Encapsulation of Simple Two-Way Active Measurement Protocol for LSPs and Pseudowires in MPLS Networks

Internet-Draft	STAMP in MPLS Networks	August 2026
Gandhi, et al.	Expires 12 February 2027	[Page]

Abstract

This document describes the procedure for encapsulating the Simple Two-Way Active Measurement Protocol (STAMP), defined in RFC 8762, and its optional extensions defined in RFC 8972, in MPLS networks. Label Switched Paths (LSPs) and Pseudowires (PWs) are used in MPLS networks for various services including carrying Layer 2 and Layer 3 data packets and may use the Control Word (CW). The procedure is also described for encapsulating STAMP test packets with or without the CW and/or an IP/UDP header for LSPs and PWs.¶

1. Introduction

The Simple Two-Way Active Measurement Protocol (STAMP) provides capabilities for measuring various metrics in IP networks [RFC8762] without the use of a control channel to pre-signal session parameters. [RFC8972] defines optional extensions for STAMP.¶

Label Switched Paths (LSPs) are used in MPLS networks for various services, including carrying Layer 2 and Layer 3 data packets. LSPs can be point-to-point or point-to-multipoint. STAMP encapsulations for point-to-multipoint LSPs are outside the scope of this document. This document specifies STAMP encapsulations for point-to-point LSPs.¶

Pseudowires (PWs) are used in MPLS networks for various services, including carrying Layer 2 and Layer 3 data packets [RFC6658]. PWs are bidirectional in nature. PWs may use the Control Word (CW) as defined in Section 3 of [RFC4385]. This document covers STAMP encapsulations for point-to-point PWs, whereas point-to-multipoint PWs are outside the scope of this document. PWs can be single-segment PWs or multi-segment PWs. This document specifies STAMP encapsulations for single-segment PWs whereas multi-segment PWs are outside the scope of this document.¶

MPLS Transport Profile (MPLS-TP) [RFC5960] is designed to use the MPLS data plane without any changes. Therefore, when STAMP is specified over an MPLS data plane, it is equally applicable to MPLS-TP networks. As specified in Section 2 of [RFC5921], "OAM and protection mechanisms, and forwarding of data packets, must be able to operate without IP forwarding support".¶

A Generic Associated Channel (G-ACh) [RFC5586] provides a mechanism to transport Operations, Administration, and Maintenance (OAM) and other control messages over the MPLS data plane. The G-ACh types identify the various OAM messages that are being transported over the channel.¶

Virtual Circuit Connectivity Verification (VCCV) is used as a Control Channel for PWs as described in [RFC5085]. A G-ACh can be used as a VCCV Control Channel as described in [RFC7708].¶

When using STAMP for MPLS and MPLS-TP for both LSPs and PWs, there are unique aspects that need to be considered concerning the use of CW, and these aspects are addressed in this document.¶

This document describes the procedure for the encapsulation of STAMP, defined in [RFC8762], and its optional extensions, defined in [RFC8972], for LSPs and PWs in MPLS networks. The procedure is also described for encapsulating STAMP test packets with or without the CW and/or an IP/UDP header for LSPs and PWs.¶

This document defines two new G-ACh types when using STAMP without an IP/UDP header. These types are independent of the PW demultiplexer type and hence applicable to both the PW label and the Layer 2 Tunneling Protocol version 3 (L2TPv3) PW demultiplexer. This document uses the existing G-ACh types for IPv4 and IPv6 when using the STAMP test packets with an IP/UDP header for the LSPs and PWs that carry CW.¶

Additional considerations for encapsulating STAMP for performance measurement of Segment Routing LSPs over the MPLS data plane are described in [I-D.ietf-spring-stamp-srpm-mpls], and are outside the scope of this document.¶

1.1. Requirements

The STAMP test packets need to be transmitted with the same label stack as that used by the LSPs and PWs to ensure proper validation of the underlay path taken by the actual data traffic. Also, the STAMP test packets need to follow the same Equal-Cost Multi-Path (ECMP) underlay path taken by the LSPs and PWs data traffic in the network. PW data traffic may be encapsulated using CW as defined in Section 3 of [RFC4385] and an IP header. As such, STAMP test packets need to be transmitted over these PWs using a G-ACh and an IP/UDP header.¶

When a STAMP test packet is transmitted to a target IP address of a STAMP Session-Reflector, it would be encapsulated for an MPLS LSP by the data plane based on the reachability of the IP address over the LSP. Hence, the STAMP test packets would be treated the same way as the data traffic forwarded over the LSP by the transit nodes along the path.¶

