Network Working Group
Request for Comments: 4719
Category: Standards Track

R. Aggarwal, Ed.
Juniper Networks
M. Townsley, Ed.
M. Dos Santos, Ed.
Cisco Systems
November 2006

Transport of Ethernet 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

Abstract

This document describes the transport of Ethernet frames over the Layer 2 Tunneling Protocol, Version 3 (L2TPv3). This includes the transport of Ethernet port-to-port frames as well as the transport of Ethernet VLAN frames. The mechanism described in this document can be used in the creation of Pseudowires to transport Ethernet frames over an IP network.

   1. Introduction ....................................................2
      1.1. Specification of Requirements ..............................2
      1.2. Abbreviations ..............................................3
      1.3. L2TPv3 Control Message Types ...............................3
      1.4. Requirements ...............................................3
   2. PW Establishment ................................................4
      2.1. LCCE-LCCE Control Connection Establishment .................4
      2.2. PW Session Establishment ...................................4
      2.3. PW Session Monitoring ......................................6
   3. Packet Processing ...............................................7
      3.1.  Encapsulation .............................................7
      3.2.  Sequencing ................................................7
      3.3.  MTU Handling ..............................................7
   4. Applicability Statement .........................................8
   5. Congestion Control .............................................10
   6. Security Considerations ........................................10
   7. IANA Considerations ............................................11
   8. Contributors ...................................................11
   9. Acknowledgements ...............................................11
   10. References ....................................................12
      10.1. Normative References .....................................12
      10.2. Informative References ...................................12

1. Introduction

The Layer 2 Tunneling Protocol, Version 3 (L2TPv3) can be used as a control protocol and for data encapsulation to set up Pseudowires (PWs) for transporting layer 2 Packet Data Units across an IP network [RFC3931]. This document describes the transport of Ethernet frames over L2TPv3 including the PW establishment and data encapsulation.

The term "Ethernet" in this document is used with the intention to include all such protocols that are reasonably similar in their packet format to IEEE 802.3 [802.3], including variants or extensions that may or may not necessarily be sanctioned by the IEEE (including such frames as jumbo frames, etc.). The term "VLAN" in this document is used with the intention to include all virtual LAN tagging protocols such as IEEE 802.1Q [802.1Q], 802.1ad [802.1ad], etc.

1.1. 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].

1.2. Abbreviations

   AC      Attachment Circuit (see [RFC3985])
   CE      Customer Edge (Typically also the L2TPv3 Remote System)
   LCCE    L2TP Control Connection Endpoint (see [RFC3931])
   NSP     Native Service Processing (see [RFC3985])
   PE      Provider Edge (Typically also the LCCE) (see [RFC3985])
   PSN     Packet Switched Network (see [RFC3985])
   PW      Pseudowire (see [RFC3985])
   PWE3    Pseudowire Emulation Edge to Edge (Working Group)

1.3. L2TPv3 Control Message Types

Relevant L2TPv3 control message types (see [RFC3931]) are listed for reference.

   SCCRQ   L2TPv3 Start-Control-Connection-Request control message
   SCCRP   L2TPv3 Start-Control-Connection-Reply control message
   SCCCN   L2TPv3 Start-Control-Connection-Connected control message
   StopCCN L2TPv3 Stop-Control-Connection-Notification control message
   ICRQ    L2TPv3 Incoming-Call-Request control message
   ICRP    L2TPv3 Incoming-Call-Reply control message
   ICCN    L2TPv3 Incoming-Call-Connected control message
   OCRQ    L2TPv3 Outgoing-Call-Request control message
   OCRP    L2TPv3 Outgoing-Call-Reply control message
   OCCN    L2TPv3 Outgoing-Call-Connected control message
   CDN     L2TPv3 Call-Disconnect-Notify control message
   SLI     L2TPv3 Set-Link-Info control message

1.4. Requirements

An Ethernet PW emulates a single Ethernet link between exactly two endpoints. The following figure depicts the PW termination relative to the NSP and PSN tunnel within an LCCE [RFC3985]. The Ethernet interface may be connected to one or more Remote Systems (an L2TPv3 Remote System is referred to as Customer Edge (CE) in this and associated PWE3 documents). The LCCE may or may not be a PE.

