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rfc:rfc3817

Network Working Group W. Townsley Request for Comments: 3817 cisco Systems Category: Informational R. da Silva

                                                       AOL Time Warner
                                                             June 2004
      Layer 2 Tunneling Protocol (L2TP) Active Discovery Relay
                   for PPP over Ethernet (PPPoE)

Status of this Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard of any kind.  Distribution of this
 memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2004).

Abstract

 The Point-to-Point Protocol (PPP) provides a standard method for
 transporting multi-protocol datagrams over point-to-point links.
 Layer Two Tunneling Protocol (L2TP), facilitates the tunneling of PPP
 packets across an intervening packet-switched network.  And yet a
 third protocol, PPP over Ethernet (PPPoE) describes how to build PPP
 sessions and to encapsulate PPP packets over Ethernet.
 L2TP Active Discovery Relay for PPPoE describes a method to relay
 Active Discovery and Service Selection functionality from PPPoE over
 the reliable control channel within L2TP.  Two new L2TP control
 message types and associated PPPoE-specific Attribute Value Pairs
 (AVPs) for L2TP are defined.  This relay mechanism provides enhanced
 integration of a specific feature in the PPPoE tunneling protocol
 with L2TP.

Table of Contents

 1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
 2.  Protocol Operation . . . . . . . . . . . . . . . . . . . . . .  2
     2.1.  PPPoE Active Discovery Stage . . . . . . . . . . . . . .  3
     2.2.  Session Establishment and Teardown . . . . . . . . . . .  4
     2.3.  PPPoE PAD Message Exchange Coherency . . . . . . . . . .  6
     2.4.  PPPoE Service Relay Capabilities Negotiation . . . . . .  8
           2.4.1.  PPPoE Service Relay Response Capability AVP. . .  8
           2.4.2.  PPPoE Service Relay Forward Capability AVP . . .  9
 3.  L2TP Service Relay Messages. . . . . . . . . . . . . . . . . .  9

Townsley & da Silva Informational [Page 1] RFC 3817 L2TP Relay for PPPoE June 2004

     3.1.  Service Relay Request Message (SRRQ) . . . . . . . . . .  9
     3.2.  Service Relay Reply Message (SRRP) . . . . . . . . . . . 10
 4.  PPPoE Relay AVP. . . . . . . . . . . . . . . . . . . . . . . . 10
 5.  Security Considerations. . . . . . . . . . . . . . . . . . . . 10
 6.  IANA Considerations. . . . . . . . . . . . . . . . . . . . . . 11
 7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
 8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     8.1.  Normative References . . . . . . . . . . . . . . . . . . 12
     8.2.  Informative References . . . . . . . . . . . . . . . . . 12
 Appendix A: PPPoE Relay in Point to Multipoint Environments. . . . 13
 Appendix B: PAD Message Exchange Coherency Examples. . . . . . . . 13
 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 16
 Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 17

1. Introduction

 PPPoE [1] is often deployed in conjunction with L2TP [2] to carry PPP
 [3] frames over a network beyond the reach of the local Ethernet
 network to which a PPPoE Host is connected.  For example, PPP frames
 tunneled within PPPoE may be received by an L2TP Access Concentrator
 (LAC) and then tunneled to any L2TP Network Server (LNS) reachable
 via an IP network.
 In addition to tunneling PPP over Ethernet, PPPoE defines a simple
 method for discovering services offered by PPPoE Access Concentrators
 (PPPoE AC) reachable via Ethernet from the PPPoE Host.  Since the
 packets used in this exchange are not carried over PPP, they are not
 tunneled with the PPP packets over L2TP, thus the discovery
 negotiation cannot extend past the LAC without adding functionality.
 This document describes a simple method for relaying PPPoE Active
 Discovery (PAD) messages over L2TP by extracting the PAD messages and
 sending them over the L2TP control channel.  After the completion of
 setup through the processing of PAD messages, PPP packets arriving
 via PPPoE are then tunneled over L2TP in the usual manner as defined
 in L2TP [2].  Thus, there are no data plane changes required at the
 LAC or LNS to support this feature.  Also, by utilizing the L2TP
 control channel, the PPPoE discovery mechanism is transported to the
 LNS reliably, before creation of any L2TP sessions, and may take
 advantage of any special treatment applied to control messages in
 transit or upon receipt.

