There are two new GET parameters for the tracker in the latest
release. They are -

key=xxxx - this is like peer id, but it's only known to the client and the
tracker. It allows clients to be behind dynamic IP. If a peer announced
a key previously, then it's accepted if and only if it gives the same key
again. If no key was given, then the fallback is checking that the IP
hasn't changed. If the IP has changed, mainline currently will give a peer
list but not change any data related to that peer, so that peers behind
dynamic IP using old clients will continue to work okay. Currently
mainline generates the value associated with key as eight random hex
values, and the tracker accepts any string from clients.

compact=1 - when a client sends this, the 'peers' return value is a single
string whose length is a multiple of 6 rather than a dict. To extract peer
information from the string, chop it into substrings of length 6. For each
substring, the first four bytes are the IP and the last two are the port,
encoded big-endian. This results in huge bandwidth savings.

Everybody developing ports should implement these keys, they're very
useful.

-Bram