[address-policy-wg] IPv6 PI resource question!

Nick,

Although your explanation of why large routing table are unwanted in the current
discussion with regards to PI space I fail to see the point. Our customers want
independence for a number of reasons -  with IPv4 they could get PI space and have
that independence. With regards to IPv6 the rules are currently different and they
cannot get PI space. As suggested on this list many a time a work around is to become
a LIR and get your own PA space. In the end these customers *will* generate new routes.
The artificial hurdle in place now is in effect only slowing down the IPv6 adaptation
in the case of our customers, it will not stop them from achieving their goals of
being independent.

Jasper

-----Original Message-----
From: address-policy-wg-admin at ripe.net [mailto:address-policy-wg-admin at ripe.net] On Behalf Of Nick Hilliard
Sent: Wednesday, February 16, 2011 11:11 PM
To: address-policy-wg at ripe.net
Subject: Re: [address-policy-wg] IPv6 PI resource question!

On 16/02/2011 19:08, Igor Ybema wrote:
> I do not understand why everyone claims that 'the routing table growth'
> is the reason not to allow too much IPv6 PI.

This is not really relevant to address-policy-wg, but it probably does need
some explanation, because there are probably people on the mailing list who
may not understand why some people get so upset about the issue.

When a packet passes through a router, the router needs to decide where to
send that packet.  The way it works is that the router examines the
destination IP address of the packet and performs a search through the
entire routing table to see what the next hop address of that packet should be.

The component on a router which does this lookup is called a "lookup
engine", and depending on how big the router is, it will be implemented in
one of a couple of different ways.

On a low-end router (e.g. Juniper J series or Cisco 7200/7300), this is
done on the main CPU.  As this is a generic purpose CPU, this means that a
router of this form will only be able to handle forwarding a certain number
of packets per second before the CPU gets to busy to handle any more.  A
router of this form is generally referred to as a "software router".

On a higher-end router (e.g. Juniper MX / M / T series, or Cisco
GSR/CRS/7600), they use dedicated hardware lookup-engines, and in a chassis
+ blade system, these lookup engines will often be located on the line
cards themselves.  The advantage of this is that you can achieve _much_
higher throughput on the router.  The disadvantage is that dedicated
hardware of this form tends to be very expensive.   A router of this form
is generally referred to as a "hardware router".

One of the more common hardware components for performing IP address
lookups is called TCAM - ternary content addressable memory.  It performs a
similar function to an associative array in a language like PHP, except
it's implemented in hardware.  It's ridiculously fast and ridiculously
expensive, and because it's so expensive, router manufacturers tend not to
put large quantities into their lookup engines.  So, a router vendor like
Cisco might create a router with a lookup engine which had enough TCAM for
256000 ipv4 addresses (e.g. C7600/SUP7203B), or 500,000 entries (e.g.
ASR1001), or they might make a line card with its own dedicated lookup
engine which could handle 1,000,000 ipv4 addresses (e.g. ASR9000, brocade XMR).

While you get very good performance from these lookup engines, you are also
constrained by the fact that if the number of prefixes on your router
exceeds the number of TCAM slots, then your router will either drop the
packets or else do the lookup on the route-processor CPU.  I.e. the moment
you hit 1,000,001 prefixes, your €200,000 C7600 with 20 x 10G ports will
turn into a software router with the performance of a C7200VXR.  This is
generally considered to be a Bad Thing.

If there are too many routes on the internet, this will cause the capacity
of these routers to be exceeded, and they will need to be upgraded with a
device with more lookup capacity.  Replacing one or two routers like this
for a small service provider is expensive, but if you have to perform a
forklift upgrade on a continental or global infrastructure, you may be
talking about hundreds of millions of € / $ / £ worth of investment.

Getting back to IPv6 PI assignments, the reason that people are so upset
about them is that an IPv6 prefix can take up to 4 times the amount of TCAM
than an ipv4 prefix.  I.e. it fills up your router's routing slots 4 times
faster.  So, it will cause serious problems in future years if there is
lots of IPv6 PI assigned to lots of end-users.

Nick




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