Support grows for Jumbo Frames

Momentum continues to build for Jumbo Frames, as a standards body and vendors begin to endorse this nonstandard technique for transferring large amounts of data over gigabit Ethernet.

The Internet Engineering Task Force (IETF) has published a working document proposing that Ethernet frames be extended beyond the standard 1.5KB limit. The document also specifies a way to designate that the frame is large in an Ethernet frame header.

The two biggest vendors of network interface cards (NICs) are also lending support to Jumbo Frames. Intel has released a software driver for its gigabit Ethernet card that lets users adjust the maximum frame size allowed. A frame could be as small as 4KB or as large as 16KB. Separately, 3Com plans to implement Jumbo Frames on its gigabit Ethernet cards in the third quarter.

Intel and 3Com join a handful of other vendors, including Fore Systems, IBM, Microsoft and Silicon Graphics, that have already declared their support for Jumbo Frames.

Jumbo Frames — with a maximum size of 9KB — were created by Alteon WebSystems to address a problem that occurs with servers attached to a gigabit Ethernet net. Every time a frame arrives at a server, an interrupt is generated, meaning the server’s processor has to divert attention to the incoming frame. At gigabit speeds, the interrupts come fast and furious.

“Even with Pentium IIIs, servers can talk at wire-speed gigabit Ethernet or they can do useful work — but it’s hard to do both,” said David Passmore, research director of Net Reference. Larger frame sizes mean fewer headers to process for any given period of time, freeing up a server processor for other tasks. However, no independent testing has been done yet to quantify the benefits of Jumbo Frames.

Although 3Com and Intel are supporting Jumbo Frames in their NICs, the companies see the technology as an interim measure before servers become able to handle gigabit Ethernet.

The vendors point out that Microsoft’s Windows 2000, for instance, will be able to pass large frames to a NIC, which can cut them into standard-sized frames and send them over a network.