Internet Engineering Task Force (IETF) Z. Ali
Request for Comments: 9259 C. Filsfils
Category: Standards Track Cisco Systems
ISSN: 2070-1721 S. Matsushima
Softbank
D. Voyer
Bell Canada
M. Chen
Huawei
June 2022
Operations, Administration, and Maintenance (OAM) in Segment Routing
over IPv6 (SRv6)
Abstract
This document describes how the existing IPv6 mechanisms for ping and
traceroute can be used in a Segment Routing over IPv6 (SRv6) network.
The document also specifies the OAM flag (O-flag) in the Segment
Routing Header (SRH) for performing controllable and predictable flow
sampling from segment endpoints. In addition, the document describes
how a centralized monitoring system performs a path continuity check
between any nodes within an SRv6 domain.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9259.
Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
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Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.
Table of Contents
1. Introduction
1.1. Requirements Language
1.2. Abbreviations
1.3. Terminology and Reference Topology
2. OAM Mechanisms
2.1. OAM Flag in the Segment Routing Header
2.1.1. OAM Flag Processing
2.2. OAM Operations
3. Security Considerations
4. Privacy Considerations
5. IANA Considerations
6. References
6.1. Normative References
6.2. Informative References
Appendix A. Illustrations
A.1. Ping in SRv6 Networks
A.1.1. Pinging an IPv6 Address via a Segment List
A.1.2. Pinging a SID
A.2. Traceroute in SRv6 Networks
A.2.1. Traceroute to an IPv6 Address via a Segment List
A.2.2. Traceroute to a SID
A.3. Hybrid OAM Using the OAM Flag
A.4. Monitoring of SRv6 Paths
Acknowledgements
Contributors
Authors' Addresses
1. Introduction
As Segment Routing over IPv6 (SRv6) [RFC8402] simply adds a new type
of Routing Extension Header, existing IPv6 OAM mechanisms can be used
in an SRv6 network. This document describes how the existing IPv6
mechanisms for ping and traceroute can be used in an SRv6 network.
This includes illustrations of pinging an SRv6 Segment Identifier
(SID) to verify that the SID is reachable and is locally programmed
at the target node. This also includes illustrations for
tracerouting to an SRv6 SID for hop-by-hop fault localization as well
as path tracing to a SID.
This document also introduces enhancements for the OAM mechanism for
SRv6 networks that allow controllable and predictable flow sampling
from segment endpoints using, e.g., the IP Flow Information Export
(IPFIX) protocol [RFC7011]. Specifically, the document specifies the
OAM flag (O-flag) in the SRH as a marking bit in the user packets to
trigger telemetry data collection and export at the segment
endpoints.
This document also outlines how the centralized OAM technique in
[RFC8403] can be extended for SRv6 to perform a path continuity check
between any nodes within an SRv6 domain. Specifically, the document
illustrates how a centralized monitoring system can monitor arbitrary
SRv6 paths by creating loopback probes that originate and terminate
at the centralized monitoring system.
1.1. 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.
1.2. Abbreviations
The following abbreviations are used in this document:
SID: Segment Identifier
SL: Segments Left
SR: Segment Routing
SRH: Segment Routing Header [RFC8754]
SRv6: Segment Routing over IPv6 [RFC8402]
PSP: Penultimate Segment Pop [RFC8986]
USP: Ultimate Segment Pop [RFC8986]
ICMPv6: Internet Control Message Protocol for the Internet Protocol
version 6 [RFC4443]
IS-IS: Intermediate System to Intermediate System
OSPF: Open Shortest Path First [RFC2328]
IGP: Interior Gateway Protocol (e.g., OSPF and IS-IS)
BGP-LS: Border Gateway Protocol - Link State [RFC8571]
1.3. Terminology and Reference Topology
The terminology and simple topology in this section are used for
illustration throughout the document.
+--------------------------| N100 |---------------------------------+
| |
| ====== link1====== link3------ link5====== link9------ ====== |
||N1||------||N2||------| N3 |------||N4||------| N5 |---||N7||
|| ||------|| ||------| |------|| ||------| |---|| ||
====== link2====== link4------ link6======link10------ ======
| | | |
---+-- | ------ | --+---
|CE1 | +-------| N6 |---------+ |CE2 |
------ link7 | | link8 ------
------
Figure 1: Reference Topology
In the reference topology:
* Node j has an IPv6 loopback address 2001:db8:L:j::/128.
* Nodes N1, N2, N4, and N7 are SRv6-capable nodes.
* Nodes N3, N5, and N6 are IPv6 nodes that are not SRv6-capable
nodes. Such nodes are referred to as "non-SRv6-capable nodes".
* CE1 and CE2 are Customer Edge devices of any data plane capability
(e.g., IPv4, IPv6, and L2).
* A SID at node j with locator block 2001:db8:K::/48 and function U
is represented by 2001:db8:K:j:U::.
* Node N100 is a controller.
* The IPv6 address of the nth link between nodes i and j at the i
side is represented as 2001:db8:i:j:in::. For example, in
Figure 1, the IPv6 address of link6 (the second link between nodes
N3 and N4) at node N3 is 2001:db8:3:4:32::. Similarly, the IPv6
address of link5 (the first link between nodes N3 and N4) at node
N3 is 2001:db8:3:4:31::.
