| Network Work group | N. Kumar, Ed. |
| Internet-Draft | C. Pignataro, Ed. |
| Intended status: Standards Track | Cisco |
| Expires: February 3, 2018 | G. Swallow |
| Individual | |
| N. Akiya | |
| Big Switch Networks | |
| S. Kini | |
| Individual | |
| H. Gredler | |
| RtBrick Inc | |
| M. Chen | |
| Huawei | |
| August 2, 2017 |
Label Switched Path (LSP) Ping/Traceroute for Segment Routing Networks with MPLS Data-plane
draft-ietf-mpls-spring-lsp-ping-04
Segment Routing architecture leverages the source routing and tunneling paradigms and can be directly applied to MPLS data plane. A node steers a packet through a controlled set of instructions called segments, by prepending the packet with a 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 Segment Routing architecture. This document illustrates the problem and describe a mechanism to perform LSP Ping and Traceroute on Segment Routing network over MPLS data plane.
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This Internet-Draft will expire on February 3, 2018.
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[I-D.ietf-spring-segment-routing] introduces and explains Segment Routing architecture that leverages the source routing and tunneling paradigms. A node steers a packet through a controlled set of instructions called segments, by prepending the packet with Segment Routing header. A detailed definition about Segment Routing architecture is available in [I-D.ietf-spring-segment-routing]
As defined in [I-D.ietf-spring-segment-routing-mpls], the Segment Routing architecture can be directly applied to MPLS data plane in a way that, the Segment identifier (Segment ID) will be of 20-bits size and Segment Routing header is the label stack.
"Detecting Multi-Protocol Label Switched (MPLS) Data Plane Failures" [RFC8029] defines a simple and efficient mechanism to detect data plane failures in Label Switched Paths (LSP) by specifying information to be carried in an MPLS "echo request" and "echo reply" for the purposes of fault detection and isolation. Mechanisms for reliably sending the echo reply are defined. The functionality defined in [RFC8029] is modeled after the ping/traceroute paradigm (ICMP echo request [RFC0792]) and is typically referred to as LSP ping and LSP traceroute. [RFC8029] supports hierarchal and stitching LSPs.
Unlike LDP or RSVP which are the other well-known MPLS control plane protocols, the basis of segment ID assignment in Segment Routing architecture is not hop-by-hop always. Depending on the type of segment ID, the assignment can be unique to the node or within the domain.
This nature of Segment Routing raises additional consideration for fault detection and 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 data plane.
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].
This document uses the terminologies defined in [I-D.ietf-spring-segment-routing], [RFC8029], and so the readers are expected to be familiar with the same.
This document defines sub-TLVs for the Target Forwarding Equivalence Class (FEC) Stack TLV and explains how they can be used to tackle below challenges.
[RFC8029] defines the OAM machinery that helps with fault detection and isolation in MPLS data-plane path with the use of various Target FEC Stack Sub-TLV that are carried in MPLS Echo Request 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 Segment Routing architecture raises a need for additional machinery for path validation. This section discusses the challenges as below:
L1
+--------+
| L2 |
R3-------R6
/ \
/ \
R1----R2 R7----R8
\ /
\ /
R4-------R5
Figure 1: Segment Routing network
The Node Segment IDs for R1, R2, R3, R4, R5, R6, R7 and R8 are 5001,
5002, 5003, 5004, 5005, 5006, 5007, 5008 respectively.
9136 --> Adjacency Segment ID from R3 to R6 over link L1.
9236 --> Adjacency Segment ID from R3 to R6 over link L2.
9124 --> Adjacency segment ID from R2 to R4.
9123 --> Adjacency Segment ID from R2 to R3.
The forwarding semantic of Adjacency Segment ID is to pop the segment ID and send the packet to a specific neighbor over a specific link. A malfunctioning node may forward packets using Adjacency Segment ID to incorrect neighbor or over incorrect link. Exposed segment ID (after incorrectly forwarded Adjacency Segment ID) might still allow such packet to reach the intended destination, although the intended strict traversal has been broken.
Assume in above topology, R1 sends traffic with segment stack as {9124, 5008} so that the path taken will be R1-R2-R4-R5-R7-R8. If the Adjacency Segment ID 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, in a different example, it may not be helpful. For example if R3, due to misprogramming, forwards packet with Adjacency Segment ID 9236 via link L1 while it is expected to be forwarded over Link L2.
