BESS Working Group A. Farrel
Internet-Draft Old Dog Consulting
Intended status: Standards Track J. Drake
Expires: November 20, 2021 E. Rosen
Juniper Networks
K. Patel
Arrcus, Inc.
L. Jalil
Verizon
May 19, 2021
Gateway Auto-Discovery and Route Advertisement for Segment Routing
Enabled Site Interconnection
draft-ietf-bess-datacenter-gateway-11
Abstract
Data centers are critical components of the infrastructure used by
network operators to provide services to their customers. Data
centers are attached to the Internet or a backbone network by gateway
routers. One data center typically has more than one gateway for
commercial, load balancing, and resiliency reasons.
Segment Routing is a protocol mechanism that can be used within a
data center, and also for steering traffic that flows between two
data center sites. In order that one data center site may load
balance the traffic it sends to another data center site, it needs to
know the complete set of gateway routers at the remote data center,
the points of connection from those gateways to the backbone network,
and the connectivity across the backbone network.
Other sites, such as access networks, also need to be connected
across backbone networks through gateways.
This document defines a mechanism using the BGP Tunnel Encapsulation
attribute to allow each gateway router to advertise the routes to the
prefixes reachable in the site to which it provides access, including
advertising them on behalf of each other gateway to the same site.
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
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working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on November 20, 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. Requirements Language . . . . . . . . . . . . . . . . . . . . 5
3. Site Gateway Auto-Discovery . . . . . . . . . . . . . . . . . 5
4. Relationship to BGP Link State and Egress Peer Engineering . 7
5. Advertising a Site Route Externally . . . . . . . . . . . . . 7
6. Encapsulation . . . . . . . . . . . . . . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
8. Security Considerations . . . . . . . . . . . . . . . . . . . 8
9. Manageability Considerations . . . . . . . . . . . . . . . . 9
9.1. Relationship to Route Target Constraint . . . . . . . . . 10
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
11.1. Normative References . . . . . . . . . . . . . . . . . . 10
11.2. Informative References . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
Data centers (DCs) are critical components of the infrastructure used
by network operators to provide services to their customers. DCs are
attached to the Internet or a backbone network by gateway routers
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(GWs). One DC typically has more than one GW for various reasons
including commercial preferences, load balancing, or resiliency
against connection or device failure.
Segment Routing (SR) [RFC8402] is a protocol mechanism that can be
used within a DC, and also for steering traffic that flows between
two DC sites. In order for a source DC (also known as an ingress DC)
that uses SR to load balance the flows it sends to a destination DC
(also known as an egress DC), it needs to know the complete set of
entry nodes (i.e., GWs) for that egress DC from the backbone network
connecting the two DCs. Note that it is assumed that the connected
set of DCs and the backbone network connecting them are part of the
same SR BGP Link State (LS) instance ([RFC7752] and
[I-D.ietf-idr-bgpls-segment-routing-epe]) so that traffic engineering
using SR may be used for these flows.
Other sites, such as access networks, also need to be connected
across backbone networks through gateways. For illustrative
purposes, consider the ingress and egress sites shown in Figure 1 as
separate ASes (noting that the sites could be implemented as part of
the ASes to which they are attached, or as separate ASes). The
various ASes that provide connectivity between the ingress and egress
sites could each be constructed differently and use different
technologies such as IP, MPLS with global table routing native BGP to
the edge, MPLS IP VPN, SR-MPLS IP VPN, or SRv6 IP VPN. That is, the
ingress and egress sites can be connected by tunnels across a variety
of technologies. This document describes how SR identifiers (SIDs)
are used to identify the paths between the ingress and egress sites.
The solution described in this document is agnostic as to whether the
transit ASes do or do not have SR capabilities. the solution uses SR
to stitch together path segments between GWs and through the ASBRs.
Thus, there is a requirement that the GWs and ASBRs are SR-capable.
The solution supports the SR path being extended into the ingress and
egress sites if they are SR-capable.
The solution defined in this document can be seen in the broader
context of site interconnection in
[I-D.farrel-spring-sr-domain-interconnect]. That document shows how
other existing protocol elements may be combined with the solution
defined in this document to provide a full system, but is not a
necessary reference for understanding this document.
