Network Working Group J. Chroboczek
Internet-Draft IRIF, University of Paris-Diderot
Obsoletes: 6126,7557 (if approved) D. Schinazi
Intended status: Standards Track Google LLC
Expires: November 8, 2019 May 7, 2019
The Babel Routing Protocol
draft-ietf-babel-rfc6126bis-09
Abstract
Babel is a loop-avoiding distance-vector routing protocol that is
robust and efficient both in ordinary wired networks and in wireless
mesh networks. This document describes the Babel routing protocol,
and obsoletes RFCs 6126 and 7557.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Features . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Limitations . . . . . . . . . . . . . . . . . . . . . . . 4
1.3. Specification of Requirements . . . . . . . . . . . . . . 5
2. Conceptual Description of the Protocol . . . . . . . . . . . 5
2.1. Costs, Metrics and Neighbourship . . . . . . . . . . . . 5
2.2. The Bellman-Ford Algorithm . . . . . . . . . . . . . . . 5
2.3. Transient Loops in Bellman-Ford . . . . . . . . . . . . . 6
2.4. Feasibility Conditions . . . . . . . . . . . . . . . . . 7
2.5. Solving Starvation: Sequencing Routes . . . . . . . . . . 8
2.6. Requests . . . . . . . . . . . . . . . . . . . . . . . . 10
2.7. Multiple Routers . . . . . . . . . . . . . . . . . . . . 10
2.8. Overlapping Prefixes . . . . . . . . . . . . . . . . . . 11
3. Protocol Operation . . . . . . . . . . . . . . . . . . . . . 12
3.1. Message Transmission and Reception . . . . . . . . . . . 12
3.2. Data Structures . . . . . . . . . . . . . . . . . . . . . 13
3.3. Acknowledgments and acknowledgment requests . . . . . . . 17
3.4. Neighbour Acquisition . . . . . . . . . . . . . . . . . . 17
3.5. Routing Table Maintenance . . . . . . . . . . . . . . . . 20
3.6. Route Selection . . . . . . . . . . . . . . . . . . . . . 24
3.7. Sending Updates . . . . . . . . . . . . . . . . . . . . . 25
3.8. Explicit Requests . . . . . . . . . . . . . . . . . . . . 28
4. Protocol Encoding . . . . . . . . . . . . . . . . . . . . . . 32
4.1. Data Types . . . . . . . . . . . . . . . . . . . . . . . 32
4.2. Packet Format . . . . . . . . . . . . . . . . . . . . . . 33
4.3. TLV Format . . . . . . . . . . . . . . . . . . . . . . . 34
4.4. Sub-TLV Format . . . . . . . . . . . . . . . . . . . . . 35
4.5. Parser state . . . . . . . . . . . . . . . . . . . . . . 35
4.6. Details of Specific TLVs . . . . . . . . . . . . . . . . 36
4.7. Details of specific sub-TLVs . . . . . . . . . . . . . . 47
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 48
6. Security Considerations . . . . . . . . . . . . . . . . . . . 49
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 49
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 50
8.1. Normative References . . . . . . . . . . . . . . . . . . 50
8.2. Informative References . . . . . . . . . . . . . . . . . 50
Appendix A. Cost and Metric Computation . . . . . . . . . . . . 51
A.1. Maintaining Hello History . . . . . . . . . . . . . . . . 51
A.2. Cost Computation . . . . . . . . . . . . . . . . . . . . 52
A.3. Metric Computation . . . . . . . . . . . . . . . . . . . 54
Appendix B. Constants . . . . . . . . . . . . . . . . . . . . . 54
Appendix C. Considerations for protocol extensions . . . . . . . 55
Appendix D. Stub Implementations . . . . . . . . . . . . . . . . 57
Appendix E. Software Availability . . . . . . . . . . . . . . . 58
Appendix F. Changes from previous versions . . . . . . . . . . . 58
F.1. Changes since RFC 6126 . . . . . . . . . . . . . . . . . 58
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F.2. Changes since draft-ietf-babel-rfc6126bis-00 . . . . . . 58
F.3. Changes since draft-ietf-babel-rfc6126bis-01 . . . . . . 58
F.4. Changes since draft-ietf-babel-rfc6126bis-02 . . . . . . 59
F.5. Changes since draft-ietf-babel-rfc6126bis-03 . . . . . . 59
F.6. Changes since draft-ietf-babel-rfc6126bis-03 . . . . . . 60
F.7. Changes since draft-ietf-babel-rfc6126bis-04 . . . . . . 60
F.8. Changes since draft-ietf-babel-rfc6126bis-05 . . . . . . 60
F.9. Changes since draft-ietf-babel-rfc6126bis-06 . . . . . . 60
F.10. Changes since draft-ietf-babel-rfc6126bis-07 . . . . . . 60
F.11. Changes since draft-ietf-babel-rfc6126bis-08 . . . . . . 60
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 61
1. Introduction
Babel is a loop-avoiding distance-vector routing protocol that is
designed to be robust and efficient both in networks using prefix-
based routing and in networks using flat routing ("mesh networks"),
and both in relatively stable wired networks and in highly dynamic
wireless networks.
1.1. Features
The main property that makes Babel suitable for unstable networks is
that, unlike naive distance-vector routing protocols [RIP], it
strongly limits the frequency and duration of routing pathologies
such as routing loops and black-holes during reconvergence. Even
after a mobility event is detected, a Babel network usually remains
loop-free. Babel then quickly reconverges to a configuration that
preserves the loop-freedom and connectedness of the network, but is
not necessarily optimal; in many cases, this operation requires no
packet exchanges at all. Babel then slowly converges, in a time on
the scale of minutes, to an optimal configuration. This is achieved
by using sequenced routes, a technique pioneered by Destination-
Sequenced Distance-Vector routing [DSDV].
More precisely, Babel has the following properties:
o when every prefix is originated by at most one router, Babel never
suffers from routing loops;
o when a single prefix is originated by multiple routers, Babel may
occasionally create a transient routing loop for this particular
prefix; this loop disappears in a time proportional to its
diameter, and never again (up to an arbitrary garbage-collection
(GC) time) will the routers involved participate in a routing loop
for the same prefix;
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o assuming bounded packet loss rates, any routing black-holes that
may appear after a mobility event are corrected in a time at most
proportional to the network's diameter.
Babel has provisions for link quality estimation and for fairly
arbitrary metrics. When configured suitably, Babel can implement
shortest-path routing, or it may use a metric based, for example, on
measured packet loss.
Babel nodes will successfully establish an association even when they
are configured with different parameters. For example, a mobile node
that is low on battery may choose to use larger time constants (hello
and update intervals, etc.) than a node that has access to wall
power. Conversely, a node that detects high levels of mobility may
choose to use smaller time constants. The ability to build such
heterogeneous networks makes Babel particularly adapted to the
unmanaged and wireless environment.
Finally, Babel is a hybrid routing protocol, in the sense that it can
carry routes for multiple network-layer protocols (IPv4 and IPv6),
whichever protocol the Babel packets are themselves being carried
over.
1.2. Limitations
Babel has two limitations that make it unsuitable for use in some
environments. First, Babel relies on periodic routing table updates
rather than using a reliable transport; hence, in large, stable
networks it generates more traffic than protocols that only send
updates when the network topology changes. In such networks,
protocols such as OSPF [OSPF], IS-IS [IS-IS], or the Enhanced
Interior Gateway Routing Protocol (EIGRP) [EIGRP] might be more
suitable.
Second, unless the optional algorithm described in Section 3.5.5 is
implemented, Babel does impose a hold time when a prefix is
retracted. While this hold time does not apply to the exact prefix
being retracted, and hence does not prevent fast reconvergence should
it become available again, it does apply to any shorter prefix that
covers it. This may make those implementations of Babel that do not
implement the optional algorithm described in Section 3.5.5
unsuitable for use in networks that implement automatic prefix
aggregation.
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1.3. 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.
2. Conceptual Description of the Protocol
Babel is a loop-avoiding distance vector protocol: it is based on the
Bellman-Ford protocol, just like the venerable RIP [RIP], but
includes a number of refinements that either prevent loop formation
altogether, or ensure that a loop disappears in a timely manner and
doesn't form again.
Conceptually, Bellman-Ford is executed in parallel for every source
of routing information (destination of data traffic). In the
following discussion, we fix a source S; the reader will recall that
the same algorithm is executed for all sources.
2.1. Costs, Metrics and Neighbourship
For every pair of neighbouring nodes A and B, Babel computes an
abstract value known as the cost of the link from A to B., written
C(A, B). Given a route between any two (not necessarily
neighbouring) nodes, the metric of the route is the sum of the costs
of all the edges along the route. The goal of the routing algorithm
is to compute, for every source S, the tree of routes of lowest
metric to S.
Costs and metrics need not be integers. In general, they can be
values in any algebra that satisfies two fairly general conditions
(Section 3.5.2).
A Babel node periodically sends Hello messages to all of its
neighbours; it also periodically sends an IHU ("I Heard You") message
to every neighbour from which it has recently heard a Hello. From
the information derived from Hello and IHU messages received from its
neighbour B, a node A computes the cost C(A, B) of the link from A to
B.
2.2. The Bellman-Ford Algorithm
Every node A maintains two pieces of data: its estimated distance to
S, written D(A), and its next-hop router to S, written NH(A).
Initially, D(S) = 0, D(A) is infinite, and NH(A) is undefined.
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Periodically, every node B sends to all of its neighbours a route
update, a message containing D(B). When a neighbour A of B receives
the route update, it checks whether B is its selected next hop; if
that is the case, then NH(A) is set to B, and D(A) is set to C(A, B)
+ D(B). If that is not the case, then A compares C(A, B) + D(B) to
its current value of D(A). If that value is smaller, meaning that
the received update advertises a route that is better than the
currently selected route, then NH(A) is set to B, and D(A) is set to
C(A, B) + D(B).
A number of refinements to this algorithm are possible, and are used
by Babel. In particular, convergence speed may be increased by
sending unscheduled "triggered updates" whenever a major change in
the topology is detected, in addition to the regular, scheduled
updates. Additionally, a node may maintain a number of alternate
routes, which are being advertised by neighbours other than its
selected neighbour, and which can be used immediately if the selected
route were to fail.
2.3. Transient Loops in Bellman-Ford
It is well known that a naive application of Bellman-Ford to
distributed routing can cause transient loops after a topology
change. Consider for example the following topology:
B
1 /|
1 / |
S --- A |1
\ |
1 \|
C
After convergence, D(B) = D(C) = 2, with NH(B) = NH(C) = A.
Suppose now that the link between S and A fails:
B
1 /|
/ |
S A |1
\ |
1 \|
C
When it detects the failure of the link, A switches its next hop to B
(which is still advertising a route to S with metric 2), and
advertises a metric equal to 3, and then advertises a new route with
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metric 3. This process of nodes changing selected neighbours and
increasing their metric continues until the advertised metric reaches
"infinity", a value larger than all the metrics that the routing
protocol is able to carry.
2.4. Feasibility Conditions
Bellman-Ford is a very robust algorithm: its convergence properties
are preserved when routers delay route acquisition or when they
discard some updates. Babel routers discard received route
announcements unless they can prove that accepting them cannot
possibly cause a routing loop.
More formally, we define a condition over route announcements, known
as the "feasibility condition", that guarantees the absence of
routing loops whenever all routers ignore route updates that do not
satisfy the feasibility condition. In effect, this makes Bellman-
Ford into a family of routing algorithms, parameterised by the
feasibility condition.
Many different feasibility conditions are possible. For example, BGP
can be modelled as being a distance-vector protocol with a (rather
drastic) feasibility condition: a routing update is only accepted
when the receiving node's AS number is not included in the update's
AS-Path attribute (note that BGP's feasibility condition does not
ensure the absence of transient "micro-loops" during reconvergence).
