Network Working Group Tom Worster
Internet Draft
Expiration Date: September 2004
Yakov Rekhter
Juniper Networks, Inc.
Eric C. Rosen, editor
Cisco Systems, Inc.
March 2004
Encapsulating MPLS in IP or Generic Routing Encapsulation (GRE)
draft-ietf-mpls-in-ip-or-gre-06.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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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 which do not have MPLS enabled in
their core routers. This draft specifies two IP-based
encapsulations, MPLS-in-IP, and MPLS-in-GRE (Generic Routing
Encapsulation). Each of these is applicable in some circumstances.
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Table of Contents
1 Specification of Requirements .......................... 2
2 Motivation ............................................. 2
3 Encapsulation in IP .................................... 3
4 Encapsulation in GRE ................................... 5
5 Common Procedures ...................................... 6
5.1 Preventing Fragmentation and Reassembly ................ 6
5.2 TTL or Hop Limit ....................................... 7
5.3 Differentiated Services ................................ 8
6 Applicability .......................................... 8
7 IANA Considerations .................................... 9
8 Security Considerations ................................ 9
8.1 Securing the Tunnel Using IPsec ........................ 9
8.2 In the Absence of IPsec ................................ 11
9 Acknowledgments ........................................ 12
10 Normative References ................................... 12
11 Informative References ................................. 13
12 Author Information ..................................... 13
13 Intellectual Property Notice ........................... 14
14 Copyright Notice ....................................... 14
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. 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 such LSP, there is a Label Switching Router (LSR) which is
the "LSP Ingress", and an LSR which 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)
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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 the top label can be replaced, without loss of
functionality, by other sorts of encapsulation headers. For example,
the top label could be replaced by an IP header or a Generic Routing
Encapsulation (GRE) header. 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.
In order 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 this knowledge may be conveyed by the application's protocol.
The means of conveying this knowledge is outside the scope of the
current document.
3. Encapsulation in IP
MPLS-in-IP messages have the following format:
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| 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 [value to be assigned by IANA], 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 a packet is received by the tunnel's receive
endpoint, the receive endpoint 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.
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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 a packet is received by the tunnel's receive endpoint,
the receive endpoint decapsulates the MPLS packet by removing the IP
header and the GRE header. 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 that are 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 to be a security feature,
functioning as a 32-bit cleartext password, but this is an extremely
weak form of security. In order to (a) facilitate high speed
implementations of the encapsulation/decapsulation procedures, and
(b) 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 correctly process packets without these optional fields.
It MAY be able to correctly process packets 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.
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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".
In the case where 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 to get fragmented (due to
"ordinary" IP fragmentation), it would have to be be reassembled by
the tunnel tail 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 to be 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
IN THE CASE WHERE 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 being used, then the tunnel MUST set the DF bit. This
prevents intermediate nodes in the tunnel from performing
fragmentation. (If IPv6 is being 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
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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 it is large packets
that are the cause of 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 properly
identify the originating sender. 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 which the tunnel head SHOULD use for the purposes
of 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 needs to get 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 form the
encapsulating IPv6 header into the TTL field of the MPLS header, but
only if that does not cause the TTL value in the MPLS header to
become larger.
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.
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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 which need to be applied to properly
support the Differentiated Services Architecture 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 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 need 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, which 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.
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7. IANA Considerations
The MPLS-in-IP encapsulation requires that IANA allocate an IP
Protocol Number, 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 using IP or GRE tunnels is the
possibility that the tunnel's receive endpoint will get a packet
which appears to be from the tunnel, but which was not actually put
into the tunnel by the tunnel's transmit endpoint. (I.e., 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 tunnel. (I.e., 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. (I.e., 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. E.g., lack of privacy for tunneled
packets is not a significant issue if the applications generating the
packets do not require privacy.
8.1. Securing the Tunnel Using IPsec
All of these security issues can be avoided if the MPLS-in-IP or
MPLS-in-GRE tunnels are secured using IPsec.
When using IPsec, 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 setting up the tunnel. If the tunnel is set up
automatically as the result, e.g., of information distributed by BGP,
then the use of BGP's MD5-based authentication mechanism is
satisfactory.
