Performance Measurement (PM) with Alternate Marking Method in Service Function Chaining (SFC) Domain
draft-mirsky-sfc-pmamm-04

Abstract

This document describes how the alternate marking method be used as the passive performance measurement method in a Service Function Chaining (SFC) domain.

Status of This Memo

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Copyright Notice

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Table of Contents

• 1. Introduction
• 2. Conventions used in this document
• 2.1. Terminology
• 2.2. Requirements Language
• 3. Mark Field in NSH Base Header
• 4. Theory of Operation
• 4.1. Single Mark Enabled Measurement
• 4.2. Double Mark Enabled Measurement
• 4.3. Residence Time Measurement with the Alternate Marking Method
• 5. IANA Considerations
• 5.1. Mark Field in NSH Base Header
• 6. Security Considerations
• 7. Acknowledgment
• 8. References
• 8.1. Normative References
• 8.2. Informative References
• Authors' Addresses

1. Introduction

[RFC7665] introduced architecture of a Service Function Chain (SFC) in the network and defined its components as classifier, Service Function Forwarder (SFF), and Service Function (SF). [RFC8321] describes passive performance measurement method, which can be used to measure packet loss, latency, and jitter on live traffic. Because this method is based on marking consecutive batches of packets the method often referred to as Alternate Marking Method (AMM).

This document defines how the alternate marking method can be used to measure packet loss and delay metrics of a service flow over e2e or any segment of the SFC.

2. Conventions used in this document

2.1. Terminology

MM: Marking Method

OAM: Operations, Administration and Maintenance

SFC: Service Function Chain

SF: Service Function

SFF: Service Function Forwarder

SFP: Service Function Path

NSH: Network Service Header

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. Mark Field in NSH Base Header

 
   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
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |Ver|O|R|    TTL    |   Length  | M |R|R|MD Type|     Proto     |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 1: NSH Base format

[RFC8300] defines the format of the Network Service Header (NSH). Figure 1, as part of NSH Base and designated for the alternate marking performance measurement method [RFC8321]. The Mark field MUST NOT be used in defining forwarding and/or quality of service treatment of an SFC packet. The Mark field MUST be used only for the performance measurement of data traffic in the SFC layer. Because the setting of the field to any value does not affect forwarding and/or quality of service treatment of a packet, the alternate marking method in SFC layer can be viewed as a real example of passive performance measurement method.

Figure 2 displays the format of the Mark field.

 
 0      
 0   1 
+-+-+-+-+
| L | D |
+-+-+-+-+

Figure 2: Mark field format

• L- Loss flag;
• D - Delay flag.

4. Theory of Operation

The marking method can be successfully used in the SFC. Without limiting any generality consider SFC presented in Figure 3. Any combination of markings, Loss and/or Delay, can be applied to a service flow by any component of the SFC at either ingress or egress point to perform node, link, segment or end-to-end measurement to detect performance degradation defect and localize it efficiently.

 
                +---+  +---+   +---+  +---+   +---+  +---+
                |SF1|  |SF2|   |SF3|  |SF4|   |SF5|  |SF6|
                +---+  +---+   +---+  +---+   +---+  +---+
                   \   /          \  /           \  /    
   +----------+   +----+         +----+         +----+    
   |Classifier|---|SFF1|---------|SFF2|---------|SFF3|
   +----------+   +----+         +----+         +----+           

Figure 3: SFC network

Using the marking method a component of the SFC creates distinct sub-flows in the particular service traffic over SFC. Each sub-flow consists of consecutive blocks that are unambiguously recognizable by a monitoring point at any component of the SFC and can be measured to calculate packet loss and/or packet delay metrics.

4.1. Single Mark Enabled Measurement

As explained in the [RFC8321], marking can be applied to delineate blocks of packets based either on the equal number of packets in a block or based on an the same time interval. The latter method offers better control as it allows better account for capabilities of downstream nodes to report statistics related to batches of packets and, at the same time, time resolution that affects defect detection interval.

If the Single Mark measurement used, then the Delay flag Figure 2 MUST be set to zero on transmit and ignored on reception by monitoring point.

The Loss flag is used to create alternate flows to measure the packet loss by switching the value of the Loss flag every N-th packet or at specified time intervals. Delay metrics MAY be calculated with the alternate flow using any of the following methods:

• First/Last Packet Delay calculation: whenever the marking, i.e., the value of Loss flag, changes a component of the SFC can store the timestamp of the first/last packet of the block. The timestamp can be compared with the timestamp of the packet that arrived in the same order through a monitoring point at a downstream component of the SFC to compute packet delay. Because timestamps collected based on order of arrival, this method is sensitive to packet loss and re-ordering of packets
• Average Packet Delay calculation: an average delay is calculated by considering the average arrival time of the packets within a single block. A component of the SFC may collect timestamps for each packet received within a single block. Average of the timestamp is the sum of all the timestamps divided by the total number of packets received. Then the difference between averages calculated at two monitoring points is the average packet delay on that segment. This method is robust to out of order packets and also to packet loss (only a small error is introduced). This method only provides a single metric for the duration of the block, and it doesn't give the minimum and maximum delay values. Highly optimized implementation of the method can reduce the duration of the block and thus overcome the limitation.

4.2. Double Mark Enabled Measurement

Double Mark method allows measurement of minimum and maximum delays for the monitored flow, but it requires more nodal and network resources. If the Double Mark method used, then the Loss flag MUST be used to create the alternate flow, i.e., mark larger batches of packets. The Delay flag MUST be used to denote single packets to measure delay jitter.

The first marking (Loss flag alternation) is needed for packet loss and also for average delay measurement. The second marking (Delay flag is put to one) creates a new set of marked packets that are fully identified over the SFC, so that a component can store the timestamps of these packets; these timestamps can be compared with the timestamps of the same packets on another element of the SFC to compute packet delay values for each packet. The number of measurements can be easily increased by changing the frequency of the second marking. But the rate of the second marking must be not too high to avoid out of order issues. This method supports the calculation of not only the average delay but also the minimum and maximum delay values and, in broader terms, to know more about the statistic distribution of delay values.

4.3. Residence Time Measurement with the Alternate Marking Method

Residence time is the variable part of the propagation delay that a packet experiences while traversing a network, e.g., SFC. Residence Time over an SFC is the sum of the nodal residence times, i.e., periods that the packet spent in each of SFFs that compose the SFC. The nodal residence time in SFC itself is the sum of sub-nodal residence times that the packet spent in each of SFs that are part of the given SFC and are mapped to the SFF. The residence time and deviation of the residence time metrics may include any combination of minimum, maximum, values over measurement period, as well as mean, median, percentile. These metrics may be used to evaluate the performance of the SFC and its elements before and during its operation.

Use of the specially marked packets simplifies residence time measurement and correlation of the measured metrics over the SFC end-to-end. For example, the alternate marking method may be used as described in Section 4.2 to identify packets in the data flow to be used to measure the residence time. The nodal and sub-nodal residence time metrics can be locally calculated and then collected using either in-band or out-band OAM mechanisms.

5. IANA Considerations

5.1. Mark Field in NSH Base Header

This document requests IANA to allocate Mark field as two bits-long field from NSH Base Header Reserved Bits [RFC8300].

This document requests IANA to register values of the Mark field of NSH as the following:

6. Security Considerations

This document lists the OAM requirement for SFC domain and does not raise any security concerns or issues in addition to ones common to networking and SFC.

7. Acknowledgment

TBD

8. References

8.1. Normative References

8.2. Informative References

Authors' Addresses