| Internet-Draft | Simple TWAMP Extensions for SR Networks | September 2022 |
| Gandhi, et al. | Expires 6 March 2023 | [Page] |
Segment Routing (SR) leverages the source routing paradigm. SR is applicable to both Multiprotocol Label Switching (SR-MPLS) and IPv6 (SRv6) forwarding planes. This document specifies RFC 8762 (Simple Two-Way Active Measurement Protocol (STAMP)) extensions for SR networks, for both SR-MPLS and SRv6 forwarding planes by augmenting the optional extensions defined in RFC 8972.¶
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Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.¶
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This Internet-Draft will expire on 6 March 2023.¶
Copyright (c) 2022 IETF Trust and the persons identified as the document authors. All rights reserved.¶
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Segment Routing (SR) leverages the source routing paradigm for Software Defined Networks (SDNs). SR is applicable to both Multiprotocol Label Switching (SR-MPLS) and IPv6 (SRv6) forwarding planes [RFC8402]. SR Policies as defined in [RFC9256] are used to steer traffic through a specific, user-defined paths using a stack of Segments. A comprehensive SR Performance Measurement (PM) toolset is one of the essential requirements to measure network performance to provide Service Level Agreements (SLAs).¶
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. [RFC8972] defines optional extensions, in the form of TLVs, for STAMP. Note that the YANG data model defined in [I-D.ietf-ippm-stamp-yang] can be used to provision the STAMP Session-Sender and STAMP Session-Reflector.¶
The STAMP test packets are transmitted along an IP path between a Session-Sender and a Session-Reflector to measure performance delay and packet loss along that IP path. It may be desired in SR networks that the same path (same set of links and nodes) between the Session-Sender and Session-Reflector is used for the STAMP test packets in both directions. This is achieved by using the STAMP [RFC8762] extensions for SR-MPLS and SRv6 networks specified in this document by augmenting the optional extensions defined in [RFC8972].¶
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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
MPLS: Multiprotocol Label Switching.¶
PM: Performance Measurement.¶
SID: Segment ID.¶
SL: Segment List.¶
SR: Segment Routing.¶
SR-MPLS: Segment Routing with MPLS forwarding plane.¶
SRv6: Segment Routing with IPv6 forwarding plane.¶
SSID: STAMP Session Identifier.¶
STAMP: Simple Two-Way Active Measurement Protocol.¶
In the reference topology shown below, the STAMP Session-Sender S1 initiates a STAMP test packet and the STAMP Session-Reflector R1 transmits a reply STAMP test packet. The reply test packet may be transmitted to the Session-Sender S1 on the same path (same set of links and nodes) or a different path in the reverse direction from the path taken towards the Session-Reflector R1.¶
The nodes S1 and R1 may be connected via a link or an SR path [RFC8402]. The link may be a physical interface, virtual link, or Link Aggregation Group (LAG) [IEEE802.1AX], or LAG member. The SR path may be an SR Policy [RFC9256] on node S1 (called head-end) with destination to node R1 (called tail-end).¶
T1 T2
/ \
+-------+ Test Packet +-------+
| | - - - - - - - - - ->| |
| S1 |=====================| R1 |
| |<- - - - - - - - - - | |
+-------+ Reply Test Packet +-------+
\ /
T4 T3
STAMP Session-Sender STAMP Session-Reflector
Reference Topology
¶
The STAMP TLV option in [RFC8972] defines the use of the 8-bit flags field common to all STAMP TLVs.¶
A one-bit flag called Verification Check (V) is defined at bit position 3 in the flags field of the STAMP TLV. A Session-Sender MUST set the V flag to 0 before transmitting an extended STAMP test packet when reply test packet is required. A Session-Reflector MUST set the V flag to 1 for any STAMP TLV that it supports that includes an instruction or request for data that cannot be followed or conflicts with the Session-Reflector processing or capability, for example, when using Stateless Session-Reflector. The V flag MUST be set to 0 by the Session-Reflector when the instruction or the request for data from the TLV in the test packet was followed.¶
A Session-Sender MUST set the V flag to 1 before transmitting an extended STAMP test packet when test packet reply is not required. A Session-Reflector MUST NOT reply and MUST drop the test packet if the Session-Reflector determined for any STAMP TLV that is supports that includes an instruction or request for data in any of the TLVs that cannot be followed or is in conflict with the Session-Reflector policy or capability, for example, when using Stateless Session-Reflector.¶
For the STAMP Session-Reflector that supports the STAMP TLV extensions defined in this document, the test packets may carry additional instructions in a TLV for the Session-Reflector to follow. In this case, the V flag provides feedback to the Session-Sender if the Session-Reflector was able to follow that instruction or reply with the requested data. For example, Session-Reflector supports the TLV and it is well-formed, the STAMP test packet including all the TLVs was successfully processed but the additional instruction or request for data in one of the TLVs was not followed. Another example is when using the "Direct Measurement" TLV defined in [RFC8972], the Stateless Session-Reflector that supports this TLV but was not able to return the requested Tx and Rx counters in the TLV. The Session-Reflector can either drop the packet or return the packet with the error back to the Session-Sender using the V flag, based on the request in the test packet and local policy.¶
