The Babel Routing ProtocolIRIF, University of Paris-DiderotCase 701475205 Paris Cedex 13Francejch@irif.frGoogle LLC1600 Amphitheatre ParkwayMountain ViewCalifornia94043USAdschinazi.ietf@gmail.comBabel 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.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.The main property that makes Babel suitable for unstable networks is
that, unlike naive distance-vector routing protocols ,
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
.More precisely, Babel has the following properties:
when every prefix is originated by at most one router, Babel never
suffers from routing loops;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;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.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 , IS-IS , or the Enhanced Interior
Gateway Routing Protocol (EIGRP) might be more
suitable.Second, unless the optional algorithm described in 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
unsuitable for use in networks that implement automatic prefix
aggregation.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
when, and only when,
they appear in all capitals, as shown here.Babel is a loop-avoiding distance vector protocol: it is based on the
Bellman-Ford protocol, just like the venerable 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.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 links 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
().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.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.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.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:
After convergence, D(B) = D(C) = 2, with NH(B) = NH(C) = A.Suppose now that the link between S and A fails:
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 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.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 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 . 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).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:
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.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:
Suppose now that the link between A and S breaks:
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 starving 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:
After S increases its sequence number, and the new sequence number is
propagated to B, we have:
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
(). Doing otherwise would cause
route oscillation while a sequence number propagates through the network,
and might even cause persistent blackholes with some exotic metrics.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.The above discussion assumes that each 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.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':
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 ().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).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:
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 ().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, router-ids could be assigned randomly, or they could be 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.)Babel protocol packets are sent in the body of a UDP datagram (as
described in 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 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 , 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 sent
before the deadline specified in the corresponding request. The particular
class of updates specified in MUST be
sent in a timely manner. The particular class of request and update TLVs
specified in SHOULD be sent in a timely
manner.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
below, an actual implementation would probably use two tables, one with
selected routes and one with fallback routes.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.A node's sequence number is a 16-bit integer that is included in route
updates sent for routes originated by this node.A node increments its sequence number (modulo 2^16) whenever it
receives a request for a new sequence number (). 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.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.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:
the local node's interface over which this neighbour is reachable;the address of the neighbouring interface;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;a history of recently received Unicast Hello packets from this neighbour;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;the expected incoming Multicast Hello sequence number for this neighbour,
an integer modulo 2^16.the expected incoming Unicast Hello sequence number for this neighbour,
an integer modulo 2^16.the outgoing Unicast Hello sequence number for this neighbour, 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
the outgoing Unicast Hello seqno for a neighbour 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.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:
the prefix (prefix, plen), where plen is the prefix length, that this
entry applies to;the router-id of a router originating this prefix;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 .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:
the source (prefix, plen, router-id) for which this route is advertised;the neighbour that advertised this route;the metric with which this route was advertised by the neighbour, or
FFFF hexadecimal (infinity) for a recently retracted route;the sequence number with which this route was advertised;the next-hop address of this route;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
.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.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:
the prefix, plen, router-id, and seqno being requested;the neighbour, if any, on behalf of which we are forwarding this
request;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.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 () and retractions (),
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.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 ).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 Unicast 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 .After receiving a Hello, or determining that it has missed one, the node
recomputes the association's cost () and
runs the route selection procedure ().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 (), 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 () and runs the route
selection procedure ().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 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 (),
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:
the cost is strictly positive;if no Hello TLVs of either kind were received recently, then the cost
is infinite;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
for examples of strategies for
computing a link's cost that are known to work well in practice.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 (), 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 . If the
feasibility condition is satisfied, then the update cannot possibly cause
a routing loop.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 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 .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:
metric is infinite; orno entry exists in the source table indexed by (prefix, plen, router-id);
oran entry (prefix, plen, router-id, seqno', metric') exists in the
source table, and either
seqno' < seqno orseqno = 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.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:
if c is infinite, then M(c, m) is infinite;M is strictly monotonic: M(c, m) > m.
