Hybrid Two-Step Performance Measurement Method

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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 the 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 to 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 the collection of network state information instantaneous to the packet processing, among them [P4.INT] and [I-D.ietf-ippm-ioam-data]. The instantaneous, i.e., in the data packet itself, collection of telemetry information simplifies the process of attribution of telemetry information to the particular monitored flow. On the other hand, this collection method impacts the data packets, potentially changing their treatment by the networking nodes. Also, the amount of information the instantaneous method collects might be incomplete because of the limited space it can be allotted. Other proposals defined methods to collect telemetry information in a separate packet from each node traversed by the monitored data flow. Examples of this approach to collecting telemetry information are [I-D.ietf-ippm-ioam-direct-export] and [I-D.song-ippm-postcard-based-telemetry]. These methods allow data collection from any arbitrary path and avoid directly impacting data packets. On the other hand, the correlation of data and the monitored flow requires that each packet with telemetry information also includes characteristic information about the monitored flow.¶

This document introduces Hybrid Two-Step (HTS) as a new method of telemetry collection that improvers accuracy of a measurement by separating the act of measuring or calculating the performance metric from the collecting and transporting this information while minimizing the overhead of the generated load in a network. HTS method extends the two-step mode of Residence Time Measurement (RTM) defined in [RFC8169] to on-path network state collection and transport. HTS allows the collection of telemetry information from any arbitrary path, does not change data packets of the monitored flow and makes the process of attribution of telemetry to the data flow simple.¶

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). Modifications to a network are determined based on the performance metric information available when 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 by:¶

• the resolution and accuracy of each measurement;¶
• predictability of both the time at which each measurement is made and the timeliness of measurement collection data delivery for use.¶

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 the egress interface. The method includes recording a local clock value on receiving the first octet of an affected message at the device ingress, and again recording the clock value on transmitting 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 time recorded can differ from the ideal case by any fixed amount. A correction can 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. Recording the departure of a packet time in the same packet may be decremental to the accuracy of the measurement because the departure time includes the variable time component (such as that associated with buffering and queuing of the packet). A similar problem may lower the 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 egress interface state. To mitigate this problem [RFC8169] defined an 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 on the path, especially 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 the client layer or risk missing network state data for the part, most probably the latter part, of the path.¶

In some networks, for example, wireless that are in the scope of [I-D.ietf-raw-use-cases], it is beneficial to collect the telemetry, including the calculated performance metrics, that reflects conditions experienced by the monitored flow at a node, other than the egress. For example, a head-end can optimize path selection based on the compounded information that reflects network conditions, resource utilization. This mode is referred to as the upstream collection and the other - downstream collection to differentiate between two modes of telemetry collection..¶

4. Theory of Operation

The HTS method consists of two phases:¶

• performing a measurement and/or obtaining network state information on a node;¶
• collecting and transporting the measurement and/or the telemetry information.¶

HTS may use an HTS Trigger carried in a data packet or a specially constructed test packet. For example, an HTS Trigger could be a packet that has IOAM Option-Type set to the "IOAM Hybrid Two-Step Option-Type" value (TBA1) allocated by IANA (see Section 6.1). The HTS Trigger also includes IOAM Namespace-ID and IOAM-Trace-Type information [I-D.ietf-ippm-ioam-data]. A packet in the flow to which the Alternate-Marking method [RFC8321] is applied can be used as an HTS Trigger. The nature of the HTS Trigger is a transport network layer-specific, and its description is outside the scope of this document. The packet that includes the HTS Trigger in this document is also referred to as the trigger packet.¶

