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Versions: 00 01 02 03 04 05 06 rfc2716                                  
     PPP Extensions Working Group                             Bernard Aboba
     INTERNET-DRAFT                                               Microsoft
     Category: Standards Track                                    Dan Simon
     <draft-ietf-pppext-eaptls-02.txt>                            Microsoft
     19 October 1997
     
     
                      PPP EAP TLS Authentication Protocol
     
     
     1.  Status of this Memo
     
     This document is an Internet-Draft.  Internet-Drafts are working docu-
     ments of the Internet Engineering Task Force (IETF),  its  areas,  and
     its  working groups.  Note that other groups may also distribute work-
     ing documents as Internet-Drafts.
     
     Internet-Drafts are draft documents valid for a maximum of six  months
     and  may  be updated, replaced, or obsoleted by other documents at any
     time.  It is inappropriate to use Internet-Drafts as  reference  mate-
     rial or to cite them other than as ``work in progress.''
     
     To  learn  the  current status of any Internet-Draft, please check the
     ``1id-abstracts.txt'' listing contained in the Internet-Drafts  Shadow
     Directories   on   ds.internic.net   (US  East  Coast),  nic.nordu.net
     (Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim).
     
     The  distribution  of  this memo is unlimited.  It is filed as <draft-
     ietf-pppext-eaptls-02.txt>, and expires May 1, 1998. Please send  com-
     ments to the authors.
     
     
     2.  Abstract
     
     The  Point-to-Point  Protocol  (PPP)  provides  a  standard method for
     transporting multi-protocol datagrams over point-to-point links.   PPP
     also  defines  an extensible Link Control Protocol (LCP), which can be
     used to negotiate authentication methods, as  well  as  an  Encryption
     Control  Protocol  (ECP),  used  to negotiate data encryption over PPP
     links, and a Compression Control Protocol  (CCP),  used  to  negotiate
     compression  methods.  The Extensible Authentication Protocol (EAP) is
     a PPP extension that provides support  for  additional  authentication
     methods within PPP.
     
     Transport  Level  Security  (TLS)  provides for mutual authentication,
     integrity-protected ciphersuite negotiation and key  exchange  between
     two  endpoints.   This  document describes how EAP-TLS, which includes
     support for fragmentation and reassembly, provides for these TLS mech-
     anisms within EAP.
     
     
     
     
     
     
     
     
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     3.  Introduction
     
     The  Extensible  Authentication Protocol (EAP), described in [5], pro-
     vides a standard mechanism for support  of  additional  authentication
     methods  within  PPP.  Through the use of EAP, support for a number of
     authentication schemes may be added, including smart cards,  Kerberos,
     Public Key, One Time Passwords, and others. To date however, EAP meth-
     ods such as [6] have focussed on authenticating a client to a  server.
     
     However,  in order to guard against rogue servers, it may be desirable
     to support mutual authentication. In addition,  since  PPP  encryption
     protocols  such as [10] and [11] assume existence of a session key, it
     is useful to have a mechanism for  session  key  establishment.  Since
     design of authentication and key management protocols is a non-trivial
     exercise, it is desirable to avoid creating new mechanisms  for  this.
     The  EAP protocol described in this document allows a PPP peer to take
     advantage of the protected ciphersuite negotiation, mutual authentica-
     tion and key management capabilities of the TLS protocol, described in
     [15].
     
     
     3.1.  Requirements language
     
     This specification uses the same words as [12] for defining  the  sig-
     nificance of each particular requirement.  These words are:
     
     
     MUST      This  word,  or  the adjectives "REQUIRED" or "SHALL", means
               that the definition is an absolute requirement of the speci-
               fication.
     
     MUST NOT  This phrase, or the phrase "SHALL NOT", means that the defi-
               nition is an absolute prohibition of the specification.
     
     SHOULD    This word, or the adjective "RECOMMENDED", means that  there
               may  exist  valid  reasons  in  particular  circumstances to
               ignore a particular item, but the full implications must  be
               understood and carefully weighed before choosing a different
               course.
     
     SHOULD NOT
               This phrase means that there may exist valid reasons in par-
               ticular   circumstances  when  the  particular  behavior  is
               acceptable or even useful, but the full implications  should
               be  understood  and the case carefully weighed before imple-
               menting any behavior described with this label.
     
     MAY       This word, or the adjective "", means that an item is  truly
               optional.  One vendor may choose to include the item because
               a particular marketplace requires it or because  the  vendor
               feels  that it enhances the product while another vendor may
               omit the  same  item.   An  implementation  which  does  not
               include a particular option MUST be prepared to interoperate
               with another implementation which does include  the  option,
     
     
     
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               though  perhaps with reduced functionality. In the same vein
               an implementation which does  include  a  particular  option
               MUST be prepared to interoperate with another implementation
               which does not include the option.(except,  of  course,  for
               the feature the option provides)
     
     An  implementation is not compliant if it fails to satisfy one or more
     of the must or must not requirements for the protocols it  implements.
     An  implementation  that  satisfies all the must, must not, should and
     should not requirements for its protocols is said to be  "uncondition-
     ally compliant"; one that satisfies all the must and must not require-
     ments but not all the should or should not requirements for its proto-
     cols is said to be "conditionally compliant."
     
     
     4.  Protocol overview
     
     
     
     4.1.  Overview of the EAP-TLS conversation
     
     As  described in [5] and [17], the EAP-TLS conversation will typically
     begin with the  authenticator  and  the  peer  negotiating  EAP.   The
     authenticator  will then typically send an EAP-Request/Identity packet
     to the peer, and the peer will respond with  an  EAP-Response/Identity
     packet to the authenticator, containing the peer's userId.
     
     From  this  point forward, while nominally the EAP conversation occurs
     between the PPP authenticator and the peer, as described in  [17]  the
     authenticator  MAY  act  as a passthrough device, with the EAP packets
     received from the peer being encapsulated for transmission to a RADIUS
     server  or backend security server. In the discussion that follows, we
     will use the term "EAP server" to denote the  ultimate  endpoint  con-
     versing with the peer.
     