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The MPLS-in-IP or MPLS-in-GRE encapsulated packets should be
considered 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 IPsec transport mode is used by the tunnel
head to secure the MPLS-in-IP packet. This is followed by an IPsec
header followed by the MPLS label stack. The IPsec header needs 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.
Recall 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 which 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 which 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.) Whether additional security, i.e., confidentiality
and/or replay protection, is required will depend upon the needs of
the applications whose data is being sent through the tunnel. If
confidentiality is not needed, then either the AH or the ESP
protocols MAY be used. If confidentiality is needed, the ESP
protocol MUST be used, and the payload must be encrypted. If ESP is
used, the tunnel tail MUST check that the source IP address of any
packet that is received on a given SA is the one that is expected.
Key distribution may be done either manually, or automatically by
means of IKE [RFC2409]. Manual key distribution is much simpler, but
also less scalable, than automatic key distribution. Which method of
key distribution is appropriate for a particular tunnel thus needs 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 MUST be used.
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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 separately secure multiple MPLS-in-IP tunnels
between a given pair of nodes, 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 the source and destination
addresses mentioned above, and the IP protocol number representing
GRE (47). If it is desired to separately secure multiple MPLS-in-GRE
tunnels between a given pair of nodes, each tunnel must have unique
pair of IP addresses.
8.2. In the Absence of IPsec
If the tunnels are not secured using 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
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 combination
of source-based and destination-based) filtering at the boundaries.
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9. Acknowledgments
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, and Pekka Savola.
10. Normative References
[RFC791] "Internet Protocol," J. Postel, Sep 1981
[RFC792] "Internet Control Message Protocol", J. Postel, Sept 1981
[RFC1191] "Path MTU Discovery", J.C. Mogul, S.E. Deering, November
1990
[RFC1981] "Path MTU Discovery for IP version 6", J. McCann, S.
Deering, J. Mogul, August 1996
[RFC2460]"Internet Protocol, Version 6 (IPv6) Specification," S.
Deering and R. Hinden, RFC 2460,Dec 1998
[RFC2463] "Internet Control Message Protocol (ICMPv6) for the
Internet Protocol Version 6 (IPv6) Specification", A. Conta, S.
Deering, December 1998
[RFC2473] "Generic Packet Tunneling in IPv6 Specification", A. Conta,
S. Deering, December 1998
[RFC2784] "Generic Routing Encapsulation (GRE)", D. Farinacci, T. Li,
S. Hanks, D. Meyer, P. Traina, March 2000
[RFC3031] "Multiprotocol Label Switching Architecture", E. Rosen, A.
Viswanathan, R. Callon, January 2001
[RFC3032] "MPLS Label Stack Encoding", E. Rosen, D. Tappan, G.
Fedorkow, Y. Rekhter, D. Farinacci, T. Li, A. Conta. January 2001
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11. Informative References
[RFC2401] "Security Architecture for the Internet Protocol", S. Kent,
R. Atkinson, November 1998
[RFC2402] "IP Authentication Header", S. Kent, R. Atkinson, November
1998
[RFC2406] "IP Encapsulating Security Payload (ESP)", S. Kent
R.Atkinson, November 1998
[RFC2409] "The Internet Key Exchange (IKE)", D. Harkins, D. Carrel,
November 1998
[RFC2475] "An Architecture for Differentiated Service", S. Blake, D.
Black, M. Carlson, E. Davies, Z. Wang, W. Weiss. December 1998
[RFC2890] "Key and Sequence Number Extensions to GRE", G. Dommety,
August 2000
[RFC2983] "Differentiated Services and Tunnels", D. Black. October
2000
[RFC3260] "New Terminology and Clarifications for Diffserv", D.
Grossman, April 2002
[RFC3270] "Multiprotocol Label Switching (MPLS) Support of
Differentiated Services", F. Le Faucheur, L. Wu, B. Davie, S. Davari,
P. Vaananen, R. Krishnan, P. Cheval, J. Heinanen. May 2002
12. Author Information
Tom Worster
Email: fsb@thefsb.org
Yakov Rekhter
Juniper Networks, Inc.
1194 N. Mathilda Ave.
Sunnyvale, CA 94089
Email: yakov@juniper.net
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Eric Rosen
Cisco Systems, Inc.
1414 Massachusetts Avenue
Boxborough, MA 01719
Email: erosen@cisco.com
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English.
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