Data traffic over the L2-Specific Sublayer (L2SS), as used in L2TPv3 PWs, carries CW but does not carry an IP/UDP header. As such, STAMP test packets need to be transmitted over these L2TPv3 PWs using a G-ACh carrying only the STAMP payload without any IP/UDP header.¶

Private Line Emulation (PLE) [RFC9801] traffic is sent over a Packet Switched Network (PSN) as Virtual Private Wire Services (VPWS) using PWs. The data packets are encapsulated with PLE CW, but they do not carry any IP header. As such, STAMP test packets need to be transmitted using the same label stack, including the VPWS PW Label as the PLE traffic [RFC9801], and encapsulated using a G-ACh but without an IP/UDP header. This allows STAMP test packets to experience the same forwarding behaviour, follow the same underlay path as the PLE traffic, and avoid different ECMP behaviour on intermediate nodes.¶

The G-ACh types allow for the demultiplexing of the VCCV Control Channel for PWs [RFC7708]. The G-ACh types for STAMP test packets with or without IP/UDP headers are also used to demultiplex the VCCV Control Channel for PWs. Signaling extensions for the VCCV Control Channel for PWs for STAMP are outside the scope of this document.¶

The G-ACh provides support for the OAM Control Channel associated with MPLS-TP [RFC5960] LSPs and PWs. The OAM Control Channel for MPLS-TP needs to be extended to encapsulate STAMP test packets (just like the delay and loss measurement packets defined in [RFC6374]). The G-ACh types for STAMP also allow for the demultiplexing of the OAM Control Channel for MPLS-TP.¶

The requirements for the encapsulation of the STAMP test packets for the LSPs and PWs in MPLS networks can be summarized as follows:¶

The G-ACh MUST support STAMP test packets with an IP/UDP header.¶
The G-ACh MUST support STAMP test packets without an IP/UDP header.¶
The G-ACh types MUST support demultiplexing of the Control Channel for STAMP test packets.¶
Session-Sender test packets MUST follow the underlay path taken by the data traffic that uses CW.¶
Session-Sender test packets MUST follow the same ECMP underlay path taken by the data traffic that uses CW and an Entropy Label defined in [RFC6790].¶
Session-Sender test packets MUST follow the same ECMP underlay path taken by the data traffic that uses CW but does not use an Entropy Label defined in [RFC6790].¶
Session-Reflector test packets MAY follow the reverse underlay path taken by Session-Sender test packets.¶
Session-Reflector test packets MAY follow the same reverse ECMP underlay path taken by Session-Sender test packets.¶

1.2. Examples of MPLS Data Traffic Use Cases

Examples of MPLS data traffic use cases for STAMP test packets with IP/UDP headers:¶

MPLS PW Data Traffic (with CW and IP header)¶
MPLS-TP PW Data Traffic (with CW and IP header)¶
MPLS LSP Data Traffic (with IP header)¶

Examples of MPLS data traffic use cases for STAMP test packets without IP/UDP headers:¶

MPLS Ethernet PW Data Traffic [RFC4448]¶
L2SS used in L2TPv3 PW Data Traffic [RFC3931]¶
Private Line Emulation [RFC9801] PW Data Traffic¶
TDM over IP [RFC5087] PW Data Traffic (with no IP header)¶
MPLS-TP LSP Data Traffic¶

2. Conventions Used in This Document

2.1. 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.¶

2.2. Abbreviations

Table 1: Abbreviations
Abbreviation	Meaning	Reference
CE	Customer Edge	[RFC4026]
CW	Control Word	[RFC4385]
ECMP	Equal-Cost Multi-Path	[RFC6790]
G-ACh	Generic Associated Channel	[RFC5586]
GAL	Generic Associated Channel Label	[RFC5586]
GTSM	Generalized TTL Security Mechanism	[RFC5082]
HMAC	Hash-based Message Authentication Code	[RFC2104]
L2SS	L2-Specific Sublayer	[RFC3931]
L2TPv3	Layer 2 Tunneling Protocol version 3	[RFC3931]
L2VPN	Layer 2 Virtual Private Network	[RFC4026]
L3VPN	Layer 3 Virtual Private Network	[RFC4026]
LSP	Label Switched Path	[RFC3032]
MPLS	Multiprotocol Label Switching	[RFC3032]
MPLS-TP	MPLS Transport Profile	[RFC5960]
OAM	Operations, Administration, and Maintenance	[RFC5586]
PE	Provider Edge	[RFC4026]
PLE	Private Line Emulation	[RFC9801]
PSN	Packet Switched Network	[RFC9801]
PW	Pseudowire	[RFC6658]
S bit	Bottom of Stack bit	[RFC3032]
SSID	STAMP Session Identifier	[RFC8972]
STAMP	Simple Two-Way Active Measurement Protocol	[RFC8762]
TC	Traffic Class	[RFC5462]
TDM	Time-Division Multiplexing	[RFC5087]
TTL	Time to Live	[RFC3032]
VCCV	Virtual Circuit Connectivity Verification	[RFC5085]
VPWS	Virtual Private Wire Service	[RFC9801]

