L2TPext Working Group V. Jain
Internet-Draft Pipal Systems
Expires: September 2002
R. Penno
S. McGeown
Nortel Networks
Ly Loi
TahoeNetworks
Marc Eaton-Brown
FrontRunner Solutions
April, 2002
L2TP Tunnel Switching
draft-ietf-l2tpext-tunnel-switching-02.txt
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Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
Abstract
L2TP "Tunnel Switching" or "Multihop" is the process of forwarding
an L2TP tunneled data link from the logical termination point of one
L2TP session onto another at an L2TP-aware intervening node. This
action has the affect of directing a session, or groups of sessions,
based on L2TP or tunneled data link characterisics.
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This document will provide some examples of where this technique
might be useful in various network environments, offer guidelines to
Tunnel Switch implementors, and define associated tunnel switching
nomenclature, and provide protocol extensions to help facilitate
tunnel switching.
Specification of Requirements
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
document, are to be interpreted as described in RFC 2119 [3].
Table of Contents
1. What is L2TP Tunnel Switching. . . . . . . . . . . . . . . 3
2. Tunnel Switching Nomenclature . . . . . . . . . . . . . . .3
3. Advantages of L2TP Tunnel Switching. . . . . . . . . . . . 4
4. Disadvantages of Tunnel Switching. . . . . . . . . . . . . 5
5. Extensions to enhance tunnel switching support. . . . . . .6
6. References . . . . . . . . . . . . . . . . . . . . . . . .11
7. Acknowledgments. . . . . . . . . . . . . . . . . . . . . .11
8. Author's Addresses. . . . . . . . . . . . . . . . . . . . 11
Full Copyright Statement. . . . . . . . . . . . . . . . . 12
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1. What is L2TP Tunnel Switching
L2TP tunneling allows processing of layer2 packets to be divorced
from the termination of layer2 circuit. L2TP tunnel switching
facilitates moving the termination of a layer2 session further to
another LNS potentially unknown to the first LAC. It does so by re-
tunneling the layer2 session over another L2TP tunnel to a different
LNS. The knowledge of whether to switch a layer2 session to another
L2TP tunnel can be static or dynamic (for example when a PPP session
is established).
_______ _______ _______ _______ _______
| L2 | | | | | | | |
| User | | LAC A | | LNS A | LAC B | | LNS B |
|_______| |_______| |_______|_______| |_______|
|------ L2- ------|
|---- L2/L2TP ----|
|-- L2 --|
|------ L2/L2TP -------|
|------- tunnel switching ------|
The figure above presents a typical tunnel switching scenario. The
user opens a layer2 (for example PPP) session to LAC A, which puts
the layer2 session into a L2TP tunnel that terminates on LNS A. If
LNS A decides to further tunnel the layer2 session, it puts the
layer2 session on another L2TP tunnel originating on LAC B and
terminating on LNS B. LNS A and LAC B reside on the same device.
The process of getting the layer2 session terminating on LNS A and
further tunneling it to another LNS, in the example above LNS B, is
called tunnel switching.
2. Tunnel Switching Nomenclature
Ingress Tunnel Aggregator (ITA): These devices are represented by
the first layer of LACs, represented in the picture by LAC A. This
is the node which terminates layer2 circuits and initiates the first
L2TP tunnel.
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Tunnel Switching Aggregator (TSA): These are the devices that act as
LNS as well as LAC for a particular layer2 session therefore it
typically re-tunnels a layer2 session to another LNS. The TSA is
composed of two parts: the TSA-LAC and the TSA-LNS.
Egress Tunnel Aggregator (ETA): These are devices that terminate the
layer2 session. They are represented in the picture by LNS B.
3. Advantages of L2TP Tunnel Switching
* Often, the administrative domain of a LAC, an ILEC or CLEC, is not
the same as that of an LNS, which is typically an ISP terminating
subscriber's Layer2 connection. In such situations, a multi tier
deployment with tunnel switching helps the LEC (ILEC or CLEC) mask
its internal network architecture from the ISPs. In particular, it
eases the configuration across different administrative domains. For
example, for every new ITA added in the system, ISP doesn't need to
reconfigure its LNSs - for them the LACs are always same (TSAs).
* L2TP tunnel switching divorces the location of "decision-maker" LNS.
