SPRING Working Group S. Litkowski
Internet-Draft Orange
Intended status: Informational M. Aissaoui
Expires: February 10, 2018 Nokia
August 9, 2017
Implementing non protected paths using SPRING
draft-litkowski-spring-non-protected-paths-02
Abstract
Segment Routing (SR) leverages the source routing paradigm. A node
can steer a packet on a specific path by prepending the packet with
an SR header. In the framework of traffic-engineering use cases, a
customer may request its service provider to implement some non
protected paths. This means that in case of a failure within the
network, fast-reroute (or similar) techniques should not be activated
for those paths. This document analyzes the different options to
implement a non protected path with Segment Routing and in a future
release will provide a recommandation on the best option.
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 [RFC2119].
Status of This Memo
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This Internet-Draft will expire on February 10, 2018.
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Copyright Notice
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Table of Contents
1. Problem statement . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements for a non protected LSP . . . . . . . . . . . . 6
2.1. ECMP considerations . . . . . . . . . . . . . . . . . . . 7
3. Options to create a non protected path with Segment Routing . 7
3.1. Using only non protected adjacency segments . . . . . . . 7
3.2. Using a combination of node segments and adjacency
segments . . . . . . . . . . . . . . . . . . . . . . . . 8
3.2.1. Adding a protection flag in the Node SID . . . . . . 8
3.2.2. Using Strict SPF Node SID . . . . . . . . . . . . . . 9
3.2.3. Using two Node-SIDs with different local policies . . 9
3.2.4. Advantages and drawbacks . . . . . . . . . . . . . . 9
3.3. Using a combination of adjacency segments and binding-SID 10
4. Comparison . . . . . . . . . . . . . . . . . . . . . . . . . 11
5. Recommended option(s) . . . . . . . . . . . . . . . . . . . . 13
6. Security Considerations . . . . . . . . . . . . . . . . . . . 13
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 13
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
9. Normative References . . . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
1. Problem statement
In some cases, a customer may prefer to react on network failures
using its own mechanism. In such cases, the customer usually has two
disjoint paths, so a path can take over the traffic in case of
failure of the other. The disjoint paths can be provided by a single
provider or by multihoming to different providers as displayed in the
figure below.
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_________________________________________
/ \
/ \
| |
| |
| |
| ***********************> |
| +------+ 10 +------+ |
CE1 ****| PE 1 | ----- R1 ---- R2 ------- | PE 2 |**** CE2
| +------+ | | +------+ |
| | | |
| | | |
| +------+ | | +------+ |
CE3 ****| PE 3 | ----- R3 ---- R4 ------- | PE 4 |**** CE4
| +------+ ***********************> +------+ |
| |
\ /
\_________________________________________/
Figure 1 - Disjoint paths provided by a single provider
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_________________________________________
/ \
/ \
| |
| |
| ***********************> |
| +------+ 10 +------+ |
CE1 ****| PE 1 | ----- R1 ---- R2 ------- | PE 2 |**** CE2
| +------+ +------+ |
| |
\ /
\_________________________________________/
_________________________________________
/ \
/ \
| |
| |
| +------+ +------+ |
CE3 ****| PE 1 | ----- R3 ---- R4 ------- | PE 2 |**** CE4
| +------+ ***********************> +------+ |
| |
\ /
\_________________________________________/
Figure 2 - Disjoint paths provided by using two providers
As the traffic protection is ensured by an end-to-end mechanism at
the customer level, the customer requests the service provider to not
protect the paths. This is particularly required to avoid both
protection mechanisms (customer level and provider level) to be
activated at the same time which may lead to unpredictable side
effects. However the service provider is allowed to restore the end-
to-end path automatically when the primary path is failing by
computing and installing a new primary path at the head-end. How the
end-to-end protection is handled is out of scope of this document and
will be under the customer responsibility.
Another use case could be a service provider selling the traffic
protection as a service option. So by default, the provided IP/MPLS
path is not protected by any fast-reroute mechanism but the customer
can subscribe to an option to activate fast-reroute for its traffic.
In the figure 3, the Customer1 service between PE1 and PE2 is
protected, in case of failure between R1 and R2, the LSP can use a
bypass through R3-R4 nodes until the convergence occurs. The
Customer2 did not subscribe to the traffic protection option. If
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R3-R4 fails, the traffic between CE3 and CE4 will be disrupted until
the convergence occurs.
