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HISTORIC

Network Working Group                                   D.L.  Mills
Request for Comments:  904                              April 1984


             Exterior Gateway Protocol Formal Specification


0.  Status of this Memo

     This RFC is the specification of the Exterior Gateway Protocol
(EGP).  This document updates RFCs 827 and 888.  This RFC specifies a
standard for the DARPA community.  Interactions between gateways of
different autonomous systems in the ARPA-Internet must follow this
protocol.

1.  Introduction

     This document is a formal specification of the Exterior Gateway
Protocol (EGP), which is used to exchange net-reachability information
between Internet gateways belonging to the same or different autonomous
systems.  The specification is intended as a reference guide for
implementation, testing and verification and includes suggested
algorithmic parameters suitable for operation over a wide set of
configurations, including the ARPANET and many local-network
technologies now part of the Internet system.

     Specifically excluded in this document is discussion on the
background, application and limitations of EGP, which have been
discussed elsewhere (RFC-827, RFC-888).  If, as expected, EGP evolves to
include topologies not restricted to tree-structures and to incorporate
full routing capabilities, this specification will be amended or
obsoleted accordingly.  However, it is expected that, as new features
are added to EGP, the basic protocol mechanisms described here will
remain substantially unchanged, with only the format and interpretation
of the Update message (see below) changed.

     Section 2 of this document describes the nomenclature used, while
Section 3 describes the state-machine model, including events, actions,
parameters and state transitions.  Section 4 contains a functional
description of the operation of the machine, together with specific
procedures and algorithms.  Appendix A describes the EGP message
formats, while Appendix B contains a summary of the minor differences
between these and the formats described in RFC-888.  Appendix C presents
a reachability analysis including a table of composite state transitions
for a system of two communicating EGP gateways.

1.1.  Summary and Overview

     EGP exists in order to convey net-reachability information between
neighboring gateways, possibly in different autonomous systems.  The
protocol includes mechanisms to acquire neighbors, monitor neighbor
reachability and exchange net-reachability information in the form of
Update messages.  The protocol is based on periodic polling using
Hello/I-Heard-You (I-H-U) message exchanges to monitor neighbor
reachability and Poll commands to solicit Update responses.

     Specification of EGP is based on a formal model consisting of a
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finite-state automaton with defined events, state transitions and
actions.  The following diagram shows a simplified graphical
representation of this machine (see Section 3.4 for a detailed state
transition table).

          +-------+
          |       |---------------+---------------+
    +---->| Idle  |               A               A
    |     |       |-----------+   |               |
    |     +-------+           |   |               |
    |       |   A     Request |   | Cease         | Cease
    | Start |   | Cease       |   |               |
    |       V   | Refuse      V   |               |
    |     +-------+ Confirm +-------+    Up   +-------+
    |     |       |-------->|       |-------->|       |
    |     | Aqsn  |         | Down  |   Down  |  Up   |
    |     |       |----+    |       |<--------|       |
    |     +-------+    |    +-------+         +-------+
    |                  |        |                 |
    | Stop             |        |                 |
    | Cease-ack        | Stop   | Stop            | Stop
    |     +-------+    |        |                 |
    |     |       |    V        V                 V
    +-----| Cease |<---+--------+-----------------+
          |       |
          +-------+

     Following is a brief summary and overview of gateway operations by
state as determined by this model.

Idle State (0)

    In the Idle state the gateway has no resources (table space)
    assigned to the neighbor and no protocol activity of any kind is in
    progress.  It responds only to a Request command or a Start event
    (system or operator initiated) and ignores all other commands and
    responses.  The gateway may optionally return a Cease-ack response
    to a Cease command in this state.

    Upon receipt of a Request command the gateway initializes the state
    variables as described in Section 3.1, sends a Confirm response and
    transitions to the Down state, if resource committments permit, or
    sends a Refuse response and returns to the Idle state if not.  Upon
    receipt of a Start event it sends a Request command and transitions
    to the Acquistion state.

Acquisition State (1)

    In the Acquisition state the gateway periodically retransmits
    Request commands.  Upon receiving a Confirm response it initializes
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    the state variables and transitions to the Down state.  Upon
    receiving a Refuse response it returns to the Idle state.  The
    gateway does not send any other commands or responses in this state,
    since not all state variables have yet been initialized.

Down State (2)

    In the Down state the gateway has received a Request command or a
    Confirm response has been received for a previously sent Request.
    The neighbor-reachability protocol has declared the neighbor to be
    down.  In this state the gateway processes Request, Cease and Hello
    commands and responds as required.  It periodically retransmits
    Hello commands if enabled.  It does not process Poll commands and
    does not send them, but may optionally process an unsolicited Update
    indication.

Up State (3)

    In the Up state the neighbor-reachability protocol has declared the
    neighbor to be up.  In this state the gateway processes and responds
    to all commands.  It periodically retransmits Hello commands, if
    enabled, and Poll commands.

Cease State (4)

    A Stop event causes a Cease command to be sent and a transition to
    the Cease state.  In this state the gateway periodically retransmits
    the Cease command and returns to the Idle state upon receiving a
    Cease-ack response or a another Stop event.  The defined state
    transitions are designed to ensure that the neighbor does with high
    probability receive the Cease command and stop the protocol.

     In following sections of this document document a state machine
which can serve as a model for implementation is described.  It may
happen that implementators may deviate from this model while conforming
to the protocol specification;  however, in order to verify conformance
to the specification, the state-machine model is intended as the
reference model.

     Although not mentioned specifically in this document, it should be
understood that all Internet gateways must include support for the
Internet Control Message Protocol (ICMP), specifically ICMP Redirect and
ICMP Destination Unreachable messages.

