Time Travels: a Closer Look at Ptp

WHITE PAPER TIME TRAVELS: A CLOSER LOOK AT PTP

TIME TRAVELS: A CLOSER LOOK AT PTP

Precision Time Protocol is a PTP – Why it’s necessary values gives the slave enough data to system requirement for the correct its internal clock to exactly latest IP media standards, The IEEE 1588 Precision Timing Protocol match the clock of the master. This including AES67 audio and (PTP) specifies a methodology for process is repeated periodically to SMPTE ST2110 video (as well as synchronizing devices to a single, ensure that each slave device maintains other popular formats such as shared clock across packet-based synchronization with the master. Figure Dante audio). This white paper networks, including Ethernet switches 1, on the next page, shows an overview describes key factors that must and IP routers. This standard, which was of this process, which is described in the be considered for successful PTP originally developed for laboratory and following paragraphs. deployments, including system industrial installations, is now architectures, PTP device fundamental for IP video/audio signal 1) The master, which knows the correct configuration, and multicast transport using SMPTE ST 2110 and IP time, begins the process by periodically network considerations. A few audio signals using AES67, among other generating a Sync message. The sync troubleshooting tips are also media applications. One of the main message is typically multicast to all of provided. uses of 1588 is for creating a common the devices on a particular subnet. It timebase for multiple video cameras contains header information that (GENLOCK), and also to synchronize identifies the master and includes a video and audio devices to a uniform precise time stamp of when that clock (time code). message left the master’s network interface. Depending on the hardware PTP Message Flow and software configuration of the master, the Sync information can be The PTP synchronization process works generated either as one message by exchanging a series of messages (one-step) or the master may issue two between a master device and a slave messages in rapid succession (two- device. During this process, the slave step). device can determine the exact amount of network delay from the master as a) Two-step operation is required well as determining if its clock is offset when the master is unable to include from the master. Knowing these two a hardware timestamp directly inside

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an outbound Sync message and therefore must send a Sync Follow-up message that contains the precise hardware timestamp.

b) One-step messages are used when the master has the ability to include a precise timestamp derived from the hardware within the Sync message. Note that the selection of one-step or two-step operation is made as a configuration selection for both the master and the slave devices.

In either case, once the slave receives a Sync (and optionally the Sync Follow- up) message, the slave will be able to determine precisely when the Sync message arrived at its hardware interface and to read the message(s) to determine the precise timestamp when the Sync message left the master’s hardware interface. The difference parameter of the slave device (which so that it is perfectly aligned to the between those two timestamps could make changing a master device master’s clock. Very tight accuracy represents the one way delay from the more difficult). The option of using levels are frequently achieved in this master to the slave, provided that there multicast or unicast technology to send process; slave clocks can often be is no offset between the slave and Delay Request messages is typically a aligned within a few nanoseconds of the master clocks. configuration parameter in the setup of master clock. slave and master devices. 2) The next step in the process is for the 3) Each time a master receives a Delay slave to send a Delay Request message Request message it must send out a Distributing the Clocks back to the master. Essentially, this is a Delay Response message to that device. message is very similar to the Sync Included in this message is the precise For a PTP network to operate properly, a message, except that the header data timestamp of when the Delay Request few general network requirements need fields are changed to show the slave message was received at the master’s to be in place. First and foremost, the device as the originator of the message. input as well as the identity of the PTP synchronization process described device that sent the Delay Request above requires a bidirectional network A Delay Request message can be sent message. Note that in systems with capable of transmitting IP packets from back using the same multicast group higher quantities of devices, the the master to the slave and back. This address as the original Sync message, or processing of a large number of delay enables all devices to receive Sync, it can be unicast directly to the Master’s request messages can become a burden Follow-Up, and Delay Response IP address. In the former case, all of the for the master. messages and to transmit Delay other devices that are members of the Requests. multicast group will receive the Once the slave device has received the message and have to parse it to Delay Response message, it has all the Each master device, or device that could determine that the message did not information necessary to calculate the potentially become a master, must have originate from the master. In the latter delay from the master and to determine the ability to receive Announce case, the slave device has to know the whether the slave’s internal clock has messages from every other master or master’s correct (non-multicast) IP any offset (positive or negative) from potential master device. Announce address; in some cases this address the master. The slave then uses these messages are sent on a regular basis needs to be set as a configuration two values to correct its internal clock from the active master and all other

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devices that receive those messages should not attempt to become masters unless they determine that they would be a better master than the current master. All devices that could potentially become the master must execute a common Best Master Clock algorithm which is defined in the IEEE 1588 specification. Once a best master clock has been selected all other potential master devices must stop sending announce messages and should receive their timing from the common best master. This process is illustrated in Figure 2.