Another simple feasibility condition, used in the Destination-
Sequenced Distance-Vector (DSDV) routing protocol [DSDV] and in the
Ad hoc On-Demand Distance Vector (AODV) protocol, stems from the
following observation: a routing loop can only arise after a router
has switched to a route with a larger metric than the route that it
had previously selected. Hence, one could decide that a route is
feasible only when its metric at the local node would be no larger
than the metric of the currently selected route, i.e., an
announcement carrying a metric D(B) is accepted by A when C(A, B) +
D(B) <= D(A). If all routers obey this constraint, then the metric
at every router is nonincreasing, and the following invariant is
always preserved: if A has selected B as its successor, then D(B) <
D(A), which implies that the forwarding graph is loop-free.
Babel uses a slightly more refined feasibility condition, derived
from EIGRP [DUAL]. Given a router A, define the feasibility distance
of A, written FD(A), as the smallest metric that A has ever
advertised for S to any of its neighbours. An update sent by a
neighbour B of A is feasible when the metric D(B) advertised by B is
strictly smaller than A's feasibility distance, i.e., when D(B) <
FD(A).
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It is easy to see that this latter condition is no more restrictive
than DSDV-feasibility. Suppose that node A obeys DSDV-feasibility;
then D(A) is nonincreasing, hence at all times D(A) <= FD(A).
Suppose now that A receives a DSDV-feasible update that advertises a
metric D(B). Since the update is DSDV-feasible, C(A, B) + D(B) <=
D(A), hence D(B) < D(A), and since D(A) <= FD(A), D(B) < FD(A).
To see that it is strictly less restrictive, consider the following
diagram, where A has selected the route through B, and D(A) = FD(A) =
2. Since D(C) = 1 < FD(A), the alternate route through C is feasible
for A, although its metric C(A, C) + D(C) = 5 is larger than that of
the currently selected route:
B
1 / \ 1
/ \
S A
\ /
1 \ / 4
C
To show that this feasibility condition still guarantees loop-
freedom, recall that at the time when A accepts an update from B, the
metric D(B) announced by B is no smaller than FD(B); since it is
smaller than FD(A), at that point in time FD(B) < FD(A). Since this
property is preserved when A sends updates, it remains true at all
times, which ensures that the forwarding graph has no loops.
2.5. Solving Starvation: Sequencing Routes
Obviously, the feasibility conditions defined above cause starvation
when a router runs out of feasible routes. Consider the following
diagram, where both A and B have selected the direct route to S:
A
1 /| D(A) = 1
/ | FD(A) = 1
S |1
\ | D(B) = 2
2 \| FD(B) = 2
B
Suppose now that the link between A and S breaks:
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A
|
| FD(A) = 1
S |1
\ | D(B) = 2
2 \| FD(B) = 2
B
The only route available from A to S, the one that goes through B, is
not feasible: A suffers from spurious starvation. At that point, the
whole subtree suffering from starvation must be reset, which is
essentially what EIGRP does when it performs a global synchronisation
of all the routers in the sarving subtree (the "active" phase of
EIGRP).
Babel reacts to starvation in a less drastic manner, by using
sequenced routes, a technique introduced by DSDV and adopted by AODV.
In addition to a metric, every route carries a sequence number, a
nondecreasing integer that is propagated unchanged through the
network and is only ever incremented by the source; a pair (s, m),
where s is a sequence number and m a metric, is called a distance.
A received update is feasible when either it is more recent than the
feasibility distance maintained by the receiving node, or it is
equally recent and the metric is strictly smaller. More formally, if
FD(A) = (s, m), then an update carrying the distance (s', m') is
feasible when either s' > s, or s = s' and m' < m.
Assuming the sequence number of S is 137, the diagram above becomes:
A
|
| FD(A) = (137, 1)
S |1
\ | D(B) = (137, 2)
2 \| FD(B) = (137, 2)
B
After S increases its sequence number, and the new sequence number is
propagated to B, we have:
A
|
| FD(A) = (137, 1)
S |1
\ | D(B) = (138, 2)
2 \| FD(B) = (138, 2)
B
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at which point the route through B becomes feasible again.
Note that while sequence numbers are used for determining
feasibility, they are not used in route selection: a node ignores the
sequence number when selecting the best route to a given destination
(Section 3.6). Doing otherwise would cause route oscillation while a
seqno propagates through the network, and might even cause persistent
blackholes with some exotic metrics.
2.6. Requests
In DSDV, the sequence number of a source is increased periodically.
A route becomes feasible again after the source increases its
sequence number, and the new sequence number is propagated through
the network, which may, in general, require a significant amount of
time.
Babel takes a different approach. When a node detects that it is
suffering from a potentially spurious starvation, it sends an
explicit request to the source for a new sequence number. This
request is forwarded hop by hop to the source, with no regard to the
feasibility condition. Upon receiving the request, the source
increases its sequence number and broadcasts an update, which is
forwarded to the requesting node.
Note that after a change in network topology not all such requests
will, in general, reach the source, as some will be sent over links
that are now broken. However, if the network is still connected,
then at least one among the nodes suffering from spurious starvation
has an (unfeasible) route to the source; hence, in the absence of
packet loss, at least one such request will reach the source.
(Resending requests a small number of times compensates for packet
loss.)
Since requests are forwarded with no regard to the feasibility
condition, they may, in general, be caught in a forwarding loop; this
is avoided by having nodes perform duplicate detection for the
requests that they forward.
2.7. Multiple Routers
The above discussion assumes that every prefix is originated by a
single router. In real networks, however, it is often necessary to
have a single prefix originated by multiple routers: for example, the
default route will be originated by all of the edge routers of a
routing domain.
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Since synchronising sequence numbers between distinct routers is
problematic, Babel treats routes for the same prefix as distinct
entities when they are originated by different routers: every route
announcement carries the router-id of its originating router, and
feasibility distances are not maintained per prefix, but per source,
where a source is a pair of a router-id and a prefix. In effect,
Babel guarantees loop-freedom for the forwarding graph to every
source; since the union of multiple acyclic graphs is not in general
acyclic, Babel does not in general guarantee loop-freedom when a
prefix is originated by multiple routers, but any loops will be
broken in a time at most proportional to the diameter of the loop --
as soon as an update has "gone around" the routing loop.
Consider for example the following topology, where A has selected the
default route through S, and B has selected the one through S':
1 1 1
::/0 -- S --- A --- B --- S' -- ::/0
Suppose that both default routes fail at the same time; then nothing
prevents A from switching to B, and B simultaneously switching to A.
However, as soon as A has successfully advertised the new route to B,
the route through A will become unfeasible for B. Conversely, as
soon as B will have advertised the route through A, the route through
B will become unfeasible for A.
In effect, the routing loop disappears at the latest when routing
information has gone around the loop. Since this process can be
delayed by lost packets, Babel makes certain efforts to ensure that
updates are sent reliably after a router-id change (Section 3.7.2).
Additionally, after the routers have advertised the two routes, both
sources will be in their source tables, which will prevent them from
ever again participating in a routing loop involving routes from S
and S' (up to the source GC time, which, available memory permitting,
can be set to arbitrarily large values).
2.8. Overlapping Prefixes
In the above discussion, we have assumed that all prefixes are
disjoint, as is the case in flat ("mesh") routing. In practice,
however, prefixes may overlap: for example, the default route
overlaps with all of the routes present in the network.
After a route fails, it is not correct in general to switch to a
route that subsumes the failed route. Consider for example the
following configuration:
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1 1
::/0 -- A --- B --- C
Suppose that node C fails. If B forwards packets destined to C by
following the default route, a routing loop will form, and persist
until A learns of B's retraction of the direct route to C. B avoids
this pitfall by installing an "unreachable" route after a route is
retracted; this route is maintained until it can be guaranteed that
the former route has been retracted by all of B's neighbours
(Section 3.5.5).
3. Protocol Operation
Every Babel speaker is assigned a router-id, which is an arbitrary
string of 8 octets that is assumed unique across the routing domain.
For example, routers-ids could be assigned randomly, or they could
derived from a link-layer address. (The protocol encoding is
slightly more compact when router-ids are assigned in the same manner
as the IPv6 layer assigns host IDs.)
3.1. Message Transmission and Reception
Babel protocol packets are sent in the body of a UDP datagram (as
described in Section 4 below). Each Babel packet consists of zero or
more TLVs. Most TLVs may contain sub-TLVs.
The source address of a Babel packet is always a unicast address,
link-local in the case of IPv6. Babel packets may be sent to a well-
known (link-local) multicast address or to a (link-local) unicast
address. In normal operation, a Babel speaker sends both multicast
and unicast packets to its neighbours.
With the exception of Hello TLVs and acknowledgments, all Babel TLVs
can be sent to either unicast or multicast addresses, and their
semantics does not depend on whether the destination is a unicast or
a multicast address. Hence, a Babel speaker does not need to
determine the destination address of a packet that it receives in
order to interpret it.
A moderate amount of jitter may be applied to packets sent by a Babel
speaker: outgoing TLVs are buffered and SHOULD be sent with a small
random delay. This is done for two purposes: it avoids
synchronisation of multiple Babel speakers across a network [JITTER],
and it allows for the aggregation of multiple TLVs into a single
packet.
The exact delay and amount of jitter applied to a packet depends on
whether it contains any urgent TLVs. Acknowledgment TLVs MUST be
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sent before the deadline specified in the corresponding request. The
particular class of updates specified in Section 3.7.2 MUST be sent
in a timely manner. The particular class of request and update TLVs
specified in Section 3.8.2 SHOULD be sent in a timely manner.
3.2. Data Structures
In this section, we give a description of the data structures that
every Babel speaker maintains. This description is conceptual: a
Babel speaker may use different data structures as long as the
resulting protocol is the same as the one described in this document.
For example, rather than maintaining a single table containing both
selected and unselected (fallback) routes, as described in
Section 3.2.6 belong, an actual implementation would probably use two
tables, one with selected routes and one with fallback routes.
3.2.1. Sequence number arithmetic
Sequence numbers (seqnos) appear in a number of Babel data
structures, and they are interpreted as integers modulo 2^16. For
the purposes of this document, arithmetic on sequence numbers is
defined as follows.
Given a seqno s and an integer n, the sum of s and n is defined by
s + n (modulo 2^16) = (s + n) MOD 2^16
or, equivalently,
s + n (modulo 2^16) = (s + n) AND 65535
where MOD is the modulo operation yielding a non-negative integer and
AND is the bitwise conjunction operation.
Given two sequence numbers s and s', the relation s is less than s'
(s < s') is defined by
s < s' (modulo 2^16) when 0 < ((s' - s) MOD 2^16) < 32768
or equivalently
s < s' (modulo 2^16) when s /= s' and ((s' - s) AND 32768) = 0.
3.2.2. Node Sequence Number
A node's sequence number is a 16-bit integer that is included in
route updates sent for routes originated by this node.
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A node increments its sequence number (modulo 2^16) whenever it
receives a request for a new sequence number (Section 3.8.1.2). A
node SHOULD NOT increment its sequence number (seqno) spontaneously,
since increasing seqnos makes it less likely that other nodes will
have feasible alternate routes when their selected routes fail.
3.2.3. The Interface Table
The interface table contains the list of interfaces on which the node
speaks the Babel protocol. Every interface table entry contains the
interface's outgoing Multicast Hello seqno, a 16-bit integer that is
sent with each Multicast Hello TLV on this interface and is
incremented (modulo 2^16) whenever a Multicast Hello is sent. (Note
that an interface's Multicast Hello seqno is unrelated to the node's
seqno.)
There are two timers associated with each interface table entry --
the multicast hello timer, which governs the sending of scheduled
Multicast Hello and IHU packets, and the update timer, which governs
the sending of periodic route updates.
3.2.4. The Neighbour Table
The neighbour table contains the list of all neighbouring interfaces
from which a Babel packet has been recently received. The neighbour
table is indexed by pairs of the form (interface, address), and every
neighbour table entry contains the following data:
o the local node's interface over which this neighbour is reachable;
o the address of the neighbouring interface;
o a history of recently received Multicast Hello packets from this
neighbour; this can, for example, be a sequence of n bits, for
some small value n, indicating which of the n hellos most recently
sent by this neighbour have been received by the local node;
o a history of recently received Unicast Hello packets from this
neighbour;
o the "transmission cost" value from the last IHU packet received
from this neighbour, or FFFF hexadecimal (infinity) if the IHU
hold timer for this neighbour has expired;
o the neighbour's expected incoming Multicast Hello sequence number,
an integer modulo 2^16.