2.  Nomenclature

     The following EGP message types are recognized in this document.
The format of each of these messages is described in Appendix A.
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        Name            Function
        ------------------------------------------------------
        Request         request acquisition of neighbor and/or
                        initialize polling variables
        Confirm         confirm acquisition of neighbor and/or
                        initialize polling variables
        Refuse          refuse acquisition of neighbor
        Cease           request de-acquisition of neighbor
        Cease-ack       confirm de-acquisition of neighbor
        Hello           request neigbor reachability
        I-H-U           confirm neigbor reachability
        Poll            request net-reachability update
        Update          net-reachability update
        Error           error

     EGP messages are classed as commands which request some action,
responses, which are sent to indicate the status of that action, and
indications, which are similar to responses, but may be sent at any
time.  Following is a list of commands along with their possible
responses.

        Command         Corresponding Responses
        ---------------------------------------
        Request         Confirm, Refuse, Error
        Cease           Cease-ack, Error
        Hello           I-H-U, Error
        Poll            Update, Error

     The Update and Error messages are classed both as responses and
indications.  When sent in reply to a previous command, either of these
messages is classed as a response.  In some circumstances an unsolicited
Update message can be sent, in which case it is classed as an
indication.  The use of the Error message other than as a response to a
previous command is a topic for further study.

3.  State Machine

     This section describes the state-machine model for EGP, including
the variables and constants which establish the state at any time, the
events which cause the state transitions, the actions which result from
these transitions and the state-transition table which defines the
behavior.

3.1.  State Variables

     The state-machine model includes a number of state variables which
establish the state of the protocol between the gateway and each of its
neighbors.  Thus, a gateway maintaining EGP with a number of neighbors
must maintain a separate set of these state variables for each neighbor.
The current state, events and actions of the state machine apply to each
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D.L. Mills


neighbor separately.

     The model assumes that system resources, including the set of state
variables, are allocated when the state machine leaves the Idle state,
either because of the arrival of a Request specifying a new neighbor
addreess, or because of a Start event specifying a new neighbor address.
When either of these events occur the values of the state variables are
initialized as indicated below.  Upon return to the Idle state all
resources, including the set of state variables, are deallocated and
returned to the system.  Implementators may, of course, elect to
dedicate resources and state variables permananently.

     Included among the set of state variables are the following which
determine the state transitions of the model.  Initial values for all of
the variables except the send sequence number S are set during the
initial Request/Confirm exchange.  The initial value for S is arbitrary.

        Name    Function
        --------------------------------------------------------------
        R       receive sequence number
        S       send sequence number
        T1      interval between Hello command retransmissions
        T2      interval between Poll command retransmissions
        T3      interval during which neighbor-reachability
                indications are counted
        M       hello polling mode
        t1      timer 1 (used to control Request, Hello and Cease
                command retransmissions)
        t2      timer 2 (used to control Poll command retransmissions)
        t3      timer 3 (abort timer)

Additional state variables may be necessary to support various timer and
similar internal housekeeping functions.  The function and management of
the cited variables are discussed in Section 4.

3.2.  Fixed Parameters

     This section defines several fixed parameters which characterize
the gateway functions.  Included is a suggested value for each parameter
based on experimental implementations in the Internet system.  These
values may or may not be appropriate for the individual configuration.

     Following is a list of time-interval parameters which control
retransmissions and other time-dependent functions.
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        Name    Value   Description
        --------------------------------------------------------------
        P1      30 sec  minimum interval acceptable between successive
                        Hello commands received
        P2      2 min   minimum interval acceptable between successive
                        Poll commands recieved
        P3      30 sec  interval between Request or Cease command
                        retransmissions
        P4      1 hr    interval during which state variables are
                        maintained in the absence of commands or
                        responses in the Down and Up states.
        P5      2 min   interval during which state variables are
                        maintained in the absence of responses in the
                        Acquisition and Cease states

     Parameters P4 and P5 are used only if the abort-timer option is
implemented.  Parameter P4 establishes how long the machine will remain
in the Down and Up states in the absence of commands or responses and
would ordinarily be set to sustain state information while the neighbor
is dumped and restarted, for example.  Parameter P5 establishes how long
the machine will remain in the Acquisition or Cease states in the
absence of responses and would ordinarily be set in the same order as
the expected value of T3 variables.

     Following is a list of other parameters of interest.

        Name    Active  Passive Description
        -----------------------------------------------
        j       3       1       neighbor-up threshold
        k       1       4       neighbor-down threshold

     The j and k parameters establish the "noise immunity" of the
neighbor-reachability protocol described later.  The values in the
Active column are suggested if the gateway elects to do hello polling,
while the values in the Passive column are suggested otherwise.

3.3.  Events

     Following is a list of events that can cause state transitions in
the model.
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Name            Event
----------------------------------------------------------------------
Up              At least j neighbor-reachability indications have been
                received within the last T3 seconds.
Down            At most k neighbor-reachabilitiy indications have been
                received within the last T3 seconds.
Request         Request command has been received.
Confirm         Confirm command has been received.
Refuse          Refuse response has been received.
Cease           Cease command has been received.
Cease-ack       Cease-ack response has been received.
Hello           Hello command has been received.
I-H-U           I-H-U response has been received.
Poll            Poll command has been received.
Update          Update response has been received.
Start           Start event has been recognized due to system or
                operator intervention.
Stop/t3         Stop event has been recognized due to (a) system or
                operator intervention or (b) expiration of the abort
                timer t3.
t1              Timer t1 has counted down to zero.
t2              Timer t2 has counted down to zero.

     There is one special event, called a neighbor-reachability
indication, which occurs when:

1.  The gateway is operating in the active mode (hello polling enabled)
    and either a Confirm, I-H-U or Update response is received.

