Internet DRAFT - draft-ietf-spring-oam-usecase
draft-ietf-spring-oam-usecase
spring R. Geib, Ed.
Internet-Draft Deutsche Telekom
Intended status: Informational C. Filsfils
Expires: June 21, 2018 C. Pignataro, Ed.
N. Kumar
Cisco Systems, Inc.
December 18, 2017
A Scalable and Topology-Aware MPLS Dataplane Monitoring System
draft-ietf-spring-oam-usecase-10
Abstract
This document describes features of an MPLS path monitoring system
and related use cases. 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. The MPLS monitoring system adds
features to the traditional MPLS Ping and LSP Trace, in a very
complementary way. 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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This Internet-Draft will expire on June 21, 2018.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology and Acronyms . . . . . . . . . . . . . . . . . . 5
2.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
2.2. Acronyms . . . . . . . . . . . . . . . . . . . . . . . . 5
3. An MPLS Topology-Aware Path Monitoring System . . . . . . . . 6
4. SR-based Path Monitoring Use Case Illustration . . . . . . . 7
4.1. Use Case 1 - LSP Dataplane Monitoring . . . . . . . . . . 7
4.2. Use Case 2 - Monitoring a Remote Bundle . . . . . . . . . 10
4.3. Use Case 3 - Fault Localization . . . . . . . . . . . . . 11
5. Path Trace and Failure Notification . . . . . . . . . . . . . 12
6. Applying SR to Monitoring non-SR based LSPs (LDP and possibly
RSVP-TE) . . . . . . . . . . . . . . . . . . . . . . . . . . 12
7. PMS Monitoring of Different Segment ID Types . . . . . . . . 13
8. Connectivity Verification Using PMS . . . . . . . . . . . . . 14
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
10. Security Considerations . . . . . . . . . . . . . . . . . . . 15
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 16
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
12.1. Normative References . . . . . . . . . . . . . . . . . . 17
12.2. Informative References . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18
1. Introduction
Network operators need to be able to monitor the forwarding paths
used to transport user packets. Monitoring packets are expected to
be forwarded in dataplane in a similar way as user packets. 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 system as a functional component called
(MPLS) Path Monitoring System, PMS. The PMS is using MPLS data plane
path monitoring capabilities. The use cases introduced here are
limited to a single Interior Gateway Protocol (IGP) MPLS domain. The
use cases of this document refer to the PMS system realised as a
separate node. Although many use cases depict the PMS as a physical
node, no assumption should be made, and the node could be virtual.
This system is defined as a functional component abstracted to have
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many realizations. The terms PMS and system are used interchangeably
in the following.
The system applies to monitoring of non Segment Routing Label
Switched Paths (LSP's) like Label Distribution Protocol (LDP) as well
as to monitoring of Segment Routed LSP's (section 7 offers some more
information). As compared to non Segment Routing approaches, Segment
Routing is expected to simplify such a monitoring system by enabling
MPLS topology detection based on IGP signaled segments. The MPLS
topology should be detected and correlated with the IGP topology,
which is too detected by IGP signaling. Thus a centralized and MPLS
topology aware monitoring unit can be realized in a Segment Routed
domain. This topology awareness can be used for Operation,
Administration, and Maintenance (OAM) purposes as described by this
document.
Benefits offered by the system:
o The system described here allows to set up an SR domain wide
centralized connectivity validation. Many operators of large
networks regard centralized monitoring system as useful..
o The MPLS Ping (or continuity check) packets never leave the MPLS
user data plane.
o SR allows the transport of MPLS path trace or connectivity
validation packets for every Label Switched Path to all nodes of
an SR domain. This use case doesn't describe new path trace
features. The system described here allows to set up an SR domain
wide centralized connectivity validation, which is useful in large
network operator domains.
o The system sending the monitoring packet is also receiving it.
The payload of the monitoring packet may be chosen freely. This
allows sending probing packets which represent customer traffic,
possibly from multiple services (e.g., small Voice over IP packet,
larger HTTP packets) and embedding of useful monitoring data
(e.g., accurate time stamps since both sender and receiver have
the same clock and sequence numbers to ease the measurement...).
o Set up of a flexible MPLS monitoring system in terms of
deployment: from one single centralized one to a set of
distributed systems (e.g., on a per region or service base), and
in terms of redundancy from 1+1 to N+1.
