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Versions: 00 01 draft-ietf-msec-tesla-for-alc-norm

RMT                                                              V. Roca
Internet-Draft                                             A. Francillon
Expires: August 28, 2006                                      S. Faurite
                                                                   INRIA
                                                       February 24, 2006


             The Use of TESLA in the ALC and NORM Protocols
              draft-faurite-rmt-tesla-for-alc-norm-01.txt

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Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This document explains how to integrate the TESLA source
   authentication and packet integrity protocol to the ALC and NORM
   content delivery protocols.  This document only considers the
   authentication/integrity of the packets generated by the session's
   sender.





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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Conventions Used in this Document  . . . . . . . . . . . .  4
     1.2.  Terminology and Notations  . . . . . . . . . . . . . . . .  4
   2.  Using TESLA with ALC and NORM  . . . . . . . . . . . . . . . .  6
     2.1.  ALC and NORM Specificities that Impact TESLA . . . . . . .  6
     2.2.  The Need for Secure Time Synchronization . . . . . . . . .  7
       2.2.1.  Direct Time Synchronization  . . . . . . . . . . . . .  7
       2.2.2.  Indirect Time Synchronization  . . . . . . . . . . . .  7
   3.  Sender Operations  . . . . . . . . . . . . . . . . . . . . . .  9
     3.1.  TESLA Parameters . . . . . . . . . . . . . . . . . . . . .  9
       3.1.1.  Key Chains . . . . . . . . . . . . . . . . . . . . . .  9
       3.1.2.  Time Interval Schedule . . . . . . . . . . . . . . . . 10
     3.2.  TESLA Messages and Authentication Tags . . . . . . . . . . 10
       3.2.1.  Bootstrap Information  . . . . . . . . . . . . . . . . 10
       3.2.2.  Direct Time Synchronization Response . . . . . . . . . 11
       3.2.3.  Authentication Tag . . . . . . . . . . . . . . . . . . 12
     3.3.  TESLA Messages and Authentication Tag Format . . . . . . . 12
       3.3.1.  Bootstrap Information Format . . . . . . . . . . . . . 12
       3.3.2.  Format of a Direct Time Synchronization Response . . . 18
       3.3.3.  Format of a Standard Authentication Tag  . . . . . . . 19
       3.3.4.  Format of an Authentication Tag with a New Key
               Chain Commitment . . . . . . . . . . . . . . . . . . . 19
       3.3.5.  Format of an Authentication Tag with an Old Chain
               Last Key Disclosure  . . . . . . . . . . . . . . . . . 20
   4.  Receiver Operations  . . . . . . . . . . . . . . . . . . . . . 21
     4.1.  Initialization of a Receiver . . . . . . . . . . . . . . . 21
       4.1.1.  Processing the Bootstrap Information Message . . . . . 21
       4.1.2.  Time Synchronization . . . . . . . . . . . . . . . . . 21
       4.1.3.  Format of a Direct Time Synchronization Request  . . . 22
     4.2.  Authentication of Received Packets . . . . . . . . . . . . 23
   5.  Integration in the ALC and NORM Protocols  . . . . . . . . . . 24
     5.1.  Authentication Header Extension Format . . . . . . . . . . 24
     5.2.  Use of Authentication Header Extensions  . . . . . . . . . 26
     5.3.  Managing Silent Periods  . . . . . . . . . . . . . . . . . 26
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 28
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 29
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 30
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 30
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 30
   Appendix A.  IANA Considerations . . . . . . . . . . . . . . . . . 32
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 33
   Intellectual Property and Copyright Statements . . . . . . . . . . 34







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1.  Introduction

   Many applications using multicast and broadcast communications
   require that each receiver be able to authenticate the source of any
   packet it receives as well as its integrity.  For instance, ALC
   [RFC3450] and NORM [RFC3940] are two Content Delivery Protocols (CDP)
   designed to transfer reliably objects (e.g. files) between a
   session's sender and several receivers.

   The NORM protocol is based on bidirectional transmissions.  Each
   receiver acknowledges data received or, in case of packet erasures,
   asks for retransmissions.  The ALC protocol defines unidirectional
   transmissions.  Reliability can be achieved by means of cyclic
   transmissions of the content within a carousel, or by the use of
   proactive Forward Error Correction codes (FEC), or by the joint use
   of these mechanisms.  Being purely unidirectional, ALC is massively
   scalable, while NORM is intrinsically limited in terms of the number
   of receivers that can be handled in a session.  Both protocols have
   in common the fact that they operate at application level, on top of
   an erasure channel (e.g. the Internet) where packets can be lost
   (erased) during the transmission.  With some use case, an attacker
   might impersonate the the ALC or NORM session's sender and inject
   forged packets to the receivers, thereby corrupting the objects
   reconstructed by the receivers.

   The situation is much more complex in case of group communications
   than it is with unicast communications.  Indeed, in the latter case a
   simple solution exist: the sender and receiver can share a secret key
   to compute a Message Authentication Code (MAC) of all messages
   exchanged.  This is no longer feasible in case of a multicast and
   broadcast communications since sharing a group key between the sender
   and all receivers and using the same MAC scheme means that any group
   member can impersonate the sender and send forged messages to other
   receivers.

   The usual solution to provide the source authentication and message
   integrity services in case of multicast and broadcast communications
   consists in relying on asymmetric cryptography and using digital
   signatures.  Yet this solution is limited by high computational costs
   and high transmission overheads.  The Timed Efficient Stream Loss-
   tolerant Authentication protocol (TESLA) is an alternative solution
   that provides the two required services, while being compatible with
   high rate transmissions over lossy channels.

