Cisco InfiniBand 4x User's Guide download pdf (Page 47)

Chapter 3. InfiniBand technology 33

InfiniBand specifies multiple transport services for data reliability. Table 3-1 describes each of

the supported services. For a given queue pair, one transport level is used.

Table 3-1 Transport services

3.1.3 Network layer

The network layer handles routing of packets from one subnet to another. (Within a subnet,

the network layer is not required.) Packets that are sent between subnets contain a global

route header (GRH). The GRH contains the 128-bit IPv6 address for the source and

destination of the packet. The packets are forwarded between subnets through a router,

based on each device’s 64-bit GUID. The router modifies the LRH with the proper local

address within each subnet. Therefore the last router in the path replaces the local ID (LID) in

the local route header (LRH) with the LID of the destination port. Within the network layer,

InfiniBand packets do not require the network layer information and header overhead when

used within a single subnet (which is a likely scenario for InfiniBand system area networks).

3.1.4 Data link layer

The data link layer encompasses packet layout, point-to-point link operations, and switching

within a local subnet. Each device within the subnet has a 16-bit LID assigned to it by the

subnet manager which is used for addressing. The data link layer forwards packets to the

device specified by the destination LID within an LRH.

Packets

There are two types of packets in the link layer: management packets and data packets.

Management packets are used for link configuration and maintenance. Device information,

such as virtual lane support, is determined with management packets.

Data packets carry up

to 4 KB of a transaction payload.

Switching

Within a subnet, packet forwarding and switching is handled at the link layer. All devices

within a subnet have a 16-bit LID assigned by the subnet manager. All packets sent within a

subnet use the LID for addressing. Link Level switching forwards packets to the device

specified by a destination LID within an LRH in the packet. The LRH is present in all packets.

QoS

QoS is supported by InfiniBand through virtual lanes (VL). These VLs are separate logical

communication links that share a single physical link. Each link can support up to 15 standard

VLs and one management lane (VL 15). VL15 is the highest priority and VL0 is the lowest.

Management packets use VL15 exclusively. Each device must support a minimum of VL0

and VL15 while other VLs are optional.

Class of service Description

Reliable connection Acknowledged - connection oriented

Reliable datagram Acknowledged - multiplexed

Unreliable connection Unacknowledged - connection oriented

Unreliable datagram Unacknowledged - connectionless

Raw datagram Unacknowledged - connectionless

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Cisco InfiniBand 4x User's Guide Page 47