Subject: understanding Echo mode

From: Lokesh B <[email-masked]>
Date: Wed, 21 Aug 2013 12:51:45 +0530

Hi,

Following is my understanding on why echo mode is required.

Because RFC 5880 does not mandate route lookup for control packets in
asynchronous mode, there can be implementations that just build BFD packet
and transmit directly on connected network interfaces without doing route
lookup for peer device. Hence echo mode is provided to test real routing by
mandating that control packet should be transmitted to peer by looping it
via forwarding plane that tests existence of route to peer machine.

Is it correct? or is there any other requirement that is kept in mind while
proposing echo mode.

Regards
-Lokesh

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From: Marc Binderberger <[email-masked]>
Date: Wed, 21 Aug 2013 09:50:47 +0200

Hello Lokesh,

> by mandating that control packet should be 
> transmitted to peer by looping it via forwarding plane that tests 
> existence of route to peer machine.

No, echo is not about control packets (in the sense of BFD control 
packets). And there is, similar to control packets, no requirement how 
router A originates the echo packet to router B (the peer). This can be 
again a transmission directly on the wire.

There are many motivations for echo. The echo packet traverses the 
"full" hardware path on the peer, it does a "routing lookup" on the 
peer - the exact meaning is implementation specific - and it allows to 
put the burden of fast detection on the router that requires it (and 
not the peer), in case of asymmetric detection times. And a few more 
things.

Regards, Marc

On Wed, 21 Aug 2013 12:51:45 +0530, Lokesh B wrote:
> asynchronous mode, there can be implementations that just build BFD 
> packet and transmit directly on connected network interfaces without 
> doing route lookup for peer device. Hence echo mode is provided to 
> test real routing by mandating that control packet should be 
> transmitted to peer by looping it via forwarding plane that tests 
> existence of route to peer machine.
> 
> Is it correct? or is there any other requirement that is kept in mind 
> while proposing echo mode.
> 
> 
> Regards
> -Lokesh

---

From: Dave Katz <[email-masked]>
Date: Wed, 21 Aug 2013 11:59:36 -0700

Yep.  And among the "more things" are the ability to do proprietary things in the echo protocol (since it is a private matter to the originating machine) and less work for the receiving end (perhaps handy when the receiving machine has a large fan-in).

--Dave