Network Working Group W. Mark Townsley Internet-Draft Carlos Pignataro Expiration Date: August 2006 Scott Wainner cisco Systems
<draft-ietf-mpls-over-l2tpv3-00.txt>Ted Seely February 2005Sprint Jeffery S. Young Alcatel February 2006 Encapsulation of MPLS over Layer 2 Tunneling Protocol Version 3 draft-ietf-mpls-over-l2tpv3-01.txt Status of this Memo By submitting this Internet-Draft, I certifyeach author represents that any applicable patent or other IPR claims of which I amhe or she is aware have been or will be disclosed, and any of which I becomehe or she becomes aware will be disclosed, in accordance with RFC 3668.Section 6 of BCP 79. 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. 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".progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt .http://www.ietf.org/1id-abstracts.html The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html . 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 payload types over IP networks. This document defines how to carry an MPLS label or labelstack and its payload over L2TPv3. This enables an application which traditionally requires an MPLS-enabled core network to utilize an L2TPv3 encapsulation over an IP network instead. Contents Status of this Memo.......................................... 1 1. Introduction.............................................. 2 2. MPLS over L2TPv3 Encoding................................. 23 3. Assigning the L2TPv3 Session ID and Cookie................ 4 4. Applicability............................................. 4 5. Security Considerations................................... 56 5.1 In the Absence of IPsec............................... 6 5.2 Context Validation.................................... 7 5.3 Securing the Tunnel with IPsec........................ 7 6. IANA Considerations....................................... 68 7. Acknowledgments........................................... 6Acknowledgements.......................................... 8 8. References................................................ 68 8.1 Normative References.................................. 68 8.2 Informative References................................ 69 9. Contacts.................................................. 6Authors' 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]. 1. Introduction This document defines how to encapsulate an MPLS label or labelstack and its payload over L2TPv3. After defining the MPLS over L2TPv3 encapsulation procedure, other MPLS over IP encapsulation options including IP, GRE and IPsec are discussed in context with MPLS over L2TPv3 in an Applicability section. This document only describes encapsulation and does not concern itself with all possible MPLS- based applications which may be enabled over L2TPv3. 2. MPLS over L2TPv3 Encoding MPLS over L2TPv3 allows tunneling of an MPLS stack [RFC3032] and its payload over an IP network utilizing the L2TPv3 encapsulation defined in [RFC3931]. The MPLS Label Stack and payload isare carried in itstheir entirety after IP (either IPv4 or IPv6) and L2TPv3. +-+-+-+-+-+-+-+-+-+-+ | IP | +-+-+-+-+-+-+-+-+-+-+ | L2TPv3 | +-+-+-+-+-+-+-+-+-+-+ | MPLS Label Stack | +-+-+-+-+-+-+-+-+-+-+ | MPLS Payload | +-+-+-+-+-+-+-+-+-+-+ Figure 2.1 MPLS StackPacket over L2TPv3/IP The L2TPv3 encapsulation carrying a single MPLS label stack entry is 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Session ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Cookie (optional, maximum 64 bits)... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Label | Label | Exp |S| TTL | Stack +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Entry Figure 2.2 MPLS labelover L2TPv3 encapsulation When encapsulating MPLS over L2TPv3, the L2TPv3 L2-Specific-Sublayer MUST NOTMAY be present. It is generally not present and hence not included in Figure 2.2. The L2TPv3 Session ID MUST be present. The Cookie MAY be present. Session ID The L2TPv3 Session ID is a 32-bit identifier field locally selected as a lookup key for the context of an L2TP Session. An L2TP Session contains necessary context for processing a received L2TP packet. At a minimum, such context contains whether the Cookie (see description below) is present, the value it was assigned, the presence and type of an L2TPv3 L2-Specific-Sublayer, as well as what type of tunneled encapsulation follows (i.e., Frame Relay, Ethernet, MPLS, etc).etc.) Cookie The L2TPv3 Cookie field contains a variable length (maximum 64 bits) randomly assigned value. It is intended to provide an additional