* 2001:db8:K:j:Xin:: is explicitly allocated as the End.X SID at
node j towards neighbor node i via the nth link between nodes i
and j. For example, 2001:db8:K:2:X31:: represents End.X at node
N2 towards node N3 via link3 (the first link between nodes N2 and
N3). Similarly, 2001:db8:K:4:X52:: represents the End.X at node
N4 towards node N5 via link10 (the second link between nodes N4
and N5). Please refer to [RFC8986] for a description of End.X
SID.
* A SID list is represented as <S1, S2, S3>, where S1 is the first
SID to visit, S2 is the second SID to visit, and S3 is the last
SID to visit along the SR path.
* (SA,DA) (S3, S2, S1; SL)(payload) represents an IPv6 packet with:
- IPv6 header with source address SA, destination address DA, and
SRH as the next header
- SRH with SID list <S1, S2, S3> with SegmentsLeft = SL
Note the difference between the < > and () symbols: <S1, S2,
S3> represents a SID list where S1 is the first SID and S3 is
the last SID to traverse. (S3, S2, S1; SL) represents the same
SID list but encoded in the SRH format where the rightmost SID
in the SRH is the first SID and the leftmost SID in the SRH is
the last SID. When referring to an SR Policy in a high-level
use case, it is simpler to use the <S1, S2, S3> notation. When
referring to an illustration of the detailed packet behavior,
the (S3, S2, S1; SL) notation is more convenient.
- (payload) represents the payload of the packet.
2. OAM Mechanisms
This section defines OAM enhancements for SRv6 networks.
2.1. OAM Flag in the Segment Routing Header
[RFC8754] describes the Segment Routing Header (SRH) and how SR-
capable nodes use it. The SRH contains an 8-bit Flags field.
This document defines the following bit in the SRH Flags field to
carry the O-flag:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| |O| |
+-+-+-+-+-+-+-+-+
Where:
O-flag: OAM flag in the SRH Flags field defined in [RFC8754].
2.1.1. OAM Flag Processing
The O-flag in the SRH is used as a marking bit in user packets to
trigger telemetry data collection and export at the segment
endpoints.
An SR domain ingress edge node encapsulates packets traversing the SR
domain as defined in [RFC8754]. The SR domain ingress edge node MAY
use the O-flag in the SRH for marking the packet to trigger the
telemetry data collection and export at the segment endpoints. Based
on local configuration, the SR domain ingress edge node may implement
a classification and sampling mechanism to mark a packet with the
O-flag in the SRH. Specification of the classification and sampling
method is outside the scope of this document.
This document does not specify the data elements that need to be
exported and the associated configurations. Similarly, this document
does not define any formats for exporting the data elements.
Nonetheless, without the loss of generality, this document assumes
that the IP Flow Information Export (IPFIX) protocol [RFC7011] is
used for exporting the traffic flow information from the network
devices to a controller for monitoring and analytics. Similarly,
without the loss of generality, this document assumes that requested
information elements are configured by the management plane through
data set templates (e.g., as in IPFIX [RFC7012]).
Implementation of the O-flag is OPTIONAL. If a node does not support
the O-flag, then it simply ignores it upon reception. If a node
supports the O-flag, it can optionally advertise its potential via
control plane protocol(s).
The following is appended to line S01 of the pseudocode associated
with the SID S (as defined in Section 4.3.1.1 of [RFC8754]) when N
receives a packet destined to S, S is a local SID, and the O-flag is
processed.
S01.1. IF the O-flag is set and local configuration permits
O-flag processing {
a. Make a copy of the packet.
b. Send the copied packet, along with a timestamp,
to the OAM process for telemetry data collection
and export. ;; Ref1
}
Ref1: To provide an accurate timestamp, an implementation should
copy and record the timestamp as soon as possible during packet
processing. Timestamp and any other metadata are not carried in
the packet forwarded to the next hop.
Please note that the O-flag processing happens before execution of
regular processing of the local SID S. Specifically, line S01.1 of
the pseudocode specified in this document is inserted between lines
S01 and S02 of the pseudocode defined in Section 4.3.1.1 of
[RFC8754].
Based on the requested information elements configured by the
management plane through data set templates [RFC7012], the OAM
process exports the requested information elements. The information
elements include parts of the packet header and/or parts of the
packet payload for flow identification. The OAM process uses
information elements defined in IPFIX [RFC7011] and Packet Sampling
(PSAMP) [RFC5476] for exporting the requested sections of the
mirrored packets.
If the penultimate segment of a segment list is a PSP SID, telemetry
data from the ultimate segment cannot be requested. This is because,
when the penultimate segment is a PSP SID, the SRH is removed at the
penultimate segment, and the O-flag is not processed at the ultimate
segment.
The processing node MUST rate-limit the number of packets punted to
the OAM process to a configurable rate. This is to avoid impacting
the performance of the OAM and telemetry collection processes.
Failure to implement the rate limit can lead to a denial-of-service
attack, as detailed in Section 3.
The OAM process MUST NOT process the copy of the packet or respond to
any Upper-Layer header (like ICMP, UDP, etc.) payload to prevent
multiple evaluations of the datagram.
The OAM process is expected to be located on the routing node
processing the packet. Although the specification of the OAM process
or the external controller operations are beyond the scope of this
document, the OAM process SHOULD NOT be topologically distant from
the routing node, as this is likely to create significant security
and congestion issues. How to correlate the data collected from
different nodes at an external controller is also outside the scope
of this document. Appendix A illustrates use of the O-flag for
implementing a hybrid OAM mechanism, where the "hybrid"
classification is based on [RFC7799].
