Subject: [Pana] NEW: [issue57] PSR retransmission strategy

From: [email-masked]
Date: Thu, 5 Feb 2004 11:41:51 +0200

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From: Yoshihiro Ohba <[email-masked]>
Date: Mon, 09 Feb 2004 14:59:49 -0800

On Thu, Feb 05, 2004 at 11:41:51AM +0200, [email-masked] wrote:
> New submission from Dan Forsberg <[email-masked]>:
> 
> From Jari Arkko:
> >      - "PSR retransmission strategy"
> > 
> >        In Section 4.2 I also had a possible issue with retransmission
> >        strategy in case PSR is lost. The document seems to say that
> > PSRs are never retransmitted but at the same time the PDIs
> > are silently discarded. This could lead to a deadlock.
> 
> Alper Yegin:
> Another issue to open.

As Jari pointed out, I think the deadlock situation can occur.

There are two cases when a PSR is sent.

- A PSR is sent with creating a PANA session state.

This cases occurs when a Cookie AVP is not included in the PSR (i.e.,
stateful discovery).  Since the PAA does create a state, the deadlock
can occur if PAA discards PDI after sending the PSR.  I think that the
PAA should retransmit the PSR in this case to avoid the deadlock.

- A PSR is sent without creating a PANA session state.

This cases occurs when a Cookie AVP is included in the PSR (i.e.,
stateless discovery).  Since the PAA does not have a state, the
deadlock cannot not occur.

What about PSA?  I think in the case where a PSR is retransmitted, the
PSA does not need to be retransmitted.

Suggested changes:

In section 4.2, change:

   "A PANA-Start-Request message is never retransmitted. A
   PANA-Start-Answer message is retransmitted based on timer in the same
   manner as other messages retransmitted at PANA-layer."
                         
to:
                                                       
   "If a PANA-Start-Request message is sent without carrying a Cookie
   AVP, the PAA MUST retransmit the message.  Otherwise, if a
   PANA-Start-Request message is sent with carrying a Cookie AVP, the
   PAA MUST NOT retransmit the message.  On the other hand, if a
   PANA-Start-Answer message is sent without carrying a Cookie AVP, the
   PaC MUST NOT retransmit the message.  Otherwise, if a
   PANA-Start-Answer message is sent with carrying
   a Cookie AVP, the PaC MUST retransmit the message."

Change the first paragraph of section 10 to:

   "The PANA protocol provides retransmissions for all the message
   exchanges except PANA-Auth-Request/Answer. PANA-Auth-Request messages
   carry EAP requests which are retransmitted by the EAP protocol
   entities when needed. The messages that need PANA-level
   retransmissions are listed below:
                                                                                
         PANA-PAA-Discover (PDI)
         PANA-Start-Request (PSR)*
         PANA-Start-Answer (PSA)**
         PANA-Bind-Request (PBR)
         PANA-Reauth-Request (PRAR)
         PANA-Termination-Request (PTR)
    
   *)  PSR that carries a Cookie AVP is not retransmitted.
   **) PSA that does not carry a Cookie AVP is not retransmitted."

Yoshihiro Ohba

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From: Yoshihiro Ohba <[email-masked]>
Date: Mon, 09 Feb 2004 14:59:49 -0800

Suggested changes:

In section 4.2, change:

to:

                                                                                
         PANA-PAA-Discover (PDI)

Yoshihiro Ohba

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From: Alper Yegin <[email-masked]>
Date: Tue, 10 Feb 2004 13:35:58 -0800

It is possible that both PAA and PaC initiate the discovery and
   initial handshake procedure at the same time, i.e., the PAA sends a
   PANA-Start-Request message while the PaC sends a PANA-PAA-Discover
   message.  To resolve the race condition, the PAA SHOULD silently
   discard the PANA-PAA-Discover message received from the PaC after it
   has sent a PANA-Start-Request message with creating a state for the
   PaC.

JARI: (Substantial) Since you said above that a PANA-Start-Request
      message is never retransmitted, what if this message is lost
      AND you at the same time silently discard all PANA-PAA-Discover
      messages? Seems like you would be stuck, or did I miss something?

Couldn't we solve this by simply PAA responding to PDI even if it already
sent an unsolicited PSR? Of course the PaC would know not to respond to all
PSRs it receives.

