rfc904.txt

RFC 相关的技术文档

                        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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D.L. Mills


             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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D.L. Mills


                                        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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D.L. Mills


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