     Once  having  received  the peer's userId, the EAP server MUST respond
     with an EAP-TLS/Start packet, which is an EAP-Request packet with EAP-
     Type=EAP-TLS, the Start (S) bit set, and no data.  The EAP-TLS conver-
     sation will then begin, with the peer sending an  EAP-Response  packet
     with EAP-Type=EAP-TLS.  The data field of that packet will encapsulate
     one or more TLS records in TLS record layer format, containing  a  TLS
     client_hello  handshake  message.  The current cipher spec for the TLS
     records will be TLS_NULL_WITH_NULL_NULL and  null  compression.   This
     current cipher spec remains the same until the change_cipher_spec mes-
     sage  signals  that  subsequent  records  will  have  the   negotiated
     attributes for the remainder of the handshake.
     
     The  client_hello  message contains the client's TLS version number, a
     sessionId, a random number, and a set of ciphersuites supported by the
     client.  The version offered by the client MUST correspond to TLS v1.0
     or later.
     
     The EAP server will then respond with an EAP-Request packet with  EAP-
     Type=EAP-TLS.  The  data  field of this packet will encapsulate one or
     
     
     
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     more TLS records. These will contain a TLS server_hello handshake mes-
     sage,  possibly followed by TLS certificate, server_key_exchange, cer-
     tificate_request, server_hello_done  and/or  finished  handshake  mes-
     sages,  and/or  a  TLS  change_cipher_spec  message.  The server_hello
     handshake message contains a TLS version number, another  random  num-
     ber,  a  sessionId,  and  a  ciphersuite.   The version offered by the
     server MUST correspond to TLS v1.0 or later.
     
     If the client's sessionId is null or unrecognized by the  server,  the
     server  MUST  choose  the sessionId to establish a new session; other-
     wise, the sessionId  will  match  that  offered by the  client,  indi-
     cating  a  resumption  of the previously established session with that
     sessionID.  The server will  also  choose  a  ciphersuite  from  those
     offered  by  the client; if the session matches the client's, then the
     ciphersuite MUST match the one negotiated during the handshake  proto-
     col execution that established the session.
     
     The  purpose  of the sessionId within the TLS protocol is to allow for
     improved efficiency in the case where a client repeatedly attempts  to
     authenticate  to  an  EAP  server within a short period of time. While
     this model was developed for use with HTTP authentication, it may also
     have application to PPP authentication (e.g. multilink).
     
     As  a result, it is left up to the peer whether to attempt to continue
     a previous session, thus shortening the  TLS  conversation.  Typically
     the  peer's  decision will be made based on the time elapsed since the
     previous authentication attempt to that EAP server. Based on the  ses-
     sionId  chosen  by  the  peer, and the time elapsed since the previous
     authentication, the EAP server will decide whether to allow  the  con-
     tinuation, or whether to choose a new session.
     
     In  the case where the EAP server and authenticator reside on the same
     device, then client will only be able to continue sessions  when  con-
     necting  to the same NAS or tunnel server. Should these devices be set
     up in a rotary or round-robin then it may not be possible for the peer
     to  know  in  advance  the authenticator it will be connecting to, and
     therefore which sessionId to attempt to reuse.  As  a  result,  it  is
     likely  that the continuation attempt will fail. In the case where the
     EAP authentication is remoted then continuation is much more likely to
     be  successful,  since  multiple  NAS  devices and tunnel servers will
     remote their EAP authentications to the same RADIUS server.
     
     If the EAP server is resuming a previously established  session,  then
     it  MUST  include only a TLS change_cipher_spec message and a TLS fin-
     ished handshake message after the server_hello message.  The  finished
     message contains the EAP server's authentication response to the peer.
     If the EAP server is not resuming a  previously  established  session,
     then it MUST include a TLS server_certificate handshake message, and a
     server_hello_done handshake message MUST be the last handshake message
     encapsulated in this EAP-Request packet.
     
     The  certificate  message  contains a public key certificate chain for
     either a key exchange public key (such as an RSA or Diffie-Hellman key
     exchange  public key) or a signature public key (such as an RSA or DSS
     
     
     
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     signature public key).  In the latter case, a TLS  server_key_exchange
     handshake  message  MUST also be included to allow the key exchange to
     take place.
     
     The certificate_request message is included when  the  server  desires
     the client to authenticate itself via public key. While the EAP server
     SHOULD require client authentication, this is not a requirement, since
     it  may be possible that the server will require that the peer authen-
     ticate via some other means.
     
     The peer MUST respond to the EAP-Request with an  EAP-Response  packet
     of  EAP-Type=EAP-TLS.   The data field of this packet will encapsulate
     one or more TLS records containing a  TLS  change_cipher_spec  message
     and  finished  handshake  message,  and possibly certificate, certifi-
     cate_verify and/or client_key_exchange  handshake  messages.   If  the
     preceding server_hello message sent by the EAP server in the preceding
     EAP-Request packet indicated the resumption  of  a  previous  session,
     then the peer MUST send only the change_cipher_spec and finished hand-
     shake messages.  The finished message contains the peer's  authentica-
     tion response to the EAP server.
     
     If  the  preceding  server_hello message sent by the EAP server in the
     preceeding EAP-Request packet did not indicate  the  resumption  of  a
     previous  session,  then  the  peer  MUST  send,  in  addition  to the
     change_cipher_spec and finished messages, a  client_key_exchange  mes-
     sage,  which  completes the exchange of a shared master secret between
     the peer and the EAP server.   If  the  EAP  server  sent  a  certifi-
     cate_request  message  in  the  preceding EAP-Request packet, then the
     peer MUST send, in addition, certificate and certificate_verify  hand-
     shake messages.  The former contains a certificate for the peer's sig-
     nature public key, while the latter contains the peer's signed authen-
     tication  response to the EAP server. After receiving this packet, the
     EAP server will verify the peer's certificate and  digital  signature,
     if requested.
     
     If  the  peer's  authentication is unsuccessful, the EAP server SHOULD
     send an EAP-Request packet with EAP-Type=EAP-TLS, encapsulating a  TLS
     record  containing  the appropriate TLS alert message.  The EAP server
     SHOULD send a TLS alert message  rather  immediately  terminating  the
     conversation  so  as to allow the peer to inform the user of the cause
     of the failure and possibly allow for a restart of the conversation.
     
