]> Cisco Systems, Inc.

Canada rgandhi@cisco.com

Cisco Systems, Inc.

cfilsfil@cisco.com

Bell Canada

daniel.voyer@bell.ca

Universita di Roma "Tor Vergata"

Italy stefano.salsano@uniroma2.it

Huawei

mach.chen@huawei.com

MPLS Working Group Segment Routing (SR) leverages the source routing paradigm. RFC 6374 specifies protocol mechanisms to enable the efficient and accurate measurement of packet loss, one-way and two-way delay, as well as related metrics such as delay variation in MPLS networks using probe messages. This document utilizes these mechanisms for Performance Delay and Loss Measurements in Segment Routing networks with MPLS data plane (SR-MPLS), for both SR-MPLS links and end-to-end paths including SR-MPLS Policies. In addition, this document defines Return Path TLV for two-way performance measurement and Block Number TLV for loss measurement.

Service provider's ability to satisfy Service Level Agreements (SLAs) depend on the ability to measure and monitor performance metrics for packet loss and one-way and two-way delay, as well as related metrics such as delay variation. The ability to monitor these performance metrics also provides operators with greater visibility into the performance characteristics of their networks, thereby facilitating planning, troubleshooting, and network performance evaluation. Segment Routing (SR) leverages the source routing paradigm and greatly simplifies network operations for Software Defined Networks (SDNs). SR is applicable to both Multiprotocol Label Switching (SR-MPLS) and IPv6 (SRv6) data planes. SR takes advantage of the Equal-Cost Multipaths (ECMPs) between source and transit nodes, between transit nodes and between transit and destination nodes. SR Policies as defined in are used to steer traffic through a specific, user-defined paths using a stack of Segments. Built-in SR Performance Measurement (PM) is one of the essential requirements to provide Service Level Agreements (SLAs). specifies protocol mechanisms to enable the efficient and accurate measurement of performance metrics in MPLS networks using probe messages. These mechanisms are also well-suited for Segment Routing networks when using MPLS data plane (SR-MPLS). also supports "direct mode" Loss Measurement (LM), which is required in SR networks. specifies the procedures to be used when sending and processing out-of-band performance measurement probe responses over an UDP return path when receiving RFC 6374 based probe queries. These procedures can be used to send out-of-band probe responses for both SR-MPLS links and Policies for one-way measurement. This document utilizes the probe-based mechanisms defined in for Performance Delay and Loss Measurements in SR networks with MPLS data plane, for both SR-MPLS links and end-to-end paths including SR-MPLS Policies. In addition, this document defines Return Path TLV for two-way performance measurement and Block Number TLV for loss measurement. The Performance Measurements (PM) for SR-MPLS links are used to compute extended Traffic Engineering (TE) metrics for delay and loss and can be advertised in the network using the routing protocol extensions defined in , , and .

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in when, and only when, they appear in all capitals, as shown here.

ACH: Associated Channel Header. DM: Delay Measurement. ECMP: Equal Cost Multi-Path. G-ACh: Generic Associated Channel (G-ACh). GAL: Generic Associated Channel (G-ACh) Label. LM: Loss Measurement. MPLS: Multiprotocol Label Switching. NTP: Network Time Protocol. PM: Performance Measurement. PSID: Path Segment Identifier. PTP: Precision Time Protocol. SID: Segment ID. SL: Segment List. SR: Segment Routing. SR-MPLS: Segment Routing with MPLS data plane. TC: Traffic Class. TE: Traffic Engineering. URO: UDP Return Object.

In the reference topology shown in Figure 1, the querier node R1 initiates a performance measurement probe query and the responder node R3 sends a probe response message for the query message received. The probe response message is sent back to the querier node R1. SR is enabled with MPLS data plane on nodes R1 and R3. The nodes R1 and R3 may be directly connected via a link enabled with MPLS or there exists a Point-to-Point (P2P) SR-MPLS path e.g. Policy on node R1 (called head-end) with destination to node R3 (called tail-end).

| | | R1 |=====================| R3 | | |<- - - - - - - - - - | | +-------+ Response +-------+ \ / T4 T3 Querier Responder ]]>

For one-way, two-way and round-trip delay measurements, the procedures defined in Section 2.4 and Section 2.6 of are used. For transmit and receive packet loss measurements, the procedures defined in Section 2.2 and Section 2.6 of are used. For Performance Measurement, probe query and response messages are sent as following: For delay measurement, the probe messages are sent in-band (on the same path as the data traffic) by the querier node, and are used to measure the delay experienced by the actual data traffic flowing on the SR-MPLS links and Policies. For loss measurement, the probe messages are sent in-band (on the same path as the data traffic) by the querier node, and are used to collect the receive traffic counters for the incoming link or incoming SID where the probe query messages are received at the responder node (using incoming link or incoming SID avoids additional state on the responder. The In-Situ Operations, Administration, and Maintenance (IOAM) mechanisms for SR-MPLS defined in are used to carry PM information in-band as part of the data traffic packets, and are outside the scope of this document.

