Fred Goldstein   k1io
				Version 2.2  2-feb-1992

	 The Radio Shortest Path First (RSPF) Routing Protocol
	    For Internet Protocol over Amateur Packet Radio



I.   Introduction and Version Notes
II.  Acquisition of router-router adjacencies
III. Acquisition of end node adjacencies
IV.  Link state propagation
V.   The Shortest Path First Spanning Tree
Appendix A:  Router Parameters
Appendix B:  Schematic Representation of RSPF Tables

[note:  Significant changes to Version 2.1 are flagged in this text
with "**".]

I.  Introduction

The packet radio environment is a very difficult one for TCP/IP to run
on.  As typically implemented in the Amateur Service, it provides very
low bandwidths within severely constrained implementations.  The most
common radio technology has been 1200 baud single-frequency operation,
although higher speeds and some full-duplex operation are also found.
Virtually all amateur packet radio TCP/IP activity (the AMPRnet) makes
use of personal computers, most with severely constrained memory address

In this environment, automatic routing of IP packets has not been the
norm.  Routing protocols designed for other environments are rarely
satisfactory.  Manual route tables are common.  Most amateur packet
radio operation to date does not even make use of IP; instead, it
converges applications directly upon a source-routed frame relaying
protocol called AX.25.  Improved routing support will be an important
factor in the success of IP.

Many IP and other wireline networks have implemented link state routing
based upon Dijkstra's "SPF" (shortest path first) spanning tree
algorithm.  A wireline implementation of SPF for IP is being
standardized as the Open SPF Interior Gateway Protocol (OSPF), and an
SPF procedure has been accepted by ISO as the standard "IS-IS" routing
protocol for OSI connectionless networks [ISO10589].  A similar (and
derivative) procedure can be applied to AMPRnet (Net 44) and perhaps to
similarly-constrained packet radio networks.  It is called Radio
Shortest Path First (RSPF);  this document describes the RSPF protocol,
version 2.2.

RSPF occupies the role traditionally referred to in TCP/IP networks as
an "Interior Gateway Protocol" (IGP), where "Gateway" means "router".
It makes use of the services of the Internet Protocol.  It is not
inconceivable that a router could use both RSPF and another IGP, or
communicate with another network using the Exterior Gateway Protocol
(EGP).  However these scenarios are not described in this document.

RSPF is intended to be implemented on nodes which serve as routers; its
implementation on end nodes is optional, and not required for the end
nodes to take advantage of routing services.  Any IP station may be an
end node giving no further consideration to routing.

I.1. Elements of RSPF

The RSPF protocol is designed for use by Internet-layer routing nodes (IP
Gateways) in a packet radio network using the Internet Protocol.  It is
comprised of four major functions:

	1) Acquisition of router-router adjacencies
	2) Acquisition of end node adjacencies
	3) Link state propagation
	4) Spanning tree route decision making.

Its net result is the automatic maintenance of a least-cost routing
table for use by IP Routing.  RSPF is optimized for the geographically
heirarchical addressing used in AMPRnet, but does not depend upon it.

RSPF is simpler than OSPF and IS-IS, as it is principally designed for
personal computer implementation within the Amateur Radio Service.  It
also adds procedures to take advantage of packet radio's
"semi-broadcast" nature.

I.2. Addressing convention

When an RSPF router communicates with an end node, it will typically
deal with a 32 bit IP address.  Routers themselves, however, also
support node group addressing (fewer than 32 bits need match).  This
follows the convention in the KA9Q NOS IP routing program, which permits
a crude form of heirarchical addressing as well as allowing portable
operations to override the defaults.  RSPF looks for the match (node or
node group) with the greatest number of matching bits.  Only if the
number of bits matched is equal, then the lower cost path will be used.

Thus a match to a full node address will override a "cheaper" path that
matches its "subnet" (node group) of 24 bits, which overrides a 16-bit
node group, etc., noting that AMPRnet node groups need not follow rigid
IP subnetting rules, and that AMPRnet is a single Class A net.  In every
case, a greater number of bits matched is considered a superior path to
a destination than one that matches fewer bits, regardless of the value
of the routing metric ("cost").

**An extension of this is the handling of manual routes, which are
routes defined by a manual entry into a table.  Manual routes provide a
useful starting point during the RSPF convergence, and are required when
RSPF implementation in a given area is limited.  As an order of
precedence, whether a route is manual or RSPF-generated should only be
used to break ties equal in both subnet size and cost; RSPF-generated
routes then take precedence.

I.3  Subnetworks

The generic term used herein for the layer directly beneath IP, creating
the adjacency between two IP nodes, is subnetwork.  A routing protocol
gains efficiency by recognizing the nature of all subnetworks that it
supports.  RSPF is intended to work across a specific set of subnetworks
that occur in packet radio.  (Note that this use of "subnetwork" is
consistent with OSI terminology; the function of the IP "subnet mask" in
other IP routing protocols is herein referred to as "node group".)

I.3.1.  Classification by topology

All subnworks may be classified according to topology.  An initial
division is made between broadcast-topology subnetworks and
general-topology subnetworks.

I.3.1.1  Broadcast topology.  A broadcast-topology subnetwork is one in
which a node may send a single packet which is propagated to all other
nodes, which receive it with high probability.  Ethernet and FDDI are
examples of broadcast-topology subnetworks.

I.3.1.2.  General topology.  A general-topology subnetwork is any that
does not offer a reasonably reliable broadcast service.  Within this
classification, an extreme case is a point-to-point subnetwork, with
only two nodes.  Examples of point-to-point subnetwork protocols are

Packet-switched networks are typically general-topology, with wireline
networks conforming to CCITT Recommendation X.25 being one example.

Another general-topology subnetwork found in packet radio is the type
implemented by "TheNet" (from NordLink) and its clones.  Its network
layer offers a datagram service to an addressed point, not a useful
broadcast service, and again with no guarantee of delivery.

I.3.1.3.  Topologies found in packet radio.  Within the amateur packet
radio world, an example of a broadcast-topology subnetwork might be a
packet "LAN" implemented using a full-duplex RF repeater.  However,
these are relatively rare; the typical single-frequency radio "LAN"
using CSMA or Aloha operation, typified by AX.25 subnetworks, fails the
test of broadcast topology.  It loses too many packets for routing to be
able to trust unacknowledged delivery.  This is often due to the "hidden
transmitter syndrome" (HTS), the details of which are beyond the scope
of this document.  However, some steps may be taken in order to make use
of the near-broadcast nature of these "LANs".  These are "semi-
broadcast" subnetworks.

I.3.2.  Adjacencies and Subnetwork Access Points

The purpose of any subnetwork is to create adjacencies between IP nodes.
Each IP node is identified at the IP layer by its IP address.  Each node
in turn identifies each of its adjacencies as a Subnetwork Access Point
(SNAcP).  A SNAcP is identified within the router by the 2-tuple
{Subnetwork address, Hardware port}.  In the case of a point-to-point
subnetwork, the subnetwork address component is null.  In the case of
some other subnetworks, it may be almost arbitrarily complex.  For
example, an AX.25 "address" may consist of a string of addresses by
which the AX.25 subnetwork performs source-routed frame relaying.

Thus the actual routing information includes 3-tuples consisting of the
IP destination address or node group as the key, followed by the two
elements of the SNAcP.  The output of RSPF is this routing table.

I.3.3. Connection-oriented vs. connectionless subnetworks

IP is a connectionless (datagram) network protocol, and supports both
connection-oriented (virtual circuit) and connectionless (datagram)
lower layers (subnets).  In a network with a high packet loss rate, the
local retransmission provided by a connection-oriented datalink may
substantially improve overall throughput.  However, in a high-speed
dedicated backbone, particularly one implemented using full-duplex radio
or wireline links, connectionless links may provide better overall
performance.  The choice of which to use is a local matter; RSPF will
work with both.