                 +---------------------------------------+
                 |                 LCCE                  |
                 +-+   +-----+   +------+   +------+   +-+
                 |P|   |     |   |PW ter|   | PSN  |   |P|
   Ethernet  <==>|h|<=>| NSP |<=>|minati|<=>|Tunnel|<=>|h|<==> PSN
   Interface     |y|   |     |   |on    |   |      |   |y|
                 +-+   +-----+   +------+   +------+   +-+
                 |                                       |
                 +---------------------------------------+
                       Figure 1: PW termination

The PW termination point receives untagged (also referred to as 'raw') or tagged Ethernet frames and delivers them unaltered to the PW termination point on the remote LCCE. Hence, it can provide untagged or tagged Ethernet link emulation service.

The "NSP" function includes packet processing needed to translate the Ethernet frames that arrive at the CE-LCCE interface to/from the Ethernet frames that are applied to the PW termination point. Such functions may include stripping, overwriting, or adding VLAN tags. The NSP functionality can be used in conjunction with local provisioning to provide heterogeneous services where the CE-LCCE encapsulations at the two ends may be different.

The physical layer between the CE and LCCE, and any adaptation (NSP) functions between it and the PW termination, are outside of the scope of PWE3 and are not defined here.

2. PW Establishment

With L2TPv3 as the tunneling protocol, Ethernet PWs are L2TPv3 sessions. An L2TP Control Connection has to be set up first between the two LCCEs. Individual PWs can then be established as L2TP sessions.

2.1. LCCE-LCCE Control Connection Establishment

The two LCCEs that wish to set up Ethernet PWs MUST establish an L2TP Control Connection first as described in [RFC3931]. Hence, an Ethernet PW Type must be included in the Pseudowire Capabilities List as defined in [RFC3931]. The type of PW can be either "Ethernet port" or "Ethernet VLAN". This indicates that the Control Connection can support the establishment of Ethernet PWs. Note that there are two Ethernet PW Types required. For connecting an Ethernet port to another Ethernet port, the PW Type MUST be "Ethernet port"; for connecting an Ethernet VLAN to another Ethernet VLAN, the PW Type MUST be "Ethernet VLAN".

2.2. PW Session Establishment

The provisioning of an Ethernet port or Ethernet VLAN and its association with a PW triggers the establishment of an L2TP session via the standard Incoming Call three-way handshake described in Section 3.4.1 of [RFC3931].

Note that an L2TP Outgoing Call is essentially a method of controlling the originating point of a Switched Virtual Circuit (SVC), allowing it to be established from any reachable L2TP-enabled device able to perform outgoing calls. The Outgoing Call model and its corresponding OCRQ, OCRP, and OCCN control messages are mainly used within the dial arena with L2TPv2 today and has not been found applicable for PW applications yet.

The following are the signaling elements needed for the Ethernet PW establishment:

a) Pseudowire Type: The type of a Pseudowire can be either "Ethernet: port" or "Ethernet VLAN". Each LCCE signals its Pseudowire type in the Pseudowire Type AVP [RFC3931]. The assigned values for "Ethernet port" and "Ethernet VLAN" Pseudowire types are captured in the "IANA Considerations" of this document. The Pseudowire Type AVP MUST be present in the ICRQ.

b) Pseudowire ID: Each PW is associated with a Pseudowire ID. The: two LCCEs of a PW have the same Pseudowire ID for it. The Remote End Identifier AVP [RFC3931] is used to convey the Pseudowire ID. The Remote End Identifier AVP MUST be present in the ICRQ in order for the remote LCCE to determine the PW to associate the L2TP session with. An implementation MUST support a Remote End Identifier of four octets known to both LCCEs either by manual configuration or some other means. Additional Remote End Identifier formats that MAY be supported are outside the scope of this document.

c) The Circuit Status AVP [RFC3931] MUST be included in ICRQ and ICRP: to indicate the circuit status of the Ethernet port or Ethernet VLAN. For ICRQ and ICRP, the Circuit Status AVP MUST indicate that the circuit status is for a new circuit (refer to N bit in Section 2.3.3). An implementation MAY send an ICRQ or ICRP before an Ethernet interface is ACTIVE, as long as the Circuit Status AVP (refer to A bit in Section 2.3.3) in the ICRQ or ICRP reflects the correct status of the Ethernet port or Ethernet VLAN link. A subsequent circuit status change of the Ethernet port or Ethernet VLAN MUST be conveyed in the Circuit Status AVP in ICCN or SLI control messages. For ICCN and SLI (refer to Section 2.3.2), the Circuit Status AVP MUST indicate that the circuit status is for an existing circuit (refer to N bit in Section 2.3.3) and reflect the current status of the link (refer to A bit in Section 2.3.3).

