wdiff draft-geib-spring-oam-usecase-02.txt draft-geib-spring-oam-usecase-03.txt

spring R. Geib, Ed.
Internet-Draft Deutsche Telekom
Intended status: Informational C. Filsfils
Expires: January 3, April 17, 2015 C. Pignataro
N. Kumar
Cisco Systems, Inc.
July 2,
October 14, 2014

Use case for a scalable and topology aware MPLS data plane monitoring
system
draft-geib-spring-oam-usecase-02
draft-geib-spring-oam-usecase-03

Abstract

This document describes features and a use case of a path monitoring
system. Segment based routing enables a scalable and simple method
to monitor data plane liveliness of the complete set of paths
belonging to a single domain. Compared with legacy MPLS ping and
path trace, MPLS topology awareness reduces management and control
plane involvement of OAM measurements while enabling new OAM
features.

Status of this Memo

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. An MPLS topology aware path monitoring system . . . . . . . . 4
3. SR based OAM path monitoring use case illustration . . . . . . . . . . . . . . 5 6
3.1. Use-case 1 - LSP dataplane liveliness detection and monitoring . . . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Use-case 2 - Monitoring a remote bundle . . . . . . . . . 8
3.3. Use-Case 3 - Fault localization . . . . . . . . . . . . . 8
4. Failure Notification from PMS to LERi . . . . . . . . . . . . 9
5. Applying SR to monitor LDP paths . . . . . . . . . . . . . . . 9
6. PMS monitoring of different Segment ID types . . . . . . . . . 9
7. Connectivity Verification using PMS . . . . . . . . . . . . . 10
8. Extensions of related standards . . . . . . . . . . . . . helpful for this use case . . 10
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
10. Security Considerations . . . . . . . . . . . . . . . . . . . 10
11. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 10 11
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
12.1. Normative References . . . . . . . . . . . . . . . . . . . 11
12.2. Informative References . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11 12

1. Introduction

It is essential for a network operator to monitor all the forwarding
paths observed by the transported user packets. The monitoring flow
must
is expected to be forwarded in dataplane in a similar way as user
packets.
Problem localization is required. Segment Routing enables forwarding of packets along pre-
defined paths and segments and thus a Segment Routed monitoring
packet can stay in dataplane while passing along one or more segments
to be monitored.

This document describes a solution to this problem statement and illustrates it with use-cases. use-cases based on data plane
path monitoring capabilities. The solution use case is described for limited to a single
IGP MPLS domain.

The solution use case applies to monitoring of LDP LSP's as well as to
monitoring of Segment Routed LSP's. As compared to LDP, Segment
Routing simplifies the
solution by is expected to simplify the use of IGP-based signalled case by enabling MPLS
topology detection based on IGP signaled segments as specified by
[ID.sr-isis]. Thus a centralised monitoring unit is and MPLS topology aware monitoring
unit can be realized in a Segment Routed domain and this domain. This topology
awareness is can be used for OAM purposes. purposes as described by this use case.
The MPLS path monitoring system described by this document can be
realised with pre-Segment based Routing (SR) technology. Making such
a pre-SR MPLS monitoring system aware of a domains complete MPLS
topology requires e.g. management plane access. To avoid the use of
stale MPLS label information, IGP must be monitored and MPLS topology
must be timely aligned with IGP topology. Obviously, enhancing IGPs
to exchange of MPLS topology information as done by SR significantly
simplifies and stabilises such an MPLS path monitoring system.

This document adopts the terminology and framework described in
[ID.sr-archi]. It further adopts the editorial simplification
explained in section 1.2 of the segment routing use-cases
[ID.sr-use].

The proposed solution use case offers several benefits for network monitoring. A
single centralized monitoring device is able to monitor the complete
set of a domains forwarding paths. OAM Monitoring packets never leave
data plane. Legacy MPLS path trace is still required. In addition to
Segment Routing related IGP extensions, also RFC 4379 function (whose specification and
features are not part of this use case) is required, if the actual
data plane of a router should be extended to support detection of checked against its control plane.
SR routed paths. They further capabilities allow to direct MPLS OAM packets from a centralized
monitoring system to any router within a domain whose path should be enhanced to support all deployed IP/MPLS entropy options.
traced.

