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rfc:rfc6126

Independent Submission J. Chroboczek Request for Comments: 6126 PPS, University of Paris 7 Category: Experimental April 2011 ISSN: 2070-1721

                     The Babel Routing Protocol

Abstract

 Babel is a loop-avoiding distance-vector routing protocol that is
 robust and efficient both in ordinary wired networks and in wireless
 mesh networks.

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for examination, experimental implementation, and
 evaluation.
 This document defines an Experimental Protocol for the Internet
 community.  This is a contribution to the RFC Series, independently
 of any other RFC stream.  The RFC Editor has chosen to publish this
 document at its discretion and makes no statement about its value for
 implementation or deployment.  Documents approved for publication by
 the RFC Editor are not a candidate for any level of Internet
 Standard; see Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc6126.

Copyright Notice

 Copyright (c) 2011 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.

Chroboczek Experimental [Page 1] RFC 6126 The Babel Routing Protocol April 2011

Table of Contents

 1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   1.1.  Features . . . . . . . . . . . . . . . . . . . . . . . . .  3
   1.2.  Limitations  . . . . . . . . . . . . . . . . . . . . . . .  4
   1.3.  Specification of Requirements  . . . . . . . . . . . . . .  4
 2.  Conceptual Description of the Protocol . . . . . . . . . . . .  4
   2.1.  Costs, Metrics, and Neighbourship  . . . . . . . . . . . .  5
   2.2.  The Bellman-Ford Algorithm . . . . . . . . . . . . . . . .  5
   2.3.  Transient Loops in Bellman-Ford  . . . . . . . . . . . . .  6
   2.4.  Feasibility Conditions . . . . . . . . . . . . . . . . . .  6
   2.5.  Solving Starvation: Sequencing Routes  . . . . . . . . . .  8
   2.6.  Requests . . . . . . . . . . . . . . . . . . . . . . . . .  9
   2.7.  Multiple Routers . . . . . . . . . . . . . . . . . . . . . 10
   2.8.  Overlapping Prefixes . . . . . . . . . . . . . . . . . . . 11
 3.  Protocol Operation . . . . . . . . . . . . . . . . . . . . . . 11
   3.1.  Message Transmission and Reception . . . . . . . . . . . . 11
   3.2.  Data Structures  . . . . . . . . . . . . . . . . . . . . . 12
   3.3.  Acknowledged Packets . . . . . . . . . . . . . . . . . . . 15
   3.4.  Neighbour Acquisition  . . . . . . . . . . . . . . . . . . 15
   3.5.  Routing Table Maintenance  . . . . . . . . . . . . . . . . 17
   3.6.  Route Selection  . . . . . . . . . . . . . . . . . . . . . 21
   3.7.  Sending Updates  . . . . . . . . . . . . . . . . . . . . . 22
   3.8.  Explicit Route Requests  . . . . . . . . . . . . . . . . . 24
 4.  Protocol Encoding  . . . . . . . . . . . . . . . . . . . . . . 27
   4.1.  Data Types . . . . . . . . . . . . . . . . . . . . . . . . 28
   4.2.  Packet Format  . . . . . . . . . . . . . . . . . . . . . . 29
   4.3.  TLV Format . . . . . . . . . . . . . . . . . . . . . . . . 29
   4.4.  Details of Specific TLVs . . . . . . . . . . . . . . . . . 30
 5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 39
 6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 39
 7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 40
   7.1.  Normative References . . . . . . . . . . . . . . . . . . . 40
   7.2.  Informative References . . . . . . . . . . . . . . . . . . 40
 Appendix A.  Cost and Metric Computation . . . . . . . . . . . . . 41
   A.1.  Maintaining Hello History  . . . . . . . . . . . . . . . . 41
   A.2.  Cost Computation . . . . . . . . . . . . . . . . . . . . . 42
   A.3.  Metric Computation . . . . . . . . . . . . . . . . . . . . 43
 Appendix B.  Constants . . . . . . . . . . . . . . . . . . . . . . 43
 Appendix C.  Simplified Implementations  . . . . . . . . . . . . . 44
 Appendix D.  Software Availability . . . . . . . . . . . . . . . . 45

Chroboczek Experimental [Page 2] RFC 6126 The Babel Routing Protocol April 2011

1. Introduction

 Babel is a loop-avoiding distance-vector routing protocol that is
 designed to be robust and efficient both in networks using prefix-
 based routing and in networks using flat routing ("mesh networks"),
 and both in relatively stable wired networks and in highly dynamic
 wireless networks.

1.1. Features

 The main property that makes Babel suitable for unstable networks is
 that, unlike naive distance-vector routing protocols [RIP], it
 strongly limits the frequency and duration of routing pathologies
 such as routing loops and black-holes during reconvergence.  Even
 after a mobility event is detected, a Babel network usually remains
 loop-free.  Babel then quickly reconverges to a configuration that
 preserves the loop-freedom and connectedness of the network, but is
 not necessarily optimal; in many cases, this operation requires no
 packet exchanges at all.  Babel then slowly converges, in a time on
 the scale of minutes, to an optimal configuration.  This is achieved
 by using sequenced routes, a technique pioneered by Destination-
 Sequenced Distance-Vector routing [DSDV].
 More precisely, Babel has the following properties:
 o  when every prefix is originated by at most one router, Babel never
    suffers from routing loops;
 o  when a 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;
 o  assuming reasonable 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

Chroboczek Experimental [Page 3] RFC 6126 The Babel Routing Protocol April 2011

 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
 wireless environment.
 Finally, Babel is a hybrid routing protocol, in the sense that it can
 carry routes for multiple network-layer protocols (IPv4 and IPv6),
 whichever protocol the Babel packets are themselves being carried
 over.

1.2. Limitations

 Babel has two limitations that make it unsuitable for use in some
 environments.  First, Babel relies on periodic routing table updates
 rather than using a reliable transport; hence, in large, stable
 networks it generates more traffic than protocols that only send
 updates when the network topology changes.  In such networks,
 protocols such as OSPF [OSPF], IS-IS [IS-IS], or the Enhanced
 Interior Gateway Routing Protocol (EIGRP) [EIGRP] might be more
 suitable.
 Second, Babel does impose a hold time when a prefix is retracted
 (Section 3.5.5).  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.  Hence, if a previously deaggregated prefix becomes
 aggregated, it will be unreachable for a few minutes.  This makes
 Babel unsuitable for use in mobile networks that implement automatic
 prefix aggregation.

1.3. Specification of Requirements

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in [RFC2119].

2. Conceptual Description of the Protocol

 Babel is a mostly loop-free distance vector protocol: it is based on
 the Bellman-Ford protocol, just like the venerable RIP [RIP], but
 includes a number of refinements that either prevent loop formation
 altogether, or ensure that a loop disappears in a timely manner and
 doesn't form again.
 Conceptually, Bellman-Ford is executed in parallel for every source
 of routing information (destination of data traffic).  In the
 following discussion, we fix a source S; the reader will recall that
 the same algorithm is executed for all sources.

Chroboczek Experimental [Page 4] RFC 6126 The Babel Routing Protocol April 2011

2.1. Costs, Metrics, and Neighbourship

 As many routing algorithms, Babel computes costs of links between any
 two neighbouring nodes, abstract values attached to the edges between
 two nodes.  We write C(A, B) for the cost of the edge from node A to
 node B.
 Given a route between any two nodes, the metric of the route is the
 sum of the costs of all the edges along the route.  The goal of the
 routing algorithm is to compute, for every source S, the tree of the
 routes of lowest metric to S.
 Costs and metrics need not be integers.  In general, they can be
 values in any algebra that satisfies two fairly general conditions
 (Section 3.5.2).
 A Babel node periodically broadcasts Hello messages to all of its
 neighbours; it also periodically sends an IHU ("I Heard You") message
 to every neighbour from which it has recently heard a Hello.  From
 the information derived from Hello and IHU messages received from its
 neighbour B, a node A computes the cost C(A, B) of the link from A to
 B.

2.2. The Bellman-Ford Algorithm

 Every node A maintains two pieces of data: its estimated distance to
 S, written D(A), and its next-hop router to S, written NH(A).
 Initially, D(S) = 0, D(A) is infinite, and NH(A) is undefined.
 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.