level of guarantee that a data packet has been directed to the proper L2TP session by the Session ID. While the Session ID may be encoded and assigned any value (perhaps optimizing for local lookup capabilities, redirection in a distributed forwarding architecture, etc.), the Cookie MUST be selected as a cryptographically random value [RFC1750],[RFC4086], with the added restriction that it not be the same as a recently used value for a given Session ID. A well-chosen Cookie will prevent inadvertent misdirection of a stray packet containing a recently reused Session ID, a Session ID that is subject to packet corruption, and protection against some specific malicious packet insertion attacks as described in more detail in Section 4 of this document. Label Stack Entry An MPLS label stack entry as defined in [RFC3032]. The optional L2-Specific-Sublayer defined in [RFC3931] is generally not present for MPLS over L2TPv3. Generic IP encapsulation procedures such as fragmentation and MTU considerations, handling of TTL, EXP and DSCP bits, etc. are the same as the "Common Procedures" for IP encapsulation of MPLS defined in Section 5 of [MPLS-IP-GRE][RFC4023] and are not reiterated here. 3. Assigning the L2TPv3 Session ID and Cookie Much like an MPLS label, the L2TPv3 Session ID and Cookie must be selected and exchanged between participating nodes before L2TPv3 can operate. These values may be configured manually, or distributed via a signaling protocol. This document concerns itself only with the encapsulation of MPLS over L2TPv3, thus the particular method of assigning the Session ID and Cookie is out of scope. 4. Applicability The methods defined [MPLS-IP-GRE],[RFC4023], [MPLS-IPSEC] and this document all describe methods for carrying MPLS over an IP network. Cases where MPLS over L2TPv3 may be applicable compared to other alternatives are discussed in this section. It is generally simpler to have one's border routers refuse to accept an MPLS packet than to configure a router to refuse to accept certain MPLS packets carried in IP or GRE to or from certain IP sources or destinations. Thus, the use of IP or GRE to carry MPLS labelspackets increases the opportunity for MPLS label spoofing attacks. L2TPv3 provides an additional level of protection against packet spoofing before allowing a packet to enter a VPN (much like IPsec provides an additional level of protection at a PE rather than relying on ACL filters). Checking the value of the L2TPv3 Cookie is similar to any sort of ACL which inspects the contents of a packet header, except that we give ourselves the luxury of "seeding" the L2TPv3 header with a very difficult to spoof value. MPLS over L2TPv3 may be favorable compared to [MPLS-IP-GRE],[RFC4023], if: Two routers are "adjacent" over an L2TPv3 tunnel that exists for some reason outside the scope of this document, and those two routers need to send MPLS packets over that adjacency. Implementation considerations dictate the use of MPLS over L2TPv3. For example, a hardware device may be able to take advantage of the L2TPv3 encapsulation for faster or distributed processing. Packet spoofing and insertion isinsertion, service integrity and resource protection are of concern.concern, especially given the fact that an IP tunnel potentially exposes the router to rogue or inappropriate IP packets from unknown or untrusted sources. IP ACLs and numbering methods may be used to protect the PEs from rogue IP sources, but may be subject to error and cumbersome to maintain at all edge points at all times. The L2TPv3 Cookie allowsprovides a simple validation, over and above that of IP ACLs,means of validating the source of an L2TPv3 packet before allowing processing to continue. This validation offers an additional level of protection over and above IP ACLs, and a validation that the Session ID was not corrupted in transit or suffered an internal lookup error upon receipt and processing. If the Cookie value is assigned and distributed automatically, it is less subject to operator error, and if selected in a cryptographically random nature, less subject to blind guesses than source IP addresses (in the event that a hacker can insert packets within a VPN core network). (The first two of the above applicability statements were adopted from [MPLS-IP-GRE])[RFC4023]) In summary, L2TPv3 can provide a balance between the limited security against IP spoofing attacks offered by [MPLS-IP-GRE][RFC4023] vs. the greater security and associated operational and processing overhead offered by [MPLS-IPSEC]. Further, MPLS over L2TPv3 may be faster in some hardware, particularly if it is already optimized to classify incoming L2TPv3 packets carrying IP framed in a variety of ways. For example, IP encapsulated by HDLC or Frame Relay over L2TPv3 may be considered not that far removed from IP encapsulated by MPLS over L2TPv3. 