Network Working Group S. Previdi, Ed.
Internet-Draft C. Filsfils, Ed.
Intended status: Informational Cisco Systems, Inc.
Expires: October 29, 2015 B. Decraene
S. Litkowski
Orange
M. Horneffer
Deutsche Telekom
R. Shakir
British Telecom
April 27, 2015
SPRING Problem Statement and Requirements
draft-ietf-spring-problem-statement-04
Abstract
The ability for a node to specify a forwarding path, other than the
normal shortest path, that a particular packet will traverse,
benefits a number of network functions. Source-based routing
mechanisms have previously been specified for network protocols, but
have not seen widespread adoption. In this context, the term
'source' means 'the point at which the explicit route is imposed' and
therefore it is not limited to the originator of the packet (i.e.:
the node imposing the explicit route may be the ingress node of an
operator's network).
This document outlines various use cases, with their requirements,
that need to be taken into account by the Source Packet Routing in
Networking (SPRING) architecture for unicast traffic. Multicast use-
cases and requirements are out of scope of this document.
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].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on October 29, 2015.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Dataplanes . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. SPRING Use Cases . . . . . . . . . . . . . . . . . . . . . . 4
3.1. IGP-based MPLS Tunneling . . . . . . . . . . . . . . . . 4
3.1.1. Example of IGP-based MPLS Tunnels . . . . . . . . . . 4
3.2. Fast Reroute . . . . . . . . . . . . . . . . . . . . . . 5
3.3. Traffic Engineering . . . . . . . . . . . . . . . . . . . 5
3.3.1. Examples of Traffic Engineering Use Cases . . . . . . 6
3.4. Interoperability with non-SPRING nodes . . . . . . . . . 12
4. Security Considerations . . . . . . . . . . . . . . . . . . . 12
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
6. Manageability Considerations . . . . . . . . . . . . . . . . 13
7. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 13
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 13
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
9.1. Normative References . . . . . . . . . . . . . . . . . . 13
9.2. Informative References . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15
1. Introduction
The ability for a node to specify a unicast forwarding path, other
than the normal shortest path, that a particular packet will
traverse, benefits a number of network functions, for example:
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Some types of network virtualization, including multi-topology
networks and the partitioning of network resources for VPNs
Network, link, path and node protection such as fast re-route
Network programmability
OAM techniques
Simplification and reduction of network signaling components
Load balancing and traffic engineering
Source-based routing mechanisms have previously been specified for
network protocols, but have not seen widespread adoption other than
in MPLS traffic engineering.
These network functions may require greater flexibility and per
packet source imposed routing than can be achieved through the use of
the previously defined methods. In the context of this document, the
term 'source' means 'the point at which the explicit route is
imposed' and therefore it is not limited to the originator of the
packet (i.e.: the node imposing the explicit route may be the ingress
node of an operator's network). Throughout this document we refer to
this definition of 'source'.
In this context, Source Packet Routing in Networking (SPRING)
architecture is being defined in order to address the use cases and
requirements described in this document.
The SPRING architecture SHOULD allow incremental and selective
deployment without any requirement of flag day or massive upgrade of
all network elements.
The SPRING architecture SHOULD allow optimal virtualization: put
policy state in the packet header and not in the intermediate nodes
along the path. Hence, the policy is completely virtualized away
from midpoints and tail-ends.
The SPRING architecture objective is not to replace existing source
routing and traffic engineering mechanisms but rather complement them
and address use cases where removal of signaling and path state in
the core is a requirement.
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2. Dataplanes
The SPRING architecture SHOULD be general in order to ease its
applicability to different dataplanes.
The IPv6 specification [RFC2460], amended by [RFC6564] and [RFC7045],
defines the Routing Extension Header which provides IPv6 source-based
routing capabilities.
The SPRING architecture SHOULD leverage the existing MPLS dataplane
without any modification and leverage IPv6 dataplane with a new IPv6
Routing Header Type (IPv6 Routing Header is defined in [RFC2460]).
3. SPRING Use Cases
3.1. IGP-based MPLS Tunneling
The source-based routing model, applied to the MPLS dataplane, offers
the ability to tunnel services (VPN, VPLS, VPWS) from an ingress PE
to an egress PE, with or without the expression of an explicit path
and without requiring forwarding plane or control plane state in
intermediate nodes.