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o the neighbour's expected incoming Unicast Hello sequence number,
an integer modulo 2^16.
o the neighbour's outgoing Unicast Hello sequence number, an integer
modulo 2^16 that is sent with each Unicast Hello TLV to this
neighbour and is incremented (modulo 2^16) whenever a Unicast
Hello is sent. (Note that a neighbour's outgoing Unicast Hello
seqno is distinct from the interface's outgoing Multicast Hello
seqno.)
There are three timers associated with each neighbour entry -- the
multicast hello timer, which is initialised from the interval value
carried by scheduled Multicast Hello TLVs, the unicast hello timer,
which is initialised from the interval value carried by scheduled
Unicast Hello TLVs, and the IHU timer, which is initialised to a
small multiple of the interval carried in IHU TLVs.
Note that the neighbour table is indexed by IP addresses, not by
router-ids: neighbourship is a relationship between interfaces, not
between nodes. Therefore, two nodes with multiple interfaces can
participate in multiple neighbourship relationships, a situation that
can notably arise when wireless nodes with multiple radios are
involved.
3.2.5. The Source Table
The source table is used to record feasibility distances. It is
indexed by triples of the form (prefix, plen, router-id), and every
source table entry contains the following data:
o the prefix (prefix, plen), where plen is the prefix length, that
this entry applies to;
o the router-id of a router originating this prefix;
o a pair (seqno, metric), this source's feasibility distance.
There is one timer associated with each entry in the source table --
the source garbage-collection timer. It is initialised to a time on
the order of minutes and reset as specified in Section 3.7.3.
3.2.6. The Route Table
The route table contains the routes known to this node. It is
indexed by triples of the form (prefix, plen, neighbour), and every
route table entry contains the following data:
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o the source (prefix, plen, router-id) for which this route is
advertised;
o the neighbour that advertised this route;
o the metric with which this route was advertised by the neighbour,
or FFFF hexadecimal (infinity) for a recently retracted route;
o the sequence number with which this route was advertised;
o the next-hop address of this route;
o a boolean flag indicating whether this route is selected, i.e.,
whether it is currently being used for forwarding and is being
advertised.
There is one timer associated with each route table entry -- the
route expiry timer. It is initialised and reset as specified in
Section 3.5.4.
Note that there are two distinct (seqno, metric) pairs associated to
each route: the route's distance, which is stored in the route table,
and the feasibility distance, stored in the source table and shared
between all routes with the same source.
3.2.7. The Table of Pending Seqno Requests
The table of pending seqno requests contains a list of seqno requests
that the local node has sent (either because they have been
originated locally, or because they were forwarded) and to which no
reply has been received yet. This table is indexed by triples of the
form (prefix, plen, router-id), and every entry in this table
contains the following data:
o the prefix, router-id, and seqno being requested;
o the neighbour, if any, on behalf of which we are forwarding this
request;
o a small integer indicating the number of times that this request
will be resent if it remains unsatisfied.
There is one timer associated with each pending seqno request; it
governs both the resending of requests and their expiry.
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3.3. Acknowledgments and acknowledgment requests
A Babel speaker may request that a neighbour receiving a given packet
reply with an explicit acknowledgment within a given time. While the
use of acknowledgment requests is optional, every Babel speaker MUST
be able to reply to such a request.
An acknowledgment MUST be sent to a unicast destination. On the
other hand, acknowledgment requests may be sent to either unicast or
multicast destinations, in which case they request an acknowledgment
from all of the receiving nodes.
When to request acknowledgments is a matter of local policy; the
simplest strategy is to never request acknowledgments and to rely on
periodic updates to ensure that any reachable routes are eventually
propagated throughout the routing domain. In order to improve
convergence speed and reduce the amount of control traffic,
acknowledgment requests MAY be used in order to reliably send urgent
updates (Section 3.7.2) and retractions (Section 3.5.5), especially
when the number of neighbours on a given interface is small. Since
Babel is designed to deal gracefully with packet loss on unreliable
media, sending all packets with acknowledgment requests is not
necessary, and NOT RECOMMENDED, as the acknowledgments cause
additional traffic and may force additional Address Resolution
Protocol (ARP) or Neighbour Discovery (ND) exchanges.
3.4. Neighbour Acquisition
Neighbour acquisition is the process by which a Babel node discovers
the set of neighbours heard over each of its interfaces and
ascertains bidirectional reachability. On unreliable media,
neighbour acquisition additionally provides some statistics that may
be useful for link quality computation.
Before it can exchange routing information with a neighbour, a Babel
node MUST create an entry for that neighbour in the neighbour table.
When to do that is implementation-specific; suitable strategies
include creating an entry when any Babel packet is received, or
creating an entry when a Hello TLV is parsed. Similarly, in order to
conserve system resources, an implementation SHOULD discard an entry
when it has been unused for long enough; suitable strategies include
dropping the neighbour after a timeout, and dropping a neighbour when
the associated Hello histories become empty (see Appendix A.2).
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3.4.1. Reverse Reachability Detection
Every Babel node sends Hello TLVs to its neighbours to indicate that
it is alive, at regular or irregular intervals. Each Hello TLV
carries an increasing (modulo 2^16) sequence number and an upper
bound on the time interval until the next Hello of the same type (see
below). If the time interval is set to 0, then the Hello TLV does
not establish a new promise: the deadline carried by the previous
Hello of the same type still applies to the next Hello (if the most
recent scheduled Hello of the right kind was received at time t0 and
carried interval i, then the previous promise of sending another
Hello before time t0 + i still holds). We say that a Hello is
"scheduled" if it carries a non-zero interval, and "unscheduled"
otherwise.
There are two kinds of Hellos: Multicast Hellos, which use a per-
interface Hello counter (the Multicast Hello seqno), and Unicast
Hellos, which use a per-neighbour counter (the Multicast Hello
seqno). A Multicast Hello with a given seqno MUST be sent to all
neighbours on a given interface, either by sending it to a multicast
address or by sending it to one unicast address per neighbour (hence,
the term "Multicast Hello" is a slight misnomer). A Unicast Hello
carrying a given seqno should normally be sent to just one neighbour
(over unicast), since the sequence numbers of different neighbours
are not in general synchronised.
Multicast Hellos sent over multicast can be used for neighbour
discovery; hence, a node SHOULD send periodic (scheduled) Multicast
Hellos unless neighbour discovery is performed by means outside of
the Babel protocol. A node MAY send Unicast Hellos or unscheduled
Hellos of either kind for any reason, such as reducing the amount of
multicast traffic or improving reliability on link technologies with
poor support for link-layer multicast.
A node MAY send a scheduled Hello ahead of time. A node MAY change
its scheduled Hello interval. The Hello interval MAY be decreased at
any time; it MAY be increased immediately before sending a Hello TLV,
but SHOULD NOT be increased at other times. (Equivalently, a node
SHOULD send a scheduled Hello immediately after increasing its Hello
interval.)
How to deal with received Hello TLVs and what statistics to maintain
are considered local implementation matters; typically, a node will
maintain some sort of history of recently received Hellos. An
example of a suitable algorithm is described in Appendix A.1.
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After receiving a Hello, or determining that it has missed one, the
node recomputes the association's cost (Section 3.4.3) and runs the
route selection procedure (Section 3.6).
3.4.2. Bidirectional Reachability Detection
In order to establish bidirectional reachability, every node sends
periodic IHU ("I Heard You") TLVs to each of its neighbours. Since
IHUs carry an explicit interval value, they MAY be sent less often
than Hellos in order to reduce the amount of routing traffic in dense
networks; in particular, they SHOULD be sent less often than Hellos
over links with little packet loss. While IHUs are conceptually
unicast, they MAY be sent to a multicast address in order to avoid an
ARP or Neighbour Discovery exchange and to aggregate multiple IHUs
into a single packet.
In addition to the periodic IHUs, a node MAY, at any time, send an
unscheduled IHU packet. It MAY also, at any time, decrease its IHU
interval, and it MAY increase its IHU interval immediately before
sending an IHU, but SHOULD NOT increase it at any other time.
(Equivalently, a node SHOULD send an extra IHU immediately after
increasing its Hello interval.)
Every IHU TLV contains two pieces of data: the link's rxcost
(reception cost) from the sender's perspective, used by the neighbour
for computing link costs (Section 3.4.3), and the interval between
periodic IHU packets. A node receiving an IHU sets the value of the
txcost (transmission cost) maintained in the neighbour table to the
value contained in the IHU, and resets the IHU timer associated to
this neighbour to a small multiple of the interval value received in
the IHU. When a neighbour's IHU timer expires, the neighbour's
txcost is set to infinity.
After updating a neighbour's txcost, the receiving node recomputes
the neighbour's cost (Section 3.4.3) and runs the route selection
procedure (Section 3.6).
3.4.3. Cost Computation
A neighbourship association's link cost is computed from the values
maintained in the neighbour table: the statistics kept in the
neighbour table about the reception of Hellos, and the txcost
computed from received IHU packets.
For every neighbour, a Babel node computes a value known as this
neighbour's rxcost. This value is usually derived from the Hello
history, which may be combined with other data, such as statistics
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maintained by the link layer. The rxcost is sent to a neighbour in
each IHU.
Since nodes do not necessarily send periodic Unicast Hellos but do
usually send periodic Multicast Hellos (Section 3.4.1), a node SHOULD
use an algorithm that yields a finite rxcost when only Multicast
Hellos are received, unless interoperability with nodes that only
send Multicast Hellos is not required.
How the txcost and rxcost are combined in order to compute a link's
cost is a matter of local policy; as far as Babel's correctness is
concerned, only the following conditions MUST be satisfied:
o the cost is strictly positive;
o if no Hello TLVs of either kind were received recently, then the
cost is infinite;
o if the txcost is infinite, then the cost is infinite.
Note that while this document does not constrain cost computation any
further, not all cost computation strategies will give good results.
See Appendix A.2 for examples of strategies for computing a link's
cost that are known to work well in practice.
3.5. Routing Table Maintenance
Conceptually, a Babel update is a quintuple (prefix, plen, router-id,
seqno, metric), where (prefix, plen) is the prefix for which a route
is being advertised, router-id is the router-id of the router
originating this update, seqno is a nondecreasing (modulo 2^16)
integer that carries the originating router seqno, and metric is the
announced metric.
Before being accepted, an update is checked against the feasibility
condition (Section 3.5.1), which ensures that the route does not
create a routing loop. If the feasibility condition is not
satisfied, the update is either ignored or prevents the route from
being selected, as described in Section 3.5.4. If the feasibility
condition is satisfied, then the update cannot possibly cause a
routing loop.
3.5.1. The Feasibility Condition
The feasibility condition is applied to all received updates. The
feasibility condition compares the metric in the received update with
the metrics of the updates previously sent by the receiving node;
updates that fail the feasibility condition, and therefore have
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metrics large enough to cause a routing loop, are either ignored or
prevent the resulting route from being selected.
A feasibility distance is a pair (seqno, metric), where seqno is an
integer modulo 2^16 and metric is a positive integer. Feasibility
distances are compared lexicographically, with the first component
inverted: we say that a distance (seqno, metric) is strictly better
than a distance (seqno', metric'), written
(seqno, metric) < (seqno', metric')
when
seqno > seqno' or (seqno = seqno' and metric < metric')
where sequence numbers are compared modulo 2^16.
Given a source (prefix, plen, router-id), a node's feasibility
distance for this source is the minimum, according to the ordering
defined above, of the distances of all the finite updates ever sent
by this particular node for the prefix (prefix, plen) and the given
router-id. Feasibility distances are maintained in the source table,
the exact procedure is given in Section 3.7.3.
A received update is feasible when either it is a retraction (its
metric is FFFF hexadecimal), or the advertised distance is strictly
better, in the sense defined above, than the feasibility distance for
the corresponding source. More precisely, a route advertisement
carrying the quintuple (prefix, plen, router-id, seqno, metric) is
feasible if one of the following conditions holds:
o metric is infinite; or
o no entry exists in the source table indexed by (prefix, plen,
router-id); or
o an entry (prefix, plen, router-id, seqno', metric') exists in the
source table, and either
* seqno' < seqno or
* seqno = seqno' and metric < metric'.
