Subject: [RAM] couple of questions about LISP

From: marcelo bagnulo braun <[email-masked]>
Date: Wed, 28 Feb 2007 23:56:59 +0100

Hi Dino,

I have a  couple of questions about LISP

First one is about how would LISP work in the following situation.

I assume we are using LISP1

Consider that we have a host A in a site a that has EID EIDA (which is 
globally routable in order to be able to bootstrap the initial 
EID-to-RLOC mapping) and RLOCA which is a different address. Suppose 
that site

We have another host B located in a different site b that has a another 
locator RLOCB. Suppose that both site a and site b has multiple ISPs 
(at least two and they have several TR, for fault tolerance reasons)

Suppose now that host B initiates a communication with Host A

Host B sends a packet to host A
The packet contains EIDA as dest add and EIDB as src addr
The packet is processed by an ITRB1 close to site b
Since we are in LISP1, the packet is forwarded as it is

the packet reaches the ETRA1 close to site a and the ETR sends a ICMP 
packet back to the ITRB1 with the RLOCA information and the mapping 
between EIDA and RLOC is stored in ITRB1.

Now bad things start :-)

First there is a failure and EIDA is no longer reachable
No problem with that, since ITRB1 has the information about alternative 
locators for host A, it will start sending tunneled packets using RLOCA 
as dest address in the outer header and the communication is preserved 
through this outage.

However, now a second bad thing happens, which is that ITRB1 fails

I assume that it is possible in LISP, that an alternative ITR ITRB2 is 
used for site B, so packets are rerouted to that backup ITRB1 for site 
b.

But the problem is that packets that host b is sending contain EIDA as 
destiantion address which is unfortunatelly unreachable at this time. 
this was not a problem before because ITRB1 had the alternative locator 
information, but it has failed. The backup router ITRB1 does not 
contain the alternative locator information for EIDa (i.e. RLOCA), so 
how can the communication survive this outage?

I mean, i guess that fault tolerance is the key feature and the LISP 
boxes cannot become a single point of failure, so i guess it should be 
possible to survive this type of situations

Second one, in LISP 1,5 and in LISP 2 how do you think communication 
between a LISP host that is using non globally routable EIDs and legacy 
hosts non served by any LISP capable router would work?

Thanks, marcelo

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