[REGRESSION] v5.15: UDP packets not fragmented after receiving ICMP "Fragmentation Needed" (works in v5.10)

[REGRESSION] v5.15: UDP packets not fragmented after receiving ICMP "Fragmentation Needed" (works in v5.10)

[CHANDRA SEKHAR REDDY]

Hi Team,

We are observing an intermittent regression in UDP fragmentation
handling between Linux kernel versions v5.10.35 and v5.15.71.

Problem description:
Our application sends UDP packets that exceed the path MTU. An
intermediate hop returns an ICMP Type 3, Code 4 (Fragmentation Needed)
message.

On v5.10.35, the kernel correctly updates the Path MTU cache, and
subsequent packets are fragmented as expected.

On v5.15.71, although the ICMP message is received by the kernel,
subsequent UDP packets are sometimes not fragmented and continue to be
dropped.

System details:

Egress interface MTU: 9192 bytes

Path MTU at intermediate hop: 1500 bytes

Kernel parameter: ip_no_pmtu_disc=0 (default)

Questions / request for feedback:

Is this a known regression in the 5.15 kernel series?

We have verified that the Path MTU cache is usually updated correctly.

Is there a way to detect or log cases where the cache is not updated?

If this issue has already been addressed, could you please point us to
the relevant fix commit so we can backport and test it?

We have reviewed several patches between v5.10.35 and v5.15.71 related
to PMTU and ICMP handling and examined the code flow,
 but have not been able to pinpoint the root cause.

Any guidance, insights, or pointers would be greatly appreciated.

Best regards,
Chandrasekharreddy C

Re: [REGRESSION] v5.15: UDP packets not fragmented after receiving ICMP "Fragmentation Needed" (works in v5.10)

[Harshit Mogalapalli]

Hi Chandra Sekhar,

 From a stable kernels point of view I have some comments.

On 18/09/25 08:54, CHANDRA SEKHAR REDDY wrote:
> Hi Team,
> 
> We are observing an intermittent regression in UDP fragmentation
> handling between Linux kernel versions v5.10.35 and v5.15.71.
> 


These both are like really old stable kernels. Can you try with the 
latest stable kernels ?

5.15.193 and 5.10.244 ?

If you could reproduce the same on 5.15.193, maybe we should try on 
different kernels to understand the cause better (i.e 5.15.0 and then 
narrow it if needed)


> Problem description:
> Our application sends UDP packets that exceed the path MTU. An
> intermediate hop returns an ICMP Type 3, Code 4 (Fragmentation Needed)
> message.
> 
> On v5.10.35, the kernel correctly updates the Path MTU cache, and
> subsequent packets are fragmented as expected.
> 
> On v5.15.71, although the ICMP message is received by the kernel,
> subsequent UDP packets are sometimes not fragmented and continue to be
> dropped.
> 
> System details:
> 
> Egress interface MTU: 9192 bytes
> 
> Path MTU at intermediate hop: 1500 bytes
> 
> Kernel parameter: ip_no_pmtu_disc=0 (default)
> 
> Questions / request for feedback:
> 
> Is this a known regression in the 5.15 kernel series?
> 
> We have verified that the Path MTU cache is usually updated correctly.
> 
> Is there a way to detect or log cases where the cache is not updated?
> 
> If this issue has already been addressed, could you please point us to
> the relevant fix commit so we can backport and test it?
> 
> We have reviewed several patches between v5.10.35 and v5.15.71 related
> to PMTU and ICMP handling and examined the code flow,
>   but have not been able to pinpoint the root cause.
> 
> Any guidance, insights, or pointers would be greatly appreciated.
> 

Thanks,
Harshit

> Best regards,
> Chandrasekharreddy C
>