Internet DRAFT - draft-cheng-lsr-ospf-adjacency-suppress
draft-cheng-lsr-ospf-adjacency-suppress
Network Working Group W. Cheng
Internet Draft L. Gong
Intended status: Standards Track China Mobile
Expires: March 17, 2024 C. Lin
M. Chen
New H3C Technologies
September 19, 2023
OSPF Adjacency Suppression
draft-cheng-lsr-ospf-adjacency-suppress-01
Abstract
This document describes a mechanism for a router to instructs its
neighbors to suppress advertising the adjacency to it until link-
state database synchronization and LSA reoriginating are complete.
This minimizes transient routing disruption when a router restarts
from unplanned outages.
Status of this Memo
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Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction...................................................2
1.1. Requirements Language.....................................3
2. Problem........................................................3
2.1. Scenario of Two Router Network............................3
2.2. Scenario of Network with More Router......................5
3. Solution.......................................................5
3.1. Sending the SA-Indicator..................................6
3.2. Receiving the SA-Indicator................................6
4. OSPF Extensions for SA-Indicator...............................7
4.1. Advertising SA-Indicator in Hello Packets with LLS........7
4.1.1. Option A: Extended Options and Flags TLV.............7
4.1.2. Option B: Reverse Metric TLV.........................7
4.1.3. Operations...........................................8
4.2. Advertising SA-Indicator in Link-local Opaque-LSAs........8
4.2.1. Option C: SA-LSA.....................................8
4.2.2. Operations...........................................9
5. Backward Compatibility........................................10
6. Security Considerations.......................................10
7. IANA Considerations...........................................10
8. References....................................................10
8.1. Normative References.....................................10
8.2. Informative References...................................10
Acknowlegements..................................................11
Authors' Addresses...............................................11
1. Introduction
A router that is restarting from unplanned outages may not have
maintained forwarding function state. Since this is not the first
time the router has started, copies of LSAs generated by this router
in its previous incarnation may exist in the link-state databases of
other routers in the network. These copies are likely to appear
"newer" than LSAs initially generated by the starting router due to
the reinitialization of LSA sequence numbers by the starting router.
So, without requesting the starting router to update its LSAs, the
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neighbors of the starting router may transition to "Full" state and
route the traffic through the starting router. This may cause
temporary blackholes to occur until the normal operation of the
update process causes the starting router to reoriginate and flood
copies of its own LSAs with higher sequence numbers.
This document describes OSPF extensions for adjacency suppression.
This OSPF protocol extension provides functionality similar to the
SA bit of Restart TLV in IS-IS [RFC8706]. With the proposed
mechanism, the starting router's neighbors will suppress advertising
an adjacency to the starting router until the starting router has
been able to propagate newer versions of LSAs, so that the temporary
blackholes can be avoided.
1.1. 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.
2. Problem
2.1. Scenario of Two Router Network
Assume that in a simple OSPF network with two routers A-B, router A
restarts from an unplanned outage, as shown in Figure 1.
An external route 10.1.1.0/24 is advertised by router A for some
connected servers. After router A restarts, that external route is
deleted and will not be advertised until data plane is ready to
transfer packets for those servers. However, The old copies of LSAs
generated by router A still exists in the link-state databases of
router B, such as the Router-LSA with adjacency A->B and the
External-LSA with 10.1.1.0. The restarting router A reinitializes
LSA sequence numbers, hence the old copies appear to be "newer".
Without requesting router A to update its LSAs, router B will
transition to "Full" state and route the traffic through router A. A
temporary blackhole occurs.
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External 10.1.1.0/24 (Down)
|
|
Router A ----------------------- Router B
(Restarting)
| --------- 1-way Hello --------> | Init
| |
ExStart | <-------- 2-way Hello --------- |
| |
| --------- 2-way Hello --------> | Exstart
| |
ExChange | <------------ DD -------------> | ExChange
| |
Loading | <-------- LS Request ---------> | Loading
| |
| <-------- LS Update ----------> | Full ---
| | ^
| --------- LS Request ---------> | |
| (Type 1, Seq M; Type 5, Seq N) | |
| | Blackhole
Full | <-------- LS Update ----------- | to 10.1.1.0
| (Type 1, Seq M, with A->B) | |
| (Type 5, Seq N, with 10.1.1.0) | |
| | v
| --------- LS Update ----------> | --------
| (Self, Reorigniate) |
(Type 1, Seq M+1, without A->B)
(Type 5, Seq N, MaxAge)
Figure 1: Restarting Scenario of Two Router Network
The above procedure can also be summarized as the following steps:
o Step 1.1: Router A restarts from unplanned outage and router B
has the old LSA of router A in its link-state database;
o Step 1.2: Router B reaches the Full state, and update its Router-
LSA to advertise the adjacency B->A;
o Step 1.3: Temporary blackhole occurs;
o Step 1.4: Router B receives the reoriginated LSAs of router A;
o Step 1.5: Temporary blackhole disappears.