Note that the feasibility condition considers the metric advertised
by the neighbour, not the route's metric; hence, a fluctuation in a
neighbour's cost cannot render a selected route unfeasible. Note
further that retractions (updates with infinite metric) are always
feasible, since they cannot possibly cause a routing loop.
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3.5.2. Metric Computation
A route's metric is computed from the metric advertised by the
neighbour and the neighbour's link cost. Just like cost computation,
metric computation is considered a local policy matter; as far as
Babel is concerned, the function M(c, m) used for computing a metric
from a locally computed link cost and the metric advertised by a
neighbour MUST only satisfy the following conditions:
o if c is infinite, then M(c, m) is infinite;
o M is strictly monotonic: M(c, m) > m.
Additionally, the metric SHOULD satisfy the following condition:
o M is left-distributive: if m <= m', then M(c, m) <= M(c, m').
Note that while strict monotonicity is essential to the integrity of
the network (persistent routing loops may arise if it is not
satisfied), left distributivity is not: if it is not satisfied, Babel
will still converge to a loop-free configuration, but might not reach
a global optimum (in fact, a global optimum may not even exist).
As with cost computation, not all strategies for computing route
metrics will give good results. In particular, some metrics are more
likely than others to lead to routing instabilities (route flapping).
In Appendix A.3, we give a number of examples of strictly monotonic,
left-distributive routing metrics that are known to work well in
practice.
3.5.3. Encoding of Updates
In a large network, the bulk of Babel traffic consists of route
updates; hence, some care has been given to encoding them
efficiently. An Update TLV itself only contains the prefix, seqno,
and metric, while the next hop is derived either from the network-
layer source address of the packet or from an explicit Next Hop TLV
in the same packet. The router-id is derived from a separate Router-
Id TLV in the same packet, which optimises the case when multiple
updates are sent with the same router-id.
Additionally, a prefix of the advertised prefix can be omitted in an
Update TLV, in which case it is copied from a previous Update TLV in
the same packet -- this is known as address compression
(Section 4.6.9).
Finally, as a special optimisation for the case when a router-id
coincides with the interface-id part of an IPv6 address, the router-
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id can optionally be derived from the low-order bits of the
advertised prefix.
The encoding of updates is described in detail in Section 4.6.
3.5.4. Route Acquisition
When a Babel node receives an update (prefix, plen, router-id, seqno,
metric) from a neighbour neigh with a link cost value equal to cost,
it checks whether it already has a route table entry indexed by
(prefix, plen, neigh).
If no such entry exists:
o if the update is unfeasible, it MAY be ignored;
o if the metric is infinite (the update is a retraction of a route
we do not know about), the update is ignored;
o otherwise, a new entry is created in the route table, indexed by
(prefix, plen, neigh), with source equal to (prefix, plen, router-
id), seqno equal to seqno and an advertised metric equal to the
metric carried by the update.
If such an entry exists:
o if the entry is currently selected, the update is unfeasible, and
the router-id of the update is equal to the router-id of the
entry, then the update MAY be ignored;
o otherwise, the entry's sequence number, advertised metric, metric,
and router-id are updated and, if the advertised metric is not
infinite, the route's expiry timer is reset to a small multiple of
the Interval value included in the update. If the update is
unfeasible, then the (now unfeasible) entry MUST be immediately
unselected. If the update caused the router-id of the entry to
change, an update (possibly a retraction) MUST be sent in a timely
manner (see Section 3.7.2).
Note that the route table may contain unfeasible routes, either
because they were created by an unfeasible update or due to a metric
fluctuation. Such routes are never selected, since they are not
known to be loop-free; should all the feasible routes become
unusable, however, the unfeasible routes can be made feasible and
therefore possible to select by sending requests along them (see
Section 3.8.2).
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When a route's expiry timer triggers, the behaviour depends on
whether the route's metric is finite. If the metric is finite, it is
set to infinity and the expiry timer is reset. If the metric is
already infinite, the route is flushed from the route table.
After the route table is updated, the route selection procedure
(Section 3.6) is run.
3.5.5. Hold Time
When a prefix P is retracted, because all routes are unfeasible or
have an infinite metric (whether due to the expiry timer or to other
reasons), and a shorter prefix P' that covers P is reachable, P'
cannot in general be used for routing packets destined to P without
running the risk of creating a routing loop (Section 2.8).
To avoid this issue, whenever a prefix P is retracted, a route table
entry with infinite metric is maintained as described in
Section 3.5.4 above. As long as this entry is maintained, packets
destined to an address within P MUST NOT be forwarded by following a
route for a shorter prefix. This entry is removed as soon as a
finite-metric update for prefix P is received and the resulting route
selected. If no such update is forthcoming, the infinite metric
entry SHOULD be maintained at least until it is guaranteed that no
neighbour has selected the current node as next-hop for prefix P.
This can be achieved by either:
o waiting until the route's expiry timer has expired
(Section 3.5.4);
o sending a retraction with an acknowledgment request (Section 3.3)
to every reachable neighbour that has not explicitly retracted
prefix P and waiting for all acknowledgments.
The former option is simpler and ensures that at that point, any
routes for prefix P pointing at the current node have expired.
However, since the expiry time can be as high as a few minutes, doing
that prevents automatic aggregation by creating spurious black-holes
for aggregated routes. The latter option is RECOMMENDED as it
dramatically reduces the time for which a prefix is unreachable in
the presence of aggregated routes.
3.6. Route Selection
Route selection is the process by which a single route for a given
prefix is selected to be used for forwarding packets and to be re-
advertised to a node's neighbours.
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Babel is designed to allow flexible route selection policies. As far
as the protocol's correctness is concerned, the route selection
policy MUST only satisfy the following properties:
o a route with infinite metric (a retracted route) is never
selected;
o an unfeasible route is never selected.
Note, however, that Babel does not naturally guarantee the stability
of routing, and configuring conflicting route selection policies on
different routers may lead to persistent route oscillation.
Route selection is a difficult problem, since a good route selection
policy needs to take into account multiple mutually contradictory
criteria; in roughly decreasing order of importance, these are:
o routes with a small metric should be preferred to routes with a
large metric;
o switching router-ids should be avoided;
o routes through stable neighbours should be preferred to routes
through unstable ones;
o stable routes should be preferred to unstable ones;
o switching next hops should be avoided.
Route selection MUST NOT take seqnos into account: a route MUST NOT
be preferred just because it carries a higher (more recent) seqno.
Doing otherwise would cause route oscillation while a new seqno
propagates through the network, possibly following multiple paths of
different latency, and might even create persistent blackholes if the
metric being used is not left-distributive Section 3.5.2.
A simple but useful strategy is to choose the feasible route with the
smallest metric, with a small amount of hysteresis applied to avoid
switching router-ids too often.
After the route selection procedure is run, triggered updates
(Section 3.7.2) and requests (Section 3.8.2) are sent.
3.7. Sending Updates
A Babel speaker advertises to its neighbours its set of selected
routes. Normally, this is done by sending one or more multicast
packets containing Update TLVs on all of its connected interfaces;
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however, on link technologies where multicast is significantly more
expensive than unicast, a node MAY choose to send multiple copies of
updates in unicast packets, especially when the number of neighbours
is small.
Additionally, in order to ensure that any black-holes are reliably
cleared in a timely manner, a Babel node sends retractions (updates
with an infinite metric) for any recently retracted prefixes.
If an update is for a route injected into the Babel domain by the
local node (e.g., it carries the address of a local interface, the
prefix of a directly attached network, or a prefix redistributed from
a different routing protocol), the router-id is set to the local
node's router-id, the metric is set to some arbitrary finite value
(typically 0), and the seqno is set to the local router's sequence
number.
If an update is for a route learned from another Babel speaker, the
router-id and sequence number are copied from the route table entry,
and the metric is computed as specified in Section 3.5.2.
3.7.1. Periodic Updates
Every Babel speaker periodically advertises all of its selected
routes on all of its interfaces, including any recently retracted
routes. Since Babel doesn't suffer from routing loops (there is no
"counting to infinity") and relies heavily on triggered updates
(Section 3.7.2), this full dump only needs to happen infrequently.
3.7.2. Triggered Updates
In addition to periodic routing updates, a Babel speaker sends
unscheduled, or triggered, updates in order to inform its neighbours
of a significant change in the network topology.
A change of router-id for the selected route to a given prefix may be
indicative of a routing loop in formation; hence, a node MUST send a
triggered update in a timely manner whenever it changes the selected
router-id for a given destination. Additionally, it SHOULD make a
reasonable attempt at ensuring that all reachable neighbours receive
this update.
There are two strategies for ensuring that. If the number of
neighbours is small, then it is reasonable to send the update
together with an acknowledgment request; the update is resent until
all neighbours have acknowledged the packet, up to some number of
times. If the number of neighbours is large, however, requesting
acknowledgments from all of them might cause a non-negligible amount
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of network traffic; in that case, it may be preferable to simply
repeat the update some reasonable number of times (say, 5 for
wireless and 2 for wired links).
A route retraction is somewhat less worrying: if the route retraction
doesn't reach all neighbours, a black-hole might be created, which,
unlike a routing loop, does not endanger the integrity of the
network. When a route is retracted, a node SHOULD send a triggered
update and SHOULD make a reasonable attempt at ensuring that all
neighbours receive this retraction.
Finally, a node MAY send a triggered update when the metric for a
given prefix changes in a significant manner, due to a received
update, because a link's cost has changed, or because a different
next hop has been selected. A node SHOULD NOT send triggered updates
for other reasons, such as when there is a minor fluctuation in a
route's metric, when the selected next hop changes, or to propagate a
new sequence number (except to satisfy a request, as specified in
Section 3.8).
3.7.3. Maintaining Feasibility Distances
Before sending an update (prefix, plen, router-id, seqno, metric)
with finite metric (i.e., not a route retraction), a Babel node
updates the feasibility distance maintained in the source table.
This is done as follows.
If no entry indexed by (prefix, plen, router-id) exists in the source
table, then one is created with value (prefix, plen, router-id,
seqno, metric).
If an entry (prefix, plen, router-id, seqno', metric') exists, then
it is updated as follows:
o if seqno > seqno', then seqno' := seqno, metric' := metric;
o if seqno = seqno' and metric' > metric, then metric' := metric;
o otherwise, nothing needs to be done.
The garbage-collection timer for the entry is then reset. Note that
the feasibility distance is not updated and the garbage-collection
timer is not reset when a retraction (an update with infinite metric)
is sent.
When the garbage-collection timer expires, the entry is removed from
the source table.
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3.7.4. Split Horizon
When running over a transitive, symmetric link technology, e.g., a
point-to-point link or a wired LAN technology such as Ethernet, a
Babel node SHOULD use an optimisation known as split horizon. When
split horizon is used on a given interface, a routing update for
prefix P is not sent on the particular interface over which the
selected route towards prefix P was learnt.
Split horizon SHOULD NOT be applied to an interface unless the
interface is known to be symmetric and transitive; in particular,
split horizon is not applicable to decentralised wireless link
technologies (e.g., IEEE 802.11 in ad hoc mode) when routing updates
are sent over multicast.
3.8. Explicit Requests
In normal operation, a node's route table is populated by the regular
and triggered updates sent by its neighbours. Under some
circumstances, however, a node sends explicit requests in order to
cause a resynchronisation with the source after a mobility event or
to prevent a route from spuriously expiring.
The Babel protocol provides two kinds of explicit requests: route
requests, which simply request an update for a given prefix, and
seqno requests, which request an update for a given prefix with a
specific sequence number. The former are never forwarded; the latter
are forwarded if they cannot be satisfied by the receiver.
3.8.1. Handling Requests
Upon receiving a request, a node either forwards the request or sends
an update in reply to the request, as described in the following
sections. If this causes an update to be sent, the update is either
sent to a multicast address on the interface on which the request was
received, or to the unicast address of the neighbour that sent the
request.
The exact behaviour is different for route requests and seqno
requests.
