Jumbo frames? Yes!
 
	 By Selina Lo
	 Alteon Networks, 2/23/98 

 Like everything else, Ethernet should evolve with time. High-speed network users have long felt the limitations of Ethernet's small frame sizes and are among the biggest proponents of Jumbo Frames - extended Ethernet frames that range in size from the standard 1,518 bytes up to 9,000 bytes.

 The 1,518-byte frame size was designed to protect against the high bit error rates of yesterday's physical-layer Ethernet components. But computer processing power has increased by an order of magnitude, and the use of switched Ethernet over unshielded twisted pair or fiber media has significantly lowered Ethernet errors.

 More importantly, the speed and capacity of today's Ethernets are pushing the processor limits of most installed servers, and more data is being transferred between servers.

 For these reasons, extending Ethernet's frame size to reduce server overhead and increase throughput has become an attractive and logical option. Independent tests have verified that the use of Jumbo Frames can deliver a 50% increase in throughput with a simultaneous 50% decrease in CPU utilization.

 Users clearly recognize the value of bigger frames, but to get them until now, they had to turn to less-mainstream technologies such as FDDI, High Performance Parallel Interface and Fibre Channel. The problem is these technologies lack the installed base, cost-effectiveness and seamless interoperability of Ethernet.

 In the past, users who demanded the efficiency of large frame sizes had to build separate workgroup networks. Today, however, applications optimized for large frame sizes can easily be integrated with existing Ethernet LANs without causing interoperability problems. For example, you can partition a logical network in which systems can exchange Jumbo Frames and mark them with IEEE 802.1Q virtual LAN tags. The extended frames will be transparent to the rest of the network.

 Adapters that implement IEEE 802.1Q can support different Ethernet frame sizes for different logical network interfaces. For example, a server could communicate with another server using Jumbo Frames while communicating with clients sitting on another VLAN or IP subnet using standard Ethernet frames - all via the same physical connection.

 So how large should an Ethernet frame be? Ethernet's 32-bit cyclic redundancy check is effective for detecting bit errors at frame sizes under 12,000 bytes, thereby drawing a logical upper limit. Within that, the optimum large frame size can be determined by an application's block size. For example, Network File System (NFS) transfers data in 8,192-byte blocks. So adding room for headers, an attractive maximum Ethernet frame size for NFS applications is 9,000 bytes.

 At the same time that traffic is increasing and ultra high-speed Gigabit Ethernet technology is emerging, conventional Ethernet's packaging of data into mini-chunks is inhibiting the ability of servers and applications to take full advantage of next-generation networks. Extending Ethernet's frame size is a long-overdue step in the quest for speed.