Subject: Re: New draft notification: draft-dz-bfd-extensions-for-vl-packets-00

From: Jeffrey Haas <[email-masked]>
Date: Fri, 18 Jun 2010 22:02:05 +0000

[Commenting on an older thread as an individual contributor]

On Tue, Mar 02, 2010 at 04:21:26PM -0800, Dave Katz wrote:
> I'll have to examine the proposal carefully, but my first reaction is  
> that you can do most of this today without changing the protocol, by  
> simply padding the packets in which BFD is carried, without changing the 
> BFD header (or the receiving implementation) at all, since the spec only 
> requires that the BFD packet fit inside of whatever it is being carried 
> within (UDP, typically).  For that matter, the spec would allow the BFD 
> packet itself to be padded (since only the lower bound of packet length 
> is supposed to be checked) but that would be more likely to run afoul of 
> out-of-spec implementations.

I had made a similar comment in my last set of comments on the BFD base spec
before it had gone to RFC.  However, rather than utilizing the control
packet mode to do this particular type of probing, I'd suggest instead that
this is an excellent application of echo mode.

As an example, echo packets could be exchanged at an initial small size, S.
The detection multiplier for purposes of probing should be set to a
relatively large value, e.g. 5.  The payload of the echo packet would
contain information that can be used to determine if a particular size of
packet was successfully looped through the remote BFD speaker.  For example,
it could contain a 4 byte number of the number of bytes of the payload along
with padding.

The system then proceeds to probe the max size by transmitting one packet
out of the 5 from the above example with a larger size at a given step
interval.  When the large packets are consistently lost due to excessive
size, the size threshold has been determined.  The actual threshold can be
determined by bisecting the space.

Practically speaking, this would also allow the system to have 2 types of
detection: forwarding detection and large packet detection.  It would be the
transmitting system's prerogative to determine if it is going to call the
large MTU path "down" or not.  Since only 1:5 (per example) of the echo
packets could be lost due to MTU the session should survive temporary loss
of other-sized packets.

-- Jeff