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Re: Sequence Number field for manually configured SAs

I agree that the current ESP draft text is confusing.

> From: "Steve Klein (254-5623)" <steveklein@vnet.ibm.com>
> I am confused on how the Sequence Number field for ESP should be handled
> for manually configured SAs, especially with respect to implicit IVs.
>...
> Based on these passages, one could assume that for manual SAs you should
> send the Sequence Number field in the ESP but do not increment any
> counters (to avoid the rollover of the field).
>
I did not make the same interpretation.  I understood the text to mean
the Sequence Number is always present and advanced, but that manually
keyed implementations not check the field, and must be aware that it
might roll over.

The Kent ESP text is manifestly unclear.


>(quoted)
>       compliant implementation SHOULD NOT provide this service in
>       conjunction with SAs that are manually keyed.
>
I disagree with the SHOULD NOT.  Indeed, a manual implementation SHOULD
ignore two consecutive packets with the same sequence -- simple packet
duplication replay protection.

If the initial manual sequence number is chosen randomly, there is at
least a small window where the receiver is protected against replay --
the bandwidth-delay time.  The receiver is not protected against
an evesdropper recording a long sequence and replay at a much later
time, however.


> Which interpretation is correct?  I assume the same interpretation would
> also apply to the handling of the Sequence Number field in the manually
> configured AH SAs.
>
Well, mine of course!

If you start at a random number, and increment, and the receiver ignores
it entirely, you have the same effect as Kent's ESP text.

And you may note:

   Replay Window
      Default: 0 (checking off).  Range: 32 to 256.

So, you can expect all peers to ignore the field, unless and until they
have been configured to do so.  Interoperability is maintained with all
previous implementations.

WSimpson@UMich.edu
    Key fingerprint =  17 40 5E 67 15 6F 31 26  DD 0D B9 9B 6A 15 2C 32
BSimpson@MorningStar.com
    Key fingerprint =  2E 07 23 03 C5 62 70 D3  59 B1 4F 5E 1D C2 C1 A2