2.  The gateway is operating in the passive mode (hello polling
    disabled) and either a Hello or Poll command is received with the
    "Up state" code in the Status field.

3.4.  State Transition Table

     The following table summarizes the state transitions that can occur
in response to the events listed above.  Transitions are shown in the
form n/a, where n is the next state and a represents the action.
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             0 Idle      1 Aqsn      2 Down       3 Up       4 Cease
          +-----------+-----------+-----------+-----------+-----------+
Up        |0          |1          |3/Poll     |3          |4          |
Down      |0          |1          |2          |2          |4          |
Request   |2/Confirm *|2/Confirm  |2/Confirm  |2/Confirm  |4/Cease    |
Confirm   |0/Cease  **|2          |2          |3          |4          |
Refuse    |0/Cease  **|0          |2          |3          |4          |
Cease     |0/Cease-ack|0/Cease-ack|0/Cease-ack|0/Cease-ack|0/Cease-ack|
Cease-ack |0          |1          |2          |3          |0          |
Hello     |0/Cease  **|1          |2/I-H-U    |3/I-H-U    |4          |
I-H-U     |0/Cease  **|1          |2/Process  |3/Process  |4          |
Poll      |0/Cease  **|1          |2          |3/Update   |4          |
Update    |0/Cease  **|1          |2          |3/Process  |4          |
Start     |1/Request  |1/Request  |1/Request  |1/Request  |4          |
Stop/t3   |0          |0          |4/Cease    |4/Cease    |0          |
t1        |0          |1/Request  |2/Hello    |3/Hello    |4/Cease    |
t2        |0          |1          |2          |3/Poll     |4          |
          +-----------+-----------+-----------+-----------+-----------+

Note *:  The transition shown applies to the case where the
neighbor-acquisition request is accepted.  The transition "0/Refuse"
applies to the case where the request is rejected.

Note **:  The Cease action shown is optional.

3.5.  State Transitions and Actions

     The following table describes in detail the transitions of the
state machine and the actions evoked.

                Next    Message
Event           State   Sent            Actions
------------------------------------------------------------------------

Idle State (0)

Request         2       Confirm         Initialize state variables and
                        Hello           reset timer t1 to T1 seconds and
                                        reset timer t3 to P5 seconds.
  (or)          0       Refuse          Return resources.
Cease           0       Cease-ack       Return resources.
Start           1       Request         Reset timer t1 to P3 seconds and
                                        reset timer t3 to P5 seconds.

Acquisition State (1)

Request         2       Confirm         Initialize state variables and
                        Hello           reset timer t1 to T1 seconds and
                                        reset timer t3 to P5 seconds.
Confirm         2       Hello           Initialize state variables and
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                                        reset timer t1 to T1 seconds and
                                        reset timer t3 to P5 seconds.
Refuse          0                       Stop timers and return
                                        resources.
Cease           0       Cease-ack       Stop timers and return
                                        resources.
Start           1       Request         Reset timer t1 to P3 seconds and
                                        reset timer t3 to P5 seconds.
Stop/t3         0                       Stop timers and return
                                        resources.
t1              1       Request         Reset timer t1 to P3 seconds.

Down State (2)
Note: Reset timer t3 to P4 seconds on receipt of a reachability
indication.

Up              3       Poll            Reset timer t2 to T2 seconds.
Request         2       Confirm         Reinitialize state variables and
                        Hello           reset timer t1 to T1 seconds and
                                        reset timer t3 to P5 seconds.
Cease           0       Cease-ack       Stop timers and return
                                        resources.
Hello           2       I-H-U
I-H-U           2                       Process neighbor-reachability
                                        info.
Start           1       Request         Reset timer t1 to P3 seconds and
                                        reset timer t3 to P5 seconds.
Stop/t3         4       Cease           Reset timer t1 to P3 seconds and
                                        reset timer t3 to P5 seconds.
t1              2       Hello           Reset timer t1 to T1 seconds.

Up State (3)
Note: Reset timer t3 to P4 seconds on receipt of a reachability
indication.

Down            2                       Stop timer t2.
Request         2       Confirm         Renitialize state variables and
                        Hello           reset timer t1 to T1 seconds and
                                        reset timer t3 to P5 seconds.
Cease           0       Cease-ack       Stop timers and return
                                        resources.
Hello           3       I-H-U
I-H-U           3                       Process neighbor-reachability
                                        info.
Poll            3       Update
Update          3                       Process net-reachability info.
Start           1       Request         Reset timer t1 to P3 seconds and
                                        reset timer t3 to P5 seconds.
Stop/t3         4       Cease           Reset timer t1 to P3 seconds and
                                        reset timer t3 to P5 seconds.
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t1              3       Hello           Reset timer t1 to T1 seconds.
t2              3       Poll            Reset timer t2 to T2 seconds.

Cease State (4)

Request         4       Cease
Cease           0       Cease-ack       Stop timers and return
                                        resources.
Cease-ack       0                       Stop timers and return
                                        resources.
Stop/t3         0                       Stop timers and return
                                        resources.
t1              4       Cease           Reset timer t1 to P3 seconds.

4.  Functional Description

     This section contains detailed descriptions of the various
procedures and algorithms used to manage the protocol.

4.1.  Managing the State Variables

     The state variables which characterize the protocol are summarized
in Section 3.1.  This section describes the detailed management of these
variables, including sequence numbers, polling intervals and timers.

4.1.1.  Sequence Numbers

     All EGP commands and replies carry a sequence number.  The state
variable R records the last sequence number received in a command from
that neighbor.  The current value of R is used as the sequence number
for all replies and indications sent to the neighbor until a command
with a different sequence number is received from that neighbor.