Chroboczek Experimental [Page 5] RFC 6126 The Babel Routing Protocol April 2011

2.3. Transient Loops in Bellman-Ford

 It is well known that a naive application of Bellman-Ford to
 distributed routing can cause transient loops after a topology
 change.  Consider for example the following diagram:
          B
       1 /|
    1   / |
 S --- A  |1
        \ |
       1 \|
          C
 After convergence, D(B) = D(C) = 2, with NH(B) = NH(C) = A.
 Suppose now that the link between S and A fails:
          B
       1 /|
        / |
 S     A  |1
        \ |
       1 \|
          C
 When it detects the failure of the link, A switches its next hop to B
 (which is still advertising a route to S with metric 2), and
 advertises a metric equal to 3, and then advertises a new route with
 metric 3.  This process of nodes changing selected neighbours and
 increasing their metric continues until the advertised metric reaches
 "infinity", a value larger than all the metrics that the routing
 protocol is able to carry.

2.4. Feasibility Conditions

 Bellman-Ford is a very robust algorithm: its convergence properties
 are preserved when routers delay route acquisition or when they
 discard some updates.  Babel routers discard received route
 announcements unless they can prove that accepting them cannot
 possibly cause a routing loop.
 More formally, we define a condition over route announcements, known
 as the feasibility condition, that guarantees the absence of routing
 loops whenever all routers ignore route updates that do not satisfy
 the feasibility condition.  In effect, this makes Bellman-Ford into a
 family of routing algorithms, parameterised by the feasibility
 condition.

Chroboczek Experimental [Page 6] RFC 6126 The Babel Routing Protocol April 2011

 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 transitory "micro-loops" during reconvergence).
 Another simple feasibility condition, used in Destination-Sequenced
 Distance-Vector (DSDV) routing [DSDV] and in Ad hoc On-Demand
 Distance Vector (AODV) routing, 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, used in
 EIGRP [DUAL].  Given a router A, define the feasibility distance of
 A, written FD(A), as the smallest metric that A has ever advertised
 for S to any of its neighbours.  An update sent by a neighbour B of A
 is feasible when the metric D(B) advertised by B is strictly smaller
 than A's feasibility distance, i.e., when D(B) < FD(A).
 It is easy to see that this latter condition is no more restrictive
 than DSDV-feasibility.  Suppose that node A obeys DSDV-feasibility;
 then D(A) is nonincreasing, hence at all times D(A) <= FD(A).
 Suppose now that A receives a DSDV-feasible update that advertises a
 metric D(B).  Since the update is DSDV-feasible, C(A, B) + D(B) <=
 D(A), hence D(B) < D(A), and since D(A) <= FD(A), D(B) < FD(A).
 To see that it is strictly less restrictive, consider the following
 diagram, where A has selected the route through B, and D(A) = FD(A) =
 2.  Since D(C) = 1 < FD(A), the alternate route through C is feasible
 for A, although its metric C(A, C) + D(C) = 5 is larger than that of
 the currently selected route:
    B
 1 / \ 1
  /   \
 S     A
  \   /
 1 \ / 4
    C

Chroboczek Experimental [Page 7] RFC 6126 The Babel Routing Protocol April 2011

 To show that this feasibility condition still guarantees loop-
 freedom, recall that at the time when A accepts an update from B, the
 metric D(B) announced by B is no smaller than FD(B); since it is
 smaller than FD(A), at that point in time FD(B) < FD(A).  Since this
 property is preserved when A sends updates, it remains true at all
 times, which ensures that the forwarding graph has no loops.

2.5. Solving Starvation: Sequencing Routes

 Obviously, the feasibility conditions defined above cause starvation
 when a router runs out of feasible routes.  Consider the following
 diagram, where both A and B have selected the direct route to S:
    A
 1 /|        D(A) = 1
  / |       FD(A) = 1
 S  |1
  \ |        D(B) = 2
 2 \|       FD(B) = 2
    B
 Suppose now that the link between A and S breaks:
    A
    |
    |       FD(A) = 1
 S  |1
  \ |        D(B) = 2
 2 \|       FD(B) = 2
    B
 The only route available from A to S, the one that goes through B, is
 not feasible: A suffers from a spurious starvation.
 At this point, the whole network must be rebooted in order to solve
 the starvation; this is essentially what EIGRP does when it performs
 a global synchronisation of all the routers in the network with the
 source (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

Chroboczek Experimental [Page 8] RFC 6126 The Babel Routing Protocol April 2011

 equally recent and the metric is strictly smaller.  More formally, if
 FD(A) = (s, m), then an update carrying the distance (s', m') is
 feasible when either s' > s, or s = s' and m' < m.
 Assuming the sequence number of S is 137, the diagram above becomes:
    A
    |
    |       FD(A) = (137, 1)
 S  |1
  \ |        D(B) = (137, 2)
 2 \|       FD(B) = (137, 2)
    B
 After S increases its sequence number, and the new sequence number is
 propagated to B, we have:
    A
    |
    |       FD(A) = (137, 1)
 S  |1
  \ |        D(B) = (138, 2)
 2 \|       FD(B) = (138, 2)
    B
 at which point the route through B becomes feasible again.
 Note that while sequence numbers are used for determining
 feasibility, they are not necessarily used in route selection: a node
 will normally ignore the sequence number when selecting a route
 (Section 3.6).

2.6. Requests

 In DSDV, the sequence number of a source is increased periodically.
 A route becomes feasible again after the source increases its
 sequence number, and the new sequence number is propagated through
 the network, which may, in general, require a significant amount of
 time.
 Babel takes a different approach.  When a node detects that it is
 suffering from a potentially spurious starvation, it sends an
 explicit request to the source for a new sequence number.  This
 request is forwarded hop by hop to the source, with no regard to the
 feasibility condition.  Upon receiving the request, the source
 increases its sequence number and broadcasts an update, which is
 forwarded to the requesting node.

Chroboczek Experimental [Page 9] RFC 6126 The Babel Routing Protocol April 2011

 Note that after a change in network topology not all such requests
 will, in general, reach the source, as some will be sent over links
 that are now broken.  However, if the network is still connected,
 then at least one among the nodes suffering from spurious starvation
 has an (unfeasible) route to the source; hence, in the absence of
 packet loss, at least one such request will reach the source.
 (Resending requests a small number of times compensates for packet
 loss.)
 Since requests are forwarded with no regard to the feasibility
 condition, they may, in general, be caught in a forwarding loop; this
 is avoided by having nodes perform duplicate detection for the
 requests that they forward.

2.7. Multiple Routers

 The above discussion assumes that every prefix is originated by a
 single router.  In real networks, however, it is often necessary to
 have a single prefix originated by multiple routers; for example, the
 default route will be originated by all of the edge routers of a
 routing domain.
 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 diagram, where A has selected the
 default route through S, and B has selected the one through S':
            1     1     1
 ::/0 -- S --- A --- B --- S' -- ::/0
 Suppose that both default routes fail at the same time; then nothing
 prevents A from switching to B, and B simultaneously switching to A.
 However, as soon as A has successfully advertised the new route to B,
 the route through A will become unfeasible for B.  Conversely, as
 soon as B will have advertised the route through A, the route through
 B will become unfeasible for A.

Chroboczek Experimental [Page 10] RFC 6126 The Babel Routing Protocol April 2011

 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).

2.8. Overlapping Prefixes

 In the above discussion, we have assumed that all prefixes are
 disjoint, as is the case in flat ("mesh") routing.  In practice,
 however, prefixes may overlap: for example, the default route
 overlaps with all of the routes present in the network.
 After a route fails, it is not correct in general to switch to a
 route that subsumes the failed route.  Consider for example the
 following configuration:
            1     1
 ::/0 -- A --- B --- C
 Suppose that node C fails.  If B forwards packets destined to C by
 following the default route, a routing loop will form, and persist
 until A learns of B's retraction of the direct route to C.  Babel
 avoids this pitfall by maintaining an "unreachable" route for a few
 minutes after a route is retracted; the time for which such a route
 must be maintained should be the worst-case propagation time of the
 retraction of the route to C.

