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+Background
+==========
+
+'usbmuxd' stands for "USB multiplexing daemon". This daemon is in charge of
+multiplexing connections over USB to an iPhone or iPod touch. To users, it means
+you can sync your music, contacts, photos, etc. over USB. To developers, it
+means you can connect to any listening localhost socket on the device. usbmuxd
+is not used for tethering data transfer, which uses a dedicated USB interface as
+a virtual network device.
+
+Multiple connections to different TCP ports can happen in parallel. An example
+(and useful) tool called 'iproxy' is included that allows you to forward
+localhost ports to the device---allows SSH over USB on jailbroken devices, or
+allowing access the lockdown daemon (and then to all of the file access, sync,
+notification and backup services running on the device).
+
+The higher-level layers are handled by libimobiledevice. 'ifuse' is then able
+to sit on top of this and mount your device's AFC filesystem share.
+
+This package contains the usbmuxd communication interface library 'libusbmuxd'.
+
+There is also a Python implementation of the client library in the python-client
+directory, and an example tcprelay.py which performs a similar function to iproxy.
+This implementation supports OSX and Windows and the new iTunes plist-based
+usbmuxd protocol, so it is portable and will run on those operating systems with
+no modification, using Apple's native usbmuxd. This is useful if you need to
+tunnel to your device from another OS in a pinch. Run python tcpclient.py --help
+for usage information.
+
+License
+=======
+
+The contents of this package are licensed under the GNU Lesser General Public
+License, version 2.1 or, at your option, any later version (see COPYING.LGPLv2.1).
+If a more permissive license is specified at the top of a source file, it takes
+precedence over this.
+
+Legal
+=====
+
+Apple, iPhone, and iPod touch are trademarks of Apple Inc., registered in the
+U.S. and other countries.
+
+Building
+========
+
+  ./autogen.sh
+  make
+  sudo make install
+
+Running (with magic)
+====================
+
+  (Unplug + replug your jailbroken device)
+  ./iproxy 2222 22 &
+  ssh -p 2222 root@localhost
+
+Hopefully you get the normal SSH login prompt. You may still lots of debugging
+output for the moment. If this is getting in the way of your ssh login, then
+run the 'ssh' command from a different xterminal or virtual console. Of course,
+you need to have OpenSSH installed on your jailbroken device for this to work.
+
+If you have iFuse, you can run "ifuse <mountpoint">. This doesn't require
+iproxy and works on all devices, jailbroken or not.
+
+Running (without magic)
+=======================
+
+If 'udev' is _not_ automatically running on your machine and picking up the new
+.rules file, you will need to start usbmuxd by hand first. Check it's running
+and that there is only one copy with 'ps aux | grep
+usbmuxd'.
+
+  sudo usbmuxd -U -v -v &
+  ./iproxy 2222 22 &
+  ssh -p 2222 root@localhost
+
+Tip: Starting SSH if disabled
+=============================
+
+If your device is rooted, but SSH isn't started and you _cannot_ (for instance,
+cracked/broken screen) get to the Services control panel on the device, then you
+can start the SSH service over the USB by mounting the (jailbroken) filesystem.
+
+You will need to mount it using 'ifuse --afc2' (to access the root directory of
+the device), and then edit:
+
+  /Library/LaunchDaemons/com.openssh.sshd.plist
+
+to _remove_ the lines:
+
+  <key>Disabled</key>
+  <true/>
+
+Reboot the device and then sshd should be running.
+
+TODO
+====
+
+The server currently assumes that the device is well-behaved and does not do a
+bunch of checks like looking for the expected SEQ and ACK numbers from it. This
+is normally not an issue, but it's annoying for debugging because lost packets
+(which shouldn't happen, but can happen if the code is buggy) mean that stuff
+gets out of sync and then might crash and burn dozens of packets later.
+
+The server needs more testing, and some optimizing.
+
+Someone should probably do some edge-case testing on the TCP stuff.
+
+The outgoing ACK handling on the server probably needs some thought. Currently,
+when there's an outstanding ACK, we send it after a timeout (to avoid sending
+a no-payload ACK packet for everything the phone sends us). However, there's
+probably a better way of doing this.
+
+Architecture information
+========================
+
+The iPhone / iPod Touch basically implements a rather strange USB networking
+system that operates at a higher level. It is of course completely proprietary.
