Connected: An Internet Encyclopedia Multihoming Requirements

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The following general rules apply to the selection of an IP source address for sending a datagram from a multihomed host.

  1. If the datagram is sent in response to a received datagram, the source address for the response SHOULD be the specific-destination address of the request. See Sections and and the "General Issues" section of [INTRO:1] for more specific requirements on higher layers.

    Otherwise, a source address must be selected.

  2. An application MUST be able to explicitly specify the source address for initiating a connection or a request.

  3. In the absence of such a specification, the networking software MUST choose a source address. Rules for this choice are described below.

There are two key requirement issues related to multihoming:

  1. A host MAY silently discard an incoming datagram whose destination address does not correspond to the physical interface through which it is received.

  2. A host MAY restrict itself to sending (non-source- routed) IP datagrams only through the physical interface that corresponds to the IP source address of the datagrams.


Internet host implementors have used two different conceptual models for multihoming, briefly summarized in the following discussion. This document takes no stand on which model is preferred; each seems to have a place. This ambivalence is reflected in the issues (A) and (B) being optional.

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Connected: An Internet Encyclopedia Multihoming Requirements