ippm R. Geib, Ed.
Internet-Draft Deutsche Telekom
Intended status: Standards Track March 11, 2019
Expires: September 12, 2019
A Connectivity Monitoring Metric for IPPM
draft-geib-ippm-connectivity-monitoring-00
Abstract
Segment Routed measurement packets can be sent along pre-determined
paths. This allows new kinds of measurements. Connectivity
monitoring allows to supervise the state of a connection or a
(sub)path from one or a few central monitoring systems. This
document specifies a suitable type-P connectivity monitoring metric.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. A brief segment routing connectivity monitoring framework . . 3
3. Singleton Definition for Type-P-Path-Connectivity-and-
Congestion . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Metric Name . . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Metric Parameters . . . . . . . . . . . . . . . . . . . . 6
3.3. Metric Units . . . . . . . . . . . . . . . . . . . . . . 7
3.4. Defintion . . . . . . . . . . . . . . . . . . . . . . . . 7
3.5. Discussion . . . . . . . . . . . . . . . . . . . . . . . 7
3.6. Methodologies . . . . . . . . . . . . . . . . . . . . . . 7
3.7. Errors and Uncertainties . . . . . . . . . . . . . . . . 9
3.8. Reporting the Metric . . . . . . . . . . . . . . . . . . 9
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
5. Security Considerations . . . . . . . . . . . . . . . . . . . 9
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
6.1. Normative References . . . . . . . . . . . . . . . . . . 10
6.2. Informative References . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
Segment Routing enables sending measurement packets along pre-
determined segment routed paths [RFC8402]. A segment routed path may
consist of pre-determined sub paths down to specific router-
interfaces. It may also consist of sub paths spanning multiple
routers, given that all segments to address a desired path are
available and known at the SR domain edge interface.
A Path Monitoring System or PMS (see [RFC8403]) is a dedicated rather
central Segment Routing domain monitoring device (as compared to a
distributed monitoring approach based on router data and functions
only). Monitoring individual sub-paths or point-to-point connections
is executed for different purposes. IGP routing exchanges hello
messages between neighbors to keep alive routing and switfly adapt to
changes. Network Operators may be interested in monitoring
connectivity and lasting congestion of interfaces or sub-paths at a
higher timescale,e.g., on the order of seconds. This is still
significantly faster than interface monitoring based on router
information, which may be collected on a minute timescale to reduce
the CPU load caused by monitoring.
The IPPM architecture was a first step to that direction [RFC2330].
Commodity IPPM solutions require dedicated measurement systems, a
large number of measurement agents and synchronised clocks.
Monitoring a domain from edge to edge by commodity IPPM solutions
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helps to increase scalability of the monitoring system, but
localising a source cause of a detected change in network behaviour
then may require network tomography methods.
A Segment Routing PMS which is part of an SR domain is IGP topology
aware, covering the IP and (if present) the MPLS layer topology
[RFC8402]. This enables to design a PMS which can steer packets
along arbitrary pre-determined concatenated sub-paths, identified by
suitable segments. Combining the SR measurement path configuration
with a priori network tomography assumptions and methods allows for
localisation of detected changes. The latter requires setting up
multiple measurement paths which share sub-paths following the
constraints derived from network tomography, and a suitable
evaluation.
This document specifies a type-p metric determining properties of an
SR path which allows to monitor connectivity and congestion of
interfaces and further allows to locate the connection or interface
which caused a change in the reported type-p metric. This document
is focussed on the MPLS layer, but the methodolgy may be applied
within SR doamins or MPLS domains in general.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. A brief segment routing connectivity monitoring framework
The Segment Routing IGP topology information consists of the IP and
(if present) the MPLS layer topology. The minimum SR topology
information are Node-Segment-Identifiers (Node-SID), identifying an
SR router. The IGP exchange of Adjacency-SIDs [I-D.draft-ietf-isis-
segment-routing-extensions], which identify local interfaces to
adjacent nodes, is optional. It is RECOMMENDED to distribute Adj-
SIDs in a domain operating a PMS to monitor connectivity as specified
below. If Adj-SIDs aren't availbale, [RFC8029] provides methods how
to steer packets along desired paths by the proper choice of an MPLS
Echo-request IP-destination address. A detailed description of
[RFC8029] methods as a replacement of Adj-SIDs is out of scope of
this document.
A round trip measurement between two adjacent nodes is a simple
method to monitor connectivity of a connecting link. If multiple
links are operational between two adjacent nodes and only a single
one fails, a single plain round trip measurement may fail to identify
which link has failed. A round trip measurement also fails to
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identify which inteface is congested, even if only a single link
connects two adjacent nodes.
Segment Routing enables the set-up of extended measurement loops.
Several different measurement loops can be set up. If these form a
partial overlay, any change in the network properties impacts more
than a single loops round trip time (or drops packets of more than
one loop). An randomly chosen paths may fail to produce unique
result patterns. A centralised monitoring approach further benefits
from keeping the number of measurement loops low, as this improves
scalability one hand and keeps the number of results to be evaluated
and correlated low.
