Subject: [Pana] Issue106: sequence number on rexmited messages must be incremented

From: Alper Yegin <[email-masked]>
Date: Mon, 16 Aug 2004 11:16:04 -0700

Hi Erik,

In your review feedback, you had said:

- the retransmission text says that a retransmitted packet has a new 
  sequence number. This makes no sense to me.

Can you please elaborate what kind of issues you are seeing with this.

Thank you.

Alper

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From: Yoshihiro Ohba <[email-masked]>
Date: Mon, 20 Sep 2004 09:28:05 -0400

If a retransmitted packet has a new sequence number, the sender would
need to re-generate a new MAC AVP for every retransmitted packet that
contains a MAC AVP, since the MAC computation covers PANA header.

This could be a performance issue at PAA which may have a large number
of retransmitted messages to different PaCs.  Also, if we don't change
the sequence number for retransmitted packets, implementation would be
much more simpler.

Yoshihiro Ohba

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From: Erik Nordmark <[email-masked]>
Date: Tue, 24 Aug 2004 10:57:39 -0700 (PDT)

> Can you please elaborate what kind of issues you are seeing with this.

Retransmissions are needed in two cases:
1. The message was lost
2. The acknowledgement of the message was lost

If a different sequence number is used for the retransmission
and #2 applies, then 2 copies of the message would be delivered
to the upper layers since PANA would have no mechanism to tell that
it has already received the message once.

  Erik

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From: Alper Yegin <[email-masked]>
Date: Tue, 24 Aug 2004 14:53:13 -0700

> it has already received the message once.

Upper layer to PANA is EAP. Retransmission of EAP carrying messages are
not handled by PANA. An EAP-carrying PANA message is retransmitted only
when EAP retransmits the same EAP message. And in that case PANA may not
even know it is a retransmission.

Since retransmission is not caused by PANA, duplicate prevention should
not be provided by PANA either. 

If message duplication due to lower layer (PANA) retransmissions is the
concern, I think that should be fine.

Alper

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From: Alper Yegin <[email-masked]>
Date: Tue, 24 Aug 2004 14:57:12 -0700

> of retransmitted messages to different PaCs.  

I'm not sure if optimizing the spec for this is justified. Are there any
such protocol examples?

> Also, if we don't change

I'm not sure about that either. The PANA layer has to detect EAP
retransmissions. Either we need to accommodate that in the PANA-EAP
interface, or PANA must inspect the EAP payload. 

Alper

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From: Yoshihiro Ohba <[email-masked]>
Date: Tue, 24 Aug 2004 18:30:36 -0400

> I'm not sure if optimizing the spec for this is justified. Are there any

IKEv2 has Message ID which does not change for retransmitted messages.
The exact copy of the original IKEv2 message is retransmitted.

> interface, or PANA must inspect the EAP payload. 

Since PANA relies on the EAP retransmission mechanism for EAP
messages, PANA-Auth-Request messages are not retransmitted by PANA.
Thus, the same transmission sequence number should not be used for
PANA-Auth-Requests messages, regardless of whether the messages are
generated as a result of EAP retransmission or not.  

Yoshihiro Ohba

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From: Erik Nordmark <[email-masked]>
Date: Tue, 24 Aug 2004 16:43:55 -0700 (PDT)

> Upper layer to PANA is EAP. 

No, there is PANA itself.
Perhaps all the PANA message semantics are idempotent, but that is far
from clear to me.

> Retransmission of EAP carrying messages are

Yes, but that isn't a PANA retransmission. That is an EAP method performing
a retransmission.

> not be provided by PANA either. 

There seems to be PANA retransmissions of non-EAP messages.
Don't those PANA messages also have sequence numbers?  
   Other PANA layer messages that require a response
   from the communicating peer are retransmitted based on timer at
   PANA-layer until a response is received (in which case the
   retransmission timer is stopped) or the number of retransmission
   reaches the maximum value (in which case the PANA session MUST be
   deleted immediately). 

  Erik

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From: Alper Yegin <[email-masked]>
Date: Tue, 24 Aug 2004 17:07:33 -0700

Yes, there are retransmissions of non-EAP messages, and they also have
sequence numbers. 

If we go back to your earlier comment:

	If a different sequence number is used for the retransmission
and #2 
	applies, then 2 copies of the message would be delivered to the
upper 
	layers since PANA would have no mechanism to tell that it has
already 
	received the message once.
 
If PaC receives a duplicate request, it must respond to it as it does
not know whether the peer received the earlier response or not. So I
think duplicate request should not be discarded, but processed.

Alper

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From: Erik Nordmark <[email-masked]>
Date: Tue, 24 Aug 2004 17:20:56 -0700 (PDT)

> think duplicate request should not be discarded, but processed.

Yes, but wouldn't it send the same response as it did the first time?

Using a new sequence number makes that quite hard, since it
can't easily tell whether the two requests were the same.

If there are PANA protocol operations that are not idempotent,
i.e. where processing the request twice results in something
different than processing it once, then it is requires that
the PANA reliability sub-layer retain the responses so that they can
be used when receiving a retransmitted request.

BTW: The use of "MAY" in section 4.11 implies that PANA isn't specified
to be robust to packet loss; for the protocol to be robust, duplicate
requests need to result in retransmitted responses.

   Erik

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From: Alper Yegin <[email-masked]>
Date: Wed, 25 Aug 2004 16:08:12 -0700

> Yes, but wouldn't it send the same response as it did the first time?

Yes, it would.

> be used when receiving a retransmitted request.

So, if I'm understanding you right: This sub-layer is supposed to retain
a copy of the last transmitted message. If this is a request message, it
is retransmitted when retransmission timer goes off. If it is an answer
message, it is retransmitted when a duplicate request is received.
Preserving the same sequence number across retransmissions make the
latter easier (at least contain it in this sub-layer, other parts of
PANA process does not have to process the message). It makes sense to
me.

> requests need to result in retransmitted responses.

We shall fix that (btw, Section 4.7 and 4.11 are duplicates (an editing
bug), and 4.7 is more up-to-date... but nevertheless it needs that fix.)

Alper