DHCPv4 Option for IPv4 routes with IPv6 nexthops

DHCPv4 Option for IPv4 routes with IPv6 nexthops NetDEF, Inc.

San Jose USA equinox@diac24.net equinox@opensourcerouting.org

Max-Planck-Institut fuer Informatik

Campus E14 Saarbruecken 66123 Germany +49 681 9325 3527 tfiebig@mpi-inf.mpg.de

Internet Dynamic Host Configuration ipv6 This draft lives at As a result of the shortage of IPv4 addresses, installations are increasingly recovering IPv4 addresses from uses where they are not strictly necessary. One such situation is in establishing next hops for IPv4 routes, replacing this use with IPv6 addresses. This document describes how to provision DHCP-configured hosts with their routes in such a situation.

Introduction more intro TBA DHCP is likely handing out individual IPv4 addresses, i.e. the subnet mask is /32 when this mechanism is relevant. To preempt any misunderstanding, this is an IPv4 DHCP option. NOT a DHCPv6 option. A particularly interesting scenario enabled by the extension described here is the complete removal of IPv4 addresses from first-hop routers acting as DHCP relays, while still providing IPv4 connectivity. In this scenario, the relay (assumed colocated with the router) has no IPv4 address to use to communicate with the client. An almost-working solution for this case is presented by with the transport method. Since this mechanism encapsulates IPv4 DHCP messages, all related IPv4 configuration can be carried - but notably there is no way to encode an IPv6 default gateway or other route. If the router and relay are not co-located, the relay may have an IPv4 address while the router does not. In this case, the option described in this document could be carried in a plain IPv4 DHCP message.

Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

Expected host behavior The option described in this document is intended to be implemented on hosts supporting IPv4 routes with IPv6 nexthops as described in . Hosts that do not support this function MUST ignore the option described in this document and behave as before. Hosts that support behavior and acquire their configuration from SHOULD implement the option described here.

Overlapping routes documents a mechanism to communicate a set of routes and their nexthops over DHCP. The original DHCP "router" option (code 3) may communicate a default router. If either of these options is used, the routes communicated may overlap. To get consistent and unsurprising behavior, this document places the follwing expectations on the host:

Routes that describe distinct destination prefixes MUST be handled independently. This includes routes that differ only in prefix length. As a result, the routing table MAY contain a mix of IPv4 routes with IPv4 nexthops as well as IPv6 nexthops. Standard longest prefix match behavior MUST be observed.
If routes with the same destination prefix are described both with previously existing methods as well as the options documented here, the route described by the latter MUST be used and the routes with IPv4 nexthops MUST be discarded. This notably includes "unreachable" routes described here; a route with an IPv4 nexthop for such a destination MUST still be discarded.
Multiple routes for the same destination prefix with different nexthops of the same address family SHOULD be combined into a single route for equal-cost multipath behavior, if the host supports this. If ECMP routes are not supported, the host MUST deterministically choose one of the routes. This MAY be done by using the first or last option as seen in DHCP packet order, or by choosing the numerically lowest or highest nexthop.

Default route The default route is expressed here as a route for 0.0.0.0/0. There is no distinct special encoding for a default gateway, any nexthop for 0.0.0.0/0 MUST be treated as if it were a default gateway.

Routes clashing with the connected subnet TODO: determine what behavior is reasonable here. (The client is likely to be given a /32 subnet mask anyway.)

Type: TBA1 (field defined in )
Length: as defined in
Routes: One or more routes encoded as described below. DHCP Servers MUST NOT emit this option with an empty payload. DHCP clients MUST ignore this option if its length is zero. It is easily possible for encoded routes and their nexthops to exceed the encodable size of 255 bytes. This is addressed by allowing multiple occurences of this option. DHCP clients MUST process all instances of this option additively. DHCP servers MAY encode configured routes in as many instances of this option as they choose; they SHOULD however ideally choose a space-efficient encoding.

Route encoding 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+ | T | prefixlen | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | IPv4 prefix (0 - 4 octets) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | IPv6 nexthop (0, 8 or 16 octets) | : ... : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

T: Determines the function and nexthop encoding for this route. Valid values and their meanings are: This encoding mechanism might be excessive. The original motivation was to have a way to tell the client that it should use the relay's link-local address as next-hop. Filling that in from the DHCP server is annoying, since the server has otherwise no need to know that address. It's not a problem if the operator configures a fixed LL on all routers (e.g. fe80::1), but without that each router would just be using its autoconf'd LL. Having an encoding for 'unreachable' routes and a shorthand for link-local addresses is really just "fluff" too.
T = 0 - use DHCP packet source: Indicates that this route should use the DHCP packet's source address as nexthop. When is in use, hosts MUST retrieve the IPv6 source address of the DHCPv6 packet carrying the DHCPV4-RESPONSE message. TODO: does it really make sense to support IPv4 here? Maybe only allow this with DHCP4o6? The IPv6 nexthop field for this route has zero length.
T = 1 - unreachable destination: Indicates that the destination prefix specified by this route is unreachable. More specific destination prefixes may still be reachable. TODO: specify fault mix (unreachable+reachable) The IPv6 nexthop field for this route has zero length.
T = 2 - link-local IPv6 nexthop: Indicates that this route uses a link-local IPv6 nexthop address, encoding only the latter 8 bytes of the address for space efficiency. The upper 8 bytes of the address are filled in with fe80::. The IPv6 nexthop field for this route is 8 octets long.
T = 3 - generic IPv6 nexthop: Indicates that this route uses the specified IPv6 nexthop, encoding the full address. The encoded address may be any type of unicast IPv4 address, including GUA, ULA and LL. The IPv6 nexthop field for this route is 16 octets long.
prefixlen: IPv4 prefix length, integer value from 0 to 32 (inclusive)
IPv4 prefix: The route's destination prefix, encoded in as few bytes necessary for the given prefixlen value. Calculate as ceil(prefixlen / 8).
IPv6 nexthop: encoding as determined by T

YANG model TBD if needed

Security Considerations TBD

Privacy Considerations TBD

IANA Considerations A codepoint from the "BOOTP Vendor Extensions and DHCP Options" registry is requested for use with the option described in . Editor note: 2 places of TBA1

References Normative References Informative References

Acknowledgements TBD, FILL IN

Example encoded options

(TBA) 01 00: A single default route (0.0.0.0/0) using the DHCP packet's source address as nexthop.
(TBA) 0a 88 0a 00 00 00 00 00 00 00 01: A single route for 10.0.0.0/8 via fe80::1.
(TBA) 15 40 d8 c0 00 02 20 01 0d b8 12 34 56 78 00 00 00 00 00 00 00 00: A route marking 0.0.0.0/0 as unreachable, and another route for 192.0.2.0/24 via 2001:db8:1234:5678::.

Editing notes (TO BE REMOVED)

-00: