Re: [ANCP] Adjacency timer

["Sanjay Wadhwa" <swadhwa at juniper.net>]

Hi Vitali
  You raise a valid question. Handling of a change in timer value in
adjacency messages is not specified. Some possible options are:

1. Allow dynamic changes in timer value without bringing down the
adjacency. ANCP draft calls for timer values to be configurable, and
explicitly allows for asymmetric timer values between peers. This makes
it possible for graceful handling of timer changes without bringing down
the session. However, allowing change in the timer value for an active
session diverges from GSMP RFC.

2. Ignore timer field in subsequent messages after the initial SYN.
Optionally, the change in timer value from the one received in initial
SYN can be detected and logged as an error, but the adjacency messages
with the changed timer value could be processed normally.

3. Consider the message with a changed timer value invalid and ignore
the message. However, ignoring periodic ACK messages with changed time
value will eventually bring down the session. 

4. Log an error and bring down the session if you observe a changed
timer value.

Options 1 and 2 seem more reasonable. I do see some value in option 1
i.e. in allowing for dynamic changes in the timer value (e.g. if the
controller is busy, it could temporarily advertise a higher value to the
peer). However, option 2 is simpler and more inline with GSMP RFC.
Thoughts?

Thanks
-Sanjay


-----Original Message-----
From: ancp-bounces at ietf.org [mailto:ancp-bounces at ietf.org] On Behalf Of
Vitali Vinokour (vvinokou)
Sent: Thursday, May 22, 2008 3:24 PM
To: ancp at ietf.org
Subject: [ANCP] Adjacency timer

Hello,

GSMP rfc specifies that "The Timer field is used to inform the receiver
of the timer value used in the adjacency protocol of the sender.  The
timer specifies the nominal time between periodic adjacency protocol
messages.  It is a constant for the duration of a GSMP session."

However, Timer field is present in all adjacency protocol messages. Is
the receiver expected to cache the remote timer value from the very
first SYN and ignore Timer field in all subsequent messages? Or is the
expected behavior to match the Timer field in each message to the
initial value and consider invalid all messages whith mismatching
values?  Neither GSMP rfc nor ANCP draft specify the behavior.

Thanks,
Vitali 
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