The source-based routing model, applied to the MPLS dataplane, offers
the ability to tunnel unicast services (VPN, VPLS, VPWS) from an
ingress PE to an egress PE, with or without the expression of an
explicit path and without requiring forwarding plane or control plane
state in intermediate nodes. p2mp and mp2mp tunnels are out of the
scope of this document.
3.1.1. Example of IGP-based MPLS Tunnels
This section illustrates an example use-case.
P1---P2
/ \
A---CE1---PE1 PE2---CE2---Z
\ /
P3---P4
Figure 1: IGP-based MPLS Tunneling
In Figure 1 above, the four nodes A, CE1, CE2 and Z are part of the
same VPN. CE2 advertises to PE2 a route to Z. PE2 binds a local
label LZ to that route and propagates the route and its label via
MPBGP to PE1 with nhop 192.168.0.2 (i.e.: the local address of PE2).
PE1 installs the VPN prefix Z in the appropriate VRF and resolves the
next-hop onto the node segment associated with PE2.
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In order to cope with the reality of current deployments, the SPRING
architecture SHOULD allow PE to PE forwarding according to the IGP
shortest path without the addition of any other signaling protocol.
The packet each PE forwards across the network will contain (within
their label stack) the necessary information derived from the
topology database in order to deliver the packet to the remote PE.
3.2. Fast Reroute
FRR technologies have been deployed by network operators in order to
cope with link or node failures through pre-computation of backup
paths.
The SPRING architecture SHOULD address the following requirements:
o support of FRR on any topology
o pre-computation and setup of backup path without any additional
signaling (other than the regular IGP/BGP protocols)
o support of shared risk constraints
o support of node and link protection
o support of microloop avoidance
Further illustrations of the problem statement for FRR are to be
found in [I-D.ietf-spring-resiliency-use-cases].
3.3. Traffic Engineering
Traffic Engineering has been addressed using IGP protocol extensions
(for resources information propagation) and RSVP-TE for signaling
explicit paths. Different contexts and modes have been defined
(single vs. multiple domains, with or without bandwidth admission
control, centralized vs. distributed path computation, etc).
In all cases, one of the major components of the TE architecture is
the soft state based signaling protocol (RSVP-TE) which is used in
order to signal and establish the explicit path. Each path, once
computed, need to be signaled and state for each path must be present
in each node traversed by the path. This incurs a scalability
problem especially in the context of SDN where traffic
differentiation may be done at a finer granularity (e.g.: application
specific). Also the amount of state needed to be maintained and
periodically refreshed in all involved nodes contributes
significantly to complexity and the number of failures cases, and
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thus increases operational effort while decreasing overall network
reliability.
The source-based routing model allows traffic engineering to be
implemented without the need of a signaling component.
The SPRING architecture SHOULD support traffic engineering,
including:
o loose or strict options
o bandwidth admission control
o distributed vs. centralized model (PCE, SDN Controller)
o disjointness in dual-plane networks
o egress peering traffic engineering
o load-balancing among non-parallel links
o Limiting (scalable, preferably zero) per-service state and
signaling on midpoint and tail-end routers.
o ECMP-awareness
o node resiliency property (i.e.: the traffic-engineering policy is
not anchored to a specific core node whose failure could impact
the service.
3.3.1. Examples of Traffic Engineering Use Cases
Here follows the description of two sets of use cases:
o Traffic Engineering without Admission Control
o Traffic Engineering with Admission Control
3.3.1.1. Traffic Engineering without Bandwidth Admission Control
In this section, we describe Traffic Engineering use-cases without
bandwidth admission control.
3.3.1.1.1. Disjointness in dual-plane networks
Many networks are built according to the dual-plane design, as
illustrated in Figure 2:
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Each access region k is connected to the core by two C routers
C1k and C2k where k refers to the region.
C1k is part of plane 1 and aggregation region k
C2k is part of plane 2 and aggregation region k
C1k has a link to C2j iff k = j.
The core nodes of a given region are directly connected.
Inter-region links only connect core nodes of the same plane.
{C1k has a link to C1j} iff {C2k has a link to C2j}.
The distribution of these links depends on the topological
properties of the core of the AS. The design rule presented
above specifies that these links appear in both core planes.
