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Versions: 00 01 02

Network Working Group                                         B. Jonglez
Internet-Draft                                                  ENS Lyon
Updates: 6126 (if approved)                                J. Chroboczek
Intended status: Experimental           PPS, University of Paris-Diderot
Expires: January 3, 2015                                    July 2, 2014


      Delay-based Metric Extension for the Babel Routing Protocol
                  draft-jonglez-babel-rtt-extension-00

Abstract

   This document defines an extension to the Babel routing protocol
   [BABEL] that uses the delay to a neighbour in metric computation and
   therefore makes it possible to prefer lower latency links to higher
   latency ones.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on January 3, 2015.

Copyright Notice

   Copyright (c) 2014 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
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   described in the Simplified BSD License.



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Table of Contents

   1.  Introduction and background . . . . . . . . . . . . . . . . .   2
   2.  Protocol operation  . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Delay estimation  . . . . . . . . . . . . . . . . . . . .   3
     2.2.  Metric computation  . . . . . . . . . . . . . . . . . . .   4
     2.3.  Stability issues  . . . . . . . . . . . . . . . . . . . .   5
   3.  Packet format . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.1.  Timestamp sub-TLV in Hello TLVs . . . . . . . . . . . . .   6
     3.2.  Timestamp sub-TLV in IHU TLVs . . . . . . . . . . . . . .   7
   4.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction and background

   The Babel routing protocol [BABEL] does not mandate a specific
   algorithm for computing metrics; existing implementations use a
   packet-loss based metric on wireless links and a simple hop-count
   metric on all other types of links.  While this strategy works
   reasonably well in many networks, it fails to select reasonable
   routes in some topologies involving tunnels or VPNs.

   Consider for example the following topology, with three routers A, B
   and D located in Paris and a fourth router located in Tokyo,
   connected through tunnels in a diamond topology.

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

   When routing traffic from A to D, it is obviously preferable to use
   the local route through B, as this is likely to provide better
   service quality and lower monetary cost than the distant route
   through C.  However, the existing implementations of Babel consider
   both routes as having the same metric, and will therefore route the
   traffic through C in roughly half the cases.



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   In this memo, we specify an extension to the Babel routing protocol
   that enables precise measurement of the round-trip time (RTT) of a
   link, and allows its usage in metric computation.  Since this causes
   a negative feedback loop, special care is needed to ensure that the
   resulting network is reasonably stable (Section 2.3).

   We believe that this protocol may be useful in other situations than
   the one described above, such as when running Babel in a congested
   wireless mesh network or over a complex link layer that performs its
   own routing; the high granularity of the timestamps used (1ms) should
   make it easier to experiment with RTT-based metrics on this kind of
   link layers.

2.  Protocol operation

   The protocol estimates the RTT to each neighbour (Section 2.1) which
   it then uses for metric computation (Section 2.2).

2.1.  Delay estimation

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

   A Babel speaker periodically sends a multicast Hello message over all
   of its interfaces.  This Hello is usually accompanied with a set of
   IHU messages, one per neighbour.

   In order to enable the computation of RTTs, a node A includes in
   every Hello that it sends a timestamp t1 according to A's clock.
   Additionally, a node B includes in the IHU it sends to A the
   timestamp t1 included in the last Hello received from A, and the
   timestamp t1' according to B's clock at which it received that Hello.
   Upon receiving B's combined Hello and IHU, node A records the
   timestamp t2 at which it received the combined packet, according to
   A's clock.  This is described in the following sequence diagram:















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

   Node A then computes the RTT as (t2 - t1) - (t2' - t1').

   This algorithm has a number of desirable properties.  First, since
   there is no requirement that t1' and t2' be equal, the protocol
   remains asynchronous -- the only change to Babel's message scheduling
   that is required is to ensure that IHUs are always sent together with
   Hellos.  Second, since only differences of timestamps according to a
   single clock are computed, it does not require synchronised clocks.
   Third, it is mostly stateless -- a node only needs to store the two
   timestamps associated with the last hello received from each
   neighbour.  Finally, since it only requires piggybacking a couple of
   timestamps on each Hello and IHU packet, it makes efficient use of
   network resources.

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

2.2.  Metric computation

   The algorithm described in the previous section allows computing an
   RTT to all neighbours.  How to map this value to a link cost is left
   to the implementation.



