MPLS Working Group N. Kumar
Internet-Draft G. Swallow
Intended status: Standards Track C. Pignataro
Expires: April 24, 2014 N. Akiya
Cisco Systems, Inc.
M. Chen
Huawei
October 21, 2013

Label Switched Path (LSP) Ping/Trace for Segment Routing Networks Using MPLS Dataplane
draft-kumar-mpls-spring-lsp-ping-00

Abstract

Segment Routing architecture leverages the source routing and tunneling paradigm and can be directly applied to MPLS dataplane. A node steers a packet through a controlled set of instructions called segments, by prepending the packet with Segment Routing header.

The segment assignment and forwarding semantic nature of Segment Routing raises additional consideration for connectivity verification and fault isolation in LSP with SPRING architecture. This document illustrates the problem and describe a mechanism to perform LSP Ping and Traceroute on Segment Routing network over MPLS dataplane.

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Table of Contents

1. Introduction

[I-D.filsfils-rtgwg-segment-routing] introduces and explains Segment Routing architecture that leverages the source routing and tunneling paradigm. A node steers a packet through a controlled set of instructions called segments, by prepending the packet with SR header. A detailed definition about Segment Routing architecture is available in draft-filsfils-rtgwg-segment-routing and different use-cases are discussed in draft-filsfils-rtgwg-segment-routing-use-cases.

The Segment Routing architecture can be directly applied to MPLS dataplane in a way that, the segment will be of 20-bits size and SR header is the label stack.

[RFC4379] describes the mechanism to perform connectivity verification and fault isolation in Label Switched Path (LSP). Unlike LDP or RSVP which are the other well-known MPLS control plane protocols, segment assignment in Segment Routing architecture is not hop-by-hop basis.

This nature of Segment Routing raises additional consideration for connectivity verification and fault isolation in Segment Routing network. This document illustrates the problem and describe a mechanism to perform LSP Ping and Traceroute on Segment Routing network over MPLS dataplane.

2. Requirements notation

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].

3. Path validation in Segment Routing networks

[RFC4379] defines the OAM machinery that helps with connectivity check and fault isolation in MPLS dataplane path with the use of various Target FEC Stack Sub-TLV that are carried in MPLS Ping packets and used by the responder for FEC validation. While it is obvious that new Sub-TLVs need to be assigned, the unique nature of SPRING architecture raises a need for additional machinery for path validation. This section discuss the challenges as below: 

				
                      L1
                  +--------+
                  |   L2   |
                  R3-------R6
                 /           \
                /             \
        R1----R2               R7----R8
                \             /   L3
                 \           /
                  R4-------R5

          Figure 1: SPRING network

    500x --> Node Segment ID for Router X 
             (Ex: 5006 is node segment ID for R6)
    9axy --> Adj Segment ID from Router X to Y over link a 
             (Ex: 9136 is Adj segment ID from R3 to R6 via link 1)

The forwarding semantic of Adjacency segment is to pop the segment and send the packet to a specific neighbor over a specific link. A malfunctioning node may forward packets using Adjacency segment to incorrect neighbor or over incorrect link. Exposed segment (after incorrectly forwarded Adjacency segment) might still allow such packet to traverse to intended destination, yet intended strict traversal has been broken.

Assume in above topology, R1 sends traffic with segment stack as {9124, 5007, 9378} so that the path taken will be R1-R2-R4-R5-R7-R8. If the adjacency segment 9124 is misprogrammed in R2 to send the packet to R1 or R3, it will still be delivered to R8 but is not via the expected path.

MPLS traceroute may help with detecting such deviation in above mentioned scenario. However, it may not be helpful if R3, due to misprogramming, forwards packet with adjacency segment 9236 via link L1 while it is expected to be forwarded over Link L2.

This document defines Target FEC Stack sub-TLVs and explains how they can be used to tackle above challenges.

4. Segment Routing Sub-TLV Format

The format of the following FEC Sub-TLVs follows the philosophy of Target FEC Stack TLV carrying FECs corresponding to each label in the label stack. When operated with the procedures defined in [RFC4379], this allows LSP ping/traceroute operations to function when Target FEC Stack TLV contains more FECs than received label stack at responder nodes.

			 	Type  Sub-Type    Value Field
				----  --------  ---------------
					1	TBD1      IPv4 Prefix Node Segment ID
						TBD2      IPv6 Prefix Node Segment ID
						TBD3      Adjacency Segment ID

Service Segments and FRR will be considered in future version.

