Subject: Re: Bridge or

From: Muneyoshi Suzuki <[email-masked]>
Date: Mon, 25 Nov 2002 16:19:05 +0900

Cheng-Yin,

It seems to me, draft-ietf-pwe3-ethernet-encap-01.txt specifies refence
model and protocol for Ethernet MAC frame transport over GRE/LSP tunnel. 
It does not emulate anything.

Please refer section 3. 

>3. Requirements for Ethernet Pseudo-Wire Emulation
>   An Ethernet PW emulates a single Ethernet link between exactly two
>   endpoints. The mechanisms described in this document are agnostic to
>   that which is beneath the "Pseudo Wire" level in Figure 2, concerning
>   itself only with the "Emulated Service" portion of the stack.
>
>   The following reference model describes the termination point of each
>   end of the PW within the PE:
>           +-----------------------------------+
>           |                PE                 |
>   +---+   +-+  +-----+  +------+  +------+  +-+
>   |   |   |P|  |     |  |PW ter|  | PSN  |  |P|
>   |   |<==|h|<=| NSP |<=|minati|<=|Tunnel|<=|h|<== From PSN
>   |   |   |y|  |     |  |on    |  |      |  |y|
>   | C |   +-+  +-----+  +------+  +------+  +-+
>   | E |   |                                   |
>   |   |   +-+  +-----+  +------+  +------+  +-+
>   |   |   |P|  |     |  |PW ter|  | PSN  |  |P|
>   |   |==>|h|=>| NSP |=>|minati|=>|Tunnel|=>|h|==> To PSN
>   |   |   |y|  |     |  |on    |  |      |  |y|
>   +---+   +-+  +-----+  +------+  +------+  +-+
>           |                                   |
>           +-----------------------------------+
>                       ^        ^
>                       |        |
>                       A        B
>           Figure 3: PW reference diagram
>
>   The PW terminates at a logical port within the PE, defined at point A
>   in the above diagram. This port provides an Ethernet MAC service that
                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
The above description clearly addresses that the PW provides Ethernet MAC
service.

>   will deliver each Ethernet packet that is received at point A,
>   unaltered, to the point A in the corresponding PE at the other end of
>   the PW.
>
>   The "NSP" function includes packet processing needed to translate the
>   Ethernet packets that arrive at the CE-PE interface to/from the
>   Ethernet packets that are applied to the PW termination point. Such
>   functions may include stripping, overwriting or adding VLAN tags,
>   physical port multiplexing and demultiplexing, PW-PW bridging, L2
>   encapsulation, shaping, policing, etc.
>
>   The points to the left of A, including the physical layer between the
>   CE and PE, and any adaptation (NSP) functions between it and the PW
>   terminations, are outside of the scope of PWE3 and are not defined
>   here.

The above two paragraphs clearly defines that the NSP terminates Ethernet
MAC frame between PE-CE.

Therefore, form a viewpoint of protocol architecture, PW reference 
mode is interpreted as shown in the following figure.

                   Bridge                   Bridge
                +-----------+            +-----------+
                |\         /|            |\         /|
                | \ Relay / |            | \ Relay / |
                |  \     /  |            |  \     /  |
                |   \   /   |            |   \   /   |
                |    \ /    |            |    \ /    |
                |     V     |            |     V     |
                | MAC |  E  |            |  E  | MAC |
                +--+--+--+--+            +--+--+--+--+
                   |     |                  |     |
      : Ethernet   |     |  GRE/LSP tunnel  |     |   Ethernet    :
+--+  :LAN segment |     |                  |     |  LAN segment  :    +--+
|CE|==:==============    +------------------+    =================:====|CE|
+--+  :                                                           :    +--+
      :               |<---------PW----------->|                  :
    Customer          A                        A          Customer
    Interface                                              Interface

The above figure shows a "Bridged LAN". Two big boxes are "Brides" and
it is a special kind of 1982-style Bridge, because it supports only 
two ports, so MAC Relay Entity can be shrank. "MAC" and "Relay" 
in a box are "MAC Entity" and "MAC Relay Entity" respectively.

"E" is an Entity that provides MAC service to the upper layer and the 
service is compatible with the MAC service defined in IEEE 802.3-2002.
In summery, it seems to me, draft-ietf-pwe3-ethernet-encap-01.txt does
not define any emulation technology, it specifies protocol for Ethernet 
MAC frame transport over GRE/LSP tunnel and the protocol Entity is
refeed as "E" in the above figure.

It does not emulate any LAN, because in IEEE 802 context, a LAN means
a LAN segment or LAN segments interconnected with repeaters, and LAN
segment means the medium connection, e.g., 1000BASE-T, between Medium 
Dependent Interfaces in a LAN. I think the PW is not intended to support
medium dependent service such as 100Mbps-to-100Mbps LAN service, because
this kind of service may not make sense.

And I also don't understand that someone claimed that today's LAN consists
of "wires". What is "wire"? 1000BASE-LX use "fiber", and furthermore,
IEEE 802.11 WG specifies "wireless LAN" :-) If it means an LAN segment,
as I described in the above, such service does not make sense.

And finally, I think, the following specification in 
draft-ietf-pwe3-ethernet-encap-01.txt is correct, but need not address
in the document.

>3.1.6. IEEE 802.3x Flow Control Interworking
>   In a standard Ethernet network, the flow control mechanism is
>   optional and typically configured between the two nodes on a point-
>   to-point link (e.g.  between the CE and the PE). IEEE 802.3x PAUSE
>   frames MUST NOT be carried across the PW. See Appendix A for notes on
>   CE-PE flow control.

This is because, Ethernet PAUSE frame must be terminated in the MAC Entity 
in the above figure. However, it is clearly addressed in IEEE 802.3-2002 and
it is purely Ethernet dependent issue, thus it is out of scope of PWE3.
In the PW section, there is no PAUSE frame, because E entity does not 
generate PAUSE frame because it is not Ethernet. So the draft need not 
specify anything for PAUSE frame.

Thanks,

Muneyoshi Suzuki