Subject: [PWE3] Signaled Interface MTU for PW

From: "Raqib Jones" <[email-masked]>
Date: Thu, 25 Aug 2005 09:28:12 -0700

in draft-ietf-pwe3-control-protocol-17.txt; the MTU value which is
signaled for a 
PW is specified as the MTU of the AC interface, excluding overhead.  In
other
words 
 
interface MTU = (AC MTU - AC overhead).
 " - Interface MTU sub-TLV type        
A 2 octet value indicating the MTU in octets. This is the Maximum       
 Transmission Unit, excluding encapsulation overhead, of the      
  egress packet interface that will be transmitting the       
 decapsulated PDU that is received from the MPLS enabled network.    
    This parameter is applicable only to PWs transporting packets and   
    is REQUIRED for these PW types. If this parameter does not match

  in both directions of a specific PW, that PW MUST NOT be enabled."
 
A few questions here on determing the AC overhead:
 
- For Ethernet Raw mode, packets through the circuit may or may not
include VLAN tags;
how should the overhead be calculated for this case?
 
- 'Ethernet VLAN' mode (or raw mode) may include additional SVLAN tags;
it's unclear whether the 
additional tags are included in the overhead.
 
- For 802.3 format, the overhead includes a few additional bytes;  are
these included in the OH
calculation?
 
- For ATM SDU mode, it is unclear how to determine the packet overhead,
since the OH is variable
(there is padding to force the packet to a multiple of 53 bytes).
 
Is there a consensus on how much OH to consider for each encapsulation
type? It would
be great if it was clear in each document that describes an
encapsulation.
 
Thanks.

	Raqib Jones 
	Hammerhead Systems Inc.

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