Subject: [lisp] Discussion about FIRMS

From: Michael Menth <[email-masked]>
Date: Tue, 04 Aug 2009 11:46:23 +0200

Dear colleagues,

thanks a lot for your interest in FIRMS at the RRG meeting last Friday. 
We still had interesting discussions after the meeting and I'd like to 
share with you the outcome.

http://www.ietf.org/proceedings/75/slides/RRG-1.pdf
http://www3.informatik.uni-wuerzburg.de/~menth/Publications/papers/Menth09-FIRMS.pdf

1) FIRMS: In case of cache misses at the ITR, packets can be forwarded 
as "data-probes" to a map-base which forwards them to the ETR. The 
map-bases can be located close to the ETR to minimize the extra delay 
due to path stretch.

Joel's comment: it is not possible to place the map-base close to the ETRs.

Michael: Yes, there are cases where this is difficult. For instance, 
when EIDs of a common prefix have different ETRs. Then, the map-base can 
be optimally placed only for a subset of them, or for all of them in a 
suboptimal way to minimize the path stretch compared to a direct route 
from ITR to ETR. In case that map-bases cannot be located near an ETR, 
the data-probes travel at least over a direct path from the ITR to the 
map-base which is not the case in many other proposals for mapping systems.

This general problem does not exist with APT's default mapper (DM) 
because it has the mapping information in the source network. The DM is 
close to the source and can tunnel the packets directly to the ETRs so 
that only minimal path stretch occurs. However, all mappings are need to 
be available in the DM which leads to a large amount of data in the DMs. 
Moreover, updates are difficult because
they need to be propagated to all DMs which takes relatively long time. 
In FIRMS, a map-base holds only a limited number of mappings and the 
mapping data need to be changed only in map-bases responsible for the 
respective EID-prefix.

Hence, optimal forwarder placement, scalability, and fast updates seem 
to be conflicting goals. FIRMS gives more importance to the latter two.

2) Joel's concern was that ITRs may become lazy and forward most traffic 
over the mapping infrastructure so that delay due to path stretch 
matters. We think differently since the ITR's interest is
to provide optimal service to the traffic which implies getting the 
mapping for a direct tunnel. In particular we think that initially 
increased packet delay is better than dropped packets. So far, these are 
opinions and the truth is probably application-dependent.

3) FIRMS does not use public infrastructure to carry map-requests.

That statement during my presentation led to misunderstandings. FIRMS 
uses a hierarchical map-base pointer (MBP) distribution network 
consisting of MBP exchange nodes (MBPXs) to provide a copy of the MBP 
table to all map-resolvers. This MBP table tells the map-resolvers where 
the distributed map-bases are. Only little traffic is carried over the 
MBPXs and in particular no map-requests so that limited capacity is 
needed for public infrastructure. This is in contrast to other mapping 
systems in literature that we have reviewed in our paper. ALT is 
designed such that public network elements are completely avoided. 
However, this also raises the question of control (see next email).

Kind regards,

    Michael

-- 
Dr. Michael Menth, Assistant Professor
University of Wuerzburg, Institute of Computer Science
Am Hubland, D-97074 Wuerzburg, Germany, room B206
phone: (+49)-931/31-86644 (new), fax: (+49)-931/888-6632
mailto:[email-masked]
http://www3.informatik.uni-wuerzburg.de/research/ngn