Certain deployments do not have decision-making capabilities on the
LAC. For example PC based LACs might not have mechanisms to be
Configured with policies a service provider wants to adopt; On the
other hand, it might not be desirable to expose such policies to
every customer LAC CPE. The decision to choose the right LNS, for
load balancing or other administrative purposes when multiple LNSs
are available might not always be best achieved by the first LAC.
Not all LACs should be expected to exhibit such rich functionality,
features and flexibility.
* L2TP tunnel switching allows using a common L2TP tunnel on the LAC
for sessions that are actually destined to different LNSs. This
enables wholesaling of layer2 sessions destined to any LNS that go
over a few tunnels.
* L2TP tunnel switching may be used to reduce the number of tunnels in
a fully meshed environment. An advantage here may be reflected in a
lesser number of LAC to LNS relationships one has to manage by
providing an aggregate point for configuration, network management,
and provisioning. This could be of particular concern when tunnels
cross administrative boundaries. An analysis of the this point is
given from three different perspectives: Entire System, ITA, and
ETA.
* Entire System
In a traditional deployment, the total number of L2TP sessions
between N LACs (ITA) and M LNSs (ETA) is N*M, assuming there is
at least one layer2 session from every LAC that needs to be
terminated on each LNS. With tunnel switching, this number can be
reduced to (#ETA + #ITA) * (#TSA).
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Of course the advantage on the reduction of tunnels in the system
only holds when the (#TSA)is less than the number of LNS1s (see
picture below).
* ITA
From the first layer of LACs point of view, the reduction in the
number of tunnels holds whenever ITA*TSA < TSA*ETA, which is true
for most deployments.
* ETA
On the other hand, the advantage on the reduction of tunnels from
the last LNS (LNS2) point of view in comparison with LNS1 is
considerable, since the M*(#TSA) is usually much less than M*N.
____ ______ ______ ______ ______
| PC | | LAC1 |__________| LNS1 | LAC2 |______________| LNS2 |
|____| |______|\ ___|______|______| |______|
\ /
\__/
___/\
____ ______ / \ ______ ______ ______
| PC | | LAC1 |_______\__| LNS1 | LAC2 |______________| LNS2 |
|____| |______| |______|______| |______|
. . . .
. . .
. / \ . .
____ ______ / \ ______ ______ ______
| PC | | LAC1 |/ \_| LNS1 | LAC2 |______________| LNS2 |
|____| |______| |______|______| |______|
4. Disadvantages of L2TP tunnel switching:
* Focal point of failure: As TSA aggregates more and more tunnels,
it becomes a focal point for failure, in comparison with ITAs having
tunnels connected to ETAs directly.
* Increased Signaling: Subscriber's might be authenticated/LCP
negotiated multiple times, because each TSA might have its own
criteria to determine if a subscriber should be authenticated or if
LCP parameters negotiated (proxy-LCP) are appropriate.
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* Session limit within an L2TP tunnel: Bundling sessions within a
single L2TP tunnel makes one deployment more likely to hit the 65k
limit inherent of the L2TP protocol faster than if you have unique
tunnels. Care should be taken while deploying L2TP tunnel switching
to not exceed this limit due to aggregation of various sessions in
limited number of tunnels.
* When an TSA <-> ETA tunnel collapses for one reason or another (link
failure, etc), the initial ITA<->TSA link continues to function
normally, even though there is nowhere for the layer2 traffic to go
once it gets to this TSA point. This causes a major disruption of
service impact, as several subscribers who were knocked off the
network will not be able to get back on the network. This creates a
"black hole" condition, which directs all new sessions to the TSA,
which has no path to the ETA. All new session attempts for this ETA
fail.
In section 5 we propose a solution to this problem
* Session source tracing. A session which passes through a TSA loses
much of its source destination information from an L2TP perspective.
Thus, if there is a problem with a given session at an LNS, it
becomes more difficult to trace the session back to the original LAC
from which it began.
In section 5 we propose a solution to this problem
* L2TP parameters lost in TSA transit. RFC2661 does not specify how
exactly to propagate information arriving in L2TP AVPs from one
session or control connection to another at a TSA. Thus, there may
be inconsistency in what information is propagated, omitted, or
replaced at each TSA.
In section XA we discuss how information gets propagated through a
tunnel switch.
5. Extensions to enhance tunnel switching support
In this section we present new AVPs that can enhance tunnel
switching support beyond what is usually deployed today and solve
some of the problems mentioned on the previous section. These
extensions are only applicable for L2TP tunnel switching.
