Network Working Group
Request for Comments: 4023
Category: Standards Track

T. Worster
Motorola, Inc.
Y. Rekhter
Juniper Networks, Inc.
E. Rosen, Ed.
Cisco Systems, Inc.
March 2005

Encapsulating MPLS in IP or Generic Routing Encapsulation (GRE)

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

Various applications of MPLS make use of label stacks with multiple entries. In some cases, it is possible to replace the top label of the stack with an IP-based encapsulation, thereby enabling the application to run over networks that do not have MPLS enabled in their core routers. This document specifies two IP-based encapsulations: MPLS-in-IP and MPLS-in-GRE (Generic Routing Encapsulation). Each of these is applicable in some circumstances.

   1.  Motivation  ..................................................  2
   2.  Specification of Requirements  ...............................  3
   3.  Encapsulation in IP  .........................................  3
   4.  Encapsulation in GRE  ........................................  4
   5.  Common Procedures  ...........................................  5
       5.1.  Preventing Fragmentation and Reassembly  ...............  5
       5.2.  TTL or Hop Limit  ......................................  6
       5.3.  Differentiated Services  ...............................  7
   6.  Applicability  ...............................................  7
   7.  IANA Considerations  .........................................  8
   8.  Security Considerations  .....................................  8
       8.1.  Securing the Tunnel with IPsec .........................  8
       8.2.  In the Absence of IPsec  ............................... 10
   9.  Acknowledgements ............................................. 11
   10. Normative References  ........................................ 11
   11. Informative References  ...................................... 12
   Authors' Addresses ............................................... 13
   Full Copyright Statement ......................................... 14

1. Motivation

In many applications of MPLS, packets traversing an MPLS backbone carry label stacks with more than one label. As described in section 3.15 of [RFC3031], each label represents a Label Switched Path (LSP). For each LSP, there is a Label Switching Router (LSR) that is the "LSP Ingress", and an LSR that is the "LSP Egress". If LSRs A and B are the Ingress and Egress, respectively, of the LSP corresponding to a packet's top label, then A and B are adjacent LSRs on the LSP corresponding to the packet's second label (i.e., the label immediately beneath the top label).

The purpose (or one of the purposes) of the top label is to get the packet delivered from A to B, so that B can further process the packet based on the second label. In this sense, the top label serves as an encapsulation header for the rest of the packet. In some cases, other sorts of encapsulation headers can replace the top label without loss of functionality. For example, an IP header or a Generic Routing Encapsulation (GRE) header could replace the top label. As the encapsulated packet would still be an MPLS packet, the result is an MPLS-in-IP or MPLS-in-GRE encapsulation.

With these encapsulations, it is possible for two LSRs that are adjacent on an LSP to be separated by an IP network, even if that IP network does not provide MPLS.

To use either of these encapsulations, the encapsulating LSR must know

the IP address of the decapsulating LSR, and

that the decapsulating LSR actually supports the particular encapsulation.

This knowledge may be conveyed to the encapsulating LSR by manual configuration, or by means of some discovery protocol. In particular, if the tunnel is being used to support a particular application and that application has a setup or discovery protocol, then the application's protocol may convey this knowledge. The means of conveying this knowledge is outside the scope of the this document.

2. Specification of Requirements

In this document, the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as described in [RFC2119].

3. Encapsulation in IP

MPLS-in-IP messages have the following format:

             +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             |                                     |
             |             IP Header               |
             |                                     |
             +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             |                                     |
             |          MPLS Label Stack           |
             |                                     |
             +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             |                                     |
             |            Message Body             |
             |                                     |
             +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

IP Header: This field contains an IPv4 or an IPv6 datagram header as defined in [RFC791] or [RFC2460], respectively. The source and destination addresses are set to addresses of the encapsulating and decapsulating LSRs, respectively.

MPLS Label Stack: This field contains an MPLS Label Stack as defined in [RFC3032].

Message Body

This field contains one MPLS message body.

The IPv4 Protocol Number field or the IPv6 Next Header field is set to 137, indicating an MPLS unicast packet. (The use of the MPLS-in- IP encapsulation for MPLS multicast packets is not supported by this specification.)

