1.. SPDX-License-Identifier: GPL-2.0 2 3===== 4IPsec 5===== 6 7 8Here documents known IPsec corner cases which need to be keep in mind when 9deploy various IPsec configuration in real world production environment. 10 111. IPcomp: 12 Small IP packet won't get compressed at sender, and failed on 13 policy check on receiver. 14 15Quote from RFC3173:: 16 17 2.2. Non-Expansion Policy 18 19 If the total size of a compressed payload and the IPComp header, as 20 defined in section 3, is not smaller than the size of the original 21 payload, the IP datagram MUST be sent in the original non-compressed 22 form. To clarify: If an IP datagram is sent non-compressed, no 23 24 IPComp header is added to the datagram. This policy ensures saving 25 the decompression processing cycles and avoiding incurring IP 26 datagram fragmentation when the expanded datagram is larger than the 27 MTU. 28 29 Small IP datagrams are likely to expand as a result of compression. 30 Therefore, a numeric threshold should be applied before compression, 31 where IP datagrams of size smaller than the threshold are sent in the 32 original form without attempting compression. The numeric threshold 33 is implementation dependent. 34 35Current IPComp implementation is indeed by the book, while as in practice 36when sending non-compressed packet to the peer (whether or not packet len 37is smaller than the threshold or the compressed len is larger than original 38packet len), the packet is dropped when checking the policy as this packet 39matches the selector but not coming from any XFRM layer, i.e., with no 40security path. Such naked packet will not eventually make it to upper layer. 41The result is much more wired to the user when ping peer with different 42payload length. 43 44One workaround is try to set "level use" for each policy if user observed 45above scenario. The consequence of doing so is small packet(uncompressed) 46will skip policy checking on receiver side. 47