IEN 196 11 September 1981














          ISSUES INVOLVING NON-ROUTING GATEWAYS



                    B. Bowman & P. Cook


        Ford Aerospace & Communications Corporation

                 Colorado Springs, Colorado

                        (303-471-9110)


                         IEN # 196



                    September 11, 1981




















IEN 196 11 September 1981


INTRODUCTION

This  note  presents  some  proposed modifications to the gateway
routing  algorithm  and  the  protocol  for  exchanging   routing
information described in IEN #109, "How To Build A Gateway".  The
described algorithm modifications were defined  as  a  result  of
problems  encountered  in  the  implementation  of the gateway to
gateway control protocol specified in IEN #  109.   The  problems
encountered  in implementing the algorithm involve the mechanisms
for  incorporating   non-routing   gateways   in   the   catenet.
Therefore,  this  discussion focuses on modifications designed to
facilitate use of non-routing gateways in  concert  with  routing
gateways in the catenet.

As  indicated  above,  the  issues  discussed  resulted  from our
implementation  of  the  IEN  #  109  algorithm.   Some   initial
implementation  questions  concerning interpretation of the rules
for assembling routing updates from non-routing gateways  led  to
our  analysis  of  the  design  of  this  routing strategy.  From
routing strategy design issues, our study progressed to questions
regarding  the  intended/optimal  role of non-routing gateways in
the catenet.

As a result, this note presents specific modifications to  IEN  #
109  necessary  for  successful  implementation and also includes
discussion of the broader issues of the role of  the  non-routing
gateway in the catenet.

Finally,  we  propose  a  modification to the routing information
exchanged in the IEN  #  109  algorithm,  which  we  believe  has
significant  advantages in reducing the logical complexity of the
algorithm and traffic between gateways.  Although we have  looked
at  design approaches of other authors in this field, we have not
done an exhaustive search  to  determine  the  "novelty"  of  our
approach.   We would also like to stress that our goal was not to
design a new routing algorithm but simply to implement the IEN  #
109  algorithm.   In  a  very  real sense, it can be said that we
backed into these routing design issues as a natural  consequence
to our implementation efforts.

I. Non-Routing Gateways

It  should  be  noted  that  any  solution  must  be  based on an
understanding regarding the  intended  role  of  the  non-routing
gateway  in  the  catenet.   Unambiguous  rules for incorporating
non-routing gateways in the routing scheme can only be  specified
when  the intended role is clearly defined.  An optimal solution,
then, would involve clarification of the role of the  non-routing
gateway   followed  by  revision  of  the  routing  specification
consistent with the defined role.

                                -2-

IEN 196 11 September 1981


IEN # 109, "How to Build a Gateway", contains a section  (Sect.9,
Page  7)  discussing  the  interface between routing gateways and
non-routing gateways and, in particular, discussing the use of  a
non-routing  gateway  by  a  routing gateway for routing packets.
Two areas of this section  are  problematic  with  respect  to  a
successful  implementation.   These  are, (1) initialization with
respect to the non-routing gateway and  (2)  computation  of  the
minimum distance vector.

A. Initialization

IEN  # 109 gives the following instructions for initializing with
respect to a non-routing neighbor gateway.

 * "For each non-routing neighbor gateway of gateway G1,  compute
the  routing  update  that  would be sent to G1 assuming that all
gateways and network connections are operational.  These  routing
updates are assembled in G1."

This rule can be interpreted in one of two ways.  The first is to
assemble the non-routing gateway's routing update showing  actual
distances to each network from the non-routing gateway.  Figure 1
shows an example of this interpretation.

In this example, Gateway (B) is a non-routing gateway and Gateway
(A)  is  a  routing  gateway.   Initially,  G(A) assembles G(B)'s
update as actual distances from G(B) to the 3 networks.  Based on
this initialization, G(A) assumes that it is a distance of 1 from
Net.#1 because the non-routing gateway provides the only path  to
Net.#1.   G(A)  also assumes that it is directly (0) connected to
Nets.#2 and #3.  G(A) builds its initial minimum distance  vector
reflecting these distances.

Assume,  then, that G(A)'s link to Net.#3 goes down at some time.
When this happens, G(A) recomputes it's minimum distance  vector.
Since  G(A)  is  no  longer connected to Net.#3, there is no path
available unless the non-routing gateway's path is used.   G(B)'s
update  indicates  a  distance  of  1 to Net.#3.  Therefore, G(A)
assumes a path to Net.#3 of distance 2 through G(B).  Of  course,
there  is  no path to Net.#3 but G(B) does not accept or transmit
routing tables so any packets addressed to  Net.#3  will  shuttle
indefinitely  (until the IP4 time to live field is decremented to
zero) from G(A) to G(B) to  G(A),  etc.   When  this  example  is
extended  to  larger catenets, the shuttling problem persists and
simply  involves  higher  distance  shuttles.   Therefore,   this
interpretation of the initialization instructions is unworkable.


