When using CLAT it doesn't make a difference whether MSS clamping is done by CLAT, NAT64, or both. I recommend doing it on both sides to ensure it gets done.
If the NAT64 gateways you use are outside of your control and doesn't do MSS clamping you will get more reliability by having your CLAT do MSS clamping.
I am not sure how much ping will help troubleshooting MTU problems when protocols can be handled differently. I think to ensure you have addressed all the possible MTU problems you'd have to test TCP, UDP, and ping separately. (As far as I recall no other protocol is required to be supported by the NAT64 RFC).
For TCP you can pretty much avoid MTU problems by using MSS clamping. And for UDP there is an extension to do fragmentation on the UDP layer rather than the IP layer. This extension is intended to work better with NAT than IP fragmentation. But it's fairly new and not widely supported.
For DNS it's recommended to switch to TCP if the payload would exceed 1280 bytes on UDP over IPv6. I don't know if other UDP based protocols also aim to avoid fragmentation.
kasperd
npub1jh…xut5y
2026-08-21 21:43:08 UTC
in reply to nevent1q…vyef
Author Public Key
npub1jhpgfu25y30c3h9xrrlxhrhge7669h3xwphjs99nau4k66ctsnyqjxut5ySeen on
wss://relay.momostr.pinkPublished at
2026-08-21 21:43:08 UTCEvent JSON
{
"id": "2ae03d20002fe7b500d22c314691eebce1f39d825237942071c6a16dc7affc16",
"pubkey": "95c284f154245f88dca618fe6b8ee8cfb5a2de26706f2814b3ef2b6d6b0b84c8",
"created_at": 1787348588,
"kind": 1,
"tags": [
[
"e",
"27f753c048715bd0e6e66ac0ec810ff9c3bf88334c48b95cc645fa6a782f944a",
"",
"reply",
"f9d775eb638c05c904dc2333bbe328b52561e2e2da42e3cbea73230079560a02"
],
[
"p",
"95c284f154245f88dca618fe6b8ee8cfb5a2de26706f2814b3ef2b6d6b0b84c8"
],
[
"e",
"a5f6d47027964ad395ce7728ab7203db1206ea499d8a6944293655a1673f8d77",
"",
"root",
"f9d775eb638c05c904dc2333bbe328b52561e2e2da42e3cbea73230079560a02"
],
[
"p",
"f9d775eb638c05c904dc2333bbe328b52561e2e2da42e3cbea73230079560a02"
],
[
"proxy",
"https://westergaard.social/objects/a821f564-d187-446d-9714-931180dbbe07",
"activitypub"
],
[
"L",
"pink.momostr"
],
[
"l",
"pink.momostr.activitypub:https://westergaard.social/objects/a821f564-d187-446d-9714-931180dbbe07",
"pink.momostr"
],
[
"-"
]
],
"content": "When using CLAT it doesn't make a difference whether MSS clamping is done by CLAT, NAT64, or both. I recommend doing it on both sides to ensure it gets done.\n\nIf the NAT64 gateways you use are outside of your control and doesn't do MSS clamping you will get more reliability by having your CLAT do MSS clamping.\n\nI am not sure how much ping will help troubleshooting MTU problems when protocols can be handled differently. I think to ensure you have addressed all the possible MTU problems you'd have to test TCP, UDP, and ping separately. (As far as I recall no other protocol is required to be supported by the NAT64 RFC).\n\nFor TCP you can pretty much avoid MTU problems by using MSS clamping. And for UDP there is an extension to do fragmentation on the UDP layer rather than the IP layer. This extension is intended to work better with NAT than IP fragmentation. But it's fairly new and not widely supported.\n\nFor DNS it's recommended to switch to TCP if the payload would exceed 1280 bytes on UDP over IPv6. I don't know if other UDP based protocols also aim to avoid fragmentation.",
"sig": "6eca091b94d2930605e66859c2809576518288eefd26e9c13bf4759c23b691bda6766516d586a266510a5532ad4d6d3baac6fc1e13fe0eb8172ece76514be416"
}