RSPF Specification - Acquisition of end node adjacencies

III. Acquisition of end node adjacencies

Four possible means of automatically determining adjacencies to end nodes are the inclusion of RSPF in end nodes, the use of connected-mode subnet links, the use of ARP, and the use of a "wiretap" algorithm (see RFC981). Unless a connection mode Data Link layer or subnetwork (with keepalive timers) is used, adjacent nodes may need to send each other messages at regular intervals (ping) to ensure that the link is still usable. A procedure is outlined below for routers and end nodes to acquire knowledge of each other.
Adjacencies may also be set manually. RSPF maintains a manual routes table which may list both individual nodes and node groups that this router will route to absent any other information. (This is required for creating node group support of end nodes.) Manual adjacencies are determined from the manual routes table. An entry in the manual route table not flagged as private should be propagated as a known adjacency. Private entries are not propagated. In the event that a private route provides connectivity to a general-topology subnetwork which notifies the router of a potential adjacency, this indirect adjacency may be propagated. (This latter detail is unproven and may warrant a flag to disable, on a system or per-manual-route basis.)

III.1. RSPF optional in end nodes

RSPF need not be activated in end nodes; this permits them to use a simple version of the TCP/IP software. A node that has RSPF support in its software but operates as an end node can also use the router-router connection procedures and simply broadcast its adjacency to the router in a one-entry bulletin with a low horizon. Such a node may also be simultaneously homed on two or more other routers, unlike true end nodes whose traffic either bypasses RSPF (using ARP) or arrives by way of its associated router.
There is no "redirect" function provided in RSPF. Since radio does not generally provide a true "broadcast" topology subnetwork, a router cannot presume that if both end nodes can hear it, that both end nodes can hear each other. If an RSPF-equipped end node hears the destination directly, it may test adjacency to that node, via ARP. If that succeeds, then it should choose on its own to route packets there directly, since that one-hop link on an interface will cost less than a two-hop link across the same interface.

III.2 End node subnet connection

If an end node knows the IP address of the router which will connect it to the network at large, it may establish a connected-mode AX.25 or other subnet connection to the router; the presence of this connection indicates that the node is reachable from that router, which then adds it to its links table and subsequent bulletins. This may, of course, require an ARP exchange in order to acquire the subnet address (eg., the AX.25 address).

III.3. Connectionless using Address Resolution Protocol

Alternately, the end node can simply use ARP and use connectionless link procedures. In this case the router should assume that the end node is available until either a rather lengthy timer expires, or the router is unable to make an ARP contact after the ARP timer expires. (A loss of reachability should not be inferred from the ARP timeout.)
Routers may periodically broadcast their availability (suggested interval: every 15 minutes) with a broadcast frame sent to the broadcast address. In AX.25, this is a UI frame sent to "QST-0". A human-readable ("unproto") message may go here, allowing individual operators to recognize routers and connect as appropriate. (No specific PDU coding is provided, as the end nodes do not use RSPF, and thus this is not really an RSPF packet. However, the RRH frame may double for this function, since its plaintext will generally be readable.) III.3.1. Promiscuous ARP
A router may also choose to use "Promiscuous ARP" to provide service to an end node which is attempting to connect with an IP address reachable by the router. In such a case the router should wait an extra interval after receiving the ARP request because the desired destination may actually be directly reachable; ARP procedures may need to be modified to provide this.

III.4. Wiretap

Another potential approach is for routers to simply listen to AX.25 traffic on the link and determine who is adjacent to whom. This is the gist of the "wiretap" algorithm in RFC981, which also finds non-adjacent nodes by taking advantage of the source routing found in AX.25 frames. Integration of wiretap into RSPF is for further study. (It is not part of RSPF 2.2.)