Network Working Group J. Chroboczek
Internet-Draft PPS, University of Paris-Diderot
Intended status: Experimental July 4, 2014
Expires: January 5, 2015
Diversity Routing for the Babel Routing Protocol
draft-chroboczek-babel-diversity-routing-00
Abstract
This document defines an extension to the Babel routing protocol that
allows routing updates to carry radio frequency information, and
therefore makes it possible to use radio diversity information for
route selection.
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Table of Contents
1. Introduction and background . . . . . . . . . . . . . . . . . 2
2. Operation of the protocol . . . . . . . . . . . . . . . . . . 3
2.1. Changes to data structures . . . . . . . . . . . . . . . 3
2.2. Receiving updates . . . . . . . . . . . . . . . . . . . . 4
2.3. Sending updates . . . . . . . . . . . . . . . . . . . . . 4
2.4. Metric computation and route selection . . . . . . . . . 5
2.5. Protocol encoding . . . . . . . . . . . . . . . . . . . . 5
3. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
Appendix A. The Z3 algorithm . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction and background
The Babel routing protocol [BABEL] does not mandate a specific
algorithm for computing metrics; Appendix A of that document suggests
using an additive integer metric. While this works well in many
topologies, it fails to take into account the possibility of
interference between radio links, which is important in multi-
frequency wireless mesh networks.
Consider for example the following topology, where the solid lines
use one radio frequency and the dashed lines another, and suppose
that the solid frequency has very slightly lower packet loss than the
dashed one:
B
/ \
/ \
A D
\ .
\ .
C
When sending data from A to D, Babel will reliably choose the solid
route through B. Howerver, this route self-interferes: when B is
sending a packet to D, it cannot simultaneously be receiving a packet
from A, which halves the effective throughput. No such issue arises
with the route through C, which should therefore be preferred.
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Interference needs to be taken into account even when it happens
between non-adjacent links. Consider the following topology:
B +++ C
/ \
/ \
A F
\ .
\ .
D +++ E
When routing data from A to F, the route through B and C has two
interfering links: if two packets are sent by A and C at roughly the
same time, a collision will occur, and both packets will need to be
resent. Again, no such issue arises with the route through D and E.
2. Operation of the protocol
The diversity protocol extension allows a Babel router to attach
information about radio frequency to the routes that it maintains --
we call this the route's "diversity information".
We assume that all links can be categorised into one of the following
categories:
o non-interfering links, e.g. wired links;
o links that have a well defined frequency, and only interfere with
other links at the same frequency; these are described by a single
channel number, an integer between 1 and 254;
o interfering links, links that interfere with all other links
except non-interfering links.
This model does not describe reality accurately, since distinct but
close radio frequencies do in fact interfere, but it works well
enough in practical networks, where a small number of discrete radio
frequencies are used.
2.1. Changes to data structures
A Babel router maintains a route table ([BABEL] Section 3.2.5). A
router implementing diversity routing has one additional field in
every route table entry:
o the diversity data, a (possibly zero-length) sequence of channel
numbers, each of which is an integer between 1 and 255.
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The diversity data is interpreted as the set of channels of the links
that would be followed by a packet sent along this route, omitting
non-interfering links. The value 255 is special -- it indicates an
interfering link.
2.2. Receiving updates
When a node receives an Update TLV, it creates or updates a routing
table entry according to [BABEL], Section 3.5.4. A node that
performs diversity routing extends the procedure given in that
section with the following procedure.
Let D be the diversity information attached to the received Update
TLV, or the one-element sequence 255 if there is no such information.
Then the routing table entry diversity information is set to D',
where:
o if the interface over which the update was received is non-
interfering, then D'=D;
o if the interface over which the update was received is tuned to
channel C, then D'=C.D;
o if the interface over which the update was received is
interfering, then D'=255.D.
Note that zero-length diversity information is different from lack of
diversity information: the latter is treated as 255 (interfering,
since no information is available) in order to ensure
interoperability with the original Babel protocol.
