Internet DRAFT - draft-ietf-teas-sr-rsvp-coexistence-rec
draft-ietf-teas-sr-rsvp-coexistence-rec
TEAS Working Group H. Sitaraman, Ed.
Internet-Draft V. Beeram
Intended status: Informational Juniper Networks
Expires: November 17, 2018 I. Minei
Google, Inc.
S. Sivabalan
Cisco Systems, Inc.
May 16, 2018
Recommendations for RSVP-TE and Segment Routing LSP co-existence
draft-ietf-teas-sr-rsvp-coexistence-rec-04.txt
Abstract
Operators are looking to introduce services over Segment Routing (SR)
LSPs in networks running Resource Reservation Protocol (RSVP-TE)
LSPs. In some instances, operators are also migrating existing
services from RSVP-TE to SR LSPs. For example, there might be
certain services that are well suited for SR and need to co-exist
with RSVP-TE in the same network. Such introduction or migration of
traffic to SR might require co-existence with RSVP-TE in the same
network for an extended period of time depending on the operator's
intent. The following document provides solution options for keeping
the traffic engineering database consistent across the network,
accounting for the different bandwidth utilization between SR and
RSVP-TE.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on November 17, 2018.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions used in this document . . . . . . . . . . . . . . 3
3. Solution options . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Static partitioning of bandwidth . . . . . . . . . . . . 4
3.2. Centralized management of available capacity . . . . . . 4
3.3. Flooding SR utilization in IGP . . . . . . . . . . . . . 4
3.4. Running SR over RSVP-TE . . . . . . . . . . . . . . . . . 5
3.5. TED consistency by reflecting SR traffic . . . . . . . . 5
4. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
5. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.1. Normative References . . . . . . . . . . . . . . . . . . 9
8.2. Informative References . . . . . . . . . . . . . . . . . 10
Appendix A. Multiplier value range . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
Introduction of SR [I-D.ietf-spring-segment-routing] in the same
network domain as RSVP-TE [RFC3209] presents the problem of
accounting for SR traffic and making RSVP-TE aware of the actual
available bandwidth on the network links. RSVP-TE is not aware of
how much bandwidth is being consumed by SR services on the network
links and hence both at computation time (for a distributed
computation) and at signaling time RSVP-TE LSPs will incorrectly
place loads. This is true where RSVP-TE paths are distributed or
centrally computed without a common entity managing both SR and RSVP-
TE computation for the entire network domain.
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The problem space can be generalized as a dark bandwidth problem to
cases where any other service exists in the network that runs in
parallel across common links and whose bandwidth is not reflected in
the available and reserved values in the traffic engineering database
(TED). In most practical instances given the static nature of the
traffic demands, limiting the available reservable bandwidth
available to RSVP-TE has been an acceptable solution. However, in
the case of SR traffic, there is assumed to be very dynamic traffic
demands and there is considerable risk associated with stranding
capacity or overbooking service traffic resulting in traffic drops.
The high level requirements to consider are:
1. Placement of SR LSPs in the same domain as RSVP-TE LSPs must not
introduce inaccuracies in the TED used by distributed or
centralized path computation engines.
2. Engines that compute RSVP-TE paths may have no knowledge of the
existence of the SR paths in the same domain.
3. Engines that compute RSVP-TE paths should not require a software
upgrade or change to their path computation logic.
4. Protocol extensions should be avoided or be minimal as in many
cases this co-existence of RSVP-TE and SR may be needed only
during a transition phase.
5. Placement of SR LSPs in the same domain as RSVP-TE LSPs that are
computed in a distributed fashion must not require migration to a
central controller architecture for the RSVP-TE LSPs.
2. Conventions used in this document
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.
3. Solution options
The following section lists SR and RSVP co-existence solution
options. A specific solution is not recommended as all solutions are
valid even though some may not satisfy all the requirements. If a
solution is acceptable for an operator based on their deployment
model then such a solution can be chosen.
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3.1. Static partitioning of bandwidth
In this model, the static reservable bandwidth of an interface can be
statically partitioned between SR and RSVP-TE and each can operate
within that bandwidth allocation and SHOULD NOT preempt each other.
While it is possible to configure RSVP-TE to only reserve up to a
certain maximum link bandwidth and manage the remaining link
bandwidth for other services, this is a deployment where SR and RSVP-
TE are separated in the same network (ships in the night) and can
lead to suboptimal link bandwidth utilization not allowing each to
consume more, if required and constraining the respective
deployments.
The downside of this approach is the inability to use the reservable
bandwidth effectively and inability to use bandwidth left unused by
the other protocol.
