Performance Measurement (PM) with Alternate Marking Method in Service Function Chaining (SFC) Domain
draft-mirsky-sfc-pmamm-00

Abstract

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

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1. Introduction

[RFC7665] introduced architecture of a Service Function Chain (SFC) in the network and defined its components as classifier, Sevice Function Forwarder (SFF), and Service Function (SF). [I-D.ietf-ippm-alt-mark] 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 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

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 [RFC2119].

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  |R|R|R|R|MD Type|     Proto     |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 1: NSH Base format

[I-D.ietf-sfc-nsh] defines format of the Network Service Header (NSH). The format of NSH Base is presented in Figure 1. [I-D.ietf-ippm-alt-mark]. The Mark field MUST NOT be used in defining forwarding and/or quality of service treatment of a SFC packet. The Mark field MUST be used only for the performance measurement of data traffic in SFC layer. Because 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 true example of passive performance measurement method.

The Figure 2 displays format of the Mark field.

 
 0      
 0   1 
+-+-+-+-+
| S | D |
+-+-+-+-+

Figure 2: Mark field format

S- Single mark method;
D - Double mark method.

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 [I-D.ietf-ippm-alt-mark], marking can be applied to delineate blocks of packets based either on equal number of packets in a block or based on equal 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 D flag MUST be set to zero on transmit and ignored by monitoring point.

The S flag is used to create alternate flows to measure the packet loss by switching value of the S flag every N-th packet or at certain 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. value of S 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 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 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 single metric for the duration of the block and it doesn't give the minimum and maximum delay values. This limitation could be overcome by reducing the duration of the block by means of an highly optimized implementation of the method.

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 S flag MUST be used to create the alternate flow, i.e. mark larger batches of packets. The D flag MUST be used to mark single packets to measure delay jitter.

The first marking (S flag alternation) is needed for packet loss and also for average delay measurement. The second marking (D flag is put to one) creates a new set of marked packets that are fully identified over the SFC, so that a componenet can store the timestamps of these packets; these timestamps can be compared with the timestamps of the same packets on another component 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 frequency of the second marking must be not too high in order to avoid out of order issues. This method is useful to have not only the average delay but also the minimum and maximum delay values and, in wider terms, to know more about the statistic distribution of delay values.

5. IANA Considerations

This document requests IANA to register format of the OAM field of NSH as the following:

OAM field of BIER Header
Bit Position	Marking	Description	Reference
0	S	Single Mark Measurement	This document
1	D	Double Mark Measurement	This document

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. Acknowledgement

TBD

8. References

8.1. Normative References

[I-D.ietf-sfc-nsh]	Quinn, P. and U. Elzur, "Network Service Header", Internet-Draft draft-ietf-sfc-nsh-12, February 2017.
[RFC2119]	Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC7665]	Halpern, J. and C. Pignataro, "Service Function Chaining (SFC) Architecture", RFC 7665, DOI 10.17487/RFC7665, October 2015.

8.2. Informative References

[I-D.ietf-ippm-alt-mark]

Fioccola, G., Capello, A., Cociglio, M., Castaldelli, L., Chen, M., Zheng, L., Mirsky, G. and T. Mizrahi, "Alternate Marking method for passive performance monitoring", Internet-Draft draft-ietf-ippm-alt-mark-04, March 2017.

Authors' Addresses

Greg Mirsky ZTE Corp. EMail: gregimirsky@gmail.com

Giuseppe Fioccola Telecom Italia EMail: giuseppe.fioccola@telecomitalia.it