2.2. OAM Operations
IPv6 OAM operations can be performed for any SRv6 SID whose behavior
allows Upper-Layer header processing for an applicable OAM payload
(e.g., ICMP, UDP).
Ping to an SRv6 SID is used to verify that the SID is reachable and
is locally programmed at the target node. Traceroute to a SID is
used for hop-by-hop fault localization as well as path tracing to a
SID. Appendix A illustrates the ICMPv6-based ping and UDP-based
traceroute mechanisms for ping and traceroute to an SRv6 SID.
Although this document only illustrates ICMPv6-based ping and UDP-
based traceroute to an SRv6 SID, the procedures are equally
applicable to other OAM mechanisms that probe an SRv6 SID (e.g.,
Bidirectional Forwarding Detection (BFD) [RFC5880], Seamless BFD
(S-BFD) [RFC7880], and Simple Two-way Active Measurement Protocol
(STAMP) probe message processing [STAMP-SR]). Specifically, as long
as local configuration allows the Upper-Layer header processing of
the applicable OAM payload for SRv6 SIDs, the existing IPv6 OAM
techniques can be used to target a probe to a (remote) SID.
IPv6 OAM operations can be performed with the target SID in the IPv6
destination address without an SRH or with an SRH where the target
SID is the last segment. In general, OAM operations to a target SID
may not exercise all of its processing depending on its behavior
definition. For example, ping to an End.X SID [RFC8986] only
validates the SID is locally programmed at the target node and does
not validate switching to the correct outgoing interface. To
exercise the behavior of a target SID, the OAM operation should
construct the probe in a manner similar to a data packet that
exercises the SID behavior, i.e. to include that SID as a transit SID
in either an SRH or IPv6 DA of an outer IPv6 header or as appropriate
based on the definition of the SID behavior.
3. Security Considerations
[RFC8754] defines the notion of an SR domain and use of the SRH
within the SR domain. The use of OAM procedures described in this
document is restricted to an SR domain. For example, similar to SID
manipulation, O-flag manipulation is not considered a threat within
the SR domain. Procedures for securing an SR domain are defined in
Sections 5.1 and 7 of [RFC8754].
As noted in Section 7.1 of [RFC8754], compromised nodes within the SR
domain may mount attacks. The O-flag may be set by an attacking node
attempting a denial-of-service attack on the OAM process at the
segment endpoint node. An implementation correctly implementing the
rate limiting described in Section 2.1.1 is not susceptible to that
denial-of-service attack. Additionally, SRH flags are protected by
the Hashed Message Authentication Code (HMAC) TLV, as described in
Section 2.1.2.1 of [RFC8754]. Once an HMAC is generated for a
segment list with the O-flag set, it can be used for an arbitrary
amount of traffic using that segment list with the O-flag set.
The security properties of the channel used to send exported packets
marked by the O-flag will depend on the specific OAM processes used.
An on-path attacker able to observe this OAM channel could conduct
traffic analysis, or potentially eavesdropping (depending on the OAM
configuration), of this telemetry for the entire SR domain from such
a vantage point.
This document does not impose any additional security challenges to
be considered beyond the security threats described in [RFC4884],
[RFC4443], [RFC0792], [RFC8754], and [RFC8986].
4. Privacy Considerations
The per-packet marking capabilities of the O-flag provide a granular
mechanism to collect telemetry. When this collection is deployed by
an operator with the knowledge and consent of the users, it will
enable a variety of diagnostics and monitoring to support the OAM and
security operations use cases needed for resilient network
operations. However, this collection mechanism will also provide an
explicit protocol mechanism to operators for surveillance and
pervasive monitoring use cases done contrary to the user's consent.
5. IANA Considerations
IANA has registered the following in the "Segment Routing Header
Flags" subregistry in the "Internet Protocol Version 6 (IPv6)
Parameters" registry:
+=====+=============+===========+
| Bit | Description | Reference |
+=====+=============+===========+
| 2 | O-flag | RFC 9259 |
+-----+-------------+-----------+
Table 1
6. References
6.1. 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>.
[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>.
[RFC8754] Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J.,
Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
(SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020,
<https://www.rfc-editor.org/info/rfc8754>.
[RFC8986] Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer,
D., Matsushima, S., and Z. Li, "Segment Routing over IPv6
(SRv6) Network Programming", RFC 8986,
DOI 10.17487/RFC8986, February 2021,
<https://www.rfc-editor.org/info/rfc8986>.
6.2. Informative References
[RFC0792] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, DOI 10.17487/RFC0792, September 1981,
<https://www.rfc-editor.org/info/rfc792>.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC2328, April 1998,
<https://www.rfc-editor.org/info/rfc2328>.
[RFC4443] Conta, A., Deering, S., and M. Gupta, Ed., "Internet
Control Message Protocol (ICMPv6) for the Internet
Protocol Version 6 (IPv6) Specification", STD 89,
RFC 4443, DOI 10.17487/RFC4443, March 2006,
<https://www.rfc-editor.org/info/rfc4443>.
[RFC4884] Bonica, R., Gan, D., Tappan, D., and C. Pignataro,
"Extended ICMP to Support Multi-Part Messages", RFC 4884,
DOI 10.17487/RFC4884, April 2007,
<https://www.rfc-editor.org/info/rfc4884>.
[RFC5476] Claise, B., Ed., Johnson, A., and J. Quittek, "Packet
Sampling (PSAMP) Protocol Specifications", RFC 5476,
DOI 10.17487/RFC5476, March 2009,
<https://www.rfc-editor.org/info/rfc5476>.
