Subject: [babel] What's up with AE 0?

From: Juliusz Chroboczek <[email-masked]>
Date: Wed, 19 Jul 2017 18:23:24 +0200

Executive summary: rfc6126bis leaves the wildcard AE mostly unchanged.

AE 0 specifies a wildcard: the only element of the AE is the 0-length byte
sequence, and represents "all the things you have".  AE 0 is allowed in
three places:

  1. in a route request, it requests a full route dump;
  2. in an Update with Metric=infinity, it retracts all the routes
     announced by the sender;
  3. in an IHU, it says "I Heard You" to anyone who receives this update.

All three applications of AE 0 remain in rfc6126bis.  We've clarified that
for implementations with extensions, (1) and (2) apply to all routes --
a wildcard request requests all routes, even the ones that require an
extension, and a wildcard retraction retracts all routes.  This needs
adding to Appendix C.

IHU(AE=0) were originally intended for implementations that are not
interested in bidirectional detection, and perform simple inverse
detection, RIP style; such implementation can just send IHU(AE=0), "I hear
everyone!".  However, no implementation ever did that -- even sbabeld
produces proper per-neighbour IHUs.

David uses IHU(AE=0) for a different purpose: he sends IHUs over unicast,
and hence doesn't need to use the Address field to constrain the receiver.
This was not intended, but looks to me like a perfectly valid
implementation.

I will edit the document to say that IHU(AE=0) SHOULD NOT be used for
multicast IHUs, and MAY be used over unicast.  (David suggests MUST NOT,
but I feel it's too strong.)

-- Juliusz

---

From: David Schinazi <[email-masked]>
Date: Wed, 19 Jul 2017 18:27:34 +0200

My initial preference was to forbid (MUST NOT) IHU_AE=0 over multicast.
Now I understand that it can be used by implementations that do not
want bidirectional detection. I'm fine with making this a SHOULD NOT
as long as we add text in the same paragraph (or close by) that explains
that using IHU_AE=0 over multicast will disable bidirectional detection.

David Schinazi

> On Jul 19, 2017, at 18:23, Juliusz Chroboczek <[email-masked]> wrote:
> 
> _______________________________________________
> babel mailing list
> [email-masked]
> https://www.ietf.org/mailman/listinfo/babel

---

From: Juliusz Chroboczek <[email-masked]>
Date: Thu, 20 Jul 2017 16:28:34 +0200

> that using IHU_AE=0 over multicast will disable bidirectional detection.

Noted and agreed.

-- Juliusz