[Bloat] Fwd: 400G forwarding - how does it work?

[Simon Leinen]

Dave Taht via Bloat writes:
> I'd like to understand more deeply, too.

"Deep" is the right word here, 'cause there is DEEP pipelining going on.

"1 packet every 6.08 nanoseconds" (per pipeline) does NOT mean that you
only have (at 1GHz clock speed) 6 cycles to spend on each packet, just
that *each stage of the pipeline* needs to be ready to accept a new
packet every 6 cycles.

So if you can distribute the work over, say, 20 pipeline stages, then
you have 120 cycles to work on each packet.

Implementing this is surely tricky! Especially as you have to ship
relatively large amounts of (packet) data to the correct output port as
part of that processing.  Definitely a major engineering feat...

Cheers,
-- 
Simon.

> ---------- Forwarded message ---------
> From: James Bensley <jwbensley+nanog at gmail.com>
> Date: Mon, Jul 25, 2022 at 5:55 AM
> Subject: 400G forwarding - how does it work?
> To: NANOG <nanog at nanog.org>


> Hi All,

> I've been trying to understand how forwarding at 400G is possible,
> specifically in this example, in relation to the Broadcom J2 chips,
> but I don't the mystery is anything specific to them...

> According to the Broadcom Jericho2 BCM88690 data sheet it provides
> 4.8Tbps of traffic processing and supports packet forwarding at 2Bpps.
> According to my maths that means it requires packet sizes of 300Bs to
> reach line rate across all ports. The data sheet says packet sizes
> above 284B, so I guess this is excluding some headers like the
> inter-frame gap and CRC (nothing after the PHY/MAC needs to know about
> them if the CRC is valid)? As I interpret the data sheet, J2 should
> supports chassis with 12x 400Gbps ports at line rate with 284B packets
> then.

> Jericho2 can be linked to a BCM16K for expanded packet forwarding
> tables and lookup processing (i.e. to hold the full global routing
> table, in such a case, forwarding lookups are offloaded to the
> BCM16K). The BCM16K documentation suggests that it uses TCAM for exact
> matching (e.g.,for ACLs) in something called the "Database Array"
> (with 2M 40b entries?), and SRAM for LPM (e.g., IP lookups) in
> something called the "User Data Array" (with 16M 32b entries?).

> A BCM16K supports 16 parallel searches, which means that each of the
> 12x 400G ports on a Jericho2 could perform an forwarding lookup at
> same time. This means that the BCM16K "only" needs to perform
> forwarding look-ups at a linear rate of 1x 400Gbps, not 4.8Tbps, and
> "only" for packets larger than 284 bytes, because that is the Jericho2
> line-rate Pps rate. This means that each of the 16 parallel searches
> in the BCM16K, they need to support a rate of 164Mpps (164,473,684) to
> reach 400Gbps. This is much more in the realm of feasible, but still
> pretty extreme...

> 1 second / 164473684 packets = 1 packet every 6.08 nanoseconds, which
> is within the access time of TCAM and SRAM but this needs to include
> some computing time too e.g. generating a key for a lookup and passing
> the results along the pipeline etc. The BCM16K has a clock speed of
> 1Ghz (1,000,000,000, cycles per second, or cycle every 1 nano second)
> and supports an SRAM memory access in a single clock cycle (according
> to the data sheet). If one cycle is required for an SRAM lookup, the
> BCM16K only has 5 cycles to perform other computation tasks, and the
> J2 chip needs to do the various header re-writes and various counter
> updates etc., so how is magic this happening?!?

> The obvious answer is that it's not magic and my understanding is
> fundamentally flawed, so please enlighten me.

> Cheers,
> James.


> -- 
> FQ World Domination pending: https://blog.cerowrt.org/post/state_of_fq_codel/
> Dave Täht CEO, TekLibre, LLC
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