The Session-Sender may need to transmit test packets to the Session-Reflector with a different destination address that is not matching an address on the Session-Reflector e.g. when the STAMP test packet is encapsulated by a tunneling protocol or an MPLS Segment List with destination IPv4 address from 127/8 range or Segment Routing Header (SRH) with destination IPv6 address ::1/128. When using IPv4 destination address from 127/8 range (e.g. for measurement in an ECMP environment), the STAMP test packet may not reach the intended Session-Reflector in an error condition, and an un-intended node may transmit reply test packet resulting in reporting of invalid measurement metrics.¶
[RFC8972] defines STAMP test packets that can include one or more optional TLVs. In this document, the TLV type (value 9) is defined for the Destination Node Address TLV for the STAMP test packet [RFC8972]. The format of the Destination Node Address TLV is shown in Figure 1:¶
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |STAMP TLV Flags| Type=9 | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ . Address . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Length field is used to decide the Address Family of the Address.¶
The STAMP TLV Flags are set using the procedures described in [RFC8972].¶
The Destination Node Address TLV is optional. The Destination Node Address TLV indicates the address of the intended Session-Reflector node of the test packet. The Destination Node Address is also used to uniquely identify the STAMP session on the Session-Reflector when the optional SSID is not sent. For security reasons (e.g., to avoid node discovery), the Session-Reflector SHOULD use the received Destination Node Address as the Source Address in the IP header of the reply test packet, instead of using its Node Address. The Session-Reflector MUST add the received Destination Node Address TLV in the reply test packet to ensure the symmetric reply test packet size and to transmit the STAMP TLV Flags to the Session-Sender.¶
A Session-Sender MUST set the V flag to 0 in the Destionation Node Address TLV before transmitting an extended STAMP test packet when test packet reply is required. A Session-Reflector that supports this TLV, MUST set the V flag in the reply test packet to 1 if the Session-Reflector determined that it is not the intended Destination as identified in the Destination Node Address TLV. Otherwise, the Session-Reflector MUST set the V flag in the Destination Node Address TLV in the reply test packet to 0.¶
A Session-Sender MUST set the V flag to 1 before transmitting an extended STAMP test packet when test packet reply is not required. A Session-Reflector that supports this TLV, MUST NOT reply and MUST drop the test packet if the Session-Reflector determined that it is not the intended Destination as identified in the Destination Node Address TLV. This behaviour may be desired in some networks for the security reasons (e.g., to avoid node discovery).¶
For end-to-end SR paths, the Session-Reflector may need to transmit the reply test packet on a specific return path. The Session-Sender can request this in the test packet to the Session-Reflector using a Return Path TLV. With this TLV carried in the Session-Sender test packet, signaling and maintaining dynamic SR network state for the STAMP sessions on the Session-Reflector are avoided.¶
There are two modes defined for the behaviors on the Session-Reflector in Section 4 of [RFC8762]. A Stateful Session-Reflector that requires configuration that must match all Session-Sender parameters, including Source Address, Destination Address, Source UDP Port, Destination UDP Port, and possibly SSID (assuming the SSID is configurable and not auto-generated). In this case, a local policy can be used to direct the test packet by creating additional states for the STAMP sessions on the Session-Reflector. In the case of promiscuous operation, the Stateless Session-Reflector will require an indication of how to return the test packet on a specific path, for example, measurement in an ECMP environment.¶
For links, the Session-Reflector may need to transmit the reply test packet on the same incoming link in the reverse direction. The Session-Sender can request this in the test packet to the Session-Reflector using a Return Path TLV.¶
[RFC8972] defines STAMP test packets that can include one or more optional TLVs. In this document, the TLV Type (value 10) is defined for the Return Path TLV that carries the return path for the Session-Sender test packet. The format of the Return Path TLV is shown in Figure 2:¶
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |STAMP TLV Flags| Type=10 | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Return Path Sub-TLVs | . . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The STAMP TLV Flags are set using the procedures described in [RFC8972].¶