We assume a common design rule found in such deployments: the inter-
plane link costs (Cik-Cjk where i<>j) are set such that the route to
an edge destination from a given plane stays within the plane unless
the plane is partitioned.
Edge Router A
/ \
/ \
/ \ Agg Region A
/ \
/ \
C1A----------C2A
| \ | \
| \ | \
| C1B----------C2B
Plane1 | | | | Plane2
| | | |
C1C--|-----C2C |
\ | \ |
\ | \ |
C1Z----------C2Z
\ /
\ / Agg Region Z
\ /
\ /
Edge Router Z
Figure 2: Dual-Plane Network and Disjointness
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In this scenario, the operator requires the ability to deploy
different strategies. For example, Edge Router A should be able to
use the three following options:
o the traffic is load-balanced across any ECMP path through the
network
o the traffic is load-balanced across any ECMP path within the
Plane1 of the network
o the traffic is load-balanced across any ECMP path within the
Plane2 of the network
Most of the data traffic from A to Z would use the first option, such
as to exploit the capacity efficiently. The operator would use the
two other choices for specific premium traffic that has requested
disjoint transport.
The SPRING architecture SHOULD support this use case with the
following requirements:
o Zero per-service state and signaling on midpoint and tail-end
routers.
o ECMP-awareness.
o Node resiliency property: the traffic-engineering policy is not
anchored to a specific core node whose failure could impact the
service.
3.3.1.1.2. Egress Peering Traffic Engineering
+------+
| |
+---D F
+---------+ / | AS 2 |\ +------+
| |/ +------+ \| Z |
A C | |
| |\ +------+ /| AS 4 |
B AS1 | \ | |/ +------+
| | +---E G
+---------+ | AS 3 |
+------+\
Figure 3: Egress peering traffic engineering
Let us assume, in the network depicted in Figure 3, that:
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C in AS1 learns about destination Z of AS 4 via two BGP paths
(AS2, AS4) and (AS3, AS4).
C may or may not be configured so to enforce next-hop-self
behavior before propagating the paths within AS1.
C may propagate all the paths to Z within AS1 (BGP add-paths,
[I-D.ietf-idr-add-paths]).
C may install in its FIB only the route via AS2, or only the route
via AS3, or both.
In that context, the SPRING architecture SHOULD allow the operator of
AS1 to apply a traffic-engineering policy such as the following one,
regardless the configured behavior of next-hop-self:
Steer 60% of the Z-destined traffic received at A via AS2 and 40%
via AS3.
Steer 80% of the Z-destined traffic received at B via AS2 and 20%
via AS3.
The SPRING architecture SHOULD allow an ingress node (i.e., an
explicit route source node) to select the exit point of a packet as
any combination of an egress node, an egress interface, a peering
neighbor, and a peering AS.
3.3.1.1.3. Load-balancing among non-parallel links
The SPRING architecture SHOULD allow a given node to load share
traffic across multiple non parallel links even if these lead to
different neighbors. This may be useful to support traffic
engineering policies.
+---C---D---+
| |
PE1---A---B-----F-----E---PE2
Figure 4: Multiple (non-parallel) Adjacencies
In the above example, the operator requires PE1 to load-balance its
PE2-destined traffic between the ABCDE and ABFE paths.
3.3.1.2. Traffic Engineering with Bandwidth Admission Control
The implementation of bandwidth admission control within a network
(and its possible routing consequence which consists in routing along
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explicit paths where the bandwidth is available) requires a capacity
planning process.
The spreading of load among ECMP paths is a key attribute of the
capacity planning processes applied to packet-based networks.
3.3.1.2.1. Capacity Planning Process
Capacity Planning anticipates the routing of the traffic matrix onto
the network topology, for a set of expected traffic and topology
variations. The heart of the process consists in simulating the
placement of the traffic along ECMP-aware shortest-paths and
accounting for the resulting bandwidth usage.
The bandwidth accounting of a demand along its shortest-path is a
basic capability of any planning tool or PCE server.
For example, in the network topology described below, and assuming a
default IGP metric of 1 and IGP metric of 2 for link GF, a 1600Mbps
A-to-Z flow is accounted as consuming 1600Mbps on links AB and FZ,
800Mbps on links BC, BG and GF, and 400Mbps on links CD, DF, CE and
EF.