+Generally speaking, this is what goes on in a typical usage scenario:
+
+0. iTunes opens a connection to usbmuxd and asks it for device notifications
+1. User inserts phone into computer
+2. usbmuxd notices the phone and pings it with a version packet
+3. phone replies
+4. usbmuxd now considers the phone to be connected and tells iTunes
+5. iTunes opens another separate connection to usbmuxd and asks it to connect
+   to, say, the afc port on the device
+6. usbmuxd sends a pseudo-TCP SYN packet to the phone
+7. the phone's kernel driver receives the SYN packet and itself opens a
+   TCP connection to localhost on the afc port
+8. the phone replies with a pseudo-TCP SYN/ACK indicating that the port is open
+   and the connection can proceed
+7. usbmuxd sends a final ACK to the phone
+8. usbmuxd replies to iTunes with a "connection successful" message
+9. any data that iTunes writes to the usbmuxd socket from now on is forwarded,
+   through pseudo-TCP, through USB, back into a more regular TCP connection to
+   localhost, to the afc daemon on the phone, and vice versa
+
+The usbmuxd protocol is a relatively simple binary message protocol documented
+here:
+
+http://wikee.iphwn.org/usb:usbmux
+
+Note that once a connection is established the UNIX socket essentially becomes
+a dedicated pipe to the TCP connction and no more high-level control is
+possible (closing the socket closes the TCP connection). Ditto for the "listen
+for devices" mode - usbmuxd will reject any commands in such mode, and the
+socket essentially becomes a dedicated device notification pipe. This means
+that you need, at minimum, TWO connections to usbmuxd to do anything useful.
+
+On Windows, usbmuxd works the same way but a TCP connection to localhost port
+27015 replaces the UNIX socket. On OSX, the UNIX socket is /var/run/usbmuxd. The
+server and client implemented here default the same /var/run/usbmuxd socket.
+
+The phone protocol operates over a pair of USB bulk endpoints. There is an outer
+layer used for packet size info and a "protocol" (version and TCP are the only
+two options), and that header is followed by TCP headers for actual data comms.
+However, the protocol isn't actual TCP, just a custom protocol which for some
+reason uses a standard TCP header and leaves most fields unused.
+
+There is no reordering or retransmission. There are no checksums, no URG, no
+PSH, no non-ACK, no FIN. What there *is* is the SEQ/ACK/window mechanism used
+for flow control, and RST is used as the only connection teardown mechanism (and
+also for "connection refused"), and the connection startup is SYN/SYNACK/ACK.
+
+Windows are constant-scaled by 8 bits. This is legal TCP as long as the
+corresponding option is negotiated. Of course, no such negotiation happens on
+this protocol.
+
+Note that, since there are no retransmissions, there is some overlap between ACK
+and window for flow control. For example, the server doesn't ever touch its
+window size, and just refuses to ACK stuff if its buffers are full and the
+client isn't reading data. The phone happily seems to stop sending stuff.
+
+Also, if the phone RSTs you out of nowhere, look at the packet payload for a
+textual error message. Note: if it claims to suffer from amnesia, that probably
+means you overflowed its input buffer by ignoring its flow control / window
+size. Go figure. Probably a logic bug in the kernel code.
+
+Note that all normal packets need to have flags set to ACK (and only ACK). There
+is no support for, erm, not-acking. Keep the ack number field valid at all
+times.
+
+The usbmuxd CONNECT request port field is byte-swapped (network-endian). This is
+even more annoying for the plist based protocol, since it's even true there
+(where the field is plain text). So even for the plain text int, you need to
+swap the bytes (port 22 becomes <integer>5632</integer>). I have no clue if this
+is the case on the new plist protocol on PPC macs (is the newer iTunes available
+for those?)
+
+There are a bunch of gotchas due to the USB framing, and this is even worse
+because implementations tend to get it wrong (i.e. libusb, and this is the
+reason for the patch). Basically, USB Bulk offers, at the low level, the ability
+to transfer packets from 0 to wMaxPacketSize (512 here) bytes, period. There is
+no other support for higher level framing of transfers. The way you do those is
+by breaking them up into packets, and the final shorter packet marks the end of
+the transfer. The critical bit is that, if the transfer happens to be divisible
+by 512, you send a zero-length packet (ZLP) to indicate the end of the transfer.
+Libusb doesn't set this option by default and the iPhone gets packets stuck to
+each other, which it doesn't like. Actually, this framing is sort of redundant
+because the usbmux packet header includes a length field, but the phone still
+wants the ZLPs or else it breaks. To make matters worse, usbdevfs imposes a max
+transfer size of 16k, so libusb breaks transfers into that size. This is okay
+for sending as long as the ZLP is only added to the last transfer (the patch
+does that), but it can easily cause nasty race conditions on RX due to libusb
+doing multiple outstanding reads at the same time and then cancelling the rest
+when shorter data arrives (but what if some data got into the other requests
+already?), so we only do 16k reads and stick them together ourselves by looking
+at the packet size header. We still depend on ZLPs being sent to end transfers
+at non-16k boundaries that are multiples of 512, but that seems to work fine. I
+guess the ZLPs might cause spurious 0-byte transfers to show up on RX if things
+line up right, but we ignore those. By the way, the maximum packet/transfer size
+is 65535 bytes due to the 16-bit length header of the usbmux protocol.