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 <[email protected]>
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!