2. Protocol Operation

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in RFC 2119 [4].

Townsley & da Silva Informational [Page 2] RFC 3817 L2TP Relay for PPPoE June 2004

 When PPPoE PAD messages are received at a PPPoE Access Concentrator,
 the messages are passed over the L2TP control connection via a newly
 defined Service Relay Request Message (SRRQ) on an established tunnel
 (Section 3.1).  When received, the PPPoE PAD message is processed at
 the L2TP node, or relayed to another L2TP node or PPPoE Access
 Concentrator.  PPPoE PAD messages sent as replies are handled in a
 similar manner over a newly defined Service Relay Reply Message
 (SRRP) (Section 3.2).

2.1. PPPoE Active Discovery Stage

 When a PPPoE Active Discovery Initiation packet (PADI) is received by
 an L2TP LAC that is providing PPPoE Service Relay, the PADI MUST be
 packaged in its entirety (including the Ethernet MAC header) within
 the PPPoE Relay AVP and transmitted over established L2TP Control
 Connection(s) associated with the interface on which the PADI
 arrived.
 The PPPoE Relay AVP is sent via the Service Relay Request Message
 (SRRQ) defined in Section 3.  The SRRQ message MUST NOT be sent to an
 L2TP node which did not include the PPPoE Service Relay Response
 Capability AVP during control connection establishment.  If no
 acceptable control connection is available or cannot be created,
 PPPoE PAD operation MUST be handled locally by some means (including
 intentionally ignoring the PPPoE PAD message, though this must be a
 deliberate act).
 It is a matter of local policy as to which control connections will
 be established for relay and associated with a given interface, and
 when the Control Connections will be established.  For instance, an
 implementation may "nail up" a control connection to a particular
 L2TP destination and associate the connection with an interface over
 which PPPoE PADI packets will arrive.  Alternatively, an
 implementation might dynamically establish a Control Connection to a
 predetermined destination upon receipt of a PADI, or upon receipt of
 a PADI from a particular source.
 Upon receipt of the SRRQ, the included PPPoE PADI message MUST be
 processed as described in [3], be relayed to another L2TP control
 connection, or be relayed to another PPPoE AC.
 After processing of a PADI, any resultant PPPoE Active Discovery
 Offer packet (PADO) MUST be encapsulated in a PPPoE Relay AVP and
 delivered via the Service Relay Reply Message (SRRP) to the sender of
 the SRRQ.

Townsley & da Silva Informational [Page 3] RFC 3817 L2TP Relay for PPPoE June 2004

 Upon receipt of an SRRP message with relayed PADO, a LAC MUST send
 the encapsulated PADO message to the corresponding PPPoE Host.  The
 source MAC address of the PADO message MUST be one which the LAC will
 respond to, perhaps requiring substitution of its own MAC address.
 In each exchange above, the PPPoE Host-Uniq TAG or AC-Cookie TAG MUST
 be used as described in Section 2.3.
 Following is an example of the PAD exchange between a PPPoE Host, LAC
 and LNS up to this point, assuming the L2TP Control Connection has
 already been established.  Examples that include AC-Cookie TAG and
 Host-Uniq TAG operation are included in the Appendix.
    PPPoE Host         LAC            Tunnel Switch            LNS
               PADI ->
                          SRRQ (w/PADI) ->      SRRQ (w/PADI) ->
                          <- SRRP (w/PADO)      <- SRRP (w/PADO)
               <- PADO

