Re: [IPsec] diet-esp - How do you know?

["Scott Fluhrer (sfluhrer)" <[e-mail]>]

The easiest way would be to assign the first few bits of the SPI to indicate the SPI size; for example, all 8 bit SPIs might be allocated to have the first two bits being 11; all 16 bit SPIs might have those two bits being 10; etc.  That way, an examination of the first few bits of the SPI would unambiguously give you the SPI size.

Obviously, this doesn’t apply to a ‘0 byte SPI’.  I have no idea how that is intended to be processed; does that mean that the decrypter is expected to just try to decrypt the packet with all the SAs he has and see which one worked?

From: IPsec <[e-mail]> On Behalf Of Daniel Migault
Sent: Tuesday, May 24, 2022 4:48 PM
To: Robert Moskowitz <[e-mail]>
Cc: Paul Wouters <paul.wouters=[e-mail]>; IPsecME WG <[e-mail]>
Subject: Re: [IPsec] diet-esp - How do you know?

The issue only comes when a gateway wants to support all sizes of SPIs 0 - 1 - 2 - 3 - 4 bytes - which is very unlikely. For a deterministic lookup, I would suggest using IP addresses and the minimum allowed byted compressed SPI.
If you use 2 - 3 bytes, the likelihood of collision might still be very low to support an additional signature check.

Yours,
Daniel

On Tue, May 24, 2022 at 4:30 PM Robert Moskowitz <[e-mail]<mailto:[e-mail]>> wrote:
That is the 'easy' part.

What does the code do when it receives an ESP packet?  How do it know that it is a diet-esp packet and apply the rules?

Next Header just says: ESP.
On 5/24/22 16:23, Daniel Migault wrote:
This is correct. IKEv2 is used both to agree on the use of Diet-ESP as well as values to be used for the compression/decompression.

Yours,
Daniel

On Tue, May 24, 2022 at 11:14 AM Paul Wouters <paul.wouters=[e-mail]<mailto:[e-mail]>> wrote:

On Sun, May 22, 2022 at 9:20 PM Robert Moskowitz <[e-mail]<mailto:[e-mail]>> wrote:
I think there is something else I am missing here.

How does the receiving system 'know' that the packet is a diet-esp packet?

https://datatracker.ietf.org/doc/html/draft-mglt-ipsecme-ikev2-diet-esp-extension-02

It's negotiated with IKEv2.

I guess the IKE stack has to signal this to the ESP implementation on what to expect when
the policy is installed ?

Paul

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Daniel Migault
Ericsson


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