Network Working Group Tom Worster
Internet Draft
Expiration Date: February 2003
Yakov Rekhter
Juniper Networks, Inc.
Eric C. Rosen, editor
Cisco Systems, Inc.
August 2002
Encapsulating MPLS in IP or GRE
draft-rosen-mpls-in-ip-or-gre-00.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
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Abstract
In various applications of MPLS, label stacks with multiple entries
are used. 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. Each of these is
applicable in some circumstances.
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Table of Contents
1 Motivation ............................................. 2
2 Encapsulation in IP .................................... 3
3 Encapsulation in GRE ................................... 4
4 Common Procedures ...................................... 4
4.1 Fragmentation, Reassembly, and MTU ..................... 4
4.2 TTL .................................................... 5
4.3 EXP and DSCP fields .................................... 5
5 Applicability .......................................... 5
6 Security Considerations ................................ 6
7 Acknowledgments ........................................ 6
8 References ............................................. 6
9 Author Information ..................................... 7
1. Motivation
In many applications of MPLS, packets traversing an MPLS backbone
carry label stacks with more than one label. As described in
[RFC3031], section 3.15, 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)
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 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.
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2. 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] and [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 Protocol Number field in an IPv4 header and the Next Header field
in an IPv6 are set as follows:
- X indicates an MPLS unicast packet,
- Y indicates an MPLS multicast packet. (The use of the MPLS-in-IP
encapsulation for MPLS multicast packets is for further study.)
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 packet of
the decapsulated packet.
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3. Encapsulation in GRE
The MPLS-in-GRE encapsulation encapsulates an MPLS packet in GRE
[RFC2784]. The packet then consists of an IP header 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). The
optional GRE checksum, key [RFC2890] and sequence number [RFC2890]
fields MUST NOT be used.
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
packet of the decapsulated packet.
4. 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".
4.1. Fragmentation, Reassembly, and MTU
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 be decapsulated. To
avoid the need for the tunnel tail to perform reassembly, the tunnel
head MUST set the Don't Fragment flag of the encapsulating IPv4
header.
The tunnel head SHOULD perform Path MTU Discovery [RFC1191] over each
MPLS-in-IP and MPLS-in-GRE tunnel.
The tunnel head MUST maintain a Tunnel MTU value for each MPLS-in-IP
or MPLS-in-GRE 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.
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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 this
encapsulation would be a packet whose size exceeds the Tunnel MTU,
then the tunnel head SHOULD use the Tunnel MTU value for the purposes
of fragmentation and PMTU discovery outside the tunnel.
4.2. TTL
The tunnel head MAY place the TTL from the MPLS label stack into the
encapsulating IP header. The tunnel tail MAY place the TTL from the
encapsulating IP header into the MPLS header, but only if that does
not cause the TTL value in the MPLS header to become smaller.
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.
4.3. EXP and DSCP fields
The tunnel head MAY consider the EXP field of the encapsulated MPLS
packet when setting the DSCP field of the encapsulating IP header.
The tunnel tail MAY modify the EXP field of the encapsulated MPLS
packet, based on consideration of the DSCP field of the encapsulating
IP 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. 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.
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- 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.
6. Security Considerations
MPLS-in-IP or MPLS-in-GRE tunnels may be secured using IPsec. If
they 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. This can be done by address filtering at the boundaries of an
administrative domain. When the tunnel head and the tunnel tail are
not in the same domain, this may become difficult, and it can even
become impossible if the packets must traverse the public Internet.
7. Acknowledgments
This draft is a combination of two previous drafts:
- draft-worster-mpls-in-ip, by Tom Worster, Paul Doolan, Yasuhiro
Katsube, Tom K. Johnson, Andrew G. Malis, and Rick Wilder
- draft-rekhter-mpls-over-gre, by Yakov Rekhter, Daniel Tappan, and
Eric Rosen
The current authors wish to thank all these authors for their
contribution.
8. References
[RFC7915] "Internet Protocol," J. Postel, Sep 1981
[RFC2460]"Internet Protocol, Version 6 (IPv6) Specification," S.
Deering and R. Hinden, RFC 2460,Dec 1998
[RFC1191] "Path MTU Discovery", J.C. Mogul, S.E. Deering, November
1990
[RFC2784] "Generic Routing Encapsulation (GRE)", D. Farinacci, T. Li,
S. Hanks, D. Meyer, P. Traina, March 2000
[RFC2890] "Key and Sequence Number Extensions to GRE", G. Dommety,
August 2000
[RFC3031] "Multiprotocol Label Switching Architecture", E. Rosen, A.
Worster, et al. [Page 6]
Internet Draft draft-rosen-mpls-in-ip-or-gre-00.txt August 2002
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
9. Author Information
Tom Worster
Email: fsb@thefsb.org
Yakov Rekhter
Juniper Networks, Inc.
1194 N. Mathilda Ave.
Sunnyvale, CA 94089
Email: yakov@juniper.net
Eric Rosen
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA, 01824
e-mail: erosen@cisco.com
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