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Network Working Group J. Dong
Internet-Draft S. Bryant
Intended status: Standards Track Huawei Technologies
Expires: January 3, 2019 Z. Li
China Mobile
T. Miyasaka
KDDI Corporation
July 2, 2018
Segment Routing for Enhanced VPN Service
draft-dong-spring-sr-for-enhanced-vpn-01
Abstract
Enhanced VPN (VPN+) is an enhancement to VPN technology to enable it
to support the needs of new applications, particularly applications
that are associated with 5G services. These applications require
better isolation and have more stringent performance requirements
than can be provided with overlay VPNs. The characteristics of an
enhanced VPN as perceived by its tenant needs to be comparable to
those of a dedicated private network. This requires tight
integration between the overlay VPN and the underlay network
resources in a scalable manner. An enhanced VPN may form the
underpin of 5G network slicing, but will also be of use in its own
right. This document describes the use of segment routing based
mechanisms to provide the enhanced VPN service with dedicated network
resources.
Requirements Language
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 RFC 2119 [RFC2119] RFC 2119 [RFC8174][when, and only when, they
appear in all capitals, as shown here.
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 https://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 January 3, 2019.
Copyright Notice
Copyright (c) 2018 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
Provisions Relating to IETF Documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Segment Routing with Resource Allocation . . . . . . . . . . 4
3. Procedures . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Topology and Resource Computation . . . . . . . . . . . . 5
3.2. Network Resource and SID Allocation . . . . . . . . . . . 6
3.3. Construction of SR Virtual Topology . . . . . . . . . . . 7
3.4. VPN Service to SR Virtual Topology Mapping . . . . . . . 8
4. Benefits of the Proposed Mechanism . . . . . . . . . . . . . 8
4.1. MPLS-TP . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.2. RSVP-TE . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.3. Classic SR . . . . . . . . . . . . . . . . . . . . . . . 10
4.4. SR with Resource Allocation . . . . . . . . . . . . . . . 10
5. Service Assurance . . . . . . . . . . . . . . . . . . . . . . 10
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
7. Security Considerations . . . . . . . . . . . . . . . . . . . 11
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
9.1. Normative References . . . . . . . . . . . . . . . . . . 11
9.2. Informative References . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
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1. Introduction
Driven largely by needs arising from the 5G mobile network design,
the concept of network slicing has gained traction. There is a need
to create a VPN with enhanced characteristics. Specifically there is
a need for a transport network supporting a set of virtual networks
each of which provides the client with dedicated (private) network
resources drawn from a shared pool. The tenant of such a network can
require a degree of isolation and performance that previously could
only be satisfied by dedicated networks. Additionally the tenant may
ask for some level of control of their virtual network e.g. to
customize the service paths in their network slices.
The enhanced VPN service (VPN+) as specified in
[I-D.bryant-rtgwg-enhanced-vpn] is targeted at new applications which
require better isolation and have more stringent performance
requirements than can be provided with existing overlay VPNs. An
enhanced VPN may form the underpin of 5G network slicing, but will
also be of use in its own right. Although different VPNs can be
associated with dedicated RSVP-TE [RFC3209] LSPs to provide some
guarantee to the service performance, such mechanisms would introduce
per-VPN per-flow states into the network, which is known to have
scalability issues and has not been the choice of most IP/MPLS
networks.
Segment Routing (SR) [I-D.ietf-spring-segment-routing] specifies a
mechanism to steer packet through an ordered list of segments. It
can achieve explicit source routing without introducing per-path
state into the network. Like RSVP-TE, SR supports source
specification of the packet path, However, currently SR does not have
the capability of reserving or identifying different network
resources for different services or customers. Although the
controller can have global view of the network utilization state and
can provision different services onto different SR paths, in the data
plane it still relies on the DiffServ QoS model to provide coarse-
grained traffic differentiation in the network. While this may be
sufficient for some traditional services, it cannot meet the
requirement of the emerging services.
This document extends the SR paradigm to use segments to identify the
different subset of resources allocated on each network elements
(links or nodes). The SR Identifiers (SIDs) associated with
particular network resources can be used to construct customized
virtual networks for different services, the SID can also be used to
steer the service traffic to be processed with the corresponding
allocated resources. This mechanism can be used to provide the
enhanced VPN service with dedicated network resources.
