Re: [OSPF] OSPF WG Minutes from IETF 95

Paul Jakma <paul@jakma.org> Thu, 14 April 2016 14:58 UTC

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Date: Thu, 14 Apr 2016 15:58:09 +0100 (BST)
From: Paul Jakma <paul@jakma.org>
To: "Acee Lindem (acee)" <acee@cisco.com>
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Subject: Re: [OSPF] OSPF WG Minutes from IETF 95
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On Wed, 13 Apr 2016, Acee Lindem (acee) wrote:

> One of the authors or myself will write an errata to clarify this
> definition.

Great. :)

> But the OSPF stub router functionality is achieved using 0xffff, so 
> your proposal is NOT backward compatible.

Why not?

Any sufficiently large metric (relative to the cost of the longest path 
in the network) will induce the desired "transit is still fine" 
behaviour in the stub-router RFC.

It turns out the 0xffff metric was never intended to be special by that 
RFC - or we wouldn't be having this discussion. It _not_ intended to 
change SPF behaviour. So, how could using a lower value cause 
compatibility issues? ??

> The reality is that the standards WILL NOT change to match your 
> interpretation.

Well, not asking you to. ;)

I'm just saying Quagga has been shipping for years with "0xffff link 
metric means its SPF will not follow such a link out of a router when 
calculating the SPF tree" behaviour. The reason was provide the 
"absolutely no-transit" type behaviour (as "discouraged, but transit is 
still OK" behaviour is achievable with many other metrics).

I'm not arguing for anything, I'm just saying the existence of such 
implementations is a reality.

For Quagga, it seems the best way forward is:

- As/when H-bit clearly is going to be a standard, Quagga can use H-bit
   + 0xffff link metrics to signal "absolutely no transit", when the
   administrator desires that behaviour.

   * This will have the desired effect with all routers that recognise
     the H-bit, and all Quagga

- If the administrator wants "discourage, but transit still OK" then
   Quagga will just set some large, non-0xffff metric in the links (e.g.
   0xfffe).

   * This will have the desired effect with all routers, Quagga old and
     new, RFC2328, pre-1583, etc., without any issues (AFAIK).

That seems the best thing we can do.

It leaves other cases where things might not quite be consistent. For 
those there'll be an admin option to control the SPF-0xffff-transit 
behaviour between the longish-standing Quagga "no-transit" behaviour and 
1583/2328. That will probably have to default to the "no-transit" 
behaviour for a while longer, but in time hopefully flip it over to 
2328.

regards,
-- 
Paul Jakma	paul@jakma.org	@pjakma	Key ID: 64A2FF6A
Fortune:
Given sufficient time, what you put off doing today will get done by itself.