2.2. Session Establishment and Teardown

 When a LAC that is providing the PPPoE Service Relay feature receives
 a valid PPPoE Active Discovery Request packet (PADR), the LAC MUST
 treat this as an action for creation of a Incoming Call Request
 (ICRQ) as defined in [2].  The resultant ICRQ message MUST contain
 the PPPoE Relay AVP containing the PADR in its entirety.
 Upon receipt of an L2TP ICRQ message, the LNS parses the PADR message
 as described in [3].  If this is an acceptable PPPoE service
 connection (e.g., the Service-Name-Error TAG would not be included in
 a PPPoE Active Discovery Session-confirmation packet (PADS)
 response), the L2TP Incoming-Call-Reply (ICRP) message that is sent
 to the LAC includes the resultant PPPoE PADS encapsulated within the
 PPPoE Relay AVP.  If the service is unacceptable, the PADS with a
 Service-Name-Error Tag is delivered via the Relay Session AVP within
 a Call-Disconnect-Notify (CDN) message, which also tears down the
 L2TP session.  The PPPoE PADS SESSION_ID in the PPPoE Relay AVP MUST
 always be zero as it will be selected and filled in by the LAC.
 Upon receipt of an ICRP with the PPPoE Relay AVP, the LAC parses the
 PADS from the AVP, inserts a valid PPPoE SESSION_ID, and responds to
 the PPPoE Host with the PADS.  The MAC address of the PADS MUST be
 the same one was utilized during the PADI/PADO exchange described
 above.  The LAC also completes the L2TP session establishment by
 sending an Incoming-Call-Connected (ICCN) to the LNS and binds the

Townsley & da Silva Informational [Page 4] RFC 3817 L2TP Relay for PPPoE June 2004

 L2TP session with the PPPoE session.  PPP data packets may now flow
 between the PPPoE session and the L2TP session in the traditional
 manner.
 If the L2TP session is torn down for any reason, the LAC MUST send a
 PPPoE Active Discovery Terminate packet (PADT) to the host to
 indicate that the connection has been terminated.  This PADT MAY be
 received from the LNS via the PPPoE Relay AVP within a CDN message if
 this was a graceful shutdown initiated by the PPPoE subsystem at the
 LNS.  As with the PADS, the SESSION_ID in the PADT message is zero
 until filled in with the proper SESSION_ID at the LAC.
 If the LAC receives a PADT from the PPPoE Host, the L2TP session MUST
 be shut down via the standard procedures defined in [2].  The PADT
 MUST be sent in the CDN message to the LNS via the PPPoE Relay AVP.
 If the PPPoE system at the LNS disconnects the session, a PADT SHOULD
 be sent in the CDN.  In the event that the LAC receives a disconnect
 from L2TP and did not receive a PADT, it MUST generate a properly
 formatted PADT and send it to the PPPoE Host as described in [3].
 Session Establishment
   PPPoE Host         LAC            Tunnel Switch            LNS
              PADR ->
                         ICRQ (w/PADR) ->
                                               ICRQ (w/PADR) ->
                                               <- ICRP (w/PADS)
                         <- ICRP (w/PADS)
              <- PADS
                           ICCN ->
                                                    ICCN ->
 Session Teardown (LNS Initiated)
   PPPoE Host         LAC            Tunnel Switch            LNS
                                                <- CDN (w/PADT)
                          <- CDN (w/PADT)
              <- PADT
 Session Teardown (Host Initiated)
   PPPoE Host         LAC            Tunnel Switch            LNS
              PADT ->
                          CDN (w/PADT) ->
                                                CDN (w/PADT) ->

Townsley & da Silva Informational [Page 5] RFC 3817 L2TP Relay for PPPoE June 2004

