Rogers explains 'shaping' policy

[James Knott]

Lennart Sorensen wrote:
> On Thu, Jun 12, 2008 at 05:41:55PM -0400, James Knott wrote:
>> I've wondered about this too. Ethernet switches generally use some form 
>> of "round robin" to ensure all ports can transmit data. This means that 
>> if you have a huge pile of data to send, after you send one packet, 
>> everyone else gets a chance to send a packet, before you can send a 
>> second packet. Now I realize that contention resolution is handled 
>> differently in cable modems, but there is such a mechanism to ensure 
>> fair access. Why does this not help to ensure fair use? Also, Rogers 
>> charges users who go over their allocated data amount. You'd think that 
>> might slow down some users. However, I can understand Rogers' concerns 
>> about the amount of bandwidth some of those peer-peer services consume.
> 
> Many protocols have to wait for the acknowledge before sending another
> chunk of data.  Large file transfer generally do not.  So you end up
> with so much file transfer traffic waiting already, that you can long
> delays between chunks of data on the interactive stuff.
> 

With TCP, the receive window limits the amount of outstanding 
unacknowledged data.  However, that's got nothing to do with what I 
described above.  The networking hardware can limit how much data is 
sent, before the next user(s) is given a chance.  This means that no 
matter how much data someone wants to send, they still have to wait for 
other users.  So, with an ethernet switch, one frame from the heavy user 
can be sent, then all other users get a chance to send a frame, if 
available and then the heavy user gets another shot etc.  If the other 
users have little to send, then the heavy user gets most of the 
bandwidth.  If the other users have lots of traffic, then the heavy user 
  gets only a smaller part of the available bandwidth.


> Of course there are ways to help that too, such as the ToS values in
> packets, which are supposed to let you prioritize low volume interactive
> traffic over high volume bulk traffic, although how many ISPs actually
> follow those values (since you have to trust that the user's
> applications mark the packets appropriately) I don't know.  Certainly
> some switches do obey those values to determine what should be forwarded
> first to a given port as do some routers.
> 

The current term is QoS for quality of service.  The old ToS byte is now 
used for "Differentiated Services Code Point" (DSCP), which can be used 
for priority.  So you'd give things such as VoIP a higher priority than 
email or file transfer.  However, to do this, all the equipment has to 
support it.





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