Internet DRAFT - draft-saad-sr-fa-link
draft-saad-sr-fa-link
SPRING Working Group T. Saad
Internet-Draft V. Beeram
Intended status: Informational C. Barth
Expires: August 19, 2021 Juniper Networks, Inc.
S. Sivabalan
Ciena Corporation.
February 15, 2021
Segment-Routing over Forwarding Adjacency Links
draft-saad-sr-fa-link-03
Abstract
Label Switched Paths (LSPs) set up in Multiprotocol Label Switching
(MPLS) networks can be used to form Forwarding Adjacency (FA) links
that carry traffic in those networks. An FA link can be assigned
Traffic Engineering (TE) parameters that allow other LSR(s) to
include it in their constrained path computation. FA link(s) can be
also assigned Segment-Routing (SR) segments that enable the steering
of traffic on to the associated FA link(s). The TE and SR attributes
of an FA link can be advertised using known protocols that carry link
state information. This document elaborates on the usage of FA
link(s) and their attributes in SR enabled networks.
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 August 19, 2021.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Forwarding Adjacency Links . . . . . . . . . . . . . . . . . 3
3.1. Creation and Management . . . . . . . . . . . . . . . . . 4
3.2. Link Flooding . . . . . . . . . . . . . . . . . . . . . . 4
3.3. Underlay LSP(s) . . . . . . . . . . . . . . . . . . . . . 5
3.4. State Changes . . . . . . . . . . . . . . . . . . . . . . 5
3.5. TE Parameters . . . . . . . . . . . . . . . . . . . . . . 5
3.6. Link Local and Remote Identifiers . . . . . . . . . . . . 6
4. Segment-Routing over FA Links . . . . . . . . . . . . . . . . 6
4.1. SR IGP Segments for FA . . . . . . . . . . . . . . . . . 7
4.1.1. Parallel Adjacencies . . . . . . . . . . . . . . . . 7
4.2. SR BGP Segments for FA . . . . . . . . . . . . . . . . . 7
4.3. Applicability to Interdomain . . . . . . . . . . . . . . 8
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
7. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 9
8. Normative References . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
To improve scalability in Multi-Protocol Label Switching (MPLS)
networks, it may be useful to create a hierarchy of LSPs as
Forwarding Adjacencies (FA). The concept of FA link(s) and FA-LSP(s)
was introduced in [RFC4206].
In Segment-Routing (SR), this is particularly useful for two main
reasons.
First, it allows the stitching of sub-path(s) so as to realize an
end-to-end SR path. Each sub-path can be represented by a FA link
that is supported by one or more underlying LSP(s). The underlying
LSP(s) that support an FA link can be setup using different
technologies- including RSVP-TE, LDP, and SR. The sub-path(s), or FA
link(s) in this case, can possibly interconnect multiple
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administrative domains, allowing each FA link within a domain to use
a different technology to setup the underlying LSP(s).
Second, it allows shortening of a large SR Segment-List by
compressing one or more slice(s) of the list into a corresponding FA
TE link that each can be represented by a single segment- see
Section 4. Effectively, it reduces the number of segments that an
ingress router has to impose to realize an end-to-end path.
The FA links are treated as normal link(s) in the network and hence
it can leverage existing link state protocol extensions to advertise
properties associated with the FA link. For example, Traffic-
Engineering (TE) link parameters and Segment-Routing (SR) segments
parameters can be associated with the FA link and advertised
throughout the network.
Once advertised in the network using a suitable protocols that
support carrying link state information, such as OSPF, ISIS or BGP
Link State (LS)), other LSR(s) in the network can use the FA TE
link(s) as well as possibly other normal TE link(s) when performing
path computation and/or when specifying the desired explicit path.
Though the concepts discussed in this document are specific to MPLS
technology, these are also extensible to other dataplane technologies
- e.g. SRv6.
2. Terminology
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. Forwarding Adjacency Links
FA Link(s) can be created and supported by underlying FA LSPs. The
FA link is of type point-to-point. FA links may be represented as
either unnumbered or numbered. The nodes connected by an FA link do
not usually establish a routing adjacency over the FA link. When FA
links are numbered with IPv4 addresses, the local and remote IPv4
addresses can come out of a /31 that is allocated by the LSR that
originates the FA-LSP. For unnumbered FA link(s), other provisions
may exist to exchange link identifier(s) between the endpoints of the
FA.
