Subject: [Ipsec] Ordering of Security Association in Transport Adjacency

From: "Salekul Islam" <[email-masked]>
Date: Sun, 4 Dec 2005 21:37:27 -0500

Hi,

I have a simple question. I am sorry if it had been answered already. I could not find it in the mailing list archive.

The IPsec architecture has recommend two transport-mode SAs like this:

    [IP1][AH][ESP][upper]

My question is why it has not recommended the other order also like this:

    [IP1][ESP][AH][upper]

Is there any cryptographical benefit or anything else?

Thank you,

Salekul

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From: Stephen Kent <[email-masked]>
Date: Mon, 5 Dec 2005 11:09:43 -0500

At 9:37 PM -0500 12/4/05, Salekul Islam wrote:
>Hi,
>
>I have a simple question. I am sorry if it had been answered 
>already. I could not find it in the mailing list archive.
>

The only motivation for using both AH and ESP in transport mode is to 
receive integrity protection for the IP header, as provided by AH. 
That dictates the order in the first example above. The other 
example, in which AH is applied on the inside, is not useful, since 
we have ESP options that offer integrity and confidentiality, which 
are more efficient that putting AH inside of ESP.

Steve

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From: Markku Savela <[email-masked]>
Date: Mon, 5 Dec 2005 19:05:54 +0200

> From: Stephen Kent <[email-masked]>

> The other > example, in which AH is applied on the inside, is not
> useful, since we have ESP options that offer integrity and
> confidentiality, which are more efficient that putting AH inside of
> ESP.

I can think one use for the latter order: you want to check the
integrity of the IP header, but don't want to show this fact to
outside snooper/hacker.