Network Working Group T. Clausen
Internet-Draft LIX, Ecole Polytechnique, France
Intended status: Informational October 27, 2008
Expires: April 30, 2009
The MANET Link Type
draft-clausen-manet-linktype-00
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Abstract
This document describes the link characteristics and properties for
links over which MANET protocols are designed to operate.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. The MANET Router . . . . . . . . . . . . . . . . . . . . . . . 4
4. MANET Interfaces . . . . . . . . . . . . . . . . . . . . . . . 5
5. MANET Network Dynamics . . . . . . . . . . . . . . . . . . . . 6
6. The MANET Link Type . . . . . . . . . . . . . . . . . . . . . 8
6.1. Connectivity: Symmetry, Transitivity, Continouity ? . . . 8
6.2. Subnet Model and Addresses . . . . . . . . . . . . . . . . 9
6.3. Multicast and Broadcast Scopes . . . . . . . . . . . . . . 10
7. The MANET Addressing Architecture . . . . . . . . . . . . . . 11
7.1. MANET Interface Configuration . . . . . . . . . . . . . . 12
7.2. MANET Addressing Architecture Characteristics . . . . . . 12
8. Security Considerations . . . . . . . . . . . . . . . . . . . 13
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
10. Informative References . . . . . . . . . . . . . . . . . . . . 13
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 14
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1. Introduction
A Mobile Ad hoc NETwork, or MANET, is commonly described as a loosely
connected set of routers with no predetermined infrastructure and
where the routers discover and maintain, even when faced with
dynamically changing topologies, a routing structure. Neither the
set of routers in the MANET nor their connections to each other can
be assumed to be pre-determined nor to be known in advance; either
may change randomly over the lifetime of the MANET.
MANETs are often constructed from routers using wireless broadcast
interfaces, such as IEEE 802.11 interfaces in ad hoc mode, in order
to establish connectivity among each other. Other network interfaces
and link types, such as Ethernet or point-top-point IP over IP
tunnels, are occasionally present between routers in a MANET, and are
then also used by MANET protocols, such as a MANET routing protocols
(e.g. [RFC3626], [RFC3561], [RFC4728], [RFC3684]) calculating
routing paths.
This presents MANET protocols with the challenge of operating not
only over well known link types such as an Ethernet or a point-to-
point IP over IP tunnel, but also to over links as formed over
wireless broadcast interfaces.
The purpose of this document is to describe a MANET Link Type which
accommodates both, such that a protocol designed for the MANET Link
Type will operate correctly both when presented with e.g. "an
Ethernet" or with "a wireless broadcast interface".
2. Terminology
Neighbor - a router B is a neighbor of a router A if B can receive
communication directly from router A, without passing through any
intermediates at the same layer (i.e. an IP router).
Wireless broadcast interface - a network interface where the medium
supports true broadcast transmissions, and where link layer
messages can be either multicast or unicast. The transmission
reachability is constrained by the radio range of the
transmittter, which can be time-varying.
Node - any device (router or host) that implements IP.
Router - a node that forwards IP packets not explicitly addressed to
itself.
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MANET Router - a router which has, at least, one MANET interface
towards a link of the MANET Link Type, as described in this
document, and which is capable of ensuring correct operation over
that link.
Host - any node that is not a router, i.e. a host does not forward
packets addressed to others. A Host runs a standard IP stack, and
is subject to no MANET Link specific assumptions.
MANET Link - a link of the MANET Link type, as described in this
document.
3. The MANET Router
The entities that are concerned by the MANET Link Type described in
this document are MANET routers. A MANET Router is a router which
has at least one, but possibly more, MANET interfaces, and zero or
more interfaces of other types and towards other networks, as
illustrated in Figure 1.
\ | /
\|/ ------- MANET Interface(s)
|
+---+----+
| |
| Manet |
| Router |
| |
+--------+
| |
| | ------- NON-MANET Interface(s)
----+ +----
Figure 1: MANET Router
MANET interfaces are the only interfaces which are exposed to links
of the MANET Link Type, as described in this document. Protocols
operating directly over these MANET interfaces are, therefore, the
only protocols which are required to be aware of the characteristics
of the MANET Link Type.