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   Obviously, the mapping should be monotonic (larger RTTs imply larger
   costs).  In addition, in order to enhance stability (Section 2.3),
   the mapping should be bounded -- above a certain RTT, all links are
   equally bad.

2.2.1.  Sample mapping

   The sample implementation of Babel uses the following function for
   mapping RTTs to link costs, parameterised by three parameters rtt-
   min, rtt-max and max-rtt-penalty:

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

   For RTTs below rtt-min, the link cost is just the nominal cost of a
   single hop, C.  Between rtt-min and rtt-max, the cost increases
   linearly; abover rtt-max, the constant value max-rtt-penalty is added
   to the nominal cost.

2.3.  Stability issues

   Using delay as an input to the routing metric in congested networks
   gives rise to a negative feedback loop: low RTT encourages traffic,
   which in turn causes the RTT to increase.  In a congested network,
   such a feedback loop can cause persistent oscillations.





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   The sample implementation of Babel uses three techniques that
   collaborate to limit the frequency of oscillations:

   o  the measured RTT is smoothed, which limits Babel's response to
      short-term RTT variations;

   o  the mapping function is bounded, which avoids switching between
      congested routes;

   o  a hysteresis algorithm is applied to the metric before route
      selection, which limits the worst-case frequency of route
      oscillations.

   These techniques are discussed in more detail in [DELAY-BASED].

3.  Packet format

   This extension defines the Timestamp sub-TLV [BABEL-EXT], whose Type
   field has value 3.  This sub-TLV can be contained within a Hello sub-
   TLV, in which case it carries a single timestamp, or within an IHU
   sub-TLV, in which case it carries two timestamps.

   Timestamps are encoded as 32-bit unsigned integers, expressed in
   units of one microsecond, counting from an arbitrary origin.
   Timestamps wrap around every 4295 seconds, or slightly more than one
   hour.

3.1.  Timestamp sub-TLV in Hello TLVs

   When contained within a Hello TLV, the Timestamp sub-TLV has the
   following format:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Type = 3    |    Length     |      Transmit timestamp       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields :

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

   Length    The length of the body, exclusive of the Type and Length
             fields.





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   Transmit timestamp  The time at which the packet containing this sub-
             TLV was sent, according to the sender's clock.

   If the Length field is larger than the expected 4 octets, the sub-TLV
   MUST be processed normally and any extra data contained in this sub-
   TLV MUST be silently ignored.

3.2.  Timestamp sub-TLV in IHU TLVs

   When contained in an IHU TLV destined for node A, the Timestamp sub-
   TLV has the following format:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Type = 3    |    Length     |        Origin timestamp       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               |        Receive timestamp      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields :

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

   Length    The length of the body, exclusive of the Type and Length
             fields.

   Origin timestamp  A copy of the transmit timestamp of the last
             Timestamp sub-TLV contained in a Hello TLV received from
             node A.

   Receive timestamp  The time at which the last Hello with a Timestamp
             sub-TLV was received from node A according to the sender's
             clock.

   If the Length field is larger than the expected 8 octets, the sub-TLV
   MUST be processed normally and any extra data contained in this sub-
   TLV MUST be silently ignored.

4.  References

   [BABEL]    Chroboczek, J., "The Babel Routing Protocol", RFC 6126,
              February 2011.






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   [BABEL-EXT]
              Chroboczek, J., "Extension Mechanism for the Babel Routing
              Protocol", Internet Draft draft-chroboczek-babel-
              extension-mechanism-01, June 2014.

   [DELAY-BASED]
              Jonglez, B. and J. Chroboczek, "A delay-based routing
              metric", March 2014.

              Available online from http://arxiv.org/abs/1403.3488

   [MILLS]    Mills, D., "DCN Local-Network Protocols", RFC 891,
              December 1983.

   [NTP]      Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network
              Time Protocol Version 4: Protocol and Algorithms
              Specification", RFC 5905, June 2010.

Authors' Addresses

   Baptiste Jonglez
   ENS Lyon
   France

   Email: baptiste.jonglez@ens-lyon.org


   Juliusz Chroboczek
   PPS, University of Paris-Diderot
   Case 7014
   75205 Paris Cedex 13
   France

   Email: jch@pps.univ-paris-diderot.fr

















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