Prefix Unreachable Announcement
draft-wang-lsr-prefix-unreachable-annoucement-06
LSR Working Group A. Wang
Internet-Draft China Telecom
Intended status: Standards Track G. Mishra
Expires: September 27, 2021 Verizon Inc.
Z. Hu
Y. Xiao
Huawei Technologies
March 26, 2021
Prefix Unreachable Announcement
draft-wang-lsr-prefix-unreachable-annoucement-06
Abstract
This document describes a mechanism to solve an existing issue with
Longest Prefix Match (LPM), that exists where an operator domain is
divided into multiple areas or levels where summarization is
utilized. This draft addresses a fail-over issue related to a multi
areas or levels domain, where a link or node down event occurs
resulting in an LPM component prefix being omitted from the FIB
resulting in black hole sink of routing and connectivity loss. This
draft introduces a new control plane convergence signaling mechanism
using a negative prefix called Prefix Unreachable Announcement (PUA),
utilized to detect a link or node down event and signal the RIB that
the event has occurred to force immediate control plane convergence.
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/.
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 September 27, 2021.
Wang, et al. Expires September 27, 2021 [Page 1]
Internet-Draft PUA March 2021
Copyright Notice
Copyright (c) 2021 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
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions used in this document . . . . . . . . . . . . . . 3
3. Scenario Description . . . . . . . . . . . . . . . . . . . . 3
3.1. Inter-Area Node Failure Scenario . . . . . . . . . . . . 4
3.2. Inter-Area Links Failure Scenario . . . . . . . . . . . . 4
4. PUA (Prefix Unreachable Advertisement) Procedures . . . . . . 5
5. MPLS and SRv6 LPM based BGP Next-hop Failure Application . . 5
6. Implementation Consideration . . . . . . . . . . . . . . . . 6
7. Deployment Considerations . . . . . . . . . . . . . . . . . . 7
8. Security Considerations . . . . . . . . . . . . . . . . . . . 7
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
10. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 7
11. Normative References . . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
As part of an operator optimized design criteria, a critical
requirement is to limit Shortest Path First (SPF) churn which occurs
within a single OSPF area or ISIS level. This is accomplished by
sub-dividing the IGP domain into multiple areas for flood reduction
of intra area prefixes so they are contained within each discrete
area to avoid domain wide flooding.
OSPF and ISIS have a default and summary route mechanism which is
performed on the OSPF area border router or ISIS L1-L2 node. The
OSPF summary route is triggered to be advertised conditionally when
at least one component prefix exists within the non-zero area. ISIS
Level-L1-L2 node as well generate a summary prefix into the level-2
backbone area for Level 1 area prefixes that is triggered to be
advertised conditionally when at least a single component prefix
Wang, et al. Expires September 27, 2021 [Page 2]
Internet-Draft PUA March 2021
exists within the Level-1 area. ISIS L1-L2 node with attach bit set
also generates a default route into each Level-1 area along with
summary prefixes generated for other Level-1 areas.
Operators have historically relied on MPLS architecture which is
based on exact match host route FEC binding for single area.
[RFC5283] LDP inter-area extension provides the ability to LPM, so
now the RIB match can now be a summary match and not an exact match
of a host route of the egress PE for an inter-area LSP to be
instantiated. SRV6 routing framework utilities the IPv6 data plane
standard IGP LPM. When operators start to migrate from MPLS LSP
based host route bootstrapped FEC binding, to SRv6 routing framework,
the IGP LPM now comes into play with summarization which will
influence the forwarding of traffic when a link or node event occurs
for a component prefix within the summary range resulting in black
hole routing of traffic.
The motivation behind this draft is based on either MPLS LPM FEC
binding, or SRv6 BGP service overlay using traditional unicast
routing (uRIB) LPM forwarding plane where the IGP domain has been
carved up into OSPF or ISIS areas and summarization is utilized. In
this scenario where a failure conditions result in a black hole of
traffic where multiple ABRs exist and either the area is partitioned
or other link or node failures occur resulting in the component
prefix host route missing within the summary range. Summarization of
inter-area types routes propagated into the backbone area for flood
reduction are made up of component prefixes. It is these component
prefixes that the PUA tracks to ensure traffic is not black hole sink
routed due to a PE or ABR failure. The PUA mechanism ensures
immediate control plane convergence with ABR or PE node switchover
when area is partitioned or ABR has services down to avoid black hole
of traffic.
