Hardware Reference
In-Depth Information
One kind of hardware solution for fast packet processing is to use a custom
ASIC
(
Application-Specific Integrated Circuit
). Such a chip is like a hardwired
program that does whatever set of processing functions it was designed for. Many
current routers uses ASICs. ASICs have many problems, however. First, they take
a long time to design and manufacture. They are also rigid, so if new functionality
is needed, a new chip is needed. Furthermore, bug management is a nightmare,
since the only way to fix one is to design, manufacture, ship, and install new chips.
They are also expensive unless the volume is so large as to allow amortizing the
development effort over a substantial number of chips.
A second solution is the
FPGA
(
Field Programmable Gate Array
), a collec-
tion of gates that can be organized into the desired circuit by rewiring them in the
field. These chips have a much shorter time to market than ASICs and can be
rewired in the field by removing them from the system and inserting them into a
special reprogramming device. On the other hand, they are complex, slow, and ex-
pensive, making them unattractive except for niche applications.
Finally, we come to
network processors
, programmable devices that can
handle incoming and outgoing packets at wire speed (i.e., in real time). A common
design is a plug-in board containing a network processor on a chip along with
memory and support logic. One or more network lines connect to the board and
are routed to the network processor. There packets are extracted, processed, and
either sent out on a different network line (e.g., for a router) or are sent out onto the
main system bus (e.g., the PCI bus) in the case of end-user device such as a PC. A
typical network processor board and chip are illustrated in Fig. 8-16.
Network processor board
SRAM
SDRAM
Network
processor
Specialized procs.
Control
CPU
Local
mem
Network
interface
SRAM
interface
SDRAM
interface
Buses
...
PCI
interf.
PPE
PPE
PPE
PPE
PPE
PPE
PPE
PPE
PCI connector
Figure 8-16.
A typical network processor board and chip.
Both SRAM and SDRAM are provided on the board and typically used in dif-
ferent ways. SRAM is faster, but more expensive, than SDRAM, so there is only a
relatively small amount of it. SRAM is used to hold routing tables and other key
