The Question
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Suppose you want to connect to a network based on the X.25 protocol. You may
want to connect an InterBridge to their backbone zone, then connect the
InterBridge via one of its modem ports to one of the ports on an X.25 pad
located in the WHTC complex. The connection is to tie into another AppleTalk
network that has been connected in a similar fashion at some other point
in the X.25 network. Your concerns are:
- how to initiate the connection between two InterBridges across the X.25
bridge.
- the amount and size of packet traffic generated by two InterBridges polling
each other's existence across the X.25 bridge.
Making the Connection
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To initiate the communication between bridges, just turn on the InterBridge.
It then polls the nearest bridge and updates its internal table of zones
available on the net from the adjacent InterBridge. It also passes its zone
address number and name to the other bridge. If a bridge is turned off, it
will rebuild the zone table at power on. After a delay in rebuilding the
table, it maintains the table on its own. Applications such as the Chooser
and InBox, which communicate over AppleTalk, can request the zone information
from the InterBridge. Bridges periodically update their tables
and will, upon receiving a new zone address or a packet referencing an
unnamed zone address with a name, will update the table to reflect the new
zone address and zone name.
There is one consideration that may cause some applications not to function
with this network. Because InterBridges use ZIP, which is "best effort"
protocol, checking for timeouts or proper data handling must be handled by the
application. Some applications, notably AlisaTalk running on a VAX, will time-
out if delays are very long across the network. You are then at the mercy of
whatever traffic and traffic problems might exist on the X.25 net.
Packet Traffic
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During the first 30 seconds of polling time, approximately 5-6 packets are
transferred between InterBridges. These packets are in ZIP (Zone Information
Protocol). The first will be a ZIP bringup packet that will be delivered out
each of the connected ports. This is followed by a ZIP GetZoneList or
GetMyZone packet. The GetZoneList packet is replied with a GetZoneList Reply
packet. The new InterBridge might send a ZIP Query for information on packets
whose addresses are not yet in its Zone Information Table (ZIT). This also
will be replied and will be in the form of a ZIP reply packet.
The packets range in size: 8 bytes for a ZIP GetZoneList, 45 bytes for a
ZIP bringup, 13 bytes plus 32 bytes for each zonename for a ZIP reply packet,
and for a GetZoneList Reply there would be 9 bytes and 32 bytes for each zone
name.
The first group of bytes contain the LAP (Link Access Protocol) header, the
DDP Datagram Delivery Protocol) header, and the ZIP (Zone Information
Protocol) header. This will be true for takedown, bringup, query, and reply
packets. The ZIP GetzoneList and GetZoneList Reply packets use ATP (AppleTalk
Transaction Protocol) header and user bytes. These transactions take place
with each connected port.
There are two AppleTalk ports and two Serial ports on the InterBridge. After
the initial polling, one packet each 10 seconds is transmitted to maintain
the connection while idling.