   This document explains how to integrate the TESLA source
   authentication and packet integrity protocol to the ALC and NORM
   content delivery protocols.  Since the FLUTE application [RFC3926] is
   built on top of ALC, it will directly benefit from the services



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   offered by TESLA at the transport layer.

   This specification only considers the authentication/integrity of the
   packets generated by the session's sender.  This specification does
   not consider the packets that will be generated by receivers, for
   instance the feedback packets of NORM.  Adding authentication/
   integrity to the packets sent by receivers is outside the scope of
   this document.  Of course, this remark does not apply to ALC where
   transmissions are purely unidirectional.

   For more information on the TESLA protocol and its principles, please
   refer to [RFC4082][Perrig.book04].  For more information on ALC, NORM
   and FLUTE, please refer to [RFC3450], [RFC3940] and [RFC3926]
   respectively, or [Neumann.ccr05].

1.1.  Conventions Used in this Document

   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].

1.2.  Terminology and Notations

   The following notations and definitions are used throughout this
   document:

   o  PRF is the Pseudo Random Function;

   o  MAC is the Message Authentication Code;

   o  HMAC is the Keyed-Hash Message Authentication Code;

   o  t_s is the sender local time value at some absolute time;

   o  t_r is the receiver local time value at the same absolute time;

   o  T_0, the start time corresponding to the beginning of the session
      (NTP timestamp);

   o  T_int, the interval duration (in milliseconds);

   o  d, the key disclosure delay (in number of intervals);

   o  D_t, the upper bound of the lag of the receiver's clock with
      respect to the clock of the sender (in sign/second/sub-second
      format);





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   o  D^0_t, the upper bound on the lag of the sender's clock with
      respect to the time reference in indirect time synchronization
      mode;

   o  D^R_t, the upper bound on the lag of the receivers's clock with
      respect to the time reference in indirect time synchronization
      mode;

   o  N is the number of keys in a key chain.  When several chains are
      used, all chains MUST have the same length, N;

   o  N_tx_old_kck is the number of intervals during which the last key
      of the old key chain SHOULD be sent, after switching to a new key
      chain and waiting for the disclosure delay d;

   o  N_tx_new_kcc is the number of intervals during which the
      commitment to a new key chain SHOULD be sent, before switching to
      the new key chain;

































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2.  Using TESLA with ALC and NORM

2.1.  ALC and NORM Specificities that Impact TESLA

   The ALC and NORM protocols have features and requirements that
   largely impact the way TESLA can be used.

   In case of ALC:

   o  ALC is massively scalable: nothing in the protocol specification
      limits the number of receivers that join an on-going session.
      Therefore an ALC session potentially includes a huge number (e.g.
      millions or more) receivers;

   o  ALC can work on top of purely unidirectional transport channels:
      this is one of the assets of ALC, and examples of unidirectional
      channels include satellites (even if a back channel might exist)
      and DVB-H systems.

   o  ALC defines an on-demand content delivery model [RFC3451] where
      receivers can arrive at any time, at their own discretion,
      download the content and leave the session.  Other models (e.g.
      push or streaming) are also defined.

   o  ALC sessions are potentially very long: with some use cases, in
      particular in an on-demand mode, a session can last several months
      during which the content is continuously transmitted within a
      carousel.  The content can either be static (e.g. in case of a
      software update) or be regularly updated (e.g. in case of a web
      site).

   Depending on the use case, some of the above features may not apply.
   For instance ALC can also be used over a bidirectional channel, or
   ALC can be used for small groups.

   In case of NORM:

   o  NORM has been designed for limited size sessions: unlike ALC, NORM
      is not massively scalable.  The reason is that NORM relies on
      feedback messages and the source may collapse if the feedback
      message rate is too high;

   o  NORM requires a bidirectional transport channel: yet the back
      channel is not necessarily a high rate channel since only low to
      medium rate control traffic will flow on it.  Networks with an
      asymmetric connectivity (e.g. a high rate satellite downlink and a
      low-rate RTC based return channel) is appropriate;




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2.2.  The Need for Secure Time Synchronization

   The security offered by TESLA relies heavily on time.  Therefore the
   session's sender and each receiver need to be time synchronized in a
   secure way.  To that purpose, two general methods exist:

   o  direct time synchronization, and

   o  indirect time synchronization.

2.2.1.  Direct Time Synchronization

   When direct time synchronization is used, each receiver asks the
   sender for a time synchronization.  The source then directly answers
   to each request, signing the reply.  The security of this
   synchronization method is guaranteed, but there are two potential
   issues:

   o  a bidirectional channel MUST exist between the source and each
      receiver,

   o  the source may collapse it the rate of requests is too high.

   Direct time synchronization may not be an issue with NORM since
   bidirectional communications already take place.  Yet direct time
   synchronization may be an issue with ALC since: there might not be
   any back channel to the session's sender and there are potentially a
   huge number of receivers.

