Hey,
with DCO and possible future hardware assisted OpenVPN acceleration we
are approaching the point where 32 bit IVs are not cutting it any more.
To illustrate the problem, some back of the envelope math here:
If we want to keep the current 3600s renogotiation interval and have a
safety margin of 30% we have about 3 million packets (2*32 * 0.7) to
work with. That translates to about 835k packets per second.
With 1300 Byte packets that translates into 8-9 Gbit/s. That is from
unrealistic any more. Current DCO implementations are already in
spitting distance to that or might even reach (for a single client
connection) that if you have extremely fast single core performance CPU.
So I think we need to consider adding 64bit IV now rather than later.
So the proposal is the following:
- add IV_PACKET_FORMAT_AEAD_V2 flag to the protocol flags. This signal
that the other side supports the new AEAD data channel packet format
that supports 64 bit IVs.
I don't think we need to extend this feature to the CBC data channel
format. I just don't see a use case where people would be able to
upgrade to a new OpenVPN version to use 64 bit IVs but not to also
change to use AEAD ciphers.
- add protocol-flag aead-packet-format-v2 This signals the client to
switch to the new data channel format.
And finally have the data channel format. Since this format is
negotiated like the cipher, there is no need to use another opcode if
keep the peer id to just 24 bit. But we might want to extend the format
to have it 8 byte aligned to also allow peer-id to be extended in the
future.
Regardless of what choice we take, this is a good opportunity to rectify
the position of the AEAD tag in our packet. Especially for hardware
implementations it is quite advantageous to have the AEAD at the end of
the packet instead of the beginning and since we need to have a new data
format, there is no reason to keep the tag at the start of the packet.
So the choice is basically
Variant A:
Bytes
1 234 5-13 13- (n+13) (n+14) - (n+30)
[OP_CODE_DATA_V2][peerid] [64 bit IV] [ payload ][ 16 byte auth tag]
Variant B:
1 3-8
[OP_CODE_DATA_V3] [ peer-id/padding] [rest identical]
Arne
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