Subject: [Pana] Lost answer problem (a new issue)

From: Yoshihiro Ohba <[email-masked]>
Date: Mon, 28 Jun 2004 20:37:37 -0400

There might be some cases in which a PaC or a PAA might need to send a
duplicate answer to a retransmitted old request (as well as a fresh
answer to a new request if any) considering that an answer message for
an old request can be lost.  Without this, a PANA session may stall in
a way that the requester keeps retransmitting the old request while
the responder may keep discarding them with waiting for a new request.
The cases are described below.

- When a PaC sends a PANA-Start-Answer message in response to a valid
PANA-Start-Request message without a Cookie AVP, the answer message
can be lost.  The PANA-Start-Request would be retransmitted.  To deal
with the case, it might be better for the PaC to return a duplicate
PANA-Start-Answer message after the PaC has been sent the first
PANA-Start-Answer message until the PaC receives a valid
PANA-Auth-Request message.

- When a PaC sends a PANA-FirstAuth-End-Answer message in response to a
valid PANA-FirstAuth-End-Request message (remember that a
PANA-FirstAuth-End exchange is used for delimiting the first and
second EAP authentications), the answer message may be lost.  The
PANA-FirstAuth-End-Request would be retransmitted (with updating
transmission sequence number).  To deal with the case, it might be
better for the PaC to return a duplicate PANA-FirstAuth-End-Answer
message (with updating transmission and received sequence numbers)
after the PaC has been sent the first PANA-FirstAuth-End-Answer
message until the PaC receives a valid PANA-Auth-Request message for
the second EAP authentication.

- When a PaC sends a PANA-Bind-Answer message in response to a valid
PANA-Bind-Request message, the answer message can be lost.  As as
result, the PAA will retransmit the PANA-Bind-Request (but with using
a different transmission sequence number).  To deal with the case, it
might be better for the PaC to return a duplicate PANA-Bind-Answer
message (but with using different transmission and received sequence
numbers) for a while after the first PANA-Bind-Request was received.
The same thing can also apply to a PANA-Address-Update-Answer message
that is sent by a PaC.

- When a PaC or a PAA sends a PANA-Termination-Answer message in
response to a valid PANA-Termination-Request message, the answer
message can be lost.  As as result, the requester will retransmit the
request (but with using a different transmission sequence number).  To
deal with the case, it might be better for the sender of the answer to
return a duplicate answer (with using different transmission and
received sequence numbers) for a while after the first
PANA-Termination-Request was received.  The same thing can also apply
to a PANA-Reauth-Answer message that is sent by a PaC or a PAA.

Those could improve robustness of the PANA protocol against loss of
answer message.

Comments?

Yoshihiro Ohba

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