Layers in CAs, Switches and Routers

Refer to Figure 6-3 on page 112.

HCA Layers

A 2-port HCA is pictured in the illustration (but recognize that it may have as few as one or as many as 255 ports). Software communicates with the HCA by issuing calls to the Verb interface. The verbs provide the interface between software and the QP message transfer engines implemented as part of the Transport Layer. Each RC, UC, and UD QP, as well as RD EECs, within the Transport Layer is set up to exchange messages through a specific HCA port. Each of the ports implements its own Network, Link, and Physical Layers.

TCA Layers

The only manner in which a TCA differs from an HCA lies in the upper layer entity that interfaces to the TCA's Transport Layer. For an HCA, it is the Verb Layer. For a TCA, the specification refers to it as an ULP (Upper Layer Protocol) and the manner in which it's implemented is outside the scope of the specification.

Switch Layers

With the exception of switch port 0, each switch port implements only the Physical and Link Layers. The Network Layer is unnecessary because a switch only handles the relay of packets within a subnet, rather than between subnets (that is the province of a router).

Switch port 0 is the switch management port. Whether or not port 0 actually implements the Physical Layer that would permit it to be connected to an external link is optional. Port 0 must implement the Transport Layer (specifically QP0—the SMI, and QP1—the GSI) to handle the processing of SMPs and GMPs. The other switch ports do not implement the Transport Layer because they always internally forward SMPs and GMPs to switch port 0 for processing and all non-management packets are forwarded through another port based on the packet's LRH:DLID field.

Router Layers

Each router port must implement the Physical and Link Layers. All ports on a router may share the Network Layer. In addition, the router must implement the Transport Layer to handle the reception and transmission of SMPs and GMPs on any of its ports.