2.2.2.  Indirect Time Synchronization

   When indirect time synchronization is used, the source and each
   receiver must synchronize securely via an external time reference.
   Several possibilities exist:

   o  sender and receivers can synchronize through a NTPv3 (Network Time
      Protocol version 3) [RFC1305] hierarchy of servers.  The
      authentication mechanism of NTPv3 MUST be used in order to
      authenticate each NTP message individually.  It prevents for
      instance an attacker to impersonate a NTP server;

   o  similarly sender and receivers can synchronize through a NTPv4
      (Network Time Protocol version 4) [I-D.ietf-ntp-ntpv4-proto]
      hierarchy of servers.  The Autokey security protocol of NTPv4 MUST
      be used in order to authenticate each NTP message individually;

   o  similarly, they can synchronize through a SNTPv4 (Simple Network
      Time Protocol version 4) [RFC2030] hierarchy of servers.  The



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      authentication features of SNTPv4 must then be used.  Note that
      TESLA only needs a loose (but secure) time synchronization, which
      is in line with the time synchronization service offered by SNTP;

   o  they can synchronize through a GPS or Galileo (or similar) device
      that also provide a high precision time reference.  This time
      reference is in general trusted, yet depending on the use case,
      this trust will or not be acceptable;

   o  they can synchronize thanks to a dedicated hardware, embedded on
      each sender and receiver, that offers a clock with a time-drift
      that is negligible in front of the TESLA time accuracy
      requirements.  This feature enables a device to synchronize its
      embedded clock with the official time reference from time to time,
      when feasible (in an extreme case once, at manufacturing time),
      and then to remain autonomous for some time, depending on the
      known maximum clock drift.

   A bidirectional channel is required by the NTP/SNTP schemes.  On the
   opposite, with the GPS/Galileo and high precision clock schemes, no
   such assumption is made.  In situations where ALC is used on purely
   unidirectional transport channels (Section 2.1), using the NTP/SNTP
   schemes is not possible.  Another aspect is the scalability
   requirement of ALC, and to a lesser extent NORM.  From this point of
   view, the above mechanisms usually do not raise any problem, unlike
   the direct synchronization schemes.  Therefore, using indirect time
   synchronization is a good candidate, in particular with ALC.

   In any case, this document does not explain in details how to achieve
   time synchronization, whether it follows a direct or indirect sheme.
   The document only provides general guidelines.  The details are
   outside the scope of this document.



















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3.  Sender Operations

3.1.  TESLA Parameters

3.1.1.  Key Chains

   The sender divides the time into uniform intervals of duration T_int.
   The sender then computes a one-way key chain of N keys, and assigns
   one key from the chain to each interval in sequence.  In order to
   compute this chain, it must first select a Primary Key, choose two
   PRF function F and F'.  Then it computes all the previous keys using
   K_{i-1} = F(K_i).  The key for MAC calculation can then be derived
   from the corresponding K_i key by K'_i=F'(K_i).  The randomness of
   the Primary key is vital to the security since no one should be able
   to guess it.

   The key chain has a finite length, N, so the TESLA session must
   finish before the end of the key chain.  But the longer the key
   chain, the higher the memory and computation required to cope with
   it.  Another solution consists in switching to a new key chain when
   necessary [Perrig.book04].

   To do so, the sender commits the new key chain with the old key
   chain.  Let's say that the old key chain stops at K_N and the new key
   chain starts at K_{N+1}.  Then the sender includes the commitment
   F(K_{N+1}) to the new key chain to packets authenticated with the old
   key chain.  This commitment SHOULD be sent during N_tx_new_kcc
   intervals before the end of the old key chain.  Since several packets
   are usually sent during a time interval, the sender should alternate
   between sending a disclosed key of the old key chain, and the
   commitment to the new key chain.

   The receiver will keep the commitment, until the key K_{N+1} is
   disclosed, at interval N+1+d.  Then the receiver will be able to test
   the validity of that key by computing F(K_{N+1}) and comparing it to
   the commitment.

   When the key chain is changed, it becomes impossible to recover a
   previous key from the old key chain.  This is a problem if the
   receiver lost the packets disclosing the last key of the old key
   chain.  A solution consists in re-sending the last key, K_N, of the
   old key chain.  This SHOULD be done during N_tx_old_kck intervals at
   the beginning of the new key chain, after the disclosure delay d.
   Since several packets are usually sent during a time interval, the
   sender should alternate between sending a disclosed key of the new
   key chain, and the last key of the old key chain.

   In some cases a receiver having experienced a very long disconnection



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   might have lost the commitment of the new chain.  Therefore this
   receiver will be unable to authenticate any packet related to the new
   chain and all the following ones.  The solution for this receiver to
   catch up consists in receiving the bootstrap information.  This can
   happen by waiting for the next periodic transmission (indirect time
   synchronization), by requesting it (direct time synchronization), or
   through an external mechanism (Section 3.2.1).

3.1.2.  Time Interval Schedule

   The sender must determine the following parameters:

   o  T_0, the start time corresponding to the beginning of the session;

   o  T_int, the interval duration, usually ranging from 100
      milliseconds to 1 second;

   o  d, the key disclosure delay (in number of intervals).  It is the
      time to wait before disclosing a key;

   o  N, the number of keys in a key chain;

   The correct choice of T_int, d, and N is crucial for the usability of
   the scheme.  For instance, a T_int*d product that is too long will
   cause excessive delay in the authentication process.  A T_int*d
   product that is is too short will cause too many packets to be
   unverifiable by some receivers.  A N*T_int product that is too small
   will cause the sender to switch too often to new key chains.  A N
   that is too long with respect to the expected session duration, if
   this latter is known, will require the sender to compute too many
   keys without using them all.  [RFC4082] (sections 3.2 and 3.6) gives
   general guidelines for initializing these parameters.

3.2.  TESLA Messages and Authentication Tags

   At a sender, TESLA produces two types of signaling information:

   o  The bootstrap information, which is a digitally signed packet
      containing all the information required to bootstrap TESLA at a
      receiver.

   o  The authentication tag, which is sent in all packets (see
      Section 5 for exceptions) and contains the MAC of the packet.

3.2.1.  Bootstrap Information

   In order to initialize the TESLA component at a receiver, the sender
   must communicate some key information.  This TESLA bootstrap



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   information MUST be securely transmitted, in particular a receiver
   must be able to check the packet source and the packet integrity
   using standard protocols.  Any digital signature will do.

