Huawei

8-2 Matam Haifa 3190501 Israel tal.mizrahi.phd@gmail.com

TU Wien

Gusshausstrasse 25/E389 Vienna 1040 Austria +43 1 58801 38813 Joachim.Fabini@tuwien.ac.at http://www.tc.tuwien.ac.at/about-us/staff/joachim-fabini/

AT&T Labs

200 Laurel Avenue South Middletown NJ 07748 United States of America +1 732 420 1571 acmorton@att.com

General NTP Working Group Timestamps Various network protocols make use of binary-encoded timestamps that are incorporated in the protocol packet format, referred to as "packet timestamps" for short. This document specifies guidelines for defining packet timestamp formats in networking protocols at various layers. It also presents three recommended timestamp formats. The target audience of this document includes network protocol designers. It is expected that a new network protocol that requires a packet timestamp will, in most cases, use one of the recommended timestamp formats. If none of the recommended formats fits the protocol requirements, the new protocol specification should specify the format of the packet timestamp according to the guidelines in this document.

Status of This Memo This document is not an Internet Standards Track specification; it is published for informational purposes. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Not all documents approved by the IESG are candidates for any level of Internet Standard; see Section 2 of RFC 7841. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at .

Copyright Notice Copyright (c) 2020 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents () in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

• .  Introduction
  • .  Background
  • .  Scope of this Document
  • .  How to Use This Document
• .  Terminology
  • .  Requirements Language
  • .  Abbreviations
  • .  Terms Used in This Document
• .  Packet Timestamp Specification Template
• .  Recommended Timestamp Formats
  • .  Using a Recommended Timestamp Format
  • .  NTP Timestamp Formats
    • .  NTP 64-Bit Timestamp Format
    • .  NTP 32-Bit Timestamp Format
  • .  The PTP Truncated Timestamp Format
• .  Synchronization Aspects
• .  Timestamp Use Cases
  • .  Example 1
  • .  Example 2
• .  Packet Timestamp Control Field
  • .  High-Level Control Field Requirements
• .  IANA Considerations
• .  Security Considerations
• . References
  • .  Normative References
  • .  Informative References
• Acknowledgments
• Authors' Addresses

Introduction

Background Timestamps are widely used in network protocols for various purposes: for logging or reporting the time of an event, for messages in delay measurement and clock synchronization protocols, and as part of a value that is unlikely to repeat (nonce) in security protocols. Timestamps are represented in the RFC series in one of two forms: text-based timestamps and packet timestamps. Text-based timestamps are represented as user-friendly strings and are widely used in the RFC series -- for example, in information objects and data models, e.g., , , and . Packet timestamps, on the other hand, are represented by a compact binary field that has a fixed size and are not intended to have a human-friendly format. Packet timestamps are also very common in the RFC series and are used, for example, for measuring delay and for synchronizing clocks, e.g., , , and .

Scope of this Document This document presents guidelines for defining a packet timestamp format in network protocols. Three recommended timestamp formats are presented. It is expected that a new network protocol that requires a packet timestamp will, in most cases, use one of these recommended timestamp formats. In some cases, a network protocol may use more than one of the recommended timestamp formats. However, if none of the recommended formats fits the protocol requirements, the new protocol specification should specify the format of the packet timestamp according to the guidelines in this document. The rationale behind defining a relatively small set of recommended formats is that it enables significant reuse; network protocols can typically reuse the timestamp format of the Network Time Protocol (NTP) or the Precision Time Protocol (PTP) , allowing a straightforward integration with an NTP- or PTP-based timer. Moreover, since accurate timestamping mechanisms are often implemented in hardware, a new network protocol that reuses an existing timestamp format can be quickly deployed using existing hardware timestamping capabilities.

How to Use This Document This document is intended as a reference for network protocol designers. When defining a network protocol that uses a packet timestamp, the recommended timestamp formats should be considered first (). If one of these formats is used, it should be referenced along the lines of the examples in Sections and . If none of the recommended formats fits the required functionality, then a new timestamp format should be defined using the template in .

