Re: [Isis-wg] [rbridge] Why is MTU discovery important?

[sgai <sgai at cisco.com>]

I support Sina Proposal

-- Silvano


On 4/7/09 2:57 PM, "Sina Mirtorabi (smirtora)" <smirtora at cisco.com> wrote:

> Radia,
> 
> The minimum size packet is what I proposed and I think it is the easiest
> solution to go forward. We should also allow for higher size packet if
> there is a mechanism to safely enable that.
> Now it is not because somebody (the highest priority Rbridge) to tell
> you what to do that you have to blindly follow, but if everybody
> announce that jumbo value then you can safely move to that value.
> So here what we can do:
> 
> Put an originatingLSPBufferSize in a TLV when generating LSP and (ISO
> 10589:2002) already recommend that. The default value would be 1450
> otherwise it is set to the configured value.
> An Rbridge can start generating a non-default size X LSP if all the
> reachable Rbridge has announce at least a value >= X, in other word the
> non-default size LSP is set to the minimum of all announced LSP size.
> 
> 
> Note that for a non-default size LSP, we rely on the administrator's
> configuration and we don't actually check the MTU path. If later there
> is a mechanism to actually check the MTU of a Rbridge link, then
> originatingLSPBufferSize could be set to the minimum MTU size of all
> local Rbridge's link. This will guarantee that the LSP size will always
> be set to the minimum MTU size of any link.
> 
> Thanks
> Sina
> 
>   
> 
> 
> 
>> 
>> OK. It's clear we should NOT pad Hellos.
>> 
>> It also seems like enough people would feel uncomfortable with not
>> doing
>> MTU discovery that we should make
>> sure there is a mechanism for that also.
>> 
>> So can we focus for a minute on the following question, "whether to
>> make
>> MTU size a forever constant,
>> or configurable in a campus", as described below.
>> 
>> The simplest option is to pick some minimum size, say 1450 bytes, put
>> that in the spec now, and forever
>> say that TRILL Hellos and LSPs cannot be bigger than that.
>> MTU discovery would be done to ensure that
>> every link can handle 1450 bytes, and not use (not report in LSPs)
>> links
>> that can't handle that.
>> 
>> However, my preference is to have an optional TLV in the LSP that says
>> "campus-wide MTU size". This TLV
>> would only be relevant if:
>> a) it is larger than 1450, and
>> b) it is in the LSP of the highest priority RBridge in the campus.
>> 
>> If we put that in, then we would be enabling TRILL campuses that
>> support
>> jumbo frames.
>> 
>> The rule is that if the highest priority RBridge says MTU size should
>> be
>> 10000 bytes, then all RBridges would adjust
>> their buffer sizes, and test their adjacencies to ensure they could
>> handle 10000 bytes, and if they don't
>> then stop reporting those links (just like they would with the simpler
>> proposal if the links
>> couldn't support 1450).
>> 
>> 
>> 
>> 
>> _______________________________________________
>> Isis-wg mailing list
>> Isis-wg at ietf.org
>> https://www.ietf.org/mailman/listinfo/isis-wg
> 
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