     To ensure that the peer receives the TLS alert message, the EAP server
     MUST  wait for the peer to reply with an EAP-Response packet. The EAP-
     Response packet sent by the peer MAY encapsulate  a  TLS  client_hello
     handshake  message, in which case the EAP server MAY allow the EAP-TLS
     conversation to be restarted, or it MAY contain an EAP-Response packet
     with  EAP-Type=EAP-TLS  and no data, in which case the EAP-Server MUST
     send an EAP-Failure packet, and terminate the conversation. It  is  up
     to the EAP server whether to allow restarts, and if so, how many times
     the conversation can be restarted. An EAP Server implementing  restart
     capability  SHOULD  impose a limit on the number of restarts, so as to
     protect against denial of service attacks.
     
     
     
     
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     If the peers authenticates successfully, the EAP server  MUST  respond
     with  an  EAP-Request packet with EAP-Type=EAP-TLS, which includes, in
     the case of a new TLS session, one or more TLS records containing  TLS
     change_cipher_spec  and  finished handshake messages.  The latter con-
     tains the EAP server's authentication response to the peer.  The  peer
     will then verify the hash in order to authenticate the EAP server.
     
     If  the  EAP server authenticates unsuccessfully, the peer MAY send an
     EAP-Response packet of EAP-Type=EAP-TLS containing a TLS Alert message
     identifying  the  reason  for  the failed authentication. The peer MAY
     send a TLS alert message rather than immediately terminating the  con-
     versation  so as to allow the EAP server to log the cause of the error
     for examination by the system administrator.
     
     To ensure that the EAP Server receives the TLS alert message, the peer
     MUST wait for the EAP-Server to reply before terminating the conversa-
     tion.  The EAP Server MUST reply  with  an  EAP-Failure  packet  since
     server authentication failure is a terminal condition.
     
     If  the  EAP  server authenticates successfully, the peer MUST send an
     EAP-Response packet of EAP-Type=EAP-TLS, and no data.  The  EAP-Server
     then MUST respond with an EAP-Success message.
     
     
     4.2.  Retry behavior
     
     As  with  other EAP protocols, the EAP server is responsible for retry
     behavior. This means that if the EAP server does not receive  a  reply
     from the peer, it MUST resend the EAP-Request for which it has not yet
     received an EAP-Response. However,  the  peer  MUST  NOT  resend  EAP-
     Response packets without first being prompted by the EAP server.
     
     For  example,  if  the  initial  EAP-TLS  start packet sent by the EAP
     server were to be lost, then the peer would not receive  this  packet,
     and  would  not  respond  to it. As a result, the EAP-TLS start packet
     would be resent by the EAP server. Once the peer received the  EAP-TLS
     start   packet,  it  would  send  an  EAP-Response  encapsulating  the
     client_hello message.  If the EAP-Response were to be lost,  then  the
     EAP  server would resend the initial EAP-TLS start, and the peer would
     resend the EAP-Response.
     
     As a result, it is possible that a peer will  receive  duplicate  EAP-
     Request messages, and may send duplicate EAP-Responses.  Both the peer
     and the EAP-Server should be engineered to handle this possibility.
     
     
     4.3.  Fragmentation
     
     A single TLS record may be up to 16384 octets in  length,  but  a  TLS
     message  may  span multiple TLS records, and a TLS certificate message
     may in principle be as long as 16MB. The  group  of  EAP-TLS  messages
     sent in a single round may thus be larger than the PPP MTU size or the
     maximum RADIUS packet size of 4096 octets, and EAP-TLS therefore needs
     to support fragmentation and reassembly. Note that in order to protect
     
     
     
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     against denial of service attacks, it may be desirable for  an  imple-
     mentation  to  set  a maximum size for one such group of TLS messages.
     Since a typical certificate chain is rarely longer than a few thousand
     octets,  and  no  other  field is likely to be anwhere near as long, a
     reasonable choice of maximum acceptable message length might be 64 KB.
     
     Since  EAP  is a simple ACK-NAK protocol, fragmentation support can be
     added in a simple manner. In EAP, fragments that are lost  or  damaged
     in  transit will be retransmitted, and since sequencing information is
     provided by the Identifier field in EAP, there is no need for a  frag-
     ment offset field as is provided in IPv4.
     
     EAP-TLS  fragmentation support is provided through addition of a flags
     octet within the EAP-Response and EAP-Request packets, as  well  as  a
     TLS  Message  Length  field  of  four octets. Flags include the Length
     included (L), More fragments (M), and EAP-TLS Start (S)  bits.  The  L
     flag  is  set  to  indicate the presence of the four octet TLS Message
     Length field, and MUST be set for the first fragment of  a  fragmented
     TLS  message or set of messages. The M flag is set on all but the last
     fragment. The S flag is set only within the EAP-TLS start message sent
     from  the EAP server to the peer. The TLS Message Length field is four
     octets, and provides the total length of the TLS  message  or  set  of
     messages  that is being fragmented; this simplifies buffer allocation.
     
     When an EAP-TLS peer receives an EAP-Request packet  with  the  M  bit
     set, it MUST respond with an EAP-Response with EAP-Type=EAP-TLS and no
     data.  The EAP server MUST wait until  it  receives  the  EAP-Response
     before  sending  another  fragment.  Similarly,  when  the  EAP server
     receives an EAP-Response with the M bit set, it MUST respond  with  an
     EAP-Request  with EAP-Type=EAP-TLS and no data. The EAP peer MUST wait
     until it receives the EAP-Request before sending another fragment.
     
     
     4.4.  Identity verification
     
     As part of the TLS negotiation, the server presents a  certificate  to
     the peer, and if mutual authentication is requested, the peer presents
     a certificate to the server.
     
     Note that since the peer has made a claim  of  identity  in  the  EAP-
     Response/Identity (MyID) packet, the EAP server SHOULD verify that the
     claimed identity corresponds to the certificate presented by the peer.
     Typically  this  will  be  accomplished  either  by placing the userId
     within the peer certificate, or by providing  a  mapping  between  the
     peer certificate and the userId using a directory service.
     
