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IETF RFC 4349
High-Level Data Link Control (HDLC) Frames over Layer 2 Tunneling Protocol, Version 3 (L2TPv3)
Last modified on Monday, February 13th, 2006
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Network Working Group C. Pignataro
Request for Comments: 4349 M. Townsley
Category: Standards Track Cisco Systems
February 2006
High-Level Data Link Control (HDLC) Frames
over Layer 2 Tunneling Protocol, Version 3 (L2TPv3)
 Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright © The Internet Society (2006).
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
High-Level Data Link Control (HDLC) frames over L2TPv3.
Pignataro & Townsley Standards Track PAGE 1 
RFC 4349 HDLC Frames over L2TPv3 February 2006
Table of Contents
1. Introduction ....................................................2
1.1. Abbreviations ..............................................2
1.2. Specification of Requirements ..............................3
2. Control Connection Establishment ................................3
3. HDLC Link 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 Circuit Status AVP for HDLC .........................6
4. Encapsulation ...................................................6
4.1. Data Packet Encapsulation ..................................6
4.2. Data Packet Sequencing .....................................7
4.3. MTU Considerations .........................................7
5. Applicability Statement .........................................8
6. Security Considerations .........................................9
7. IANA Considerations .............................................9
7.1. Pseudowire Type ............................................9
7.2. Result Code AVP Values .....................................9
8. Acknowledgements ................................................9
9. References .....................................................10
9.1. Normative References ......................................10
9.2. Informative References ....................................10
1. Introduction
[RFC 3931] defines a base protocol for Layer 2 Tunneling over IP
networks. This document defines the specifics necessary for
tunneling HDLC Frames over L2TPv3. Such emulated circuits are
referred to as HDLC Pseudowires (HDLCPWs).
Protocol specifics defined in this document for L2TPv3 HDLCPWs
include those necessary for simple point-to-point (e.g., between two
L2TPv3 nodes) frame encapsulation, and for simple interface up and
interface down notifications.
The reader is expected to be very familiar with the terminology and
protocol constructs defined in [RFC 3931].
1.1 Abbreviations
HDLC High-Level Data Link Control
HDLCPW HDLC Pseudowire
LAC L2TP Access Concentrator (see [RFC 3931])
LCCE L2TP Control Connection Endpoint (see [RFC 3931])
PW Pseudowire
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RFC 4349 HDLC Frames over L2TPv3 February 2006
1.2. 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 [RFC 2119].
2. Control Connection Establishment
In order to tunnel an HDLC link over IP using L2TPv3, an L2TPv3
Control Connection MUST first be established as described in
[RFC 3931]. The L2TPv3 SCCRQ Control Message and corresponding SCCRP
Control Message MUST include the HDLC Pseudowire Type of 0x0006 (see
Section 7, "IANA Considerations"), in the Pseudowire Capabilities
List as defined in 5.4.3 of [RFC 3931]. This identifies the control
connection as able to establish L2TP sessions to support HDLC
Pseudowires (HDLCPWs).
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 [RFC 3931], though the
unstructured Hostname AVP [RFC 3931] MAY be used to identify LCCEs as
well.
3. HDLC Link Status Notification and Session Establishment
This section specifies how the status of an HDLC interface is
reported between two LCCEs, and the associated L2TP session creation
and deletion that occurs.
3.1. L2TPv3 Session Establishment
Associating an HDLC serial interface with a PW and its transition to
"Ready" or "Up" results in the establishment of an L2TP session via
the standard three-way handshake described in Section 3.4.1 of
[RFC 3931]. For purposes of this discussion, the action of locally
associating an interface running HDLC with a PW by local
configuration or otherwise is referred to as "provisioning" the HDLC
interface. The transition of the interface to "ready" or "up" will
be referred to as the interface becoming ACTIVE. The transition of
the interface to "not-ready" or "down" will be referred to as the
interface becoming INACTIVE.
Pignataro & Townsley Standards Track PAGE 3
RFC 4349 HDLC Frames over L2TPv3 February 2006
An LCCE MAY initiate the session immediately upon association with an
HDLC interface or wait until the interface becomes ACTIVE before
attempting to establish an L2TP session. Waiting until the interface
transitions to ACTIVE may be preferred, as it delays allocation of
resources until absolutely necessary.
The Pseudowire Type AVP defined in Section 5.4.4 of [RFC 3931],
Attribute Type 68, MUST be present in the ICRQ messages and MUST
include the Pseudowire Type of 0x0006 for HDLCPWs.
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 HDLCPWs.
Following is an example of the L2TP messages exchanged for an HDLCPW
that is initiated after an HDLC interface is provisioned and becomes
ACTIVE.
LCCE (LAC) A LCCE (LAC) B
------------------ ------------------
HDLC Interface Provisioned
HDLC Interface Provisioned
HDLC Interface ACTIVE
ICRQ (status = 0x03) ---->
HDLC Interface 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 interface is
provisioned and becomes ACTIVE. The Circuit Status AVP indicates
that this link is ACTIVE and New (0x03). The Remote End ID AVP
[RFC 3931] MUST be present in the ICRQ in order to identify the HDLC
link (together with the identity of the LCCE itself as defined in
Section 2) with which to associate the L2TP session. The Remote End
ID AVP defined in [RFC 3931] 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.
