[j-nsp] LDP Max PDU Length

[Eric Van Tol]

Hi all,
I'm performing some MPLS testing between a J2300 and a Telco Systems
TM-100 switch.  I'm attempting to get an LDP session established between
the two devices and it's not working, failing on the Juniper with a "Bad
PDU Length" message.  The Max PDU Length from the Juniper shows it to be
a value of 4096.  The TM100 shows, in the LDP Initialization Message,
that the Max PDU is 1440.  

It's my understanding from RFC3036 that the Max PDU is negotiated
between the two neighbors and the smaller of the two is used.  However,
it goes on to say that the receiving node can reject the PDU length if
it is too small or too large.

My questions:

1.  How exactly is the max PDU length determined - is it based of the
MTU of the physical medium?
2.  If it's an arbitrary value, why wouldn't the value then be
negotiated to 1440?
3.  What exactly determines that a received PDU length is
"unacceptable"?

Thanks,
Eric