3.8.1.1. Route Requests
When a node receives a route request for a given prefix, it checks
its route table for a selected route to this exact prefix. If such a
route exists, it MUST send an update (over unicast or over
multicast); if such a route does not exist, it MUST send a retraction
for that prefix.
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When a node receives a wildcard route request, it SHOULD send a full
route table dump. Full route dumps MAY be rate-limited, especially
if they are sent over multicast.
3.8.1.2. Seqno Requests
When a node receives a seqno request for a given router-id and
sequence number, it checks whether its route table contains a
selected entry for that prefix. If a selected route for the given
prefix exists, it has finite metric, and either the router-ids are
different or the router-ids are equal and the entry's sequence number
is no smaller (modulo 2^16) than the requested sequence number, the
node MUST send an update for the given prefix. If the router-ids
match but the requested seqno is larger (modulo 2^16) than the route
entry's, the node compares the router-id against its own router-id.
If the router-id is its own, then it increases its sequence number by
1 (modulo 2^16) and sends an update. A node MUST NOT increase its
sequence number by more than 1 in response to a seqno request.
Otherwise, if the requested router-id is not its own, the received
request's hop count is 2 or more, and the node is advertising the
prefix to its neighbours, the node selects a neighbour to forward the
request to as follows:
o if the node has one or more feasible routes toward the requested
prefix with a next hop that is not the requesting node, then the
node MUST forward the request to the next hop of one such route;
o otherwise, if the node has one or more (not necessarily feasible)
routes to the requested prefix with a next hop that is not the
requesting node, then the node SHOULD forward the request to the
next hop of one such route.
In order to actually forward the request, the node decrements the hop
count and sends the request in a unicast packet destined to the
selected neighbour.
A node SHOULD maintain a list of recently forwarded seqno requests
and forward the reply (an update with a seqno sufficiently large to
satisfy the request) in a timely manner. A node SHOULD compare every
incoming seqno request against its list of recently forwarded seqno
requests and avoid forwarding it if it is redundant (i.e., if it has
recently sent a request with the same prefix, router-id and a seqno
that is not smaller modulo 2^16).
Since the request-forwarding mechanism does not necessarily obey the
feasibility condition, it may get caught in routing loops; hence,
requests carry a hop count to limit the time during which they remain
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in the network. However, since requests are only ever forwarded as
unicast packets, the initial hop count need not be kept particularly
low, and performing an expanding horizon search is not necessary. A
single request MUST NOT be duplicated: it MUST NOT be forwarded to a
multicast address, and it MUST NOT be forwarded to multiple
neighbours. However, if a seqno request is resent by its originator,
the subsequent copies MAY be forwarded to a different neighbour than
the initial one.
3.8.2. Sending Requests
A Babel node MAY send a route or seqno request at any time, to a
multicast or a unicast address; there is only one case when
originating requests is required (Section 3.8.2.1).
3.8.2.1. Avoiding Starvation
When a route is retracted or expires, a Babel node usually switches
to another feasible route for the same prefix. It may be the case,
however, that no such routes are available.
A node that has lost all feasible routes to a given destination but
still has unexpired unfeasible routes to that destination MUST send a
seqno request; if it doesn't have any such routes, it MAY still send
a seqno request. The router-id of the request is set to the router-
id of the route that it has just lost, and the requested seqno is the
value contained in the source table plus 1.
If the node has any (unfeasible) routes to the requested destination,
then it MUST send the request to at least one of the next-hop
neighbours that advertised these routes, and SHOULD send it to all of
them; in any case, it MAY send the request to any other neighbours,
whether they advertise a route to the requested destination or not.
A simple implementation strategy is therefore to unconditionally
multicast the request over all interfaces.
Similar requests will be sent by other nodes that are affected by the
route's loss. If the network is still connected, and assuming no
packet loss, then at least one of these requests will be forwarded to
the source, resulting in a route being advertised with a new sequence
number. (Due to duplicate suppression, only a small number of such
requests will actually reach the source.)
In order to compensate for packet loss, a node SHOULD repeat such a
request a small number of times if no route becomes feasible within a
short time. In the presence of heavy packet loss, however, all such
requests might be lost; in that case, the mechanism in the next
section will eventually ensure that a new seqno is received.
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3.8.2.2. Dealing with Unfeasible Updates
When a route's metric increases, a node might receive an unfeasible
update for a route that it has currently selected. As specified in
Section 3.5.1, the receiving node will either ignore the update or
unselect the route.
In order to keep routes from spuriously expiring because they have
become unfeasible, a node SHOULD send a unicast seqno request when it
receives an unfeasible update for a route that is currently selected.
The requested sequence number is computed from the source table as in
Section 3.8.2.1 above.
Additionally, since metric computation does not necessarily coincide
with the delay in propagating updates, a node might receive an
unfeasible update from a currently unselected neighbour that is
preferable to the currently selected route (e.g., because it has a
much smaller metric); in that case, the node SHOULD send a unicast
seqno request to the neighbour that advertised the preferable update.
3.8.2.3. Preventing Routes from Expiring
In normal operation, a route's expiry timer never triggers: since a
route's hold time is computed from an explicit interval included in
Update TLVs, a new update (possibly a retraction) should arrive in
time to prevent a route from expiring.
In the presence of packet loss, however, it may be the case that no
update is successfully received for an extended period of time,
causing a route to expire. In order to avoid such spurious expiry,
shortly before a selected route expires, a Babel node SHOULD send a
unicast route request to the neighbour that advertised this route;
since nodes always send either updates or retractions in response to
non-wildcard route requests (Section 3.8.1.1), this will usually
result in the route being either refreshed or retracted.
3.8.2.4. Acquiring New Neighbours
In order to speed up convergence after a mobility event, a node MAY
send a unicast wildcard request after acquiring a new neighbour.
Additionally, a node MAY send a small number of multicast wildcard
requests shortly after booting. Note however that doing that
carelessly can cause serious congestion when a whole network is
rebooted, especially on link layers with high per-packet overhead
(e.g., IEEE 802.11).
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4. Protocol Encoding
A Babel packet is sent as the body of a UDP datagram, with network-
layer hop count set to 1, destined to a well-known multicast address
or to a unicast address, over IPv4 or IPv6; in the case of IPv6,
these addresses are link-local. Both the source and destination UDP
port are set to a well-known port number. A Babel packet MUST be
silently ignored unless its source address is either a link-local
IPv6 address or an IPv4 address belonging to the local network, and
its source port is the well-known Babel port. It MAY be silently
ignored if its destination address is a global IPv6 address.
In order to minimise the number of packets being sent while avoiding
lower-layer fragmentation, a Babel node SHOULD attempt to maximise
the size of the packets it sends, up to the outgoing interface's MTU
adjusted for lower-layer headers (28 octets for UDP over IPv4, 48
octets for UDP over IPv6). It MUST NOT send packets larger than the
attached interface's MTU adjusted for lower-layer headers or 512
octets, whichever is larger, but not exceeding 2^16 - 1 adjusted for
lower-layer headers. Every Babel speaker MUST be able to receive
packets that are as large as any attached interface's MTU adjusted
for lower-layer headers or 512 octets, whichever is larger. Babel
packets MUST NOT be sent in IPv6 Jumbograms.
In order to avoid global synchronisation of a Babel network and to
aggregate multiple TLVs into large packets, a Babel node SHOULD
buffer every TLV and delay sending a packet by a small, randomly
chosen delay [JITTER]. In order to allow accurate computation of
packet loss rates, this delay MUST NOT be larger than half the
advertised Hello interval.
4.1. Data Types
4.1.1. Interval
Relative times are carried as 16-bit values specifying a number of
centiseconds (hundredths of a second). This allows times up to
roughly 11 minutes with a granularity of 10ms, which should cover all
reasonable applications of Babel.
4.1.2. Router-Id
A router-id is an arbitrary 8-octet value. A router-id MUST NOT
consist of either all zeroes or all ones.
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4.1.3. Address
Since the bulk of the protocol is taken by addresses, multiple ways
of encoding addresses are defined. Additionally, a common subnet
prefix may be omitted when multiple addresses are sent in a single
packet -- this is known as address compression (Section 4.6.9).
Address encodings:
o AE 0: wildcard address. The value is 0 octets long.
o AE 1: IPv4 address. Compression is allowed. 4 octets or less.
o AE 2: IPv6 address. Compression is allowed. 16 octets or less.
o AE 3: link-local IPv6 address. Compression is not allowed. The
value is 8 octets long, a prefix of fe80::/64 is implied.
The address family associated to an address encoding is either IPv4
or IPv6; it is undefined for AE 0, IPv4 for AE 1, and IPv6 for AEs 2
and 3.
4.1.4. Prefixes
A network prefix is encoded just like a network address, but it is
stored in the smallest number of octets that are enough to hold the
significant bits (up to the prefix length).
4.2. Packet Format
A Babel packet consists of a 4-octet header, followed by a sequence
of TLVs (the packet body), optionally followed by a second sequence
of TLVs (the packet trailer).
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Magic | Version | Body length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Packet Body ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
| Packet Trailer...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
Fields :
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Magic The arbitrary but carefully chosen value 42 (decimal);
packets with a first octet different from 42 MUST be
silently ignored.
Version This document specifies version 2 of the Babel protocol.
Packets with a second octet different from 2 MUST be
silently ignored.
Body length The length in octets of the body following the packet
header (excluding the Magic, Version and Body length
fields, and excluding the packet trailer).
Packet Body The packet body; a sequence of TLVs.
Packet Trailer The packet trailer; another sequence of TLVs.
The packet body and trailer are both sequences of TLVs. The packet
body is the normal place to store TLVs; the packet trailer only
contains specialised TLVs that do not need to be protected by
cryptographic security mechanisms. When parsing the trailer, the
receiver MUST ignore any TLV unless its definition explicitly states
that it is allowed to appear there. Among the TLVs defined in this
document, only Pad1 and PadN are allowed in the trailer; since these
TLVs are ignored in any case, an implementation MAY silently ignore
the packet trailer without even parsing it, unless it implements at
least one extension that defines TLVs that are allowed to appear in
the trailer.
4.3. TLV Format
With the exception of Pad1, all TLVs have the following structure:
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 | Length | Payload...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
Fields :
Type The type of the TLV.
Length The length of the body, exclusive of the Type and Length
fields. If the body is longer than the expected length of
a given type of TLV, any extra data MUST be silently
ignored.
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Payload The TLV payload, which consists of a body and, for selected
TLV types, an optional list of sub-TLVs.
TLVs with an unknown type value MUST be silently ignored.
4.4. Sub-TLV Format
Every TLV carries an explicit length in its header; however, most
TLVs are self-terminating, in the sense that it is possible to
determine the length of the body without reference to the explicit
Length field. If a TLV has a self-terminating format, then it MAY
allow a sequence of sub-TLVs to follow the body.
Sub-TLVs have the same structure as TLVs. With the exception of
PAD1, all TLVs have the following structure:
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 | Length | Body...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
Fields :
Type The type of the sub-TLV.
Length The length of the body, in octets, exclusive of the Type
and Length fields.
Body The sub-TLV body, the interpretation of which depends on
both the type of the sub-TLV and the type of the TLV within
which it is embedded.
The most-significant bit of the sub-TLV, called the mandatory bit,
indicates how to handle unknown sub-TLVs. If the mandatory bit is
not set, then an unknown sub-TLV MUST be silently ignored, and the
rest of the TLV processed normally. If the mandatory bit is set,
then the whole enclosing TLV MUST be silently ignored (except for
updating the parser state by a Router-Id, Next-Hop or Update TLV, see
Section 4.6.7, Section 4.6.8, and Section 4.6.9).
4.5. Parser state
Babel uses a stateful parser: a TLV may refer to data from a previous
TLV. The parser state consists of the following pieces of data:
o for each address encoding that allows compression, the current
default prefix; this is undefined at the start of the packet, and
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is updated by each Update TLV with the Prefix flag set
(Section 4.6.9);
o for each address family (IPv4 or IPv6), the current next-hop; this
is the source address of the enclosing packet for the matching
address family at the start of a packet, and is updated by each
Next-Hop TLV (Section 4.6.8);
o the current router-id; this is undefined at the start of the
packet, and is updated by each Router-ID TLV (Section 4.6.7) and
by each Update TLV with Router-Id flag set.
