Re: WGLC for draft-ietf-bfd-large-packets

Jeffrey Haas <jhaas@pfrc.org> Thu, 26 September 2019 19:46 UTC

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Date: Thu, 26 Sep 2019 15:49:48 -0400
From: Jeffrey Haas <jhaas@pfrc.org>
To: "Les Ginsberg (ginsberg)" <ginsberg@cisco.com>
Cc: "Ketan Talaulikar (ketant)" <ketant@cisco.com>, "Reshad Rahman (rrahman)" <rrahman@cisco.com>, "rtg-bfd@ietf.org" <rtg-bfd@ietf.org>
Subject: Re: WGLC for draft-ietf-bfd-large-packets
Message-ID: <20190926194948.GA22700@pfrc.org>
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Les,

On Tue, Sep 24, 2019 at 10:48:51PM +0000, Les Ginsberg (ginsberg) wrote:
> A few more thoughts - maybe these are more helpful than my previous comments - maybe not. I am sure you will let me know.
> 
> Protocol extensions allowing negotiation and/or advertisement of support for larger PDUs may well be useful - but let's agree that it is desirable to deploy this without protocol extensions just to keep the interoperability bar low.
> 
> My primary angst is with the following paragraph in Section 3:
> 
> "It is also worthy of note that even if an implementation can function
>    with larger transport PDUs, that additional packet size may have
>    impact on BFD scaling.  Such systems may support a lower transmission
>    interval (bfd.DesiredMinTxInterval) when operating in large packet
>    mode.  This interval may depend on the size of the transport PDU."
> 
> Given long experience that CPU use correlates more highly with number of packets than with number of bytes, the first sentence would seem to be weakly supported.
> Given the previously mentioned concerns about detection time, the second sentence seems to compromise the value of the extension.

My experience is largely identical to yours.

The motivation for mentioning anything at all here is TANSTAAFL[1], and
we've already had people ask about possible impacts.  And, as we discussed
previously in the thread we shall inevitably get asked about it during TSV
review in IESG.

The primary reason this is a "may" in the non-RFC 2119 sense is that our
experience also suggests that when the scaling impacts are primarily pps
rather than bps that this feature will likely have no major impact on
implementations beyond your valid concerns about exercising bugs.

I suspect had this not been mentioned at all, you would have been happier.
But you're not the target audience for this weak caveat.

> What might be better?
> 
> 1)Some statement that MTU isn't necessarily a consistent value for all systems connected to an interface - which can impact the results when large BFD packets are used. Implementations might then want to consider supporting "bfd-mtu" configuration and/or iterating across a range of packet sizes to determine what works and what doesn't.

I'm not clear what you intend by this statement.

Are you asking that we emphasize the use case in a different way?  The
Introduction currently states: 
  "However,
   some applications may require that the Path MTU [RFC1191] between
   those two systems meets a certain minimum criteria.  When the Path
   MTU decreases below the minimum threshold, those applications may
   wish to consider the path unusable."

I'm also unclear what "Implementations" may refer to here.  BFD?  An
arbitrary user application?  If the latter, the application may not have
strict control over the generation of a given PDU size; e.g. TCP
applications.

> 2)Use of both padded and unpadded packets in combination with draft-ietf-bfd-stability to determine whether a BFD failure is due to padding or a generic forwarding failure.
> 
> Either of these suggestions are really "diagnostic modes" which may help diagnose a problem but aren't meant to be used continuously as part of fast failure detection.

We could certainly add a paragraph or two as an application note about using
this for BFD stability purposes as well.

-- Jeff