Simple Two-Way Direct Loss Measurement Procedure

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Table of Contents

1. Introduction

Many Service Provider Service Level Agreements (SLAs) depend on the ability to measure performance loss metric experienced by the Customer data traffic flow. Accurate Customer data packet loss can be measured by using a Direct Loss Measurement (DLM) procedure. Currently there is no efficient active measurement procedure available for accurate data packet loss detection in IP networks. Note that an approach for conducting packet loss measurement in an IP network is documented in [RFC7680]. This approach requires clock synchronization between the measurement points and lacks support for accurate data packet loss measurement.¶

[ITU-Y1731] defines procedures for performance loss monitoring for Ethernet-based networks. Specifically, the Loss Measurement Message (LMM) defined in Section 9.12 of [ITU-Y1731] can be used for accurate frame loss measurement as described in Appendix II of that document. The procedure is specific to the Ethernet-based networks and does not apply to the IP networks.¶

The Simple Two-Way Active Measurement Protocol (STAMP) provides capabilities for the measurement of various performance metrics in IP networks [RFC8762] without the use of a control channel to pre-signal session parameters. The STAMP can be used for (synthetic or inferred) packet loss measurement based on the Sequence Number in the test packets, however, this method can only provide approximate packet loss metrics.¶

[RFC8972] defines optional extensions for STAMP. The STAMP test packet with the "Direct Measurement" TLV (Type 5) [RFC8972] can be used for combined timestamps and data packet counters collection. This method, however, has the following limitations when used for detecting data packet loss:¶

• For hardware-based implementation (e.g., in an ASIC), the optional "Direct Measurement" TLV adds unnecessary processing overhead on the Session-Reflector as not all STAMP Session-Sender test packets carry the "Direct Measurement" TLV and also there can be multiple TLV Types present. This also means that the location of the transmit counter is not at the fixed location in the STAMP test packets.¶
• The STAMP "Direct Measurement" TLV does not support 64-bit counters, counters for bytes, counters per traffic class.¶
• The STAMP "Direct Measurement" TLV also does not identify the Block Number of the Direct Measurement, which is required for Alternate-Marking Method (AMM) [RFC9341] for data packet loss measurement.¶

This document defines Simple Two-Way Direct Loss Measurement (DLM) procedure that can be used for Alternate-Marking Method [RFC9341] for detecting accurate data packet loss in a network. Specifically, DLM probe packets are defined for both unauthenticated and authenticated modes and they are efficient for hardware-based implementation (e.g., in an ASIC).¶

2. Conventions Used in This Document

                     C1                    C2
                    /                       \
           +-------+     DLM Probe Packet    +-------+
           |       | - - - - - - - - - - - ->|       |
           |   S1  |=========================|   R1  |
           |       |<- - - - - - - - - - - - |       |
           +-------+  DLM Reply Probe Packet +-------+
                    \                       /
                     C4                    C3

         Session-Sender                  Session-Reflector

                      Reference Topology

3. Overview

For accurate data packet loss detection, the DLM probe packets are transmitted by the Session-Sender over UDP transport, and are used to collect the transmit and receive counters for the data traffic flow under measurement. The DLM reply probe packets are transmitted by the Session-Reflector to collect the transmit and receive counters for the data traffic flow under measurement in the reverse direction.¶

The DLM probe packets carry user-configured destination UDP port. The destination UDP port 862 is not used for the DLM probe packets. The user-configured destination UDP port follows the guidelines described in Section 4.1 of [RFC8762]. Different destination UDP port is used for DLM probe packets than the STAMP test packets defined in [RFC8762]. Hence, the Session-Sender and the Session-Reflector do not require backwards compatibility and support for STAMP.¶

A DLM session is identified by the 4-tuple (source and destination IP addresses, source and destination UDP port numbers). A DLM Session-Sender MAY generate a locally unique Sender Session Identifier (SSID). The SSID is a two-octet, non-zero unsigned integer. The SSID generation policy is implementation specific. An implementation MUST NOT assign the same identifier to different DLM sessions. A Session-Sender uses the SSID to identify a DLM session.¶

The DLM Session-Reflector operates in the Stateless mode.¶

In this document, the examples of DLM probe packets are shown with UDP header, however, the probe packets can be encapsulated with a different header based on the transport protocol used in the network.¶

4. Session-Sender Direct Loss Measurement Probe Packet

In this document, base Session-Sender DLM probe packet formats are defined as shown in Figure 1 and Figure 2 for unauthenticated and authenticated modes, respectively. They are stand-alone DLM probe packet formats to carry the counters for the data traffic flow under measurement. The DLM probe packet formats are similar to the base STAMP test packet formats (for example the locations of the Counters vs. STAMP Timestamps).¶

