IPv6 Routing Header - How does it work?

[Vriz]

I see. Actually, I'm not trying to state a route for my packet to go.
It's just that I would like to find out the route my packet has taken from a
source to a destination.
If I'm not wrong, I think traceroute in linux does that. However, I would
like to know how it is done.
And if my packet has travelled from a source to a destination using a
specific route, does that mean it's going to be more or less unchanged for
my other packets (same source and destination)?
Thank you.



----- Original Message -----
From: "Patrick R. McManus" <mcmanus at ducksong.com>
To: "Vriz" <vriztll at hotmail.com>
Cc: "__Boston Linux Mail List" <discuss at blu.org>
Sent: Thursday, April 18, 2002 9:10 PM
Subject: Re: IPv6 Routing Header - How does it work?


> [Vriz: Thu, Apr 18, 2002 at 09:26:55AM +0800]
> > Thanks Patrick. May I know if there's any way (using ICMP?) for me at
the
> > source node to find out what is the shortest route to the destination
node?
> > I mean the intermediate nodes addresses.
> > Thank you.
>
> The basic answer to your question is no. First of all, "shortest" can
> be defined many different ways (fiber miles, miles weighted by medium,
> congestion, link latency, link bandwidth, etc..) More importantly,
> policy plays a very strong role in route decisions as well (cheapest
> link, links that need balanced traffic ratios, hauling to peer vs
> hot-potato transit, etc..)
>
> And each network that your packet travels makes all of the above
> decisions independently of the other networks.. This makes predicting
> paths (much less optimal paths!) very hard. Indeed, the nature of
> stateless routing says that when one router passes the packet on he
> has no idea what the next one will do with it - there is no concept of
> end-to-end path from a router's point of view - only next hop.
>
> you might make some progress by searching for "bgp looking glass" on
> google.
>
> you seem to be implying that you want to use loose source routing to
> manually select a more efficient path for your traffic. Don't do that
> - it won't work for numerous reasons. Even if you weren't filtered
> (any you likely would be - by either a customer border router or the
> host itself), static routes on a global scale are a very bad
> idea. They can't react to changes in topology caused by severe
> congestion or reachability - that's why we have dynamic routing
> protocols like bgp, is-is and ospf. source routes only really make
> sense as a diagnostic tool.
>
> at this point though, this doesn't have much to do with linux or unix.
>
> -Patrick
>
>
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