Network Working Group W. Palter
Request for Comments: 3437 zev.net
Category: Standards Track W. Townsley
Cisco Systems
December 2002
Layer-Two Tunneling Protocol Extensions for
PPP Link Control Protocol Negotiation
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This document defines extensions to the Layer Two Tunneling Protocol
(L2TP) for enhanced support of link-specific Point to Point Protocol
(PPP) options. PPP endpoints typically have direct access to the
common physical media connecting them and thus have detailed
knowledge about the media that is in use. When the L2TP is used, the
two PPP peers are no longer directly connected over the same physical
media. Instead, L2TP inserts a virtual connection over some or all
of the PPP connection by tunneling PPP frames over a packet switched
network such as IP. Under some conditions, an L2TP endpoint may need
to negotiate PPP Link Control Protocol (LCP) options at a location
which may not have access to all of the media information necessary
for proper participation in the LCP negotiation. This document
provides a mechanism for communicating desired LCP options between
L2TP endpoints in advance of PPP LCP negotiation at the far end of an
L2TP tunnel, as well as a mechanism for communicating the negotiated
LCP options back to where the native PPP link resides.
Palter & Townsley Standards Track [Page 1]
RFC 3437 L2TP Extensions for PPP LCP Negotiation December 2002
Table of Contents
1. Introduction............................................... 21.1 Specification of Requirements........................... 32. LCP Options From LAC to LNS................................ 32.1 LCP Want Options (iccn, occn)........................... 42.2 LCP Allow Options (iccn, occn).......................... 62.3 LCP Options From LNS to LAC............................. 73. Security Considerations.................................... 84. IANA Considerations........................................ 85. Normative References....................................... 86. Author's Addresses......................................... 97. Full Copyright Statement................................... 10
L2TP [RFC2661] provides a very limited amount of guidance to the LNS
as to what type of interface a tunneled PPP session arrived on at an
LAC. Such information is limited to whether the interface was
"synchronous" or "asynchronous", "digital" or "analog." These
indications provide some guidance when negotiating PPP LCP at the
LNS, but they are not as robust as they could be.
This document defines a more robust way to inform the LAC of LCP
negotiated options, and provides guidance to the LNS on the limits
and values that the LAC requires during LCP negotiation. Deep
knowledge of PPP [RFC1661] and L2TP [RFC2661] are expected for the
remainder of this document.
L2TP Proxy LCP allows options to be negotiated where the native PPP
link resides, thus circumventing issues with ACCM, Alternate FCS, and
other LCP Options that the LNS would not necessarily know how to
properly negotiate without access to the physical media for the
native PPP connection, interface type, or configuration. However,
use of Proxy LCP introduces other problems as well as there are
options within LCP PPP negotiation which should be set or adjusted by
the LNS, such as the PPP Authentication Type and MRU. Finally, the
PPP Client may reinitiate LCP negotiation at any time, and unless the
LAC is sniffing every PPP data packet it forwards, it would not be
aware that this is even occurring.
LCP options may be classified into roughly three different categories
with respect to their affect on L2TP; (1) options which affect
framing in a way that the LAC may need to know about or handle
specifically (e.g., ALT-FCS, ACCM, MRU), (2) options that are mostly
transparent to the LAC (e.g., AUTH-TYPE), and (3) options that the
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RFC 3437 L2TP Extensions for PPP LCP Negotiation December 2002
LAC may wish to influence because they are dependent on the media
type (ACFC, PFC). We are most concerned with options that fall into
category (1) and (3).
This document defines new AVPs to allow the LAC and the LNS to
communicate complete LCP information in order to react accordingly.
LCP option information is structured in the same way as the Proxy LCP
AVPs are in [RFC2661]. This essentially involves encapsulation of a
PPP LCP Configure-Request or Configure-Ack packet within an L2TP AVP.
In this document, several words are used to signify the requirements
of the specification. These words are often capitalized. 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 [RFC2119].
The LAC may utilize the following AVPs within an ICCN or OCCN message
in order to influence the LNS to negotiate LCP in a specific manner.
