One document matched: draft-ietf-l2tpext-pwe3-fr-03.txt
Differences from draft-ietf-l2tpext-pwe3-fr-02.txt
Network Working Group W. Mark Townsley
Internet-Draft George Wilkie
Category: Standards Track Skip Booth
<draft-ietf-l2tpext-pwe3-fr-03.txt> Jed Lau
March 2004 Stewart Bryant
cisco Systems
Frame-Relay over L2TPv3
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
The Layer 2 Tunneling Protocol, Version 3, (L2TPv3) defines a
protocol for tunneling a variety of data link protocols over IP
networks. This document describes the specifics of how to tunnel
Frame-Relay over L2TPv3, including frame encapsulation, virtual-
circuit creation, deletion, and line status change notification.
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Contents
Status of this Memo.......................................... 1
1. Introduction.............................................. 2
1.1 Abbreviations......................................... 3
2. Control Connection Establishment.......................... 3
3. PVC Status Notification and Session Establishment......... 3
3.1 L2TPv3 Session Establishment.......................... 3
3.2 L2TPv3 Session Teardown............................... 5
3.3 L2TPv3 Session Maintenance............................ 5
3.4 Use of the Circuit Status AVP for Frame-Relay......... 6
4. Encapsulation............................................. 6
4.1 Data Packet Encapsulation............................. 6
4.2 Data Packet Sequencing................................ 7
5. Security Considerations................................... 8
6. IANA Considerations....................................... 8
7. Acknowledgments........................................... 8
8. References................................................ 8
8.1 Normative References.................................. 8
8.2 Informative References................................ 8
9. Contacts.................................................. 9
Specification of Requirements
In this document, several words are used to signify the requirements
of the specification. These words are often capitalized. 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 [RFC2119].
1. Introduction
[L2TPv3] defines a base protocol for Layer 2 Tunneling over IP
networks. This document defines the specifics necessary for tunneling
Frame-Relay over L2TPv3. Such emulated circuits are referred to as
Frame-Relay Pseudowires (FRPWs).
Protocol specifics defined in this document for L2TPv3 FRPWs include
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those necessary for frame encapsulation, PVC creation, deletion, and
status change notification. Support for Switched Virtual Circuits
(SVCs) and Switched/soft Permanent Virtual Circuits (SPVCs) are
outside the scope of this document.
The reader is expected to be very familiar with the terminology and
protocol constructs defined in [L2TPv3].
1.1 Abbreviations
FR Frame-Relay
FRPW Frame-Relay Pseudo-Wire
LCCE L2TP Control Connection Endpoint (See [L2TPv3])
PVC Permanent virtual circuit
PW Pseudo-Wire
VC Virtual circuit
2. Control Connection Establishment
In order to tunnel a Frame-Relay circuit over IP using L2TPv3, an
L2TPv3 Control Connection MUST first be established as described in
[L2TPv3]. The L2TPv3 SCCRQ Control Message and corresponding SCCRP
Control Message MUST include the Frame-Relay PW Type of TBD1 (See
IANA Considerations Section), in the Pseudo Wire Capabilities List as
defined in 5.4.3 of [L2TPv3]. This identifies the control connection
as able to establish L2TP sessions to support Frame-Relay Pseudo-
Wires (FRPWs).
An LCCE MUST be able to uniquely identify itself in the SCCRQ and
SCCRP messages via a globally unique value. By default, this is
advertised via the structured Router ID AVP [L2TPv3], though the
unstructured Hostname AVP [L2TPv3] MAY be used if both endpoints
support an application (as defined by the Application Code AVP
[L2TPv3]) to identify LCCEs via this value.
3. PVC Status Notification and Session Establishment
This section specifies how the status of a PVC is reported between
two LCCEs. This includes what should happen when a PVC is created,
deleted or when it changes state between ACTIVE and INACTIVE.
3.1 L2TPv3 Session Establishment
PVC creation (provisioning) results in establishment of an L2TP
session via the standard three-way handshake described in section
3.4.1 of [L2TPv3]. An LCCE MAY initiate the session immediately upon
PVC creation, or wait until the PVC state transitions to ACTIVE
before attempting to establish a session for the PVC. Waiting until
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the PVC transitions to ACTIVE may be preferred as it delays
allocation of L2TP resources until absolutely necessary.
The Circuit Status AVP (see Section 4) MUST be present in the ICRQ
and ICRP messages, and MAY be present in the SLI message for FRPWs.
Following is an example of the L2TP messages exchanged for an FRPW
which is initiated after a new PVC is provisioned and becomes ACTIVE.
