One document matched: draft-jonglez-babel-rtt-extension-00.txt
Network Working Group B. Jonglez
Internet-Draft ENS Lyon
Updates: 6126 (if approved) J. Chroboczek
Intended status: Experimental PPS, University of Paris-Diderot
Expires: January 3, 2015 July 2, 2014
Delay-based Metric Extension for the Babel Routing Protocol
draft-jonglez-babel-rtt-extension-00
Abstract
This document defines an extension to the Babel routing protocol
[BABEL] that uses the delay to a neighbour in metric computation and
therefore makes it possible to prefer lower latency links to higher
latency ones.
Status of This Memo
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Table of Contents
1. Introduction and background . . . . . . . . . . . . . . . . . 2
2. Protocol operation . . . . . . . . . . . . . . . . . . . . . 3
2.1. Delay estimation . . . . . . . . . . . . . . . . . . . . 3
2.2. Metric computation . . . . . . . . . . . . . . . . . . . 4
2.3. Stability issues . . . . . . . . . . . . . . . . . . . . 5
3. Packet format . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Timestamp sub-TLV in Hello TLVs . . . . . . . . . . . . . 6
3.2. Timestamp sub-TLV in IHU TLVs . . . . . . . . . . . . . . 7
4. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction and background
The Babel routing protocol [BABEL] does not mandate a specific
algorithm for computing metrics; existing implementations use a
packet-loss based metric on wireless links and a simple hop-count
metric on all other types of links. While this strategy works
reasonably well in many networks, it fails to select reasonable
routes in some topologies involving tunnels or VPNs.
Consider for example the following topology, with three routers A, B
and D located in Paris and a fourth router located in Tokyo,
connected through tunnels in a diamond topology.
+------------+
| A (Paris) +---------------+
+------------+ \
/ \
/ \
/ \
+------------+ +------------+
| B (Paris) | | C (Tokyo) |
+------------+ +------------+
\ /
\ /
\ /
+------------+ /
| D (Paris) +---------------+
+------------+
When routing traffic from A to D, it is obviously preferable to use
the local route through B, as this is likely to provide better
service quality and lower monetary cost than the distant route
through C. However, the existing implementations of Babel consider
both routes as having the same metric, and will therefore route the
traffic through C in roughly half the cases.
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In this memo, we specify an extension to the Babel routing protocol
that enables precise measurement of the round-trip time (RTT) of a
link, and allows its usage in metric computation. Since this causes
a negative feedback loop, special care is needed to ensure that the
resulting network is reasonably stable (Section 2.3).
We believe that this protocol may be useful in other situations than
the one described above, such as when running Babel in a congested
wireless mesh network or over a complex link layer that performs its
own routing; the high granularity of the timestamps used (1ms) should
make it easier to experiment with RTT-based metrics on this kind of
link layers.
2. Protocol operation
The protocol estimates the RTT to each neighbour (Section 2.1) which
it then uses for metric computation (Section 2.2).
2.1. Delay estimation
The RTT to a neighbour is estimated using an algorithm due to Mills
[MILLS], originally developed for the HELLO routing protocol and
later used in NTP [NTP].
A Babel speaker periodically sends a multicast Hello message over all
of its interfaces. This Hello is usually accompanied with a set of
IHU messages, one per neighbour.
In order to enable the computation of RTTs, a node A includes in
every Hello that it sends a timestamp t1 according to A's clock.
Additionally, a node B includes in the IHU it sends to A the
timestamp t1 included in the last Hello received from A, and the
timestamp t1' according to B's clock at which it received that Hello.
Upon receiving B's combined Hello and IHU, node A records the
timestamp t2 at which it received the combined packet, according to
A's clock. This is described in the following sequence diagram:
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A B
| |
t1 + |
|\ |
| \ |
| \ | Hello(t1)
| \ |
| \ |
| \|
| + t1'
| |
| |
| |
| + t2'
| /|
| / |
| / |
| / | Hello(t2')
| / | IHU(t1, t1')
|/ |
t2 + |
| |
v v
Node A then computes the RTT as (t2 - t1) - (t2' - t1').
This algorithm has a number of desirable properties. First, since
there is no requirement that t1' and t2' be equal, the protocol
remains asynchronous -- the only change to Babel's message scheduling
that is required is to ensure that IHUs are always sent together with
Hellos. Second, since only differences of timestamps according to a
single clock are computed, it does not require synchronised clocks.
Third, it is mostly stateless -- a node only needs to store the two
timestamps associated with the last hello received from each
neighbour. Finally, since it only requires piggybacking a couple of
timestamps on each Hello and IHU packet, it makes efficient use of
network resources.