**Connection-oriented subnetworks are assumed here to operate in assured
mode, as is provided with LAP-B.  No connection-oriented non-assured
subnets, such as wireline Frame Relay, are contemplated for packet radio
RSPF use at this time.

The reliability of the routing information, however, may be somewhat
greater with connection-oriented datalink procedures.  This occurs
both because the link will have more reliable propagation of network
state information, and because these will give more rapid indication of
a physical link failure.  In a true broadcast-topology subnetwork,
connectionless operation should suffice.   Much of RSPF's uniqueness
comes from permitting connectionless operation over unreliable media, a
combination that appears to be in much demand within the AMPRnet

I.4. Relationship to other protocols

All packets specific to RSPF shall be exchanged inside IP packets using
a protocol identifier which, pending formal assignment of one, shall be
73.  Such IP packets shall be sent with a time to live (TTL) value of 1.
If broadcast procedures are used over AX.25, connectionless AX.25 UI
frames shall be sent, with a broadcast address "QST-0" in the AX.25
address and **an IP address ending in [.255], indicating multicast.
This permits different ports on a router to have different broadcast
addresses.  (A router can also "read the mail" on passing radio packets
not addressed to it; such procedures are for further study.)  Equivalent
procedures may be developed for other subnetworks.

Note that in this document, "subnetwork" and "data link" are synonymous,
and refer to the layer over which IP packets are exchanged.

I.5.  Change history

I.5.1.  Changes for Version 2.1. (October, 1989)

RSPF draft 2.0 was released in June, 1988, as the first nearly-
implementable version.  It was first implemented in September, 1989 by
Anders Klemets.  Version 2.1 reflects changes whose need was discovered
during this implementation.  These changes are both editorial and, in a
few cases, substantive.

The format of the Routing Update packet was slightly modified.  In order
to prevent fragments of two or more different routing update messages
from being erroneously merged, an Envelope ID was added to each such
packet, with the same Envelope ID on all fragments of a multi-packet
message.  The term for such a message is now "envelope";  it contains
one or more "bulletins", each of which originated from a single router.

There are no longer separate packet types for Full Routing Update and
Partial Routing Update.  Instead, they are distinguished by the value of
the subsequence number, which is always 0 for Full Routing Updates and
is never 0 for Partial Routing Updates.  A given envelope may contain
both types of bulletin.

Cost is now set on the basis of receiver instead of transmitter.  This
permits the automatic link quality adjustment to operate on the basis
of locally-received traffic.

It is explicitly stated that upon creation of a new router-router
adjacency, the routers exchage full routing information.  This allows
routers to initialize themselves with a reasonably complete map of the

1.5.2.  Changes for Version 2.2 (January, 1992)

Version 2.1 was actually implemented and deployed in several locations,
providing a useful input to this version.  The changes in Version 2.2
are intended to be compatible, at the message level, with Version 2.1;
however, the internal organization is different.  This should preserve
interoperability in the face of several changes.  Some descriptive
changes were made in this document in order to make it less dependent
upon any one implementation or network.

The major change is the generalization of the subnetwork and the SNAcP;
this allows arbitrary subnetworks to be converged under RSPF.  Behavior
of "private" and "manual" routes is also now explicitly described.

The behavior of RSPF in acquiring new adjacencies is also clarified with
a new state machine.  A connectionless adjacency is never put into
general use until it is successfully tested.  This fixes an ambiguity in
Version 2.1 which led to the immediate creation of a "suspect" route
upon receipt of a Router-Router Hello message, even if it failed
successive pings.

Sequence number behavior for bulletins is also clarified, with a new
initialization procedure.  The number space is now linear and finite.

II.  Acquisition of router-router adjacencies

For RSPF to operate correctly, all routers must remain reasonably
current as to the state of the network at large.  This is handled by the
propagation of "bulletin" messages among routers.  End nodes need not
concern themselves with this; they will normally communicate through one
selected router at any given time, for all (non-adjacent) destinations
(not seen by ARP or other lower-layer procedures).  End nodes can also,
of course, connect to each other directly, bypassing RSPF.

Each router maintains an adjacencies table.  Each router's adjacency
table lists the following information for all other nodes, both routers
and end nodes, from which it directly receives packets over a

	Adjacent node IP address (i.e.,
	Adjacent node subnetwork (eg.,AX.25) address (eg., K1IO-0)
	Subnet (hardware) used (i.e., AX0)
	Subnet cost (i.e., 4)
	Number of packets heard since last RRH update (i.e., 50)
	Packet sequence number in last RRH update (i.e., 2593)
	Time of last RRH update (i.e., 2130).
        **Link state (tentative, good, suspect, lost)
        **Type of service (connection-oriented, connectionless)

           Table II.1.  Adjacencies table.

II.1.  Router-router hello

For the backbone to create its topology automatically, there must be a
way for routers to discover each other with minimal overhead.  For this
purpose, a router-router hello (RRH) message is provided.  Periodically,
based upon the value of timer "rrhtimer", the router sends out the RRH
message to the subnet multicast address and IP multicast address.  RSPF
makes no assumption of reciprocity (that links are bidirectional);
receipt of an RRH packet provides the receiver with information about a
potential one-way (received) adjacency.

It is noted, however, that while the route testing procedure does not
require reciprocity of subnetworks for "ping" testing, some
implementations will require a successful two-way ARP exchange before
the ICMP Echo packet can be sent.  This often results in some
requirement for reciprocity in practice, even though it is not a
characteristic of RSPF.

II.2.  Connection-oriented procedure

If a router uses connection-oriented subnet procedures on a given
interface, then when a router receives an RRH packet on such an
interface, it checks to see if it already has a link to that packet's
originator in its own **adjacencies table.  If not, and the interface is
of a broadcast nature (reliable or unreliable), it waits a random
period of time (example for AX.25:  within the range of 0 to 10 times
the link's value of T1, DWAIT or SLOTTIME, and in any case much longer
than the timers used within a CSMA or Aloha subnet such as AX.25) and
then attempts to establish a connection in the usual manner.  For
example, in an AX.25 subnet, the router initiates the connection with
SABM; the distant router responds with the usual UA (link established)
or DM (link rejected).  Failure to initiate a connection (i.e., receive
a UA) terminates this procedure; no adjacency is added.

If a two-way connection has been established, then both routers add the
link to their adjacency tables.  While the existence of this route is
set reciprocally, the cost of each side of the route is set separately
for each side of the connection.  Cost is propagated in the routing
update (link state) packet.  Thus the adjacency between two routers is
not actually used for real traffic until the first routing update packet
exchange has taken place.  (This may be useful in dealing with the
situation where a UA is lost during link initiation; procedures for
coping with this known LAP-B problem are for further study.)

Loss of an adjacency may then be noted by the loss of the subnet
connection.  When this occurs, the router removes the adjacency from its
adjacency table and follows the "bad news" procedures outlined below for
link state propagation.  (If this cannot be determined by the
implementation, then "ping" procedures as in 1.3.2 below may be useful.)

II.3.  Connectionless procedure

If a router uses connectionless datalink procedures to its own
adjacencies (most suitable to low-loss links such as broadcast-topology
or reliable general-topology subnetworks), then when a router receives
an RRH packet, it checks to see if this adjacency is already in its
adjacency table.

II.3.1.  Acquisition of connectionless adjacencies

**In practice, reliable broadcast topology is unusual in the packet
radio arena.  Thus, the acquisition of a new adjacency begins by setting
the link status to "**tentative".  A tentative-state adjacency is tested
by attempting to exchange a packet (ICMP echo) with it, up to a settable
"maxping" number of times.  If successful, the link state is raised to
"good".  If unsuccessful, the adjacency is returned to null, ignored,
and not added to any tables.  **If the destination is already reachable
via a different route, then this prior route should not be discarded; it
should be returned to use if the newly-tested adjacency fails, or if the
new adjacency has a higher cost.  (This may be implemented with a "don't
route" function in IP, bypassing the existing IP route table, passing
normally-hidden subnet information to and from RSPF.)