   The bootstrap information can be sent in point to point after a
   direct synchronization request/response exchange.  The bootstrap
   information can also be broadcast to all receivers, for instance
   periodically, either in indirect time synchronization mode, or in
   direct synchronization mode when a large number of clients arrive at
   the same time.

   More specifically, the periodic broadcast of the bootstrap
   information message will be required when:

   o  the ALC session uses an ``on-demand'' mode, clients arriving at
      their own discretion;

   o  some clients experience an intermittent connectivity.  This is
      particularly true when several key chains are used in an ALC or
      NORM session, since there is a risk that some receivers loose all
      the commitments to the new key chain.

   A balance must be found between the signaling overhead and the
   maximum initial waiting time at the receiver before starting the
   delayed authentication process.  A frequency of a few seconds for the
   transmission of this bootstrap information is often a reasonable
   value.

   The bootstrap information message will be broadcast a limited number
   of times, at the beginning of the session, in other cases.  This is
   true in particular with ALC or NORM sessions in ``push'' mode, when
   all clients have a high probability of receiving at least one packet.
   An extreme case consists in sending the bootstrap information only
   once.

   In some use cases, the bootstrapping information MAY be sent to
   receivers through an external mechanism, for instance in a way
   similar to [I-D.ietf-msec-bootstrapping-tesla].  How to do that is
   outside the scope of this document.

3.2.2.  Direct Time Synchronization Response

   In direct time synchronization mode, receivers will send request
   messages to the session's sender and include their local time, t_r
   (Section 4.1.2).  Upon receipt of this request, the sender records
   its local time, t_s, and sends a response message that contains both
   t_r and t_s ([RFC4082], section 3.3.1).  This message is unicast to
   the receiver.  This direct time synchronization response message MUST



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   be securely transmitted, in particular the receiver must be able to
   check the packet source and the packet integrity using standard
   protocols.  Any digital signature will do.

   The Direct time synchronization messages are distinct from the
   Bootstrap Information message.  Therefore, if a large number of
   receivers try to initialize their TESLA component at the same time (a
   reasonable assumption in "push" mode), a single Bootstrap Information
   message can be broadcast to all of them.  Otherwise, a separate
   Bootstrap Information message can be broadcast to each client after
   the direct time synchronization response message.

   The same session might include receivers that use either time
   synchronization mode.  A common Bootstrap Information message enables
   both kinds of receivers to initialize their TESLA component.

3.2.3.  Authentication Tag

   Every authenticated packet must have an authentication tag,
   containing the MAC of the message and either a disclosed key or a
   commitment to a new key chain.

   The computation of the MAC, MAC(K_i, M), includes the ALC or NORM
   header (with the various header extensions) and the payload when
   applicable.  The UDP/IP/MAC headers are not included.  During this
   computation, the MAC(K_i, M) field of the authentication tag (see
   Section 3.3.3 Section 3.3.4 Section 3.3.5) MUST be set to 0.

3.3.  TESLA Messages and Authentication Tag Format

   This section specifies the format of the various kinds of TESLA
   messages and authentication tags sent by the session's sender.

3.3.1.  Bootstrap Information Format

















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     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  KC PRF type  | MAC PRF type  | HMAC func type| Signature type|
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |      K_j Key length           |      Signature length         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |# cert |rsv|B|A|      d        |             T_int             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                               N                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          Id j of K_j                          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    ~                              K_j              +-+-+-+-+-+-+-+-+
    |                                               |   Padding     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                              T_0                              +
    |                         (NTP timestamp)                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    ~                     Signature extension                       ~
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |P|                                                             |
    ~-+             D^0_t (optional, present if A==1)               ~
    |         (NTP timestamp diff, with P==1 if positive)           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    ~          NTP extension (optional, present if B==1)            ~
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 1: Bootstrap information format.

   The format of the bootstrap information is depicted in Figure 1.

   o  The key chain PRF type is the reference number of the F function
      used to calculate the key chain (Appendix A).

   o  The MAC PRF type is the reference number of the F' function used
      to derive the MAC key from the key chain (Appendix A).

   o  The HMAC function type is the reference number of the function
      used to compute the HMAC of the packets (Appendix A).





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   o  Signature type is the reference number of the digital signature
      used to authenticate this bootstrap information (Appendix A).

   o  # of certs is the number of certificates present in the signature
      extension.

   o  A is a flag indicating whether or not the P flag and D^0_t field
      are present (A==1) or not (A==0).  The P flag and D^0_t field are
      needed in indirect time synchronization mode.

   o  d is an unsigned integer that defines the key disclosure delay (in
      number of intervals). d MUST be greater or equal to 2.

   o  T_int is an unsigned integer that defines the interval duration
      (in milliseconds) one interval.

   o  K_j Key length is the length in bits of key K_j.

   o  Signature length is the number of bytes of the signature included
      in the signature extension.

   o  N is the number of keys of the key chain.

   o  Id j of K_j is an unsigned integer corresponding to the index of
      the interval of the key released in this bootstrap information.
      For performance reasons, the sender SHOULD always send a bootstrap
      information with the highest Id j possible since this will reduce
      the number of computation for the receivers that join later.

   o  K_j is the key corresponding to the interval j.  If i is the
      current interval we MUST have: j < i - d.

   o  T_0 is the start time corresponding to the beginning of the
      session, i.e. interval 0.  It is a NTP timestamp.

   o  The signature extension is described in Section 3.3.1.1.  It's
      format depends on the "# of certs" field.

   o  P is optional.  It is a flag indicating whether the D^0_t NTP
      timestamp difference is positive (P==1) or negative (P==0).  It is
      only used in indirect time synchronization mode when the A flag is
      1.

   o  D^0_t is optional.  It is the upper bound on the lag of the
      sender's clock with respect to the time reference.  When several
      time references are specified (e.g. several NTP servers), then
      D^0_t is the maximum upper bound on the lag with each time
      reference.  D^0_t is composed of two unsigned integers, as with



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      NTP timestamps: the first 31 bits give the time difference in
      seconds (the MSB is used by the P flag); the remaining 32 bits
      give the sub-second time difference.  It is only used in indirect
      time synchronization mode when flag A==1.

