AT&T IoT Devices
  • Basics
  • Devices
  • Modules
  • Get Certified
  • FAQ
  • Contact
 

Maximum Transmission Unit (MTU) Settings for FirstNet Devices

The MTU settings for FirstNet devices are different than other IoT devices.

What is MTU?

  • MTU is Maximum Transmission Unit. It is a measurement of the largest IP (Internet Protocol) packet that can be carried on a network. MTU includes all headers and the actual payload - including the TCP (Transmission Control Protocol) header of 40 bytes.
  • The network MTU used by FirstNet devices will be lowered to 1342 bytes. This is different from the MTU used in AT&T's network where the MTU is 1430 bytes.

What are the MTU requirements for FirstNet devices?

  • The MTU for FirstNet devices must be dynamically negotiated with the FirstNet network. FirstNet devices will start receiving their MTU from the network as early as 3Q-2021. Two protocols are used to deliver the MTU settings to the device:
    • PCO - Protocol Configuration Option. This is defined in 3GPP 24.008 and is an optional field in the set up messages between the device and the FirstNet network. PCO is used to negotiate the MTU setting for IPv4 devices
    • RA - Router Advertisement is sent by the network to the device. RA will broadcast the MTU setting for IPv6 devices.
  • The MTU must also be statically set to 1342 bytes. A static setting is used in the event a device is not capable of PCO/RA or if this negotiation fails for any reason.

What should I do to make sure my device is compliant?

  • If your devices uses the module's TCP/IP stack:
    • Integrate a module firmware into your device that is compliant with the above requirements.
    • Process a maintenance release for your device - you can onboard this at the AT&T Project One web site.
  • If your device has a TCP/IP stack that is independent of the module:
    • Integrate the operating system's TCP/IP stack so it uses the MTU derived from the embedded modem (note: modem must also comply with the above requirements).
    • Process a maintenance release for your device - you can onboard this at the AT&T Project One web site.

What are the consequences of non-compliance?

  • Your device will experience packet fragmentation and reduced throughput if it is not compliant with these MTU requirements by 1Q-2022.
  • IoT customer solutions
  • IoT platforms and services
  • Network solutions
  • Partnership program
  • Business support
  • IoT Device Catalog
  • About AT&T
  • News
  • Networking exchange blog
  • Business community
  • Youtube channel

Connect with AT&T

Business community

  • Legal policy center
  • Privacy policy
  • Terms of use
  • Broadband details
  • Your choices
  • Advertising choices
  • Accessibility
  • Your Privacy Choices California Consumer Privacy Act (CCPA) Opt-Out Icon

© AT&T Intellectual Property. All rights reserved. AT&T, Globe logo,Mobilizing Your World, and DIRECTTV are registered trademarks of AT&T Intellectual Property and/or AT&T affiliated companies. All other marks are the property of their respective owners.