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a=Ol13hO9ccFRV9qXi2t6ftBPywas=:19 a=xqWC_Br6kY4A:10 a=IkcTkHD0fZMA:10 a=yJojWOMRYYMA:10 a=GoEa3M9JfhUA:10 a=6EyYywLGfX6WYdpxRyMA:9 a=QEXdDO2ut3YA:10 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 >