     Similarly,  the  peer  MUST verify the validity of the EAP server cer-
     tificate, and SHOULD also examine the EAP server name presented in the
     certificate,  in  order  to  determine  whether  the EAP server can be
     trusted. Please note that in the case where the EAP authentication  is
     remoted that the EAP server will not reside on the same machine as the
     authenticator, and therefore the name in the EAP server's  certificate
     cannot  be expected to match that of the intended destination. In this
     case, a more appropriate  test  might  be  whether  the  EAP  server's
     
     
     
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     certificate is signed by a CA controlling the intended destination and
     whether the EAP server exists within a target sub-domain.
     
     
     4.5.  Key derivation
     
     Since the normal TLS keys are used in  the  handshake,  and  therefore
     should not be used in a different context, new encryption keys must be
     derived from the TLS master secret for use with PPP  encryption.   For
     both  peer  and EAP server, the derivation proceeds as follows:  given
     the master secret negotiated by the TLS  handshake,  the  pseudorandom
     function  (PRF) defined in the specification for the version of TLS in
     use, and the value random defined as the concatenation  of  the  hand-
     shake  message  fields client_hello.random and server_hello.random (in
     that order), the value PRF(master  secret,  "client  EAP  encryption",
     random) is computed up to 128 bytes, and the value PRF("", "client EAP
     encryption", random) is computed up to 64 bytes (where "" is an  empty
     string).   The  peer  encryption key (the one used for encrypting data
     from peer to EAP server) is obtained  by  truncating  to  the  correct
     length  the  first  32  bytes  of  the  first  PRF of these two output
     strings.  The EAP server encryption key (the one used  for  encrypting
     data from EAP server to peer), if different from the client encryption
     key, is obtained by truncating to the correct  length  the  second  32
     bytes  of  this same PRF output string.  The client authentication key
     (the one used for  computing  MACs  for  messages  from  peer  to  EAP
     server),  if used, is obtained by truncating to the correct length the
     third 32 bytes of this same PRF output string.  The EAP server authen-
     tication  key  (the  one used for computing MACs for messages from EAP
     server to peer), if used, and if different from the  peer  authentica-
     tion  key,  is obtained by truncating to the correct length the fourth
     32 bytes of this same PRF output string.  The peer initialization vec-
     tor  (IV), used for messages from peer to EAP server if a block cipher
     has been specified, is obtained by truncating to  the  cipher's  block
     size  the  first  32  bytes  of the second PRF output string mentioned
     above.  Finally, the server initialization vector (IV), used for  mes-
     sages from peer to EAP server if a block cipher has been specified, is
     obtained by truncating to the cipher's block size the second 32  bytes
     of this second PRF output.
     
     The use of these encryption and authentication keys is specific to the
     PPP encryption mechanism used, such as those defined in [10] and [11].
     Additional  keys  or  other  non-secret  values  (such  as IVs) can be
     obtained as needed for future PPP encryption methods by extending  the
     outputs of the PRF beyond 128 bytes and 64 bytes, respectively.
     
     
     4.6.  ECP negotiation
     
     Since  TLS  supports ciphersuite negotiation, peers completing the TLS
     negotiation will also have selected a ciphersuite, which includes  key
     strength,  encryption  and  hashing methods. As a result, a subsequent
     Encryption Control Protocol (ECP) conversation, if it  occurs,  has  a
     predetermined result.
     
     
     
     
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     In  order to ensure agreement between the EAP-TLS ciphersuite negotia-
     tion and the subsequent ECP negotiation (described in [7]), during ECP
     negotiation the PPP peer MUST offer only the ciphersuite negotiated in
     EAP-TLS.  This ensures that the PPP authenticator MUST accept the EAP-
     TLS  negotiated  ciphersuite in order for the conversation to proceed.
     Should the authenticator not accept  the  EAP-TLS  negotiated  cipher-
     suite, then the peer MUST send an LCP terminate and disconnect.
     
     Please  note  that as described in [17], it cannot be assumed that the
     PPP authenticator and EAP server are located on the  same  machine  or
     that  the  authenticator understands the EAP-TLS conversation that has
     passed through it. Thus if the peer offers a  ciphersuite  other  than
     the one negotiated in EAP-TLS there is no way for the authenticator to
     know how to respond correctly.
     
     
     4.7.  CCP negotiation
     
     TLS as described in [15] supports compression as well  as  ciphersuite
     negotiation.  However,  TLS only provides support for a limited number
     of compression types which do not overlap with the  compression  types
     used in PPP. As a result, during the EAP-TLS conversation the EAP end-
     points MUST NOT request or negotiate  compression.  Instead,  the  PPP
     Compression  Control  Protocol (CCP), described in [16] should be used
     to negotiate the desired compression scheme.
     
     
     4.8.  Examples
     
     In the case where the EAP-TLS mutual authentication is successful, the
     conversation will appear as follows:
     
     Authenticating Peer     Authenticator
     -------------------     -------------
                             <- PPP LCP Request-EAP
                             auth
     PPP LCP ACK-EAP
     auth ->
                             <- PPP EAP-Request/
                             Identity
     PPP EAP-Response/
     Identity (MyID) ->
                             <- PPP EAP-Request/
                             EAP-Type=EAP-TLS
                             (TLS Start)
     PPP EAP-Response/
     EAP-Type=EAP-TLS
     (TLS client_hello)->
                             <- PPP EAP-Request/
                             EAP-Type=EAP-TLS
                             (TLS server_hello,
                              TLS certificate,
                         [TLS server_key_exchange,]
                         [TLS certificate_request,]
     
     
     
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                          TLS server_hello_done)
     PPP EAP-Response/
     EAP-Type=EAP-TLS
     (TLS certificate,
      TLS client_key_exchange,
     [TLS certificate_verify,]
      TLS change_cipher_spec,
      TLS finished) ->
                             <- PPP EAP-Request/
                             EAP-Type=EAP-TLS
                             (TLS change_cipher_spec,
                          TLS finished)
     PPP EAP-Response/
     EAP-Type=EAP-TLS ->
                             <- PPP EAP-Success
     PPP Authentication
     Phase complete,
     NCP Phase starts
     
