Maximum MTU supported by Cloudflare's 1.1.1.1

Hi There,

I am interseted to know if it is possible to utilize jumbo frames or achieve an MTU 1500 or above when interacting with the 1.1.1.1 service.

I tried pinging Cloudflare DNS 1.1.1.1 only stuck at MTU 1472 through my provider. Does it not support mtu 1500?

ping 1.1.1.1 size 1500 source 119.110.127.25
PING 1.1.1.1 (1.1.1.1): 1500 bytes of data
^C
— 1.1.1.1 ping statistics —
3 packets sent, 0 packets received, 100% packet loss

ping 1.1.1.1 size 1472 source 119.110.127.25
PING 1.1.1.1 (1.1.1.1): 1472 bytes of data
— 1.1.1.1 ping statistics —
3 packets sent, 3 packets received, 0% packet loss
min/avg/max/stddev round-trip = 12,268/12,354/12,478/0,090 ms

Thanks

If you ping a payload size of 1472, the packet is 1500 bytes as there is a 28 byte header (for IPv4).

2 Likes

@sjr is right. Furthermore, I’ve put your post in GPT-4 and got the following answer:

The issue you’re experiencing with MTU (Maximum Transmission Unit) sizes and the inability to ping Cloudflare’s DNS service at 1.1.1.1 with a packet size of 1500 bytes seems to be related to the way MTU sizes are handled in networking.

  1. MTU Size Limitations: The standard MTU for Ethernet networks is 1500 bytes. However, the actual MTU size that can be used depends on the entire path the packets travel through the network. If any link in the network path has an MTU smaller than the packet size, the packet will need to be fragmented to pass through that link, or it will be dropped if the “Don’t Fragment” bit is set in the packet header.
  2. Your Test Results: You mentioned that pinging 1.1.1.1 works with a packet size of 1472 bytes but not with 1500 bytes. This suggests that the maximum MTU size supported somewhere along the path to 1.1.1.1 is less than 1500 bytes. The size of 1472 bytes, when combined with the 28 bytes of IP and ICMP header, equals 1500 bytes, which fits the standard Ethernet MTU.
  3. Cloudflare’s 1.1.1.1 Service: Cloudflare’s DNS service should be capable of handling standard Ethernet MTU sizes. The issue likely lies with the MTU sizes supported by intermediate networks or your own network configuration.
  4. Jumbo Frames: Jumbo frames are larger than the standard Ethernet frame and can carry up to 9000 bytes of payload. However, using jumbo frames requires that all devices on the network path support them. If any device does not support jumbo frames, they will either need to fragment the packet (if allowed) or drop it.
  5. Troubleshooting: To resolve this, you can:
  • Check your own network equipment (router, switches) to ensure they are configured to support an MTU of 1500 bytes.
  • Contact your ISP to see if they impose any MTU size restrictions in their network.
  • Use traceroute or similar tools to check at which point in the path to 1.1.1.1 packets are being dropped or need fragmentation.

The fact that you can successfully ping with an MTU of 1472 bytes but not with 1500 bytes suggests a limitation in the MTU size somewhere along the network path, rather than a specific issue with Cloudflare’s 1.1.1.1 service.

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