Subject: [lisp] On the two usages of Map-Requests

From: Olivier Bonaventure <[email-masked]>
Date: Wed, 27 May 2009 14:57:23 +0200

Hello,

In the LISP architecture, the mapping system is used for two related but
different purposes :

Discovery usage : This is the first usage of the Map-Request messages.
When an ITR needs to send a packet towards an EID for which it does not
knw any mapping, it will send a mapping request through the mapping
system and will receive a Map-Reply message that provides the mapping
that it can insert in its mapping cache.

Update usage : This is the second usage of Map-Request messages. Even if
the ITR already knows a mapping for a given EID, it may need to send
Map-Request messages to one or several of the ETRs that are responsible
for this EID. Typical examples include updating the mapping upon
expiration of the mapping lifetime, verification of the reachability of
a remote ETR, reaction to reception of a packet containing the SMR bit
set, ...

I think that in the discussion about the mapping systems that it would
be useful to distinguish between these two roles of the LISP Map-Request
messages because there two usages can be potentially different. For
example, Map-Request used for update will likely be more frequent than
Map-Requests used for Discovery, Map-Requests used for update may
include additional information compared to the Map-Request that are used
 for discovery, Map-Requests used for update could have a shorter
retransmission timer than Map-Request used for discovery, ... Another
example is the rate limit imposed on Map-Request. The current draft
recommends : "Map-Requests MUST be rate-limited.  It is recommended that
a Map-Request for the same EID-prefix be sent no more than once per
second." This is an important requirement for Map-Requests that are used
for discovery. However, Map-Requests that are used for update could have
 a different rate-limit. For example, when sending Map-Requests to
update the mapping of an EID prefix reachable via two RLOCs, then it
could be acceptable to send a Map-Request to both RLOCs at the same time.

Olivier

-- 
http://inl.info.ucl.ac.be , UCLouvain, Belgium

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From: Dino Farinacci <[email-masked]>
Date: Wed, 27 May 2009 23:55:13 -0700

> that it can insert in its mapping cache.

Right.

> a remote ETR, reaction to reception of a packet containing the SMR bit

Yes, but if an ITR wants to send a Map-Request as a keepalive or to  
find more recent mapping, it can do so by sending to the locator  
address of one of the ETRs. So this doesn't directly use the mapping  
system.

Likewise, SMRs are from ETR to ITR so they too don't use the mapping  
system directly.

> Map-Requests used for Discovery, Map-Requests used for update may

For the same mapping entry? I don't think that is true, but you may  
have another more application for it.

> could be acceptable to send a Map-Request to both RLOCs at the same  

I don't know what you mean exactly by using a Map-Request for an  
update. Do you mean in response to SMRs? If so, the rate-limiting is  
happening on the SMR source.

Dino

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From: Olivier Bonaventure <[email-masked]>
Date: Fri, 29 May 2009 21:33:24 +0200

Dino,

> Yes, but if an ITR wants to send a Map-Request as a keepalive or to find
> more recent mapping, it can do so by sending to the locator address of
> one of the ETRs. So this doesn't directly use the mapping system.

Yes, discovery Map_requests will be send via the mapping system while
update Map-Requests can be sent directly to the ETRs

> Likewise, SMRs are from ETR to ITR so they too don't use the mapping

Agreed.

> For the same mapping entry? I don't think that is true, but you may have
> another more application for it.

Assume a Mapping entry with a TTL of 5 minutes. When the ITR uses the
prefix for the first time, it will send the request to the mapping
system. However, when it needs to re-confirm the mapping at the
expiration of its TTL, it can send the Map-Request directly to the ETR
without using the mapping system.

> I don't know what you mean exactly by using a Map-Request for an update.

I mean that the ITR sending the Map-Request already knows the mapping
and wants to verify that the mapping did not change or reconfirm it
after a TTL expiration.

Assume a site with two ETRs. If an ITR needs to revalidate the mapping
of this site, it could send a Map-Request to both ETRs to obtain a
faster response.

Olivier

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From: Dino Farinacci <[email-masked]>
Date: Fri, 29 May 2009 16:07:44 -0700

>> find

Agree.

> without using the mapping system.

Yes, but you assume all the security is sending the Map-Request on the  
ALT. I was stating that the nonce  usage adds even more value.

>> update.

Okay.

> of this site, it could send a Map-Request to both ETRs to obtain a

Not worth the overhead. What if there are 8 ETRs and how do you know  
which ones are the better ones to send to.

Dino