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Layer Two Tunneling Protocol Extensions

Document Charter Layer Two Tunneling Protocol Extensions WG (l2tpext) Snapshot
Title Layer Two Tunneling Protocol Extensions
Last updated 1999-10-21
State Approved
WG State Concluded
IESG Responsible AD Deborah Brungard
Charter edit AD (None)
Send notices to (None)

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 
  ( 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.