Subject: Re: [PATCH net-next 3/3] xfrm: Restrict "level use" for IPComp configuration

From: Fan Du <[email-masked]>
Date: Fri, 13 Dec 2013 17:16:03 +0800

On 2013年12月10日 21:11, Steffen Klassert wrote:
> On Tue, Dec 10, 2013 at 10:39:51AM +0800, Fan Du wrote:
>>
>>
>> On 2013年12月09日 18:38, Steffen Klassert wrote:
>>>
>>> I think this will make a lot of people unhappy. It was never required
>>> to set 'optional' for ipcomp, and I'd bet that most users don't set
>>> it for ipcomp. I understand the problem, but we can't fix it like that.
>>
>> Instead of making this check, what about wire 'optional' to 1? it doesn't
>> breaking existing script.
>
> But it might change what a user expects to happen.
>
>>
>> Do you have any other way to cure this problem other than 'optional'.
>>
>
> I think the user can 'fix' the problem himself by setting 'optional'.
> This has also the advantage that he is aware about the change. Maybe
> this should be documented somewhere.
>

I suspect adding a WARN in here is not good, so how about below doc looks
like?

Documentation/networking/ipsec.txt

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
    MTU.

    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
payload length.

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.

-- 
浮沉随浪只记今朝笑

--fan