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Versions: 00 01 02 03 04 05 06 rfc2716                                  
     PPP Extensions Working Group                             Bernard Aboba
     INTERNET-DRAFT                                               Microsoft
     Category: Informational                                      Dan Simon
     <draft-ietf-pppext-eaptls-03.txt>                            Microsoft
     21 April 1998
     
     
                      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 view the entire list of current Internet-Drafts, please check
     the "1id-abstracts.txt" listing contained in the Internet-Drafts
     Shadow Directories on ftp.is.co.za (Africa), ftp.nordu.net
     (Northern Europe), ftp.nis.garr.it (Southern Europe), munnari.oz.au
     (Pacific Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu
     (US West Coast).
     
     The  distribution  of  this memo is unlimited.  It is filed as <draft-
     ietf-pppext-eaptls-03.txt>, and expires November 1, 1998. Please  send
     comments 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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     INTERNET-DRAFT                                           21 April 1998
     
     
     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,  it  may  be  desirable to support mutual authentication, and
     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 secure key management protocols is non-
     trivial,  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
     [13].
     
     
     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,
               though perhaps with reduced functionality. In the same  vein
     
     
     
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     INTERNET-DRAFT                                           21 April 1998
     
     
               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 [15], 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 [15] 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 Identity, 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
     more  TLS  records.  These  will  contain a TLS server_hello handshake
     
     
     
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     INTERNET-DRAFT                                           21 April 1998
     
     
     message, possibly followed by  TLS  certificate,  server_key_exchange,
     certificate_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
     signature  public key).  In the latter case, a TLS server_key_exchange
     
     
     
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     INTERNET-DRAFT                                           21 April 1998
     
     
     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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     INTERNET-DRAFT                                           21 April 1998
     
     
     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,  the
     maximum RADIUS packet size of 4096 octets, or even the Multilink Maxi-
     mum Received Reconstructed Unit (MRRU).  As  described  in  [16],  the
     
     
     
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     INTERNET-DRAFT                                           21 April 1998
     
     
     multilink  MRRU is negotiated via the Multilink MRRU LCP option, which
     includes an MRRU length field of two  octets,  and  thus  can  support
     MRRUs as large as 64 KB.
     
     However,  note  that in order to protect against reassembly lockup and
     denial of service attacks, it may be desirable for  an  implementation
     to set a maximum size for one such group of TLS messages. Since a typ-
     ical 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.
     
     If this value is chosen, then fragmentation can  be  handled  via  the
     multilink  PPP  fragmentation mechanisms described in [16]. While this
     is desirable, there may be cases in which multilink or  the  MRRU  LCP
     option  cannot  be  negotiated. As a result, an EAP-TLS implementation
     MUST provide its own support for fragmentation and reassembly.
     
     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.   This  serves as a fragment ACK. The EAP server MUST wait until
     it receives the EAP-Response before sending another fragment. In order
     to  prevent  errors  in  processing  of fragments, the EAP server MUST
     increment the Identifier field for each fragment contained  within  an
     EAP-Request,  and  the  peer MUST include this Identifier value in the
     fragment ACK contained within the EAP-Reponse. Retransmitted fragments
     will contain the same Identifier value.
     
     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.  This  serves as a fragment ACK. The EAP peer MUST wait until it
     receives the EAP-Request before sending another fragment.  In order to
     prevent errors in the processing of fragments, the EAP server MUST use
     increment the Identifier value for each fragment ACK contained  within
     an EAP-Request, and the peer MUST include this Identifier value in the
     subsequent fragment contained within an EAP-Reponse.
     
     
     
     
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     INTERNET-DRAFT                                           21 April 1998
     
     
     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  cer-
     tificate  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
     
     
     
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     INTERNET-DRAFT                                           21 April 1998
     
     
     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.
     