2.3. Sending updates
A Babel node sends updates in various circumstances, described in
[BABEL], Section 3.7. A node performing diversity routing attaches
diversity data to every update that it send. This diversity data is
computed as follows:
o if the update is for a locally redistributed route, then the value
is implementation-dependent (zero-length diversity information is
a good choice);
o if the update is for a route in the Babel route table, then the
diversity information is taken from the route table.
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2.4. Metric computation and route selection
How the diversity data is used for metric computation and/or route
selection is left to the implementation, as long as it obeys the
rules given in Sections 3.5.2 and 3.6 of [BABEL]. In particular, the
strict monotonicity requirement implies that a non-interfering hop
must be taken into account in the resulting metric -- it cannot be
simply ignored.
An algorithm that has been found to work relatively well in practice
is given in Appendix A.
2.5. Protocol encoding
We define one new sub-TLV which is attached to Update TLVs and
contains a sequence of channel numbers.
2.5.1. Encoding of channel numbers
A channel number is encoded as a one-octet integer. The following
values are used by the current implementation:
0 This value is reserved, MUST NOT be sent, and MUST be silently
filtered out on reception;
1-14 IEEE 802.11b channels;
36-165 IEEE 802.11a channels;
255 used to represent an interfering link.
Other values are not currently used, and MAY be used by mutual
agreement to represent radio frequencies not covered by the above.
2.5.2. The Diversity sub-TLV
Diversity data is carried in a Diversity sub-TLV [BABEL-EXT] that is
carried by Update TLVs. The sub-TLV contains a sequence of octets
that directly encode the diversity data from the route table.
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 = 2 | Length | Channel 1 | Channel 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Channel 3 | ...
+-+-+-+-+-+-+-+-+-+-+-+-+-
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Fields :
Type Set to 2 to indicate a Diversity Information sub-TLV.
Length The length of the body, exclusive of the Type and Length
fields.
Channel n A channel number, or 255 if the hop is assumed to interfere
with all other hops, as described in the previous section.
3. References
[BABEL] Chroboczek, J., "The Babel Routing Protocol", RFC 6126,
February 2011.
[BABEL-EXT]
Chroboczek, J., "Extension Mechanism for the Babel Routing
Protocol", Internet Draft draft-chroboczek-babel-
extension-mechanism-01, June 2014.
Appendix A. The Z3 algorithm
In this section, we describe the Z3 algorithm, a particular instance
of diversity routing that has seen some modest deployment and that
appears to work reasonably well in practice while being extremely
easy to implement.
The Z3 algorithm works by announcing a slightly smaller metric than
the metric it uses for route selection when announcing over a non-
interfering link. In effect, a Z3 router maintains two metrics for
each route: the noninterfering metric, which is announced on links
that can be proven to not interfere with the route being announced,
and the interfering metric, which is used for route selection and
announced over all other links.
More precisely, upon receiving an update with metric M over a link
with cost C, the interfering metric is set to C+M, as suggested in
Appendix A of [BABEL]. The non-interfering metric is set to
alpha*C+M, where 0<alpha<1 is called the diversity factor (with
rounding biased upwards in order to ensure strict monotonicity).
Let D be the diversity data of route R, and L be a link. We say that
R interferes with L when one of the following is true:
o L is a non-interfering link (e.g. an Ethernet); or
o L is a radio interface tuned to channel C, and neither C nor 255
is an element of D.
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When we announce R over L, we announce the interfering metric if R
interferes with L, and the non-interfering metric otherwise.
The metric that Z3 yields is non-isotonic; hence, Z3 Babel does not
necessarily converge to a set of minimum-metric routes. In fact, the
set of minimum-metric routes might not even be a tree in the general
case. The author believes that Z3 Babel converges to a Nash
equilibrium, but this appears to be a difficult property to prove.
Author's Address
Juliusz Chroboczek
PPS, University of Paris-Diderot
Case 7014
75205 Paris Cedex 13
France
Email: jch@pps.univ-paris-diderot.fr
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