2.3. STAMP Reference Topology

In the STAMP reference topology shown in Figure 1, there exists an LSP or a PW to transport data between Provider Edge (PE) endpoints S1 and R1. The STAMP Session-Sender on PE node S1 initiates a Session-Sender test packet, and the STAMP Session-Reflector on PE node R1 transmits a reply Session-Reflector test packet. The Session-Reflector test packet may be transmitted to the STAMP Session-Sender node S1 on the same path (same set of links and nodes) in the reverse direction of the path taken towards the Session-Reflector node R1.¶


                 |<-------- Pseudowire ------->|
                 |<-------- LSP -------------->|
                 |                             |
                 |     T1                T2    |
                 |    /                   \    |
             +-------+      Test Packet    +-------+
             |       | - - - - - - - - - ->|       |
             |   S1  |=====================|   R1  |
             |       |<- - - - - - - - - - |       |
             +-------+  Reply Test Packet  +-------+
                      \                   /
                       T4                T3

         STAMP Session-Sender        STAMP Session-Reflector
         Provider Edge Endpoint      Provider Edge Endpoint

Figure 1: STAMP Reference Topology using LSP and PW

T1 is a transmit timestamp, and T4 is a receive timestamp added by node S1. T2 is a receive timestamp, and T3 is a transmit timestamp added by node R1.¶

The STAMP test packets are used for both one-way and round-trip performance metrics, like delay, delay variation, and packet loss [RFC8972].¶

3. Overview

The STAMP Session-Sender and Session-Reflector test packets defined in [RFC8972] are encapsulated and transmitted over the LSPs and PWs in MPLS networks. The base STAMP test packets can be encapsulated using an IP/UDP header and may use destination UDP port 862 [RFC8762]. The source UDP port is chosen by the Session-Sender.¶

3.1. G-ACh Types for STAMP

STAMP test packets are encapsulated over G-ACh in two formats: Format-1 (with an IP/UDP header) and Format-2 (without an IP/UDP header).¶

For encapsulating the STAMP test packets over a G-ACh with IP/UDP headers (in Format-1), IPv4 and IPv6 channel types [RFC4385] are used for both Session-Sender and Session-Reflector test packets. The destination UDP port number in the Session-Sender and Session-Reflector test packets distinguishes the test packets. The IP version (IPv4 or IPv6) MUST match the IP version used for the LSPs and PWs being measured. When an LSP carries both IPv4 and IPv6 data traffic, the IP version used in the STAMP test packet MUST match the IP version of the specific data traffic flow being measured.¶

For encapsulating the STAMP test packets over a G-ACh without adding IP/UDP headers (in Format-2), two new channel types are defined in this document: one for the Session-Sender test packets and one for the Session-Reflector test packets. The different channel types are required for the Session-Sender and Session-Reflector test packets as the STAMP test packets do not have a way to discriminate between them.¶

3.2. Using STAMP for LSPs and PWs

The STAMP test packets are encapsulated with an MPLS header using the same label stack as the PW data traffic (including the PW label) and a G-ACh header (instead of the CW used by the data traffic). The encapsulation allows STAMP test packets to follow the same path as the PW data traffic and provides the same ECMP behaviour on the intermediate nodes.¶

Similarly, the STAMP test packets are encapsulated with an IP/UDP header without a G-ACh header, and with an MPLS header using the same label stack as the MPLS LSP and MPLS-TP LSP data traffic that contains an IP header, without CW. The label stack may include the L2 or L3 VPN label for the service carried over the LSP. The encapsulation provides the STAMP test packets with the same ECMP behaviour on the intermediate nodes.¶

The OAM Control Channel traffic between two PE endpoints is not forwarded past the PE endpoints towards Customer Edge (CE) devices; instead, the OAM messages are intercepted at the PE endpoints for exception processing in the control plane. [RFC5085] defines mechanisms for the VCCV Control Channel to carry OAM messages for PWs.¶

The "In-band VCCV for Control Word with 0001b as first nibble (Type 1)" defined in Section 5.1.1 of [RFC5085] MUST be added when measuring PWs with CW to avoid different ECMP hashing behaviour.¶

The method for "TTL Expiry VCCV (Type 3)" defined in Section 5.1.3 of [RFC5085] allows the termination of OAM messages on the remote PE endpoint nodes. This method is applied to the STAMP test packets to force the test packets to be processed on Session-Sender and Session-Reflector control planes by adding the PW label with a TTL value of 1.¶