     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 onversation 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 [15], 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 [13] 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 [14] 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
     
     
     
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     INTERNET-DRAFT                                           21 April 1998
     
     
     -------------------     -------------
                             <- 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,
      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/
     
     
     
     Aboba & Simon                                                [Page 10]


     INTERNET-DRAFT                                           21 April 1998
     
     
                             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
                             (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
     
     
     
     Aboba & Simon                                                [Page 11]


     INTERNET-DRAFT                                           21 April 1998
     
     
     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)->
                             <- 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/
     
     
     
     Aboba & Simon                                                [Page 12]


     INTERNET-DRAFT                                           21 April 1998
     
     
     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,
      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
     
     
     
     Aboba & Simon                                                [Page 13]


     INTERNET-DRAFT                                           21 April 1998
     
     
                             (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:
     
     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 EA-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)
     
     
     
     Aboba & Simon                                                [Page 14]


     INTERNET-DRAFT                                           21 April 1998
     
     
     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
     (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
     
     
     
     Aboba & Simon                                                [Page 15]


     INTERNET-DRAFT                                           21 April 1998
     
     
     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
     
        13 - EAP TLS
     
     Data
     
        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
     
     
     
     Aboba & Simon                                                [Page 16]


     INTERNET-DRAFT                                           21 April 1998
     
     
        13 - 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
        (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
     
     
     
     
     Aboba & Simon                                                [Page 17]


     INTERNET-DRAFT                                           21 April 1998
     
     
        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
     
        13 - 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
        (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
     
     
     
     Aboba & Simon                                                [Page 18]


     INTERNET-DRAFT                                           21 April 1998
     
     
     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.
     
     
     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  [15],  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 [15], 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.
     
     
     
     
     
     Aboba & Simon                                                [Page 19]


     INTERNET-DRAFT                                           21 April 1998
     
     
     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
     
     It is envisaged that EAP-TLS will be used primarily  with  dialup  PPP
     connections.  However,  there  are  also  circumstances  in  which PPP
     encryption may be used along with Layer 2 tunneling protocols such  as
     PPTP and L2TP.
     
     In mandatory layer 2 tunneling, a PPP peer makes a connection to a NAS
     or router which tunnels the PPP packets to  a  tunnel  server.   Since
     with  mandatory  tunneling  a PPP peer cannot tell whether its packets
     are being tunneled, let alone whether the network device  is  securing
     the  tunnel, if security 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, PPP
     encryption provides a convenient means to ensure the privacy of  pack-
     ets transiting between the client and the tunnel server.
     
     There  also may be circumstances in which PPP encryption may be desir-
     able even where voluntary tunneling is being used.  In voluntary  tun-
     neling,  the  client initiates the tunnel to the tunnel server without
     assistance  from  a  network  device.   Routers  implementing  Network
     Address  and  Port Translation (NAPT) are now growing rapidly in popu-
     larity.  Where NAPT is turned on, IPSEC cannot be used to  secure  the
     outer  layer  of  a client-initiated layer 2 tunnel, since the address
     and port translated packet will then fail the authentication check. By
     contrast,  Layer 2 tunnels utilizing PPP encryption may pass unimpeded
     through a NAT.
     
     
     7.  Acknowledgments
     
     Thanks to Terence Spies, Glen Zorn and Narendra Gidwani  of  Microsoft
     for useful discussions of this problem space.
     
     
     8.  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.
     
     
     
     Aboba & Simon                                                [Page 20]


     INTERNET-DRAFT                                           21 April 1998
     
     
     [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. Blunk, J. Vollbrecht.  "PPP Extensible Authentication Protocol
     (EAP)." RFC 2284, Merit Network, Inc., March 1998.
     
     [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] T. Dierks, C. Allen.  "The TLS Protocol  Version  1.0."  Internet
     draft  (work  in  progress)  draft-ietf-tls-protocol-05.txt, Consensus
     Development, November 1997.
     
     [14] D. Rand.  "The PPP Compression Control Protocol." RFC 1962,  Nov-
     ell, June 1996.
     
     [15]  P.  Calhoun,  A.C. Rubens, B. Aboba.  "Extensible Authentication
     Protocol Support in RADIUS." Internet draft (work in progress), draft-
     ietf-radius-eap-04.txt,  Sun  Microsystems,  Merit Network, Microsoft,
     March 1998.
     
     [16]  K. Sklower, B. Lloyd, G. McGregor, D. Carr, T.  Coradetti,  "The
     PPP Multilink Protocol (MP)", RFC 1990, August 1996.
     
     
     9.  Authors' Addresses
     
     Bernard Aboba
     Microsoft Corporation
     One Microsoft Way
     Redmond, WA 98052
     
     
     
     
     Aboba & Simon                                                [Page 21]


     INTERNET-DRAFT                                           21 April 1998
     
     
     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
     
     Pc
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     Aboba & Simon                                                [Page 22]