7 Network Working Group B. Aboba
8 Requests for Commments: 2716 D. Simon
9 Category: Experimental Microsoft
13 PPP EAP TLS Authentication Protocol
17 This memo defines an Experimental Protocol for the Internet
18 community. It does not specify an Internet standard of any kind.
19 Discussion and suggestions for improvement are requested.
20 Distribution of this memo is unlimited.
24 Copyright (C) The Internet Society (1999). All Rights Reserved.
28 The Point-to-Point Protocol (PPP) provides a standard method for
29 transporting multi-protocol datagrams over point-to-point links. PPP
30 also defines an extensible Link Control Protocol (LCP), which can be
31 used to negotiate authentication methods, as well as an Encryption
32 Control Protocol (ECP), used to negotiate data encryption over PPP
33 links, and a Compression Control Protocol (CCP), used to negotiate
34 compression methods. The Extensible Authentication Protocol (EAP) is
35 a PPP extension that provides support for additional authentication
38 Transport Level Security (TLS) provides for mutual authentication,
39 integrity-protected ciphersuite negotiation and key exchange between
40 two endpoints. This document describes how EAP-TLS, which includes
41 support for fragmentation and reassembly, provides for these TLS
42 mechanisms within EAP.
46 The Extensible Authentication Protocol (EAP), described in [5],
47 provides a standard mechanism for support of additional
48 authentication methods within PPP. Through the use of EAP, support
49 for a number of authentication schemes may be added, including smart
50 cards, Kerberos, Public Key, One Time Passwords, and others. To date
51 however, EAP methods such as [6] have focussed on authenticating a
58 Aboba & Simon Experimental [Page 1]
60 RFC 2716 PPP EAP TLS Authentication Protocol October 1999
63 However, it may be desirable to support mutual authentication, and
64 since PPP encryption protocols such as [9] and [10] assume existence
65 of a session key, it is useful to have a mechanism for session key
66 establishment. Since design of secure key management protocols is
67 non-trivial, it is desirable to avoid creating new mechanisms for
68 this. The EAP protocol described in this document allows a PPP peer
69 to take advantage of the protected ciphersuite negotiation, mutual
70 authentication and key management capabilities of the TLS protocol,
73 2.1. Requirements language
75 In this document, the key words "MAY", "MUST, "MUST NOT", "optional",
76 "recommended", "SHOULD", and "SHOULD NOT", are to be interpreted as
81 3.1. Overview of the EAP-TLS conversation
83 As described in [5], the EAP-TLS conversation will typically begin
84 with the authenticator and the peer negotiating EAP. The
85 authenticator will then typically send an EAP-Request/Identity packet
86 to the peer, and the peer will respond with an EAP-Response/Identity
87 packet to the authenticator, containing the peer's userId.
89 From this point forward, while nominally the EAP conversation occurs
90 between the PPP authenticator and the peer, the authenticator MAY act
91 as a passthrough device, with the EAP packets received from the peer
92 being encapsulated for transmission to a RADIUS server or backend
93 security server. In the discussion that follows, we will use the term
94 "EAP server" to denote the ultimate endpoint conversing with the
97 Once having received the peer's Identity, the EAP server MUST respond
98 with an EAP-TLS/Start packet, which is an EAP-Request packet with
99 EAP-Type=EAP-TLS, the Start (S) bit set, and no data. The EAP-TLS
100 conversation will then begin, with the peer sending an EAP-Response
101 packet with EAP-Type=EAP-TLS. The data field of that packet will
102 encapsulate one or more TLS records in TLS record layer format,
103 containing a TLS client_hello handshake message. The current cipher
104 spec for the TLS records will be TLS_NULL_WITH_NULL_NULL and null
105 compression. This current cipher spec remains the same until the
106 change_cipher_spec message signals that subsequent records will have
107 the negotiated attributes for the remainder of the handshake.
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116 RFC 2716 PPP EAP TLS Authentication Protocol October 1999
119 The client_hello message contains the client's TLS version number, a
120 sessionId, a random number, and a set of ciphersuites supported by
121 the client. The version offered by the client MUST correspond to TLS
124 The EAP server will then respond with an EAP-Request packet with
125 EAP-Type=EAP-TLS. The data field of this packet will encapsulate one
126 or more TLS records. These will contain a TLS server_hello handshake
127 message, possibly followed by TLS certificate, server_key_exchange,
128 certificate_request, server_hello_done and/or finished handshake
129 messages, and/or a TLS change_cipher_spec message. The server_hello
130 handshake message contains a TLS version number, another random
131 number, a sessionId, and a ciphersuite. The version offered by the
132 server MUST correspond to TLS v1.0 or later.
