Internet DRAFT - draft-balls-ccamp-relax-loop-check
draft-balls-ccamp-relax-loop-check
CCAMP S. Balls, Ed.
Internet-Draft J. Hardwick
Updates: 3209 (if approved) Metaswitch
Intended status: Informational C. Margaria
Expires: May 23, 2013 Nokia Siemens Networks
November 19, 2012
Relaxing RSVP Loop Checking
draft-balls-ccamp-relax-loop-check-02
Abstract
This specification relaxes the rules governing loop checking within
RSVP. These were originally defined in RFC3209 and are too strict
for the requirements of today's data planes.
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 May 23, 2013.
Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. General Overview . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Example in WDM networks . . . . . . . . . . . . . . . . . 3
2.2. Example using Connectivity Matrices . . . . . . . . . . . 4
2.3. Example with additional label restrictions . . . . . . . . 5
2.4. Example In Distributed Networks . . . . . . . . . . . . . 6
2.5. Example with Ingress Protection . . . . . . . . . . . . . 7
3. Existing workaround . . . . . . . . . . . . . . . . . . . . . 7
4. Solution . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.2. Assumptions and limitations . . . . . . . . . . . . . . . 7
4.3. General Rules . . . . . . . . . . . . . . . . . . . . . . 7
4.4. RRO handling . . . . . . . . . . . . . . . . . . . . . . . 7
4.5. ERO handling . . . . . . . . . . . . . . . . . . . . . . . 8
4.6. Interface handling . . . . . . . . . . . . . . . . . . . . 8
4.7. Signalling . . . . . . . . . . . . . . . . . . . . . . . . 9
4.8. Error Handling . . . . . . . . . . . . . . . . . . . . . . 9
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.1. Normative References . . . . . . . . . . . . . . . . . . . 9
8.2. Informative References . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
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1. Introduction
Generalized MPLS (GMPLS) Traffic Engineering (TE) Label Switched
Paths (LSPs) are prohibited from passing through a single node more
than once. Today's data planes are such that allowing spiral paths
through a control plane node should be allowed in order to set up
LSPs.
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 [RFC2119].
2. General Overview
With today's data planes it is acceptable for a single data flow
(LSP) to pass through a single control plane node on more than one
occasion on the path from source to destination. Currently control
plane protocols will prevent such a path being managed in the control
plane as they explicitly detect this as a loop. However, this may
not necessarily be a loop in the data plane and it is desirable for
such LSPs to be able to be managed in the same way as non-looping
LSPs. This document refers to such LSPs as spiralling LSPs.
2.1. Example in WDM networks
In WDM networks it can be necessary to route the data via an
additional box in order to fulfil regeneration or wavelength
conversion requirements. For example, consider the following simple
example.
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+-----+ +-----+ +-----+
| | Link 1 | | Link 2 | |
| A |----------| B |----------| C |
| | | | | |
+-----+ +-----+ +-----+
| |
| |
Link 3 | | Link 4
| |
| |
+-----+
| |
| D |
| |
+-----+
Figure 1
If node B cannot perform wavelength conversion but Link 1 and Link 2
do not have a common free wavelength then the only way to set up a
path from node A to node C will be via node D. This requires two
passes through node B which to RSVP looks like a loop, but is a
spiral.
2.2. Example using Connectivity Matrices
In any type of network a specific node may have connectivity
restrictions that limit the output ports available given the input
ports. Connectivity Matrices are described in [RFC6163] For example,
given the above network, where node B has the following connectivity
restrictions.
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+-------+
| |
1 | B | 2
----|-\ /-|----
| | | |
| | | |
+-------+
| |
3 | | 4
| |
Figure 2
As in the above example, the only way to set up a path from node A to
node C will be via node D. This requires two passes through node B
which to RSVP looks like a loop, but is a sprial.
2.3. Example with additional label restrictions
Connections between ports on a node may be restricted based on
labels. Consider the following network.
+-----+ +-----+ +-----+
| | Link 1 | | Link 2 | |
| A |----------| B |----------| C |
| | | | | |
+-----+ +-----+ +-----+
\ |
\ |
\ Link 3 |
\ |
\ +-----+
\ | |
Link 4 +-------| D |
| |
+-----+
Figure 3
This network has the following properties.
o Node A is electro-optical outputting Lambda 1 and can switch
Lambda 2.
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o Node D can convert between Lambda 1 and Lambda 2.
o Link 1 and Link 3 have Lambda 1 available.
o All links have Lambda 2 available.
To setup a path from A to C in this network, the LSP must pass
through Link 1 twice: once using Lambda 1 and once using Lambda 2.
This results in the path A-B-D-A-B-C being taken which requires two
passes through node A. This looks like a loop, but due to the
different lambdas used on each pass is a spiral.
2.4. Example In Distributed Networks
In networks where the control plane and data plane are physically
distinct, it is possible that a single control plane element will be
controlling multiple data plane elements. This is the case now in
some ASON networks, and will increasingly be the case with the move
towards SDN networks. Consider the following network.
+-----+
Control | |
/--------------------| CP |----\
| | | |
| +-----+ |
| +-----+ | +-----+
| | | | | |
| | CP | | | CP |
| | | | | |
| +-----+ | +-----+
| | | |
+-----+ +-----+ +-----+ +-----+
| | Data | | Data | | Data | |
| A |----------| B |----------| D |----------| C |
| | | | | | | |
+-----+ +-----+ +-----+ +-----+
Figure 4
CP are the control planes instances, with A, B, D and C the data
plane. Since data nodes A and D are managed by one control plane, an
LSP from A to C would appear as a loop, where it is clear that this
is not the case in the data plane. As different interfaces are being
used the control plane could treat such an LSP as a spiral.
