I’ve recently taken part in a few conversations about jumbo frames and thought that perhaps you might be considering them for your network. If so, please allow me to give you a little more information that can help you decide if your network can and should support jumbo frames.
Jumbo Frames: The Basics
A jumbo frame is any Ethernet frame larger than 1518 bytes, the maximum size of a traditional Ethernet frame. Jumbo frames can reach a maximum of 9018 bytes.
The benefits of jumbo frames are two-fold. First, jumbo frames can increase the efficiency of your network, allowing you to carry more payload with less Ethernet framing overhead. Second, jumbo frames can encapsulate a single traditional Ethernet frame—up to the maximum 1518 bytes—within an Ethernet frame.
Challenges to Implementing Jumbo Frames
There are challenges that come with implementing jumbo frames:
- Every device along the jumbo frame’s path must support jumbo frames.
Many Ethernet devices, especially older equipment, cannot support jumbo frames. If a switch receives a jumbo frame but does not support jumbo frames, the switch will silently discard the “malformed” frame.
- Traffic engineering analysis is required to ensure that the size and quantity of jumbo frames will not adversely affect existing services.
When designing a network, it is essential to understand the traffic characteristics for each service that is carried over that network. These characteristics include expected traffic volume and packet size as well as application sensitivity to packet loss and delay. A successfully implemented traffic engineering plan will assure the quality of all services if all services are behaving as expected. If you implement jumbo frames, you may be surprised to know that you will be changing the nature of packet latency on your network. Therefore, you must revisit your traffic engineering plans to ensure quality of experience.
Jumbo Frames in the Network
As an example of adversely affected services, consider a hypothetical T1 circuit emulation service (CES) that can tolerate up to 2 msec of jitter end-to-end.
- In the initial traffic engineering study you determine that you can deliver sub 2 msec jitter for CES as long as CES is marked at a higher priority than other service traffic and the path has less than 165 Gigabit Ethernet hops in it.
- This is based on the worst case scenario where there is one full 1518 byte packet in transmission ahead of the CES packet at each hop along the path.
- The delay variation of 12 microseconds per GE hop is cumulative and after 165 hops, exceeds the CES application's jitter tolerance of 2 msec.
- With jumbo frames, the maximum packet size may increase to as much as 9018 bytes which represents 72 microseconds of delay per GE hop. This ends up reducing the number of GE hops in the CES path to 27.
One solution is to reengineer the network to account for the introduction of jumbo frames in the GE network. A better solution might be to reengineer the network for the introduction of 10GE. Not only does 10GE hardware inherently support jumbo frames but the 10GE interfaces are 10 times faster than 1GE interfaces. A single 9018 byte packet can be transmitted out a 10 Gigabit interface in 7.2 microseconds, increasing the number of network hops that my hypothetical CES service can traverse to 270!
Got some thoughts about jumbo frames? Maybe some plans to implement them in your network? Tell me about them.
Best wishes,
Buck
micah_ruge
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