Network Working Group Luca Martini Internet Draft Eric C. Rosen Expiration Date: 0 2004 Cisco Systems, Inc. Giles Heron Toby Smith Andrew G. Malis Laurel Networks, Inc. Tellabs Yeongil Seo KT Technology Lab 0 2004 Encapsulation Methods for Transport of PPP/HDLC Over IP and MPLS Networks draft-ietf-pwe3-hdlc-ppp-encap-mpls-04.txt Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Abstract A Pseudowire (PW) can be used to carry PPP, or HDLC Protocol Data Units over an IP or MPLS network without terminating the PPP/HDLC protocol. This enables service providers to offer "emulated" HDLC, or PPP link services over existing IP or MPLS networks. This document specifies the encapsulation of PPP/HDLC PDUs within a pseudowire. Martini, et al. [Page 1] Internet Draft draft-ietf-pwe3-hdlc-ppp-encap-mpls-04.txt 0 2004 Table of Contents 1 Specification of Requirements .......................... 2 2 Intellectual Property Statement ........................ 2 3 Introduction ........................................... 3 4 General encapsulation method ........................... 5 4.1 The Control Word ....................................... 5 4.1.1 Setting the sequence number ............................ 6 4.1.2 Processing the sequence number ......................... 6 4.2 MTU Requirements ....................................... 7 5 Protocol-Specific Details .............................. 8 5.1 HDLC ................................................... 8 5.2 PPP .................................................... 8 6 Using an MPLS Label as the Demultiplexer Field ......... 9 6.1 MPLS Shim EXP Bit Values ............................... 9 6.2 MPLS Shim S Bit Value .................................. 9 7 Security Considerations ................................ 9 8 Full Copyright Statement ............................... 9 9 References ............................................. 10 10 Author Information ..................................... 11 1. Specification of Requirements 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. Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of Martini, et al. [Page 2] Internet Draft draft-ietf-pwe3-hdlc-ppp-encap-mpls-04.txt 0 2004 such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf- ipr@ietf.org. By submitting this Internet-Draft, I certify that any applicable patent or other IPR claims of which I am aware have been disclosed, or will be disclosed, and any of which I become aware will be disclosed, in accordance with RFC 3668. 3. Introduction A PPP/HDLC Pseudowire (PW) allows PPP/HDLC Protocol Data Units (PDUs) to be carried over an IP network or an MPLS network. In addressing the issues associated with carrying a PPP/HDLC PDU over a PSN, this document assumes that a Pseudowire (PW) has been set up by some means outside the scope of this document. This may be via manual configuration, or a signaling protocol such as that defined in [1] or [7]. As described in [8], this PW may be tunneled through an MPLS, IPv4 or IPv6 PSN. The following figure describe the reference models which are derived from [8] to support the HDLC/PPP PW emulated services. Martini, et al. [Page 3] Internet Draft draft-ietf-pwe3-hdlc-ppp-encap-mpls-04.txt 0 2004 |<-------------- Emulated Service ---------------->| | | | |<------- Pseudo Wire ------>| | | | | | | | |<-- PSN Tunnel -->| | | | V V V V | V AC +----+ +----+ AC V +-----+ | | PE1|==================| PE2| | +-----+ | |----------|............PW1.............|----------| | | CE1 | | | | | | | | CE2 | | |----------|............PW2.............|----------| | +-----+ ^ | | |==================| | | ^ +-----+ ^ | +----+ +----+ | | ^ | | Provider Edge 1 Provider Edge 2 | | | | | | Customer | | Customer Edge 1 | | Edge 2 | | | | native HDLC/PPP service native HDLC/PPP service Figure 1: PWE3 HDLC/PPP Interface Reference Configuration This document specifies the emulated PW encapsulation for PPP, and HDLC. Although different layer 2 protocols require different information to be carried in this encapsulation, an attempt has been made to make the encapsulation as common as possible for all layer 2 protocols. Other layer 2 protocols are described in separate documents. [4] [5] [6] This document also specifies the way in which the demultiplexer field is added to the emulated PW encapsulation when an MPLS label is used as the demultiplexer field. QoS related issues are not discussed in this document. For the purpose of this document PE1 will be defined as the ingress router, and PE2 as the egress router. A layer 2 PDU will be received at PE1, encapsulated at PE1, transported, decapsulated at PE2, and transmitted out of PE2. Martini, et al. [Page 4] Internet Draft draft-ietf-pwe3-hdlc-ppp-encap-mpls-04.txt 0 2004 4. General encapsulation method 4.1. The Control Word There are three requirements that may need to be satisfied when transporting layer 2 protocols over an IP or MPLS backbone: -i. Sequentiality may need to be preserved. -ii. Small packets may need to be padded in order to be transmitted on a medium where the minimum transport unit is larger than the actual packet size. -iii. Control bits carried in the header of the layer 2 frame may need to be transported. When carrying HDLC/PPP over an IP or MPLS backbone sequentiality may need to be preserved. The OPTIONAL control