Subject: RE: [MSEC] draft-lehtovirta-srtp-rcc-03.txt

From: "Karl Norrman \(KI/EAB\)" <[email-masked]>
Date: Tue, 6 Jun 2006 23:09:47 +0200

Hello! 

The problem in 3 and 4 is that the receiver does not yet
know what is an "old" packet, i.e., the receiver has not
initialized the highest received SEQ (s_l) when the first
packet arrives.

This figure describes case 3 (assume that the SRTP packet with
SEQ = 0xffff is delayed and that those with SEQ = 0, 1, 2
are lost).

  Sender                            Receiver
ROC   SEQ                         ROC    s_l
0     0xfffe

0     0xffff --+
               |
1     0x0000   |  
               | (MIKEY ROC=1)
1     0x0001 --|--------------->  1      0x????
               |(SRTP SEQ=0xffff)
1     0x0002   +--------------->  1      0xffff
                (SRTP SEQ=0x0003)
1     0x0003 ------------------>  2      0x0003 

If there is no integrity protection, the receiver would deduce
that the ROC should be increased by one when receiving the last
packet in the figure.
As you point out, if there *is* integrity protection, the packet
with SEQ = 0xffff would be dropped and the ROC would not be incorrectly
increased when the last packet arrives.

The mechanism in RFC3711 can unfortunately not be applied in case 3
and 4 since it assumes that the receiver has reasonable guess of
the ROC and s_l, and s_l is in these cases not known.

BR
Karl

> -----Original Message-----
> From: Fries, Steffen [mailto:[email-masked]] 
> Sent: den 6 juni 2006 13:02
> To: Karl Norrman (KI/EAB)
> Cc: [email-masked]
> Subject: RE: [MSEC] draft-lehtovirta-srtp-rcc-03.txt
> 
>  Hi Karl,
> 
> I just read over the updated draft. The only thing that was 
> not quite clear to me was the point three in the 
> introduction. If the receiver gets an "old" packet, would'nt 
> it be recognized by the integrity check (if applied)? Same to 
> point 4, I would have expected, that the mechanism described 
> in RFC3711 works in this scenario (packet loss smaller than
> 2^15 packets). The increment/decrement of the (local) ROC 
> should only be done after a successful integrity check.
> 
> 
> Regards
> 	Steffen
> 
> 
> 
> > -----Original Message-----
> > From: [email-masked] 
> > [mailto:[email-masked]] On Behalf Of Karl Norrman 
> > (KI/EAB)
> > Sent: Sunday, June 04, 2006 5:32 PM
> > To: [email-masked]; [email-masked]
> > Subject: [MSEC] draft-lehtovirta-srtp-rcc-03.txt
> > 
> > Hello!
> > 
> > An update of the draft is now available at:
> > http://www.ietf.org/internet-drafts/draft-lehtovirta-srtp-rcc-03.txt
> > 
> > We are planning to request for it to be published, and 
> Lakshminath has 
> > kindly volunteered to help with shepherding.
> > 
> > Please provide any further comments within one week.
> > 
> > Regards,
> > Karl
> > _______________________________________________
> > msec mailing list
> > [email-masked]
> > http://six.pairlist.net/mailman/listinfo/msec
> > 
> 
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