8.8. PACKET TERMINATION

A packet is terminated in one of two ways:

• The beginning of a new packet marks the end of a previous packet.

• The end of a packet may be marked with one of the following: an end-of-packet (eop); restart-from-retry; link-request; or stomp control symbol.

The stomp control symbol is used if a transmitting processing element detects a problem with the transmission of a packet. It may choose to cancel the packet by sending the stomp control symbol instead of terminating it in a different, possibly system-fatal, manner such as corrupting the CRC value.

The restart-from-retry control symbol can cancel the current packet. It may also be transmitted on an idle link. This control symbol is used to command the receiver to start accepting packets again after the receiver has retried a packet and entered a stopped state.

The link-request control symbol can cancel the current packet. It may also be transmitted on an idle link. It can be used by software for system observation and maintenance, and it can be used by software or hardware to enable the receiver to start accepting packets after the receiver has refused a packet owing to a transmission error.

A port receiving a cancelled packet is required to drop the packet. The cancellation of a packet does not result in the generation of any errors. If the packet was cancelled because the sender received a packet-not-accepted control symbol, the error that caused the packet-not-accepted control symbol to be sent should be reported in the normal manner.

If a port receiving a cancelled packet has not previously acknowledged the packet and is not in an 'Input-stopped' state (Retry-stopped or Error-stopped), the port should immediately enter the Input Retry-stopped state and follow the Packet Retry mechanism, if the packet was cancelled with a control symbol other than a restart-from-retry or a link-request/input-status control symbol. As part of the Packet Retry mechanism, the port sends a packet-retry control symbol to the sending port, indicating that the cancelled packet was not accepted.

Figure 8.11 shows an example of an end-of-packet control symbol marking the end of a packet. The stomp, link-request, and restart-from-retry control symbol cases would look similar.

Figure 8.11. End-of-packet control symbol marking end of packet (16-bit port)