2. Conventions used in this document
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. Scenario Description
Figure 1 illustrates the topology scenario when OSPF or ISIS is
running in multi areas or multi levels domain. R0-R4 are routers in
backbone area, S1-S4,T1-T4 are internal routers in area 1 and area 2
respectively. R1 and R3 are area border routers or ISIS Level 1-2
border nodes between area 0 and area 1. R2 and R4 are area border
routers between area 0 and area 2.
Wang, et al. Expires September 27, 2021 [Page 3]
Internet-Draft PUA March 2021
S1/S4 and T2/T4 PEs peer to customer CEs for overlay VPNs. Ps1/Ps4
is the loopback0 address of S1/S4 and Pt2/Pt4 is the loopback0
address of T2/T4.
+---------------------+------+--------+-----+--------------+
| +--+ +--+ ++-+ ++-+ +-++ + -+ +--+|
| |S1+--------+S2+---+R1+---|R0+----+R2+---+T1+--------+T2||
| +-++Ps1 +-++ ++-+ +--+ +-++ ++++ Pt2 +-++|
| | | | | || | |
| | | | | || | |
| +-++Ps4 +-++ ++-+ +-++ ++++ Pt4+-++|
| |S4+--------+S3+---+R3+-----------+R4+---+T3+--------+T4||
| +--+ +--+ ++-+ +-++ ++-+ +--+|
| | | |
| | | |
| Area 1 | Area 0 | Area 2 |
+---------------------+---------------+--------------------+
Figure 1: OSPF Inter-Area Prefix Unreachable Announcement Scenario
3.1. Inter-Area Node Failure Scenario
If the area border router R2/R4 does the summary action, then one
summary address that cover the prefixes of area 2 will be announced
to area 0 and area 1, instead of the detail address. When the node
T2 is down, Pt2 bgp next hop becomes unreachable while the LPM
summary prefix continues to be advertised into the backbone area.
Except the border router R2/R4, the other routers within area 0 and
area 1 do not know the unreachable status of the Pt2 bgp next hop
prefix. Traffic will continue to forward LPM match to prefix Pt2 and
will be dropped on the ABR or Level 1-2 border node resulting in
black hole routing and connectivity loss. Customer overlay VPN dual
homed to both S1/S4 and T2/R4, traffic will not be able to fail-over
to alternate egress PE T4 bgp next hop Pt4 due to the summarization.
3.2. Inter-Area Links Failure Scenario
In a link failure scenario, if the link between T1/T2 and T1/T3 are
down, R2 will not be able to reach node T2. But as R2 and R4 do the
summary announcement, and the summary address covers the bgp next hop
prefix of Pt2, other nodes in area 0 area 1 will still send traffic
to T2 bgp next hop prefix Pt2 via the border router R2, thus black
hole sink routing the traffic.
In such a situation, the border router R2 should notify other routers
that it can't reach the prefix Pt2, and lets the other ABRs(R4) that
can reach prefix Pt2 advertise one specific route to Pt2, then the
Wang, et al. Expires September 27, 2021 [Page 4]
Internet-Draft PUA March 2021
internal routers will select R4 as the bypass router to reach prefix
Pt2.
4. PUA (Prefix Unreachable Advertisement) Procedures
[RFC7794] and [I-D.ietf-lsr-ospf-prefix-originator] draft both define
one sub-tlv to announce the originator information of the one prefix
from a specified node. This draft utilizes such TLV for both OSPF
and ISIS to signal the negative prefix in the perspective PUA when a
link or node goes down.
ABR detects link or node down and floods PUA negative prefix
advertisement along with the summary advertisement according to the
prefix-originator specification. The ABR or ISIS L1-L2 border node
has the responsibility to add the prefix originator information when
it receives the Router LSA from other routers in the same area or
level.
When the ABR or ISIS L1-L2 border node generates the summary
advertisement based on component prefixes, the ABR will announce one
new summary LSA or LSP which includes the information about this down
prefix, with the prefix originator set to NULL. The number of PUAs
is equivalent to the number of links down or nodes down. The LSA or
LSP will be propagated with standard flooding procedures.
