HPack Demo

A Demo project on how hpack process
This project used python-hyper/hpack as the hpack encoder/decoder
And used http2jp/hpack-test-case for the test case

Install and Run

git clone https://github.com/chhsiao90/hpack-demo.git
pip install -r requirement.txt

# Run the simplest case
python demo.py story.json

# Select a case from hpack-test-case/raw-data and run it
python demo.py

Core concept on hpack

Encoder will use an index from static/dynamic table to represent a key-value header
Encoder will index key-value header into dynamic table if it hadn't bean defined
Decoder will decompressed index to original key-value header by searching static/dynamic table
Deocder will set up dynamic table if new key-value received

Core object on hpack

Encoder and Decoder

Encoder crompressed the original header into binary format
Decoder decompressed the binary format message back to original header
Each server and client have one encoder and one decoder
Each encoder corresponde to a decoder
- Client->Encoder => Server->Decoder
- Server->Encoder => Client->Decoder
Encoder and Decoder is stateful because it contains a dynamic table

Static Table

A list of key-value pair
It's hardcode and fixed
All decoder and encoder had one static table

Dynamic Table

A list of key-value pair
Used to cache header content had been used
Each decoder and encoder had one dynamic table
Server decoder's dynamic table will same as client encoder's dynamic table
Client decoder's dynamic table will same as server encoder's dynamic table

In binary

First byte

1XXXXXXX: Indexd Header
01XXXXXX: Literal Header with Indexing
0000XXXX: Literal Header without Indexing
0001XXXX: Literal Header never Indexing

Indexing

Get Index Value

Convert the remain bits of first byte to integer.
If all of the remain bits of first byte is 1, add next bytes till the byte that contains 0

Index = 0

No entry in static table or dynamic table
following content would be Name

0 < Index <= 61

Has entry in static table
following content would be value

Index > 61

Has entry in dynamic table
following content would be value

Length

Summarize every byte till the byte that contains 0

Name

First Bit

1 = Huffman
0 = Non-Huffman

Get Length

Check Length Block

Following Bytes

Following Length bytes is the name

Value