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2. Segment Routing with Resource Allocation
In segment routing, several types of segments are defined to
represent either topological elements or service instructions. Node
segment and adjacency segment are two major types of topological
segments. Some other types of segments may be associated with
specific service functions for service chaining purpose. However, so
far the segments are not associated with network resources for the
QoS purpose.
In order to support the enhanced VPNs which require guaranteed
performance and isolation from other services in the network, the
overlay VPN needs to been integrated with underlay networks. This
requires that some dedicated network resources be allocated to the
enhanced VPN. When segment routing is used to build enhanced VPNs,
it is necessary to associate the segments with network resources.
By extending the segment routing paradigm, different subsets of
network resources are allocated from each network element, and
associated with different SIDs. On one particular link, multiple
adjacency segment identifiers (Adj-SIDs) can be allocated, each of
which is associated with a set of resource allocated from the link,
such as bandwidth, queues, etc. For one particular node, multiple
node-SIDs can be allocated, each of which may be associated with a
set of resource allocated from the node, such as the processing
resources. This per-segment resource allocation complies to the SR
paradigm, which avoids introducing per-path state into the network.
Different groups of SIDs associated with network resources can be
used to build the virtual underlay networks for different enhanced
VPNs, this provides the required isolation between enhanced VPNs.
The adj-SIDs are used to steer traffic of different enhanced VPNs
into different link resources on each hop. The node SIDs can be used
to steer traffic of different enhanced VPNs into different node
resources. The node SIDs can also be used to build loose SR paths
for different enhanced VPNs, in which case the node SIDs are used by
the transit nodes to steer traffic to use the link resources
allocated for the corresponding enhanced VPN. Note that in this case
Penultimate Hop Popping (PHP) MUST be disabled, so that each node
could use the node-SID to identify the enhanced VPN and the
corresponding network resources allocated.
3. Procedures
This section describes the procedures of provisioning an enhanced VPN
service based on segment routing with resource allocation.
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According to the requirement of an enhanced VPN service, the network
controller calculates a sub-topology of the underlay network to
support this enhanced VPN. Within the sub-topology, the network
resources needed on each network element can also be determined. The
network resources are allocated in a per-segment manner, and are
associated with different node-SIDs and adj-SIDs. The group of the
node-SIDs and adj-SIDs allocated for the enhanced VPN will be used by
network nodes and the network controller to build a SR virtual
topology, which is used as the virtual underlay of the enhanced VPN
service. The extensions to IGP protocol to distribute the SIDs
associated with resources allocated for a virtual network topology is
specified in [I-D.dong-lsr-sr-enhanced-vpn].
Assume that customer requests for an enhanced VPN service from the
network operator. The fundamental requirement is that customer A's
service does not experience interference from other service in the
network, such as other VPN services, or the non-VPN services in the
network. The detailed requirements can be described using following
characteristics:
o Service topology: the service sites and the connectivity between
them
o Service bandwidth: the bandwidth requirement between service sites
o Isolation: the level of isolation from other services in the
network
o Reliability: whether fast repair or end-to-end protection is
needed or not.
o Latency and jitter constraints.
3.1. Topology and Resource Computation
It is assumed that a centralized network controller is responsible
for the provisioning of enhanced VPNs. The controller needs to
collect the information of network connectivity, network resources,
network performance and other relevant network state of the underlay
network. This can be done using either the IGP [RFC5305] [RFC3630]
[RFC7471] [RFC7810] or BGP-LS [RFC7752] [I-D.ietf-idr-te-pm-bgp].
Based on the network information collected from the underlay network,
the controller can compute the best way to meet the requirements of a
new tenant whilst maintaining the needs of the existing tenants that
are using the same network. The output of the computation is a sub-
topology of the underlay network, along with the network resources
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need to be allocated on each network element (e.g. links and nodes)
in the sub-topology.
3.2. Network Resource and SID Allocation
According to the computation result of section 3.1, the network
controller instructs the network devices involved in the sub-topology
of the enhanced VPN to allocate the required network resources. This
can be done with either PCEP [RFC5440] or Netconf/YANG [RFC6241]
[RFC7950] with necessary extensions. The network resources are
allocated in a per-segment manner. In addition, dedicated segment
identifiers, e.g. node-SIDs and adj-SIDs are also allocated to
represent the network resources allocated for each enhanced VPN on
each network segment.
