Subject: QoS query

From: [email-masked] (Jonathan C Day)
Date: 6 Mar 1998 17:26:55 GMT

Hi,
	This is more out of curiosity than anything. Is there any way a router can
'self-QoS'? By this, I mean the following: Can a router intelligently drop packets
where it would otherwise overload a given route, and feed that information back up
the multicast tree, /without/ the end-users (senders or receivers) explicitly
reserving bandwidth? My understanding of 'conventional' QoS (such as RSVP) is that
the sender needs to explicitly request bandwidth. But obviously this is going to
be limited if you don't know who's going to be receiving and what the network is
like there. It also relies on end-users having IP stacks & s/ware that support
the protocol. Also, 'conventional' QoS seem to rely on intermediate routers
supporting the same QoS methods. I could be mistaken there, though.

	A fictional example might help explain what I mean here:

You have one source (A), two routers (B and C), and three receiving machines (D,
E and F).

A ---> B ---> C ---> D
        \      \
         \      \
          --> E  --> F

The connection between A and B can handle (speed of transmission)
The connection between B and C can handle 100 Kb/s
The connection between C and D can handle 14.4 Kb/s
The connection between C and F can handle 33.6 Kb/s
The connection between B and E can handle 50.0 Kb/s

The fastest rate C needs to receive is 33.6 (max of 14.4 & 33.6), since none of
its active connections can handle anything faster. Excess is intelligently dropped.
The fastest B needs to transmit to C is 33.6 (min of 100 & 33.6), so the fastest B
needs to receive is 50.0 (max of 50.0 & 33.6). B could then advise A that it
needn't transmit faster than 50.0, since that's the fastest anyone can receive,
assuming A can receive such notices.

Jonathan Day