[RFC5837] Atlas, A., Ed., Bonica, R., Ed., Pignataro, C., Ed., Shen,
N., and JR. Rivers, "Extending ICMP for Interface and
Next-Hop Identification", RFC 5837, DOI 10.17487/RFC5837,
April 2010, <https://www.rfc-editor.org/info/rfc5837>.
[RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,
<https://www.rfc-editor.org/info/rfc5880>.
[RFC7011] Claise, B., Ed., Trammell, B., Ed., and P. Aitken,
"Specification of the IP Flow Information Export (IPFIX)
Protocol for the Exchange of Flow Information", STD 77,
RFC 7011, DOI 10.17487/RFC7011, September 2013,
<https://www.rfc-editor.org/info/rfc7011>.
[RFC7012] Claise, B., Ed. and B. Trammell, Ed., "Information Model
for IP Flow Information Export (IPFIX)", RFC 7012,
DOI 10.17487/RFC7012, September 2013,
<https://www.rfc-editor.org/info/rfc7012>.
[RFC7799] Morton, A., "Active and Passive Metrics and Methods (with
Hybrid Types In-Between)", RFC 7799, DOI 10.17487/RFC7799,
May 2016, <https://www.rfc-editor.org/info/rfc7799>.
[RFC7880] Pignataro, C., Ward, D., Akiya, N., Bhatia, M., and S.
Pallagatti, "Seamless Bidirectional Forwarding Detection
(S-BFD)", RFC 7880, DOI 10.17487/RFC7880, July 2016,
<https://www.rfc-editor.org/info/rfc7880>.
[RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
July 2018, <https://www.rfc-editor.org/info/rfc8402>.
[RFC8403] Geib, R., Ed., Filsfils, C., Pignataro, C., Ed., and N.
Kumar, "A Scalable and Topology-Aware MPLS Data-Plane
Monitoring System", RFC 8403, DOI 10.17487/RFC8403, July
2018, <https://www.rfc-editor.org/info/rfc8403>.
[RFC8571] Ginsberg, L., Ed., Previdi, S., Wu, Q., Tantsura, J., and
C. Filsfils, "BGP - Link State (BGP-LS) Advertisement of
IGP Traffic Engineering Performance Metric Extensions",
RFC 8571, DOI 10.17487/RFC8571, March 2019,
<https://www.rfc-editor.org/info/rfc8571>.
[RFC9197] Brockners, F., Ed., Bhandari, S., Ed., and T. Mizrahi,
Ed., "Data Fields for In Situ Operations, Administration,
and Maintenance (IOAM)", RFC 9197, DOI 10.17487/RFC9197,
May 2022, <https://www.rfc-editor.org/info/rfc9197>.
[STAMP-SR] Gandhi, R., Ed., Filsfils, C., Voyer, D., Chen, M.,
Janssens, B., and R. Foote, "Performance Measurement Using
Simple TWAMP (STAMP) for Segment Routing Networks", Work
in Progress, Internet-Draft, draft-ietf-spring-stamp-srpm-
03, 1 February 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-spring-
stamp-srpm-03>.
Appendix A. Illustrations
This appendix shows how some of the existing IPv6 OAM mechanisms can
be used in an SRv6 network. It also illustrates an OAM mechanism for
performing controllable and predictable flow sampling from segment
endpoints. How the centralized OAM technique in [RFC8403] can be
extended for SRv6 is also described in this appendix.
A.1. Ping in SRv6 Networks
The existing mechanism to perform the reachability checks, along the
shortest path, continues to work without any modification. Any IPv6
node (SRv6-capable or non-SRv6-capable) can initiate, transit, and
egress a ping packet.
The following subsections outline some additional use cases of ICMPv6
ping in SRv6 networks.
A.1.1. Pinging an IPv6 Address via a Segment List
If an SRv6-capable ingress node wants to ping an IPv6 address via an
arbitrary segment list <S1, S2, S3>, it needs to initiate an ICMPv6
ping with an SR header containing the SID list <S1, S2, S3>. This is
illustrated using the topology in Figure 1. The user issues a ping
from node N1 to a loopback of node N5 via segment list
<2001:db8:K:2:X31::, 2001:db8:K:4:X52::>. The SID behavior used in
the example is End.X, as described in [RFC8986], but the procedure is
equally applicable to any other (transit) SID type.
Figure 2 contains sample output for a ping request initiated at node
N1 to a loopback address of node N5 via segment list
<2001:db8:K:2:X31::, 2001:db8:K:4:X52::>.
> ping 2001:db8:L:5:: via segment list 2001:db8:K:2:X31::,
2001:db8:K:4:X52::
Sending 5, 100-byte ICMPv6 Echos to B5::, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 0.625
/0.749/0.931 ms
Figure 2: Sample Ping Output at an SRv6-Capable Node
All transit nodes process the echo request message like any other
data packet carrying an SR header and hence do not require any
change. Similarly, the egress node does not require any change to
process the ICMPv6 echo request. For example, in the example in
Figure 2:
* Node N1 initiates an ICMPv6 ping packet with the SRH as follows:
(2001:db8:L:1::, 2001:db8:K:2:X31::) (2001:db8:L:5::,
2001:db8:K:4:X52::, 2001:db8:K:2:X31::, SL=2, NH = ICMPv6)(ICMPv6
Echo Request).
