| Audio/Video Transport Core Maintenance | A.M. Williams |
| Internet-Draft | Audinate |
| Intended status: Standards Track | K. Gross |
| Expires: January 02, 2013 | AVA Networks |
| R. van Brandenburg | |
| H.M. Stokking | |
| TNO | |
| July 3, 2012 |
RTP Clock Source Signalling
draft-ietf-avtcore-clksrc-00
NTP timestamps are used by several RTP protocols for synchronisation and statistical measurement. This memo specifies SDP signalling identifying NTP timestamp clock sources and SDP signalling identifying the media clock sources in a multimedia session.
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 RFC 2119 [RFC2119].
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 02, 2013.
Copyright (c) 2012 IETF Trust and the persons identified as the document authors. All rights reserved.
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RTP protocols use NTP format timestamps to facilitate media stream synchronisation and for providing estimates of round trip time (RTT) and other statistical parameters.
Information about media clock timing exchanged in NTP format timestamps may come from a clock which is synchronised to a global time reference, but this cannot be assumed nor is there a standardised mechanism available to indicate that timestamps are derived from a common reference clock. Therefore, RTP implementations typically assume that NTP timestamps are taken using unsynchronised clocks and must compensate for absolute time differences and rate differences. Without a shared reference clock, RTP can time align flows from the same source at a given receiver using relative timing, however tight synchronisation between two or more different receivers (possibly with different network paths) or between two or more senders is not possible.
High performance AV systems often use a reference media clock distributed to all devices in the system. The reference media clock is often distinct from the reference clock used to provide timestamps. A reference media clock may be provided along with an audio or video signal interface, or via a dedicated clock signal (e.g. genlock or audio word clock). If sending and receiving media clocks are known to be synchronised to a common reference clock, performance can improved by minimising buffering and avoiding rate conversion.
This specification defines SDP signalling of timestamp clock sources and media reference clock sources.
Timestamp clock source and reference media clock signalling benefit applications requiring synchronised media capture or playout and low latency operation.
Examples include, but are not limited to:
The definitions of streams, sources and levels of information in SDP descriptions follow the definitions found in Source-Specific Media Attributes in the Session Description Protocol (SDP) [RFC5576].
The NTP timestamps used by RTP are taken by reading a local real-time clock at the sender or receiver. This local clock may be synchronised to another clock (time source) by some means or it may be unsynchronised. A variety of methods are available to synchronise local clocks to a reference time source, including network time protocols (e.g. NTP [RFC5905]) and radio clocks like GPS [IS-GPS-200F].
The following sections describe and define SDP signalling, indicating whether and how the local timestamping clock in an RTP sender/receiver is synchronised to a reference clock.
Two or more local clocks that are sufficiently synchronised will produce timestamps for a given RTP event can be used as if they cam from the same clock. Providing they are sufficiently synchronised, timestamps produced in one RTP sender/receiver can be directly compared to a local clock in another RTP sender/receiver. The timestamps produced by synchronized local clocks in two or more RTP senders/receivers can be directly compared.
The accuracy of synchronization required is application dependent. See Applications [applications] section for a discussion of applications and their corresponding requirements. To serve as a reference clock, clocks must minimally be syntonized (exactly frequency matched) to one another.
Sufficient synchronization can typically be achieving by using a network time protocol (e.g. NTP, 802.1AS, IEEE 1588-2008) to synchronize all devices to a single master clock.
Another approach is to use clocks providing a global time reference (e.g. GPS, Galileo). This concept may be used in conjunction with network time protocols as some protocols (e.g. PTP, NTP) allow master clocks to indicate explicitly that they are "traceable" back to a global time reference.
A single NTP server is identified by hostname (or IP address) and an optional port number. If the port number is not indicated, it is assumed to be the standard NTP port (123).
Two or more NTP servers may be listed at the same level in the session description to indicate that they are interchangeable. RTP senders/receivers can use any of the listed NTP servers to govern a local clock that is equivalent to a local clock slaved to a different server.
The IEEE 1588 Precision Time Protocol (PTP) family of clock synchronisation protocols provides a shared reference clock in an network - typically a LAN. IEEE 1588 provides sub-microsecond synchronisation between devices on a LAN and typically locks within seconds at startup. With support from Ethernet switches, IEEE 1588 protocols can achieve nanosecond timing accuracy in LANs. Network interface chips and cards supporting hardware time-stamping of timing critical protocol messages are also available.
