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| 2 |
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| 3 | =====
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| 4 | IPsec
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| 5 | =====
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| 6 |
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| 7 |
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| 8 | Here documents known IPsec corner cases which need to be keep in mind when
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| 9 | deploy various IPsec configuration in real world production environment.
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| 10 |
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| 11 | 1. IPcomp:
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| 12 | Small IP packet won't get compressed at sender, and failed on
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| 13 | policy check on receiver.
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| 14 |
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| 15 | Quote from RFC3173::
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| 16 |
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| 17 | 2.2. Non-Expansion Policy
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| 18 |
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| 19 | If the total size of a compressed payload and the IPComp header, as
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| 20 | defined in section 3, is not smaller than the size of the original
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| 21 | payload, the IP datagram MUST be sent in the original non-compressed
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| 22 | form. To clarify: If an IP datagram is sent non-compressed, no
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| 23 |
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| 24 | IPComp header is added to the datagram. This policy ensures saving
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| 25 | the decompression processing cycles and avoiding incurring IP
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| 26 | datagram fragmentation when the expanded datagram is larger than the
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| 27 | MTU.
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| 28 |
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| 29 | Small IP datagrams are likely to expand as a result of compression.
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| 30 | Therefore, a numeric threshold should be applied before compression,
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| 31 | where IP datagrams of size smaller than the threshold are sent in the
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| 32 | original form without attempting compression. The numeric threshold
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| 33 | is implementation dependent.
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| 34 |
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| 35 | Current IPComp implementation is indeed by the book, while as in practice
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| 36 | when sending non-compressed packet to the peer (whether or not packet len
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| 37 | is smaller than the threshold or the compressed len is larger than original
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| 38 | packet len), the packet is dropped when checking the policy as this packet
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| 39 | matches the selector but not coming from any XFRM layer, i.e., with no
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| 40 | security path. Such naked packet will not eventually make it to upper layer.
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| 41 | The result is much more wired to the user when ping peer with different
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| 42 | payload length.
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| 43 |
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| 44 | One workaround is try to set "level use" for each policy if user observed
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| 45 | above scenario. The consequence of doing so is small packet(uncompressed)
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| 46 | will skip policy checking on receiver side.
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