rfc891.txt

RFC 相关的技术文档


DCN Local-Network Protocols                                        Page 16
D.L. Mills

DECREMENT-HOLD Event
    This event is evoked once per second to decrement the value of HOLD.

    1.  If the value of HOLD is zero, do nothing;  otherwise, decrement its
        value.

READ-CLOCK Procedure

    This procedure is called by a client process.  It returns the apparent
    time-of-day computed as the integer part of the sum CLK.CLOCK plus
    CLK.COUNT.  Note that the precision of the value returned is limited to
    +-1 millisecond, so that client processes must expect the apparent
    time to "run backward" occasionally due to drift corrections.  When
    this happens the backward step will never be greater than one
    millisecond and will never occur more often than twice per second.
    
    1.  In the case of line clocks CLK.COUNT is always zero, while in
        the case of programmable clocks the hardware must be
        interrogated to extract the value of CLK.COUNT.  If following
        interrogation a counter-overflow condition is evident, add
        CLOCK-TICK to CLK.CLOCK and interrogate the hardware again.
        
    2.  When the value of CLK.COUNT has been determined compute the sum
        CLK.COUNT + CLK.CLOCK.  If this sum exceeds the number of
        milliseconds in 24 hours (86,400,000), reduce CLK.CLOCK by
        86,400,000, set HOLD-INTERVAL -> HOLD, set CLOCK-VALID (bit 15
        of DATE) to one, roll over DATE to the next calendar day and
        start over.  If not, return the integer part of the sum as the
        apparent time-of-day. 
        
        The CLOCK-VALID bit is set to insure that a master-clock update is
        received at least once per day.  Note that, in the case of
        uncompensated crystal oscillators of the type commonly used as the
        1000-Hz time base, a drift of several parts per million can be
        expected, which would result in a time drift of several tenths of a
        second per day, if not corrected.

SET-CLOCK Procedure
    This procedure is called by a client process.  It sets a time-of-day
    correction factor in milliseconds.  The argument represents a 32-bit
    signed fixed-point quantity with decimal point to the right of bit
    0 that is to be added to CLK.CLOCK so that READ-CLOCK subsequently
    returns the actual time-of-day.  
    
    1.  If the correction factor is in the range -2**(16-ADJUST-FRACTION) to
        +2**(16-ADJUST-FRACTION) - 1 (about +-128 milliseconds with the
        suggested value of ADJUST-FRACTION), the value of the argument
        replaces CLK.DELTA and the procedure is complete.  If not, add the
        value of the sign-extended argument to CLK.CLOCK and set CLK.DELTA to
        zero.  In addition, set HOLD-INTERVAL -> HOLD, since this
        represents a relatively large step-change in apparent time.
        The value of HOLD represents the remaining number of seconds
        in which timestamps should be considered invalid and is used
        by the HELLO process to suppress roundtrip delay calculations
        which might involve invalid timestamps. 

DCN Local-Network Protocols                                        Page 17
D.L. Mills

        

3.3.  HELLO Process

     The HELLO process maintains clock synchronization with a neighbor
HELLO process using the HELLO protocol.  It also participates in the
routing algorithm.  There is one HELLO process and one set of local
state variables for each link connecting the host to one of its
neighbors.

3.3.1.  Local variables

HLO.BROADCAST
    This is a one-bit switch state variable.  When set to zero a
    point-to-point link is assumed.  When set to one a broadcast (e.g.
    Ethernet) link is assumed.

HLO.KEEP-ALIVE
    This is an eight-bit counter state variable used to indicate whether the
    link is up.  It is initialized with a value of zero.

HLO.LENGTH
    This is a 16-bit integer state variable used to record the length in
    octets of the last HELLO message sent.

HLO.NEIGHBOR-ADDRESS
    This is a 32-bit integer state variable used to contain the neighbor host
    Internet address.

HLO.PID
    This is an eight-bit integer state variable used to identify the
    net-output process associated with this HELLO process.  It is initialized
    by the kernel when the process is created and remains unchanged
    thereafter.

HLO.POLL
    This is a one-bit switch state variable.  When set the HELLO process
    spontaneously sends HELLO messages.  When not set the HELLO process
    responds to HELLO messages, but does not send them spontaneously.