Node-SIDs: Node-SIDs:
r:101 r:102
g:201 Adj-SIDs: g:202
b:301 r:1001:1G r:1001:1G b:302
+-----+ g:2001:2G g:2001:2G +-----+
| A | b:3001:1G b:3001:1G | B |Adj-SIDs:
| +------------------------+ + r:1003:1G
Adj-SIDs +--+--+ +--+--+\g:2003:2G
r:1002:1G| r:1002:1G| \
g:2002:2G| g:2002:2G| \ r:1001:1G
b:3002:3G| b:3002:2G| \g:2001:2G
| | \ +-----+ Node-SIDs:
| | \+ E | r:105
| | /+ | g:205
r:1001:1G| r:1002:1G| / +-----+
g:2001:2G| g:2002:2G| /r:1002:1G
b:3001:3G| b:3002:2G| / g:2002:2G
+--+--+ +--+--+ /
| | | |/r:1003:1G
| C +------------------------+ D + g:2003:2G
+-----+ r:1002:1G r:1001:1G +-----+
Node-SIDs: g:2002:1G g:2001:1G Node-SIDs:
r:103 b:3002:2G b:3001:2G r:104
g:203 g:204
b:303 b:304
Figure 1. SID identify resources allocated for different virtual networks
Figure 1 shows a network fragment of enhanced VPN supported by SR.
In this example, there are three virtual topologies created for
enhanced VPNs red (r) , green (g) and blue (b). The red and green
topologies consist of nodes A, B, C, D, and E with all their
interconnecting links, whilst the blue topology only consists of
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nodes A, B, C and D with all their interconnecting links. Each node
allocates a dedicated adjacency SID for each link participating in a
particular topology. Each node is also allocated with a dedicated
node SID for each topology it participates in. The adj-SIDs are
associated with the link resources (e.g. bandwidth) allocated for
each topology, so that the adj-SIDs can be used to steer service of
different enhanced VPNs into different set of reserved resources in
the data plane. The node-SIDs can be associated with dedicated nodal
resources allocated for each topology. In addition, the node-SIDs of
different topologies can be used to build loose SR path within each
virtual topology, and steer service of different enhanced VPNs into
the different set of reserved resources in the data plane.
In Figure 1, the notation x:nnnn:y that in topology colour x, the
adj-SID nnnn will steer the packet over that link which has a total
bandwidth of y assigned to that topology. Thus the note r:1002:1G in
link C->D says that the red topology over link C->D has a reserved
bandwidth of 1Gb/s and will used by a packet arriving at node C with
an adj-SID 1002 at the top of the label stack.
3.3. Construction of SR Virtual Topology
Each network node SHOULD advertise the allocated network resources
and the associated SIDs for the enhanced VPN into the network. The
detailed mechanism and extensions to IGP are described in
[I-D.dong-lsr-sr-enhanced-vpn]. The allocated network resources and
the associated SIDs for the enhanced VPN needs to be distributed to
the network controller for state synchronization and global
optimization for each virtual topology. This can be done using
either BGP-LS [RFC7752] or Netconf/YANG [RFC6241][RFC7950] with
necessary extensions. With the collected network resource and SIDs
information, the controller and network nodes are able to construct
the SR virtual topologies using the node-SIDs and adj-SIDs allocated
for enhanced VPNs. Unlike classical segment routing in which network
resources are shared by all services and customers, the SR virtual
topologies map to dedicated resource allocated in the underlay, so
that they can be used to meet the service requirement of enhanced VPN
and provide the required isolation from other services in the same
network.
Figure 2 shows the virtual SR topologies created from the underlay
network in Figure 1.
