Network Working Group Wei Luo
Internet Draft Cisco Systems, Inc.
Jan 2004
L2VPN Extensions for L2TP
draft-ietf-l2tpext-l2vpn-00.txt
Status of this Memo
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Abstract
The Layer 2 Tunneling Protocol (L2TP) provides a standard method for
setting up and managing L2TP sessions to tunnel a variety of L2
protocols. One of the reference models supported by L2TP describes
the use of an L2TP session to connect two Layer 2 circuits attached
to a pair of peering LACs, which is a basic form of Layer 2 Virtual
Private Network (L2VPN). This document defines the protocol
extensions for L2TP to set up different types of L2VPN in a unified
fashion.
Specification of Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC 2119].
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Table of Contents
Status of this Memo.......................................... 1
1. Introduction.............................................. 2
2. Network Reference Model................................... 3
3. Forwarder Identifier...................................... 4
4. Protocol Components....................................... 4
4.1 Control Messages...................................... 4
4.2 Existing AVPs for L2VPN............................... 4
4.3 New AVPs for L2VPN.................................... 5
5. Signaling Procedures...................................... 7
5.1 Overview.............................................. 7
5.2 Pseudowire Tie Detection.............................. 7
5.3 Generic Algorithm..................................... 8
6. IANA Considerations....................................... 11
7. Security Considerations................................... 11
8. References................................................ 11
9. Authors' Address.......................................... 12
1. Introduction
[L2TPv3] defines a dynamic tunneling mechanism to carry multiple L2
protocols besides PPP (as originally defined in [RFC 2661]) over a
packet-based network. The baseline protocol supports various types
of applications, which have been highlighted in the different L2TP
reference models in [L2TPv3]. L2VPN applications are typically in
the scope of the LAC-LAC reference model.
This document discusses the commonalities as well as differences
among L2VPN applications with respect to utilizing L2TPv3 as the
signaling protocol.
The acronym "L2TP" refers to L2TPv3 or L2TP in general in this
document.
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2. Network Reference Model
In the LAC-LAC reference model, a LAC serves as a cross-connect
between attachment circuits and L2TP sessions. Each L2TP session
acts as an emulated circuit, also known as pseudowire. A pseudowire
is used to bind two "forwarders" together. For different L2VPN
applications, different types of forwarders are defined.
In the L2VPN framework [L2VPN FW], a LAC is a Provider Edge (PE)
device. LAC and PE are interchangable terms in the context of this
document. Remote systems in the LAC-LAC reference model are Customer
Edge (CE) devices.
+----+ L2 +----+ +----+ L2 +----+
| CE |------| PE |....[core network]....| PE |------| CE |
+----+ +----+ +----+ +----+
|<- emulated service ->|
|<----------------- L2 service -------------->|
L2VPN Network Reference Model
In a simple cross-connect application, an attachment circuit is a
forwarder directly bound to a pseudowire. It is a one-to-one
mapping. Traffic received from the attachment circuit on a local PE
is forwarded to the remote PE through the pseudowire. When the
remote PE receives traffic from the pseudowire, it forwards the
traffic to the corresponding attachment circuit on its end. The
forwarding decision is based on the attachment circuit or pseudowire
demultiplexing identifier.
With Virtual Private LAN Service (VPLS), a Virtual Switching Instance
(VSI) is a forwarder connected to one or more attachment circuits and
pseudowires. A single pseudowire is used to connect a pair of VSIs
on two peering PEs. Traffic received from an attachment circuit or a
pseudowire is first forwarded to the corresponding VSI based on the
attachment circuit or pseudowire demutiplexing identifier. The VSI
performs additional lookup to determine where to further forward the
traffic.
With Virtual Private Wire Service (VPWS), attachment circuits are
grouped into "colored pools". Each pool is a forwarder and connected
through a network of point-to-point cross-connect. The data
forwarding perspective is identical to the cross-connect application.
However, constructing colored pools involves more complicated
signaling procedures.