Following the IP header is an MPLS packet, as specified in [RFC3032]. This encapsulation causes MPLS packets to be sent through "IP tunnels". When the tunnel's receive endpoint receives a packet, it decapsulates the MPLS packet by removing the IP header. The packet is then processed as a received MPLS packet whose "incoming label" [RFC3031] is the topmost label of the decapsulated packet.

4. Encapsulation in GRE

The MPLS-in-GRE encapsulation encapsulates an MPLS packet in GRE [RFC2784]. The packet then consists of an IP header (either IPv4 or IPv6), followed by a GRE header, followed by an MPLS label stack as specified in [RFC3032]. The protocol type field in the GRE header MUST be set to the Ethertype value for MPLS Unicast (0x8847) or Multicast (0x8848).

This encapsulation causes MPLS packets to be sent through "GRE tunnels". When the tunnel's receive endpoint receives a packet, it decapsulates the MPLS packet by removing the IP and the GRE headers. The packet is then processed as a received MPLS packet whose "incoming label" [RFC3031] is the topmost label of the decapsulated packet.

[RFC2784] specifies an optional GRE checksum, and [RFC2890] specifies optional GRE key and sequence number fields. These optional fields are not very useful for the MPLS-in-GRE encapsulation. The sequence number and checksum fields are not needed, as there are no corresponding fields in the native MPLS packets being tunneled. The GRE key field is not needed for demultiplexing, as the top MPLS label of the encapsulated packet is used for that purpose. The GRE key field is sometimes considered a security feature, functioning as a 32-bit cleartext password, but this is an extremely weak form of security. In order (a) to facilitate high-speed implementations of the encapsulation/decapsulation procedures and (b) to ensure interoperability, we require that all implementations be able to operate correctly without these optional fields.

More precisely, an implementation of an MPLS-in-GRE decapsulator MUST be able to process packets correctly without these optional fields. It MAY be able to process packets correctly with these optional fields.

An implementation of an MPLS-in-GRE encapsulator MUST be able to generate packets without these optional fields. It MAY have the capability to generate packets with these fields, but the default state MUST be that packets are generated without these fields. The encapsulator MUST NOT include any of these optional fields unless it is known that the decapsulator can process them correctly. Methods for conveying this knowledge are outside the scope of this specification.

5. Common Procedures

Certain procedures are common to both the MPLS-in-IP and the MPLS- in-GRE encapsulations. In the following, the encapsulator, whose address appears in the IP source address field of the encapsulating IP header, is known as the "tunnel head". The decapsulator, whose address appears in the IP destination address field of the decapsulating IP header, is known as the "tunnel tail".

   If IPv6 is being used (for either MPLS-in-IPv6 or MPLS-in-GRE-in-
   IPv6), the procedures of [RFC2473] are generally applicable.

5.1. Preventing Fragmentation and Reassembly

If an MPLS-in-IP or MPLS-in-GRE packet were fragmented (due to "ordinary" IP fragmentation), the tunnel tail would have to reassemble it before the contained MPLS packet could be decapsulated. When the tunnel tail is a router, this is likely to be undesirable; the tunnel tail may not have the ability or the resources to perform reassembly at the necessary level of performance.

Whether fragmentation of the tunneled packets is allowed MUST be configurable at the tunnel head. The default value MUST be that packets are not fragmented. The default value would only be changed if it were known that the tunnel tail could perform the reassembly function adequately.

THE PROCEDURES SPECIFIED IN THE REMAINDER OF THIS SECTION ONLY APPLY IF PACKETS ARE NOT TO BE FRAGMENTED.

Obviously, if packets are not to be fragmented, the tunnel head MUST NOT fragment a packet before encapsulating it.

If IPv4 is used, then the tunnel MUST set the DF bit. This prevents intermediate nodes in the tunnel from performing fragmentation. (If IPv6 is used, intermediate nodes do not perform fragmentation in any event.)

The tunnel head SHOULD perform Path MTU Discovery ([RFC1191] for IPv4, or [RFC1981] for IPv6).