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1001 1001 2001 2001 3001 3001
101---------102 201---------202 301---------302
| | \1003 | | \2003 | |
1002| 1002| \ 1001 2002| 2002| \ 2001 3002| 3002|
| | 105 | | 205 | |
1001| 1002| / 1002 2001| 2002| / 2002 3001| 3002|
| | / 1003 | | / 2003 | |
103---------104 203---------204 303---------304
1002 1001 1002 2001 3002 3001
Topology Red Topology Green Topology Blue
Figure 2. SR virtual topologies using different groups of SIDs
3.4. VPN Service to SR Virtual Topology Mapping
The services of an enhanced VPN customer would be provisioned using
the customized SR virtual topology as the underlay. This ensures
that services of different enhanced VPNs will only use the network
resources allocated and will not interfere with each other. For each
enhanced VPN customer, the service paths can be customized for
different services within the SR virtual topology, and the allocated
network resources are shared by different services of the same
enhanced VPN customer.
For example, to create a strict path along the path A-B-D-E in the
red topology in Figure 2, the SR label stack imposed to the service
packet would be (1001, 1002, 1003). For the same strict path in
green topology, the SR label stack would be (2001, 2002, 2003). In
the case where we wish to construct a loose path A-D-E in the green
topology, the service packet SHOULD be imposed with the SR label
stack (201, 204, 205). At node A the packet is sent towards D via
either node B or C using the link and node resources allocated for
the green topology. At node D the packet is forwarded to E using the
link and node resource allocated for the green topology. Similarly,
a packet for the loose path A-D-E in the red topology would arrive at
node A with the SID stack (101, 104, 105).
4. Benefits of the Proposed Mechanism
Compared with existing mechanisms, The proposed mechanism described
in this document provides several key characteristics:
o Flexibility and scalability
o Lower state maintenance overhead and fewer protocols types
o Better isolation and performance than Classic SR due to allocation
of resources in the underlay to specific services.
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This SR based mechanism can provide the required isolation between
different enhanced VPNs, without introducing per-path state into the
network. For each enhanced VPN, the resource allocation is done in a
per-segment manner, which aligns with the segment routing paradigm.
This provides better scalability compared to RSVP-TE based per-flow
resource reservation.
In addition to isolation, the SR based mechanism also allows resource
sharing between different services of the same enhanced VPN customer.
This gives the customer more flexibility and control in service
planning and provisioning, the experience would be similar to using a
dedicated private network. The performance of critical services
flows in a particular enhanced VPN can be further ensured using the
mechanisms defined in [DetNet].
The detailed comparison with other candidates technologies are given
in the following sections.
4.1. MPLS-TP
MPLS-TP could be enhanced to include the allocation of specific
resources along the path to a specific LSP. This would require that
the SDN system set up and maintain every resource at every path for
every customer, and map this to the LSP in the data plane, hence at
every hop unique LSP label is needed for each path. Whilst this
would be a way to produce a proof of concept for network slicing of
an MPLS underlay, delegation would be difficult, resulting in a high
overhead and high touch system. This leads to scaling concerns. The
number of labels needed at any node would be the total number of
services passing through that node. Experience with early pseudowire
designs shows that this can lead to scaling issues.
4.2. RSVP-TE
RSVP-TE would have the same scaling concern as MPLS-TP in terms of
the number of LSPs that need to be maintained being equal to the
number of service passing through any given node. Additionally it
would have the two RSVP disadvantages that basic SR seeks to address:
o The use of additional protocol (RSVP) for path establishment in
addition to the routing protocol used to discover the topology and
the network resources.
o The overhead of the soft-state maintenance associated with RSVP.
The impact of this overhead would be exacerbated by the increased
number of end to end paths requiring state maintenance.
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4.3. Classic SR
Classic SR minimizes the number of control protocols compared to
RSVP-TE to just the routing protocol. It also attempts to minimize
the core state by pushing state into the packet, although limited
number of binding SIDs may be required to overcome the limitations in
the ability of some nodes to push large label stacks. However,
classic SR does not have any resource allocation and identification
mechanism below the level of link, and none at node level, which
restricts the extent to which some particular tenant traffic can be
isolated from other traffic in the network.
4.4. SR with Resource Allocation
The approach described in this document seeks to achieve a compromise
between the state limitations of classic TE system and the lack of
resource awareness in classic SR.
Specifically, by segmenting the path and allocating network resources
to each element of the virtual network topologies, the operator can
choose the granularity of resource to path binding within a virtual
topology. In network segments where resource is scarce such that the
service requirement cannot be delivered, the SR approach is able to
allocate specific resources to a particular service. By contrast, in
other parts of the network where resource is plentiful, the resource
may be shared by a number of services. The decision to do this is in
the hands of the operator. Because of the segmented nature of the
path, resource aggregation is possible in a way that is not possible
with RSVP-TE and MPLS-TP due to the use of dedicated label to
identify each end-to-end path.
