Understanding MTU Size in Cellular IoT Connectivity

When deploying IoT devices on cellular networks, one of the most common yet overlooked causes of packet loss and poor performance is an incorrectly configured MTU (Maximum Transmission Unit).

Unlike wired networks, where an MTU of 1500 bytes is typical, cellular connectivity introduces tunneling and additional headers that reduce the effective MTU. For IoT developers, understanding these limitations is critical to ensuring devices stay connected reliably.

What Is MTU and Why It Matters

• MTU (Maximum Transmission Unit): The maximum size of a packet that can be sent without fragmentation.
• MSS (Maximum Segment Size): The MTU minus TCP/IP headers.

When MTU is mismatched between device and network, packets may be dropped, retransmitted, or fragmented. This leads to higher latency, wasted bandwidth, and degraded performance — especially problematic for IoT devices transmitting small but frequent bursts of data.

For battery-powered IoT devices, repeated retransmissions can also significantly increase power consumption, shortening device lifespan.

MTU by Carrier (VPLMN Examples)

Mobile operators in the U.S. typically define the MTU when establishing the PDP (Packet Data Protocol) context. Here are observed defaults:

• AT&T: 1430 bytes
• T-Mobile: Up to 1500 bytes, but commonly 1420 bytes
• Verizon: 1428 bytes

Because of tunneling overhead, real-world MTU values are usually in the 1340–1440 range.

Header Overhead

Cellular networks often apply encapsulation methods such as IPsec or MPLS, which consume additional bytes in the packet. Typical overheads include:

• TCP header: 20 bytes
• IP header: 20 bytes
• ICMP header: 8 bytes (when used)
• IPsec header: 62 bytes

This means the theoretical maximum MSS (data payload) is often 1300–1380 bytes to avoid fragmentation.

Path MTU Discovery (PMTUD)

Most modern devices use Path MTU Discovery (PMTUD) to automatically adjust MTU size. Here’s how it works:

1. Device sends packets with the DF (Don’t Fragment) flag set.
2. If a router encounters a packet larger than the path MTU, it drops it and returns an ICMP “Fragmentation Needed” message.
3. Device lowers its MTU to match.

In theory, this should prevent fragmentation. In practice, some IoT devices fail to properly lower MTU, resulting in packet loss.

How to Test MTU Size

You can test MTU using ping with the “don’t fragment” flag:

ping x.x.x.x -s 1452 -f

This sends a 1460-byte packet (1452 + 8-byte ICMP header). If the network MTU is smaller, packets will be dropped and the device should reduce MTU accordingly.

Example:

• Tunnel Interface max MTU: 1476 (1500 – 24 byte GRE header)
• Actual max MTU on wire: ~1360

Industry Insights: Why Networks Lower MTU

From operator experience and 3GPP recommendations:

• Networks often use IPsec/MPLS, which adds overhead.
• Lower MTUs (e.g., 1340–1430) are chosen to ensure compatibility across roaming partners.
• IoT developers should design conservatively for MSS in the 1300–1380 range.

Lessons Learned & Recommendations

• Never assume a 1500-byte MTU on cellular — real-world is lower.
• Test across multiple carriers (AT&T, Verizon, T-Mobile) for your IoT device.
• Use PMTUD but verify it actually works with your modem and firmware.
• Configure your IoT stack for a safe MSS around 1300–1380 bytes to minimize risk of packet loss.

How Simplex Wireless Helps

At Simplex Wireless, we’ve tested hundreds of devices across AT&T, T-Mobile, and Verizon networks. Our expertise allows us to guide you on topics such as MTU misconfiguration, which can silently degrade performance in IoT deployments.

• Learn more about our IoT SIM cards
• Explore our IoT connectivity solutions for global projects
• Check out our article on enabling BIP/STK for SGP.32 eSIM management

With 191-country coverage, carrier-grade technology, and Atlanta-based support, Simplex Wireless helps you deploy IoT projects with confidence.

This article was curated by Raimo Järvenpää, CPO, Simplex Wireless