3.2. Centralized management of available capacity
In this model, a central controller performs path placement for both
RSVP-TE and SR LSPs. The controller manages and updates its own view
of the in-use and the available capacity. As the controller is a
single common entity managing the network it can have a unified and
consistent view of the available capacity at all times.
A practical drawback of this model is that it requires the
introduction of a central controller managing the RSVP-TE LSPs as a
prerequisite to the deployment of any SR LSPs. Therefore, this
approach is not practical for networks where distributed TE with
RSVP-TE LSPs is already deployed, as it requires a redesign of the
network and is not backwards compatible. This does not satisfy
requirement 5.
Note that it is not enough for the controller to just maintain the
unified view of the available capacity, it must also perform the path
computation for the RSVP-TE LSPs, as the reservations for the SR LSPs
are not reflected in the TED.
3.3. Flooding SR utilization in IGP
Using techniques in [RFC7810], [RFC7471] and [RFC7823], the SR
utilization information can be flooded in IGP-TE and the RSVP-TE path
computation engine (CSPF) can be changed to consider this
information. This requires changes to the RSVP-TE path computation
logic and would require upgrades in deployments where distributed
computation is done across the network.
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This does not fit with requirements 3 and 4 mentioned earlier.
3.4. Running SR over RSVP-TE
SR can run over dedicated RSVP-TE LSPs that carry only SR traffic.
In this model, the LSPs can be one-hop or multi-hop and can provide
bandwidth reservation for the SR traffic based on functionality such
as auto-bandwidth. The model of deployment would be similar in
nature to running LDP over RSVP-TE. This would allow the TED to stay
consistent across the network and any other RSVP-TE LSPs will also be
aware of the SR traffic reservations. In this approach, non-SR
traffic MUST NOT take the SR-dedicated RSVP-TE LSPs, unless required
by policy.
The drawback of this solution is that it requires SR to rely on RSVP-
TE for deployment. Furthermore, the accounting accuracy/frequency of
this method is dependent on performance of auto-bandwidth for RSVP-
TE. Note that for this method to work, the SR-dedicated RSVP-TE LSPs
must be set up with the best setup and hold priorities in the
network.
3.5. TED consistency by reflecting SR traffic
The solution relies on dynamically measuring SR traffic utilization
on each TE interface and reducing the bandwidth allowed for use by
RSVP-TE. It is assumed that SR traffic receives precedence in terms
of the placement on the path over RSVP traffic (that is, RSVP traffic
can be preempted from the path in case of insufficient resources).
This is logically equivalent to SR traffic having the best preemption
priority in the network. Note that this does not necessarily mean
that SR traffic has higher QoS priority, in fact, SR and RSVP traffic
may be in the same QoS class.
Reducing the bandwidth allowed for use by RSVP-TE can be explored
using the three parameters available in IGP-TE ([RFC5305],[RFC3630]),
namely Maximum-Link-Bandwidth, Maximum-Reservable-Bandwidth and
Unreserved-Bandwidth.
o Maximum-Link-Bandwidth: This parameter can be adjusted to
accommodate the bandwidth required for SR traffic with cascading
impacts on Maximum-Reservable-Bandwidth and Unreserved-Bandwidth.
However, changing the maximum bandwidth for the TE link will
prevent any compute engine for SR or RSVP from determining the
real static bandwidth of the TE link. Further, when the Maximum-
Reservable-Bandwidth is derived from the Maximum-Link-Bandwidth,
its definition changes since Maximum-Link-Bandwidth will account
for the SR traffic.
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o Unreserved-Bandwidth: SR traffic could directly adjust the
Unreserved-Bandwidth, without impacting Maximum-Link-Bandwidth or
Maximum-Reservable-Bandwidth. This model is equivalent to the
option described in Section 3.4. Furthermore this would result in
overloading IGP-TE advertisements to directly reflect both RSVP-TE
bandwidth bookings and SR bandwidth measurements.
o Maximum-Reservable-Bandwidth: As the preferred option, SR traffic
could adjust the Maximum-Reservable-Bandwidth, with cascading
impact on the Unreserved-Bandwidth.
The following methodology can be used at every TE node for this
solution, using the following parameters:
o T: Traffic statistics collection time interval
o k: The number of traffic statistics samples that can provide a
smoothing function to the statistics collection. The value of k
is a constant integer multiplier greater or equal to 1.
o N: Traffic averaging calculation (adjustment) interval such that N
= k * T
o Maximum-Reservable-Bandwidth: The maximum available bandwidth for
RSVP-TE.
