p2p is a software package for advertising and discovering content on the local network (LAN). It is built on top of HTTP and mDNS/DNS-SD.
p2p is written specifically for Chrome OS and uses services and conventions specific to that OS. It is not intended to be portable to other OSes.
Content to be shared with other devices on the LAN is to stored in an encrypted part of the stateful partition. The following directory is used:
In the following this directory shall be referred to as
Files to be shared are advertised via a DNS-SD service with the service type
The address and port number advertised in the DNS-SD service refer to a HTTP server running on the same machine. To simplify firewall management, port 16725 (“AU”) is always used but in the future this may by dynamic.
Any file with a
.p2p file extension in
$(P2P_DIR) is advertised as a TXT record in a DNS-SD response where its value is the length of the file (decimal encoding). For example, if the file
$(P2P_DIR)/some_payload.bin.p2p exists and is 123456789 bytes long, the DNS-SD response will advertise a TXT record
id_ prefix is there just for namespacing purposes). By convention, this allows any client to download the file using the URL
PORT is taken from the DNS-SD response. Additionally, to allow sharing files that are not completely downloaded yet, one can set the
user.cros-p2p-filesize extended attribute with the final size. This will make the HTTP server block until content is available. Note that the mDNS/DNS-SD response always returns the size of the file on disk - this is to allow a peer to pick the peer with e.g. the most downloaded bytes.
Note that the size set in the
user.cros-p2p-filesize extended attribute - if present - is always constant (because it contains the final size of the file). In contrast, the size conveyed via the mDNS/DNS-SD response reflects the actual size of the file which grows as more and more data is being downloaded. As such, this value is not necessarily constant and in fact, by virtue of how mDNS/DNS-SD works, changes to the value are propagated on the local network. To conserve bandwidth with frequently changing files, file size changes are propagated at most every ten seconds.
$(P2P_DIR) directory is the sole interface for other software on the local system to share files with other peers. For example, if the
update_engine program (used for updating Chrome OS) is downloading an payload it can create a file, say
$(P2P_DIR)/some_update_xyz.bin.p2p.tmp and start writing to it as it downloads the rest of the payload. When the
update_engine program has verified that the file is authentic (by e.g. checking a cryptographic signature in the beginning of the file) and it knows the final size, it can
user.cros-p2p-filesizexattr to the size
and, hey presto, the file some_
update_xyz.bin is now shared with the rest of the local network.
In addition to advertising files, the
_cros_p2p._tcp DNS-SD service will also advertise the current number of HTTP connections in the num-connections TXT attribute. This can used (in a cooperative manner) to limit the number of simultaneous downloads in the LAN.
p2p package is comprised of three programs.
The primary purpose of this program is to monitor the
$(P2P_DIR) directory and advertise
.p2p files via mDNS/DNS-SD (it uses the Avahi package to do this). When there is a non-zero number of .p2p files, it will start the
p2p-http-server program and when the number of
.p2p files drop to zero it will terminate kill the instance.
This program is run as a daemon (long-running process) and is usually started via an Upstart job,
p2p, to ensure the firewall is properly configured (specifically, opening the TCP port that the HTTP server will listen on), its dependencies (e.g. Avahi) has started and that the program is launched with appropriate privileges (using minijail0 to drop privileges).
The purpose of this program is to serve
.p2p files via HTTP. It is started and stopped by the
p2p-server server, when needed.
This is a simple program to discover content available on the LAN. Given a file identifier, it looks on the LAN for
_cros_p2p._tcp DNS-SD services. If one or more peers have the file,
p2p-client picks one of them and prints the URL on stdout.
p2pUser and Group
p2p user (and
p2p group) is used to run
p2p-http-server without root privileges.
$(P2P_DIR) is owned by root and its permissions are
rwxr-xr-x, that is
In the future a more sophisticated ACL scheme may be introduced to allow known unprivileged programs (say,
update_engine) to share content.
(TODO: write me)