5.1 Tunnel Set Dependency
A new object L2TP Dependency should be defined to maintain a
relationship between the ITA to TSA tunnels and the TSA to ETA
tunnels.
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This object can be utilized by ITA to route away L2TP sessions from
failures in the TSA to ETA connection. Information about failures
between TSA and ETA should be provided to the ITA through a new set
of AVPs defined below.
5.2 Tunnel Dependency Load AVP (All Control Messages)
The Tunnel Dependency Load AVP, Attribute Type XS, indicates the
capacity to carry L2TP sessions from TSA to ETA for certain
"service profile" (e.g., a ISP or Domain Name)
The Attribute Value field for this AVP has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Number of Services Profiles |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SPN Length | Service Profile Name ... (arbitrary length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Maximum Load |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Current Load |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Number of Services Profiles indicates the number of
occurrences of the tuple (Service Profile Name, Maximum Load,
Current Load).
The Service Profile Name is up to 256 bytes long, but MUST be at
least 1 octet. This name should be as broadly unique as possible,
because the tunnels between ITA to TSA can contain sessions for
different service profiles.
The Maximum Load indicates the Maximum reference capacity for a
service profile. It could be the number of maximum tunnels
supported by the system at a point in time, maximum amount of
bandwidth, or some other metric that reflects ratio
The Current Load indicated the current capacity of the system, it
could be the number of active tunnels at a point in time, amount
of utilized bandwidth, or some other metric that reflects a
meaningful ratio.
This AVP MAY be hidden (the H-bit can be either 0 or 1). The
M-bit for this AVP MUST be set to 0.
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5.3 Loop Prevention AVP
The Loop Prevention AVP, Attribute Type TBD, can be used to detect
the existence of loops in the TSA network.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Number of Nodes |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HN Length | HostName ... (arbitrary length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IP address of the Node |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Number of Nodes indicates the occurrences of the tuple (Host
Name, IP Address). Each tuple identifies a node in the tunnel-
switched-path.
The Hostname MUST be the same as that used on the Hostname AVP
when the tunnel was established.
The IP address field represents the IP address which was used to
establish the tunnel.
This AVP is updated by LAC when a new tunnel is being
established. It adds (Hostname, IP address) tuple to the existing
AVP and increments Number of Nodes.
5.4 CDN Messages and L2TP Tunnel Switching
The Call-Disconnect-Notify (CDN) message is an L2TP control message
sent either by the LAC or LNS to request disconnection of a specific
call within the L2TP tunnel. Its purpose is to inform the peer of
the disconnection and the reason why it occurred.
As an indication to its use, an Incoming-Call-Request message is
generated by the LAC when the subscriber's call is detected. The LAC
selects a Session ID and sends the Incoming-Call-Request to the LNS;
it then waits for a response from the LNS - keeping the subscribers
call waiting. It is at this point that the LNS may decline to accept
the call
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It is also understood that if the call is terminated before the LNS
accepts it, a Call-Disconnect-Notify is sent by the Calling LAC to
indicate this condition, and is understood to be catered for within
current L2TP implementation. Any CDN messages originating from the
LAC are therefore omitted from the scope of this proposal, however
the Calling LAC must interpret the CDN messages received from the
LNS correctly, and must take the appropriate steps to ensure that
the intention of the CDN messages are carried out as expected.
In the case where an L2TP Tunnel Switch has successfully extended
the Control Connection to the ETA, and it returns general CDN
messages during session establishment, any such messages may be
passed transparently through to the ITA or may be interpreted by the
TSA; such scenarios are included.
The following is proposed with regards of tunnel switching and CDN
messages:
o To define the circumstances that warrants the ETA to send general
protocol CDN messages to the LAC over and above all other
signaling mechanisms defined in RFC2661.
o To include decisions that may be undertaken by an TSA.
o To include assurances that multiple TSA peering is supported.
5.5 Scenarios for generating CDN messages
The proposed causes and recommended LNS generated CDN messages for
each scenario are documented here.
5.5.1 LNS specific CDN Message
Here the TSA-LNS or Peering LNS cannot accept another Session and
signals to the downstream LAC.
o The TSA-LNS or Peering LNS reaches a pre-determined threshold,
this may be administrative or it may be a system function. As a
result, CDN Code-7 is generated by the LNS and passed to the
downstream LAC.
