nanog mailing list archives
Re: Waste will kill ipv6 too
From: Nick Hilliard <nick () foobar org>
Date: Fri, 29 Dec 2017 15:23:07 +0000
My wild guess is if we'd just waited a little bit longer to formalize IPng we'd've more seriously considered variable length addressing with a byte indicating how many octets in the address even if only 2 lengths were immediately implemented (4 and 16.)Actually, that got heaved over the side fairly early in the IPng discussion, because nobody who was building silicon last century wanted to deal with arbitrary-length addresses. The IPv4 header had source and destination addresses on 4-byte boundaries for good reasons which still held true when we did the IPv6 headers.It's rather interesting how parsing of variable length addresses was thought to be way too complicated - while parsing of IPv6 extension header chains of unknown length was okay.
variable length addressing was thrown out at the time because the OSI model showed that it created a good deal of trouble for no overriding benefit. Processing variable length addresses was found to be as difficult as processing the longest length allowed. In practice, NSAP addresses were limited to 20 octets, but in theory they could be up to 255. IPv6 extension headers, on the other hand, weren't considered a major imposition at the time because almost all routers in the mid 1990s were cpu switched rather than handled on asics. Walking through TLVs is relatively straightforward to handle from an algorithmic point of view, but it creates pain when dealing with forwarding lookup engines because extension headers means inspecting more header data when calculating the next-hop when you're dealing with ECMP / LAGs. This is harder when dealing with hardware/offloaded lookup engines because more header information needs to be copied from the interface to the lookup engine, which has a cost. It's not insurmountable, just more expensive from a hardware point of view, which means that lots of cheaper kit doesn't do this well, or in some cases, at all. Nick
Current thread:
- Re: Waste will kill ipv6 too, (continued)
- Re: Waste will kill ipv6 too Ricky Beam (Dec 28)
- Re: Waste will kill ipv6 too Mark Andrews (Dec 28)
- Re: Waste will kill ipv6 too Owen DeLong (Dec 28)
- Re: Waste will kill ipv6 too bzs (Dec 28)
- Re: Waste will kill ipv6 too Owen DeLong (Dec 28)
- Re: Waste will kill ipv6 too bzs (Dec 28)
- Re: Waste will kill ipv6 too Large Hadron Collider (Dec 28)
- Re: Waste will kill ipv6 too valdis . kletnieks (Dec 28)
- Re: Waste will kill ipv6 too sthaug (Dec 29)
- Re: Waste will kill ipv6 too Mikael Abrahamsson (Dec 29)
- Re: Waste will kill ipv6 too Nick Hilliard (Dec 29)
- Re: Waste will kill ipv6 too Owen DeLong (Dec 29)
- Re: Waste will kill ipv6 too Nick Hilliard (Dec 29)
- Re: Waste will kill ipv6 too William Herrin (Dec 29)
- Re: Companies using public IP space owned by others for internal routing james machado (Dec 17)
- Re: Companies using public IP space owned by others for internal routing Harry McGregor (Dec 17)
- Re: Companies using public IP space owned by others for internal routing Tom Carter (Dec 18)
- Re: Companies using public IP space owned by others for internal routing Harald Koch (Dec 18)
- Re: Companies using public IP space owned by others for internal routing Mark Andrews (Dec 18)