_________________________________________
/ \
/ \
| Protected LSP |
| ***********************> |
| +------+ 10 +------+ |
CE1 ****| PE 1 | ----- R1 ---- R2 ------- | PE 2 |**** CE2 Customer1
| +------+ |* *| +------+ |
| |* *| |
| |* *| |
| +------+ |********| +------+ |
CE3 ****| PE 3 | ----- R3 ---- R4 ------- | PE 4 |**** CE4 Customer2
| +------+ ***********************> +------+ |
| Non protected LSP |
\ /
\_________________________________________/
Figure 3 - Provider selling traffic protection as an option
A service provider may also propose a traffic protection service
based on path protection rather than local repair on each transit
node. In the figure 4, on PE1, two LSPs were created to ensure the
customer traffic protection between PE1 and PE2. The primary LSP is
used to carry the traffic in the nominal situation. The protection
LSP is built as disjoint from the primary LSP and may be
preestablished (from controlplane and/or dataplane point of view).
When the primary LSP fails, PE1 is responsible to switch the traffic
to the protection LSP. As the protection is provided by PE1, both
primary and protection LSPs should be setup as non protected so
transit nodes will not activate any local-repair mechanism for those
LSPs.
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_________________________________________
/ \
/ \
| Primary LSP |
| ***********************> |
| +------+ 10 +------+ |
CE1 ****| PE 1 | ----- R1 ---- R2 ------- | PE 2 |**** CE2 Customer1
| +------+ | | +------+ |
| *\ | | /> |
| *\ | | /* |
| *\ | | /* |
| *+- R3 ---- R4 ----+* |
| ******************* |
| Protection LSP |
\ /
\_________________________________________/
Figure 4 - Provider selling traffic protection as an option
A segment-routing path is expressed as a list of segment identifiers
(SID) from different types (Node-SID, Adj-SID, Binding-SID ...). In
order to ensure that the segment routing path is not protected, we
need to ensure that it does not contain any segment representing a
protected path. As an example, in the Figure 1, we consider a path
from PE1 to PE2 expressed with the following segment list:
{Adj_R1R3,Node_R2,Adj_R2PE2}. If we want to ensure that this path is
not protected, we need to ensure that the segment represented by
Adj_R1R3 represents a non protected segment, as well as the segments
Node_R2 and Adj_R2PE2.
The segment routing path may be computed by a Path Computation
Element (PCE). In order to fulfil the non protected path constraint,
the PCE needs to be aware of the available SIDs in the network and
their protection status.
Several techniques may be used to represent a non protected path with
a segment identifier. We propose to analyze the different options.
2. Requirements for a non protected LSP
o A non protected LSP SHOULD follow a primary path defined based on
the constraints of the LSP. This path can be the shortest path
(as per the IGP metric) or a more constrained path (explicit path)
to fulfil for example a bandwidth, latency or disjointness
requirement.
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o Upon a failure, a non protected LSP SHOULD be reestablished over a
new suitable non-protected path that still fulfils the constraints
of the LSP.
o Upon a failure (link, node, srlg...), the traffic of a non
protected LSP MUST NOT use any local-repair or any local-rerouting
mechanism on transit nodes.
o The computation of a new primary path for the LSP will be handled
by the computation node responsible of this LSP (it could be the
head-end or a PCE).
o Upon any other traffic-engineering topology change (metric change,
overload status change, bandwidth change, latency change...), the
non protected LSP MAY be reoptimized to a better path.
2.1. ECMP considerations
When equal cost paths are available within the end-to-end path,
implementations may reuse a fast-reroute like mechanism in the
dataplane, so when one of the outgoing interface fails, the dataplane
switches traffic immediately to the remaining outgoing interfaces in
the ECMP set. This behavior is usually hardcoded and cannot be
disabled. Based on this assumption, a non protected LSP SHOULD avoid
ECMPs.
3. Options to create a non protected path with Segment Routing
3.1. Using only non protected adjacency segments
A node can advertise multiple adjacency segments for a particular
link with different properties. The non-protected property is
already defined as part of the protocol encodings
([I-D.ietf-isis-segment-routing-extensions],
[I-D.ietf-ospf-segment-routing-extensions] and
[I-D.gredler-idr-bgp-ls-segment-routing-extension]) through the B
flag. However, from an implementation perspective, advertising a
protected adjacency segment, a non protected adjacency segment or
both for each link is optional.