Three flavours of IEEE 1588 are in use today:
Each IEEE 1588 clock is identified by a globally unique EUI-64 called a "ClockIdentity". A slave clock using one of the IEEE 1588 family of network time protocols acquires the ClockIdentity/EUI-64 of the grandmaster clock that is the ultimate source of timing information for the network. A master clock which is itself slaved to another master clock passes the grandmaster ClockIdentity through to its slaves.
Several instances of the IEEE 1588 protocol may operate independently on a single network, forming distinct PTP network protocol domains, each of which may have a different grandmaster clock. As the IEEE 1588 standards have developed, the definition of PTP domains has changed. IEEE 1588-2002 identifies protocol subdomains by a textual name, but IEEE 1588-2008 identifies protocol domains using a numeric domain number. 802.1AS is a Layer-2 profile of IEEE 1588-2008 supporting a single numeric clock domain (0).
When PTP subdomains are signalled via SDP, senders and receivers SHOULD check that both grandmaster ClockIdentity and PTP subdomain match when determining clock equivalence.
The PTP protocols employ a distributed election protocol called the "Best Master Clock Algorithm" (BMCA) to determine the active clock master. The clock master choices available to BMCA can be restricted or favourably biased by setting stratum values, preferred master clock bits, or other parameters to influence the election process. In some systems it may be desirable to limit the number of possible PTP clock masters to avoid re-signalling timestamp clock sources when the clock master changes.
Global reference clocks provide a source of traceable time, typically via a hardware radio receiver interface. Examples include GPS and Galileo. Apart from the name of the reference clock system, no further identification is required.
At the time of writing, it is common for RTP senders/receivers not to synchronise their local timestamp clocks to a shared master. An unsynchronised clock such as a quartz oscillator is identified as a "local" reference clock.
In some systems, all RTP senders/receivers may use a timestamp clock synchronised to a reference clock that is not provided by one of the methods listed above. Examples may include the reference time information provided by digital television or cellular services. These sources are identified as "private" reference clocks. All RTP senders/receivers in a session using a private reference clock are assumed to have a mechanism outside this specification confirming that their local timestamp clocks are equivalent.
A timestamp clock source may be labelled "traceable" if it is known to be sourced from a global time reference such as TAI or UTC. Providing adjustments are made for differing time bases, timestamps taken using clocks synchronised to a traceable time source can be directly compared even if the clocks are synchronised to different sources or via different mechanisms.
Since all NTP and PTP servers providing traceable time can be directly compared, it is not necessary to identify traceable time servers by protocol address or other identifiers.
Network time protocol services periodically exchange timestamped messages between servers and clients. Assuming RTP sender/receiver clocks are based on commonly available quartz crystal hardware which is subject to drift, tight synchronisation requires frequent exchange of synchronisation messages.
Unfortunately, in some implementations, it is not possible to control the frequency of synchronisation messages nor is it possible to discover when the last synchronisation message occurred. In order to provide a measure of confidence that the timestamp clock is sufficiently synchronised, an optional timestamp may be included in the SDP clock source signalling. In addition, the frequency of synchronisation message may also be signalled.
The optional timestamp and synchronisation frequency parameters provide an indication of synchronisation quality to the receiver of those parameters. If the synchronisation confidence timestamp is far from the timestamp clock at the receiver of the parameters, it can be assumed that synchronisation has not occurred recently or the timestamp reference clock source cannot be contacted. In this case, the receiver can take action to prevent unsynchronised playout or may fall back to assuming that the timestamp clocks are not synchronised.
Synchronisation frequency is expressed as a signed (two's-compliment) 8-bit field which is the base-2 logarithm of the frequency in Hz. The synchronisation frequencies represented by this field range from 2^-128 Hz to 2^+127 Hz. The field value of 0 corresponds to an update frequency of 1 Hz.
Specification of the timestamp reference clock source may be at any or all levels (session, media or source) of an SDP description (see level definitions [definitions] earlier in this document for more information).
Timestamp clock source signalling included at session-level provides default parameters for all RTP sessions and sources in the session description. More specific signalling included at the media level overrides default session level signalling. Further, source-level signalling overrides timestamp clock source signalling at the enclosing media level and session level.
If timestamp clock source signalling is included anywhere in an SDP description, it must be properly defined for all levels in the description. This may simply be achieved by providing default signalling at the session level.
Timestamp reference clock parameters may be repeated at a given level (i.e. for a session or source) to provide information about additional servers or clock sources. If the attribute is repeated at a given level, all clocks described at that level are assumed to be equivalent. Traceable clock sources MUST NOT be mixed with non-traceable clock sources at any given level. Unless synchronisation confidence information is available for each of the reference clocks listed at a given level, it SHOULD only be included with the first reference clock source attribute at that level.