* Node N2, which is an SRv6-capable node, performs the standard SRH
processing. Specifically, it executes the End.X behavior
indicated by the 2001:db8:K:2:X31:: SID and forwards the packet on
link3 to node N3.
* Node N3, which is a non-SRv6-capable node, performs the standard
IPv6 processing. Specifically, it forwards the echo request based
on DA 2001:db8:K:4:X52:: in the IPv6 header.
* Node N4, which is an SRv6-capable node, performs the standard SRH
processing. Specifically, it observes the End.X behavior
(2001:db8:K:4:X52::) and forwards the packet on link10 towards
node N5. If 2001:db8:K:4:X52:: is a PSP SID, the penultimate node
(node N4) does not, should not, and cannot differentiate between
the data packets and OAM probes. Specifically, if
2001:db8:K:4:X52:: is a PSP SID, node N4 executes the SID like any
other data packet with DA = 2001:db8:K:4:X52:: and removes the
SRH.
* The echo request packet at node N5 arrives as an IPv6 packet with
or without an SRH. If node N5 receives the packet with an SRH, it
skips SRH processing (SL=0). In either case, node N5 performs the
standard ICMPv6 processing on the echo request and responds with
the echo reply message to node N1. The echo reply message is IP
routed.
A.1.2. Pinging a SID
The ping mechanism described above can also be used to perform SID
reachability checks and to validate that the SID is locally
programmed at the target node. This is explained in the following
example. The example uses ping to an End SID, as described in
[RFC8986], but the procedure is equally applicable to ping any other
SID behaviors.
Consider the example where the user wants to ping a remote SID
2001:db8:K:4::, via 2001:db8:K:2:X31::, from node N1. The ICMPv6
echo request is processed at the individual nodes along the path as
follows:
* Node N1 initiates an ICMPv6 ping packet with the SRH as follows:
(2001:db8:L:1::, 2001:db8:K:2:X31::) (2001:db8:K:4::,
2001:db8:K:2:X31::; SL=1; NH=ICMPv6)(ICMPv6 Echo Request).
* Node N2, which is an SRv6-capable node, performs the standard SRH
processing. Specifically, it executes the End.X behavior
indicated by the 2001:db8:K:2:X31:: SID on the echo request
packet. If 2001:db8:K:2:X31:: is a PSP SID, node N4 executes the
SID like any other data packet with DA = 2001:db8:K:2:X31:: and
removes the SRH.
* Node N3, which is a non-SRv6-capable node, performs the standard
IPv6 processing. Specifically, it forwards the echo request based
on DA = 2001:db8:K:4:: in the IPv6 header.
* When node N4 receives the packet, it processes the target SID
(2001:db8:K:4::).
* If the target SID (2001:db8:K:4::) is not locally instantiated and
does not represent a local interface, the packet is discarded
* If the target SID (2001:db8:K:4::) is locally instantiated or
represents a local interface, the node processes the Upper-Layer
header. As part of the Upper-Layer header processing, node N4
responds to the ICMPv6 echo request message with an echo reply
message. The echo reply message is IP routed.
A.2. Traceroute in SRv6 Networks
The existing traceroute mechanisms, along the shortest path, continue
to work without any modification. Any IPv6 node (SRv6-capable or a
non-SRv6-capable) can initiate, transit, and egress a traceroute
probe.
The following subsections outline some additional use cases of
traceroute in SRv6 networks.
A.2.1. Traceroute to an IPv6 Address via a Segment List
If an SRv6-capable ingress node wants to traceroute to an IPv6
address via an arbitrary segment list <S1, S2, S3>, it needs to
initiate a traceroute probe with an SR header containing the SID list
<S1, S2, S3>. The user issues a traceroute from node N1 to a
loopback of node N5 via segment list <2001:db8:K:2:X31::,
2001:db8:K:4:X52::>. The SID behavior used in the example is End.X,
as described in [RFC8986], but the procedure is equally applicable to
any other (transit) SID type. Figure 3 contains sample output for
the traceroute request.
> traceroute 2001:db8:L:5:: via segment list 2001:db8:K:2:X31::,
2001:db8:K:4:X52::
Tracing the route to 2001:db8:L:5::
1 2001:db8:2:1:21:: 0.512 msec 0.425 msec 0.374 msec
DA: 2001:db8:K:2:X31::,
SRH:(2001:db8:L:5::, 2001:db8:K:4:X52::, 2001:db8:K:2:X31::, SL=2)
2 2001:db8:3:2:31:: 0.721 msec 0.810 msec 0.795 msec
DA: 2001:db8:K:4:X52::,
SRH:(2001:db8:L:5::, 2001:db8:K:4:X52::, 2001:db8:K:2:X31::, SL=1)
3 2001:db8:4:3::41:: 0.921 msec 0.816 msec 0.759 msec
DA: 2001:db8:K:4:X52::,
SRH:(2001:db8:L:5::, 2001:db8:K:4:X52::, 2001:db8:K:2:X31::, SL=1)
4 2001:db8:5:4::52:: 0.879 msec 0.916 msec 1.024 msec
DA: 2001:db8:L:5::
Figure 3: Sample Traceroute Output at an SRv6-Capable Node
In the sample traceroute output, the information displayed at each
hop is obtained using the contents of the "Time Exceeded" or
"Destination Unreachable" ICMPv6 responses. These ICMPv6 responses
are IP routed.