     Implementors are free to manage the sequence numbers of the
commands sent;  however, it is suggested that a separate send state
variable S be maintained for each EGP neighbor and that its value be
incremented just before the time an Poll command is sent and at no other
times.  The actions upon receipt of a response or indication with
sequence number not equal to S is not specified;  however, it is
recommended these be discarded.

4.1.1.  Polling Intervals

     As part of the Request/Confirm exchange a set of polling intervals
are established including T1, which establishes the interval between
Hello command retransmissions, and T2, which establishes the interval
between Poll retransmissions.

     Each gateway configuration is characterized by a set of fixed
parameters, including P1, which specifies the minimum polling interval
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at which it will respond to Hello commands, and P2, which specifies the
minimum polling interval at which it will respond to Poll commands.  P1
and P2 are inserted in the Hello Interval (S1) and Poll Interval (S2)
fields, respectively, of Request commands and Confirm responses.

     A gateway receiving a Request command or Confirm response uses the
S1 and S2 fields in the message to calculate its own T1 and T2 state
variables, respectively.  Implementors are free to perform this
calculation in arbitrary ways;  however, the following constraints must
be observed:

1.  If T1 < S1 the neighbor may discard Hello commands.  If T2 < S2 the
    neighbor may discard Poll commands.

2.  The time window T3 in which neighbor-reachability indications are
    counted is dependent on T1.  In the case where two neighbors select
    widely differing values for their T3 state variables, the
    neighbor-reachability algorithm may not work properly.  This can be
    avoided if T1 > max(P1, S1).

3.  If either S1 or S2 or both are unacceptable for some reason (e.g.
    exceed useful limits), the neighbor may either send a Refuse
    response or declare a Stop event, depending on state.

     It is suggested that T3 be computed as four times the value of T1,
giving a window of four neighbor-reachability indications, which has
been found appropriate in the experimental implementations.
Implementors may choose to make T3 a fixed parameter in those cases
where the path between the neighbors has well-known characteristics.

     Note that, if a gateway attempts to send Hello commands near the
rate max(P1, S1) or Poll commands near the rate max(P2, S2), the
neighbor may observe their succeeding arrivals to violate the polling
restrictions due to bunching in the net.  For this reason the gateway
should send at rates somewhat below these.  Just how much below these
rates is appropriate depends on many factors beyond the scope of this
specification.

4.1.3.  Hello Polling Mode

     The neighbor-reachability algorithm can be used in either the
active or passive mode.  In the active mode Hello commands are sent
periodically along with Poll commands, with reachability determined by
the corresponding I-H-U and Update responses.  In the passive mode Hello
commands are not sent and I-H-U responses are not expected.
Reachability is then determined from the Status field of received Hello
or Poll commands or Update responses.

     The M state variable specifies whether the gateway operates in the
active or passive mode.  At least one of the two neighbors sharing the
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protocol must operate in the active mode;  however, the
neighbor-reachability protocol is designed to work even if both
neighbors operate in the active mode.  The value of M is determined from
the Status field of a Request command or Confirm response.  The sender
sets this field according to whether the implementation supports the
active mode, passive mode or both:

                Status  Sender capabilities
                --------------------------------
                0       either active or passive
                1       active only
                2       passive only

     The receiver inspects this field and sets the value of M according
to its own capabilities as follows:

                Status  Receiver capabilites
                field   0       1       2
                -------------------------------
                0       *       active  passive
                1       passive active  passive
                2       active  active  **

     In the case of "*" the mode is determined by comparing the
autonomous system numbers of the neigbors.  The neighbor with the
smallest such number assumes active mode, while the other neighbor
assumes passive mode.  In the case of "**" the neighbor may either send
a Refuse response or declare a Stop event, depending on state.

4.1.4.  Timers

     There are three timers defined in the state machine:  t1, used to
control retransmission of Request, Hello and Cease messages, t2, used to
control retransmission of Poll commands, and t3, which serves as an
abort-timer mechanism should the protocol hang indefinately.  The timers
are set to specified values upon entry to each state and count down to
zero.

     In the case of t1 and t2 state-dependent events are declared when
the timer counts down to zero, after which the timer is reset to the
specified value and counts down again.  In the case of t3 a Stop event
is declared when the timer counts down to zero.  Implementors may choose
not to implement t3 or, if so, may choose to implement it only in
certain states, with the effect that Request, Hello and/or Cease
commands may be retransmitted indefinately.

     The following table shows the initial values for each of the timers
in each state.  A missing value indicates the timer is not used in that
state.  Note that timer t3 is set to P4 upon receipt of a
neighbor-reachability indication when in either the Down or Up states.
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                Idle    Aqsn    Down    Up      Cease
        Timer   0       1       2       3       4
        ---------------------------------------------
        t1              P3      T1              P3
        t2                              T2
        t3              P5      P5              P5

4.2.  Starting and Stopping the Protocol

     The Start and Stop events are intrinsic to the system environment
of the gateway.  They can be declared as the result of the gateway
process being started and stopped by the operator, for example.  A Start
event has meaning only in some states;  however, a Stop event has
meaning in all states.

     In all except the Idle state the abort timer t3 is presumed
running.  This timer is initialized at P5 seconds upon entry to any
state and at P4 seconds upon receipt of a neighbor-reachability
indication in the Down and Up states.  If it expires a Stop event is
declared.  A Stop event can also be declared by an intrinsic system
action such as a resource problem or operator command.

     If the abort timer is not implemented a manually-initiated Stop
event can be used to stop the protocol.  If this is done in the Down or
Up states, the machine will transition to the Cease state and emit a
Cease command.  If the neighbor does not respond to this command the
machine will stay in the Cease state indefinately;  however, a second
Stop event can be used in this state to force a transition to the Idle
state.