2.3. PPPoE PAD Message Exchange Coherency

 PPPoE PAD messages will arrive from multiple ethernet interfaces and
 be relayed across multiple L2TP control connections.  In order to
 track which PAD messages must be sent where, we utilize the Host-Uniq
 TAG and AC-Cookie TAG.  Each are used in the same manner, depending
 on which PAD message is being sent or replied to.  Both take
 advantage of the fact that any PAD message sent as a reply to another
 PAD message MUST echo these TAGs in their entirety [3].
 For purposes of this discussion, it is useful to define two
 "directions" which PAD messages will traverse during a relayed PPPoE
 PAD message exchange.  Thus, for the following example,
                   "Upstream" ----------------------->
          PPPoE Host ------ LAC ----- Tunnel Switch ------ LNS
                   <--------------------- "Downstream"
 PAD messages being sent from the PPPoE Host, through the LAC, Tunnel
 Switch, and LNS, are defined to be traversing "Upstream." PAD
 messages being sent in the opposite direction are defined to be
 traversing "Downstream."
 Consider further, the following observation for this example:
 PAD messages that are sent Upstream: PADI, PADR, PADT
 PAD messages that are sent Downstream: PADO, PADS, PADT
 Also, there is a request/response connection between the PADI and
 PADO which must be linked with some common value.  Similarly, there
 is a request/response connection between PADO and PADR.  The PADS is
 sent on its own with no response, but must be delivered to the sender
 of the PADR.  The PADT must be sent with the same SESSION_ID as
 established in the PADS.
 The goal for PAD message exchange coherency is to ensure that the
 connections between the PADI/PADO, PADO/PADR, and PADR/PADS and
 PADS/PADT all remain intact as the PAD messages are relayed from node
 to node.
 The basic mechanism for ensuring this for PADI, PADO, and PADR
 messages is the AC-Cookie TAG and Host-Uniq TAG.  Both of these TAGs
 are defined as arbitrary data which must be echoed in any message
 sent as a response to another message.  This is the key to tying
 these PAD messages together at each hop.  The following two rules
 makes this possible:

Townsley & da Silva Informational [Page 6] RFC 3817 L2TP Relay for PPPoE June 2004

    For PAD messages that are sent Upstream, a new Host-Uniq TAG MUST
    be inserted at each relaying node before the PAD message is
    forwarded.  There SHOULD be at most one Host-Uniq TAG per PAD
    message.
    For PAD messages being sent Downstream, a new AC-Cookie TAG MUST
    be inserted at each relaying node before the PAD message is
    forwarded.  There SHOULD be at most one AC-Cookie TAG per PAD
    message.  Additionally, for an LNS receiving multiple PAD messages
    from upstream, there SHOULD be at most one PAD message forwarded
    downstream per received SRRP Message.  In other words, there
    SHOULD be exactly one PPPoE Relay AVP per L2TP SRRP Message.
 The exception here is the PADS, which cannot carry an AC-Cookie TAG
 (and, thankfully, doesn't need to), and the PADT.  We will discuss
 these later in this section.  Using the above rules, PADI, PADO, and
 PADR messages may be relayed through an arbitrary number of nodes,
 each inserting its own value to link a message response that it might
 receive.
 In order to implement this exchange without tying up resources at
 each L2TP node, it is desirable to not require ephemeral state at
 each node waiting for a message response from each forwarded PAD
 message.  This is achievable if one is willing to be very intelligent
 about the values that will be sent in the PPPoE TAGs used for message
 coherency.  Given that the TAGs are of arbitrary size and composition
 and are always echoed in their entirety, one may use the information
 here to map any next relay hop information.  For example, the L2TP
 Tunnel ID (Control Connection ID) could be encoded in the TAG in
 order to identify where to relay the message when it arrives.  If one
 chooses this method, the encoding MUST incorporate some method of
 encryption and authentication of the value.  Note that this is a
 relatively simple proposition given that it is only the source of the
 encrypted and data that will ever need to decrypt and authenticate
 the value upon receipt (thus, no key exchanges are necessary, and any
 of a myriad of algorithms may be chosen).  Note that individual TAGs
 MUST never exceed 255 octets in length, and the length of an entire
 PPPoE message MUST never exceed the maximum segment size of the
 underlying ethernet.  In the event that a TAG exceeds 255 octets in
 length, a compression scheme which may include storage of state at an
 L2TP node may be necessary before constructing a new TAG.
 The PADS and PADT messages do not rely on the AC-Cookie TAG or Host-
 Uniq TAG for directing to the proper node.  As described in Section
 2.2, the L2TP session is created upon receipt of a valid PADR at the
 L2TP LAC.  Since the PADS is sent as an AVP on this message exchange,