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3.1. Creation and Management
In general, the creation/termination of an FA link and its FA-LSP is
driven either via configuration on the LSR at the head-end of the
adjacency, or dynamically using suitable North Bound Interface (NBI)
protocol, e.g. Netconf, gRPC, PCEP, etc.
The following FA-LSP attributes may be configured, including:
bandwidth and resource colors, and other constraints. The path taken
by the FA-LSP may be either computed by the LSR at the head-end of
the FA-LSP, or externally by a PCE and furnished to the headend.
The attributes of the FA link can be inherited from the underlying
LSP(s) that induced its creation. In general, for dynamically
provisioned FAs, a policy-based mechanism may be needed to associate
link attributes to those of the FA-LSPs.
When the FA link is supported by bidirectional FA LSP(s), a pair of
FA link(s) are advertised from each endpoint of the FA. These are
usually referred to as symmetrical link(s).
3.2. Link Flooding
Multiple protocols exist that can exchange link state information in
the network. For example, when advertising TE link(s) and their
attribute(s) using OSPF and ISIS protocols, the respective extensions
are defined in [RFC3630] and [RFC5305]. Also, when exchanging such
information in BGP protocol, extensions for BGP link state are
defined in [RFC7752] and [RFC8571]. The same protocol encodings can
be used to advertise FA(s) as TE link(s). As a result, the FA TE
link(s) and other normal TE link(s) will appear in the TE link state
database of any LSR in the network, and can be used for computing
end-to-end TE path(s).
When IGP protocols are used to advertise link state information about
FA links, the FA link(s) can appear in both the TE topology, as well
as the IGP topology. The use of FA link in the IGP topology may
result in undesirable routing loops. A router SHOULD leverage
exisitng mechanisms to exclude the FA link from the IGP Shortest Path
First (SPF) computations, and to restrict its use within the TE
topology for traffic engineered paths computation.
For example, when using ISIS to carry FA link state information,
[RFC5305] section 3 describes a way to restrict the link to the TE
topology by setting the IGP link metric to maximum (2^24 - 1).
Alternatively, when using OSPF, the FA link(s) can be advertised
using TE Opaque LSA(s) only, and hence, strictly show up in the TE
topology as described in [RFC3630] .
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3.3. Underlay LSP(s)
The LSR that hosts an FA link can setup the underlying LSP(s) using
different technologies - e.g. RSVP-TE, LDP, and SR.
The FA link can be supported by one or more underlay LSP(s) that
terminate on the same remote endpoint. The underlay path(s) can be
setup using different signaling technologies, e.g. using RSVP-TE,
LDP, SR, etc. When multiple LSP(s) support the same FA link, the
attributes of the FA link can be derived from the aggregate
properties of each of the underlying LSP(s).
3.4. State Changes
The state of an FA TE link reflects the state of the underlying LSP
path that supports it. The TE link is assumed operational and is
advertised as long as the underlying LSP path is valid. When all
underlying LSP paths are invalidated, the FA TE link advertisement is
withdrawn.
3.5. TE Parameters
The TE metrics and TE attributes are used by path computation
algorithms to select the TE link(s) that a TE path traverses. When
advertising an FA link in OSPF or ISIS, or BGP-LS, the following TE
parameters are defined:
TE Path metrics: the FA link advertisement can include information
about TE, IGP, and other performance metrics (e.g. delay, and
loss). The FA link TE metrics, in this case, can be derived from
the underlying path(s) that support the FA link by producing the
path accumulative metrics. When multiple LSP(s) support the same
FA link, then the higher accumulative metric amongst the LSP(s) is
inherited by the FA link.
Resource Class/Color: An FA link can be assigned (e.g. via
configuration) a specific set of admin-groups. Alternatively, in
some cases, this can be derived from the underlying path affinity
- for example, the underlying path strictly includes a specific
admin-group.
SRLGs: An FA advertisement could contain the information about the
Shared Risk Link Groups (SRLG) for the path taken by the FA LSP
associated with that FA. This information may be used for path
calculation by other LSRs. The information carried is the union
of the SRLGs of the underlying TE links that make up the FA LSP
path. It is possible that the underlying path information might
change over time, via configuration updates or dynamic route
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modifications, resulting in the change of the union of SRLGs for
the FA link. If multiple LSP(s) support the same FA link, then it
is expected all LSP(s) have the same SRLG union - note, that the
exact paths need not be the same.
