| Network Working Group | G. Mirsky |
| Internet-Draft | ZTE Corp. |
| Intended status: Standards Track | I. Meilik |
| Expires: September 13, 2017 | Broadcom |
| March 12, 2017 |
Support of IEEE-1588 time stamp format in Two-Way Active Measurement Protocol (TWAMP)
draft-ietf-ippm-twamp-time-format-05
This document describes an OPTIONAL feature for active performance measurement protocols allowing use of the Precision Time Protocol time stamp format defined in IEEE-1588v2-2008, as an alternative to the Network Time Protocol that is currently used.
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One-Way Active Measurement Protocol (OWAMP) [RFC4656] defines that only the NTP [RFC5905] format of a time stamp can be used in OWAMP-Test protocol. Two-Way Active Measurement Protocol (TWAMP) [RFC5357] adopted the OWAMP-Test packet format and extended it by adding a format for a reflected test packet. Both the sender's and reflector's packets time stamps are expected to follow the 64-bit long NTP format [RFC5905]. NTP, when used over Internet, typically achieves clock accuracy of about 5ms to 100ms. Surveys conducted recently suggest that 90% devices achieve accuracy of better than 100 ms and 99% - better than 1 sec. It should be noted that NTP synchronizes clocks on the control plane, not on data plane. Distribution of clock within a node may be supported by independent NTP domain or via interprocess communication in multiprocessor distributed system. Any of the mentioned solutions will be subject to additional queuing delays that negatively affect data plane clock accuracy.
Precision Time Protocol (PTP) [IEEE.1588.2008] has gained wide support since the development of OWAMP and TWAMP. PTP, using on-path support and other mechanisms, allows sub-microsecond clock accuracy. PTP is now supported in multiple implementations of fast forwarding engines and thus accuracy achieved by PTP is the accuracy of clock in data plane. An option to use a more accurate clock as a source of time stamps for IP performance measurements is one of this specification's advantages. Another advantage is realized by simplification of hardware in data plane. To support OWAMP or TWAMP test protocol time stamps must be converted from PTP to NTP. That requires resources, use of micro-code or additional processing elements, that are always limited. To address this, this document proposes optional extensions to Control and Test protocols to support use of IEEE-1588v2 time stamp format as optional alternative to the NTP time stamp format.
One of the goals of this specification is not only to allow end-points of a test session to use timestamp format other than NTP but to support backwards compatibility with nodes that do not yet support this extension.
IPPM: IP Performance Measurement
NTP: Network Time Protocol
PTP: Precision Time Protocol
TWAMP: Two-Way Active Measurement Protocol
OWAMP: One-Way Active Measurement Protocol
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 [RFC2119].
OWAMP connection establishment follows the procedure defined in Section 3.1 of [RFC4656] and additional steps in TWAMP described in Section 3.1 of [RFC5357]. In these procedures, the Modes field has been used to identify and select specific communication capabilities. At the same time the Modes field has been recognized and used as extension mechanism [RFC6038]. The new feature requires one bit position for Server and Control-Client to negotiate which timestamp format can be used in some or all test sessions invoked with this control connection. The end-point of the test session, Session-Sender and Session-Receiver or Session-Reflector, that supports this extension MUST be capable to interpret NTP and PTPv2 timestamp formats. If the end-point does not support this extension, then the value of PTPv2 Timestamp flag MUST be 0 because it is in Must Be Zero field. If the value of PTPv2 Timestamp flags is 0, then the advertising node can use and interpret only NTP timestamp format.
Use of PTPv2 Timestamp flags is discussed in the following sub-sections. For details on the assigned values and bit positions see the Section 3.
In OWAMP-Test [RFC4656] the Session-Receiver and/or Fetch-Client interpret collected timestamps. Thus, the Server uses the Modes field timestamp format to indicate which formats the Session-Receiver is capable to interpret. The Control-Client inspects values set by the Server for timestamp formats and sets values in the Modes field of the Set-Up-Response message according to timestamp formats Session-Sender can use. The rules of setting timestamp flags in Modes field in server greeting and Set-Up-Response messages and interpreting them are as follows:
If OWAMP-Control uses Fetch-Session commands, then selection and use of one or another timestamp format is local decision for both Session-Sender and Session-Receiver.
In TWAMP-Test [RFC5357] the Session-Sender interprets collected timestamps. Hence, in the Modes field a Server advertises timestamp formats that the Session-Reflector can use in TWAMP-Test message. The choice of the timestamp format to be used by the Session-Sender is a local decision. The Control-Client inspects the Modes field and sets timestamp flags values to indicate which format will be used by the Session-Reflector. The rules of setting and interpreting flag values are as follows:
Participants of a test session need to indicate which timestamp format being used. The specification is to use Z field in Error Estimate defined in Section 4.1.2 of [RFC4656]. The new interpretation of the Error Estimate is in addition to it specifying error estimate and synchronization, Error Estimate indicates format of a collected timestamp. And this specification changes the semantics of the Z bit field, the one between S and Scale fields, to be referred as Timestamp format and value MUST be set per the following:
As result of this value of the Z field from Error Estimate, Sender Error Estimate or Send Error Estimate and Receive Error Estimate SHOULD NOT be ignored and MUST be used when calculating delay and delay variation metrics based on collected timestamps.
This document does not specify how Session-Sender and Session-Reflector in TWAMP Light mode are informed of timestamp format to be used. It is assumed that, for example, configuration could be used to direct Session-Sender and Session-Reflector respectively to use timestamp format per their capabilities and rules listed in Section 2.2.
The TWAMP-Modes registry defined in [RFC5618].
IANA is requested to reserve a new PTPv2 Timestamp as follows:
| Value | Description | Semantics | Reference |
|---|---|---|---|
| TBA1 (proposed 256) | PTPv2 Timestamp Capability | bit position TBA2 (proposed 8) | This document |
Use of particular format of a timestamp in test session does not appear to introduce any additional security threat to hosts that communicate with OWAMP and/or TWAMP as defined in [RFC4656], [RFC5357] respectively. The security considerations that apply to any active measurement of live networks are relevant here as well. See the Security Considerations sections in [RFC4656] and [RFC5357].
The authors would like to thank Lakshmikanthan and Suchit Bansal for their insightful suggestions. The authors would like to thank David Allan for his thorough review and thoughtful comments.