Questions about SETTINGS and SETTINGS ACK

All,

Quick questions for my implementation about SETTINGS: 

1. Can an endpoint have more than one SETTINGS frame in flight? That is, can an endpoint send a second SETTINGS frame without waiting for the corresponding ACK?  The spec doesn't forbid this, but does talk about having the sender time out after some reasonable period of time.

2. If multiple SETTINGS frames can be "in flight", how does the sender know *which* settings are being acknowledged when it gets a SETTINGS frame back with the ACK bit set?

3. If multiple SETTINGS frames cannot be in flight, does the receiver issue a connection error of type PROTOCOL_ERROR?

Thanks!

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Michael Sweet, Senior Printing System Engineer, PWG Chair