Configuring Path MTU Auto Discovery

Usage Scenario

The link-layer MTUs of different networks that a communication path traverses may vary. The smallest MTU on the path is the most important factor that influences the communication between the two ends of the path. The smallest MTU on the communication path is called the path MTU.

The path MTU varies with the selected path and therefore may change. In addition, the path MTU in one direction may be inconsistent with that in the reverse direction. Enabling path MTU auto discovery allows a device to discover the path MTU from the transmit end to the receive end. TCP encapsulates IP packets based on the path MTU when transmitting BGP messages.

In Figure 1, path MTU auto discovery is not enabled, and a BGP peer relationship is established between Device A and Device C. After Device A sends 9000-byte BGP messages to Device C, Device B fragments the messages upon reception by encapsulating one IP header, one Layer 2 frame header, and one Layer 2 frame trailer in each fragment, which reduces transmission efficiency. If a fragment of a message is lost, the message becomes invalid.

Figure 1 Network without path MTU auto discovery

In Figure 2, path MTU auto discovery is enabled, and a BGP peer relationship is established between Device A and Device C. Device A sends BGP messages to Device C based on the path MTU (6000 bytes). In this example, Device A sends a 6000-byte BGP message and a 3000-byte BGP message. In this case, Device B does not fragment the messages upon reception, which improves transmission efficiency.

Figure 2 Network with path MTU auto discovery

The path MTU auto discovery function affects TCP MSS calculation.

• If path MTU auto discovery is not enabled:

  • On the sending end, the calculation formula of TCP MSS is as follows: MSS = MIN { CFGMSS, MTU-40 }.

  • On the receive end:

    If the device supports SYNCOOKIE, the calculation formula of MSS is as follows: MSS = MIN { MIN { CFGMSS, MTU-40 } , internally-defined MSS value }.

    If the device does not support SYNCOOKIE, the calculation formula of MSS is as follows: MSS = MIN { CFGMSS, MTU-40 }.

• After path MTU auto discovery is enabled:

  • On the sending end, the local MSS value is updated only when the device sends packets in which the MSS value is greater than the path MTU. The calculation formula of TCP MSS is as follows: MSS = MIN { MIN { CFGMSS, MTU-40 }, PMTU-40 }.

  • On the receive end:

    If the device supports SYNCOOKIE, the calculation formula of TCP MSS is as follows: MSS = MIN { MIN { MIN { CFGMSS, MTU-40 }, internally-defined MSS value }, PMTU-40 }.

    If the device does not support SYNCOOKIE, the calculation formula of TCO MSS is as follows: MSS = MIN { MIN { CFGMSS, MTU-40 }, PMTU-40 }.

The parameters in the calculation formula are described as follows:

• CFGMSS: MIN { APPMSS, CLICFGMSS }
  • APPMSS: Specifies the MSS configured using the peer tcp-mss command.
  • CLICFGMSS: Specifies the maximum MSS configured using the tcp max-mss mss-value command.
• MTU-40: Specifies the interface MTU minus 40.
• PMTU-40: Specifies the path MTU minus 40.
• internally-defined MSS value: Specifies an internally-defined MSS value, which can be 216, 460, 952, 1400, 2900, 4900, 7900, or 9500. Upon receipt of a packet, the receive-end device uses the internally-defined MSS which is smaller than but close to the MSS of the received packet.

Pre-configuration Tasks

Before configuring path MTU auto discovery, configure basic BGP functions.