The Return Path TLV is optional. The Session-Sender MUST only insert one Return Path TLV in the STAMP test packet. The Session-Reflector that supports this TLV, MUST only process the first Return Path TLV in the test packet and ignore other Return Path TLVs if present, and it MUST add the received Return Path TLV (including all Sub-TLVs) in the reply test packet to ensure the symmetric reply test packet size and to transmit the STAMP TLV Flags to the Session-Sender. The Session-Reflector that supports this TLV MUST reply using the Return Path received in the Session-Sender test packet. In the case where the Session-Reflector does not support this TLV, the procedure defined in [RFC8762] is followed by the Session-Reflector.¶
A Session-Sender MUST set the V flag to 0 before transmitting an extended STAMP test packet when test packet reply is required. A Session-Reflector that supports this TLV, MUST set the V flag in the reply test packet to 1 if the Session-Reflector determined that it cannot use the return path in the test packet to transmit the reply test packet. Otherwise, the Session-Reflector MUST set the V flag in the reply test packet to 0.¶
A Session-Sender MUST set the V flag to 1 before transmitting an extended STAMP test packet when test packet reply is not required. A Session-Reflector that supports this TLV, MUST NOT reply and MUST drop the test packet if the Session-Reflector determined that it cannot use the return path in the test packet to transmit the reply test packet.¶
The Return Path TLV contains one or more Sub-TLVs to carry the information for the requested return path. A Return Path Sub-TLV can carry Return Path Control Code, Return Path IP Address or Return Path Segment List.¶
The STAMP Sub-TLV Flags are set using the procedures described in [RFC8972].¶
When Return Path Sub-TLV is present in the Session-Sender test packet, the Session-Reflector that supports this TLV, MUST transmit reply test packet using the return path information specified in the Return Path Sub-TLV.¶
A Return Path TLV MUST NOT contain both Control Code Sub-TLV as well as Return Address or Return Segment List Sub-TLV.¶
The format of the Return Path Control Code Sub-TLV is shown in Figure 3. The Type of the Return Path Control Code Sub-TLV is defined as following:¶
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |STAMP TLV Flags| Type=1 | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Control Code | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Control Code Flags (32-bit): Defined as follows.¶
When Control Code flag is set to 0x0 in the Session-Sender test packet, the Session-Reflector does not transmit reply test packet to the Session-Sender and terminates the STAMP test packet. Only the one-way measurement is applicable in this case. Optionally, the Session-Reflector may locally stream performance metrics via telemetry using the information from the received test packet. All other Return Path Sub-TLVs MUST be ignored in this case.¶
When Control Code flag is set to 0x1 in the Session-Sender test packet, the Session-Reflector transmits the reply test packet over the same incoming link where the test packet is received in the reverse direction towards the Session-Sender. All other Return Path Sub-TLVs MUST be ignored in this case.¶
The STAMP reply test packet may be transmitted to the Session-Sender to a different destination address on the Session-Sender using Return Path TLV. For this, the Session-Sender can specify in the test packet the receiving destination node address for the Session-Reflector reply test packet. When transmitting the STAMP test packet to a different destination address, the Session-Sender MUST follow the procedure defined in Section 4.3 of [RFC8762].¶
The format of the Return Address Sub-TLV is shown in Figure 4. The Address Family field indicates the type of the address, and it SHALL be set to one of the assigned values in the "IANA Address Family Numbers" registry. The Type of the Return Address Sub-TLV is defined as following:¶
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |STAMP TLV Flags| Type=2 | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Reserved | Address Family | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ . Address . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The format of the Segment List Sub-TLVs in the Return Path TLV is shown in Figures 5, 6, and 7. The segment entries MUST be in network order. The Segment List Sub-TLV can be one of the following Types:¶
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |STAMP TLV Flags| Type=3 | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Segment(1) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ . . . . . . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Segment(n) (bottom of stack) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |STAMP TLV Flags| Type=4 | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Segment(1) | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ . . . . . . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Segment(n) (bottom of stack) | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |STAMP TLV Flags| Type=5 | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | LB Length | LN Length | Fun. Length | Arg. Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Segment(1) | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ . . . . . . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | LB Length | LN Length | Fun. Length | Arg. Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Segment(n) (bottom of stack) | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The SR-MPLS Label Stack contains a list of 32-bit Label Stack Entry (LSE) that includes a 20-bit label value, 8-bit TTL value, 3-bit TC value and 1-bit EOS field. An SR-MPLS Label Stack Sub-TLV may carry only Binding SID Label [I-D.ietf-pce-binding-label-sid] of the Return SR-MPLS Policy.¶