This entails that protocols which are not intended to operate over
MANET interfaces are not required to be able to handle the
characteristics of the MANET Link Type.
In particular, any node connected to a MANET router over an interface
other than a MANET interface, will see the MANET router as it would
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see any other IP router. For example, in Figure 2 the hosts
connected to the MANET router via the Ethernet link will simply
perceive an Ethernet link with hosts and a router, oblivious to if
the router is a MANET router or not.
\ | /
\|/ ------- MANET Interface(s)
|
+---+----+
| |
| Manet |
| Router |
| |
+--------+
|
| ------- Ethernet Interface
|
+-----------------------+
| | | |
+---+ +---+ +---+ +---+
| H | | H | | H | | H |
+---+ +---+ +---+ +---+
Figure 2: MANET Router with Hosts on an Ethernet
Isolated by an IP hop, hosts on the Ethernet link in Figure 2 are not
exposed to the particularities of the MANET Link Type - similarly to
how, say, hosts on an Ethernet connected to an OSPF router with NBMA
interfaces are not exposed to the particularities of the NBMA Link
Type.
4. MANET Interfaces
An interface is, according to [RFC4862], "a node's attachment to a
link", indicating that an interface is a unique point of attachment
to a single link. It therefore follows that a MANET interface is a
MANET routers point of attachment to a MANET Link.
A MANET interface is often a wireless broadcast interface, as
illustrated in Figure 3, in which 5 MANET interfaces are connected to
the same MANET link and form a simple MANET. If these MANET
interfaces are wireless broadcast interfaces, their transmission
range is limited, as indicated.
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-------+------- ---+------- Transmission
------+------- -------+------- -------+------- Ranges
\|/ \|/ \|/ \|/ \|/
| | | | |
+-+-+ +-+-+ +-+-+ +-+-+ +-+-+
| A | | B | | C | | D | | E |
+---+ +---+ +---+ +---+ +---+
Figure 3: MANET Interface Transmission Ranges
In this simple network, a transmission by the node C reaches only
nodes B and D on the MANET link, due to the limited transmission
range of the wireless broadcast interface. Similarly, a transmission
from node B reaches only nodes A and C and a transmission by node D
reaches only node E - the latter might be due to environmental
interference or obstacles, transmission power levels or antenna
properties of node C or D (e.g. directional antennas on either or
both of C and D).
Note that the "Ethernet-like" interface characteristics that are
usually assumed, where all Ethernet interfaces on the same link can
reach each other, is a special case of this; and an Ethernet
interface would be a perfectly acceptable MANET interface.
This example in Figure 3 exhibits some of the characteristics of a
MANET Link: connectivity on a link can not be assumed to be
symmetric, nor can it be assumed to be transitive. These MANET Link
Type characteristics are detailed in Section 6.
5. MANET Network Dynamics
In a MANET, the set of participating MANET routers may change,
possibly frequently, over time, as can the relative position of the
MANET routers change as the network evolves. More specifically, the
set of MANET interfaces attached to a given MANET Link may change
over time, and a MANET interface may change its position on a MANET
Link, which may change the set of neighbors of that MANET interface.
The simple network in Figure 3 may evolve over time, as illustrated
in Figure 4, where at time t1 node A disappears from the MANET Link
and node C and D moves out of radio range from each other and are no
longer able to communicate. At time t2, node F appears on the MANET
Link, at a position where nodes C and D are within its radio range.
The set of MANET interfaces which can be reached by a transmission
from any MANET interface on the MANET Link may therefore also change
over time. In particular, the ability for a MANET interface to
receive a transmission from another MANET interface on the same MANET
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Link and at a given point in time does not necessarily indicate that
such is also possible in the future.