Comparing two potential grandmaster clocks requires an A vs. B comparison based on multiple criteria. Table 1 shows an ordered list of the criteria that are used each time two clocks are compared. The process works by going matched pair should get preference in a through each individual criteria starting Within the list of criteria shown in working system. Also note that criterion from the top of the list and comparing Table 1, two priorities can be configured 6 is the ultimate tiebreaker: as long as clock A vs. clock B. if the two clocks are by the network administrator. Priority 1 the two clocks do not have identical matched identically for a given criteria is an eight bit field that can be used to MAC addresses (which they shouldn’t) then the comparison continues by going adjust specific devices to be high or low there will always be one clock selected to the next item on the list. As soon as priority; note that this criteria (since it is over the other. one clock is found to be preferred to the at the top of the list) is more important other clock, the process stops and that than all of the others. Priority 2 (which Criteria 2, 3 and 4 are related to the clock becomes the grandmaster. The is shown in the fifth row of Figure 3) is a quality of a particular clock and the other clock automatically stops sending secondary 8-bit priority field that can be different levels are defined in the IEEE announce messages and remains silent used to break ties between clocks that 1588 specification. Without going into as long as it continues to receive match in all of categories one through excessive amounts of detail, a clock that announce messages periodically from four. This criterion would be a good is connected to a better quality source, the selected grandmaster. place to define which clock of a one that has more accurate internal

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electronics, and one that is more stable Transparent clocks sit in the path that that devices that are connected to these will be selected over other clocks. Note PTP (and all other) network traffic ports can synchronize directly to this that some of the values used in these passes through and do as much as they switch. The primary benefit of BCs in a fields made the specific to a particular can to make themselves invisible to the network is to reduce the amount of profile of clock usage. In particular, traffic Figure( 3). This is accomplished traffic coming from slave devices to a SMPTE has defined several additional by first calculating the amount of time master. Remember that master devices clock classes (criterion 2) that go in nanoseconds that a PTP message will regularly receive Delay Request beyond what is in the basic IEEE spends in transit going through a TC messages from every one of their slave standard. device (in other works, the exact latency devices, and must send an individual of the Ethernet switch). This value is Delay Response message to each one. then added to the correction field that is As the number of slave devices that Transparent and Boundary contained in PTP message headers. depend on a single master grows, the Clocks When the message reaches its amount of message processing time destination, the receiving device can also grows. If this burden gets too large, Inside PTP networks, Ethernet switches compensate for the TC latency using the the master device will have trouble provide connections between devices, value of this field. handing the load. Hence, BCs are a including those between grandmasters common feature of large PTP networks. and slave devices. These switches can Boundary clocks act as both a slave and from slave devices to a master. be operated in one of three different a master in order to propagate PTP modes – Boundary, Transparent, or messages to large populations of slave Remember that master devices will non-PTP-aware. The latter state, which devices (Figure 3). An Ethernet switch regularly receive Delay Request may be used on small networks when that is set up to be a BC will normally messages from every one of their slave there is no other choice, is not have one port that is configured as a devices, and must send an individual recommended. When switches are not PTP slave, and that port will be in the Delay Response message to each one. PTP-aware, the transit time for PTP same network as the grandmaster or As the number of slave devices that messages can vary, and this will affect another master clock. All or some of the depend on a single master grows, the the accuracy of the downstream clocks. other ports in the switch can then be amount of message processing time Each time a PTP Sync or Delay Request configured as BC Master ports, meaning also grows. If this burden gets too large, message is delayed for even a short time, it can cause an inaccurate delay or offset value to be calculated, thereby perturbing the slave’s clock and affecting it’s stability. Even with moderate traffic loading, an unmanaged Ethernet switch can be a source of instability in a PTP distribution system.