2.3. PW Session Monitoring

2.3.1. Control Connection Keep-alive

The working status of a PW is reflected by the state of the L2TPv3 session. If the corresponding L2TPv3 session is down, the PW associated with it MUST be shut down. The Control Connection keep- alive mechanism of L2TPv3 can serve as a link status monitoring mechanism for the set of PWs associated with a Control Connection.

2.3.2. SLI Message

In addition to the Control Connection keep-alive mechanism of L2TPv3, Ethernet PW over L2TP makes use of the Set-Link-Info (SLI) control message defined in [RFC3931]. The SLI message is used to signal Ethernet link status notifications between LCCEs. This can be useful to indicate Ethernet interface state changes without bringing down the L2TP session. Note that change in the Ethernet interface state will trigger an SLI message for each PW associated with that Ethernet interface. This may be one Ethernet port PW or more than one Ethernet VLAN PW. 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 is the initial ICRQ, ICRP, and CDN messages that establish and tear down the L2TP session itself. 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).

2.3.3. Use of Circuit Status AVP for Ethernet

Ethernet PW reports circuit status with the Circuit Status AVP defined in [RFC3931]. 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 A (Active) bit indicates whether the Ethernet interface is ACTIVE (1) or INACTIVE (0).

The N (New) bit indicates whether the circuit status is for a new (1) Ethernet circuit or an existing (0) Ethernet circuit.

3. Packet Processing

3.1. Encapsulation

The encapsulation described in this section refers to the functionality performed by the PW termination point depicted in Figure 1, unless otherwise indicated.

The entire Ethernet frame, without the preamble or frame check sequence (FCS), is encapsulated in L2TPv3 and is sent as a single packet by the ingress LCCE. This is done regardless of whether or not a VLAN tag is present in the Ethernet frame. For Ethernet port- to-port mode, the remote LCCE simply decapsulates the L2TP payload and sends it out on the appropriate interface without modifying the Ethernet header. For Ethernet VLAN-to-VLAN mode, the remote LCCE MAY rewrite the VLAN tag. As described in Section 1, the VLAN tag modification is an NSP function.

The Ethernet PW over L2TP is homogeneous with respect to packet encapsulation, i.e., both ends of the PW are either untagged or tagged. The Ethernet PW can still be used to provide heterogeneous services using NSP functionality at the ingress and/or egress LCCE. The definition of such NSP functionality is outside the scope of this document.

The maximum length of the Ethernet frame carried as the PW payload is irrelevant as far as the PW is concerned. If anything, that value would only be relevant when quantifying the faithfulness of the emulation.

3.2. Sequencing

Data packet sequencing MAY be enabled for Ethernet PWs. The sequencing mechanisms described in [RFC3931] MUST be used for signaling sequencing support.

3.3. MTU Handling

With L2TPv3 as the tunneling protocol, the IP packet resulting from the encapsulation is M + N bytes longer than the Ethernet frame without the preamble or FCS. Here M is the length of the IP header along with associated options and extension headers, and 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,  for L2TPv3 data messages over IP;
         N = 16-28, for L2TPv3 data messages over UDP;
         (N does not include the IP header).

Fragmentation in the PSN can occur when using Ethernet over L2TP, unless proper configuration and management of MTU sizes are in place between the Customer Edge (CE) router and Provider Edge (PE) router, and across the PSN. This is not specific only to Ethernet over L2TPv3, and the base L2TPv3 specification [RFC3931] provides general recommendations with respect to fragmentation and reassembly in Section 4.1.4. "PWE3 Fragmentation and Reassembly" [RFC4623] expounds on this topic, including a fragmentation and reassembly mechanism within L2TP itself in the event that no other option is available. Implementations MUST follow these guidelines with respect to fragmentation and reassembly.

4. Applicability Statement

The Ethernet PW emulation allows a service provider to offer a "port-to-port"-based Ethernet service across an IP Packet Switched Network (PSN), while the Ethernet VLAN PW emulation allows an "VLAN- to-VLAN"-based Ethernet service across an IP Packet Switched Network (PSN).

