fields, Source Address and Destination Address. A Destination's 12
bit Switch Address is the same for all Sources. Switches commonly
have address lookup tables to map 12 bit logical addresses to
physical ports. This mode is used for networking.
Control fields in the I-field are:
L The "Locally Defined" bit, when set, indicates that the I-field
is not in the standard format. The meaning of bits 30-0 are
locally defined.
W The Width bit, when set, requests a 64 bit connection through
the switch. It is meaningless if Cable B is not installed at
the Source. If W is set and either the Source or the requested
Destination has no Cable B to the switch, the switch rejects
the connection. Otherwise the switch connects both Cable A and
Cable B if W is set, or Cable A only if W is clear. This
feature is useful if both Source and Destination
implementations can selectively disable or enable Cable B on
each new connection.
D The Direction bit, when set, reverses the sense of the Source
Address and Destination Address fields. In other words, D=1
means that the Source Address is in bits 0-11 and the
Destination Address is in bits 12-23. This bit was defined to
give devices a simple way to route return messages. It is not
useful for LAN operations.
PS The Path Selection field determines whether the I-field
contains Source Route or Address information, and in Logical
Address mode, whether the switch may select from multiple
possible routes to the destination. The value "01" selects
Logical Address mode and fixed routes.
C The Camp-on bit requests the switch not to reject the
connection if the selected Destination is busy (connected to
another Source) but wait and make the connection when the
Destination is free.
15 Appendix B -- How to Build a Practical HIPPI LAN
"IP on HIPPI" describes the network host's view of a HIPPI local area
network without providing much information on the architecture of the
network itself. Here we describe a network constructed from
available HIPPI components, having the following characteristics:
1. A tree structure with a central HIPPI-SC compliant hub and
optional satellite switches
2. Each satellite is connected to the hub by just one bidirectional
HIPPI link.
3. Serial HIPPI or transparent fiber optic HIPPI extender devices
may be used in any link.
4. Some satellites may be a particular switch product which is not
HIPPI-SC compliant.
5. Host systems are attached either directly to the hub or to
satellites, by single bidirectional links in which both HIPPI cables
go to the same numbered switch port.
Switch Address Management
Switch addresses use a flat address space. The 12-bit address is
subdivided into 6 bits of switch number and 6 bits of port number.
11 5 0
+-----------------------+-----------------------+
| Switch Number | Port Number |
+-----------------------+-----------------------+
Logical Address Construction
Switches may be numbered arbitrarily. A given host's address
consists of the number of the switch it is directly attached to and
the physical port number on that switch to which its input channel is
attached.
In the singly-connected tree structure, there is exactly one path
between any pair of hosts. Since each satellite must be connected
directly to the hub, the maximum length of this path is three hops,
and the minimum length is one. Each HIPPI-SC compliant switch is
programmed to map each of the host switch addresses to the
appropriate output port: either the port to which the host is
directly attached or a port that is linked to another switch in the
path to it.
Special Treatment of Nonstandard Switches
There is one commercially available switch that was designed
before the drafting of HIPPI-SC and is not fully compliant. It is
in common use, so it is worth making some special provisions to
allow its use in a HIPPI LAN. This switch supports only the
Source Route mode of addressing with a four bit right shift that
can be disabled by a hardware switch on each input port.
Addresses cannot be mapped. The switch does not support the "W",
"D", or "PS" fields of the I-field; it ignores their contents.
Use of this switch as a satellite will require a slight deviation
from normal I-field usage by the hosts that are directly attached
to it. Hosts attached to standard switches are not affected.
For a destination connected to a non compliant satellite, the
satellite uses only the least significant four bits of the I-field
as the address. Since the address contains the destination's
physical port number in the least significant bits, its port will
be selected. Nonstandard switches should be set to disable I-
field shifting at the input from the hub, so that the destination
host will see its correct switch address in the I-field when
performing self-address discovery. I-field shifting must be
enabled on the satellite for each input port to which a host is
attached.
Hosts attached to nonstandard satellites must deviate from the
normal I-field usage when connecting to hosts on another switch.
It is suggested that all host implementations have this capability
as long as the nonstandard switches remain in use. The host must
know, by some manual configuration method, that it is connected to
a nonstandard switch, and it must have its "link port" number;
that is, the number of the port on the satellite that is connected
to the hub.
The normal I-field format for a 32-bit connection, per the
document, is this:
31 26 23 11 0
+---------+---+-+-----------------------+-----------------------+
|0 0 0 0 0|x 1|C| Unused | Destination Address |
+---------+---+-+-----------------------+-----------------------+
The special I-field format is:
31 26 24 15 4 3 0
+---------+---+-+---------------+-----------------------+-------+
|0 0 0 0 0|x 1|C| Unused | Destination Address | Link |
+---------+---+-+---------------+-----------------------+-------+
This I-field is altered by shifting the lower 24 bits left by four
and adding the link port number. Camp-on is optional, and the PS
field is set to 01 or 11 (the host's option) as if the switch
supported logical address mode. All other I-field bits are set to
zero. When the host requests a connection with this I-field, the
switch selects a connection through the link port to the hub, and
shifts the lower 24 bits of the I-field right by four bits. The link
port number is discarded and the I-field passed through to the hub is
a proper HIPPI-SC I-field selecting logical address mode.
A host on a nonstandard satellite may use the special I-field format
for all connection requests. If connecting to another host on the
same satellite, this will cause the connection to take an
unnecessarily long path through the hub and back. If an optimization
is desired, the host can be given additional information to allow it
to use the standard I-field format when connecting to another host on
the same switch. This information could consist of a list of the
other hosts on the same switch, or the details of address formation,
along with the switch number of the local satellite, which would
allow the host to analyze the switch address to determine whether or
not the destination is on the local switch. This optimization is
fairly complicated and may not always be worthwhile.