134 If the client's sessionId is null or unrecognized by the server, the
135 server MUST choose the sessionId to establish a new session;
136 otherwise, the sessionId will match that offered by the client,
137 indicating a resumption of the previously established session with
138 that sessionID. The server will also choose a ciphersuite from those
139 offered by the client; if the session matches the client's, then the
140 ciphersuite MUST match the one negotiated during the handshake
141 protocol execution that established the session.
143 The purpose of the sessionId within the TLS protocol is to allow for
144 improved efficiency in the case where a client repeatedly attempts to
145 authenticate to an EAP server within a short period of time. While
146 this model was developed for use with HTTP authentication, it may
147 also have application to PPP authentication (e.g. multilink).
149 As a result, it is left up to the peer whether to attempt to continue
150 a previous session, thus shortening the TLS conversation. Typically
151 the peer's decision will be made based on the time elapsed since the
152 previous authentication attempt to that EAP server. Based on the
153 sessionId chosen by the peer, and the time elapsed since the previous
154 authentication, the EAP server will decide whether to allow the
155 continuation, or whether to choose a new session.
157 In the case where the EAP server and authenticator reside on the same
158 device, then client will only be able to continue sessions when
159 connecting to the same NAS or tunnel server. Should these devices be
160 set up in a rotary or round-robin then it may not be possible for the
161 peer to know in advance the authenticator it will be connecting to,
162 and therefore which sessionId to attempt to reuse. As a result, it is
163 likely that the continuation attempt will fail. In the case where the
164 EAP authentication is remoted then continuation is much more likely
165 to be successful, since multiple NAS devices and tunnel servers will
166 remote their EAP authentications to the same RADIUS server.
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172 RFC 2716 PPP EAP TLS Authentication Protocol October 1999
175 If the EAP server is resuming a previously established session, then
176 it MUST include only a TLS change_cipher_spec message and a TLS
177 finished handshake message after the server_hello message. The
178 finished message contains the EAP server's authentication response to
179 the peer. If the EAP server is not resuming a previously established
180 session, then it MUST include a TLS server_certificate handshake
181 message, and a server_hello_done handshake message MUST be the last
182 handshake message encapsulated in this EAP-Request packet.
184 The certificate message contains a public key certificate chain for
185 either a key exchange public key (such as an RSA or Diffie-Hellman
186 key exchange public key) or a signature public key (such as an RSA or
187 DSS signature public key). In the latter case, a TLS
188 server_key_exchange handshake message MUST also be included to allow
189 the key exchange to take place.
191 The certificate_request message is included when the server desires
192 the client to authenticate itself via public key. While the EAP
193 server SHOULD require client authentication, this is not a
194 requirement, since it may be possible that the server will require
195 that the peer authenticate via some other means.
197 The peer MUST respond to the EAP-Request with an EAP-Response packet
198 of EAP-Type=EAP-TLS. The data field of this packet will encapsulate
199 one or more TLS records containing a TLS change_cipher_spec message
200 and finished handshake message, and possibly certificate,
201 certificate_verify and/or client_key_exchange handshake messages. If
202 the preceding server_hello message sent by the EAP server in the
203 preceding EAP-Request packet indicated the resumption of a previous
204 session, then the peer MUST send only the change_cipher_spec and
205 finished handshake messages. The finished message contains the
206 peer's authentication response to the EAP server.
208 If the preceding server_hello message sent by the EAP server in the
209 preceeding EAP-Request packet did not indicate the resumption of a
210 previous session, then the peer MUST send, in addition to the
211 change_cipher_spec and finished messages, a client_key_exchange
212 message, which completes the exchange of a shared master secret
213 between the peer and the EAP server. If the EAP server sent a
214 certificate_request message in the preceding EAP-Request packet, then
215 the peer MUST send, in addition, certificate and certificate_verify
216 handshake messages. The former contains a certificate for the peer's
217 signature public key, while the latter contains the peer's signed
218 authentication response to the EAP server. After receiving this
219 packet, the EAP server will verify the peer's certificate and digital
220 signature, if requested.