It is worth noting, that topology changes in the network may affect
the FA link underlying LSP path(s), and hence, can dynamically change
the TE metrics and TE attributes of the FA links.
3.6. Link Local and Remote Identifiers
It is possible for the FA link to be numbered or unnumbered.
[RFC4206] describes a procedure for identifying a numbered FA TE link
using IPv4 addresses.
For unnumbered FA link(s), the assignment and handling of the local
and remote link identifiers is specified in [RFC3477]. The LSR at
each end of the unnumbered FA link assigns an identifier to that
link. This identifier is a non-zero 32-bit number that is unique
within the scope of the LSR that assigns it. There is no a priori
relationship between the identifiers assigned to a link by the LSRs
at each end of that link.
The FA link is a unidirectional and point-to-point link. Hence, the
combination of link local identifier and advertising node can
uniquely identify the link in the TED. In some cases, however, it is
desirable to associate the forward and reverse FA links in the TED.
In this case, the combination of link local and remote identifier can
identify the pair of forward and reverse FA link(s). The LSRs at the
two end points of an unnumbered link can exchange with each other the
identifiers they assign to the link. Exchanging the identifiers may
be accomplished by configuration, or by means of protocol extensions.
For example, when the FA link is established over RSVP-TE FA LSP(s),
then RSVP extensions have been introduced to exchange the FA link
identifier in [RFC3477]. Other protocol extensions pertaining to
specific link state protocols, and LSP setup technologies will be
discussed in a separate document.
If the link remote identifier is unknown, the value advertised is set
to 0 [RFC5307].
4. Segment-Routing over FA Links
The Segment Routing (SR) architecture [RFC4206] describes that an IGP
adjacency can be formed over a FA link - in which the remote node of
an IGP adjacency is a non-adjacent IGP neighbor.
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In Segment-Routing (SR), the adjacency that is established over a
link can be assigned an SR Segment [RFC8402]. For example, the Adj-
SID allows to strictly steer traffic on to the specific adjacency
that is associated with the Adj-SID.
4.1. SR IGP Segments for FA
Extensions have been defined to ISIS [RFC8667] and OSPF [RFC8665] in
order to advertise the the Adjacency-SID associated with a specific
IGP adjacency. The same extensions apply to adjacencies over FA
link. A node can bind an Adj-SID to an FA data-link. The Adj-SID
dictates the forwarding of packets through the specific FA link or FA
link(s) identified by the Adj-SID, regardless of its IGP/SPF cost.
When the FA link Adj-SID is supported by a single underlying LSP that
is associated with a binding label or SID, the same binding label can
be used for the FA link Adj-SID. For example, if the FA link is
supported by an SR Policy that is assigned a Binding SID B, the Adj-
SID of the FA link can be assigned the same Binding SID B.
When the FA link Adj-SID is supported by multiple underlying LSP(s)
or SR Policies - each having its own Binding label or SID, an
independent FA link Adj-SID is allocated and bound to the multiple
underlying LSP(s).
4.1.1. Parallel Adjacencies
Adj-SIDs can also be used in order to represent a set of parallel FA
link(s) between two endpoints.
When parallel FA links are associated with the same Adj-SID, a
"weight" factor can be assigned to each link and advertised with the
Adj-SID advertised with each FA link. The weight informs the ingress
(or an SDN/orchestration system) about the load-balancing factor over
the parallel adjacencies.
4.2. SR BGP Segments for FA
BGP segments are allocated and distributed by BGP. The SR
architecture [RFC8402] defines three types of BGP segments for Egress
Peer Engineering (EPE): PeerNode SID, PeerAdj SID, and PeerSet SID.
The applicability of each of the three types to FA links is discussed
below:
o PeerNode SID: a BGP PeerNode segment/SID is a local segment. At
the BGP node advertising, the forwarding semantics are:
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* SR operation: NEXT.
* Next-Hop: forward over any FA link associated with the segment
that terminates on remote endpoint.
o PeerAdj SID: a BGP PeerAdj segment/SID is a local segment. At the
BGP node advertising it, the forwarding semantics are:
* SR operation: NEXT.
