Required support

[mnot]

We should specify what implementations are required to support if they choose to implement this spec, e.g.:

• group labels with at least n characters
• at least n groups per resource
• at least n groups per invalidation header

[mnot]

Fastly - docs

• 1024 bytes for each group label
• 16,384 max bytes for whole header

Cloudflare - docs

• no max length on a label in header; 1024 bytes in API
• 16k overall limit on whole set of tags

Akamai - docs

• 128 characters per group label
• 128 groups per response

[mnot]

Based on those limits and being conservative, we could say that an implementation should support at least 128 groups, each with a 128 character label.

[mnot]

After feedback during LC and list discussion, we settled on 32x32; #3016
See thread starting at:
https://www.w3.org/mid/4282B150-F40F-47B6-B1F3-37DB3F4586D0@gbiv.com