Pignataro & Townsley Standards Track PAGE 4
RFC 4349 HDLC Frames over L2TPv3 February 2006
As with the ICRQ, the ICRP is sent only after the associated HDLC
interface transitions to ACTIVE as well. If LCCE B had not been
provisioned for the interface identified in the ICRQ, a CDN would
have been immediately returned indicating that the associated link
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 an HDLC interface
is ACTIVE, as long as the Circuit Status AVP reflects that the link
is INACTIVE and an SLI is sent when the HDLC interface 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 link is removed (unprovisioned) at either LCCE, the
associated L2TP session MUST be torn down via the CDN message defined
in Section 3.4.3 of [RFC 3931].
General Result Codes regarding L2TP session establishment are defined
in [RFC 3931]. Additional HDLC result codes are defined as follows:
20 - HDLC Link was deleted permanently (no longer provisioned)
21 - HDLC Link has been INACTIVE for an extended period of time
3.3. L2TPv3 Session Maintenance
HDLCPWs over L2TP make use of the Set Link Info (SLI) control message
defined in [RFC 3931] to signal HDLC link status notifications between
PEs. The SLI message is a single message that is sent over the L2TP
control channel, signaling the interface state change.
The SLI message MUST be sent any time there is a status change of any
values identified in the Circuit Status AVP. The only exceptions to
this are the initial ICRQ, ICRP, and CDN messages, which establish
and teardown the L2TP session itself. The SLI message may be sent
from either PE 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 facility defined in Section 4.4 of [RFC 3931] for session
keepalive. This gives all sessions basic dead peer and path
detection between PEs.
Pignataro & Townsley Standards Track PAGE 5
RFC 4349 HDLC Frames over L2TPv3 February 2006
3.4. Use of Circuit Status AVP for HDLC
HDLC reports Circuit Status with the Circuit Status AVP defined in
[RFC 3931], 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 SHOULD be set to one (1) if the Circuit Status
indication is for a new HDLC circuit; to zero (0) otherwise.
The A (Active) bit indicates whether the HDLC interface is ACTIVE (1)
or INACTIVE (0).
4. Encapsulation
4.1. Data Packet Encapsulation
HDLCPWs use the default encapsulations defined in [RFC 3931] for
demultiplexing, sequencing, and flags. The HDLCPW Type over L2TP is
intended to operate in an "interface to interface" or "port to port"
fashion, passing all HDLC data and control PDUs over the PW. The
HDLC PDU is stripped of flags and trailing FCS, bit/byte unstuffing
is performed, and the remaining data, including the address, control,
and protocol fields, is transported over the PW.
Since all packets are passed in a largely transparent manner over the
HDLCPW, any protocol that has HDLC-like framing may utilize the
HDLCPW mode, including PPP, Frame-Relay ("port to port" Frame-Relay
transport), X.25 (LAPB), etc. In such cases, the negotiations and
signaling of the specific protocols transported over the HDLCPW take
place between the Remote Systems. A non-exhaustive list of examples
and considerations of this transparent nature include:
o When the HDLCPW transports Point-to-Point Protocol (PPP)
traffic, PPP negotiations (Link Control Protocol, optional
authentication, and Network Control Protocols) are performed
between Remote Systems, and LCCEs do not participate in these
negotiations.
Pignataro & Townsley Standards Track PAGE 6
RFC 4349 HDLC Frames over L2TPv3 February 2006
o When the HDLCPW transports Frame-Relay traffic, PVC status
management procedures (Local Management Interface) take place
between Remote Systems, and LCCEs do not participate in LMI.
Additionally, individual Frame-Relay virtual-circuits are not
visible to the LCCEs, and the FECN, BECN, and DE bits are
transported transparently.
o When the HDLCPW transports X.25 (LAPB) traffic, LCCEs do not
function as either LAPB DCE or DTE devices.
On the other hand, exceptions include cases where direct access to
the HDLC interface is required, or modes that operate on the flags,
FCS, or bit/byte unstuffing that is performed before sending the HDLC
PDU over the PW. An example of this is PPP ACCM negotiation.
4.2. Data Packet Sequencing
Data Packet Sequencing MAY be enabled for HDLCPWs. The sequencing
mechanisms described in Section 4.6.1 of [RFC 3931] MUST be used for
signaling sequencing support. HDLCPWs over L2TP MUST request the
presence of the L2TPv3 Default L2-Specific Sublayer defined in
Section 4.6 of [RFC 3931] when sequencing is enabled, and MAY request
its presence at all times.
4.3. MTU Considerations
With L2TPv3 as the tunneling protocol, the packet resulting from the
encapsulation is N bytes longer than the HDLC frame without the 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 [RFC 3931].