In addition to monitoring paths, problem localization is required.
Topology awareness is an important feature of link state IGPs
deployed by operators of large networks. MPLS topology awareness
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combined with IGP topology awareness enables a simple and scalable
data plane based monitoring mechanism. Faults can be localized:
o by capturing the Interior Gateway Protocol (IGP) topology and
analyzing IGP messages indicating changes of it.
o by correlation between different SR based monitoring probes.
o by setting up an MPLS traceroute packet for a path (or Segment) to
be tested and transporting it to a node to validate path
connectivity from that node on.
MPLS OAM offers flexible traceroute (connectivity verification)
features to detect and execute data paths of an MPLS domain. By
utilizing the Equal Cost Multipath (ECMP) related tool set offered,
e.g., by RFC 8029 [RFC8029], a SR based MPLS monitoring system can be
enabled to:
o detect how to route packets along different ECMP routed paths.
o construct Ping packets, which can be steered to paths whose
connectivity is to be checked, also if ECMP is present.
o limit the MPLS label stack of such a Ping packet checking
continuity of every single IGP-Segment to the maximum number of 3
labels. A smaller label stack may also be helpful, if any router
interprets a limited number of packet header bytes to determine an
ECMP path along which to route a packet.
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 any server residing at a
single interface of the domain to be monitored. The PMS doesn't need
to support the complete MPLS routing or control plane. It needs to
be capable to learn and maintain an accurate MPLS and IGP topology.
MPLS Ping and traceroute packets need to be set up and sent with the
correct segment stack. The PMS further must be able to receive and
decode returning Ping or Traceroute packets. Packets from a variety
of protocols can be used to check continuity. These include Internet
Control Message Protocol [RFC0792] [RFC4443] [RFC4884] [RFC4950],
Bidirectional Forwarding Detection (BFD) [RFC5884], Seamless
Bidirectional Forwarding Detection (S-BFD) [RFC7880] [RFC7881] (see
Section 3.4 of [RFC7882]), and MPLS LSP Ping [RFC8029]. They can
also have any other OAM format supported by the PMS. As long as the
packet used to check continuity returns back to the server while no
IGP change is detected, the monitored path can be considered as
validated. If monitoring requires pushing a large label stack, a
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software based implementation is usually more flexible than an
hardware based one. Hence router label stack depth and label
composition limitations don't limit MPLS OAM choices.
[I-D.ietf-mpls-spring-lsp-ping] discusses SR OAM applicability and
MPLS traceroute enhancements adding functionality to the use cases
described by this document.
The document describes both use cases and a standalone monitoring
framework. The monitoring system re-uses existing IETF OAM protocols
and leverage Segment Routing (Source Routing) to allow a single
device to send, have exercised, and receive its own probing packets.
As a consequence, there are no new interoperability considerations.
Standard Track is not required and Informational status is
appropriate
2. Terminology and Acronyms
2.1. Terminology
Continuity Check
is defined in Section 2.2.7 of RFC 7276 [RFC7276].
Connectivity Verification
is defined in Section 2.2.7 of RFC 7276 [RFC7276].
MPLS topology
The MPLS topology of an MPLS domain is the complete set of MPLS-
and IP-address information and all routing and data plane
information required to address and utilize every MPLS path
within this domain from an MPLS Path Monitoring System attached
to this MPLS domain at an arbitrary access. This document
assumes availability of the MPLS topology (which can be detected
with available protocols and interfaces). None of the use cases
will describe how to set it up.
This document further adopts the terminology and framework described
in [I-D.ietf-spring-segment-routing].
