What is HPACK Compression in APNs

Question

thank you for reading my question. I have read numerous documents about HTTP/2 and tried to understand most important concepts for constructing a APNs server-side API in Java.

• So far, I have a working code for creating Payload and initiating ssl handshake with APNs.
• I am stuck with figuring out a way to send Push Notification to APNs, specifically sending a HTTP/2 POST request to the Apple server.

APNs requires the use of HPACK (header compression for HTTP/2), which prevents repeated header keys and values. APNs maintains a small dynamic table for HPACK. To help avoid filling up the APNs HPACK table and necessitating the discarding of table data, encode headers in the following way—especially when sending a large number of streams:

If I have understood correctly, I need to use Huffman Encoding for the header compression. If I am wrong on this, please correct me.

The :path value should be encoded as a literal header field without indexing

The authorization request header, if present, should be encoded as a literal header field without indexing

I read RFC 7541 for this, but have no idea what they are talking about. Please note that I am trying to understand the system and requirements to gain knowledge through this post, not just answers to the specific questions.

The appropriate encoding to employ for the apns-id, apns-expiration, and apns-collapse-id request headers differs depending on whether it is part of the initial or a subsequent POST operation, as follows:

The first time you send these headers, encode them with incremental indexing to allow the header names to be added to the dynamic table

Subsequent times you send these headers, encode them as literal header fields without indexing

What do they mean when they say, "encode them with incremental indexing to allow the header names to be added to the dynamic table" and "Subsequent times you send these headers, encode them as literal header fields without indexing" afterwards. I guess understanding one of two literal header fields with/out indexing will help me comprehend this better.

Thank you again for reading the question and please help me with this!!