Network Working Group W. Mark Townsley
Internet-Draft George Wilkie
Category: Standards Track Skip Booth
Expiration Date: March 2006 Stewart Bryant
Cisco Systems
Jed Lau
September 2005
Frame-Relay over L2TPv3
draft-ietf-l2tpext-pwe3-fr-07.txt
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2005). 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 status change notification.
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Contents
Status of this Memo.......................................... 1
1. Introduction.............................................. 3
1.1 Abbreviations......................................... 3
2. Control Connection Establishment.......................... 3
3. PVC Status Notification and Session Establishment......... 4
3.1 L2TPv3 Session Establishment.......................... 4
3.2 L2TPv3 Session Teardown............................... 6
3.3 L2TPv3 Session Maintenance............................ 6
3.4 Use of the Circuit Status AVP for Frame-Relay......... 7
3.5 Frame-Relay Header Length AVP......................... 7
4. Encapsulation............................................. 8
4.1 Data Packet Encapsulation............................. 8
4.2 Data Packet Sequencing................................ 9
4.3 MTU Considerations.................................... 10
5. Applicability Statement................................... 10
6. Security Considerations................................... 11
7. IANA Considerations....................................... 11
7.1 Pseudowire Type....................................... 11
7.2 Result Code AVP Values................................ 11
7.3 Control Message Attribute Value Pairs (AVPs).......... 12
8. Acknowledgments........................................... 12
9. References................................................ 12
9.1 Normative References.................................. 12
9.2 Informative References................................ 12
10. Authors' Addresses....................................... 13
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].
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1. Introduction
[RFC3931] 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
operating in a "virtual circuit to virtual circuit" mode include
those necessary for frame encapsulation, PVC creation, deletion, and
status change notification. Frame-Relay traffic may also be
transported in a "port to port" or "interface to interface" fashion
using HDLC Pseudowires as defined in [HDLC]. 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 [RFC3931].
1.1 Abbreviations
FR Frame-Relay
FRPW Frame-Relay Pseudowire
LCCE L2TP Control Connection Endpoint (See [RFC3931])
PVC Permanent virtual circuit
PW Pseudowire
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
[RFC3931]. The L2TPv3 SCCRQ Control Message and corresponding SCCRP
Control Message MUST include the Frame-Relay DLCI PW Type of 0x0001
(See IANA Considerations Section), in the Pseudowire Capabilities
List as defined in 5.4.3 of [RFC3931]. This identifies the control
connection as able to establish L2TP sessions to support Frame-Relay
Pseudowires (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 [RFC3931], though the
unstructured Hostname AVP [RFC3931] MAY be used to identify LCCEs as
well.
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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. When
emulating a Frame-Relay service, if the procedures for PVC status
management defined in [Q933] Annex A are being used between an LCCE
and the attached Remote System, an LCCE MUST participate in them (see
Section 3.3).
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 [RFC3931]. 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
the PVC transitions to ACTIVE may be preferred as it delays
allocation of L2TP resources until absolutely necessary.
The Pseudowire Type AVP defined in Section 5.4.4 of [RFC3931],
Attribute Type 68, MUST be present in the ICRQ messages and MUST
include the Frame-Relay DLCI PW Type of 0x0001 for FRPWs.
The Circuit Status AVP (see Section 3.4) MUST be present in the ICRQ
and ICRP messages, and MAY be present in the SLI message for FRPWs.
The Frame-Relay Header Length AVP (see Section 3.5) MAY be present in
the ICRQ and ICRP messages.
Following is an example of the L2TP messages exchanged for an FRPW
which is initiated after a new PVC is provisioned and becomes ACTIVE.
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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 [RFC3931] 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 [RFC3931] 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. If so,
the period and maximum number of retries MUST be configurable.
An Implementation MAY send an ICRQ or ICRP before a PVC is ACTIVE, as
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.
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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 [RFC3931].
General Result Codes regarding L2TP session establishment are defined
in [RFC3931]. Additional Frame-Relay result codes are defined as
follows:
17: FR PVC was deleted permanently (no longer provisioned)
18: FR PVC has been INACTIVE for an extended period of time
19: Mismatched FR Header Length
3.3 L2TPv3 Session Maintenance
FRPW over L2TP makes use of the Set Link Info (SLI) control message
defined in [RFC3931] 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 are 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).