     ECP negotiation
     
     CCP negotiation
     
     In the case where the EAP-TLS mutual authentication is successful, and
     fragmentation is required, the conversation will appear as follows:
     
     Authenticating Peer     Authenticator
     -------------------     -------------
                             <- PPP LCP Request-EAP
                             auth
     PPP LCP ACK-EAP
     auth ->
                             <- PPP EAP-Request/
                             Identity
     PPP EAP-Response/
     Identity (MyID) ->
                             <- PPP EAP-Request/
                             EAP-Type=EAP-TLS
                             (TLS Start, S bit set)
     PPP EAP-Response/
     EAP-Type=EAP-TLS
     (TLS client_hello)->
                             <- PPP EAP-Request/
                             EAP-Type=EAP-TLS
                             (TLS server_hello,
                              TLS certificate,
                         [TLS server_key_exchange,]
                         [TLS certificate_request,]
                          TLS server_hello_done)
                             (Fragment 1: L, M bits set)
     PPP EAP-Response/
     EAP-Type=EAP-TLS ->
                             <- PPP EAP-Request/
                             EAP-Type=EAP-TLS
     
     
     
     Aboba & Simon                                                [Page 10]


     INTERNET-DRAFT                                         19 October 1997
     
     
                             (Fragment 2: M bit set)
     PPP EAP-Response/
     EAP-Type=EAP-TLS ->
                             <- PPP EAP-Request/
                             EAP-Type=EAP-TLS
                             (Fragment 3)
     PPP EAP-Response/
     EAP-Type=EAP-TLS
     (TLS certificate,
      TLS client_key_exchange,
     [TLS certificate_verify,]
      TLS change_cipher_spec,
      TLS finished)(Fragment 1:
      L, M bits set)->
                              <- PPP EAP-Request/
                             EAP-Type=EAP-TLS
     PPP EAP-Response/
     EAP-Type=EAP-TLS
     (Fragment 2)->
                            <- PPP EAP-Request/
                             EAP-Type=EAP-TLS
                             (TLS change_cipher_spec,
                          TLS finished)
     PPP EAP-Response/
     EAP-Type=EAP-TLS ->
                             <- PPP EAP-Success
     PPP Authentication
     Phase complete,
     NCP Phase starts
     
     ECP negotiation
     
     CCP negotiation
     
     In the case where the server authenticates to the client successfully,
     but  the  client fails to authenticate to the server, the conversation
     will appear as follows:
     
     Authenticating Peer     Authenticator
     -------------------     -------------
                             <- PPP LCP Request-EAP
                             auth
     PPP LCP ACK-EAP
     auth ->
                             <- PPP EAP-Request/
                             Identity
     PPP EAP-Response/
     Identity (MyID) ->
                             <- PPP EAP-Request/
                             EAP-Type=EAP-TLS
                             (TLS Start)
     PPP EAP-Response/
     EAP-Type=EAP-TLS
     (TLS client_hello)->
     
     
     
     Aboba & Simon                                                [Page 11]


     INTERNET-DRAFT                                         19 October 1997
     
     
                             <- PPP EAP-Request/
                             EAP-Type=EAP-TLS
                             (TLS server_hello,
                              TLS certificate,
                         [TLS server_key_exchange,]
                          TLS certificate_request,
                          TLS server_hello_done)
     PPP EAP-Response/
     EAP-Type=EAP-TLS
     (TLS certificate,
      TLS client_key_exchange,
      TLS certificate_verify,
      TLS change_cipher_spec,
      TLS finished) ->
                             <- PPP EAP-Request
                             EAP-Type=EAP-TLS
                             (TLS Alert message)
     PPP EAP-Response/
     EAP-Type=EAP-TLS ->
                             <- PPP EAP-Failure
                             (User Disconnected)
     
     In the case where server authentication is unsuccessful, the conversa-
     tion will appear as follows:
     
     Authenticating Peer     Authenticator
     -------------------     -------------
                             <- PPP LCP Request-EAP
                             auth
     PPP LCP ACK-EAP
     auth ->
                             <- PPP EAP-Request/
                             Identity
     PPP EAP-Response/
     Identity (MyID) ->
                             <- PPP EAP-Request/
                             EAP-Type=EAP-TLS
                             (TLS Start)
     PPP EAP-Response/
     EAP-Type=EAP-TLS
      (TLS client_hello)->
                             <- PPP EAP-Request/
                             EAP-Type=EAP-TLS
                             (TLS server_hello,
                              TLS certificate,
                         [TLS server_key_exchange,]
                         [TLS certificate_request,]
                          TLS server_hello_done)
     PPP EAP-Response/
     EAP-Type=EAP-TLS
      (TLS certificate,
      TLS client_key_exchange,
     [TLS certificate_verify,]
      TLS change_cipher_spec,
     
     
     
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     INTERNET-DRAFT                                         19 October 1997
     
     
      TLS finished) ->
                             <- PPP EAP-Request/
                             EAP-Type=EAP-TLS
                             (TLS change_cipher_spec,
                              TLS finished)
     PPP EAP-Response/
     EAP-Type=EAP-TLS
     (TLS Alert message) ->
                             <- PPP EAP-Failure
                             (User Disconnected)
     
     In  the  case where a previously established session is being resumed,
     and both sides authenticate successfully, the conversation will appear
     as follows:
     
     Authenticating Peer     Authenticator
     -------------------     -------------
                             <- PPP LCP Request-EAP
                             auth
     PPP LCP ACK-EAP
     auth ->
                             <- PPP EAP-Request/
                             Identity
     PPP EAP-Response/
     Identity (MyID) ->
                             <- PPP EAP-Request/
                             EAP-Request/
                             EAP-Type=EAP-TLS
                             (TLS Start)
     PPP EAP-Response/
     EAP-Type=EAP-TLS
     (TLS client_hello)->
                             <- PPP EAP-Request/
                             EAP-Type=EAP-TLS
                             (TLS server_hello,
                              TLS change_cipher_spec,
                          TLS finished)
     PPP EAP-Response/
     EAP-Type=EAP-TLS
     (TLS change_cipher_spec,
      TLS finished) ->
                             <- PPP EAP-Success
     PPP Authentication
     Phase complete,
     NCP Phase starts
     