The tunnel head MUST maintain a "Tunnel MTU" for each tunnel; this is the minimum of (a) an administratively configured value, and, if known, (b) the discovered Path MTU value minus the encapsulation overhead.

If the tunnel head receives, for encapsulation, an MPLS packet whose size exceeds the Tunnel MTU, that packet MUST be discarded. However, silently dropping such packets may cause significant operational problems; the originator of the packets will notice that his data is not getting through, but he may not realize that large packets are causing packet loss. He may therefore continue sending packets that are discarded. Path MTU discovery can help (if the tunnel head sends back ICMP errors), but frequently there is insufficient information available at the tunnel head to identify the originating sender properly. To minimize problems, it is advised that MTUs be engineered to be large enough in practice to avoid fragmentation.

In some cases, the tunnel head receives, for encapsulation, an IP packet, which it first encapsulates in MPLS and then encapsulates in MPLS-in-IP or MPLS-in-GRE. If the source of the IP packet is reachable from the tunnel head, and if the result of encapsulating the packet in MPLS would be a packet whose size exceeds the Tunnel MTU, then the value that the tunnel head SHOULD use for fragmentation and PMTU discovery outside the tunnel is the Tunnel MTU value minus the size of the MPLS encapsulation. (That is, the Tunnel MTU value minus the size of the MPLS encapsulation is the MTU that is to be reported in ICMP messages.) The packet will have to be discarded, but the tunnel head should send the IP source of the discarded packet the proper ICMP error message as specified in [RFC1191] or [RFC1981].

5.2. TTL or Hop Limit

The tunnel head MAY place the TTL from the MPLS label stack into the TTL field of the encapsulating IPv4 header or the Hop Limit field of the encapsulating IPv6 header. The tunnel tail MAY place the TTL from the encapsulating IPv4 header or the Hop Limit from the encapsulating IPv6 header into the TTL field of the MPLS header, but only if this does not increase the TTL value in the MPLS header.

Whether such modifications are made, and the details of how they are made, will depend on the configuration of the tunnel tail and the tunnel head.

5.3. Differentiated Services

The procedures specified in this document enable an LSP to be sent through an IP or GRE tunnel. [RFC2983] details a number of considerations and procedures that have to be applied to support the Differentiated Services Architecture properly in the presence of IP- in-IP tunnels. These considerations and procedures also apply in the presence of MPLS-in-IP or MPLS-in-GRE tunnels.

Accordingly, when a tunnel head is about to send an MPLS packet into an MPLS-in-IP or MPLS-in-GRE tunnel, the setting of the DS field of the encapsulating IPv4 or IPv6 header MAY be determined (at least partially) by the "Behavior Aggregate" of the MPLS packet. Procedures for determining the Behavior Aggregate of an MPLS packet are specified in [RFC3270].

Similarly, at the tunnel tail, the DS field of the encapsulating IPv4 or IPv6 header MAY be used to determine the Behavior Aggregate of the encapsulated MPLS packet. [RFC3270] specifies the relation between the Behavior Aggregate and the subsequent disposition of the packet.

6. Applicability

The MPLS-in-IP encapsulation is the more efficient, and it would generally be regarded as preferable, other things being equal. There are, however, some situations in which the MPLS-in-GRE encapsulation may be used:

Two routers are "adjacent" over a GRE tunnel that exists for some reason that is outside the scope of this document, and those two routers have to send MPLS packets over that adjacency. As all packets sent over this adjacency must have a GRE encapsulation, the MPLS-in-GRE encapsulation is more efficient than the alternative, that would be an MPLS-in-IP encapsulation which is then encapsulated in GRE.

Implementation considerations may dictate the use of MPLS-in- GRE. For example, some hardware device might only be able to handle GRE encapsulations in its fastpath.

7. IANA Considerations

The IANA has allocated IP Protocol Number 137 for MPLS-in-IP encapsulation, as described in section 3. No future IANA actions will be required. The MPLS-in-GRE encapsulation does not require any IANA action.