C-----D
/ \ \
A---B +--E--F--Z
\ /
G------+
Figure 5: Capacity Planning an ECMP-based demand
ECMP is extremely frequent in SP, Enterprise and DC architectures and
it is not rare to see as much as 128 different ECMP paths between a
source and a destination within a single network domain. It is a key
efficiency objective to spread the traffic among as many ECMP paths
as possible.
This is illustrated in the below network diagram which consists of a
subset of a network where already 5 ECMP paths are observed from A to
M.
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C
/ \
B-D-L--
/ \ / \
A E \
\ M
\ G /
\ / \ /
F K
\ /
I
Figure 6: ECMP Topology Example
When the capacity planning process detects that a traffic growth
scenario and topology variation would lead to congestion, a capacity
increase is triggered and if it cannot be deployed in due time, a
traffic engineering solution is activated within the network.
A basic traffic engineering objective consists of finding the
smallest set of demands that need to be routed off their shortest
path to eliminate the congestion, then to compute an explicit path
for each of them and instantiating these traffic-engineered policies
in the network.
The SPRING architecture SHOULD offer a simple support for ECMP-based
shortest path placement as well as for explicit path policy without
incurring additional signaling in the domain. This includes:
o the ability to steer a packet across a set of ECMP paths
o the ability to diverge from a set of ECMP shortest paths to one or
more paths not in the set of shortest paths
3.3.1.2.2. SDN/SR use-case
The SDN use-case lies in the SDN controller, (e.g.: Stateful PCE as
described in [I-D.ietf-pce-stateful-pce].
The SDN controller is responsible to control the evolution of the
traffic matrix and topology. It accepts or denies the addition of
new traffic into the network. It decides how to route the accepted
traffic. It monitors the topology and upon topological change,
determines the minimum traffic that should be rerouted on an
alternate path to alleviate a bandwidth congestion issue.
The algorithms supporting this behavior are a local matter of the SDN
controller and are outside the scope of this document.
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The means of collecting traffic and topology information are the same
as what would be used with other SDN-based traffic-engineering
solutions (e.g. [RFC7011] and [I-D.ietf-idr-ls-distribution].
The means of instantiating policy information at a traffic-
engineering head-end are the same as what would be used with other
SDN-based traffic-engineering solutions (e.g.:
[I-D.ietf-i2rs-architecture], [I-D.crabbe-pce-pce-initiated-lsp] and
[I-D.ietf-pce-segment-routing]).
In the context of Centralized-Based Optimization and the SDN use-
case, here are the functionalities that the SPRING architecture
SHOULD deliver:
Explicit routing capability with or without ECMP-awareness.
No signaling hop-by-hop through the network.
State is only maintained at the policy head-end. No state is
maintained at mid-points and tail-ends.
Automated guaranteed FRR for any topology.
Optimum virtualization: the policy state is in the packet header
and not in the intermediate nodes along the path. The policy is
completely virtualized away from midpoints and tail-ends.
Highly responsive to change: the SDN Controller only needs to
apply a policy change at the head-end. No delay is introduced due
to programming the midpoints and tail-end along the path.
3.4. Interoperability with non-SPRING nodes
SPRING nodes MUST inter-operate with non-SPRING nodes and in both
MPLS and IPv6 dataplanes.
4. Security Considerations
There is an assumed trust model such that the source imposing an
explicit route on a packet is assumed to be allowed to do so. In
such context trust boundaries should strip explicit routes from a
packet.
For each data plane technology that SPRING specifies, a security
analysis MUST be provided showing how protection is provided against
an attacker disrupting the network by for example, maliciously
injecting SPRING packets.
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5. IANA Considerations
This document does not request any IANA allocations.
6. Manageability Considerations
The SPRING WG SHOULD provide OAM and the management needed to manage
SPRING enabled networks. The SPRING procedures MAY also be used as a
tool for OAM in SPRING enabled networks.
OAM use cases and requirements are described in
[I-D.geib-spring-oam-usecase] and
[I-D.kumar-spring-sr-oam-requirement].
