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How complicated is IPv6 for ISPs?

I think there's quite a few practical issues that people might not always consider.
  • IPv6 can take considerably more CPU power to process on the provided router than IPv4, so could impact router costs (as it requires more cycles to process the bigger address), or could skew average speed figures.
Address matching is a relatively small part of the cost of routing. It's certainly true that software IPv4 stacks have had more optimisation work done than v6 stacks, but there's no fundamental reason why v6 can't perform as well, e.g. with trie data structures. The flip side is that with v6 you don't need to do NAT, which involves updating header checksums.

Routers which forward in hardware are a different matter: it's a question of whether the CAM can match v6 addresses. If it can, then there's no difference in CPU requirement. If it can't, then the router needs replacing. Any equipment bought in the last 5-10 years should be IPv6 capable, and if it's older than that, it's due for an upgrade anyway.

Since v6 headers are larger, you can expect user-observed speedtest figures to be ~1.5% lower (since the user part of the datagram is smaller for a same-sized MTU).

  • The kit (routers/bngs/etc) in the ISP might have limited memory/fib/rib and suddenly having IPv6 routes to every customer as well as IPv4 might force them to have to upgrade their equipment sooner
In the default-free zone there are over 900K IPv4 routes, and ~180K IPv6 routes. This could be an issue for core routers, but access routers generally have default route and local infrastructure routes only.

  • Their backend systems might be too rigid to support IPv6
Definitely an issue, but not a huge piece of work to fix. A bigger issue IMO is the perceived extra training and work for their support staff, but BT and Sky have proved this is not a problem in practice.

  • They'll have to establish all of their IPv4 peering sessions with everyone again for IPv6, if they don't then they may find IPv6 traffic will cost them more than IPv4 traffic
Setting up peering sessions is a one-off activity, as is updating their monitoring systems.

There are some cases where the other networks will peer on v4 but not v6, or vice versa.

More annoying is that Google and Hurricane Electric announce their v4 routes to transit providers, but not their v6 routes - they are trying to get off transit and force everyone to peer. But Cogent (tier 1 provider) will not peer with anyone they don't consider to be a tier 1. Hence if you buy Cogent transit, you can't reach Google and HE over IPv6.
 
If you're routing in software then you may well have SIMD, vector instructions, 128 bit registers so can do nice, rapid bitwise operations on those which saves time. Once you've worked out the routing at initial session start you can happily use a look up table

ASIC-based routers really should have IPv6 ready to go - relatively cheap Broadcom stuff can handle accelerated forwarding of v6 so no excuse for carrier-grade stuff not to.
 
I think there's quite a few practical issues that people might not always consider.
  • IPv6 can take considerably more CPU power to process on the provided router than IPv4, so could impact router costs (as it requires more cycles to process the bigger address), or could skew average speed figures.
This is not true. IPv6 headers are fixed length and the behaviour around extension headers is well-defined, so it is actually considerably easier for a router to parse IPv6 headers within a fixed amount of memory and using less CPU cycles. Additionally, there is also no longer a header checksum (so no need to burn CPU cycles on those) and there is no fragmentation in IPv6 whatsoever so that complexity is removed too.
 
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