                               -3-

IEN 196 11 September 1981




     FIGURE 1:   INITIALIZATION OF NON-ROUTING GATEWAY UPDATE
                 "ACTUAL DISTANCE" METHOD


CATENET CONFIGURATION



 __________              __________              __________
|          |            |          |            |          |
|  NET #1  |----G(B)----|  NET #2  |----G(A)----|  NET #3  |
|__________|            |__________|            |__________|


where
    G(A) = Gateway A:  Routing Gateway
    G(B) = Gateway B:  Non-Routing Gateway


G(A) DISTANCE MATRIX AFTER INITIALIZATION

                          NETWORKS
   GATEWAYS              1    2    3


      A                  1    0    0
      B                  0    0    1



G(A) DISTANCE MATRIX (ASSUMING LOSS OF CONNECTIVITY TO NET.#3)

                          NETWORKS
   GATEWAYS              1    2    3


      A                  1    0    2
      B                  0    0    1





                               -4-

IEN 196                                         11 September 1981


The  other  possible interpretation of the IEN # 109 instructions
is to assemble the non-routing gateway's  update  showing  actual
distances  to each network from the non-routing gateway only when
the non-routing gateway is as close to or closer to  the  network
than   the   gateway   assembling   the   routing  update.   This
interpretation  incorporates  a  rule  from  Section  7,  page  7
regarding  computation  of  routing  updates.   Figure 2 shows an
example of this interpretation.

In this example, Gateway (B is a non-routing gateway and Gateways
A and C are routing gateways.  The example is presented from  the
perspective  of  Gateway  A.   Gateway  B's update is initialized
showing 0 distance to Nets.#1 and #2.  A distance of 1  is  shown
to  Net.#3  because Gateway B is as close to Net.#3 as is Gateway
A.  An infinite distance is shown to Net.#4 because Gateway A  is
closer  to Net.#4 than Gateway B.  The example shows receipt of a
routing update from Gateway C which shows Gateway  C's  distances
calculated  according  to  the rules for preparation of a routing
update.

Gateway  A  calculates  its minimym distance vector as 0 distance
from Nets.#2 and #4 because they are directly connected, distance
1  from Net.#1 because of the path through Gateway B and distance
1 from Net.#3 because of the path through Gateway C.

Assuming  that Gateway C loses connectivity to Network #3 at some
time, Gateway C would send Gateway A a new routing update showing
infinite   distance   to   Network  #3.   Gateway  A  would  then
recalculate its minimum distance vector.  G(A) would assume  that
the only path to Net.#3 is through the non-routing gateway (G(B))
and that this is a path of distance 2.  This occurs because  G(B)
neither accepts nor transmits routing updates so it is unaware of
the connectivity loss and G(A) is assembled with  G(B)'s  routing
update  so  there is no provision for changes to this information
in G(A).  The situation is complicated by  the  fact  that  G(A),
upon  recalculation of its minimum distance vector, will transmit
a routing update to G(C) showing the distance of 2 from  G(A)  to
Network  #3.   G(C)  will recompute its distance vector to show a
path of distance 3 from  G(C)  to  Network  #3.   Since  no  path
actually exists and all 3 Gateways believe a path exists, packets
addressed to Net.#3 will shuttle indefinitely (until the IP4 time
to live field is decremented to zero).

Thus it seems that Thus it seems that both interpretations of the
IEN  #  109  instruction for assembling the non-routing gateway's
update present serious problems.


                               -5-

IEN 196                                         11 September 1981




     FIGURE 2:   INITIALIZATION OF NON-ROUTING GATEWAY UPDATE
                 "TAILORED UPDATE" METHOD


CATENET CONFIGURATION


                                                 __________
 __________              __________             |          |
|          |            |          |----G(A)----|  NET #4  |
|  NET #1  |----G(B)----|  NET #2  |            |__________|
|          |            |          |             __________
|          |            |          |            |          |
|__________|            |__________|----G(C)----|  NET #3  |
                                                |__________|



where
   G(A) = Gateway A:  Routing Gateway
   G(B) = Gateway B:  Non-Routing Gateway
   G(C) = Gateway C:  Routing Gateway


G(A) DISTANCE MATRIX AFTER INITIALIZATION

                          NETWORKS
   GATEWAYS              1    2    3    4


      A                  1    0    1    0
      B                  0    0    1   INF
      C                  1    0    0   INF



G(A) DISTANCE MATRIX (ASSUMING LOSS OF CONNECTIVITY TO NET.#3)

                          NETWORKS
   GATEWAYS              1    2    3    4


      A                  1    0    2    0
      B                  0    0    1   INF
      C                  1    0   INF  INF




                               -6-

IEN 196                                         11 September 1981


B.  Computation of the Minimum Distance Vector

IEN # 109 goes on  to  provide  the  following  instructions  for
calculating the minimum distance vector when non-routing gateways
are included.