The Ethernet or Ethernet VLAN PW emulation has the following characteristics in relationship to the respective native service:

Ethernet PW connects two Ethernet port ACs, and Ethernet VLAN PW connects two Ethernet VLAN ACs, which both support bi-directional transport of variable-length Ethernet frames. The ingress LCCE strips the preamble and FCS from the Ethernet frame and transports the frame in its entirety across the PW. This is done regardless of the presence of the VLAN tag in the frame. The egress LCCE receives the Ethernet frame from the PW and regenerates the preamble and FCS before forwarding the frame to the attached Remote System (see Section 3.1). Since FCS is not being transported across either Ethernet or Ethernet VLAN PWs, payload integrity transparency may be lost. To achieve payload integrity transparency on Ethernet or Ethernet VLAN PWs using L2TP over IP or L2TP over UDP/IP, the L2TPv3 session can utilize IPsec as specified in Section 4.1.3 of [RFC3931].

While architecturally [RFC3985] outside the scope of the L2TPv3 PW itself, if VLAN tags are present, the NSP may rewrite VLAN tags on ingress or egress from the PW (see Section 3.1).

The Ethernet or Ethernet VLAN PW only supports homogeneous Ethernet frame type across the PW; both ends of the PW must be either tagged or untagged. Heterogeneous frame type support achieved with NSP functionality is outside the scope of this document (see Section 3.1).

Ethernet port or Ethernet VLAN status notification is provided using the Circuit Status AVP in the SLI message (see Sections 2.3.2 and 2.3.3). Loss of connectivity between LCCEs can be detected by the L2TPv3 keep-alive mechanism (see Section 2.3.1 of this document and Section 4.4 of [RFC3931]). The LCCE can convey these indications back to its attached Remote System.

The maximum frame size that can be supported is limited by the PSN MTU minus the L2TPv3 header size, unless fragmentation and reassembly is used (see Section 3.3 of this document and Section 4.1.4 of [RFC3931]).

The Packet Switched Network may reorder, duplicate, or silently drop packets. Sequencing may be enabled in the Ethernet or Ethernet VLAN PW for some or all packets to detect lost, duplicate, or out-of-order packets on a per-session basis (see Section 3.2).

The faithfulness of an Ethernet or Ethernet VLAN PW may be increased by leveraging Quality-of-Service (QoS) features of the LCCEs and the underlying PSN. For example, for Ethernet 802.1Q [802.1Q] VLAN transport, the ingress LCCE MAY consider the user priority field (i.e., 802.1p) of the VLAN tag for traffic classification and QoS treatments, such as determining the Differentiated Services (DS) field [RFC2474] of the encapsulating IP header. Similarly, the egress LCCE MAY consider the DS field of the encapsulating IP header when rewriting the user priority field of the VLAN tag or queuing the Ethernet frame before forwarding the frame to the Remote System. The mapping between the user priority field and the IP header DS field as well as the Quality-of-Service model deployed are application specific and are outside the scope of this document.

5. Congestion Control

As explained in [RFC3985], the PSN carrying the PW may be subject to congestion, with congestion characteristics depending on PSN type, network architecture, configuration, and loading. During congestion, the PSN may exhibit packet loss that will impact the service carried by the Ethernet or Ethernet VLAN PW. In addition, since Ethernet or Ethernet VLAN PWs carry a variety of services across the PSN, including but not restricted to TCP/IP, they may or may not behave in a TCP-friendly manner prescribed by [RFC2914] and thus consume more than their fair share.

Whenever possible, Ethernet or Ethernet VLAN PWs should be run over traffic-engineered PSNs providing bandwidth allocation and admission control mechanisms. IntServ-enabled domains providing the Guaranteed Service (GS) or DiffServ-enabled domains using EF (expedited forwarding) are examples of traffic-engineered PSNs. Such PSNs will minimize loss and delay while providing some degree of isolation of the Ethernet or Ethernet VLAN PW's effects from neighboring streams.

LCCEs SHOULD monitor for congestion (by using explicit congestion notification or by measuring packet loss) in order to ensure that the service using the Ethernet or Ethernet VLAN PW may be maintained. When severe congestion is detected (for example, when enabling sequencing and detecting that the packet loss is higher than a threshold), the Ethernet or Ethernet VLAN PW SHOULD be halted by tearing down the L2TP session via a CDN message. The PW may be restarted by manual intervention or by automatic means after an appropriate waiting time. Note that the thresholds and time periods for shutdown and possible automatic recovery need to be carefully configured. This is necessary to avoid loss of service due to temporary congestion and to prevent oscillation between the congested and halted states.