**An alternative to ICMP echo for AX.25 and other broadcast and
semi-broadcast subnets is to directly use ARP to test the adjacency.  In
practice, ICMP will require ARP anyway.  This option raises questions of
layering, but may solve other problems, such as the existence of a prior
IP route when the available IP layer does not support "don't route".

II.3.2.  Loss of connectionless adjacencies

Loss of an adjacency may be noted by timeout; if no RRH message is
received, and no frames have been received from the adjacent router for
a period of time "suspect timer" (initial suggestion:  slightly over
twice the maximum interval "rrhtimer" between RRH messages), then the
adjacency state becomes "suspect".  The router should attempt (a
settable number of times "maxping") to exchange a packet (ICMP echo)
with the suspect adjacency;  if unsuccessful (after the usual number of
retries), the adjacency is marked "lost".  It may also be marked lost if
other attempts to send data through that router fail, such that the
implementation determines that there is a high probability that the link
is lost.  (However, no obvious means of doing this is noted.  Thus the
"ping" is more likely to be required.)

II.3.3.  Link states

For purposes of route table generation and link state propagation, a
link whose state is "tentative" or "lost" is "bad";  bad routes are not
used in creating the routing table.  Tentative routes are not
propagated;  newly bad ("lost") routes are propagated by the link state
("bad news") propagation procedure, and then dropped.  Links whose state
is "good" or "suspect" are used for route table generation and are

Note that if a link is connection-oriented, there may be no need for
either a suspect or tentative state; these links are either "good",
"lost", or "null".

**Table II-1.  State transition table for adjacencies.

   (Null) ----------> Tentative -----------> Good <----\ traffic or
           RRH heard     |   ping succeeds    \  \-----/ RRH heard
                          \                    \--------->Null      \---| no RRH/tfc heard
                           ping fails  \            v

II.4.  Manual procedure for general-topology subnetworks

A general-topology subnetwork may have adjacencies which cannot be
detected by means of a broadcast-oriented (RRH) procedure.  These links
cannot be automatically added, and alternative procedures are required.
These links may be connection-oriented or connectionless; the link state
transition procedure is determined accordingly.

The most general way to handle this is to insert these adjacencies
manually.  The state of these adjacencies is determined the same way as
with links that are created by hearing RRH messages.  Note that when an
adjacency is manually-initiated across a general-topology subnet, the
initiating side of the link takes action, while the responding side
should accept the incoming connection request, and both sides see the
link.  Thus only one side of the adjacency actually needs to have a
manual entry.  If a connection already exists to another node across a
subnetwork, then there is no need to initiate a second connection simply
because a manual entry exists.

**In some cases (such as source-routed frame relay "digipeating" over
AX.25), a manual route can be entered to a destination whose first hop,
at least, crosses a broadcast-style subnetwork.  This type of adjacency
may require manual intervention in the ARP function as well.

II.4.1.  Semi-automatic discovery

In addition to manually-created links across a subnetwork, there exist
cases in which the RSPF router is also a subnet router, and is thus
privy to routing exchange information generated by that subnetwork.  (An
example is a node which implements both RSPF and "TheNet".)  If the
subnetwork provides sufficient information such that support of RSPF by
another node can be inferred (i.e., by a distinctive subnetwork
address), then RSPF may sequentially initiate appropriate procedures to
determine whether or not a useful adjacency exists.

Translation of routing metrics into an RSPF link metric is handled on a
case-by-case basis.  Specific convergence procedures for any such
subnetwork require further study.

Table II-2.  Coding of the RRH PDU.

				  1	 	  2
 |0              |8              |6              |4              |
 | RSPF Version #| Type (RRH)    | Checksum                      |
 |         Full IP Address of sending router                     |
 |  last packet sent seq. #      |  flags        |
 |        plaintext                                              |...


  An RSPF Router-Router Hello packet is sent within IP with a protocol
  ID of .  Each RRH packet contains the following

   RSPF Version Number:  Version number of this protocol "22".
** This packet format is identical with version 21; RSPF22 should
   accept "21" as valid.  Future versions "30" and above may be
   incompatible.  (Accept anything 2x on receive.)

    Type:  Value of 3 for RRH.

    Checksum:  IP-style checksum

    Address:  Full IP address of sending router

    Last packet sent sequence number:  An integer (mod 65536)
    incremented by 1 for every frame sent by the subnet across
    this interface.  This value allows receiving entities using
    connectionless procedures to use the automatic link quality
    measurement technique described in II.5.

    Flags:  The low-order bit is 1 if connectionless datalink is
    preferred on this interface; 0 if connection-oriented is preferred.
    (Set by system management based upon anticipated link quality.)
    Other bits are reserved (sent 0; ignored on receive).  Note that
    some implementations do not support this; "preferred" does not
    mean "mandatory".

    Plaintext:  An optional text message (length may be up to maximum
    size remaining in datalink PDU).  This might serve, for example,
    to "broadcast" the router's existence to persons who might be
    "reading the mail" (monitoring a radio channel promiscuously).
    **It may be noted that a very short RRH message can be received
    intact across a subnetwork that otherwise might not be capable of
    supporting maximum-size packets with reasonable reliability.  This
    should be taken into account when setting the value of Plaintext.
    An explicit minimum is not, however, stated.

II.5.  Automatic link quality measurement
(This procedure was not tested in the implementation of RSPF 2.1, and is
therefore considered highly experimental!)
A connectionless link or subnetwork may have very reliable, or very
sporadic, performance.  Since there is no procedure for ensuring the
reliability of packets sent over a connectionless link, a high rate of
packet loss may occur without being detected by the routers.  If this
loss is high enough, another route may become a better choice; a high
enough packet loss rate may be enough to mark a link as "down".  The
automatic link quality measurement procedure allows links which are not
yet "down", but whose performance is substandard, to be noted.

Every router shall maintain, for each link, a count of all packets sent
over each link.  Every time an RRH message is sent, it includes the
current value of this counter (modulo 65536).  Every router also
maintains, in its adjacency table, a count of the total number of
packets received from said adjacency since the last RRH message, and the
value of that counter as received in the last RRH message.

Upon receipt of an RRH message, the recipient router compares the value
of the received packet counter with the last received value in the
adjacency table.  The difference (taking into account wrap-around at the
modulus) is compared with the number of packets received since the last
RRH message.  (This works even if an RRH message is lost.)  This packet
loss ratio is then maintained as a guideline to determine link quality.
If link quality falls below a settable threshhold, the link state is
changed to suspect.  Timestamp can also be used to compute packet
arrival rate.

Connection-oriented data links presumably deliver 100% of attempted
packets.  A high-quality connectionless link, such as Ethernet/LLC1,
will come close to this.  However, single-frequency packet radio links
are prone to packet loss for several reasons, including hidden
transmitters, lack of collision detection, and rf interference.  The
packet loss ratio is useful in setting link cost, and may also be used
to determine whether a link should use connectionless or
connection-oriented procedures.

If a router reports, in its link update packets, that a given link has a
cost of _n_, then its adjacencies (and only its adjacencies) may apply
the packet loss ratio to adjust the cost which they maintain in their
link state tables.  These adjusted costs, rather than the received
costs, may then be propagated to other routers.

Such procedures should be applied sparingly, as each change must be
propagated and could, if used too frequently **or inconsistently across
a network, result in network-wide instability.  A suggested
(experimental) algorithm is as follows:  A percentage threshhold x is
set, as is a cost-adjustment factor y.  If fewer than x% of packets are
received during a measurement interval, the cost of that adjacency is
multipled by y.  For example, if x is 80 and y is 1.5, then an adjacency
with a nominal assigned cost of 4 will be up-costed to 6 if only 70% of
packets are received, but will be restored to 4 if 80% or more are
received during the next measurement interval.  The measurement interval
is the time between RRH messages, which typically should precede routing
update messages.

