draft-ietf-idr-bgp-bestpath-selection-criteria-00.txt   draft-ietf-idr-bgp-bestpath-selection-criteria-01.txt 
IDR Working Group Rajiv Asati IDR Working Group Rajiv Asati
Internet Draft Cisco Systems Internet Draft Cisco Systems
Intended status: Informational Intended status: Informational
August 28, 2009 August 28, 2009
BGP Bestpath Selection Criteria BGP Bestpath Selection Criteria
draft-ietf-idr-bgp-bestpath-selection-criteria-00.txt draft-ietf-idr-bgp-bestpath-selection-criteria-01.txt
Status of this Memo Status of this Memo
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and restrictions with respect to this document. and restrictions with respect to this document.
Abstract Abstract
BGP specification [RFC4271] prescribes 'BGP next-hop reachability' as BGP specification [RFC4271] prescribes 'BGP next-hop reachability' as
one of the key 'Route Resolvability Condition' that must be satisfied one of the key 'Route Resolvability Condition' that must be satisfied
before the BGP bestpath candidate selection. This condition, however, before the BGP bestpath candidate selection. This condition, however,
may not be sufficient (as explained in the Appendix section) and may not be sufficient (as explained in the Appendix section) and
desire further granularity. desire further granularity.
Conventions used in this document This document defines enhances the "Route Resolvability Condition" to
facilitate the next-hop to be resolved in the chosen data plane.
In examples, "C:" and "S:" indicate lines sent by the client and
server respectively.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC-2119 [RFC2119].
Table of Contents Table of Contents
1. Introduction...................................................3 1. Introduction...................................................3
2. Route Resolvability Condition - Modification...................3 2. Specification Language.........................................3
3. Conclusions....................................................4 3. Route Resolvability Condition - Modification...................3
4. Security Considerations........................................4 4. Conclusions....................................................4
5. IANA Considerations............................................5 5. Security Considerations........................................5
6. Acknowledgments................................................5 6. IANA Considerations............................................5
7. Appendix.......................................................5 7. Acknowledgments................................................5
8. References.....................................................8 8. Appendix.......................................................5
Author's Addresses................................................8 9. References.....................................................8
Author's Addresses................................................9
1. Introduction 1. Introduction
As per BGP specification [RFC4271], when a router receives a BGP As per BGP specification [RFC4271], when a router receives a BGP
path, BGP must qualify it as the valid candidate prior to the BGP path, BGP must qualify it as the valid candidate prior to the BGP
bestpath selection using the 'Route Resolvability Condition' bestpath selection using the 'Route Resolvability Condition'
(section#9.1.2.1 of RFC4271]. After the path gets qualified as the (section#9.1.2.1 of RFC4271]. After the path gets qualified as the
bestpath candidate, it becomes eligible to be the bestpath, and may bestpath candidate, it becomes eligible to be the bestpath, and may
get advertised out to the neigbhor(s), if it became the bestpath. get advertised out to the neigbhor(s), if it became the bestpath.
However, in BGP networks that utilize data plane protocol other than However, in BGP networks that utilize data plane protocol other than
IP, such as MPLS etc. to forward the received traffic towards the IP, such as MPLS [RFC3031] etc. to forward the received traffic
next-hop, the above qualification condition may not be sufficient. In towards the next-hop, the above qualification condition may not be
fact, this may expose the BGP networks to experience traffic sufficient. In fact, this may expose the BGP networks to experience
blackholing i.e. traffic loss, due to malfunctioning of the chosen traffic blackholing i.e. traffic loss, due to malfunctioning of the
data plane protocol to the next-hop. This is explained further in the chosen data plane protocol to the next-hop. This is explained further
Appendix section. in the Appendix section.
This document defines further granularity to the "Route Resolvability This document defines further granularity to the "Route Resolvability
Condition" by (a) resolving the BGP next-hop reachability in the Condition" by (a) resolving the BGP next-hop reachability in the
forwarding database of a particular data plane protocol, and (b) forwarding database of a particular data plane protocol, and (b)
optionally including the BGP next-hop "path availability" check. optionally including the BGP next-hop "path availability" check.
2. Route Resolvability Condition - Modification The goal is to enable BGP to select the bestpaths based on whether or
not the corresponding nexthop can be resolved in the valid data
plane.