This specification offers no congestion control and is not TCP friendly [TFRC]. Future works for PW congestion control (being studied by the PWE3 Working Group) will provide congestion control for all PW types including Ethernet and Ethernet VLAN PWs.

6. Security Considerations

Ethernet over L2TPv3 is subject to all of the general security considerations outlined in [RFC3931].

7. IANA Considerations

The signaling mechanisms defined in this document rely upon the following Ethernet Pseudowire Types (see Pseudowire Capabilities List as defined in 5.4.3 of [RFC3931] and L2TPv3 Pseudowire Types in 10.6 of [RFC3931]), which were allocated by the IANA (number space created as part of publication of [RFC3931]):

      Pseudowire Types
      ----------------
      0x0004  Ethernet VLAN Pseudowire Type
      0x0005  Ethernet Pseudowire Type

8. Contributors

The following is the complete list of contributors to this document.

Rahul Aggarwal
Juniper Networks

Xipeng Xiao
Riverstone Networks

W. Mark Townsley
Stewart Bryant
Maria Alice Dos Santos
Cisco Systems

Cheng-Yin Lee
Alcatel

Tissa Senevirathne
Consultant

Mitsuru Higashiyama
Anritsu Corporation

9. Acknowledgements

This RFC evolved from the document, "Ethernet Pseudo Wire Emulation Edge-to-Edge". We would like to thank its authors, T.So, X.Xiao, L. Anderson, C. Flores, N. Tingle, S. Khandekar, D. Zelig and G. Heron for their contribution. We would also like to thank S. Nanji, the author of "Ethernet Service for Layer Two Tunneling Protocol", for writing the first Ethernet over L2TP document.

Thanks to Carlos Pignataro for providing a thorough review and helpful input.

10. References

10.1. Normative References

   [RFC3931]  Lau, J., Townsley, M., and 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.
   
   [RFC4623]  Malis, A. and M. Townsley, "Pseudowire Emulation Edge-to-
              Edge (PWE3) Fragmentation and Reassembly", RFC 4623,
              August 2006.

10.2. Informative References

   [RFC3985]  Bryant, S. and P. Pate, "Pseudo Wire Emulation Edge-to-
              Edge (PWE3) Architecture", RFC 3985, March 2005.
   
   [RFC2914]  Floyd, S., "Congestion Control Principles", BCP 41, RFC
              2914, September 2000.
   
   [RFC2474]  Nichols, K., Blake, S., Baker, F., and D. Black,
              "Definition of the Differentiated Services Field (DS
              Field) in the IPv4 and IPv6 Headers", RFC 2474, December
              1998.
   
   [802.3]    IEEE, "IEEE std 802.3 -2005/Cor 1-2006 IEEE Standard for
              Information Technology - Telecommuincations and
              Information Exchange Between Systems - Local and
              Metropolitan Area Networks", IEEE Std 802.3-2005/Cor
              1-2006 (Corrigendum to IEEE Std 802.3-2005)
   
   [802.1Q]   IEEE, "IEEE standard for local and metropolitan area
              networks virtual bridged local area networks", IEEE Std
              802.1Q-2005 (Incorporates IEEE Std 802.1Q1998, IEEE Std
              802.1u-2001, IEEE Std 802.1v-2001, and IEEE Std 802.1s-
              2002)
   
   [802.1ad]  IEEE, "IEEE Std 802.1ad - 2005 IEEE Standard for Local and
              metropolitan area networks - virtual Bridged Local Area
              Networks, Amendment 4: Provider Bridges", IEEE Std
              802.1ad-2005 (Amendment to IEEE Std 8021Q-2005)
   
   [TFRC]     Handley, M., Floyd, S., Padhye, J., and J. Widmer, "TCP
              Friendly Rate Control (TFRC): Protocol Specification", RFC
              3448, January 2003.

Author Information

Rahul Aggarwal
Juniper Networks
1194 North Mathilda Avenue
Sunnyvale, CA 94089

EMail:

          rahul@juniper.net

W. Mark Townsley
Cisco Systems
7025 Kit Creek Road
PO Box 14987
Research Triangle Park, NC 27709

EMail:

          mark@townsley.net

Maria Alice Dos Santos
Cisco Systems
170 W Tasman Dr
San Jose, CA 95134

EMail:

          mariados@cisco.com

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