Segment Routing enables the set-up of extended measurement loops.
Several different measurement loops can be set up. If these form a
partial overlay, any change in the network properties impacts more
than a single loops round trip time (or drops packets of more than
one loop). An randomly chosen paths may fail to produce unique
result patterns. A centralised monitoring approach further benefits
from keeping the number of measurement loops low, as this improves
scalability one hand and keeps the number of results to be evaluated
and correlated low.
An example SR domain is shown below. The PMS shown should monitor
the connectivity of all 6 links between nodes L100 and L200 one one
side and the connected nodes L050, L060 and L070 on the other side.
The round trip times per measurement loop are assumed to exhibit
unique delays.
+---+ +----+ +----+
|PMS| |L100|-----|L050|
+---+ +----+\ /+----+
| / \ \_/_____
| / \ / \+----+
+----+/ \/_ +----|L060|
|L300| / |/ +----+
+----+\ / /\_
\ / / \
\+----+ / +----+
|L200|-----|L070|
+----+ +----+
Connectivity verification with a PMS
Figure 1
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The SID values are picked for convenient reading only. Node-SID: 100
identifies L100, Node-SID: 300 identifies L300 and so on. Adj-SID
10050: Adjacency L100 to L050, Adj-SID 10060: Adjacency L100 to L060,
Adj-SID 60200: Adjacency L60 to L200
This requires 6 measurement paths, each of which has the following
properties:
o It follows a single round trip from one Ln00 to one L0m0 (e.g.,
between L100 and L050).
o It passes two more links between that Ln00 one more L0m0 and the
other Ln00 (e.g., between L100 and L060 and then L060 to L200)
o Every link is passed by a single round trip per measurement loop
only once and only once unidirectional by two other loops, and the
latter two pass along opposing directions (that's three loops
passing each single link, e.g., one having a round trip L100 to
L050 and back, a second passing L100 to L050 only and a third loop
passing L050 to L100 only).
This results in 6 measurement loops for the given example (the start
and end of each measurement loop is PMS to L300 to L100 or L200 and a
similar sub-path on the return leg. It is ommitted here for
brevity):
1. L100 -> L050 -> L100 -> L060 -> L200
2. L100 -> L060 -> L100 -> L070 -> L200
3. L100 -> L070 -> L100 -> L050 -> L200
4. L200 -> L050 -> L200 -> L060 -> L100
5. L200 -> L060 -> L200 -> L070 -> L100
6. L200 -> L070 -> L200 -> L050 -> L100
The measurement loops set up as shown have the following properties:
o Any single complete loss of connectivity caused by a failing
single link briefly between any Ln00 and any L0m0 node disturbs
(and changes the measured delay) of three loops.
o Whereas any congested single interface between any Ln00 and any
L0m0 node only impacts the measured delay of two measurement
loops.
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A closer look reveals that each single event of interest for the
proposed metric, which are a loss of connectivity or a case of
congestion, uniquely only impacts a single a-priori determinable set
of measurement loops. If, e.g., connectivity is lost between L200
and L050, measurement loops (3), (4) and (6) indicate a change in the
measured delay.
As a second example, if the interface L070 to L100 is congested,
measurement loops (3) and (5) indicate a change in the measured
delay. Without listing all events, all cases of single losses of
connectivity or single events of congestion influence only delay
measurements of a unique set of measurement loops.
3. Singleton Definition for Type-P-Path-Connectivity-and-Congestion
3.1. Metric Name
Type-P-Path-Connectivity-and-Congestion
3.2. Metric Parameters
o Src, the IP address of a source host
o Dst, the IP address of a destination host if IP routing is
applicable; in the case of MPLS routing, a diagnostic address as
specified by [RFC8029]
o T, a time
o lambda, a rate in reciprocal seconds
o L, a packet length in bits. The packets of a Type P packet stream
from which the sample Path-Connectivity-and-Congestion metric is
taken MUST all be of the same length.
o MLA, a Monitoring Loop Address information ensuring that a
singleton passes a single sub-path_a to be monitored
bidirectional, a sub-path_b to be monitored unidirectional and a
sub-path_c to be monitored unidirectional, where sub-path_a, -_b
and -_c MUST NOT be identical.
o P, the specification of the packet type, over and above the source
and destination addresses
o DS, a constant time interval between two type-P packets
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3.3. Metric Units
A sequence of consecutive time values.
3.4. Defintion
A moving average of AV time values per measurement path is compared
by a change point detection algorithm. The temporal packet spacing
value DS represents the smallest period within which a change in
connectivity or congestion may be detected.
A single loss of connectivity of a sub-path between two nodes affects
three different measurement paths. Depending on the value chosen for
DS, packet loss might occur (note that the moving average evaluation
needs to span a longer period than convergence time; alternatively,
packet-loss visible along the three measurement paths may serve as an
evaluation criterium). After routing convergence the type-p packets
along the three measurement paths show a change in delay.