LCCE (LAC) A LCCE (LAC) B
------------------ ------------------
FR PVC Provisioned
FR PVC Provisioned
FR PVC ACTIVE
ICRQ (status = 0x03) ---->
FR PVC ACTIVE
<---- ICRP (status = 0x03)
L2TP session established,
OK to send data into tunnel
ICCN ----->
L2TP session established,
OK to send data into tunnel
In the example above, an ICRQ is sent after the PVC is created and
becomes ACTIVE. The Circuit Status AVP indicates that this PVC is
ACTIVE and New (0x03). The Remote End ID AVP [L2TPv3] must be present
in the ICRQ in order to identify the PVC (together with the identity
of the LCCE itself as defined in section 2) to associate the L2TP
session with. The Remote End ID AVP defined in [L2TPv3] is of opaque
form and variable length, though one MUST at a minimum support use of
an unstructured four-octet value that is known to both LCCEs (either
by direct configuration, or some other means). The exact method of
how this value is configured, retrieved, discovered, or otherwise
determined at each LCCE is outside the scope of this document.
As with the ICRQ, the ICRP is sent only after the FR PVC transitions
to ACTIVE as well. If LCCE B had not been provisioned for the PVC
identified in the ICRQ, a CDN would have been immediately returned
indicating that the circuit was not provisioned or available at this
LCCE. LCCE A should then exhibit a periodic retry mechanism. The
period and maximum number of retries MUST be configurable.
An Implementation MAY send an ICRQ or ICRP before a PVC is ACTIVE, as
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long as the Circuit Status AVP reflects that the PVC is INACTIVE and
an SLI is sent when the PVC becomes ACTIVE (see Section 3.3).
The ICCN is the final stage in the session establishment, confirming
the receipt of the ICRP with acceptable parameters to allow
bidirectional traffic.
3.2 L2TPv3 Session Teardown
In the event a PVC is deleted (unprovisioned) at either LCCE, the
associated L2TP session MUST be torn down via the CDN message defined
in Section 3.4.3 of [L2TPv3].
General Result Codes regarding L2TP session establishment are defined
in [L2TPv3]. Additional Frame-Relay result codes are defined as
follows:
TBD2: FR PVC was deleted permanently (no longer provisioned)
TBD3: FR PVC has been INACTIVE for an extended period of time
3.3 L2TPv3 Session Maintenance
FRPW over L2TP makes use of the Set Link Info (SLI) control message
defined in [L2TPv3] to signal Frame-Relay link status notifications
between LCCEs. This includes ACTIVE or INACTIVE notifications of the
VC, or any other parameters that may need to be shared between the
tunnel endpoints or LCCEs in order to provide proper PW emulation.
The SLI message is a single message that is sent over the L2TP
control channel signaling the state change. Since the message is
delivered reliably, there is no additional response or action
required of the PW subsytem to ensure that the state change
notification was received by the tunnel peer.
The SLI message MUST be sent any time there is a circuit status
change which may be reported by any values identified in the Circuit
Status AVP. The only exception to this is the initial ICRQ, ICRP and
CDN messages which establish and teardown the L2TP session itself
when the PVC is created or deleted. The SLI message may be sent from
either LCCE at any time after the first ICRQ is sent (and perhaps
before an ICRP is received, requiring the peer to perform a reverse
Session ID lookup).
All sessions established by a given control connection utilize the
L2TP Hello factility defined in Section 4.4 of [L2TPv3] for session
keepalive. This gives all sessions basic dead peer and path detection
between LCCEs.
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3.4 Use of the Circuit Status AVP for Frame-Relay
Frame-relay circuit status is reported via the Circuit Status AVP
defined in [L2TPv3]. For reference, this AVP is shown below:
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |A|N|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Value is a 16 bit mask with the two least significant bits
defined and the remaining bits reserved for future use. Reserved bits
MUST be set to 0 when sending, and ignored upon receipt.
The A (Active) bit indicates whether the FR PVC is ACTIVE (1) or
INACTIVE (0).
The N (New) bit indicates whether the circuit status indication is
for a new FR PVC (1) or an existing FR PVC (0).
4. Encapsulation
4.1 Data Packet Encapsulation
The FR PDU is transported in its entirety, excluding the opening and
closing HDLC flags and the FCS. Bit stuffing is undone. The L2TPv3
Session Header is that as defined in [L2TPv3]. If sequencing or other
features require presence of an L2-Specific Sublayer, the Default
format defined in section 4.6 of [L2TPv3] MUST be used.