If Differentiated-Service (Diffserv)-aware MPLS Traffic
Engineering (DS-TE) [RFC4124] is enabled, the Maximum-Reservable-
Bandwidth SHOULD be interpreted as the aggregate bandwidth
constraint across all Class-Types independent of the Bandwidth
Constraints model.
o Initial Maximum-Reservable-Bandwidth: The Maximum-reservable-
bandwidth for TE when no SR traffic or RSVP-TE reservations exist
on the interface.
o RSVP-unreserved-bandwidth-at-priority-X: Maximum-Reservable-
Bandwidth - sum of (existing reservations at priority X and all
priorities better than X)
o SR traffic threshold percentage: The percentage difference of
traffic demand that when exceeded can result in a change to the
RSVP-TE Maximum-Reservable-Bandwidth
o IGP-TE update threshold: Specifies the frequency at which IGP-TE
updates should be triggered based on TE bandwidth updates on a
link
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o M: An optional multiplier that can be applied to the SR traffic
average. This multiplier provides the ability to grow or shrink
the bandwidth used by SR. Appendix A offers further guidance on
M.
At every interval T, each node SHOULD collect the SR traffic
statistics for each of its TE interfaces. The measured SR traffic
includes all labelled SR traffic and any traffic entering the SR
network over that TE interface. Further, at every interval N, given
a configured SR traffic threshold percentage and a set of collected
SR traffic statistics samples across the interval N, the SR traffic
average (or any other traffic metric depending on the algorithm used)
over this period is calculated. This method of sampling traffic
statistics and adjusting bandwidth reservation accordingly is similar
to how bandwidth gets adjusted for auto-bandwidth RSVP-TE LSPs.
If the difference between the new calculated SR traffic average and
the current SR traffic average (that was computed in the prior
adjustment) is at least SR traffic threshold percentage, then two
values MUST be updated:
o New Maximum-Reservable-Bandwidth = Initial Maximum-Reservable-
Bandwidth - (new SR traffic average * M)
o New RSVP-unreserved-bandwidth-at-priority-X = New Maximum-
Reservable-Bandwidth - sum of (existing reservations at priority X
and all priorities better than X)
A DS-TE LSR that advertises Bandwidth Constraints TLV should update
the bandwidth constraints for class-types based on operator policy.
For example, when Russian Dolls Model (RDM) [RFC4127] is in use, then
only BC0 may be updated. Whereas, when Maximum Allocation Model
(MAM) [RFC4125] is in use, then all BCs may be updated equally such
that the total value updated is equal to the newly calculated SR
traffic average.
Note that the computation of the new RSVP-unreserved-bandwidth-at-
priority-X MAY result in RSVP-TE LSPs being hard or soft preempted.
Such preemption will be based on relative priority (e.g. low to high)
between RSVP-TE LSPs. The IGP-TE update threshold SHOULD allow for
more frequent flooding of unreserved bandwidth. From an operational
point of view, an implementation SHOULD be able to expose both the
configured and the actual values of the Maximum-Reservable-Bandwidth.
If LSP preemption is not acceptable, then the RSVP-TE Maximum-
Reservable-Bandwidth cannot be reduced below what is currently
reserved by RSVP-TE on that interface. This may result in bandwidth
not being available for SR traffic. Thus, it is required that any
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external controller managing SR LSPs SHOULD be able to detect this
situation (for example by subscribing to TED updates [RFC7752]) and
SHOULD take action to reroute existing SR paths.
Generically, SR traffic (or any non-RSVP-TE traffic) should have its
own priority allocated from the available priorities. This would
allow SR to preempt other traffic according to the preemption
priority order.
In this solution, the logic to retrieve the statistics, calculating
averages and taking action to change the Maximum-Reservable-Bandwidth
is an implementation choice, and all changes are local in nature.
However, note that this is a new network trigger for RSVP-TE
preemption and thus is a consideration for the operator.
The above solution offers the advantage of not introducing new
network-wide mechanisms especially during scenarios of migrating to
SR in an existing RSVP-TE network and reusing existing protocol
mechanisms.
4. Acknowledgements
The authors would like to thank Steve Ulrich for his detailed review
and comments.
5. Contributors
The following individuals contributed to this document:
Chandra Ramachandran
Juniper Networks
Email: csekar@juniper.net
Raveendra Torvi
Juniper Networks
Email: rtorvi@juniper.net
Sudharsana Venkataraman
Juniper Networks
Email: sudharsana@juniper.net
Martin Vigoureux
Nokia
Email: martin.vigoureux@nokia.com
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6. IANA Considerations
This draft does not have any request for IANA.
7. Security Considerations
This document describes solution options for the co-existence of
RSVP-TE and SR LSPs in the same administrative domain. The security
considerations for SR are described in
[I-D.ietf-spring-segment-routing]. The security considerations
pertaining to RSVP-TE are described in [RFC5920]. The security
considerations of each architecture are typically unaffected by the
presence of the other. However, when RSVP-TE and SR LSPs co-exist,
it is possible for a hijacked SR traffic stream to maliciously
consume sufficient bandwidth and cause disruption to RSVP-TE LSPs.