A congestion of a single interface of a sub-path connecting two nodes
affects two different measurement paths. The the type-p packets
along the two congested measurement paths show an additional change
in delay.
3.5. Discussion
Detection of a multiple losses of monitored sub-path connectivity or
congestion of a multiple monitored sub-paths may be possible. These
cases have not been investigated, but may occur in the case of Shared
Risk Link Groups. Monitoring Shared Risk LinkGroups and sub-paths
with multiple failures abd congestion is not within scope of this
document.
3.6. Methodologies
For the given type-p, the methodology is as follows:
o The set of measurement paths MUST be routed in a way that each
single loss of connectivity and each case of single interface
congestion of one of the sub-paths passed by a type-p packet
creates a unique pattern of type-p packets belonging to a subset
of all configured measurement paths indicate a change in the
measured delay. As a minimum, each sub-path to be monitored MUST
be passed
o
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* by one measurement_path_1 and its type-p packet in
bidirectional direction
* by one measurement_path_2 and its type-p packet in "downlink"
direction
* by one measurement_path_3 and its type-p packet in "uplink"
direction
o "Uplink" and "Downlink" have no architectural relevance. The
terms are chosen to express, that the packets of
measurement_path_2 and measuremnt_path_3 pass the monitored sub-
path unidirectional in opposing direction. Measuremnt_path_1,
measurement_path_2 and measurement_path_3 MUST NOT be identical.
o All measurement paths SHOULD terminate between identical sender
and receiver interfaces. It is recommended to connect the sender
and receiver as closely to the paths to be monitored as possible.
Each intermediate sub-path between sender and receiver one one
hand and sub-paths to be monitored is an additional source of
errors requiring separate monitoring.
o Segment Routed domains supporting Node- and Adj-SIDs should enable
the monitoring path set-up as specified. Other routing protocols
may be used as well, but the monitoring path set up might be
complex or impossible.
o Pre-compute how the two and three measurement path delay changes
correlate to sub-path connectivity and congestion patterns.
Absolute change valaues aren't required, a simultaneous change of
two or three particular measurement paths is.
o Ensure that the temporal resolution of the measurement clock
allows to reliably capture a unique delay value for each
configured measurement path while sub-path connectivity is
complete and no congestion is present.
o Synchronised clocks are not strictly required, as the metric is
evaluating differences in delay. Changes in clock synchronisation
SHOULD NOT be close to the time interval within which changes in
connectivity or congestion should be monitored.
o At the Src host, select Src and Dst IP addresses, and address
information to route the type-p packet along one of the configured
measurement path. Form a test packet of Type-P with these
addresses.
o Configure the Dst host access to receive the packet.
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o At the Src host, place a timestamp, a sequence number and a unique
identifier of the measurement path in the prepared Type-P packet,
and send it towards Dst.
o Capture the one-way delay and determine packet-loss by the metrics
specified by [RFC7679] and [RFC7680] respectively and store the
result for the path.
o If two or three subpaths indicate a change in delay, report a
change in connectivity or congestion status as pre-computed above.
o If two or three sub paths indicate a change in delay, report a
change in connectivity or congestion status as pre-computed above.
Note that monitoring 6 sub paths requires setting up 6 monitoring
paths as shown in the figure above.
3.7. Errors and Uncertainties
Sources of error are:
o Measurement paths whose delays don't indicate a change after sub-
path connectivity changed.
o A timestamps whose resolution is missing or inacurrate at the
delays measured for the different monitoring paths.
o Multiple occurrences of sub path connectivity and congestion.
o Loss of connectivity and congestion along sub-paths connecting the
measurement device(s) with the sub-paths to be monitored.
3.8. Reporting the Metric
The metric reports loss of connectivity of monitored sub-path or
congestion of an interface and identifies the sub-path and the
direction of traffic in the case of congestion.
4. IANA Considerations
If standardised, the metric will require an entry in the IPPM metric
registry.
5. Security Considerations
This draft specifies how to use methods specified or described within
[RFC8402] and [RFC8403]. It does not introduce new or additional SR
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features. The security considerations of both references apply here
too.
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>.
[RFC7679] Almes, G., Kalidindi, S., Zekauskas, M., and A. Morton,
Ed., "A One-Way Delay Metric for IP Performance Metrics
(IPPM)", STD 81, RFC 7679, DOI 10.17487/RFC7679, January
2016, <https://www.rfc-editor.org/info/rfc7679>.
[RFC7680] Almes, G., Kalidindi, S., Zekauskas, M., and A. Morton,
Ed., "A One-Way Loss Metric for IP Performance Metrics
(IPPM)", STD 82, RFC 7680, DOI 10.17487/RFC7680, January
2016, <https://www.rfc-editor.org/info/rfc7680>.
[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>.
[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>.
6.2. Informative References
[RFC2330] Paxson, V., Almes, G., Mahdavi, J., and M. Mathis,
"Framework for IP Performance Metrics", RFC 2330,
DOI 10.17487/RFC2330, May 1998,
<https://www.rfc-editor.org/info/rfc2330>.
[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>.
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Author's Address
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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