For those using InterBridges to connect an AppleTalk network to an X.25
backbone, this article concerns the rate and size of packet traffic generated
by the idle polling of InterBridges across the net, as well as some information
on how to initialize this polling.
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 one is a ZIP bringup packet that comes out of
each of the connected ports. ZIP bringup is followed by a ZIP GetZoneList or
GetMyZone packet. The GetZoneList is followed by a GetZoneList Reply
packet. A newly started InterBridge sends out a ZIP Query for information on
packets whose addresses are not yet in its Zone Information Table (ZIT). This
ZIP query also is answered with 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 holds 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 2 AppleTalk ports and 2 Serial ports on
the InterBridge. After the initial polling, 1 packet gets transmitted every ten
seconds maintains the connection while idling.
To start the bridges communicating, simply turn one on. The InterBridge polls
its nearest neighbor, asks it to update its own internal table of zones, then
gives the neighbor bridge its zone information. If a bridge happens to be
turned off, the bridge's zone table gets rebuilt at power on, which, from that
point on, it maintains on its own. Applications communicating over AppleTalk
request the zone information from the InterBridge for the user. Bridges
periodically update their tables, and upon receiving a new zone address or a
packet referencing an unnamed zone address with a name, the bridge updates its
table to reflect the new zone name and address.
There is a problem that may cause some applications not to function
on this type of network. Since InterBridges use ZIP, which is "best effort"
protocol, this means checking for timeouts and 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. These programs
operate at the mercy of any other traffic on the X.25 net.