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Introduction
5
Protocol
The protocol section is used to indicate the protocol being used by the transport layer. This could be TCP, UDP, or
something else.
Time to live
The time to live section is used to prevent a frame from traversing the network forever. This field contains a number
(maximum 255) that is set when the frame is originally generated. Each time the frame is passed through the
bridge/router, the bridge/router will decrement the time to live by two. When the time to live reaches zero, the
frame is discarded.
Header Checksum
The header checksum is used to verify the data in the IP header. The IP header is recalculated each time a frame is
passed through a router. The recalculation is necessary because the time to live field is changed.
Fragmentation
Fragmentation occurs when an IP frame must be split up into smaller IP frames. When the originating device
generates the IP frame, the device is not aware of all the paths the frame must traverse to get to the destination
device. If the IP frame is to pass through a network that has small packet capabilities, the IP frame must be split up
and reassembled at the destination device. Each of the fragments is assigned a fragment offset value, which
determines where the fragment fits into the original IP frame.
The P2600 router will accept fragmented frames directed to itself and reassemble them, but it will not fragment
frames.
Options
There are various options that may be set for any IP frame.
Source Routing
Source routing is used to predetermine the path that the IP frame must travel through the network. There are two
types of source routing: strict source routing and loose source routing.
Strict source routing will contain a list of IP addresses of routers that must be used when the IP frame is sent through
the network. Strict source routing is used mainly to provide some type of data security. Once the IP frame has
reached the destination station, the destination station will take the list of IP addresses from the options field, reverse
them, and use them for a strict route back to the originating station.
Loose source routing will also contain a list of IP address of routers to be used on the path to the destination station.
However, the IP frame may pass through other intermediate routers to get to the next IP address in the loose source
routing list.
Route Recording
Route recording simply keeps a list of all the IP addresses of the routers that the IP frame has passed through on its
way to the destination station.
Time Stamps
The time stamp option is used to record the time at which the IP frame passed through each router on its way to the
destination station.