2.2. Acronyms
ECMP Equal-Cost Multi-Path
IGP Interior Gateway Protocol
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LER Label Edge Router
LSP Label Switched Path
LSR Label Switching Router
OAM Operations, Administration, and Maintenance
PMS Path Monitoring System
RSVP-TE Resource ReserVation Protocol-Traffic Engineering
SID Segment Identifier
SR Segment Routing
SRGB Segment Routing Global Block
3. An MPLS Topology-Aware Path Monitoring System
Any node at least listening to the IGP of an SR domain is MPLS
topology aware (the node knows all related IP addresses, SR SIDs and
MPLS labels). An MPLS PMS which is able to learn the IGP LSDB
(including the SID's) is able to execute arbitrary chains of label
switched paths. To monitor an MPLS SR domain, a PMS needs to set up
a topology data base of the MPLS SR domain to be monitored. It may
be used to send ping type packets to only check continuity along such
a path chain based on the topology information only. In addition,
the PMS can be used to trace MPLS Label Switched Path and thus verify
their connectivity and correspondence between control and data plane,
respectively. The PMS can direct suitable MPLS traceroute packets to
any node along a path segment.
Let us describe how the PMS constructs a labels stack to transport a
packet to LER i, monitor its path to LER j and then receive the
packet back.
The PMS may do so by sending packets carrying the following MPLS
label stack information:
o Top Label: a path from PMS to LER i, which 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
Adjacency-SID. If the shortest path from LER i to LER j is
supposed to be monitored, the Node-SID (LER j) can be used.
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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).
The PMS should be physically connected to a router which is part of
the SR domain. It must be able to send and receive MPLS packets via
this interface. As mentioned above, routing protocol support isn't
required and the PMS itself doesn't have to be involved in IGP or
MPLS routing. A static route will do. The option to connect a PMS
to an MPLS domain by a tunnel may be attractive to some operators.
MPLS so far separates networks securely by avoiding tunnel access to
MPLS domains. Tunnel based access of a PMS to an MPLS domain is out
of scope of this document, as it implies additional security aspects.
4. SR-based Path Monitoring Use Case Illustration
4.1. Use Case 1 - LSP Dataplane Monitoring
Figure 1 shows an example of this functional component as a system,
which can be physical or virtual.
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+---+ +----+ +-----+
|PMS| |LSR1|-----|LER i|
+---+ +----+ +-----+
| / \ /
| / \__/
+-----+/ /|
|LER m| / |
+-----+\ / \
\ / \
\+----+ +-----+
|LSR2|-----|LER j|
+----+ +-----+
Example of a PMS based LSP dataplane monitoring
Figure 1
For the sake of simplicity, let's assume that all the nodes are
configured with the same SRGB [I-D.ietf-spring-segment-routing].
Let's assign the following Node SIDs to the nodes of the figure: PMS
= 10, LER i = 20, LER j = 30.
The aim is to set up a continuity check of the path between LER i and
LER j. As has been said, the monitoring packets are to be sent and
received by the PMS. Let's assume the design aim is to be able to
work with the smallest possible SR label stack. In the given
topology, a fairly simple option is to perform an MPLS path trace, as
specified by RFC 8029 [RFC8029] (using the Downstream (Detailed)
Mapping information resulting from a path trace). The starting point
for the path trace is LER i and the PMS sends the MPLS path trace
packet to LER i. The MPLS echo reply of LER i should be sent to the
PMS. As a result, IP destination address choices are detected, which
are then used to target any one of the ECMP routed paths between LER
i and LER j by the MPLS ping packets to later check path continuity.
The Label stack of these ping packets doesn't need to consist of more
than 3 labels. Finally, the PMS sets up and sends packets to monitor
connectivity of the ECMP routed paths. The PMS does this by creating
a measurement packet with the following label stack (top to bottom):
20 - 30 - 10. The ping packets reliably use the monitored path, if
the IP-address information which has been detected by the MPLS trace
route is used as the IP destination address (note that this IP
address isn't used or required for any IP routing).
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
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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 (i.e., a continuity check
only along the path to LER i must succeed). If desired, an MPLS
trace route may be used to exactly detect the data plane path
taken for this MPLS Segment. It is usually sufficient to just
apply any of the existing Shortest Path routed paths.
o If ECMP is deployed, separate continuity checks monitoring all
possible paths which a packet may use between LER i and LER j may
be desired. This can be done by applying an MPLS trace route
between LER i and LER j. Another option is to use SR routing, but
this will likely require additional label information within the
label stack of the ping packet. Further, if multiple links are
deployed between two nodes, SR methods to address each individual
path require an Adj-SID to be assigned to each single interface.