Both Transparent Clocks (TCs) and Boundary Clocks (BCs) are used in networks to properly distribute PTP messages to multiple devices. In fact, both types of devices may be present at different layers of the PTP clock hierarchy as shown in Figure 4. With properly configured and functioning TCs and BCs, it is possible to get thousands of devices all synchronized to a common master clock with accuracies in the nanosecond range.

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the master device will have trouble handing the load. Hence, BCs are a common feature of large PTP networks.

Redundant Systems

Most modern media networks use redundant hardware for critical system infrastructure. This practice, of course, should extend to the PTP clock distribution system as well as to the large scale Ethernet switches used to interconnect devices within these systems. Having fully redundant Ethernet backbones makes a good deal of sense and can be accommodated with the correct PTP architecture. Figure 4 shows a simplified diagram of a media network with redundant Ethernet switches along with redundant PTP timing sources. Configuring PTP Domains and match unique requirements. For In this example network, two separate Profiles example, PTP technology can be used to grandmasters are available to generate synchronize operations of an electric redundant PTP signals, although only A PTP domain is used to identify utility power grid that stretches over one will be active at any time. Low port masters and slaves that are going to hundreds of kilometers, generating long count transparent Ethernet switches are work together. Only devices that are in message transmission times. A suitable used to distribute the grandmaster the same domain are able to operate profile for that application could use outputs to redundant enterprise-class together. Masters and slaves that different polling times and message Ethernet switches which may act as receive PTP messages from other timeouts as compared to a media boundary clocks. In the event of a domains will ignore them. The domain network inside a single building. switchover from the active to the number is configured into every PTP Fortunately, PTP profiles can be defined standby network, having a grandmaster device and it is carried in the header of to meet performance requirements of distributed across both systems will every PTP message. This feature of the each application independently. allow the slave devices to continue to IEEE 1588 specification allows one Message timing is one area where PTP operate without a clock or master network to support multiple different profiles come into play. A PTP profile discontinuity. Note that it is extremely PTP systems simultaneously, but it is will define the Announce message important that the grandmaster devices hard to conceive of an application frequency, the Sync message frequency, are able to see messages from each where that would be useful in a modern and the Delay Request message other and thereby permit Announce media network. (Note that in version 1 frequency. More frequent messages are messages from the active grandmaster of the IEEE 1588 specification that the desired for some applications, because to propagate to the standby domains were named instead of they lead to quicker convergence of grandmaster. Careful network setup numbered; however, for the current slave clocks to the masters, but they may be required prevent Ethernet traffic version of 1588 only numbers are used.) have the drawback of increasing the loops from forming. amount of PTP message traffic on the A PTP profile defines many of the key network. operational characteristics of a PTP network. Different profiles can be used Carefully balancing the frequency of for different applications, allowing messages against the convergence time system performance to be fine-tuned to is essential for having a well-

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TABLE 2. PROFILE COMPATIBILITY BETWEEN AES AND SMPTE STANDARDS Parameter Name AES 67 Range SMPTE 2059 Range Proposed Value domainNumber 0 to 255 0 to 127 0

logAnnounceInterval 0 to 4 -3 to 1 0 (1sec)

announceReceiptTimeout 2 to 10 2 to 10 3 (3sec)

logSincInterval -4 to 1 -7 to -1 -3 (.125 sec)

logMinDelayReqInterval -3 to 2 -3 to 2 -3 (.125 sec) (based on logSyncInterval = -3) functioning, stable network. include a device that will act as a with multiple switches and an IGMP Interestingly, the AES 67 standard and querier that regularly sends out IGMP querier generating the necessary the SMPTE ST 2059 standard defined Membership Queries in order to Membership Query messages. two different ranges of values for these stimulate end devices to send various message frequencies . A study Membership Reports. On a pure layer 2 During the transition from traditional was done in 2016 to find a common network, which is often the case for video equipment to IP and PTP based operating point for both standards and Media Networks, an IGMP querier is video equipment there will undoubtedly the results of that study are shown in required. Only one querier should be be circumstances when legacy devices Table 2 above. active at any time in each subnet. Many that do not support PTP timing need to modern multicast-aware switches have be synchronized with new equipment. Network Considerations a built-in querier function that can be There are two ways of dealing with this enabled on various ports using the situation. The first way involves using a A few network aspects that are appropriate switch configuration dual function master (with both PTP somewhat peculiar to PTP systems commands. Figure 5 shows a network and standard synch outputs) connected deserve a quick mention. These are the multicast querier, the use of combined networks that have both legacy (non- PTP) devices and PTP-enabled ones, and the issue of port speed mismatches.

