IPPM Working Group G. Mirsky
Internet-Draft ZTE Corp.
Intended status: Informational W. Lingqiang
Expires: January 3, 2019 G. Zhui
ZTE Corporation
July 2, 2018

Hybrid Two-Step Performance Measurement Method


Development of, and advancements in, automation of network operations brought new requirements for measurement methodology. Among them is the ability to collect instant network state as the packet being processed by the networking elements along its path through the domain. This document introduces a new hybrid measurement method, referred to as hybrid two-step, as it separates the act of measuring and/or calculating performance metric from the act of collecting and transporting network state.

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Table of Contents

1. Introduction

Successful resolution of challenges of automated network operation, as part of, for example, overall service orchestration or data center operation, relies on a timely collection of accurate information that reflects the state of network elements on an unprecedented scale. Because performing the analysis and act upon the collected information requires considerable computing and storage resources, the network state information is unlikely to be processed by network elements themselves but will be relayed into the data storage facilities, e.g. data lakes. The process of producing, collecting network state information also referred in this document as network telemetry, and transporting it for post-processing should work equally well with data flows or injected in the network test packets. RFC 7799 [RFC7799] describes a combination of elements of passive and active measurement as a hybrid measurement.

Several technical methods have been proposed to enable collection of network state information instantaneous to the packet processing, among them [P4.INT] and [I-D.ietf-ippm-ioam-data].

This document introduces Hybrid Two-Step (HTS) as a new hybrid measurement method that separates measuring or calculating performance metric from the collecting and transporting this information. The Hybrid Two-Step method extends the two-step mode of Residence Time Measurement (RTM) defined in [RFC8169] to on-path network state collection and transport.

2. Conventions used in this document

2.1. Terminology

RTM Residence Time Measurement

ECMP Equal Cost Multipath

MTU Maximum Transmission Unit

HTS Hybrid Two-Step

Network telemetry - the process of collecting and reporting of network state

2.2. 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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

3. Problem Overview

Performance measurements are meant to provide data that characterize conditions experienced by traffic flows in the network and possibly trigger operational changes (e.g. - re-route of flows, or changes in resource allocations). Changes to a network are determined based on the performance metric information available at the time that a change is to be made. The correctness of this determination is based on the quality of the collected metrics data. The quality of collected measurement data is defined is defined by:

Consider the case of delay measurement that relies on collecting time of packet arrival at the ingress interface and time of the packet transmission at egress interface. The method may be to record a local clock value on receiving the first octet of an affected message at the device ingress, and again to record the clock value on sending the first byte of the same message at the device egress. In this ideal case, the difference between the two recorded clock times corresponds to the time that the message spent in traversing the device. In practice, the times actually recorded can differ from the ideal case by any fixed amount and a correction may then be applied to compute the same time difference taking into account the known fixed time associated with the actual measurement. In this way, the resulting time difference reflects any variable delay associated with queuing.

Depending on the implementation, it may be a challenge to compute the difference between message arrival and departure times and - on the fly - add the necessary residence time information to the same message. And that task may become even more challenging if the packet is encrypted. Implementations SHOULD NOT record a message departure time that may be significantly inaccurate in an effort to include a correlated/computed delay value, in the same message, as a result of estimating the departure time while including any variable time component (such as that associated with buffering and queuing of messages). A similar problem may cause a lower quality of, for example, information that characterizes utilization of the egress interface. If unable to obtain the data consistently, without variable delays for additional processing, information may not accurately reflect the state at the egress interface. To mitigate this problem [RFC8169] defined RTM two-step mode.

Another challenge associated with methods that collect network state information into the actual data packet is the risk to exceed the Maximum Transmission Unit (MTU) size, particularly if the packet traverses overlay domains or VPNs. Since the fragmentation is not available at the transport network, operators may have to reduce MTU size advertised to client layer or risk missing network state data for the part, most probably the latter part, of the path.

4. Theory of Operation

The HTS method consists of the two phases:

HTS uses HTS Control message to define types of measurement or network state data collection requested from a node. HTS Control message may be inserted into the data packet, as meta-data or shim, or be transmitted in a specially constructed test packet.

To collect measurement and network state data from the nodes HTS method uses the follow-up packet. The node that creates the HTS Control message also originates the HTS follow-up packet. The follow-up packet contains characteristic information, copied from the data packet, sufficient for participating nodes to associate it with the original packet. The exact composition of the characteristic information is specific for each transport network and its definition is outside the scope of this document. The follow-up packet also uses the same encapsulation as the data packet. If not payload but only network information used to load-balance flows in equal cost multipath (ECMP), use of the network encapsulation identical to the data packet should guarantee that the follow-up packet remains in-band, i.e. traverses the same set of network elements, with the original data packet. Only one outstanding follow-up packet may be on the node for the given path. That means that if the node receives HTS Control message for the flow on which it still waits for the follow-up packet to the previous HTS Control message, the node will originate the follow-up packet to transport the former set of the network state data and transmit it before it transmits the follow-up packet with the latest set of network state information.

5. IANA Considerations

This document doesn't have any IANA requirements. The section may be deleted before the publication.

6. Security Considerations

Nodes that practice HTS method are presumed to share a trust model that depends on the existence of a trusted relationship among nodes. This is necessary as these nodes are expected to correctly modify the specific content of the data in the follow-up packet, and the degree to which HTS measurement is useful for network operation depends on this ability. In practice, this means that those portions of messages that contain the network state data cannot be covered by either confidentiality or integrity protection. Though there are methods that make it possible in theory to provide either or both such protections and still allow for intermediate nodes to make detectable but authenticated modifications, such methods do not seem practical at present, particularly for protocols that used to measure latency and/or jitter.

The ability to potentially authenticate and/or encrypt the network state data for scenarios both with and without the participation of intermediate nodes that participate in HTS measurement is left for further study.

While it is possible for a supposed compromised node to intercept and modify the network state information in the follow-up packet, this is an issue that exists for nodes in general - for any and all data that may be carried over the particular networking technology - and is therefore the basis for an additional presumed trust model associated with an existing network.

7. Acknowledgments


8. References

8.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017.

8.2. Informative References

[I-D.ietf-ippm-ioam-data] Brockners, F., Bhandari, S., Pignataro, C., Gredler, H., Leddy, J., Youell, S., Mizrahi, T., Mozes, D., Lapukhov, P., Chang, R., daniel.bernier@bell.ca, d. and J. Lemon, "Data Fields for In-situ OAM", Internet-Draft draft-ietf-ippm-ioam-data-03, June 2018.
[P4.INT] "In-band Network Telemetry (INT)", P4.org Specification, October 2017.
[RFC7799] Morton, A., "Active and Passive Metrics and Methods (with Hybrid Types In-Between)", RFC 7799, DOI 10.17487/RFC7799, May 2016.
[RFC8169] Mirsky, G., Ruffini, S., Gray, E., Drake, J., Bryant, S. and A. Vainshtein, "Residence Time Measurement in MPLS Networks", RFC 8169, DOI 10.17487/RFC8169, May 2017.

Authors' Addresses

Greg Mirsky ZTE Corp. EMail: gregimirsky@gmail.com
Wang Lingqiang ZTE Corporation No 19 ,East Huayuan Road Beijing , 100191 P.R.China Phone: +86 10 82963945 EMail: wang.lingqiang@zte.com.cn
Guo Zhui ZTE Corporation No 19 ,East Huayuan Road Beijing , 100191 P.R.China Phone: +86 10 82963945 EMail: guo.zhui@zte.com.cn