[babel] What's up with AE 0?

[Juliusz Chroboczek <[e-mail]>]

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