4.1. IPv4 Prefix Node Segment ID

The format is as below:


   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
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                         IPv4 Prefix                           |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |Prefix Length  |      Resv     |     Protocol  |  SID Length   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                       Node Segment ID                         |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |             Reserved          |                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
  |                Advertising Node Identifier                    |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

IPv4 Prefix

Prefix Length

Protocol

SID Length

Node Segment ID

Advertising Node Identifier

4.2. IPv6 Prefix Node Segment ID

The format is as below:


   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
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                                                               |
  |                         IPv6 Prefix                           |
  |                                                               |
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |Prefix Length  |      Resv     |     Protocol  |  SID Length   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                        Node Segment ID                        |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |             Reserved          |                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
  |                Advertising Node Identifier                    |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

IPv6 Prefix

Prefix Length

Protocol

SID Length

Node Segment ID

Advertising Node Identifier

4.3. IGP Adjacency Segment ID

The format is as below: 

   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
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                      Local Interface ID                       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                      Remote Interface ID                      |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |   Protocol    |    SID Length |                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
  |                Advertising Node Identifier                    |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                 IGP Adjacency Segment ID                      |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Local Interface ID

Remote Interface ID

Protocol

SID Length

Advertising Node Identifier

IGP Adjacency Segment ID

5. Extension to Downstream Mapping TLV

In an echo reply, the Downstream Mapping TLV [RFC4379] is used to report for each interface over which a FEC could be forwarded. For an FEC, there are multiple protocols that may be used to distribute label mapping. The "Protocol" field of the Downstream Mapping TLV is used to return the protocol that is used to distribute a specific a label. The following protocols are defined in section 3.2 of [RFC4379]: 

   Protocol #        Signaling Protocol
   ----------        ------------------
            0        Unknown
            1        Static
            2        BGP
            3        LDP
            4        RSVP-TE

With segment routing, OSPF or ISIS can be used for label distribution, this document adds two new protocols as follows: 

   Protocol #        Signaling Protocol
   ----------        ------------------
            5        OSPF
            6        ISIS

6. Procedures

This section describes aspects of LSP ping/traceroute operations that require further considerations beyond [RFC4379].

6.1. FECs in Target FEC Stack TLV

When LSP echo request packets are generated by an initiator, FECs carried in Target FEC Stack TLV may need to or desire to have deviating contents. This document outlines expected Target FEC Stack TLV construction mechanics by initiator for known scenarios.

6.2. FEC Stack Change TLV

The network node which advertised the node segment ID is responsible for generating FEC Stack Change TLV of &pop& operation for node segment ID, regardless of if PHP is enabled or not.

The network node that is immediate downstream of the node which advertised the adjacency segment ID is responsible for generating FEC Stack Change TLV of &pop& operation for adjacency segment ID.

6.3. PHP, Adjacency SID Pop, Implicit NULL

Forwarding behavior of node segment ID PHP is equivalent to usage of implicit Null in MPLS protocols that embraces downstream label allocation scheme. Adjacency segment ID is also similar in a sense that it can be thought as nexthop destined locally allocated segment that has PHP enabled. Procedures described in Section 4.4 of [RFC4379] relies on Stack-D and Stack-R explicitly having Implicit Null value. It may simplify implementations to reuse Implicit Null for node segment ID PHP and adjacency segment ID cases. However, it is technically incorrect for Implicit Null value to externally appear. Therefore, implicit Null MUST NOT be placed in Stack-D and Interface and Label Stack TLV for node segment ID PHP and adjacency segment ID cases.

6.4. Segment Protocol Check

7. IANA Considerations

To be Updated.

8. Security Considerations

To be Updated.

9. Acknowledgement

The authors would like to thank Stefano Previdi for his review and comments.

10. Contributing Authors

Tarek Saad
Cisco Systems
Email: tsaad@cisco.com

Siva Sivabalan
Cisco Systems
Email: msiva@cisco.com

11. References

11.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[I-D.filsfils-rtgwg-segment-routing] Filsfils, C., Previdi, S., Bashandy, A., Decraene, B., Litkowski, S., Horneffer, M., Milojevic, I., Shakir, R., Ytti, S., Henderickx, W., Tantsura, J. and E. Crabbe, "Segment Routing Architecture", Internet-Draft draft-filsfils-rtgwg-segment-routing-00, June 2013.
[RFC4379] Kompella, K. and G. Swallow, "Detecting Multi-Protocol Label Switched (MPLS) Data Plane Failures", RFC 4379, February 2006.
[RFC6424] Bahadur, N., Kompella, K. and G. Swallow, "Mechanism for Performing Label Switched Path Ping (LSP Ping) over MPLS Tunnels", RFC 6424, November 2011.

11.2. Informative References

[RFC6425] Saxena, S., Swallow, G., Ali, Z., Farrel, A., Yasukawa, S. and T. Nadeau, "Detecting Data-Plane Failures in Point-to-Multipoint MPLS - Extensions to LSP Ping", RFC 6425, November 2011.
[RFC6291] Andersson, L., van Helvoort, H., Bonica, R., Romascanu, D. and S. Mansfield, "Guidelines for the Use of the "OAM" Acronym in the IETF", BCP 161, RFC 6291, June 2011.

Authors' Addresses

Nagendra Kumar Cisco Systems, Inc. 7200 Kit Creek Road Research Triangle Park, NC 27709 US EMail: naikumar@cisco.com
George Swallow Cisco Systems, Inc. 1414 Massachusetts Ave Boxborough, MA 01719 US EMail: swallow@cisco.com
Carlos Pignataro Cisco Systems, Inc. 7200 Kit Creek Road Research Triangle Park, NC 27709-4987 US EMail: cpignata@cisco.com
Nobo Akiya Cisco Systems, Inc. 2000 Innovation Drive Kanata, ON K2K 3E8 Canada EMail: nobo@cisco.com
Mach(Guoyi) Chen Huawei EMail: mach.chen@huawei.com