If these AVPs are supported by the LNS, they should override any
suggestions for LCP options implied by the Bearer Type or Framing
Type AVPs.
These AVPs may coexist with the Proxy LCP and Proxy Authentication
AVPs (Proxy AVPs) defined in the base L2TP specification. If Proxy
AVPs are received, the LNS may choose to accept these parameters, or
renegotiate LCP with the options suggested by the AVPs defined in
this document. If the LAC wishes to force negotiation of LCP by the
LNS, it should simply omit all Proxy AVPs during call initialization.
By default, the AVPs defined in this document are not mandatory (M-
bit is set to zero). However, if an implementation needs to strongly
enforce adherence to the options defined within the AVPs, it MAY set
the M-bit to 1, thus forcing the peer to discontinue the session if
it does not support this AVP. This is NOT recommended unless it is
known that the result of operating without these extensions is
completely unacceptable.
If the AVPs in sections 2.1 and 2.2 are sent to the LNS, the LAC MUST
be prepared to accept the AVPs as defined in section 2.3.
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RFC 3437 L2TP Extensions for PPP LCP Negotiation December 2002
The LCP Allow Options AVP, Attribute Type 49, contains a list of
options that the LAC wants to be negotiated by the LNS.
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 | LCP Configure-Req ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... LCP Configure-Req (continued) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 (before hiding) of this AVP is 6 plus the length of the
LCP Configure Request.
The AVP SHOULD be present in the following messages: ICCN, OCCN
The LCP Configure-Req Value for this AVP is identical to the
information field of a PPP LCP Configure-Req Packet (much like a
Proxy LCP AVP in [RFC2661]). It is sent from the LAC to the LNS, and
is intended to guide PPP LCP negotiations at an LNS. In some cases,
each individual PPP LCP option carried in this AVP maps to a desired
value (e.g., MRU) and in some cases it maps to a specific option that
is desired to be enabled (e.g., ACFC). The LNS should use these
suggestions when building its initial Configure-Request.
The following chart defines some of the more common LCP options that
may be included in this AVP with guidance on how to handle them at
the LAC and LNS. This table is provided for some of the more common
or problematic LCP options. It is not intended to be an exhaustive
representation of all LCP options available.
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RFC 3437 L2TP Extensions for PPP LCP Negotiation December 2002
LCP Want Option LAC Action LNS Action
MRU LAC provides a LNS SHOULD begin LCP
negotiation maximum
value with this value.
However, it MAY reduce
MRU if necessary.
ACCM LAC Provides a mask LNS SHOULD begin LCP
negotiation with this
value. LNS may add
bit(s) while
negotiating.
PFC LAC provides PFC LNS SHOULD begin LCP
on the link type negotiation if it is
desired with
the link type this value.
(e.g. AHDLC)
ACFC LAC provides ACCOMP LNS SHOULD begin LCP
if it is desired on negotiation with this
the link type value.
(e.g. AHDLC)
FCS-ALT LAC indicates required LNS SHOULD begin
values for the link negotiation with this
type value. Note that this
value is of no
consequence to the LNS
as FCS is stripped at
the LAC, however some
PPP media types require
this option.
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RFC 3437 L2TP Extensions for PPP LCP Negotiation December 2002
The LCP Allow Options AVP, Attribute Type 50 contains a list of
options that the LAC will allow to be negotiated by the LNS.
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 | LCP Configure-Ack ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... LCP Configure-Ack (continued) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 (before hiding) of this AVP is 6 plus the length of the
LCP Configure Request.
The AVP MAY be present in the following messages: ICCN, OCCN
The LCP Configure-Ack Value for this AVP is identical to the
information field of a PPP LCP Configure-Req Packet (much like a
Proxy LCP AVP in [RFC2661]). It is sent from the LAC to the LNS, and
is intended to guide PPP LCP negotiations at an LNS. In some cases,
each individual PPP LCP option carried in this AVP maps to a maximum
value (e.g., MRU), while in others it maps to an option that is
permitted by the LAC (e.g., ACFC). If the option is not included
here, it can be assumed by the LNS that the LAC does not understand
how to perform that particular option at the link layer (and would
thus Configure-Reject that option). Information in this AVP should
be utilized when building PPP Configure-Ack, Configure-Reject and
Configure-Nak messages.