VCCV Type 2 is also referred to as the "MPLS Router Alert Label" [RFC5085]. This method could result in a different ECMP hashing behaviour, and thus result in the STAMP test packets taking a path that differs from that of the actual data traffic under test [RFC5085]. Hence, the use of VCCV Type 2 for STAMP when measuring PW traffic is not supported by the procedures defined in this document.¶

The procedure to encapsulate STAMP test packets for PWs is also applicable to MPLS LSPs and MPLS-TP LSPs when using CW. For measuring the data traffic over MPLS LSPs using an IP header, STAMP test packets in Format-1 are transmitted. For measuring the data traffic over MPLS-TP LSPs, not using an IP header, STAMP test packets in Format-2 are transmitted with a TTL value of 1 in the ultimate LSP label in the MPLS header.¶

The IPv4 Time to Live (TTL), IPv6 Hop Limit, and Generalized TTL Security Mechanism (GTSM) procedures from [RFC5082] also apply to the encapsulation of STAMP test packets; hence, the IPv4 TTL, non-ultimate MPLS label TTL, and IPv6 Hop Limit MUST be set to 255. Note that the TTL in the ultimate PW label or LSP label is set separately to 1 when using the TTL Expiry method (VCCV Type 3) described above.¶

The G-ACh label (GAL) defined in [RFC5586] also applies to the G-ACh types defined in this document with STAMP test packets without an IP/UDP header (Format-2). This use case is similar to the use case for MPLS-TP LSP performance measurement defined in [RFC6374]. As specified in Section 4.2 of [RFC5586], the GAL MUST NOT be used with PWs in MPLS-TP networks. Therefore, Format-2 encapsulation using GAL does not apply to MPLS-TP PWs.¶

The GAL applies to Format-1 using the G-ACh channel type for IPv4 (0x0021) or IPv6 (0x0057) as described in [RFC5586].¶

3.3. Applicability of Control Channel Types to STAMP for LSPs and PWs

Control Channel Types defined in [RFC5085] are applicable to STAMP test packets for LSPs and PWs as shown in Table 2:¶

Table 2: Control Channel Types for LSPs and PWs
Control Channel Type	Control Channel Name	STAMP Header Format	G-ACh Type
Type 1	In-band: Control Word with 0001b as first nibble	Format-1 (IP/UDP Headers)	IPv4 G-ACh (0x0021) and IPv6 G-ACh (0x0057)
Type 1	In-band: Control Word with 0001b as first nibble	Format-2 (No IP/UDP Headers)	STAMP G-ACh (TBA1 and TBA2)
Type 2	Out-of-band: MPLS Router Alert Label	Not supported	Not supported
Type 3	TTL Expiry: Label with TTL as 1	Format-1 (IP/UDP Headers)	IPv4 G-ACh (0x0021) and IPv6 G-ACh (0x0057)
Type 3	TTL Expiry: Label with TTL as 1	Format-2 (No IP/UDP Headers)	STAMP G-ACh (TBA1 and TBA2)

4. Session-Sender Test Packet

STAMP Session-Sender test packets are transmitted for an LSP or a PW using an MPLS header with or without an IP/UDP header. For PWs, Session-Sender test packets are transmitted using the label stack of the PW, including the PW label and the G-ACh. For LSPs, Session-Sender test packets are transmitted using the label stack of the LSP with or without a G-ACh.¶

4.1. Session-Sender Test Packet with IP/UDP Header

The content of an example STAMP Session-Sender test packet for an LSP or a PW encapsulated using a G-ACh and an IP/UDP header 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                Label(1)               | TC  |0|      TTL      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 .                                                               .
 .                                                               .
 .                                                               .
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    PW Label or Ultimate LSP Label     | TC  |1|      1        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |0 0 0 1|Version|    Reserved   | IPv4 (0x0021) or IPv6 (0x0057)|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | IP Header                                                     |
 .  Source IP Address                                            .
 .     = Session-Sender IPv4 or IPv6 Address                     .
 .  Destination IP Address                                       .
 .     = Session-Reflector IPv4 or IPv6 Address                  .
 .  IPv4 Protocol or IPv6 Next Header = UDP (17)                 .
 .                                                               .
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | UDP Header                                                    |
 .  Source Port = As chosen by Session-Sender                    .
 .  Destination Port = User-configured Destination Port or 862   .
 .                                                               .
 +---------------------------------------------------------------+
 | Payload = Test Packet as specified in Section 3 of RFC 8972   |
 .           in Figure 1 and Figure 3                            .
 .                                                               .
 +---------------------------------------------------------------+