Alper

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From: Yoshihiro Ohba <[email-masked]>
Date: Tue, 10 Feb 2004 14:42:28 -0800

> sent an unsolicited PSR? Of course the PaC would know not to respond to all

That is possible.  But I think (timer-based) retransmission is needed
anyway for unsolicited PSR.  In addition, when PSR carries
EAP-Request, timer-based retransmission of PSR should not occur in
PANA.  Instead, PAA should rely on retransmission at EAP layer, like
PANA-Auth-Request message.

Yoshihiro Ohba

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From: Yoshihiro Ohba <[email-masked]>
Date: Tue, 10 Feb 2004 14:42:28 -0800

Yoshihiro Ohba

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From: Alper Yegin <[email-masked]>
Date: Thu, 12 Feb 2004 11:45:56 -0800

OK, I thought a bit more about this.

Unless we want to rely on the PaC re-doing whatever it did to trigger the
unsolicited PSR, it is reasonable to rely on retransmission of PSR.

In that case, how about:

a- PDI is always retransmitted
b- PSR and PSA are never retransmitted at the PANA layer when they carry EAP
AVP. (PSR gets retransmitted when the EAP layer retransmits)
c- PSA is retransmitted in solicited case (no EAP payload)
d- PSR is retransmitted in unsolicited case (no EAP payload)

Note 1: In case there are multiple PAAs on the link, the PSR retransmission
(at PANA layer or due to EAP retransmission) creates inefficiency, because
the PaC will pick one PAA and ignore others until they give up.

Note 2: For the sake of simplifying the protocol, we could combine (c) and
(d) and say: "PSR is retransmitted (no EAP payload)." Though, per Note 1,
this would create additional inefficiency.

Comments?

Alper

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From: Alper Yegin <[email-masked]>
Date: Thu, 12 Feb 2004 11:45:56 -0800

In that case, how about:

Comments?

Alper

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From: Yoshihiro Ohba <[email-masked]>
Date: Thu, 12 Feb 2004 11:16:49 -0800

> the PaC will pick one PAA and ignore others until they give up.

For solicited case, yes.  For unsolicited case, it depends on whether 
the EP that found the presence of PaC informs multiple PAAs or not.
If the EP informs one PAA only, then the inefficiency does not apply.

> this would create additional inefficiency.

Yes.  So which do you suggest, combining (c) and (d) or separating?

Yoshihiro Ohba

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From: Yoshihiro Ohba <[email-masked]>
Date: Thu, 12 Feb 2004 11:16:49 -0800

Yoshihiro Ohba

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From: Alper Yegin <[email-masked]>
Date: Thu, 12 Feb 2004 12:34:13 -0800

> If the EP informs one PAA only, then the inefficiency does not apply.

Good point. 

> Yes.  So which do you suggest, combining (c) and (d) or separating?

I'm inclined to say combined.

Alper

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From: Alper Yegin <[email-masked]>
Date: Thu, 12 Feb 2004 12:34:13 -0800

Good point. 

Alper

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From: Yoshihiro Ohba <[email-masked]>
Date: Thu, 12 Feb 2004 12:43:32 -0800

Doesn't the combining approach conflict with stateless discovery?

Yoshihiro Ohba

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From: Yoshihiro Ohba <[email-masked]>
Date: Thu, 12 Feb 2004 12:43:32 -0800

Yoshihiro Ohba

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From: Alper Yegin <[email-masked]>
Date: Thu, 12 Feb 2004 16:39:37 -0800

> Doesn't the combining approach conflict with stateless discovery?

Hmmm, yes it does. Are you suggesting we keep (c) and (d) as is?

The unsolicited case will still be stateful, due to (d).
Now solicited case where EAP AVP is not carried in PSR-PSA is the only one
that can keep the PAA stateless (until PaC engages in authentication).

Alper

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From: Alper Yegin <[email-masked]>
Date: Thu, 12 Feb 2004 16:39:37 -0800

Alper

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From: Yoshihiro Ohba <[email-masked]>
Date: Thu, 12 Feb 2004 15:52:13 -0800

> Hmmm, yes it does. Are you suggesting we keep (c) and (d) as is?

Yes.

> that can keep the PAA stateless (until PaC engages in authentication).

Correct.

Yoshihiro Ohba

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From: Yoshihiro Ohba <[email-masked]>
Date: Thu, 12 Feb 2004 15:52:13 -0800

Yes.

Correct.

Yoshihiro Ohba