Terminology

Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

Abbreviations

NTP

Network Time Protocol

PTP

Precision Time Protocol

TAI

International Atomic Time

UTC

Coordinated Universal Time

Terms Used in This Document

Timestamp:

A value that represents a point in time, corresponding to an event that occurred or is scheduled to occur.

Timestamp error:

The difference between the timestamp value and the value of a reference clock at the time of the event that the timestamp was intended to indicate.

Timestamp format:

The specification of a timestamp, which is represented by a set of attributes that unambiguously defines the syntax and semantics of a timestamp.

Timestamp accuracy:

The mean over an ensemble of measurements of the timestamp error.

Timestamp precision:

The variation over an ensemble of measurements of the timestamp error.

Timestamp resolution:

The minimal time unit used for representing the timestamp.

Packet Timestamp Specification Template This document recommends using the timestamp formats defined in . In cases where these timestamp formats do not satisfy the protocol requirements, the timestamp specification should clearly state the reasons for defining a new format. Moreover, it is recommended to derive the new timestamp format from an existing timestamp format, either a timestamp format from this document or any other previously defined timestamp format. The timestamp specification must unambiguously define the syntax and semantics of the timestamp. The current section defines the minimum set of attributes, but it should be noted that in some cases, additional attributes or aspects will need to be defined in the timestamp specification. This section defines a template for specifying packet timestamps. A timestamp format specification MUST include at least the following aspects:

Timestamp syntax:

Size:

The number of bits (or octets) used to represent the packet timestamp field. If the timestamp is comprised of more than one field, the size of each field is specified. Network order (big endian) is assumed by default; if this is not the case, then this section explicitly specifies the endianity.

Timestamp semantics:

Units:

The units used to represent the timestamp. If the timestamp is comprised of more than one field, the units of each field are specified. If a field is limited to a specific range of values, this section specifies the permitted range of values.

Resolution:

The timestamp resolution; the resolution is equal to the timestamp field unit. If the timestamp consists of two or more fields using different time units, then the resolution is the smallest time unit.

Wraparound:

The wraparound period of the timestamp; any further wraparound-related considerations should be described here.

Epoch:

The origin of the timescale used for the timestamp; the moment in time used as a reference for the timestamp value. For example, the epoch may be based on a standard time scale, such as UTC. Another example is a relative timestamp, in which the epoch could be the time at which the device using the timestamp was powered up and is not affected by leap seconds (see the next attribute).

Leap seconds:

This subsection specifies whether the timestamp is affected by leap seconds. If the timestamp is affected by leap seconds, then it represents the time elapsed since the epoch minus the number of leap seconds that have occurred since the epoch.

Synchronization aspects:

The specification of a network protocol that makes use of a packet timestamp is expected to include the synchronization aspects of using the timestamp. While the synchronization aspects are not strictly part of the timestamp format specification, these aspects provide the necessary context for using the timestamp within the scope of the protocol. In some cases, timestamps are used without synchronization, e.g., a timestamp that indicates the number of seconds since power-up. In such cases, the Synchronization Aspects section will specify that the timestamp does not correspond to a synchronized time reference and may discuss how this affects the usage of the timestamp. Further details about synchronization aspects are discussed in .

Recommended Timestamp Formats This document defines a set of recommended timestamp formats. Clearly, different network protocols may have different requirements and constraints; consequently, they may use different timestamp formats. The choice of a specific timestamp format for a given protocol may depend on various factors. A few examples of factors that may affect the choice of the timestamp format include the following:

• Timestamp size: While some network protocols use a large timestamp field, in some cases, there may be constraints with respect to the timestamp size, affecting the choice of the timestamp format.
• Resolution: The time resolution is another factor that may directly affect the selected timestamp format. A potentially important factor in this context is extensibility; it may be desirable to allow a timestamp format to be extensible to a higher resolution by extending the field. For example, the resolution of the NTP 32-bit timestamp format can be improved by extending it to the NTP 64-bit timestamp format in a straightforward way.
• Wraparound period: The length of the time interval in which the timestamp is unique may also be an important factor in choosing the timestamp format. Along with the timestamp resolution, these two factors determine the required number of bits in the timestamp.
• Common format for multiple protocols: If there are two or more network protocols that use timestamps and are often used together in typical systems, using a common timestamp format should be preferred if possible. For example, if the network protocol that is being defined typically runs on a PC, then an NTP-based timestamp format may allow easier integration with an NTP-synchronized timer. In contrast, a protocol that is typically deployed on a hardware-based platform may make better use of a PTP-based timestamp, allowing more efficient integration with a PTP-synchronized timer.

