Re: [Lsr] Mirja Kühlewind's No Objection on draft-ietf-ospf-ospfv2-hbit-11: (with COMMENT)

Alvaro Retana <> Mon, 02 December 2019 17:18 UTC

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From: Alvaro Retana <>
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Subject: Re: [Lsr] =?utf-8?q?Mirja_K=C3=BChlewind=27s_No_Objection_on_draft-i?= =?utf-8?q?etf-ospf-ospfv2-hbit-11=3A_=28with_COMMENT=29?=
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On December 2, 2019 at 10:40:05 AM, Mirja Kühlewind wrote:



> 1) This sentence in section 3:
> "An OSPFv2 router advertising a router-LSA with the H-bit
> set indicates that it MUST NOT be used as a transit router (see
> Section 4) by other OSPFv2 routers in the area supporting the
> functionality."
> Isn't the MUST here too restrictive? What if the router is the part of the
> only path to a certain host? Or is the assumption that this host is some kind
> of endhost/deadend, but then it should never be on the shortest path anyway,
> or?
> Later on you say
> "When the H-bit is set, the OSPFv2 router is a Host (non-transit)
> router and is incapable of forwarding transit traffic."
> However, there might also be routers that don't understand the H bit and
> therefore will route traffic over this host anyway, no?

Completely avoiding transit traffic is the goal of the H-bit, which is
the reason/justification of the "MUST NOT".  Hence it being called the

rfc6987 defines the "best effort" functionality that would be
equivalent to "SHOULD NOT": if no alternate path exits then the path
through the router can still be used.

§8 (in the third bullet) mentions the case where a rogue router can
partition a network by setting the H-Bit...and §5 talks about the
mitigation in mixed environments, where the recommendation (third
bullet) results in the rfc6987 behavior if not all the routers
advertise support.

> 2) Shouldn't this document update RFC2328, given section 4 and this sentence
> in section 2: "If the H-bit is set then the calculation of the shortest-
> path tree for an area, as described in section 16.1 of [RFC2328], is
> modified by including a check to verify that transit vertices DO NOT
> have the H-bit set (see Section 4)."
> (And why is DO NOT in upper letters?)

Because the H-bit is an optional feature and not intended to be
supported by all OSPFv2 routers, then the formal Update is not needed.

> 3) Is there an attack that by spoofing the H-bit an attacker could influence
> the routing such that traffic is router over a malicious node instead? I guess
> there are multiple ways to impact the routing that way and this might not be
> specific to the H bit but maybe still worth thinking about it once more...?

Yes.  By using the H-bit the traffic is directed away from the node,
which would force the traffic through another path, including a
specific node.  Similar to the last bullet in §8, this action would be
indistinguishable from the proper use of the H-bit, or from simply
shutting down an interface...

Thanks for the review!