word defined here addresses this requirement. Implementations MUST support sending no control word, and MAY support sending a control word. In all cases the egress router must be aware of whether the ingress router will send a control word over a specific virtual circuit. This may be achieved by configuration of the routers, or by signaling, for example as defined in [1]. The control word is defined as follows: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 0|0 0 0 0|B|E| Length | Sequence Number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 2: MPLS PWE3 Control Word In the above diagram the first 4 bits are the PID as defined in [8] B and E are fragmentation bits and their functionality is specified in [9]. The next 4 bits provide space for carrying protocol specific flags. These are not used for HDLC/PPP and they They MUST be set to 0 when transmitting, and MUST be ignored upon receipt. The next 6 bits provide a length field, which is used as follows: If the packet's length (defined as the length of the layer 2 payload plus the length of the control word) is less than 64 bytes, the length field MUST be set to the packet's length. Otherwise the length field MUST be set to zero. The value of the length field, if non- zero, can be used to remove any padding. When the packet reaches the Martini, et al. [Page 5] Internet Draft draft-ietf-pwe3-hdlc-ppp-encap-mpls-04.txt 0 2004 service provider's egress router, it may be desirable to remove the padding before forwarding the packet. The next 16 bits provide a sequence number that can be used to guarantee ordered packet delivery. The processing of the sequence number field is OPTIONAL. The sequence number space is a 16 bit, unsigned circular space. The sequence number value 0 is used to indicate an unsequenced packet. 4.1.1. Setting the sequence number For a given PW, and a pair of routers PE1 and PE2, if PE1 supports frame sequencing then the following procedures should be used: - the initial frame transmitted on the PW MUST use sequence number 1 - subsequent frames MUST increment the sequence number by one for each frame - when the transmit sequence number reaches the maximum 16 bit value (65535) the sequence number MUST wrap to 1 If the transmitting router PE1 does not support sequence number processing, then the sequence number field in the control word MUST be set to 0. 4.1.2. Processing the sequence number If a router PE2 supports receive sequence number processing, then the following procedures should be used: When a PW is initially set up, the "expected sequence number" associated with it MUST be initialized to 1. When a frame is received on that PW, the sequence number should be processed as follows: - if the sequence number on the frame is 0, then the frame passes the sequence number check - otherwise if the frame sequence number >= the expected sequence number and the frame sequence number - the expected sequence number < 32768, then the frame is in order. Martini, et al. [Page 6] Internet Draft draft-ietf-pwe3-hdlc-ppp-encap-mpls-04.txt 0 2004 - otherwise if the frame sequence number < the expected sequence number and the expected sequence number - the frame sequence number >= 32768, then the frame is in order. - otherwise the frame is out of order. If a packet is in order then, it can be delivered immediately. If the packet is in order, then the expected sequence number MUST be set using the algorithm: expected_sequence_number := frame_sequence_number + 1 mod 2**16 if (expected_sequence_number = 0) then expected_sequence_number := 1; Packets which are received out of order MAY be dropped or reordered at the discretion of the receiver. A simple extension of the above processing algorithm can be used to detect lost packets. If a router PE2 does not support receive sequence number processing, then the sequence number field MAY be ignored. 4.2. MTU Requirements The network MUST be configured with an MTU that is sufficient to transport the largest encapsulation frames. If MPLS is used as the tunneling protocol, for example, this is likely to be 12 or more bytes greater than the largest frame size. Other tunneling protocols may have longer headers and require larger MTUs. The methodology described in [9] MAY be used to fragment encapsulated frames that exeeed the PSN MTU. However if [9] is not used then if the ingress router determines that an encapsulated layer 2 PDU exceeds the MTU of the PSN tunnel through which it must be sent, the PDU MUST be dropped. If an egress router receives an encapsulated layer 2 PDU whose payload length (i.e., the length of the PDU itself without any of the encapsulation headers), exceeds the MTU of the destination layer 2 interface, the PDU MUST be dropped. Martini, et al. [Page 7] Internet Draft draft-ietf-pwe3-hdlc-ppp-encap-mpls-04.txt 0 2004 5. Protocol-Specific Details 5.1. HDLC HDLC mode provides port to port transport of HDLC encapsulated traffic. The HDLC PDU is transported in its entirety, including the HDLC address, control and protocol fields, but excluding HDLC flags and the FCS. Bit/Byte stuffing is undone. The control word is OPTIONAL. If the control word is used then the flag bits in the control word are not used, and MUST be set to 0 when transmitting, and MUST be ignored upon receipt. The HDLC mode is suitable for port to port transport of Frame Relay UNI or NNI traffic. It must be noted, however, that this mode is transparent to the FECN, BECN and DE bits. Since all packets are passed in a largely transparent manner over the HDLC PW, any protocol which has HDLC-like framing may utilize the HDLC PW mode, including PPP, Frame-Relay, X.25, etc. 