The following chart defines some of the more common LCP options that
may be included in this AVP with guidance on how to handle them at
the LAC and LNS. This table is provided for illustration purposes
for some of the more common or problematic LCP options. It is not
intended to be an exhaustive representation of all LCP options
available.
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RFC 3437 L2TP Extensions for PPP LCP Negotiation December 2002
LCP Allow Option LAC Action LNS Action
MRU LAC provides a LNS may accept reduction
maximum value MRU as requested.
ACCM LAC Provides a mask LNS may accept bit(s)
defined here. Note that
if ACCM is missing it is
assumed that it is not
applicable to the link
type.
PFC LAC provides PFC LNS may accept PFC.
if it is allowed on
the link type
(e.g. AHDLC)
ACFC LAC provides ACFC LNS may accept ACFC.
if it is allowed on
the link type
(e.g. AHDLC)
FCS-ALT LAC indicates valid Negotiation this option
values for the link is of no consequence to
type the LNS as the FCS is
stripped at the LAC.
However, the LNS SHOULD
only accept FCS-ALT types
listed here (more than
one
value may be present).
In order to communicate negotiated LCP parameters from the LNS to the
LAC, the format of two existing messages in [RFC2661] are used.
These are:
Last Sent LCP Confreq (IETF L2TP Attribute 27)
Last Received LCP Confreq (IETF L2TP Attribute 28)
These AVPs are sent from the LAC to the LNS to support Proxy LCP
negotiation. In order to report negotiated LCP parameters from the
LNS to the LAC, two messages of precisely the same format are
defined:
LNS Last Sent LCP Confreq (IETF L2TP Attribute 51)
LNS Last Received LCP Confreq (IETF L2TP Attribute 52)
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RFC 3437 L2TP Extensions for PPP LCP Negotiation December 2002
When LCP negotiation is completed by the LNS, a Set-Link-Info control
message MUST be sent with these AVPs contained within. These AVPs
MUST contain the last sent and last received (with respect to the
LNS) LCP packets.
Rather than simply using the old Attribute values in the SLI Message,
new AVP Attribute types are defined for these messages due to the
fact that some existing L2TP implementations might check for what
could seem like misplacement of known AVP types and generate a false
error condition.
There are no known additional significant threats incurred by the
mechanisms described in this document.
This document defines additional L2TP AVPs that identify link
characteristics and interface information of a tunneled PPP link. If
these values were snooped, a rogue individual may have access to more
information about a given network or topology. Given that these same
values may be negotiated over the tunneled link in PPP LCP packets
anyway, this is no more information than is potentially transmitted
today, it is just in a different form.
This document requires four new L2TP "AVP Attribute" numbers to be
assigned by IANA.
49, Section 2.1, LCP Want Options
50, Section 2.2, LCP Allow Options
51, Section 2.3, LNS Last Sent LCP Confreq
52, Section 2.3, LNS Last Received LCP Confreq
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC1661] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51,
RFC 1661, July 1994.
[RFC 2661] Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn, G.
and B. Palter, "Layer Two Tunneling Layer Two Tunneling
Protocol (L2TP)", RFC 2661, August 1999.
Palter & Townsley Standards Track [Page 8]
RFC 3437 L2TP Extensions for PPP LCP Negotiation December 2002
W. Mark Townsley
Cisco Systems
7025 Kit Creek Road
PO Box 14987
Research Triangle Park, NC 27709
EMail: mark@townsley.net
Bill Palter
EMail: palter.ietf@zev.net
Palter & Townsley Standards Track [Page 9]
RFC 3437 L2TP Extensions for PPP LCP Negotiation December 2002
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Palter & Townsley Standards Track [Page 10]