A Segment ID can represent a service based instruction. A Segment Routing header can have label stack entries where the label represents a service to be applied along the path. Since these labels are part of the label stack, they can influence the path taken by a packet and consequently have implications on MPLS OAM. Service Label is left for future study.
The format of the following Segment ID 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 [RFC8029], this allows LSP ping/traceroute operations to function when Target FEC Stack TLV contains more FECs than received label stack at responder nodes.
Three new sub-TLVs are defined for Target FEC Stack TLVs (Type 1), Reverse-Path Target FEC Stack TLV (Type 16) and Reply Path TLV (Type 21).
sub-Type Value Field
-------- ---------------
34 IPv4 IGP-Prefix Segment ID
35 IPv6 IGP-Prefix Segment ID
36 IGP-Adjacency Segment ID
Service Segments and FRR will be considered in future version.
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 | Protocol | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
IPv4 Prefix
Prefix Length
Protocol
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 | Protocol | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
IPv6 Prefix
Prefix Length
Protocol
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Adj. Type | Protocol | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Local Interface ID (4 or 16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Remote Interface ID (4 or 16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
| Advertising Node Identifier (4 or 6 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
| Receiving Node Identifier (4 or 6 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Adj. Type (Adjacency Type)
Protocol
Local Interface ID
Remote Interface ID
Advertising Node Identifier
Receiving Node Identifier
In an echo reply, the Downstream Mapping TLV [RFC8029] is used to report for each interface over which a FEC could be forwarded. For a FEC, there are multiple protocols that may be used to distribute label mapping. The "Protocol" field of the Downstream Detailed Mapping TLV is used to return the protocol that is used to distribute a specific a label. The following protocols are defined in Section 3.4.1.2 of [RFC8029]:
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
This section describes aspects of LSP Ping and traceroute operations that require further considerations beyond [RFC8029].
When LSP echo request packets are generated by an initiator, FECs carried in Target FEC Stack TLV may need to have deviating contents. This document outlines expected Target FEC Stack TLV construction mechanics by initiator for known scenarios.
Section 3.4.1.3 of [RFC8029] defines FEC Stack Change sub-TLV that a router must include when the FEC stack changes.
The network node which advertised the Node Segment ID is responsible for generating FEC Stack Change sub-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 sub-TLV of &pop& operation for Adjacency Segment ID.
The forwarding semantic of Node Segment ID with PHP flag is equivalent to usage of implicit Null in MPLS protocols. Adjacency Segment ID is also similar in a sense that it can be thought as next hop destined locally allocated segment that has PHP enabled. Procedures described in Section 4.4 of [RFC8029] 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.
This section updates the procedure defined in Step 6 of Section 4.4. of [RFC8029]
}
}
}
}
}
}
}
LSP Traceroute operation can properly traverse every hop of Segment Routing network in Uniform Model described in [RFC3443]. If one or more LSRs employ Short Pipe Model described in [RFC3443], then LSP Traceroute may not be able to properly traverse every hop of Segment Routing network due to absence of TTL copy operation when outer label is popped. Short Pipe being the most commonly used model. The following TTL manipulation technique MAY be used when Short Pipe model is used.
When tracing a LSP according to the procedures in [RFC8029] the TTL is incremented by one in order to trace the path sequentially along the LSP. However when a source routed LSP has to be traced there are as many TTLs as there are labels in the stack. The LSR that initiates the traceroute SHOULD start by setting the TTL to 1 for the tunnel in the LSP's label stack it wants to start the tracing from, the TTL of all outer labels in the stack to the max value, and the TTL of all the inner labels in the stack to zero. Thus a typical start to the traceroute would have a TTL of 1 for the outermost label and all the inner labels would have TTL 0. If the FEC Stack TLV is included it should contain only those for the inner stacked tunnels. The Return Code/Subcode and FEC Stack Change TLV should be used to diagnose the tunnel as described in [RFC8029]. When the tracing of a tunnel in the stack is complete, then the next tunnel in the stack should be traced. The end of a tunnel can be detected from the "Return Code" when it indicates that the responding LSR is an egress for the stack at depth 1. Thus the traceroute procedures in [RFC8029] can be recursively applied to traceroute a source routed LSP.