If the nodes in the area receive the PUA flood from all of its ABR
routers, they will start BGP convergence process if there exist BGP
session on this PUA prefix. The PUA creates a forced fail over
action to initiate immediate control plane convergence switchover to
alternate egress PE. Without the PUA forced convergence the down
prefix will yield black hole routing resulting in loss of
connectivity.
When only some of the ABRs can't reach the failure node/link, as that
described in Section 3.2, the ABR that can reach the PUA prefix
should advertise one specific route to this PUA prefix. The internal
routers within another area can then bypass the ABRs that can't reach
the PUA prefix, to reach the PUA prefix.
5. MPLS and SRv6 LPM based BGP Next-hop Failure Application
In an MPLS or SR-MPLS service provider core, scalability has been a
concern for operators which have split up the IGP domain into
multiple areas to avoid SPF churn. Normally, MPLS FEC binding for
LSP instantiation is based on egress PE exact match of a host route
Looback0. [RFC5283] LDP inter-area extension provides the ability to
LPM, so now the RIB match can now be a summary match and not an exact
match of host route of the egress PE for an inter-area LSP to be
Wang, et al. Expires September 27, 2021 [Page 5]
Internet-Draft PUA March 2021
instantiated. The caveat related to this feature that has prevented
operators from using the [RFC5283] LDP inter-area extension concept
is that when the component prefixes are now hidden in the summary
prefix, and thus the visibility of the BGP next-hop attribute is
lost.
In a case where a PE is down, and the [RFC5283] LDP inter-area
extension LPM summary is used to build the LSP inter-area, the LSP
remains partially established black hole on the ABR performing the
summarization. This major gap with [RFC5283] inter-area extension
forces operators into a workaround of having to flood the BGP next-
hop domain wide. In a small network this is fine, however if you
have 1000s PEs and many areas, the domain wide flooding can be
painful for operators as far as resource usage memory consumption and
computational requirements for RIB / FIB / LFIB label binding control
plane state. The ramifications of domain wide flooding of host
routes is described in detail in [RFC5302] domain wide prefix
distribution with 2 level ISIS Section 1.2 - Scalability. As SRv6
utilizes LPM, this problem exists as well with SRv6 when IGP domain
is broken up into areas and summarization is utilized.
PUA is now able to provide the negative prefix component flooded
across the backbone to the other areas along with the summary prefix,
which is now immediately programmed into the RIB control plane. MPLS
LSP exact match or SRv6 LPM match over fail over path can now be
established to the alternate egress PE. No disruption in traffic or
loss of connectivity results from PUA. Further optimizations such as
LFA and BFD can be done to make the data plane convergence hitless.
The PUA solution applies to MPLS or SR-MPLS where LDP inter-area
extension is utilized for LPM aggregate FEC, as well a SRv6 IPv6
control plane LPM match summarization of BGP next hop.
6. Implementation Consideration
Considering the balances of reachable information and unreachable
information announcement capabilities, the implementation of this
mechanism should set one MAX_Address_Announcement (MAA) threshold
value that can be configurable. Then, the ABR should make the
following decisions to announce the prefixes:
1. If the number of unreachable prefixes is less than MAA, the ABR
should advertise the summary address and the PUA.
2. If the number of reachable address is less than MAA, the ABR
should advertise the detail reachable address only.
3. If the number of reachable prefixes and unreachable prefixes
exceed MAA, then advertise the summary address with MAX metric.
Wang, et al. Expires September 27, 2021 [Page 6]
Internet-Draft PUA March 2021
7. Deployment Considerations
To support the PUA advertisement, the ABRs should be upgraded
according to the procedures described in Section 4. The PEs that
want to accomplish the BGP switchover that described in Section 3.1
and Section 5 should also be upgraded to act upon the receive of the
PUA message. Other nodes within the network can ignore such PUA
message if they don't care or don't support.