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.)

IV.  Link state propagation

Link state information is propagated between routers within bulletin
envelopes, which are sequences of packets containing partial or full
copies of the sending node's link state table.  Both point-to-point and
broadcast procedures are provided.

IV.1.  Optional multicast/broadcast

Packet radio is inherently a broadcast medium.  Packet radio networks,
however, do not necessarily offer a reliable broadcast service, even if
they happen to use a broadcast physical medium.  It is still possible to
exploit the broadcast nature of the medium.  RSPF link state propagation
procedures allow but do not require such multicasting.

If the link uses connectionless procedures for user data packet
exchange, then broadcast procedures should be used for link state packet
exchange.  The converse may not necessarily be true:  The threshhold of
loss that militates against connectionless transmission of user data may
be more stringent than that which call for non-broadcast transmission of
link state packets.  Note that RSPF specifically permits its broadcast
procedures to be used over subnetworks that do not have the reliability
of true broadcast-topology subnetworks.  This reduces the channel
utilization on radio links.

IV.2.  Routing update bulletins

Routing updates are passed along from router to router via routing
update bulletins, which are broadcast within routing update envelopes.
Bulletin propagation is designed to make it highly likely that every
node within a given "horizon" eventually receives every routing update
message sent out by a given node.

IV.2.1.  Sequence numbers

Every router originates information about changes in its own
adjacencies, as well as periodic retranmissions of its entire list of
adjacencies.  These bulletins are then propagated among other routers.
The router whose adjacency information is being broadcast is called the
_reporting router_; this may be some hops away from the routers which
forward it to their own adjacencies.  Each reporting router's bulletins
(adjacency updates) contain sequence numbers (and in some cases, a
subsequence number).  These sequence and subsequence numbers are
generated by the reporting router and propagated; they are not changed
when a bulletin is relayed.  They are incrememted by 1 every time a new
one is generated.

IV.2.1.1.  Restored sequence numbers

**If RPSF is restarted, after having been run previously run (i.e., in the
event of a system restart) before all knowledge of that router has timed
out of the network, then sequence numbers beginning with 1 could cause
the network to fail to converge, as these bulletins will always appear
obsolete.  A procedure is needed for routers to "catch up" with its
previous sequence number if it is still in the network.

**When a router first goes into service, its sequence number is
initialized at 1 (or another low nonzero number).  The router sends RRH
before it sends its first bulletin.  This enables it to first receive an
update from its own adjacencies.  Even if it sends a bulletin before
receiving one from its adjacencies, sending a bulletin with a sequence
number of 1 will prompt the adjacency to update it.

Whenever a router receives a bulletin, it examines the contents to see
if its own router number is included as a reporting router.  If so, then
it resets its own sequence number to 1 greater than the sequence number
received.  This enables the router to resume its sequence number
generation at a higher number than where it left off.  A value of 0 is
never used for reporting.  However, a router shall respond to receipt of
a bulletin with a sequence number of 0 with an update containing the
current sequence number.  (The 0 is thus reserved for use as a "poll".)

IV.2.1.2.  Sequence number exhaust

**Modular (circular) numbers are not used in RPSF Version 2.2.  Thus a
router may eventually exhaust its 16-bit number space, if it is in
continuous operation long enough (nearly two years, given 15-minute
updates).  Should this occur, the router may reinitialize itself by
halting all bulletin origination for a period long enough for the entire
network to "forget" about that router's existence.  Pending further
study, a period of two hours is recommended for RSPF in a typical packet
radio environment.

IV.2.2  Subsequence numbers

Bulletins may also carry change information incremental to previous
bulletins.  These carry the same sequence number as the full routing
update bulletin to which they are reporting incremental changes; each
such partial routing update bulletin has a subsequence number.  The
subsequence number is zero for a full routing update bulletin.

IV.2.3.  Horizon

Every bulletin also has a horizon value, set by the reporting router,
associated with each of its constituent links.  (A given reporting
router may have more than one constituent link, if it is a multi-port
router.)  Every time a bulletin is propagated, each horizon value is
decremented by 1.  When it hits zero, the bulletin is not propagated
further.  Note that for horizon purposes, a bulletin's individual
constituent links may have separate horizons; when a link's horizon hits
zero, other links' adjacency information from the same reporting router
may continue to be propogated.

It should also be noted that if a given link has adjacencies with
different horizons, these must be treated as separate links, since
horizon is reported as a characteristic of a link.  Such a circumstance
may occur, for example, when a router serves a node group.  Adjacencies
within the node group are typically propagated with short horizons,
since they are only of local interest (i.e., to other nodes in or near
that node group).  Similarly, the node group itself is propagated with a
long horizon, across a backbone.  However, adjacencies outside the node
group may be propagated with long horizons, as they would not otherwise
be reached across a backbone dependent upon node groups for long-haul

IV.2.4  Routers table

Every router maintains within memory a routers table containing one
tuple for every other router on the network, excepting those which are
unseen because of a horizon.  (An extreme case of this exception is an
end node running RSPF with a horizon of 1, whose existence is only known
to its own adjacencies.)  This tuple contains the following elements:

	IP Address (router number)
	Last received bulletin sequence number
	Last received bulletin subsequence number
	Timestamp for when the data was received.
	Horizon remaining in bulletin when received

This table is used to coordinate the receipt and transmission of
bulletins, using either broadcast or non-broadcast procedures.  **If a
router chooses to use multiple IP addresses (as is commonplace on the
Internet where different interfaces may use different addresses), only
one IP address is used by each router for propagating link state
information.   This is used as the router number.

IV.3.  Flooding without congestion

A procedure is used to "flood" the network with link state information.
This is designed to minimize the total amount of information
transmitted, while maintaining an up-to-date data base on each router.

IV.3.1.  Upon initialization of adjacencies

Bulletins are forwarded in a routing update envelope which may contain
one or more reporting routers' bulletins (adjacency lists), which are
flooded to the network.  A bulletin envelope may actually concatenate
multiple reporting routers' bulletins; this is in fact the typical case,
when routing update packets are exchanged on schedule, and when a given
router acquires a new adjacency.  However, partial routing updates (good
news and bad news) may be sent at any time.

The first time an adjacency is acquired, the two routers perform a full
routing update with each other, exchanging their complete link lists.
Once this is complete, the routers perform the SPF algorithm and compute
new routing tables.

IV.3.2.  Preventing loops and retransmissions

A bulletin from reporting router X (which lists all of X's adjacencies)
is sent, initially by X, to every adjacent (to X) router A.  This router
A passes it along to all of its own adjacencies B, etc.  This flooding
is controlled such that no reporting router's adjacency update is sent
more than once between the same pair of routers.

After router A sends its bulletin envelope to B, the recipient router B
then looks at the sequence number of each reporting router X's adjacency
bulletin within that envelope, and for each reporting router's bulletin,
compares the sequence number of the just-received adjacency bulletin
with the highest sequence number previously originated from X.  If the
just-received bulletin is newer (higher number), then it [**for further
study: sends a positive acknowledgement to A, and] joins in the flooding
to its own adjacencies.  The horizon is, of course, decremented.

If the bulletin from X has the same number as was stored in B, B checks
the horizon left in the received bulletin against the horizon left when
it previously received the bulletin.  In the event that the latest
bulletin received had a shorter (lower number) horizon left than the
earlier one, it simply ignores [and acknowledges] the (redundant)
message.  But if the reporting router X's horizon left is greater for
the new copy of the bulletin, router B propagates it as if it were a new
bulletin. This way, if the bulletin happened to first arrive via a
roundabout path, it won't accidentally fail to reach the intended end of
its range. (Summary:  Newer or longer-horizon bulletins are both passed
along.  Same age or older (by sequence number) or same or lower horizon
will not be propagated.)

