Internet Engineering Task Force (IETF) B. Jonglez

Internet Engineering Task Force (IETF) B. Jonglez
Request for Comments: 9616 ENS Lyon
Category: Standards Track J. Chroboczek
ISSN: 2070-1721 IRIF, Université Paris Cité
September 2024

Delay-Based Metric Extension for the Babel Routing Protocol

Abstract

This document defines an extension to the Babel routing protocol that
measures the round-trip time (RTT) between routers and makes it
possible to prefer lower-latency links over higher-latency ones.

Status of This Memo

This is an Internet Standards Track document.

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). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9616.

Copyright Notice

Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.

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Table of Contents

1. Introduction
1.1. Applicability
2. Specification of Requirements
3. RTT Sampling
3.1. Data Structures
3.2. Protocol Operation
3.3. Wrap-Around and Node Restart
3.4. Implementation Notes
4. RTT-Based Route Selection
4.1. Smoothing
4.2. Cost Computation
4.3. Hysteresis
5. Backwards and Forwards Compatibility
6. Packet Format
6.1. Timestamp Sub-TLV in Hello TLVs
6.2. Timestamp Sub-TLV in IHU TLVs
7. IANA Considerations
8. Security Considerations
9. References
9.1. Normative References
9.2. Informative References
Acknowledgements
Authors' Addresses

1. Introduction

The Babel routing protocol [RFC8966] 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.

+------------+
| A (Paris) +---------------+
+------------+ \
/ \
/ \
/ \
+------------+ +------------+
| B (Paris) | | C (Tokyo) |
+------------+ +------------+
\ /
\ /
\ /
+------------+ /
| D (Paris) +---------------+
+------------+

Figure 1: Four Routers in a Diamond Topology

For example, consider the topology described in Figure 1, with three
routers A, B, and D located in Paris and a fourth router C located in
Tokyo, connected through tunnels in a diamond topology. 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; therefore, they will route the traffic through C in
roughly half the cases.

In the first part of this document (Section 3), we specify an
extension to the Babel routing protocol that produces a sequence of
accurate measurements of the round-trip time (RTT) between two Babel
neighbours. These measurements are not directly usable as an input
to Babel's route selection procedure since they tend to be noisy and
to cause a negative feedback loop, which might give rise to frequent
oscillations. In the second part (Section 4), we define an algorithm
that maps the sequence of RTT samples to a link cost that can be used
for route selection.

1.1. Applicability

The extension defined in Section 3 provides a sequence of accurate
but potentially noisy RTT samples. Since the RTT is a symmetric
measure of delay, this protocol is only applicable in environments
where the symmetric delay is a good predictor of whether a link
should be taken by routing traffic, which might not necessarily be
the case in networks built over exotic link technologies.

The extension makes minimal requirements on the nodes. In
particular, it does not assume synchronised clocks, and only requires
that clock drift be negligible during the time interval between two
Hello TLVs. Since that is on the order of a few seconds, this
requirement is met even with cheap crystal oscillators, such as the
ones used in consumer electronics.

The algorithm defined in Section 4 depends on a number of assumptions
about the network. The assumption with the most severe consequences
is that all links below a certain RTT (rtt-min in Section 4.2) can be
grouped in a single category of "good" links. While this is the case
in wide-area overlay networks, it makes the algorithm inapplicable in
networks where distinguishing between low-latency links is important.

There are other assumptions, but they are less likely to limit the
algorithm's applicability. The algorithm assumes that all links
above a certain RTT (rtt-max in Section 4.2) are equally bad, and
they will only be used as a last resort. In addition, in order to
avoid oscillations, the algorithm is designed to react slowly to RTT
variations, thus causing suboptimal routing for seconds or even
minutes after an RTT change; while this is a desirable property in
fixed networks, as it avoid excessive route oscillations, it might be
an issue with networks with high rates of node mobility.

2. Specification of Requirements

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

3. RTT Sampling

3.1. Data Structures

We assume that every Babel speaker maintains a local clock that
counts microseconds from an arbitrary origin. We do not assume that
clocks are synchronised: clocks local to distinct nodes need not
share a common origin. The protocol will eventually recover if the
clock is stepped, so clocks need not persist across node reboots.

Every Babel speaker maintains a Neighbour Table, described in
Section 3.2.4 of [RFC8966]. This extension extends every entry in
the Neighbour Table with the following data:

* the Origin Timestamp, a 32-bit timestamp (modulo 2^32) according
to the neighbour's clock;

* the Receive Timestamp, a 32-bit timestamp (modulo 2^32) according
to the local clock.

Both values are initially undefined.

3.2. Protocol Operation

The RTT to a neighbour is estimated using an algorithm due to Mills
[RFC891], originally developed for the HELLO routing protocol and
later used in NTP [RFC5905].