In an IPv6 domain, a MPLS like tree trace functionality addition to monitoring paths, problem localization is desirable. required.
Faults can be localized:

o by IGP LSA analysis.

o by correlation between different SR based monitoring probes.

o by any MPLS traceroute and adapted ping messages.

The proposed solution requires topology awareness as well as a
suitable security architecture. method (possibly in combination with SR
based path stacks).

Topology awareness is an essential part of link state IGPs. Adding
MPLS topology awareness to an IGP speaking device hence enables a
simple and scaleable scalable data plane based monitoring mechanism.

MPLS OAM offers flexible features to recognise an execute data paths
of an MPLS domain. By utilsing the ECMP related tool set of offered
e.g. by RFC 4379 [RFC4379], a segment based routing LSP monitoring
system may:

o easily detect ECMP functionality and properties of paths at data
level.

o construct monitoring packets executing desired paths also if ECMP
is present.

o limit the MPLS label stack of an OAM packet to a minmum of 3
labels.

MPLS OAM supports detection and execution of ECMP paths quite smart.
This document is foscused on MPLS path monitoring.

Alternatively, any path may be executed by building suitable label
stacks. This allows path execution without ECMP awareness.

The MPLS path monitoring system may be a specialised system any server residing at a
single interface of the domain to be monitored. It doesn't have to
support any specialised protocol stack, it just should be capable of
understanding the topology and building the probe packet with the
right segment stack. As long as measurement packets return to this
or another interface to connecting such a
specialised OAM system, server, the MPLS monitoring system is
servers are the single
entity entities pushing monitoring packet label
stacks. Concerns about router If the depth of label stacks to be pushed by a PMS are of
concern for a domain, a dedicated server based path monitoring
architecture allows limiting monitoring related label stack pushing capabilities don't apply in this case. pushes to
these servers.

First drafts discussing requirements, extensions of RFC4379 SR OAM requirements and possible solutions to
allow SR usage as described by this document
are at hand, have been submitted
already, see [ID.sr-4379ext] and [ID.sr-oam_detect].

2. An MPLS topology aware path monitoring system

An MPLS path monitoring system (PMS) which is able to learn the IGP
LSDB (including the SID's) is able to build a measurement packet
which executes every execute arbitrary chain chains of
label switched paths. A It can send pure monitoring packets along such
a path chain or it can direct suitable MPLS OAM packets to any node
along a path segment. Segment Routing here is used as a means of
adding label stacks and hence transport to standard MPLS OAM packets,
which then detect correspondence of control and data plane of this
(or any other addressed) path. Any node connected to an SR domain is
MPLS topology aware (the node knows all related IP
adresses, MPLS addresses, SR SIDs
and MPLS labels). Thus a PMS connected to an MPLS SR domain just
needs to set up a topology data base for monitoring purposes.

Let us describe how the PMS can check the liveliness of the MPLS constructs a labels stack to transport path between a
packet to LER i and i, monitor the path of it to LER j and then monitor it. receive the
packet back.

The PMS may do so by sending packets carrying the following MPLS
label stack infomation:

o Top Label: a path from PMS to LER i This is expressed as Node SID
of LER i.

o Next Label: the path that needs to be monitored from LER i to LER
j. If this path is a single physical interface (or a bundle of
connected interfaces), it can be expressed by the related AdjSID.
If the shortest path from LER i to LER j is supposed to be
monitored, the Node-SID (LER j) can be used. Another option is to
insert a list of segments expressing the desired path (hop by hop
as an extreme case). If LER i pushes a stack of Labels based on a
SR policy decision and this stack of LSPs is to be monitored, the
PMS needs an interface to collect the information enabling it to
address this SR created path.

o Next Label or address: the path back to the PMS. Likely, no
further segment/label is required here. Indeed, once the packet
reaches LER j, the 'steering' part of the solution is done and the
probe just needs to return to the PMS. This is best achieved by
popping the MPLS stack and revealing a probe packet with PMS as
destination address (note that in this case, the source and
destination addresses could be the same). If an IP address is
applied, no SID/label has to be assigned to the PMS (if it is a
host/server residing in an IP subnet outside the MPLS domain).