Pignataro & Townsley Standards Track PAGE 7
RFC 4349 HDLC Frames over L2TPv3 February 2006
5. Applicability Statement
HDLC Pseudowires support a "port to port" or "interface to interface"
deployment model operating in a point-to-point fashion. In addition
to the transport of HDLC frames, a natural application of HDLCPWs
allows for the transport of any protocol using an HDLC-like framing.
The HDLCPW emulation over a packet-switched network (PSN) has the
following characteristics in relationship to the native service:
o HDLC data and control fields are transported transparently (see
Section 4.1). The specific negotiations and signaling of the
protocol being transported are performed between Remote Systems
transparently, and the LCCE does not participate in them.
o The trailing FCS (Frame Check Sequence) containing a CRC (Cyclic
Redundancy Check) is stripped at the ingress LCCE and not
transported over HDLCPWs. It is therefore regenerated at the
egress LCCE (see Section 4.1). This means that the FCS may not
accurately reflect errors on the end-to-end HDLC link. Errors
or corruption introduced in the HDLCPW payload during
encapsulation or transit across the packet-switched network may
not be detected. This lack of integrity-check transparency may
not be of concern if it is known that the inner payloads or
upper protocols transported perform their own error and
integrity checking. To allow for payload integrity-checking
transparency on HDLCPWs using L2TP over IP or L2TP over UDP/IP,
the L2TPv3 session can utilize IPSec as specified in Section
4.1.3 of [RFC 3931].
o HDLC link status notification is provided using the Circuit
Status AVP in the SLI message (see Section 3.4).
o The length of the resulting L2TPv3 packet is longer than the
encapsulated HDLC frame without flags and FCS (see Section 4.3),
with resulting MTU and fragmentation implications discussed in
Section 4.1.4 of [RFC 3931].
o The packet-switched network may reorder, duplicate, or silently
drop packets. Sequencing may be enabled in the HDLCPW for some
or all packets to detect lost, duplicate, or out-of-order
packets on a per-session basis (see Section 4.2).
o The faithfulness of an HDLCPW may be increased by leveraging
Quality of Service features of the LCCEs and the underlying PSN.
Pignataro & Townsley Standards Track PAGE 8
RFC 4349 HDLC Frames over L2TPv3 February 2006
6. Security Considerations
HDLC over L2TPv3 is subject to the security considerations defined in
[RFC 3931]. Beyond the considerations when carrying other data link
types, there are no additional considerations specific to carrying
HDLC.
7. IANA Considerations
7.1. Pseudowire Type
The signaling mechanisms defined in this document rely upon the
allocation of an HDLC Pseudowire Type (see Pseudowire Capabilities
List as defined in 5.4.3 of [RFC 3931] and L2TPv3 Pseudowire Types in
10.6 of [RFC 3931]) by the IANA (number space created as part of
publication of [RFC 3931]). The HDLC Pseudowire Type is defined in
Section 2 of this specification:
L2TPv3 Pseudowire Types
-----------------------
0x0006 - HDLC Pseudowire Type
7.2. Result Code AVP Values
This number space is managed by IANA as described in section 2.3 of
[BCP0068]. Two 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
-----------------------------------------
20 - HDLC Link was deleted permanently (no longer provisioned)
21 - HDLC Link has been INACTIVE for an extended period of time
8. Acknowledgements
Thanks to Sudhir Rustogi and George Wilkie for valuable input. Maria
Alice Dos Santos provided helpful review and comment. Many thanks to
Mark Lewis for providing review and clarifying comments during IETF
Last Call.
Pignataro & Townsley Standards Track PAGE 9
RFC 4349 HDLC Frames over L2TPv3 February 2006
9. References
9.1. Normative References
[RFC 3931] Lau, J., Townsley, M., and I. Goyret, "Layer Two Tunneling
Protocol - Version 3 (L2TPv3)", RFC 3931, March 2005.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
9.2. Informative References
[BCP0068] Townsley, W., "Layer Two Tunneling Protocol (L2TP)
Internet Assigned Numbers Authority (IANA) Considerations
Update", BCP 68, RFC 3438, December 2002.
Authors' Addresses
Carlos Pignataro
Cisco Systems
7025 Kit Creek Road
PO Box 14987
Research Triangle Park, NC 27709
EMail: cpignata@cisco.com
W. Mark Townsley
Cisco Systems
7025 Kit Creek Road
PO Box 14987
Research Triangle Park, NC 27709
EMail: mark@townsley.net
Pignataro & Townsley Standards Track PAGE 10
RFC 4349 HDLC Frames over L2TPv3 February 2006
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Copyright © The Internet Society (2006).
This document is subject to the rights, licenses and restrictions
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Pignataro & Townsley Standards Track PAGE 11
High-Level Data Link Control (HDLC) Frames over Layer 2 Tunneling Protocol, Version 3 (L2TPv3)
RFC TOTAL SIZE: 22888 bytes
PUBLICATION DATE: Monday, February 13th, 2006
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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