3. Protocol Operation

 Every Babel speaker is assigned a router-id, which is an arbitrary
 string of 8 octets that is assumed unique across the routing domain.
 We suggest that router-ids should be assigned in modified EUI-64
 format [ADDRARCH].  (As a matter of fact, the protocol encoding is
 slightly more compact when router-ids are assigned in the same manner
 as the IPv6 layer assigns host IDs.)

3.1. Message Transmission and Reception

 Babel protocol packets are sent in the body of a UDP datagram.  Each
 Babel packet consists of one or more TLVs.

Chroboczek Experimental [Page 11] RFC 6126 The Babel Routing Protocol April 2011

 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 (this is the usual case) or to a
 (link-local) unicast address.  In normal operation, a Babel speaker
 sends both multicast and unicast packets to its neighbours.
 With the exception of Hello TLVs and acknowledgements, all Babel TLVs
 can be sent to either unicast or multicast addresses, and their
 semantics does not depend on whether the destination was a unicast or
 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 is applied to packets sent by a Babel
 speaker: outgoing TLVs are buffered and SHOULD be sent with a small
 random delay.  This is done for two purposes: it avoids
 synchronisation of multiple Babel speakers across a network [JITTER],
 and it allows for the aggregation of multiple TLVs into a single
 packet.
 The exact delay and amount of jitter applied to a packet depends on
 whether it contains any urgent TLVs.  Acknowledgement TLVs MUST be
 sent before the deadline specified in the corresponding request.  The
 particular class of updates specified in Section 3.7.2 MUST be sent
 in a timely manner.  The particular class of request and update TLVs
 specified in Section 3.8.2 SHOULD be sent in a timely manner.

3.2. Data Structures

 Every Babel speaker maintains a number of data structures.

3.2.1. Sequence Number

 A node's sequence number is a 16-bit integer that is included in
 route updates sent for routes originated by this node.  A node
 increments its sequence number (modulo 2^16) whenever it receives a
 request for a new sequence number (Section 3.8.1.2).
 A node SHOULD NOT increment its sequence number (seqno)
 spontaneously, since increasing seqnos makes it less likely that
 other nodes will have feasible alternate routes when their selected
 routes fail.

3.2.2. The Interface Table

 The interface table contains the list of interfaces on which the node
 speaks the Babel protocol.  Every interface table entry contains the
 interface's Hello seqno, a 16-bit integer that is sent with each

Chroboczek Experimental [Page 12] RFC 6126 The Babel Routing Protocol April 2011

 Hello TLV on this interface and is incremented (modulo 2^16) whenever
 a Hello is sent.  (Note that an interface's Hello seqno is unrelated
 to the node's seqno.)
 There are two timers associated with each interface table entry --
 the Hello timer, which governs the sending of periodic Hello and IHU
 packets, and the update timer, which governs the sending of periodic
 route updates.

3.2.3. The Neighbour Table

 The neighbour table contains the list of all neighbouring interfaces
 from which a Babel packet has been recently received.  The neighbour
 table is indexed by pairs of the form (interface, address), and every
 neighbour table entry contains the following data:
 o  the local node's interface over which this neighbour is reachable;
 o  the address of the neighbouring interface;
 o  a history of recently received Hello packets from this neighbour;
    this can, for example, be a sequence of n bits, for some small
    value n, indicating which of the n hellos most recently sent by
    this neighbour have been received by the local node;
 o  the "transmission cost" value from the last IHU packet received
    from this neighbour, or FFFF hexadecimal (infinity) if the IHU
    hold timer for this neighbour has expired;
 o  the neighbour's expected Hello sequence number, an integer modulo
    2^16.
 There are two timers associated with each neighbour entry -- the
 hello timer, which is initialised from the interval value carried by
 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 fairly common
 situation when wireless nodes with multiple radios are involved.

3.2.4. The Source Table

 The source table is used to record feasibility distances.  It is
 indexed by triples of the form (prefix, plen, router-id), and every
 source table entry contains the following data:

Chroboczek Experimental [Page 13] RFC 6126 The Babel Routing Protocol April 2011

 o  the prefix (prefix, plen), where plen is the prefix length, that
    this entry applies to;
 o  the router-id of a router originating this prefix;
 o  a pair (seqno, metric), this source's feasibility distance.
 There is one timer associated with each entry in the source table --
 the source garbage-collection timer.  It is initialised to a time on
 the order of minutes and reset as specified in Section 3.7.3.

3.2.5. The Route Table

 The route table contains the routes known to this node.  It is
 indexed by triples of the form (prefix, plen, neighbour), and every
 route table entry contains the following data:
 o  the source (prefix, plen, router-id) for which this route is
    advertised;
 o  the neighbour that advertised this route;
 o  the metric with which this route was advertised by the neighbour,
    or FFFF hexadecimal (infinity) for a recently retracted route;
 o  the sequence number with which this route was advertised;
 o  the next-hop address of this route;
 o  a boolean flag indicating whether this route is selected, i.e.,
    whether it is currently being used for forwarding and is being
    advertised.
 There is one timer associated with each route table entry -- the
 route expiry timer.  It is initialised and reset as specified in
 Section 3.5.4.

3.2.6. The Table of Pending Requests

 The table of pending 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 prefixes, and every
 entry in this table contains the following data:
 o  the prefix, router-id, and seqno being requested;

Chroboczek Experimental [Page 14] RFC 6126 The Babel Routing Protocol April 2011

 o  the neighbour, if any, on behalf of which we are forwarding this
    request;
 o  a small integer indicating the number of times that this request
    will be resent if it remains unsatisfied.
 There is one timer associated with each pending request; it governs
 both the resending of requests and their expiry.

3.3. Acknowledged Packets

 A Babel speaker may request that any neighbour receiving a given
 packet reply with an explicit acknowledgement within a given time.
 While the use of acknowledgement requests is optional, every Babel
 speaker MUST be able to reply to such a request.
 An acknowledgement MUST be sent to a unicast destination.  On the
 other hand, acknowledgement requests may be sent to either unicast or
 multicast destinations, in which case they request an acknowledgement
 from all of the receiving nodes.
 When to request acknowledgements is a matter of local policy; the
 simplest strategy is to never request acknowledgements and to rely on
 periodic updates to ensure that any reachable routes are eventually
 propagated throughout the routing domain.  For increased efficiency,
 we suggest that acknowledged packets should be used in order to send
 urgent updates (Section 3.7.2) 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
 acknowledgement requests is not necessary, and not even recommended,
 as the acknowledgements cause additional traffic and may force
 additional Address Resolution Protocol (ARP) or Neighbour Discovery
 exchanges.

3.4. Neighbour Acquisition

 Neighbour acquisition is the process by which a Babel node discovers
 the set of neighbours heard over each of its interfaces and
 ascertains bidirectional reachability.  On unreliable media,
 neighbour acquisition additionally provides some statistics that MAY
 be used in link quality computation.

3.4.1. Reverse Reachability Detection

 Every Babel node sends periodic Hellos over each of its interfaces.
 Each Hello TLV carries an increasing (modulo 2^16) sequence number
 and the interval between successive periodic packets sent on this
 particular interface.

Chroboczek Experimental [Page 15] RFC 6126 The Babel Routing Protocol April 2011

 In addition to the periodic Hello packets, a node MAY send
 unscheduled Hello packets, e.g., to accelerate link cost estimation
 when a new neighbour is discovered, or when link conditions have
 suddenly changed.
 A node MAY change its Hello interval.  The Hello interval MAY be
 decreased at any time; it SHOULD NOT be increased, except immediately
 before sending a Hello packet.  (Equivalently, a node SHOULD send an
 unscheduled 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.  A
 possible algorithm is described in Appendix A.1.
 After receiving a Hello, or determining that it has missed one, the
 node recomputes the association's cost (Section 3.4.3) and runs the
 route selection procedure (Section 3.6).

