IBM 9077 manual Direction Bit, 1.7 L, VU, and W Bits, IP Routing and the I-field, 137

4.5.1.6 Direction Bit

HIPPI hosts set the direction bit (D). This bit determines how a switch or router reads the 24-bits of destination address information. Figure 49 on page 135 and the previous descriptions of source routing and logical addressing have the destination address information organized as if the host has set the destination bit to 0.

If the destination bit is set to 1, the information in the 24-bit destination addressing is read starting from the left, and the logical addresses of host A and B change places. The destination bit makes it easy for source and destination hosts to reply and reverse transfer data to one another, or it can serve as a means to trace where a connection originates.

4.5.1.7 L, VU, and W Bits

The L and VU control bits are not used by the GRF, and the HIPPI media cards do not support double-wide HIPPI connections.

It will reject the connection if the W bit is set on.

4.5.1.8 IP Routing

In an IP connection, data coming from a HIPPI I/O channel is formatted into standard IP packets. Embedded in the front of each IP datagram is the IP header. The media card reads the header only if the information in the HIPPI I-field indicates an IP connection.

The header contains the Internet address of the host sending the datagram and the Internet address of the target IP media host for which the datagram is intended. This target host can be attached to any media that supports IP, or be reached via that attached media. Because the GRF is a router, it creates and updates an IP routing table that describes paths to destination addresses. This is the basis of IP routing.

Each GRF media card holds a copy of this IP routing table. When processing an IP connection, a HIPPI media card “opens” the datagram’s header, reads the address of the target host, and decides to which GRF media card the IP datagram is transferred.

4.5.1.9 IP Routing and the I-field

A HIPPI host’s I-field table can be used to direct the GRF HIPPI media card to do IP routing.

In the I-field, the PS bit needs to be set to 01 or 11 and a designated destination logical address (0xfc0) must be placed in bits 0–11. The mapping

Configuration of IP-Forwarding Media Cards 137