If any router B receives from router A a bulletin (from any reporting
router X) that contains a lower sequence number than one that had
previously arrived at B from said X, then it can be presumed that A has
"obsolete" information.  B replies by sending a bulletin to A with the
latest link state information for that node X.  As a corollary, a router
may poll for information about a given reporting router by sending a
routing update bulletin for that reporting router with a sequence number
**of 0.  Said bulletin may contain zero links' information.

IV.4.  Point-to-point bulletin envelope exchange

A router may acquire routing information from adjacent routers via
point-to-point procedures which treat every adjacency as a separate
logical data link.  (Such a procedure also works, of course, over
point-to-point links such as wirelines.)  This tends to have the highest
reliability, since connection-oriented data link control procedures can
be used to ensure the accuracy and completeness of the data passed over
the link.  It has the disadvantage of requiring separate transmission of
the same data to different adjacent nodes on the same radio channel.

Bulletin envelopes are thus exchanged (when connection-oriented is
selected) periodically (based upon timers) and when new information is
received (over a new adjacency, and when good and bad news arrive).
Delivery of these bulletins is considered to be generally reliable.

IV.5.  Broadcast bulletin propagation

When a router is adjacent to several other routers via the same
broadcast (i.e., packet radio) channel, retransmission of routing
bulletins to each adjacency makes additional use of bandwidth, which may
be a scarce resource.  A broadcast procedure may be used to pass the
bulletin along that link.  Note that broadcast propagation of bulletins
may be combined with non-broadcast propagation, on a link by link basis.
Although packet radio is a broadcast medium, it is not a perfect one:
The reliability of multicast packets is not as high as for wireline
LANs.  This leads to the need for a unique broadcast "flooding"
technique.  Note that in a true broadcast-topology subnetwork, these
procedures add little channel overhead, so these procedures are
applicable there as well.

IV.5.1.  AX.25 subnetworks

**At this time, only the AX.25 subnetwork is widely used in AMPRnet
while providing a multicast/broadcast service.  Similar procedures may
be adapted for use elsewhere.

A routing update bulletin is broadcast to the Layer 2 multicast AX.25
address using the IP multicast address.  Individual recipient routers
may or may not receive the entire bulletin, since there is no
acknowledgement provided.

In the previously described case of a non-broadcast message sent via a
connection-oriented datalink, the entire routing update bulletin can
be assumed to have been received intact.  Thus, if a given reporting
router has originated a new complete list of its adjacencies (new
sequence numbers, subsequence numbers are 0), then any adjacency not
included is presumed to have ceased to exist.  Such a presumption is
not always possible when a bulletin was received via unacknowledged
broadcast, as the message might have been received in part.  This
should not make the partially received bulletin unusable.

A bulletin envelope is transmitted in one or more packets, each of
which constitutes a numbered fragment of the whole bulletin envelope.
Within the transmitted bulletin envelope, each individual reporting
router's bulletin begins with a node-header which contains the number
of links being reported on.  Within each bulletin, each link is
identified by a link header, each of which specifies the number of
adjacencies being reported on (for that link).  Since each IP packet
that makes up a bulletin contains a fragment number, it is also
possible to determine whether or not a fragment was lost.  Each packet
also contains an envelope-ID, which prevents accidental mis-assembly
of different bulletins that may arrive close in time.

In connection-oriented non-broadcast procedures, a routing update
bulletin (not a partial one with a subsequence numbers>0) provides a
complete list of adjacencies known to the sending node, except where the
horizon is exceeded.  Absence of a previouly-known adjacency then
implies that the adjacency has been lost.  However, that assumption does
not apply to fragmented bulletins received in part, which can occur with
broadcast procedures: If a fragment was lost before reaching the end of
a given reporting router's bulletin within the bulletin envelope, then
the absence of a previously-known adjacency in the received bulletin
does not mean that the adjacency was lost.

RSPF procedures dictate that routing update bulletins with a subsequence
number of zero are presumed to be complete lists of adjacencies from
their originators; higher subsequence numbers represent incremental
changes only.  In the broadcast procedures, a routing update bulletin
with a subsequence number of zero, if received only in part, is
treated as a partial routing update bulletin.  If it received in full,
it is treated as a full routing update bulletin.

Thus, the recipient compares the sequence number with the previously
received sequence number from that reporting router.  If it is higher
than the previously received one, then its adjacencies are used.  If
it was received in full, and had a subsequence number of 0, then its
previous adjacencies are replaced (removed if not contained in the
latest full routing update bulletin).  If it was not received in full,
or if its subsequence number was not zero, then its previous adjacencies
are left intact unless explicitly changed by the received bulletin.

If a bulletin is received in full, then the routers table is updated
with the latest sequence and/or subsequence number, horizon, and
timestamp.  If it is received in part, then these entries are not
updated.  After the bulletin has been completely transmitted, a
recipient node which has received a partial update from a reporting node
may poll for that node's latest information, by **originating a bulletin
naming the reporting router in question, specifying sequence number 0,
with zero links for that reporting router.  (This is the same polling
procedure used for non-broadcast links.)

Note that if a fragment is lost, a reporting router whose node-header is
in the lost fragment will of course remain unchanged in the recipient's
data base.  Furthermore, any data received in subsequent fragments,
prior to a node-header, will also be meaningless.  The last adjacency
of the last link in a reporting router's bulletin will have the Last
flag set to 1, signaling that following the address, a node header
follows.  Note also that the first node-header within an envelope is
pointed to by the sync byte in the envelope header.  The last flag and
sync byte should match or an error should be noted.

**IV.5.2.  Reconstructing bulletins from multiple fragments

If the resent bulletin is the same one as previously received in part,
and it too is received in part, then if all of the previously received
fragments were stored, the receiving router may attempt to reconstruct
the entire bulletin from the two (or more) fragmented transmissions.
Once an entire bulletin has been reconstructed, the receiving router may
treat this as a complete update.  (This procedure is optional.)

**IV.5.3.  Non-broadcast unreliable subnets

If an adjacency is established across a general-topology subnet that
does not offer reliable packet delivery (eg., TheNet packet level), then
broadcast procedures should be used to exchange routing information
across the subnet between the two adjacencies.  If the entire bulletin
is received intact, then it should be used; if it is received in part,
the received portions may be used, and the recipient may poll for a
retransmission as in IV.5.1 and IV.5.2 above.

IV.6.  Routing update bulletin packets

A routing update bulletin envelope (Table IV.1) may contain several
different reporting routers' updated link state information,
concatenated into one message, with a different sequence number for each
source (reporting router).  One of the sources, of course, may be the
local router.  Each router's link state information is further broken
down by link, which allows its backbone routing information to be
propogated separately from its local end node adjacency information.

IV.6.1.  Node group reduction of adjacency list

If an end node establishes a connection with a router whose node group
default addresses (based on the significant bit count) already include
that end station, no bulletin need mention that adjacency, as packets to
that end station will already be routed correctly.  Node groups are
defined manually.

IV.6.2.  Incremental changes (good news and bad news)

Bulletins that only report changes in state come in two flavors.  Good
news bulletins inform other routers that an adjacency has been noted.
bad news bulletins inform them that an adjacency has been lost.
Theoretically, a router could send out a new full routing update
bulletin every time it gained or lost an adjacency. However, the use of
shorter good news and bad news packets, which do not contain a full
routing update, allow the network to remain relatively current with less
transmitted traffic.

Good news and bad news packets are propogated like other packets, but
are not originated by the same rules.  While full routing bulletins are
originated based upon a timer (rspftimer), good news packets are
transmitted immediately upon receipt and initiated immediately after an
adjacency is initialized.  This enables new links to be useful quickly.
Bad news, however, should not travel as fast:  A node should cache any
bad news message for a time (**recommendation for this timer:
rspftimer/16) while waiting for the link to come back up.  This helps
keep the network stable; if the node receives a packet destined for the
lost destination, it may send an ICMP "host unreachable" message to the
originator of the packet, unless it can reroute the packet itself (as
may happen with the loss of a backbone link when others still exist).

