2.3.5 Layer Two Tunneling Protocol Extensions (l2tpext)

Last Modified: 2003-07-21

Mark Townsley <mark@townsley.net>
Internet Area Director(s):
Thomas Narten <narten@us.ibm.com>
Margaret Wasserman <mrw@windriver.com>
Internet Area Advisor:
Thomas Narten <narten@us.ibm.com>
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General Discussion: l2tpext@ietf.org
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Description of Working Group:
This group is responsible for extensions to the Layer 2 Tunneling
Protocol.  Examples of L2TP "extensions" include any changes to the
L2TP encapsulation, control messages, or new AVPs sent in IETF
standard control messages.

I. L2TP Background:

L2TP (RFC2661) provides a means for tunneling PPP over IP. Because PPP
can effectivly carry any traffic (e.g., IP (RFC 1332), IPX (RFC 1552),
etc.) it can effectively be used to tunnel arbitrary protocols over
IP. L2TP provides:

- An extensible control protocol for dynamic setup, maintenance, and
  teardown of multiple layer 2 tunnels between two logical endpoints.

- An encapsulation method for tunneling PPP frames between each
  endpoint. This includes multiplexing of multiple, discrete, PPP
  streams between each endpoint.

L2TP looks (in most ways) like just another point-to-point link to PPP
and may thereby take advantage of the work done for any protocol
for use over a traditional PPP WAN link. It should be noted, however,
that the ability to dynamically establish a PPP connection between any
two IP connected endpoints brings new applications and challenges of
scale to existing PPP implementations and protocol definitions that
be considered.

As high-speed broadband access to the home replaces traditional dialup
infrastructure, L2TP has been utilized as one standard method for
aggregation and delivery of PPP connections over packet networks. Thus,
rather than a relatively small scale or low speed circuit-switched
connection such as an analog modem or ISDN connection at the L2TP
Access Concentrator (LAC), we see PPP being received over ATM PVCs
which are generally higher speed and "always-on" vs. temporally
connected.  Further, there are non-IETF standard PPP tunneling
protocols that have been developed and deployed, including PPPoE
(RFC 2516) and the 3GPP2 Wireless GPRS Tunneling Protocol Standard
(http://www.3gpp.org) that interface with L2TP at various points in the
network.  While it is unfortunate that there is more than one standard
method for tunneling PPP defined, each of these have their own
installed bases and specific application-driven nuances. Proper
integration with these various tunneling methods as they "hand-off" to
the L2TP portion of the network must be ensured.

II. L2TP Interaction with PWE3 for Pseudo-Wire Transport:

In addition to tunneling PPP, L2TPEXT will develop protocol extensions
necessary for the tunneling of specific "pseudo-wires" as defined in
the PWE3 WG. Specific milestone identification for this activity is
currently subject to ongoing work and results from PWE3.

III. WG Activities

The Working Group is currently focussed on the following activities:

- RFC2661 bundles data transport, protocol signaling, and PPP
  emulation methods into a single document. This working group will
  separate RFC2661 into stand-alone documents for greater
  modularity. This will consist of a base L2TP document defining
  common tunneling constructs and encapsulation, and a PPP document
  defining the use of these constructs for tunneling of PPP sessions
  as defined in RFC2661. Documents for tunneling of pseudo-wires
  defined in PWE3 will be forthcoming as well.

  As RFC2661 is rewritten to separate the tunnel setup and maintenance
  sections for support of new applications and increased modularity,
  some modifications to the base protocol may be necessary. This
  includes addition of a Pseudo Wire AVP to identify the pseudo wire
  being carried (with PPP identified as 0). In all cases, these will
  follow the extensible models offered in the L2TP base protocol
  design, with as much attention to backwards compatibility as
  possible given the new requirements.

In addition to its broader scope, L2TPEXT has ongoing work to complete
from its inception as a tunneling protocol for PPP only. While RFC2661
will ultimately be made obsolete by a new L2TP base specification and
companion PPP over L2TP specification, documents based on RFC2661
which do not require this new degree of modularity will still be
published in the near term. These include:

- Identification of specific parameters and modes of IPsec in order to
  aid interoperability when IPsec is used to secure L2TP traffic.

- An L2TP MIB for network management.

- An L2TP Differentiated Services Extension to negotiate DSCP
  parameters to be set for packets associated with a given L2TP
  tunnel, sessions within a tunnel, or L2TP control traffic which may
  need differentiated QoS settings.

- Extensions to L2TP for additional or more robust control information
  for informational or operational purposes as deemed necessary based
  on operational experience. These include better transfer of L2TP PPP
  LCP Information between tunnel endpoints when such state needs to be
  shared, PPP Disconnect codes within L2TP control messages for better
  debugging, and L2TP session information for enhanced logging,
  billing, and error reporting.

- Standard methods for operation over such packet networks such as
  Frame Relay and AAL5.