2. Specification Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Route Resolvability Condition - Modification
This document proposes two amendments to 'Route Resolvability This document proposes two amendments to 'Route Resolvability
Condition', which is defined in RFC4271, in consideration for a Condition', which is defined in RFC4271, in consideration for a
particular data plane protocol. particular data plane protocol:
1) The next-hop reachability SHOULD be resolved in a particular data 1) The next-hop reachability (check) SHOULD be resolved in a
plane protocol. forwarding database of a particular data plane protocol.
For example, if a BGP IPv4/v6 or VPNv4/v6 path wants to use MPLS data For example, if a BGP IPv4/v6 or VPNv4/v6 path wants to use
plane to the next-hop, as determined by the policy, then the BGP MPLS data plane to the next-hop, as determined by the policy,
'next-hop reachability' should be resolved using the MPLS data plane. then the BGP 'next-hop reachability' should be resolved using
In another example, if BGP path wants to use the IP data plane to the the MPLS forwarding database. In another example, if BGP path
next-hop, as determined by the policy, then BGP 'next-hop wants to use the IP data plane to the next-hop, as determined
reachability' should be resolved using the IP data plane. The latter by the policy, then BGP 'next-hop reachability' should be
example covers MPLS-in-IP encapsulation techniques such as [RFC4817], resolved using the IP forwarding database. The latter example
[RFC4023] etc. relates to MPLS-in-IP encapsulation techniques such as
[RFC4817], [RFC4023] etc.
The selection of particular data plane is a matter of a policy, and The selection of particular data plane is a matter of a policy, and
is outside the scope of this document. A dynamic signaling such as is outside the scope of this document. It is envisioned that the
BGP encapsulation SAFI (or tunnel encap attribute) [RFC5512] may be policy would exist for either per-neighbor or per-SAFI or both. A
used to convey the data plane protocol chosen by the policy. dynamic signaling such as BGP encapsulation SAFI (or tunnel encap
attribute) [RFC5512] may be used to convey the data plane protocol
chosen by the policy.
This check may be limited to confirming the availability of the valid This check is about confirming the availability of the valid
forwarding entry for the next-hop in the forwarding database of the forwarding entry for the next-hop in the forwarding database of the
chosen data plane protocol. chosen data plane protocol.
2) The 'path availability' check for the BGP next-hop MAY be 2) The 'path availability' check for the BGP next-hop MAY be
performed. This criterion checks for the functioning path to the performed. This criterion checks for the functional data plane
next-hop in a particular data plane protocol. path to the next-hop in a particular data plane protocol.
The path availability check may be performed by any of the OAM data- The path availability check may be performed by any of the OAM data-
plane liveness mechanisms associated with the data plane that is used plane liveness mechanisms associated with the data plane that is used
to reach the Next Hop. The data plane protocol for this criterion to reach the Next Hop. The data plane protocol for this criterion
must be the same as the one selected by the previous criterion (#1). MUST be the same as the one selected by the previous criterion (#1).
The mechanism(s) to perform the "path availability" check and the The mechanism(s) to perform the "path availability" check and the
selection of particular data plane are a matter of a policy and selection of particular data plane are a matter of a policy and
outside the scope of this document. outside the scope of this document.
For example, if a BGP VPNv4 path wants to use the MPLS as the data For example, if a BGP VPNv4 path wants to use the MPLS as the
plane protocol to the next-hop, then MPLS path availability to the data plane protocol to the next-hop, then MPLS path
next-hop should be evaluated i.e. liveness of MPLS LSP to the next- availability to the next-hop should be evaluated i.e. liveness
hop should be validated. of MPLS LSP to the next-hop should be validated.
3. Conclusions 4. Conclusions
Both amendments discussed in section 2 provide further clarity and Both amendments discussed in section 2 provide further clarity and
granularity to help the BGP speaker to either continue to advertise a granularity to help the BGP speaker to either continue to advertise a
BGP path's reachability or withdraw the BGP path's reachability, BGP path's reachability or withdraw the BGP path's reachability,
based on the consideration for the path's next-hop reachability based on the consideration for the path's next-hop reachability
and/or availability in a particular data plane. and/or availability in a particular data plane.
4. Security Considerations The intention of this document is to help operators to build BGP
networks that can avoid self-blackholing.
5. Security Considerations
This draft doesn't impose any additional security constraints. This draft doesn't impose any additional security constraints.
5. IANA Considerations 6. IANA Considerations
None. None.