Because good news and bad news messages represent changes to the last
full link state bulletin propogated, but do not purport to completely
represent a node's link states, they carry bulletin subsequence
numbers.  These use the last bulletin sequence number originated by the
reporting router, but the sub-sequence value increments from 1. (A full
link state packet has a sub-sequence value of 0, and resets the
subsequence counter.)

IV.6.3.  Routes to nearby destinations

Sometimes more than one router will serve the same area (determined by
the significant bits' matching), and they will need to know which one has
the better path to a given station.  These adjacency messages may only
require a short horizon, as will good news and bad news packets which
refer to end nodes going on and off the air.   Higher horizons are
needed for backbone routers.

**This distinction allows routers to define their service area and role
within a network by appropriate horizon adjustment.  A router that only
uses short horizons may be useful for providing connectivity within a
constrained geographic area, typically encompassing one or a small
number of node groups, in which not all end users have full connectivity
to a single router.  Such a router will set its horizon_link, reporting
on other routers, to a low value.  A router that serves as a backbone
node, propagating node groups across a wider horizon, will have a high
horizon_group, reporting node groups, value.  (**Horizon_link and
horizon_group values are usually set the same.  Horizon_portable is
usually set to the same value as horizon_group and horizon_link;
horizon_local is set to a low value, so that even a backbone router will
not propagate intra-node group information across the backbone.)

Table IV.1.  Routing update (link state packet) bulletin envelope:

				  1	 	  2
 |0              |8              |6              |4              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ----
 | RSPF Version #| Type          | fragment #    | fragment total| packet
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ -hdr
 |       Checksum                | sync byte     | # nodes below |
 |  Envelope-ID                  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ----
 |         Reporting node #1 full IP Router-Address              | node
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ -hdr
 |  Node 1 bulletin  sequence #  | sub-sequence #| # links       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ----
 | horizon left   |  ERP factor  |  link cost    |  #adjacencies | link
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ _-hdr_
 |significant bits|
 |              Adjacent node(s) 1,1,1 IP address                | adj.
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---
 |significant bits|
 |             Adjacent node(s) 1,1,2 IP address                 | adj.
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---
 |significant bits|
 |             Adjacent node(s) 1,1,n IP address                 |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---
 | horizon left   |  ERP factor  |  link cost    |  #adjacencies | link
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---
 |significant bits|
 |             Adjacent node(s) 1,2,1 IP address                 | adj.
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---
 |significant bits|
 |             Adjacent node(s) 1,2,n IP address                 | adj.
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---
 |         Reporting node #2 full IP Address                     | node
 |  Node 2 bulletin sequence #   | sub-sequence #|  # links      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---
 | horizon left  |  ERP factor   |  link cost    |  #adjacencies | link
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---
 |significant bits|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ adj.
 |             Adjacent node(s) 2,1,1 IP address                 |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---
 |significant bits|
 |             Adjacent node(s) 2,1,2 IP address                 |
 | horizon left    |  ERP factor |  link cost    |  #adjacencies |
 |significant bits|
 |             Adjacent node(s) 2,2,1 IP address                 |
 |significant bits|
 |             Adjacent node(s) 2,2,n IP address                 |
 |         Reporting node #n full IP address                     |
 |  Node n bulletin sequence #   | sub-sequence #|   # links     |


  An RSPF bulletin packet is sent within IP with a type of .  Each routing update envelope
  contains an envelope packet header that contains:

    RSPF Version Number:  Version number of the protocol (22).

    Type:  (Value 1 for Routing Update Bulletin Envelope)

    Fragment Number:  States which fragment, in a segmented message,
    this is, beginning at 1.  Non-fragmented messages use 1.
    Fragment total:  Total number of fragments in message; 1 if not

    Checksum:  IP-style checksum.

    Sync byte:  Which octet in this packet (counting from this
    byte as byte 0) is the beginning of the first node-header.  If 0,
    this fragment has no node-header.  Non-fragmented messages
    use a value of 4 (because 3 bytes follow in packet header).

    Number of nodes reporting:  The number of reporting routers in the
    messages that follows (this packet or a sequence of packets forming
    the envelope).

  The node-header (for each reporting router) contains 8 octets:

    Reporting router #n full IP router address:  The IP address of
    the router whose adjacencies are being reported below.  (Note
    that if a router uses separate IP addresses on its links, it
    should still adopt a single one as its router address.)

    Bulletin sequence number:  When a bulletin is passed along, this
    number is NOT changed; every new bulletin from a given Reporting
    router has a value 1 higher than the previous bulletin from that
    reporting router.
    Sub-sequence number:  Good news and bad news packets representing
    incremental changes from the last full report increment this value
    by 1; it is 0 for full bulletins.

    but not necessarily, representing different types of link in a
    mulitiport gateway) being reported below; the following four octets
    are the header for each link.

 [For each reporting router, adjacencies are reported separately by
  link cost.  This is the received cost value for that data link, after
  any adjustment that may be based upon packet loss ratio.  Adjacencies
  are also reported separately by horizon, even if the cost is the same.
  It does not matter at this point if there are multiple RF or other
  links if their cost and horizon are the same.  Likewise, separate
  received costs or horizons on one link will be treated separately
  and such adjacencies will be grouped under separate link headers

    Horizon left:  This number is decremented every time a routing update
    bulletin is passed along; when it reaches 0, it is not passed further.

    Link cost:  A "figure of merit" for each link, rising with
    slower/poorer links.  This is the number whose total is minimized
    by SPF.  The range is 1-127.  As a starting point, a 56000 bps fdx
    backbone link might have a value of 2, a 4800bps hdx link a value
    of 5, a 1200bps hdx link a value of 10 and a 300 bps hf "wormhole"
    a value of 20.  An Ethernet or high-speed (1 Mbps+) link might
    then have a value of 1.  A value of 255 denotes the loss of a
    link; this is found in Bad News packets.  These values should be
    coordinated network-wide;  adjusting them will change the way
    packets are routed by RSPF.

    Number of adjacencies:  The number of adjacencies to be listed for that
    reporting node.

    ERP Factor:  Used for "approximating" a route; contains the number of
    significant bits for which a given node can be presumed to be a valid
    router, even if it doesn't report in detail.  A low factor = wider
    coverage area; thus ERP of 16 means that if NO other match is found for
    a given address, this router will try to handle it if it matches 16
    bits.  Basically a handle for future enhancements; its use will not
    be required in this release of RSPF.

  For each adjacency of the given link cost, the following is provided:

    Significant bits: The number of bits used for node group routing
    purposes.  Usually 32 but may be lower if a given link purports to
    serve all end nodes in an area defined using the most-matched-bits
    node group convention.  Higher numbers of bits matched take a higher
    priority than least cost.  This uses the low-order 5 bits of the

    Last-flag:  If this is the last adjacency in the list for this
    reporting router, this value is 1; otherwise it is 0.  (This
    occupies the high-order bit of the significant bits octet.)

    Full IP address: The full IP address for this node.

Note that the n,n,n vector within the bulletin has three fields in the
above representation: Reporting router within bulletin envelope, link
cost/horizon within reporting router's bulletin, and reporting adjacency
with that link cost/horizon.

IV.7.  Fragmentation

In a moderate to large network, a full routing update can easily exceed
the maximum size of an AX.25 frame or IP packet.  The RSPF Routing
Update message, however, may be sent in fragments.  The IP fragmentation
function is not used for this; bulletins are not assumed to be
terminated by a packet boundary.  Each fragment is, however, numbered in
the packet header, along with an indication of the number of the total
number of fragments in that envelope.