Using a Recommended Timestamp Format A specification that uses one of the recommended timestamp formats should specify explicitly that this is a recommended timestamp format and point to the relevant section in the current document.

NTP Timestamp Formats

NTP 64-Bit Timestamp Format The Network Time Protocol (NTP) 64-bit timestamp format is defined in . This timestamp format is used in several network protocols, including , , and . Since this timestamp format is used in NTP, it should be preferred in network protocols that are typically deployed in concert with NTP. The format is presented in this section according to the template defined in .

NTP 64-Bit Timestamp Format 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Seconds | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Fraction | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Timestamp field format:

Seconds:

Specifies the integer portion of the number of seconds since the epoch.

Size:

32 bits.

Units:

Seconds.

Fraction:

Specifies the fractional portion of the number of seconds since the epoch.

Size:

32 bits.

Units:

The unit is 2^-32 seconds, which is roughly equal to 233 picoseconds.

Epoch:

The epoch is 1 January 1900 at 00:00 UTC. Note: As pointed out in , strictly speaking, UTC did not exist prior to 1 January 1972, but it is convenient to assume it has existed for all eternity. The current epoch implies that the timestamp specifies the number of seconds since 1 January 1972 at 00:00 UTC plus 2272060800 (which is the number of seconds between 1 January 1900 and 1 January 1972).

Leap seconds:

This timestamp format is affected by leap seconds. The timestamp represents the number of seconds elapsed since the epoch minus the number of leap seconds. Thus, during and possibly before and/or after the occurrence of a leap second, the value of the timestamp may temporarily be ambiguous, as further discussed in .

Resolution:

The resolution is 2^-32 seconds.

Wraparound:

This time format wraps around every 2³² seconds, which is roughly 136 years. The next wraparound will occur in the year 2036.

NTP 32-Bit Timestamp Format The Network Time Protocol (NTP) 32-bit timestamp format is defined in . This timestamp format is used in and . This timestamp format should be preferred in network protocols that are typically deployed in concert with NTP. The 32-bit format can be used either when space constraints do not allow the use of the 64-bit format or when the 32-bit format satisfies the resolution and wraparound requirements. The format is presented in this section according to the template defined in .

NTP 32-Bit Timestamp Format 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Seconds | Fraction | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Timestamp field format:

Seconds:

Specifies the integer portion of the number of seconds since the epoch.

Size:

16 bits.

Units:

Seconds.

Fraction:

Specifies the fractional portion of the number of seconds since the epoch.

Size:

16 bits.

Units:

The unit is 2^-16 seconds, which is roughly equal to 15.3 microseconds.

Epoch:

The epoch is 1 January 1900 at 00:00 UTC. Note: As pointed out in , strictly speaking, UTC did not exist prior to 1 January 1972, but it is convenient to assume it has existed for all eternity. The current epoch implies that the timestamp specifies the number of seconds since 1 January 1972 at 00:00 UTC plus 2272060800 (which is the number of seconds between 1 January 1900 and 1 January 1972).

Leap seconds:

This timestamp format is affected by leap seconds. The timestamp represents the number of seconds elapsed since the epoch minus the number of leap seconds. Thus, during and possibly before and/or after the occurrence of a leap second, the value of the timestamp may temporarily be ambiguous, as further discussed in .

Resolution:

The resolution is 2^-16 seconds.

Wraparound:

This time format wraps around every 2¹⁶ seconds, which is roughly 18 hours.