Figure 2: Example Session-Sender Test Packet with G-ACh and IP/UDP Header

The destination address in the IP header of a STAMP test packet can be one of the following when adding an MPLS encapsulation for an LSP or a PW.¶

A routable IPv4 address¶
A routable IPv6 address¶
An IPv4 address from the 127/8 range¶
An IPv6 address from the Dummy IPv6 Prefix address 100:0:0:1::/64 block [RFC9780] [IANA-IPv6-REG]¶

Examples of implementations are:¶

An implementation using a routable IP address as the destination address during the initial forwarding step, before the STAMP test packet gets forwarded into the MPLS LSP or PW.¶
An implementation using a non-routable IP address as the destination address while adding both an IP header and an MPLS encapsulation in the same forwarding step.¶

The G-ACh header [RFC5586] with the channel type for IPv4 or IPv6 MUST immediately follow the bottom of the label stack. The payload contains the STAMP Session-Sender test packet defined in [RFC8972].¶

The STAMP Session-Sender test packet G-ACh header contains the following fields:¶

Version: The Version field is set to 0, as defined in [RFC4385].¶
Reserved: Reserved bits MUST be set to zero upon transmission and ignored upon receipt.¶
Channel Type: G-ACh type for IPv4 header (0x0021) or IPv6 header (0x0057) [RFC4385].¶

4.2. Session-Sender Test Packet without IP/UDP Header

The content of an example STAMP Session-Sender test packet for an LSP or a PW encapsulated using GAL and a G-ACh without an IP/UDP header is shown in Figure 3.¶

  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                Label(1)               | TC  |0|      TTL      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 .                                                               .
 .                                                               .
 .                                                               .
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    PW Label or Ultimate LSP Label     | TC  |0|      1        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    GAL                                | TC  |1|      1        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |0 0 0 1|Version|    Reserved   | STAMP Sender G-ACh (TBA1)     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Payload = Test Packet as specified in Section 3 of RFC 8972   |
 .           in Figure 1 and Figure 3                            .
 .                                                               .
 +---------------------------------------------------------------+

Figure 3: Example Session-Sender Test Packet with GAL and G-ACh without IP/UDP Header

The G-ACh header [RFC5586] with the new STAMP Session-Sender channel type (value TBA1) MUST immediately follow the bottom of the label stack. The payload contains the STAMP Session-Sender test packet defined in [RFC8972].¶

The STAMP channel type allows the identification of the encapsulated STAMP payload when demultiplexing G-ACh.¶

The STAMP Session-Sender test packet G-ACh header contains the following fields:¶

Version: The Version field is set to 0, as defined in [RFC4385].¶
Reserved: Reserved bits MUST be set to zero upon transmission and ignored upon receipt.¶
Channel Type: G-ACh type for STAMP Session-Sender packet (TBA1).¶

5. Session-Reflector Test Packet

The STAMP Session-Reflector reflects the test packet back to the Session-Sender using the same channel in the reverse direction of the LSP or PW on which it was received. The Session-Reflector has enough information to reflect the received test packet to the Session-Sender using the LSP or PW context.¶

The STAMP Session-Reflector test packet is transmitted on the same path in the reverse direction of the LSP or the PW.¶

The STAMP test packet can be transmitted using an MPLS header with or without an IP/UDP header. The Session-Reflector test packet is sent with an IP/UDP header if the Session-Sender test packet is received with an IP/UDP header; otherwise, it is sent without an IP/UDP header.¶

The Session-Reflector can use the PW label or the ultimate LSP label in the received packet to find the LSP or the PW in the reverse direction. If the received packet context is a PW, the Session-Reflector uses the reverse-direction PW label stack and G-ACh to transmit the Session-Reflector test packet. The reverse-direction PW label stack is determined by the Session-Reflector through static configuration or the signaling protocol used to establish the PW. If the received packet context is an LSP, the Session-Reflector uses the reverse-direction LSP label stack, with or without a G-ACh as applicable, to transmit the Session-Reflector test packet. When a G-ACh is used, the Session-Reflector test packet uses the same G-ACh as that received in the Session-Sender test packet.¶

5.1. Session-Reflector Test Packet with IP/UDP Header

The content of an example STAMP Session-Reflector test packet for an LSP or a PW encapsulated using a G-ACh and an IP/UDP header is shown in Figure 4.¶