Since the parser state is separate from the bulk of Babel's state,
and since for correct parsing it must be identical across
implementations, it is updated before checking for mandatory TLVs:
parsing a TLV MUST update the parser state even if the TLV is
otherwise ignored due to an unknown mandatory sub-TLV.
None of the TLVs that modify the parser state are allowed in the
packet trailer; hence, an implementation may choose to use a
dedicated stateless parser to parse the packet trailer.
4.6. Details of Specific TLVs
4.6.1. Pad1
0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| Type = 0 |
+-+-+-+-+-+-+-+-+
Fields :
Type Set to 0 to indicate a Pad1 TLV.
This TLV is silently ignored on reception. It is allowed in the
packet trailer.
4.6.2. PadN
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 = 1 | Length | MBZ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
Fields :
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Type Set to 1 to indicate a PadN TLV.
Length The length of the body, exclusive of the Type and Length
fields.
MBZ Set to 0 on transmission.
This TLV is silently ignored on reception. It is allowed in the
packet trailer.
4.6.3. Acknowledgment Request
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 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Nonce | Interval |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This TLV requests that the receiver send an Acknowledgment TLV within
the number of centiseconds specified by the Interval field.
Fields :
Type Set to 2 to indicate an Acknowledgment Request TLV.
Length The length of the body, exclusive of the Type and Length
fields.
Reserved Sent as 0 and MUST be ignored on reception.
Nonce An arbitrary value that will be echoed in the receiver's
Acknowledgment TLV.
Interval A time interval in centiseconds after which the sender will
assume that this packet has been lost. This MUST NOT be 0.
The receiver MUST send an Acknowledgment TLV before this
time has elapsed (with a margin allowing for propagation
time).
This TLV is self-terminating, and allows sub-TLVs.
4.6.4. Acknowledgment
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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 | Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This TLV is sent by a node upon receiving an Acknowledgment Request.
Fields :
Type Set to 3 to indicate an Acknowledgment TLV.
Length The length of the body, exclusive of the Type and Length
fields.
Nonce Set to the Nonce value of the Acknowledgment Request that
prompted this Acknowledgment.
Since nonce values are not globally unique, this TLV MUST be sent to
a unicast address.
This TLV is self-terminating, and allows sub-TLVs.
4.6.5. Hello
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 = 4 | Length | Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Seqno | Interval |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This TLV is used for neighbour discovery and for determining a
neighbour's reception cost.
Fields :
Type Set to 4 to indicate a Hello TLV.
Length The length of the body, exclusive of the Type and Length
fields.
Flags The individual bits of this field specify special handling
of this TLV (see below).
Seqno If the Unicast flag is set, this is the value of the
sending node's outgoing Unicast Hello seqno for this
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neighbour. Otherwise, it is the sending node's outgoing
Multicast Hello seqno for this interface.
Interval If non-zero, this is an upper bound, expressed in
centiseconds, on the time after which the sending node will
send a new scheduled Hello TLV with the same setting of the
Unicast flag. If this is 0, then this Hello represents an
unscheduled Hello, and doesn't carry any new information
about times at which Hellos are sent.
The Flags field is interpreted as follows:
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|U|X|X|X|X|X|X|X|X|X|X|X|X|X|X|X|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
o U (Unicast) flag (8000 hexadecimal): if set, then this Hello
represents a Unicast Hello, otherwise it represents a Multicast
Hello;
o X: all other bits MUST be sent as 0 and silently ignored on
reception.
Every time a Hello is sent, the corresponding seqno counter MUST be
incremented. Since there is a single seqno counter for all the
Multicast Hellos sent by a given node over a given interface, if the
Unicast flag is not set, this TLV MUST be sent to all neighbors on
this link, which can be achieved by sending to a multicast
destination, or by sending multiple packets to the unicast addresses
of all reachable neighbours. Conversely, if the Unicast flag is set,
this TLV MUST be sent to a single neighbour, which can achieved by
sending to a unicast destination. In order to avoid large
discontinuities in link quality, multiple Hello TLVs SHOULD NOT be
sent in the same packet.
This TLV is self-terminating, and allows sub-TLVs.
4.6.6. IHU
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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 = 5 | Length | AE | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Rxcost | Interval |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address...
+-+-+-+-+-+-+-+-+-+-+-+-
An IHU ("I Heard You") TLV is used for confirming bidirectional
reachability and carrying a link's transmission cost.
Fields :
Type Set to 5 to indicate an IHU TLV.
Length The length of the body, exclusive of the Type and Length
fields.
AE The encoding of the Address field. This should be 1 or 3
in most cases. As an optimisation, it MAY be 0 if the TLV
is sent to a unicast address, if the association is over a
point-to-point link, or when bidirectional reachability is
ascertained by means outside of the Babel protocol.
Reserved Sent as 0 and MUST be ignored on reception.
Rxcost The rxcost according to the sending node of the interface
whose address is specified in the Address field. The value
FFFF hexadecimal (infinity) indicates that this interface
is unreachable.
Interval An upper bound, expressed in centiseconds, on the time
after which the sending node will send a new IHU; this MUST
NOT be 0. The receiving node will use this value in order
to compute a hold time for this symmetric association.
Address The address of the destination node, in the format
specified by the AE field. Address compression is not
allowed.
Conceptually, an IHU is destined to a single neighbour. However, IHU
TLVs contain an explicit destination address, and MAY be sent to a
multicast address, as this allows aggregation of IHUs destined to
distinct neighbours into a single packet and avoids the need for an
ARP or Neighbour Discovery exchange when a neighbour is not being
used for data traffic.
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IHU TLVs with an unknown value in the AE field MUST be silently
ignored.
This TLV is self-terminating, and allows sub-TLVs.
4.6.7. Router-Id
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 = 6 | Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Router-Id +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A Router-Id TLV establishes a router-id that is implied by subsequent
Update TLVs. This TLV sets the router-id even if it is otherwise
ignored due to an unknown mandatory sub-TLV.
Fields :
Type Set to 6 to indicate a Router-Id TLV.
Length The length of the body, exclusive of the Type and Length
fields.
Reserved Sent as 0 and MUST be ignored on reception.
Router-Id The router-id for routes advertised in subsequent Update
TLVs. This MUST NOT consist of all zeroes or all ones.
This TLV is self-terminating, and allows sub-TLVs.
4.6.8. Next Hop
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 = 7 | Length | AE | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next hop...
+-+-+-+-+-+-+-+-+-+-+-+-
A Next Hop TLV establishes a next-hop address for a given address
family (IPv4 or IPv6) that is implied in subsequent Update TLVs.
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This TLV sets up the next-hop for subsequent Update TLVs even if it
is otherwise ignored due to an unknown mandatory sub-TLV.
Fields :
Type Set to 7 to indicate a Next Hop TLV.
Length The length of the body, exclusive of the Type and Length
fields.
AE The encoding of the Address field. This SHOULD be 1 (IPv4)
or 3 (link-local IPv6), and MUST NOT be 0.
Reserved Sent as 0 and MUST be ignored on reception.
Next hop The next-hop address advertised by subsequent Update TLVs,
for this address family.
When the address family matches the network-layer protocol that this
packet is transported over, a Next Hop TLV is not needed: in the
absence of a Next Hop TLV in a given address family, the next hop
address is taken to be the source address of the packet.
Next Hop TLVs with an unknown value for the AE field MUST be silently
ignored.
This TLV is self-terminating, and allows sub-TLVs.
4.6.9. Update
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 = 8 | Length | AE | Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Plen | Omitted | Interval |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Seqno | Metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix...
+-+-+-+-+-+-+-+-+-+-+-+-
An Update TLV advertises or retracts a route. As an optimisation, it
can optionally have the side effect of establishing a new implied
router-id and a new default prefix.
Fields :
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Type Set to 8 to indicate an Update TLV.
Length The length of the body, exclusive of the Type and Length
fields.
AE The encoding of the Prefix field.
Flags The individual bits of this field specify special handling
of this TLV (see below).
Plen The length of the advertised prefix.
Omitted The number of octets that have been omitted at the
beginning of the advertised prefix and that should be taken
from a preceding Update TLV in the same address family with
the Prefix flag set.
Interval An upper bound, expressed in centiseconds, on the time
after which the sending node will send a new update for
this prefix. This MUST NOT be 0. The receiving node will
use this value to compute a hold time for the route table
entry. The value FFFF hexadecimal (infinity) expresses
that this announcement will not be repeated unless a
request is received (Section 3.8.2.3).
Seqno The originator's sequence number for this update.
Metric The sender's metric for this route. The value FFFF
hexadecimal (infinity) means that this is a route
retraction.
Prefix The prefix being advertised. This field's size is
(Plen/8 - Omitted) rounded upwards.
The Flags field is interpreted as follows:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|P|R|X|X|X|X|X|X|
+-+-+-+-+-+-+-+-+
o P (Prefix) flag (80 hexadecimal): if set, then this Update
establishes a new default prefix for subsequent Update TLVs with a
matching address encoding within the same packet, even if this TLV
is otherwise ignored due to an unknown mandatory sub-TLV;
o R (Router-Id) flag (40 hexadecimal): if set, then this TLV
establishes a new default router-id for this TLV and subsequent
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Update TLVs in the same packet, even if this TLV is otherwise
ignored due to an unknown mandatory sub-TLV. This router-id is
computed from the first address of the advertised prefix as
follows:
* if the length of the address is 8 octets or more, then the new
router-id is taken from the 8 last octets of the address;
* if the length of the address is smaller than 8 octets, then the
new router-id consists of the required number of zero octets
followed by the address, i.e., the address is stored on the
right of the router-id. For example, for an IPv4 address, the
router-id consists of 4 octets of zeroes followed by the IPv4
address.
o X: all other bits MUST be sent as 0 and silently ignored on
reception.
The prefix being advertised by an Update TLV is computed as follows:
o the first Omitted octets of the prefix are taken from the previous
Update TLV with the Prefix flag set and the same address encoding,
even if it was ignored due to an unknown mandatory sub-TLV;
o the next (Plen/8 - Omitted) rounded upwards octets are taken from
the Prefix field;
o the remaining octets are set to 0. If AE is 3 (link-local IPv6),
Omitted MUST be 0)
If the Metric field is finite, the router-id of the originating node
for this announcement is taken from the prefix advertised by this
Update if the Router-Id flag is set, computed as described above.
Otherwise, it is taken either from the preceding Router-Id packet, or
the preceding Update packet with the Router-Id flag set, whichever
comes last, even if that TLV is otherwise ignored due to an unknown
mandatory sub-TLV.
The next-hop address for this update is taken from the last preceding
Next Hop TLV with a matching address family (IPv4 or IPv6) in the
same packet even if it was otherwise ignored due to an unknown
mandatory sub-TLV; if no such TLV exists, it is taken from the
network-layer source address of this packet.
If the metric field is FFFF hexadecimal, this TLV specifies a
retraction. In that case, the router-id, next-hop and seqno are not
used. AE MAY then be 0, in which case this Update retracts all of
the routes previously advertised by the sending interface. If the
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metric is finite, AE MUST NOT be 0. If the metric is infinite and AE
is 0, Plen and Omitted MUST both be 0.
Update TLVs with an unknown value in the AE field MUST be silently
ignored.
This TLV is self-terminating, and allows sub-TLVs.
4.6.10. Route Request
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 = 9 | Length | AE | Plen |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix...
+-+-+-+-+-+-+-+-+-+-+-+-
A Route Request TLV prompts the receiver to send an update for a
given prefix, or a full route table dump.
Fields :
Type Set to 9 to indicate a Route Request TLV.
Length The length of the body, exclusive of the Type and Length
fields.
AE The encoding of the Prefix field. The value 0 specifies
that this is a request for a full route table dump (a
wildcard request).
Plen The length of the requested prefix.
Prefix The prefix being requested. This field's size is Plen/8
rounded upwards.
A Request TLV prompts the receiver to send an update message
(possibly a retraction) for the prefix specified by the AE, Plen, and
Prefix fields, or a full dump of its route table if AE is 0 (in which
case Plen MUST be 0 and Prefix is of length 0).