In the sample traceroute output, the information for link3 is
returned by node N3, which is a non-SRv6-capable node. Nonetheless,
the ingress node is able to display SR header contents as the packet
travels through the non-SRv6-capable node. This is because the "Time
Exceeded" ICMPv6 message can contain as much of the invoking packet
as possible without the ICMPv6 packet exceeding the minimum IPv6 MTU
[RFC4443]. The SR header is included in these ICMPv6 messages
initiated by the non-SRv6-capable transit nodes that are not running
SRv6 software. Specifically, a node generating an ICMPv6 message
containing a copy of the invoking packet does not need to understand
the extension header(s) in the invoking packet.
The segment list information returned for the first hop is returned
by node N2, which is an SRv6-capable node. Just like for the second
hop, the ingress node is able to display SR header contents for the
first hop.
There is no difference in processing of the traceroute probe at an
SRv6-capable and a non-SRv6-capable node. Similarly, both
SRv6-capable and non-SRv6-capable nodes may use the address of the
interface on which probe was received as the source address in the
ICMPv6 response. ICMPv6 extensions defined in [RFC5837] can be used
to display information about the IP interface through which the
datagram would have been forwarded had it been forwardable, the IP
next hop to which the datagram would have been forwarded, the IP
interface upon which the datagram arrived, and the sub-IP component
of an IP interface upon which the datagram arrived.
The IP address of the interface on which the traceroute probe was
received is useful. This information can also be used to verify if
SIDs 2001:db8:K:2:X31:: and 2001:db8:K:4:X52:: are executed correctly
by nodes N2 and N4, respectively. Specifically, the information
displayed for the second hop contains the incoming interface address
2001:db8:2:3:31:: at node N3. This matches the expected interface
bound to End.X behavior 2001:db8:K:2:X31:: (link3). Similarly, the
information displayed for the fourth hop contains the incoming
interface address 2001:db8:4:5::52:: at node N5. This matches the
expected interface bound to the End.X behavior 2001:db8:K:4:X52::
(link10).
A.2.2. Traceroute to a SID
The mechanism to traceroute an IPv6 address via a segment list
described in the previous section can also be used to traceroute a
remote SID behavior, as explained in the following example. The
example uses traceroute to an End SID, as described in [RFC8986], but
the procedure is equally applicable to tracerouting any other SID
behaviors.
Please note that traceroute to a SID is exemplified using UDP probes.
However, the procedure is equally applicable to other implementations
of traceroute mechanism. The UDP encoded message to traceroute a SID
would use the UDP ports assigned by IANA for "traceroute use".
Consider the example where the user wants to traceroute a remote SID
2001:db8:K:4::, via 2001:db8:K:2:X31::, from node N1. The traceroute
probe is processed at the individual nodes along the path as follows:
* Node N1 initiates a traceroute probe packet as follows
(2001:db8:L:1::, 2001:db8:K:2:X31::) (2001:db8:K:4::,
2001:db8:K:2:X31::; SL=1; NH=UDP)(Traceroute probe). The first
traceroute probe is sent with the hop-count value set to 1. The
hop-count value is incremented by 1 for each subsequent traceroute
probe.
* When node N2 receives the packet with hop-count = 1, it processes
the hop-count expiry. Specifically, node N2 responds with the
ICMPv6 message with type "Time Exceeded" and code "hop limit
exceeded in transit". The ICMPv6 response is IP routed.
* When node N2 receives the packet with hop-count > 1, it performs
the standard SRH processing. Specifically, it executes the End.X
behavior indicated by the 2001:db8:K:2:X31:: SID on the traceroute
probe. If 2001:db8:K:2:X31:: is a PSP SID, node N2 executes the
SID like any other data packet with DA = 2001:db8:K:2:X31:: and
removes the SRH.
* When node N3, which is a non-SRv6-capable node, receives the
packet with hop-count = 1, it processes the hop-count expiry.
Specifically, node N3 responds with the ICMPv6 message with type
"Time Exceeded" and code "Hop limit exceeded in transit". The
ICMPv6 response is IP routed.
* When node N3, which is a non-SRv6-capable node, receives the
packet with hop-count > 1, it performs the standard IPv6
processing. Specifically, it forwards the traceroute probe based
on DA 2001:db8:K:4:: in the IPv6 header.
* When node N4 receives the packet with DA set to the local SID
2001:db8:K:4::, it processes the End SID.
* If the target SID (2001:db8:K:4::) is not locally instantiated and
does not represent a local interface, the packet is discarded.
* If the target SID (2001:db8:K:4::) is locally instantiated or
represents a local interface, the node processes the Upper-Layer
header. As part of the Upper-Layer header processing, node N4
responds with the ICMPv6 message with type "Destination
Unreachable" and code "Port Unreachable". The ICMPv6 response is
IP routed.
Figure 4 displays a sample traceroute output for this example.
> traceroute 2001:db8:K:4:X52:: via segment list 2001:db8:K:2:X31::
Tracing the route to SID 2001:db8:K:4:X52::
1 2001:db8:2:1:21:: 0.512 msec 0.425 msec 0.374 msec
DA: 2001:db8:K:2:X31::,
SRH:(2001:db8:K:4:X52::, 2001:db8:K:2:X31::; SL=1)
2 2001:db8:3:2:21:: 0.721 msec 0.810 msec 0.795 msec
DA: 2001:db8:K:4:X52::,
SRH:(2001:db8:K:4:X52::, 2001:db8:K:2:X31::; SL=0)
3 2001:db8:4:3:41:: 0.921 msec 0.816 msec 0.759 msec
DA: 2001:db8:K:4:X52::,
SRH:(2001:db8:K:4:X52::, 2001:db8:K:2:X31::; SL=0)
Figure 4: Sample Output for Hop-by-Hop Traceroute to a SID
A.3. Hybrid OAM Using the OAM Flag
This section illustrates a hybrid OAM mechanism using the O-flag.
