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