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Re: [RPRWG] CRC check in each node?




Hi Mike and all

If we try and look away from how LAN and Ethernet is done today and just concentrate on what to be solved. The far easiest way to provide error statistics for a given flow is to not remove the packets. This will work even with different system vendors on the same ring. And I can not imagine that any carrier will not want the possibility to have error statistics per customer/SLA. This IS different from etehernet and LANs - there is no concept of carrier capabilities and isn't that one of the reasons for wanting RPR in the metro?

I do not understand why anyone wants to remove the packets - for bandwidth optimization? I do not beleive that it is important to optimize bandwidth for the case of errored packets. I would rather optimize in order to avoid errored packets. Fault location? Can be solved differently (see below).

Header error check:
Needed for validation the control information used within RPR itself. Packets with errored headers are discarded, since we do not know what to do with them or what customer/SLA it belongs to.

Payload error check:
Only monitoring. Can be used for determing where errors are injected (fault location). Also needed to keep track of performance statistics on the interfaces. Can be accompanied with an error indication telling that this packet is already discovered to be in error further upstream. This would increase the fault location efficiency. Draw back is that it must be placed in 1) the header, meaning a need for store and forward or 2) in the end of the packets, do we share it with the CRC? or 3) a special faulty CRC value (e.g., the correct CRC inverted) to be inserted instead of the received value.

I do not want to post an oppinion on store-and-forward as a concept, I just do not think that the argument for using it should be error location or packet discarding since error location that can be solved differently and discarding errored packets is not (in my view) a desired behaviour.

Robin

Mike Takefman wrote:

> All,
>
> An interesting point from 802.1D 1998 is the following.
>
> "Note that the frame is completely received
> before it is relayed as the Frame Check Sequence (FCS)
> is to be calculated and the frame discarded if in
> error."
>
> It strikes me that if an 802.1D bridge must not relay a packet
> with a bad FCS, there is nothing wrong with removing it in
> the transit path of the 802.17 MAC (assuming that it can be
> done). We are not changing the operation that will
> occur, just making it happen sooner.
>
> I did not see an example of guaranteeing deliverly of
> a packet with "good" address and "bad" contents in current 802
> networks. The FCS will alias bad address and bad contents and
> since .1D specifies that packets with FCS errors will be removed
> I do not understand how anything with an FCS error continues to
> flow to an end station unless the LAN is a single segment.
>
> I would appreciate a pointer to the relevant part of the standard
> that describes how to do this type of operation across multiple
> segments.
>
> thanks,
>
> mike