 *  "The  gateway,  G(A),  first  computes  its minimum distance
    vector using only the routing updates  from  neighbors  that
    participate  in  routing.   If  the  minimum distance to any
    network is infinity, i.e., the network  is  unreachable  via
    any  of  the  routing gateways, then the minimum distance to
    that  network  is  re-computed  using  the  routing   update
    compiled for the non-routing neighbor gateway."

This rule appears inefficient if we look at the example shown  in
Figure  3.   In  this example, Gateway B is a non-routing gateway
and Gateways A and  C  are  routing  gateways.   The  example  is
presented  from the perspective of Gateway A and the focus of the
example is on the distance to Network #1.

G(A)  is  assembled  with  the  update  of  G(B) (the non-routing
gateway) and G(B) shows a distance of 0 from Net.#1 since  it  is
directly  connected.  In our example, G(A) has received an update
from G(C).  G(C) shows a distance of 1 to  Net.#1  based  on  its
path through G(B).  According to the rule, G(A) has to prefer the
route through G(C) since G(C) is a routing neighbor and  G(B)  is
non-routing.   Therefore,  G(A)  assumes  a path of distance 2 to
Net.#1 and G(A) will route  to  G(C)  any  packets  addressed  to
Net.#1.   On receipt of the packet addressed to Net.#1, G(C) will
route the packet to G(B) where it will be delivered to Net.#1.

This  solution  is  not  unworkable  but  it does cause delay and
inefficiency by forcing an additonal transfer with respect to the
optimal  path.  In a geographically dispersed network, this delay
and additional network traffic could become significant.


                               -7-

IEN 196                                         11 September 1981



     FIGURE 3:   COMPUTATION OF THE MINIMUM DISTANCE VECTOR



CATENET CONFIGURATION


                                                 __________
 __________              __________             |          |
|          |            |          |----G(A)----|  NET #4  |
|  NET #1  |----G(B)----|  NET #2  |            |__________|
|          |            |          |             __________
|          |            |          |            |          |
|__________|            |__________|----G(C)----|  NET #3  |
                                                |__________|



where
   G(A) = Gateway A:  Routing Gateway
   G(B) = Gateway B:  Non-Routing Gateway
   G(C) = Gateway C:  Routing Gateway


G(A) DISTANCE MATRIX

                          NETWORKS
   GATEWAYS              1    2    3    4


      A                  2    0    1    0
      B                  0    0    1   INF
      C                  1    0    0    1




                               -8-

IEN 196                                         11 September 1981


C.  Proposed Solution to Non-Routing Gateway Problems

Role Clarification

As indicated above, a solution to the problems  in  incorporation
of non-routing gateways must address clarification of the role of
the non-routing gateway in the catenet.  Three possible roles for
the non-routing gateway have been identified and are discussed in
this section.  In  general,  we  see  potential  for  non-routing
gateways   to  be  used  to  provide  1)  the  "only"  route,  2)
"redundant"  routes  or  3)  "parallel"  routes.   Use   of   the
non-routing  gateway to provide "redundant" and "parallel" routes
is discussed in Section D.

Figure  4  illustrates  use of the non-routing gateway to provide
the "only" route.  This  role  basically  restricts  the  use  of
non-routing  gateways  to  allow  (i.e.,  recognize)  them in the
catenet only when the non-routing  gateway  provides  the  "only"
connection to a network or network branches.

Although other possible roles will be discussed , this particular
role  definition  is  the  one  we  recommend.  Using non-routing
gateways where they provide the "only" possible route  seems  the
most  logical  use  of  these gateways and also lends itself most
easily  to  unambiguous  rule  definition.   The  mechanisms  for
incorporating  non-routing  gateways  consistent  with the "only"
route role are described in the following paragraphs.














                               -9-

IEN 196                                         11 September 1981




     FIGURE 4:   NON-ROUTING GATEWAY AS "ONLY" ROUTE


CATENET CONFIGURATION



 __________              __________
|          |            |          |
|  NET #1  |----G(R)----|  NET #3  |
|__________|            |__________|
      |                       |
      |                       |
    G(R)                      |
      |                       |
      |                       |
 __________                   |
|          |                  |
|  NET #2  |----G(R)----------|
|__________|
      |
      |
    G(NR)
      |
      |
 __________
|          |
|  NET #4  |
|__________|
      |
      |
   etc. (network branch)



   G(R) = Routing Gateway
   G(NR) = Non-Routing Gateway


NOTE:   In this configuration, the non-routing gateway provides
        the only connection to Network #4.


                              -10-

IEN 196                                         11 September 1981


Rules for "Only" Route Role

It appears that the initialization and route computation problems
associated with non-routing neighbor  gateways  described  above,
could  be  solved  by a few rule changes.  The proposed new rules
are as follows:

 *  IEN # 109 Rule:

    For   each  non-routing  neighbor  gateway  of  gatewayG(A),
    compute the routing  update  that  would  be  sent  to  G(A)
    assuming  that  all  gateways  and  network  connections are
    operational.  These routing updates are assembled in G(A).

