|.. SPDX-License-Identifier: GPL-2.0
|Here documents known IPsec corner cases which need to be keep in mind when
|deploy various IPsec configuration in real world production environment.
| Small IP packet won't get compressed at sender, and failed on
| policy check on receiver.
|Quote from RFC3173::
| 2.2. Non-Expansion Policy
| If the total size of a compressed payload and the IPComp header, as
| defined in section 3, is not smaller than the size of the original
| payload, the IP datagram MUST be sent in the original non-compressed
| form. To clarify: If an IP datagram is sent non-compressed, no
| IPComp header is added to the datagram. This policy ensures saving
| the decompression processing cycles and avoiding incurring IP
| datagram fragmentation when the expanded datagram is larger than the
| Small IP datagrams are likely to expand as a result of compression.
| Therefore, a numeric threshold should be applied before compression,
| where IP datagrams of size smaller than the threshold are sent in the
| original form without attempting compression. The numeric threshold
| is implementation dependent.
|Current IPComp implementation is indeed by the book, while as in practice
|when sending non-compressed packet to the peer (whether or not packet len
|is smaller than the threshold or the compressed len is larger than original
|packet len), the packet is dropped when checking the policy as this packet
|matches the selector but not coming from any XFRM layer, i.e., with no
|security path. Such naked packet will not eventually make it to upper layer.
|The result is much more wired to the user when ping peer with different
|One workaround is try to set "level use" for each policy if user observed
|above scenario. The consequence of doing so is small packet(uncompressed)
|will skip policy checking on receiver side.