BESS Working Group A. Farrel
Internet-Draft Old Dog Consulting
Intended status: Standards Track J. Drake
Expires: October 17, 2021 E. Rosen
Juniper Networks
K. Patel
Arrcus, Inc.
L. Jalil
Verizon
April 15, 2021
Gateway Auto-Discovery and Route Advertisement for Segment Routing
Enabled Domain Interconnection
draft-ietf-bess-datacenter-gateway-10
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.
Segment Routing may also be operated in other domains, such as access
networks. Those domains 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 in the Segment Routing domains to which it provides access,
and also to advertise on behalf of each other gateway to the same
Segment Routing domain.
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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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 . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 5
3. SR Domain Gateway Auto-Discovery . . . . . . . . . . . . . . 5
4. Relationship to BGP Link State and Egress Peer Engineering . 7
5. Advertising an SR Domain Route Externally . . . . . . . . . . 7
6. Encapsulation . . . . . . . . . . . . . . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
8. Security Considerations . . . . . . . . . . . . . . . . . . . 8
9. Manageability Considerations . . . . . . . . . . . . . . . . 9
9.1. Relationship to Route Target Constraint . . . . . . . . . 9
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
11.1. Normative References . . . . . . . . . . . . . . . . . . 10
11.2. Informative References . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
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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
(GWs). One DC typically has more than one GW for various reasons
including commercial preferences, load balancing, or resiliency
against connection of 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 (ingress) DC that uses SR to
load balance the flows it sends to a destination (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.
SR may also be operated in other domains, such as access networks.
Those domains also need to be connected across backbone networks
through gateways. For illustrative purposes, consider the Ingress
and Egress SR Domains shown in Figure 1 as separate ASes. The
various ASes that provide connectivity between the Ingress and Egress
Domains 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.
Suppose that there are two gateways, GW1 and GW2 as shown in
Figure 1, for a given egress SR domain and that they each advertise a
route to prefix X which is located within the egress SR domain 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 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.
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----------------- ---------------------
| Ingress | | Egress ------ |
| SR Domain | | SR Domain |Prefix| |
| | | | X | |
| | | ------ |
| -- | | --- --- |
| |GW| | | |GW1| |GW2| |
-------++-------- ----+-----------+-+--
| \ | / |
| \ | / |
| -+------------- --------+--------+-- |
| ||ASBR| ----| |---- |ASBR| |ASBR| | |
| | ---- |ASBR+------+ASBR| ---- ---- | |
| | ----| |---- | |
| | | | | |
| | ----| |---- | |
| | AS1 |ASBR+------+ASBR| AS2 | |
| | ----| |---- | |
| --------------- -------------------- |
--+-----------------------------------------------+--
| |ASBR| |ASBR| |
| ---- AS3 ---- |
| |
-----------------------------------------------------
Figure 1: Example Segment Routing Domain 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 SR domains 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 [I-D.ietf-idr-tunnel-encaps]
as follows:
When a GW to a given SR domain advertises a route to a prefix X
within that SR domain, it will include a Tunnel Encapsulation
attribute that contains the union of the Tunnel Encapsulation
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attributes advertised by each of the GWs to that SR domain,
including itself.
In other words, each route advertised by a GW identifies all of the
GWs to the same SR domain (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 SR domain. 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 SR domain
can now see all possible paths to X in the egress SR domain
regardless of which route is propagated to it, and it can choose one,
or balance traffic flows as it sees fit.
The solution defined in this document can be seen in the broader
context of SR domain 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.
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. SR Domain Gateway Auto-Discovery
To allow a given SR domain'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 SR domain. That
identifier is common across all GWs to the domain (i.e., the same
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identifier is used by all GWs to the same SR domain), and unique
across all SR domains that are connected (i.e., across all GWs to
all SR domains that are interconnected).
o A route target ([RFC4360]) is attached to each GW's auto-discovery
route and has its value set to the SR domain identifier.
o Each GW constructs an import filtering rule to import any route
that carries a route target with the same SR domain identifier
that the GW itself uses. This means that only these GWs will
import those routes, and that all GWs to the same SR domain will
import each other's routes and will learn (auto-discover) the
current set of active GWs for the SR domain.
The auto-discovery route that each GW advertises consists of the
following:
o An IPv4 or IPv6 NLRI containing one of the GW's loopback addresses
(that is, with an AFI/SAFI pair that is one of 1/1, 2/1, 1/4, or
2/4).
o A Tunnel Encapsulation attribute [I-D.ietf-idr-tunnel-encaps]
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
SHOULD 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 SR domain's active GWs (including itself) in every route
advertised externally to that SR domain. 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 SR domain operator decides to terminate the gateway's activity,
it withdraws the advertisements described above. This means that
remote gateways at other sites will stop seeing advertisements from
this gateway.
Note that if a GW is (mis)configured with a different SR domain
identifier from the other GWs to the same domain 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 SR domain
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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 SR
domain 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 an SR Domain Route Externally
When a packet destined for prefix X is sent on an SR TE path to a GW
for the SR domain containing X (that is, the packet is sent in the
Ingress Domain on an SR TE path that describes the path including
within the Egress Domain), it needs to carry the receiving GW's label
for X such that this label 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 [I-D.ietf-idr-tunnel-encaps] for X.
Alternatively, if the GWs for a given SR domain are configured to
allow remote GWs to perform SR TE through that SR domain for a prefix
X, then each GW computes an SR TE path through that SR domain to X
from each of the currently active GWs, and places each in an MPLS
label stack sub-TLV [I-D.ietf-idr-tunnel-encaps] 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 label stack is constructed in this case, and how the
advertisements would work.
6. Encapsulation
If the GWs for a given SR domain are configured to allow remote GWs
to send them a packet in that SR domain'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
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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 domains: that may mean disclosing
information that is normally contained within a domain. So it needs
to be understood that normal security paradigms based on the
boundaries of domains are weakened. 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 domains.
Particular exposures resulting from this work include:
o Gateways to a domain will know about all other gateways to the
same domain. This feature applies within a domain and so is not a
substantial exposure, but it does mean that if the BGP exchanges
within a domain can be snooped or if a gateway can be subverted
then an attacker may learn the full set of gateways to a domain.
This would facilitate more effective attacks on that domain.
o The existence of multiple gateways to a domain becomes more
visible across the backbone and even into remote domains. This
means that an attacker is able to prepare a more comprehensive
attack than exists when only the locally attached backbone network
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(e.g., the AS that hosts the domain) 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 domain that does not have external BGP peering (i.e.,
is not really a domain 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 domain) 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-domain nodes.
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 domain
interconnection, but are not new protocol vulnerabilities so much as
new exposures of information that SHOULD be protected against using
existing protocol mechanisms. 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 an SR domain identifier. The same identifier MUST be
assigned to every GW to the same domain, and each domain MUST have a
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
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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 Gyan Mishra for review comments, and to
Robert Raszuk for useful discussions.
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.
[I-D.ietf-idr-tunnel-encaps]
Patel, K., Velde, G., Sangli, S., and J. Scudder, "The BGP
Tunnel Encapsulation Attribute", draft-ietf-idr-tunnel-
encaps-21 (work in progress), January 2021.
[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>.
[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>.
[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>.
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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>.
[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>.
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-16
(work in progress), June 2019.
[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>.
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[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>.
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
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Luay Jalil
Verizon
Email: luay.jalil@verizon.com
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