Especially when router B has many more LSAs than router A, the time
between Step 1.2 and Step 1.4 will be prolonged, and the impact of
blackhole could be more significant.
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In addition to external routes, other types of routes which have old
copies on neighbor may have the same problem during restarting.
2.2. Scenario of Network with More Router
Assume that there are more routers in the network, as shown in the
following figure. Router C represents the rest of the network
attached to router B.
+-----+ +-----+ +-----+
|Rtr A|--------|Rtr B|--------|Rtr C|
+-----+ +-----+ +-----+
(Restarting) (Rest of the Network)
Figure 2: Restarting Scenario of Network with More Routers
From the perspective of router C, a temporary blackhole may also
occur when the following order comes:
o Step 2.1: Router A restarts from unplanned outage and router C
has the old LSA of router A in its link-state database;
o Step 2.2: Router C receives the new Router-LSA of B advertising
the adjacency B->A;
o Step 2.3: Temporary blackhole occurs;
o Step 2.4: Router C receives the reoriginated LSAs of router A
without the adjacency A->B;
o Step 2.5: Temporary blackhole disappears.
The above procedure is likely to occur under certain conditions,
such as packet loss, out of order, MinLSInterval, or MinLSArrival,
because the sequence of the flooding process cannot be controlled
precisely.
3. Solution
The solution proposed in this document is to allow the restarting
router to control the timing for its neighbor to advertise adjacency
after FULL state.
o Step 3.1: The restarting router signals suppressing adjacency to
its neighbor;
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o Step 3.2: The neighboring router suppresses the advertisement of
the adjacency to the starting router (even if it transitions to
the FULL state during this period);
o Step 3.3: The restarting router reoriginates and floods its own
LSAs;
o Step 3.4: The restarting router stops signaling suppressing
adjacency to its neighbor;
o Step 3.5: The neighboring router advertises the adjacency to the
restarting router.
The proposed solution is similar with the mechanism of the SA bit of
Restart TLV in IS-IS [RFC8706]. The OSPF signaling of suppressing
adjacency is called the SA-Indicator, which will be specified in
Section 4.
3.1. Sending the SA-Indicator
When a router is starting, it starts a timer T-SA and sends the SA-
Indicator to its neighbors. After the synchronization of link-state
database and the reoriginating of its own LSAs are complete (with
additional delay), the timer T-SA is canceled.
When the timer T-SA has expired or been canceled, the starting
router MUST clear the SA-Indicator.
3.2. Receiving the SA-Indicator
When a router receives an SA-Indicator, if there exists on this
interface an adjacency in the FULL state with the same Router ID,
then the router MUST suppress advertisement of the adjacency to the
neighbor in its own LSAs. In the case of broadcast and NBMA links,
the Designated Routers are responsible for the suppressing of
adjacency advertisement.
Until the SA-Indicator is cleared, the adjacency advertisement MUST
continue to be suppressed. During that period, if the neighbor
transitions to the FULL state, the new adjacency MUST NOT be
advertised.
Besides, a router that suppresses advertisement of an adjacency MUST
NOT use this adjacency when performing its SPF calculation.
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4. OSPF Extensions for SA-Indicator
This Section defines the extensions for OSPF protocol to advertise
SA-Indicator (Suppressing Adjacency Indicator). The advertising of
SA-Indicator has several options. Section 4.1 describes how to
advertise SA-Indicator in Hello packets with LLS [RFC5613]. Section
4.2 describes how to advertise SA-Indicator in link-local Opaque-
LSAs [RFC5250].
4.1. Advertising SA-Indicator in Hello Packets with LLS
There are two possible positions in the OSPF LLS [RFC5613] to carry
the SA-Indicator, which are specified in Section 4.1.1 and 4.1.2.
