Internet-Draft | Path Tracing in SR-MPLS networks | May 2024 |
Filsfils, et al. | Expires 14 November 2024 | [Page] |
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 [INT], [RFC9197], [I-D.song-opsawg-ifit-framework], and [I-D.kumar-ippm-ifa].¶
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 [I-D.filsfils-spring-path-tracing].¶
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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 [INT], [RFC9197], [I-D.song-opsawg-ifit-framework], and [I-D.kumar-ippm-ifa].¶
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 [RFC8660], as well as SRv6 [RFC8986]. This document defines the Path Tracing specification for the SR-MPLS dataplane. The SRv6 dataplane is detailed in [I-D.filsfils-spring-path-tracing].¶
The following terms used within this document are defined in [RFC6790], [RFC8402], [RFC8754], [RFC8986], [I-D.decraene-mpls-slid-encoded-entropy-label-id] and [I-D.filsfils-spring-path-tracing]: 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 Section 6.1 of this document.¶
SEL: Structured Entropy Label as defined in [I-D.decraene-mpls-slid-encoded-entropy-label-id].¶
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.¶
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.¶
For each configured PT Probing Instance, according to the probe-rate, the PT Source generates a PT probe packet as follows:¶
S01. Generate a new packet S02. Push an SRH PT-TLV S03. Set the session ID field of the SRH PT-TLV as per PT Probing Instance configuration S04. Set the Sequence Number field of SRH PT-TLV and increase local counter S05. Push an MPLS HbH-PT header S06. Set all bits of MCD Stack of the MPLS HbH-PT header to zero S07. Set the VER field of the MPLS HbH-PT to 0x2 S08. Set the value of Opt Data Len field as per the PT Probing Instance configuration S09. Push an MPLS Structured Entropy Label (SEL) S10. Set the PTI flag in the ELC field of the SEL S11. Set the value of the SEL entropy field as per the PT Probing Instance configuration S12. Set Bottom of Stack bit (S) of the SEL to 1 S13. Push an MPLS Entropy Indicator Label (ELI) S14. Push an MPLS TEF Label as per the PT Probing Instance configuration S15. Set the TC and TTL value of the TEF Label as per PT Probing Instance configuration S16. Push an SR-MPLS transport Label stack as per the PT Probing Instance configuration S17. Set the TC and TTL value of the SR-MPLS transport Labels as per PT Probing Instance configuration S18. Add padding bytes after SRH PT-TLV to reach the desired packet size as per the MTU sweeping range configuration in the PT Probing Instance configuration S19. Perform MPLS lookup using the topmost label to determine the Outgoing Interface (IFACE-OUT) S20. Record Transmit 64-bit timestamp (SRC.T64) in the T64 field of the SRH PT-TLV S21. Record IFACE-OUT ID (SRC.OIF) in the IF_ID field of the SRH PT-TLV S22. Record IFACE-OUT Load (SRC.OIL) in the IF_LD field of the SRH PT-TLV S23. Forward the packet via IFACE-OUT¶
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.¶
When an MPLS LSR router receives an MPLS packet with SEL, the MPLS LSR router processes the SEL as follows:¶
S01. When processing SEL { S02. Use Entropy field to compute ECMP hash and decide IFACE-OUT S03. IF (SEL[ELC].PTI == 1 and SEL[BOS] == 1) { S04. Compute the Midpoint MCD for IFACE-OUT S05. Locate the MPLS HbH-PT immediately after SEL S06. MPLS_HbH-PT.MCD_Stack[3:Opt_Data_Len -1] = MPLS_HbH-PT.MCD_Stack[0:Opt_Data_Len -4] //Shift MCD Stack 3Bytes to the right S07. MPLS_HbH-PT.MCD_Stack[0:2] = MCD[0:2] //i.e., Push the MCD at the beginning of the Stack S08. } S09. }¶
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.¶
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.¶
S01. Record Rx 64-bit timestamp (SNK.T64) S02. Record incoming interface ID (Sink.IIF) S03. Record incoming interface Load (Sink.IIL) S04. Push a new IPv6 header S05. Set the IPv6 SA to the Sink node loopback S06. Set the IPv6 DA to the first SID in the SRv6 SID List S07. Set the IPv6 Next Header field to 43 (SRH) S08. Append an SRH S09. Set the SRH Next Header field to 137 (MPLS) S10. Write the SID list in the SRH S11. Append an SRH PT-TLV S12. Set the session ID field of the SRH PT-TLV to zero S13. Set the Sequence Number field of the SRH PT-TLV to zero S14. Write Sink.T64 in the T64 field of the SRH PT-TLV S15. Write Sink.IIF in the IF_ID field of the SRH PT-TLV S16. Write Sink.IIL in the IF_LD field of the SRH PT-TLV S17. Perform an IPv6 lookup and forward the 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.¶
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:¶
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | VER | RSVD | Opt Data Len | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | | ~ MCD Stack ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 1: IPv6 Hop-by-Hop Path Tracing Option 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.¶
Insignificant MTU overhead:¶
PT has the lowest MTU overhead compared to alternative solutions such as [INT], [RFC9197], [I-D.song-opsawg-ifit-framework], and [I-D.kumar-ippm-ifa].¶
Linerate and HW friendliness:¶
Scalable Fine-grained Timestamp:¶
Scalable Load measurement¶