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
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].
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.
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
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) ->
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:
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,
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
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.
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