  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                Label(1)               | TC  |0|      TTL      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 .                                                               .
 .                                                               .
 .                                                               .
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    PW Label or Ultimate LSP Label     | TC  |1|      1        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |0 0 0 1|Version|    Reserved   | IPv4 (0x0021) or IPv6 (0x0057)|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | IP Header                                                     |
 .  Source IP Address                                            .
 .     = As chosen by Session-Reflector                          .
 .  Destination IP Address                                       .
 .     = Source IP Address from Session-Sender Test Packet       .
 .  IPv4 Protocol or IPv6 Next Header = UDP (17)                 .
 .                                                               .
 +---------------------------------------------------------------+
 | UDP Header                                                    |
 .  Source Port = As chosen by Session-Reflector                 .
 .  Destination Port                                             .
 .     = Source Port from Session-Sender Test Packet             .
 .                                                               .
 +---------------------------------------------------------------+
 | Payload = Test Packet as specified in Section 3 of RFC 8972   |
 .           in Figure 2 and Figure 4                            .
 .                                                               .
 +---------------------------------------------------------------+

Figure 4: Example Session-Reflector Test Packet with G-ACh and IP/UDP Header

The G-ACh header [RFC5586] with the channel type IPv4 or IPv6 MUST immediately follow the bottom of the label stack. The payload contains the STAMP Session-Reflector test packet defined in [RFC8972].¶

The STAMP Session-Reflector test packet MUST use the IP/UDP information from the received test packet when an IP/UDP header is present in the received test packet.¶

The STAMP Session-Reflector test packet G-ACh header contains the following fields:¶

Version: The Version field is set to 0, as defined in [RFC4385].¶
Reserved: Reserved bits MUST be set to zero upon transmission and ignored upon receipt.¶
Channel Type: G-ACh type for IPv4 header (0x0021) or IPv6 header (0x0057) [RFC4385].¶

5.2. Session-Reflector Test Packet without IP/UDP Header

The content of an example STAMP Session-Reflector test packet for an LSP or a PW encapsulated using GAL and a G-ACh without an IP/UDP header is shown in Figure 5.¶

  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                Label(1)               | TC  |0|      TTL      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 .                                                               .
 .                                                               .
 .                                                               .
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    PW Label or Ultimate LSP Label     | TC  |0|      1        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    GAL                                | TC  |1|      1        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |0 0 0 1|Version|    Reserved   | STAMP Reflector G-ACh (TBA2)  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Payload = Test Packet as specified in Section 3 of RFC 8972   |
 .           in Figure 2 and Figure 4                            .
 .                                                               .
 +---------------------------------------------------------------+

Figure 5: Example Session-Reflector Test Packet with GAL and G-ACh without IP/UDP Header

The G-ACh header [RFC5586] with the new STAMP Session-Reflector channel type (value TBA2) MUST immediately follow the bottom of the label stack. The payload contains the STAMP Session-Reflector test packet defined in [RFC8972].¶

The STAMP channel type allows the identification of the encapsulated STAMP payload when demultiplexing G-ACh.¶

The STAMP Session-Reflector test packet G-ACh header contains the following fields:¶

Version: The Version field is set to 0, as defined in [RFC4385].¶
Reserved: Reserved bits MUST be set to zero upon transmission and ignored upon receipt.¶
Channel Type: G-ACh type for STAMP Session-Reflector packet (TBA2).¶

6. Operational Considerations

The operational considerations specified in Section 5 of [RFC8762] also apply to the procedure described in this document. Further, the operation and management of performance measurement based on STAMP specified in Section 3 of [RFC8762] also apply to the procedure described in this document.¶

An operator may wish to only add MPLS encapsulation in STAMP test packets destined to addresses within the MPLS administrative domain based on some local policy.¶

The encapsulation procedure defined in this document uses a G-ACh for STAMP test packets to follow the path of the data packets with CW. This assumes that intermediate nodes apply the same ECMP hashing behaviour to STAMP test packets with G-ACh as to data packets with CW when using the same label stack.¶

6.1. Considerations for Broken LSPs

Forwarding STAMP test packets on a broken LSP would lead to the STAMP session being down when all packets on the LSP are dropped. Otherwise, when the packets are incorrectly forwarded by MPLS or IP to the egress node (hosting the STAMP Session-Reflector), it could lead to an invalid measurement of the LSP, for example, if the packets followed a different path than the LSP. A non-routable IPv4/IPv6 destination address, described in Section 4.1, avoids IP forwarding of the Session-Sender test packets to the egress node on a different path than the LSP. However, there is a potential risk of receiving Session-Reflector test packets from an unintended STAMP Session-Reflector hosted on the node where the broken LSP terminates, since the STAMP Session-Reflector may not know that the test packets were received due to a broken LSP. In this case, network analytics would detect invalid measurements reported by STAMP over a broken LSP path.¶

Further, the destination IP address-based filtering SHOULD be provisioned on the edges of the MPLS administrative domain to prevent the IP forwarded STAMP test packets for a broken LSP within the domain from leaking outside that domain. A non-routable IPv4/IPv6 destination address, described in Section 4.1, MAY be used in STAMP test packets to help avoid this.¶

