RFC1582 - Extensions to RIP to Support Demand Circuits( 七 )

sends a valid "triggered" message, the destination should be marked
as supporting the mechanism once more (to allow for the next hop
router"s configuration being changed). It should be sent a triggered
request and a triggered response to obtain and propagate up-to-date
routing information.
2.8 Fragmentation
If a routing update is sufficiently large, the information MUST be
fragmented over several triggered response packets:
o Each fragment MUST be individually acknowledged with a triggered
acknowledgement packet.
The sender of the routing update MUST periodically retransmit
fragments which have not been acknowledged (or until the
destination is marked as not supporting the mechanism).
o A router receiving fragments MUST re-assemble them before
updating its routing database.
o If all fragments are not received within four times the
retransmit period, they MUST be discarded.
A triggered request packet MUST then be sent to the originator
of the routing update.
On receiving the triggered request packet, the originator of the
routing update MUST retransmit ALL fragments.
o If a fragment with an updated sequence number is received, ALL
fragments with the earlier sequence number MUST be discarded.
An updated sequence number is defined as any sequence number
that is different. There is no concept of the value of the
sequence number conveying its age.
Fragmentation timer values are covered in section 7.
2.9 Preventing Queue Overload
In order to prevent too many routing messages being queued at a WAN
interface, the routing task MAY operate a scheme whereby
"broadcasting" of a triggered request or triggered response to a WAN
interface is staggered. All routing requests or routing responses
are not sent to ALL next hop routers on the interface in a single
batch:
o The routing task should limit the number of outstanding triggered
request messages for which a triggered response has not been
received.
o The routing task should limit the number of outstanding triggered
response messages for which a triggered acknowledgement has not
been received.
As outstanding messages are appropriately acknowledged, further
messages can be sent out to other next hop routers, until all next
hop routers have been sent the message and have acknowledged it.
The maximum number of outstanding messages transmitted without
acknowledgement is a function of the link speed and the number of
other routing protocols operating the triggered update mechanism.
Messages should always be acknowledged immediately (even if it causes
the limit to be exceeded), since a connection is almost certainly
available. This has the potential benefit of allowing the VC to
close sooner (on its idle timer).
Sending all triggered request fragments to a destination at once is
also beneficial.
3. IP Routing Information Protocol Version 1
This section should be read in conjunction with reference [1].
IP RIP is a UDP-based protocol which generally sends and receives
datagrams on UDP port number 520.
To support the mechanism outlined in this proposal the packet format
for RIP version 1 [1] is modified as shown in Figure 2.
Every Routing Information Protocol datagram contains the following:
COMMAND Commands supported in RIP Version 1 are: request (1),
response (2), traceon (3), traceoff (4), SUN reserved (5).
The fields sequence number, fragment number and number of
fragments MUST NOT be included in packets with these
command values.
The following new commands (with values in brackets) are
required:
TRIGGERED REQUEST (6)
A request for the responding system to send all of its