An LCCE participating in the procedures for PVC status management
defined in [Q933] Annex A, MUST transmit an SLI message including the
Circuit Status AVP (see Section 3.4) when it detects a change in the
status for a particular local FR PVC (i.e., when it detects a
service-affecting condition or the clearing of such condition). An
LCCE receiving an SLI message indicating a change in the status of a
particular FRPW SHOULD generate corresponding updates for the FR PVC
towards the Remote System as defined in [Q933] Annex A.
All sessions established by a given control connection utilize the
L2TP Hello facility defined in Section 4.4 of [RFC3931] for session
keepalive. This gives all sessions basic dead peer and path detection
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between LCCEs.
3.4 Use of the Circuit Status AVP for Frame-Relay
Frame-relay circuit status is reported via the Circuit Status AVP
defined in [RFC3931], Attribute Type 71. 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 |N|A|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 N (New) bit indicates whether the Circuit Status indication is
for a new FR PVC (1) or an existing FR PVC (0).
The A (Active) bit indicates whether the FR PVC is ACTIVE (1) or
INACTIVE (0).
3.5 Frame-Relay Header Length AVP
The "Frame-Relay Header Length AVP", Attribute type 85, indicates the
number of bytes in the Frame Relay header. The two peer LCCEs MUST
agree on the length of the Frame Relay header.
This AVP is exchanged during session negotiation (in ICRQ, ICRP). If
the other LCCE supports a different Frame Relay header length, the
associated L2TP session MUST be torn down via CDN message with result
code 19 (see Section 3.2).
If the Frame-Relay Header Length AVP is not signaled, it MUST be
assumed that the peer uses a 2-byte Frame Relay header.
The Attribute Value field for this AVP has the following format:
Frame-Relay Header Length (ICRQ, ICRP)
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Frame Relay Header Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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The Frame Relay Header Length Type is a 2-octet unsigned integer with
the following values defined in this document:
2 - Two-octet Frame Relay Header
4 - Four-octet Frame Relay Header
This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for this
AVP MAY be set to 0, but MAY vary (see Section 5.2 of [RFC3931]).
The length (before hiding) of this AVP is 8.
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 [RFC3931]. If sequencing or
other features require presence of an L2-Specific Sublayer, the
Default format defined in Section 4.6 of [RFC3931] 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
(referred to as Address Field in [Q922]) 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|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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):
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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.
The encapsulation of Frame Relay frames with Two-octet FR Header is
REQUIRED. The encapsulation of Frame Relay frames with Four-octet FR
Header is OPTIONAL. The encapsulation of Frame Relay frames with
Three-octet FR Header is ouside the scope of this document.
4.2 Data Packet Sequencing
Data Packet Sequencing MAY be enabled for FRPWs. The sequencing
mechanisms described in [RFC3931] 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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4.3 MTU Considerations
With L2TPv3 as the tunneling protocol, the packet resulted from the
encapsulation is N bytes longer than Frame-Relay frame without the
opening and closing HDLC flags or FCS. The value of N depends on the
following fields:
L2TP Session Header:
Flags, Ver, Res - 4 octets (L2TPv3 over UDP only)
Session ID - 4 octets
Cookie Size - 0, 4 or 8 octets
L2-Specific Sublayer - 0 or 4 octets (i.e., using sequencing)
Hence the range for N in octets is:
N = 4-16, L2TPv3 data messages are over IP;
N = 16-28, L2TPv3 data messages are over UDP;
(N does not include the IP header).
The MTU and fragmentation implications resulting from this are
discussed in Section 4.1.4 of [RFC3931].
5. Applicability Statement
The Frame Relay PW emulation described in this document allows a
service provider to offer a Frame Relay PVC based service across an
IP packet switched network (PSN). A Frame Relay port based service
can be offered using [HDLC].
The FRPW emulation has the following characteristics in relationship
to the native service:
o There is a one-to-one mapping between a Frame Relay PVC and an
FRPW, supporting bi-directional transport of variable length
frames. The Frame Relay frame is transported in its entirety,
including the DLCI and the C/R, FECN, BECN, and DE bits, but
excluding the opening and closing flags and the FCS. The egress
LCCE re-writes the DLCI and regenerates the FCS.
o Two and Four octet address fields are supported. The length is
negotiated between LCCEs during session establishment (see
Section 3.5).
o The availability or unavailability of the PVC is signalled
between LCCEs using the Circuit Status AVP (see Section 3.4).