The PTP Truncated Timestamp Format The Precision Time Protocol (PTP) uses an 80-bit timestamp format. The truncated timestamp format is a 64-bit field, which is the 64 least significant bits of the 80-bit PTP timestamp. Since this timestamp format is similar to the one used in PTP, this timestamp format should be preferred in network protocols that are typically deployed in PTP-capable devices. The PTP truncated timestamp format was defined in and is used in several protocols, such as , , , and .

PTP Truncated Timestamp Format 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Seconds | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Nanoseconds | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Timestamp field format:

Seconds:

Specifies the integer portion of the number of seconds since the epoch.

Size:

32 bits.

Units:

Seconds.

Nanoseconds:

Specifies the fractional portion of the number of seconds since the epoch.

Size:

32 bits.

Units:

Nanoseconds. The value of this field is in the range 0 to (10⁹)-1.

Epoch:

The PTP epoch is 1 January 1970 00:00:00 TAI.

Leap seconds:

This timestamp format is not affected by leap seconds.

Resolution:

The resolution is 1 nanosecond.

Wraparound:

This time format wraps around every 2³² seconds, which is roughly 136 years. The next wraparound will occur in the year 2106.

Synchronization Aspects A specification that defines a new timestamp format or uses one of the recommended timestamp formats should include a Synchronization Aspects section. Note that the recommended timestamp formats defined in this document () do not include the synchronization aspects of these timestamp formats, but it is expected that specifications of network protocols that make use of these formats should include the synchronization aspects. Examples of a Synchronization Aspects section can be found in . The Synchronization Aspects section should specify all the assumptions and requirements related to synchronization. For example, the synchronization aspects may specify whether nodes populating the timestamps should be synchronized among themselves and whether the timestamp is measured with respect to a central reference clock such as an NTP server. If time is assumed to be synchronized to a time standard such as UTC or TAI, it should be specified in this section. Further considerations may be discussed in this section, such as the required timestamp accuracy and precision. Another aspect that should be discussed in this section is leap second considerations. The timestamp specification template () specifies whether the timestamp is affected by leap seconds. It is often the case that further details about leap seconds will need to be defined in the Synchronization Aspects section. Generally speaking, a leap second is a one-second adjustment that is occasionally applied to UTC in order to keep it aligned with solar time. A leap second may be either positive or negative, i.e., the clock may either be shifted one second forward or backward. All leap seconds that have occurred up to the publication of this document have been in the backward direction, and although forward leap seconds are theoretically possible, the text throughout this document focuses on the common case, which is the backward leap second. In a timekeeping system that considers leap seconds, the system clock may be affected by a leap second in one of three possible ways:

• The clock is turned backwards one second at the end of the leap second.
• The clock is frozen during the duration of the leap second.
• The clock is slowed down during the leap second and adjacent time intervals until the new time value catches up. The interval for this process, commonly referred to as "leap smear", can range from several seconds to several hours before, during, and/or after the occurrence of the leap second.

The way leap seconds are handled depends on the synchronization protocol and is thus not specified in this document. However, if a timestamp format is defined with respect to a timescale that is affected by leap seconds, the Synchronization Aspects section should specify how the use of leap seconds affects the timestamp usage.

Timestamp Use Cases Packet timestamps are used in various network protocols. Typical applications of packet timestamps include delay measurement, clock synchronization, and others. The following table presents a (non-exhaustive) list of protocols that use packet timestamps and the timestamp formats used in each of these protocols. Protocols That Use Packet Timestamps

	Recommended Formats			Other
Protocol	NTP 64-Bit	NTP 32-Bit	PTP Trunc.
NTP	+
OWAMP	+
TWAMP TWAMP	+ +		+
TRILL			+
MPLS			+
TCP				+
RTP	+			+
IPFIX				+
BinaryTime				+
	+	+
		+	+

The rest of this section presents two hypothetical examples of network protocol specifications that use one of the recommended timestamp formats. The examples include the text that specifies the information related to the timestamp format.

Example 1

Timestamp:

The timestamp format used in this specification is the NTP 64-bit format, as described in of RFC 8877.