3.4.2. Bidirectional Reachability Detection

 In order to establish bidirectional reachability, every node sends
 periodic IHU ("I Heard You") TLVs to each of its neighbours.  Since
 IHUs carry an explicit interval value, they MAY be sent less often
 than Hellos in order to reduce the amount of routing traffic in dense
 networks; in particular, they SHOULD be sent less often than Hellos
 over links with little packet loss.  While IHUs are conceptually
 unicast, they SHOULD be sent to a multicast address in order to avoid
 an ARP or Neighbour Discovery exchange and to aggregate multiple IHUs
 in 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.
 Every IHU TLV contains two pieces of data: the link's rxcost
 (reception cost) from the sender's perspective, used by the neighbour
 for computing link costs (Section 3.4.3), and the interval between
 periodic IHU packets.  A node receiving an IHU updates the value of
 the sending neighbour's txcost (transmission cost), from its
 perspective, to the value contained in the IHU, and resets this
 neighbour's IHU timer to a small multiple of the value received in
 the IHU.
 When a neighbour's IHU timer expires, its txcost is set to infinity.

Chroboczek Experimental [Page 16] RFC 6126 The Babel Routing Protocol April 2011

 After updating a neighbour's txcost, the receiving node recomputes
 the neighbour's cost (Section 3.4.3) and runs the route selection
 procedure (Section 3.6).

3.4.3. Cost Computation

 A neighbourship association's link cost is computed from the values
 maintained in the neighbour table -- namely, 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.
 How the txcost and rxcost are combined in order to compute a link's
 cost is a matter of local policy; as far as Babel's correctness is
 concerned, only the following conditions MUST be satisfied:
 o  the cost is strictly positive;
 o  if no hellos were received recently, then the cost is infinite;
 o  if the txcost is infinite, then the cost is infinite.
 Note that while this document does not constrain cost computation any
 further, not all cost computation strategies will give good results.
 We give a few examples of strategies for computing a link's cost that
 are known to work well in practice in Appendix A.2.

3.5. Routing Table Maintenance

 Conceptually, a Babel update is a quintuple (prefix, plen, router-id,
 seqno, metric), where (prefix, plen) is the prefix for which a route
 is being advertised, router-id is the router-id of the router
 originating this update, seqno is a nondecreasing (modulo 2^16)
 integer that carries the originating router seqno, and metric is the
 announced metric.
 Before being accepted, an update is checked against the feasibility
 condition (Section 3.5.1), which ensures that the route does not
 create a routing loop.  If the feasibility condition is not
 satisfied, the update is either ignored or treated as a retraction,
 depending on some other conditions (Section 3.5.4).  If the
 feasibility condition is satisfied, then the update cannot possibly
 cause a routing loop, and the update is accepted.

Chroboczek Experimental [Page 17] RFC 6126 The Babel Routing Protocol April 2011

3.5.1. The Feasibility Condition

 The feasibility condition is applied to all received updates.  The
 feasibility condition compares the metric in the received update with
 the metrics of the updates previously sent by the receiving node;
 updates with finite metrics large enough to cause a loop are
 discarded.
 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 (p, plen, 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 (p, plen) carrying the router-id id.  Feasibility
 distances are maintained in the source table; the exact procedure is
 given in Section 3.7.3.
 A received update is feasible when either it is a retraction (its
 metric is FFFF hexadecimal), or the advertised distance is strictly
 better, in the sense defined above, than the feasibility distance for
 the corresponding source.  More precisely, a route advertisement
 carrying the quintuple (prefix, plen, router-id, seqno, metric) is
 feasible if one of the following conditions holds:
 o  metric is infinite; or
 o  no entry exists in the source table indexed by (id, prefix, plen);
    or
 o  an entry (prefix, plen, router-id, seqno', metric') exists in the
    source table, and either
  • seqno' < seqno or
  • seqno = seqno' and metric < metric'.

Chroboczek Experimental [Page 18] RFC 6126 The Babel Routing Protocol April 2011

 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.

3.5.2. Metric Computation

 A route's metric is computed from the metric advertised by the
 neighbour and the neighbour's link cost.  Just like cost computation,
 metric computation is considered a local policy matter; as far as
 Babel is concerned, the function M(c, m) used for computing a metric
 from a locally computed link cost and the metric advertised by a
 neighbour MUST only satisfy the following conditions:
 o  if c is infinite, then M(c, m) is infinite;
 o  M is strictly monotonic: M(c, m) > m.
 Additionally, the metric SHOULD satisfy the following condition:
 o  M is isotonic: 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 appear if it is not
 satisfied), isotonicity is not: if it is not satisfied, Babel will
 still converge to a locally optimal routing table, but might not
 reach a global optimum (in fact, such a global optimum may not even
 exist).
 As with cost computation, not all strategies for computing route
 metrics will give good results.  In particular, some metrics are more
 likely than others to lead to routing instabilities (route flapping).
 In Appendix A.3, we give a number of examples of strictly monotonic,
 isotonic routing metrics that are known to work well in practice.

3.5.3. Encoding of Updates

 In a large network, the bulk of Babel traffic consists of route
 updates; hence, some care has been given to encoding them
 efficiently.  An Update TLV itself only contains the prefix, seqno,
 and metric, while the next hop is derived either from the network-
 layer source address of the packet or from an explicit Next Hop TLV
 in the same packet.  The router-id is derived from a separate
 Router-Id TLV in the same packet, which optimises the case when
 multiple updates are sent with the same router-id.
 Additionally, a prefix of the advertised prefix can be omitted in an
 Update TLV, in which case it is copied from a previous Update TLV in
 the same packet -- this is known as address compression [PACKETBB].

Chroboczek Experimental [Page 19] RFC 6126 The Babel Routing Protocol April 2011

 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 Section 4.4.

3.5.4. Route Acquisition

 When a Babel node receives an update (id, prefix, seqno, metric) from
 a neighbour neigh with a link cost value equal to cost, it checks
 whether it already has a routing table entry indexed by (neigh, id,
 prefix).
 If no such entry exists:
 o  if the update is unfeasible, it is ignored;
 o  if the metric is infinite (the update is a retraction), the update
    is ignored;
 o  otherwise, a new route table entry is created, indexed by (neigh,
    id, prefix), with seqno equal to seqno and an advertised metric
    equal to the metric carried by the update.
 If such an entry exists:
 o  if the entry is currently installed and the update is unfeasible,
    then the behaviour depends on whether the router-ids of the two
    entries match.  If the router-ids are different, the update is
    treated as though it were a retraction (i.e., as though the metric
    were FFFF hexadecimal).  If the router-ids are equal, the update
    is ignored;
 o  otherwise (i.e., if either the update is feasible or the entry is
    not currently installed), then the entry's sequence number,
    advertised metric, metric, and router-id are updated and, unless
    the advertised metric is infinite, the route's expiry timer is
    reset to a small multiple of the Interval value included in the
    update.
 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 routing table is updated, the route selection procedure
 (Section 3.6) is run.

Chroboczek Experimental [Page 20] RFC 6126 The Babel Routing Protocol April 2011

3.5.5. Hold Time

 When a prefix p is retracted, because all routes are unfeasible, too
 old, or have an infinite metric, and a shorter prefix p' that covers
 p is reachable, p' cannot in general be used for routing packets
 destined to p without running the risk of creating a routing loop
 (Section 2.8).
 To avoid this issue, whenever a prefix is retracted, a routing table
 entry with infinite metric is maintained as described in
 Section 3.5.4 above, and packets destined for that prefix MUST NOT be
 forwarded by following a route for a shorter prefix.  The infinite
 metric entry is maintained until it is superseded by a feasible
 update; if no such update arrives within the route hold time, the
 entry is flushed.

3.6. Route Selection

 Route selection is the process by which a single route for a given
 prefix is selected to be used for forwarding packets and to be
 re-advertised to a node's neighbours.
 Babel is designed to allow flexible route selection policies.  As far
 as the protocol's correctness is concerned, the route selection
 policy MUST only satisfy the following properties:
 o  a route with infinite metric (a retracted route) is never
    selected;
 o  an unfeasible route is never selected.
 Note, however, that Babel does not naturally guarantee the stability
 of routing, and configuring conflicting route selection policies on
 different routers may lead to persistent route oscillation.
 Defining a good route selection policy for Babel is an open research
 problem.  Route selection can take into account multiple mutually
 contradictory criteria; in roughly decreasing order of importance,
 these are:
 o  routes with a small metric should be preferred over routes with a
    large metric;
 o  switching router-ids should be avoided;
 o  routes through stable neighbours should be preferred over routes
    through unstable ones;

Chroboczek Experimental [Page 21] RFC 6126 The Babel Routing Protocol April 2011

 o  stable routes should be preferred over unstable ones;
 o  switching next hops should be avoided.
 A simple strategy is to choose the feasible route with the smallest
 metric, with a small amount of hysteresis applied to avoid switching
 router-ids.
 After the route selection procedure is run, triggered updates
 (Section 3.7.2) and requests (Section 3.8.2) are sent.