6. Acknowledgments 7. Acknowledgments
Yakov Rekhter provided critical suggestions and feedback to improve Yakov Rekhter provided critical suggestions and feedback to improve
this document. Thanks to John Scudder and Chandrashekhar Appanna for this document. Thanks to John Scudder and Chandrashekhar Appanna for
contributing to the discussions that formed the basis of this contributing to the discussions that formed the basis of this
document. Thanks to Ilya Varlashkin, who made the case to revive this document. Thanks to Ilya Varlashkin and Michael Benjamin, who made
document and provided useful feedback. the case to revive this document and provided useful feedback. Also
thanks to Keyur Patel for his feedback.
This document was prepared using 2-Word-v2.0.template.dot. This document was prepared using 2-Word-v2.0.template.dot.
7. Appendix 8. Appendix
7.1. Problem Applicability 8.1. Problem Applicability
In IP networks using BGP, a router would continue to attract traffic In IP networks using BGP, a router would continue to attract traffic
by advertising the BGP prefix reachability to neighbor(s) as long as by advertising the BGP prefix reachability to neighbor(s) as long as
the router had a route to the next-hop in its routing table, but the router had a route to the next-hop in its routing table, but
independent of whether the router has a functional forwarding path to independent of whether the router has a functional forwarding path to
the next-hop. This may cause the forwarded traffic to be dropped the next-hop. This may cause the forwarded traffic to be dropped
inside the IP network. inside the IP network.
In MPLS or MPLS VPN networks [RFC4364], the same problem is observed In MPLS or MPLS VPN networks [RFC4364], the same problem is observed
if the functional MPLS LSP to the next-hop is not available (due to if the functional MPLS LSP to the next-hop is not available (due to
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Table entry, or the missing MPLS Forwarding table entry, or LDP Table entry, or the missing MPLS Forwarding table entry, or LDP
binding defect, or down LDP session between the P routers (with binding defect, or down LDP session between the P routers (with
independent label distribution control) etc. In such a situation, it independent label distribution control) etc. In such a situation, it
is clear that the CE1->CE2 traffic inserted into the MPLS network by is clear that the CE1->CE2 traffic inserted into the MPLS network by
PE1 will get dropped inside the MPLS network. PE1 will get dropped inside the MPLS network.
It is undesirable to have PE1 continue to convey to the CE1 router It is undesirable to have PE1 continue to convey to the CE1 router
that PE1 (and the MPLS network) is still the next-hop for the remote that PE1 (and the MPLS network) is still the next-hop for the remote
VPN reachability, without being sure of the corresponding LSP health. VPN reachability, without being sure of the corresponding LSP health.
7.1.1. Multi-Homed VPN Site 8.1.1. Multi-Homed VPN Site
If the remote VPN site is dual-homed to both PE2 and PE3, then PE1 If the remote VPN site is dual-homed to both PE2 and PE3, then PE1
may learn two VPNv4 paths to the prefix a.b.c.d. via PE2 and PE3 may learn two VPNv4 paths to the prefix a.b.c.d. via PE2 and PE3
routers, as shown below in Figure 2. PE1 may select the bestpath for routers, as shown below in Figure 2. PE1 may select the bestpath for
the prefix a.b.c.d via PE2 (say, for which the PE1->PE2 LSP is mal- the prefix a.b.c.d via PE2 (say, for which the PE1->PE2 LSP is mal-
functioning) and advertise that bestpath to CE1 in the context of functioning) and advertise that bestpath to CE1 in the context of
figure 2. figure 2.
<------MP-BGP------> <------MP-BGP------>
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<------MP-BGP------> <------MP-BGP------>
CE1~~~~~~~~~PE1~~~MPLS Network~~~PE2~~~~~~~~CE2~~ CE1~~~~~~~~~PE1~~~MPLS Network~~~PE2~~~~~~~~CE2~~
\ / ^ \ / ^
\~~~~~~~~~~PE3~~~~~~~/ ^ \~~~~~~~~~~PE3~~~~~~~/ ^
^ ^
a.b.c.d a.b.c.d
Figure 2 MPLS VPN Network - CE2 Dual-Homing Figure 2 MPLS VPN Network - CE2 Dual-Homing
This causes CE1 to likely send the traffic destined to prefix a.b.c.d This causes CE1 to likely send the traffic destined to prefix a.b.c.d
to the PE1 router, which forwards the traffic over the malfunctioning to the PE1 router, which forwards the traffic over the malfunctioning
LSP to PE2. It is clear that this MPLS encapsulated VPN traffic ends LSP to PE2. It is clear that this MPLS encapsulated VPN traffic ends
up getting dropped or blackholed somewhere inside the MPLS network. up getting dropped or blackholed somewhere inside the MPLS network.