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. 5.2. PPP PPP mode provides point to point transport of PPP encapsulated traffic, as specified in [3]. The PPP PDU is transported in its entirety, including the protocol field (whether compressed using PFC or not), but excluding any media-specific framing information, such as HDLC address and control fields or FCS. Since media-specific framing is not carried the following options will not operate correctly if the PPP peers attempt to negotiate them: - Frame Check Sequence (FCS) Alternatives - Address-and-Control-Field-Compression (ACFC) - Asynchronous-Control-Character-Map (ACCM) Note also that PW LSP Interface MTU negotiation as specified in [1] is not affected by PPP MRU advertisement. Thus if a PPP peer sends a PDU with a length in excess of that negotiated for the PW tunnel that PDU will be discarded by the ingress router. The control word is OPTIONAL. If the control word is used then the flag bits in the control word are not used, and MUST be set to 0 when transmitting, and MUST be ignored upon receipt. Martini, et al. [Page 8] Internet Draft draft-ietf-pwe3-hdlc-ppp-encap-mpls-04.txt 0 2004 6. Using an MPLS Label as the Demultiplexer Field To use an MPLS label as the demultiplexer field, a 32-bit label stack entry [2] is simply prepended to the emulated PW encapsulation, and hence will appear as the bottom label of an MPLS label stack. This label may be called the "PW label". The particular emulated pseudo- wire identified by a particular label value must be agreed by the ingress and egress LSRs, either by signaling (e.g, via the methods of [1]) or by configuration. Other fields of the label stack entry are set as follows. 6.1. MPLS Shim EXP Bit Values If it is desired to carry Quality of Service information, the Quality of Service information SHOULD be represented in the EXP field of the PW label. If more than one MPLS label is imposed by the ingress LSR, the EXP field of any labels higher in the stack SHOULD also carry the same value. 6.2. MPLS Shim S Bit Value The ingress LSR, PE1, MUST set the S bit of the PW label to a value of 1 to denote that the PW label is at the bottom of the stack. 7. Security Considerations The ethernet pseudowire type is subject to all of the general security considerations discussed in [8][1]. This document specifies only encapsulations, and not the protocols that may be used to carry the encapsulated packets across the PSN. Each such protocol may have its own set of security issues, but those issues are not affected by the encapsulations specified herein. 8. Full Copyright Statement Copyright (C) The Internet Society (2004). This document is subject to the rights, licenses and restrictions contained in BCP 78 and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE Martini, et al. [Page 9] Internet Draft draft-ietf-pwe3-hdlc-ppp-encap-mpls-04.txt 0 2004 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 9. References [1] "Transport of Layer 2 Frames Over MPLS", Martini, L., et al., draft-ietf-pwe3-control-protocol-06.txt, ( work in progress ) [2] "MPLS Label Stack Encoding", E. Rosen, Y. Rekhter, D. Tappan, G. Fedorkow, D. Farinacci, T. Li, A. Conta. RFC3032 [3] "The Point-to-Point Protocol (PPP)", RFC 1661. [4] "Encapsulation Methods for Transport of ATM Cells/Frame Over IP and MPLS Networks", draft-ietf-pwe3-atm-encap-02.txt ( work in progress ) [5] "Encapsulation Methods for Transport of Ethernet Frames Over IP/MPLS Networks", draft-ietf-pwe3-ethernet-encap-06.txt. ( work in progress ) [6] "Frame Relay over Pseudo-Wires", draft-ietf-pwe3-frame-relay- 01.txt. ( work in progress ) [7] "Layer Two Tunneling Protocol (Version 3) "L2TPv3"J. Lau, M. Townsley, A. Valencia, G. Zorn, I. Goyret, G. Pall, A. Rubens, B. Palter, , work in progress, draft-ietf-l2tpext-l2tp-base-12.txt, March 2004. [8] "PWE3 Architecture" Bryant, et al., draft-ietf-pwe3-arch-07.txt ( work in progress ), June 2003. [9] "PWE3 Fragmentation and Reassembly", A. Malis,W. M. Townsley, draft-ietf-pwe3-fragmentation-05.txt ( work in progress ) February 2004 Martini, et al. [Page 10] Internet Draft draft-ietf-pwe3-hdlc-ppp-encap-mpls-04.txt 0 2004 10. Author Information Luca Martini Cisco Systems, Inc. 9155 East Nichols Avenue, Suite 400 Englewood, CO, 80112 e-mail: lmartini@cisco.com Giles Heron Tellabs Abbey Place 24-28 Easton Street High Wycombe Bucks HP11 1NT UK e-mail: giles.heron@tellabs.com Eric C. Rosen Cisco Systems, Inc. 1414 Massachusetts Avenue Boxborough, MA 01719 E-mail: erosen@cisco.com Toby Smith Laurel Networks, Inc. Omega Corporate Center 1300 Omega Drive Pittsburgh, PA 15205 e-mail: tob@laurelnetworks.com Andrew G. Malis Tellabs 90 Rio Robles Dr. San Jose, CA 95134 e-mail: Andy.Malis@tellabs.com Yeongil Seo 463-1 KT Technology Lab Jeonmin-dong Yusung-gu Daegeon, Korea email: syi1@kt.co.kr Martini, et al. [Page 11] Internet Draft draft-ietf-pwe3-hdlc-ppp-encap-mpls-04.txt 0 2004 Martini, et al. [Page 12]