Control Word

[javier-ramos]

Hi, Reviewing the Table 7-3 and other pages of ciscopress Layer 2 VPN Architectures (Luo, Pignataro, Bokotey, Chan) , I see that Control word always is 0 for EoMPLS. When may be useful the control word on EoMPLS?

[swaroop.potdar]

Hi Javier,

Based on PWE3 guidelines the ATOM(Draft Martini) draft has been derived.

Originally first 4 bits were to be set to 0 to indicate PW data,next 12 bits were reserved for drafts based on these guidelines to implement the reserved bits for specific funtionality. And last 16 bits are specifically meant for signalling.

a) Now in Draft Martini, first 4 bits have been implemented as it is and are always set to 0.

b) Of the next 12 bits, the first four have been defined for protocol specific flags,

c) next 2 set to 0,

d) small payload is taken care by padding which is done by adding the length field of 6 bits,

e) and the last 16 bits have been used for sequencing as it was defined in PWE3.

Now from EoMPLS persepctive accroding to the draft,

a) Now Ethernet as a Media doesnt have any sequencing inbuilt and passes on the same to upper layers, and also from EoMPLS perspective the received layer 2 frame at the ingress is never fragmented so sequencing bits allocated would not be of much help. So can be ignored.

b)The protocol specific 4 bytes also dont have much role to play as Ethernet doesnt have any inherent congestion control mechanisms, neither different variants of ethernet which can be encoded to be sent across to the other side.

So the draft hasnt scoped much to be used of control word for ethernet, so its set to 0

indicating control word not present.

Now having discussed this, and thinking a little wild, EoMPLS can use the 4 bits to indicate the ethertype,

though this has not been scoped as 0800 is more prevalent. Or else also may be trying to implement mechanism

like flow-control, encoding pasue-frame kind of mechsnism for on wire flow or congestion control :-)

HTH-Cheers,

Swaroop

[datanocvsnl]

That is a some neat explanation.

Brgds,

[javier-ramos]

... then never uses it? or uses it whenever possible.?

I undertood you, but the book explain that Control Word for EoMPLS control word is disbled only "if" peer PE does not support.

This may be a relative point of view... the trick is what is the point of view of writer.

[swaroop.potdar · Accepted solution]

1)The draft only says control word is Optional for Ethernet or Vlan over MPLS.

And can be signalled or configured between two routers to support or not support.

2) In Cisco the flexibility for us in configuring it has been excluded and Control Word has been kept in not supported state. As currently we dont need much information apart from the right VCID and VPN label to transmit the received Layer 2 traffic.

So the book as may be read by people later on in future as well, has made a careful statement saying if the remote PE suports it, as it may so happen that in future if there is a need to encode certain paparemeter which need to be transmitted to the other side. Then those can be sent via the control word. Little unlikely for now, but cant be ruled in in medium term future.

HTH-Cheers,

Swaroop