As described in Section 2.9.1 of , probe query and response messages flow over the MPLS Generic Associated Channel (G-ACh). A probe message for SR-MPLS links contains G-ACh Label (GAL) (with S=1). The GAL is followed by an Associated Channel Header (ACH), which identifies the message type, and the message payload following the ACH as shown in Figure 2. The probe messages are routed over the links for both delay and loss measurement using the same procedure described in . For SR-MPLS links, the TTL value is set to 1 in the SR-MPLS header for one-way and two-way measurement modes.

As described in Section 2.9.1 of , probe query and response messages flow over the MPLS Generic Associated Channel (G-ACh). A probe message for an end-to-end SR-MPLS Policy measurement contains SR-MPLS label stack , with the G-ACh Label (GAL) at the bottom of the stack (with S=1). The GAL is followed by an Associated Channel Header (ACH), which identifies the message type, and the message payload following the ACH as shown in Figure 3. For SR-MPLS Policies, the TTL value is set to 255 in the SR-MPLS header.

The SR-MPLS label stack can be empty (as shown in Figure 2) to indicate Implicit NULL label case. For SR-MPLS Policy performance measurement, in order to ensure that the probe query message is processed by the intended responder node, Destination Address TLV (Type 129) MAY be sent in the probe query message. The responder node only returns Success in Control Code if it is the intended destination for the probe query. Otherwise, it MUST return 0x15: Error - Invalid Destination Node Identifier .

In one-way performance measurement mode , the querier node can receive "out-of-band" probe responses by properly setting the UDP Return Object (URO) TLV in the probe query message. The URO TLV (Type=131) is defined in and includes the UDP-Destination-Port and IP Address. In particular, if the querier node sets its own IP address in the URO TLV, the probe response is sent back by the responder node to the querier node. In addition, the "control code" in the probe query message is set to "out-of-band response requested". In this delay measurement mode, as per Reference Topology, timestamps T1 and T2 are collected by the probes to measure one-way delay. The one-way mode is applicable to both SR-MPLS links and Policies.

In two-way performance measurement mode , when using a bidirectional SR path , the probe response message is sent back to the querier node in-band (on the same path as the data traffic) on the reverse direction SR-MPLS link or associated SR-MPLS Policy using a message with format similar to their probe query message. In this case, the "control code" in the probe query message is set to "in-band response requested". In this delay measurement mode, as per Reference Topology, all timestamps T1, T2, T3, and T4 are collected by the probes. All four timestamps are used to measure two-way delay. The two-way mode is applicable to both SR-MPLS links and Policies. Specifically, the probe response message is sent back on the incoming physical interface where the probe query message is received. This is useful for example, in case of two-way measurement mode for link delay. The Path Segment Identifier (PSID) of the forward SR-MPLS Policy in the probe query can be used to find the associated reverse SR-MPLS Policy to send the probe response message for two-way measurement of SR-MPLS Policy unless when using the Return Path TLV.

The Loopback measurement mode defined in Section 2.8 of can be used to measure round-trip delay for a bidirectional SR-MPLS path . The probe query messages in this case carries the return Path label stack as part of the MPLS header. The GAL is still carried at the bottom of the label stack (with S=1). The responder node does not process the probe messages and generate response messages, and hence Loopback Request object (Type 3) is not required for SR. In this delay measurement mode, as per Reference Topology, the timestamps T1 and T4 are collected by the probes. Both these timestamps are used to measure round-trip delay.