      *  ----- Editor's note: a first alternative would be to use
         floating point arithmetic, IEEE754 for carrying D^0_t.  NTP
         timestamp difference is usually performed with double floating
         point arithmetic internally (at least in TESLA and NTPv4), so
         it makes sense.  Is single-precision (32-bit) sufficient or
         should double-precision (64-bit) be used?  A second alternative
         would be to use a signed integer representing the difference in
         sub-second units (e.g. in milliseconds).  This is simple but it
         requires NTP timestamp/ms conversions on both sides. -----

   o  The NTP extension is optional is described in Section 3.3.1.2.
      Its presence can be detected by the total length of the signature.

3.3.1.1.  Signature Extension Format

   The signature extension format when the "# of certs" field is
   strictly greater than 0 (2 in this example) is:

     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | cert 1 type   | cert 1 length |                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
    |                                                               |
    ~                         Certificate 1         +-+-+-+-+-+-+-+-+
    |                                               |   Padding     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | cert 2 type   | cert 2 length |                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
    |                                                               |
    ~                         Certificate 2         +-+-+-+-+-+-+-+-+
    |                                               |    Padding    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    ~                           Signature           +-+-+-+-+-+-+-+-+
    |                                               |    Padding    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 2: Signature extension format when #cert==2.

   In Figure 2:





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   o  Type of certificate identifies the algorithm used for the
      certificate (see Appendix A).

   o  The certificate length is the length in bytes of the certificate.

   o  The certificate field contains a certificate signed by an external
      authority and that certifies the sender's public key.  This field
      is padded (with 0) up to a multiple of 32 bits.

   o  The signature is a digital signature using the type and length
      specified in the main part of the bootstrap information message.
      This field is padded (with 0) up to a multiple of 32 bits.

   The signature extension format when the "# of certs" field is zero
   (i.e. when no certificate is provided) is:

     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    ~                           Signature           +-+-+-+-+-+-+-+-+
    |                                               |    Padding    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 3: Signature extension format when #cert==0.

   In Figure 3:

   o  The signature is a digital signature using the type and length
      specified in the main part of the bootstrap information message.
      This field is padded (with 0) up to a multiple of 32 bits.

3.3.1.2.  NTP Extension Format

   In some use cases, when NTP or SNTP is used in indirect
   synchronization mode, the session's sender must have a way to
   indicate to receivers one or more NTP server that will act as time
   reference (Section 4.1.2).













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     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | total length  | # of entries  |  reserved (0) |   cert type   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | FQDN 1 length | cert 1 length |                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
    |                                                               |
    ~                  NTP Server 1 FQDN            +-+-+-+-+-+-+-+-+
    |                                               |   Padding     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    ~               NTP Certificate 1 (optional)    +-+-+-+-+-+-+-+-+
    |                                               |   Padding     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | FQDN 2 length | cert 2 length |                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
    |                                                               |
    ~                  NTP Server 2 FQDN            +-+-+-+-+-+-+-+-+
    |                                               |   Padding     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    ~               NTP Certificate 2 (optional)    +-+-+-+-+-+-+-+-+
    |                                               |   Padding     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 4: Optional NTP information format, when two NTP servers are
   specified

   Figure 4 shows the optional NTP information format, when two NTP
   servers are specified:

   o  The total length is the total length in units of 32 bit words of
      this NTP information extension;

   o  The # of entries is the number of NTP entries;

   o  Type of certificates identifies the algorithm used for all the
      certificates that may be provided (see Appendix A).

   o  The FQDN length is the number of bytes of the NTP server fully
      qualified domain name;

   o  The NTP server FQDN is a string containing the NTP server Fully
      Qualified Domain Name (e.g. "ntp.foo.bar.").  This field is padded
      (with 0) up to a multiple of 32 bits;





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   o  The NTP Certificate is optional.  The content delivery server can
      use it to self-certify the NTP public key.  The certificate length
      indicates whether this field is present or not.  This field is
      padded (with 0) up to a multiple of 32 bits.

      ----- Editor's note: Providing only NTP Server FQDN is perhaps too
      limitative.  It should be possible to use either a FQDN or an
      IPv4/IPv6 address. -----

3.3.2.  Format of a Direct Time Synchronization Response


     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                     t_s (NTP timestamp)                       +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                     t_r (NTP timestamp)                       +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |# cert |                    reserved (0)                       |
    +-------+-------------------------------------------------------+
    |                                                               |
    ~                     Signature extension                       ~
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 5: Format of a Direct Time Synchronization Response

   The response to a direct time synchronization request contains the
   following information:

   o  t_s, the sender local time value when receiving the direct time
      synchronization request message;

   o  t_r, the receiver local time value contained in the direct time
      synchronization request message;

   o  # of certs is the number of certificates present in the signature
      extension.

   o  The signature extension is described in Section 3.3.1.1.  It's
      format depends on the "# of certs" field.





