation Offset: If a datagram is fragmented, this field is useful. It is useful when reconstructing a
datagram from its fragments.

Time to Live: We know that
datagram travels through one
or more routers before
reaching its destination. In
case of Network problems,
some of the routes to the final
destination may not be
available due to hardware
failures, link failure or
congestion. In that condition
the datagram may be sent
through a different route. This
can continue for a long time if
the problems aren’t
resolved quickly. To avoid this, the datagram sender initializes this field to some number. As the datagram
travels through routers this field is decremented each time. If the value becomes zero or negative, it is
immediately discarded. No attempt is made to forward it to the next hop. This avoids the datagram travel an
infinite amount of time and hence preventing network congestion. After all the datagrams are reached to
their destination, the TCP finds out the missing datagram and will request for its retransmission. Thus, IP is not
responsible for the error-free, timely and insequence delivery of the entire message--- it is done by TCP.

Protocol: This field identifies
the transport protocol
running on top of IP. After the
datagram is constructed from
its fragments, it has to be
passed on to the upper layer
software piece.
Source Address: This field
contains the 32-bit IP address
of the sender.
Destination Address: This field
contains the 32-bit IP address
of the final destination.
Options: This field contains optional information such as routing details, timing, management and alignment.
For instance, it can store the information about the exact route that the datagram has taken. However, most
of the time, the space in this field is not sufficient for all these details, therefore, it is not used very often.

Tcp header/IP Header/Authentication header

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