Additionally, the metric SHOULD satisfy the following condition:
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
, we give a number of examples
of strictly monotonic, left-distributive routing metrics that are known to
work well in practice.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 ().Finally, as a special optimisation for the case when a router-id
coincides with the interface-id part of an IPv6 address, the router-id can
optionally be derived from the low-order bits of the advertised prefix.The encoding of updates is described in detail in
.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:
if the update is unfeasible, it MAY be ignored;if the metric is infinite (the update is a retraction of a route we
do not know about), the update is ignored;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:
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;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 ).
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 ).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
() is run.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 ().To avoid this issue, whenever a prefix P is retracted, a route table
entry with infinite metric is maintained as described in 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:
waiting until the route's expiry timer has expired ();sending a retraction with an acknowledgment request () 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.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.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:
a route with infinite metric (a retracted route) is never selected;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:
routes with a small metric should be preferred to routes with a large
metric;switching router-ids should be avoided;routes through stable neighbours should be preferred to routes
through unstable ones;stable routes should be preferred to unstable ones;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 .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
() and requests
() are sent.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; 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 .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
(), this full dump only needs to happen
infrequently.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 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 ).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:
if seqno > seqno', then seqno' := seqno, metric' := metric;if seqno = seqno' and metric' > metric, then metric' := metric;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.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.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.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.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.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.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:
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;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 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.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 ().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.When a route's metric increases, a node might receive an unfeasible
update for a route that it has currently selected. As specified in
, 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 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.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
(), this will usually result in
the route being either refreshed or retracted.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).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 . In order to allow accurate computation of packet loss
rates, this delay MUST NOT be larger than half the advertised Hello
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.A router-id is an arbitrary 8-octet value. A router-id MUST NOT
consist of either all zeroes or all ones.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 ().Address encodings:
AE 0: wildcard address. The value is 0 octets long.AE 1: IPv4 address. Compression is allowed. 4 octets or less.AE 2: IPv6 address. Compression is allowed. 16 octets or less.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.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).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).Fields :
The arbitrary but carefully chosen value 42 (decimal);
packets with a first octet different from 42 MUST be silently ignored.This document specifies version 2 of the Babel
protocol. Packets with a second octet different from 2 MUST be silently
ignored.The length in octets of the body following the
packet header (excluding the Magic, Version and Body length fields, and
excluding the packet trailer).The packet body; a sequence of TLVs.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.With the exception of Pad1, all TLVs have the following structure:Fields :
The type of the TLV.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.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.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:Fields :
The type of the sub-TLV.The length of the body, in octets, exclusive of the
Type and Length fields.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 ,
, and ).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:
for each address encoding that allows compression, the current
default prefix; this is undefined at the start of the packet, and is
updated by each Update TLV with the Prefix flag set
();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
();the current router-id; this is undefined at the start of the packet,
and is updated by each Router-ID TLV ()
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.Fields :
Set to 0 to indicate a Pad1 TLV.This TLV is silently ignored on reception. It is allowed in the packet
trailer.Fields :
Set to 1 to indicate a PadN TLV.The length of the body, exclusive of the Type and
Length fields.Set to 0 on transmission.This TLV is silently ignored on reception. It is allowed in the packet
trailer.This TLV requests that the receiver send an Acknowledgment TLV