4.1.1. Option A: Extended Options and Flags TLV
The SA-Indicator can be carried in the Type 1 Extended Options and
Flags TLV [RFC5613] as a new SA-bit. This bit is defined for the LLS
block included in Hello packets and instructs the receiver to
suppress advertising an adjacency to the sender.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 1 | 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extended Options and Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Extended Options Bit:
0x00000001: LR-bit
0x00000002: RS-bit
0x00000004: I-bit
0x00000008: F-bit
0x00000010: B-bit
0x00000020: FR-bit
TBD : SA-bit
Figure 3: Format of the Extended Options and Flags TLV
4.1.2. Option B: Reverse Metric TLV
The SA-Indicator can be carried in the Type 19 Reverse Metric TLV
[RFC9339] as a new SA-bit. This bit is defined for the LLS block
included in Hello packets and instructs the receiver to suppress
advertising an adjacency to the sender.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 19 | 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MTID | Flags | Reverse Metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Flags:
0x00000001: H-bit
0x00000002: O-bit
TBD : SA-bit
Figure 4: Format of the Reverse Metric TLV
4.1.3. Operations
o Set the SA-Indicator: Send Hello packets containing the LLS block
with the Extended Options and Flags TLV or Reverse Metric TLV
that has the SA-bit set.
o Unset the SA-Indicator: Send Hello packets with the SA-bit clear.
4.2. Advertising SA-Indicator in Link-local Opaque-LSAs
The restarting router can originate link-local Opaque-LSAs, called
the SA-LSAs as defined in Section 4.2.1, to advertise the SA-
Indicator to its neighbors.
4.2.1. Option C: SA-LSA
The SA-LSA is a link-local scoped Opaque-LSA. The Opaque Type is TBA
and the Opaque ID equal to 0. SA-LSAs are originated by a router
that wishes the receiver to suppress advertising an adjacency to the
originator.
Each SA-LSA has an LS age field set to 0 when the LSA is first
originated; the current value of the LS age then indicates how long
ago the restarting router made its request for suppressing
adjacency. The body of the LSA is TLV-encoded.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS age | Options | 9 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TBA | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS checksum | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- TLVs -+
| ... |
Figure 5: Format of the SA-LSA
The format of the TLVs within the body of a SA-LSA is as following:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The following is the list of TLVs that can appear in the body of a
SA-LSA:
o IP interface address (Type=1, length=4). The router's IP
interface address on the subnet associated with the SA-LSA.
Required on broadcast, NBMA and Point-to-MultiPoint segments,
where the neighbor uses the IP interface address to identify the
restarting router.
DoNotAge is never set in a SA-LSA.
SA-LSAs have link-local scope because they only need to be seen by
the router's direct neighbors.
4.2.2. Operations
o Set the SA-Indicator: Originate the SA-LSA.
o Unset the SA-Indicator: Flush the SA-LSA.
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5. Backward Compatibility
The described technique requires cooperation from neighboring
routers. If a router does not support this technique, it will ignore
the SA-Indicator and advertise the adjacency when the neighbor
transitions to the FULL state. As a result, the behavior would be
the same as without this specification.
6. Security Considerations
TBD.
7. IANA Considerations
TBD.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5250] Berger, L., Bryskin, I., Zinin, A., and R. Coltun, "The
OSPF Opaque LSA Option", RFC 5250, DOI 10.17487/RFC5250,
July 2008, <https://www.rfc-editor.org/info/rfc5250>.
[RFC5613] Zinin, A., Roy, A., Nguyen, L., Friedman, B., and D.
Yeung, "OSPF Link-Local Signaling", RFC 5613, DOI
10.17487/RFC5613, August 2009, <https://www.rfc-
editor.org/info/rfc5613>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, May 2017
[RFC9339] Talaulikar, K., Psenak, P., and H. Johnston, " OSPF
Reverse Metric", RFC 9339, DOI 10.17487/RFC9339, December
2022, <https://www.rfc-editor.org/info/rfc9339>.
8.2. Informative References
[RFC8706] Ginsberg, L., and P. Wells, "Restart Signaling for IS-IS",
RFC 8706, DOI 10.17487/RFC8706, February 2020,
<https://www.rfc-editor.org/info/rfc8706>.
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Acknowlegements
The authors would like to acknowledge Les Ginsberg for highlighting
the problems on remote routers.
Authors' Addresses
Weiqiang Cheng
China Mobile
China
Email: chengweiqiang@chinamobile.com
Liyan Gong
China Mobile
China
Email: gongliyan@chinamobile.com
Changwang Lin
New H3C Technologies
China
Email: linchangwang.04414@h3c.com
Mengxiao Chen
New H3C Technologies
China
Email: chen.mengxiao@h3c.com
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