5. Security Considerations The L2TPv3 Cookie does not provide protection via encryption. However,There are three main concerns when used with a sufficiently random 64-bit value whichtransporting MPLS labeled traffic between PEs using IP tunnels. The first is kept secret from a hacker,the L2TPv3 Cookiepossibility that a third party may be used asinsert packets into a simple yet effectivepacket source authentication check whichstream between two PEs. The second is quite resistent to brute forcethat a third party might view the packet stream between two PEs. The third is that a third party may alter packets in a stream between two PEs. The security requirements of the applications whose traffic is being sent through the tunnel characterizes how significant these issues are. Operators may use multiple methods to mitigate the risk including access lists, authentication, encryption, and context validation. Operators needs to consider the cost to mitigate the risk. Security is also discussed as part of the applicability discussion in section 4 of this document. 5.1 In the Absence of IPsec If the tunnels are not secured with IPsec, then some other method should be used to ensure that packets are decapsulated and processed by the tunnel tail only if those packets were encapsulated by the tunnel head. If the tunnel lies entirely within a single administrative domain, address filtering at the boundaries can be used to ensure that no packet with the IP source address of a tunnel endpoint or with the IP destination address of a tunnel endpoint can enter the domain from outside. However, when the tunnel head and the tunnel tail are not in the same administrative domain, this may become difficult, and filtering based on the destination address can even become impossible if the packets must traverse the public Internet. Sometimes only source address filtering (but not destination address filtering) is done at the boundaries of an administrative domain. If this is the case, the filtering does not provide effective protection at all unless the decapsulator of MPLS over L2TPv3 validates the IP source address of the packet. Additionally, the "Data Packet Spoofing" considerations in Section 8.2. of [RFC3931] and the "Context Validation" considerations in Section 5.2 of this document apply. Those two sections highlight the benefits of the L2TPv3 Cookie. 5.2 Context Validation The L2TPv3 Cookie does not provide protection via encryption. However, when used with a sufficiently random 64-bit value which is kept secret from a hacker, the L2TPv3 Cookie may be used as a simple yet effective packet source authentication check which is quite resistant to brute force packet spoofing attacks. It also alleviates the need to rely solely on filter lists based on a list of valid source IP addresses, and thwarts attacks which could benefit by spoofing a permitted source IP address. The L2TPv3 Cookie provides a means of validating the currently assigned Session ID on the packet flow, providing context protection, and may be deemed complimentary to securing the tunnel utilizing IPsec. In the absence of cryptographic security on the data plane (such as that provided by IPsec), the L2TPv3 Cookie provides a simple method of validating the Session ID lookup performed on each L2TPv3 packet. If the Cookie is sufficiently random and remains unknown to an attacker (that is, the attacker has no way to predict Cookie values or monitor traffic between PEs) then the Cookie provides an additional measure of protection against malicious spoofed packets inserted at the PE over and above that of typical IP address and port ACLs. 5.3 Securing the Tunnel with IPsec L2TPv3 tunnels may also be secured using IPsec.IPsec, as specified in Section 4.1.3. of [RFC3931]. IPSec may provide authentication, privacy protection, integrity checking and replay protection. These functions may be deemed necessary by the operator. When using IPsec, the tunnel head andhead and the tunnel tail should be treated as the endpoints of a Security Association. A single IP address of the tunnel head is used as the source IP address, and a single IP address of the tunnel tail is used as the destination IP address. The means by which each node knows the proper