Re: [homenet] Genart last call review of draft-ietf-homenet-babel-profile-05

Juliusz Chroboczek <> Wed, 21 February 2018 00:42 UTC

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Date: Wed, 21 Feb 2018 01:42:05 +0100
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From: Juliusz Chroboczek <>
To: Stewart Bryant <>
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Subject: Re: [homenet] Genart last call review of draft-ietf-homenet-babel-profile-05
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> I am the assigned Gen-ART reviewer for this draft.

Thanks for your comments, Stewart.

>       if implementations use conflicting route selection policies,
>       persistent oscillations might occur.
SB> Is this consistent with the statement earlier in the para that
SB> " Distinct
SB> implementations of RFC 6126bis Babel will interoperate, in the
SB> sense that they will maintain a set of loop-free forwarding paths"?

Yes, it is consistent.  Please see Section 3.6 of rfc6126bis.

In short, Babel guarantees that the forwarding graph remains loop-free
at all times (which is somewhat weaker than what you'd expect -- in
particular, it does not entail that a packet will never pass the same
router more than once).  It does not make any guarantees about the
stability of the forwarding graph if the route selection policy is
completely crazy.

As far as I am aware, the problem of defining the class of non-crazy route
selection policies is an open research problem.  The best we can do, to
the best of my knowledge, is give examples of policies that do not cause

This is not unlike BGP, which does an excellent job avoiding loops, but
does not prevent persisten oscillations in the presence of crazy policies.
Interestingly enough, a numbre of papers indicate that this is not
a problem in the Internet, and that router operators are pretty good at
defining reasonable BGP policies.  I believe the same is true of Babel.

>  Since IPv6 has some
>       features that make implementations somewhat simpler and more
>       reliable (notably link-local addresses), we require carrying
>       control data over IPv6.
SB> Earlier you said that IPv4 also had Link Local addresses, so how
SB> can link local addresses be the deciding selection criteria? Is there
SB> something technically better about IPv6 LL?

No, I didn't -- I'm trying to be consistent, and use LL to mean fe80::/64.
I believe the issue is in Section 1, where I say

   traffic is carried over either link-local IPv6 or IPv4

where what I mean is

   either link-local IPv6 carries traffic, or IPv4 carries traffic.

I'm not a native speaker, and I'll be grateful if you can suggest a better

> Minor issues:

>       Rationale: support for wireless transit links is a "killer
>       feature" of Homenet, something that is requested by our users and
>       easy to explain to our bosses.  In the absence of dynamically

SB> Not sure explicability to your boss counts for much as a basis for
SB> a feature an international standard.

I think this paragraph is helpful for implementors -- it helps people
explain to their bosses why we're bothering with link-quality estimation
when we've done routing protocols with no link-quality estimation for the
last fifty years or so.  (The Fuzzball LSI-11 router had link-quality
estimation, but that was in the 1980s.)  Still, if you find the tone too
informal, I'm open to reformulating.

> Abstract

>    This document defines the subset of the Babel routing protocol and
>    its extensions that a Homenet router must implement, as well as the
>    interactions between HNCP and Babel.

SB> HNCP needs to be expanded Both need a reference, but the reference
SB> needs to be expanded i.e. RFC7788 not [RFC7788]

Is this consistent with the last sentence of RFC 7322 Section 4.3?

-- Juliusz