Townsley & da Silva Informational [Page 7] RFC 3817 L2TP Relay for PPPoE June 2004

 its coherency may be secured via the L2TP session itself.  Similarly
 for the PADT, as it is carried in the L2TP disconnect message (CDN)
 for the L2TP session.
 Clients are supposed to treat an AC-Cookie TAG as an opaque object.
 They differentiate PADOs only by MAC address, Service-Name TAG(s) and
 by AC-Name TAG(s).  If an LAC sends multiple PADOs, they should
 contain different AC-Name TAGs.
 Furthermore, a node performing PPPoE L2TP Relay (such as an LAC)
 SHOULD attempt to distinguish or rate limit retransmitted PADx
 messages (perhaps via the source MAC address and/or arriving
 interface of the message) in order to limit the overloading of L2TP.
 Examples of this operation for a number of scenarios and
 considerations for certain deployment situations may be found in the
 Appendix of this document.

2.4. PPPoE Service Relay Capabilities Negotiation

 If the extensions defined in this document are present and configured
 for operation on a given Control Connection, the AVPs listed in this
 section MUST be present in the Start-Control-Connection-Request
 (SCCRQ) or Start-Control-Connection-Reply (SCCRP) messages during
 control connection setup.

2.4.1. PPPoE Service Relay Response Capability AVP

 The PPPoE Service Relay Response Capability AVP, Attribute Type 56,
 indicates to an L2TP peer that the PPPoE Service Relay (SRRQ, SRRP)
 messages and the PPPoE Relay AVP will be processed and responded to
 when received.
  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |M|H| rsvd  |      Length       |           Vendor ID           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |         Attribute Type        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 The Vendor ID is the IETF Vendor ID of 0.
 This AVP MAY be hidden (the H bit MAY be 0 or 1).
 The M bit for this AVP may be set to 0 or 1.  If the sender of this
 AVP does not wish to establish a connection to a peer which does not

Townsley & da Silva Informational [Page 8] RFC 3817 L2TP Relay for PPPoE June 2004

 understand this L2TP extension, it SHOULD set the M bit to 1,
 otherwise it MUST be set to 0.
 The Length of this AVP is 6.
 The AVP may be present in the following messages: SCCRQ, SCCRP

2.4.2. PPPoE Service Relay Forward Capability AVP

 The PPPoE Service Relay Forward Capability AVP, Attribute Type 57,
 indicates to an L2TP peer that PPPoE Service Relay (SRRQ, SRRP)
 messages and the PPPoE Relay AVP may be sent by this L2TP peer.
  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |M|H| rsvd  |      Length       |           Vendor ID           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |         Attribute Type        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 The Vendor ID is the IETF Vendor ID of 0.
 This AVP MAY be hidden (the H bit MAY be 0 or 1).
 The M bit for this AVP may be set to 0 or 1.  If the sender of this
 AVP does not wish to establish a connection to a peer which does not
 understand this L2TP extension, it SHOULD set the M bit to 1,
 otherwise it MUST be set to 0.
 The Length of this AVP is 6.
 The AVP may be present in the following messages: SCCRQ, SCCRP

3. L2TP Service Relay Messages

 This section identifies two new L2TP messages used to deliver PPPoE
 PADI and PADO messages.

3.1. Service Relay Request Message (SRRQ)

 The Service Relay Request Message (SRRQ), Message Type 18, is sent by
 an LAC to relay requests for services.  This document defines one new
 AVP that may be present to request service in section 2.  Further
 service relay mechanisms may also use this message in a similar
 context.  Discussion of other service relay mechanisms are outside
 the scope of this document.