     ECP negotiation
     
     CCP negotiation
     
     In  the  case where a previously established session is being resumed,
     and the server authenticates to the client successfully but the client
     fails  to  authenticate to the server, the conversation will appear as
     follows:
     
     
     
     Aboba & Simon                                                [Page 13]


     INTERNET-DRAFT                                         19 October 1997
     
     
     Authenticating Peer     Authenticator
     -------------------     -------------
                             <- PPP LCP Request-EAP
                             auth
     PPP LCP ACK-EAP
     auth ->
                             <- PPP EAP-Request/
                             Identity
     PPP EAP-Response/
     Identity (MyID) ->
                             <- PPP EAP-Request/
                             EAP-Request/
                             EAP-Type=EAP-TLS
                             (TLS Start)
     PPP EAP-Response/
     EAP-Type=EAP-TLS
     (TLS client_hello) ->
                             <- PPP EAP-Request/
                             EAP-Type=EAP-TLS
                             (TLS server_hello,
                              TLS change_cipher_spec,
                          TLS finished)
     PPP EAP-Response/
     EAP-Type=EAP-TLS
     (TLS change_cipher_spec,
      TLS finished) ->
                             <- PPP EAP-Request
                             EAP-Type=EAP-TLS
                             (TLS Alert message)
     PPP EAP-Response
     EAP-Type=EAP-TLS ->
                              <- PPP EAP-Failure
                              (User Disconnected)
     
     In the case where a previously established session is  being  resumed,
     and  the  server authentication is unsuccessful, the conversation will
     appear as follows:
     
     Authenticating Peer     Authenticator
     -------------------     -------------
                             <- PPP LCP Request-EAP
                             auth
     PPP LCP ACK-EAP
     auth ->
                             <- PPP EAP-Request/
                             Identity
     PPP EAP-Response/
     Identity (MyID) ->
                             <- PPP EAP-Request/
                             EAP-Request/
                             EAP-Type=EAP-TLS
                             (TLS Start)
     PPP EAP-Response/
     EAP-Type=EAP-TLS
     
     
     
     Aboba & Simon                                                [Page 14]


     INTERNET-DRAFT                                         19 October 1997
     
     
     (TLS client_hello)->
                             <- PPP EAP-Request/
                             EAP-Type=EAP-TLS
                             (TLS server_hello,
                              TLS change_cipher_spec,
                          TLS finished)
     PPP EAP-Response/
     EAP-Type=EAP-TLS
     (TLS Alert message) ->
                             <- PPP EAP-Failure
                             (User Disconnected)
     
     
     5.  Detailed description of the EAP-TLS protocol
     
     
     
     5.1.  PPP EAP TLS Packet Format
     
     A summary of the PPP EAP TLS Request/Response packet format  is  shown
     below.  The fields are transmitted from left to right.
     
      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Code      |   Identifier  |            Length             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Type      |        Data...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     
     Code
     
        1 - Request
        2 - Response
     
     Identifier
     
        The  identifier  field  is one octet and aids in matching responses
        with requests.
     
     Length
     
        The Length field is two octets and indicates the length of the  EAP
        packet  including  the  Code,  Identifier,  Length,  Type, and Data
        fields.  Octets outside the range of the  Length  field  should  be
        treated  as Data Link Layer padding and should be ignored on recep-
        tion.
     
     Type
     
        ? - EAP TLS
     
     Data
     
     
     
     
     Aboba & Simon                                                [Page 15]


     INTERNET-DRAFT                                         19 October 1997
     
     
        The format of the Data field is determined by the Code field.
     
     
     5.2.  PPP EAP TLS Request Packet
     
     A summary of the PPP EAP TLS Request packet  format  is  shown  below.
     The fields are transmitted from left to right.
     
     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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Code      |   Identifier  |            Length             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Type      |     Flags     |      TLS Message Length
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     TLS Message Length        |       TLS Data...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     
     Code
     
        1
     
     Identifier
     
        The  identifier  field  is one octet and aids in matching responses
        with requests.  The  identifier  field  MUST  be  changed  on  each
        Request packet.
     
     Length
     
        The  Length field is two octets and indicates the length of the EAP
        packet including  the  Code,  Identifier,  Length,  Type,  and  TLS
        Response fields.
     
     Type
     
        ? - EAP TLS
     
     Flags
     
        0 1 2 3 4 5 6 7 8
        +-+-+-+-+-+-+-+-+
        |L M S R R R R R|
        +-+-+-+-+-+-+-+-+
     
        L = Length included
        M = More fragments
        S = EAP-TLS start
        R = Reserved
     
        The  L bit (length included) is set to indicate the presence of the
        four octet TLS Message Length field, and MUST be set for the  first
        fragment  of a fragmented TLS message or set of messages. The M bit
        (more fragments) is set on all but the last  fragment.  The  S  bit
     
     
     
     Aboba & Simon                                                [Page 16]


     INTERNET-DRAFT                                         19 October 1997
     
     
        (EAP-TLS  start) is set in an EAP-TLS Start message. This differen-
        tiates the EAP-TLS Start message from a fragment acknowledgement.
     
     TLS Message Length
     
        The TLS Message Length field is four octets, and is present only if
        the  L  bit is set. This field provides the total length of the TLS
        message or set of messages that is being fragmented.
     
     TLS data
     
        The TLS data consists of the encapsulated TLS packet in TLS  record
        format.
     
     
     5.3.  PPP EAP TLS Response Packet
     
     A  summary  of  the PPP EAP TLS Response packet format is shown below.
     The fields are transmitted from left to right.
     
     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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Code      |   Identifier  |            Length             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Type      |     Flags     |      TLS Message Length
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     TLS Message Length        |       TLS Data...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     
     Code
     
        2
     
     Identifier
     
        The identifier field is one octet and  MUST  match  the  Identifier
        field from the corresponding request.
     
     Length
     
        The  Length field is two octets and indicates the length of the EAP
        packet including the Code, Identifier, Length, Type, and  TLS  data
        fields.
     