It is desirable to force PE1 to select an alternate bestpath via that It is desirable to force PE1 to select an alternate bestpath via that
next-hop (such as PE3), whose LSP is correctly functioning. next-hop (such as PE3), whose LSP is correctly functioning.
7.1.2. Single-Homed VPN Site with Site-to-Site Backup Connectivity 8.1.2. Single-Homed VPN Site with Site-to-Site Backup Connectivity
The local VPN site may have a backup/dial-up link available at the CE The local VPN site may have a backup/dial-up link available at the CE
router, but the backup link will not even be activated as long as the router, but the backup link will not even be activated as long as the
CE's routing table continues to point to the PE router as the next- CE's routing table continues to point to the PE router as the next-
hop (over the MPLS/VPN network). hop (over the MPLS/VPN network).
<------MP-BGP------> <------MP-BGP------>
CE1~~~~~~~~~PE1~~~MPLS Network~~~PE2~~~~~~~~CE2~~ CE1~~~~~~~~~PE1~~~MPLS Network~~~PE2~~~~~~~~CE2~~
\ / ^ \ / ^
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PE-CE link, CE1 will not be able to activate the backup link (barring PE-CE link, CE1 will not be able to activate the backup link (barring
any tracking functionality) to the remote VPN site. any tracking functionality) to the remote VPN site.
In summary, if PE1 could appropriately qualify the BGP VPNv4 In summary, if PE1 could appropriately qualify the BGP VPNv4
bestpath, then the VPN traffic outage could likely be avoided. Even bestpath, then the VPN traffic outage could likely be avoided. Even
if the VPN site was not multi-homed, it is desirable to force PE1 to if the VPN site was not multi-homed, it is desirable to force PE1 to
withdraw the path from CE1 to improve the CE-to-CE convergence. This withdraw the path from CE1 to improve the CE-to-CE convergence. This
document proposes a mechanism to achieve the optimal BGP behavior at document proposes a mechanism to achieve the optimal BGP behavior at
PE. PE.
8. References 8.1.3. 6PE or 6VPE
8.1. Normative References This problem is very much applicable to the MPLS network that is
providing either 6PE [RFC4978] or 6VPE [] service to transport IPv6
packets over the MPLS network.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4364] Rosen E. and Rekhter Y., "BGP/MPLS IP Virtual Private [RFC4364] Rosen E. and Rekhter Y., "BGP/MPLS IP Virtual Private
Networks (VPNs)", RFC4364, February 2006. Networks (VPNs)", RFC4364, February 2006.
[RFC4271] Rekhter, Y., Li T., and Hares S.(editors), "A Border [RFC4271] Rekhter, Y., Li T., and Hares S.(editors), "A Border
Gateway Protocol 4 (BGP-4)", RFC 4271, January 2006 Gateway Protocol 4 (BGP-4)", RFC 4271, January 2006
8.2. Informative References 9.2. Informative References
[RFC3031] Rosen, et al., "Multiprotocol Label Switching
Architecture", RFC3031, Jan 2001.
[RFC5512] Rosen, E., Mohapatra, P., "BGP Encapsulation SAFI and BGP
Tunnel Encapsulation Attribute", RFC5512, April 2009.
[RFC4023] Rosen, et al., "Encapsulating MPLS in IP or Generic Routing [RFC4023] Rosen, et al., "Encapsulating MPLS in IP or Generic Routing
Encapsulation", RFC4023, March 2005. Encapsulation", RFC4023, March 2005.
[RFC4817] Townsley, et al., "Encapsulation of MPLS over Layer 2 [RFC4817] Townsley, et al., "Encapsulation of MPLS over Layer 2
Tunneling Protocol Version 3", RFC4817, Nov 2006. Tunneling Protocol Version 3", RFC4817, Nov 2006.
[RFC5512] Rosen, E., Mohapatra, P., "BGP Encapsulation SAFI and BGP [RFC4978] De Clercq, et al., "Connecting IPv6 Islands over IPv4 MPLS
Tunnel Encapsulation Attribute", RFC5512, April 2009. Using IPv6 Provider Edge Routers", RFC4978, Feb 2007.
[RFC4659] De Clercq, et al., "BGP-MPLS IP VPN Extension for IPv6
VPN", RFC4659, Sep 2006.
Author's Addresses Author's Addresses
Rajiv Asati Rajiv Asati
Cisco Systems Cisco Systems
7025 Kit Creek Road 7025 Kit Creek Road
RTP, NC 27560 USA RTP, NC 27560 USA
Email: rajiva@cisco.com Email: rajiva@cisco.com
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