Townsley & da Silva Informational [Page 9] RFC 3817 L2TP Relay for PPPoE June 2004

3.2. Service Relay Reply Message (SRRP)

 The Service Relay Reply Message (SRRP), Message Type 19, is sent by
 an LAC to relay responses of requests for services.  This document
 defines one new AVP that may be present as a response to a request
 for service in section 2.  Further service relay mechanisms may also
 use this message in a similar context.  Discussion of other service
 relay mechanisms are outside the scope of this document.

4. PPPoE Relay AVP

 The PPPoE Relay AVP, Attribute Type 55, carries the entire PADI,
 PADO, PADR, PADS and PADT messages within, including Ethernet MAC
 source and destination addresses.  This is the only AVP necessary for
 relay of all PAD messages via L2TP.
  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |M|H| rsvd  |      Length       |           Vendor ID           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |         Attribute Type        |       PPPoE PAD Message ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                  ... (Until end of message is reached)          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 The Vendor ID is the IETF Vendor ID of 0.
 This AVP MAY be hidden (the H bit MAY be 0 or 1).
 The M bit for this AVP may be set to 0 or 1.  If the sender of this
 AVP does not wish to establish a connection to a peer which does not
 understand this L2TP extension, it SHOULD set the M bit to 1,
 otherwise it MUST be set to 0.
 The Length of this AVP is 6 plus the length of the PPPoE PAD Message.
 The AVP may be present in the following messages: SRRQ, SRRP, ICRQ,
 ICRP, ICCN, and CDN.

5. Security Considerations

 PPPoE has a number of known security weaknesses that are not
 described here.  For example, an intruder between a PPPoE Host and a
 PPPoE AC who can observe or modify PPPoE Active Discovery traffic has
 numerous opportunities for denial of service and other attacks.  The
 use of the L2TP extensions described here makes it possible to tunnel
 PPPoE discovery packets between the LAC and LNS, extending the path

Townsley & da Silva Informational [Page 10] RFC 3817 L2TP Relay for PPPoE June 2004

 which the PPPoE Active Discovery packets are transported.  There are
 two possible implications of this.  First, the tunneled packets may
 now be observable by an intruder having access to traffic along the
 L2TP tunnel path.  This MAY make information regarding service
 offerings or host identity easier to obtain to a rogue party given
 that it is being sent over a wider variety of media, and presumably
 over a longer distance and/or more hops or administrative domains.
 Whether this information could be used for malicious purposes depends
 on the information contained within, but it is conceivable that this
 could be sensitive information, and this mechanism increases the
 possibility that this information would be presented to an
 interloper.  Second, it may also be possible for an intruder to
 modify PPPoE Active Discovery traffic while it is being carried
 within L2TP control messages.
 There are at least two methods defined to help thwart this inspection
 or modification by an unauthorized individual.  One of the two MUST
 be used if the service discovery information is considered to be
 sensitive and is traversing an untrusted network.  The first
 suggested method is AVP hiding described in [2].  This may be used to
 hide the contents of the packets in transit, though offers no
 integrity protection against modification of data in the AVP.  The
 second and more secure method is protecting L2TP with IPsec as
 defined in [6].

6. IANA Considerations

 This document requires three new "AVP Attribute" (attribute type)
 numbers to be assigned through IETF Consensus [5] as indicated in
 Section 10.1 of [2].
    1. PPPoE Relay AVP (section 4.0)
    2. PPPoE Relay Response Capability AVP  (section 2.4.1)
    3. PPPoE Relay Forward Capability AVP  (section 2.4.2)
 This document requires two new "Message Type" numbers to be assigned
 through IETF Consensus [5] as indicated in Section 10.2 of [2].
    1. Service Relay Request Message (SRRQ) (Section 3.1)
    2. Service Relay Reply Message (SRRP) (Section 3.2)
 There are no additional requirements on IANA to manage numbers in
 this document or assign any other numbers.

