Add interceptor to aggregate CCFB reports

[mengelbart]

Description

This interceptor keeps track of outgoing RTP packets and reads incoming CCFB (RFC8888) RTCP packets. For each incoming CCFB packet, it creates a report of the included acknowledgments and stores it in the interceptor attributes. Applications reading RTCP packets from it can read the report, e.g., to perform bandwidth estimation.

This is a PoC to improve bandwidth estimation in Pion. The current GCC implementation is hard to use and test. Giving applications access to the feedback reports could simplify and decouple the actual bandwidth estimation from the interceptors.

TODO

• Add unit tests
• Implement TWCC converter (currently, only RFC 8888 is supported)
• Remove old entries from history
• Document the semantics of the generated reports (currently, it generates a report for every packet included in the feedback packet, including the sequence number, size, departure timestamp, status (received or not), arrival timestamp, and ECN bits.

[codecov]

Codecov Report

Attention: Patch coverage is 83.75000% with 52 lines in your changes missing coverage. Please review.

Project coverage is 72.26%. Comparing base (c06f448) to head (1072695).

Files with missing lines	Patch %	Lines
pkg/ccfb/interceptor.go	77.44%	22 Missing and 8 partials ⚠️
pkg/ccfb/twcc_receiver.go	70.83%	21 Missing ⚠️
internal/test/mock_stream.go	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #300      +/-   ##
==========================================
+ Coverage   71.38%   72.26%   +0.87%     
==========================================
  Files          79       84       +5     
  Lines        4526     4842     +316     
==========================================
+ Hits         3231     3499     +268     
- Misses       1158     1199      +41     
- Partials      137      144       +7     

Flag	Coverage Δ
go	72.13% <83.75%> (+0.88%)	⬆️
wasm	70.42% <83.43%> (+0.93%)	⬆️

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[Sean-Der]

This is so exciting! I am glad to see you back @mengelbart :)

@jech and @JoeTurki are getting deep into this world if you are looking for people to work with/quick reviews

[jech]

Could you please give some background? Why not simply provide a function that parses the reports?

To my untrained eyes, it looks like your interceptor aims to allow a client to do:

_, attr, _ := track.ReadRTCP()
data := attr.Get(CCFBAttributesKey)

Without an interceptor, the same code would look like this:

p, _, _ := track.ReadRTCP()
var data CCFBReportPacket
data.Unmarshal(p.Data)

Now, perhaps I'm biased against interceptors (which are causing me a fair amount of suffering), but the second code looks clearer to me. What am I missing?

[mengelbart]

The interceptor also keeps track of outgoing RTP packets, timestamps them, and matches RTCP reports to the history of sent packets. The report you get from the attributes then contains departure and arrival timestamps for each packet that was reported on in the RTCP. Without the interceptor, you have to keep track of the departure timestamps yourself.