[e2e] IP options inserted in transit

Joe Touch touch at ISI.EDU
Thu Aug 7 10:11:16 PDT 2003

Craig Partridge wrote:
> Hi folks:
> I've been reading through some of the IP options text in RFCs 791 and 1122
> and I can't seem to find a definitive answer to the following question:
>     Can a router (or other intermediate device) add and remove IP options
>     from a datagram?

Hi, Craig,

RFC791 specifically discusses the insertion and deletion of some options 
- notably NOP. So in principle it's in there.

However, there are implications on the MTU that inserting an option 
might violate in ways that might create black-holes. I.e., any router 
that inserts an option, IMO, "MUST" follow P-MTU-preserving guidelines 
from, e.g., RFC2003.

I would expect that such an option is probably not very useful unless 
coupled with source routing, since the router using (& consuming) that 
option would have to be pinned. This isn't required for existing 
options, since all meaningful ones (except 'end' and 'nop') do not have 
explicit 'may add or delete' clauses.

That said, it might be more useful/consistent to consider implementing 
such options as end-to-end on a new outer IP header, i.e., as a property 
of a 2003-style tunnel, rather than as options inserted on-the-fly. 
Especially since the option really is end-to-end - where 'end's are the 
tunnel endpoints ;-)


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