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228 RFC 2716 PPP EAP TLS Authentication Protocol October 1999
231 If the peer's authentication is unsuccessful, the EAP server SHOULD
232 send an EAP-Request packet with EAP-Type=EAP-TLS, encapsulating a TLS
233 record containing the appropriate TLS alert message. The EAP server
234 SHOULD send a TLS alert message rather immediately terminating the
235 conversation so as to allow the peer to inform the user of the cause
236 of the failure and possibly allow for a restart of the conversation.
238 To ensure that the peer receives the TLS alert message, the EAP
239 server MUST wait for the peer to reply with an EAP-Response packet.
240 The EAP-Response packet sent by the peer MAY encapsulate a TLS
241 client_hello handshake message, in which case the EAP server MAY
242 allow the EAP-TLS conversation to be restarted, or it MAY contain an
243 EAP-Response packet with EAP-Type=EAP-TLS and no data, in which case
244 the EAP-Server MUST send an EAP-Failure packet, and terminate the
245 conversation. It is up to the EAP server whether to allow restarts,
246 and if so, how many times the conversation can be restarted. An EAP
247 Server implementing restart capability SHOULD impose a limit on the
248 number of restarts, so as to protect against denial of service
251 If the peers authenticates successfully, the EAP server MUST respond
252 with an EAP-Request packet with EAP-Type=EAP-TLS, which includes, in
253 the case of a new TLS session, one or more TLS records containing TLS
254 change_cipher_spec and finished handshke messages. The latter
255 contains the EAP server's authentication response to the peer. The
256 peer will then verify the hash in order to authenticate the EAP
259 If the EAP server authenticates unsuccessfully, the peer MAY send an
260 EAP-Response packet of EAP-Type=EAP-TLS containing a TLS Alert
261 message identifying the reason for the failed authentication. The
262 peer MAY send a TLS alert message rather than immediately terminating
263 the conversation so as to allow the EAP server to log the cause of
264 the error for examination by the system administrator.
266 To ensure that the EAP Server receives the TLS alert message, the
267 peer MUST wait for the EAP-Server to reply before terminating the
268 conversation. The EAP Server MUST reply with an EAP-Failure packet
269 since server authentication failure is a terminal condition.
271 If the EAP server authenticates successfully, the peer MUST send an
272 EAP-Response packet of EAP-Type=EAP-TLS, and no data. The EAP-Server
273 then MUST respond with an EAP-Success message.
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284 RFC 2716 PPP EAP TLS Authentication Protocol October 1999
289 As with other EAP protocols, the EAP server is responsible for retry
290 behavior. This means that if the EAP server does not receive a reply
291 from the peer, it MUST resend the EAP-Request for which it has not
292 yet received an EAP-Response. However, the peer MUST NOT resend EAP-
293 Response packets without first being prompted by the EAP server.
295 For example, if the initial EAP-TLS start packet sent by the EAP
296 server were to be lost, then the peer would not receive this packet,
297 and would not respond to it. As a result, the EAP-TLS start packet
298 would be resent by the EAP server. Once the peer received the EAP-TLS
299 start packet, it would send an EAP-Response encapsulating the
300 client_hello message. If the EAP-Response were to be lost, then the
301 EAP server would resend the initial EAP-TLS start, and the peer would
302 resend the EAP-Response.