With the solution option specified in Section 3.5, the impact to
RSVP-TE traffic can be controlled and paths re-routed. Some latent
risk of disruption still remains because this solution option relies
on taking statistics samples and adopting to new traffic flows only
after the adjustment period. The defensive mechanisms described in
the base SR security framework should be employed to guard against
situations that result in SR traffic hijacking or denial of service.
8. References
8.1. 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-15 (work
in progress), January 2018.
[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>.
[RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,
<https://www.rfc-editor.org/info/rfc3209>.
[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>.
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8.2. Informative References
[RFC3630] Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering
(TE) Extensions to OSPF Version 2", RFC 3630,
DOI 10.17487/RFC3630, September 2003,
<https://www.rfc-editor.org/info/rfc3630>.
[RFC4124] Le Faucheur, F., Ed., "Protocol Extensions for Support of
Diffserv-aware MPLS Traffic Engineering", RFC 4124,
DOI 10.17487/RFC4124, June 2005,
<https://www.rfc-editor.org/info/rfc4124>.
[RFC4125] Le Faucheur, F. and W. Lai, "Maximum Allocation Bandwidth
Constraints Model for Diffserv-aware MPLS Traffic
Engineering", RFC 4125, DOI 10.17487/RFC4125, June 2005,
<https://www.rfc-editor.org/info/rfc4125>.
[RFC4127] Le Faucheur, F., Ed., "Russian Dolls Bandwidth Constraints
Model for Diffserv-aware MPLS Traffic Engineering",
RFC 4127, DOI 10.17487/RFC4127, June 2005,
<https://www.rfc-editor.org/info/rfc4127>.
[RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic
Engineering", RFC 5305, DOI 10.17487/RFC5305, October
2008, <https://www.rfc-editor.org/info/rfc5305>.
[RFC5920] Fang, L., Ed., "Security Framework for MPLS and GMPLS
Networks", RFC 5920, DOI 10.17487/RFC5920, July 2010,
<https://www.rfc-editor.org/info/rfc5920>.
[RFC7471] Giacalone, S., Ward, D., Drake, J., Atlas, A., and S.
Previdi, "OSPF Traffic Engineering (TE) Metric
Extensions", RFC 7471, DOI 10.17487/RFC7471, March 2015,
<https://www.rfc-editor.org/info/rfc7471>.
[RFC7752] Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
S. Ray, "North-Bound Distribution of Link-State and
Traffic Engineering (TE) Information Using BGP", RFC 7752,
DOI 10.17487/RFC7752, March 2016,
<https://www.rfc-editor.org/info/rfc7752>.
[RFC7810] Previdi, S., Ed., Giacalone, S., Ward, D., Drake, J., and
Q. Wu, "IS-IS Traffic Engineering (TE) Metric Extensions",
RFC 7810, DOI 10.17487/RFC7810, May 2016,
<https://www.rfc-editor.org/info/rfc7810>.
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[RFC7823] Atlas, A., Drake, J., Giacalone, S., and S. Previdi,
"Performance-Based Path Selection for Explicitly Routed
Label Switched Paths (LSPs) Using TE Metric Extensions",
RFC 7823, DOI 10.17487/RFC7823, May 2016,
<https://www.rfc-editor.org/info/rfc7823>.
Appendix A. Multiplier value range
The following is a suggestion for the range of values for M:
M is a per-node positive real number that ranges from 0 to 2 with a
default of 1 and may be expressed as a percentage.
o If M < 1, then the SR traffic average is being understated, which
can result in the link getting full even though Maximum-
Reservable-Bandwidth does not reach zero.
o If M > 1, then the SR traffic average is overstated, thereby
resulting in the Maximum-Reservable-Bandwidth reaching zero before
the link gets full. If the reduction of Maximum-Reservable-
Bandwidth becomes a negative value, then a value of zero SHOULD be
used and advertised.
Authors' Addresses
Harish Sitaraman (editor)
Juniper Networks
1133 Innovation Way
Sunnyvale, CA 94089
US
Email: hsitaraman@juniper.net
Vishnu Pavan Beeram
Juniper Networks
10 Technology Park Drive
Westford, MA 01886
US
Email: vbeeram@juniper.net
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Ina Minei
Google, Inc.
1600 Amphitheatre Parkway
Mountain View, CA 94043
US
Email: inaminei@google.com
Siva Sivabalan
Cisco Systems, Inc.
2000 Innovation Drive
Kanata, Ontario K2K 3E8
Canada
Email: msiva@cisco.com
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