-------+------- ---+-------
------+------- -------+------- -------+-------
\|/ \|/ \|/ \|/ \|/
| | | | |
+-+-+ +-+-+ +-+-+ +-+-+ +-+-+ t0
| A | | B | | C | | D | | E |
+---+ +---+ +---+ +---+ +---+
-------+------- ---+-------
------+------- -------+-------
\|/ \|/ \|/ \|/
| | | |
+-+-+ +-+-+ +-+-+ +-+-+ t1
| B | | C | | D | | E |
+---+ +---+ +---+ +---+
-------+------- ---+-------
------+------- -------+------- -------+-------
\|/ \|/ \|/ \|/ \|/
| | | | |
+-+-+ +-+-+ +-+-+ +-+-+ +-+-+ t2
| B | | C | | F | | D | | E |
+---+ +---+ +---+ +---+ +---+
Figure 4: MANET Network Dynamics
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6. The MANET Link Type
[Thaler] enumerates a set of properties, which are commonly assumed
by applications and upper layer protocols, and notes that these
assumptions are becoming increasingly less true. The MANET Link Type
is an example of a Link Type which challenges these assumptions.
The MANET Link Type characteristics can be summarized as follows:
o connectivity can not be assumed to be symmetric within a MANET
Link;
o connectivity can not be assumed to be transitive within a MANET
Link;
o connectivity can not be assumed to be continuous within a MANET
Link;
o the point of attachment to a MANET Link determines the view of
that MANET link;
o multicast and broadcast can not be assumed to work across a MANET
Link;
o a subnet is smaller than a MANET Link;
o addresses can not be assumed to be part of an on-link subnet on a
MANET Link.
It is important to note that the MANET Link Type is non-prescriptive,
i.e. it does not *require* a link to have these characteristics in
order for MANET protocols to operate correctly over it, however
protocols designed for the MANET Link Type are *required* to be able
to operate correctly also when presented with these link
characteristics.
This, in particular, entails that for example an Ethernet would be
perfectly acceptable as a MANET Link and that MANET protocols would
operate correctly when presented with an Ethernet.
6.1. Connectivity: Symmetry, Transitivity, Continouity ?
As indicated in Section 4, MANET interfaces on a MANET Link may not
all be neighbors, i.e. may not be able to communicate directly
between each other without intermediate retransmissions,
specifically:
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o node A being a neighbor of node B does not necessarily imply that
node B is also a neighbor of node A;
o node A being a neighbor of nodes B and C does not necessarily
imply that nodes B and C are neighbors.
Furthermore, as indicated in Section 5, neighbors may change over,
specifically:
o the ability for a MANET interface to receive a transmission from
another MANET interface on the same MANET Link and at a given
point in time does not necessarily indicate that such is also
possible in the future.
6.2. Subnet Model and Addresses
[Thaler] observes that "a subnet is smaller than, or equal to a
link", specifically that "destinations with addresses in the same
subnet can be reached with TTL (or Hop Count) = 1". On a MANET Link,
a transmission with TTL (or Hop Count) = 1 can be received only by
MANET interfaces which are neighbors of the sending MANET interface.
The first observation is, that "subnet", "reachability without
decrementing TTL" and "addresses within a subnet" are intimately
related, and assume that "all interfaces with addresses within the
same subnet are neighbors".
-------+------- ---+------- Transmission
------+------- -------+------- -------+------- Ranges
\|/ \|/ \|/ \|/ \|/
| | | | |
+-+-+ +-+-+ +-+-+ +-+-+ +-+-+
| A | | B | | C | | D | | E |
+---+ +---+ +---+ +---+ +---+
Figure 5: MANET Subnet Challenge
Considering node C on the MANET Link as depicted in Figure 5, this
router can reach the MANET interfaces of node B and D in a single
transmission and with a TTL (or Hop Count) = 1. Nodes B, C and D
could, therefore, be candidates for belonging to the same subnet.