This method is based on control plane information - a connectivity
verification based on MPLS traceroute seems to be a fairly good
option to deal with ECMP and validation of control and data plane
correlation.
o The path LER j to PMS must be available (i.e., a continuity check
only along the path from LER j to PMS must succeed). If desired,
an MPLS trace route may be used to exactly detect the data plane
path taken for this MPLS Segment. It is usually sufficient to
just apply any of the existing Shortest Path routed paths.
Once the MPLS paths (Node-SIDs) and the required information to deal
with ECMP have been detected, the path continuity between LER i and
LER j can be monitored by the PMS. Path continuity monitoring by
ping packets does not require RFC 8029 [RFC8029] MPLS OAM
functionality. All monitoring packets stay on dataplane, hence path
continuity monitoring does not require control plane interaction in
any LER or LSR of the domain. To ensure consistent interpretation of
the results, the PMS should be aware of any changes in IGP or MPLS
topology or ECMP routing. While the description given here
pronouncing path connectivity checking as a simple basic application,
others like checking continuity of underlying physical infrastructure
or delay measurements may be desired. In both cases, a change in
ECMP routing which is not caused by an IGP or MPLS topology change
may not be desirable. A PMS therefore should also periodically
verify connectivity of the SR paths which are monitored for
continuity.
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Determining a path to be executed prior to a measurement may also be
done by setting up a label stack including all Node-SIDs along that
path (if 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 RFC 8029
[RFC8029] connectivity verification and, if there's only one physical
connection between all nodes, the approach 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, or multiple physical
connections exist between any two nodes, all Adj-SIDs along that path
should be part of the label stack.
While a single PMS can detect the complete MPLS control and data
plane topology, a reliable deployment requires two separated PMS.
Scalable permanent surveillance of a set of LSPs could require
deployment of several PMS. 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 limited effort and a unique
possibility to scale a flexible monitoring solution as required by
the operator (the number of PMS deployed is independent of the
locations of the origin and destination of the monitored paths). The
PMS can be enabled to send MPLS OAM packets with the label stacks and
address information identical to those of the monitoring packets to
any node of the MPLS domain. The routers of the monitored domain
should support MPLS LSP Ping RFC 8029 [RFC8029]. They may also
incorporate the additional enhancements defined in
[I-D.ietf-mpls-spring-lsp-ping] to incorporate further MPLS trace
route features. ICMP Ping based continuity checks don't require
router control plane activity. Prior to monitoring a path, MPLS OAM
may be used to detect ECMP dependent forwarding of a packet. A PMS
may be designed to learn the IP address information required to
execute a particular ECMP routed path and interfaces along that path.
This allows to monitor these paths with label stacks reduced to a
limited number of Node-SIDs resulting from SPF routing. The PMS does
not require access to LSR / LER management- or data-plane information
to do so.
4.2. Use Case 2 - Monitoring a Remote Bundle
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+---+ _ +--+ +-------+
| | { } | |---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
In the figure, R1 addresses Link "x" Lx by the Adjacency SID 99x,
while R2 addresses Link 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).
PMS sends the probe to its connected 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.
As has been mentioned in section 5.1, the PMS must be able monitor
continuity of the path PMS to R2 (Node-SID 72) as well as continuity
from R1 to the PMS. If both are given and packets are lost,
forwarding on one of the three interfaces connecting R1 to R2 must be
disturbed.
4.3. Use Case 3 - Fault Localization
In the previous example, a uni-directional fault on the middle link
in direction of R2 to R1 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 succeed while the second would fail.
Correlation of the measurements reveals that the only difference is
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using the Adjacency SID 663 of the middle link from R2 to R1 in the
non successful measurement. Assuming the second probe has been
routed correctly, the problem is that for some (possibly unknown)
reason SR packets to be forwarded from R2 via the interface
identified by Adjacency SID 663 are lost.
The example above only illustrates a method to localize a fault by
correlated continuity checks. Any operational deployment requires a
well designed engineering to allow for the desired non ambiguous
diagnosis on the monitored section of the SR network. 'Section' here
could be a path, a single physical interface, the set of all links of
a bundle or an adjacency of two nodes, just to name a few.
