Subject: RE: ICMPv3: Message Source Address Determination..

From: [email-masked]
Date: Sat, 19 Jun 2004 14:47:02 -0500

Thanks Derek.  

Good that we decided to keep that paragraph in the draft.

Regards
Mukesh

-----Original Message-----
From: ext Derek Fawcus [mailto:[email-masked]]
Sent: Saturday, June 19, 2004 9:57 AM
To: Gupta Mukesh (Nokia-NET/MtView)
Cc: [email-masked]
Subject: Re: ICMPv3: Message Source Address Determination..

On Thu, May 13, 2004 at 02:43:44PM -0500, [email-masked] wrote:
> Hi All,
> 
> [Please express your opinion.  It is needed to make
> the informed decision.]
> 
> Pekka raised a concern about the usability of one of the
> methods (section 2.2 (c)) to select the source address 
> of the ICMPv3 packet.  I want to know if anyone has 
> implemented it ?  or how useful and practical this 
> method is in your opinion ?
> 
> The decision that I want to make is to whether we should
> keep it in the draft or remove it.

I've not been keeping up with the list recently,  hence the
late response.  But yes,  I implemented this.

The box in question try's to determine the source address
based what (I determined) would be the most useful.  If an
suitable an address couldn't be found one the appropriate
interface,  a set of fallbacks would result in another
(potentially less useful) address being chosen.

e.g. packet filtering - depending upon if it was an ingress or
     egress filter,  I'd choose an address of the incoming or
     outgoing interface.

DF

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From: Derek Fawcus <[email-masked]>
Date: Sat, 19 Jun 2004 21:15:13 +0100

> Good that we decided to keep that paragraph in the draft.

Well from a practical point of view it wouldn't matter if it
was removed or not.  I just thought I'd point out that there
is (at least) one implementation which does this.

The issue being that a node can not in general control which
particular router would generate such an ICMP,  or what route
the packet would follow,  so it can't reasonably rely upon
any specific source address being picked.

  [ Ignoring source routing options,  and prior knowledge ]

So frankly,  in the absense of configuration overriding any
adress selection,  the router can pick any address it owns
as the source of the ICMP packet.  The paragraph in question
was really a hint at a quality of implementation issue.

Now if the router does pick the source address so as to be "most
useful",  it's only really useful in connection with people manually
debugging specific packet trace issues.

DF