5. Service Assurance
In order to provide service assurance it is necessary to instrument
the network at multiple levels. The network operator needs to
ascertain that the underlay is operating correctly. A tenant needs
to ascertain that their services are correctly operating. In
principle these can use existing techniques. These are well known
problems and solutions either exist or are in development to address
them.
However new work is needed to instrument the virtual network
topologies that are created. Such instrumentation needs to operate
without causing disruption to other services using the network.
Given the sensitivity of some applications care needs to be taken to
ensure that the instrumentation itself does not cause disruption
either to the service being instrumented or to another service.
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6. IANA Considerations
This document makes no request of IANA.
Note to RFC Editor: this section may be removed on publication as an
RFC.
7. Security Considerations
The normal security considerations of VPNs are applicable and it is
assumed that industry best practise is applied to an enhanced VPN.
The security considerations of segment routing are applicable and it
is assumed that these are applied to an enhanced VPN that uses SR.
Some applications of enhanced VPNs are sensitive to packet latency,
and the enhanced VPNs provisioned to carry their traffic have latency
SLAs. By disrupting the latency of such traffic an attack can be
directly targeted at the customer application, or can be targeted at
the network operator by causing them to violate their service level
agreement and thus causing them commercial consequences. Dynamic
attacks of this sort are not something that networks have
traditionally guarded against, and networking techniques need to be
developed to defend against this type of attack. By rigorously
policing ingress traffic and carefully provisioning the resources
provided to critical services this type of attack can be prevented.
However case needs to be taken where it is necessary to provide
shared resources, and when the network needs to be reconfigured as
part of ongoing maintenance or in response to a failure.
It is important that steps are taken to ensure that details of the
underlay are not exposed to third parties to minimise the possibility
that an exploit be developed as a result of exploiting a shared
resource.
8. Acknowledgements
The authors would like to thank Mach Chen, Zhenbin Li for the
discussion and suggestions to this document.
9. References
9.1. Normative References
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[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>.
[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>.
9.2. Informative References
[DetNet] "DetNet WG", 2016,
<https://datatracker.ietf.org/wg/detnet>.
[I-D.bryant-rtgwg-enhanced-vpn]
Bryant, S., Dong, J., Li, Z., and T. Miyasaka, "Enhanced
Virtual Private Networks (VPN+)", draft-bryant-rtgwg-
enhanced-vpn-02 (work in progress), March 2018.
[I-D.dong-lsr-sr-enhanced-vpn]
Dong, J. and S. Bryant, "IGP Extensions for Segment
Routing based Enhanced VPN", draft-dong-lsr-sr-enhanced-
vpn-00 (work in progress), June 2018.
[I-D.ietf-idr-te-pm-bgp]
Ginsberg, L., Previdi, S., Wu, Q., Tantsura, J., and C.
Filsfils, "BGP-LS Advertisement of IGP Traffic Engineering
Performance Metric Extensions", draft-ietf-idr-te-pm-
bgp-10 (work in progress), March 2018.
[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>.
[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>.
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[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>.
[RFC5440] Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation
Element (PCE) Communication Protocol (PCEP)", RFC 5440,
DOI 10.17487/RFC5440, March 2009,
<https://www.rfc-editor.org/info/rfc5440>.
[RFC6241] Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
and A. Bierman, Ed., "Network Configuration Protocol
(NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
<https://www.rfc-editor.org/info/rfc6241>.
[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>.
[RFC7950] Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
RFC 7950, DOI 10.17487/RFC7950, August 2016,
<https://www.rfc-editor.org/info/rfc7950>.
Authors' Addresses
Jie Dong
Huawei Technologies
Email: jie.dong@huawei.com
Stewart Bryant
Huawei Technologies
Email: stewart.bryant@gmail.com
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Zhenqiang Li
China Mobile
Email: li_zhenqiang@hotmail.com
Takuya Miyasaka
KDDI Corporation
Email: ta-miyasaka@kddi.com
Dong, et al. Expires January 3, 2019 [Page 14]
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