Note that clock source parameters may change from time to time, for example, as a result of a PTP clock master election. The SIP [RFC3261] protocol supports re-signalling of updated SDP information, however other protocols may require additional notification mechanisms.
Figure 1 shows the ABNF [RFC2234] grammar for the SDP reference clock source information.
timestamp-refclk = "a=ts-refclk:" clksrc [ SP sync-confidence ] CRLF
clksrc = ntp / ptp / gps / gal / local / private
ntp = "ntp=" ntp-server-addr
ntp-server-addr = host [ ":" port ]
ntp-server-addr =/ "traceable" )
ptp = "ptp=" ptp-version ":" ptp-gmid [":" ptp-domain]
ptp-version = "IEEE1588-2002"
ptp-version =/ "IEEE1588-2008"
ptp-version =/ "IEEE802.1AS-2011"
ptp-gmid = EUI64
ptp-gmid =/ "traceable"
ptp-domain = ptp-domain-name / ptp-domain-nmbr
ptp-domain-name = "domain-name=" 16ptp-domain-char
ptp-domain-char = %x21-7E / %x00
; allowed characters: 0x21-0x7E (IEEE 1588-2002)
ptp-domain-nmbr = "domain-nmbr=" %x00-7f
; allowed number range: 0-127 (IEEE 1588-2008)
gps = "gps"
gal = "gal"
local = "local"
private = "private" [ ":" "traceable" ]
sync-confidence = sync-timestamp [SP sync-frequency]
sync-timestamp = sync-date SP sync-time SP sync-UTCoffset
sync-date = 4DIGIT "-" 2DIGIT "-" 2DIGIT
; yyyy-mm-dd (e.g., 1982-12-02)
sync-time = 2DIGIT ":" 2DIGIT ":" 2DIGIT "." 3DIGIT
; 00:00:00.000 - 23:59:59.999
sync-UTCoffset = ( "+" / "-" ) 2DIGIT ":" 2DIGIT
; +HH:MM or -HH:MM
sync-frequency = 2HEXDIG
; If N is the field value, HZ=2^(N-127)
host = hostname / IPv4address / IPv6reference
hostname = *( domainlabel "." ) toplabel [ "." ]
toplabel = ALPHA / ALPHA *( alphanum / "-" ) alphanum
domainlabel = alphanum
=/ alphanum *( alphanum / "-" ) alphanum
IPv4address = 1*3DIGIT "." 1*3DIGIT "." 1*3DIGIT "." 1*3DIGIT
IPv6reference = "[" IPv6address "]"
IPv6address = hexpart [ ":" IPv4address ]
hexpart = hexseq / hexseq "::" [ hexseq ] / "::" [ hexseq ]
hexseq = hex4 *( ":" hex4)
hex4 = 1*4HEXDIG
port = 1*DIGIT
EUI64 = 7(2HEXDIG "-") 2HEXDIG
Figure 2 shows an example SDP description with a timestamp reference clock source defined at the session level.
v=0 o=jdoe 2890844526 2890842807 IN IP4 10.47.16.5 s=SDP Seminar i=A Seminar on the session description protocol u=http://www.example.com/seminars/sdp.pdf e=j.doe@example.com (Jane Doe) c=IN IP4 224.2.17.12/127 t=2873397496 2873404696 a=recvonly a=ts-refclk:ntp=traceable m=audio 49170 RTP/AVP 0 m=video 51372 RTP/AVP 99 a=rtpmap:99 h263-1998/90000
Figure 3 shows an example SDP description with timestamp reference clock definitions at the media level overriding the session level defaults. Note that the synchronisation confidence timestamp appears on the first attribute at the media level only.
v=0 o=jdoe 2890844526 2890842807 IN IP4 10.47.16.5 s=SDP Seminar i=A Seminar on the session description protocol u=http://www.example.com/seminars/sdp.pdf e=j.doe@example.com (Jane Doe) c=IN IP4 224.2.17.12/127 t=2873397496 2873404696 a=recvonly a=ts-refclk:local m=audio 49170 RTP/AVP 0 a=ts-refclk:ntp=203.0.113.10 2011-02-19 21:03:20.345+01:00 a=ts-refclk:ntp=198.51.100.22 m=video 51372 RTP/AVP 99 a=rtpmap:99 h263-1998/90000 a=ts-refclk:ptp=IEEE802.1AS-2011:39-A7-94-FF-FE-07-CB-D0
Figure 4 shows an example SDP description with a timestamp reference clock definition at the source level overriding the session level default.
