Tuesday, September 26, 2017

Request: Less Dynamic Prefix, Mr./Ms. ISP

by Craig Miller

There are two types of Global Prefixes, one that is provider independent (PI), allowing one to switch ISPs and keep the same prefix, and the other is provider aggregatable (PA), where the upstream provider allocates a prefix from their block.

PIs are great, and one need only go to your local RIR (Regional Internet Registry) to request/pay for a prefix block. But realistically, only larger organizations will go this route. Many smaller organizations and homes will use PA prefixes.

PA made easy

Through the magic of DHCPv6-PD (DHCPv6 with Prefix Delegation), allocating a prefix (either a /64 or less) to a small business or home is easy. Modern routers will make the PD request, and advertise the allocated prefix into the SOHO/home LAN, and IPv6 end to end connectivity is available.

ISPs are used to having their customers have a dynamic DHCPv4 address. But with many ISPs, a router reboot will result in the same IPv4 address, since the router is using the same MAC address to make the request.

With DHCPv6, a DUID (DHCP Unique Identifier) is used rather than a MAC address. But similar to the MAC address in DHCPv4, the DUID does not change between router reboots, and therefore DHCPv6 requests can receive the same external IPv6 address on the router.

PA Prefix disconnect

Alas, this is not the same for PA prefixes. Inside the ISP, there seems to be no connection between DHCPv6 address allocated and PD prefix allocated to the customer. This results in a semi-static outside IPv6 address on the router, and a very dynamic (changing with every connection/reboot) PD prefix in the customer's LAN.

A very dynamic LAN prefix causes some challenges, such as:

  • downstream routers, may not update to the new prefix in a timely manner causing unknown network outages which are mysteriously fixed by rebooting
  • Some DNS sserver configuration requires a Global Address, if the SOHO/home is using its own DNS server, or a DNS service other than the ISPs, this configuration may require updating with each new PD prefix
  • Network servers on the LAN will have changing IPv6 addressesj, making file sharing, and other network services difficult
  • Firewall configuration, allowing external access

Some of these issues can be mitigated by using a ULA (Unique Local Address) prefix on the LAN in addition to the ISPs very dynamic PA GUA prefix. But that requires more IPv6 knowledge than just plug in play.

Please, a less dynamic PA prefix

Dear Mr./Ms. ISP, I would like to have less dynamic PA addresses. As a customer, I would like to have the address prefix (assigned via PD) linked with my DUID and DHCPv6 address records. At the end of the day, we all want IPv6 to be simpler for the customer.

* note, a provider provides a PA address via DHCPv6-PD (Prefix Delegation)


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  2. I have noticed in the UK, that IPv6 on most ISPs tends to be "sticky" (Sky and BT), even on ISPs that reallocated a new IPv4 on every reconnect.

    But in order for the IPv6 allocation to be "sticky" you must ensure that you don't drop the lease when the WAN interface is restarted (e.g. router reboot). OpenWRT has an option just for this that seemed to work for me. This is "norealease":

    config interface 'wan6'
    option ifname 'eth0.2'
    option proto 'dhcpv6'
    option norelease '1'

    After this BT always gave me the save delegation.

    But why oh why, don't they just give static allocation on IPv6. I'm guessing extra revenue on so called business class accounts (which gave fixed IPv4 addresses, and they want this for IPv6) is the reason.

    I have personally now switched to an ISP that gives fixed IPv4 and IPv6 PD, to avoid this nastiness you point out.