As described in Section 4, the ABR will advertise the PUA message
once it detects there is link or node down within the summary
address. In order to reduce the unnecessary advertisements of PUA
messages on ABRs, the ABRs should support the configuration of the
protected prefixes. Based on such information, the ABR will only
advertise the PUA message when the protected prefixes(for example,
the loopback addresses of PEs that run BGP) that within the summary
address is missing.
The advertisement of PUA message should only last one configurable
period to allow the services that run on the failure prefixes are
converged or switchover. If one prefix is missed before the PUA
mechanism takes effect, the ABR will not declare its absence via the
PUA mechanism.
8. Security Considerations
Advertisement of PUA information follow the same procedure of
traditional LSA. The action based on the PUA is clearly defined in
this document for ABR or Level1/2 router and the receiver that run
BGP.
There is no changes to the forward behavior of other internal
routers.
9. IANA Considerations
This document has no IANA actions.
10. Acknowledgement
Thanks Peter Psenak, Les Ginsberg, Acee Lindem, Shraddha Hegde,
Robert Raszuk, Tonly Li, Jeff Tantsura, Tony Przygienda and Bruno
Decraene for their suggestions and comments on this draft.
Wang, et al. Expires September 27, 2021 [Page 7]
Internet-Draft PUA March 2021
11. Normative References
[I-D.ietf-lsr-ospf-prefix-originator]
Wang, A., Lindem, A., Dong, J., Psenak, P., and K.
Talaulikar, "OSPF Prefix Originator Extensions", draft-
ietf-lsr-ospf-prefix-originator-07 (work in progress),
October 2020.
[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>.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC2328, April 1998,
<https://www.rfc-editor.org/info/rfc2328>.
[RFC5283] Decraene, B., Le Roux, JL., and I. Minei, "LDP Extension
for Inter-Area Label Switched Paths (LSPs)", RFC 5283,
DOI 10.17487/RFC5283, July 2008,
<https://www.rfc-editor.org/info/rfc5283>.
[RFC5302] Li, T., Smit, H., and T. Przygienda, "Domain-Wide Prefix
Distribution with Two-Level IS-IS", RFC 5302,
DOI 10.17487/RFC5302, October 2008,
<https://www.rfc-editor.org/info/rfc5302>.
[RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
<https://www.rfc-editor.org/info/rfc5340>.
[RFC5709] Bhatia, M., Manral, V., Fanto, M., White, R., Barnes, M.,
Li, T., and R. Atkinson, "OSPFv2 HMAC-SHA Cryptographic
Authentication", RFC 5709, DOI 10.17487/RFC5709, October
2009, <https://www.rfc-editor.org/info/rfc5709>.
[RFC7770] Lindem, A., Ed., Shen, N., Vasseur, JP., Aggarwal, R., and
S. Shaffer, "Extensions to OSPF for Advertising Optional
Router Capabilities", RFC 7770, DOI 10.17487/RFC7770,
February 2016, <https://www.rfc-editor.org/info/rfc7770>.
[RFC7794] Ginsberg, L., Ed., Decraene, B., Previdi, S., Xu, X., and
U. Chunduri, "IS-IS Prefix Attributes for Extended IPv4
and IPv6 Reachability", RFC 7794, DOI 10.17487/RFC7794,
March 2016, <https://www.rfc-editor.org/info/rfc7794>.
Wang, et al. Expires September 27, 2021 [Page 8]
Internet-Draft PUA March 2021
[RFC7981] Ginsberg, L., Previdi, S., and M. Chen, "IS-IS Extensions
for Advertising Router Information", RFC 7981,
DOI 10.17487/RFC7981, October 2016,
<https://www.rfc-editor.org/info/rfc7981>.
Authors' Addresses
Aijun Wang
China Telecom
Beiqijia Town, Changping District
Beijing 102209
China
Email: wangaj3@chinatelecom.cn
Gyan Mishra
Verizon Inc.
Email: gyan.s.mishra@verizon.com
Zhibo Hu
Huawei Technologies
Huawei Bld., No.156 Beiqing Rd.
Beijing 100095
China
Email: huzhibo@huawei.com
Yaqun Xiao
Huawei Technologies
Huawei Bld., No.156 Beiqing Rd.
Beijing 100095
China
Email: xiaoyaqun@huawei.com
Wang, et al. Expires September 27, 2021 [Page 9]