The Ethernet traffic mix-up

Ethernet communications convergence conumdrum

Broken CD with wrench

Expert Clinic The possibility of converging storage traffic, with its restrictive profile, on to general messy Ethernet LAN traffic is now a distinct possibility. What are the underlying problems and how are they being dealt with and countered?

Given that they have been dealt with how should you think about convergence and what general principles should you have in mind as you start a convergence project?

Three experts tell you how they think it is. FIrst up is network architect Greg Ferro.

Greg Ferro - Network Architect and Senior Engineer/Designer.

Gregg Ferro

When mixing Storage and Data traffic in a common network fabric, it’s important to understand that storage networking was intended to create a dedicated channel from server to array and that data networking is dynamic.

A SCSI connection expects dedicated bandwidth and zero contention like a cable between the motherboard and the HDD. When SCSI was moved into a network protocol, the standards didn’t change this. The standard used the concept of a “channel” and specified fibre optic cable for reliability so the protocol was called Fibre Channel. Fibre Channel builds a network “fabric” where each switch has state about every device connected to the network, and each host “logs in” and notifies the fabric about its configuration and status. Fibre Channel uses an XON/XOFF protocol that only sends data when the remote host signals back to the sender that it can receive data.

A data network has no knowledge of the hosts or that path that the data will take, paths through the system are dynamically determined and the protocols that control the forwarding are loosely controlled and can change without the knowledge of the host. The first Ethernet frame is received by a switch and then flooded to all ports to attempt to discover where the end host is located. All deliver.

The first Ethernet frame is received by a switch and then flooded to all ports to attempt to discover where the end host is located.

How, then, does a Data Network handle storage traffic and data at the same time ? By creating conditions where storage traffic is prioritised across the Ethernet Fabric to ensure that it emulates a channel. At every point in the data network, the designer specifies configuration for a specific traffic flow to be prioritised through the network fabric. This isn’t new to data networks as Voice Over IP / IP Telephony has already driven much of the underlying Quality of Service technology.

Therefore the art of the converging storage and data onto a single network relies on detecting different traffic types and then forwarding them differently, according to their needs. Thus, storage traffic must be lossless, reliable and low latency. Data traffic needs bandwidth and flexibility. These are not incompatible. The underlying need to share traffic means that Ethernet switches have adopted many of the features of Storage Fabrics. So much so that we now call Data Centre networks “Ethernet Fabrics” as standard terminology.

Greg Ferro describes himself as Human Infrastructure for Cisco and Data Networking. He works freelance as a Network Architect and Senior Engineer/Designer, mostly in the United Kingdom and previously in Asia Pacific region. He is currently focussing on Data Centre, Security and Application Networking technologies and spending a lot of time pondering design models, building operational excellence and creating business outcomes.


Next up is Duncan Hughes from Brocade, who reckons FCoE is usable and great for virtualised server environments.

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