address of the other is outside the scope of this document. However, if a control protocol is used to set up the tunnels, such control protocol MUST have an authentication mechanism, and this MUST be used when the tunnel is set up. If the tunnel tail should be treatedis set up automatically as the endpointsresult of, for example, information distributed by BGP, then the use of a Security Association.BGP's MD5- based authentication mechanism can serve this purpose. The MPLS over L2TPv3 encapsulated packets should be considered as originating at the tunnel head and as being destined for the tunnel tail; IPsec transport mode shouldSHOULD thus be used. Note that the tunnel tail and the tunnel head are LSP adjacencies (label distribution adjacencies, see [RFC3031]), which means that the topmost label of any packet sent through the tunnel must be one that was distributed by the tunnel tail to the tunnel head. The tunnel tail MUST know precisely which labels it has distributed to the tunnel heads of IPsec-secured tunnels. Labels in this set MUST NOT be distributed by the tunnel tail to any LSP adjacencies other than those that are tunnel heads of IPsec-secured tunnels. If an MPLS packet is received without an IPsec encapsulation, and if its topmost label is in this set, then the packet MUST be discarded. Securing L2TPv3 using IPsec MUST provide authentication and integrity. (Note that the authentication and integrity provided will apply to the entire MPLS packet, including the MPLS label stack.) Consequently, the implementation MUST support ESP with null encryption. ESP with encryption MAY be supported if a source requires confidentiality. If ESP is used, the tunnel tail MUST check that the source IP address of any packet received on a given SA is the one expected. Key distribution may be done either manually or automatically. Security is also discussed as partautomatically by means of the applicability discussionIKE [RFC2409]. Manual keying MUST be supported. If automatic keying is implemented, IKE in section 4 ofmain mode with preshared keys MUST be supported. A particular application may escalate this document.requirement and request implementation of automatic keying. Manual key distribution is much simpler, but also less scalable, than automatic key distribution. If replay protection is regarded as necessary for a particular tunnel, automatic key distribution should be configured. 6. IANA Considerations There are no IANA considerations for this document. 7. AcknowledgmentsAcknowledgements Thanks to Robert Raszuk, Clarence Filsfils and Eric Rosen for their review of this document. Some text was adopted from [MPLS-IP-GRE].[RFC4023]. 8. References 8.1 Normative References [RFC3931] J.Lau, M.J., Townsley, M., and I. Goyret, "Layer Two Tunneling Protocol (Version 3)", work in progress, draft-ietf-l2tpext-l2tp-base-15.txt, December 2004.- 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. [MPLS-IP-GRE] T.[RFC4023] Worster, Y.T., Rekhter, Y., and E. Rosen, "Encapsulating MPLS in IP or Generic Routing Encapsulation (GRE)", work in progress, draft-ietf-mpls-in-ip-or-gre-08.txt, June 2004.RFC 4023, March 2005. 8.2 Informative References [RFC2547] E. Rosen, Y. Rekhter, "BGP/MPLS VPNs", RFC 2547, March 1999.[RFC3032] R.Rosen, et. al.,E., Tappan, D., Fedorkow, G., Rekhter, Y., Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack Encoding,"Encoding", RFC 3032, January 2001. [RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness Requirements for Security", BCP 106, RFC 4086, June 2005. [MPLS-IPSEC] E.Rosen, J. De Clercq, O/ Paridaens, Y. T'Joens, C. Sargor, "UseE., "Architecture for the Use of PE-PE IPsec Tunnels in RFC2547BGP/MPLS IP VPNs", workdraft-ietf-l3vpn-ipsec-2547-05 (work in progress, draft-ietf-l3vpn-ipsec-2547-01.txt,progress), August 2003.2005. [RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)", RFC 2409, November 1998. [RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol Label Switching Architecture", RFC 3031, January 2001. 9. ContactsAuthors' Addresses W. Mark Townsley cisco Systems EMail: firstname.lastname@example.org Carlos Pignataro cisco Systems 7025 Kit Creek Road PO Box 14987 Research Triangle Park, NC 27709 EMail: email@example.com Scott Wainner cisco Systems 13600 Dulles Technology Drive Herndon, VA 20171 EMail: firstname.lastname@example.org Ted Seely Sprint EMail: email@example.com Jeffrey S. Young Alcatel EMail: Jeffrey.S.Young@alcatel.com 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. 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