In order to permit parsing of the incoming fragments by routers who
are using unacknowledged broadcast information (with the high
likelihood of lost fragments), every bulletin's packet header contains a
sync byte indicator.  This indicates which byte, where the next byte in
the header is byte 1, is the beginning of a node header.  If a previous
fragment was lost, the receiver should ignore the number of bytes
indicated in the sync byte before resuming parsing of the packet.  (If
the fragment does not exceed 255 bytes, this will always be sufficient.
It is recognized that long packets may not be able to use this mechanism
reliably, and a value of "0" should be used to indicate that no sync is
possible within this fragment.)

Each routing update bulletin envelope takes the form of a three-
dimensional list, with the dimensions being reporting router, link and
adjacency. A given bulletin envelope may report on link state from one
or more remote nodes, as well as from the sending node.  Each node may
have one or more links; each link may have one or more adjacencies.

Packets may not be fragmented within adjacencies, but may be
fragmented after an adjacency's address and before the next adjacency's
significant bits field.  (This presents a 5-octet field that should
not be fragmented.)  The next fragment, in a new packet, simply begins
where the last one left off; the receiver knows how much more to
expect based upon the node and link count in the respective
node-header and link-header.

A router may not start sending a new Routing Update message of any kind
to any given IP address until all fragments of a previous message to
that address have been transmitted.  This applies both to point to
point (non-multicast address) and multicast procedures.

IV.8.  Bulletin Timers

The timers used for bulletin updates must be a compromise between
maintaing the network's current state and the traffic required to do
so.  An initial suggestion:  Good news messages should be initiated
within a few seconds and bad news should wait at least **rspftimer/16,
with relatively short horizons on the bulletins (i.e., the routers
within the region).  Full routing updates with normal horizons should be
sent out every rspftimer interval (typically 30 minutes).  If the
network is small, more frequent updates may be possible; too frequent
updates risk choking the network with update traffic.

V.  The Shortest Path First spanning tree algorithm

As a routing node comes onto the network, it acquires over time a
complete list of adjacencies between other nodes on the network except
as limited by the "horizon".  Each adjacency (point to point link)
within the entire known network has a "cost" associated with it.  It
should be noted that adjacencies, for the purposes of this algorithm,
are "logical" and not necessarily physical;  if a subnetwork link
occurs below IP (as in AX.25 digipieating or TheNet), the two ends of
the link are still adjacent.  (Adjacency at the IP internet layer is
based upon subnetworks, which may contain their own links.)

Cost is set for the receive side of each link.  If the receiver and
transmitter do not agree on cost, the network shall apply different
routes for packets going in opposite directions between the same two
end nodes.  (This is not a problem.  In a radio environment, one
should NOT assume reciprocity across a link.)

V.1.  Tables

V.1.1.  Link State Table

Each router builds a _link state table_ (or links table) that lists, for
every known link in the network (from every reporting router), the two
ends and the cost of that end of the link.  The ends are listed by an IP
router address and, for the destination IP router address, a number of
significant bits.  This is what is updated by the bulletins and by good
news/bad news messages.  Adjacent links also are merged in from the
adjacencies table.

	Source IP address	Dest. IP addr/bits	Cost		5		10

V.1.2.  Paths table

The goal of the SPF algorithm within RSPF is to build a _paths table_
which lists, for every reachable node (or node group approximation of
fewer than 32 bits) on the network, that node address (or node group
address and number of significant bits), the adjacent node used to get
there (i.e., where you blast the packets to next), and the total cost of
the path.  This is done by building a spanning tree with the router doing
the computation (the home router) as the root of the tree.  The paths
table thus lists the best way across the tree from the home router to all
known destinations.

V.1.2.1.  Route table

**It should be noted that the only implementation of RSPF to date is
within the KA9Q NOS package.  This includes a route table which roughly
corresponds to the paths table, trusting ARP to provide subnetwork
information.  However, RSPF routing makes nominal use of a logical route
table which includes both the paths table entries and the subnetwork
address required to reach the first adjacency.  The structure of this
table's implementation need not be defined here.  It may, for example, be
a logical union of the paths table relation (providing IP addresses) with
ARP and related subnetwork table relations, using IP address as the
common key.  (Thus there is no RSPF route table per se.  This corresponds
to the logical route table in RSPF 2.1.) Or RSPF may supersede these
tables with a separate route table including required hardware and
subnetwork address information.

Entries in the route table include:

        Destination IP Address
        Destination IP Address number of bits (0-32)
        Selected adjacency hardware interface
        Selected adjacency subnetwork address string
        Adjacency IP address

V.1.3.  Trial table

Every router contains, for the purposes of building the tree, a
_trial table_ that has three entries:  Destination address/bits,
adjacent node, and cost of this path.  The paths table additionally
has one more entry, the _parent_ node, which is the last hop
before the destination.  Thus in a path A -> B -> C -> D -> E, home
router A views E as the destination, D as the parent, and B as the
adjacency.  Note that in the path from A to B, A is the parent node.

V.2.  Building the paths table

Begin building the paths table by using the home router as the first
node on the paths table.  The cost to reach yourself is always 0, so
make the first entry on the paths table as follows:  Source=self,
destination=self, parent=self, and cost=0.  From here on in, parent
is always (by definition of the SPF spanning tree) the node most
recently added to the paths table.

	Destination	Adjacent 	Parent		Cost	5	10	15

		Paths Table showing relationship between
	destination, parent and adjacent nodes, where the home
	node is and is three hops away;
	all hops here have a cost of 5.

V.2.1.  Scanning the links

The home router now scans its links table looking for all nodes (routers
and end nodes) that are adjacent to the current parent node, except
for links to nodes which are already on the paths table.  (It is
generally fastest to build the paths table by scanning the links table
in order of increasing link cost.)  Each of these new nodes is added
to the trial table, except for the parent node (which is already on
the paths table, so it can't possibly have a shorter path).  The value
of cost placed on the trial table is the cost of the link from the
parent to the destination plus the cost from home to the parent node
(which value is already on the paths table).

A node may only appear once in the trial table at any given time.  If
an adjacency is found to a node that is already on the trial table, a
new entry replaces the existing entry if and only if the new total
cost is lower.  If the cost is higher, it is ignored.  (If the costs
are equal, then a "tiebreaker" is determined by treating the
lower-numbered IP address as the lower cost.  This will simply make
the spanning tree more deterministic in case of tie.)  Thus the trial
table always contains the lowest cost path to each destination found so

Once all of the links from the parent node have had their chance to go
onto the trial table, scan the entire trial table for the _one_ entry
with the lowest total cost.  This not need be a link from that parent
node!  In case of tie, pick the lower IP address (again just to be
deterministic).  Move this node to the paths table;  guaranteed,
there'll be no cheaper path to that node!  The adjacency used for that
node in the paths table is the adjacency to its parent node.  Note
that the parent _must_ already be on the paths table since that's the
source of the parent; you're working your way outward.

This new addition to the paths table becomes the new parent node.  Repeat
the procedure above (from the beginning of V.2.1) until there are no
nodes left on the trial table.  This means you've completed the spanning
tree and have found the least-cost path to every other node.

One of the router parameters is maximum_cost.  If the cost to a given
parent node exceeds this value, the procedure stops, since all subsequent
entries in the route table will have a higher cost.  This value has some
semblane to the time-to-live parameter of the IP packet:  It makes little
sense to compute a routing table to nodes that cannot be reached within
time-to-live.  (Ideally, ttl will be implemented using a timer rather
than a node counter, but this is difficult to do with some of the packet
radio data link controller implementations; vis., TNCs.)

A router should recalculate its routes periodically based upon the
current links table.  How frequently depends upon the CPU load involved.
Initial estimates are that it should be recalculated after receipt of
every routing update bulletin:  Partial calculations do not save
enough CPU time to make them worthwhile.

