OSPF-MANET Broadcast: Use of the OSPF-MANET Interface in Single-Hop Broadcast Networks
How to run the OSPF-MANET interface on a network where every node reaches every other directly, and how to detect that situation.
Retana, A. and S. Ratliff, "Use of the OSPF-MANET Interface in Single-Hop Broadcast Networks", RFC 7137, DOI 10.17487/RFC7137, February 2014, <https://www.rfc-editor.org/info/rfc7137>.
@misc{rfc7137,
series = {Request for Comments},
number = 7137,
howpublished = {RFC 7137},
publisher = {RFC Editor},
doi = {10.17487/RFC7137},
url = {https://www.rfc-editor.org/info/rfc7137},
author = {A. Retana and S. Ratliff},
title = {{Use of the OSPF-MANET Interface in Single-Hop Broadcast Networks}},
pagetotal = 8,
year = 2014,
month = feb,
} RFC text
Contents
Internet Engineering Task Force (IETF) A. Retana
Request for Comments: 7137 S. Ratliff
Updates: 5820 Cisco Systems, Inc.
Category: Experimental February 2014
ISSN: 2070-1721
Use of the OSPF-MANET Interface in Single-Hop Broadcast Networks
Abstract
This document describes the use of the OSPF-MANET interface in
single-hop broadcast networks. It includes a mechanism to
dynamically determine the presence of such a network and specific
operational considerations due to its nature.
This document updates RFC 5820.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.
This document defines an Experimental Protocol for the Internet
community. This document is a product of the Internet Engineering
Task Force (IETF). It represents the consensus of the IETF
community. It has received public review and has been approved for
publication by the Internet Engineering Steering Group (IESG). Not
all documents approved by the IESG are a candidate for any level of
Internet Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7137.
Copyright Notice
Copyright (c) 2014 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
(http://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
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Single-Hop Broadcast Networks . . . . . . . . . . . . . . 3
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
3. Single-Hop Network Operation . . . . . . . . . . . . . . . . 4
3.1. Use of Router Priority . . . . . . . . . . . . . . . . . 4
3.2. Unsynchronized Adjacencies . . . . . . . . . . . . . . . 5
4. Single-Hop Network Detection . . . . . . . . . . . . . . . . 6
4.1. Transition from Multi-Hop to Single-Hop Mode . . . . . . 6
4.2. Transition from Single-Hop to Multi-Hop Mode . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
7.1. Normative References . . . . . . . . . . . . . . . . . . 7
7.2. Informative References . . . . . . . . . . . . . . . . . 8
1. Introduction
The OSPF-MANET interface [RFC5820] uses the point-to-multipoint
adjacency model over a broadcast media to allow the following:
o All router-to-router connections are treated as if they were
point-to-point links.
o The link metric can be set on a per-neighbor basis.
o Broadcast and multicast can be accomplished through the Layer 2
broadcast capabilities of the media.
It is clear that the characteristics of the MANET interface can also
be beneficial in other types of network deployments -- specifically,
in single-hop broadcast capable networks that may have a different
cost associated with any pair of nodes.
This document updates [RFC5820] by describing the use of the MANET
interface in single-hop broadcast networks; this consists of its
simplified operation by not requiring the use of overlapping relays
as well as introducing a new heuristic for smart peering using the
Router Priority.
1.1. Single-Hop Broadcast Networks
The OSPF extensions for MANETs assume the ad hoc formation of a
network over bandwidth-constrained wireless links, where packets may
traverse several intermediate nodes before reaching their destination
(multi-hop paths on the interface). By contrast, a single-hop
broadcast network (as considered in this document) is one that is
structured in such a way that all the nodes in it are directly
connected to each other. An Ethernet interface is a good example of
the connectivity model.
Furthermore, the single-hop networks considered may have different
link metrics associated to the connectivity between a specific pair
of neighbors. The OSPF broadcast model [RFC2328] can't accurately
describe these differences. A point-to-multipoint description is
more appropriate given that each node can reach every other node
directly.
