Network Working Group Carlos Pignataro
Internet-Draft W. Mark Townsley
Category: Standards Track Cisco Systems
Expiration Date: March 2006
September 2005
HDLC Frames over L2TPv3
draft-ietf-l2tpext-pwe3-hdlc-07.txt
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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
High-Level Data Link Control (HDLC) frames over L2TPv3.
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Contents
Status of this Memo.......................................... 1
1. Introduction.............................................. 3
1.1 Abbreviations......................................... 3
2. Control Connection Establishment.......................... 3
3. HDLC Link Status Notification and Session Establishment... 3
3.1 L2TPv3 Session Establishment.......................... 4
3.2 L2TPv3 Session Teardown............................... 6
3.3 L2TPv3 Session Maintenance............................ 6
3.4 Use of Circuit Status AVP for HDLC.................... 6
4. Encapsulation............................................. 7
4.1 Data Packet Encapsulation............................. 7
4.2 Data Packet Sequencing................................ 8
4.3 MTU Considerations.................................... 8
5. Applicability Statement................................... 8
6. Security Considerations................................... 9
7. IANA Considerations....................................... 9
7.1 Pseudowire Type....................................... 9
7.2 Result Code AVP Values................................ 10
8. Acknowledgments........................................... 10
9. References................................................ 10
9.1 Normative References.................................. 10
9.2 Informative References................................ 10
10. Authors' Addresses....................................... 10
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
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 simple interface up and
interface down notifications.
The reader is expected to be very familiar with the terminology and
protocol constructs defined in [RFC3931].
1.1 Abbreviations
HDLC High-Level Data Link Control
HDLCPW HDLC Pseudowire
LAC L2TP Access Concentrator (See [RFC3931])
LCCE L2TP Control Connection Endpoint (See [RFC3931])
PW Pseudowire
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
[RFC3931]. The L2TPv3 SCCRQ Control Message and corresponding SCCRP
Control Message MUST include the HDLC Pseudowire Type of 0x0006 (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 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 [RFC3931], though the
unstructured Hostname AVP [RFC3931] 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.
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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
[RFC3931]. 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
interfacing becoming INACTIVE.
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 [RFC3931],
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,
ICRP messages and MAY be present in the SLI message for HDLCPWs.
Following is an example of the L2TP messages exchanged for an HDLCPW
which is initiated after an HDLC interface is provisioned and becomes
ACTIVE.
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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 [RFC3931]
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) 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 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.
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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 [RFC3931].
General Result Codes regarding L2TP session establishment are defined
in [RFC3931]. 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 [RFC3931] 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 exception 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 [RFC3931] for session
keepalive. This gives all sessions basic dead peer and path detection
between PEs.
3.4 Use of Circuit Status AVP for HDLC
HDLC reports Circuit Status with 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 SHOULD be set to one (1) if the Circuit Status
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indication is for a new HDLC circuit, 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 [RFC3931] 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, transported over the PW.
Since all packets are passed in a largely transparent manner over the
HDLCPW, any protocol which 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.
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 which 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.
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4.2 Data Packet Sequencing
Data Packet Sequencing MAY be enabled for HDLCPWs. The sequencing
mechanisms described in Section 4.6.1 of [RFC3931] 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 [RFC3931] when sequencing is enabled, and MAY request
its presence at all times.
4.3 MTU Considerations
With L2TPv3 as the tunneling protocol, the packet resulted from the
encapsulation is N bytes longer than 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 [RFC3931].
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
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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 accross 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
[RFC3931].
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
encpsulated HDLC frame without flags and FCS (see Section 4.3),
with resulting MTU and fragmentation implications discussed in
Section 4.1.4 of [RFC3931].
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 the underlying
PSN.
6. Security Considerations
HDLC over L2TPv3 is subject to the security considerations defined in
[RFC3931]. There are no additional considerations specific to
carrying HDLC that are not present carrying other data link types.
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 [RFC3931] and L2TPv3 Pseudowire Types in
10.6 of [RFC3931]) by the IANA (number space created as part of
publication of [RFC3931]). The HDLC Pseudowire Type is defined in
Section 2 of this specification:
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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. Acknowledgments
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.
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.
9.2 Informative References
[BCP0068] Townsley, W., Layer Two Tunneling Protocol (L2TP)
Internet Assigned Numbers Authority (IANA)
Considerations Update", RFC3438, BCP0068, December 2002
10. Authors' Addresses
Carlos Pignataro
Cisco Systems
7025 Kit Creek Road
PO Box 14987
Research Triangle Park, NC 27709
cpignata@cisco.com
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W. Mark Townsley
Cisco Systems
7025 Kit Creek Road
PO Box 14987
Research Triangle Park, NC 27709
mark@townsley.net
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