Subject: Re: [lisp] Comments on draft-fuller-lisp-alt-04

From: [email-masked] (Noel Chiappa)
Date: Sat, 7 Feb 2009 09:34:07 -0500 (EST)

> From: David Meyer <[email-masked]>

    >> 2. The types of messages that are transmitted on the ALT-topology are
    >> unclear. From my reading of the text, it seems that the ALT-topology
    >> .. is only used to forward two types of messages : - Data Probes - Map
    >> Requests
    >> However, at several places in the paper .. you also include Map
    >> Replies in the types of messages that travel through the ALT topology.
    >> At other places you indicate more clearly that Map replies are sent on
    >> the normal topology.

    > .. thanks for spotting that.

So, I'm curious - which is it? Do replies go via the ALT from ETR to ITR, or
do they go direct from ETR to ITR?

	Noel

---

From: "Joel M. Halpern" <[email-masked]>
Date: Sat, 07 Feb 2009 09:54:04 -0500

The document claims that replies can go both ways.

In fact, it would require additional unspecified mechanisms to make 
returning the replies over the ALT work.  And in my opinion it is a bad 
idea anyway.

Joel

Noel Chiappa wrote:
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---

From: Dino Farinacci <[email-masked]>
Date: Sat, 7 Feb 2009 23:40:05 -0800

> do they go direct from ETR to ITR?

Direct from ETR to ITR by default.

There is an option where the ITR originated Map-Request that is  
received by the first-hop ALT router could modify the "ITR RLOC" field  
in the payload to point to it's own RLOC. So the Map-Reply that is  
sent by the ETR can return to the first-hop ALT router so it could  
cache it. And then subsequent Map-Requests could be replied to by the  
first-hop ALT router flagged as non-authoritative.

We haven't implemented this because it makes it harder to update  
mappings when ALT routers are caching.

Another aspect is that the mapping data in a Map-Request *could* be  
cached by ALT routers as they are forwarding them towards ETRs so  
return traffic can have a 1 tunnel hop for reduced latency.

These are ideas we could experiment with incrementally if we need to.

Dino

---

From: Vince Fuller <[email-masked]>
Date: Mon, 9 Feb 2009 12:31:56 -0800

>> So, I'm curious - which is it? Do replies go via the ALT from ETR to  
>> ITR, or> do they go direct from ETR to ITR?
>
> There is an option where the ITR originated Map-Request that is received 
> by the first-hop ALT router could modify the "ITR RLOC" field in the 
> payload to point to it's own RLOC. So the Map-Reply that is sent by the 
> ETR can return to the first-hop ALT router so it could cache it. And then 
> subsequent Map-Requests could be replied to by the first-hop ALT router 
> flagged as non-authoritative.
>
> cached by ALT routers as they are forwarding them towards ETRs so return 
> traffic can have a 1 tunnel hop for reduced latency.
>

To reinforce what Dino wrote: under normal circumstances, a Map-Request sent
by an ITR in to the ALT results in a Map-Reply sent directly from the
andwering ETR back to the ITR. The Map-Reply is not sent over the ALT.
This is what is implemented today and is the approach we expect to take.

I thought I fixed all references to Map-Replies being sent over the ALT when
I updated from -03 to -04 but it would seem that I missed at least one. This
is my mistake and I'll fix that (and incorporate other comments) for -05.

As Dino mentioned, we have been considering whether it would be useful to
sent Map-Replies over the ALT and have a way to experiment with doing so;
I will make a note of when updating to -05.

	--Vince