within the number of centiseconds specified by the Interval field.Fields :
Set to 2 to indicate an Acknowledgment Request TLV.The length of the body, exclusive of the Type and
Length fields.Sent as 0 and MUST be ignored on
reception.An arbitrary value that will be echoed in the
receiver's Acknowledgment TLV.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.This TLV is sent by a node upon receiving an Acknowledgment Request.Fields :
Set to 3 to indicate an Acknowledgment TLV.The length of the body, exclusive of the Type and
Length fields.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.This TLV is used for neighbour discovery and for determining a
neighbour's reception cost.Fields :
Set to 4 to indicate a Hello TLV.The length of the body, exclusive of the Type and
Length fields.The individual bits of this field specify special
handling of this TLV (see below).If the Unicast flag is set, this is the value of the
sending node's outgoing Unicast Hello seqno for this neighbour. Otherwise,
it is the sending node's outgoing Multicast Hello seqno for this interface.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:
U (Unicast) flag (8000 hexadecimal): if set, then this Hello
represents a Unicast Hello, otherwise it represents a Multicast Hello;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.An IHU ("I Heard You") TLV is used for confirming bidirectional
reachability and carrying a link's transmission cost.Fields :
Set to 5 to indicate an IHU TLV.The length of the body, exclusive of the Type and
Length fields.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.Sent as 0 and MUST be ignored on reception.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.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.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.IHU TLVs with an unknown value in the AE field MUST be silently
ignored.This TLV is self-terminating, and allows sub-TLVs.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 :
Set to 6 to indicate a Router-Id TLV.The length of the body, exclusive of the Type and
Length fields.Sent as 0 and MUST be ignored on reception.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.A Next Hop TLV establishes a next-hop address for a given address
family (IPv4 or IPv6) that is implied in subsequent Update TLVs. 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 :
Set to 7 to indicate a Next Hop TLV.The length of the body, exclusive of the Type and
Length fields.The encoding of the Address field. This SHOULD be
1 (IPv4) or 3 (link-local IPv6), and MUST NOT be 0.Sent as 0 and MUST be ignored on reception.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.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 :
Set to 8 to indicate an Update TLV.The length of the body, exclusive of the Type and
Length fields.The encoding of the Prefix field.The individual bits of this field specify special
handling of this TLV (see below).The length of the advertised prefix.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.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 ().The originator's sequence number for this update.The sender's metric for this route. The value FFFF
hexadecimal (infinity) means that this is a route retraction.The prefix being advertised. This field's size is
(Plen/8 - Omitted) rounded upwards.The Flags field is interpreted as follows:
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;R (Router-Id) flag (40 hexadecimal): if set, then this TLV establishes
a new default router-id for this TLV and subsequent 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.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:
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;the next (Plen/8 - Omitted) rounded upwards octets are taken from the
Prefix field;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 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.A Route Request TLV prompts the receiver to send an update for a given
prefix, or a full route table dump.Fields :
Set to 9 to indicate a Route Request TLV.The length of the body, exclusive of the Type and
Length fields.The encoding of the Prefix field. The value 0 specifies
that this is a request for a full route table dump (a wildcard
request).The length of the requested 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.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 :
Set to 10 to indicate a Seqno Request TLV.The length of the body, exclusive of the Type and
Length fields.The encoding of the Prefix field. This MUST NOT be 0.The length of the requested prefix.The sequence number that is being requested.The maximum number of times that this TLV may be
forwarded, plus 1. This MUST NOT be 0.Sent as 0 and MUST be ignored on reception.The Router-Id that is being requested. This MUST
NOT consist of all zeroes or all ones.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 specified by the
Seqno field. If this request cannot be satisfied locally, then it is
forwarded according to the rules set out in
.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.Fields :
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.Fields :
Set to 1 to indicate a PadN sub-TLV.The length of the body, in octets, exclusive of the
Type and Length fields.Set to 0 on transmission.This sub-TLV is silently ignored on reception. It is allowed within
any TLV that allows sub-TLVs.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:TypeNameReference0Pad1this document1PadNthis document112-126Reserved for Experimental Usethis document127Reserved for expansion of the type spacethis document240-254Reserved for Experimental Usethis document255Reserved for expansion of the type spacethis documentExisting 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 . The initial values in this registry
are as follows:BitNameReference0Unicastthis document1-15UnassignedIANA is instructed to replace all references to RFCs 6126 and 7557
in all of the registries mentioned above by references to this document.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
and HMAC-based authentication . 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.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, Joao Sobrinho and Martin Vigoureux.
