Re: [RRG] Which Side to Control Ingress Link Selection?

[Scott Brim <swb@employees.org>]

Excerpts from Christian Vogt at 11:41:50 +0200 on Thu 6 Mar 2008:
> one question that I think is worth being discussed more closely on
> this list: Who should have control over which ingress link packets
> reach a multi-homed edge network?
> 
> (a) the recipient edge network
> 
> (b) the sending edge network
> 
> (c) the sending edge network when initiating a packet exchange, and
>     the recipient edge network subsequently

(a) is agreed, and you're pondering (c) ...

> There are two main options to enable recipient edge networks to
> select their ingress links:
> 
> (a) Synchronization between indirection routers in the sending edge
>     network.  The recipient edge network must then send its
>     reachability information to only one indirection router of the
>     sending edge network, and this would likely be the indirection
>     router that handles packets in the other direction.
> 
> (b) Sending reachability information to all indirection routers of a
>     sending edge network.

The problem with (b) is that the destination site will not know all of
the source site's egress points.  Ingress and egress points do not
have to be the same.  The destination site can only respond to queries
it receives.  It is up to the source site if it wants to coordinate
among its egress points.

> Two comments:
> 
> - Option (a) does not necessarily require continuous
>   synchronization.  It may be realized through on-demand
>   synchronization, such as by multicasting reachability information
>   amongst indirection routers once received from a remote edge
>   network.
> 
> - Option (b) has the nice property that it puts the burden of
>   "synchro- nizing" indirection routers to the edge network that has
>   a benefit from it, namely, to the edge network that is selecting
>   its ingress link.

But the problem being solved is optimizing the source site's egress
point.  The destination site already solved its problem: traffic
enters where it wants.  In any case I don't think this idea, of having
all sites maintain knowledge of all other sites' interconnect points,
is going to scale very well.

> Would be interesting to hear other folk's opinions on who should
> have control over which ingress link packets reach a multi-homed
> edge network, especially from folks working at providers or
> operators of multi-homed edge networks.

I like Bill's answer, with a few extra details above.

swb

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