5. Multicast > Error Management of Multicast Packets
120
Tsi576 User Manual
June 6, 2016
Integrated Device Technology
www.idt.com
5.4
Error Management of Multicast Packets
Multicast packets have four sources of error - packet TEA, packet STOMPing, exceeding the
maximum latency time, and exceeding the time-to-live timeout.
5.4.1
Packet TEA
A multicast packet at the head of the ingress queue is subject to TEA. The TEA function does not
differentiate between multicast and unicast packets. The assertion of TEA for a packet at the head of
the ingress queue causes the packet to be immediately dropped from the queue. The packet is not
forwarded to the destination port (see
“Multicast Work Queue” on page 107
).
5.4.2
Multicast Packet Stomping
Transfers from the ingress port through the multicast work queue into the broadcast buffers operate in
cut-through mode. It is possible for the ingress port’s link partner to stomp a packet, or terminate it due
to some other error condition. It is also possible for the ingress port to detect a CRC or some other error
with the packet. To handle these situations, the Tsi576 supports stomping of a packet while it is being
transferred from ingress port to broadcast buffer.
When a packet arrives at the Tsi576 and a stomp control symbol is received part way though the packet,
the packet is still multicast to the egress ports. However, the egress ports stomp the packet when they
transmit the packet.
5.4.3
Multicast Maximum Latency Timer
It is expected that the time-outs required for multicast packets in many systems are shorter than that
required to enforce time-to-live time-outs for unicast packets, so a separate timer known as the
maximum latency timer is used. The maximum latency value is similar in concept to the timer, but
separate from it. This allows systems designers to enforce latency rules for multicast traffic that are
different from that of unicast traffic in the system.
Each broadcast buffer has a separate maximum latency timer to enforce latency limits for a packet and
has the capacity to hold a single maximum size packets or up to eight smaller packets (totaling up to,
but not more than, 280 bytes). The maximum latency timer starts counting when the packet is
completely received by the broadcast buffer. The multicast latency timer has a maximum period of
0xFFFFFF*6.4ns. The multicast latency timer is programmed using the
Latency Counter CSR” on page 398
The maximum latency timer is associated only with the packet at the head of the broadcast buffer. The
multicast latency timer is held in reset when a packet is being transferred from the broadcast buffer to
the egress port.