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3.3.3.  Format of a Standard Authentication Tag


     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                         Id i of K_i                           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    ~                    Disclosed Key K_{i-d}      +-+-+-+-+-+-+-+-+
    |                                               |   Padding     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    ~                        MAC(K_i, M)            +-+-+-+-+-+-+-+-+
    |                                               |   Padding     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 6: Format of the authentication tag

   Figure 6 shows the format of the authentication tag:

   o  The Id i is the index of the key used for computing the MAC of
      this packet.

   o  The disclosed key MUST be the key used for interval i-d.

   o  MAC(K_i, M) is the message authentication code of the current
      packet, including the ALC or NORM header (including the header
      extensions), plus the payload when applicable.

3.3.4.  Format of an Authentication Tag with a New Key Chain Commitment

   During the last N_tx_new_kcc intervals of the current key chain, the
   sender MUST send a commitment to the next key chain.  This is done by
   replacing the disclosed key of the authentication tag with the new
   key chain commitment, F(K_{N+1}).  Figure 7 shows the corresponding
   format.














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     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                         Id i of K_i                           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    ~              New Key Commitment F(K_{N+1})    +-+-+-+-+-+-+-+-+
    |                                               |   Padding     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    ~                        MAC(K_i, M)            +-+-+-+-+-+-+-+-+
    |                                               |   Padding     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 7: Format of the authentication tag with a new key chain
   commitment

3.3.5.  Format of an Authentication Tag with an Old Chain Last Key
        Disclosure

   During the first N_tx_old_kcc intervals of the new key chain after
   the disclosing interval, d, the sender MUST send a commitment to the
   old key chain.  This is done by replacing the disclosed key of the
   authentication tag with the last key of the old chain.  Figure 8
   shows the corresponding 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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                         Id i of K_i                           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    ~                  Last Key of Old Chain, K_N   +-+-+-+-+-+-+-+-+
    |                                               |   Padding     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    ~                        MAC(K_i, M)            +-+-+-+-+-+-+-+-+
    |                                               |   Padding     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 8: Format of the authentication tag with an old chain last key
   disclosure








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4.  Receiver Operations

4.1.  Initialization of a Receiver

   A receiver must be initialized before being able to authenticate the
   source of incoming packets.  Two actions must be performed:

   o  receive and process a bootstrap information message, and

   o  calculate an upper bound of the sender's local time, and to that
      purpose, he must perform time synchronization.

4.1.1.  Processing the Bootstrap Information Message

   A receiver must first receive a packet containing the bootstrap
   information, digitally signed by the sender, and verify its
   signature.  Because the packet is signed, the receiver also needs to
   know the public key of the sender.  The present document does not
   specify how the public key of the sender is communicated reliably and
   in a secure way to all possible receivers.  Once the bootstrap
   information has been verified, the receiver can initialize its TESLA
   component.  The receiver SHOULD then ignore the following bootstrap
   information messages, if any.  There is an exception though: when a
   new key chain is used and a receiver missed all the commitments for
   this new key chain, then this latter SHOULD process any new Bootstrap
   information message.

   Before TESLA has been initialized, a receiver MUST ignore all packets
   other than the bootstrap information message.  Yet, a receiver MAY
   buffer incoming packets, recording the reception time of each packet,
   and proceed with delayed authentication later, once the receiver will
   be fully initialized.  In that case, the buffer must be carefully
   sized.

4.1.2.  Time Synchronization

   First of all, the receiver must know whether the ALC or NORM session
   relies on direct or indirect synchronization.  This information is
   communicated by an out-of-band mechanism (for instance when
   describing the various parameters of a FLUTE session in case of ALC).
   In some cases, both mechanisms might be acceptable in the same
   session.

4.1.2.1.  Direct Time Synchronization

   In case of a direct time synchronization, a receiver MUST first
   synchronize with the sender.  To that purpose, the receiver sends a
   direct time synchronization request message.  This message includes



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   the local time (NTP timestamp) at the receiver when sending the
   message.  This timestamp will be integrated in the sender's response.
   Figure 9 details the direct time synchronization message format.

4.1.2.2.  Indirect Time Synchronization

   With the indirect time synchronization method, the sender MAY provide
   in its bootstrap information, the URL or IP address of the NTP
   server(s) he trusts along with an OPTIONAL certificate for each NTP
   server.  When several NTP servers are specified, a receiver MUST
   choose one of them.  This document does not specify how the choice is
   made, but for the sake of scalability, the clients SHOULD NOT use the
   same server if several possibilities are offered.  The NTP
   synchronization between the NTP server and the receiver MUST be
   authenticated, either using the certificate provided by the content
   delivery server, or another certificate the client may obtain for
   this NTP server.

   Then the receiver computes the time offset between itself and the NTP
   server chosen.  Note that the receiver does not need to update the
   local time, since this operation would often require some privileges,
   computing the time offset is sufficient.

   Since the offset between the server and the time reference is
   indicated in the bootstrap information message, the receiver can now
   calculate an upper bound of the sender's local time.

4.1.3.  Format of a Direct Time Synchronization 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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                     t_r (NTP timestamp)                       +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 9: Format of a Direct Time Synchronization Request

   The direct time synchronization request (Figure 9) contains the
   following information:

   o  t_r, the receiver local time value when sending this direct time
      synchronization request message;






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4.2.  Authentication of Received Packets

   The receiver can now authenticate incoming packets.  To that purpose,
   he MUST follow different steps ([RFC4082] section 3.5):

   1.  The receiver parses the different packet headers.  If the TESLA
       authentication tag is not present, the receiver MUST reject the
       packet.