5. Path Trace and Failure Notification
Sometimes forwarding along a single path indeed doesn't work, while
the control plane information is healthy. Such a situation may occur
after maintenance work within a domain. An operator may perform on
demand-tests, but execution of automated PMS path trace checks may be
set up too (scope may be limited to a subset of important end-to-end
paths crossing the router or network section after completion of the
maintenance work there). Upon detection of a path which can't be
used, the operator needs to be notified. A check ensuring that re-
routing event is differed from a path facing whose forwarding
behavior doesn't correspond to the control plane information is
necessary (but out of scope of this document).
Adding an automated problem solution to the PMS features only makes
sense, if the root cause of the symptom appears often, can be assumed
to be non-ambiguous by its symptoms, can be solved by a pre-
determined chain of commands and the automated PMS reaction not doing
any collateral damage. A closer analysis is out of scope of this
document.
The PMS is expected to check control plane liveliness after a path
repair effort was executed. It doesn't matter whether the path
repair was triggered manually or by an automated system.
6. Applying SR to Monitoring non-SR based LSPs (LDP and possibly RSVP-
TE)
The MPLS path monitoring system described by this document can be
realized with non-Segment Routing (SR) based technology. Making such
a non-SR MPLS monitoring system aware of a domain's complete MPLS
topology requires, e.g., management plane access to the routers of
the domain to be monitored or set up of a dedicated tLDP tunnel per
router to set up an LDP adjacency. To avoid the use of stale MPLS
label information, the IGP must be monitored and MPLS topology must
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be timely aligned with IGP topology. Enhancing IGPs to exchange of
MPLS topology information as done by SR significantly simplifies and
stabilizes such an MPLS path monitoring system.
A SR based PMS connected to a MPLS domain consisting of LER and LSR
supporting SR and LDP or RSVP-TE in parallel in all nodes may use SR
paths to transmit packets to and from start and end points of non-SR
based LSP paths to be monitored. In the example given in figure 1,
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 or RSVP-TE label identifying the path or tunnel,
respectively from LER i 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.
An implementation report on a PMS operating in an LDP domain is given
in [I-D.leipnitz-spring-pms-implementation-report]. In addition,
this report compares delays measured with a single PMS to the results
measured by three standard conformant Measurement Agents ([RFC6808]
connected to an MPLS domain at three different sites). The delay
measurements of the PMS and the IPPM Measurement Agents were compared
based on a statistical test in [RFC6576]. The Anderson Darling
k-sample test showed that the PMS round-trip delay measurements are
equal to those captured by an IPPM conformant IP measurement system
for 64 Byte measurement packets with 95% confidence.
The authors are not aware of similar deployment for RSVP-TE.
Identification of tunnel entry- and transit-nodes may add complexity.
They are not within scope of this document.
7. PMS Monitoring of Different Segment ID Types
MPLS SR topology awareness should allow the PMS to monitor liveliness
of SIDs related to interfaces within the SR and IGP domain,
respectively. Tracing a path where an SR capable node assigns an
Adj-SID for a non-SR-capable node may fail. This and other backward
compatibility with non Segment Routing devices are discussed by
[I-D.ietf-mpls-spring-lsp-ping].
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To match control plane information with data plane information for
all relevant types of Segment IDs, [I-D.ietf-mpls-spring-lsp-ping]
enhances MPLS OAM functions defined by RFC 8029 [RFC8029].
8. Connectivity Verification Using PMS
While the PMS based use cases explained in Section 5 are sufficient
to provide continuity check between LER i and LER j, it may not help
perform connectivity verification.
+---+
|PMS|
+---+
|
|
+----+ +----+ +-----+
|LSRa|-----|LSR1|-----|LER i|
+----+ +----+ +-----+
| / \ /
| / \__/
+-----+/ /|
|LER m| / |
+-----+\ / \
\ / \
\+----+ +-----+
|LSR2| |LER j|
+----+ +-----+
Connectivity verification with a PMS
Figure 3
Let's assign the following Node SIDs to the nodes of the figure: PMS
= 10, LER i = 20, LER j = 30, LER m = 40. PMS is intended to
validate the path between LER m and LER j. In order to validate this
path, PMS will send the probe packet with label stack of (top to
bottom): {40} {30} {10}. Imagine any of the below forwarding entry
misprogrammed situation:
o LSRa receiving any packet with top label 40 will POP and forwards
to LSR1 instead of LER m.
o LSR1 receiving any packet with top label 30 will pop and forward
to LER i instead of LER j.