    New Rule:

    F or each non-routing  neighbor  gateway  of  gateway  G(A),
    compute  the  routing  update  that  would  be  sent to G(A)
    assuming that  all  gateways  and  network  connections  are
    operational.   In  computing  these  routing  updates,  show
    actual  (non-infinite)  distances  to  networks  which   are
    reachable  only  through  the non-routing gateway.  Networks
    which  are  reachable  by  a  routing   gateway   shall   be
    initialized  with  distance  equal  infinity.  These routing
    updates are assembled in G(A).

    IEN # 109 Rule:

    T he gateway, G(A),  first  computes  its  minimum  distance
    vector  using  only  the routing updates from neighbors that
    participate in routing.  If  the  minimum  distance  to  any
    network  is  infinity, (i.e., the network is unreachable via
    any of the routing gateways), then the minimum  distance  to
    that   network  is  re-computed  using  the  routing  update
    compiled for the non-routing neighbor gateway.

    New Rule:

    The gateway,  G(A),  first  computes  its  minimum  distance
    vector using all the routing updates (including updates from
    both routing and non-routing gateways).

These  proposed  rule  changes, taken together, cause non-routing
gateways to be used only when they provide the  only  path  to  a
network  or  string  of networks.  These rules also eliminate the
possibility of selecting any other path (i.e., through a  routing
gateway)  when  a  non-routing  gateway  provides  the only path.
Thus, non-routing gateways are used for packet routing if and
only if they provide the only route to a destination.



                              -11-

IEN 196                                         11 September 1981


Figure 5 shows how the new rules solve the problems identified in
Examples  1,  2,  and  3.   In  this  example,  Gateway  B  is  a
non-routing Gateway and Gateways A and C  are  routing  gateways.
The  example is presented from the perspective of Gateway A.  The
G(A) Distance Matrix after initialization shows that the  problem
depicted in Figure 3 (computation of the minimum distance vector)
is resolved through application of  the  new  rules.   Since  the
minimum  distance  vector  is  calculated  based  equally  on the
routing and non-routing gateway updates, the shortest  path  (and
only  path)  to  Network  #1 is selected by both routing gateways
(G(A) and G(C)).

The  G(A)  Distance Matrix resulting from loss of connectivity to
Net.#4  shows   that   the   problem   depicted   in   Figure   1
(Initialization   of  non-routing  gateway  update)  is  resolved
through use of the new rules.  Since  the  assembled  non-routing
gateway  update  no  longer  shows paths to networks reachable by
routing gateways, G(A) is able to determine that  Net.#4  is  now
unreachable  and  does  not  attempt  to  route traffic to Net.#4
through Gateway (B).

The  G(A)  Distance Matrix resulting from loss of connectivity to
Net.#3  shows   that   the   problem   depicted   in   Figure   2
(Initialization  of  non-routing Gateway update) is also resolved
through application of the new rules.  This problem  is  resolved
because  the  assembled non-routing gateway update no longer show
paths to networks reachable  by  routing  gateways.   Thus,  when
Net.#3 is declared unreachable by Gateway C, G(A) determines that
Net.#3 is unreachable by all gateways.
















                              -12-

IEN 196                                         11 September 1981

     FIGURE 5:   PROPOSED SOLUTION

CATENET CONFIGURATION
                                                 __________
 __________              __________             |          |
|          |            |          |----G(A)----|  NET #4  |
|  NET #1  |----G(B)----|  NET #2  |            |__________|
|          |            |          |             __________
|          |            |          |            |          |
|__________|            |__________|----G(C)----|  NET #3  |
                                                |__________|

where

   G(A) = Gateway A:  Routing Gateway
   G(B) = Gateway B:  Non-Routing Gateway
   G(C) = Gateway C:  Routing Gateway

G(A) DISTANCE MATRIX AFTER INITIALIZATION

                          NETWORKS
   GATEWAYS              1    2    3    4

      A                  1    0    1    0
      B                  0   INF  INF  INF
      C                  1    0    0   INF


G(A) DISTANCE MATRIX (ASSUMING LOSS OF CONNECTIVITY TO NET.#4)

                          NETWORKS
   GATEWAYS              1    2    3    4

      A                  1    0    1   INF
      B                  0   INF  INF  INF
      C                  1    0    0   INF


G(A) DISTANCE MATRIX (ASSUMING LOSS OF CONNECTIVITY TO NET.#3)

                          NETWORKS
   GATEWAYS              1    2    3    4

      A                  1    0   INF   0
      B                  0   INF  INF  INF
      C                  1    0   INF  INF


                              -13-

IEN 196                                         11 September 1981


D.  Other Possible Approaches

Redundant Role

Restricting the use of non-routing gateways  in  the  catenet  to
locations  where  they  provide  the  only  configured  path  and
adoption of the corresponding rule  changes  is  our  recommended
solution  to  the  implementation  problems we've identified.  In
reviewing IEN # 109 and it's predecessor IEN #  30,  we  find  it
difficult  to  determine  the  intended  role  of the non-routing
gateways.