In summary, the single-hop broadcast interfaces considered in this
document have the following characteristics:
o direct connectivity between all the nodes
o different link metrics that may exist per-neighbor
o broadcast/multicast capabilities
2. Requirements 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. Single-Hop Network Operation
The operation of the MANET interface doesn't change when implemented
on a single-hop broadcast interface. However, the operation of some
of the proposed enhancements can be simplified. Explicitly, the
Overlapping Relay Discovery Process SHOULD NOT be executed, and the
A-bit SHOULD NOT be set by any of the nodes, so that the result is an
empty set of Active Overlapping Relays.
This document describes the use of already defined mechanisms and
requires no additional on-the-wire changes.
3.1. Use of Router Priority
Smart peering [RFC5820] can be used to reduce the burden of requiring
a full mesh of adjacencies. In short, a new adjacency is not
required if reachability to the node is already available through the
existing shortest path tree (SPT). In general, the reachability is
verified on a first-come-first-served basis; i.e., in a typical
network, the neighbors with which a FULL adjacency is set up depend
on the order of discovery.
The state machine for smart peering allows for the definition of
heuristics, beyond the SPT reachability, to decide whether or not it
considers a new adjacency to be of value. This section describes one
such heuristic to be used in Step (3) of the state machine, in place
of the original one in Section 3.5.3.2 of [RFC5820].
The Router Priority (as defined in OSPFv2 [RFC2328] and OSPFv3
[RFC5340]) is used in the election of the (Backup) Designated Router,
and can be configured only in broadcast and Non-Broadcast Multi-
Access (NBMA) interfaces. The MANET interface is a broadcast
interface using the point-to-multipoint adjacency model; this means
that no (Backup) Designated Router is elected. For its use with the
MANET interface, the Router Priority is defined as:
Router Priority
An 8-bit unsigned integer. Used to determine the precedence of
which router(s) to establish a FULL adjacency with during the
Smart Peering selection process. When more than one router
attached to a network is present, the one with the highest
Router Priority takes precedence. If there is still a tie, the
router with the highest Router ID takes precedence.
The heuristic for the state machine for smart peering is described
as:
(3) |
,'''''''''''''''''''''''''''''''''''''''''''''''''''''''''|
| ............................ |
| |Determine if the number of| |
| |existing adjacencies is < | |
| |the maximum configured | |
| |value | |
| '`'''''''\'''''''''''''''/'' |
| \ / |
| ................\.........../.............. |
| |Determine if the neighbor has the highest| |
| |(Router Priority, Router ID) combination | |
| ''''''''''''`'''/'''''''\'''''''''''''''''' |
| / \ |
'`'''''''''''''''''''''/'''''''''''\'''''''''''''''''''''''
Smart Peering Algorithm
In order to avoid churn in the selection and establishment of the
adjacencies, every router SHOULD wait until the ModeChange timer
(Section 4) expires before running the state machine for smart
peering. Note that this wait should cause the selection process to
consider all the nodes on the link, instead of being triggered based
on receiving a Hello message from a potential neighbor. The nodes
selected using this process are referred to simply as "smart peers".
It is RECOMMENDED that the maximum number of adjacencies be set to 2.
3.2. Unsynchronized Adjacencies
An unsynchronized adjacency [RFC5820] is one for which the database
synchronization is postponed, but that is announced as FULL because
SPT reachability can be proven. A single-hop broadcast network has a
connectivity model in which all the nodes are directly connected to
each other. This connectivity results in a simplified reachability
check through the SPT: the adjacency to a specific peer MUST be
advertised as FULL by at least one smart peer.
The single-hop nature of the interface allows then the advertisement
of the reachable adjacencies as FULL without additional signaling.
Flooding SHOULD be enabled for all the unsynchronized adjacencies to
take advantage of the broadcast nature of the media. As a result,
all the nodes in the interface will be able to use all the LSAs
received.