Note: if the PMS is an IP host not connected to the MPLS domain, the
PMS can send its probe with the list of SIDs/Labels onto a suitable
tunnel provding providing an MPLS access to a router which is part of the
monitored MPLS domain.

3. SR based OAM path monitoring use case illustration

3.1. Use-case 1 - LSP dataplane liveliness detection and monitoring

+---+ +----+ +-----+
|PMS| |LSR1|-----|LER i|
+---+ +----+ +-----+
| / \ /
| / \__/
+-----+/ /|
|LER m| / |
+-----+\ / \
\ / \
\+----+ +-----+
|LSR2|-----|LER j|
+----+ +-----+

Example of a PMS based LSP dataplane liveness detection and monitoring

Figure 1

For the sake of simplicity, let's assume that all the nodes are
configured with the same SRGB [ID.sr-archi]. [ID.sr-archi], as described by section
1.2 of [ID.sr-use].

Let's assign the following Node SIDs to the nodes of the figure: PMS
= 10, LER i = 20, LER j = 30.

The aim

To be able to work with the smallest possible SR label stack, first A
suitable MPLS OAM method is used to check liveliness of detect the ECMP routed path
between LER i to LER j which is to be monitored (and the required
address information to direct a packet along it). Afterwards the PMS
sets up and sends packets to monitor availability of that path afterwards. the detected
path. The PMS does this by creating a measurement packet with the
following label stack (top to bottom): 20 - 30 - 10. The packet will
only reliably use the monitored path, if the label and address
information used in combination with the MPLS OAM method of choice is
identical to that of the monitoring packet.

LER m forwards the packet received from the PMS to LSR1. Assuming
Pen-ultimate Hop Popping to be deployed, LSR1 pops the top label and
forwards the packet to LER i. There the top label has a value 30 and
LER i forwards it to LER j. This will be done transmitting the
packet via LSR1 or LSR2. The LSR will again pop the top label. LER
j will forward the packet now carrying the top label 10 to the PMS
(and it will pass a LSR and LER m).

A few observations on the example given in figure 1:

o The path PMS to LER i must be available. This path must be
detectable, but it is usually sufficient to apply an SPF based
path.

o If ECMP is deployed, it may be desired to measure along both
possible paths, paths which a packet may use between LER i and LER j. To
do so, in a first step the PMS sends MPLS OAM packets mechanism chosen to execute detect ECMP must reveal
the required information (an example is a so called tree trace trace)
between LER i and LER j and stores the IP
destination addresses required to execute each detected path. j. This method of dealing with ECMP based
load balancing paths requires the smallest SR label stacks if long term
monitoring of paths is applied after the tree trace completion.

o The path LER j to PMS to must be be available. This path must be
detectable, but it is usually sufficient to apply an SPF based
path.

Once the MPLS paths (Node SIDs) and the required IP address information to deal
with ECMP has been detected, the paths of LER i to LER j can be
monitored by the PMS. Monitoring doesn't itself does not require MPLS OAM functionality, it is
purely based
functionality. All monitoring packets stay on forwarding. dataplane, hence path
monitoring does no longer require control plane interaction in any
LER or LSR of the domain. To ensure reliable results, the PMS should
be aware of any changes in IGP or MPLS topology. Further changes in
ECMP functionality at LER i will impact results. Either the PMS
should be notified of such changes or they should be limited to
planned maintenance. After a topology change, a suitable MPLS OAM will
mechanism may be useful to detect the impact of the change.

Determining a path to be executed prior to a measurement may also be
done by setting up a label stack including all node Node SIDs along that
path (if LER1 LSR1 has Node SID 40 in the example and it should be passed
between LER i and LER j, the label stack is 20 - 40 - 30 - 10). The
advantage of this method is, that it does not involve MPLS OAM
functionality and it is independent of ECMP functionalities. The
method still is able to monitor all link combinations of all paths of
an MPLS domain. If correct forwarding along the desired paths has to
be checked, RFC4739 functionality should some suitable MPLS OAM mechanism may be applied also in
this case.