V.3.  Filling in the routing table

**The route table in NOS (the KA9Q et al implementations of IP)
contains, for each entry, the destination address, number of bits,
interface, gateway and metric.  RSPF 2.2 conceptually replaces this with
a manual route table (essentially what exists in NOS without RSPF) and a
route table which RSPF generates itself; when RSPF is activated, this
latter table is used for packet routing.

**The route table is filled in from the paths table after each time the
SPF algorithm has finished.  It takes entries from both the paths table
and the manual route table.  Order of precedence is important here.
Manual routes have lower precedence than routes taken from the paths
table, given an equal cost, but a lower cost manual route will override a
higher-cost paths table entry.

Since the routing table will be periodically recalculated from
scratch, implementation **requires two separate route tables, one "in
progress" and one "in service".  When the route calculation is
complete, the "in progress" table becomes "in service" and the old one
is cleared for reuse.  This allows packet forwarding to continue while
the completed paths table is being converted into the route table.
**Calculation of the paths table and route table can require substantial
CPU resources, and should not take precedence over packet forwarding.

V.4.  Manual routes

A manual route table may also be maintained.  This takes second
precedence to RSPF-generated entries in generating the route table.
Manual routes are useful defaults before RSPF generates routing entries,
for destinations not reported on by RSPF, for creating node group
adjacencies, and for interworking with other routing protocols.  **Note
that manual routes in RSPF 2.2 are (at least logically) not simply
entries in the initial routing table, but are permanent (until changed
manually) entries in a separate manual route table which is merged into
the RSPF-generated route table as the last stage of each calculation of
the route table.  (As an implementation option, the manual route table
could conceivably be interleaved with the route table, with a "manual"
flag on these entries, but the manual entries behave differently.)

**Note that all manual routes are considered as adjacencies.  This is
obviously wrong at times, but cannot be determined automatically.  Thus
the private flag is provided.

Entries in the manual route table include:

        Destination IP Address
        Destination IP Address number of bits (0-32)
        Selected adjacency hardware interface
        Selected adjacency subnetwork address string
        Adjacency IP address
        Flag:  Private/non-private

V.4.1.  Private routes

Any manual route may be flagged as private.  These routes are used in
calculating the route table, but are never propagated to other nodes.
Default routes should always be defined as private, as they do not denote
known adjacencies, only values that this node will use absent better

**V.5.  Secondary storage of routing information

This section is for study.  It may be unnecessary because adjacencies
will provide a full report to a router upon initialization of an

Real implementations of RSPF, especially under single-tasking operating
systems (such as MS/PC/DR-DOS), will not always run continuously without
interruption.  Nor will all end systems.  When RPSF starts running, it
has an empty links table, and depends upon manual routes until receiving
links information from its adjacencies.  On a packet radio network,
given the low bandwidths, this convergence may take excessively long.

An implementation may work around this by storing, in non-volatile media
(eg., hard disk) the information needed to quickly resume operation.
This file (whose internal format is a local matter) should contain a
timestamp and the last sequence number and subsequence number originated
by this node, followed by the contents of the links table.  This file
should be stored after a new bulletin (with sequence or subsequence number)
is transmitted.  Only one copy of this file need be stored.  Other tables
may be optionally stored, for diagnostic purposes, but only the links
table is required.

When RSPF begins to run, it looks for this file.  If present, it checks
the timestamp.   If the timestamp is recent (suggested maximum age:  no
older than twice the rspftimer), then the file should be read into the
paths table.


The author would like the thank the participants in the Internet
tcp-group for their assistance in preparing this, and to the various
radio amateurs who have tested RSPF 2.1 on the air, for their
assistance.  Special thanks to Anders Klemets SM0RGV for drafting the
first implementation of RSPF2.1, and to Mike Bilow N1BEE for creating a
stable implementation as well as for his assitance in editing this

Appendix A.  Router parameters

Every router must set a number of parameters in order to properly
operate.  While RSPF builds its routing matrix automatically, overall
network efficiency and stability may require some fine-tuning based
upon experience.  These include parameters set for each data link
or subnetwork layer entity (i.e., each radio channel) and for the
router in general.  Commands given below are not necessarily the
statements used in real implementations.

Link/subnet settings:

   Set Link cost:  This is the cost parameter based upon the link speed
    and type.  Since the overall cost of the end-to-end path is
    minimized by the RSPF spanning tree, link cost should be set to
    arrive at the best overall network performance.  The legal range
    is 1-127.  This is sent in routing update bulletins.

Node settings:

  Add/Delete Node group: [IPaddr]/bits {cost}.  This allows a node to
   announce its ability to serve a group of nodes, identified using
   the standard NOS convention of address/significant bits.  Thus a
   node group setting of []/25 will match all nodes from
   [] to [].  Cost is optional; if set, this
   cost to will be propogated to reach such nodes; otherwise, the
   link's default is used.  This is fed into the manual route table.
   **A given router may support multiple node groups; this
   facilitatates operation near the boundary of address-subnet
   regions.  Such a router may use a lower node group cost for its own
   node group than for adjacent (or nearby) ones, so that it is not a
   favored route to other groups.  High costs for other node groups are
   still useful because they define whether an adjacency is governed by
   horizon local or horizon portable.  (Use of a Private flag for this
   function, instead of propagating a higher cost, is for further

  Set horizon link:   This sets the horizon value for the node's
   routing bulletins apropos 32-bit addresses of other adjacent
   routers.  This should be high enough to propogate across the
   backbone, if a backbone router.

  Set horizon group:  This sets the horizon value for the node's
   routing bulletins apropos node group addresses (fewer than 32
   bits matched) nominally served (providing end user adjacencies) by
   this router.  Usually matches the horizon link value.

  Set horizon local:  This sets the horizon vaue for the node's
   routing bulletins apropos full link addresses (32 bits) to
   non-routers within the router's node group area.  This is set to
   a low value so that only other routers serving the same or
   overlapping node group(s) will receive these messages.

  Set horizon portable:  This sets the horizon value for the
   node's routing bulletins apropos full link addresses (32 bits)
   to non-router adjacencies not within a supported node group.  This
   allows portable end nodes to have their location in the network
   propogated farther than the local horizon; this is usually set the
   same as horizon group.

  Set RRH timer:  This sets the time, in seconds, between
   router-router hello (RRH) broadcasts.  Initial suggestion:  900.

  Set RSPF timer:  This sets the time, in seconds, between newly
   originated link state bulletins.  Initial suggestion:  900.

  Set suspect timer:  This sets the time, in seconds, after which
   a  connectionless adjacency is "suspect" if no packets are
   received from it.  The route is then tested (e.g., pinged).  Initial
   suggestion:  2000.

  Set suspect count (maxping):  This sets the number of times an ICMP
   echo (ping) should be sent after a node is suspect, before it is
   removed from the adjacency list.  Initial suggestion:  3.

APPENDIX B.  Schematic representation of RSPF tables.

             ---------------------------------------------------/ ADJAC.        LINKS                     PATHS     | (SNAcP)
    SUBNETS-->+----+       +------+   [SPF]          +-----+    |
              |II. |------>|V.1.1 |              --->|V.1.2|    |
    RRH IN--->|    |       |      |    TRIAL    /    |     |    |
              +----+       |      |-->+-----+  /     +-----+    |
       BULLETINS<--------->+------+   |V.1.3|-/least    |       |
        |                   ^         |     |  cost     V       |
      +------+        non-  |         +-----+       +-------+   |
      |IV.2.4|      private |                       |V.1.2.1|  /
      |      |  +-------+--/                        |       |<-
      +------+  | V.4   |-------------------------->|       |
      ROUTERS   |       |    all manual routes      +-------+
                +-------+                             ROUTE
               MANUAL ROUTE

        Figure B.  Information flow showing relationship between
        tables.  Numbers in tables refer to sections above which
        introduce each table.