8. Security Considerations

The main security problem faced when IP or GRE tunnels are used is the possibility that the tunnel's receive endpoint will get a packet that appears to be from the tunnel, but that was not actually put into the tunnel by the tunnel's transmit endpoint. (The specified encapsulations do not by themselves enable the decapsulator to authenticate the encapsulator.) A second problem is the possibility that the packet will be altered between the time it enters the tunnel and the time it leaves. (The specified encapsulations do not by themselves assure the decapsulator of the packet's integrity.) A third problem is the possibility that the packet's contents will be seen while the packet is in transit through the tunnel. (The specification encapsulations do not ensure privacy.) How significant these issues are in practice depends on the security requirements of the applications whose traffic is being sent through the tunnel. For example, lack of privacy for tunneled packets is not a significant issue if the applications generating the packets do not require privacy.

Because of the different potential security requirements, deployment scenarios, and performance considerations of different applications using the described encapsulation mechanism, this specification defines IPsec support as OPTIONAL. Basic implementation requirements if IPsec is implemented are described in section 8.1. If IPsec is not implemented, additional mechanisms may have to be implemented and deployed. Those are discussed in section 8.2.

8.1. Securing the Tunnel with IPsec

All of these security issues can be avoided if the MPLS-in-IP or MPLS-in-GRE tunnels are secured with IPsec. Implementation requirements defined in this section apply if IPsec is implemented.

When IPsec is used, the tunnel head and the tunnel tail should be treated as the endpoints of a Security Association. For this purpose, a single IP address of the tunnel head will be used as the source IP address, and a single IP address of the tunnel tail will be 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. If a control protocol is used to set up the tunnels (e.g., to inform one tunnel endpoint of the IP address of the other), the control protocol MUST have an authentication mechanism, and this MUST be used when the tunnel is set up. If the tunnel is set up automatically as the result of, for example, information distributed by BGP, then the use of BGP's MD5-based authentication mechanism is satisfactory.

The MPLS-in-IP or MPLS-in-GRE encapsulated packets should be viewed as originating at the tunnel head and as being destined for the tunnel tail; IPsec transport mode SHOULD thus be used.

The IP header of the MPLS-in-IP packet becomes the outer IP header of the resulting packet when the tunnel head uses IPsec transport mode to secure the MPLS-in-IP packet. This is followed by an IPsec header, followed by the MPLS label stack. The IPsec header has to set the payload type to MPLS by using the IP protocol number specified in section 3. If IPsec transport mode is applied on a MPLS-in-GRE packet, the GRE header follows the IPsec header.

At the tunnel tail, IPsec outbound processing recovers the contained MPLS-in-IP/GRE packet. The tunnel tail then strips off the encapsulating IP/GRE header to recover the MPLS packet, which is then forwarded according to its label stack.

Note that the tunnel tail and the tunnel head are LSP adjacencies, 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.

An IPsec-secured MPLS-in-IP or MPLS-in-GRE tunnel MUST provide authentication and integrity. (Note that the authentication and integrity will apply to the entire MPLS packet, including the MPLS label stack.) Thus, the implementation MUST support ESP will 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 by means of IKE [RFC2409]. Manual keying MUST be supported. If automatic keying is implemented, IKE in main mode with preshared keys

MUST be supported. A particular application may escalate this requirement and request implementation of automatic keying.

Manual key distribution is much simpler, but also less scalable, than automatic key distribution. Therefore, which method of key distribution is appropriate for a particular tunnel has to be carefully considered by the administrator (or pair of administrators) responsible for the tunnel endpoints. If replay protection is regarded as necessary for a particular tunnel, automatic key distribution should be configured.

If the MPLS-in-IP encapsulation is being used, the selectors associated with the SA would be the source and destination addresses mentioned above, plus the IP protocol number specified in section 3. If it is desired to secure multiple MPLS-in-IP tunnels between a given pair of nodes separately, each tunnel must have unique pair of IP addresses.

If the MPLS-in-GRE encapsulation is being used, the selectors associated with the SA would be the source and destination addresses mentioned above, and the IP protocol number representing GRE (47). If it is desired to secure multiple MPLS-in-GRE tunnels between a given pair of nodes separately, each tunnel must have unique pair of IP addresses.