v=0 o=jdoe 2890844526 2890842807 IN IP4 10.47.16.5 s=SDP Seminar i=A Seminar on the session description protocol u=http://www.example.com/seminars/sdp.pdf e=j.doe@example.com (Jane Doe) c=IN IP4 224.2.17.12/127 t=2873397496 2873404696 a=recvonly a=ts-refclk:local m=audio 49170 RTP/AVP 0 m=video 51372 RTP/AVP 99 a=rtpmap:99 h263-1998/90000 a=ssrc:12345 ts-refclk:ptp=IEEE802.1AS-2011:39-A7-94-FF-FE-07-CB-D0
The media clock source for a stream determines the timebase used to advance the RTP timestamps included in RTP packets. The media clock may be asynchronously generated by the sender, it may be generated in fixed relationship to the reference clock or it may be generated with respect to another stream on the network (which is presumably being received by the sender).
In the simplest sender implementation, the sender generates media by sampling audio or video according to a free-running local clock. The RTP timestamps in media packets are advanced according to this media clock and packet transmission is typically timed to regular intervals on this timeline. The sender may or may not include an NTP timestamp in sender reports to allow mapping of this asynchronous media clock to a reference clock.
The asynchronously generated media clock is the assumed mode of operation when there is no signalling of media clock source. Alternatively, asynchronous media clock me be signaled.
A media clock may be directly derived from a reference clock. For this case it is required that a reference clock be specified. The signalling indicates a media clock offset value at the epoch (time of origin) of the reference clock. A rate for the media clock may also be specified. If include, the rate specification here overrides that specified or implied by the media description. If omitted, the rate is assumed to be the exact rate used by the media format. For example, the media clock for an 8 kHz G.711 audio stream will advance exactly 8000 units for each second advance in the reference clock from which it is derived.
A rate modifier may optionally be expressed as the ratio of two integers. This provision is useful for accommodating certain "oddball rates" associated with NTSC video (see Figure 7).
The media clock for an outgoing stream may be generated based on the media clock received with an incoming stream. In this case, the signalling identifies the session and the stream source. The received media clock may be converted to a real-time clock which can then be used to generate outgoing media clocks. In this way, the format of the reference stream does not need to match the format of the outgoing stream.
A reference stream can be either another RTP stream or AVB stream based on the IEEE 1722 standard. An RTP stream is identified by destination IP address (for a multicast stream) or source IP address (for a unicast stream), destination port number and CNAME of the source.
An IEEE 1722 stream is identified by its StreamID, an EUI-64.
Specification of the media clock source may be at any or all levels (session, media or source) of an SDP description (see level definitions (Section 3) earlier in this document for more information).
Media clock source signalling included at session level provides default parameters for all RTP sessions and sources in the session description. More specific signalling included at the media level overrides default session level signalling. Further, source-level signalling overrides media clock source signalling at the enclosing media level and session level.
Media clock source signalling may be present or absent on a per-stream basis. In the absence of media clock source signals, receivers assume an asynchronous media clock generated by the sender.
Media clock source parameters may be repeated at a given level (i.e. for a session or source) to provide information about additional clock sources. If the attribute is repeated at a given level, all clocks described at that level are comparable clock sources and may be used interchangeably.
Figure 5 shows the ABNF [RFC2234] grammar for the SDP media clock source information.
timestamp-mediaclk = "a=mediaclk:" mediaclock
mediaclock = refclk / rtp / streamid / sender
refclk = "offset=" 1*DIGIT [ SP "rate=" 1*DIGIT "/" 1*DIGIT ]
rtp = "rtp=" nettype SP addrtype SP connection-address SP port SP cname
streamid = "IEEE1722=" EUI64
sender = "sender"
cname = non-ws-string
nettype = token
;typically "IN"
addrtype = token
;typically "IP4" or "IP6"
token-char = %x21 / %x23-27 / %x2A-2B / %x2D-2E / %x30-39 / %x41-5A
/ %x5E-7E
token = 1*(token-char)
connection-address = multicast-address / unicast-address
unicast-address = IP4-address / IP6-address / FQDN / extn-addr
multicast-address = IP4-multicast / IP6-multicast / FQDN / extn-addr
IP4-multicast = m1 3( "." decimal-uchar ) "/" ttl [ "/" integer ]
; IPv4 multicast addresses may be in the
; range 224.0.0.0 to 239.255.255.255
m1 = ("22" ("4"/"5"/"6"/"7"/"8"/"9")) / ("23" DIGIT )
IP6-multicast = hexpart [ "/" integer ]
; IPv6 address starting with FF
FQDN = 4*(alpha-numeric / "-" / ".")