However a transmission by node B can not reach node D without being
retransmitted by node C and so a transmission from node B with a TTL
(or Hop Count) = 1 will not reach node D. Furthermore, due to the
limited transmission range of node D, node D can reach neither of
nodes B and C -- and so, nodes B, C and D can not belong to the same
subnet.
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This leads to the second observation, that the only set of addresses
which on a MANET Link can be guaranteed reachable in a single
transmission with TTL (or Hop count) = 1 are those of the
transmitting interface. The only addresses which can belong to the
same subnet on a MANET Link are, therefore, the addresses assigned to
the same MANET interface.
While the MANET interfaces of nodes B and C in Figure 5 may not be
configured with addresses from within the same subnet, these may
still communicate e.g. as point-to-point links where the two
endpoints have addresses from unrelated address spaces.
6.3. Multicast and Broadcast Scopes
IPv4 Limited Broadcast (255.255.255.255) and IPv4 and IPv6 Link Local
Multicast (FFx2::) are specified to not be forwarded [RFC3927]
[RFC4291] [RFC3330]. As a consequence, on a MANET Link:
o the scope within which a Limited Broadcast or a Link Local
Multicast transmission can be received is limited to that of the
transmitting MANET interface and its neighbors.
In other words, the broadcast and link local multicast scope of a
MANET interface on a MANET Link is the MANET interfaces which are
within transmission range. In figure Figure 6, the broadcast and
multicast scope of node C is, as indicated, the MANET interfaces of
nodes B and D; the broadcast and multicast scope of node D is the
MANET interface of node E.
---+------- Multicast
-------+------- Scopes
\|/ \|/ \|/ \|/ \|/
| | | | |
+-+-+ +-+-+ +-+-+ +-+-+ +-+-+
| A | | B | | C | | D | | E |
+---+ +---+ +---+ +---+ +---+
Figure 6: MANET Broadcast and Multicast Scope
Symmetrically, a broadcast or a link local multicast can be received
by any MANET interface within transmission range of the transmitter,
whether those are the receivers intended or not. Due to the fact
that connectivity can not be assumed to be symmetric, the transmitter
may not a priori know which MANET interfaces have received the
broadcast or link local multicast, nor can it be assumed that the
recipients a posteriori can signal that they received the broadcast
or link local multicast.
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7. The MANET Addressing Architecture
This section presents an addressing architecture model for MANETs,
which preserves the integrity of the conventional IP addressing
architecture while allowing for the particularities of the MANET Link
Type. In particular, the applications and protocols running on hosts
are not exposed to a MANET Link. Only MANET interfaces of MANET
routers are required to be aware of the MANET Link Type, and to be
configured according to the characteristics of the MANET Link Type.
A MANET router is a router with at least one MANET interface towards
a MANET Link and, possibly, with zero or more other interfaces
towards other routers or hosts. The MANET router may, as any router
be delegated zero or more prefixes, which it may assign, integrally
or as subnet prefixes, to any links of its non-MANET interfaces,
which are configured accordingly. Hosts and routers on these non-
MANET interfaces may be assigned addresses from within these prefixes
according to the address (auto)configuration mechanisms governing
these (non-MANET) links, such as [RFC4862] and [RFC2131].
Considering the example in Figure 7, the MANET router is delegated
the prefix p::/48. Subnet-prefixes p:1::/62, p:2::/62 and p:3::/63
from p::/48 are derived and assigned to the non-MANET links.
Interfaces on these links are configured with addresses from within
the subnet prefix of that link, as usual.
M
A
\ | / N
\|/ ------- MANET Interface(s) E
| T
+- --+--- -+
| |
Manet | p::/48
.........................| Router |..............................
| |
+- ------ -+ N
p:1::/62 | | | p:3::/62 o
+-------+-------+-------+ | +-------+-------+-------+ n
| | | | | | | | | |
+---+ +---+ +---+ +---+ | +---+ +---+ +---+ +---+ M
| H | | H | | H | | H | | | H | | H | | H | | H | A
+---+ +---+ +---+ +---+ | +---+ +---+ +---+ +---+ N
| E
-------------+---------------- T
p:2::/62
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Figure 7: MANET Router and Prefixes Example
The configuration of MANET interface(s) of the MANET router requires
special attention, and is detailed in Section 7.1.