This TLV is self-terminating, and allows sub-TLVs.
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4.6.11. Seqno Request
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 = 10 | Length | AE | Plen |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Seqno | Hop Count | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Router-Id +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix...
+-+-+-+-+-+-+-+-+-+-+
A Seqno Request TLV prompts the receiver to send an Update for a
given prefix with a given sequence number, or to forward the request
further if it cannot be satisfied locally.
Fields :
Type Set to 10 to indicate a Seqno Request message.
Length The length of the body, exclusive of the Type and Length
fields.
AE The encoding of the Prefix field. This MUST NOT be 0.
Plen The length of the requested prefix.
Seqno The sequence number that is being requested.
Hop Count The maximum number of times that this TLV may be forwarded,
plus 1. This MUST NOT be 0.
Reserved Sent as 0 and MUST be ignored on reception.
Router-Id The Router-Id that is being requested. This MUST NOT
consist of all zeroes or all ones.
Prefix The prefix being requested. This field's size is Plen/8
rounded upwards.
A Seqno Request TLV prompts the receiving node to send a finite-
metric Update for the prefix specified by the AE, Plen, and Prefix
fields, with either a router-id different from what is specified by
the Router-Id field, or a Seqno no less (modulo 2^16) than what is
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specified by the Seqno field. If this request cannot be satisfied
locally, then it is forwarded according to the rules set out in
Section 3.8.1.2.
While a Seqno Request MAY be sent to a multicast address, it MUST NOT
be forwarded to a multicast address and MUST NOT be forwarded to more
than one neighbour. A request MUST NOT be forwarded if its Hop Count
field is 1.
This TLV is self-terminating, and allows sub-TLVs.
4.7. Details of specific sub-TLVs
4.7.1. Pad1
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| Type = 0 |
+-+-+-+-+-+-+-+-+
Fields :
Type Set to 0 to indicate a Pad1 sub-TLV.
This sub-TLV is silently ignored on reception. It is allowed within
any TLV that allows sub-TLVs.
4.7.2. PadN
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 = 1 | Length | MBZ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
Fields :
Type Set to 1 to indicate a PadN sub-TLV.
Length The length of the body, in octets, exclusive of the Type
and Length fields.
MBZ Set to 0 on transmission.
This sub-TLV is silently ignored on reception. It is allowed within
any TLV that allows sub-TLVs.
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5. IANA Considerations
IANA has registered the UDP port number 6696, called "babel", for use
by the Babel protocol.
IANA has registered the IPv6 multicast group ff02::1:6 and the IPv4
multicast group 224.0.0.111 for use by the Babel protocol.
IANA has created a registry called "Babel TLV Types". The values in
this registry are not changed by this specification.
IANA has created a registry called "Babel sub-TLV Types". Due to the
addition of a Mandatory bit to the Babel protocol, the values in the
"Babel sub-TLV Types" registry are amended as follows:
+---------+-----------------------------------------+---------------+
| Type | Name | Reference |
+---------+-----------------------------------------+---------------+
| 0 | Pad1 | this document |
| | | |
| 1 | PadN | this document |
| | | |
| 112-126 | Reserved for Experimental Use | this document |
| | | |
| 127 | Reserved for expansion of the type | this document |
| | space | |
| | | |
| 240-254 | Reserved for Experimental Use | this document |
| | | |
| 255 | Reserved for expansion of the type | this document |
| | space | |
+---------+-----------------------------------------+---------------+
Existing assignments in the "Babel sub-TLV Types" registry in the
range 2 to 111 are not changed by this specification. The values 224
through 239, previously reserved for Experimental Use, are now
unassigned.
IANA has created a registry called "Babel Flags Values". IANA is
instructed to rename this registry to "Babel Update Flags Values",
with its contents unchanged.
IANA is instructed to create a new registry called "Babel Hello Flags
Values". The allocation policy for this registry is Specification
Required [RFC8126]. The initial values in this registry are as
follows:
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+------+------------+---------------+
| Bit | Name | Reference |
+------+------------+---------------+
| 0 | Unicast | this document |
| | | |
| 1-15 | Unassigned | |
+------+------------+---------------+
IANA is instructed to replace all references to RFCs 6126 and 7557 in
all of the registries mentioned above by references to this document.
6. Security Considerations
As defined in this document, Babel is a completely insecure protocol.
Any attacker can misdirect data traffic by advertising routes with a
low metric or a high seqno. This issue can be solved either by a
lower-layer security mechanism (e.g., link-layer security or IPsec),
or by deploying a suitable authentication mechanism within Babel
itself. There are currently two such mechanisms: Babel over DTLS
[BABEL-DTLS] and HMAC-based authentication [BABEL-HMAC]. Both
mechanisms ensure integrity of messages and prevent message replay,
but only DTLS supports asymmetric keying and message confidentiality.
HMAC is simpler and does not depend on DTLS, and therefore its use is
RECOMMENDED whenever both mechanisms are applicable.
The information that a Babel node announces to the whole routing
domain is often sufficient to determine a mobile node's physical
location with reasonable precision. The privacy issues that this
causes can be mitigated somewhat by using randomly chosen router-ids
and randomly chosen IP addresses, and changing them periodically.
When carried over IPv6, Babel packets are ignored unless they are
sent from a link-local IPv6 address; since routers don't forward
link-local IPv6 packets, this provides protection against spoofed
Babel packets being sent from the global Internet. No such natural
protection exists when Babel packets are carried over IPv4.
7. Acknowledgments
A number of people have contributed text and ideas to this
specification. The authors are particularly indebted to Matthieu
Boutier, Gwendoline Chouasne, Margaret Cullen, Donald Eastlake, Toke
Hoiland-Jorgensen and Joao Sobrinho. Earlier versions of this
document greatly benefited from the input of Joel Halpern. The
address compression technique was inspired by [PACKETBB].
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8. References
8.1. Normative References
[BABEL-DTLS]
Decimo, A., Schinazi, D., and J. Chroboczek, "Babel
Routing Protocol over Datagram Transport Layer Security",
Internet Draft draft-ietf-babel-dtls-04, February 2019.
[BABEL-HMAC]
Do, C., Kolodziejak, W., and J. Chroboczek, "HMAC
authentication for the Babel routing protocol", Internet
Draft draft-ietf-babel-hmac-04, March 2019.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, June 2017.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017.
8.2. Informative References
[DSDV] Perkins, C. and P. Bhagwat, "Highly Dynamic Destination-
Sequenced Distance-Vector Routing (DSDV) for Mobile
Computers", ACM SIGCOMM'94 Conference on Communications
Architectures, Protocols and Applications 234-244, 1994.
[DUAL] Garcia Luna Aceves, J., "Loop-Free Routing Using Diffusing
Computations", IEEE/ACM Transactions on Networking 1:1,
February 1993.
[EIGRP] Albrightson, B., Garcia Luna Aceves, J., and J. Boyle,
"EIGRP -- a Fast Routing Protocol Based on Distance
Vectors", Proc. Interop 94, 1994.
[ETX] De Couto, D., Aguayo, D., Bicket, J., and R. Morris, "A
high-throughput path metric for multi-hop wireless
networks", Proc. MobiCom 2003, 2003.
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[IS-IS] "Information technology -- Telecommunications and
information exchange between systems -- Intermediate
System to Intermediate System intra-domain routeing
information exchange protocol for use in conjunction with
the protocol for providing the connectionless-mode network
service (ISO 8473)", ISO/IEC 10589:2002, 2002.
[JITTER] Floyd, S. and V. Jacobson, "The synchronization of
periodic routing messages", IEEE/ACM Transactions on
Networking 2, 2, 122-136, April 1994.
[OSPF] Moy, J., "OSPF Version 2", RFC 2328, April 1998.
[PACKETBB]
Clausen, T., Dearlove, C., Dean, J., and C. Adjih,
"Generalized Mobile Ad Hoc Network (MANET) Packet/Message
Format", RFC 5444, February 2009.
[RIP] Malkin, G., "RIP Version 2", RFC 2453, November 1998.
Appendix A. Cost and Metric Computation
The strategy for computing link costs and route metrics is a local
matter; Babel itself only requires that it comply with the conditions
given in Section 3.4.3 and Section 3.5.2. Different nodes may use
different strategies in a single network and may use different
strategies on different interface types. This section describes the
strategies used by the sample implementation of Babel.
The sample implementation of Babel sends periodic Multicast Hellos,
and never sends Unicast Hellos. It maintains statistics about the
last 16 received Hello TLVs of each kind (Appendix A.1), computes
costs by using the 2-out-of-3 strategy (Appendix A.2.1) on wired
links, and ETX (Appendix A.2.2) on wireless links. It uses an
additive algebra for metric computation (Appendix A.3.1).
A.1. Maintaining Hello History
For each neighbour, the sample implementation of Babel maintains two
sets of Hello history, one for each kind of Hello, and an expected
sequence number, one for Multicast and one for Unicast Hellos. Each
Hello history is a vector of 16 bits, where a 1 value represents a
received Hello, and a 0 value a missed Hello. For each kind of
Hello, the expected sequence number, written ne, is the sequence
number that is expected to be carried by the next received Hello from
this neighbour.
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Whenever it receives a Hello packet of a given kind from a neighbour,
a node compares the received sequence number nr for that kind of
Hello with its expected sequence number ne. Depending on the outcome
of this comparison, one of the following actions is taken:
o if the two differ by more than 16 (modulo 2^16), then the sending
node has probably rebooted and lost its sequence number; the whole
associated neighbour table entry is flushed and a new one is
created;
o otherwise, if the received nr is smaller (modulo 2^16) than the
expected sequence number ne, then the sending node has increased
its Hello interval without us noticing; the receiving node removes
the last (ne - nr) entries from this neighbour's Hello history (we
"undo history");
o otherwise, if nr is larger (modulo 2^16) than ne, then the sending
node has decreased its Hello interval, and some Hellos were lost;
the receiving node adds (nr - ne) 0 bits to the Hello history (we
"fast-forward").
The receiving node then appends a 1 bit to the Hello history and sets
ne to (nr + 1). If the Interval field of the received Hello is not
zero, it resets the neighbour's hello timer to 1.5 times the
advertised Interval (the extra margin allows for delay due to
jitter).
Whenever either Hello timer associated to a neighbour expires, the
local node adds a 0 bit to this neighbour's Hello history, and
increments the expected Hello number. If both Hello histories are
empty (they contain 0 bits only), the neighbour entry is flushed;
otherwise, the relevant hello timer is reset to the value advertised
in the last Hello of that kind received from this neighbour (no extra
margin is necessary in this case, since jitter was already taken into
account when computing the timeout that has just expired).
A.2. Cost Computation
This section discusses how to compute costs based on Hello history.
A.2.1. k-out-of-j
K-out-of-j link sensing is suitable for wired links that are either
up, in which case they only occasionally drop a packet, or down, in
which case they drop all packets.
The k-out-of-j strategy is parameterised by two small integers k and
j, such that 0 < k <= j, and the nominal link cost, a constant K >=
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1. A node keeps a history of the last j hellos; if k or more of
those have been correctly received, the link is assumed to be up, and
the rxcost is set to K; otherwise, the link is assumed to be down,
and the rxcost is set to infinity.
Since Babel supports two kinds of Hellos, a Babel node performs k-
out-of-j twice for each neighbour, once on the Unicast and once on
the Multicast Hello history. If either of the instances of k-out-
of-j indicates that the link is up, then the link is assumed to be
up, and the rxcost is set to K; if both instances indicate that the
link is down, then the link is assumed to be down, and the rxcost is
set to infinity. In other words, the resulting rxcost is the minimum
of the rxcosts yielded by the two instances of k-out-of-j link
sensing.
The cost of a link performing k-out-of-j link sensing is defined as
follows:
o cost = FFFF hexadecimal if rxcost = FFFF hexadecimal;
o cost = txcost otherwise.
A.2.2. ETX
Unlike wired links, which are bimodal (either up or down), wireless
links exhibit continuous variation of the link quality. Naive
application of hop-count routing in networks that use wireless links
for transit tends to select long, lossy links in preference to
shorter, lossless links, which can dramatically reduce throughput.