Both  papers indicate that non-routing gateways are to be used to
forward traffic only when  they  provide  the  only  route  to  a
network.   This  seems to indicate that the initial intent was to
use non-routing gateways in the role defined above (i.e.   "only"
route  role).   On the other hand, it's possible that the authors
actually intended to use the non-routing gateways to provide  the
only  configured  path  and  to provide backup routes to networks
connected  by  routing  gateways.   This  role  for   non-routing
gateways  is what we have called the "redundant " role.  Figure 6
illustrates this use  of  non-routing  gateways.   This  role  is
basically an addition to the "only" route role.

Here the non-routing gateway can also be configured  in  parallel
with  a  routing gateway but the non-routing path is only used if
connectivity through the routing  gateway  is  lost.   Thus,  the
non-routing  gateway is used as a backup for the routing gateway.
As  long  as  the  routing  gateway  and  its   connections   are
functional,  the  non-routing  gateway is not used to forward any
traffic.














749/

                              -14-

IEN 196                                         11 September 1981




     FIGURE 6:   NON-ROUTING GATEWAY AS "REDUNDANT" ROUTE



CATENET CONFIGURATION



 __________              __________
|          |----G(NR)---|          |
|  NET #1  |            |  NET #2  |
|__________|----G(R)----|__________|
      |                       |
      |                       |
    G(R)                    G(R)
      |                       |
      |                       |
 __________________________________
|                                  |
|           NET #3                 |
|__________________________________|


WHERE
   G(R) = Routing Gateway
   G(NR) = Non-Routing Gateway








                              -15-

IEN 196                                         11 September 1981



It seems that this second role interpretation  is  implementable.
Basically, the corresponding rules would be:

 1)  Assemble routing update showing direct network  connections
     and  actual  distances  to  networks  reachable only by the
     non-routing gateway.

 2)  Calculate minimum distance vector using non-routing gateway
     paths only when they provide the only route.

Implementing   this   role  concept,  then,  involves  the  added
complexity of the two stage minimum distance  vector  calculation
(with  bias toward routing gateway paths).  While this may be the
role anticipated by the authors of IEN # 109 and #  30,  we  feel
the  backup  role  is  of  little  added value for the additional
complexity involved.  It seems unlikely to us that networks would
buy  a  processor  which was to be idle the majority of the time.
Instead, if need existed for a backup, it seems a routing gateway
would  be a better choice and cost efficient as it could increase
bandwidth as well as providing redundancy.

Parallel Role

At the other end of the spectrum, it is possible that some  group
might wish to use non-routing gateways in any configuration where
routing gateways can be used.  In this role, which  we  call  the
"parallel"   role,  non-routing  gateways  are  used  to  provide
additional  (parallel)  paths  and  thus   effectively   increase
bandwidth.   This role differs from the "redundant" route role in
that non-routing gateways are used to forward traffic  even  when
they  do  not  provide  the only path.  It is this facility which
allows the effective increase in real-time bandwidth.

Whether  this intended role is implementable is an open question.
Obviously, it would  be  more  complex  to  implement  and  would
basically require a redefinition of some of the ground rules.  We
rejected this possible role application because it  significantly
reduces  catenet  reliability  and seems to defeat the whole idea
behind the routing algorithm function.  If  non-routing  gateways
are  used  routinely  to forward traffic in parallel with routing
gateways, connectivity  changes  occuring  with  the  non-routing
gateways  are  unreported  and therefore the non-routing gateways
could easily become traffic sinks.






                              -16-

IEN 196                                         11 September 1981

We  felt that the only possibility for using non-routing gateways
to increase bandwidth would be to implement the role outside  the
gateway-gateway   control   protocol.   This  function  could  be
implemented as a host function where there was  an  understanding
between hosts on two networks that traffic would be split on some
basis  between  routing  and  non-routing  gateways.   Thus,  the
decision  to  sacrifice catenet reliability in favor of increased
traffic flow would  be  made  by  the  parties  involved  without
involving    any    gateway   control   function   awareness   or
participation.

