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+Here documents known IPsec corner cases which need to be keep in mind when
+deploy various IPsec configuration in real world production environment.
+1. IPcomp: 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 large 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.