3.7. Sending Updates

 A Babel speaker advertises to its neighbours its set of selected
 routes.  Normally, this is done by sending one or more multicast
 packets containing Update TLVs on all of its connected interfaces;
 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 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., the address of a local interface, the prefix of a
 directly attached network, or redistributed from a different routing
 protocol), the router-id is set to the local 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 routing table
 entry, and the metric is computed as specified in Section 3.5.2.

3.7.1. Periodic Updates

 Every Babel speaker periodically advertises all of its selected
 routes on all of its interfaces, including any recently retracted
 routes.  Since Babel doesn't suffer from routing loops (there is no
 "counting to infinity") and relies heavily on triggered updates
 (Section 3.7.2), this full dump only needs to happen infrequently.

3.7.2. Triggered Updates

 In addition to the 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.

Chroboczek Experimental [Page 22] RFC 6126 The Babel Routing Protocol April 2011

 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 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 acknowledgement 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
 acknowledgements 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, either due to a
 received update or because a link cost has changed.  A node SHOULD
 NOT send triggered updates for other reasons, such as when there is a
 minor fluctuation in a route's metric, when the selected next hop
 changes, or to propagate a new sequence number (except to satisfy a
 request, as specified in Section 3.8).

3.7.3. Maintaining Feasibility Distances

 Before sending an update (prefix, plen, router-id, seqno, metric)
 with finite metric (i.e., not a route retraction), a Babel node
 updates the feasibility distance maintained in the source table.
 This is done as follows.
 If no entry indexed by (prefix, plen, router-id) exists in the source
 table, then one is created with value (prefix, plen, router-id,
 seqno, metric).
 If an entry (prefix, plen, router-id, seqno', metric') exists, then
 it is updated as follows:
 o  if seqno > seqno', then seqno' := seqno, metric' := metric;

Chroboczek Experimental [Page 23] RFC 6126 The Babel Routing Protocol April 2011

 o  if seqno = seqno' and metric' > metric, then metric' := metric;
 o  otherwise, nothing needs to be done.
 The garbage-collection timer for the modified entry is then reset.
 Note that the garbage-collection timer is not reset when a retraction
 is sent.

3.7.4. Split Horizon

 When running over a transitive, symmetric link technology, e.g., a
 point-to-point link or a wired LAN technology such as Ethernet, a
 Babel node SHOULD use an optimisation known as split horizon.  When
 split horizon is used on a given interface, a routing update is not
 sent on this particular interface when the advertised route was
 learnt from a neighbour over the same interface.
 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).

3.8. Explicit Route Requests

 In normal operation, a node's routing table is populated by the
 regular and triggered updates sent by its neighbours.  Under some
 circumstances, however, a node sends explicit requests 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 a neighbour.

3.8.1. Handling Requests

 Upon receiving a request, a node either forwards the request or sends
 an update in reply to the request, as described in the following
 sections.  If this causes an update to be sent, the update is either
 sent to a multicast address on the interface on which the request was
 received, or to the unicast address of the neighbour that sent the
 update.
 The exact behaviour is different for route requests and seqno
 requests.

Chroboczek Experimental [Page 24] RFC 6126 The Babel Routing Protocol April 2011

3.8.1.1. Route Requests

 When a node receives a route request for a prefix (prefix, plen), it
 checks its route table for a selected route to this exact prefix.  If
 such a route exists, it MUST send an update; if such a route does
 not, it MUST send a retraction for that prefix.
 When a node receives a wildcard route request, it SHOULD send a full
 routing table dump.

3.8.1.2. Seqno Requests

 When a node receives a seqno request for a given router-id and
 sequence number, it checks whether its routing table contains a
 selected entry for that prefix; if no such entry exists, or the entry
 has infinite metric, it ignores the request.
 If a selected route for the given prefix exists, and either the
 router-ids are different or the router-ids are equal and the entry's
 sequence number is no smaller than the requested sequence number, it
 MUST send an update for the given prefix.
 If the router-ids match but the requested seqno is larger 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 and sends an update.  A node MUST NOT increase
 its sequence number by more than 1 in response to a route request.
 If the requested router-id is not its own, the received request's hop
 count is 2 or more, and the node has a route (not necessarily a
 feasible one) for the requested prefix that does not use the
 requestor as a next hop, the node SHOULD forward the request.  It
 does so by decreasing the hop count and sending the request in a
 unicast packet destined to a neighbour that advertises the given
 prefix (not necessarily the selected neighbour) and that is distinct
 from the neighbour from which the request was received.
 A node SHOULD maintain a list of recently forwarded requests and
 forward the reply in a timely manner.  A node SHOULD compare every
 incoming request against its list of recently forwarded requests and
 avoid forwarding it if it is redundant.
 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 for 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

Chroboczek Experimental [Page 25] RFC 6126 The Babel Routing Protocol April 2011

 low, and performing an expanding horizon search is not necessary.  A
 request MUST NOT be forwarded to a multicast address, and it MUST be
 forwarded to a single neighbour only.

3.8.2. Sending Requests

 A Babel node MAY send a route or seqno request at any time, to a
 multicast or a unicast address; there is only one case when
 originating requests is required (Section 3.8.2.1).

3.8.2.1. Avoiding Starvation

 When a route is retracted or expires, a Babel node usually switches
 to another feasible route for the same prefix.  It may be the case,
 however, that no such routes are available.
 A node that has lost all feasible routes to a given destination MUST
 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.
 Such a request SHOULD be multicast over all of the node's attached
 interfaces.  Similar requests will be sent by other nodes that are
 affected by the route's loss and will be forwarded by neighbouring
 nodes up to the source.  If the network is connected, and there is no
 packet loss, this will result in a route being advertised with a new
 sequence number.  (Note that, 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.  Under heavy packet loss, however, all such requests may
 be lost; in that case, the second mechanism in the next section will
 eventually ensure that a new seqno is received.

3.8.2.2. Dealing with Unfeasible Updates

 When a route's metric increases, a node might receive an unfeasible
 update for a route that it has currently selected.  As specified in
 Section 3.5.1, the receiving node will either ignore the update or
 retract the route.
 In order to keep routes from spuriously expiring because they have
 become unfeasible, a node SHOULD send a unicast seqno request
 whenever it receives an unfeasible update for a route that is
 currently selected.  The requested sequence number is computed from
 the source table as above.

Chroboczek Experimental [Page 26] RFC 6126 The Babel Routing Protocol April 2011

 Additionally, a node SHOULD send a unicast seqno request whenever it
 receives an unfeasible update from a currently unselected neighbour
 that is "good enough", i.e., that would lead to the received route
 becoming selected were it feasible.

3.8.2.3. Preventing Routes from Expiring

 In normal operation, a route's expiry timer should never trigger:
 since a route's hold time is computed from an explicit interval
 included in Update TLVs, a new update 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 retractions in response to non-wildcard route
 requests (Section 3.8.1.1), this will usually result in either the
 route being refreshed or a retraction being received.

3.8.2.4. Acquiring New Neighbours

 In order to speed up convergence after a mobility event, a node MAY
 send a unicast wildcard request after acquiring a new neighbour.
 Additionally, a node MAY send a small number of multicast wildcard
 requests shortly after booting.

4. Protocol Encoding

 A Babel packet is sent as the body of a UDP datagram, with network-
 layer hop count set to 1, destined to a well-known multicast address
 or to a unicast address, over IPv4 or IPv6; in the case of IPv6,
 these addresses are link-local.  Both the source and destination UDP
 port are set to a well-known port number.  A Babel packet MUST be
 silently ignored unless its source address is either a link-local
 IPv6 address, or an IPv4 address belonging to the local network, and
 its source port is the well-known Babel port.  Babel packets MUST NOT
 be sent as IPv6 Jumbograms.
 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/IPv4, 48 octets
 for UDP/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-

Chroboczek Experimental [Page 27] RFC 6126 The Babel Routing Protocol April 2011

 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.
 In order to avoid global synchronisation of a Babel network and to
 aggregate multiple TLVs into large packets, a Babel node MUST buffer
 every TLV and delay sending a UDP packet by a small, randomly chosen
 delay [JITTER].  In order to allow accurate computation of packet
 loss rates, this delay MUST NOT be larger than half the advertised
 Hello interval.