     Type
     
        ? - EAP TLS
     
     Flags
     
        0 1 2 3 4 5 6 7 8
        +-+-+-+-+-+-+-+-+
        |L M S R R R R R|
     
     
     
     Aboba & Simon                                                [Page 17]


     INTERNET-DRAFT                                         19 October 1997
     
     
        +-+-+-+-+-+-+-+-+
     
        L = Length included
        M = More fragments
        S = EAP-TLS start
        R = Reserved
     
        The  L bit (length included) is set to indicate the presence of the
        four octet TLS Message Length field, and MUST be set for the  first
        fragment  of a fragmented TLS message or set of messages. The M bit
        (more fragments) is set on all but the last  fragment.  The  S  bit
        (EAP-TLS  start) is set in an EAP-TLS Start message. This differen-
        tiates the EAP-TLS Start message from a fragment acknowledgement.
     
     TLS Message Length
     
        The TLS Message Length field is four octets, and is present only if
        the  L  bit is set. This field provides the total length of the TLS
        message or set of messages that is being fragmented.
     
     TLS data
     
        The TLS data consists of the encapsulated TLS packet in TLS  record
        format.
     
     
     6.  Security issues
     
     
     6.1.  Certificate revocation
     
     Since  the  EAP server is on the Internet during the EAP conversation,
     the EAP server is capable of following a certificate chain or  verify-
     ing  whether the peer's certificate has been revoked. In contrast, the
     peer may or may not have Internet connectivity, and thus while it  can
     validate the EAP server's certificate based on a pre-configured set of
     CAs, it may not be able  to  follow  a  certificate  chain  or  verify
     whether the EAP server's certificate has been revoked.
     
     In  the  case  where the peer is initiating a voluntary Layer 2 tunnel
     using PPTP or L2TP, the peer will typically already have a PPP  inter-
     face  and Internet connectivity established at the time of tunnel ini-
     tiation. As a result, during the EAP conversation  it  is  capable  of
     checking for certificate revocation.
     
     However,  in  the case where the peer is initiating an intial PPP con-
     versation, it will not have Internet connectivity and is therefore not
     capable of checking for certificate revocation until after NCP negoti-
     ation completes and the peer has access to the Internet. In this case,
     the  peer  SHOULD check for certificate revocation after connecting to
     the Internet.
     
     
     
     
     
     
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     INTERNET-DRAFT                                         19 October 1997
     
     
     6.2.  Separation of the EAP server and PPP authenticator
     
     As a result of the EAP-TLS conversation, the EAP endpoints will  mutu-
     ally  authenticate,  negotiate a ciphersuite, and derive a session key
     for subsequent use in PPP encryption. Since the peer  and  EAP  client
     reside  on the same machine, it is necessary for the EAP client module
     to pass the session key to the PPP encryption module.
     
     The situation may be more complex on the PPP authenticator.  As  noted
     in  [17],  the  PPP  authenticator  may  or may not reside on the same
     machine as the EAP server. For example, when RADIUS/EAP is  used,  the
     EAP  server  may be a backend security server, or a module residing on
     the RADIUS server.
     
     In the case where the EAP server and PPP authenticator reside on  dif-
     ferent machines, there are several implications for security. Firstly,
     the mutual authentication defined in EAP-TLS will  occur  between  the
     peer  and  the EAP server, not between the peer and the authenticator.
     This means that as a result of the EAP-TLS  conversation,  it  is  not
     possible  for  the  peer to validate the identity of the NAS or tunnel
     server that it is speaking to.
     
     As described in [17], when EAP/RADIUS is used to encapsulate EAP pack-
     ets, the Signature attribute is required in EAP/RADIUS Access-Requests
     sent from the NAS or tunnel server to the  RADIUS  server.  Since  the
     Signature  attribute involves a keyed-MD5 hash, it is possible for the
     RADIUS server to verify the integrity of the Access-Request as well as
     the NAS or tunnel server's identity. Similarly, Access-Challenge pack-
     ets sent from the RADIUS server to the NAS are also authenticated  and
     integrity protected using a keyed-MD5 hash, enabling the NAS or tunnel
     server to determine the integrity of the packet and verify  the  iden-
     tity  of  the  RADIUS server. Moreover, EAP-TLS packets in transit are
     integrity protected and authenticated end-to-end via  TLS  mechanisms,
     so  that they cannot be successfully modified by a rogue NAS or tunnel
     server.
     
     The second issue that arises in the case of  an  EAP  server  and  PPP
     authenticator  residing  on different machines is that the session key
     negotiated between the peer and EAP server will need to be transmitted
     to  the  authenticator.  Therefore a mechanism needs to be provided to
     transmit the session key from the EAP server to the  authenticator  or
     tunnel  server  that  needs  to use the key. The specification of this
     transit mechanism is outside the scope of this document.
     
     
     6.3.  Relationship of PPP encryption to other security mechanisms
     
     PPP encryption currently plays an important role in  the  securing  of
     Layer 2 tunneling protocols such as PPTP, described in [13], and L2TP,
     described in [14]. While it may be envisaged that security  mechanisms
     such  as  IPSEC  will  eventually become ubiquitous, it will take some
     time for vendors to add IPSEC capabilities to their  devices,  and  in
     any  case  legacy  authenticator devices or routers may not be able to
     support IPSEC without being upgraded.  As a result, it is likely  that
     
     
     
     Aboba & Simon                                                [Page 19]


     INTERNET-DRAFT                                         19 October 1997
     
     
     non-IPSEC  capable  devices  will  persist in operational networks for
     quite some time.
     
     In an environment where IPSEC is not ubiquitous, in Layer 2  tunneling
     protocols a role remains for PPP encryption. Since with mandatory tun-
     neling a PPP peer cannot tell whether its packets are being  tunneled,
     let  alone  whether the authenticator is securing the tunnel, if secu-
     rity is required then the client must make its  own  arrangements.  In
     the case where all endpoints cannot be relied upon to implement IPSEC,
     TLS, or another suitable security protocol, then PPP  encryption  pro-
     vides  a  very convenient means to ensure the privacy of packets tran-
     siting between the client and the tunnel server.
     
