route(8): clarify -prefixlen description

[ngie]

Summary

route(8): clarify -prefixlen description

Try to reword -prefixlen section to more clearly and accurately describe
how the -prefixlen modifier works.

While here, fix a word that igor considered a typo: aggregatable addresses
is a valid technical term per RFC-2374, but it was superseded by the
technical term "aggregator" in RFC-3587.

(I couldn't figure out where in the code the -prefixlen 64/aggregator part
was enforced, only the number 128, for the max prefixlen. I could use some
help tracking down where the value, 64, is enforced.)

MFC after: 1 week

[hrs]

The 64-bit default prefixlen was removed at rev.277241. I did not notice it but I do not think it is a correct change because it has several bad side-effects. I will take a look at route.c by this weekend.

For the sentence describing the default prefix length, "aggregator addresses" is a bit odd. The intent of the original sentence was "64 is the default value because the aggregatable GUA structure in RFC 2374 assumes 64-bit prefix length for an IPv6 GUA on a single link". "aggregator" is a field of an IPv6 address and "aggregatable address" is a name of the IPv6 address structure itself. These two are different from each other. However, the concept of aggregatable address is now obsolete by RFC 3587 as you pointed out. "aggregator" is also an obsolete term now. So if we really want to clarify it, I would say "the default prefixlen is 64 because IPv6 unicast addresses except those that start with binary value 000 usually have interface IDs that are 64 bit long." Or just dropping "for aggregator addresses" in the proposed sentence also works.

[ngie]

Follow up to finding from hrs

Remove "aggregator address" and instead state that the prefixlen is 64
when the destination argument isn't default.

[ngie]

In D10087#208586, @hrs wrote:

The 64-bit default prefixlen was removed at rev.277241. I did not notice it but I do not think it is a correct change because it has several bad side-effects. I will take a look at route.c by this weekend.

For the sentence describing the default prefix length, "aggregator addresses" is a bit odd. The intent of the original sentence was "64 is the default value because the aggregatable GUA structure in RFC 2374 assumes 64-bit prefix length for an IPv6 GUA on a single link". "aggregator" is a field of an IPv6 address and "aggregatable address" is a name of the IPv6 address structure itself. These two are different from each other. However, the concept of aggregatable address is now obsolete by RFC 3587 as you pointed out. "aggregator" is also an obsolete term now. So if we really want to clarify it, I would say "the default prefixlen is 64 because IPv6 unicast addresses except those that start with binary value 000 usually have interface IDs that are 64 bit long." Or just dropping "for aggregator addresses" in the proposed sentence also works.

Thank you very much for the in-depth description. I'm glad I brought this to the expert's attention :).

[ngie]

Ping (is there anything I can do to improve this further)?

[ngie]

Ping-ping?

[wblock]

sbin/route/route.8

296 ↗

(On Diff #26525)

Please start new sentences on new lines.

[ngie]

sbin/route/route.8

296 ↗

(On Diff #26525)

Thanks for the feedback -- will do!

[ngie]

Updating per feedback from @wblock.

[ngie]

@wblock: could you please re-review this change?

[ngie]

... or @crees?

[0mp]

I believe it looks fine from the mdoc(7) point of view.

[ngie]

@emaste/@jtl: can you please review/approve this diff?

[crees]

Docs approval not necessarily needed, but given for a green tick there.

[emaste]

Approved by: emaste