304 As a result, it is possible that a peer will receive duplicate EAP-
305 Request messages, and may send duplicate EAP-Responses. Both the
306 peer and the EAP-Server should be engineered to handle this
311 A single TLS record may be up to 16384 octets in length, but a TLS
312 message may span multiple TLS records, and a TLS certificate message
313 may in principle be as long as 16MB. The group of EAP-TLS messages
314 sent in a single round may thus be larger than the PPP MTU size, the
315 maximum RADIUS packet size of 4096 octets, or even the Multilink
316 Maximum Received Reconstructed Unit (MRRU). As described in [2], the
317 multilink MRRU is negotiated via the Multilink MRRU LCP option, which
318 includes an MRRU length field of two octets, and thus can support
319 MRRUs as large as 64 KB.
321 However, note that in order to protect against reassembly lockup and
322 denial of service attacks, it may be desirable for an implementation
323 to set a maximum size for one such group of TLS messages. Since a
324 typical certificate chain is rarely longer than a few thousand
325 octets, and no other field is likely to be anwhere near as long, a
326 reasonable choice of maximum acceptable message length might be 64
329 If this value is chosen, then fragmentation can be handled via the
330 multilink PPP fragmentation mechanisms described in [2]. While this
331 is desirable, there may be cases in which multilink or the MRRU LCP
332 option cannot be negotiated. As a result, an EAP-TLS implementation
333 MUST provide its own support for fragmentation and reassembly.
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340 RFC 2716 PPP EAP TLS Authentication Protocol October 1999
343 Since EAP is a simple ACK-NAK protocol, fragmentation support can be
344 added in a simple manner. In EAP, fragments that are lost or damaged
345 in transit will be retransmitted, and since sequencing information is
346 provided by the Identifier field in EAP, there is no need for a
347 fragment offset field as is provided in IPv4.
349 EAP-TLS fragmentation support is provided through addition of a flags
350 octet within the EAP-Response and EAP-Request packets, as well as a
351 TLS Message Length field of four octets. Flags include the Length
352 included (L), More fragments (M), and EAP-TLS Start (S) bits. The L
353 flag is set to indicate the presence of the four octet TLS Message
354 Length field, and MUST be set for the first fragment of a fragmented
355 TLS message or set of messages. The M flag is set on all but the last
356 fragment. The S flag is set only within the EAP-TLS start message
357 sent from the EAP server to the peer. The TLS Message Length field is
358 four octets, and provides the total length of the TLS message or set
359 of messages that is being fragmented; this simplifies buffer
362 When an EAP-TLS peer receives an EAP-Request packet with the M bit
363 set, it MUST respond with an EAP-Response with EAP-Type=EAP-TLS and
364 no data. This serves as a fragment ACK. The EAP server MUST wait
365 until it receives the EAP-Response before sending another fragment.
366 In order to prevent errors in processing of fragments, the EAP server
367 MUST increment the Identifier field for each fragment contained
368 within an EAP-Request, and the peer MUST include this Identifier
369 value in the fragment ACK contained within the EAP-Reponse.
370 Retransmitted fragments will contain the same Identifier value.
372 Similarly, when the EAP server receives an EAP-Response with the M
373 bit set, it MUST respond with an EAP-Request with EAP-Type=EAP-TLS
374 and no data. This serves as a fragment ACK. The EAP peer MUST wait
375 until it receives the EAP-Request before sending another fragment.
376 In order to prevent errors in the processing of fragments, the EAP
377 server MUST use increment the Identifier value for each fragment ACK
378 contained within an EAP-Request, and the peer MUST include this
379 Identifier value in the subsequent fragment contained within an EAP-
382 3.4. Identity verification
384 As part of the TLS negotiation, the server presents a certificate to
385 the peer, and if mutual authentication is requested, the peer
386 presents a certificate to the server.
388 Note that since the peer has made a claim of identity in the EAP-
389 Response/Identity (MyID) packet, the EAP server SHOULD verify that
390 the claimed identity corresponds to the certificate presented by the
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396 RFC 2716 PPP EAP TLS Authentication Protocol October 1999
399 peer. Typically this will be accomplished either by placing the
400 userId within the peer certificate, or by providing a mapping between
401 the peer certificate and the userId using a directory service.
403 Similarly, the peer MUST verify the validity of the EAP server
404 certificate, and SHOULD also examine the EAP server name presented in
405 the certificate, in order to determine whether the EAP server can be
406 trusted. Please note that in the case where the EAP authentication is
407 remoted that the EAP server will not reside on the same machine as
408 the authenticator, and therefore the name in the EAP server's
409 certificate cannot be expected to match that of the intended
410 destination. In this case, a more appropriate test might be whether
411 the EAP server's certificate is signed by a CA controlling the
412 intended destination and whether the EAP server exists within a
417 Since the normal TLS keys are used in the handshake, and therefore
418 should not be used in a different context, new encryption keys must
419 be derived from the TLS master secret for use with PPP encryption.