 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        Transmit Timestamp                     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        Transmit Counter (C1)                  |
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |X|B|T|S|BlockNumber|   DSCP    | SSID                          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 |                                                               |
 |                        MBZ (28 octets)                        |
 |                                                               |
 |                                                               |
 |                                                               |
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        Transmit Timestamp                     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        MBZ (12 octets)                        |
 |                                                               |
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        Transmit Counter (C1)                  |
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |X|B|T|S|BlockNumber|   DSCP    | SSID                          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 |                        MBZ (68 octets)                        |
 .                                                               .
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 |                        HMAC (16 octets)                       |
 |                                                               |
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Fields are defined as the following:¶

Transmit Timestamp (32-bit): This is 32-bit nano-sec field of the PTPv2 timestamp on transmit side. This field may carry Sequence Number instead of PTPv2 timestamp.¶

Transmit Counter (64-bit): The number of packets or octets transmitted by the Session-Sender in the DLM probe packet. The counter is always written at the well-known fixed location in the DLM probe packet. This is an important property for hardware-based implementation (e.g., in an ASIC), e.g., for point-to-point links and circuits. Counter is for the data traffic flow under measurement.¶

XBTS Flags (3-bit): The meanings of the Flag bits are:

X: Extended counter format indicator. Indicates the use of extended (64-bit) counter values. Initialized to 1 upon creation (and prior to transmission) of a DLM probe packet. Set to 0 when the DLM probe packet is transmitted or received over an interface that writes 32-bit counter values.¶

B: Octet (byte) count. When set to 1, indicates that the Counter fields represent octet counts. The octet count applies to all packets within the DLM scope, and the octet count of a packet transmitted or received includes the total length of that packet (but excludes headers, labels, or framing of the channel itself). When set to 0, indicates that the Counter fields represent packet counts.¶

T: Traffic-class-specific measurement indicator. Set to 1 when the DLM session is scoped to data packets of a particular traffic class (DSCP value), and 0 otherwise. When set to 1, the DSCP field of the DLM probe packet indicates the measured traffic class.¶

S: Sequence Number indicator. When set to 1, it indicates that the Transmit Timestamp field contains Sequence Number (instead of PTPv2 timestamp).¶

DSCP (6-bit): DSCP of the data traffic flow being measured when T flag is set.¶

Block Number (6-bit): The Direct Loss Measurement using Alternate-Marking Method [RFC9341] requires collecting Block Number of the counters for the data traffic flow under measurement. To be able to correlate the transmit and receive counters of the matching Block Number, the Block Number of the counters carried in the DLM probe packets.¶

SSID (16-bit): DLM Sender Session Identifier.¶

HMAC: The use of the HMAC field is described in Section 4.4 of [RFC8762]. HMAC uses its own key and the mechanism to distribute the HMAC key is outside the scope of this document.¶

MBZ: Must be Zero. It MUST be all zeroed on the transmission and MUST be ignored on receipt.¶

5. Session-Reflector Direct Loss Measurement Probe Packet

The Session-Reflector receives the DLM Session-Sender probe packet and verifies it. If the DLM probe packet is validated, the Session-Reflector that supports this specification prepares and transmits the DLM reply probe packet. In this document, Session-Reflector DLM reply probe packet formats are defined as shown in Figure 3 and Figure 4, for unauthenticated and authenticated modes, respectively.¶

 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        Transmit Timestamp                     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        Transmit Counter (C3)                  |
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |X|B|T|S|BlockNumber|   DSCP    | SSID                          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        Receive Counter (C2)                   |
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        Session-Sender Transmit Timestamp      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        Session-Sender Counter (C1)            |
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |FLAGS  |SS-BlockNum| SS-DSCP   | MBZ (2 octets)                |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |Ses-Sender TTL |        MBZ                                    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        Transmit Timestamp                     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        MBZ (12 octets)                        |
 |                                                               |
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        Transmit Counter (C3)                  |
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |X|B|T|S|BlockNumber|   DSCP    | SSID                          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        MBZ (4 octets)                         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        Receive Counter (C2)                   |
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        MBZ (8 octets)                         |
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        Session-Sender Transmit Timestamp      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        MBZ (12 octets)                        |
 |                                                               |
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        Session-Sender Counter (C1)            |
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |FLAGS  |SS-BlockNum| SS-DSCP   | MBZ (2 octets)                |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        MBZ (4 octets)                         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |Ses-Sender TTL |                                               |
 +-+-+-+-+-+-+-+-+                                               +
 |                        MBZ (15 octets)                        |
 |                                                               |
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 |                        HMAC (16 octets)                       |
 |                                                               |
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Fields are defined as the following:¶

Transmit Timestamp (32-bit): This is 32-bit nano-sec field of the PTPv2 timestamp on transmit side. This field may carry Sequence Number instead of PTPv2 timestamp.¶

Transmit Counter (64-bit): The number of packets or octets transmitted by the Session-Reflector in the DLM reply probe packet. Counter is for the reverse direction data traffic flow under measurement. The Session-Reflector writes the Transmit Counter at the same location in the DLM reply probe packet as the Session-Sender DLM probe packet. This is an important property for hardware-based implementation (e.g., in an ASIC).¶