Synchronization aspects:

It is assumed that the nodes that run this protocol are synchronized to UTC using a synchronization mechanism that is outside the scope of this document. In typical deployments, this protocol will run on a machine that uses NTP for synchronization. Thus, the timestamp may be derived from the NTP-synchronized clock, allowing the timestamp to be measured with respect to the clock of an NTP server. Since the NTP time format is affected by leap seconds, the current timestamp format is similarly affected. Thus, the value of a timestamp during and possibly before and/or after a leap second may be temporarily inaccurate.

Example 2

Timestamp:

The timestamp format used in this specification is the PTP truncated format, as described in of RFC 8877.

Synchronization aspects:

It is assumed that the nodes that run this protocol are synchronized among themselves. Nodes may be synchronized to a global reference time. Note that if PTP is used for synchronization, the timestamp may be derived from the PTP-synchronized clock, allowing the timestamp to be measured with respect to a PTP grandmaster clock.

Packet Timestamp Control Field In some cases, it is desirable to have a control field that describes the structure, format, content, and properties of timestamps. Control information about the timestamp format can be conveyed in some protocols using a dedicated control plane protocol or may be made available at the management plane, for example, using a YANG data model. An optional control field allows some of the control information to be attached to the timestamp. An example of a packet timestamp control field is the Error Estimate field, defined by , which is used in the One-Way Active Measurement Protocol (OWAMP) and Two-Way Active Measurement Protocol (TWAMP) . The Root Dispersion and Root Delay fields in the NTP header are two examples of fields that provide information about the timestamp precision. Another example of an auxiliary field is the Correction Field in the PTP header ; its value is used as a correction to the timestamp and may be assigned by the sender of the PTP message and updated by transit nodes (Transparent Clocks) in order to account for the delay along the path. This section defines high-level guidelines for defining packet timestamp control fields in network protocols that can benefit from such timestamp-related control information. The word "requirements" is used in its informal context in this section.

High-Level Control Field Requirements A control field for packet timestamps must offer an adequate feature set and fulfill a series of requirements to be usable and accepted. The following list captures the main high-level requirements for timestamp fields.

1. Extensible Feature Set: Protocols and applications depend on various timestamp characteristics. A timestamp control field must support a variable number of elements (components) that either describe or quantify timestamp-specific characteristics or parameters. Examples of potential elements include timestamp size, encoding, accuracy, leap seconds, reference clock identifiers, etc.
2. Size: Essential for an efficient use of timestamp control fields is the trade-off between supported features and control field size. Protocols and applications may select the specific control field elements that are needed for their operation from the set of available elements.
3. Composition: Applications may depend on specific control field elements being present in messages. The status of these elements may be either mandatory, conditional mandatory, or optional, depending on the specific application and context. A control field specification must support applications in conveying or negotiating (a) the set of control field elements along with (b) the status of any element (i.e., mandatory, conditional mandatory, or optional) by defining appropriate data structures and identity codes.
4. Category: Control field elements can characterize either static timestamp information (e.g., timestamp size in bytes and timestamp semantics: NTP 64-bit format) or runtime timestamp information (e.g., estimated timestamp accuracy at the time of sampling: 20 microseconds to UTC). For efficiency reasons, it may be meaningful to support separation of these two concepts: while the former (static) information is typically valid throughout a protocol session and may be conveyed only once, at session establishment time, the latter (runtime) information augments any timestamp instance and may cause substantial overhead for high-traffic protocols.

Proposals for timestamp control fields will be defined in separate documents and are out of scope of this document.

IANA Considerations This document has no IANA actions.