Townsley & da Silva Informational [Page 11] RFC 3817 L2TP Relay for PPPoE June 2004

7. Acknowledgements

 Thanks to Vinay Shankarkumar for valuable review, comment, and
 implementation.
 Thanks to David Skoll and a number of others on pppoe@ipsec.org for
 providing very helpful discussion about their PPPoE implementations.
 Thanks to Ross Wheeler, Louis Mamakos, and David Carrel for providing
 valuable clarifications of PPPoE [1] while designing this protocol.

8. References

8.1. Normative References

 [1] Mamakos, L., Lidl, K., Evarts, J., Carrel, D., Simone, D. and R.
     Wheeler, "A Method for Transmitting PPP Over Ethernet (PPPoE)",
     RFC 2516, February 1999.
 [2] Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn, G. and B.
     Palter, "Layer Two Tunneling Protocol 'L2TP'", RFC 2661, August
     1999.
 [3] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51, RFC
     1661, July 1994.
 [4] Bradner, S., "Key words for use in RFCs to Indicate Requirement
     Levels", BCP 14, RFC 2119, March 1997.
 [5] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
     Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.

8.2. Informative References

 [6] Patel, B., Aboba, B., Dixon, W., Zorn, G. and S. Booth, "Securing
     L2TP Using IPsec," RFC 3193, November 2001.

Townsley & da Silva Informational [Page 12] RFC 3817 L2TP Relay for PPPoE June 2004

Appendix A: PPPoE Relay in Point to Multipoint Environments

 The PPPoE PADI message in its native form, is sent as a broadcast
 message on an Ethernet link.  Thus, more than one AC concentrator
 could conceivably receive and respond to this message.  Similarly, a
 PPPoE interface could be associated with more than one L2TP Control
 Connection, in order to query multiple LNSs with potentially varying
 service profiles, as well as to load balance requests.
 As the PADI message is propagated, one may choose to replicate the
 message to multiple Control Connections in order to mimic the
 behavior of the PADI being sent on an ethernet link with multiple ACs
 attached.  If the number of replicated nodes is large, and the number
 of hops deep, then an unmanageable "fan-out" of PADI propagation may
 occur.  Thus, care should be taken here to only replicate messages to
 multiple Control Connections when it is absolutely necessary.
 The only case where it is seems necessary to replicate messages to
 multiple destinations is in the case where each destination is known
 to have varying service policies that all need to be advertised to a
 PPPoE Host for its gathering and selection.  At the time of this
 writing, the authors know of no PPPoE Host implementations that take
 advantage of this ability (instead, responding to only a single PPPoE
 PADO).  This, of course, is subject to change if and when PPPoE
 implementations are advanced to this stage.
 In cases where multiple Control Connections may exist to multiple
 LNSs for load balancing purposes, L2TP Service Relay should take
 measures to try one Control Connection at a time, rather than
 broadcasting to all Control Connections simultaneously.

Appendix B: PAD Message Exchange Coherency Examples

 Example 1: "PPPoE Relay With Multiple LNSs"
                      ,--- LNS1
                     /
         Host --- LAC
                     \
                      `--- LNS2
 This example assumes that there is good reason to send a copy of the
 PADI to both LNSs (e.g., each LNS may have a different service
 profile to offer).

