FCIP Flow Control and QoS

This section summarizes the flow-control and QoS mechanisms supported by FCIP.

FCIP Flow Control

FCIP does not provide any flow-control mechanisms of its own. The only FCIP flow-control functionality of note is the mapping function between FC and TCP/IP flow-control mechanisms. FCIP vendors have implemented various proprietary features to augment FCIP performance. Most notable are the FCP_XFER_RDY IU spoofing techniques. In some cases, even the FCP_RSP IU is spoofed. For more information about FCIP flow control, readers are encouraged to consult IETF RFC 3821 and the ANSI T11 FC-BB-3 specification.

FCIP QoS

FCIP does not provide any QoS mechanisms of its own. However, RFC 3821 requires the FC Entity to specify the IP QoS characteristics of each new TCP connection to the FCIP Entity at the time that the TCP connection is requested. In doing so, no requirement exists for the FC Entity to map FC QoS mechanisms to IP QoS mechanisms. This may be optionally accomplished by mapping the value of the Preference subfield or the Priority subfield in the CS_CTL/Priority field of the FC header to an IntServ/RSVP request or a DiffServ DSCP value. FCIP links are not established dynamically in response to received FC frames, so the FC Entity needs to anticipate the required service levels prior to FC frame reception. One method to accommodate all possible FC QoS values is to establish one TCP connection for each of the seven traffic classes identified by the IEEE 802.1D-2004 specification. The TCP connections can be aggregated into one or more FCIP links, or each TCP connection can be associated with an individual FCIP link. The subsequent mapping of FC QoS values onto the seven TCP connections could then be undertaken in a proprietary manner. Many other techniques exist, and all are proprietary. For more information about FCIP QoS, readers are encouraged to consult IETF RFC 3821 and the ANSI T11 FC-BB-3 specification.