Internet DRAFT - draft-jonglez-babel-rtt-extension
draft-jonglez-babel-rtt-extension
Network Working Group B. Jonglez
Internet-Draft ENS Lyon
Updates: 6126 (if approved) J. Chroboczek
Intended status: Experimental IRIF, University of Paris-Diderot
Expires: September 12, 2019 March 11, 2019
Delay-based Metric Extension for the Babel Routing Protocol
draft-jonglez-babel-rtt-extension-02
Abstract
This document defines an extension to the Babel routing protocol that
uses symmetric delay in metric computation and therefore makes it
possible to prefer lower latency links to higher latency ones.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Protocol operation . . . . . . . . . . . . . . . . . . . . . 3
2.1. Delay estimation . . . . . . . . . . . . . . . . . . . . 3
2.2. Metric computation . . . . . . . . . . . . . . . . . . . 5
2.3. Stability issues . . . . . . . . . . . . . . . . . . . . 6
2.4. Backwards and forwards compatibility . . . . . . . . . . 6
3. Packet format . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Timestamp sub-TLV in Hello TLVs . . . . . . . . . . . . . 7
3.2. Timestamp sub-TLV in IHU TLVs . . . . . . . . . . . . . . 7
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
6.1. Normative References . . . . . . . . . . . . . . . . . . 8
6.2. Informative References . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
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) +---------------+
+------------+
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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.
In this document, 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 (Section 3.4.1 of [BABEL]). This Hello is usually
accompanied with a set of IHU messages, one per neighbour
(Section 3.4.2 of [BABEL]).
In order to enable the computation of RTTs, a node A SHOULD include
in every Hello that it sends a timestamp t1 (according to A's clock).
When a node B receives A's Hello, it records in its neighbour table
the timestamp t1 as well as the time t1' according to its own (B's)
clock at which it received the packet.
When B later sends an IHU to A, it SHOULD attach to the IHU the
timestamps t1 and t1' which it has stored in its neighbour table.
Additionally, it SHOULD ensure that the packet within which the IHU
is sent contains a Hello TLV with an associated timestamp t2'
(according to B's clock). Symmetrically, A will record in its
neighbour table the timestamp t2' as well as the time t2 (according
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to A's clock) at which it has received the Hello. This is
illustrated in the following sequence diagram:
A B
| |
t1 + |
|\ |
| \ |
| \ | Hello(t1)
| \ |
| \ |
| \|
| + t1'
| |
| |
| |
| + t2'
| /|
| / |
| / |
| / | Hello(t2')
| / | IHU(t1, t1')
|/ |
t2 + |
| |
v v
A then estimates the RTT between A and B 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
is the requirement that a packet containing an IHU also contains a
Hello. Second, since only differences of timestamps according to a
single clock are computed, it does not require synchronised clocks.
Third, it requires very little additional state -- a node only needs
to store the two timestamps associated with the last hello received
from each neighbour. Finally, since it only requires piggybacking
one or two timestamps on each Hello and IHU packet, it makes
efficient use of network resources.
In principle, this algorithm is inaccurate 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 clock drift is unlikely to happen at that time scale.
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2.2. Metric computation
The algorithm described in the previous section allows computing an
RTT to every neighbour. How to map this value to a link cost is a
local implementation matter.
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. Example metric computation
The current 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.
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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.
The current 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].
2.4. Backwards and forwards compatibility
This protocol extension stores the data that it requires within sub-
TLVs of Babel's Hello and IHU TLVs. As discussed in Section 4 of
[BABEL-EXT], implementations that do not understand this extension
will silently ignore the sub-TLVs while parsing the rest of the TLVs
that they contain. In effect, this extension supports building
hybrid networks consisting of extended and unextended routers, and
while such networks might suffer from sub-optimal routing, they will
not suffer from blackholes or routing loops.
If a sub-TLV defined in this extension is longer than expected, the
additional data is silently ignored. This provision is made in order
to allow a future version of this document to extend the packet
format with additional data.
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.
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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.
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.
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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. IANA Considerations
IANA is instructed to add the following entry to the "Babel Sub-TLV
Types" registry:
+------+-----------+-----------------+
| Type | Name | Reference |
+------+-----------+-----------------+
| 3 | Timestamp | (this document) |
+------+-----------+-----------------+
5. Security Considerations
This extension merely adds additional timestamping data to two of the
TLVs sent by a Babel router, and does not significantly change the
security properties of the Babel protocol.
6. References
6.1. Normative References
[BABEL] Chroboczek, J., "The Babel Routing Protocol", RFC 6126,
February 2011.
[BABEL-EXT]
Chroboczek, J., "Extension Mechanism for the Babel Routing
Protocol", RFC 7557, May 2015.
6.2. Informative References
[DELAY-BASED]
Jonglez, B. and J. Chroboczek, "A delay-based routing
metric", March 2014.
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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
IRIF, University of Paris-Diderot
Case 7014
75205 Paris Cedex 13
France
Email: jch@irif.fr
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