420 For both peer and EAP server, the derivation proceeds as follows:
421 given the master secret negotiated by the TLS handshake, the
422 pseudorandom function (PRF) defined in the specification for the
423 version of TLS in use, and the value random defined as the
424 concatenation of the handshake message fields client_hello.random and
425 server_hello.random (in that order), the value PRF(master secret,
426 "client EAP encryption", random) is computed up to 128 bytes, and the
427 value PRF("", "client EAP encryption", random) is computed up to 64
428 bytes (where "" is an empty string). The peer encryption key (the
429 one used for encrypting data from peer to EAP server) is obtained by
430 truncating to the correct length the first 32 bytes of the first PRF
431 of these two output strings. TheEAP server encryption key (the one
432 used for encrypting data from EAP server to peer), if different from
433 the client encryption key, is obtained by truncating to the correct
434 length the second 32 bytes of this same PRF output string. The
435 client authentication key (the one used for computing MACs for
436 messages from peer to EAP server), if used, is obtained by truncating
437 to the correct length the third 32 bytes of this same PRF output
438 string. The EAP server authentication key (the one used for
439 computing MACs for messages from EAP server to peer), if used, and if
440 different from the peer authentication key, is obtained by truncating
441 to the correct length the fourth 32 bytes of this same PRF output
442 string. The peer initialization vector (IV), used for messages from
443 peer to EAP server if a block cipher has been specified, is obtained
444 by truncating to the cipher's block size the first 32 bytes of the
445 second PRF output string mentioned above. Finally, the server
446 initialization vector (IV), used for messages from peer to EAP server
450 Aboba & Simon Experimental [Page 8]
452 RFC 2716 PPP EAP TLS Authentication Protocol October 1999
455 if a block cipher has been specified, is obtained by truncating to
456 the cipher's block size the second 32 bytes of this second PRF
459 The use of these encryption and authentication keys is specific to
460 the PPP encryption mechanism used, such as those defined in [9] and
461 [10]. Additional keys or other non-secret values (such as IVs) can
462 be obtained as needed for future PPP encryption methods by extending
463 the outputs of the PRF beyond 128 bytes and 64 bytes, respectively.
467 Since TLS supports ciphersuite negotiation, peers completing the TLS
468 negotiation will also have selected a ciphersuite, which includes key
469 strength, encryption and hashing methods. As a result, a subsequent
470 Encryption Control Protocol (ECP) conversation, if it occurs, has a
471 predetermined result.
473 In order to ensure agreement between the EAP-TLS ciphersuite
474 negotiation and the subsequent ECP negotiation (described in [6]),
475 during ECP negotiation the PPP peer MUST offer only the ciphersuite
476 negotiated inEAP-TLS. This ensures that the PPP authenticator MUST
477 accept the EAP-TLS negotiated ciphersuite in order for the
478 onversation to proceed. Should the authenticator not accept the
479 EAP-TLS negotiated ciphersuite, then the peer MUST send an LCP
480 terminate and disconnect.
482 Please note that it cannot be assumed that the PPP authenticator and
483 EAP server are located on the same machine or that the authenticator
484 understands the EAP-TLS conversation that has passed through it. Thus
485 if the peer offers a ciphersuite other than the one negotiated in
486 EAP-TLS there is no way for the authenticator to know how to respond
491 TLS as described in [12] supports compression as well as ciphersuite
492 negotiation. However, TLS only provides support for a limited number
493 of compression types which do not overlap with the compression types
494 used in PPP. As a result, during the EAP-TLS conversation the EAP
495 endpoints MUST NOT request or negotiate compression. Instead, the PPP
496 Compression Control Protocol (CCP), described in [13] should be used
497 to negotiate the desired compression scheme.