XBTS Flags (3-bit): The XBTS Flags for the reverse direction data traffic flow under measurement set using the same procedure defined for the Session-Sender DLM probe packet.¶

DSCP (6-bit): Set for the reverse direction data traffic flow under measurement using the same procedure defined for the Session-Sender DLM probe packet.¶

Block Number (6-bit): Set for the reverse direction data traffic flow under measurement using the same procedure defined for the Session-Sender DLM probe packet.¶

SSID: SSID is the exact copy of the SSID in the received Session-Sender DLM probe packet.¶

Receive Counter (64-bit): The number of packets or octets received at the Session-Reflector. It is written by the Session-Reflector in the DLM reply probe packet. Counter is for the data traffic flow under measurement.¶

Session-Sender Counter (64-bit): This is the exact copy of the Transmit Counter from the received Session-Sender DLM probe packet.¶

Session-Sender Transmit Timestamp (32-bit): This is the exact copy of the Transmit Timestamp from the received Session-Sender DLM probe packet.¶

Session-Sender Block Number: This is the exact copy of the Block Number from the received Session-Sender DLM probe packet.¶

Session-Sender FLAGS: This is the exact copy of the XBTS Flags from the received Session-Sender DLM probe packet.¶

Session-Sender DSCP: This is the exact copy of the DSCP from the received Session-Sender DLM probe packet.¶

Session-Sender TTL: The Session-Sender TTL field is one octet long, and its value is the copy of the TTL field in IPv4 (or Hop Limit in IPv6) from the received Session-Sender DLM probe packet.¶

6. Data Loss Calculation

Using the Counters C1, C2, C3 and C4 as per reference topology, from the nth and (n-1)th DLM probe packets, packet loss and byte loss for the data traffic flow can be calculated as follows:¶

Transmit Loss TxL[ n-1, n] = (C1[ n] - C1[ n-1]) - (C2[ n] - C2[ n-1])¶

Receive Loss RxL[ n-1, n] = (C3[ n] - C3[ n-1]) - (C4[ n] - C4[ n-1])¶

The Total Transmit and Receive Loss are calculated as follows:¶

Total Transmit Loss = TxL[ 1, 2] + TxL[ 2, 3] + ...¶

Total Receive Loss = RxL[ 1, 2] + RxL[ 2, 3] + ...¶

These values are updated each time a DLM reply probe packet is received and processed at the Session-Sender, and they represent the Total Transmit and Total Receive Loss since the DLM session was initiated. When computing the values TxL[n-1,n] and RxL[n-1,n], the possibility of counter wrap must be taken into account.¶

When using Alternate-Marking Method, all Counters used for loss calculation belongs to the same Block Number, as described in Section 3.1 of [RFC9341].¶

7. Optional Extensions

There are currently no optional (TLV) extensions defined for the DLM probe packets.¶

8. Integrity Protection and Confidentiality Protection

The integrity protection and confidentiality protection specified in [RFC8762] also apply to the procedures defined in this document.¶

9. Operational Considerations

The operational considerations specified in [RFC8762] also apply to the procedures defined in this document.¶

10. Security Considerations

The DLM protocol defined in this document is intended for deployment in a single operator network domain. As such, the Session-Sender address and Session-Reflector address are provisioned by the operator for the DLM session. It is assumed that the operator has verified the integrity of the path and identity of the far-end Session-Reflector.¶

If desired, attacks can be mitigated by performing basic validation and sanity checks, at the Session-Sender, of the counter fields (e.g., packet loss is not negative) in received reply probe packets. The minimal state associated with these protocols also limits the extent of measurement disruption that can be caused by a corrupt or invalid probe packet to a single probe cycle.¶

The DLM protocol uses UDP port that could become a target of denial of service (DoS) or could be used to aid on-path attacks. Thus, the security considerations and measures to mitigate the risk of the attack documented in Section 6 of [RFC8545] equally apply to the STAMP extensions in this document.¶

The security considerations specified in [RFC8762] and [RFC8972] also apply to the protocol defined in this document. Specifically, the message integrity protection using HMAC, as defined in [RFC8762] Section 4.4, also apply to the procedure described in this document.¶

11. IANA Considerations

This document has no IANA actions.¶

Acknowledgments

The authors would like to thank Greg Mirsky, Tianran Zhou, Gyan Mishra, Zhenqiang Li, Reshad Rahman, Cheng Li, and Yali Wang for the comments on Direct Loss Measurement. The authors would like to thank Pierpaolo Loreti, Stefano Salsano, and the team for the Open Source implementation of SRv6-PM Loss Monitoring and its publications in [SRV6-PM-TNSM] and [SRV6-PM-IEEE]. The authors would like to acknowledge the earlier work on the loss measurement using TWAMP described in draft-xiao-ippm-twamp-ext-direct-loss.¶

Authors' Addresses