4. Single-Hop Network Detection
A single-hop network is one in which all the nodes are directly
connected. Detection of such an interface can be easily done at
every node by comparing the speaker's 1-hop neighbors with its 2-hop
neighborhood. If for every 1-hop neighbor, the set of 2-hop
neighbors contains the whole set of the remaining 1-hop neighbors,
then the interface is a single-hop network; this condition is called
the Single-Hop Condition.
A new field is introduced in the MANET interface data structure. The
name of the field is SingleHop, and it is a flag indicating whether
the interface is operating in single-hop mode (as described in
Section 3). The SingleHop flag is set when the node meets the
Single-Hop Condition on the interface. If the Single-Hop Condition
is no longer met, then the SingleHop flag MUST be cleared.
A new timer is introduced to guide the transition of the interface
from/to multi-hop mode (which is the default mode described in
[RFC5820]) to/from single-hop mode:
o ModeChange: Every time a node changes the state of the SingleHop
flag for the interface, the corresponding ModeChange timer MUST be
set. The ModeChange timer represents the length of time in
seconds that an interface SHOULD wait before changing between
multi-hop and single-hop modes. It is RECOMMENDED that this timer
be set to Wait Time [RFC2328].
The following sections describe the steps to be taken to transition
between interface modes.
4.1. Transition from Multi-Hop to Single-Hop Mode
Detection of the Single-Hop Condition triggers the transition into
single-hop mode by setting both the SingleHop flag and the ModeChange
timer.
Once the ModeChange timer expires, the heuristic defined in
Section 3.1 MAY be executed to optimize the set of adjacencies on the
interface. Note that an adjacency MUST NOT transition from FULL to
2-Way unless the simplified reachability check (Section 3.2) can be
verified.
4.2. Transition from Single-Hop to Multi-Hop Mode
Not meeting the Single-Hop Condition triggers the transition into
multi-hop mode by clearing the SingleHop flag and setting the
ModeChange timer. The A-bit MUST be set if the Single-Hop condition
is no longer met because of one of the following cases:
o an increase in the set of 1-hop neighbors, without the
corresponding increase of the 2-hop neighborhood
o a decrease of the 2-hop neighborhood while maintaining all the
previous 1-hop neighbors
Once the ModeChange timer expires, the multi-hop operation described
in [RFC5820] takes over.
Note that the cases listed above may result in the interface either
gaining or losing a node before the ModeChange timer expires. In
both cases, the heuristic defined in Section 3.1 MAY be executed to
optimize the set of adjacencies on the interface.
In the case that a node joins the interface, the Designated Router
and Backup Designated Router fields in the Hello packet [RFC2328] MAY
be used to inform the new node of the identity (Router ID) of the
current smart peers (and avoid the optimization).
5. Security Considerations
No new security concerns beyond the ones expressed in [RFC5820] are
introduced in this document.
6. Acknowledgements
The authors would like to thank Anton Smirnov, Jeffrey Zhang, Alia
Atlas, Juan Antonio Cordero, Richard Ogier, and Christer Holmberg for
their comments.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.
[RFC5820] Roy, A. and M. Chandra, "Extensions to OSPF to Support
Mobile Ad Hoc Networking", RFC 5820, March 2010.
7.2. Informative References
[RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
for IPv6", RFC 5340, July 2008.
Authors' Addresses
Alvaro Retana
Cisco Systems, Inc.
7025 Kit Creek Rd.
Research Triangle Park, NC 27709
USA
EMail: aretana@cisco.com
Stan Ratliff
Cisco Systems, Inc.
7025 Kit Creek Rd.
Research Triangle Park, NC 27709
USA
EMail: sratliff@cisco.com
AI ELI5
On this page
§1Why Use the MANET Interface Here
The OSPF-MANET interface (RFC 5820) treats every router-to-router connection as a point-to-point link, allows a link metric per neighbor, and uses the layer-2 broadcast and multicast of the media. The authors argue these traits help in other networks too, specifically single-hop broadcast networks where all nodes are directly connected but the cost between pairs of nodes may differ. An Ethernet interface is given as a good example of the connectivity model.