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508 RFC 2716 PPP EAP TLS Authentication Protocol October 1999
513 In the case where the EAP-TLS mutual authentication is successful,
514 the conversation will appear as follows:
516 Authenticating Peer Authenticator
517 ------------------- -------------
518 <- PPP LCP Request-EAP
536 [TLS server_key_exchange,]
537 [TLS certificate_request,]
538 TLS server_hello_done)
542 TLS client_key_exchange,
543 [TLS certificate_verify,]
544 TLS change_cipher_spec,
548 (TLS change_cipher_spec,
562 Aboba & Simon Experimental [Page 10]
564 RFC 2716 PPP EAP TLS Authentication Protocol October 1999
567 In the case where the EAP-TLS mutual authentication is successful,
568 and fragmentation is required, the conversation will appear as
571 Authenticating Peer Authenticator
572 ------------------- -------------
573 <- PPP LCP Request-EAP
583 (TLS Start, S bit set)
591 [TLS server_key_exchange,]
592 [TLS certificate_request,]
593 TLS server_hello_done)
594 (Fragment 1: L, M bits set)
599 (Fragment 2: M bit set)
608 TLS client_key_exchange,
609 [TLS certificate_verify,]
610 TLS change_cipher_spec,
611 TLS inished)(Fragment 1:
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620 RFC 2716 PPP EAP TLS Authentication Protocol October 1999
628 (TLS change_cipher_spec,
640 In the case where the server authenticates to the client
641 successfully, but the client fails to authenticate to the server, the
642 conversation will appear as follows:
644 Authenticating Peer Authenticator
645 ------------------- -------------
646 <- PPP LCP Request-EAP
664 [TLS server_key_exchange,]
665 TLS certificate_request,
666 TLS server_hello_done)
670 TLS client_key_exchange,
674 Aboba & Simon Experimental [Page 12]
676 RFC 2716 PPP EAP TLS Authentication Protocol October 1999
679 TLS certificate_verify,
680 TLS change_cipher_spec,
684 (TLS change_cipher_spec,
696 In the case where server authentication is unsuccessful, the
697 conversation will appear as follows:
699 Authenticating Peer Authenticator
700 ------------------- -------------
701 <- PPP LCP Request-EAP
719 [TLS server_key_exchange,]
720 [TLS certificate_request,]
721 TLS server_hello_done)
725 TLS client_key_exchange,
726 [TLS certificate_verify,]
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732 RFC 2716 PPP EAP TLS Authentication Protocol October 1999
735 TLS change_cipher_spec,
739 (TLS change_cipher_spec,
743 (TLS change_cipher_spec,
749 (TLS Alert message) ->
753 In the case where a previously established session is being resumed,
754 and both sides authenticate successfully, the conversation will
757 Authenticating Peer Authenticator
758 ------------------- -------------
759 <- PPP LCP Request-EAP
777 TLS change_cipher_spec
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788 RFC 2716 PPP EAP TLS Authentication Protocol October 1999
793 (TLS change_cipher_spec,
804 In the case where a previously established session is being resumed,
805 and the server authenticates to the client successfully but the
806 client fails to authenticate to the server, the conversation will
809 Authenticating Peer Authenticator
810 ------------------- -------------
811 <- PPP LCP Request-EAP
825 (TLS client_hello) ->
829 TLS change_cipher_spec,
833 (TLS change_cipher_spec,
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844 RFC 2716 PPP EAP TLS Authentication Protocol October 1999
852 In the case where a previously established session is being resumed,
853 and the server authentication is unsuccessful, the conversation will
856 Authenticating Peer Authenticator
857 ------------------- -------------
858 <- PPP LCP Request-EAP
876 TLS change_cipher_spec,
880 (TLS change_cipher_spec,
886 (TLS Alert message) ->
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900 RFC 2716 PPP EAP TLS Authentication Protocol October 1999
903 4. Detailed description of the EAP-TLS protocol
905 4.1. PPP EAP TLS Packet Format
907 A summary of the PPP EAP TLS Request/Response packet format is shown
908 below. The fields are transmitted from left to right.
911 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
912 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
913 | Code | Identifier | Length |
914 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
916 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
925 The identifier field is one octet and aids in matching responses
930 The Length field is two octets and indicates the length of the EAP
931 packet including the Code, Identifier, Length, Type, and Data
932 fields. Octets outside the range of the Length field should be
933 treated as Data Link Layer padding and should be ignored on
942 The format of the Data field is determined by the Code field.
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956 RFC 2716 PPP EAP TLS Authentication Protocol October 1999
959 4.2. PPP EAP TLS Request Packet
961 A summary of the PPP EAP TLS Request packet format is shown below.
962 The fields are transmitted from left to right.
965 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
966 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
967 | Code | Identifier | Length |
968 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
969 | Type | Flags | TLS Message Length
970 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
971 | TLS Message Length | TLS Data...
