Jumbo frames are like a double quarter pounder, more meat with less bun overhead. (Jumbo frames typically refer to Ethernet frames with larger than the standard 1500 byte payload and are a feature of Gigabit Ethernet.)
It sounds great in principle but there are caveats. For jumbo frames to work without issues the end devices and all the components that connect them have to support jumbo frames. If they don’t, at best you get fragmentation or a negotiation of frame size back down, potentially degrading performance. At worst somewhere along the path the packet gets dropped.
Here is a quick look at the behaviour of jumbo frames using two devices connected to the same Nortel 8600. Client A was sending frames to Client B:
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As you can see the results are quite varied.
Case 1:
is the perfect scenario where both clients and the path in between support jumbo frames and all the applications work.
Case 2:
is the worst scenario (or maybe best of the worst because sometimes it is easier that when it breaks – it all breaks). Both clients support jumbo frames so that is what they try to send. However part of the network does not support jumbo frames so they get dropped in the network. The clients know frames are being dropped but they don’t know why… so they just keep resending jumbo frames.
Cases 3:
is a mixed bag of outcomes that can be a pain to trouble shoot. Client A supports jumbo frames while Client B and part of the network do not. The results give some insight into the behaviour of network protocols. FTP and Windows transfers use TCP with has a mechanism to negotiate packet size. Client A proposes a large size and Client B comes back with the smaller one of which it is capable. Lower size wins and this is what is used for the conversation (denoted by Std in the table). Ping uses ICMP and there is no mechanism to negotiate packet size. Client A just goes ahead and sends the jumbo packet. Client B won’t be able to handle it but it gets dropped in the network anyway.
Case 4 is similar to Case 3, with the alteration that now the network can handle jumbo frames. While the results are the same, the subtle difference is that in this case with the Ping test Client B may get the large ICMP packet but it can’t deal with it and will see it as an error.
So, bigger may not always be better. If you are implementing jumbo frames make sure you are looking at the whole path that frame will travel.
There are other considerations with jumbo frames such as which devices support them and what happens when you introduce a router that can fragment large packets. If there is interest in these topics we can address them with another post.
Troy.