4.1. Data Types

4.1.1. Interval

 Relative times are carried as 16-bit values specifying a number of
 centiseconds (hundredths of a second).  This allows times up to
 roughly 11 minutes with a granularity of 10 ms, which should cover
 all reasonable applications of Babel.

4.1.2. Router-Id

 A router-id is an arbitrary 8-octet value.  Router-ids SHOULD be
 assigned in modified EUI-64 format [ADDRARCH].

4.1.3. Address

 Since the bulk of the protocol is taken by addresses, multiple ways
 of encoding addresses are defined.  Additionally, a common subnet
 prefix may be omitted when multiple addresses are sent in a single
 packet -- this is known as address compression [PACKETBB].
 Address encodings:
 o  AE 0: wildcard address.  The value is 0 octets long.
 o  AE 1: IPv4 address.  Compression is allowed. 4 octets or less.
 o  AE 2: IPv6 address.  Compression is allowed. 16 octets or less.
 o  AE 3: link-local IPv6 address.  The value is 8 octets long, a
    prefix of fe80::/64 is implied.
 The address family of an address is either IPv4 or IPv6; it is
 undefined for AE 0, IPv4 for AE 1, and IPv6 for AE 2 and 3.

Chroboczek Experimental [Page 28] RFC 6126 The Babel Routing Protocol April 2011

4.1.4. Prefixes

 A network prefix is encoded just like a network address, but it is
 stored in the smallest number of octets that are enough to hold the
 significant bits (up to the prefix length).

4.2. Packet Format

 A Babel packet consists of a 4-octet header, followed by a sequence
 of TLVs.
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Magic     |    Version    |        Body length            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   Packet Body ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-
 Fields :
 Magic     The arbitrary but carefully chosen value 42 (decimal);
           packets with a first octet different from 42 MUST be
           silently ignored.
 Version   This document specifies version 2 of the Babel protocol.
           Packets with a second octet different from 2 MUST be
           silently ignored.
 Body length  The length in octets of the body following the packet
              header.
 Body      The packet body; a sequence of TLVs.
 Any data following the body MUST be silently ignored.

4.3. TLV Format

 With the exception of Pad1, all TLVs have the following structure:
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Type      |    Length     |     Body...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

Chroboczek Experimental [Page 29] RFC 6126 The Babel Routing Protocol April 2011

 Fields :
 Type      The type of the TLV.
 Length    The length of the body, exclusive of the Type and Length
           fields.  If the body is longer than the expected length of
           a given type of TLV, any extra data MUST be silently
           ignored.
 Body      The TLV body, the interpretation of which depends on the
           type.
 TLVs with an unknown type value MUST be silently ignored.

4.4. Details of Specific TLVs

4.4.1. Pad1

 0
 0 1 2 3 4 5 6 7
 +-+-+-+-+-+-+-+-+
 |   Type = 0    |
 +-+-+-+-+-+-+-+-+
 Fields :
 Type      Set to 0 to indicate a Pad1 TLV.
 This TLV is silently ignored on reception.

4.4.2. PadN

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    Type = 1   |    Length     |      MBZ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
 Fields :
 Type      Set to 1 to indicate a PadN TLV.
 Length    The length of the body, exclusive of the Type and Length
           fields.
 MBZ       Set to 0 on transmission.
 This TLV is silently ignored on reception.

Chroboczek Experimental [Page 30] RFC 6126 The Babel Routing Protocol April 2011

4.4.3. Acknowledgement Request

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    Type = 2   |    Length     |          Reserved             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            Nonce              |          Interval             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 This TLV requests that the receiver send an Acknowledgement TLV
 within the number of centiseconds specified by the Interval field.
 Fields :
 Type      Set to 2 to indicate an Acknowledgement Request TLV.
 Length    The length of the body, exclusive of the Type and Length
           fields.
 Reserved  Sent as 0 and MUST be ignored on reception.
 Nonce     An arbitrary value that will be echoed in the receiver's
           Acknowledgement TLV.
 Interval  A time interval in centiseconds after which the sender will
           assume that this packet has been lost.  This MUST NOT be 0.
           The receiver MUST send an acknowledgement before this time
           has elapsed (with a margin allowing for propagation time).

4.4.4. Acknowledgement

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    Type = 3   |    Length     |            Nonce              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 This TLV is sent by a node upon receiving an Acknowledgement Request.
 Fields :
 Type      Set to 3 to indicate an Acknowledgement TLV.
 Length    The length of the body, exclusive of the Type and Length
           fields.

Chroboczek Experimental [Page 31] RFC 6126 The Babel Routing Protocol April 2011

 Nonce     Set to the Nonce value of the Acknowledgement Request that
           prompted this Acknowledgement.
 Since nonce values are not globally unique, this TLV MUST be sent to
 a unicast address.

4.4.5. Hello

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    Type = 4   |    Length     |          Reserved             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            Seqno              |          Interval             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 This TLV is used for neighbour discovery and for determining a link's
 reception cost.
 Fields :
 Type      Set to 4 to indicate a Hello TLV.
 Length    The length of the body, exclusive of the Type and Length
           fields.
 Reserved  Sent as 0 and MUST be ignored on reception.
 Seqno     The value of the sending node's Hello seqno for this
           interface.
 Interval  An upper bound, expressed in centiseconds, on the time
           after which the sending node will send a new Hello TLV.
           This MUST NOT be 0.
 Since there is a single seqno counter for all the Hellos sent by a
 given node over a given interface, this TLV MUST be sent to a
 multicast destination.  In order to avoid large discontinuities in
 link quality, multiple Hello TLVs SHOULD NOT be sent in the same
 packet.

Chroboczek Experimental [Page 32] RFC 6126 The Babel Routing Protocol April 2011

4.4.6. IHU

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    Type = 5   |    Length     |       AE      |    Reserved   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            Rxcost             |          Interval             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |       Address...
 +-+-+-+-+-+-+-+-+-+-+-+-
 An IHU ("I Heard You") TLV is used for confirming bidirectional
 reachability and carrying a link's transmission cost.
 Fields :
 Type      Set to 5 to indicate an IHU TLV.
 Length    The length of the body, exclusive of the Type and Length
           fields.
 AE        The encoding of the Address field.  This should be 1 or 3
           in most cases.  As an optimisation, it MAY be 0 if the TLV
           is sent to a unicast address, if the association is over a
           point-to-point link, or when bidirectional reachability is
           ascertained by means outside of the Babel protocol.
 Reserved  Sent as 0 and MUST be ignored on reception.
 Rxcost    The rxcost according to the sending node of the interface
           whose address is specified in the Address field.  The value
           FFFF hexadecimal (infinity) indicates that this interface
           is unreachable.
 Interval  An upper bound, expressed in centiseconds, on the time
           after which the sending node will send a new IHU; this MUST
           NOT be 0.  The receiving node will use this value in order
           to compute a hold time for this symmetric association.
 Address   The address of the destination node, in the format
           specified by the AE field.  Address compression is not
           allowed.
 Conceptually, an IHU is destined to a single neighbour.  However, IHU
 TLVs contain an explicit destination address, and it SHOULD be sent
 to a multicast address, as this allows aggregation of IHUs destined

Chroboczek Experimental [Page 33] RFC 6126 The Babel Routing Protocol April 2011

 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 for the AE field MUST be silently
 ignored.

4.4.7. Router-Id

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    Type = 6   |    Length     |          Reserved             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 +                           Router-Id                           +
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 A Router-Id TLV establishes a router-id that is implied by subsequent
 Update TLVs.
 Fields :
 Type      Set to 6 to indicate a Router-Id TLV.
 Length    The length of the body, exclusive of the Type and Length
           fields.
 Reserved  Sent as 0 and MUST be ignored on reception.
 Router-Id The router-id for routes advertised in subsequent Update
           TLVs

4.4.8. Next Hop

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    Type = 7   |    Length     |      AE       |   Reserved    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |       Next hop...
 +-+-+-+-+-+-+-+-+-+-+-+-
 A Next Hop TLV establishes a next-hop address for a given address
 family (IPv4 or IPv6) that is implied by subsequent Update TLVs.