7. Security Considerations

The procedures defined in this document are intended for deployment in a single network administrative domain. As such, the Session-Sender address, Session-Reflector address, and IP and MPLS forward and return paths are provisioned by the operator for the STAMP session. It is assumed that the operator has verified the integrity of the IP and MPLS forward and return paths used to transmit STAMP test packets.¶

The security considerations specified in [RFC8762] and [RFC8972] also apply to the procedure described in this document. Specifically, the message integrity protection using HMAC, as defined in Section 4.4 of [RFC8762], also applies to the procedure described in this document. The measures specified in Section 7 of [RFC8762] to mitigate attacks using the registered UDP port number also apply.¶

Routers that support G-ACh are subject to the same security considerations as defined in [RFC4385] and [RFC5586].¶

The message throttling mechanisms described in the security considerations in Section 10 of [RFC5085] to protect against potential (deliberate or unintentional) attacks also apply to the procedure described in this document.¶

If desired, attacks can be mitigated by performing basic validation checks (such as whether timestamp T2 is later than timestamp T1 in the STAMP Reference Topology shown in Figure 1, when the Session-Sender and Session-Reflector clocks are synchronized) in received Session-Reflector test packets at the Session-Sender. The minimal state associated with this protocol also limits the extent of measurement disruption that can be caused by a corrupt or invalid test packet to a single test cycle.¶

An attacker can send a forged STAMP test packet to the ingress or egress node of an LSP, causing the STAMP session to be terminated prematurely. To mitigate these threats, operators SHOULD filter STAMP test packets at the edges of the MPLS administrative domain.¶

Furthermore, implementations SHOULD NOT assign STAMP Session Identifiers (SSIDs) [RFC8972] in a predictable manner. To avoid predictability, implementations can leverage a Cryptographically Secure Pseudorandom Number Generator [NIST-CSPRNG].¶

The STAMP test packets received via a PW or an LSP are processed in the context of that PW or LSP, and the encapsulations defined in this document do not introduce a mechanism for cross-service OAM interactions.¶

Table 3: STAMP G-ACh Types
Value	Description	Reference
TBA1	STAMP Session-Sender G-ACh Type	This document
TBA2	STAMP Session-Reflector G-ACh Type	This document

9. References

9.1. Normative References

[RFC2119]: Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/info/rfc2119>.
[RFC3032]: Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y., Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack Encoding", RFC 3032, DOI 10.17487/RFC3032, January 2001, <https://www.rfc-editor.org/info/rfc3032>.
[RFC4385]: Bryant, S., Swallow, G., Martini, L., and D. McPherson, "Pseudowire Emulation Edge-to-Edge (PWE3) Control Word for Use over an MPLS PSN", RFC 4385, DOI 10.17487/RFC4385, February 2006, <https://www.rfc-editor.org/info/rfc4385>.
[RFC5082]: Gill, V., Heasley, J., Meyer, D., Savola, P., Ed., and C. Pignataro, "The Generalized TTL Security Mechanism (GTSM)", RFC 5082, DOI 10.17487/RFC5082, October 2007, <https://www.rfc-editor.org/info/rfc5082>.
[RFC5085]: Nadeau, T., Ed. and C. Pignataro, Ed., "Pseudowire Virtual Circuit Connectivity Verification (VCCV): A Control Channel for Pseudowires", RFC 5085, DOI 10.17487/RFC5085, December 2007, <https://www.rfc-editor.org/info/rfc5085>.
[RFC5586]: Bocci, M., Ed., Vigoureux, M., Ed., and S. Bryant, Ed., "MPLS Generic Associated Channel", RFC 5586, DOI 10.17487/RFC5586, June 2009, <https://www.rfc-editor.org/info/rfc5586>.
[RFC6790]: Kompella, K., Drake, J., Amante, S., Henderickx, W., and L. Yong, "The Use of Entropy Labels in MPLS Forwarding", RFC 6790, DOI 10.17487/RFC6790, November 2012, <https://www.rfc-editor.org/info/rfc6790>.
[RFC8174]: Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8762]: Mirsky, G., Jun, G., Nydell, H., and R. Foote, "Simple Two-Way Active Measurement Protocol", RFC 8762, DOI 10.17487/RFC8762, March 2020, <https://www.rfc-editor.org/info/rfc8762>.
[RFC8972]: Mirsky, G., Min, X., Nydell, H., Foote, R., Masputra, A., and E. Ruffini, "Simple Two-Way Active Measurement Protocol Optional Extensions", RFC 8972, DOI 10.17487/RFC8972, January 2021, <https://www.rfc-editor.org/info/rfc8972>.