Suppose that there are two gateways, GW1 and GW2 as shown in
Figure 1, for a given egress site and that they each advertise a
route to prefix X which is located within the egress site with each
setting itself as next hop. One might think that the GWs for X could
be inferred from the routes' next hop fields, but typically it is not
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the case that both routes get distributed across the backbone: rather
only the best route, as selected by BGP, is distributed. This
precludes load balancing flows across both GWs.
----------------- ---------------------
| Ingress | | Egress ------ |
| Site | | Site |Prefix| |
| | | | X | |
| | | ------ |
| -- | | --- --- |
| |GW| | | |GW1| |GW2| |
-------++-------- ----+-----------+-+--
| \ | / |
| \ | / |
| -+------------- --------+--------+-- |
| ||ASBR| ----| |---- |ASBR| |ASBR| | |
| | ---- |ASBR+------+ASBR| ---- ---- | |
| | ----| |---- | |
| | | | | |
| | ----| |---- | |
| | AS1 |ASBR+------+ASBR| AS2 | |
| | ----| |---- | |
| --------------- -------------------- |
--+-----------------------------------------------+--
| |ASBR| |ASBR| |
| ---- AS3 ---- |
| |
-----------------------------------------------------
Figure 1: Example Site Interconnection
The obvious solution to this problem is to use the BGP feature that
allows the advertisement of multiple paths in BGP (known as Add-
Paths) [RFC7911] to ensure that all routes to X get advertised by
BGP. However, even if this is done, the identity of the GWs will be
lost as soon as the routes get distributed through an Autonomous
System Border Router (ASBR) that will set itself to be the next hop.
And if there are multiple Autonomous Systems (ASes) in the backbone,
not only will the next hop change several times, but the Add-Paths
technique will experience scaling issues. This all means that the
Add-Paths approach is limited to sites connected over a single AS.
This document defines a solution that overcomes this limitation and
works equally well with a backbone constructed from one or more ASes
using the Tunnel Encapsulation attribute [RFC9012] as follows:
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When a GW to a given site advertises a route to a prefix X within
that site, it will include a Tunnel Encapsulation attribute that
contains the union of the Tunnel Encapsulation attributes
advertised by each of the GWs to that site, including itself.
In other words, each route advertised by a GW identifies all of the
GWs to the same site (see Section 3 for a discussion of how GWs
discover each other). I.e., the Tunnel Encapsulation attribute
advertised by each GW contains multiple Tunnel TLVs, one or more from
each active GW, and each Tunnel TLV will contain a Tunnel Egress
Endpoint Sub-TLV that identifies the GW for that Tunnel TLV.
Therefore, even if only one of the routes is distributed to other
ASes, it will not matter how many times the next hop changes, as the
Tunnel Encapsulation attribute will remain unchanged.
To put this in the context of Figure 1, GW1 and GW2 discover each
other as gateways for the egress site. Both GW1 and GW2 advertise
themselves as having routes to prefix X. Furthermore, GW1 includes a
Tunnel Encapsulation attribute which is the union of its Tunnel
Encapsulation attribute and GW2's Tunnel Encapsulation attribute.
Similarly, GW2 includes a Tunnel Encapsulation attribute which is the
union of its Tunnel Encapsulation attribute and GW1's Tunnel
Encapsulation attribute. The gateway in the ingress site can now see
all possible paths to X in the egress site regardless of which route
is propagated to it, and it can choose one, or balance traffic flows
as it sees fit.
2. 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 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Site Gateway Auto-Discovery
To allow a given site's GWs to auto-discover each other and to
coordinate their operations, the following procedures are
implemented:
o Each GW is configured with an identifier for the site. That
identifier MUST be the same across all GWs to the site (i.e., the
same identifier is used by all GWs to the same site), and MUST be
unique across all sites that are connected (i.e., across all GWs
to all sites that are interconnected).
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o A route target ([RFC4360]) MUST be attached to each GW's auto-
discovery route (defined below) and its value MUST be set to the
site identifier.
o Each GW MUST construct an import filtering rule to import any
route that carries a route target with the same site identifier
that the GW itself uses. This means that only these GWs will
import those routes, and that all GWs to the same site will import
each other's routes and will learn (auto-discover) the current set
of active GWs for the site.