II.  Routing Update Calculations

In the process of designing the gateway routing software IAW  IEN
# 109, we determined that a subset of the routing algorithm rules
dictated a logically  cumbersome  implementation.   These  rules,
from IEN # 109 are as follows:

 A.  "The gateway  reports  its  distance  to  a  network  to  a
     neighbor only if it is as close or closer to a network than
     its neighbor."  (p.7)

 B.  "The  gateway  maintains  a copy of the most recent routing
     updates that it sent to each of its neighbors.  The gateway
     computes  the  new  routing  updates to send to each of its
     neighbors.  If any of these routing updates  are  different
     than  the  preceding  updates,  then  the gateway sends new
     routing updates to its neighbors."  (p.7)

Rule  A  calls  for "tailoring" routing updates for each neighbor
such that the transmitting gateway reports its distance  relative
to  the  connectivity  of  each  recipient gateway.  Thus, when a
gateway calculates its routing update, it  first  calculates  its
minimum distance vector and then compares that vector to the last
routing update from each  neighbor.   From  this  comparison,  it
creates  the  "tailored"  routing  update for each neighbor which
reflects not only the  transmitting  gateway's  connectivity  but
also  the  connectivity of each neighbor.  The comparison process
is performed as follows:







                              -17-

IEN 196                                         11 September 1981


        DO (For each neighbor)

           DO (For each network)

              IF (Transmitting gateway is as close or closer to

                 the network)

                   Report actual distance

              ELSE (Report infinity distance)

           ENDDO

        ENDDO

Rule  B requires that, having performed the calculations based on
Rule A resulting in routing  updates  for  each  neighbor,  these
updates  be  compared  to the updates last sent to each neighbor.
If any of the updates are different from those last sent, all the
updates are transmitted.  If there are no changes to any neighbor
since the last transmittal, no updates are sent.   Implementation
of  this  rule  requires  that copies of the last updates sent be
maintained at the gateway.

The  basis for these rules, particularly Rule A, is prevention of
shuttling which could occur if only one path existed to a network
and  that  network became disconnected.  If gateways were allowed
to  report  actual  distances  rather  than  infinity,  then  two
gateways  could,  because of their dependent or relative distance
relationship, begin step increments of  their  distances  to  the
disconnected  network  eventually  approaching  infinity and thus
shuttle packets until the IP4 time to live field  is  decremented
to zero.

This shuttling problem is,  in  fact,  a  natural  outcome  of  a
routing   scheme   which   involves   the  exchange  of  distance
information only.   Thus,  the  problem  can  be  solved  by  the
addition  of special rules such as Rule #1 above or can be solved
by  going  to  a  routing  scheme  which  exchanges  connectivity
information (distance and path).








                              -18-

IEN 196                                         11 September 1981


After  studying  the  current  solution  of  exchanging  relative
distance  vectors,  it  seems  that  going  to a routing scheme
involving exchange of both connectivity information and  distance
provides  a  simpler  and  more  efficient  solution.   The added
information  exchange  proposed  is  more  than  offset  by   the
computational  simplicity  and  the  reduction  in routing update
traffic afforded.

The  routing  scheme  proposed  provides for including in routing
updates both the  actual  distance  vector  (as  opposed  to  the
current tailored relative distance vector) and the routing table.

Figure 7 is a configuration used as the basis for illustration of
the  updates.   To see how these routing updates are constructed,
examine the update from G3 in Figure 8 (Step 3).  G3 is  directly
connected to Nets.#2 and #26, so G3 shows a distance of 0 to each
with G3 as the "next" gateway address for routing.  G3 is 1  away
from Net.#1 and this path to Net.#1 is through G2.  Similarly, G3
is 1 away from Nets.#3 and #27 and these paths are through G5 and
G6 respectiviely.  G3 is also a distance of 1 from Net.#10.


























                              -19-

IEN 196                                         11 September 1981




     FIGURE 7:   CATENET CONFIGURATION



 __________              __________
|          |            |          |
|  NET #1  |            |  NET #4  |
|__________|            |__________|
      |                      |
      |                      |
     G2                     G4
      |                      |
      |                      |
 __________             __________
|          |           |          |
|  NET #26 |-----G1----|  NET #10 |
|__________|----       |__________|
      |        |             |
      |        |             |
     G3        |------------G5
      |                      |
      |                      |
 __________             __________
|          |           |          |
|  NET #2  |           |  NET #3  |
|__________|           |__________|
      |
      |
     G6
      |
      |
 __________
|          |
|  NET #27 |
|__________|



NOTE:  Gateways G1 - G6 are all routing gateways.