The OSPF broadcast model (RFC 2328) cannot describe those differing metrics accurately, so a point-to-multipoint description fits better. This document updates RFC 5820 with a simplified operation, which does not need overlapping relays, and a new heuristic for smart peering using Router Priority.
If everyone in a room can talk to everyone but some friends are farther away, a list of individual distances describes things better than one big group label.
§2Operation in Single-Hop Mode
The operation of the MANET interface does not change on a single-hop interface, but some enhancements can be simplified. The Overlapping Relay Discovery Process SHOULD NOT be run, and the A-bit SHOULD NOT be set by any node, so the set of Active Overlapping Relays is empty. The document uses already defined mechanisms and needs no additional on-the-wire changes.
An unsynchronized adjacency is one whose database synchronization is postponed but which is announced as FULL because reachability through the shortest path tree can be proven. In a single-hop network the check is simple: the adjacency to a peer MUST be advertised as FULL by at least one smart peer. Flooding SHOULD be enabled for all unsynchronized adjacencies so that all nodes can use all the LSAs received.
Because everyone is already close, the extra helper steps are skipped. Friends only need to be told someone can reach each person, and then the news reaches all.
§3Smart Peering with Router Priority
Smart peering reduces the need for a full mesh of adjacencies: a new adjacency is not required if the node is already reachable through the existing shortest path tree. This document gives a heuristic to use in step 3 of the smart peering state machine, in place of the original one in RFC 5820.
For the MANET interface, Router Priority is an 8-bit unsigned integer that decides which routers to form a FULL adjacency with. The router with the highest Router Priority takes precedence, and a tie goes to the highest Router ID. The heuristic checks that the number of adjacencies is below the configured maximum and that the neighbor has the highest (Router Priority, Router ID) combination. Routers SHOULD wait for the ModeChange timer to expire before running it, to avoid churn. The selected nodes are called smart peers, and a maximum of 2 adjacencies is RECOMMENDED.
Instead of everyone holding hands with everyone, each person holds hands with only the top-ranked couple of friends, and ties are settled by who has the higher ID number.
§4Detecting Single-Hop Networks and Changing Mode
Each node compares its 1-hop neighbors with its 2-hop neighborhood. If, for every 1-hop neighbor, the 2-hop set contains all the remaining 1-hop neighbors, the interface is single-hop. This is the Single-Hop Condition. A new SingleHop flag in the interface data structure is set when it holds and MUST be cleared when it stops holding.
A new ModeChange timer MUST be set whenever the flag changes. It is the time an interface SHOULD wait before switching modes, and it is RECOMMENDED to be set to Wait Time from RFC 2328.
Going to single-hop mode, once the timer expires the Router Priority heuristic MAY be run, and an adjacency MUST NOT go from FULL to 2-Way unless the simplified reachability check can be verified. Going back to multi-hop mode, the A-bit MUST be set if the condition fails because the 1-hop set grew without the 2-hop set growing, or because the 2-hop neighborhood shrank while keeping all earlier 1-hop neighbors. After the timer, the multi-hop operation of RFC 5820 takes over. When a node joins, the Designated Router and Backup Designated Router fields in the Hello packet MAY be used to tell it who the current smart peers are.
Each kid checks if all their friends can also see each other. If so, they use the simple rules. If someone moves out of sight, they wait a bit and switch back to the careful rules.
- Multi-hop mode start
- Single-hop mode
| From | To | Event |
|---|---|---|
| Multi-hop mode | Single-hop mode | Single-Hop Condition met: set SingleHop flag and ModeChange timer |
| Single-hop mode | Multi-hop mode | Condition no longer met: clear flag, set timer |