Subject: WG consensus action: ACCEPT LL18 Host with LL address MUST ARP for all addresses

From: Erik Guttman <[email-masked]>
Date: Tue, 23 Sep 2003 12:11:29 +0200

Action: Accept new text, as modified by discussion

The new text should read:

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.

    A multi-homed host needs to select an outgoing interface whether or
    not the destination is a Link-Local IPv4 address. Details of that
    process are beyond the scope of this specification. After selecting
    an interface, the multi-homed host should send packets involving
    Link-Local IPv4 addresses as specified in this document, as if the
    selected interface were the host's only interface. See Section 3 for
    further discussion of multi-homed hosts.

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 IPv4 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.

    In the case of a device with a single interface and only a Link-Local
    IPv4 address, this requirement can be paraphrased as "ARP for
    everything".

    In many network stacks, achieving this "ARP for everything" behaviour
    may be as simple as having no primary IP router configured, having
    the primary IP router address configured to 0.0.0.0, or having the
    primary IP router address set to be the same as the host's own Link-
    Local IPv4 address.  For suggested behavior in multi-homed hosts, see
    Section 3.