Re: WG action: 1 week to discuss LL18: Host

The first paragraph of Philip's suggested 2.6.2 overlaps with the last
paragraph of 2.6.1.  This suggests combining them.  How about this?

2.6.1 Source Address Usage

Since each interface on a host may have a Link-Local IPv4 address in
addition to zero or more other addresses configured by other means
(e.g. manually or via a DHCP server), a host may have to make a
choice about what source address to use when it sends a packet or
initiates a TCP connection.

The host SHOULD use a routable address in preference to a Link-Local
IPv4 address except for communication to peers for which the host has an
existing TCP connection at the time in which the host obtained a
routable address configuration.  For more details, see Section 1.7.

If a host is multihomed, it must first determine which interface
to use to send any packet, whether or not the destination is a
Link-Local address.   For a multi-homed host, the decision as to which
source address to use is also more difficult.  This specification does not
define how these decisions are made.  Rather the issues are explained and
advice is given on how to address known problems (see Section 3).

2.6.2 Forwarding Rules

Whichever interface is used, if the destination address is in the
169.254/16 prefix (including the 169.254.255.255 broadcast address), then
the sender MUST ARP for the destination address and then send its packet
directly to the destination on the same physical link.  This MUST be done
whether the interface is configured with a Link-local or a routable IPv4
address.

In many network stacks, achieving this functionality may be as simple as
adding a routing table entry indicating that 169.254/16 is directly
reachable on the local link.

The host MUST NOT send a packet with a Link-Local IPv4 destination
address to any router for forwarding.

If the destination address is a unicast address outside the 169.254/16
prefix, then the host SHOULD use an appropriate routable IPv4 source
address, if it can.  If for any reason the host chooses to send the packet
with a Link-Local source address (e.g. no routable address is available on
the selected interface), then it MUST ARP for the destination address and
then send its packet, with a Link-Local IPv4 source address and a routable
destination IPv4 address, directly to its destination on the same physical
link.  The host MUST NOT send the packet to any router for forwarding.