| Network Working Group | M.P.H. Petit-Huguenin |
| Internet-Draft | Stonyfish, Inc. |
| Updates: 3550 (if approved) | February 15, 2011 |
| Intended status: Standards Track | |
| Expires: August 19, 2011 |
Support for multiple clock rates in an RTP session
draft-petithuguenin-avtext-multiple-clock-rates-00
This document clarifies the RTP specification when different clock rates are used in an RTP session. It also provides guidance on how to interoperate with legacy RTP implementations that use multiple clock rates.
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The clock rate is a parameter of the payload format. It is often defined as been the same as the sampling rate but it is not always the case (see e.g. the G722 and MPA audio codecs in [RFC3551]).
An RTP sender can switch between different payloads during the lifetime of an RTP session and because clock rates are defined by payload types, it is possible that the clock rate also varies during an RTP session. RTP [RFC3550] lists using multiple clock rates as one of the reasons to not use different payloads on the same SSRC but unfortunately this advice was not always followed and some RTP implementations change the payload in the same SSRC even if the different payloads use different clock rates.
This creates three problems:
Table 1 contains a non-exhaustive list of fields in RTCP packets that uses a clock rate as unit:
| Field name | RTCP packet type | Reference |
|---|---|---|
| RTP timestamp | SR | [RFC3550] |
| Interarrival jitter | RR | [RFC3550] |
| min_jitter | XR Summary Block | [RFC3611] |
| max_jitter | XR Summary Block | [RFC3611] |
| mean_jitter | XR Summary Block | [RFC3611] |
| dev_jitter | XR Summary Block | [RFC3611] |
| Interarrival jitter | IJ | [RFC5450] |
| RTP timestamp | SMPTETC | [RFC5484] |
| Jitter | RSI Jitter Block | [RFC5760] |
| Median jitter | RSI Stats Block | [RFC5760] |
This document changes the RTP specification by recommending to use a different SSRC for each clock rate in most of the cases.
The following sections describe the various ways legacy RTP implementations behave when multiple clock rates are used. Legacy RTP refers to RFC 3550 without the modifications introduced by this document.
One way of managing multiple clock rates is to use a different SSRC for each different clock rate, as in this case there is no ambiguity on the clock rate used by fields in the RTCP packets. This method also seems to be the original intent of RTP as can be deduced from points 2 and 3 of section 5.2 of RFC 3550.
On the other hand changing the SSRC can be a problem for some implementations designed to work only with unicast IP addresses, where having multiple SSRCs is considered a corner case. Lip synchronization can also be a problem in the interval between the beginning of the new stream and the first RTCP SR packet. This is not different than what happen at the beginning of the RTP session but it can be more annoying for the end-user.
The simplest way of managing multiple clock rates is to use the same SSRC for all the payload types regardless of the clock rates.
Unfortunately there is no clear definition on how the RTP timestamp should be calculated in this case. The following subsections present two variants.
The most common method of calculating the RTP timestamp ensures that the value increases monotonically. The formula used by this method is as follow:
timestamp = previous_timestamp + (current_capture_time - previous_capture_time) * current_clock_rate
The problem with this method is that the jitter calculation on the receiving side gives invalid result during the transition between two clock rates, as shown in Table 2. The capture and arrival time are in seconds, starting at the beginning of the capture of the first packet; clock rate is in Hz; the RTP timestamp does not include the random offset; the transit, jitter, and average jitter use the clock rate as unit.
| Capt. time | Clock rate | RTP timestamp | Arrival time | Transit | Jitter | Average jitter |
|---|---|---|---|---|---|---|
| 0 | 8000 | 0 | 0.1 | 800 | ||
| 0.02 | 8000 | 160 | 0.12 | 800 | 0 | 0 |
| 0.04 | 8000 | 320 | 0.14 | 800 | 0 | 0 |
| 0.06 | 8000 | 480 | 0.16 | 800 | 0 | 0 |
| 0.08 | 16000 | 800 | 0.18 | 2080 | 480 | 30 |
| 0.1 | 16000 | 1120 | 0.2 | 2080 | 0 | 28 |
| 0.12 | 16000 | 1440 | 0.22 | 2080 | 0 | 26 |
| 0.14 | 8000 | 1600 | 0.24 | 320 | 720 | 70 |
| 0.16 | 8000 | 1760 | 0.26 | 320 | 0 | 65 |
Calculating the correct transit time on the receiving side can be done by using the following formulas:
The main problem with this method, in addition to the fact that the jitter calculation described in RFC 3550 cannot be used, is that is it dependent on the previous RTP packets, packets that can be reordered or lost in the network. But it seems that this is what most implementations are using.