A Babel speaker periodically sends Hello messages to its neighbours
(Section 3.4.1 of [RFC8966]). Additionally, it occasionally sends a
set of IHU ("I Heard You") messages, at most one per neighbour
(Section 3.4.2 of [RFC8966]).

A B
| |
t1 + |
|\ |
| \ |
| \ | Hello(t1)
| \ |
| \ |
| \|
| + t1'
| |
| | RTT = (t2 - t1) - (t2' - t1')
| |
| + t2'
| /|
| / |
| / |
| / | Hello(t2')
| / | IHU(t1, t1')
|/ |
t2 + |
| |
v v

Figure 2: Mills' Algorithm

In order to enable the computation of RTTs, a node A MUST include, in
every Hello that it sends, a timestamp t1 (according to A's local
clock), as illustrated in Figure 2. When a node B receives A's
timestamped Hello, it computes the time t1' at which the Hello was
received (according to B's local clock). It then MUST record the
value t1 in the Origin Timestamp field of the Neighbour Table entry
corresponding to A and the value t1' in the Receive Timestamp field
of the Neighbour Table entry.

When B sends an IHU to A, it checks whether both timestamps are
defined in the Neighbour Table. If that is the case, then it MUST
ensure that its IHU TLV is sent in a packet that also contains a
timestamped Hello TLV (either a normally scheduled Hello or an
unscheduled Hello, see Section 3.4.1 of [RFC8966]). It MUST include
in the IHU both the Origin Timestamp and the Receive Timestamp stored
in the Neighbour Table.

Upon receiving B's packet, A computes the time t2 (according to its
local clock) at which it was received. Node A MUST then verify that
it contains both a Hello TLV with timestamp t2' and an IHU TLV with
two timestamps t1 and t1'. If that is the case, A computes the
value:

RTT = (t2 - t1) - (t2' - t1')

(where all computations are done modulo 2^32), which is a measurement
of the RTT between A and B. (A then stores the values t2' and t2 in
its Neighbour Table, as B did before.)

This algorithm has a number of desirable properties:

1. The algorithm is symmetric: A and B use the same procedures for
timestamping packets and computing RTT samples, and both nodes
produce one RTT sample for each received (Hello, IHU) pair.

2. Since there is no requirement that t1' and t2' be equal, the
protocol is asynchronous: the only change to Babel's message
scheduling is the requirement that a packet containing an IHU
also contain a Hello.

3. Since the algorithm only ever computes differences of timestamps
according to a single clock, it does not require synchronised
clocks.

4. The algorithm requires very little additional state: a node only
needs to store the two timestamps associated with the last hello
received from each neighbour.

5. Since the algorithm only requires piggybacking one or two
timestamps on each Hello and IHU TLV, it makes efficient use of
network resources.

In principle, this algorithm is inaccurate in the presence of clock
drift (i.e., when A's clock and B's clock are running at different
frequencies). However, t2' - t1' is usually on the order of a few
seconds, and significant clock drift is unlikely to happen at that
time scale.

In order for RTT values to be consistent between implementations,
timestamps need to be computed at roughly the same point in the
network stack. Transmit timestamps SHOULD be computed just before
the packet is passed to the network stack (i.e., before it is
subjected to any queueing delays); receive timestamps SHOULD be
computed just after the packet is received from the network stack.

3.3. Wrap-Around and Node Restart

Timestamp values are a count of microseconds stored as a 32-bit
unsigned integer; thus, they wrap around every 71 minutes or so.
What is more, a node may occasionally reboot and restart its clock at
an arbitrary origin. For these reasons, very old timestamps or
nonsensical timestamps MUST NOT be used to yield RTT samples.

The following algorithm can be used to discard obsolete samples.
When a node receives a packet containing a Hello and an IHU, it
compares the current local time t2 with the Origin Timestamp
contained in the IHU; if the Origin Timestamp appears to be in the
future, or if it is in the past by more than a time T (the value T =
3 minutes is recommended), then the timestamps are still recorded in
the Neighbour Table, but they are not used for computation of an RTT
sample.

Similarly, the node compares the Hello's timestamp with the Receive
Timestamp recorded in the Neighbour Table; if the Hello's timestamp
appears to be older than the recorded timestamp, or if it appears to
be more recent by an interval larger than the value T, then the
timestamps are not used for computation of an RTT sample.

3.4. Implementation Notes

The accuracy of the computed RTT samples depends on Transmit
Timestamps being computed as late as possible before a packet
containing a Hello TLV is passed to the network stack, and Receive
Timestamps being computed as early as possible after reception of a
packet containing a (Hello, IHU) pair. We have found the following
implementation strategy to be useful.