The auto-discovery route that each GW advertises consists of the
following:
o An IPv4 or IPv6 Network Layer Reachability Information (NLRI)
[RFC4760] containing one of the GW's loopback addresses (that is,
with an AFI/SAFI pair that is one of IPv4/NLRI used for unicast
forwarding (1/1), IPv6/NLRI used for unicast forwarding (2/1),
IPv4/NLRI with MPLS Labels (1/4), or IPv6/NLRI with MPLS Labels
(2/4)).
o A Tunnel Encapsulation attribute [RFC9012] containing the GW's
encapsulation information encoded in one or more Tunnel TLVs.
To avoid the side effect of applying the Tunnel Encapsulation
attribute to any packet that is addressed to the GW itself, the GW
MUST use a different loopback address for packets intended for it.
As described in Section 1, each GW will include a Tunnel
Encapsulation attribute with the GW encapsulation information for
each of the site's active GWs (including itself) in every route
advertised externally to that site. As the current set of active GWs
changes (due to the addition of a new GW or the failure/removal of an
existing GW) each externally advertised route will be re-advertised
with a new Tunnel Encapsulation attribute which reflects current set
of active GWs.
If a gateway becomes disconnected from the backbone network, or if
the site operator decides to terminate the gateway's activity, it
MUST withdraw the advertisements described above. This means that
remote gateways at other sites will stop seeing advertisements from
this gateway. Note that if the routing within a site is broken (for
example, such that there is a route from one GW to another, but not
in the reverse direction), then it is possible that incoming traffic
will be routed to the wrong GW to reach the destination prefix - in
this degraded network situation, traffic may be dropped.
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Note that if a GW is (mis)configured with a different site identifier
from the other GWs to the same site then it will not be auto-
discovered by the other GWs (and will not auto-discover the other
GWs). This would result in a GW for another site receiving only the
Tunnel Encapsulation attribute included in the BGP best route; i.e.,
the Tunnel Encapsulation attribute of the (mis)configured GW or that
of the other GWs.
4. Relationship to BGP Link State and Egress Peer Engineering
When a remote GW receives a route to a prefix X, it uses the Tunnel
Egress Endpoint Sub-TLVs in the containing Tunnel Encapsulation
attribute to identify the GWs through which X can be reached. It
uses this information to compute SR Traffic Engineering (SR TE) paths
across the backbone network looking at the information advertised to
it in SR BGP Link State (BGP-LS)
[I-D.ietf-idr-bgp-ls-segment-routing-ext] and correlated using the
site identity. SR Egress Peer Engineering (EPE)
[I-D.ietf-idr-bgpls-segment-routing-epe] can be used to supplement
the information advertised in BGP-LS.
5. Advertising a Site Route Externally
When a packet destined for prefix X is sent on an SR TE path to a GW
for the site containing X (that is, the packet is sent in the ingress
site on an SR TE path that describes the whole path including those
parts that are within the egress site), it needs to carry the
receiving GW's SID for X such that this SID rises to the top of the
stack before the GW completes its processing of the packet. To
achieve this, each Tunnel TLV in the Tunnel Encapsulation attribute
contains a Prefix SID sub-TLV [RFC9012] for X. As defined in
[RFC9012], the Prefix SID sub-TLV is only for IPv4/IPV6 labelled
unicast routes, so the solution described in this document only
applies to routes of those types. If the use of the Prefix SID sub-
tlv for routes of other types is defined in the future, further
documents will be needed to describe their use.
Alternatively, if MPLS SR is in use and if the GWs for a given site
are configured to allow remote GWs to perform SR TE through that site
for a prefix X, then each GW computes an SR TE path through that site
to X from each of the currently active GWs, and places each in an
MPLS label stack sub-TLV [RFC9012] in the SR Tunnel TLV for that GW.
Please refer to Section 7 of
[I-D.farrel-spring-sr-domain-interconnect] for worked examples of how
the SID stack is constructed in this case, and how the advertisements
would work.
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6. Encapsulation
If the GWs for a given site are configured to allow remote GWs to
send them a packet in that site's native encapsulation, then each GW
will also include multiple instances of a Tunnel TLV for that native
encapsulation in externally advertised routes: one for each GW and
each containing a Tunnel Egress Endpoint sub-TLV with that GW's
address. A remote GW may then encapsulate a packet according to the
rules defined via the sub-TLVs included in each of the Tunnel TLVs.
