This document describes the extensions I've made to the DC protocol, to be
able to support extra functionality. Those are of course subject to change 
and discussion but I'll try to keep it as up to date as possible. Other 
clients are most welcome to follow this scheme and use the features 
presented here as they wish. The main scheme stems from a discussion 
sometime ago on the javadc-devel list on sourceforge...

Note; I assume familiarity with the dc protocol as used in neomodus dc 
1.00091...

The main idea behind the protocol is that when setting up the connection, 
each client tells the other one about all commands it understands, and 
after that, use those commands as pleased...to be able to do this we need 
to add a command to the protocol that tells the other client about the 
features it supports. We also need to make sure that only clients that
have a fair chance of understanding this command (to avoid unneccesary 
data being sent and to avoid confusing other clients). The scheme I've 
chosen is based on the $Lock information being sent upon setting up a 
connection, where a client that supports the extension command sends a 
special string in the lock, to let the other one know that it supports 
the extended commands. When having found such a client, it sends 
information about all features it supports before sending a correctly 
computed $Key.

On a side note; DC++ requires commands to come in a certain order to be 
able to efficiently use resources and will ignore any commands out of 
order (such as multiple $Send commands). This also improves the 
determinism of the protocol.


*** Client-Client communication ***

Feature: Supports
Since: 0.17

Purpose: To identify which features a certain client supports

Usage: "$Supports <feature1> <feature2> ... <featureN>|"
<featurename> = Case sensitive string describing the feature that a 
certain client supports. This should be the name of the command being 
sent, i e if a client supports a command "$GetBZListLen|", featurename 
should be GetBZListLen, unless of course, this is not a feature that 
requires sending extra data. Feature names may (obviously) not contain 
spaces. Empty $Supports should not be sent.

Should be sent: After receiving $Lock but before sending $Key

Notes: In the initial discussions, we thought about adding version 
information (something like "$Supports <name> <version>|"), but I've 
decided not to use this because 1) There could be problems with old 
versions, i e client x supports version 1.2, does that mean that it 
supports version 1.1 and 1.0 as well? 2) It's really easy to add a new 
version, i e "$Supports GetBZListLen GetBZListLen2|" to advertise an 
upgraded $GetBZListLen command.
This command should only be sent if the $Lock starts with the exact string 
EXTENDEDPROTOCOL", to avoid confusing clients that don't understand it
anyway. DC++ also uses the Pk part of the lock to identify exact client 
version, so a full $Lock string from a dc++ client is 
"$Lock EXTENDEDPROTOCOLxxxx Pk=DCPLUSPLUS0.164xxxx" where xxxx denotes 
some random data (not beginning with a number...=).

--------------------------------------------------------------------

Feature: MiniSlots
Since: 0.304

Usage: This allows the other client to use a free slot for small files / 
file list. $Supports is needed because nmdc closes the file list browser
when the connection is broken, which it becomes when the client on the
other side tries to download its first file and fails because it has no
real slot (there were lots of nmdc users bitching about this...).

--------------------------------------------------------------------

Feature: GetZBlock
Since: 0.307
Deprecated - replaced by ADCGet

Usage: Instead of $Get and $Send, use 
"$GetZBlock <start> <numbytes> <filename>|" where <start> is the 0-based
(yes, 0-based, not like get that's 1-based) starting index of the file 
used, <numbytes> is the number of bytes to send and <filename> obviously 
is the filename. The other client then responds 
"$Sending <bytes>|<compressed data>", if the sending is ok or 
"$Failed <errordescription>|" if it isn't. If everything's ok, the data is 
sent until the whole uncompressed length has been sent. <bytes> specifies
how many uncompressed bytes will be sent, not compressed, as the sending
client doesn't know how well the file will compress. $Sending is 
needed to be able to distinguish the failure command from file data. Only 
one roundtrip is done for each block though, minimizing the need for 
maintaining states.

Compression: Compression is done using ZLib (v 1.1.4 in DC++ 0.21's case), 
using dynamic compression level. The compression level can of course be 
changed by the implementator to reduce CPU usage, or even just store 
compression in the case of non-compressible files, which then works as 
adler32 check of the transferred data.

Support of $GetZBlock also implies support for $UGetZBlock.  The syntax
and semantics of $UGetZBlock are the same as the $GetZBlock, but the
filename must be given in utf-8 encoding.

