[c-nsp] vpn

[Brian McMahon]

Quoth Arturo Servin:
>     My mistake, the right question is:
> 
> I also have a question. You can use AH or ESP but not both at the 
>>same time. ESP will authenticate and encrypt while AH will only authenticates. 
>>Is it correct?

No.  You *can* use AH and ESP together, at least as far as the protocols 
go.  It's just that, for most purposes, you wouldn't want to.  The only 
thing that AH (and therefore AH+ESP) does that ESP doesn't is to extend 
authentication to cover the outermost IP header.

Using Ye Olde Typewriter Graphics, here's how it looks.  (Use monospaced 
viewing font for best results.)  "e" is for encrypted, "a" is for 
authenticated.

ESP:

                          |< - - encrypted - - >|
+-----------+------------+eeeeeeeeeeeeeeee+eeeee--------+
| IP header | ESP header | Protected data | ESP trailer |
+-----------+aaaaaaaaaaaa+aaaaaaaaaaaaaaaa+aaaaa--------+
             |< - - - - authenticated - - - - ->|

AH:

+-----------+-----------+----------------+
| IP header | AH header | Protected data |
+aaaaaaaaaaa+aaaaaaaaaaa+aaaaaaaaaaaaaaaa+
|< - - - - - - authenticated - - - - - ->|

(Adapted from figures 3.4 and 3.5 in the Doraswamy/Harkins IPsec book, 
2nd edition.  RFC2406 distinguishes between the ESP trailer (protected) 
and the ESP auth field (not), which is probably more detail than most 
people really want or need.)

So if, for example, "unable to survive NAT" is on your list of required 
features, AH+ESP is your solution.  8-)

Translating to the wonderfully baroque IOS config syntax, you implement 
AH+ESP by specifying both AH and ESP transforms in the same transform 
set (for example, crypto ipsec transform-set paranoid ah-sha-hmac 
esp-des esp-sha-hmac).

-- 
Brian McMahon <brian dot mcmahon at cabrillo dot edu>
Instructor, Cisco Networking Academy
Cabrillo College, Aptos, California