Obviously, the

In theory at least, a single PMS is able to check and monitor data plane liveliness
availability of all LSPs in the domain. The PMS may be a router, but
could also be dedicated monitoring system. If measurement system
reliability is an issue, more than a single PMS may be connected to
the MPLS domain.

Monitoring an MPLS domain by a PMS based on SR offers the option of
monitoring complete MPLS domains with little effort and very
excellent scalability. Data plane failure detection by circulating
monitoring packets can be executed at any time. The PMS further
executes
could be enabled to send MPLS OAM functions everywhere in packets with the label stacks and
address information identical to those of the monitoring packets to
any node of the MPLS domain. It does not require access to LSR/LER
management interfaces or their control plane to do so. MPLS
traceroutes as specified above should be executed only during off
peak times (and then with limited parallel MPLS ping/trace load).

3.2. Use-case 2 - Monitoring a remote bundle

+---+ _ +--+ +-------+
| | { } | |---991---L1---662---| |
|PMS|--{ }-|R1|---992---L2---663---|R2 (72)|
| | {_} | |---993---L3---664---| |
+---+ +--+ +-------+

SR based probing of all the links of a remote bundle

Figure 2

R1 adresses addresses Lx by the Adjacency SID 99x, while R2 adresses addresses Lx by
the Adjacency SID 66(x+1).

In the above figure, the PMS needs to assess the dataplane
availability of all the links within a remote bundle connected to
routers R1 and R2.

The monitoring system retrieves the SID/Label information from the
IGP LSDB and appends the following segment list/label stack: {72,
662, 992, 664} on its IP probe (whose source and destination
addresses are the address of the PMS).

MS sends the probe to its connected router. If the connected router
is not SR compliant, a tunneling technique can be used to tunnel the
probe and its MPLS stack to the first SR router. The MPLS/SR domain
then forwards the probe to R2 (72 is the Node SID of R2). R2
forwards the probe to R1 over link L1 (Adjacency SID 662). R1
forwards the probe to R2 over link L2 (Adjacency SID 992). R2
forwards the probe to R1 over link L3 (Adjacency SID 664). R1 then
forwards the IP probe to PMS as per classic IP forwarding.

3.3. Use-Case 3 - Fault localization

In the previous example, a uni-directional fault on the middle link
from R1 to R2 would be localized by sending the following two probes
with respective segment lists:

o 72, 662, 992, 664

o 72, 663, 992, 664

The first probe would fail while the second would succeed.
Correlation of the measurements reveals that the only difference is
using the Adjacency SID 662 of the middle link from R1 to R2 in the
non successful measurement. Assuming the second probe has been
routed correctly, the fault must have been occurring in R2 which
didn't forward the packet to the interface identified by its
Adjacency SID 662.

4. Failure Notification from PMS to LERi

PMS on detecting any failure in the path liveliness MAY may use any out-
of-band mechanism to signal te\he the failure to LERi. LER i. This document does
not not propose any specific mechanism and Operators operators can choose any
existing or new approach.

Alternately, the Operator may log the failure in local monitoring
system and take necessary action by manual intervention.

5. Applying SR to monitor LDP paths

A SR based PMS connected to a MPLS domain consisting of LER and LSR
supporting SR and LDP in parrallel parallel in all nodes may use SR paths to
transmit packets to and from start and end points of LDP paths to be
monitored. In the above example, the label stack top to bottom may
be as follows, when sent by the PMS:

o Top: SR based Node-SID of LER i at LER m.

o Next: LDP label identifying the path to LER j at LER i.

o Bottom: SR based Node-SID identifying the path to the PMS at LER j

While the mixed operation shown here still requires the PMS to be
aware of the LER LDP-MPLS topology, the PMS may learn the SR MPLS
topology by IGP and use this information.