It is important to note that even if an adjacency segment has the B
flag set (protected), it remains up to a local policy of the
advertising router to implement the protection or not.
If both protected and non protected Adj-SID are advertised, every
node in the network (including PCEs) can be aware of the adjacency
segments protection property. When a non protected path is
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requested, the path computation module can choose to encode the path
with a list a non protected adjacency segments only.
One of the advantage of using only adjacency segments is the
insurance that the traffic will never go transiently outside the path
defined by the computation module responsible of the path. This
solution is fully compliant with the requirements sets in Section 2.
One of the drawbacks of using only adjacency segments is the
resulting label stack depth as each hop should require a segment in
the stack: crossing 15 nodes, means stacking 15 labels to encode the
SR tunnel. Having such a deep stack may be a problem for current
hardwares and softwares for either pushing the stack (because the
head end is limited in the number of labels it can push) or
loadbalancing flows on transit nodes (as deep packet inspection or
entropy label look up may be difficult with a deep label stack).
Another drawback of advertising both protected and non protected
adjacency segments is the additional controlplane and dataplane
resource consumption used in the network. As the adjacency SIDs have
a local significance, this resource consumption can be considered as
negligeable from a data plane point of view. From a control plane
point of view, this can also be considered as negligeable with the
current CPU and memory usually available on routers.
3.2. Using a combination of node segments and adjacency segments
Using a combination of node segments and adjacency segments is the
usual way of creating a segment routing path. However the well known
Node-SID (algorithm type Shortest Path) may be protected by a local-
repair mechanism by any transit node or may use ECMPs which may be a
problem when used for a non protected path. Protecting a particular
Node-SID is a matter of a local policy configuration on every node.
The following discusses a number of possible approaches.
3.2.1. Adding a protection flag in the Node SID
As for adjacency segments, a new flag may be added in the Prefix-SID
to encode the willingness of protection. Each node will then
advertise two Node-SIDs (using SPF algorithm), one with the
protection flag set, the other without the protection flag set. The
same discussion regarding ECMP is also applicable here.
The remaining flag space in the Prefix-SID is small, so adding a new
flag requires analysis but this should not be considered as a
showstopper.
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3.2.2. Using Strict SPF Node SID
[I-D.ietf-spring-segment-routing] defines a Strict Shortest Path
algorithm which mandates that the packet is forwarded according to
ECMP-aware SPF algorithm and instructs any router in the path to
ignore any possible local policy overriding SPF decision. The use of
a local-repair for a strict SPF Node-SID is allowed as long as the
FRR mechanism enforces the post convergence path to the destination.
This solution does not bring any benefit compared to the regular
Node-SID (as it has similar properties).
3.2.3. Using two Node-SIDs with different local policies
Having two instances of the Node-SID (protected and not protected) is
a requirement when using Node-SID in protected and non protected
paths. The protection of a Node-SID is a matter of a local policy
configuration on every node in the network. A service provider may
configure two Node-SIDs per node and may adjust the local-repair on
every node to protect one Node-SID but not the other. As the
protection of the Node-SID is inherited from the protection of the
associated prefix, the service provider will need to deploy a new set
of prefixes to all nodes to deploy the new set of Node-SIDs. Then it
will need to maintain the local-repair policy on every node to ensure
that the prefixes associated to the non protected Node-SID are not
using the local-repair.
The path computation engine (head-end or PCE) must be aware of the
policy defined by the service provider so it can select the right
SIDs/prefixes when computing a path.
3.2.4. Advantages and drawbacks
One advantage of combining adjacency and node segments is the
reduction of the label stack size.
The drawbacks are the increase of the controlplane and dataplane
resource consumption. Whereas having two adjacency SIDs introduces a
negligeable impact, having two nodes SIDs increases controlplane and
dataplane processing as each node in the network will have to install
an MPLS->MPLS and IP->MPLS entry for each additional Node-SID. The
regular IP convergence time of the network may be doubled in the
worst case while the newly deployed node-SIDs are only used for
traffic-engineering applications. One of the other drawback is that
a Node-SID may be transiently rerouted on a path that does not fit
the constraints anymore if a transit node converges faster than the
head-end: this concern is not new and applies to all traffic-
engineering use cases. Note that there is a high chance for a
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transit node to reroute faster than the head-end as it has usually
less computations to run (SPF+CSPFs) and less prefixes to rewrite; it
may also run less features leaving more CPU slots for IGP
reconvergence. The transient rerouting of the Node-SID may lead to
microloops in the network that may impact the customer traffic.