Without loss of the generality, the illustration assumes node N100 is
a centralized controller.
This illustration is different from the "in situ OAM" defined in
[RFC9197]. This is because in situ OAM records operational and
telemetry information in the packet as the packet traverses a path
between two points in the network [RFC9197]. The illustration in
this subsection does not require the recording of OAM data in the
packet.
The illustration does not assume any formats for exporting the data
elements or the data elements that need to be exported. The
illustration assumes system clocks among all nodes in the SR domain
are synchronized.
Consider the example where the user wants to monitor sampled IPv4 VPN
999 traffic going from CE1 to CE2 via a low-latency SR Policy P
installed at node N1. To exercise a low-latency path, the SR Policy
P forces the packet via segments 2001:db8:K:2:X31:: and
2001:db8:K:4:X52::. The VPN SID at node N7 associated with VPN 999
is 2001:db8:K:7:DT999::. 2001:db8:K:7:DT999:: is a USP SID. Nodes
N1, N4, and N7 are capable of processing the O-flag, but node N2 is
not capable of processing the O-flag. Node N100 is the centralized
controller capable of processing and correlating the copy of the
packets sent from nodes N1, N4, and N7. Node N100 is aware of O-flag
processing capabilities. Node N100, with help from nodes N1, N4, and
N7, implements a hybrid OAM mechanism using the O-flag as follows:
* A packet P1 is sent from CE1 to node N1. The packet is:
P1: (IPv4 header)(payload)
* Node N1 steers packet P1 through the SR Policy P. Based on local
configuration, node N1 also implements logic to sample traffic
steered through SR Policy P for hybrid OAM purposes.
Specification for the sampling logic is beyond the scope of this
document. Consider the case where packet P1 is classified as a
packet to be monitored via the hybrid OAM. Node N1 sets the
O-flag during the encapsulation required by SR Policy P. As part
of setting the O-flag, node N1 also sends a timestamped copy of
packet P1 to a local OAM process. The packet is:
P1: (2001:db8:L:1::, 2001:db8:K:2:X31::) (2001:db8:K:7:DT999::,
2001:db8:K:4:X52::, 2001:db8:K:2:X31::; SL=2; O-flag=1;
NH=IPv4)(IPv4 header)(payload)
The local OAM process sends a full or partial copy of packet P1 to
node N100. The OAM process includes the recorded timestamp,
additional OAM information (like incoming and outgoing interface),
and any applicable metadata. Node N1 forwards the original packet
towards the next segment 2001:db8:K:2:X31::.
* When node N2 receives the packet with the O-flag set, it ignores
the O-flag. This is because node N2 is not capable of processing
the O-flag. Node N2 performs the standard SRv6 SID and SRH
processing. Specifically, it executes the End.X behavior
[RFC8986] indicated by the 2001:db8:K:2:X31:: SID and forwards
packet P1 over link3 towards node N3. The packet is:
P1: (2001:db8:L:1::, 2001:db8:K:4:X52::) (2001:db8:K:7:DT999::,
2001:db8:K:4:X52::, 2001:db8:K:2:X31::; SL=1; O-flag=1;
NH=IPv4)(IPv4 header)(payload)
* When node N3, which is a non-SRv6-capable node, receives packet
P1, it performs the standard IPv6 processing. Specifically, it
forwards packet P1 based on DA 2001:db8:K:4:X52:: in the IPv6
header.
* When node N4 receives packet P1, it processes the O-flag. The
packet is:
P1: (2001:db8:L:1::, 2001:db8:K:4:X52::) (2001:db8:K:7:DT999::,
2001:db8:K:4:X52::, 2001:db8:K:2:X31::; SL=1; O-flag=1;
NH=IPv4)(IPv4 header)(payload)
As part of processing the O-flag, it sends a timestamped copy of
the packet to a local OAM process. Based on local configuration,
the local OAM process sends a full or partial copy of packet P1 to
node N100. The OAM process includes the recorded timestamp,
additional OAM information (like incoming and outgoing interface,
etc.), and any applicable metadata. Node N4 performs the standard
SRv6 SID and SRH processing on the original packet P1.
Specifically, it executes the End.X behavior indicated by the
2001:db8:K:4:X52:: SID and forwards packet P1 over link10 towards
node N5. The packet is:
P1: (2001:db8:L:1::, 2001:db8:K:7:DT999::) (2001:db8:K:7:DT999::,
2001:db8:K:4:X52::, 2001:db8:K:2:X31::; SL=0; O-flag=1;
NH=IPv4)(IPv4 header)(payload)
* When node N5, which is a non-SRv6-capable node, receives packet
P1, it performs the standard IPv6 processing. Specifically, it
forwards the packet based on DA 2001:db8:K:7:DT999:: in the IPv6
header.
