InfiniBand Network Architecture

Problem: Fast CA Port Can Cause Problems

Refer to Figure 24-1 on page 591. The port on CA X is capable of transmitting data at 30Gb/s. If it were permitted to transmit back-to-back packets to a destination (such as CA Y or CA Z), the result would be as follows:

1. The switch ingress port's VL receive buffer [see “Virtual Lanes” on page 119 and “QoS within the Subnet: SL and VLs” on page 617] selected by the packet's LRH:SL would fill up very rapidly.

2. The ingress port's Link Layer would very quickly fill up the egress port's selected VL transmit buffer.

3. The speed at which the egress port's Link Layer can unload its VL transmit buffer is severely limited by its link speed (in the example, either 3x or 12x slower than the ingress port's link speed). ...