Source stacking can be optionally used to apply services on the packet at a LSR along the path, where a label in the stack is used to trigger service application. A data plane failure detection and isolation mechanism should provide its functionality without applying these services. This is mandatory for services that are stateful, though for stateless services [RFC8029] could be used as-is. It MAY also provide a mechanism to detect and isolate faults within the service function itself.
How a node treats Service label is outside the scope of this document and will be included in this or a different document later.
[I-D.ietf-spring-segment-routing-ldp-interop] describes how Segment Routing operates in network where SR-capable and non-SR-capable nodes coexist. In such networks, there may not be any FEC mapping in the responder when the Initiator is SR-capable while the responder is not (or vice-versa). But this is not different from RSVP and LDP interop scenarios. When LSP Ping is triggered, the responder will set the FEC-return-code to Return 4, "Replying router has no mapping for the FEC at stack-depth".
Similarly when SR-capable node assigns Adj-SID for non-SR-capable node, LSP traceroute may fail as the non-SR-capable node is not aware of "IGP Adjacency Segment ID" sub-TLV and may not reply with FEC Stack change. This may result in any further downstream nodes to reply back with Return-code as 4, "Replying router has no mapping for the FEC at stack-depth".
IANA is requested to assign three new Sub-TLVs from "Sub-TLVs for TLV Types 1, 16 and 21" sub-registry from the "Multi-Protocol Label Switching (MPLS) Label Switched Paths (LSPs) Ping Parameters" [IANA-MPLS-LSP-PING] registry.
Sub-Type Sub-TLV Name Reference
-------- ----------------- ------------
34 IPv4 IGP-Prefix Segment ID Section 5.1 of this document
35 IPv6 IGP-Prefix Segment ID Section 5.2 of this document
36 IGP-Adjacency Segment ID Section 5.3 of this document
IANA is requested to create a new "Protocol" registry under the "Multi-Protocol Label Switching (MPLS) Label Switched Paths (LSPs) Ping Parameters" registry. Code points in the range of 0-250 will be assigned by Standards Action. The range of 251-254 are reserved for experimental use and will not be assigned. The initial entries into the registry will be.
Value Meaning Reference
---------- ---------------- ------------
0 Unknown Section 3.4.1.2 of RFC8029
1 Static Section 3.4.1.2 of RFC8029
2 BGP Section 3.4.1.2 of RFC8029
3 LDP Section 3.4.1.2 of RFC8029
4 RSVP-TE Section 3.4.1.2 of RFC8029
5 OSPF Section 6 of this document
6 ISIS Section 6 of this document
7-250 Unassigned
251-254 Experimental use This document
255 Reserved This document
IANA is requested to assign a new Return Code from the "Multi-Protocol Label Switching (MPLS) Label Switched Paths (LSPs) Ping Parameters" in "Return Codes" Sub-registry.
Value Meaning Reference
---------- ----------------- ------------
TBD7 Mapping for this FEC is not associated Section 7.4 of
with the incoming interface this document
Note to the RFC Editor (please remove before publication): IANA has made early allocation for sub-type 34, 35 and 35. The early allocation expires 2017-09-15.
This document defines additional Sub-TLVs and follows the mechanism defined in [RFC8029]. So all the security consideration defined in [RFC8029] will be applicable for this document and in addition it does not impose any security challenges to be considered.
The authors would like to thank Stefano Previdi, Les Ginsberg, Balaji Rajagopalan, Harish Sitaraman, Curtis Villamizar, Pranjal Dutta, Lizhong Jin, Tom Petch, Victor Ji and Mustapha Aissaoui for their review and comments.
The authors wold like to thank Loa Andersson for his comments and recommendation to merge drafts.
The following are key contributors to this document:
| [I-D.ietf-spring-segment-routing-ldp-interop] | Filsfils, C., Previdi, S., Bashandy, A., Decraene, B. and S. Litkowski, "Segment Routing interworking with LDP", Internet-Draft draft-ietf-spring-segment-routing-ldp-interop-08, June 2017. |
| [IANA-MPLS-LSP-PING] | IANA, "Multi-Protocol Label Switching (MPLS) Label Switched Paths (LSPs) Ping Parameters" |
| [RFC0792] | Postel, J., "Internet Control Message Protocol", STD 5, RFC 792, DOI 10.17487/RFC0792, September 1981. |