HLO.TIMESTAMP
    This is a 32-bit integer temporary variable used to record the time of
    arrival of a HELLO message.

HLO.TSP
    This is a 16-bit signed integer state variable used in roundtrip delay
    calculations.


DCN Local-Network Protocols                                        Page 18
D.L. Mills


3.3.2.  Parameters

HELLO-INTERVAL
    This is an integer which defines the interval in seconds between HELLO
    messages.  It ranges from about eight to a maximum of 30 seconds,
    depending on line speed.

HOLD-DOWN-INTERVAL
    This is an integer which defines the interval in seconds a host will be
    considered up following receipt of a HELLO message indicating that
    host is up.  A value of 120 is suggested.
    
KEEP-ALIVE-INTERVAL
    This is an integer which defines the interval, in units of
    HELLO-INTERVAL, that a HELLO process will consider the link up.  A
    value of four is suggested.
    
MAXDELAY
    This is an integer which defines the maximum roundtrip delay in
    seconds on a path to any reachable host.  A value of 30 is suggested.
    
MINDELAY
    This is an integer which defines the minimum switching threshold in
    milliseconds below which routes will not be changed.  A value of 100 is
    suggested.

3.3.3.  Events and Procedures

INPUT-PACKET Event
    When a packet arrives the net-input process first sets HLO.TIMESTAMP to
    the value returned by the READ-CLOCK procedure, then checks the
    packet for valid local leader, IP header format and checksum.  If
    the protocol field in the IP header indicates a HELLO message, the
    packet is passed to the HELLO process.  If any errors are found
    the packet is dropped. 
    
    The HELLO process first checks the packet for valid HELLO header format
    and checksum.  If any errors are found the packet is dropped.  Otherwise,
    it proceeds as follows:

    1.  If PKT.SOURCE is equal to LOCAL-ADDRESS, then the line to the
        neighbor host is looped.  If this is a broadcast link
        (HLO.BROADCAST is set to one), then ignore this nicety;  if
        not, this is considered an error and further processing is
        abandoned.  Note that, in special configurations involving
        other systems (e.g.  ARPANET IMPs and gateways) it may be
        useful to use looped HELLO to monitor connectivity.  The DCN
        implementation provides this feature, but is not described here.
        
    2.  Set KEEP-ALIVE-INTERVAL -> HLO.KEEP-ALIVE.  This indicates the
        maximum number of HELLO intervals the HLO.TSP field is
        considered valid. 


DCN Local-Network Protocols                                        Page 19
D.L. Mills


    3.  Set PKT.TIMESTAMP - HLO.TIMESTAMP -> HLO.TSP.  This is part of the
        roundtrip delay calculation.  The value of HLO.TSP will be
        updated and returned to the neighbor in the next HELLO message
        transmitted.  Next, compute the raw roundtrip delay and offset:
        HLO.TIMESTAMP - PKT.TSP -> DELAY and HLO.TSP + DELAY/2 -> OFFSET. 
        Note:  in the case of a broadcast link (HLO.BROADCAST set to one) set
        DELAY to zero.

    4.  Perform this step only in the case of non-broadcast links
        (HLO.BROADCAST set to zero).  If PKT.SOURCE is not equal to
        HLO.NEIGHBOR-ADDRESS, then a new neighbor has appeared on this
        link. Set PKT.SOURCE -> HLO.NEIGHBOR ADDRESS, MAXDELAY ->
        DELAY and proceed to the next step.  This will force the line
        to be declared down and result in a hold-down cycle.
        Otherwise, if either PKT.TSP is zero or HOLD is nonzero, then
        the DELAY calculation is invalid and further processing is
        abandoned.  Note that a hold-down cycle is forced in any 
        case if a new neighbor is recognized.

