[mif] Hybrid Access Problem

Margaret Wasserman <margaretw42@gmail.com> Tue, 11 November 2014 04:08 UTC

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From: Margaret Wasserman <margaretw42@gmail.com>
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Subject: [mif] Hybrid Access Problem
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Let me see if I can start a technical discussion here...

If you have an ISP today that has two different access networks available, such as one 3GPP network and one DSL network, how do(es) the gateway box(es) (connected to both networks) decide where it should send each packet it receives?  I think there are several possible cases here;

(1) There is one gateway attached to both outbound networks.  This has two sub-cases:

(1.1) Hosts behind the single gateway have separate addresses on the two attached networks.  In that case, the gateway is constrained to send packets over the outgoing network that uses the address prefix that matches the source address, or the packets will be thrown away by ingress filters on the other side.  So, the network choice is made by the hosts.

(1.2) Hosts behind the gateway have only one address each.  This has two sub-cases:

(1.2.1) The gateway has separate addresses on the two networks, and does some sort of translation from internal to external addresses in the prefix of the "right" outgoing link.

(1.2.2) The gateway has only one IP address that is somehow shared across the two links.

(2)  There are two gateways, each attached to a single outbound network.  In this case, hosts will always have separate addresses for the two networks, and will need to make a decision about which outbound network to use. 

Cases 1.1 and 2 are essentially the same from a host standpoint, in that the host needs to make a network choice.  This is a problem we have discussed in MIF -- What sort of information does/should the host need to make that choice, and how is that information communicated to the host?  

Case 1.2.1 is a typical case of how NAT (or NPTv6) can be used for multi-homing.  The IETF generally prefers to avoid recommending solutions that use NAT, but do we have a better answer?

Case 1.2.2 becomes a layer 2 problem and is probably outside the scope of the IETF.

Are there cases that I am missing here?

Margaret