When a Hello TLV is buffered for transmission, we insert a PadN sub-
TLV (Section 4.7.2 of [RFC8966]) with a length of 4 octets within the
TLV. When the packet is ready to be sent, we check whether it
contains a 4-octet PadN sub-TLV; if that's the case, we overwrite the
PadN sub-TLV with a Timestamp sub-TLV with the current time, and send
out the packet.

Conversely, when a packet is received, we immediately compute the
current time and record it with the received packet. We then process
the packet as usual and use the recorded timestamp in order to
compute an RTT sample.

The protocol is designed to survive the clock being reset when a node
reboots; on POSIX systems, this makes it possible to use the
CLOCK_MONOTONIC clock for computing timestamps. If CLOCK_MONOTONIC
is not available, CLOCK_REALTIME may be used, since the protocol is
able to survive the clock being occasionally stepped.

4. RTT-Based Route Selection

The protocol described above yields a series of RTT samples. While
these samples are fairly accurate, they are not directly usable as an
input to the route selection procedure, for at least three reasons:

1. In the presence of bursty traffic, routers experience transient
congestion, which causes occasional spikes in the measured RTT.
Thus, the RTT signal may be noisy and require smoothing before it
can be used for route selection.

2. Using the RTT signal for route selection gives rise to a negative
feedback loop. When a route has a low RTT, it is deemed to be
more desirable; this causes it to be used for more data traffic,
which may lead to congestion, which in turn increases the RTT.
Without some form of hysteresis, using RTT for route selection
would lead to oscillations between parallel routes, which would
lead to packet reordering and negatively affect upper-layer
protocols (such as TCP).

3. Even in the absence of congestion, the RTT tends to exhibit some
variation. If the RTTs of two parallel routes oscillate around a
common value, using the RTT as input to route selection will
cause frequent routing oscillations, which, again, indicates the
need for some form of hysteresis.

In this section, we describe an algorithm that integrates smoothing
and hysteresis. It has been shown to behave well both in simulation
and experimentally over the Internet [DELAY-BASED] and is RECOMMENDED
when RTT information is being used for route selection. The
algorithm is structured as follows:

* the RTT values are first smoothed in order to avoid instabilities
due to outliers (Section 4.1);

* the resulting smoothed samples are mapped to a cost using a
bounded, non-linear mapping, which avoids instabilities at the
lower and upper end of the RTT range (Section 4.2);

* a hysteresis filter is applied in order to limit the amount of
oscillation in the middle of the RTT range (Section 4.3).

4.1. Smoothing

The RTT samples provided by Mills' algorithm are fairly accurate, but
noisy: experiments indicate the occasional presence of individual
samples that are much larger than the expected value. Thus, some
form of smoothing SHOULD be applied in order to avoid instabilities
due to occasional outliers.

An implementation MAY use the exponential average algorithm, which is
simple to implement and appears to yield good results in practice
[DELAY-BASED]. The algorithm is parameterised by a constant α, where
0 < α < 1, which controls the amount of smoothing being applied. For
each neighbour, it maintains a smoothed value RTT, which is initially
undefined. When the first sample RTT0 is measured, the smoothed
value is set to the value of RTT0. At each new sample RTTn, the
smoothed value is set to a weighted average of the previous smoothed
value and the new sample:

RTT := α RTT + (1 - α) RTTn

The smoothing constant α SHOULD be between 0.8 and 0.9; the value
0.836 is the RECOMMENDED default.

4.2. Cost Computation

The smoothed RTT value obtained in the previous step needs to be
mapped to a link cost, suitable for input to the metric computation
procedure (Section 3.5.2 of [RFC8966]). Obviously, the mapping
should be monotonic (larger RTTs imply larger costs). In addition,
the mapping should be constant beyond a certain value (all very bad
links are equally bad) so that congested links do not contribute to
routing instability. The mapping should also be constant around 0,
so that small oscillations in the RTT of low-RTT links do not
contribute to routing instability.

cost
^
|
|
| C + max-rtt-penalty
| +---------------------------
| /.
| / .
| / .
| / .
| / .
| / .
| / .
| / .
| / .
| / .
C +------------+ .
| . .
| . .
| . .
| . .
0 +---------------------------------------------------->
0 rtt-min rtt-max RTT

Figure 3: Mapping from RTT to Link Cost

Implementations SHOULD use the mapping described in Figure 3, which
is parameterised by three parameters: rtt-min, rtt-max, and max-rtt-
penalty. For RTT values below rtt-min, the link cost is just the
nominal cost C of a single hop. Between rtt-min and rtt-max, the
cost increases linearly; above rtt-max, the constant value max-rtt-
penalty is added to the nominal cost.

