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

"Bernie Volz (volz)" <volz@cisco.com> Wed, 04 November 2020 12:14 UTC

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From: "Bernie Volz (volz)" <volz@cisco.com>
To: "ianfarrer@gmx.com" <ianfarrer@gmx.com>
CC: Jen Linkova <furry13@gmail.com>, Michael Richardson <mcr+ietf@sandelman.ca>, dhcwg <dhcwg@ietf.org>, v6ops list <v6ops@ietf.org>
Thread-Topic: [dhcwg] [v6ops] [EXTERNAL] Re: Question to DHCPv6 Relay Implementors regarding draft-ietf-dhc-dhcpv6-pd-relay-requirements
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Subject: Re: [dhcwg] [v6ops] [EXTERNAL] Re: Question to DHCPv6 Relay Implementors regarding draft-ietf-dhc-dhcpv6-pd-relay-requirements
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Hi ... looks good but perhaps MAC address is too Ethernet specific and just link-layer address would be better?

- Bernie

On Oct 29, 2020, at 12:24 PM, "ianfarrer@gmx.com" <ianfarrer@gmx.com> wrote:


Hi,

Sorry for the delay in reply, I’ve been out of the office for the last few weeks for various reasons.

Here’s a new wording proposal incorporating Jen & Bernie’s suggestions:

R-4
To prevent routing loops, the relay SHOULD implement a configurable policy to drop packets
received on a DHCP-PD client facing interface with a destination address in a prefix delegated
to a client connected to that interface, as follows:  For point-to-point links, when the packet’s
ingress and egress interfaces match. For multi-access links, when the packet’s ingress and
egress interface match, and the source MAC and next-hop MAC addresses match. An
ICMPv6 Type 1, Code 6 (Destination Unreachable, reject route to destination) error message MAY
be sent as per [RFC4443], section 3.1.  The ICMP policy SHOULD be configurable.

Thanks,
Ian

On 15. Oct 2020, at 03:51, Jen Linkova <furry13@gmail.com<mailto:furry13@gmail.com>> wrote:

On Wed, Oct 14, 2020 at 12:44 AM Bernie Volz (volz) <volz@cisco.com<mailto:volz@cisco.com>> wrote:
If not, perhaps we just say:

R-4
To prevent routing loops, the relay SHOULD implement a configurable policy to drop traffic received from an uplink interface as follows:

I'm not sure 'from an uplink interface' makes sense. In the case of a
routing loop caused by an amnesiac DHCP-PD client it would be a
downstream interface.
The scenario when such traffic arrives from an uplink interface is
'the uplink router believes the prefix is delegated to the client but
the relay does not have a route pointing to the client so it sends
traffic back' - so more likely 'an amnesiac relay' case.

For point-to-point links, when the packet's ingress and egress interfaces match. For multi-access links, when the packet's ingress and egress interface match, and the source MAC and next-hop MAC addresses match. An ICMPv6 Type 1, Code 6 (Destination Unreachable, reject route to
destination) error message MAY be sent as per [RFC4443], section 3.1.  The ICMP policy SHOULD be configurable.

- Bernie

-----Original Message-----
From: ianfarrer@gmx.com<mailto:ianfarrer@gmx.com> <ianfarrer@gmx.com<mailto:ianfarrer@gmx.com>>
Sent: Tuesday, October 13, 2020 9:16 AM
To: Michael Richardson <mcr+ietf@sandelman.ca<mailto:mcr+ietf@sandelman.ca>>; Jen Linkova <furry13@gmail.com<mailto:furry13@gmail.com>>
Cc: Bernie Volz (volz) <volz@cisco.com<mailto:volz@cisco.com>>; dhcwg <dhcwg@ietf.org<mailto:dhcwg@ietf.org>>; 6man <ipv6@ietf.org<mailto:ipv6@ietf.org>>; v6ops list <v6ops@ietf.org<mailto:v6ops@ietf.org>>
Subject: Re: [dhcwg] [v6ops] [EXTERNAL] Re: Question to DHCPv6 Relay Implementors regarding draft-ietf-dhc-dhcpv6-pd-relay-requirements

Hi,

Thanks for all of the discussion on this. We’ve reworked the requirement as follows:

R-4
To prevent routing loops, the relay SHOULD implement a configurable policy to drop client traffic as follows:  For point-to-point links, when the packet's ingress and egress interfaces match. For multi-access links, when the packet's ingress and egress interface match, and the source MAC and next-hop MAC addresses match. An ICMPv6 Type 1, Code 6 (Destination Unreachable, reject route to
destination) error message MAY be sent back to the client.  The ICMP policy SHOULD be configurable.

Thanks,
Ian

On 9. Oct 2020, at 17:41, Michael Richardson <mcr+ietf@sandelman.ca<mailto:mcr+ietf@sandelman.ca>> wrote:


Jen Linkova <furry13@gmail.com<mailto:furry13@gmail.com>> wrote:
I think there is confusion re: the scenario we are talking about.
I've attached the diagram for the case which concerns me.
So:
- The Relay R has an interface eth0 connected to a switch S.
- Devices A and B are connected to the same switch and using R as a
default gateway.
- The prefix 2001:db8::/56 was delegated to a client A via the relay R.

a friendly amendment to your example to aid in human comprehension:
   } - The prefix 2001:db8:0000:0123:/64 was delegated to a client A via the relay R.
   }  - R installs a route for 2001:db8:0000:0123:/64 towards A via eth0.

- The device B (which has an address NOT from the delegated prefix,
but from another /64 assigned to that common link, let's sat
2001:db8:cafe::/64) sends a packet to an address from the delegated

now, my brain can more clearly see that 2001:db8:cafe::/64 is not
within 2001:db8:0000:0123:/64, while I had to use a few extra brain
cells to see that it wasn't in that ::/56 :-)

What I'd expect to happen (with DHCP-PD or without - e.g. if R has a
static route towards A, not a dynamic route produced by PD):
- the packet is sent to A. Well, if A does not have a route to
2001:db8::42 then indeed a routing loop might happen. But if A does
have a route, the packet will be delivered.

What seems to be required by R4:
- R detects that the packet is received via eth0 and needs to be sent
back to eth0. R4 seems to require such packets to be dropped.
So if B would never be able to communicate to any address in the
delegated prefix, right?

Am I missing anything?

I think that you got it right.

Perhaps the missing piece of the rule is don’t send it back to where it came from, based on link layer addresses (or link if point-to-point).

Yes. If R4 was saying 'drop the packet if it comes from the same
link-layer address you are going to send it back' - it would make
total sense. But I don't think routers do *that*.

Yes, if we made the check on L2 address, then it would work.
And I agree that routers are exactly doing that.

I think that it also works if B is a router with additional interfaces
downstream, unless there are multiple paths.

--
Michael Richardson <mcr+IETF@sandelman.ca<mailto:mcr+IETF@sandelman.ca>>   . o O ( IPv6 IøT consulting )
         Sandelman Software Works Inc, Ottawa and Worldwide

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--
SY, Jen Linkova aka Furry