7.1. MANET Interface Configuration
As described in Section 6.2, on a MANET Link the only addresses which
can be guaranteed to be reachable with TTL (or Hop Count) = 1, and
therefore can be admitted to be within the same subnet, are the
addresses assigned to the sending MANET interface. Consequently,
MANET interfaces must be configured such that:
o no two MANET interfaces appear within the same subnet, i.e. with
the same prefix and prefix length.
This can be, and is commonly, accomplished by configuring MANET
interfaces with a /32 (IPv4) or a /128 (IPv6) address, e.g. as an
unnumbered interface, borrowing a single IP address from a non-MANET
interface of the MANET router.
It is worth noting that prefix lengths shorter than /128 (IPv6) or
/32 (IPv4) are possible on MANET interfaces, as long as the prefixes
are unique to a single MANET interface. Note that the above
statements are not an exception, but simply a clarification that
MANET are no different from other networks in this respect.
7.2. MANET Addressing Architecture Characteristics
The MANET addressing architecture presented in this section makes a
clear separation between the role of MANET router and host in a
MANET, recognizing that:
o MANET Link Type characteristics are only exposed to MANET
interfaces of MANET-aware routers, running appropriate protocols;
o routers and hosts, and more generally networks/subnets, on non-
MANET interface(s) are not subject to the particularities of the
MANET Link Type but are isolated herefrom by an IP hop;
o applications and protocols on hosts and routers, and more
generally networks/subnets, on non-MANET interfaces run
unmodified.
Note that this addressing architecture is similar to how routing in
the existing Internet is structured. Routers run their routing
protocol over router interconnects with various characteristics to
which only the routing protocols are privy. On the other hand, hosts
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connect to routers over interfaces with well-defined characteristics.
8. Security Considerations
This document does not currently present any security considerations.
9. IANA Considerations
This document does not have any IANA actions
10. Informative References
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, March 1997.
[RFC3330] IANA, IANA., "Special-Use IPv4 Addresses", RFC 3330,
September 2002.
[RFC3561] Perkins, C., Belding-Royer, E., and S. Das, "Ad hoc On-
Demand Distance Vector (AODV) Routing", RFC 3561,
July 2003.
[RFC3626] Clausen, T. and P. Jacquet, "The Optimized Link State
Routing Protocol", RFC 3626, October 2003.
[RFC3684] Ogier, R., Templin, f., and M. Lewis, "Topology
Dissemination Based on Reverse-Path Forwarding", RFC 3684,
February 2004.
[RFC4728] Johnson, D., Hu, Y., and D. Maltz, "The Dynamic Source
Routing Protocol (DSR) for Mobile Ad Hoc Networks for
IPv4", RFC 4728, February 2007.
[RFC3927] Cheshire, S., Aboba, B., and E. Guttman, "Dynamic
Configuration of IPv4 Link-Local Addresses", RFC 3927,
May 2005.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[RFC4862] Narten, T., Thomson, S., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862, September 2007.
[Thaler] Thaler, D., "Evolution of the IP Model", Work In
Progress draft-thaler-ip-model-evolution-01.txt,
July 2008.
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Appendix A. Acknowledgments
This document is greatly inspired from discussions with Dave Thaler
(Microsoft), Jari Arkko (Ericsson), Mark Townsley (Cisco), Ian
Chakeres (Motorola).
Christopher Dearlove (BAE Systems) and Emmanuel Baccelli (INRIA) both
provided reviews and insightful comments on early iterations of this
text.
Author's Address
Thomas Heide Clausen
LIX, Ecole Polytechnique, France
Phone: +33 6 6058 9349
EMail: T.Clausen@computer.org
URI: http://www.thomasclausen.org/
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