From: finn@isi.edu (Greg Finn)
Newsgroups: comp.dcom.cell-relay
Subject: Re: Variable Length Issues
Date: 20 Nov 92 21:39:12 GMT
Reply-To: finn@dalek.isi.edu (Greg Finn)
Organization: USC-Information Sciences Institute


>But, having the capability for store and forward allows switches to
>be connected in random mesh patterns without having to build the
>cut through operation through an arbitrary number of nodes.

	I agree.  The topology of practical LANs is irregular.
Store-and-forward is useful when the hardware routing is insufficient.
For example, a dimensionally ordered X-then-Y (x,y) source route
allows only a single x-to-y bend.  Arbitrary routes are created by
composing those as a series, storing and reforwarding packets at
intermediary routing nodes when additional corners need to be turned
in the complete source-to-destination path.

	Reading between your lines, you seem also to raise the
question of the difficulty of building long cut-through routes.  That
leads to a store-and-forward problem for cut-through, when blockage
temporarily occurs on the path, requiring the storing of the packet at
an intermediate node for Kermani-Kleinrock "classical" cut-through,
but not for all types of cut-through.

	With hop-by-hop flow control that operates at the unit level
of several bytes, it becomes possible for the routers along the path
to block a packet in place on its path until the blockage at the head
of the partially completed path disappears.  This technique, called
wormhole routing, was developed at Caltech by Prof. Chuck Seitz and
his students.  Using wormhole, the routing chips do not need internal
cut-through packet buffers, just several FIFO bytes per channel.

>Fragmentation allows slow speed devices to share the same switching 
>bandwidth with higher speed devices, without bridging and allows arbitrary
>mesh networks.  This is valid even in single site installations.

>Variable length with cut-through requires all devices to have channels 
>at the same high rate to prevent congestion.  How do you design the same 
>LAN to serve low cost PCs or high performance stations without 
>wasting the technical gains by adding bridges.  Bridges are now
>quite common, and they include all the management hassles.

	Cut-through variations can operate on asynchronous channels.
That allows one to be freed from the pesky issue of clock timing and
clock skew.  The path then operates at the speed of the slowest router
in the path.

	Place FCFS arbitration into each router chip.  This was done.
At each router an orderly and locally fair assignment of channel
resources to blocked packets takes place.

	To address the problem of slow hosts one interfaces to the
network via packet buffers and DMA's the packets into and out of ones
local router at whatever rate its DMA engine can operate, subject to
the flow-control restriction imposed by the router.  The ones we use
operate at 320 and 400 Mb/s, limited by bus clock rate and SRAM
affordability.  The router operates at much higher rates.

	The greatest advantage of bridging is the intelligent
isolation of LAN sub-sets where the base technology uses a broadcast
medium.  That issue is moot when using point-to-point channels.
Concentrators, as opposed to bridges, may retain their cost advantage
where the interface to the medium is expensive, such as fiber.

>Ah, but the most valuable (monetary) user traffic is often 
>the WAN traffic, like distributed sales and inventory.

	Are you arguing that one should adopt ATM in LANs because ATM
is used for the costly long-haul traffic?  Why doesn't everyone use
the T1 or T3 data-link layers in LANs today then?  Most commercial
long-haul traffic is carried that way.  There are millions of T1
circuits for each ATM link.  ... I suggest the use of gateways.

>Finally, what about the argument for dual priority and support for
>short low latency messages?

	Assignment of paths and priorities is a higher-layer resource
allocation issue.  Call setup in ATM isn't performed by the ATM cell
router chips.  Similar mechanisms can exist in LANs that adopt a
source-routed variable-length data-link layer.  Multiple routes
between two hosts across mesh routers and bandwidth reservation are
tools that can be used to ensure quality of service.  You have a
routing authority issue the source routes.

	Bandwidth reservation on 500 Mb/s channels and multi-Gb/s
switches is less of a problem than it is for 155 Mb/s channels.  That
sounds smug, but reservation is a smaller problem when resources are
more abundant.  I am speaking here about LANs, not WANs.  Traffic
aggregation at WAN switches makes reservation a much more important
issue.

	This is akin to the old datagram vs. virtual-circuit argument.
The virtual circuit machinery and call setup adds costs in
hardware/software to a network.  Those costs can sometimes be traded
off against faster operation for datagrams, either in faster links or
faster host transmission/reception processes.  It's an old debate and
one that our LAN demonstrates is still alive and well at gigabit
rates.
--
Gregory Finn	(310) 822-1511
Information Sciences Institute, Marina Del Rey, CA 90292