Loss of connectivity between LCCEs can be detected by the L2TPv3
keepalive mechanism (see Section 4.4 in [RFC3931]). These
indications can be used to determine the PVC status to be
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signalled through [Q933] procedures at the Frame Relay
interface.
o The maximum frame size that can be supported is limited by the
PSN MTU, unless fragmentation and reassembly is used (see
Section 4.1.4 of [RFC3931]).
o Sequencing may be enabled on the FRPW to ensure frames are
delivered in order (see Section 4.2)
o Quality of Service characteristics such as throughput (CIR),
committed burst size (bc), excess burst size (be) and priority
can be provided by leveraging Quality of Service features of the
LCCEs and the underlying PSN.
6. Security Considerations
Frame Relay over L2TPv3 is subject to the security considerations
defined in [RFC3931]. There are no additional considerations specific
to carrying Frame Relay that are not present carrying other data link
types.
7. IANA Considerations
7.1 Pseudowire Type
The following value for the Frame Relay DLCI PW Type (see Pseudowire
Capabilities List as defined in 5.4.3 of [RFC3931] and L2TPv3
Pseudowire Types in 10.6 of [RFC3931]) is allocated by the IANA
(number space already created as part of publication of [RFC3931]):
L2TPv3 Pseudowire Types
-----------------------
0x0001 - Frame Relay DLCI Pseudowire Type
7.2 Result Code AVP Values
This number space is managed by IANA as described in section 2.3 of
[BCP0068]. Three new L2TP Result Codes for the CDN message appear in
section 3.2. The following is a summary:
Result Code AVP (Attribute Type 1) Values
-----------------------------------------
17 - PVC was deleted permanently (no longer provisioned)
18 - PVC has been INACTIVE for an extended period of time
19 - Mismatched FR Header Length
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7.3 Control Message Attribute Value Pairs (AVPs)
This number space is managed by IANA as described in section 2.2 of
[BCP0068]. An additional AVP Attribute, specified in section 3.5,
was allocated for this specification:
Control Message Attribute Value Pairs
-------------------------------------
85 - Frame-Relay Header Length
8. Acknowledgments
The first Frame Relay over L2TP document was published as
draft-vasavada-l2tpext-fr-svctype-00.txt, "Frame Relay Service Type
for L2TP", 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.
Carlos Pignataro helped with review and editing of the document.
During IETF Last Call, Mark Lewis provided thorough review and
comments.
9. References
9.1 Normative References
[RFC3931] J. Lau, M. Townsley, I. Goyret, "Layer Two Tunneling
Protocol - Version 3 (L2TPv3)", RFC 3931, March 2005.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[HDLC] C. Pignataro, M. Townsley, "HDLC Frames over L2TPv3",
work in progress, draft-ietf-l2tpext-pwe3-hdlc-06.txt,
June 2005.
9.2 Informative References
[BCP0068] Townsley, W., "Layer Two Tunneling Protocol (L2TP)
Internet Assigned Numbers Authority (IANA)
Considerations Update", RFC3438, BCP0068, December 2002
[Q922] ITU-T Recommendation Q.922, "ISDN Data Link Layer
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Specification for Frame Mode Bearer Services", ITU,
Geneva, 1992.
[Q933] ITU-T Recommendation Q.933, "Signalling specifications
for frame mode switched and permanent virtual connection
control and status monitoring", ITU, Geneva, 2003.
10. Authors' Addresses
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
jedlau@gmail.com
Skip Booth
Cisco Systems
7025 Kit Creek Road
PO Box 14987
Research Triangle Park, NC 27709
ebooth@cisco.com
Stewart Bryant
Cisco Systems
250 Longwater Ave
Green Park
Reading RG2 6GB
United Kingdom
stbryant@cisco.com
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Copyright Statement
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This document is subject to the rights, licenses and restrictions
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Acknowledgment
Funding for the RFC Editor function is currently provided by the
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Townsley, et al. Standards Track [Page 14]