One of the great advances in multicasting technology was the development of IGMP snooping within Ethernet switches. This technology enables switches to listen to IGMP messages that flow between end devices and the network routers and then restrict delivery of multicast messages to only those switch ports which have devices that are participating in a specific multicast. For this technology to work, each end device must periodically generate and IGMP Membership Report message that the switch can observe. To facilitate this process, multicast networks typically

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to a sync distribution network of the PTP devices are supposed to take over Troubleshooting Questions kind that have existed in broadcast as the PTP master whenever studios for many years. The other way announcements from another master Here is a list of some questions that uses a small adapter connected to the are not received, thus indicating that may be useful to answer when PTP system that provides a local sync another master is not available.) A good troubleshooting a PTP network: for each device that requires the legacy way to observe whether or not this is signal. Either method can be made to happening is by looking at the traffic on 1. Is the slave device in a network work, and both methods may be used the PTP multicast address and to see if segment that is included in the during the transition from legacy Announce messages are coming from master’s multicast? equipment to PTP-enabled devices. more than one source, or from a source 2. Are both the slave and the Port speed mismatches occur when that should not have become the master using the same PTP network connections that pass through master. One way to avoid this situation domain? switches or other devices that change is to configure all of the ordinary 3. Are both the slave and the Ethernet bit rates. For example, devices as slave-only so that they do not master set for either one-step connecting a 100 Mbit device to a one try to take over as master even if or two-step? gigabit port through the use of a format announcement messages do not arrive. 4. Are there any network converter creates a port speed asymmetries (100BaseT <-> mismatch that results in a clock offset Another area where problems have GigE for example) along the (phase error) of about four been encountered in laboratory tests is path between the slave and the microseconds . The problem occurs in mixed IGMPv2 and IGMPv3 networks. master? because the speed of packet transit in IGMPv3 has a valuable added feature 5. Can delay request messages one direction does not match the speed called Source Specific Multicasting, reach the master from the of packet transit in the other direction, which can be useful in large networks slave? thereby causing an offset in the PTP with many different sources. However, 6. Are delay response messages calculations. This can be corrected many current generation devices will being sent multicast or unicast? either by removing the offending device only support IGMPv2, which can cause 7. Has a multicast querier been or by configuring the affected Ethernet headaches on network installations. In set up for the PTP multicast, so switch ports to compensate for the particular, the default behavior for that snooping data inside timing offset. IGMPv3 devices is to revert to operating switches becomes properly in IGMPv2 mode whenever messages populated? Troubleshooting are received from IGMPv2 devices. The 8. Does the Grandmaster have a best advice is to choose either version 2 lower (better) priority value Installing a PTP network can sometimes or version 3 and ensure that every than other devices on network? be a bit tricky, with many different device on the network fully supports configuration parameters that need to that version and is configured be set properly in order for the system accordingly. to work. The good news is that once a network has been set up and is working properly, it will generally perform quite well for distributing accurate clock signals to all of the devices that are present on the network.

One of the first steps in troubleshooting a network is determining if a problem is occurring. One common failure syndrome is for a device to try to take over as a master, which is generally caused by the device not receiving Announce messages from the active master on the network. (Remember that

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About the Author

Rafael Fonseca, Vice President Product Management

Rafael brings to Artel significant experience in executive management, product management and marketing, sales, and sales operations at established and startup companies. Previously, Rafael was Vice President of Product Management, Marketing, and Sales Engineering and member of the founding team at Cedar Point Communications, acquired by GENBAND specializing in technology, telecommunications, and multimedia IP solutions. Early in his career, Rafael was a member of Bell Laboratories technical team responsible for research and development in the areas of networking synchronization and equalization for data communication and optical networking systems. He holds a M.S.E.E. from Cornell University, a B.S. in Electrical Engineering from the University of Puerto Rico at Mayaguez, and a MBA Certification from Wharton School of Business.

Rafael has authored several articles and spoken at industry venues on the operational costs of delivering next generation services, media transport, audio over IP, and other topics.