RUDP API & implementation details

I committed the latest version of RUDP. After many changes in design and implementation I’ve got something that I’m satisfied with.

The API is very simple – packets can be sent and received on 32 individual channels:

bool send(uint8_t channel_idx, uint8_t const* data, size_t size);
bool try_sending(uint8_t channel_idx, uint8_t const* data, size_t size);

try_sending is like send but fails if the previous send on the same channel didn’t finish yet. This can happen with the video frames which take some time to compress when a big keyframe arrives and there’s a context switch in the middle of the send. Since the camera data callback is async, it might try to send again too fast – so it uses try_sending to avoid getting blocked in the send call.

bool receive(uint8_t channel_idx, std::vector<uint8_t>& data);

This call will fill the data vector with a packet from channel idx and return true if succeeds.

Each of these channels has a few parameters that control the trade offs between bandwidth, latency, reliability and speed.

The most important parameter is the one that controls delivery confirmations: Send_Params::is_reliable. Reliable packets keep getting resent until the other end confirms or cancels them – or until Send_Params::cancel_after time runs out. Silkopter uses reliable packets for the remote control data,  calibration data and pid params (the silk::Comm_Message enum).

Unreliable packets are sent only once even if they get lost on the way. They are used for telemetry and the video stream since this data gets old very fast – 33ms for both video and telemetry.
A useful parameter for unreliable packets is Send_Params::cancel_on_new_data. When true, new data cancels all existing unsent data from the same channel. This is very useful for low bandwidth when video frames can take longer than 33ms to send. Another parameter – at the receiving end this time is Receive_Params::max_receive_time which indicates how long to wait for a packet. Useful for the video stream in case frame X is not ready but frame X+1 is already available. In case a packet is skipped due to this parameter, a cancel request is sent to the other end to indicate that the receiver is not interested in this data anymore. This saves quite some bandwidth.

Zlib compression can be set for each channel – and it’s on for all channels in silkopter – including the video stream where it saves between 3 and 10% of the frame size at a ~10% CPU cost.

 

Internally, packets are split in fragments of MTU size (currently 1KB). Each fragment is identified by an ID + Fragment IDX – so fragments from the same  packet share the ID.

The first fragment has a different header then the rest.

Fragments are sent as datagrams, same as pings, confirmations and cancel requests.
A datagram has a small header (5 bytes) containing the crc of all the data and the type of the datagrams. Based on the type the header can be casted to a specialized header.
The crc is actually a murmur hash of the datagram data and I’m not sure it’s really needed as UDP has it’s own crc but  better be safe than sorry. It’s very fast anyway and doesn’t even show up in the profiler.

The datagrams are managed by a pool using intrusive pointers to avoid allocations of the datagram data (a std::vector) or the ref_count (in case of using std::shared_ptr).

My test so far was this:
With both the silkopter and the GS far from the access point, I’m sending a video stream of enough bandwidth to choke the wifi. This is ~400KB/s in my current test scenario. Then I’m pushing data through a reliable channel at ~10-20 packets per second amounting to 6KB/s. So far all the data got through in less than 2-300ms which is 2-3x my max RTT. Pretty happy with the result considering that in my previous setup – TCP for reliable data + UDP for video I was getting 2-3 seconds lag even next to the access point is some worst cases.

 

The only thing missing are handling the case when one end restarts. This is problematic so far because RUDP keeps track of the last valid received packet ID and ignores packets with IDs smaller than this. So when one of the ends restarts – all its packets are ignored uptil it reaches the ID of the last packet before the restart… Not good.

 

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2 thoughts on “RUDP API & implementation details

  1. It’s good to have a second CRC for network packets. I’ve heard it’s possible to have enough errors so that the CRC matches the original one. So using a better hash function is a big gain over a weak wifi link.

    For the IDs issue, you could send some beacons to tell the other side at which ID it should restart, if it wants to resume a previous session.

    Nice results 🙂

    1. Well the UDP checksum is optional it seems so I’ll keep the murmur hash.
      The beacons idea is good but it implies that the beacons have to be confirmed by the other end. Once beacons are sent, all packages have to be ignored until beacon confirmation comes.
      Thanks!

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