For two-way performance measurement, the responder node needs to send the probe response message on a specific return path. The querier node can request in the probe query message to the responder node to send a response message back on a given return path (e.g. co-routed path for two-way measurement). This way the responder does not require any additional state for return path. For one-way performance measurement, the querier node address may not be reachable via IP route from the responder node. The querier node in this case needs to send its reachability path information to the responder node. defines DM and LM probe query messages that can include one or more optional TLVs. New TLV Type (TBA1) is defined in this document for Return Path to carry return path information in the probe query messages (in the payload). The format of the Return Path TLV is shown in Figure 4:

The Return Path TLV contains a Sub-TLV to carry the return path. The format of the Segment-List Sub-TLV is shown in Figure 5. The Label entries MUST be in network order. The Segment List Sub-TLV in the Return Path TLV can be one of the following Types: Type (value 1): SR-MPLS Label Stack of the Return Path Type (value 2): SR-MPLS Binding SID of the Reverse SR Policy

The Return Path TLV is a Mandatory TLV Type. The querier node MUST only insert one Return Path TLV in the probe query message. The Return Path TLV MUST carry only one Return Path Sub-TLV. The responder node MUST only process the first Return Path TLV and first Return Path Sub-TLV in the probe query message and ignore other Return Path TLVs if present. The responder node MUST send probe response message back on the return path specified in the Return Path TLV and MUST NOT add Return Path TLV in the probe response message.

As defined in , MPLS DM probe query and response messages use Associated Channel Header (ACH) (value 0x000C for delay measurement) , which identifies the message type, and the message payload following the ACH. For both SR-MPLS links and end-to-end Policies measurements, the same MPLS DM ACH value is used. The DM message payload as defined in Section 3.2 of is used for SR-MPLS delay measurement, for both SR-MPLS links and end-to-end Policies.

The Section 3.4 of defines timestamp format that can be used for delay measurement. The IEEE 1588 Precision Time Protocol (PTP) timestamp format is used by default as described in Appendix A of , with hardware support recommended in Segment Routing networks.

The LM protocol can perform two distinct kinds of loss measurement as described in Section 2.9.8 of . In inferred mode, LM will measure the loss of specially generated test messages in order to infer the approximate data plane loss level. Inferred mode LM provides only approximate loss accounting. In direct mode, LM will directly measure data plane packet loss. Direct mode LM provides perfect loss accounting, but may require hardware support. For both of these modes of LM, Path Segment Identifier (PSID) is used for accounting received traffic on the egress node of the SR-MPLS Policy as shown in Figure 6. Different values of PSID can be used to measure packet loss per SR-MPLS Policy, per Candidate Path or per Segment List of the SR Policy.

As defined in , MPLS LM probe query and response messages use Associated Channel Header (ACH) (value 0x000A for direct loss measurement or value 0x000B for inferred loss measurement), which identifies the message type, and the message payload following the ACH. For both SR-MPLS links and end-to-end Policies measurements, the same MPLS LM ACH value is used. The LM message payload as defined in Section 3.1 of is used for SR-MPLS loss measurement, for both SR-MPLS links and end-to-end Policies.

The loss measurement using Alternate-Marking method defined in requires to color the data traffic. To be able to correlate the transmit and receive traffic counters of the matching color, the Block Number (or color) of the traffic counters is carried by the probe query and response messages for loss measurement. defines probe query and response messages that can include one or more optional TLVs. New TLV Type (value TBA2) is defined in this document to carry the Block Number (8-bit) of the traffic counters in the probe query and response messages for loss measurement. The format of the Block Number TLV is shown in Figure 7:

The Block Number TLV is a Mandatory TLV Type. The querier node MUST only insert one Block Number TLV in the probe query message and the responder node in the probe response message MUST return the first Block Number TLV from the probe query messages and ignore other Block Number TLVs if present.

As defined in , Combined DM+LM probe query and response messages use Associated Channel Header (ACH) (value 0x000D for direct loss and delay measurement or value 0x000E for inferred loss and delay measurement), which identifies the message type, and the message payload following the ACH. For both SR-MPLS links and end-to-end Policies measurements, the same MPLS ACH value is used. The message payload as defined in Section 3.3 of is used for SR-MPLS combined delay and loss measurement, for both SR-MPLS links and end-to-end Policies.

The Point-to-Multipoint (P2MP) SR-MPLS path that originates from a root node terminates on multiple destinations called leaf nodes (e.g. P2MP SR-MPLS Policy or P2MP Transport ). The procedures for delay and loss measurement described in this document for P2P SR-MPLS Policies are also equally applicable to the P2MP SR-MPLS Policies. The procedure for one-way measurement is defined as following: The querier root node sends probe query messages using the Tree-SID defined in for the P2MP SR-MPLS Policy as shown in Figure 8. The probe query messages can contain the replication SID as defined in . Each responder leaf node adds the "Source Address" TLV (Type 130) with its IP address in the probe response messages. This TLV allows the querier root node to identify the responder leaf nodes of the P2MP SR-MPLS Policy. The P2MP root node measures the delay and loss performance for each P2MP leaf node of the end-to-end P2MP SR-MPLS Policy.