                              -20-

IEN 196                                         11 September 1981




     FIGURE 8:   ROUTING UPDATES BASED ON NEW ROUTING METHOD
                 (SEE FIGURE 7 FOR CONFIGURATION)


          NETWORK DISTANCES/"NEXT" NEIGHBOR ADDRESS(ES)



STEP   SOURCE   DEST.           1    2    3    4    10    26    27


 1.     G1      ALL  Distance  INF  INF  INF  INF    0     0    INF
                      "Next"                        G1    G1

 2.     G2      ALL  Distance   0    1    1    2     1     0     2
                      "Next"   G2   G3   G5   G1    G1    G2    G3

 3.     G2      ALL  Distance   1    0    1    2     1     0     1
                      "Next"   G2   G3   G5   G1    G1    G3    G6
                                              G5    G5

 4.     G4      ALL  Distance   2    2    1    0     0     1     3
                      "Next"   G1   G1   G5   G4    G4    G1    G1
                               G5   G5                    G5    G5

 5.   G5(10)    ALL  Distance   1    1    0    1     0     0     2
                      "Next"   G2   G3   G5   G4    G5    G5    G3

 6.   G5(26)    ALL  Distance   1    1    0    1     0     0     2
                      "Next"   G2   G3   G5   G4    G5    G5    G3

 7.     G1      ALL  Distance   1    1    1    1     0     0     2
                      "Next"   G2   G3   G5   G4    G1    G1    G3

            Assume loss of connectivity by G2 to Net.#1

 8.     G2      ALL  Distance  INF   1    1    2     1     0     2
                      "Next"        G3   G5   G5    G5    G2    G3
                                              G1    G1

 9.     G1      ALL  Distance  INF   1    1    1     0     0     2
                      "Next"        G3   G5   G4    G1    G1    G3


NOTE:   G5(10) is gateway 5 on network 10.  A gateway which is a
        neighbor on two networks is treated as two gateways.  Thus
        G5 is identified as G5(10) and G5(26) by Gateway 1.


                              -21-

IEN 196                                         11 September 1981




     FIGURE 9:   ROUTING UPDATES BASED ON IEN # 109 METHOD
                 (SEE FIGURE 7 FOR CONFIGURATION)




                         NETWORK DISTANCES


SOURCE      DEST.     1    2    3    4    10    26    27


  G1         ALL     INF  INF  INF  INF    0     0    INF
  G2         G1       0    1    1    2     1     0     2
  G1         G2      INF  INF  INF  INF    0     0    INF
  G1         G3       1    2    2    3     0     0     3
  G1         G4       1    2    2    3     0     0     3
  G1         G5(10)   1    2    2    3     0     0     3
  G1         G5(26)   1    2    2    3     0     0     3
  G3         G1       1    0    1    2    INF    0     1
  G1         G2      INF   1   INF  INF    0     0     2
  G1         G3       1   INF  INF   2     0     0    INF
  G1         G4       1    1    2    2     0     0     2
  G1         G5(10)   1    1    2    2     0     0     2
  G1         G5(26)   1    1    2    2     0     0     2
  G4         G1      INF  INF   1    0     0    INF   INF
  G1         G2      INF   1   INF   1     0     0     2
  G1         G3       1   INF  INF   1     0     0    INF
  G1         G4       1    1   INF  INF    0     0     2
  G1         G5(10)   1    1    2    1     0     0     2
  G1         G5(26)   1    1    2    1     0     0     2
  G5(10)     G1       1    1    0    1     0     0     2
  G5(26)     G1       1    1    0    1     0     0     2
  G1         G2      INF   1    1    1     0     0     2
  G1         G3       1   INF   1    1     0     0    INF
  G1         G4       1    1    1   INF    0     0     2
  G1         G5(10)   1    1   INF   1     0     0     2
  G1         G5(26)   1    1   INF   1     0     0     2
           Assume loss of connectivity by G2 to Net.#1.
  G2         G1      INF   1    1   INF   INF    0     2
  G1         G2       7    1    1    1     0     0     2
  G1         G3      INF  INF   1    1     0     0    INF
  G1         G4      INF   1    1   INF    0     0     2
  G1         G5(10)  INF   1   INF   1     0     0     2
  G1         G5(26)  INF   1   INF   1     0     0     2


                              -22-

IEN 196                                         11 September 1981


However,  in  this  case,  G3 has two paths of distance 1 through
which it can reach Net.#10.   G3  can  go  through  G1  to  reach
Net.#10  in  1 hop or it can go through G5.  Since both paths are
equal and minimum distance, both  are  reported  in  the  routing
update.

When routing updates are constructed to include both distance and
connectivity  information  as  described  above, gateways receive
enough information  to  perform  path  checks.   This  capability
enables gateways to avoid shuttling problems.

The performance of path checks is illustrated in  Figure  7.   In
Figure  8,  step 8, G2 sends a routing update which shows that G2
has lost connectivity to Net.#1 (i.e., distance =inf.).  G1  then
recomputes  its  minimum  distance  and  routing  with respect to
Net.#1.  In these calculations G1 wants to find the shortest path
but also wants to ensure that it is a "true" path.  Therefore, G1
looks at the first path of distance 1.  This path is provided  by
G3.   However,  G3's  "next" neighbor address on this path is G2.
G2 now shows a path of infinity to Net.#1, so G1 rejects  the  G3
route.  In examining the other routes of distance 1 (i.e., G5(10)
and G5(26)), G1 finds that they all go through G2 which G1  knows
is disconnected.  Thus, all the paths of distance 1 are rejected.