In principle, this protocol is incorrect in the presence of clock
drift (i.e. when A's and B's clocks are running at different
frequencies). However, t2' - t1' is usually on the order of seconds,
and significant drift is unlikely to happen at this time scale.
2.2. Metric computation
The algorithm described in the previous section allows computing an
RTT to all neighbours. How to map this value to a link cost is left
to the implementation.
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Obviously, the mapping should be monotonic (larger RTTs imply larger
costs). In addition, in order to enhance stability (Section 2.3),
the mapping should be bounded -- above a certain RTT, all links are
equally bad.
2.2.1. Sample mapping
The sample implementation of Babel uses the following function for
mapping RTTs to link costs, parameterised by three parameters rtt-
min, rtt-max and max-rtt-penalty:
cost
^
|
|
| C + max-rtt-penalty
| +---------------------------
| /.
| / .
| / .
| / .
| / .
| / .
| / .
| / .
| / .
| / .
C +------------+ .
| . .
| . .
| . .
| . .
0 +---------------------------------------------------->
0 rtt-min rtt-max RTT
For RTTs below rtt-min, the link cost is just the nominal cost of a
single hop, C. Between rtt-min and rtt-max, the cost increases
linearly; abover rtt-max, the constant value max-rtt-penalty is added
to the nominal cost.
2.3. Stability issues
Using delay as an input to the routing metric in congested networks
gives rise to a negative feedback loop: low RTT encourages traffic,
which in turn causes the RTT to increase. In a congested network,
such a feedback loop can cause persistent oscillations.
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The sample implementation of Babel uses three techniques that
collaborate to limit the frequency of oscillations:
o the measured RTT is smoothed, which limits Babel's response to
short-term RTT variations;
o the mapping function is bounded, which avoids switching between
congested routes;
o a hysteresis algorithm is applied to the metric before route
selection, which limits the worst-case frequency of route
oscillations.
These techniques are discussed in more detail in [DELAY-BASED].
3. Packet format
This extension defines the Timestamp sub-TLV [BABEL-EXT], whose Type
field has value 3. This sub-TLV can be contained within a Hello sub-
TLV, in which case it carries a single timestamp, or within an IHU
sub-TLV, in which case it carries two timestamps.
Timestamps are encoded as 32-bit unsigned integers, expressed in
units of one microsecond, counting from an arbitrary origin.
Timestamps wrap around every 4295 seconds, or slightly more than one
hour.
3.1. Timestamp sub-TLV in Hello TLVs
When contained within a Hello TLV, the Timestamp sub-TLV has the
following format:
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 = 3 | Length | Transmit timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields :
Type Set to 3 to indicate a Timestamp sub-TLV.
Length The length of the body, exclusive of the Type and Length
fields.
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Transmit timestamp The time at which the packet containing this sub-
TLV was sent, according to the sender's clock.
If the Length field is larger than the expected 4 octets, the sub-TLV
MUST be processed normally and any extra data contained in this sub-
TLV MUST be silently ignored.
3.2. Timestamp sub-TLV in IHU TLVs
When contained in an IHU TLV destined for node A, the Timestamp sub-
TLV has the following format:
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 = 3 | Length | Origin timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | Receive timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields :
Type Set to 3 to indicate a Timestamp sub-TLV.
Length The length of the body, exclusive of the Type and Length
fields.
Origin timestamp A copy of the transmit timestamp of the last
Timestamp sub-TLV contained in a Hello TLV received from
node A.
Receive timestamp The time at which the last Hello with a Timestamp
sub-TLV was received from node A according to the sender's
clock.
If the Length field is larger than the expected 8 octets, the sub-TLV
MUST be processed normally and any extra data contained in this sub-
TLV MUST be silently ignored.
4. References
[BABEL] Chroboczek, J., "The Babel Routing Protocol", RFC 6126,
February 2011.
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[BABEL-EXT]
Chroboczek, J., "Extension Mechanism for the Babel Routing
Protocol", Internet Draft draft-chroboczek-babel-
extension-mechanism-01, June 2014.
[DELAY-BASED]
Jonglez, B. and J. Chroboczek, "A delay-based routing
metric", March 2014.
Available online from http://arxiv.org/abs/1403.3488
[MILLS] Mills, D., "DCN Local-Network Protocols", RFC 891,
December 1983.
[NTP] Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network
Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, June 2010.
Authors' Addresses
Baptiste Jonglez
ENS Lyon
France
Email: baptiste.jonglez@ens-lyon.org
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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