Dear developers, I'm trying to understand the different behavior between `ip_finish_output` and `ip6_finish_output`, when deciding whether to do fragmentation or not. `ip_finish_output` calls `ip_fragment` when `skb->len` exceeds the destination mtu; In addition to this mtu check, `ip6_finish_output` also checks if `skb->len > IP6CB(skb)->frag_max_size`. The relevant code is here https://elixir.bootlin.com/linux/latest/source/net/ipv4/ip_output.c#L310 https://elixir.bootlin.com/linux/latest/source/net/ipv6/ip6_output.c#L151 As far as I know, `frag_max_size` prevents the forwarding routine from sending packets longer than the maximum fragment received after defragmentation. I'm wondering why `ip_finish_output` doesn't check similarily for `IPCB(skb)->frag_max_size`, especially when `ip_fragment` and `ip_do_fragment`, called (indirectly) by `ip_finish_output`, both cap the output mtu by this frag_max_size. Many thanks in advance! If I'm sending to the wrong person, or wrong mailing list, please let me know. It's my first time trying to ask questions to Linux developers, and sorry for the disturbance. Currently I'm not subscribed to any mailing list, but I will if necessary. Thank you for making Linux great ;) Sincerely, Wenxin Wang