Information Technology Reference
In-Depth Information
the next address. It is loose as the gateways are allowed to use any route to get to the next
specified address in the routing table.
It must be copied on fragmentation and occurs, at the most, once in a datagram.
Strict source and record route (Type = 137)
The SSRR is similar to the LSRR, but the routing must follow, exactly, the addresses in the
routing table. It thus cannot use any intermediate routes to get to these addresses. Its format
is
+--------+--------+--------+---------...--------+
|10001001| Length | Pointer| Route data |
+--------+--------+--------+---------...--------+
Record route (type = 7)
The record route option records the route of an internet datagram. It can thus be used by such
utilities as Traceroute. Its format is
+--------+--------+--------+---------...--------+
|00000111| Length | Pointer| Route data |
+--------+--------+--------+---------...--------+
where
•
Length - this is a single byte which contains the number of bytes in the option field.
•
Pointer - this is a pointer, which is relative to this option, into the route data which indi-
cates the byte at which the next address should be added to. The smallest value is 4.
•
Route data - contains a list of the route which a datagram has taken. Each entry has 4
bytes. The originating host must reserve enough area for the total number of addresses in
the routing table, as the size of this option does not change as it transverses over the
Internet. If there is a problem adding the address then an ICMP Parameter Problem can
be sent back to the source host.
It is not copied on fragmentation, and goes in the first fragment only. In addition, it occurs, at
the most, once in a datagram.
Internet timestamp (type = 68)
The Internet timestamp option records a timestamp for each gateway along the route of a
datagram. It allows the source host to trace the time that each part of the route takes. Its for-
mat is
+--------+--------+--------+--------+
|01000100| Length | Pointer|Ov |Flg|
+--------+--------+--------+--------+
| internet address |
+--------+--------+--------+--------+
| timestamp |
+--------+--------+--------+--------+
| . |
.
where
