Wormhole IP over ATM
by Manolis Katevenis
Speeding up the Internet is of capital importance; this will need
some hardware assistance. Hardware switches operate best on fixed-size
quanta, while IP packets, on the other hand, have a variable size.
It is not the first time, though, that hardware is called upon
to route variable-size packets at high speed: Wormhole Routers
did precisely that, in the eighties, for multiprocessor inter-connection
networks. The same techniques are applicable today to the Internet:
we introduced a novel, wormhole-like way to route IP over ATM,
we simulated its performance, we built a first prototype, and
we are now proceeding to testing it in real systems.
IP is the uncontested protocol for data communications. At the
same time, ATM technology finds widespread use, owing to its fixed-size
“cells” that allow high-speed hardware switching, owing to the
small size of these cells that allows fast preemption and hence
low latency, and owing to its quality-of-service (QoS) architecture.
IP can fruitfully run on top of ATM, thus getting the best of
both worlds. This has been done in software, on general-purpose
computers; we do it in hardware, at a lower cost and with a lower
latency.
ATM has some similarity with wormhole routing, the most popular
multiprocessor interconnection network technique of the eighties.
Just like virtual channels in wormhole routing carry packets segmented
into “flits”, a number of hardware-managed VC’s in ATM can carry
IP packets segmented into cells according to AAL-5; each VC is
dedicated to one packet for the duration of that packet, and is
afterwards reassigned to another packet, in hardware. This was
proposed by Barnett in 1997 and was named connectionless ATM.
We, at ICS-FORTH in Heraklion, Crete, Greece, modified the Barnett
proposal to make it applicable to existing ATM equipment: we proposed
a novel single-input, single-output Wormhole IP Router, that functions
as a VP/VC translation filter between ATM subnetworks.
Our wormhole IP routing filter has a number of advantages: (i)
it works together with standard, existing ATM equipment; (ii)
it allows the co-existence and integration of both IP and native
ATM traffic in the same networks; (iii) the quality of service
of native ATM traffic can stay unaffected by the added IP traffic,
while IP can benefit from ATM’s QoS capabilities; (iv) for IP
traffic, the system operates equivalently to a network of low-latency
gigabit IP routers, while being a lot less expensive; (v) packet
routing delay is minimized owing to virtual-cut-through routing
- segmentation and reassembly delays at intermediate routers are
eliminated; (vi) packet routing delay is minimized for all packets
- not just for the rest of the packets after a flow has been recognized,
as in IP switching; (vii) the number of pre-established connections
(labels) is small and fixed, and does not grow with the size of
the network (as in tag switching), yet all packets are routed
through pre-established connections. Based on actual internet
traces, we showed by simulation that a few tens of hardware-managed
VC’s per outgoing VP suffice for all but 0.01% or less of the
packets.
We have built a first prototype of a bi-directional wormhole IP
routing filter with two OC-3 ports (155 Mbps in each direction),
which is shown in the photograph. Two 16-MByte DRAM SIMM’s plug
into the sockets on the right; they hold the two-level IP routing
table and the ATM connection table. The datapath, the control
FSM, and the VCout free list (bit map) are contained in one FPGA.
The routing delay is fixed, equal to about one cell time (a couple
of micro-seconds), for all IP packets.
We are currently testing this prototype by connecting it to our
ATM and IP network. Further development is needed, especially
on the software side, and we are seeking partners for that and
for commercially exploiting the technology.
For more information, see http://archvlsi.ics.forth.gr/wormholeIP/
Please contact:
Manolis Katevenis - ICS-FORTH
Tel: +30 81 391664
E-mail: katevenis@ics.forth.gr