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2.5. Example with Ingress Protection
If performing ingress protection with an off-forwarding-path backup
node, as described by [I-D.torvi-mpls-rsvp-ingress-protection], then
the ingress node will see a Path message for the same session twice.
Preventing a data plane loop, but allowing a spiral is also required
in this case.
3. Existing workaround
In current networks it is possible to support such paths either
through management configuration at each node, or splitting the path
into two or more signalling sessions. In the above examples this can
be achieved with one session from A to D, and a second session from D
to C. It would also require management on node D to join the data
paths together. It is desirable that a single signalling session can
be used to set up such paths, thus only requiring management input at
the ingress.
4. Solution
4.1. Overview
To support such networks, the rules governing RSVP loop checking are
relaxed to allow spirals, but still prevent loops. No changes to
protocol messages are made.
4.2. Assumptions and limitations
These changes are only applicable to GMPLS out of band signalling
when using point to point data links.
4.3. General Rules
The following rules govern the changes in behaviour that allow RSVP
loop checking to be relaxed while still setting up non-looping data
paths in RSVP.
o For each pass through the control plane node, the pair of inbound
and outbound data interfaces and labels must be different.
4.4. RRO handling
Section 4.4.4 of [RFC3209] states that RSVP must reject a Path
message if the receiving router is already in the RRO. This is now
relaxed to allow such a condition provided a different interface-
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label pair is used in each case. If the router has existing session
state for a received Path message, and it MUST verify that the newly
requested data path (input and output interface and label) is
different from the existing data path(s) for that session, and the
existing data path(s) is (are) present earlier in the RRO. If this
is not the case, the router MUST return a "Routing problem" PathErr
message with the error value "loop detected".
In order to carry out this checking correctly, specific interfaces
and labels SHOULD be recorded in the RRO. If this is not the case,
each node can only verify the path is acceptable against local state
and should not reject the RRO if the local state is valid.
It is allowable for local policy to exist to limit the number of
different paths through a router in a single LSP instance. If this
limit is exceeded the router SHOULD return a "Routing problem"
PathErr message with the error value "loop detected". This local
policy is not intended to be advertised in routing. It is present as
a backstop to protect against malicious Path messages consuming all
resources on the router.
4.5. ERO handling
Sections 4.3.4.1 and 4.3.5 of [RFC3209] also state that RSVP must
detect and avoid loops. This checking is also relaxed to allow
spirals in the cases stated above. Again, local policy can limit the
number of different paths through a router in a single LSP instance.
A router may "look ahead" in the ERO to determine such local policy
will be exceeded in advance of it happening and SHOULD return a
"Routing problem" PathErr message with the error value "loop
detected" in such a case.
When calculating or expanding an ERO a router may include multiple
entries through a single router. If the ERO contains loose hops that
form a loop, and a node determines a non-looping route is available,
it MAY remove the loop from the ERO.
4.6. Interface handling
As stated in the general rules, an implementation supporting multiple
passes through a node must ensure that for each pass the input and
output interfaces and labels are different.
Internally, this means that if a Path message is received using a
different input interface this may no longer mean the LSP has been
rerouted upstream. Implementations must check the RRO to determine
the correct behaviour when processing such a Path message. Care must
be taken to handle valid cases where the incoming label can change.
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4.7. Signalling
For the avoidance of doubt, no new signalling is being defined in
this draft.
The behaviour of refresh or error messages is unchanged and should
therefore be sent along the looped path (if present). Nodes SHOULD
NOT shortcut the loop.
4.8. Error Handling
How to behave when receiving a PathErr with error value "loop
detected" is out of scope of this draft and is a local implementation
decision. For example, it may choose to try and recalculate the path
mandating that the error node is avoided, or does not support
looping.
5. Acknowledgements
With thanks to Jonathan Sadler and Yimin Shen for their input when
discussing this draft.
6. IANA Considerations
This memo includes no request to IANA.
7. Security Considerations
In principle these changes to RSVP pose no security exposures over
and above [RFC3209]. However, by allowing loops a single LSP can now
consume multiple resources. As suggested local policy can limit the
number of paths and thus the resource a single LSP can consume.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC 3209, December 2001.
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8.2. Informative References
[I-D.torvi-mpls-rsvp-ingress-protection]
Atlas, A., Torvi, R., and M. Jork, "Ingress Protection for
RSVP-TE p2p and p2mp LSPs",
draft-torvi-mpls-rsvp-ingress-protection-00 (work in
progress), July 2012.
[RFC6163] Lee, Y., Bernstein, G., and W. Imajuku, "Framework for
GMPLS and Path Computation Element (PCE) Control of
Wavelength Switched Optical Networks (WSONs)", RFC 6163,
April 2011.
Authors' Addresses
Steve Balls (editor)
Metaswitch
100 Church Street
Enfield, EN2 6BQ
UK
Phone: +44 208 366 1177
Email: steve.balls@metaswitch.com
Jonathan Hardwick
Metaswitch
100 Church Street
Enfield, EN2 6BQ
UK
Phone: +44 208 366 1177
Email: jonathan.hardwick@metaswitch.com
Cyril Margaria
Nokia Siemens Networks
St Martin Strasse 76
Munich, 81541
Germany
Phone: +49 89 5159 16934
Email: cyril.margaria@nsn.com
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