--------------------------------------------------------------------

Feature: XmlBZList
Since: 0.307

Usage: Supporing this means supporting utf-8 XML file lists with the 
following general structure:

<FileList Version="1" Generator="dc client name and version">
 <Directory Name="xxx">
  <Directory Name="yyy">
   <File Name="zzz" Size="1"/>
  </Directory>
 </Directory>
</FileList>

In each directory, including the root, the name of the entity must be
case-insensitive unique in that level of the hierarchy. 

Other fields may be added as necessary. DC++ for instance adds the
TTH attribute to each file it knows the TTH root of in base32 encoding.

The file list is available as "files.xml.bz2" (vs MyList.DcLst), and is
compressed using bzip2.

To retrieve unicode files from the file list, the client may also support
the above GetZBlock and its utf-8 derivatives.  Support for XmlBZList
implies support for $UGetBlock, so files are guaranteed to be retrievable.

$UGetBlock follows $UGetZBlock semantics, but without compressing 
the data. The <bytes> parameter of $Sending specifies how many bytes
will be sent.

Don't touch Version. Add your own, with a different name, if you feel 
compelled.

Don't trust Generator to determine features of the file list. It's there
mainly for debugging and informative purposes.

--------------------------------------------------------------------

Feature: ADCGet
Since: 0.402

Usage: 

"$ADCGET <type> <filename> <startpos> <bytes> <flag0>...<flagN>"
A complete replacement for all binary data transfers using an ADC-like syntax.
See the ADC specs to see how GET behaves. (http://dcplusplus.sf.net/ADC.html)

--------------------------------------------------------------------

Feature: TTHL
Since: 0.402

Usage:

Supporting this means supporting the upload of tth leaf data. Instead
of transfering the file itself, the TTH data of all leaves is transferred
in binary. The size transferred back is the number of bytes of leaf data,
from this and the file size the receiving client can calculate which
level (tree depth) the sending client is offering. The receiver should
obviosly check that the received leaf data is correct by rebuilding the
tree and checking that it's recorded root matches.

--------------------------------------------------------------------

Feature: TTHF
Since: 0.402

Usage:

Supporting this means supporting file identification by TTH root. This
means supporting downloads by TTH root instead of share directory and
name. The advantage is that moved files can still be found by the
downloader without requeuing the file.

The extension adds a namespace "TTH" before the ADC file root. A TTHF
filename has the following syntax:

TTH/<TTH root in base32, 192 binary bits>

i e the TTH namespace consists of TTH root values directly under the
"TTH/" root.

The naming scheme is valid in all types (i e also for getting TTH leaves)

--------------------------------------------------------------------

Feature: ZLIG
Since: 0.402

Usage:

Supporting this means that zlib compressed $ADCGET transfers are supported.

--------------------------------------------------------------------

*** Client-Hub communication ***

Feature: Supports
Since: 0.300

Usage: "$Supports <feature1> <feature2> ... <featureN>|"
This is mostly analogous to the client-client communcation part. The hub 
must send a $Lock that begins with "EXTENDEDPROTOCOL", which will prompt 
the client to answer with $Supports. Then, when a hub receives "$Supports" 
it _must_ answer with "$Supports" to tell the client about its features 
(which may differ from the ones the client supports). See client-client 
part for details. The hub must not send "EXTENDEDPROTOCOL" unless it 
supports at least one feature.

Should be sent: After $Lock, before $Key

--------------------------------------------------------------------

Feature: UserCommand
Since: 0.300

Usage: See http://dcplusplus.sf.net/wiki/

--------------------------------------------------------------------

Feature: NoGetINFO
Since: 0.302

Usage: This indicates that hub doesn't need $GetINFO's to send out $MyINFO's
at connect. This is a sort of light-weight QuickList that's very easy to
implement and does half of QuickList's job.

--------------------------------------------------------------------

Feature: UserIP2
Since: 0.305

Usage: This indicates support for $UserIP, see http://dcplusplus.sf.net/wiki/

--------------------------------------------------------------------

Feature: NoHello
Since: 0.305

Usage: This indicates that the client doesn't need neither $Hello nor $NickList
when connecting to a hub and receiving its users (i e $myinfo for each user is
enough). It still accepts $Hello though (to add bots to the user list),
and will still send getnicklist to indicate that it indeed is interested
in the user list. It still requires a $hello to be sent about itself during login!
--------------------------------------------------------------------

Feature: GetZBlock
Since: 0.307

Usage: This indicates that the client is capable of compressed 
transfers, and has the option to use them enabled.

--------------------------------------------------------------------

Feature: TTHSearch
Since: 0.307

Usage: This indicates that the client supports searching for queued 
files by TTH, if there is one available to it.  TTH searching is more 
CPU efficient than the previous method. 
