IPv6 Router Advertisements

Regarding the prefix option,

The prefix information option contains bits that indicate whether the prefix is to be used for on-link determination or for address configuration.

When the L bit is set,you can use the prefix for on-link determination,when not set you cannot determine any info about on-link and off-link.
The Valid lifetime indicates how long a prefix is valid for purposes of on-link determination.

Can someone please explain what is meant by this on-link off-link determination.

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Comments

suffah

First time I've ever heard of the two terms. According to RFC 2461, the definitions of "on-link" and "off-link" are as follows:

RFC2461 wrote:

on-link - an address that is assigned to an interface on a
specified link. A node considers an address to be on-
link if:

- it is covered by one of the link's prefixes, or

- a neighboring router specifies the address as
the target of a Redirect message, or

- a Neighbor Advertisement message is received for
the (target) address, or

- any Neighbor Discovery message is received from
the address.

off-link - the opposite of "on-link"; an address that is not
assigned to any interfaces on the specified link.

kryolla

on link is when you obtain an ipv6 address through autoconfig via the router advertisement and off link is via dhcpv6.

Here I have 2 ipv6 addresses and only 1 is used by hosts for autoconfig

ICMPv6-ND: Sending RA to FF02::1 on FastEthernet0/0
ICMPv6-ND: MTU = 1500
ICMPv6-ND: prefix = FC00:1:0:58::/64 onlink
ICMPv6-ND: 240/240 (valid/preferred)
ICMPv6-ND: prefix = FC00:1:0:85::/64 onlink autoconfig
ICMPv6-ND: 240/240 (valid/preferred)

EdTheLad

kryolla wrote: »

on link is when you obtain an ipv6 address through autoconfig via the router advertisement and off link is via dhcpv6.

Here I have 2 ipv6 addresses and only 1 is used by hosts for autoconfig

ICMPv6-ND: Sending RA to FF02::1 on FastEthernet0/0
ICMPv6-ND: MTU = 1500
ICMPv6-ND: prefix = FC00:1:0:58::/64 onlink
ICMPv6-ND: 240/240 (valid/preferred)
ICMPv6-ND: prefix = FC00:1:0:85::/64 onlink autoconfig
ICMPv6-ND: 240/240 (valid/preferred)

Don't think so, an on-link destination address is an address that doesn't require a default gateway i.e. next-hop router to be reached.Onlink is on the local segment Offline is off the local segment and requires a next-hop address.

As per rfc 4861.

on-link - an address that is assigned to an interface on a
specified link. A node considers an address to be on-
link if:

- it is covered by one of the link's prefixes (e.g.,
as indicated by the on-link flag in the Prefix
Information option), or

- a neighboring router specifies the address as the
target of a Redirect message, or

- a Neighbor Advertisement message is received for
the (target) address, or

- any Neighbor Discovery message is received from
the address.

off-link - the opposite of "on-link"; an address that is not
assigned to any interfaces on the specified link.

kryolla

EdTheLad wrote: »

Don't think so, an on-link destination address is an address that doesn't require a default gateway i.e. next-hop router to be reached.Onlink is on the local segment Offline is off the local segment and requires a next-hop address.

.

I dont think so a local address starts with FE80 and cant be given via a router advertisment. I think you are confused here. Next-hop is only used on routing updates and not on router advertisments for ICMPv6 ND.

on-link - an address that is assigned to an interface on a
specified link. A node considers an address to be on-
link if:

- it is covered by one of the link's prefixes (e.g.,
as indicated by the on-link flag in the Prefix
Information option), or

This is what autoconfig does via router advertisments. So are you saying my debug is wrong when it says on link LOL. Dude you need to read up more on IPv6 and not be so quick to correct people, Your 2 statments dont make sense

EdTheLad

kryolla wrote: »

I dont think so a local address starts with FE80 and cant be given via a router advertisment. I think you are confused here. Next-hop is only used on routing updates and not on router advertisments for ICMPv6 ND.

This is what autoconfig does via router advertisments. So are you saying my debug is wrong when it says on link LOL. Dude you need to read up more on IPv6 and not be so quick to correct people, Your 2 statments dont make sense

I wasn't talking about the link-local address, the onlink address is a global unicast address that is configured on th local segment i.e. the local interface of the local router or on the adjacent connected interface of a neighboring device.

Take a read of rfc 4943 for some enlightenment.
Your debug gives no information, the two addresses say onlink.
Stateful and Stateless autoconfig addresses can be onlink or offlink.

Stateless autoconfig uses RA's which contain a prefix with prefix options, one of the prefix options is an "ON-Link Flag" which is used to tell hosts if a prefix is onlink or offlink.If onlink systems with addresses within this prefix are reachable on the subnet in question without help from a router.

So my friend maybe it is you who needs to do some IPv6 reading.

kryolla

Your debug gives no information, the two addresses say onlink

If you dont know with IPv6 you can have more than 1 global unicast address and give peferential treatment on which one the host will use for autoconfig. You still havent proved anything about on-link and off-link, maybe you should go lab it up and show proof

EdTheLad

We are not discussing about multiple ipv6 addresses per interface, purely the difference between onlink and offlink.
Here's another rfc for you.
draft-wbeebee-on-link-and-off-link-determination-02 - IPv6 Subnet Model
Read section 2.2 which in plain english dismisses what you wrote earlier.