In any of these above situation, the probe packet will be delivered
back to PMS leading to a falsified path liveliness indication by the
PMS.
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Connectivity Verification functions helps us to verify if the probe
is taking the expected path. For example, PMS can intermittently
send the probe packet with label stack of (top to bottom):
{40;ttl=255} {30;ttl=1} {10;ttl=255}. The probe packet may carry
information about LER m which could be carried in Target FEC Stack in
case of MPLS Echo Request or Discriminator in case of Seamless BFD.
When LER m receives the packet, it will punt due to TTL expiry and
sends a positive response. In the above mentioned misprogramming
situation, LSRa will forwards to LSR1 which will send a negative
response to PMS as the information in probe does not match the local
node. PMS can do the same for bottom label as well. This will help
perform connectivity verification and ensure that the path between
LER m and LER j is working as expected.
9. IANA Considerations
This memo includes no request to IANA.
10. Security Considerations
The PMS builds packets with intent of performing OAM tasks. It uses
address information based on topology information, rather than a
protocol.
The PMS allows the insertion of traffic into non-SR domains. This
may be required in the case of an LDP domain attached to the SR
domain, but it can be used to maliciously insert traffic in the case
of external IP domains and MPLS based VPNs.
To prevent a PMS from inserting traffic into an MPLS VPN domain, one
or more sets of label ranges may be reserved for service labels
within an SR domain. The PMS should be configured to reject usage of
these service label values. In the same way, misuse of IP
destination addresses is blocked if only IP-destination address
values conforming to RFC 8029 [RFC8029] are settable by the PMS.
To limit potential misuse, access to a PMS needs to be authorized and
should be logged. OAM supported by a PMS requires skilled personnel
and hence only experts requiring PMS access should be allowed to
access such a system. It is recommended to directly attach a PMS to
an SR domain. Connecting a PMS to an SR domain by a tunnel is
technically possible, but adds further security issues. A tunnel
based access of a PMS to an SR domain is not recommended.
Use of stale MPLS or IGP routing information could cause a PMS
monitoring packet to leave the domain where it originated. PMS
monitoring packets should not be sent using stale MPLS or IGP routing
information. To carry out a desired measurement properly, the PMS
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must be aware of and respect the actual route changes, convergence
events, as well as the assignment of Segment IDs relevant for
measurements. At a minimum, the PMS must be able to listen to IGP
topology changes, or pull routing and segment information from
routers signaling topology changes.
Traffic insertion by a PMS may be unintended, especially if the IGP
or MPLS topology stored locally are in stale state. As soon as the
PMS has an indication, that its IGP or MPLS topology are stale, it
should stop operations involving network sections whose topology may
not be accurate. Note however that it is a task of an OAM system to
discover and locate network sections having where forwarding behavior
is not matching control plane state. As soon as a PMS or an operator
of a PMS has the impression that the PMS topology information is
stale, measures need to be taken to refresh the topology information.
These measures should be part of the PMS design. Matching forwarding
and control plane state by periodically automated execution of RFC
8029 [RFC8029] mechanisms may be such a feature. Whenever network
maintenance tasks are performed by operators, the PMS topology
discovery should be started asynchronously after network maintenance
has been finished.
A PMS loosing network connectivity or crashing must remove all IGP
and MPLS topology information prior to restarting operation.
A PMS may operate routine measurements on large scale. Care must be
taken to avoid unintended traffic insertion after topology changes
which result , e.g., in changes of label assignments to routes or
interfaces within a domain. If the labels concerned are part of the
label stack composed by the PMS for any measurement packet and their
state is stale, the measurement initially needs to be stopped. Set
up and operation of routine measurements may be automated. Secure
automated PMS operation requires a working automated detection and
recognition of stale routing state.
11. Acknowledgements
The authors would like to thank Nobo Akiya for his contribution.
Raik Leipnitz kindly provided an editorial review. The authors would
also like to thank Faisal Iqbal for an insightful review and a useful
set of comments and suggestions. Finally, Bruno Decraene's shepherd
review led to a clarified document.
