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3.9. Using Jumbo Frames

By default, Ethernet interfaces in cOS Stream operate with a default maximum packet payload size of 1500 bytes. By changing the global packet buffer size for cOS Stream, this maximum payload size can be increased for all interfaces. The size can be increased up to a maximum value of 9 kilobytes (9216 bytes), which corresponds to a maximum payload of 9000 bytes.

This ability to handle extra large packet sizes is a feature which is sometimes referred to as jumbo frames.

[Note] Note: Only virtual environments support jumbo frames

The jumbo frames feature is applicable only to virtual environments. Clavister hardware products do not support the feature.

Jumbo Frames Can Increase Throughput

Changing the packet buffer size to a value that matches the larger packet size of surrounding network equipment can increase overall firewall throughput since large packets may no longer need to be fragmented. However, increasing the buffer size will also increase the memory resources used.

It should also be noted that if packets much less than the increased packet buffer size need to be frequently processed, this could reduce throughput, so understanding the expected packet sizes is important.

Setting Up an Increased Packet Buffer Size

The following steps should be used to increase the packet buffer size:

  1. Change the global setting PacketBufferSize to a new value. For example, the following command will increase the buffer size for the system to the maximum value of 9 kilobytes: 

    System:/> set Settings InterfaceSettings PacketBufferSize=9kB

    The value 9kB is a predefined value for this setting which can be used instead of specifying the number 9216. The other predefined values are 4kB (4096 bytes), and the value Default which is the standard size of 1522 bytes.

  2. Change the MTU on interfaces that will transmit the larger packet size. For example: 

    System:/> set Interface EthernetInterface if1 MTU=9000

    Note that an MTU of 9000 would be used with the maximum packet buffer size of 9 kilobytes.

  3. Increase the maximum packet length for the relevant protocols. For example, to change the maximum TCP payload: 

    System:/> set TCPSettings TCPMSSMax=8960

    To change IP payloads: 

    System:/> set LengthLimSettings MaxTCPLen=8980

    System:/> set LengthLimSettings MaxOSPFLen=8980

    System:/> set LengthLimSettings MaxOtherSubIPLen=8980

  4. Following activation of these configuration changes, the system must be restarted so that memory is correctly reallocated.

Verified Features

The following cOS Stream features have been verified to function correctly with a larger packet buffer size:

Verified Ethernet Interface Hardware

The following Ethernet interfaces have been verified to function with the packet buffer sizes of 4 kilobytes and 9 kilobytes.

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