     A Cease command received in any state will cause the gateway to
immediately send the Cease-ack response and transition to the Idle
state.  This causes the protocol to be stopped and all system resources
committed to the gateway process to be released.  The interval between
the time the gateway enters the Idle state as the result of receiving a
Cease command and the time when it next sends a Request command to
resume the protocol is not specified;  however, it is recommended this
interval be at least P5 seconds.

     It may happen that the Cease-ack response is lost in the network,
causing the neighbor to retransmit the Cease response indefinately, at
least if it has not implemented the abort-timer option.  In order to
reduce the likelihood of this happening, it is suggested that a gateway
in the Idle state be prepared to reply to a Cease command with a
Cease-ack response whenever possible.

4.3.  Determining Neighbor Reachability

     The purpose of the neighbor-reachability algorithm is to confirm
that the neighbor can safely be considered operational and capable of
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providing reliable net-reachability information.  An equally important
purpose is to filter noisy reachability information before sending it on
to the remainder of the Internet gateway system, thus avoiding
unneccesary reachability changes.

     As described above, a gateway operating in the active mode sends
periodic Hello commands and listens for I-H-U responses in order to
determine neighbor-reachability indications.  A gateway operating in the
passive mode determines reachability indications by means of the Status
field in received Hello commands.  Poll commands and Update responses
can be used in lieu of Hello commands and I-H-U responses respectively,
since they contain the same Status-field information.

     The neighbor-reachability algorithm runs continuously while the
gateway is in the Down and Up states and operates as follows.  Define a
moving window in time starting at the present and extending backwards
for t seconds.  Then count the number n of neighbor-reachability
indications which have occured in that window.  If n increases to j,
then declare a Up event.  If n decreases to k, then declare a Down
event.  The number n is set to zero upon entering the Down state from
any state other than the Up state.

     The window t in this algorithm is defined as T3 seconds, the value
of which is suggested as four times T1, which itself is determined
during the Request/Confirm exchange.  For proper operation of the
algorithm only one neighbor-reachability indication is significant in
any window of T1 seconds and additional ones are ignored.  Note that the
only way n can increase is as the result of a new neighbor-reachability
indication and the only way it can decrease is as the result of an old
neighbor-reachability indication moving out of the window.

     The behavior of the algorithm described above and using the
suggested fixed parameters j and k differs depending on whether the
gateway is operating in the active or passive mode.  In the active mode
(j = 3, k = 1 and T3/T1 = 4), once the neighbor has been declared down
it will be forced down for at least two T1 intervals and, once it has
been declared up it will be forced up for at least two T1 intervals.  It
will not change state unless at least three of the last four
determinations of reachability have indicated that change.

     In the passive mode (j = 1, k = 4 and T3/T1 = 4), the neighbor will
be considered up from the first time the Status field of a Hello or Poll
command or Update response indicates "Up state" until four successive T1
intervals have passed without such indication.  This design, suggested
by similar designs used in the ARPANET, has proven effective in the
experimental implementations, but may need to be adjusted for other
configurations.

     It is convenient for the active gateway to send Hello commands at a
rate of one every T1 seconds and substitute a Poll command for a Hello
Exterior Gateway Protocol Formal Specification                   Page 15
D.L. Mills


command approximately once every T2 seconds, with the
neighbor-reachability indication generated by the corresponding I-H-U or
Update responses.  Its passive neighbor generates neighbor-reachability
indications from the Status field of received Hello and Poll commands
and Update responses.


     Implementors may find the following model useful in the
understanding and implementation of this algorithm.  Consider an n-bit
shift register that shifts one bit to the right each T1-second interval.
If a neighbor-reachability indication was received during the preceeding
T1-second interval a one bit is shifted into the register at the end of
the interval;  otherwise, a zero bit is shifted.  A table of 2**n
entries indexed by the contents of the register can be used to calculate
the number of one bits, which can then be used to declare the
appropriate event to the state machine.  A value of n equal to four has
been found useful in the experimental implementations.

4.4.  Determining Network Reachability

     Network reachability information is encoded into Update messages in
the form of lists of nets and gateways.  The IP Source Address field of
the Poll command is used to specify a network common to the autonomous
systems of each of the neighbors, which is usually, but not necessarily,
the one common to the neighbors themselves.  The Update response
includes a list of gateways on the common net.  Associated with each
gateway is a list of the networks reachable via that gateway together
with corresponding hop counts.

     It is important to understand that, at the present state of
development as described in RFC-827 and RFC-888, the EGP architectural
model restricts the interpretation of "reachable" in this context.  This
consideration, as well as the implied topological restrictions, are
beyond the scope of discussion here.  The reader is referred to the RFCs
for further discussion.

     Two types of gateway lists can be included in the Update response,
the format of which is described in Appendix A.  Both lists include only
those gateways directly connected to the net specified in the IP Source
Network field of the last-received Poll command.  The internal list
includes some or all of the gateways in the same autonomous system as
the sender, together with the nets which are reachable via these
gateways, with the sending gateway listed first.  A net is reachable in
this context if a path exists to that net including only gateways in the
system.  The external list includes those gateways in other autonomous
systems known to the sender.  It is important to realize that the hop
counts do not represent a routing metric and are comparable between
different gateways only if those gateways belong to the same autonomous
system;  that is, are in the internal list.

Exterior Gateway Protocol Formal Specification                   Page 16
D.L. Mills


     According to the current system architectural model, only gateways
belonging to a designated system, called the core system, may include
the external list in their Update responses.  All other gateways may
include only those gateways belonging to the same system and can claim
reachability for a particular net only if that net is reachable in the
same system.

