3.7 Classes of Service

To ensure proper data transport, applications may require different levels of delivery guarantees, bandwidth, and connectivity between communicators. Data streaming for a tape backup application, for example, implies a sustained, dedicated connection and full-bandwidth availability. Alternatively, with intermittent activity, online transaction processing (OLTP) may not require dedicated bandwidth but would benefit from acknowledgments for each transfer. To accommodate these requirements, Fibre Channel standards provide five classes of delivery service. Not all of these classes are currently implemented in shipping products.

Class 1 service defines a dedicated connection between two devices for example, a file server and a disk array with acknowledgment of frame delivery. The communicating partners establish a Class 1 connection and then assume full bandwidth and a constant circuit until the connection is taken down. Because the Class 1 circuit usually excludes others from intervening in the conversation, there is no danger that one of the partners will be busied out during an exchange. The dedicated connection also ensures that all frames will be received in the same order they were sent, something that speeds reassembly of data blocks at the FC-2 level.

Class 1 service places an additional design burden on the Fibre Channel switches. A Class 1 connection between a server and a disk that are communicating via a switch consumes switch routing resources. The switch must maintain a dedicated channel between the communicating ports and is not allowed to intervene. Thus, multiple Class 1 connections by multiple pairs would quickly impede the switch's ability to serve other requests to those ports. Analogous to a phone switch, Class 1 service provides a camp on facility, which, if supported by the switch, allows a contending request to be queued against a busy port. As soon as the current Class 1 connection is ended, the new request can be served. You need additional buffering on the switch as well as logic to monitor the queue and timeout values. Class 1 service also defines a stacked connect feature, which allows a Class 1 participant to queue additional connect requests to the switch. Because of the added complexity and cost associated with Class 1 functionality, these features are not widely implemented by Fibre Channel switch vendors.

Class 1 service can also be implemented in a nonswitched topology such as arbitrated loop. Typically, however, loop implementations employ the more democratic Class 3 service.

Class 2 service does not require a dedicated connection between talking pairs, but it does provide acknowledgment of frame delivery. In switch environments, the connectionless nature of Class 2 allows the switch to forward frames as resources allow without dedicating bandwidth to a transaction. Traffic congestion or multiple routes can therefore result in frames being received in random order, something that would require additional processing overhead for reassembly into data blocks. To alleviate this possibility, a Class 2 connection can request in-order delivery. In arbitrated loop environments, a Class 2 transfer ensures in-order delivery of frames because only the communicating partners occupy the loop for the duration of the transaction.

Class 2 is suited to mission-critical applications that demand a high degree of data integrity at the transport level. Acknowledgment of frame delivery and notification of nondelivery add an extra validation without involving upper-level protocols. Class 2 is also suited to smaller data transactions with only bursty traffic requirements. Intermittent updates to a relational database, for example, do not require a dedicated connection but may need verification via frame acknowledgments.

Like Class 2, Class 3 service is connectionless, but unlike Class 2, there is no notification of delivery. Class 3 is similar to datagram services in LAN topologies (for example, User Datagram Protocol/Internet Protocol, or UDP/IP) and relies on upper-level protocols to recover if a transport error occurs. To a certain extent, Class 3 service sacrifices reliability for reduced overhead and higher performance. In well-constructed SANs, the reliability factor may not be a pressing issue. Class 3, for example, is widely used in arbitrated loop environments, even for mission-critical applications. This is possible because of the temporary dedicated connections between partners that a loop topology provides. Frames are received in order, and, with the selection of appropriate products, the low occurrence of bit errors results in reliable frame transport. In a heavily burdened switch topology, Class 3's inherent laissez-faire attitude toward frame delivery is more apparent. A switch may discard Class 3 frames under congested conditions and, because there is no acknowledgment mechanism, trigger a more lengthy recovery chain reaction at the FC-4 upper-level protocol.

You should consider this difference between loop and switch Class 3 handling when designing SANs for specific applications. Some Class 3 implementations are more reliably deployed on loop segments alone, whereas others, depending on bandwidth allocation requirements, can be reasonably configured with a combination of loop segments and switch topologies.

For multiclass applications, Fibre Channel provides an optional intermix feature. Intermix allows any unused bandwidth in a Class 1 connection to be utilized by Class 2 and Class 3 frames. The operative descriptor is unused, because Class 1 by definition guarantees full bandwidth when Class 1 frames are present. If implemented, intermix is useful for applications such as tape backup because Class 2 or 3 frames could be used to set up the next transaction while a Class 1 connection is still active.

Two additional delivery classes have been defined, but not widely implemented, for specialized applications. Class 4 service introduces the concept of virtual circuits to Fibre Channel architecture. Like Class 1, Class 4 service is connection-oriented. But instead of allocating the entire bandwidth between communicating pairs, Class 4 lets you assign fractional bandwidth as well as different quality of service (QoS) parameters for each connection. Additionally, you can establish multiple virtual circuits between a single pair or multiple pairs of communicators. The mandated quality of service for each virtual circuit ensures that time-sensitive applications, such as real-time video, always have bandwidth available for transmission. The functionality described by Class 4 service poses considerable design challenges to switch architects. The switch must maintain and monitor potentially hundreds of virtual circuits, with variable QoS parameters for each one. This complexity may explain why Class 4 capability exists primarily as a Fibre Channel standard and not as a standard feature of switch products. Fibre Channel Class 5 is as yet undefined.

Class 6 service has been defined to provide multicast service with acknowledged delivery. Multicast is a requirement for video broadcast applications based on a central video server and multiple video recipients. This function can also be provided by Class 3 datagram service if acknowledgment is not required. Depending on the number of target devices in a multicast group, the number of acknowledgments flooding back to an initiator might overwhelm the topology. Class 6 alleviates this problem by placing a multicast server into the configuration. The multicast server is the focal point for all acknowledgments and, for each multicast transaction, returns a single confirmation to the initiator.

Although Class 4 and Class 6 services are described in granular detail in Fibre Channel documentation, this does not mean that these features have been implemented in tangible products. Fibre Channel switches, for example, typically list support for Class 2 and Class 3 services only. Support for other functionality is driven, like everything else, by market demand. The fact that Class 4 and Class 6 have emerged as Fibre Channel standards, however, reflects the influence of market requirements on the technology. It also indicates that cooperation between customers and vendors is driving the higher functionality seen in products.

An additional delivery class, known as Class F, is specifically for switch-to-switch communications. Similar to Class 2, Class F traffic is connectionless and requires acknowledgment of delivery or failure of delivery. Class F is restricted to Fibre Channel switch expansion ports (E_Ports) and is used to exchange routing, name service, and notifications between switches joined into a single fabric. E_Ports are discussed later in this chapter.