12. References
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12.1. Normative References
[I-D.ietf-spring-segment-routing]
Filsfils, C., Previdi, S., Ginsberg, L., Decraene, B.,
Litkowski, S., and R. Shakir, "Segment Routing
Architecture", draft-ietf-spring-segment-routing-13 (work
in progress), October 2017.
[RFC7276] Mizrahi, T., Sprecher, N., Bellagamba, E., and Y.
Weingarten, "An Overview of Operations, Administration,
and Maintenance (OAM) Tools", RFC 7276,
DOI 10.17487/RFC7276, June 2014,
<https://www.rfc-editor.org/info/rfc7276>.
12.2. Informative References
[I-D.ietf-mpls-spring-lsp-ping]
Kumar, N., Pignataro, C., Swallow, G., Akiya, N., Kini,
S., and M. Chen, "Label Switched Path (LSP) Ping/
Traceroute for Segment Routing IGP Prefix and Adjacency
SIDs with MPLS Data-plane", draft-ietf-mpls-spring-lsp-
ping-13 (work in progress), October 2017.
[I-D.leipnitz-spring-pms-implementation-report]
Leipnitz, R. and R. Geib, "A scalable and topology aware
MPLS data plane monitoring system", draft-leipnitz-spring-
pms-implementation-report-00 (work in progress), June
2016.
[RFC0792] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, DOI 10.17487/RFC0792, September 1981,
<https://www.rfc-editor.org/info/rfc792>.
[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>.
[RFC4950] Bonica, R., Gan, D., Tappan, D., and C. Pignataro, "ICMP
Extensions for Multiprotocol Label Switching", RFC 4950,
DOI 10.17487/RFC4950, August 2007,
<https://www.rfc-editor.org/info/rfc4950>.
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[RFC5884] Aggarwal, R., Kompella, K., Nadeau, T., and G. Swallow,
"Bidirectional Forwarding Detection (BFD) for MPLS Label
Switched Paths (LSPs)", RFC 5884, DOI 10.17487/RFC5884,
June 2010, <https://www.rfc-editor.org/info/rfc5884>.
[RFC6576] Geib, R., Ed., Morton, A., Fardid, R., and A. Steinmitz,
"IP Performance Metrics (IPPM) Standard Advancement
Testing", BCP 176, RFC 6576, DOI 10.17487/RFC6576, March
2012, <https://www.rfc-editor.org/info/rfc6576>.
[RFC6808] Ciavattone, L., Geib, R., Morton, A., and M. Wieser, "Test
Plan and Results Supporting Advancement of RFC 2679 on the
Standards Track", RFC 6808, DOI 10.17487/RFC6808, December
2012, <https://www.rfc-editor.org/info/rfc6808>.
[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>.
[RFC7881] Pignataro, C., Ward, D., and N. Akiya, "Seamless
Bidirectional Forwarding Detection (S-BFD) for IPv4, IPv6,
and MPLS", RFC 7881, DOI 10.17487/RFC7881, July 2016,
<https://www.rfc-editor.org/info/rfc7881>.
[RFC7882] Aldrin, S., Pignataro, C., Mirsky, G., and N. Kumar,
"Seamless Bidirectional Forwarding Detection (S-BFD) Use
Cases", RFC 7882, DOI 10.17487/RFC7882, July 2016,
<https://www.rfc-editor.org/info/rfc7882>.
[RFC8029] Kompella, K., Swallow, G., Pignataro, C., Ed., Kumar, N.,
Aldrin, S., and M. Chen, "Detecting Multiprotocol Label
Switched (MPLS) Data-Plane Failures", RFC 8029,
DOI 10.17487/RFC8029, March 2017,
<https://www.rfc-editor.org/info/rfc8029>.
Authors' Addresses
Ruediger Geib (editor)
Deutsche Telekom
Heinrich Hertz Str. 3-7
Darmstadt 64295
Germany
Phone: +49 6151 5812747
Email: Ruediger.Geib@telekom.de
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Clarence Filsfils
Cisco Systems, Inc.
Brussels
Belgium
Email: cfilsfil@cisco.com
Carlos Pignataro (editor)
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
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