Security Considerations A network protocol that uses a packet timestamp MUST specify the security considerations that result from using the timestamp. This section provides an overview of some of the common security considerations of using timestamps. Any metadata that is attached to control or data packets, and specifically packet timestamps, can facilitate network reconnaissance; by passively eavesdropping on timestamped packets, an attacker can gather information about the network performance and the level of synchronization between nodes. In some cases, timestamps could be spoofed or modified by on-path attackers, thus attacking the application that uses the timestamps. For example, if timestamps are used in a delay measurement protocol, an attacker can modify en route timestamps in a way that manipulates the measurement results. Integrity protection mechanisms, such as Message Authentication Codes (MACs), can mitigate such attacks. The specification of an integrity protection mechanism is outside the scope of this document as, typically, integrity protection will be defined on a per-network-protocol basis and not specifically for the timestamp field. Another potential threat that can have a similar impact is delay attacks. An attacker can maliciously delay some or all of the en route messages, with the same harmful implications as described in the previous paragraph. Mitigating delay attacks is a significant challenge; in contrast to spoofing and modification attacks, the delay attack cannot be prevented by cryptographic integrity protection mechanisms. In some cases, delay attacks can be mitigated by sending the timestamped information through multiple paths, allowing detection of and resistance to an attacker that has access to one of the paths. In many cases, timestamping relies on an underlying synchronization mechanism. Thus, any attack that compromises the synchronization mechanism can also compromise protocols that use timestamping. Attacks on time protocols are discussed in detail in .