The downstream LAC should change the state of the upstream peer to
'busy' and apply an administrative hold-down. Thereafter the TSA-LAC
or ITA should try all possible LNS peers - if there are no
available LNS peers the CDN Code should be passed to the downstream
LAC by the TSA-LNS only. The Calling LAC may choose to clear the
call.
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5.5.2 TSA-LAC specific CDN Message
This scenario only affects the TSA-LAC, which cannot establish
another session due to it reaching the aggregate session limit.
o If the TSA-LAC exceeds max-sessions then it may signal to the
TSA-LNS to generate a CDN Code-4 for the downstream LAC.
The downstream LAC should change the state of the upstream peer to
'busy' and apply an administrative hold-down. Thereafter the TSA-LAC
or Calling LAC should try all possible LNS peers - if there are no
available LNS peers the CDN Code should be passed to the downstream
LAC by the TSA-LNS only. The Calling LAC may choose to clear the
call.
5.5.3 Calling LAC Control Connection failures
If there is a problem for the LAC to establish an L2TP tunnel,
because the Control Connection to the LNS is down, then a general
CDN message may or may not be appropriate depending upon the LAC
type. Three scenarios are mentioned here.
o The Calling LAC cannot establish a Control Connection with the
Peering LNS.
If the Calling LAC cannot continue with a session because there is
no Control Channel then it should try another L2TP peer. The Calling
LAC should record the unavailability of the Peering LNS and mark it
as unavailable for a period of time that is determined by the
Administrator.
o The TSA-LAC cannot establish a Control Connection with the
upstream LNS
If the TSA-LAC cannot continue with a session because there is no
Control Channel then a CDN Code-1 message may be generated by the
TSA-LNS to signal to the upstream LAC to try another L2TP peer.
The TSA-LAC should change the state of the upstream peer to 'dead'
and apply an administrative hold-down. Thereafter the TSA-LAC should
try all possible LNS peers - if there are no available LNS peers the
CDN Code should be passed to the downstream LAC by the TSA-LNS only.
o Calling LAC receives CDN Code-1
If the Calling LAC receives CDN Code-1, then it should try another
L2TP peer. The Calling LAC should record the unavailability of the
upstream LNS and mark it as unavailable for a period of time that is
determined by the Administrator. The Calling LAC will thereafter
clear the call.
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5.5.4 LNS Session Failure
Unable to accept the incoming call the peer LNS may return the
correct CDN message defined above or it may be unaware of these
requirements.
The following are therefore best provided by implementation since
there are a number of options that may be selected.
* The Administrator may choose to interpret all CDN messages
generated by the upstream LNS. This is typically because the LNS
employs the CDN Codes defined above (and may be implemented as the
default option).
* The Administrator may choose to ignore the CDN messages generated
by the upstream LNS, and by so doing may alert the downstream LAC
to mark it as unavailable for a period of time. This is typically
due to the upstream LNS being unaware of the CDN Codes defined
above.
* The Administrator may choose to relay any CDN messages generated
by the upstream LNS transparently through to the downstream LAC.
This caters for the scenario where the TSA is not interpreting the
CDN Codes correctly or the topology does not lend itself to the
TSA intercepting CDN messages.
6. References
[RFC 2661] W. Townsley, A. Valencia, A. Rubens, G. Pall, G. Zorn,
B. Palter, "Layer 2 Tunnel Protocol (L2TP)", RFC2661,
August 1999.
[RFC 1661] Simpson, W., "The Point-to-Point Protocol (PPP)", STD
51, RFC 1661, July 1994.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
7. Acknowledgments
Thanks to W. Mark Townsley for his valuable comments
8. Author's Addresses
Vipin Jain
Pipal Systems
2903 Bunker Hill Lane,
Santa Clara CA 95054
Phone: 408-970-4700
Email: vipin@pipalsys.com
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Reinaldo Penno
Nortel Networks, Inc.
2305 Mission College Boulevard
Building SC9
Santa Clara, CA 95054
Email: rpenno@nortelnetworks.com
Ly Loi
Tahoe Networks, Inc.
3052 Orchard Drive
San Jose, CA 95134
Phone: +1 408.944.8630
Email: lll@tahoenetworks.com
Marc Eaton-Brown
FrontRunner Solutions, Ltd.
131 Finsbury Pavement
Moorgate
London EC2A 1NT
Phone: +44 7989 498337
Email: mebrown@frontrunner.eu.com
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The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
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This document and the information contained herein is provided on an
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