Why Ethernet Needs Jumbo Frames


Why you should care

Whether or not Gigabit Ethernet (and beyond) should support frame sizes (i.e. packets) larger than 1500 bytes has been a topic of great debate. With the explosive growth of Gigabit ethernet, the impact of this decision is critically important and will affect Internet performance for years to come.

Most of the debate about jumbo frames has focused on local area network performance and the impact that frame size has on host processing requirements, interface cards, memory, etc. But what is less well known, and of critical concern for high performance computing, is the impact that frame size has on wide area network performance. This document discusses why you should care, and about the largely ignored but important impact that frame size has on the wide area performance of TCP.

How jumbo is a jumbo frame anyway?

Ethernet has used 1500 byte frame sizes since it was created (around 1980). To maintain backward compatibility, 100 Mbps ethernet used the same size, and today “standard” gigabit ethernet is also using 1500 byte frames. This is so a packet to/from any combination of 10/100/1000 Mbps ethernet devices can be handled without any layer two fragmentation or reassembly.

“Jumbo frames” extends ethernet to 9000 bytes. Why 9000? First because ethernet uses a 32 bit CRC that loses its effectiveness above about 12000 bytes. And secondly, 9000 was large enough to carry an 8 KB application datagram (e.g. NFS) plus packet header overhead. Is 9000 bytes enough? It’s a lot better than 1500, but for pure performance reasons there is little reason to stop there. At 64 KB we reach the limit of an IPv4 datagram, while IPv6 allows for packets up to 4 GB in size. For ethernet however, the 32 bit CRC limit is hard to change, so don’t expect to see ethernet frame sizes above 9000 bytes anytime soon.

How can jumbo frames and 1500 byte frames coexist?

Two basic approaches exist:

  • On a port by port basis, where everything “downstream” from a given port is known to support jumbo frames.
  • Using 802.1q Virtual LANs, where jumbo frame and non-jumbo frame devices are segregated to different VLANs.

What frame sizes are actually being used?

The above graph is from a study[1] of traffic on the InternetMCI backbone in 1998. It shows the distribution of packet sizes flowing over a particular backbone OC3 link. There is clearly a wall at 1500 bytes (the ethernet limit), but there is also traffic up to the 4000 byte FDDI MTU. But here is a more surprising fact: while the number of packets larger than 1500 bytes appears small, more than 50% of the bytes were carried by such packets because of their larger size. Also, the above traffic was limited by FDDI interfaces (thus the 4000 byte limit). Many high performance flows have been achieved over ATM WAN’s offering 9180 byte MTU paths.

Local performance issues

Smaller frames usually mean more CPU interrupts and more processing overhead for a given data transfer size. Often the per-packet processing overhead sets the limit of TCP performance in the LAN environment. The above graph, from a white paper[2] by Alteon is an often cited study showing an example where jumbo frames provided 50% more throughput with 50% less CPU load than 1500 byte frames.

Such local overhead can be reduced by improved system design, offloading work to the NIC interface cards, etc. But however you feel about these often debated local performance issues, it is the WAN that we are most concerned about here.