The probe query messages can also be sent using the scheme defined for P2MP Transport using Chain Replication that may contain Bud SID as defined in . The considerations for two-way mode for performance measurement for P2MP SR-MPLS Policy (e.g. for bidirectional SR-MPLS path) are outside the scope of this document.

An SR-MPLS Policy can have ECMPs between the source and transit nodes, between transit nodes and between transit and destination nodes. Usage of Anycast SID by an SR-MPLS Policy can result in ECMP paths via transit nodes part of that Anycast group. The probe messages need to be sent to traverse different ECMP paths to measure performance delay of each of the ECMP path of an SR-MPLS Policy. Forwarding plane has various hashing functions available to forward packets on specific ECMP paths. For SR-MPLS Policy, sweeping of entropy label values can be used in probe messages to take advantage of the hashing function in forwarding plane to influence the ECMP path taken by them. The considerations for performance loss measurement for different ECMP paths of an SR-MPLS Policy are outside the scope of this document.

The extended TE metrics for SR-MPLS link delay and loss computed using the performance measurement procedures described in this document can be advertised in the routing domain as follows: For OSPF, ISIS, and BGP-LS, protocol extensions defined in , , and are used, respectively for advertising the extended TE link metrics in the network. The advertised delay-variance metric is computed as specified in Section 4.2 of . The extended TE link one-way delay metrics can also be computed using two-way delay measurement or round-trip delay measurement from loopback mode by dividing the measured delay values by 2. The extended TE link delay and loss metrics are advertised for Layer 2 bundle members in OSPF and ISIS using the same mechanisms defined in and , respectively.

The procedures defined in this document are backwards compatible with the procedures defined in at both querier and responder nodes. If the responder does not support the new Mandatory TLV Types defined in this document, it MUST return Error 0x17: Unsupported Mandatory TLV Object as per .

This document describes the procedures for performance delay and loss measurement for SR-MPLS networks, for both SR-MPLS links and end-to-end Policies using the mechanisms defined in and . This document does not introduce any additional security considerations other than those covered in , , , , and .

IANA is requested to allocate a value for the following Mandatory Block Number TLV Type for to be carried in the probe query and response messages from the "MPLS Loss/Delay Measurement TLV Object" registry contained within the "Generic Associated Channel (G-ACh) Parameters" registry set: Type TBA2: Block Number TLV IANA is also requested to allocate a value for the following Mandatory Return Path TLV Type for to be carried in the probe query messages from the "MPLS Loss/Delay Measurement TLV Object" registry contained within the "Generic Associated Channel (G-ACh) Parameters" registry set: Type TBA1: Return Path TLV IANA is requested to create the “Return Path Sub-TLV” sub-registry as part of the Return Path TLV registry. All code points in the range 1 through 127 in this registry shall be allocated according to the "IETF Review" procedure as specified in . Code points in the range 128 through 239 in this registry shall be allocated according to the "First Come First Served" procedure as specified in . Remaining code points are allocated according to : Value Description Reference 0 Reserved This document 1 - 127 Unassigned This document 128 - 239 Unassigned This document 240 - 249 Experimental This document 250 - 254 Private Use This document 255 Reserved This document This document defines the following new values in the Return Path Sub-TLV sub-registry: Value Description Reference 1 SR-MPLS Label Stack of the Return Path This document 2 SR-MPLS Binding SID of the Reverse SR Policy This document

&RFC2119; &RFC6374; &RFC7876; &RFC8174; IEEE &RFC5481; &RFC6790; &RFC7679; &RFC7471; &RFC8126; &RFC8321; &RFC8402; &RFC8570; &RFC8571; &RFC8668; &I-D.ietf-spring-segment-routing-policy; &I-D.ietf-pim-sr-p2mp-policy; &I-D.ietf-spring-sr-replication-segment; &I-D.ietf-pce-binding-label-sid; &I-D.ietf-spring-mpls-path-segment; &I-D.ietf-lsr-ospf-l2bundles; &I-D.ietf-pce-sr-bidir-path; &I-D.shen-spring-p2mp-transport-chain; &I-D.gandhi-mpls-ioam-sr;

The authors would like to thank Thierry Couture for the discussions on the use-cases for the performance measurement in segment routing networks. Authors would like to thank Patrick Khordoc for implementing the mechanisms defined in this document. The authors would like to thank Greg Mirsky for providing many useful comments and suggestions. The authors would also like to thank Stewart Bryant, Sam Aldrin, Tarek Saad, and Rajiv Asati for their review comments. Thanks to Huaimo Chen, Yimin Shen, and Xufeng Liu for MPLS-RT expert review.