   2.  Then proceed with the TESLA safe test: (1) check that the key
       used to compute the MAC of this packet has not already been
       disclosed, and (2) check the disclosed key by computing the
       necessary number of PRF functions to obtain a previously safe
       disclosed key.  If any of these two tests fail, the receiver MUST
       reject the packet.

   3.  Then, according to the [RFC3451], when applicable, perform
       congestion control even if the packet has not yet been
       authenticated.  If this feature leads to a potential DoS attack,
       it does not compromise the security features offered by TESLA and
       enables a rapid reaction in front of congestion problems.

   4.  Then buffer the packet for a later authentication, once the
       corresponding key will be received or deduced from another key.

   5.  If the disclosed key is a new one, then the receiver can
       authenticate previously stored packets using this key or any key
       derived from this one.

   6.  If a packet fails to be authenticated, then this packet MUST be
       rejected.

   7.  If a packet is successfully authenticated, then the receiver
       continues processing it.

      ----- Editor's note: [RFC4082] explains that unauthenticated
      packets SHOULD be destroyed, and if not this is at the own risk of
      the receiver.  We choose the other strategy, requiring that unsafe
      packets be destroyed when the client decides to use TESLA.  But
      the client can at any time choose to continue an ALC or NORM
      session in unsafe mode, ignoring TESLA extensions. -----










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5.  Integration in the ALC and NORM Protocols

5.1.  Authentication Header Extension Format

   The integration of TESLA in ALC or NORM is similar and relies on the
   header extension mechanism defined in both protocols.  More precisely
   we further specify the EXT_AUTH=1 header extension defined in
   [RFC3451].

   Several fields are added in addition to the HET (Header Extension
   Type) and HEL (Header Extension Length) fields (Figure 10).


     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |   HET (=1)    |      HEL      |Scheme |Version|  Resvd  |Type |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    ~                                                               ~
    |                            Content                            |
    ~                                                               ~
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 10: Format of the TESLA EXT_AUTH header extension.

   The fields of the TESLA EXT_AUTH header extension are:

   Scheme (Authentication Scheme) field (4 bits):

      "Scheme" identifies the source authentication scheme or protocol
      in use.  The value 0 is reserved for TESLA.

   Version field (4 bits):

      "Version" identifies the version number of the TESLA
      authentication scheme.  The value 0 is reserved for the current
      specification.

   Resvd (Reserved) field (5 bits):

      The "resvd" field is not used in the current specification and
      MUST be set to zero by the sender.

   Type field(3 bits):





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      The "Type" field identifies the type of TESLA information carried
      in this header extension.  This specification defines the
      following types:

      *  0: bootstrap information, sent by the sender periodically or
         after a direct synchronization request;

      *  1: authentication information for the on-going key chain, sent
         by the sender along with each packet;

      *  2: authentication information along with a new key chain
         commitment, sent by the sender when approaching the end of a
         key chain;

      *  3: authentication information along with an old key chain
         commitment, sent by the sender some time after moving to a new
         key chain;

      *  4: direct time synchronization request, sent by a NORM
         receiver.  This type of message is invalid in case of an ALC
         session since ALC is restricted to unidirectional
         transmissions.  Yet an external mechanism may provide the
         direct time synchronization functionality;

      *  5: direct synchronization response, sent by a NORM sender.
         This type of message is invalid in case of an ALC session since
         ALC is restricted to unidirectional transmissions.  Yet an
         external mechanism may provide the direct time synchronization
         functionality;

   Content (variable length, multiple of 32 bits):

      This is the TESLA information carried in the header extension,
      whose type is given by the "Type" field.

   All receivers MUST recognize EXT_AUTH but MAY NOT be able to parse
   its content, for instance because they do not include the TESLA
   building block.  In that case these receivers MUST ignore the
   EXT_AUTH extensions.  In case of NORM, the packets sent by receivers
   MAY contain a direct synchronization request but MUST NOT contain any
   of the other four TESLA EXT_AUTH header extensions.

   All authentication schemes using the EXT_AUTH header extension MUST
   reserve the same 4 bit "Scheme" field after the HET/HEL fields.  This
   way, several authentication schemes can be used in the same ALC or
   NORM session.  For instance, in case of NORM, TESLA can be used for
   the downstream traffic while another scheme is used for the upstream
   traffic.



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5.2.  Use of Authentication Header Extensions

   Each packet sent by the sessions's sender MUST contain exactly one
   TESLA EXT_AUTH header extension.

   The "bootstrap information" TESLA EXT_AUTH (Type=1) MUST be sent in a
   stand-alone control packet, rather than packets containing
   application data.  The reason is the large size of this bootstrap
   information which largely increases the maximum ALC/LCT or NORM
   header size.  By having the bootstrap information header extension in
   stand-alone packets, the maximum payload size of data packets is only
   affected by the unavoidable authentication tag, not by any additional
   large header extension sent at a low frequency.  With NORM, the
   "bootstrap information" TESLA EXT_AUTH MUST be sent in a NORM_INFO
   message.  With ALC, the "bootstrap information" TESLA EXT_AUTH MUST
   be sent in a control packet, i.e. containing no encoding symbol.

   The three "authentication information" TESLA EXT_AUTH (Type=2, 3, or
   4) MUST be attached to the ALC or NORM packet (data or control
   packet) that they protect.

   With NORM, the direct synchronization request extension header
   (Type=5) is sent by a receiver in a (TBD) NORM packet (see editor's
   note below).  There is no authentication information header extension
   in this case since this draft only considers the authentication/
   integrity of the packets generated by the session's sender.

