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Tip of the Month

November 2004

Jumbo-Size Me!

We mentioned briefly how runt Ethernet frames are less than 64 bytes and no frame is larger than 1518 bytes in the June issue. It is very easy to keep those numbers in mind while forgetting the size of jumbo frames (9,000 bytes) which nodes may be using on the gigabit segments of your network. While the default maximum size remains at 1518 bytes on a mixed Ethernet network, jumbo frames are allowed for applications requiring larger datagrams (such as NFS). Do not expect the jumbo frame size to change must beyond 9,000 while we continue to float our networks on IPv4 because the 32 bit CRC loses its effectiveness after 12,000 bytes. An exception might be the use of a different CRC method to support the maximum MTU size of 65,535 bytes.

For a real-world example, let’s look at one potential cause of a slow server. Keep in mind the source link speeds of all the nodes while capturing traffic, especially if you are capturing off a SPAN port. While the SPAN port is a single link speed to your analyzer, the originating port speeds may vary.

Always check the IP header for information about possible fragmentation. A filter can be created to capture packets based on the fragmentation flags.(look in our archives for filtering tips). If you see a value of “010” which means “Do Not Fragment” and your packets are traversing a router, the router is bound to drop packets larger than 1518 bytes and send and ICMP packet back to the client. It is then up the client to correct the MTU and resend the packet. If a client allows fragmentation and the router needs to break up the packet to forward it to a port with a lower MTU, the fragmentation bits will be non-zero and the receiver must reassemble the packets in the correct order.

In general it’s best that the TCP MSS (maximum segment size) and Ethernet MTU values match end-to-end. In most cases this can be overcome via clients that “auto” correct their MTU based on sending packets with the “Do Not Fragment” bit set.

Perform protocol analysis with your analyzer to be sure!