8.2. 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 forwarded 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 an MPLS-in-IP or MPLS-in-GRE validates the IP source address of the packet. This document does not require that the decapsulator validate the IP source address of the tunneled packets, but it should be understood that failure to do so presupposes that there is effective destination-based (or a combination of source-based and destination-based) filtering at the boundaries.

9. Acknowledgements

This specification combines prior work on encapsulating MPLS in IP, by Tom Worster, Paul Doolan, Yasuhiro Katsube, Tom K. Johnson, Andrew G. Malis, and Rick Wilder, with prior work on encapsulating MPLS in GRE, by Yakov Rekhter, Daniel Tappan, and Eric Rosen. The current authors wish to thank all these authors for their contribution.

Many people have made valuable comments and corrections, including Rahul Aggarwal, Scott Bradner, Alex Conta, Mark Duffy, Francois Le Feucheur, Allison Mankin, Thomas Narten, Pekka Savola, and Alex Zinin.

10. Normative References

   [RFC791]  Postel, J., "Internet Protocol", STD 5, RFC 791, September
   
             1981.
   
   [RFC792]  Postel, J., "Internet Control Message Protocol", STD 5, RFC
             792, September 1981.
   
   [RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
             November 1990.
   
   [RFC1981] McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
             for IP version 6", RFC 1981, August 1996.
   
   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.
   
   [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
             (IPv6) Specification", RFC 2460, December 1998.
   
   [RFC2463] Conta, A. and S. Deering, "Internet Control Message
             Protocol (ICMPv6) for the Internet Protocol Version 6
             (IPv6) Specification", RFC 2463, December 1998.
   
   [RFC2473] Conta, A. and S. Deering, "Generic Packet Tunneling in IPv6
             Specification", RFC 2473, December 1998.
   
   [RFC2784] Farinacci, D., Li, T., Hanks, S., Meyer, D., and P. Traina,
             "Generic Routing Encapsulation (GRE)", RFC 2784, March
             2000.
   
   [RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
             Label Switching Architecture", RFC 3031, January 2001.
   
   [RFC3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
             Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
             Encoding", RFC 3032, January 2001.

11. Informative References

   [RFC2401] Kent, S. and R. Atkinson, "Security Architecture for the
             Internet Protocol", RFC 2401, November 1998.
   
   [RFC2402] Kent, S. and R. Atkinson, "IP Authentication Header", RFC
             2402, November 1998.
   
   [RFC2406] Kent, S. and R. Atkinson, "IP Encapsulating Security
             Payload (ESP)", RFC 2406, November 1998.
   
   [RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange
             (IKE)", RFC 2409, November 1998.
   
   [RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
             and W. Weiss, "An Architecture for Differentiated Service",
             RFC 2475, December 1998.
   
   [RFC2890] Dommety, G., "Key and Sequence Number Extensions to GRE",
             RFC 2890, September 2000.
   
   [RFC2983] Black, D., "Differentiated Services and Tunnels", RFC 2983,
             October 2000.
   
   [RFC3260] Grossman, D., "New Terminology and Clarifications for
             Diffserv", RFC 3260, April 2002.
   
   [RFC3270] Le Faucheur, F., Wu, L., Davie, B., Davari, S., Vaananen,
             P., Krishnan, R., Cheval, P., and J. Heinanen, "Multi-
             Protocol Label Switching (MPLS) Support of Differentiated
             Services", RFC 3270, May 2002.

Authors' Addresses

Tom Worster
Motorola, Inc.
120 Turnpike Road
Southborough, MA 01772

   EMail: tom.worster@motorola.com

Yakov Rekhter
Juniper Networks, Inc.
1194 N. Mathilda Ave.
Sunnyvale, CA 94089

EMail:

          yakov@juniper.net

Eric Rosen
Cisco Systems, Inc.
1414 Massachusetts Avenue
Boxborough, MA 01719

EMail:

          erosen@cisco.com

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