Chroboczek Experimental [Page 34] RFC 6126 The Babel Routing Protocol April 2011

 Fields :
 Type      Set to 7 to indicate a Next Hop TLV.
 Length    The length of the body, exclusive of the Type and Length
           fields.
 AE        The encoding of the Address field.  This SHOULD be 1 or 3
           and MUST NOT be 0.
 Reserved  Sent as 0 and MUST be ignored on reception.
 Next hop  The next-hop address advertised by subsequent Update TLVs,
           for this address family.
 When the address family matches the network-layer protocol that this
 packet is transported over, a Next Hop TLV is not needed: in that
 case, the next hop 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.

4.4.9. Update

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    Type = 8   |    Length     |       AE      |    Flags      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Plen      |    Omitted    |            Interval           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |             Seqno             |            Metric             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |      Prefix...
 +-+-+-+-+-+-+-+-+-+-+-+-
 An Update TLV advertises or retracts a route.  As an optimisation,
 this can also have the side effect of establishing a new implied
 router-id and a new default prefix.
 Fields :
 Type      Set to 8 to indicate an Update TLV.
 Length    The length of the body, exclusive of the Type and Length
           fields.
 AE        The encoding of the Prefix field.

Chroboczek Experimental [Page 35] RFC 6126 The Babel Routing Protocol April 2011

 Flags     The individual bits of this field specify special handling
           of this TLV (see below).  Every node MUST be able to
           interpret the flags with values 80 and 40 hexadecimal;
           unknown flags MUST be silently ignored.
 Plen      The length of the advertised prefix.
 Omitted   The number of octets that have been omitted at the
           beginning of the advertised prefix and that should be taken
           from a preceding Update TLV with the flag with value 80
           hexadecimal set.
 Interval  An upper bound, expressed in centiseconds, on the time
           after which the sending node will send a new update for
           this prefix.  This MUST NOT be 0 and SHOULD NOT be less
           than 10.  The receiving node will use this value to compute
           a hold time for this routing table entry.  The value FFFF
           hexadecimal (infinity) expresses that this announcement
           will not be repeated unless a request is received
           (Section 3.8.2.3).
 Seqno     The originator's sequence number for this update.
 Metric    The sender's metric for this route.  The value FFFF
           hexadecimal (infinity) means that this is a route
           retraction.
 Prefix    The prefix being advertised.  This field's size is (Plen/8
           - Omitted) rounded upwards.
 The Flags field is interpreted as follows:
 o  if the bit with value 80 hexadecimal is set, then this Update
    establishes a new default prefix for subsequent Update TLVs with a
    matching address family within the same packet;
 o  if the bit with value 40 hexadecimal is set, then the low-order 8
    octets of the advertised prefix establish a new default router-id
    for this TLV and subsequent Update TLVs in the same packet.
 The prefix being advertised by an Update TLV is computed as follows:
 o  the first Omitted octets of the prefix are taken from the previous
    Update TLV with flag 80 hexadecimal set and the same address
    family;
 o  the next (Plen/8 - Omitted) (rounded upwards) octets are taken
    from the Prefix field;

Chroboczek Experimental [Page 36] RFC 6126 The Babel Routing Protocol April 2011

 o  the remaining octets are set to 0.
 If the Metric field is finite, the router-id of the originating node
 for this announcement is taken from the low-order 8 octets of the
 prefix advertised by this Update if the bit with value 40 hexadecimal
 is set in the Flags field.  Otherwise, it is taken either from the
 preceding Router-Id packet, or the preceding Update packet with flag
 40 hexadecimal set, whichever comes last.
 The next-hop address for this update is taken from the last preceding
 Next Hop TLV with a matching address family in the same packet; 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 current router-id and the Seqno are
 not used.  AE MAY then be 0, in which case this Update retracts all
 of the routes previously advertised on this interface.
 Update TLVs with an unknown value for the AE field MUST be silently
 ignored.

4.4.10. Route Request

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    Type = 9   |    Length     |      AE       |     Plen      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |      Prefix...
 +-+-+-+-+-+-+-+-+-+-+-+-
 A Route Request TLV prompts the receiver to send an update for a
 given prefix, or a full routing table dump.
 Fields :
 Type      Set to 9 to indicate a Route Request TLV.
 Length    The length of the body, exclusive of the Type and Length
           fields.
 AE        The encoding of the Prefix field.  The value 0 specifies
           that this is a request for a full routing table dump (a
           wildcard request).
 Plen      The length of the requested prefix.

Chroboczek Experimental [Page 37] RFC 6126 The Babel Routing Protocol April 2011

 Prefix    The prefix being requested.  This field's size is Plen/8
           rounded upwards.
 A Request TLV prompts the receiving node to send an update message
 for the prefix specified by the AE, Plen, and Prefix fields, or a
 full dump of its routing table if AE is 0 (in which case Plen MUST be
 0, and Prefix is of length 0).  A Request may be sent to a unicast
 address if it is destined to a single node, or to a multicast address
 if the request is destined to all of the neighbours of the sending
 interface.

4.4.11. Seqno Request

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    Type = 10  |    Length     |      AE       |    Plen       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |             Seqno             |  Hop Count    |   Reserved    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 +                          Router-Id                            +
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   Prefix...
 +-+-+-+-+-+-+-+-+-+-+
 A Seqno Request TLV prompts the receiver to send an Update for a
 given prefix with a given sequence number, or to forward the request
 further if it cannot be satisfied locally.
 Fields :
 Type      Set to 10 to indicate a Seqno Request message.
 Length    The length of the body, exclusive of the Type and Length
           fields.
 AE        The encoding of the Prefix field.  This MUST NOT be 0.
 Plen      The length of the requested prefix.
 Seqno     The sequence number that is being requested.
 Hop Count The maximum number of times that this TLV may be forwarded,
           plus 1.  This MUST NOT be 0.

Chroboczek Experimental [Page 38] RFC 6126 The Babel Routing Protocol April 2011

 Prefix    The prefix being requested.  This field's size is Plen/8
           rounded upwards.
 A Seqno Request TLV prompts the receiving node to send an 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 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 Section 3.8.1.2.
 While a Seqno Request MAY be sent to a multicast address, it MUST NOT
 be forwarded to a multicast address and MUST NOT be forwarded to more
 than one neighbour.  A request MUST NOT be forwarded if its Hop Count
 field is 1.

5. IANA Considerations

 IANA has registered the UDP port number 6697, called "babel", for use
 by the Babel protocol.
 IANA has registered the IPv6 multicast group ff02:0:0:0:0:0:1:6 and
 the IPv4 multicast group 224.0.0.111 for use by the Babel protocol.

6. Security Considerations

 As defined in this document, Babel is a completely insecure protocol.
 Any attacker can attract data traffic by advertising routes with a
 low metric.  This particular issue can be solved either by lower-
 layer security mechanisms (e.g., IPsec or link-layer security), or by
 appending a cryptographic key to Babel packets; the provision of
 ignoring any data contained within a Babel packet beyond the body
 length declared by the header is designed for just such a purpose.
 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.

Chroboczek Experimental [Page 39] RFC 6126 The Babel Routing Protocol April 2011

7. References

7.1. Normative References

 [ADDRARCH]  Hinden, R. and S. Deering, "IP Version 6 Addressing
             Architecture", RFC 4291, February 2006.
 [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.