7. Contributors
The following individuals substantially contributed to the content of
this documents:
Ruediger Geib
Deutsche Telekom
Heinrich Hertz Str. 3-7
Darmstadt 64295
DE
Email: Ruediger.Geib@telekom.de
Robert Raszuk
Mirantis Inc.
615 National Ave.
94043 Mt View, CA
US
Email: robert@raszuk.net
8. Acknowledgements
The authors would like to thank Yakov Rekhter for his contribution to
this document.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
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[RFC6564] Krishnan, S., Woodyatt, J., Kline, E., Hoagland, J., and
M. Bhatia, "A Uniform Format for IPv6 Extension Headers",
RFC 6564, April 2012.
[RFC7045] Carpenter, B. and S. Jiang, "Transmission and Processing
of IPv6 Extension Headers", RFC 7045, December 2013.
9.2. Informative References
[I-D.crabbe-pce-pce-initiated-lsp]
Crabbe, E., Minei, I., Sivabalan, S., and R. Varga, "PCEP
Extensions for PCE-initiated LSP Setup in a Stateful PCE
Model", draft-crabbe-pce-pce-initiated-lsp-03 (work in
progress), October 2013.
[I-D.geib-spring-oam-usecase]
Geib, R., Filsfils, C., Pignataro, C., and N. Kumar, "Use
case for a scalable and topology aware MPLS data plane
monitoring system", draft-geib-spring-oam-usecase-04 (work
in progress), March 2015.
[I-D.ietf-i2rs-architecture]
Atlas, A., Halpern, J., Hares, S., Ward, D., and T.
Nadeau, "An Architecture for the Interface to the Routing
System", draft-ietf-i2rs-architecture-09 (work in
progress), March 2015.
[I-D.ietf-idr-add-paths]
Walton, D., Retana, A., Chen, E., and J. Scudder,
"Advertisement of Multiple Paths in BGP", draft-ietf-idr-
add-paths-10 (work in progress), October 2014.
[I-D.ietf-idr-ls-distribution]
Gredler, H., Medved, J., Previdi, S., Farrel, A., and S.
Ray, "North-Bound Distribution of Link-State and TE
Information using BGP", draft-ietf-idr-ls-distribution-10
(work in progress), January 2015.
[I-D.ietf-pce-segment-routing]
Sivabalan, S., Medved, J., Filsfils, C., Crabbe, E.,
Lopez, V., Tantsura, J., Henderickx, W., and J. Hardwick,
"PCEP Extensions for Segment Routing", draft-ietf-pce-
segment-routing-03 (work in progress), April 2015.
[I-D.ietf-pce-stateful-pce]
Crabbe, E., Minei, I., Medved, J., and R. Varga, "PCEP
Extensions for Stateful PCE", draft-ietf-pce-stateful-
pce-11 (work in progress), April 2015.
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[I-D.ietf-spring-resiliency-use-cases]
Francois, P., Filsfils, C., Decraene, B., and R. Shakir,
"Use-cases for Resiliency in SPRING", draft-ietf-spring-
resiliency-use-cases-01 (work in progress), March 2015.
[I-D.kumar-spring-sr-oam-requirement]
Kumar, N., Pignataro, C., Akiya, N., Geib, R., Mirsky, G.,
and S. Litkowski, "OAM Requirements for Segment Routing
Network", draft-kumar-spring-sr-oam-requirement-03 (work
in progress), March 2015.
[RFC7011] Claise, B., Trammell, B., and P. Aitken, "Specification of
the IP Flow Information Export (IPFIX) Protocol for the
Exchange of Flow Information", STD 77, RFC 7011, September
2013.
Authors' Addresses
Stefano Previdi (editor)
Cisco Systems, Inc.
Via Del Serafico, 200
Rome 00142
Italy
Email: sprevidi@cisco.com
Clarence Filsfils (editor)
Cisco Systems, Inc.
Brussels
BE
Email: cfilsfil@cisco.com
Bruno Decraene
Orange
FR
Email: bruno.decraene@orange.com
Stephane Litkowski
Orange
FR
Email: stephane.litkowski@orange.com
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Martin Horneffer
Deutsche Telekom
Hammer Str. 216-226
Muenster 48153
DE
Email: Martin.Horneffer@telekom.de
Rob Shakir
British Telecom
London
UK
Email: rob.shakir@bt.com
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