* Next-Hop: forward over the specific FA link to the remote
endpoint to which the segment is related.
o PeerSet SID: a BGP PeerSet segment/SID is a local segment. At the
BGP node advertising it, the semantics are:
* SR operation: NEXT.
* Next-Hop: load-balance across any of the FA links to any remote
endpoint in the related set. The group definition is a policy
set by the operator.
4.3. Applicability to Interdomain
In order to determine the potential to establish a TE path through a
series of interconnected domains or multi-domain network, it is
necessary to have available a certain amount of TE information about
each network domain. This need not be the full set of TE information
available within each network but does need to express the potential
of providing such TE connectivity.
Topology abstraction is described in [RFC7926]. Abstraction allows
applying a policy to the available TE information within a domain so
to produce selective information that represents the potential
ability to connect across the domain. Thus, abstraction does not
necessarily offer all possible connectivity options, but presents a
general view of potential connectivity according to the policies that
determine how the domain's administrator wants to allow the domain
resources to be used.
Hence, the domain may be constructed as a mesh of border node to
border node TE FA links. When computing a path for an LSP that
crosses the domain, a computation point can see which domain entry
points can be connected to which others, and with what TE attributes.
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5. IANA Considerations
This document has no IANA actions.
6. Security Considerations
TBD.
7. Acknowledgement
The authors would like to thank Peter Psenak for reviewing and
providing valuable feedback on this document.
8. Normative References
[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>.
[RFC3477] Kompella, K. and Y. Rekhter, "Signalling Unnumbered Links
in Resource ReSerVation Protocol - Traffic Engineering
(RSVP-TE)", RFC 3477, DOI 10.17487/RFC3477, January 2003,
<https://www.rfc-editor.org/info/rfc3477>.
[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>.
[RFC4206] Kompella, K. and Y. Rekhter, "Label Switched Paths (LSP)
Hierarchy with Generalized Multi-Protocol Label Switching
(GMPLS) Traffic Engineering (TE)", RFC 4206,
DOI 10.17487/RFC4206, October 2005,
<https://www.rfc-editor.org/info/rfc4206>.
[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>.
[RFC5307] Kompella, K., Ed. and Y. Rekhter, Ed., "IS-IS Extensions
in Support of Generalized Multi-Protocol Label Switching
(GMPLS)", RFC 5307, DOI 10.17487/RFC5307, October 2008,
<https://www.rfc-editor.org/info/rfc5307>.
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[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>.
[RFC7926] Farrel, A., Ed., Drake, J., Bitar, N., Swallow, G.,
Ceccarelli, D., and X. Zhang, "Problem Statement and
Architecture for Information Exchange between
Interconnected Traffic-Engineered Networks", BCP 206,
RFC 7926, DOI 10.17487/RFC7926, July 2016,
<https://www.rfc-editor.org/info/rfc7926>.
[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>.
[RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
July 2018, <https://www.rfc-editor.org/info/rfc8402>.
[RFC8571] Ginsberg, L., Ed., Previdi, S., Wu, Q., Tantsura, J., and
C. Filsfils, "BGP - Link State (BGP-LS) Advertisement of
IGP Traffic Engineering Performance Metric Extensions",
RFC 8571, DOI 10.17487/RFC8571, March 2019,
<https://www.rfc-editor.org/info/rfc8571>.
[RFC8665] Psenak, P., Ed., Previdi, S., Ed., Filsfils, C., Gredler,
H., Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
Extensions for Segment Routing", RFC 8665,
DOI 10.17487/RFC8665, December 2019,
<https://www.rfc-editor.org/info/rfc8665>.
[RFC8667] Previdi, S., Ed., Ginsberg, L., Ed., Filsfils, C.,
Bashandy, A., Gredler, H., and B. Decraene, "IS-IS
Extensions for Segment Routing", RFC 8667,
DOI 10.17487/RFC8667, December 2019,
<https://www.rfc-editor.org/info/rfc8667>.
Authors' Addresses
Tarek Saad
Juniper Networks, Inc.
Email: tsaad@juniper.net
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Vishnu Pavan Beeram
Juniper Networks, Inc.
Email: vbeeram@juniper.net
Colby Barth
Juniper Networks, Inc.
Email: cbarth@juniper.net
Siva Sivabalan
Ciena Corporation.
Email: ssivabal@ciena.com
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