Peer

Tiny project demonstrating reliable group communication Prerequisites:

sudo apt install -Y g++ cmake make gcovr

Build

Build the Peer release binary as Peer/release/Peer:

cd Peer
mkdir release
cd release
cmake -DCMAKE_BUILD_TYPE=Release ..
make -j

Execute

Usage: ../../build/Peer [-i <peerId>] [-a <ipaddr>] [-p <udpPort>] [-c <configFile>] [-l <logFile>] [-e <errorInject>]
   <peerId>        unique peer id in the range [0..65534], default is 1.
   <ipaddr>        local IPV4 address, default is 127.0.0.1.
   <udpPort>       local udp port in the range [1025..65534], default is 4201.
   <configFile>    path to an already existing configuration file, default is ./peer.cfg.
   <logFile>       path to log file. If none is provided stdout/stderr is used.
   <errorInject>   string of format <peer id>:<msg seq#>:<bit offset> to inject a bit error on the given offset in the specified message of the given peer.

Peer/peer.cfg contains example configuration data. After the Peer process started, it creates a named pipe, e.g. /tmp/peer_pipe_<peerId> and listens for user commands, e.g.

echo send foobar >/tmp/peer_pipe_1
echo inject 1:10:33 >/tmp/peer_pipe_1 # injects bit flip on msg 10 of peer 1 at bit offset 33
echo stop >/tmp/peer_pipe_1

The "stop" command terminates the Peer process and removes the named pipe.

Test and Coverage

Build the Peer test binary in Peer/debug/Peer:

cd Peer
mkdir debug
cd debug
cmake -DCMAKE_BUILD_TYPE=Debug ..
make -j

Execute Tests

Either ctest --output-on-failure or ./PeerTest.

Generate Coverage Report

make coverage

generates a Peer/debug/coverage.html indicating covered/not covered parts of the code.

Project Assumptions and Decisions

IP Version Support

Only IPV4 is supported

Udp Packet Format

We use:

Message Datagram:

• 2 Bytes Peer-Id (Network Byte Order)
• 2 Bytes Sequence Number (Network Byte Order)
• 1..n Bytes Message
• 2 Bytes RFC 1071 (Network Byte Order)

Messages may contain arbitrary byte sequences

ACK Datagram:

• 2 Bytes Peer-Id (Network Byte Order)
• 2 Bytes Sequence Number (Network Byte Order)
• 2 Bytes RFC 1071 (Network Byte Order)

Accepted Sequence Numbers

Anything in the range [expectedSeqNr, expectedSeqNr + 9], including wrap-around at 2^16-1.

Timeouts/Repeats

• One second timeout w/o ACK before resending the frame
• At most 4 transmissions, after 4rd transmission timeout, we give up

Message Resends

We assume for resending that the original MessageId [Peer-Id, Seq#] is used, otherwise, peers cannot distinguish if the Message is a new one, or a resent one.