The FR header is defined in [Q922], however the notation used differs
from that used in IETF specifications. For reference the FR header in
IETF notation is:
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| hi dlci |C|0|lo dlci|F|B|D|1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Two-octet FR Header
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| hi dlci |C|0| dlci |F|B|D|0| dlci |0| dlci_lo |0|1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Four-octet FR Header
C/R (bit 6)
FR frame C/R (command/response) bit [Q922].
F - FECN (bit 12):
FR FECN (Forward Explicit Congestion Notification) bit [Q922].
B - BECN (bit 13):
FR BECN (Backward Explicit Congestion Notification) bit [Q922].
D - DE (bit 14)
FR DE bit indicates the discard eligibility [Q922].
Usage of the C/R, FECN, BECN and DE bits is as specified in [Q922].
The C/R bit is conveyed transparently. Its value MUST NOT be changed
by the LCCE.
The FECN bit MAY be set by the LCCE to notify the receiving end-user
that the frames it recieves have encountered congestion. The end-user
may use this indication for destination controlled transmit rate
adjustment. The bit must never be cleared by the LCCE. If the LCCE
does not support FECN it shall pass the bit unchanged.
The BECN bit MAY be set by the LCCE to notify the receiving end-user
that frames it transmits may encounter congestion. The end-user may
use this indication to adjust its transmit rate. The bit must never
be cleared by the LCCE. If the LCCE does not support BECN it shall
pass the bit unchanged.
The DE bit MAY be set by a policing function on the LCCE to indicate
that this frame SHOULD be discarded in preference to other frames in
a congestion situation. The bit must never be cleared by the LCCE. If
the LCCE does not support DE it shall pass the bit unchanged.
4.2 Data Packet Sequencing
Data Packet Sequencing MAY be enabled for FRPWs. The sequencing
mechanisms described in [L2TPv3] MUST be used for signaling
sequencing support. FRPW over L2TP MUST request the presence of the
L2TPv3 Default L2-Specific Sublayer when sequencing is enabled, and
MAY request its presence at all times.
If the FRPW is known to be carrying data which does not require
packet order to be strictly maintained (such as IP), then packet
sequencing for the FRPW SHOULD NOT be enabled.
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5. Security Considerations
Frame Relay over L2TPv3 is subject to the security considerations
defined in [L2TPv3]. There are no additional considerations specific
to carrying Frame Relay that are not present carrying other data link
types.
6. IANA Considerations
The following value needs assignment by IANA (number space should be
created as part of publication of [L2TPv3]):
TBD1 - Frame Relay Pseudowire Type (see Pseudo Wire Capabilities
List as defined in 5.4.3 of [L2TPv3]).
Two new L2TP Result Codes appear in section 3.2 which need assignment
by IANA as described in section 9.1 of [BCP0068].
TBD2 - PVC was deleted permanently (no longer provisioned)
TBD3 - PVC has been INACTIVE for an extended period of time
7. Acknowledgments
The first Frame Relay over L2TP document was published as "Frame
Relay Service Type for L2TP," draft-vasavada-l2tpext-fr-svctype-
00.txt in Feburary of 2001 by Nishit Vasavada, Jim Boyle, Chris
Garner, Serge Maskalik, and Vijay Gill. This document is
substantially different, but the basic concept of carrying Frame
Relay over L2TP is the same.
Thanks to Lloyd Wood for a razor-sharp review.
8. References
8.1 Normative References
[L2TPv3] J. Lau, M. Townsley, I. Goyret, "Layer Two Tunneling
Protocol (Version 3)", work in progress,
draft-ietf-l2tpext-l2tp-base-10.txt, August 2003.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
8.2 Informative References
[BCP0068] Townsley, W., Layer Two Tunneling Protocol (L2TP) Internet
Assigned Numbers Authority (IANA) Considerations Update",
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RFC3438, BCP0068, December 2002
[Q922] ITU-T Recommendation Q.922, ISDN Data Link Layer
Specification for Frame Mode Bearer Services, ITU, Geneva, 1992.
9. Contacts
W. Mark Townsley
cisco Systems
7025 Kit Creek Road
PO Box 14987
Research Triangle Park, NC 27709
mark@townsley.net
George Wilkie
cisco Systems
96 Commercial Street
Edinburgh, EH6 6LX
United Kingdom
gwilkie@cisco.com
Jed Lau
cisco Systems
170 W. Tasman Drive
San Jose, CA 95134
jedlau@cisco.com
Skip Booth
cisco Systems
7025 Kit Creek Road
PO Box 14987
Research Triangle Park, NC 27709
ebooth@cisco.com
Stewart Bryant
cisco Systems
Uxbridge UB11 1BL
United Kingdom
stbryant@cisco.com
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