Layer Two Tunneling Protocol Extensions
Layer Two Tunneling Protocol Extensions WG
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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
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.