972 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
980 The Identifier field is one octet and aids in matching responses
981 with requests. The Identifier field MUST be changed on each
986 The Length field is two octets and indicates the length of the EAP
987 packet including the Code, Identifier, Length, Type, and TLS
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1012 RFC 2716 PPP EAP TLS Authentication Protocol October 1999
1015 The L bit (length included) is set to indicate the presence of the
1016 four octet TLS Message Length field, and MUST be set for the first
1017 fragment of a fragmented TLS message or set of messages. The M bit
1018 (more fragments) is set on all but the last fragment. The S bit
1019 (EAP-TLS start) is set in an EAP-TLS Start message. This
1020 differentiates the EAP-TLS Start message from a fragment
1025 The TLS Message Length field is four octets, and is present only
1026 if the L bit is set. This field provides the total length of the
1027 TLS message or set of messages that is being fragmented.
1031 The TLS data consists of the encapsulated TLS packet in TLS record
1034 4.3. PPP EAP TLS Response Packet
1036 A summary of the PPP EAP TLS Response packet format is shown below.
1037 The fields are transmitted from left to right.
1040 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
1041 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1042 | Code | Identifier | Length |
1043 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1044 | Type | Flags | TLS Message Length
1045 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1046 | TLS Message Length | TLS Data...
1047 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1055 The Identifier field is one octet and MUST match the Identifier
1056 field from the corresponding request.
1060 The Length field is two octets and indicates the length of the EAP
1061 packet including the Code, Identifir, Length, Type, and TLS data
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1068 RFC 2716 PPP EAP TLS Authentication Protocol October 1999
1087 The L bit (length included) is set to indicate the presence of the
1088 four octet TLS Message Length field, and MUST be set for the first
1089 fragment of a fragmented TLS message or set of messages. The M bit
1090 (more fragments) is set on all but the last fragment. The S bit
1091 (EAP-TLS start) is set in an EAP-TLS Start message. This
1092 differentiates the EAP-TLS Start message from a fragment
1097 The TLS Message Length field is four octets, and is present only
1098 if the L bit is set. This field provides the total length of the
1099 TLS message or set of messages that is being fragmented.
1103 The TLS data consists of the encapsulated TLS packet in TLS record
1122 Aboba & Simon Experimental [Page 20]
1124 RFC 2716 PPP EAP TLS Authentication Protocol October 1999
1129 [1] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", STD
1130 51, RFC 1661, July 1994.
1132 [2] Sklower, K., Lloyd, B., McGregor, G., Carr, D. and T. Coradetti,
1133 "The PPP Multilink Protocol (MP)", RFC 1990, August 1996.
1135 [3] Simpson, W., Editor, "PPP LCP Extensions", RFC 1570, January
1138 [4] Rivest, R. and S. Dusse, "The MD5 Message-Digest Algorithm", RFC
1141 [5] Blunk, L. and J. Vollbrecht, "PPP Extensible Authentication
1142 Protocol (EAP)", RFC 2284, March 1998.
1144 [6] Meyer, G., "The PPP Encryption Protocol (ECP)", RFC 1968, June
1147 [7] National Bureau of Standards, "Data Encryption Standard", FIPS
1148 PUB 46 (January 1977).
1150 [8] National Bureau of Standards, "DES Modes of Operation", FIPS PUB
1153 [9] Sklower, K. amd G. Meyer, "The PPP DES Encryption Protocol,
1154 Version 2 (DESE-bis)", RFC 2419, September 1998.
1156 [10] Hummert, K., "The PPP Triple-DES Encryption Protocol (3DESE)",
1157 RFC 2420, September 1998.
1159 [11] Bradner, S., "Key words for use in RFCs to Indicate Requirement
1160 Levels", BCP 14, RFC 2119, March 1997.
1162 [12] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
1163 2246, November 1998.
1165 [13] Rand, D., "The PPP Compression Control Protocol", RFC 1962, June
1178 Aboba & Simon Experimental [Page 21]
1180 RFC 2716 PPP EAP TLS Authentication Protocol October 1999
1183 6. Security Considerations
1185 6.1. Certificate revocation
1187 Since the EAP server is on the Internet during the EAP conversation,
1188 the server is capable of following a certificate chain or verifying
1189 whether the peer's certificate has been revoked. In contrast, the
1190 peer may or may not have Internet connectivity, and thus while it can
1191 validate the EAP server's certificate based on a pre-configured set
1192 of CAs, it may not be able to follow a certificate chain or verify
1193 whether the EAP server's certificate has been revoked.
