McDATA StorageWorks 2/140 - Director Switch Planning Manual page 162

Implementing SAN Internetworking Solutions
4
Router Connectivity through mFCP
mFCP provides connectivity (through a GbE-bandwidth IRL)
between two Eclipse 2640 SAN Routers. mFCP is similar to Fibre
Channel protocol (FCP) but implements user datagram protocol
(UDP) for open systems interconnection (OSI) Layer 4 transport.
mFCP links are used for path failover in high-availability mSANs.
NOTE: The Eclipse 1620 SAN Router does not support mFCP and
must be deployed in mSANs as a single-router configuration.
The UDP transport protocol is fast and easy to implement, but unlike
transmission control protocol (TCP), UDP is connectionless,
best-effort, and does not guarantee order or delivery of packets. UDP
does not offer services such as packet reordering, retransmission of
lost packets, or detection of duplicate packets. Therefore, only direct,
high-reliability fiber-optic cable connections between SAN routers
are supported.
An mFCP link typically connects routers over short distances in a
data center or campus. However, mFCP links can connect routers in a
metropolitan area using wavelength division multiplexing (WDM)
equipment or dark (unused) fiber. WDM and dark fiber are
considered direct connections.
SAN routers and the UDP over GbE connection support the Institute
of Electrical and Electronics Engineers (IEEE) 802.3x Ethernet flow
control standard. Flow control prevents buffers from overflowing
and dropping packets.
A UDP over GbE connection eliminates protocol overhead (eight
bytes for UDP versus 20 bytes for TCP) and potential performance
problems. The header is smaller and does not have windowing
mechanisms that require resources to manage, and there is no
buffering of segments until notification of receipt. The connection
also uses 8B/10B bit-level encoding derived from Fibre Channel
specifications, resulting in a low bit-error rate. Flow control, low
overhead, and a low bit-error rate allow the mFCP connection to
approach the reliability of a Fibre Channel connection.
While a SAN router IRL is limited to GbE speed, multiple IRLs can be
combined using IEEE 802.3AD link aggregation standard. Up to four
links can be aggregated between two SAN routers.
McDATA Products in a SAN Environment - Planning Manual
4-20