In the live streaming there are many things happening between the camera's lens and the screen of the video player, all of which contribute to a delay that is generally referred to as "latency". This overall latency includes the time it takes for the camera frame grabber device to pass frames to the encoder, encoding, multiplexing, sending over the network, splitting, decoding and then finally displaying.
In SRT, however, "latency" is defined as only the delay introduced by sending
over the network. It's the time between the moment when the srt_sendmsg2
function is called at the sender side up to the moment when the srt_recvmsg2
function is called at the receiver side. This SRT latency is the actual time difference
between these two events.
SRT employs a TimeStamp Based Packet Delivery (TSBPD) mechanism with strict goal of keeping the time interval between two consecutive packets on the receiver side identical to what they were at the sender side. This requires introducing an extra delay that should define when exactly the packet can be retrieved by the receiver application -- if the packet arrives early, it must wait in the receiver buffer until the delivery time. This time for a packet N is roughly defined as:
PTS[N] = ETS[N] + LATENCY(option)
where ETS[N] is the time when the packet would arrive, if all delays
from the network and the processing software on both sides are identical
to what they were for the very first received data packet. This means that
for the very first packet ETS[0] is equal to this packet's arrival time.
For every following packet the delivery time interval should be equal to the
that packet's declared scheduling time interval.
SRT provides two socket options SRTO_PEERLATENCY and SRTO_RCVLATENCY.
While they have "latency" in their names, they do not define the true time
interval between the srt_sendmsg2 and srt_recvmsg2 calls for the same
packet. They are only used to add an extra delay (at the receiver side) to
the time when the packet "should" arrive (ETS). This extra delay is used to
compensate for two things:
-
an extra network delay (that is, if the packet arrived later than it "should have arrived"), or
-
a packet retransmission.
Note that many of the values included in these formulas are not controllable and some cannot be measured directly. In many cases there are measured values that are sums of other values, but the component values can't be extracted.
There are two values that we can obtain at the receiver side:
-
ATS: actual arrival time, which is the time when the UDP packet has been extracted through the
recvmsgsystem call. -
TS: time recorded in the packet header, set on the sender side and extracted from the packet at the receiver side
Note that the timestamp in the packet's header is 32-bit, which gives
it more or less 2.5 minutes to roll over. Therefore timestamp
rollover is tracked and a segment increase is performed in order to keep an
eye on the overall actual time. For the needs of the formula definitions
it must be stated that TS is the true difference between the connection
start time and the time when the sending time has been declared when
the sender application is calling any of the srt_send* functions
(see srt_sendmsg2 for details).
To understand the latency components we need also other definitions:
-
ETS (Expected Time Stamp): The packet's expected arrival time, when it "should" arrive according to its timestamp
-
PTS (Presentation Time Stamp): The packet's play time, when SRT gives the packet to the
srt_recvmsg2call (that is, it sets up the IN flag in epoll and resumes the blocked function call, if it was in blocking mode). -
STS (Sender Time Stamp): The time when the packet was scheduled for sending at the sender side (if you don't use the declared time, by default it's the monotonic time used when this function is called).
-
RTS (Receiver Time Stamp): The same as STS, but calculated at the receiver side. The only way to extract it is by using some initial statements.
The "true latency" for a particular packet in SRT can be simply defined as:
TD = PTS - STS
Note that this is a stable definition (independent of the packet),
but this value is not really controllable. So let's define the PTS
for a packet x:
PTS[x] = ETS[x] + LATENCY + DRIFT
where LATENCY is the negotiated latency value (out of the
SRTO_RCVLATENCY on the agent and SRTO_PEERLATENCY on the peer)
and DRIFT will be described later (for simplification you can
state it's initially 0).
These components undergo the following formula:
ETS[x] = start_time + TS[x]
Note that it's not possible to simply define a "true" latency based on STS because the sender and receiver are two different machines that can only see one another through the network. Their clocks are separate, and can even run at different or changing speeds, and the only visible phenomena happen when packets arrive at the receiver machine. However, the formula above does allow us to define the start time because we state the following for the very first data packet:
ETS[0] = ATS[0]
This means that from this formula we can define the start time:
start_time = ATS[0] - TS[0]
Therefore we can state that if we have two identical clocks on both machines with identical time bases and speeds, then:
ATS[x] = program_delay[x] + network_delay[x] + STS[x]
Note that two machines communicating over a network do not typically have a common clock base. Therefore, although this formula is correct, it involves components that can neither be measured nor captured at the receiver side.
This formula for ATS doesn't apply to the real latency, which is based strictly
on ETS. But you can apply this formula for the very first arriving packet,
because in this case they are equal: ATS[0] = ETS[0].
Therefore this formula is true for the very first packet:
ETS[0] = prg_delay[0] + net_delay[0] + STS[0]
We know also that the TS set on the sender side is:
TS[x] = STS[x] - snd_connect_time
Taking both formulas for ETS together:
ETS[x] = start_time + TS[x] = prg_delay[0] + net_delay[0] + snd_connect_time + TS[x]
we have then:
start_time = prg_delay[0] + net_delay[0] + snd_connect_time
IMPORTANT: start_time is not the time of arrival of the first packet,
but that time taken backwards by using the delay already recorded in TS. As TS should
represent the delay towards snd_connect_time, start_time should be simply the same
as snd_connect_time, just on the receiver side, and so shifted by the
first packet's delays of prg_delay and net_delay.
So, as we have the start time defined, the above formulas:
ETS[x] = start_time + TS[x]PTS[x] = ETS[x] + LATENCY + DRIFT
now define the packet delivery time as:
PTS[x] = start_time + TS[x] + LATENCY + DRIFT
and after replacing the start time we have:
PTS[x] = prg_delay[0] + net_delay[0] + snd_connect_time + TS[x] + LATENCY + DRIFT
and from the TS formula we get STS, so we replace it:
PTS[x] = prg_delay[0] + net_delay[0] + STS[x] + LATENCY + DRIFT
We can now get the true network latency in SRT by moving STS to the other side:
PTS[x] - STS[x] = prg_delay[0] + net_delay[0] + LATENCY + DRIFT
The DRIFT is a measure of the variance over time of the base time. To simplify the calculations above, DRIFT is considered to be 0, which is the initial state. In time, however, it changes based on the value of the Arrival Time Deviation:
ATD[x] = ATS[x] - ETS[x]
The drift is then calculated as:
DRIFT[x] = average(ATD[x-N] ... ATD[x])
The value of the drift is tracked over an appropriate number of samples. If
it exceeds a threshold value, the drift value is applied to modify the
base time. However, as you can see from the formula for ATD, the drift is
simply taken from the actual time when the packet arrived, and the time
when it would have arrived if the prg_delay and net_delay values were
exactly the same as for the very first packet. ATD then represents the
changes in these values. There can be two main factors that could result
in having this value as non-zero:
-
A phenomenon has been observed in several types of networks where the very first packet arrives quickly, but as subsequent data packets come in regularly, the network delay slightly increases and then remains fixed for a long time at this increased value. This phenomenon can be mitigated by having a reliable value for RTT. Once the increase is observed a special factor could be applied to decrease the positive value of the drift. This isn't currently implemented. This phenomenon also isn't observed in every network, especially those covering longer distances.
-
The clock speed on both machines (sender and receiver) isn't exactly the same, which means that if you decipher the ETS basing on the TS, over time it may result in values that even precede the STS (suggesting a negative network delay) or that have an enormous delay (with ATS exceeding PTS). This is actually the main reason for tracking the drift.