     There also may be circumstances in which PPP encryption may be  desir-
     able  even if IPSEC is available. Routers implementing Network Address
     Translation (NAT) are now growing rapidly in popularity.  Where NAT is
     turned on, IPSEC cannot be used to secure the outer layer of a client-
     initiated layer 2 tunnel, since the  address  translated  packet  will
     then  fail the authentication check. By contrast, Layer 2 tunnels uti-
     lizing PPP encryption may pass unimpeded through a NAT.
     
     
     7.  Copyright notice
     
     Copyright (C) The Internet Society, 1997. All Rights Reserved.
     
     This document and translations of it may be copied  and  furnished  to
     others,  and  derivative works that comment on or otherwise explain it
     or assist in its implmentation may be prepared, copied, published  and
     distributed,  in  whole  or  in part, without restriction of any kind,
     provided that the  above  copyright  notice  and  this  paragraph  are
     included on all such copies and derivative works.  However, this docu-
     ment itself may not be modified in any way, such as  by  removing  the
     copyright notice or references to the Internet Society or other Inter-
     net organizations, except as needed for  the   purpose  of  developing
     Internet standards in which case the procedures for copyrights defined
     in the Internet Standards process must be followed, or as required  to
     translate it into languages other than English.
     
     The  limited  permissions  granted above are perpetual and will not be
     revoked by the Internet Society or its successors or assigns.
     
     This document and the information contained herein is provided  on  an
     "AS  IS"  basis  and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
     TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT
     NOT  LIMITED  TO  ANY  WARRANTY THAT THE USE OF THE INFORMATION HEREIN
     WILL NOT INFRINGE  ANY  RIGHTS  OR  ANY  IMPLIED  WARRANTIES  OF  MER-
     CHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
     
     
     8.  Acknowledgments
     
     Thanks  to  Terence Spies, Glen Zorn and Narendra Gidwani of Microsoft
     for useful discussions of this problem space.
     
     
     
     Aboba & Simon                                                [Page 20]


     INTERNET-DRAFT                                         19 October 1997
     
     
     9.  References
     
     [1] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)." STD  51,
     RFC 1661, Daydreamer, July 1994.
     
     [2]  Sklower, K., Lloyd, B., McGregor, G., Carr, D., and T. Coradetti,
     "The PPP Multilink Protocol (MP)." RFC 1990, UC Berkeley, August 1996.
     
     [3]  Simpson,  W., Editor, "PPP LCP Extensions." RFC 1570, Daydreamer,
     January 1994.
     
     [4] R. Rivest, S. Dusse.  "The  MD5  Message-Digest  Algorithm."   RFC
     1321,  MIT  Laboratory  for  Computer Science, RSA Data Security Inc.,
     April 1992.
     
     [5] L. J. Blunk, J. R.  Vollbrecht.   "PPP  Extensible  Authentication
     Protocol  (EAP)." Work in progress, draft-ietf-pppext-eap-auth-02.txt,
     Merit Network, Inc., June 1996.
     
     [6] W. T. Whelan, "PPP EAP RSA Public  Key  Authentication  Protocol."
     Work  in progress, draft-ietf-pppext-eaprsa-04.txt, Cabletron Systems,
     February 1997.
     
     [7] Meyer, G., "The PPP Encryption Protocol (ECP)." RFC  1968,  Spider
     Systems. June 1996
     
     [8] National Bureau of Standards, "Data Encryption Standard." FIPS PUB
     46 (January 1977).
     
     [9] National Bureau of Standards, "DES Modes of Operation."  FIPS  PUB
     81 (December 1980).
     
     [10]  K. Sklower, G. Meyer.  "The PPP DES Encryption Protocol, Version
     2  (DESE-bis)"  Work   in   progress,   draft-ietf-pppext-des-encrypt-
     v2-00.txt, University of California, Berkeley, Shiva, July 1997.
     
     [11]  K.  Hummert.   "The  PPP Triple-DES Encryption Protocol (3DESE)"
     Work in progress, draft-ietf-pppext-3des-encrypt-00.txt, Nentec  GmbH,
     July 1997.
     
     [12]  S.  Bradner.  "Key words for use in RFCs to Indicate Requirement
     Levels."  RFC 2119, Harvard University, March 1997.
     
     [13] K. Hamzeh, G. S. Pall, J. Taarud,  W.  Verthein,  W.  A.  Little.
     "Point-to-Point  Tunneling  Protocol -- PPTP." Internet draft (work in
     progress),   draft-ietf-pppext-pptp-02.txt,   Ascend   Communications,
     Microsoft, Copper Mountain Networks, U.S. Robotics, July 1997.
     
     [14]  K.  Hamzeh,  T.  Kolar, M. Littlewood, G. S. Pall, B. Palter, J.
     Taarud, W. M. Townsley, A. Valencia, W. Verthein.  "Layer Two  Tunnel-
     ing  Protocol  L2TP."  Internet  draft  (work in progress) draft-ietf-
     pppext-l2tp-06.txt, Ascend Communications, Cisco  Systems,  Microsoft,
     Copper Mountain Networks, IBM, U.S. Robotics, August 1997.
     
     
     
     
     Aboba & Simon                                                [Page 21]


     INTERNET-DRAFT                                         19 October 1997
     
     
     [15]  T.  Dierks,  C. Allen.  "The TLS Protocol Version 1.0." Internet
     draft (work  in  progress)  draft-ietf-tls-protocol-03.txt,  Consensus
     Development, May 1997.
     
     [16]  D. Rand.  "The PPP Compression Control Protocol." RFC 1962, Nov-
     ell, June 1996.
     
     [17] P. Calhoun, A.C. Rubens, B.  Aboba.   "Extensible  Authentication
     Protocol Support in RADIUS." Internet draft (work in progress), draft-
     ietf-radius-eap-02.txt,  US  Robotics  Access  Corp.,  Merit  Network,
     Microsoft, May, 1997.
     
     
     10.  Authors' Addresses
     
     Bernard Aboba
     Microsoft Corporation
     One Microsoft Way
     Redmond, WA 98052
     
     Phone: 425-936-6605
     EMail: bernarda@microsoft.com
     
     Dan Simon
     Microsoft Corporation
     One Microsoft Way
     Redmond, WA 98052
     
     Phone: 425-936-6711
     EMail: dansimon@microsoft.com
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     Aboba & Simon                                                [Page 22]