An alternate way of generating the RTP timestamps is to use the following formula:
timestamp = capture_time * clock_rate
With this formula, the jitter calculation is correct but the RTP timestamp values are no longer increasing monotonically as shown in Table 3. RFC 3550 states that "[t]he sampling instant MUST be derived from a clock that increments monotonically[...]" but nowhere says that the RTP timestamp must increment monotonically.
| Capt. time | Clock rate | RTP timestamp | Arrival time | Transit | Jitter | Average jitter |
|---|---|---|---|---|---|---|
| 0 | 8000 | 0 | 0.1 | 800 | ||
| 0.02 | 8000 | 160 | 0.12 | 800 | 0 | 0 |
| 0.04 | 8000 | 320 | 0.14 | 800 | 0 | 0 |
| 0.06 | 8000 | 480 | 0.16 | 800 | 0 | 0 |
| 0.08 | 16000 | 1280 | 0.18 | 1600 | 0 | 0 |
| 0.1 | 16000 | 1600 | 0.2 | 1600 | 0 | 0 |
| 0.12 | 16000 | 1920 | 0.22 | 1600 | 0 | 0 |
| 0.14 | 16000 | 2240 | 0.24 | 1600 | 0 | 0 |
| 0.16 | 16000 | 2560 | 0.26 | 1600 | 0 | 0 |
| 0.14 | 8000 | 1120 | 0.24 | 800 | 0 | 0 |
| 0.16 | 8000 | 1280 | 0.26 | 800 | 0 | 0 |
The advantage with this method is that it works with the jitter calculation described in RFC 3550, a long as the correct clock rates are used.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].
An RTP Sender with RTCP turned off (i.e. by setting the RS and RR bandwidth modifiers defined in [RFC3556] to 0) SHOULD use a different SSRC for each different clock rate but MAY use different clock rates on the same SSRC as long as the RTP timestamp is calculated as described in Section 2.2.2.
An RTP Sender with RTCP turned on MUST use a different SSRC for each different clock rate. An RTCP BYE MUST be sent and a new SSRC MUST be used if the clock rate switches back to a value already seen in the RTP stream.
To accelerate lip synchronization, the next compound RTCP packet sent by the RTP sender MUST contain multiple SR packets, the first one containing the mapping for the current clock rate and the next SR packets containing the mapping for the other clock rates seen during the last period.
The RTP extension defined in [RFC6051] MAY be used to accelerate the synchronization.
An RTP Receiver MUST be able to handle clock rate changes either on the same SSRC (Section 2.1) or on different SSRC (Section 2.2.2).
An RTP Receiver MUST be able to handle a compound RTCP packet with multiple SR packets.
For interoperability with legacy RTP implementations, an RTP receiver MAY use the information in two consecutive SR packets to calculate the clock rate used, i.e. if Ni is the NTP timestamp for the SR packet i, Ri the RTP timestamp for the SR packet i and Nj and Rj the NTP timestamp and RTP timestamp for the previous SR packet j, then the clock rate can be guessed as the closest to (Ri - Rj) / (Ni - Nj).
The next subsections analyze the various combinations between legacy RTP implementations and RTP implementations that follow this document specifications.
Because a specific clock rate is associated to a specific SSRC, there is no ambiguity in the RTP timestamp received in the RTP packet or SR packet or in the jitter sent in the RR packet.
The new RTP Receiver will not be able to rebuild the correct RTP timestamp so the jitter will be incorrect. Note that this is not different than if a legacy RTP Receiver is used.
TBD
Because a specific clock rate is associated to a specific SSRC, there is no ambiguity in the RTP timestamp received in the RTP packet or SR packet or in the jitter sent in the RR packet. Some legacy RTP implementations may have problems when receiving multiple SR packets.
Because a specific clock rate is associated to a specific SSRC, there is no ambiguity in the RTP timestamp received in the RTP packet or SR packet or in the jitter sent in the RR packet.
Because this combination is used only when no RTCP packets are exchanged, there is no problem interpreting the RTCP field units. Some legacy RTP implementations may have problems if the jitter clock rates are not correctly managed.
Because this combination is used only when no RTCP packets are exchanged, there is no problem interpreting the RTCP field units.
TBD
No IANA considerations.
Thanks to Colin Perkins and Ali C. Begen for their comments, suggestions and questions that helped to improve this document.
Thanks to Robert Sparks and the attendants of SIPit 26 for the survey on multiple clock rates interoperability.
This document was written with the xml2rfc tool described in [RFC2629].
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. |
| [RFC3550] | Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, July 2003. |
An alternate way of fixing the multiple clock rates issue was proposed in [uRTR]. This document proposed to define a unified clock rate, but the proposal was rejected at IETF 61.
This section must be removed before publication as an RFC.