     The interval between successive Poll commands T2 is determined
during the Request/Confirm exchange.  However, the specification permits
at most one unsolicited Update indication between succeeding Poll
commands received from the neighbor.  It is the intent of the model here
that an Update indication is sent (a) upon entry to the Up state and (b)
when a change in the reachability data base is detected, subject to this
limitation.

     Occasionally it may happen that a Poll command or Update response
is lost in the network, with the effect that net-reachability
information may not be available until after another T2 interval.  As an
implementation option, the gateway sending a Poll command and not
receiving an Update response after T1 seconds may send another Poll.
The gateway receiving this Poll may either (a) send an Update response
if it never received the original Poll for that interval, (b) send a
second Update response (which counts as the unsolicited Update
indication mentioned in the preceeding paragraph) or (c) send an Error
response or not respond at all in other cases.

4.5.  Error Messages

     Error messages can be used to report problems such as described in
Appendix A in connection with the Error Response/Indication message
format.  In general, an Error message is sent upon receipt of another
command or response with bad format, content or ordering, but never in
response to another Error message.  Receipt of an Error message should
be considered advisory and not result in change of state, except
possibly to evoke a Stop event.
Exterior Gateway Protocol Formal Specification                   Page 17
D.L. Mills


Appendix A.  EGP Message Formats

     The formats for the various EGP messages are described in this
section.  All EGP messages include a ten-octet header of six fields,
which may be followed by additional fields depending on message type.
The format of the header is shown below along with a description of its
fields.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | EGP Version # |     Type      |     Code      |    Status     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |        Checksum               |       Autonomous System #     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |        Sequence #             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

EGP Version #           assigned number identifying the EGP version
                        (currently 2)

Type                    identifies the message type

Code                    identifies the message code (subtype)

Status                  contains message-dependent status information

Checksum                The EGP checksum is the 16-bit one's complement
                        of the one's complement sum of the EGP message
                        starting with the EGP version number field. When
                        computing the checksum the checksum field itself
                        should be zero.

Autonomous System #     assigned number identifying the particular
                        autonomous system

Sequence #              send state variable (commands) or receive state
                        variable (responses and indications)

     Following is a description of each of the message formats.  Note
that the above description applies to all formats and will not be
repeated.
Exterior Gateway Protocol Formal Specification                   Page 18
D.L. Mills


A.1.  Neighbor Acquisition Messages

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | EGP Version # |     Type      |     Code      |    Status     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |        Checksum               |       Autonomous System #     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |        Sequence #             |          Hello Interval       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |        Poll Interval          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Note:  the Hello Interval and Poll Interval fields are present only in
Request and Confirm messages.

Type                    3

Code                    0       Request command
                        1       Confirm response
                        2       Refuse response
                        3       Cease command
                        4       Cease-ack response

Status (see below)      0       unspecified
                        1       active mode
                        2       passive mode
                        3       insufficient resources
                        4       administratively prohibited
                        5       going down
                        6       parameter problem
                        7       protocol violation

Hello Interval          minimum Hello command polling interval (seconds)

Poll Interval           minumum Poll command polling interval (seconds)

Following is a summary of the assigned Status codes along with a list of
scenarios in which they might be used.
Exterior Gateway Protocol Formal Specification                   Page 19
D.L. Mills


Code    Status                  Scenarios
-------------------------------------------------------------------
0       unspecified             when nothing else fits

1       active mode             Request/Confirm only

2       passive mode            Request/Confirm only

3       insufficient resources  1. out of table space
                                2. out of system resources

4       administratively        1. unknown Autonomous System
        prohibited              2. use another gateway

5       going down              1. operator initiated Stop
                                2. abort timeout

6       parameter problem       1. nonsense polling parameters
                                2. unable to assume compatible mode

7       protocol violation      1. Invalid command or response
                                   received in this state
Exterior Gateway Protocol Formal Specification                   Page 20
D.L. Mills


A.2. Neighbor Reachability Messages

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | EGP Version # |     Type      |     Code      |    Status     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    Checksum                   |    Autonomous System #        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      Sequence #               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type                    5

Code                    0       Hello command
                        1       I-H-U response

Status                  0       indeterminate
                        1       Up state
                        2       Down state
Exterior Gateway Protocol Formal Specification                   Page 21
D.L. Mills


A.3. Poll Command

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | EGP Version # |    Type       |     Code      |    Status     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         Checksum              |       Autonomous System #     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         Sequence #            |           Reserved            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       IP Source Network                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type                    2

Code                    0

Status                  0       indeterminate
                        1       Up state
                        2       Down state

IP Source Network       IP network number of the network about which
                        reachability information is being requested
                        (coded as 1, 2 or 3 octets, left justified with
                        trailing zeros)
Exterior Gateway Protocol Formal Specification                   Page 22
D.L. Mills


A.4. Update Response/Indication

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | EGP Version # |    Type       |     Code      |    Status     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    Checksum                   |       Autonomous System #     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    Sequence #                 | # of Int Gwys | # of Ext Gwys |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       IP Source Network                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Gateway 1 IP address (without network #)      | (1-3 octets)
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  # Distances  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Distance 1   |   # Nets      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   net 1,1,1   ||||||||||||||||||||||||||||||||| (1-3 octets)
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   net 1,1,2   ||||||||||||||||||||||||||||||||| (1-3 octets)
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Distance 2   |   # Nets      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   net 1,2,1   ||||||||||||||||||||||||||||||||| (1-3 octets)
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   net 1,2,2   ||||||||||||||||||||||||||||||||| (1-3 octets)
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Gateway  n IP address (without network #)         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  # Distances  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Distance 1   |  # Nets       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   net n,1,1   |||||||||||||||||||||||||||||||||  (1-3 octets)
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   net n,1,2   |||||||||||||||||||||||||||||||||  (1-3 octets)
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Distance 2   |  # Nets       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   net n,2,1   |||||||||||||||||||||||||||||||||  (1-3 octets)
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   net n,2,2   |||||||||||||||||||||||||||||||||  (1-3 octets)
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           ...
Exterior Gateway Protocol Formal Specification                   Page 23
D.L. Mills


