Re: [dhcwg] [EXTERNAL] Re: [v6ops] Re: Question to DHCPv6 Relay Implementors regarding draft-ietf-dhc-dhcpv6-pd-relay-requirements

"Templin (US), Fred L" <> Wed, 14 October 2020 19:19 UTC

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From: "Templin (US), Fred L" <>
To: Michael Richardson <>
CC: "" <>, Jen Linkova <>, dhcwg <>, v6ops list <>, 6man <>
Thread-Topic: [EXTERNAL] Re: [dhcwg] [v6ops] Re: Question to DHCPv6 Relay Implementors regarding draft-ietf-dhc-dhcpv6-pd-relay-requirements
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Subject: Re: [dhcwg] [EXTERNAL] Re: [v6ops] Re: Question to DHCPv6 Relay Implementors regarding draft-ietf-dhc-dhcpv6-pd-relay-requirements
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Hi Michael,

> -----Original Message-----
> From: Michael Richardson []
> Sent: Wednesday, October 14, 2020 11:58 AM
> To: Templin (US), Fred L <>
> Cc:; Jen Linkova <>om>; dhcwg <>rg>; v6ops list <>rg>; 6man
> <>
> Subject: Re: [EXTERNAL] Re: [dhcwg] [v6ops] Re: Question to DHCPv6 Relay Implementors regarding draft-ietf-dhc-dhcpv6-pd-relay-
> requirements
> Templin (US), Fred L <> wrote:
>     > Michael, what I was referring to below as "failure" is the proxy case when
>     > there is an L2 proxy P between the client and relay (e.g.,
>     > RFC489). There
> RFC4389 describes an ND Proxy.
> Is that really an L2 proxy?

Yes, I believe it is an L2 proxy.

> It seems like it also must be contain either an L2-bridge, or must have the
> L3-routing table entries if it would really be capable of passing DHCPv6-PD
> prefixes through it.

The only thing it has that includes L3 information is neighbor cache entries that
keep track of the client's actual L2 address on the downstream link segment,
but rewrites the client's L2 address to its own L2 address when forwarding
onto an upstream link segment. (In the reverse direction, it receives packets
destined to its own L2 address but the client's L3 address on the upstream
link segment, then rewrites the L2 address to the client's L2 address when
forwarding onto the downstream link segment.)

> Can you explain how such a device would normally work for a client device
> A,B,C,D doing DHCPv6-PD through it?

Sure. A sends a DHCPv6 Solicit using its IPv6 link-local address as the source,
and its L2 address as the link-layer source. The proxy converts the link-layer
source to its own L2 address when forwarding the DHCPv6 solicit onto the
upstream link. When the DHCPv6 Reply comes back, the IPv6 destination is
that of client A, but the link-layer destination is the L2 address of the proxy.
The proxy then converts the L2 destination to the address of client A and
forwards it on to the client.

> And is the failure one where the router "R" fails to drop traffic it should,
> one where the router "R" drops traffic that it shouldn't?

I was thinking more along the lines of the latter; if the only way that A has
for talking to B, C, D, etc. is by going through R, it wouldn't work if R was
unconditionally dropping everything.

Thanks - Fred
> --
> Michael Richardson <>   . o O ( IPv6 IøT consulting )
>            Sandelman Software Works Inc, Ottawa and Worldwide