Code change:
This is a work in progress, but ready for an initial review. I will add more commits to the branch for dealing with IANA registration of the header field. I also need to do some massaging of Section 7.2 to account for changes there. I'll have a follow-up pull request that deals with learning the server TTL. I think that the simplest thing to do would be to have the server echo the header field, using a lower value if it doesn't support it. Application servers can discover the maximum by sending a large value, like . The alternative is a declarative push service policy, but I want to encourage an adaptive policy with respect to TTL.
An alternative to echoing the TTL is just using the header field, as is currently described. I think that might actually be sufficient. That still leaves user agents unable to determine whether they need a full resync though. We need something for .
Looking at the semantics of and in relation to and , I think that it would be unwise to use them here. Current plan is to echo .
It was less clear that this was needed than was, but the idea is that it would be good for a user agent (or maybe an application server too) to be able to learn the maximum TTL that a push service was willing to maintain.
Discussed today: conclusion was to have subscriptions expire if the server is unwilling to maintain them over a long period of inactivity.
It should be fine if we let subscriptions expire if the server is unwilling to maintain them over a long period of inactivity.
There is a related failure case in the current draft related to acknowledgements: The push server MUST push a response with a status code of 5XX (TBD) if the user agent fails to acknowledge the receipt of the push message or the push server fails to deliver the message prior to its expiration.
Discussed. Discovery of maximum TTL is not required at this time.
This is just for the request from the application server to the push service.

Related discussion:
text: 7.2. <del> Push services typically store messages for some time to allow for limited recovery from transient faults.  If a push message is stored, but not delivered, the push service can indicate the probable duration of storage by including expiration information in the response to the push request. </del> <ins> Storage of push messages based on the TTL header field comprises a potentially significant amount of storage for a push service.  A push service is not obligated to store messages indefinitely.  A push service is able to indicate how long it intends to retain a message to an application server using the TTL header field (see ttl). </ins> <del> A push service is not obligated to store messages indefinitely.  If a user agent is not actively monitoring for push messages, those messages can be lost or overridden by newer messages on the same subscription. </del> <ins> A user agent that does not actively monitor for push messages will not receive messages that expire during that interval. </ins> <del> Push messages that were stored and not delivered to a user agent are </del> <ins> Push messages that are stored and not delivered to a user agent are </ins> delivered when the user agent recommences monitoring.  Stored push messages SHOULD include a Last-Modified header field (see Section 2.2 of RFC7232) indicating when delivery was requested by an application