Next, G1 looks at the route of distance 2  provided  by  G4.   G4
shows  two  "next"  addresses,  G1  and  G5,  G1 rejects the path
through "next" address G1 since this path would obviously  result
in  shuttling.   G1  then traces the route through "next" address
G5.  Looking at the routing update provided by G5, G1  sees  that
G5  intends  to route traffic addressed to Net.#1 through G2.  G2
has already been identified as a dead-end.  As  G1  has  examined
all  known  routes  to  Net.#1  and found no acceptable route, G1
determines that it is infinity distance to  Net.#1  and  sends  a
routing  update to all its neighbors showing infinity distance to
Net.#1 (step 9).

The rules involved in the routing scheme described in the example
plus a few additional rules of the proposed  routing  scheme  are
defined as follows:

 A.  On transmission of routing updates, the gateway reports its
     actual   distance   (minimum  distance  vector)  from  each
     network.   In  addition,  for  each  network,  the  gateway
     reports  the  "next"  gateway address through which it will
     route any packets addressed to the network.  If  more  than
     one  path  of the same minimum distance exists, the gateway
     will  report  all   possible   "next"   gateway   addresses
     associated  with  that  network.  In IEN # 109 terminology,
     then, each gateway exchanges its "minimum distance  vector"
     and  its  "routing table (list of neighbor gateways through
     which to send traffic to each network)."

                              -23-

IEN 196                                         11 September 1981


 B.  On  transmission  of  routing  updates,  the gateway sets a
     timer The  transmitting  gateway  does  not  recompute  its
     minimum  distance  vector  or  send any new routing updates
     prior to timer expiration.

 C.  Gateways calculate their routing updates and routing tables
     on the basis of the minimum distance rule.  Having selected
     the  minimum distance path to a network, the gateway checks
     the "next" gateway address.

     1.  If  the  "next"  gateway address is this gateway this
         path  is  considered  unacceptable   and   the   next
         candidate path is checked.

     2.  If the "next" gateway  address  is  other  than  this
         gateway,  use  the  information in the routing update
         matrix to determine if this path (distance  of  1  or
         greater)  is  through  a  gateway  which has reported
         "infinite" distance to the network in  question.   If
         any  step  of  the  path is through a gateway showing
         "infinite" distance  to  the  network,  the  path  is
         considered  unacceptable  and the next candidate path
         is checked.

Figure   8   shows   the  routing  updates  transmitted  for  the
configuration shown in Figure 7 when routing is performed IAW IEN
#  109.   Figure  8 shows the routing updates transmitted for the
same configuration (Figure 7) when the proposed routing scheme is
used.  Both examples are worked from the perspective of G1.

The  examples  show  the  differences  in  the  routing   updates
generated  under  each  rule.   In  both  cases, packets would be
routed correctly.  In the proposed routing scheme (see Figure 8),
fewer  routing  updates  are  transmitted  with  the same results
obtained.  This reduced traffic is due,  in  part,  to  the  rule
requiring   the   gateway   to   observe   a  time-delay  between
transmission of  routing  packets.   This  rule  is  particularly
useful  when  bringing  a  gateway  on-line because it allows the
gateway to  receive  updates  from  all  or  almost  all  of  its
neighbors  before responding with its next updates.  In addition,
the gateway is able to respond to the  loss  of  connectivity  to
Net.#1 correctly based on the update input from G2 (see Figure 8)
following the new method whereas, following the  old  rules  (see
Figure  9),  G1  must  receive  updates from all neighbors before
correctly determining that it has no path to Net.#1.


                              -24-

IEN 196                                         11 September 1981


Under  the  proposed  method,  the  gateway  has  a  more complex
algorithm  to  calculate  its  routing  table.    However,   once
calculated,  the gateway does not need to "tailor" the updates to
each neighbor but sends the same information  to  all  neighbors.
Also,  the  gateway  does  not maintain a copy of the last update
transmitted to determine whether there is new information  to  be
transmitted  since  it  transmits  updates  only when its routing
table changes.  Under IEN #  109,  the  gateway  is  required  to
maintain  copies  of  each  update last sent to each neighbor and
compose the new "tailored" updates  on  an  individual  basis  to
those last sent.

Finally, the proposed routing method can be correctly implemented
without  requiring  any  special  rules  regarding  treatment  of
non-routing neighbors.   The  path  checks  built  into  the  new
routing  scheme  preclude  the shuttling problems described above
which could have occured if non-routing gateways were not treated
as exceptions.








References

 1)  V.   Strazisar, "Gateway Dynamic Routting", IEN # 25, Bolt,
     Beranek and Newman, January 25, 1978.

 2)  V.    Strazisar,   R.    Perlman,   "Gateway   Routing,  An
     Implementation Specification", IEN # 30, April 11, 1978.

 3)  V.   Strazisar, "How to Build a Gateway", IEN # 109, August
     31, 1979.













                              -25-

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