Type                    1

Code                    0

Status                  0       indeterminate
                        1       Up state
                        2       Down state
                        128     unsolicited message bit

# of Int Gwys           number of interior gateways appearing in this
                        message

# of Ext Gwys           number of exterior gateways appearing in this
                        message

IP Source Network       IP network number of the network about which
                        reachability information is being supplied
                        (coded as 1, 2 or 3 octets, left justified with
                        trailing zeros)

Gateway IP addresses    IP address (without network number) of the
                        gateway block (coded as 1, 2 or 3 octets)

# of Distances          number of distances in the gateway block

Distances               numbers depending on autonomous system
                        architecture

# of Nets               number of nets at each distance

Nets                    IP network number reachable via the gateway
Exterior Gateway Protocol Formal Specification                   Page 24
D.L. Mills


A.5. Error Response/Indication

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | EGP Version # |    Type       |     Code      |    Status     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    Checksum                   |       Autonomous System #     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       Sequence #              |          Reason               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                     Error Message Header                      |
     |            (first three 32-bit words of EGP header)           |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type                    8

Code                    0

Status                  0       indeterminate
                        1       Up state
                        2       Down state
                        128     unsolicited message bit

Reason (see below)      0       unspecified
                        1       bad EGP header format
                        2       bad EGP data field format
                        3       reachability info unavailable
                        4       excessive polling rate
                        5       no response

Error Message Header    first three 32-bit words of EGP header

Following is a summary of the assigned Reason codes along with a list of
scenarios in which they might be used.
Exterior Gateway Protocol Formal Specification                   Page 25
D.L. Mills


Code    Reason                  Scenarios
------------------------------------------------------------------------
0       unspecified             when nothing else fits

1       bad EGP header format   1. bad message length
                                2. invalid Type, Code or Status fields

                                Notes: The recipient can determine which
                                of the above hold by inspecting the EGP
                                header included in the message. An
                                instance of a wrong EGP version or bad
                                checksum should not be reported, since
                                the original recipient can not trust the
                                header format. An instance of an unknown
                                autonomous system should be caught at
                                acquistion time.

2       bad EGP data field      1. nonsense polling rates
        format                     (Request/Confirm)
                                2. invalid Update message format
                                3. response IP Net Address field does
                                   not match command (Update)

                                Notes: An instance of nonsense polling
                                intervals (e.g. too long to be useful)
                                specified in a Request or Confirm should
                                result in a Refuse or Cease with this
                                cause specified.

3       reachability info       1. no info available on net specified in
        unavailable                IP Net Address field (Poll)

4       excessive polling rate  1. two or more Hello commands received
                                   within minimum specified polling
                                   interval
                                2. two or more Poll commands received
                                   within minimum specified polling
                                   interval
                                3. two or more Request commands received
                                   within some (reasonably short)
                                   interval

                                Notes: The recipient can determine which
                                of the above hold by inspecting the EGP
                                header included in the message.

5       no response             1. no Update received for Poll within
                                   some (reasonably long) interval
Exterior Gateway Protocol Formal Specification                   Page 26
D.L. Mills


Appendix B.  Comparison with RFC-888

     Minor functional enhancements are necessary in the RFC-888 message
formats to support certain features assumed of the state-machine model,
in particular the capability to request a neighbor to suppress Hello
commands.  In addition, the model suggests a mapping between its states
and certain status and error indications which clarifies and generalizes
the interpretation.

     All of the header fields except the Status field (called the
Information field at some places in RFC-888) remain unchanged.  The
following table summarizes the suggested format changes in the Status
field for the various messages by (Type, Code) class.

Class   Messages                Status Codes
-------------------------------------------------------------------
3,0     Request                 0       unspecified
3,1     Confirm                 1       active mode
3,2     Refuse                  2       passive mode
3,3     Cease                   3       insufficient resources
3,4     Cease-ack               4       administratively prohibited
                                5       going down
                                6       parameter problem

5,0     Hello                   0       indeterminate
5,1     I-H-U                   1       Up state
2,0     Poll                    2       Down state
1,0     Update                  128     unsolicited message bit
8,0     Error

The changes from RFC-888 are as follows:

1.  The status codes have been combined in two classes, one for those
    messages involved in starting and stopping the protocol and the
    other for those messages involved in maintaining the protocol and
    exchanging reachability information.  Some messages of either class
    may not use all the status codes assigned.

2.  The status codes for the Request and Confirm indicate whether the
    sender can operate in active or passive mode.  In RFC-888 this field
    must be zero;  however, RFC-888 does not specify any mechanism to
    decide how the neighbors poll each other.

3.  The status codes for the Cease, Refuse and Cease-ack have the same
    interpretation.  This provides a clear and unambiguous indication
    when the protocol is terminated due to an unusual situation, for
    instance if the NOC dynamically repartitions the ARPANET.  The
    assigned codes are not consistent with RFC-888, since the codes for
    the Refuse and Cease were assigned conflicting values;  however, the
    differences are minor and should cause no significant problems.
Exterior Gateway Protocol Formal Specification                   Page 27
D.L. Mills


4.  The status codes for the Hello, I-H-U, Poll, Update and Error have
    the same interpretation.  Codes 0 through 2 are mutually exclusive
    and are chosen solely on the basis of the state of the sender.  In
    the case of the Update (and possibly Error) one of these codes can
    be combined with the "unsolicited bit," which corresponds to code
    128.  In RFC-888 this field is unused for the Poll and Error and may
    contain only zero or 128 for the Update, so that the default case is
    to assume that reciprocal reachability cannot be determined by these
    messages.