; fully qualified domain name as specified
; in RFC 1035 (and updates)
IP4-address = b1 3("." decimal-uchar)
b1 = decimal-uchar
; less than "224"
; The following is consistent with RFC 2373 [30], Appendix B.
IP6-address = hexpart [ ":" IP4-address ]
hexpart = hexseq / hexseq "::" [ hexseq ] / "::" [ hexseq ]
hexseq = hex4 *( ":" hex4)
hex4 = 1*4HEXDIG
; Generic for other address families
extn-addr = non-ws-string
non-ws-string = 1*(VCHAR/%x80-FF)
;string of visible characters
port = 1*DIGIT
EUI64 = 7(2HEXDIG "-") 2HEXDIG
Figure 6 shows an example SDP description 8 channels of 24-bit, 48 kHz audio transmitted as a multicast stream. Media clock is derived directly from an IEEE 1588-2008 reference.
v=0 o=- 1311738121 1311738121 IN IP4 192.168.1.1 c=IN IP4 239.0.0.2/255 s= t=0 0 m=audio 5004 RTP/AVP 96 a=rtpmap:96 L24/48000/8 a=sendonly a=ts-refclk:ptp=IEEE1588-2008:39-A7-94-FF-FE-07-CB-D0:0 a=mediaclk:offset=963214424
Figure 7 shows an example SDP description 2 channels of 24-bit, 44056 kHz NTSC "pull-down" media clock derived directly from an IEEE 1588-2008 reference clock
v=0 o=- 1311738121 1311738121 IN IP4 192.168.1.1 c=IN IP4 239.0.0.2/255 s= t=0 0 m=audio 5004 RTP/AVP 96 a=rtpmap:96 L24/44100/2 a=sendonly a=ts-refclk:ptp=IEEE1588-2008:39-A7-94-FF-FE-07-CB-D0:0 a=mediaclk:offset=963214424 rate=1000/1001
Figure 8 shows the same 48 kHz audio transmission from Figure 6 with media clock derived from another RTP multicast stream. The stream providing the media clock must use the same reference clock as this stream that references it.
v=0 o=- 1311738121 1311738121 IN IP4 192.168.1.1 c=IN IP4 224.2.228.230/32 s= t=0 0 m=audio 5004 RTP/AVP 96 a=rtpmap:96 L24/48000/2 a=sendonly a=ts-refclk:ptp=IEEE1588-2008:39-A7-94-FF-FE-07-CB-D0:0 a=mediaclk:rtp=IN IP4 239.0.0.1 5004 00:60:2b:20:12:if
Figure 9 shows the same 48 kHz audio transmission from Figure 6 with media clock derived from an IEEE 1722 AVB stream. The stream providing the media clock must be synchronized with the IEEE 1588-2008 reference clock used by this stream.
v=0 o=- 1311738121 1311738121 IN IP4 192.168.1.1 c=IN IP4 224.2.228.230/32 s= t=0 0 m=audio 5004 RTP/AVP 96 a=rtpmap:96 L24/48000/2 a=sendonly a=ts-refclk:ptp=IEEE1588-2008:39-A7-94-FF-FE-07-CB-D0:0 a=mediaclk:IEEE1722=38-D6-6D-8E-D2-78-13-2F
The SDP attribute "ts-refclk" defined by this document is registered with the IANA registry of SDP Parameters as follows:
SDP Attribute ("att-field"):
Attribute name: ts-refclk
Long form: Timestamp reference clock source
Type of name: att-field
Type of attribute: session, media and source level
Subject to charset: no
Purpose: See section 4 of this document
Reference: This document
Values: see this document and registrations below
The attribute has an extensible parameter field and therefore a registry for these parameters is required. This document creates an IANA registry called the Timestamp Reference Clock Source Parameters Registry. It contains the six parameters defined in Figure 1: "ntp", "ptp", "gps", "gal", "local", "private".
The SDP attribute "mediaclk" defined by this document is registered with the IANA registry of SDP Parameters as follows:
SDP Attribute ("att-field"):
Attribute name: mediaclk
Long form: Media clock source
Type of name: att-field
Type of attribute: session and media level
Subject to charset: no
Purpose: See section 6 of this document
Reference: This document
Values: see this document and registrations below
The attribute has an extensible parameter field and therefore a registry for these parameters is required. This document creates an IANA registry called the Media Clock Source Parameters Registry. It contains the three parameters defined in Figure 5: "refclk", "ssrc", "sender".
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