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 thereby take advantage of the work done for any protocol 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 must 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.
|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|
|Jul 01||  ||Submit L2TP PPP Disconnect Information to IESG for consideration as a Proposed Standard|
|Jul 01||  ||Submit L2TP ATM extensions to IESG for consideration as a Proposed Standard|
|Jul 01||  ||Submit L2TP MIB to IESG for consideration as a Proposed Standard|
|Jul 01||  ||Submit L2TP Link Information to IESG for consideration as a Proposed Standard|
|Jul 01||  ||Submit L2TP Session Info to IESG for consideration as a Proposed Standard|
|Aug 01||  ||Submit L2TP Header Compression to IESG for consideration as a Proposed Standard|
|Aug 01||  ||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|
|Aug 01||  ||Submit initial Internet-Draft of L2TP Base Specification|
|Aug 01||  ||Submit initial Internet-Draft of PPP over L2TP|
|RFC3070||PS||Layer Two Tunneling Protocol (L2TP) over Frame Relay|
|RFC3145||PS||L2TP Disconnect Cause Information|
|RFC3193||PS||Securing L2TP using IPSEC|