The value rtt-min should be slightly larger than the RTT of a local,
uncongested link. The value rtt-max should be the RTT above which a
link should be avoided if possible, either because it is a long-
distance link or because it is congested; reducing the value of rtt-
max improves stability, but prevents the protocol from discriminating
between high-latency links. As for max-rtt-penalty, it controls how
much the protocol will penalise long-distance links. The default
values rtt-min = 10 ms, rtt-max = 120 ms, and max-rtt-penalty = 150
are RECOMMENDED.

4.3. Hysteresis

Even after applying a bounded mapping from smoothed RTT to a cost
value, the cost may fluctuate when a link's RTT is between rtt-min
and rtt-max. Implementations SHOULD use a robust hysteresis
algorithm, such as the one described in Appendix A.3 of [RFC8966].

5. 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 Appendix D of
[RFC8966], 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; while such
networks might suffer from sub-optimal routing, they will not suffer
from routing loops or other pathologies.

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 protocol to extend the packet
format with additional data, for example high-precision or absolute
timestamps.

6. Packet Format

This extension defines the Timestamp sub-TLV whose Type field has the
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 (modulo 2^32),
expressed in units of one microsecond, counting from an arbitrary
origin. Timestamps wrap around every 4295 seconds, or roughly 71
minutes (see also Section 3.3).

6.1. 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| (continued) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type: Set to 3 to indicate a Timestamp sub-TLV.

Length: The length of the body in octets, 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 (the first 4 octets are interpreted as
described above) and any extra data contained in this sub-TLV MUST be
silently ignored. If the Length field is smaller than the expected 4
octets, then this sub-TLV MUST be ignored (and the remainder of the
enclosing TLV processed as usual).

6.2. Timestamp Sub-TLV in IHU TLVs

When contained in an IHU 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 | Origin Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| (continued) | Receive Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| (continued) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type: Set to 3 to indicate a Timestamp sub-TLV.

Length: The length of the body in octets, 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
the node to which the enclosing IHU TLV applies.

Receive Timestamp: The time, according to the sender's clock, at
which the last timestamped Hello TLV was received from the
node to which the enclosing IHU TLV applies.

If the Length field is larger than the expected 8 octets, the sub-TLV
MUST be processed normally (the first 8 octets are interpreted as
described above), and any extra data contained in this sub-TLV MUST
be silently ignored. If the Length field is smaller than the
expected 8 octets, then this sub-TLV MUST be ignored (and the
remainder of the enclosing TLV processed as usual).

7. IANA Considerations

IANA has added the following entry to the "Babel Sub-TLV Types"
registry:

+======+===========+===========+
| Type | Name | Reference |
+======+===========+===========+
| 3 | Timestamp | RFC 9616 |
+------+-----------+-----------+

Table 1

8. Security Considerations

This extension adds timestamping data to two of the TLVs sent by a
Babel router. By broadcasting the value of a reasonably accurate
local clock, these additional data might make a node more susceptible
to timing attacks.

Broadcasting an accurate time raises privacy issues. The timestamps
used by this protocol have an arbitrary origin; therefore, they do
not leak a node's boot time or time zone. However, having access to
accurate timestamps could allow an attacker to determine the physical
location of a node. Nodes might avoid disclosure of location
information by not including Timestamp sub-TLVs in the TLVs that they
send, which will cause their neighbours to fall back to hop-count
routing.

9. References

9.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC8966] Chroboczek, J. and D. Schinazi, "The Babel Routing
Protocol", RFC 8966, DOI 10.17487/RFC8966, January 2021,
<https://www.rfc-editor.org/info/rfc8966>.

9.2. Informative References

[DELAY-BASED]
Jonglez, B., Boutier, M., and J. Chroboczek, "A delay-
based routing metric", DOI 10.48550/arXiv.1403.3488, March
2014, <http://arxiv.org/abs/1403.3488>.

[RFC891] Mills, D., "DCN Local-Network Protocols", STD 44, RFC 891,
DOI 10.17487/RFC0891, December 1983,
<https://www.rfc-editor.org/info/rfc891>.

[RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
<https://www.rfc-editor.org/info/rfc5905>.

Acknowledgements

The authors are indebted to Jean-Paul Smets, who prompted the
investigation that originally lead to this protocol. We are also
grateful to Donald Eastlake, 3rd, Toke Høiland-Jørgensen, Maria
Matejka, David Schinazi, Pascal Thubert, Steffen Vogel, and Ondřej
Zajiček.

Authors' Addresses

Baptiste Jonglez
ENS Lyon
France
Email: [email protected]

Juliusz Chroboczek
IRIF, Université Paris Cité
Case 7014
75205 Paris Cedex 13
France
Email: [email protected]