Earlier versions of this document
greatly benefited from the input of Joel Halpern. The address compression
technique was inspired by .Key words for use in RFCs to Indicate Requirement LevelsAmbiguity of Uppercase vs Lowercase in RFC 2119 Key WordsHMAC authentication for the Babel routing protocolBabel Routing Protocol over Datagram Transport Layer SecurityThe synchronization of periodic routing messagesHighly Dynamic Destination-Sequenced Distance-Vector Routing
(DSDV) for Mobile ComputersRIP Version 2Loop-Free Routing Using Diffusing ComputationsOSPF Version 2Information 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)EIGRP -- a Fast Routing Protocol Based on Distance VectorsA high-throughput path metric for multi-hop wireless networksGeneralized Mobile Ad Hoc Network (MANET) Packet/Message
FormatGuidelines for Writing an IANA Considerations Section in RFCsThe strategy for computing link costs and route metrics is a local
matter; Babel itself only requires that it comply with the conditions given
in and .
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 (),
computes costs by using the 2-out-of-3 strategy () on
wired links, and ETX () on wireless links. It uses an
additive algebra for metric computation ().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.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:
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;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");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).This section discusses how to compute costs based on Hello history.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 ≥ 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:
cost = FFFF hexadecimal if rxcost = FFFF hexadecimal;cost = txcost otherwise.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 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
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).As described in , the metric
advertised by a neighbour is combined with the link cost to yield
a metric.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 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
(), the value k must be greater than -c.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 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 (). 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.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:
increasing the version number in the packet header;defining new TLVs;defining new sub-TLVs (with or without the mandatory bit set);defining new AEs;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 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.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.
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.The sample implementation of Babel is available from
.Changed UDP port number to 6696.Consistently use router-id rather than id.Clarified that the source garbage collection timer is reset after
sending an update even if the entry was not modified.In section "Seqno Requests", fixed an erroneous "route request".In the description of the Seqno Request TLV, added the description of
the Router-Id field.Made router-ids all-0 and all-1 forbidden.Added security considerations.Integrated the format of sub-TLVs.Mentioned for each TLV whether it supports sub-TLVs.Added .Added a mandatory bit in sub-TLVs.Changed compression state to be per-AF rather than per-AE.Added implementation hint for the routing table.Clarified how router-ids are computed when bit 0x40 is set in Updates.Relaxed the conditions for sending requests, and tightened the
conditions for forwarding requests.Clarified that neighbours should be acquired at some point, but it
doesn't matter when.Added Unicast Hellos.Added unscheduled (interval-less) Hellos.Changed Appendix A to consider Unicast and unscheduled Hellos.Changed Appendix B to agree with the reference implementation.Added optional algorithm to avoid the hold time.Changed the table of pending seqno requests to be indexed by router-id
in addition to prefixes.Relaxed the route acquisition algorithm.Replaced minimal implementations by stub implementations.Added acknowledgments section.Clarified that all the data structures are conceptual.Made sending and receiving Multicast Hellos a SHOULD, avoids expressing
any opinion about Unicast Hellos.Removed opinion about Multicast vs. Unicast Hellos (Appendix A.4).Made hold-time into a SHOULD rather than MUST.Clarified that Seqno Requests are for a finite-metric Update.Clarified that sub-TLVs Pad1 and PadN are allowed within any TLV that
allows sub-TLVs.Updated IANA Considerations.Updated Security Considerations.Renamed routing table back to route table.Made buffering outgoing updates a SHOULD.Weakened advice to use modified EUI-64 in router-ids.Added information about sending requests to Appendix B.A number of minor wording changes and clarifications.Minor editorial changes.Renamed isotonicity to left-distributivity.Minor clarifications to unicast hellos.Updated requirements boilerplate to RFC 8174.Minor editorial changes.Added information about the packet trailer, now that it is used by
draft-ietf-babel-hmac.Added references to security documents.Added list of obsoleted drafts to the abstract.Updated references.Added recommendation that route selection should not take seqnos into
account.Editorial changes only.