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3. Forwarder Identifier
A forwarder identifier is assigned to each forwarder on a given PE
and is unique in the context of the PE. It is defined as the
concatenation of an Attachment Group Identifier (AGI) and an
Attachment Individual Identifier (AII), denoted as <AGI, AII>. The
AGI is used to group a set of forwarders together for signaling
purpose. An AII is used to distinguish forwarders within a group.
AII can be unique at a per platform or per group basis.
As far as the signaling procedures are concerned, a forwarder
identifier is an arbitrary string of bytes. It is up to
implementations to decide the values for AGI and AII.
When connecting two forwarders together, both MUST have the same AGI
as part of their forwarder identifiers. The AII of the source
forwarder is known as the Source AII (SAII), and the AII of the
target forwarder is known as the Target AII (TAII). Therefore,
source forwarder and target forwarder can be denoted as <AGI, SAII>
and <AGI, TAII> respectively.
4. Protocol Components
4.1 Control Messages
L2TP defines two sets of session management procedures: incoming call
and outgoing call. Even though it is entirely possible to use the
outgoing call procedures to signaling L2VPNs, the incoming call
procedures has some advantages in terms of the relevance of the
semantics. [PWE3L2TP] gives more details on why the incoming call
procedures are more appropriate for setting up pseudowires.
The signaling procedures for L2VPNs described in the following
sections are based on the Incoming Call procedures.
4.2 Existing AVPs for L2VPN
Router ID
The Router ID sent in SCCRQ and SCCRP during control connection setup
establishes the unique identity of each PE.
Pseudowire Capabilities List
The Pseudowire Capabilities List sent in the SCCRQ and SCCRP
indicates the pseudowire types supported by the sending PE. It
merely serves as an advertisement to the receiving PE. Its content
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should not affect the control connection setup.
Before a local PE initiates a session of a particular pseudowire type
to a remote PE, it MUST examine whether the remote PE has advertised
this pseudowire type in this AVP, and SHOULD NOT attempt to initiate
the session if the intended pseudowire type is not supported by the
remote PE.
Pseudowire Type
The Pseudowire Type sent in ICRQ signals the intended pseudowire type
to the receiving PE. The receiving PE checks it against its local
pseudowire capabilities list. If it finds a match, it responds with
an ICRP without a Pseudowire Type AVP, which implicitly acknowledges
its acceptance of the intended pseudowire. If it does not find a
match, it MUST respond with a CDN with an "unsupported pseudowire
type" result code.
Pseudowire Control Encapsulation
The Pseudowire Control Encapsulation can be sent in ICRQ, ICRP, and
ICCN. If the receiving PE supports the specified control
encapsulation, it MUST include it in its data packets sent to the
sending PE. Otherwise, it MUST reject the connection by sending a
CDN to the sending PE.
Circuit Status
The Circuit Status is sent in both ICRQ and ICRP to inform the
receiving PE about the circuit status on the sending PE. It can also
be sent in ICCN and SLI to update the status.
Remote End Identifier
The TAII value is encoded in the Remote End ID AVP and sent in ICRQ
along with the optional AGI to instruct the receiving PE to bind the
proposed pseudowire to the forwarder that matches the specified
forwarder identifier.
4.3 New AVPs for L2VPN
Attachment Group Identifier
The AGI AVP, Attribute Type TBA, is an identifier used to associate a
forwarder to a logical group. The AGI AVP is used in conjunction
with the Local End ID AVP and Remote End ID AVP, which encode the
SAII and TAII respectively, to identify a specific forwarder. When
the AGI AVP is omitted in the control messages or contains a zero-
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length value, the forwarders are considered using the default AGI.
Note that there is only one designated default AGI value for all
forwarders.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|H|0|0|0|0| Length | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TBA | AGI (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The AGI field is a variable-length field. This AVP MAY be present in
ICRQ.
This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for this
AVP SHOULD be set to 0. The Length (before hiding) of this AVP is 6
plus the length of the AGI field.
Local End ID
The Local End ID AVP, Attribute Type TBA, encodes the SAII value.
The SAII may also be used in conjunction with the TAII to detect
pseudowire ties. When it is omitted in the control messages, it is
assumed that it has the same value as the TAII.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|H|0|0|0|0| Length | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TBA | SAII (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The SAII field is a variable-length field. This AVP MAY be present
in ICRQ.