The HTS method uses the HTS Follow-up packet, referred to as the follow-up packet, to collect measurement and network state data from the nodes. The node that creates the HTS Trigger also generates the HTS Follow-up packet. In some use cases, e.g., when HTS is used to collect the telemetry, including performance metrics, calculated based on a series of measurements, an HTS follow-up packet can be originated without using the HTS Trigger. The follow-up packet contains characteristic information sufficient for participating HTS nodes to associate it with the monitored data flow. The characteristic information can be obtained using the information of the trigger packet or constructed by a node that originates the follow-up packet. As the follow-up packet is expected to traverse the same sequence of nodes, one element of the characteristic information is the information that determines the path in the data plane. For example, in a segment routing domain [RFC8402], a list of segment identifiers of the trigger packet is applied to the follow-up packet. And in the case of the service function chain based on the Network Service Header [RFC8300], the Base Header and Service Path Header of the trigger packet will be applied to the follow-up packet. Also, when HTS is used to collect the telemetry information in an IOAM domain, the IOAM trace option header [I-D.ietf-ippm-ioam-data] of the trigger packet is applied in the follow-up packet. The follow-up packet also uses the same network information used to load-balance flows in equal-cost multipath (ECMP) as the trigger packet, e.g., IPv6 Flow Label [RFC6437] or an entropy label [RFC6790]. The exact composition of the characteristic information is specific for each transport network, and its definition is outside the scope of this document.¶

Only one outstanding follow-up packet MUST be on the node for the given path. That means that if the node receives an HTS Trigger for the flow on which it still waits for the follow-up packet to the previous HTS Trigger, the node will originate the follow-up packet to transport the former set of the network state data and transmit it before it sends the follow-up packet with the latest collection of network state information.¶

The following sections describe the operation of HTS nodes in the downstream mode of collecting the telemetry information. In the upstream mode, the bahavior of HTS nodes, in general, identical with the exception that the HTS Trigger packet does not precede the HTS Follow-up packet.¶

     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    ~                      Transport Network                        ~
    |                        Encapsulation                          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |Ver|HTS Shim L |     Flags     |Sequence Number|   Reserved    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        HTS Max Length                         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                    Telemetry Data Profile                     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    ~                     Telemetry Data TLVs                       ~
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

5. Authentication in HTS

Telemetry information may be used to drive network operation, closing the control loop for self-driving, self-healing networks. Thus it is critical to provide a mechanism to protect the telemetry information collected using the HTS method. This document defines an optional authentication of a Telemetry Data TLV that protects the collected information's integrity.¶

The format of the Authentication sub-TLV is displayed in Figure 4.¶

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Authentic. Type|   HMAC Type   |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                            Digest                             |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

where fields are defined as follows:¶

• Authentication Type - is a one-octet-long field, value TBA2 allocated by IANA Section 6.2.¶
• Length - two-octet-long field, set equal to the length of the Digest field in octets.¶
• HMAC Type - is a one-octet-long field that identifies the type of the HMAC and the length of the digest and the length of the digest according to the HTS HMAC Type sub-registry (see Section 6.4).¶
• Digest - is a variable-length field that carries HMAC digest of the text that includes the encompassing TLV.¶

This specification defines the use of HMAC-SHA-256 truncated to 128 bits ([RFC4868]) in HTS. Future specifications may define the use in HTS of more advanced cryptographic algorithms or the use of digest of a different length. HMAC is calculated as defined in [RFC2104] over text as the concatenation of the Sequence Number field of the follow-up packet (see Figure 1) and the preceding data collected in the Telemetry Data TLV. The digest then MUST be truncated to 128 bits and written into the Digest field. Distribution and management of shared keys are outside the scope of this document. In the HTS authenticated mode, the Authentication sub-TLV MUST be present in each Telemetry Data TLV. HMAC MUST be verified before using any data in the included Telemetry Data TLV. If HMAC verification fails, the system MUST stop processing corresponding Telemetry Data TLV and notify an operator. Specification of the notification mechanism is outside the scope of this document.¶

6. IANA Considerations

7. Security Considerations

Nodes that practice the 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 either confidentiality or integrity protection cannot cover those portions of messages that contain the network state data. 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 yet authenticated modifications, such methods do not seem practical at present, particularly for protocols that used to measure latency and/or jitter.¶

This document defines the use of authentication (Section 5) to protect the integrity of the telemetry information collected using the HTS method. Privacy protection can be achieved by, for example, sharing the IPsec tunnel with a data flow that generates information that is collected using HTS.¶

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 all data that to be carried over the particular networking technology - and is therefore the basis for an additional presumed trust model associated with an existing network.¶

Authors' Addresses