Node-SIDs are subject to ECMP and a local-repair mechanism may be
implemented for equal cost paths with no way to disable it. If the
requirement of preventing any local-repair or ECMP is strict, the
path computation engine needs to prevent the usage of all Node-SIDs
or needs to detect that a particular Node-SID will be subject to ECMP
and enforce the usage of additional adjacency SIDs to break the ECMP.
In any case, more adjacency-SIDs will be required in the stack to
avoid the ECMP, leading to a deeper label stack.
3.3. Using a combination of adjacency segments and binding-SID
[I-D.ietf-spring-segment-routing] defines the binding segment with
multiple use cases. One of the use case of the binding segment is to
advertise a tunnel as a segment. When a computation engine computes
a non protected path and if the resulting label stack using only non
protected adjacency segments is too deep for the network, an external
component may create shortcuts in the network by creating a binding
segment representing a list of non protected adjacency segments.
PE1--P1 P6 P10--PE2
\ / \ /
P4-P5 P7 P9
\ /
P8
Figure 3 - Use of Binding SID
In the example above, the path from PE1 to PE2 must be expressed with
the stack: {Adj_P1P4,Adj_P4P5,Adj_P5P6,Adj_P6P7,Adj_P7P8,Adj_P8P9,Adj
_P9P10,Adj_P10PE2}. This stack is too deep due to the limitations of
the network. An external component may create a binding Binding1 on
P5 that represents the non protected path (P5->P6->P7->P8->P9->P10).
When the binding is created and advertised in the topology, the
computation engine can use this binding SID in a path, resulting for
a PE1 to PE2 path to the stack:
{Adj_P1P4,Adj_P4P5,Binding1,Adj_P10PE2}. The usage of the binding
SID in the stack allowed to reduce its size to an acceptable value.
One advantage of combining adjacency and binding segments is the
reduction of the label stack size. The label stack size can be
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reduced to a small amount of labels at some price (creating some
states on transit nodes).
The drawbacks are the increase of the controlplane and dataplane
resource consumption. This controlplane and dataplane resource
consumption are variable and will be linked to the intelligence of
the external controller and computation engines and especially how
the placement of the bindings is done to maximize the sharing between
LSPs. Moreover any optimization try in the binding segment may
introduce churn in the network controlplane (Make Before Break can be
used to ensure that dataplane is not affected). Programming a
binding-SID on a transit node is feasible only if the programming
node has the necessary protocol sessions to do so. When a head-end
router is performing a path computation, it is usually not the case.
When a controller (PCE) is used, it may not have a session to all
LSRs in the network, as only edge nodes may require a path
computation. The controller may be limited for the placement of the
binding SID to the nodes it has a protocol session with (it cannot
setup a PCEP session by itself). A full deployment of protocol
sessions with the controller may not be feasible for technical
reasons (scaling, ...) or economical reasons. A potential mitigation
could be to allow protocol sessions to be setup dynamically (when
requirement comes) to an authorized subset of nodes in the network:
some protocol modifications may be necessary to allow this behavior.
4. Comparison
The following table tries to summarize the various solution pros/cons
within a comparison table:
o Solution 1: using adjacency-SIDs only
o Solution 2: using adjacency-SIDs + Node-SIDs with strict SPF
algorithm
o Solution 3: using adjacency-SIDs + Node-SIDs with new protection
flag
o Solution 4: using adjacency-SIDs + two regular Node-SIDs with a
different policy
o Solution 5: using adjacency-SIDs + Binding-SIDs
We consider a network with N nodes and L links, with an average of l
links per node.