For that reason, a routing protocol designed to support wireless
links must perform some form of link-quality estimation.
ETX [ETX] is a simple link-quality estimation algorithm that is
designed to work well with the IEEE 802.11 MAC. By default, the
IEEE 802.11 MAC performs ARQ and rate adaptation on unicast frames,
but not on multicast frames, which are sent at a fixed rate with no
ARQ; therefore, measuring the loss rate of multicast frames yields a
useful estimate of a link's quality.
A node performing ETX link quality estimation uses a neighbour's
Multicast Hello history to compute an estimate, written beta, of the
probability that a Hello TLV is successfully received. Beta can be
computed as the fraction of 1 bits within a small number (say, 6) of
the most recent entries in the Multicast Hello history, or it can be
an exponential average, or some combination of both approaches.
Let alpha be MIN(1, 256/txcost), an estimate of the probability of
successfully sending a Hello TLV. The cost is then computed by
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cost = 256/(alpha * beta)
or, equivalently,
cost = (MAX(txcost, 256) * rxcost) / 256.
Since the IEEE 802.11 MAC performs ARQ on unicast frames, unicast
frames do not provide a useful measure of link quality, and therefore
ETX ignores the Unicast Hello history. Thus, a node performing ETX
link-quality estimation will not route through neighbouring nodes
unless they send periodic Multicast Hellos (possibly in addition to
Unicast Hellos).
A.3. Metric Computation
As described in Section 3.5.2, the metric advertised by a neighbour
is combined with the link cost to yield a metric.
A.3.1. Additive Metrics
The simplest approach for obtaining a monotonic, left-distributive
metric is to define the metric of a route as the sum of the costs of
the component links. More formally, if a neighbour advertises a
route with metric m over a link with cost c, then the resulting route
has metric M(c, m) = c + m.
A multiplicative metric can be converted into an additive one by
taking the logarithm (in some suitable base) of the link costs.
A.3.2. External Sources of Willingness
A node may want to vary its willingness to forward packets by taking
into account information that is external to the Babel protocol, such
as the monetary cost of a link, the node's battery status, CPU load,
etc. This can be done by adding to every route's metric a value k
that depends on the external data. For example, if a battery-powered
node receives an update with metric m over a link with cost c, it
might compute a metric M(c, m) = k + c + m, where k depends on the
battery status.
In order to preserve strict monotonicity (Section 3.5.2), the value k
must be greater than -c.
Appendix B. Constants
The choice of time constants is a trade-off between fast detection of
mobility events and protocol overhead. Two implementations of Babel
with different time constants will interoperate, although the
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resulting convergence time will most likely be dictated by the slower
of the two.
Experience with the sample implementation of Babel indicates that the
Hello interval is the most important time constant: a mobility event
is detected within 1.5 to 3 Hello intervals. Due to Babel's reliance
on triggered updates and explicit requests, the Update interval only
has an effect on the time it takes for accurate metrics to be
propagated after variations in link costs too small to trigger an
unscheduled update or in the presence of packet loss.
At the time of writing, the sample implementation of Babel uses the
following default values:
Multicast Hello Interval: 4 seconds.
IHU Interval: the advertised IHU interval is always 3 times the
Multicast Hello interval. IHUs are actually sent with each Hello
on lossy links (as determined from the Hello history), but only
with every third Multicast Hello on lossless links.
Unicast Hello Interval: the sample implementation never sends
scheduled Unicast Hellos;
Update Interval: 4 times the Multicast Hello interval.
IHU Hold Time: 3.5 times the advertised IHU interval.
Route Expiry Time: 3.5 times the advertised update interval.
Source GC time: 3 minutes.
Request timeout: initially 2 seconds, doubled every time a request
is resent, up to a maximum of three times.
The amount of jitter applied to a packet depends on whether it
contains any urgent TLVs or not (Section 3.1). Urgent triggered
updates and urgent requests are delayed by no more than 200ms;
acknowledgments, by no more than the associated deadline; and other
TLVs by no more than one-half the Multicast Hello interval.
Appendix C. Considerations for protocol extensions
Babel is an extensible protocol, and this document defines a number
of mechanisms that can be used to extend the protocol in a backwards
compatible manner:
o increasing the version number in the packet header;
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o defining new TLVs;
o defining new sub-TLVs (with or without the mandatory bit set);
o defining new AEs;
o using the packet trailer.
This appendix is intended to guide designers of protocol extensions
in chosing a particular encoding.
The version number in the Babel header should only be increased if
the new version is not backwards compatible with the original
protocol.
In many cases, an extension could be implemented either by defining a
new TLV, or by adding a new sub-TLV to an existing TLV. For example,
an extension whose purpose is to attach additional data to route
updates can be implemented either by creating a new "enriched" Update
TLV, by adding a non-mandatory sub-TLV to the Update TLV, or by
adding a mandatory sub-TLV.
The various encodings are treated differently by implementations that
do not understand the extension. In the case of a new TLV or of a
sub-TLV with the mandatory bit set, the whole TLV is ignored by
implementations that do not implement the extension, while in the
case of a non-mandatory sub-TLV, the TLV is parsed and acted upon,
and only the unknown sub-TLV is silently ignored. Therefore, a non-
mandatory sub-TLV should be used by extensions that extend the Update
in a compatible manner (the extension data may be silently ignored),
while a mandatory sub-TLV or a new TLV must be used by extensions
that make incompatible extensions to the meaning of the TLV (the
whole TLV must be thrown away if the extension data is not
understood).
Experience shows that the need for additional data tends to crop up
in the most unexpected places. Hence, it is recommended that
extensions that define new TLVs should make them self-terminating,
and allow attaching sub-TLVs to them.
Adding a new AE is essentially equivalent to adding a new TLV: Update
TLVs with an unknown AE are ignored, just like unknown TLVs.
However, adding a new AE is more involved than adding a new TLV,
since it creates a new set of compression state. Additionally, since
the Next Hop TLV creates state specific to a given address family, as
opposed to a given AE, a new AE for a previously defined address
family must not be used in the Next Hop TLV if backwards
compatibility is required. A similar issue arises with Update TLVs
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with unknown AEs establishing a new router-id (due to the Router-Id
flag being set). Therefore, defining new AEs must be done with care
if compatibility with unextended implementations is required.
The packet trailer is intended to carry cryptographic signatures that
only cover the packet body; storing the cryptographic signatures in
the packet trailer avoids clearing the signature before computing a
hash of the packet body, and makes it possible to check a
cryptographic signature before running the full, stateful TLV parser.
Hence, only TLVs that don't need to be protected by cryptographic
security protocols should be allowed in the packet trailer. Any such
TLVs should be easy to parse, and in particular should not require
stateful parsing.
Appendix D. Stub Implementations
Babel is a fairly economic protocol. Updates take between 12 and 40
octets per destination, depending on the address family and how
successful compression is; in a double-stack flat network, an average
of less than 24 octets per update is typical. The route table
occupies about 35 octets per IPv6 entry. To put these values into
perspective, a single full-size Ethernet frame can carry some 65
route updates, and a megabyte of memory can contain a 20000-entry
route table and the associated source table.
Babel is also a reasonably simple protocol. The sample
implementation consists of less than 12 000 lines of C code, and it
compiles to less than 120 kB of text on a 32-bit CISC architecture;
about half of this figure is due to protocol extensions and user-
interface code.
Nonetheless, in some very constrained environments, such as PDAs,
microwave ovens, or abacuses, it may be desirable to have subset
implementations of the protocol.
There are many different definitions of a stub router, but for the
needs of this section a stub implementation of Babel is one that
announces one or more directly attached prefixes into a Babel network
but doesn't reannounce any routes that it has learnt from its
neighbours. It may either maintain a full routing table, or simply
select a default gateway amongst any one of its neighbours that
announces a default route. Since a stub implementation never
forwards packets except from or to directly attached links, it cannot
possibly participate in a routing loop, and hence it need not
evaluate the feasibility condition or maintain a source table.
No matter how primitive, a stub implementation MUST parse sub-TLVs
attached to any TLVs that it understands and check the mandatory bit.
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It MUST answer acknowledgment requests and MUST participate in the
Hello/IHU protocol. It MUST also be able to reply to seqno requests
for routes that it announces and SHOULD be able to reply to route
requests.
Experience shows that an IPv6-only stub implementation of Babel can
be written in less than 1000 lines of C code and compile to 13 kB of
text on 32-bit CISC architecture.
Appendix E. Software Availability
The sample implementation of Babel is available from
<https://www.irif.fr/~jch/software/babel/>.
Appendix F. Changes from previous versions
F.1. Changes since RFC 6126
o Changed UDP port number to 6696.
o Consistently use router-id rather than id.
o Clarified that the source garbage collection timer is reset after
sending an update even if the entry was not modified.
o In section "Seqno Requests", fixed an erroneous "route request".
o In the description of the Seqno Request TLV, added the description
of the Router-Id field.
o Made router-ids all-0 and all-1 forbidden.
F.2. Changes since draft-ietf-babel-rfc6126bis-00
o Added security considerations.
F.3. Changes since draft-ietf-babel-rfc6126bis-01
o Integrated the format of sub-TLVs.
o Mentioned for each TLV whether it supports sub-TLVs.
o Added Appendix C.
o Added a mandatory bit in sub-TLVs.
o Changed compression state to be per-AF rather than per-AE.
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o Added implementation hint for the routing table.
o Clarified how router-ids are computed when bit 0x40 is set in
Updates.
o Relaxed the conditions for sending requests, and tightened the
conditions for forwarding requests.
o Clarified that neighbours should be acquired at some point, but it
doesn't matter when.
F.4. Changes since draft-ietf-babel-rfc6126bis-02
o Added Unicast Hellos.
o Added unscheduled (interval-less) Hellos.
o Changed Appendix A to consider Unicast and unscheduled Hellos.
o Changed Appendix B to agree with the reference implementation.
o Added optional algorithm to avoid the hold time.
o Changed the table of pending seqno requests to be indexed by
router-id in addition to prefixes.
o Relaxed the route acquisition algorithm.
o Replaced minimal implementations by stub implementations.
o Added acknowledgments section.
F.5. Changes since draft-ietf-babel-rfc6126bis-03
o Clarified that all the data structures are conceptual.
o Made sending and receiving Multicast Hellos a SHOULD, avoids
expressing any opinion about Unicast Hellos.
o Removed opinion about Multicast vs. Unicast Hellos (Appendix A.4).
o Made hold-time into a SHOULD rather than MUST.
o Clarified that Seqno Requests are for a finite-metric Update.
o Clarified that sub-TLVs Pad1 and PadN are allowed within any TLV
that allows sub-TLVs.
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o Updated IANA Considerations.
o Updated Security Considerations.
o Renamed routing table back to route table.
o Made buffering outgoing updates a SHOULD.
o Weakened advice to use modified EUI-64 in router-ids.
o Added information about sending requests to Appendix B.
o A number of minor wording changes and clarifications.
F.6. Changes since draft-ietf-babel-rfc6126bis-03
Minor editorial changes.
F.7. Changes since draft-ietf-babel-rfc6126bis-04
o Renamed isotonicity to left-distributivity.
o Minor clarifications to unicast hellos.
o Updated requirements boilerplate to RFC 8174.
o Minor editorial changes.
F.8. Changes since draft-ietf-babel-rfc6126bis-05
o Added information about the packet trailer, now that it is used by
draft-ietf-babel-hmac.
F.9. Changes since draft-ietf-babel-rfc6126bis-06
o Added references to security documents.
F.10. Changes since draft-ietf-babel-rfc6126bis-07
o Added list of obsoleted drafts to the abstract.
o Updated references.
F.11. Changes since draft-ietf-babel-rfc6126bis-08
o Added recommendation that route selection should not take seqnos
into account.
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Authors' Addresses
Juliusz Chroboczek
IRIF, University of Paris-Diderot
Case 7014
75205 Paris Cedex 13
France
Email: jch@irif.fr
David Schinazi
Google LLC
1600 Amphitheatre Parkway
Mountain View, California 94043
USA
Email: dschinazi.ietf@gmail.com
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