6. PMS monitoring of different Segment ID types

MPLS SR topology awareness should allow the SID to monitor liveliness
of most types of SIDs (this may not be recommendable if a SID
identifies an inter domain interface).

To match control plane information with data plane information, MPLS
OAM functions as defined by e.g. RFC4379 should be enhanced to allow
collection of data relevant to check all relevant types of Segment
IDs.

7. Connectivity Verification using PMS

While the PMS based use cases explained in Section 3 is are sufficient
to provide Continuity continuity check between LER i and LER j, it may not help
perform connectivity verification. So in some cases like data plane
programming corruption, it is possible that a transit node between
LER i and LER j erroneously remove removes the top segment ID and forward forwards a
monitoring packet to the PMS based on the bottom segment ID leading
to a falsified path liveliness
to indication by the PMS.

There are various method to perform basic connectivity verification
like intermittely setting the TTL to 1 in bottom label so LER j
selectively perform connectivity verification. A detailed
explanation will Other methods are
possible and may be added in later version. when requirements and solutions are
specified.

8. Extensions of related standards helpful for this use case

The following activities are welcome enhancements supporting this use
case, but they are not part of it:

RFC4379 functions should be extended to support Flow- and Entropy
Label based ECMP. Further, an RFC4379 like functionality may be
desirable for IPv6 networks.

9. IANA Considerations

This memo includes no request to IANA.

10. Security Considerations

As mentioned in the introduction, a PMS monitoring packet should
never leave the domain where it originated. It therefore should
never use stale MPLS or IGP routing information. Further, asigning assigning
different label ranges for different purposes may be useful. A well
known global service level range may be excluded for utilisation
within PMS measurement packets. These ideas shoulddn't shouldn't start a
discussion. They rather should point out, that such a discussion is
required when SR based OAM mechanisms like a SR are standardised.

11. Acknowledgement

The authors would like to thank Nobo Akiya for his cotribution. contribution.

12. References

12.1. Normative References

[RFC4379] Kompella, K. and G. Swallow, "Detecting Multi-Protocol
Label Switched (MPLS) Data Plane Failures", RFC 4379,
February 2006.

12.2. Informative References

[ID.sr-4379ext]
IETF, "Label Switched Path (LSP) Ping/Trace for Segment
Routing Networks Using MPLS Dataplane", IETF, http://
datatracker.ietf.org/doc/draft-kumar-mpls-spring-lsp-
ping/, 2013.

[ID.sr-archi]
IETF, "Segment Routing Architecture", IETF, https://
datatracker.ietf.org/doc/
draft-filsfils-rtgwg-segment-routing/, 2013.
draft-filsfils-spring-segment-routing/, 2014.

[ID.sr-isis]
IETF, "IS-IS Extensions for Segment Routing", IETF, http:
//datatracker.ietf.org/doc/
draft-previdi-isis-segment-routing-extensions/, 2013. 2014.

[ID.sr-oam_detect]
IETF, "Detecting Multi-Protocol Label Switching (MPLS)
Data Plane Failures in Source Routed LSPs", IETF, http:/
/datatracker.ietf.org/doc/draft-kini-spring-mpls-lsp-
ping/, 2013.

[ID.sr-use]
IETF, "Segment Routing Use Cases", IETF, http://
datatracker.ietf.org/doc/
draft-filsfils-rtgwg-segment-routing-use-cases/, 2013.

Authors' Addresses

Ruediger Geib (editor)
Deutsche Telekom
Heinrich Hertz Str. 3-7
Darmstadt, 64295
Germany

Phone: +49 6151 5812747
Email: Ruediger.Geib@telekom.de

Clarence Filsfils
Cisco Systems, Inc.
Brussels,
Belgium

Phone:
Email: cfilsfil@cisco.com

Carlos Pignataro
Cisco Systems, Inc.
7200 Kit Creek Road
Research Triangle Park, NC 27709-4987
US

Email: cpignata@cisco.com

Nagendra Kumar
Cisco Systems, Inc.
7200 Kit Creek Road
Research Triangle Park, NC 27709
US

Email: naikumar@cisco.com