References Normative References In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References IEEE IEEE ITU-T This memo defines the set of Initial Entries for the IANA Performance Metrics Registry. The set includes: UDP Round-trip Latency and Loss, Packet Delay Variation, DNS Response Latency and Loss, UDP Poisson One-way Delay and Loss, UDP Periodic One-way Delay and Loss, ICMP Round-trip Latency and Loss, and TCP round-trip Latency and Loss. Work in Progress This document provides a recommended default allocation for the Network Service Header (NSH) MD Type 1 fixed length context header when NSH is used for Service Function Chaining within a data center. Work in Progress This document defines a date and time format for use in Internet protocols that is a profile of the ISO 8601 standard for representation of dates and times using the Gregorian calendar. This memorandum describes RTP, the real-time transport protocol. RTP provides end-to-end network transport functions suitable for applications transmitting real-time data, such as audio, video or simulation data, over multicast or unicast network services. RTP does not address resource reservation and does not guarantee quality-of- service for real-time services. The data transport is augmented by a control protocol (RTCP) to allow monitoring of the data delivery in a manner scalable to large multicast networks, and to provide minimal control and identification functionality. RTP and RTCP are designed to be independent of the underlying transport and network layers. The protocol supports the use of RTP-level translators and mixers. Most of the text in this memorandum is identical to RFC 1889 which it obsoletes. There are no changes in the packet formats on the wire, only changes to the rules and algorithms governing how the protocol is used. The biggest change is an enhancement to the scalable timer algorithm for calculating when to send RTCP packets in order to minimize transmission in excess of the intended rate when many participants join a session simultaneously. [STANDARDS-TRACK] The One-Way Active Measurement Protocol (OWAMP) measures unidirectional characteristics such as one-way delay and one-way loss. High-precision measurement of these one-way IP performance metrics became possible with wider availability of good time sources (such as GPS and CDMA). OWAMP enables the interoperability of these measurements. [STANDARDS-TRACK] The One-way Active Measurement Protocol (OWAMP), specified in RFC 4656, provides a common protocol for measuring one-way metrics between network devices. OWAMP can be used bi-directionally to measure one-way metrics in both directions between two network elements. However, it does not accommodate round-trip or two-way measurements. This memo specifies a Two-Way Active Measurement Protocol (TWAMP), based on the OWAMP, that adds two-way or round-trip measurement capabilities. The TWAMP measurement architecture is usually comprised of two hosts with specific roles, and this allows for some protocol simplifications, making it an attractive alternative in some circumstances. [STANDARDS-TRACK] This document describes the structure, content, construction, and semantics of language tags for use in cases where it is desirable to indicate the language used in an information object. It also describes how to register values for use in language tags and the creation of user-defined extensions for private interchange. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. The Network Time Protocol (NTP) is widely used to synchronize computer clocks in the Internet. This document describes NTP version 4 (NTPv4), which is backwards compatible with NTP version 3 (NTPv3), described in RFC 1305, as well as previous versions of the protocol. NTPv4 includes a modified protocol header to accommodate the Internet Protocol version 6 address family. NTPv4 includes fundamental improvements in the mitigation and discipline algorithms that extend the potential accuracy to the tens of microseconds with modern workstations and fast LANs. It includes a dynamic server discovery scheme, so that in many cases, specific server configuration is not required. It corrects certain errors in the NTPv3 design and implementation and includes an optional extension mechanism. [STANDARDS-TRACK] This document specifies a new ASN.1 type for representing time: BinaryTime. This document also specifies an alternate to the signing-time attribute for use with the Cryptographic Message Syntax (CMS) SignedData and AuthenticatedData content types; the binary-signing-time attribute uses BinaryTime. CMS and the signing-time attribute are defined in RFC 5652. [STANDARDS-TRACK] Many service provider service level agreements (SLAs) depend on the ability to measure and monitor performance metrics for packet loss and one-way and two-way delay, as well as related metrics such as delay variation and channel throughput. This measurement capability also provides operators with greater visibility into the performance characteristics of their networks, thereby facilitating planning, troubleshooting, and network performance evaluation. This document specifies protocol mechanisms to enable the efficient and accurate measurement of these performance metrics in MPLS networks. [STANDARDS-TRACK] This document introduces a collection of common data types to be used with the YANG data modeling language. This document obsoletes RFC 6021. This document specifies the IP Flow Information Export (IPFIX) protocol, which serves as a means for transmitting Traffic Flow information over the network. In order to transmit Traffic Flow information from an Exporting Process to a Collecting Process, a common representation of flow data and a standard means of communicating them are required. This document describes how the IPFIX Data and Template Records are carried over a number of transport protocols from an IPFIX Exporting Process to an IPFIX Collecting Process. This document obsoletes RFC 5101. This document specifies a set of TCP extensions to improve performance over paths with a large bandwidth * delay product and to provide reliable operation over very high-speed paths. It defines the TCP Window Scale (WS) option and the TCP Timestamps (TS) option and their semantics. The Window Scale option is used to support larger receive windows, while the Timestamps option can be used for at least two distinct mechanisms, Protection Against Wrapped Sequences (PAWS) and Round-Trip Time Measurement (RTTM), that are also described herein. This document obsoletes RFC 1323 and describes changes from it. As time and frequency distribution protocols are becoming increasingly common and widely deployed, concern about their exposure to various security threats is increasing. This document defines a set of security requirements for time protocols, focusing on the Precision Time Protocol (PTP) and the Network Time Protocol (NTP). This document also discusses the security impacts of time protocol practices, the performance implications of external security practices on time protocols, and the dependencies between other security services and time synchronization. Performance Monitoring (PM) is a key aspect of Operations, Administration, and Maintenance (OAM). It allows network operators to verify the Service Level Agreement (SLA) provided to customers and to detect network anomalies. This document specifies mechanisms for Loss Measurement and Delay Measurement in Transparent Interconnection of Lots of Links (TRILL) networks. I-JSON (short for "Internet JSON") is a restricted profile of JSON designed to maximize interoperability and increase confidence that software can process it successfully with predictable results. This document describes an OPTIONAL feature for active performance measurement protocols that allows use of the Precision Time Protocol timestamp format defined in IEEE 1588v2, as an alternative to the Network Time Protocol that is currently used.

Acknowledgments The authors thank , , , , , , , , , , , , , and other members of the NTP Working Group for their many helpful comments. The authors gratefully acknowledge and the people from the Network Time Foundation for sharing their thoughts and ideas.

Authors' Addresses Huawei

8-2 Matam Haifa 3190501 Israel tal.mizrahi.phd@gmail.com

TU Wien

Gusshausstrasse 25/E389 Vienna 1040 Austria +43 1 58801 38813 Joachim.Fabini@tuwien.ac.at http://www.tc.tuwien.ac.at/about-us/staff/joachim-fabini/

AT&T Labs

200 Laurel Avenue South Middletown NJ 07748 United States of America +1 732 420 1571 acmorton@att.com