1195 In the case where the peer is initiating a voluntary Layer 2 tunnel
1196 using PPTP or L2TP, the peer will typically already have a PPP
1197 interface and Internet connectivity established at the time of tunnel
1198 initiation. As a result, during the EAP conversation it is capable
1199 of checking for certificate revocation.
1201 However, in the case where the peer is initiating an intial PPP
1202 conversation, it will not have Internet connectivity and is therefore
1203 not capable of checking for certificate revocation until after NCP
1204 negotiation completes and the peer has access to the Internet. In
1205 this case, the peer SHOULD check for certificate revocation after
1206 connecting to the Internet.
1208 6.2. Separation of the EAP server and PPP authenticator
1210 As a result of the EAP-TLS conversation, the EAP endpoints will
1211 mutually authenticate, negotiate a ciphersuite, and derive a session
1212 key for subsequent use in PPP encryption. Since the peer and EAP
1213 client reside on the same machine, it is necessary for the EAP client
1214 module to pass the session key to the PPP encryption module.
1216 The situation may be more complex on the PPP authenticator, which may
1217 or may not reside on the same machine as the EAP server. In the case
1218 where the EAP server and PPP authenticator reside on different
1219 machines, there are several implications for security. Firstly, the
1220 mutual authentication defined in EAP-TLS will occur between the peer
1221 and the EAP server, not between the peer and the authenticator. This
1222 means that as a result of the EAP-TLS conversation, it is not
1223 possible for the peer to validate the identity of the NAS or tunnel
1224 server that it is speaking to.
1226 The second issue is that the session key negotiated between the peer
1227 and EAP server will need to be transmitted to the authenticator.
1228 Therefore a mechanism needs to be provided to transmit the session
1229 key from the EAP server to the authenticator or tunnel server that
1230 needs to use the key. The specification of this transit mechanism is
1234 Aboba & Simon Experimental [Page 22]
1236 RFC 2716 PPP EAP TLS Authentication Protocol October 1999
1239 outside the scope of this document.
1241 6.3. Relationship of PPP encryption to other security mechanisms
1243 It is envisaged that EAP-TLS will be used primarily with dialup PPP
1244 connections. However, there are also circumstances in which PPP
1245 encryption may be used along with Layer 2 tunneling protocols such as
1248 In compulsory layer 2 tunneling, a PPP peer makes a connection to a
1249 NAS or router which tunnels the PPP packets to a tunnel server.
1250 Since with compulsory tunneling a PPP peer cannot tell whether its
1251 packets are being tunneled, let alone whether the network device is
1252 securing the tunnel, if security is required then the client must
1253 make its own arrangements. In the case where all endpoints cannot be
1254 relied upon to implement IPSEC, TLS, or another suitable security
1255 protocol, PPP encryption provides a convenient means to ensure the
1256 privacy of packets transiting between the client and the tunnel
1261 Thanks to Terence Spies, Glen Zorn and Narendra Gidwani of Microsoft
1262 for useful discussions of this problem space.
1264 8. Authors' Addresses
1267 Microsoft Corporation
1272 EMail: bernarda@microsoft.com
1276 Microsoft Corporation
1281 EMail: dansimon@microsoft.com
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1292 RFC 2716 PPP EAP TLS Authentication Protocol October 1999
1295 9. Full Copyright Statement
1297 Copyright (C) The Internet Society (1999). All Rights Reserved.
1299 This document and translations of it may be copied and furnished to
1300 others, and derivative works that comment on or otherwise explain it
1301 or assist in its implementation may be prepared, copied, published
1302 and distributed, in whole or in part, without restriction of any
1303 kind, provided that the above copyright notice and this paragraph are
1304 included on all such copies and derivative works. However, this
1305 document itself may not be modified in any way, such as by removing
1306 the copyright notice or references to the Internet Society or other
1307 Internet organizations, except as needed for the purpose of
1308 developing Internet standards in which case the procedures for
1309 copyrights defined in the Internet Standards process must be
1310 followed, or as required to translate it into languages other than
1313 The limited permissions granted above are perpetual and will not be
1314 revoked by the Internet Society or its successors or assigns.
1316 This document and the information contained herein is provided on an
1317 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
1318 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
1319 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
1320 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
1321 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
1325 Funding for the RFC Editor function is currently provided by the
1346 Aboba & Simon Experimental [Page 24]