[PATCH] xfrm: fix tunnel model fragmentation behavior

[Steffen Klassert]

On Mon, Feb 21, 2022 at 01:16:48PM +0800, Lina Wang wrote:
> in tunnel mode, if outer interface(ipv4) is less, it is easily to let 
> inner IPV6 mtu be less than 1280. If so, a Packet Too Big ICMPV6 message 
> is received. When send again, packets are fragmentized with 1280, they
> are still rejected with ICMPV6(Packet Too Big) by xfrmi_xmit2().
> 
> According to RFC4213 Section3.2.2:
>          if (IPv4 path MTU - 20) is less than 1280
>                  if packet is larger than 1280 bytes
>                          Send ICMPv6 "packet too big" with MTU = 1280.
>                          Drop packet.
>                  else
>                          Encapsulate but do not set the Don't Fragment
>                          flag in the IPv4 header.  The resulting IPv4
>                          packet might be fragmented by the IPv4 layer
>                          on the encapsulator or by some router along
>                          the IPv4 path.
>                  endif
>          else
>                  if packet is larger than (IPv4 path MTU - 20)
>                          Send ICMPv6 "packet too big" with
>                          MTU = (IPv4 path MTU - 20).
>                          Drop packet.
>                  else
>                          Encapsulate and set the Don't Fragment flag
>                          in the IPv4 header.
>                  endif
>          endif
> Packets should be fragmentized with ipv4 outer interface, so change it.
> 
> After it is fragemtized with ipv4, there will be double fragmenation.
> No.48 & No.51 are ipv6 fragment packets, No.48 is double fragmentized, 
> then tunneled with IPv4(No.49& No.50), which obey spec. And received peer
> cannot decrypt it rightly.
> 
> 48              2002::10	2002::11 1296(length) IPv6 fragment (off=0 more=y ident=0xa20da5bc nxt=50) 
> 49   0x0000 (0) 2002::10	2002::11 1304	      IPv6 fragment (off=0 more=y ident=0x7448042c nxt=44)
> 50   0x0000 (0)	2002::10	2002::11 200	      ESP (SPI=0x00035000) 
> 51		2002::10	2002::11 180	      Echo (ping) request 
> 52   0x56dc     2002::10	2002::11 248	      IPv6 fragment (off=1232 more=n ident=0xa20da5bc nxt=50)
> 
> esp_noneed_fragment has fixed above issues. Finally, it acted like below:
> 1   0x6206 192.168.1.138   192.168.1.1 1316 Fragmented IP protocol (proto=Encap Security Payload 50, off=0, ID=6206) [Reassembled in #2]
> 2   0x6206 2002::10	   2002::11    88   IPv6 fragment (off=0 more=y ident=0x1f440778 nxt=50)
> 3   0x0000 2002::10	   2002::11    248  ICMPv6    Echo (ping) request 
> 
> Signed-off-by: Lina Wang <lina.wang at mediatek.com>

We have two commits in the ipsec tree that address a very similar
issue. That is:

commit 6596a0229541270fb8d38d989f91b78838e5e9da
xfrm: fix MTU regression

and

commit a6d95c5a628a09be129f25d5663a7e9db8261f51
Revert "xfrm: xfrm_state_mtu should return at least 1280 for ipv6"

Can you please doublecheck that the issue you are fixing still
exist in the ipsec tree?

Thanks!