7.2. Informative References

 [DSDV]      Perkins, C. and P. Bhagwat, "Highly Dynamic Destination-
             Sequenced Distance-Vector Routing (DSDV) for Mobile
             Computers", ACM SIGCOMM'94 Conference on Communications
             Architectures, Protocols and Applications 234-244, 1994.
 [DUAL]      Garcia Luna Aceves, J., "Loop-Free Routing Using
             Diffusing Computations", IEEE/ACM Transactions on
             Networking 1:1, February 1993.
 [EIGRP]     Albrightson, B., Garcia Luna Aceves, J., and J. Boyle,
             "EIGRP -- a Fast Routing Protocol Based on Distance
             Vectors", Proc. Interop 94, 1994.
 [ETX]       De Couto, D., Aguayo, D., Bicket, J., and R. Morris, "A
             high-throughput path metric for multi-hop wireless
             networks", Proc. MobiCom 2003, 2003.
 [IS-IS]     "Information technology -- Telecommunications and
             information exchange between systems -- Intermediate
             System to Intermediate System intra-domain routeing
             information exchange protocol for use in conjunction with
             the protocol for providing the connectionless-mode
             network service (ISO 8473)", ISO/IEC 10589:2002.
 [JITTER]    Floyd, S. and V. Jacobson, "The synchronization of
             periodic routing messages", IEEE/ACM Transactions on
             Networking 2, 2, 122-136, April 1994.
 [OSPF]      Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.
 [PACKETBB]  Clausen, T., Dearlove, C., Dean, J., and C. Adjih,
             "Generalized Mobile Ad Hoc Network (MANET) Packet/Message
             Format", RFC 5444, February 2009.
 [RIP]       Malkin, G., "RIP Version 2", STD 56, RFC 2453,
             November 1998.

Chroboczek Experimental [Page 40] RFC 6126 The Babel Routing Protocol April 2011

Appendix A. Cost and Metric Computation

 The strategy for computing link costs and route metrics is a local
 matter; Babel itself only requires that it comply with the conditions
 given in Sections 3.4.3 and 3.5.2.  Different nodes MAY use different
 strategies in a single network and MAY use different strategies on
 different interface types.  This section gives a few examples of such
 strategies.
 The sample implementation of Babel maintains statistics about the
 last 16 received Hello TLVs (Appendix A.1), computes costs by using
 the 2-out-of-3 strategy (Appendix A.2.1) on wired links, and ETX
 [ETX] on wireless links.  It uses an additive algebra for metric
 computation (Appendix A.3.1).

A.1. Maintaining Hello History

 For each neighbour, the sample implementation of Babel maintains a
 Hello history and an expected sequence number.  The Hello history is
 a vector of 16 bits, where a 1 value represents a received Hello, and
 a 0 value a missed 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 from a neighbour, a node compares
 the received sequence number nr with its expected sequence number ne.
 Depending on the outcome of this comparison, one of the following
 actions is taken:
 o  if the two differ by more than 16 (modulo 2^16), then the sending
    node has probably rebooted and lost its sequence number; the
    associated neighbour table entry is flushed;
 o  otherwise, if the received nr is smaller (modulo 2^16) than the
    expected sequence number ne, then the sending node has increased
    its Hello interval without our noticing; the receiving node
    removes the last (ne - nr) entries from this neighbour's Hello
    history (we "undo history");
 o  otherwise, if nr is larger (modulo 2^16) than ne, then the sending
    node has decreased its Hello interval, and some Hellos were lost;
    the receiving node adds (nr - ne) 0 bits to the Hello history (we
    "fast-forward").

Chroboczek Experimental [Page 41] RFC 6126 The Babel Routing Protocol April 2011

 The receiving node then appends a 1 bit to the neighbour's Hello
 history, resets the neighbour's Hello timer, and sets ne to (nr + 1).
 It then resets the neighbour's Hello timer to 1.5 times the value
 advertised in the received Hello (the extra margin allows for the
 delay due to jitter).
 Whenever the 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 the Hello history is empty (it
 contains 0 bits only), the neighbour entry is flushed; otherwise, it
 resets the neighbour's Hello timer to the value advertised in the
 last Hello received from this neighbour (no extra margin is necessary
 in this case).

A.2. Cost Computation

A.2.1. k-out-of-j

 K-out-of-j link sensing is suitable for wired links that are either
 up, in which case they only occasionally drop a packet, or down, in
 which case they drop all packets.
 The k-out-of-j strategy is parameterised by two small integers k and
 j, such that 0 < k <= j, and the nominal link cost, a constant K >=
 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.
 The cost of such a link is defined as
 o  cost = FFFF hexadecimal if rxcost = FFFF hexadecimal;
 o  cost = txcost otherwise.

A.2.2. ETX

 The Estimated Transmission Cost metric [ETX] estimates the number of
 times that a unicast frame will be retransmitted by the IEEE 802.11
 MAC, assuming infinite persistence.
 A node uses a neighbour's Hello history to compute an estimate,
 written beta, of the probability that a Hello TLV is successfully
 received.  The rxcost is defined as 256/beta.
 Let alpha be MIN(1, 256/txcost), an estimate of the probability of
 successfully sending a Hello TLV.  The cost is then computed by

Chroboczek Experimental [Page 42] RFC 6126 The Babel Routing Protocol April 2011

    cost = 256/(alpha * beta)
 or, equivalently,
    cost = (MAX(txcost, 256) * rxcost) / 256.

A.3. Metric Computation

A.3.1. Additive Metrics

 The simplest approach for obtaining a monotonic, isotonic 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 to an additive one by taking
 the logarithm (in some suitable base) of the link costs.

A.3.2. External Sources of Willingness

 A node may want to vary its willingness to forward packets by taking
 into account information that is external to the Babel protocol, such
 as the monetary cost of a link, the node's battery status, CPU load,
 etc.  This can be done by adding to every route's metric a value k
 that depends on the external data.  For example, if a battery-powered
 node receives an update with metric m over a link with cost c, it
 might compute a metric M(c, m) = k + c + m, where k depends on the
 battery status.
 In order to preserve strict monotonicity (Section 3.5.2), the value k
 must be greater than -c.

Appendix B. Constants

 The choice of time constants is a trade-off between fast detection of
 mobility events and protocol overhead.  Two implementations of Babel
 with different time constants will interoperate, although the
 resulting convergence time will most likely be dictated by the
 slowest of the two implementations.
 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.

Chroboczek Experimental [Page 43] RFC 6126 The Babel Routing Protocol April 2011

 At the time of writing, the sample implementation of Babel uses the
 following default values:
    Hello Interval: 4 seconds on wireless links, 20 seconds on wired
    links.
    IHU Interval: the advertised IHU interval is always 3 times the
    Hello interval.  IHUs are actually sent with each Hello on lossy
    links (as determined from the Hello history), but only with every
    third Hello on lossless links.
    Update Interval: 4 times the 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.
 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 200 ms; other TLVs are delayed
 by no more than one-half the Hello interval.

Appendix C. Simplified Implementations

 Babel is a fairly economic protocol.  Route updates take between 12
 and 40 octets per destination, depending on how successful
 compression is; in a double-stack mesh network, an average of less
 than 24 octets 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 routing table and the associated
 source table.
 Babel is also a reasonably simple protocol.  The sample
 implementation consists of less than 8000 lines of C code, and it
 compiles to less than 60 kB of text on a 32-bit CISC architecture.
 Nonetheless, in some very constrained environments, such as PDAs,
 microwave ovens, or abacuses, it may be desirable to have subset
 implementations of the protocol.
 A parasitic implementation is one that uses a Babel network for
 routing its packets but does not announce any of the routes that it
 has learnt from its neighbours.  (This is slightly more than a
 passive implementation, which doesn't even announce routes to
 itself.)  It may either maintain a full routing table or simply

Chroboczek Experimental [Page 44] RFC 6126 The Babel Routing Protocol April 2011

 select a gateway amongst any one of its neighbours that announces a
 default route.  Since a parasitic implementation never forwards
 packets, it cannot possibly participate in a routing loop; hence, it
 need not evaluate the feasibility condition and need not maintain a
 source table.
 A parasitic implementation MUST answer acknowledgement requests and
 MUST participate in the Hello/IHU protocol.  Finally, it MUST be able
 to reply to seqno requests for routes that it announces and SHOULD be
 able to reply to route requests.

Appendix D. Software Availability

 The sample implementation of Babel is available from
 <http://www.pps.jussieu.fr/~jch/software/babel/>.

Author's Address

 Juliusz Chroboczek
 PPS, University of Paris 7
 Case 7014
 75205 Paris Cedex 13,
 France
 EMail: jch@pps.jussieu.fr

Chroboczek Experimental [Page 45]

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