    5.  If processing reaches this point the DELAY and OFFSET
        variables can be assumed valid as well as the remaining data
        in the packet.  First, if DELAY < MINDELAY, set MINDELAY ->
        DELAY.  This avoids needless path switching when the
        difference in delays is not significant and has the effect
        that on low-delay links the routing algorithm degenerates to 
        min-hop rather than min-delay.  Then set HLO.PID -> PID.  There are
        two cases:

        Case 1:  PKT.NHOSTS is zero.
            This will be the case when the neighbor host has just come up or
            is on a different net or subnet.  Set NEIGHBOR-ADDRESS -> ADDRESS
            and call the ROUTE procedure, which will return the host
            ID.  Then call the UPDATE procedure.  In the case of
            errors, do nothing but return.
            
        Case 2:  PKT.NHOSTS is nonzero.
            This is the case when the neighbor host is on the same net or
            subnet.  First, save the values of DELAY and OFFSET in temporary
            variables F and G.  Then, for each value of HID from zero to
            NHOSTS-1 consider the corresponding PKT.HOSTS-TABLE entry and do
            the following:  Set F + PKT.HOST-TABLE.DELAY -> DELAY and
            G + PKT.HOST-TABLE.OFFSET -> OFFSET and call the UPDATE procedure.
            This completes processing.

        ROUTE Procedure
            This procedure returns the host ID in HID of the host represented
            by the global variable ADDRESS.

    1.  First, determine if the host represented by ADDRESS is on the same
        local net as LOCAL-ADDRESS.  For the purposes of this
        comparison bits 0-7 and 16-31 are compared for class-A nets
        and bits 8-31 are compared for class-B and class-C nets.  This
        provides for a subnet capability, where the bits 0-7 and 16-23
        (class-A) or 8-15 (class-B) are used as a subnet number.

DCN Local-Network Protocols                                        Page 20
D.L. Mills

        
        Case 1:  The host is on the same net or subnet.
            Extract the address field of ADDRESS, subtract ADDRESS-OFFSET and
            store the result in HID.  If 0 <= HID < NHOSTS, the procedure
            completes normally;  otherwise it terminates in an error
            condition.

        Case 2:  The host is not on the same net or subnet.
            Search the NET-TABLE for a match of the net fields of
            LOCAL-ADDRESS and NET-TABLE.NET.  If found set
            NET-TABLE.HID -> HID and return normally.  If the NET-TABLE.NET
            field is zero, indicating the last entry in the table, set
            HET-TABLE.HID -> HID and return normally.  Note that, in the case
            of hosts including GGP/EGP gateway modules, if no match is found
            the procedure terminates in an error condition.

UPDATE Procedure
    This procedure updates the entry of HOST-TABLE indicated by HID using
    three global variables:  DELAY, OFFSET and PID.  Its purpose is to update
    the HOST-TABLE entry corresponding to host ID HID.  In the following all
    references are to this entry.

    1.  If PID is not equal to HOST-TABLE.PID, the route to host HID is not
        via the net-output process associated with this HELLO process.  In
        this case, if DELAY + MINDELAY > HOST-TABLE.DELAY, the path is longer
        than one already in HOST-TABLE, so the procedure does nothing.

    2.  This step is reached only if either the route to host HID is via the
        net-output process associated with this HELLO process or the newly
        reported path to this host is shorter by at least MINDELAY.  
        There are two cases:

        Case 1:  HOST-TABLE.DELAY < MAXDELAY.
            The existing path to host HID is up and this is a point-to-point
            link (HLO.BROADCAST is set to zero).  If DELAY < MAXDELAY the
            newly reported path is also up.  Proceed to the next step.
            Otherwise, initiate a hold-down cycle by setting
            MAXDELAY -> HOST-TABLE.DELAY and
            HOLD-DOWN-INTERVAL -> HOST-TABLE.TTL and return.

        Case 2:  HOST-TABLE.DELAY >= MAXDELAY.
            The existing path to host HID is down.  If DELAY < MAXDELAY and
            HOST-TABLE.TTL is zero, the hold-down period has expired and the
            newly reported path has just come up.  Proceed to the next step.
            Otherwise simply return.

    3.  In this step the HOST-DELAY entry is updated.  Set
        DELAY -> HOST-TABLE.DELAY, HOLD-DOWN-INTERVAL -> HOST-TABLE.TTL and
        HLO.PID -> HOST-TABLE.PID.