]> Cisco Systems, Inc.

Belgium cf@cisco.com

Cisco Systems, Inc.

Italy ahabdels@cisco.com

Cisco Systems, Inc.

Spain pcamaril@cisco.com

Broadcom

Israel israel.meilik@broadcom.com

Goldman Sachs

USA michael.j.valentine@gs.com

Deutsche Telekom

Germany Ruediger.Geib@telekom.de

Colt Technology Services

UK Jonathan.Desmarais@colt.net

General SPRING Internet-Draft Path Tracing provides a record of the packet path as a sequence of interface ids. In addition, it provides a record of end-to-end delay, per-hop delay, and load on each interface that forwards the packet. Path Tracing has the lowest MTU overhead compared to alternative proposals such as , , , and . Path Tracing supports fine grained timestamp. It has been designed for linerate hardware implementation in the base pipeline. This document defines the Path Tracing specification for the SR-MPLS dataplane. The Path Tracing specification for the SRv6 dataplane is defined in .

Introduction Path Tracing provides a record of the packet path as a sequence of interface ids. In addition, it provides a record of end-to-end delay, per-hop delay, and load on each interface that forwards the packet. Path Tracing has the lowest MTU overhead compared to alternative proposals such as , , , and . Path Tracing supports fine grained timestamp. It has been designed for linerate hardware implementation in the base pipeline. Path Tracing is applicable to both SR-MPLS , as well as SRv6 . This document defines the Path Tracing specification for the SR-MPLS dataplane. The SRv6 dataplane is detailed in .

Terminology The following terms used within this document are defined in , , , , and : Segment Routing (SR), SR Domain, Segment Identifier (SID), SR-MPLS SID, SR Policy, Segment Routing Header (SRH), SR source node, transit node, SR Endpoint, SA, DA, EL, ELI, ELC, PT, PT Probing Instance, PT Source, PT Midpoint, PT Sink, RC, MCD, SRH PT-TLV, TEF. The following terms are used in this document as defined below: MPLS HbH-PT: MPLS Hop-by-Hop Path Tracing Option used for Path Tracing. It contains a stack of MCDs. It is defined in of this document. SEL: Structured Entropy Label as defined in . TEF Label: MPLS Label bound to Timestamp, Encapsulation and Forward (TEF) behavior. The allocation of the TEF Label is out of scope of this document. PTI: PT Indicator is a flag bit used to indicate the presence of the MPLS HbH-PT after the BoS Label and triggers PT behavior at a PT Midpoint.

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 when, and only when, they appear in all capitals, as shown here.

PT Source Node Dataplane Behavior For each configured PT Probing Instance, according to the probe-rate, the PT Source generates a PT probe packet as follows:

Notes: The pseudocode describes local processing at a node. An implementation of the pseudocode is compliant as long as the externally observable wire protocol is as described in the pseudocode.

PT Midpoint Node Dataplane Behavior When an MPLS LSR router receives an MPLS packet with SEL, the MPLS LSR router processes the SEL as follows:

Notes: The PT Midpoint behavior MUST be implemented in the normal pipeline to experience the regular datapath (i.e., linerate). Offloading the processing of this option to either the slow-path or a co-processors is not acceptable and yields invalid results.

PT Sink Node Dataplane Behavior We define a new MPLS Label bound to an SRv6 Policy with Timestamp, Encapsulation and Forward ("TEF Label" for short). When Node N receives an MPLS packet with topmost Label is TEF Label, N performs the TEF behavior to the MPLS packet.

Notes: The pseudocode describes local processing at a node. An implementation of the pseudocode is compliant as long as the externally observable wire protocol is as described in the pseudocode.

PT Headers

MPLS Hop-by-Hop Path Tracing Option We define a new header called MPLS Hop-by-Hop Path Tracing option ("MPLS HbH-PT" for short). The header is used to collect the MCD of each PT Midpoint on the packet path. The MPLS HbH-PT has the following format:

Where: VER: In MPLS, the first nibble after the Label stack indicates the packet IP protocol version. VER is set to 0x2. RSVD: Reserved 4-bits. Currently not used. Opt Data Len: carries the length of MCD stack (in bytes). Used by PT Midpoint to determine the MCD stack shift value. MCD Stack: used to collect the MCDs from PT Midpoints Note: The MPLS Hop-by-Hop Path Tracing option has a variable length. The operator, upon configuring the Source node behavior, MUST select an option length that is supported by all the routers in the network.

Benefits Insignificant MTU overhead: PT has the lowest MTU overhead compared to alternative solutions such as , , , and . Linerate and HW friendliness: Designed for linerate hardware implementation, using the regular forwarding pipeline. No offloading to co-processors whose databases might defer from forwarding pipeline. Leverages mature hardware capabilities (basic shift operation); no packet resizing at every node along the path Scalable Fine-grained Timestamp: 64-bits timestamp at PT SRC and PT SNK 8-bits truncated timestamp at PT Midpoint leveraging flexible per-outgoing-link template allowing diverse link types in the same measurement (e.g., DC, metro, WAN) Scalable Load measurement

Security Considerations TBD

IANA Considerations TBD

&RFC2119; &RFC6790; &RFC8174; &RFC8402; &RFC8660; &RFC8754; &RFC8986; &I-D.filsfils-spring-path-tracing; &I-D.decraene-mpls-slid-encoded-entropy-label-id; &RFC9197; &I-D.kumar-ippm-ifa; &I-D.song-opsawg-ifit-framework;

Contributors Cisco Systems, Inc.

USA jisu@cisco.com

Cisco Systems, Inc.

Canada rgandhi@cisco.com

Broadcom

Israel shay.zadok@broadcom.com

Broadcom

Israel mark.yufit@broadcom.com

Colt Technology Services

Belgium Bart.Janssens@colt.net