+----------+--------+-----------+------------+------------+---------+
| Criteria | Soluti | Solution | Solution 3 | Solution 4 | Solutio |
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| | on 1 | 2 | | | n 5 |
+----------+--------+-----------+------------+------------+---------+
| Label | One | Reduced | Reduced | Reduced | Reduced |
| stack | label | | | | |
| size | per | | | | |
| | hop | | | | |
| | | | | | |
| Controlp | Neglig | Potential | + 2*N | + 2*N | Adds |
| lane | ible | additiona | entries in | entries in | states |
| | | l computa | RIB | RIB | in the |
| | | tion + | | | LSRs |
| | | 2*N | | | |
| | | entries | | | |
| | | in RIB | | | |
| | | | | | |
| Dataplan | +l ent | +2*N | +2*N | +2*N | Variabl |
| e | ries | entries | entries | entries | e |
| | | | | | |
| IP conve | None | Double | Double | Double | None |
| rgence | | | | | |
| time | | | | | |
| | | | | | |
| Computat | Needs | Needs to | Needs to | Needs to | Needs |
| ion | to | select | select | select | to |
| engine | select | Adj-SIDs | Adj-SIDs | Adj-SIDs | select |
| | Adj- | with B=0 | with B=0 | with B=0 | Adj- |
| | SIDs | and Node- | and Node- | and needs | SIDs |
| | with | SIDs with | SIDs with | to | with |
| | B=0 | strict | B=0 | understand | B=0 and |
| | | SPF | | policy | place |
| | | | | from the | the |
| | | | | SP to | binding |
| | | | | select the | SID in |
| | | | | right | a smart |
| | | | | Node-SIDs | way |
| | | | | | |
| Protocol | None | None | Need a new | None | None |
| | | | flag | | |
| | | | | | |
| ECMP avo | Suppor | Supported | Supported | Supported | Support |
| idance | ted | at the | at the | at the | ed |
| | | price of | price of | price of | |
| | | increasin | increasing | increasing | |
| | | g the | the label | the label | |
| | | label | stack | stack | |
| | | stack | | | |
| | | | | | |
| Requirem | Yes | Partially | Partially | Partially | Yes |
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| ents ful | | (allows E | (allows EC | (allows EC | |
| filment | | CMP+trans | MP+transie | MP+transie | |
| | | ient rero | nt | nt | |
| | | uting) | rerouting) | rerouting) | |
| Others | None | None | None | None | Require |
| | | | | | s a con |
| | | | | | troller |
| | | | | | with se |
| | | | | | ssions |
| | | | | | to all |
| | | | | | nodes |
| | | | | | (even t |
| | | | | | ransit) |
+----------+--------+-----------+------------+------------+---------+
Comparison of solutions
5. Recommended option(s)
Based on the analysis in Section 4, we only have two solutions that
fulfill the requirements expressed in Section 2: usage of adjacency-
SIDs only, usage of a combination of adjacency SIDs and binding SIDs.
As using only Adjacency-SIDs may reduce today the possibility of
creating a path (due to the hardware/software limitations), authors
would like to encourage the usage of a combination of adjacency-SIDs
and binding-SIDs (Section 3.3) as a short-term solution.
However this approach has also several drawbacks, but authors think
that these drawbacks can be reduced by enhancing existing protocols.
As a long term solution, authors would like to encourage vendors to
support the ability for a node to push a significant number of
labels, up to the full network diameter.
6. Security Considerations
TBD.
7. Acknowledgements
Authors would like to thank Bruno Decraene for his valuable comments.
8. IANA Considerations
N/A
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9. Normative References
[I-D.gredler-idr-bgp-ls-segment-routing-extension]
Gredler, H., Ray, S., Previdi, S., Filsfils, C., Chen, M.,
and J. Tantsura, "BGP Link-State extensions for Segment
Routing", draft-gredler-idr-bgp-ls-segment-routing-
extension-02 (work in progress), October 2014.
[I-D.ietf-isis-segment-routing-extensions]
Previdi, S., Filsfils, C., Bashandy, A., Gredler, H.,
Litkowski, S., Decraene, B., and j. jefftant@gmail.com,
"IS-IS Extensions for Segment Routing", draft-ietf-isis-
segment-routing-extensions-13 (work in progress), June
2017.
[I-D.ietf-ospf-segment-routing-extensions]
Psenak, P., Previdi, S., Filsfils, C., Gredler, H.,
Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
Extensions for Segment Routing", draft-ietf-ospf-segment-
routing-extensions-18 (work in progress), July 2017.
[I-D.ietf-spring-segment-routing]
Filsfils, C., Previdi, S., Decraene, B., Litkowski, S.,
and R. Shakir, "Segment Routing Architecture", draft-ietf-
spring-segment-routing-12 (work in progress), June 2017.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
Authors' Addresses
Stephane Litkowski
Orange
Email: stephane.litkowski@orange.com
Mustapha Aissaoui
Nokia
Email: mustapha.aissaoui@nokia.com
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