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 1) a. Host sends PADI via broadcast MAC address to LAC
    b. LAC replicates the PADI message and forwards a copy to LNS1
       Host-Uniq = R1 (assigned)
    c. LAC replicates the PADI message and forwards a copy to LNS2
       Host-Uniq = R2 (assigned)
 2) a. LNS1 responds with PADO to LAC
       Host-Uniq = R1 (echoed)
       AC-Cookie = C1 (assigned)
    b. LNS1 responds with PADO to LAC
       Host-Uniq = R2 (echoed)
       AC-Cookie = C2 (assigned)
    c. LAC forwards both PADO messages to Host with source MAC set to
       MAC address of LAC.  PADO from (2a) is assigned new AC-Cookie
       C1' and PADO from (2b) is given AC-Cookie C2'
 3) a. Host sends PADR to MAC address of LAC (choosing one)
       AC-Cookie = C1' (echoed)
    b. LAC knows to forward PADR to LNS1 based on C1'
       AC-Cookie = C1 (echoed)
 4) Session Establishment at the LAC commences, with further PAD
    messages carried within the context of the L2TP session itself.
    No need to inspect the AC-Cookie TAG or Host-Uniq TAG from this
    point forward in order to direct messages properly.
 Example 2: "PPPoE Relay With L2TP Tunnel-Switching"
         Host --- LAC ---- LNS1 ---- LNS2
 1) a. Host sends PADI to LAC.
    b. LAC sends PADI to LNS1
       Host-Uniq = R1 (assigned)
    c. LNS1 sends PADI to LNS2
       Host-Uniq =  R2 (assigned)
 2) a. LNS2 responds to LNS1 with PADO
       Host-Uniq = R2 (echoed)
       AC-Cookie = C1 (assigned)

Townsley & da Silva Informational [Page 14] RFC 3817 L2TP Relay for PPPoE June 2004

    b. LNS1 relays PADO to LAC
       Host-Uniq = R1 (echoed)
       AC-Cookie = C1' (assigned)
    c. LAC sends PADO to Host
       AC-Cookie = C1'' (assigned)
 3) a. Host sends PADR to MAC address of LAC
       AC-Cookie = C1'' (echoed)
    b. LAC sends PADR to LNS1
       AC-Cookie = C1' (echoed)
    c. LNS1 sends PADR to LNS2
       AC-Cookie = C1 (echoed)
 4) Session Establishment at the LAC, LNS1 and LNS2 commences, with
    further PAD messages carried within the context of the L2TP
    session itself.  No need to inspect the AC-Cookie TAG or Host-Uniq
    TAG from this point forward in order to direct messages properly.
 Example 3: "PPPoE Relay With Multiple PPPoE ACs"
                               ,--- AC1
                              /
         Host --- LAC ---- LNS
                              \
                               `--- AC2
 In this example, AC1 and AC2 are PPPoE access concentrators on a
 broadcast domain.  Sequence of operation is as follows.
 1) a. Host sends PADI to LAC.
    b. LAC sends PADI to LNS
       Host-Uniq = R1 (assigned)
    c. LNS broadcasts PADI to AC1 and AC2
       Host-Uniq = R2 (assigned)
 2) a. AC1 sends PADO to LNS
       Host-Uniq = R2 (echoed)
       AC-Cookie = C1 (assigned)
    b. AC2 sends PADO to LNS
       Host-Uniq = R2 (echoed)
       AC-Cookie = C2 (assigned)

Townsley & da Silva Informational [Page 15] RFC 3817 L2TP Relay for PPPoE June 2004

    c. LNS sends two PADOs to LAC
       Host-Uniq = R1 (echoed)
       AC-Cookie (assigned) = C1' and C2', respectively
    d. LAC sends two PADOs to Host
       Host-Uniq = R1
       AC-Cookie (assigned) = C1'' and C2'', respectively
 3) a. Host sends PADR with to LAC to select service from AC2.
       AC-Cookie = C2'' (echoed)
    b. LAC sends PADR to LNS         AC-Cookie = C2' (echoed)
    c. LAC sends PADR to AC2
       AC-Cookie = C1 (echoed)
 4) Session Establishment at the LAC, LNS and AC2 commences, with
    further PAD messages carried within the context of the L2TP
    session or PPPoE session itself. No need to inspect
    the AC-Cookie TAG or Host-Uniq TAG from this point forward in
    order to direct messages properly.

Authors' Addresses

 W. Mark Townsley
 cisco Systems
 7025 Kit Creek Road
 Research Triangle Park, NC 27709
 EMail: mark@townsley.net
 Ron da Silva
 AOL Time Warner
 12100 Sunrise Valley Dr
 Reston, VA 20191
 EMail: rdasilva@va.rr.com

Townsley & da Silva Informational [Page 16] RFC 3817 L2TP Relay for PPPoE June 2004

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