* When node N7 receives packet P1, it processes the O-flag. The
packet is:
P1: (2001:db8:L:1::, 2001:db8:K:7:DT999::) (2001:db8:K:7:DT999::,
2001:db8:K:4:X52::, 2001:db8:K:2:X31::; SL=0; O-flag=1;
NH=IPv4)(IPv4 header)(payload)
As part of processing the O-flag, it sends a timestamped copy of
the packet to a local OAM process. The local OAM process sends a
full or partial copy of packet P1 to node N100. The OAM process
includes the recorded timestamp, additional OAM information (like
incoming and outgoing interface, etc.), and any applicable
metadata. Node N7 performs the standard SRv6 SID and SRH
processing on the original packet P1. Specifically, it executes
the VPN SID indicated by the 2001:db8:K:7:DT999:: SID and, based
on lookup in table 100, forwards packet P1 towards CE2. The
packet is:
P1: (IPv4 header)(payload)
* Node N100 processes and correlates the copy of the packets sent
from nodes N1, N4, and N7 to find segment-by-segment delays and
provide other hybrid OAM information related to packet P1. For
segment-by-segment delay computation, it is assumed that clocks
are synchronized across the SR domain.
* The process continues for any other sampled packets.
A.4. Monitoring of SRv6 Paths
In the recent past, network operators demonstrated interest in
performing network OAM functions in a centralized manner. [RFC8403]
describes such a centralized OAM mechanism. Specifically, [RFC8403]
describes a procedure that can be used to perform path continuity
checks between any nodes within an SR domain from a centralized
monitoring system. However, while [RFC8403] focuses on SR networks
with MPLS data plane, this document describes how the concept can be
used to perform path monitoring in an SRv6 network from a centralized
controller.
In the reference topology in Figure 1, node N100 uses an IGP protocol
like OSPF or IS-IS to get a view of the topology within the IGP
domain. Node N100 can also use BGP-LS to get the complete view of an
inter-domain topology. The controller leverages the visibility of
the topology to monitor the paths between the various endpoints.
Node N100 advertises an End SID [RFC8986] 2001:db8:K:100:1::. To
monitor any arbitrary SRv6 paths, the controller can create a
loopback probe that originates and terminates on node N100. To
distinguish between a failure in the monitored path and loss of
connectivity between the controller and the network, node N100 runs a
suitable mechanism to monitor its connectivity to the monitored
network.
The following example illustrates loopback probes in which node N100
needs to verify a segment list <2001:db8:K:2:X31::,
2001:db8:K:4:X52::>:
* Node N100 generates an OAM packet (2001:db8:L:100::,
2001:db8:K:2:X31::)(2001:db8:K:100:1::, 2001:db8:K:4:X52::,
2001:db8:K:2:X31::, SL=2)(OAM Payload). The controller routes the
probe packet towards the first segment, which is
2001:db8:K:2:X31::.
* Node N2 executes the End.X behavior indicated by the
2001:db8:K:2:X31:: SID and forwards the packet (2001:db8:L:100::,
2001:db8:K:4:X52::)(2001:db8:K:100:1::, 2001:db8:K:4:X52::,
2001:db8:K:2:X31::, SL=1)(OAM Payload) on link3 to node N3.
* Node N3, which is a non-SRv6-capable node, performs the standard
IPv6 processing. Specifically, it forwards the packet based on DA
2001:db8:K:4:X52:: in the IPv6 header.
* Node N4 executes the End.X behavior indicated by the
2001:db8:K:4:X52:: SID and forwards the packet (2001:db8:L:100::,
2001:db8:K:100:1::)(2001:db8:K:100:1::, 2001:db8:K:4:X52::,
2001:db8:K:2:X31::, SL=0)(OAM Payload) on link10 to node N5.
* Node N5, which is a non-SRv6-capable node, performs the standard
IPv6 processing. Specifically, it forwards the packet based on DA
2001:db8:K:100:1:: in the IPv6 header.
* Node N100 executes the standard SRv6 END behavior. It
decapsulates the header and consumes the probe for OAM processing.
The information in the OAM payload is used to detect missing
probes, round-trip delay, etc.
The OAM payload type or the information carried in the OAM probe is a
local implementation decision at the controller and is outside the
scope of this document.
Acknowledgements
The authors would like to thank Joel M. Halpern, Greg Mirsky, Bob
Hinden, Loa Andersson, Gaurav Naik, Ketan Talaulikar, and Haoyu Song
for their review comments.
Contributors
The following people contributed to this document:
Robert Raszuk
Bloomberg LP
Email: robert@raszuk.net
John Leddy
Individual
Email: john@leddy.net
Gaurav Dawra
LinkedIn
Email: gdawra.ietf@gmail.com
Bart Peirens
Proximus
Email: bart.peirens@proximus.com
Nagendra Kumar
Cisco Systems, Inc.
Email: naikumar@cisco.com
Carlos Pignataro
Cisco Systems, Inc.
Email: cpignata@cisco.com
Rakesh Gandhi
Cisco Systems, Inc.
Email: rgandhi@cisco.com
Frank Brockners
Cisco Systems, Inc.
Email: fbrockne@cisco.com
Darren Dukes
Cisco Systems, Inc.
Email: ddukes@cisco.com
Cheng Li
Huawei
Email: chengli13@huawei.com
Faisal Iqbal
Individual
Email: faisal.ietf@gmail.com
Authors' Addresses
Zafar Ali
Cisco Systems
Email: zali@cisco.com
Clarence Filsfils
Cisco Systems
Email: cfilsfil@cisco.com
Satoru Matsushima
Softbank
Email: satoru.matsushima@g.softbank.co.jp
Daniel Voyer
Bell Canada
Email: daniel.voyer@bell.ca
Mach(Guoyi) Chen
Huawei
Email: mach.chen@huawei.com