This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for this
AVP SHOULD be set to 0. The Length (before hiding) of this AVP is 6
plus the length of the SAII field.
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5. Signaling Procedures
5.1 Overview
Assume a PE assigns a forwarder identifier to one of its local
forwarders, and knows it needs to set up a pseudowire to a remote
forwarder on a remote PE that has a certain Forwarder ID. This
knowledge can be obtained either through manual configuration or some
auto-discovery procedure.
Before establishing the intended pseudowire, each pair of peering PEs
exchanges control connection messages to establish a control
connection. Each advertises its supported pseudowire types in the
Pseudowire Capabilities List AVP.
After the control connection is established, the local PE examines
whether the remote PE supports the pseudowire type it intends to set
up. Only if the remote PE supports the intended pseudowire type, it
should initiate a pseudowire connection request.
When the local PE receives an ICRQ for a pseudowire connection, it
examines the forwarder identifier encoded in the AGI, SAII, and TAII
in order to determine the following:
- whether it has a local forwarder with the forwarder identifier
value specified in the ICRQ,
- whether the remote forwarder with the forwarder identifier
specified in the ICRQ is allowed to connect with this local
forwarder.
If both conditions are met, it sends an ICRP to the remote PE to
accept the connection request. If either of the two conditions
fails, it sends a CDN to the remote PE to reject the connection
request.
5.2 Pseudowire Tie Detection
Conceivably in the network reference models, as either PE may
initiate a pseudowire to another PE at any time, the PEs could end up
initiating a pseudowire to each other simultaneously. In order to
avoid setting up duplicated pseudowires between two forwarders, each
PE must be able to independently detect such a pseudowire tie. The
following procedures need to be followed to detect a tie:
If both TAII and SAII are present in the ICRQ, the receiving PE
compares the TAII and SAII against the SAII and TAII itself
previously sent to the sending PE. If the received TAII matches the
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sent SAII and the received SAII matches the sent TAII, a tie is
detected.
If only the TAII is present in the ICRQ, the SAII is assumed to have
the same value as the TAII. The receiving PE compares the received
TAII with the SAII that it previously sent to the sending PE. If the
SAII in that ICRQ is also omitted, then the value encoded in the sent
TAII is used for comparison. If they match, a tie is detected.
Once a tie has been discovered, the standard L2TP tie breaking
procedure is used to disconnect the duplicated pseudowire.
5.3 Generic Algorithm
The following uses a generic algorithm to illustrate the protocol
interactions when constructing an L2VPN using L2TP signaling.
Each PE first forms a list, SOURCE_FORWARDERS, consisting of all
local forwarders of a given VPN. Then it puts all local forwarders
that need to be interconnected and all remote forwarders of the same
VPN into another list, TARGET_FORWARDERS. The formation of the
network topology depends on the content in the SOURCE_FORWARDERS and
TARGET_FORWARDERS list. These two lists can be constructed by manual
configuration and/or some auto-discovery procedure.
The algorithm is used to set up pseudowires among all the forwarders
that intend to be interconnected by iterating through each source and
target forwarder. An L2VPN is formed upon finishing the algorithm in
every participating PE of this L2VPN.
1. Pick the next forwarder, from SOURCE_FORWARDERS. If no
forwarder is available for processing, the processing is
complete.
2. Pick the next forwarder, from TARGET_FORWARDERS. If no
forwarder is available for processing, go back to step 1.
3. If the two forwarders are associated with different Router
IDs, a pseudowire must be established between them. Proceed
to step 6.
4. Compare the <AGI, AII> values of the two forwarders. If
they match, the source and target forwarders are the same,
so no more action is necessary. Go back to step 2.
5. As the source and target forwarders both reside on the local
PE, no pseudowire is needed. The PE simply creates a local
cross-connect between the two forwarders. Go back to step 2.
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6. As the source and target forwarders reside on different PEs,
a pseudowire must be established between them. The PE first
examines if the source forwarder has already established a
pseudowire to the target forwarder. If so, go back to step 2.
