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

From: "Fries, Steffen" <[email-masked]>
Date: Wed, 7 Jun 2006 08:17:10 +0200

Hi Karl,

thank you for the explanation. That answers my question. I agree, if the
receiver has no resonable guess of s_1 he will run into problems. As
this was my only question, the draft looks fine from my point of view.

Regards
	Steffen

> -----Original Message-----
> From: Karl Norrman (KI/EAB) [mailto:[email-masked]] 
> Sent: Tuesday, June 06, 2006 11:10 PM
> To: Fries, Steffen
> Cc: [email-masked]
> Subject: RE: [MSEC] draft-lehtovirta-srtp-rcc-03.txt
> 
> 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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