Subject: [L2tpext] A question on RFC 2684

From: Nurit Sprecher <[email-masked]>
Date: Mon, 13 May 2002 16:40:21 +0200

I have a question on RFC 2684 and I’ll appreciate if someone can clarify it
to me.

In VC Multiplexing encapsulation format of Bridged Protocols, the payload
format is defined as follows: 

             Payload Format for Bridged Ethernet/802.3 PDUs
                  +-------------------------------+
                  |         PAD 0x00-00           |
                  +-------------------------------+
                  |    MAC destination address    |
                  +-------------------------------+
                  |                               |
                  |   (remainder of MAC frame)    |
                  |                               |
                  +-------------------------------+
                  | LAN FCS (VC dependent option) |
                  +-------------------------------+

It is defined also that “the PDUs of bridged protocols MUST be carried in
the Payload of the AAL5 CPCS-PDU exactly as described in section 5.2, except
that only the fields after the PID field MUST be included”.  

I wonder if the PAD filed (0x00-00) is always required to be in the payload
as it appears in the figure, or it depends on the appearance of the LAN FCS
field in the PDU like in the LLC encapsulation format case and deined in
section 5.2.
Specifically, the question is: should I expect the 0x00-00 to always appear
in the PDU, or should I expect it only when LAN FCS is supported (which is
VC dependent option).

Thanks in advance, Nurit.

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