      ----- Editor's note: what type of NORM packet should be used to
      that purpose?  NORM_REPORT is one possibility. -----

   With ALC, the direct synchronization request information cannot be
   included in an ALC packet, since ALC is restricted to unidirectional
   transmissions, from the session's sender to the receivers.  An
   external mechanism, out of the scope of this document, must be used
   with ALC for carrying direct synchronization requests to the
   session's sender.

5.3.  Managing Silent Periods

   An ALC or NORM sender may stop transmitting packet for some time, for
   various reasons.  It can be the end of the session and all packets
   have already been sent.  The use scenario may consist in a succession
   of busy periods, when fresh objects are available, and silent
   periods.  In both cases, this is an issue since the authentication of
   the packets sent during the last d intervals requires that the
   associated keys be revealed, which can only take place after d
   additional intervals.  To resolve this boundary problem, the
   session's sender MUST sent null packets containing the TESLA EXT_AUTH



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   header extension along with the authentication information (Type=2)
   for at least d intervals after the end of the regular ALC or NORM
   packet transmissions.  The transmission rate of these null packets
   must be sufficient to guaranty that each receiver receives at least
   that containing the last key with a sufficiently high probability.














































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6.  Security Considerations

   The security of the TESLA protocol is discussed in [RFC4082].
   Security considerations specific to its use in ALC and NORM remain
   TBD...














































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7.  Acknowledgments

   The authors are grateful to David L. Mills.  MORE TO COME...
















































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8.  References

8.1.  Normative References

   [RFC1305]  Mills, D., "Network Time Protocol (Version 3)
              Specification, Implementation", RFC 1305, March 1992.

   [RFC2030]  Mills, D., "Simple Network Time Protocol (SNTP) Version 4
              for IPv4, IPv6 and OSI", RFC 2030, October 1996.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", RFC 2119, BCP 14, March 1997.

   [RFC3450]  Luby, M., Gemmell, J., Vicisano, L., Rizzo, L., and J.
              Crowcroft, "Asynchronous Layered Coding (ALC) Protocol
              Instantiation", RFC 3450, December 2002.

   [RFC3451]  Luby, M., Gemmell, J., Vicisano, L., Rizzo, L., Handley,
              M., and J. Crowcroft, "Layered Coding Transport (LCT)
              Building Block", RFC 3451, December 2002.

   [RFC3926]  Paila, T., Luby, M., Lehtonen, R., Roca, V., and R. Walsh,
              "FLUTE - File Delivery over Unidirectional Transport",
              RFC 3926, October 2004.

   [RFC3940]  Adamson, B., Bormann, C., Handley, M., and J. Macker,
              "Negative-acknowledgment (NACK)-Oriented Reliable
              Multicast (NORM) Protocol", RFC 3940, November 2004.

   [RFC4082]  Perrig, A., Song, D., Canetti, R., Tygar, J., and B.
              Briscoe, "Timed Efficient Stream Loss-Tolerant
              Authentication (TESLA): Multicast Source Authentication
              Transform Introduction", RFC 4082, June 2005.

8.2.  Informative References

   [I-D.ietf-msec-bootstrapping-tesla]
              Fries, S. and H. Tschofenig, "Bootstrapping TESLA",
              draft-ietf-msec-bootstrapping-tesla-03 (work in progress),
              January 2006.

   [I-D.ietf-ntp-ntpv4-proto]
              Burbank, J., "The Network Time Protocol Version 4 Protocol
              Specification", draft-ietf-ntp-ntpv4-proto-01 (work in
              progress), October 2005.

   [Neumann.ccr05]
              Neumann, C., Roca, V., and R. Walsh, "Large Scale Content



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              Distribution Protocols",  ACM Computer Communication
              Review (CCR), Vol. 35 No. 5, October 2005.

   [Perrig.book04]
              Perrig, A. and J. Tygar, "Secure Broadcast Communication
              in Wired and Wireless Networks", Kluwer Academic
              Publishers ISBN 0-7923-7650-1, 2004.












































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Appendix A.  IANA Considerations

   This document requires an IANA registration for the following
   attributes:

   Cryptographic pseudo-random function, TESLA-PRF:

                     The currently defined values are:

                         +--------------+-------+
                         | PRF function | Value |
                         +--------------+-------+
                         |   HMAC-SHA1  |   0   |
                         +--------------+-------+

   Cryptographic message authentication code (MAC):

                     The currently defined values are:

                         +--------------+-------+
                         | MAC function | Value |
                         +--------------+-------+
                         |   HMAC-SHA1  |   0   |
                         +--------------+-------+

   Signature type:

                     The currently defined values are:

              +------------------------------------+-------+
              |           Signature type           | Value |
              +------------------------------------+-------+
              | PKCS #1: RSA Cryptography Standard |   0   |
              +------------------------------------+-------+

   Certificate type:

                     The currently defined values are:

              +------------------------------------+-------+
              |          Certificate type          | Value |
              +------------------------------------+-------+
              | PKCS #1: RSA Cryptography Standard |   0   |
              +------------------------------------+-------+







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Authors' Addresses

   Vincent Roca
   INRIA
   655, av. de l'Europe
   Zirst; Montbonnot
   ST ISMIER cedex  38334
   France

   Email: vincent.roca@inrialpes.fr
   URI:   http://planete.inrialpes.fr/~roca/


   Aurelien Francillon
   INRIA
   655, av. de l'Europe
   Zirst; Montbonnot
   ST ISMIER cedex  38334
   France

   Email: aurelien.francillon@inrialpes.fr
   URI:   http://planete.inrialpes.fr/~francill/


   Sebastien Faurite
   INRIA
   655, av. de l'Europe
   Zirst; Montbonnot
   ST ISMIER cedex  38334
   France





















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