Subject: Importance of Identifier field in IP header in sending OSPF packet.

From: kamatchi soundaram <[email-masked]>
Date: Thu, 27 Jun 2002 16:59:28 -0700

Hi all,

   I need some clarification for the requirement of assigning the Identifier
feild of IP header in the case of OSPF packet sending.

Basically IP uses an Identifier field in its IP Header, which will be
uniquely assigned for the all IP packets generated by a particular source to
a particular destination (RFC 791 says this). May be the assigning will be
based on the source and destination address pair or a meger unique number
starting from 1 to 2 power 16. This identifier is significant in the case of
fragmentation and reassembly.

Now the following four are my questions.
1) In the case of OSPF all packets except the packets to virtual links will
be transmitted maximum one hop. And since OSPF itself takes care about the
outgoing interface MTU size when the packet is constructed, then there won't
be any fragmentation at IP level. So, in that case, will the IP identifier
field is still updated by IP with an unique value.

2) And what is general implementation? whether ip identifier is updated or
not given any preference in the case of OSPF packet?

3) In the case of OSPF packets to virtual links, it goes to many hops from
source to destination as a unicast packet. So, in that case, the identifier
field seems to be needed, for ressembly and fragmentation. So, what will be
the case in this scenario?

4) Can i send identifier field as zero always  irrespective of whether the
packet is going through virtual links or other links??

It would be highly appreciable if i get some valuable explanations for the
above questions.

thanks,
--sundar--

---

From: Anton Smirnov <[email-masked]>
Date: Fri, 28 Jun 2002 10:05:24 +0200

Sundar,

> field is still updated by IP with an unique value.
generally speaking, this is incorrect. Better say that good implementation
might try to follow this approach but even then it is not always possible.
One example is if router LSA (of any router in the area) is bigger than MTU
on the link. It is not something very common but happens in some designs. In
this case Update packet by definition cannot fit into MTU without
fragmentation.

> not given any preference in the case of OSPF packet?
general implementation is to separate IP I/O subsystem and OSPF subsystem as
one of its users. IP Identifier field is not something OSPF should worry
about at all. It should send whatever datagrams it wants (though it should
consult outgoing interface MTU and make informed decision) and receive
already reassembled datagrams.

> packet is going through virtual links or other links??
I would say no.