An SRv6 Segment List Sub-TLV and Structured SRv6 Segment List Sub-TLV may carry only Binding SID [I-D.ietf-pce-binding-label-sid] of the Return SRv6 Policy.¶
A Structured SRv6 Segment List Sub-TLV is used carry the structure and behavior for SRv6 SIDs [RFC8986] used in the Return SRv6 path as shown in Figure 7. The structure is intended for informational use by the control and management planes. The fields in the structure of the Sub-TLV are defined as follows [RFC8986]:¶
In Structured SRv6 Segment List Sub-TLV, the sum of all four sizes MUST be less than or equal to 128 bits. If the sum of all four sizes is larger than 128 bits, the Sub-TLV MUST NOT be used by the Session-Reflector.¶
The Session-Sender MUST only insert one Segment List Return Path Sub-TLV in the test packet. The Session-Reflector MUST only process the first Segment List Return Path Sub-TLV in the test packet and ignore other Segment List Return Path Sub-TLVs if present.¶
Note that in addition to P2P SR paths, the Return Segment List Sub-TLV is also applicable to P2MP SR paths. For example, for P2MP SR paths, it may only carry the Node Segment Identifier of the Session-Sender in order for the reply test packet to follow an SR path to the Session-Sender.¶
The usage of STAMP protocol is intended for deployment in limited domains [RFC8799]. As such, it assumes that a node involved in STAMP protocol operation has previously verified the integrity of the path and the 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 timestamp fields in received reply test packets. The minimal state associated with these protocols also limits the extent of measurement disruption that can be caused by a corrupt or invalid test packet to a single test cycle.¶
The security considerations specified in [RFC8762] and [RFC8972] also apply to the extensions 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.¶
STAMP uses the well-known UDP port number that could become a target of denial of service (DoS) or could be used to aid man-in-the-middle (MITM) 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 STAMP extensions defined in this document may be used for potential "proxying" attacks. For example, a Session-Sender may specify a return path that has a destination different from that of the Session-Sender. But normally, such attacks will not happen in an SR domain where the Session-Senders and Session-Reflectors belong to the same domain. In order to prevent using the extension defined in this document for proxying any possible attacks, the return path has destination to the same node where the forward path is from. The Session-Reflector may drop the Session-Sender test packet when it cannot determine whether the Return Path has the destination to the Session-Sender. That means, the Session-Sender should choose a proper source address according to the specified Return Path to help the Session-Reflector to make that decision.¶
IANA has created the "STAMP TLV Types" registry for [RFC8972]. IANA has early allocated a value for the Destination Address TLV Type and a value for the Return Path TLV Type from the IETF Review TLV range of the same registry.¶
| Value | Description | Reference |
|---|---|---|
| 9 (Early Allocation) | Destination Node Address | This document |
| 10 (Early Allocation) | Return Path | This document |
IANA is requested to create a sub-registry for "Return Path Sub-TLV Type". All code points in the range 1 through 175 in this registry shall be allocated according to the "IETF Review" procedure as specified in [RFC8126]. Code points in the range 176 through 239 in this registry shall be allocated according to the "First Come First Served" procedure as specified in [RFC8126]. Remaining code points are allocated according to Table 2:¶
| Value | Description | Reference |
|---|---|---|
| 1 - 175 | IETF Review | This document |
| 176 - 239 | First Come First Served | This document |
| 240 - 251 | Experimental Use | This document |
| 252 - 254 | Private Use | This document |
IANA is requested to allocate the values for the following Sub-TLV Types from this registry.¶
| Type | Description | Reference |
|---|---|---|
| 0 | Reserved | This document |
| 1 | Return Path Control Code | This document |
| 2 | Return Address | This document |
| 3 | SR-MPLS Label Stack of the Return Path | This document |
| 4 | SRv6 Segment List of the Return Path | This document |
| 5 | Structured SRv6 Segment List of the Return Path | This document |
| 255 | Reserved | This document |
IANA has created the "STAMP TLV Flags" subregistry. IANA has early allocated the following bit position in the "STAMP TLV Flags" subregistry.¶
| Bit Position | Symbol | Description | Reference |
|---|---|---|---|
| 3 (Early Allocation) | V | Verification Check | This document |
The authors would like to thank Thierry Couture for the discussions on the use-cases for Performance Measurement in Segment Routing. The authors would also like to thank Greg Mirsky, Mike Koldychev, Gyan Mishra, Tianran Zhou, Al Mortons, Reshad Rahman, Zhenqiang Li, Frank Brockners, and Cheng Li for providing comments and suggestions.¶