- L2TP defines a base encapsulation for operation in typical
  environments for tunneling PPP at the time RFC2661 was being
  developed. In cases where bandwidth cost is at a premium, the size
  of the L2TP header becomes more significant. L2TP will define a
  compressed version of the L2TP header for these environments that
  takes advantage of the L2TP control plane to establish operational
  parameters allowing removal of information from individual packets.
Goals and Milestones:
Done  Submit L2TP over Frame Relay to IESG for consideration as a Proposed Standard
Done  Submit L2TP Security to IESG for consideration as a Proposed Standard
Done  Submit L2TP PPP Disconnect Information to IESG for consideration as a Proposed Standard
Done  Submit L2TP ATM extensions to IESG for consideration as a Proposed Standard
Done  Submit L2TP MIB to IESG for consideration as a Proposed Standard
Done  Submit L2TP Link Information to IESG for consideration as a Proposed Standard
Done  Submit L2TP Session Info to IESG for consideration as a Proposed Standard
Done  Submit L2TP Differentiated Services to IESG for consideration as a Proposed Standard
Aug 01  Submit L2TP over AAL5 to IESG for consideration as a Proposed Standard
Done  Submit initial Internet-Draft of L2TP Base Specification
Done  Submit initial Internet-Draft of PPP over L2TP
Aug 03  Submit final Internet-Draft of L2TPv3 Base Specification to IESG
Aug 03  Submit L2TP Header Compression to IESG for consideration as a Proposed Standard
Sep 03  Submit Internet-Draft of HDLC over L2TPv3 to IESG
Sep 03  Submit Internet-Draft of Frame Relay over L2TPv3 to IESG
Nov 03  Submit Internet-Draft of Ethernet over L2TPv3 to IESG
Nov 03  Submit Internet-Draft of PPP over L2TPv3 to IESG
  • - draft-ietf-l2tpext-sesinfo-04.txt
  • - draft-ietf-l2tpext-l2tphc-06.txt
  • - draft-ietf-l2tpext-mcast-03.txt
  • - draft-dasilva-l2tp-relaysvc-06.txt
  • - draft-ietf-l2tpext-l2tp-base-10.txt
  • - draft-ietf-l2tpext-tunnel-switching-04.txt
  • - draft-ietf-l2tpext-pwe3-fr-02.txt
  • - draft-ietf-l2tpext-failover-02.txt
  • - draft-ietf-l2tpext-pwe3-hdlc-01.txt
  • - draft-ietf-l2tpext-l2tpmib-base-01.txt
  • - draft-ietf-l2tpext-pwe3-ethernet-00.txt
  • Request For Comments:
    Layer Two Tunneling Protocol (L2TP) over Frame Relay (RFC 3070) (12940 bytes)
    L2TP Disconnect Cause Information (RFC 3145) (17421 bytes)
    Securing L2TP using IPSEC (RFC 3193) (63804 bytes)
    Layer-Two Tunneling Protocol Extensions for PPP Link Control Protocol Negotiation (RFC 3437) (20820 bytes)
    Layer Two Tunnelling Protocol : ATM access network extensions (RFC 3301) (42756 bytes)
    L2TP Over AAL5 (RFC 3355) (25114 bytes)
    Layer Two Tunneling Protocol 'L2TP' Management Information Base (RFC 3371) (139974 bytes)
    L2TP IP Differentiated Services Extension (RFC 3308) (21536 bytes)
    L2TP IANA Considerations Update (RFC 3438) (9135 bytes)
    Signalling of Modem-On-Hold status in Layer 2 Tunneling Protocol (L2TP) (RFC 3573) (22758 bytes)

    Current Meeting Report

    IETF-57, Vienna
    Tuesday 7/16/03 3:15pm
    L2TPEXT minutes
    Mark - Agenda bashing
    Sasha Vainshtein - Graceful restart update on behalf of authors
    Sasha presented draft. Pat Calhoun asked questions about how one might 
    avoid session hi-jacking, and if there was forwarding info sent that could be 
    used maliciously. Somewhat incorherent discussion ensued, and it was 
    decided to take the issue offline.
    Mark - Sequence numbers are assumed to be check pointed, not lost??
    Sasha - Sessions are terminated on linecards w/circuits, with control 
    plane on different part of the system. Wanted psuedo wire traffic to not be 
    impacted by control plane reboots..
    Mark - So this draft does not target failover to another line card at all.
    Sasha - Right..
    Mark - What happens when control plane restart for 
    non-established sessions?
    Sasha - Control connection broken, non-completed sessions need to be 
    re-established after control returns.
    Mark - Hmm for next steps on doc? no response for or against doc...
    Mark - L2TPv3 updates (please see slides)
    Note, incorrect version was accidentally sent prior to meeting, correct 
    version was sent to list with edits.
    Mark - WG overview & draft status (see slides)


    L2TPv3 Update
    Graceful Restart Mechanism for L2TPv3