Re: [8023-CMSG] Questions
Brad,
I think you did miss the mark, particularly with:
"Considering that Ethernet doesn't know in advance about the provisioning
of the network and does not care about which packets it delays or drops,
then it is likely that 802.1 and the upper layers can do all the
priorities or differentiated services that they want but will see
diminishing returns as the load on the network increases."
I would agree with, "Ethernet doesn't know in advance about the provisioning
of the network", but 802.1D bridges certainly do care about which frames are
delayed or dropped. Bridges define the use of 8 queues per output port, and
frames are marked with 8 levels of priority. Although strict priority
scheduling is the only queue draining algorithm specified in the standard,
others are explicitly allowed, and most vendors implement varieties that
provide very good latency and bandwidth guarantees. Furthermore, a great
many bridges are able to assign priorities to 802.3 frames based on criteria
such as IP DSCP code points.
In short, ethernet is *far* from "best effort".
-- Norm
Booth, Bradley wrote:
> My apologies in advanced if the answers are obvious, but I've been so
> focused on cabling and physical layer the last couple of weeks, so I'm a
> bit brain dead to upper layer stuff.
>
> There has been some talk about differentiated services and priorities
> associated with 802.1 and the upper layers. Here are my questions:
> 1) If the network is overprovisioned (available bandwidth >= maximum instantaneous throughput), then am I correct in assuming that
> these differentiated services and priorities operate just fine because
> the upper layer protocols within the switches have sufficient
> bandwidth? Should I also assume that the available bandwidth is based
> upon what the end stations (adapters, servers, etc.) can handle?
> 2) If the network is not overprovisioned (either in the switches or
> adapters), then is it fair to assume that these differentiated services
> and priorities will provide diminishing returns as throughput increases
> over the available bandwidth?
>
> I keep coming back to the statement others have made that 802.1 or the
> upper layers can handle this, but I cannot help think that would only be
> true for an overprovisioned network. Considering that Ethernet doesn't
> know in advance about the provisioning of the network and does not care
> about which packets it delays or drops, then it is likely that 802.1 and
> the upper layers can do all the priorities or differentiated services
> that they want but will see diminishing returns as the load on the
> network increases.
>
> This would seem to me like going out and buying a Formula 1 race car to
> use to drive to work in Silicon Valley. A lot of money in fuel and
> equipment only to sit on 101 during rush hour(s).
>
> Am I off the mark here?
>
> Thanks,
> Brad
>