7. If no pseudowire is already established between the source and
target forwarders, the local PE obtains the address of the
remote PE, and establishes a control connection to the remote
PE if one does not already exist.
8. The local PE sends an ICRQ to the remote PE. The AGI, TAII,
and SAII values are encoded in the AGI AVP, the Remote End ID
AVP, and the Local End ID AVP respectively.
9. If the local PE receives a response corresponding to the
ICRQ it just sent, proceed to step 10. Otherwise, if the
local PE receives an ICRQ from the same remote PE, proceed
to step 11.
10. The local PE receives a response from the remote PE. If
it is a CDN, go back to step 2. If it's an ICRP, the local
PE binds the source forwarder to the pseudowire and sends
an ICCN to the remote PE. Go back to step 2.
11. If the local PE receives an ICRQ from the same remote PE,
it needs to perform session tie detection, as described in
Section 5.2. If a session tie is detected, the PE performs
tie breaking.
12. If the local PE loses the tie breaker, it sends a CDN with
the result code that indicates the disconnection is due to
losing the tie breaker. Proceed to step 14.
13. If the local PE wins the tie breaker, it ignores the remote
PE's ICRQ, but acknowledges receipt of the control message
and continues waiting for the response from the remote PE.
Go to step 10.
14. The local PE determines whether it should accept the
connection request, as described in the Section 5.1.
If it accepts the ICRQ, it sends an ICRP to the remote PE.
15. The local PE receives a response from the remote PE. If
it is a CDN, go back to step 2. If it is an ICCN, the local
PE binds the source forwarder to the pseudowire, go back
to step 2.
The following diagram illustrates the above procedure:
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---------> Pick Next
| Source Forwarder
| |
| |
| v N
| Found Source Forwarder? ----------> End
| |
| Y |
| v
| Pick Next <--------------------------------
| Target Forwarder |
| | |
| | |
| N v |
-------- Found Target Forwarder? |
| |
Y | |
v Y Y |
Same Router ID? ------> Same Forwarder ID? ------|
| | |
N | N | |
| v |
| Create Local -------|
v Cross-connect |
Pseudowire Already Y |
Established Between -------------------------------|
Source and Target? |
| |
N | |
v |
Local Initiates Pseudowire |
Connection Request to Remote |
| |
| |
v |
-------> Local Wait for Message |
| ----- from Remote -------------- |
| | | |
| | | |
| v v |
| Local Receives Pseudowire Local Receives Pseudowire |
| Connection Request Connection Response |
| from Remote from Remote |
| | | |
| | | |
| v v N |
| Perform Pseudowire Connection Accepted? --------|
| Tie Detection | |
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| | Y | |
| | v |
| | Local Binds Source ---------|
| | Forwarder to Pseudowire |
| | |
| v N N |
| Tie Detected? -----> Accept Remote -----> Reject ------|
| | Connection Request? Remote Request |
| Y | ^ | |
| v | | Y |
| Perform Tie Breaking | ------> Local Binds ----
| | | Source Forwarder
| | | to Pseudowire
| v N |
| Won Tie Breaking? ------> Disconnect
| | Local Connection
| Y |
| v
------ Ignore Remote
Connection Request
6. IANA Considerations
This document defines two new L2TP AVPs to be maintained by the IANA.
7. Security Considerations
The signaling procedures described in this document does not incur
additional security considerations that L2TP already provisions.
8. References
[L2TPv3] J. Lau et. al., "Layer Two Tunneling Protocol (version3)",
draft-ietf-l2tpext-l2tp-base-04.txt, November 2002
[L2VPN FW] L. Andersson et. al., "PPVPN L2 Framework",
draft-ietf-l2vpn-l2-framework-03.txt, October 2003
[PWE3L2TP] W. Townsley, "Pseudowires and L2TPv3",
draft-townsley-pwe3-l2tpv3-00.txt, June 2002
[RFC 2661] W. Townsley et. al., "Layer 2 Tunnel Protocol (L2TP)",
RFC 2661, August 1999.
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9. Authors' Address
Wei Luo
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
Email: luo@cisco.com
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