9.2. Informative References

[RFC2104]: Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-Hashing for Message Authentication", RFC 2104, DOI 10.17487/RFC2104, February 1997, <https://www.rfc-editor.org/info/rfc2104>.
[RFC3931]: Lau, J., Ed., Townsley, M., Ed., and I. Goyret, Ed., "Layer Two Tunneling Protocol - Version 3 (L2TPv3)", RFC 3931, DOI 10.17487/RFC3931, March 2005, <https://www.rfc-editor.org/info/rfc3931>.
[RFC4026]: Andersson, L. and T. Madsen, "Provider Provisioned Virtual Private Network (VPN) Terminology", RFC 4026, DOI 10.17487/RFC4026, March 2005, <https://www.rfc-editor.org/info/rfc4026>.
[RFC4448]: Martini, L., Ed., Rosen, E., El-Aawar, N., and G. Heron, "Encapsulation Methods for Transport of Ethernet over MPLS Networks", RFC 4448, DOI 10.17487/RFC4448, April 2006, <https://www.rfc-editor.org/info/rfc4448>.
[RFC5087]: Stein, Y., Shashoua, R., Insler, R., and M. Anavi, "Time Division Multiplexing over IP (TDMoIP)", RFC 5087, DOI 10.17487/RFC5087, December 2007, <https://www.rfc-editor.org/info/rfc5087>.
[RFC5462]: Andersson, L. and R. Asati, "Multiprotocol Label Switching (MPLS) Label Stack Entry: "EXP" Field Renamed to "Traffic Class" Field", RFC 5462, DOI 10.17487/RFC5462, February 2009, <https://www.rfc-editor.org/info/rfc5462>.
[RFC5921]: Bocci, M., Ed., Bryant, S., Ed., Frost, D., Ed., Levrau, L., and L. Berger, "A Framework for MPLS in Transport Networks", RFC 5921, DOI 10.17487/RFC5921, July 2010, <https://www.rfc-editor.org/info/rfc5921>.
[RFC5960]: Frost, D., Ed., Bryant, S., Ed., and M. Bocci, Ed., "MPLS Transport Profile Data Plane Architecture", RFC 5960, DOI 10.17487/RFC5960, August 2010, <https://www.rfc-editor.org/info/rfc5960>.
[RFC6374]: Frost, D. and S. Bryant, "Packet Loss and Delay Measurement for MPLS Networks", RFC 6374, DOI 10.17487/RFC6374, September 2011, <https://www.rfc-editor.org/info/rfc6374>.
[RFC6658]: Bryant, S., Ed., Martini, L., Swallow, G., and A. Malis, "Packet Pseudowire Encapsulation over an MPLS PSN", RFC 6658, DOI 10.17487/RFC6658, July 2012, <https://www.rfc-editor.org/info/rfc6658>.
[RFC7708]: Nadeau, T., Martini, L., and S. Bryant, "Using a Generic Associated Channel Label as a Virtual Circuit Connectivity Verification Channel Indicator", RFC 7708, DOI 10.17487/RFC7708, November 2015, <https://www.rfc-editor.org/info/rfc7708>.
[RFC9780]: Mirsky, G., Mishra, G., and D. Eastlake 3rd, "Bidirectional Forwarding Detection (BFD) for Multipoint Networks over Point-to-Multipoint MPLS Label Switched Paths (LSPs)", RFC 9780, DOI 10.17487/RFC9780, May 2025, <https://www.rfc-editor.org/info/rfc9780>.
[RFC9801]: Gringeri, S., Whittaker, J., Leymann, N., Schmutzer, C., Ed., and C. Brown, "Private Line Emulation over Packet Switched Networks", RFC 9801, DOI 10.17487/RFC9801, July 2025, <https://www.rfc-editor.org/info/rfc9801>.
[I-D.ietf-spring-stamp-srpm-mpls]: Gandhi, R., Filsfils, C., Janssens, B., Chen, M., and R. F. Foote, "Performance Measurement Using Simple Two-Way Active Measurement Protocol (STAMP) for Segment Routing over the MPLS Data Plane", Work in Progress, Internet-Draft, draft-ietf-spring-stamp-srpm-mpls-04, 22 July 2026, <https://datatracker.ietf.org/doc/html/draft-ietf-spring-stamp-srpm-mpls-04>.
[NIST-CSPRNG]: NIST Special Publication 800-90A Revision 1, "Recommendation for Random Number Generation Using Deterministic Random Bit Generators, Revision 1", June 2015.
[IANA-IPv6-REG]: IANA, "IANA IPv6 Special-Purpose Address Registry", <https://www.iana.org/assignments/iana-ipv6-special-registry>.