Asymmetrical routing opinions/debate

Scott Morris swm at emanon.com
Mon Jan 14 16:04:01 UTC 2008


Routing in general is based of the premise of "my decision, my control" and
therefore you have some (albeit limited) controls about how YOU can
influence someone else's routing decision.

So any time you have more than one connection to the collective ('Net) then
you simply run the risk of you make one decision to send a packet out a
particular link, but a bunch of other people make decisions about routing as
well and it may very well come back another path.

Presumably you have your IP addressing as a constant.  If you are NATting,
you may have some interesting problems with this, but that would be a design
problem on your end.  Same with stateful firewalls.

>From an appplication viewpoint though, it really shouldn't make any
difference.  Packet goes out.  Packet comes back.  Life is good.

In short though, you have some choices with this, but they are all design
choices on your end.  If you want to be multihomed, this is the way life is.

HTH,

Scott 

-----Original Message-----
From: owner-nanog at merit.edu [mailto:owner-nanog at merit.edu] On Behalf Of Drew
Weaver
Sent: Monday, January 14, 2008 10:31 AM
To: nanog at merit.edu
Subject: Asymmetrical routing opinions/debate


        Pardon me if I am using the wrong term, I am using the term
Asymmetrical routing to describe a scenario in which a request packet enters
a network via one path and the response packet exits the network via a
different path.

For example an ICMP ping request enters a network via ISP A and the reply
leaves via ISP B (due to multi-homing on both networks, and or some kind of
manual or automatic 'tweaking' of route preferences on one end or the
other).

I haven't noticed too many instances of this causing huge performance
problems, but I have noticed some, has anyone noticed any instances in the
real world where this has actually caused performance gains over symmetrical
routing? Also in a multi-homed environment is there any way to automatically
limit or control the amount of Asymmetrical routing which takes place?
(should you?) I have read a few papers [what few I could find] and they are
conflicted about whether or not it is a real problem for performance of
applications although I cannot see how it wouldn't be. Has there been any
real community consensus on this issue published that I may have overlooked?

Thank you,
-Drew








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