Round-trip estimation

1. A method performed by a first network device, the first network device being configured to transmit a data stream to a second network device over a network at a bit rate, the method comprising:

transmitting a series of first packets to the second network device over the network, each of the first packets having an associated transmission time and comprising a unique identification value, wherein the series of first packets comprises a first Real-time Transport Control Protocol, RTCP, packet and one or more Real-time Transport Protocol, RTP, packets, and wherein the unique identification value of each of the one or more RTP packets is a sequence number,

receiving, from the second network device, a second packet, the second packet indicating receipt of at least one of the first RTCP packet of the series of first packets by the second network device, the second packet having a receipt time T 4 at the first network device, wherein the second packet is a second RTCP packet further comprising a highest sequence number of an RTP packet of the series of first packets received at the second network device,

determining a standard round-trip time in dependence on at least:

a transmission time T 1 of the first RTCP packet of the series of first packets, and

a receipt time T 2 of the first RTCP packet of the series of first packets at the second network device,

the transmission time T 3 of the second RTCP packet from the second network device, and

the receipt time T 4 of the second RTCP packet, and calculated as T 4 −(T 3 −T 2 )−T 1 ,

identify that for a first packet having a transmission time T 1a >T 1 no indication of receipt has been received at T 4 in the second RTCP packet,

during a period after the receipt time T 4 of the second RTCP packet until before receipt of a next second RTCP packet:

determining a current round-trip time in dependence on:

a transmission time T 1a of an oldest RTP packet of the series of first packets for which no indication of receipt has been received at the first network device, where T 1 <T 1a <T 4 , and

the receipt time T 4 of the second RTCP packet, and

wherein the current round-trip time is calculated as T 4 −T 1a ,

determining whether an unused network bandwidth between the first network device and the second network device exists in dependence on the current round-trip time and standard round-trip time, wherein no unused network bandwidth exists when the standard round-trip time exceeds a first threshold and/or the current round-trip time exceeds a second threshold, and

updating the bit rate for the transmitted data stream in dependence on the existence of the unused network bandwidth.

2. The method of claim 1 , wherein the second threshold is greater than the first threshold.

3. The method of claim 1 , wherein updating the bit rate for the transmitted data stream in dependence on the existence of the unused network bandwidth comprises:

decreasing the bit rate for the data stream when the unused network bandwidth does not exist.

4. The method of claim 1 , wherein updating the bit rate for the transmitted data stream in dependence on the existence of the unused network bandwidth comprises:

increasing the bit rate for the data stream when the unused network bandwidth exists.

5. The method of claim 1 , wherein updating the bit rate for the transmitted data stream in dependence on the existence of the unused network bandwidth comprises:

decreasing the bit rate of the data stream when the standard round-trip time and/or the current round-trip time is increased.

6. The method of claim 1 , wherein the data stream comprises at least one of a video stream and audio stream.

7. The method of claim 6 , wherein updating the bit rate for the transmitted data stream comprises updating at least one of a target bit rate, average bit rate, resolution, colour depth, frame rate, sampling frequency, bit depth, and channel count.

8. The method of claim 1 , further comprising:

determining a used network bandwidth between the first network device and the second network device based on a total size of data packets transmitted between the first network device and the second network device per second;

determining a network throughput value in dependence on the used network bandwidth, the network throughput value being an amount of data the first network device is able to deliver to the second network device per second;

determining a total size of data packets buffered in the network based on a total size of data packets transmitted by the first network device and on a total size of data packets delivered to the second network device;

determining a reserved bandwidth required to deliver data packets buffered in the network to the second network device within a time interval reasonable to empty buffers in the network;

determining a remaining bandwidth for the data stream based on the network throughput value and the reserved bandwidth, and possibly based on a bandwidth used by an additional data stream; and

updating the bit rate for the data stream in dependence of the determined remaining bandwidth for the data stream.

9. A first network device configured to transmit a data stream to a second network device over a network at a bit rate, the first network device further comprising a processor and a memory and being configured to:

transmit a series of first packets to the second network device over the network, each of the first packets having an associated transmission time and comprising a unique identification value, wherein the series of first packets comprises a first Real-time Transport Control Protocol, RTCP, packet and one or more Real-time Transport Protocol, RTP, packets, and wherein the unique identification value of each of the one or more RTP packets is a sequence number,

receive, from the second network device, a second packet, the second packet indicating receipt of a last one of the first RTCP packet of the series of first packets by the second network device, the second packet having a receipt time T 4 at the first network device, wherein the second packet is a second RTCP packet further comprising a highest sequence number of an RTP packet of the series of first packets received at the second network device,

determine a standard round-trip time in dependence on at least:

a transmission time T 1 of the first RTCP packet of the series of first packets, and

a receipt time T 2 of the first RTCP packet of the series of first packets at the second network device,

the transmission time T 3 of the second RTCP packet from the second network device, and

the receipt time T 4 of the second RTCP packet, and

calculated as T 4 −(T 3 −T 2 )−T 1 ,

identify that for a first packet having a transmission time T 1a >T 1 no indication of receipt has been received at T 4 in the second RTCP packet,

during a period after the receipt time T 4 of the second RTCP packet until before receipt of a next second RTCP packet:

determine a current round-trip time in dependence on:

a transmission time T 1a of an oldest RTP packet of the series of first packets for which no indication of receipt has been received at the first network device, where T 1 <T 1a <T 4 , and

the receipt time T 4 of the second RTCP, and

wherein the current round-trip time is calculated as T 4 −T 1a ,

determine whether an unused network bandwidth between the first network device and the second network device exists in dependence on the standard round-trip time and current round-trip time, wherein no unused network bandwidth exists when the standard round-trip time exceeds a first threshold and/or the current round-trip time exceeds a second threshold, and

update the bit rate for the transmitted data stream in dependence on the existence of the unused network bandwidth.