7. IANA Considerations
IANA maintains a registry called "Border Gateway Protocol (BGP)
Parameters" with a sub-registry called "BGP Tunnel Encapsulation
Attribute Tunnel Types." The registration policy for this registry
is First-Come First-Served [RFC8126].
IANA previously assigned the value 17 from this sub-registry for "SR
Tunnel", referencing this document. IANA is now requested to mark
that assignment as deprecated. IANA may reclaim that codepoint at
such a time that the registry is depleted.
8. Security Considerations
From a protocol point of view, the mechanisms described in this
document can leverage the security mechanisms already defined for
BGP. Further discussion of security considerations for BGP may be
found in the BGP specification itself [RFC4271] and in the security
analysis for BGP [RFC4272]. The original discussion of the use of
the TCP MD5 signature option to protect BGP sessions is found in
[RFC5925], while [RFC6952] includes an analysis of BGP keying and
authentication issues.
The mechanisms described in this document involve sharing routing or
reachability information between sites: that may mean disclosing
information that is normally contained within a site. So it needs to
be understood that normal security paradigms based on the boundaries
of sites are weakened and interception of BGP messages may result in
information being disclosed to third parties. Discussion of these
issues with respect to VPNs can be found in [RFC4364], while
[RFC7926] describes many of the issues associated with the exchange
of topology or TE information between sites.
Particular exposures resulting from this work include:
o Gateways to a site will know about all other gateways to the same
site. This feature applies within a site and so is not a
substantial exposure, but it does mean that if the BGP exchanges
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within a site can be snooped or if a gateway can be subverted then
an attacker may learn the full set of gateways to a site. This
would facilitate more effective attacks on that site.
o The existence of multiple gateways to a site becomes more visible
across the backbone and even into remote sites. This means that
an attacker is able to prepare a more comprehensive attack than
exists when only the locally attached backbone network (e.g., the
AS that hosts the site) can see all of the gateways to a site.
For example, a Denial of Service attack on a single GW is
mitigated by the existence of other GWs, but if the attacker knows
about all the gateways then the whole set can be attacked at once.
o A node in a site that does not have external BGP peering (i.e., is
not really a site gateway and cannot speak BGP into the backbone
network) may be able to get itself advertised as a gateway by
letting other genuine gateways discover it (by speaking BGP to
them within the site) and so may get those genuine gateways to
advertise it as a gateway into the backbone network. This would
allow the malicious node to attract traffic without having to have
secure BGP peerings with out-of-site nodes.
o An external party intercepting BGP messages anywhere between sites
may learn information about the functioning of the sites and the
locations of end points. While this is not necessarily a
significant security or privacy risk, it is possible that the
disclosure of this information could be used by an attacker.
o If it is possible to modify a BGP message within the backbone, it
may be possible to spoof the existence of a gateway. This could
cause traffic to be attracted to a specific node and might result
in black-holing of traffic.
All of the issues in the list above could cause disruption to site
interconnection, but are not new protocol vulnerabilities so much as
new exposures of information that SHOULD be protected against using
existing protocol mechanisms such as securing the TCP sessions over
which the BGP messages flow. Furthermore, it is a general
observation that if these attacks are possible then it is highly
likely that far more significant attacks can be made on the routing
system. It should be noted that BGP peerings are not discovered, but
always arise from explicit configuration.
9. Manageability Considerations
The principal configuration item added by this solution is the
allocation of a site identifier. The same identifier MUST be
assigned to every GW to the same site, and each site MUST have a
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different identifier. This requires coordination, probably through a
central management agent.
It should be noted that BGP peerings are not discovered, but always
arise from explicit configuration. This is no different from any
other BGP operation.
9.1. Relationship to Route Target Constraint
In order to limit the VPN routing information that is maintained at a
given route reflector, [RFC4364] suggests the use of "Cooperative
Route Filtering" [RFC5291] between route reflectors. [RFC4684]
defines an extension to that mechanism to include support for
multiple autonomous systems and asymmetric VPN topologies such as
hub-and-spoke. The mechanism in RFC 4684 is known as Route Target
Constraint (RTC).
An operator would not normally configure RTC by default for any AFI/
SAFI combination, and would only enable it after careful
consideration. When using the mechanisms defined in this document,
the operator should consider carefully the effects of filtering
routes. In some cases this may be desirable, and in others it could
limit the effectiveness of the procedures.
