Subject: [ANCP] graceful restart comments

From: [email-masked]
Date: Mon, 07 Aug 2006 11:44:28 +0200

Hi,

some comments on the graceful restart draft:

The draft does not make any distinction between planned and unplanned 
restarts. Would it not be better to focus on planned restarts to begin 
with? With the current solution, if for instance the uplink of the AN 
fails, then the NAS thinks that the AN is still forwarding packets. The 
NAS will only detect after "restarting time" that the forwarding plane 
is not up anymore and during this time, the NAS was always faulty 
assuming that the forwarding plane was up. Therefore it is better to 
focus first on planned restarts: just before the planned restart, the 
restarting device sends the Graceful restart TLV with the waiting time 
to the other side, then it restarts. For planned restarts, there is not 
much risk for the other side to faulty assuming that the forwarding 
plane is up while this is clearly not the case for unplanned restarts. 
This also means that the GR-TLV has to be sent just before the planned 
restart. When the GR-TLV is sent, is left vague in the draft but I 
understand that currently the draft assumes that it has to be sent after 
ANCP session establishement.

Paragraph 3-4 of introduction: a failure of the AN uplink or a failure 
of an intermediate node will make the ANCP data invallid and not is not 
valid anymore as said in these paragraphs.

Last paragraph of section 3: The restarting neighbor MUST (and not 
SHOULD) advertise the duration.... why is it not a MUST?

The same section says that the ANCP neighbor has to wait for "restarting 
time" before setting up the adjancency again. Why is there a need for 
this? The neigbor should constantly retry to reestablish the session, 
and if it fails to reestablish the session, and to resyncronize the 
state within "restarting time" then the graceful restart procedure is 
ended. So there is no need to have a seperate "wait time". Just a 
"restart time" is sufficient but it has to be redefined.

When the AN is restarting, then how should this work? It is always the 
AN that has to establish the session and not the NAS. Because of this 
a-symmetry in the protocol, it is better to make the graceful restart 
also asymmetric.

When the wait time expires (section 4.1), the graceful restart procedure 
is aborted but it looks that aborting the procedure looks rather 
drastic: tearing down the session and start all over again. Why would 
you do this? Then we have the following sequence: the session breaks, 
comes up again, graceful restart procedure starts, timers expires, 
session down again, comes up again, and then to normal procedure. If the 
timer expires, I would say that this should be the same as receiving the 
"END-OF-STATE" message and all stale state is removed but the session 
stays up.

First paragraph of section 4.2 uses a different definition of the 
"restart time" because the draft says that after "restarting time" the 
session re-establishement starts according to section 3. Section 4.2. is 
more in line with what I said above and "wait time" has to be removed.

Editorial: remove first paragraph of the introduction, it does not 
belong here and is partially duplicate with the second paragraph.

regards,
Stefaan

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