5.  Some of the reachability codes defined in RFC-888 have been removed
    as not applicable.
Exterior Gateway Protocol Formal Specification                   Page 28
D.L. Mills


Appendix C.  Reachability Analysis

     The following table shows the state transitions which can occur in
a system of two neighboring EGP gateways.  Besides being useful in the
design and verification of the protocol, the table is useful for
implementation and testing.

     The system of two neighboring EGP gateways is modelled as a
finite-state automaton constructed as the cartesian product of two state
machines as defined above.  Each state of this machine is represented as
[i,j], where i and j are states of the original machine.  Each line of
the table shows one state transition of the machine in the form:

                        [i1,j1] -> [i2,j2]  E  A

which specifies the machine in state [i1,j1] presented with event E
transitions to state [i2,j2] and generates action A.  Multiple actions
are separated by the "/" symbol.  The special symbol "*" represents the
set of lines where all "*"s in the line take on the (same) values 0 - 4
in turn.

     The table shows only those transitions which can occur as the
result of events arriving at one of the two neighbors.  The full table
includes a duplicate set of lines for the other neighbor as well, with
each line derived from a line of the table below using the
transformation:

         [i1,j1] -> [i2,j2]  E  A  =>  [j1,i1] -> [j2,i2]  E  A

State    State  Event           Actions
---------------------------------------------------
[*,4] -> [0,4]  Cease           Cease-ack

[0,1] -> [2,1]  Request         Confirm/Hello/Up/t1
[0,1] -> [0,1]  Request         Refuse
[0,*] -> [1,*]  Start           Request/t1

[1,1] -> [2,1]  Request         Confirm/Hello/Up/t1
[1,2] -> [2,2]  Confirm         Hello/Up/t1
[1,3] -> [2,3]  Confirm         Hello/Up/t1
[1,0] -> [0,0]  Refuse          Null
[1,*] -> [1,*]  Start           Request/r1
[1,*] -> [0,*]  Stop            Null
[1,*] -> [1,*]  t1              Request/t1

[2,1] -> [3,1]  Up              Down/Hello/Poll/t1/t2
[2,1] -> [2,1]  Request         Confirm/Hello/Up/t1
[2,2] -> [2,2]  Hello           I-H-U
[2,3] -> [2,3]  Hello           I-H-U
[2,2] -> [2,2]  I-H-U           Process
Exterior Gateway Protocol Formal Specification                   Page 29
D.L. Mills


[2,3] -> [2,3]  I-H-U           Process
[2,*] -> [1,*]  Start           Request/r1
[2,*] -> [4,*]  Stop            Cease/t1
[2,1] -> [2,1]  t1              Hello/t1
[2,2] -> [2,2]  t1              Hello/t1
[2,3] -> [2,3]  t1              Hello/t1

[3,1] -> [2,1]  Down            Null
[3,2] -> [2,2]  Down            Null
[3,3] -> [2,3]  Down            Null
[3,1] -> [2,1]  Request         Confirm/Hello/Up/t1
[3,2] -> [3,2]  Hello           I-H-U
[3,3] -> [3,3]  Hello           I-H-U
[3,2] -> [3,2]  I-H-U           Process
[3,3] -> [3,3]  I-H-U           Process
[3,3] -> [3,3]  Poll            Update
[3,3] -> [3,3]  Update          Process
[3,*] -> [1,*]  Start           Request/r1
[3,*] -> [4,*]  Stop            Cease/t1
[3,1] -> [3,1]  t1              Hello/t1
[3,2] -> [3,2]  t1              Hello/t1
[3,3] -> [3,3]  t1              Hello/t1
[3,1] -> [3,1]  t2              Poll/t2
[3,2] -> [3,2]  t2              Poll/t2
[3,3] -> [3,3]  t2              Poll/t2

[4,1] -> [4,1]  Request         Cease
[4,*] -> [0,*]  Cease           Cease-ack
[4,0] -> [0,0]  Cease-ack       Null
[4,*] -> [0,*]  Stop            Null
[4,*] -> [4,*]  t1              Cease/t1

     In the state-machine model defined in this document all states of
the above machine are reachable;  however, some are reachable only in
extreme cases when one neighbor crashes, for example.  In the common
case where only one of the neighbors initiates and terminates the
protocol and neither one crashes, for example, not all states are
reachable.  Following is a matrix showing the states which can be
reached in this case, where the neighbor that initiates and terminates
the protocol is called the active gateway and the other the passive
gateway.
Exterior Gateway Protocol Formal Specification                   Page 30
D.L. Mills


                                Passive Gateway
Active     0 Idle      1 Aqsn      2 Down      3 Up        4 Cease
Gateway   +-----------+-----------+-----------+-----------+-----------+
0 Idle    |stable     |           |           |           |unstable   |
1 Aqsn    |unstable   |unstable   |unstable   |unstable   |unstable   |
2 Down    |           |           |stable     |unstable   |           |
3 Up      |           |           |unstable   |stable     |           |
4 Cease   |unstable   |unstable   |unstable   |unstable   |unstable   |
          +-----------+-----------+-----------+-----------+-----------+

     In the above matrix the blank entries represent unreachable states,
while those marked unstable represent transient states which cannot
persist for long, due to retransmission of Request and Hello messages,
for example.


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