Subject: Re: [rrg] rrg Digest, Vol 16, Issue 27

From: wei zhang <[email-masked]>
Date: Tue, 19 Jan 2010 21:21:58 +0800

Hi, Lixia
Thank you for your comments.
Please check my justification inline.

> Date: Mon, 18 Jan 2010 23:27:45 -0800
> From: Lixia Zhang <[email-masked]>
> Subject: [rrg] a quick critique on 2-phased mapping
> To: [email-masked]
> Message-ID: <[email-masked]>
> Content-Type: text/plain; charset=US-ASCII; format=flowed; delsp=yes
>
> This is a simple idea on how to scale mapping. However personally I
> feel the design is too incomplete to be considered a serious input to
> RRG. Take the following 2 issues as example:
>
The design is simple indeed but not incomplete. It focuses on mapping
mechanism and can work with many Id/Loc separation proposals(e.g.
LISP) nicely. If you need map and encap/rewrite as a whole package I
can supplement the rest part, but I think the supplementary can hardly
attribute new ideas other than LISP/IVIP etc. I am afraid of
distracting readers' attention from too many complicated details.
I believe the scalability of mapping in the ID/Loc separation scheme
is extremely important and relatively independent of
tunneling/rewriting techniques. The routing scalability problem will
be shifted to mapping system if the scalable routing presumably
acquires mapping beforehand.

> First, in this 2-phase scheme, an AS is essentially the unit of
> destinations (i.e. sending ITRs find out destination AS D, then send
> data to one of of D's ETR).  This does not offer much choice for
> traffic engineering.
>
This mapping scheme will not influence intra-domain TE within an AS.
If an AS expects some inbound traffics to certain prefixes come in
from desired ETRs, it is in the case of interdomain TE.
2-phased mapping guarantees optimized route in the core(BGP routing
with RLoc). If the destination AS D needs to enforce its interdomain
TE objective in the core, there will be a contradictory between the
core and edge. Therefore any gain of the "edge" means undesired cost
of the "core". Any way if the AS concerns more about the interdomain
TE, the ETRs can be configured to redirect special traffic to a
specific inlet of the AS. But this configuration is not an essential
part of the mapping system.
 Generally speaking it is reasonable to keep the granularity of
mapping on AS level, which will  reduce the mapping dynamics
efficiently.

> Second, there is no consideration whatsoever on failure detection and
> handling.
>
> Lixia
Since the 2-phased mapping scheme takes the advantages of DNS and BGP
at the phase I and II respectively. Therefore there is no special
failure detection/handling design besides DNS
and BGP inherent failure recovery mechanism.
Even though the mapping information in the DNS server or mapping cache
in the ITR can be obsolete and causes routing failure, ETRs will send
destination unreachable message to ITRs to triggre immediate mapping
queries. Or when an ETR is down, the BGP update will inform the ETR
failure to ITRs. ITRs will choose alternative ETRs to the destination
AS or send icmp packets to sources. The ETR reachability detection and
routing failure recovery can be implemented variably from case to
case. However it will not have substantial impact on the  functioning
of 2-phased mapping scheme.