Internet DRAFT - draft-hegde-lsr-ospf-better-idbx
draft-hegde-lsr-ospf-better-idbx
Network Working Group S. Hegde
Internet-Draft A. Przygienda
Intended status: Informational Juniper
Expires: 18 April 2024 A. Lindem
LabN Consulting LLC
16 October 2023
Improved OSPF Database Exchange Procedure
draft-hegde-lsr-ospf-better-idbx-01
Abstract
When an OSPF router undergoes restart, previous instances of LSAs
belonging to that router may remain in the databases of other routers
in the OSPF domain until such LSAs are aged out. Hence, when the
restarting router joins the network again, neighboring routers re-
establish adjacencies while the restarting router is still bringing-
up its interfaces and adjacencies and generates LSAs with sequence
numbers that may be lower than the stale LSAs. Such stale LSAs may
be interpreted as bi-directional connectivity before the initial
database exchanges are finished and genuine bi-directional LSA
connectivity exists. Such incorrect interpretation may lead to,
among other things, transient traffic packet drops. This document
suggests improvements in the OSPF database exchange process to
prevent such problems due to stale LSA utilization. The solution
does not preclude changes in the existing standard but presents an
extension that will prevent this scenario.
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.
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/.
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Internet-Drafts are draft documents valid for a maximum of six months
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This Internet-Draft will expire on 18 April 2024.
Copyright Notice
Copyright (c) 2023 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/
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Please review these documents carefully, as they describe your rights
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Solution . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Example . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
4. Security Considerations . . . . . . . . . . . . . . . . . . . 5
5. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 5
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
6.1. Informative References . . . . . . . . . . . . . . . . . 5
6.2. Normative References . . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 5
1. Introduction
When an OSPF [RFC2328] router restarts, its stale LSAs are left in
the database of other routers in the OSPF domain until the LSAs are
aged out either intentionally or by the LSA age elapsing. The stale
Router LSA can contain links to all the neighbors that had Full
adjacencies before the router restarted.
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----------C--------
| |
A-----B E-----F
| |
--------D---------
Figure 1: OSPF Network
Figure 1 shows a very simple OSPF network. In case of C undergoing
restart that does not generate purges, the other routers in the
domain will hold the stale LSA of Router C in their database. The
stale LSA may have links to B and E, which represents the topology of
C before it went down. When C restarts again, it initiates the
database exchange process with B and E. B and E may have C's stale
LSA with a higher sequence number in their database than the new ones
originated by C and hence assume this is latest copy, successively
bringing up the adjacency with C, and transitioning to Full state.
Based on C's Stale LSA having LSA links to B and E, the Shortest Path
First (SPF) back-link check is satisfied and B and E update their
routing table to point to C. This may cause C to drop this traffic
as C may not have all its previous adjacencies up and all LSAs in
place to correctly compute the necessary routes.
2. Solution
The database exchange procedure from [RFC2328] section 7.2 is
extended with additional constraints to prevent an OSPF router from
transitioning to Full state when it has stale LSAs originated by the
database exchange neighbor in its Link State Database (LSDB).
During Database exchange, when a router receives an LSA from the
neighbor for which such neighbor is the originator of the LSA and the
sequence number of the LSA is smaller than the sequence number of its
own database copy, the receiving router marks its database copy as
stale. This document proposes to create a new LSA list called "Stale
LSA List". This list is created on a per neighbor basis and
resembles the "LS Request List", the difference being LS Request
messages are not sent for stale LSAs. The router creates a "Stale
LSA List" for this neighbor and adds stale LSA to the Stale LSA List
for the neighbor. This LSA MUST NOT be removed from the Stale LSA
list and the adjacency FSM MUST NOT transition to Full state until an
LSA with a sequence number greater than its own database copy is
received (or strictly speaking, a "more recent" LSA). The Stale LSA
List cleanup procedures follow the LSRequest list cleanup procedures
as described in [RFC2328]
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2.1. Example
Figure 2 provides an example of C restarting having originated an LSA
with sequence number Y before. After restarting C originates the
same LSA with sequence number X where X < Y since it is not aware of
existence of version X yet.
C-----------------E
DBD: LSA A origin:C ------->
Sequence:X
DBD: LSA A Origin:C
<------ Sequence:Y
LSReq LSA A,
origin C ----->
Modified Procedure
<------ Add LSA A to Stale LSA List
<------- LSA A, Origin C, Seq:Y
C re-originating self LSA
LSA A, Origin C, -------> Remove Stale
Seq:Y+1 LSA A, Origin C
From Stale LSA List,
Bring adjacency to Full state if
both LS Request list and Stale LSA
list are empty.
Figure 2: Modified Database Exchange Procedure
As shown in figure Figure 2 above, E originates LSReq with Sequence
number X but waits until the LSA with sequence number Y+1 (or
strictly speaking, an LSA that compares as newer to the one it holds)
arrives. As the LSA is still in the Stale LSA List, the adjacency
will remain in Loading state and will not move to Full state. All
the neighbors of the restarting routers hold the neighbor FSM in
Loading state and do not transition to Full state until the stale LSA
is replaced with the new LSA with higher sequence number than the
stale LSA. This ensures that other routers in the network do not
compute a path through the restarting router since they cannot
satisfy the bi-directionality condition in SPF computations.
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3. IANA Considerations
No IANA Considerations
4. Security Considerations
5. Contributors
6. References
6.1. Informative References
6.2. 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>.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC2328, April 1998,
<https://www.rfc-editor.org/info/rfc2328>.
[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>.
Authors' Addresses
Shraddha Hegde
Juniper
India
Email: shraddha@juniper.net
Tony Przygienda
Juniper
1133 Innovation Way
Sunnyvale, CA
United States of America
Email: prz@juniper.net
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Acee Lindem
LabN Consulting LLC
301 Midenhall Way
Cary, North Carolina
United States of America
Email: acee.ietf@gmail.com
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