10. Acknowledgements
Thanks to Bruno Rijsman, Stephane Litkowski, Boris Hassanov, Linda
Dunbar, Ravi Singh, and Daniel Migault for review comments, and to
Robert Raszuk for useful discussions. Gyan Mishra provided a helpful
GenArt review, and John Scudder made helpful comments during IESG
review.
11. References
11.1. Normative References
[I-D.ietf-idr-bgpls-segment-routing-epe]
Previdi, S., Talaulikar, K., Filsfils, C., Patel, K., Ray,
S., and J. Dong, "BGP-LS extensions for Segment Routing
BGP Egress Peer Engineering", draft-ietf-idr-bgpls-
segment-routing-epe-19 (work in progress), May 2019.
[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>.
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[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006,
<https://www.rfc-editor.org/info/rfc4271>.
[RFC4360] Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended
Communities Attribute", RFC 4360, DOI 10.17487/RFC4360,
February 2006, <https://www.rfc-editor.org/info/rfc4360>.
[RFC4760] Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
"Multiprotocol Extensions for BGP-4", RFC 4760,
DOI 10.17487/RFC4760, January 2007,
<https://www.rfc-editor.org/info/rfc4760>.
[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
June 2010, <https://www.rfc-editor.org/info/rfc5925>.
[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>.
[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>.
[RFC9012] Patel, K., Van de Velde, G., Sangli, S., and J. Scudder,
"The BGP Tunnel Encapsulation Attribute", RFC 9012,
DOI 10.17487/RFC9012, April 2021,
<https://www.rfc-editor.org/info/rfc9012>.
11.2. Informative References
[I-D.farrel-spring-sr-domain-interconnect]
Farrel, A. and J. Drake, "Interconnection of Segment
Routing Domains - Problem Statement and Solution
Landscape", draft-farrel-spring-sr-domain-interconnect-05
(work in progress), October 2018.
[I-D.ietf-idr-bgp-ls-segment-routing-ext]
Previdi, S., Talaulikar, K., Filsfils, C., Gredler, H.,
and M. Chen, "BGP Link-State extensions for Segment
Routing", draft-ietf-idr-bgp-ls-segment-routing-ext-18
(work in progress), April 2021.
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[RFC4272] Murphy, S., "BGP Security Vulnerabilities Analysis",
RFC 4272, DOI 10.17487/RFC4272, January 2006,
<https://www.rfc-editor.org/info/rfc4272>.
[RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February
2006, <https://www.rfc-editor.org/info/rfc4364>.
[RFC4684] Marques, P., Bonica, R., Fang, L., Martini, L., Raszuk,
R., Patel, K., and J. Guichard, "Constrained Route
Distribution for Border Gateway Protocol/MultiProtocol
Label Switching (BGP/MPLS) Internet Protocol (IP) Virtual
Private Networks (VPNs)", RFC 4684, DOI 10.17487/RFC4684,
November 2006, <https://www.rfc-editor.org/info/rfc4684>.
[RFC5291] Chen, E. and Y. Rekhter, "Outbound Route Filtering
Capability for BGP-4", RFC 5291, DOI 10.17487/RFC5291,
August 2008, <https://www.rfc-editor.org/info/rfc5291>.
[RFC6952] Jethanandani, M., Patel, K., and L. Zheng, "Analysis of
BGP, LDP, PCEP, and MSDP Issues According to the Keying
and Authentication for Routing Protocols (KARP) Design
Guide", RFC 6952, DOI 10.17487/RFC6952, May 2013,
<https://www.rfc-editor.org/info/rfc6952>.
[RFC7911] Walton, D., Retana, A., Chen, E., and J. Scudder,
"Advertisement of Multiple Paths in BGP", RFC 7911,
DOI 10.17487/RFC7911, July 2016,
<https://www.rfc-editor.org/info/rfc7911>.
[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>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[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>.
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Authors' Addresses
Adrian Farrel
Old Dog Consulting
Email: adrian@olddog.co.uk
John Drake
Juniper Networks
Email: jdrake@juniper.net
Eric Rosen
Juniper Networks
Email: erosen52@gmail.com
Keyur Patel
Arrcus, Inc.
Email: keyur@arrcus.com
Luay Jalil
Verizon
Email: luay.jalil@verizon.com
Farrel, et al. Expires November 20, 2021 [Page 13]