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

Alexandre Petrescu <> Wed, 07 October 2020 10:38 UTC

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From: Alexandre Petrescu <>
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Date: Wed, 7 Oct 2020 12:38:51 +0200
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Subject: Re: [dhcwg] Question to DHCPv6 Relay Implementors regarding draft-ietf-dhc-dhcpv6-pd-relay-requirements
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There are few things I could comment on here...

Le 07/10/2020 à 12:25, a écrit :
> Hi,
> We are currently finishing WGLC for this draft. It describes 
> requirements for a 'DHCPv6 Delegating Relay' - this is a router 
> functioning as the L3 edge and DHCPv6 relay (only) with prefix 
> delegation. This is a common deployment scenario, but RFC3633/8415 only 
> really describes PD using a Delegating Router - i.e the L3 edge also 
> functions as a DHCPv6 server with no relay. When the relay and server 
> functions are performed by separate devices a number of problems with 
> how relays behave have
> been observed, so this document addresses them.
> During WGLC for this, Ole raised a comment related to one of the routing 
> requirements:
> R-4:    If the relay has learned a route for a delegated prefix via a
>             given interface, and receives traffic on this interface with
>             a destination address within the delegated prefix (that is
>             not an on-link prefix for the relay), then it MUST be
>             dropped.

'within' the delegated prefix?  How is the operation of finding whether 
an address is within a prefix implemented?  REmember the 64bit limit is 
not frozen and as such a longest prefix match operation might be the 
only possible; such an operation is however not offering the result one 
might expect.  Lpm is good for matching an address among a few prefixes, 
but is not good for deciding whether an address is 'within' a given prefix.

2. there is a new RFC now that suggests addition of an NC entry in 
router's table such that incoming packets to that Host dont get dropped. 
  Such an operation would contradict R-4.

   This is to prevent routing loops.  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.
> The problem that this is trying to solve is:
> 3.5.  Forwarding Loops between Client and Relay
>     If the client loses information about a prefix that it is delegated
>     while the lease entry and associated route is still active in the
>     delegating relay, then the relay will forward traffic to the client
>     which the client will return to the relay

Will a client return a packet to relay?  Or will it generate an ICMP 
error carrying the packet?

  (which is the client's
>     default gateway (learnt via an RA).  The loop will continue until
>     either the client is successfully reprovisioned via DHCP, or the lease
>     ages out in the relay.

I would dare to wonder whether such loop has been seen with a packet 
capture tool such as wireshark?  Or is it that we worry that such a loop 
might appear?  Both are valid points to consider.

> Ole’s comment: "And I would also be happy if we could have some 
> implementors chime in with a "we are happy and able to implement this 
> requirement”.”
> We would appreciate any feedback on this, especially from anyone with 
> experience implementing DHCPv6 relays with PD.

I do have some experience implementing relays and server doing PD, but 
it was some years ago.  The server was doing PD and the relay  just 
adding a route.  I dont remember having seen loops, but I have not tried 
the scenario described above (expiration of prefix lease).


> Thanks,
> Ian
> Current draft version: 
> <>
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