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IETF RFC 4817
Encapsulation of MPLS over Layer 2 Tunneling Protocol Version 3
Last modified on Monday, March 26th, 2007
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Network Working Group M. Townsley
Request for Comments: 4817 C. Pignataro
Category: Standards Track S. Wainner
Cisco Systems
T. Seely
Sprint Nextel
J. Young
March 2007
Encapsulation of MPLS over Layer 2 Tunneling Protocol Version 3
 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
Copyright © The IETF Trust (2007).
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 stack and its payload
over the L2TPv3 data encapsulation. This enables an application that
traditionally requires an MPLS-enabled core network, to utilize an
L2TPv3 encapsulation over an IP network instead.
Townsley, et al. Standards Track PAGE 1 
RFC 4817 MPLS over L2TPv3 March 2007
Table of Contents
1. Introduction ....................................................2
1.1. Specification of Requirements ..............................2
2. MPLS over L2TPv3 Encoding .......................................2
3. Assigning the L2TPv3 Session ID and Cookie ......................4
4. Applicability ...................................................4
5. Congestion Considerations .......................................6
6. Security Considerations .........................................6
6.1. In the Absence of IPsec ....................................7
6.2. Context Validation .........................................7
6.3. Securing the Tunnel with IPsec .............................8
7. Acknowledgements ................................................9
8. References .....................................................10
8.1. Normative References ......................................10
8.2. Informative References ....................................10
1. Introduction
This document defines how to encapsulate an MPLS label stack and its
payload inside the L2TPv3 tunnel payload. After defining the MPLS
over L2TPv3 encapsulation procedure, other MPLS over IP encapsulation
options, including IP, Generic Routing Encapsulation (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 that may be
enabled over L2TPv3.
1.1. 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 [RFC 2119].
2. MPLS over L2TPv3 Encoding
MPLS over L2TPv3 allows tunneling of an MPLS stack [RFC 3032] and its
payload over an IP network, utilizing the L2TPv3 encapsulation
defined in [RFC 3931]. The MPLS Label Stack and payload are carried
in their entirety following IP (either IPv4 or IPv6) and L2TPv3
headers.
Townsley, et al. Standards Track PAGE 2
RFC 4817 MPLS over L2TPv3 March 2007
+-+-+-+-+-+-+-+-+-+-+
| IP |
+-+-+-+-+-+-+-+-+-+-+
| L2TPv3 |
+-+-+-+-+-+-+-+-+-+-+
| MPLS Label Stack |
+-+-+-+-+-+-+-+-+-+-+
| MPLS Payload |
+-+-+-+-+-+-+-+-+-+-+
Figure 2.1 MPLS Packet 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 over L2TPv3 encapsulation
When encapsulating MPLS over L2TPv3, the L2TPv3 L2-Specific Sublayer
MAY be present. It is generally not present; hence, it is 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.)
Townsley, et al. Standards Track PAGE 3
RFC 4817 MPLS over L2TPv3 March 2007
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 [RFC 4086], 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 [RFC 3032].
The optional L2-Specific Sublayer (defined in [RFC 3931]) is generally
not present for MPLS over L2TPv3.
Generic IP encapsulation procedures, such as fragmentation and MTU
considerations, handling of Time to Live (TTL), EXP, and
Differentiated Services Code Point (DSCP) bits, etc. are the same as
the "Common Procedures" for IP encapsulation of MPLS defined in
Section 5 of [RFC 4023] 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 (if present) is out of scope.
4. Applicability
The methods defined in [RFC 4023], [MPLS-IPSEC], and this document all
describe methods for carrying MPLS over an IP network. Cases where
MPLS over L2TPv3 is comparable to other alternatives are discussed in
this section.
Townsley, et al. Standards Track PAGE 4
RFC 4817 MPLS over L2TPv3 March 2007
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 packets
increases the likelihood that an MPLS label-spoofing attack will
succeed. L2TPv3 provides an additional level of protection against
packet spoofing before allowing a packet to enter a Virtual Private
Network (VPN) (much like IPsec provides an additional level of
protection at a Provider Edge (PE) router rather than relying on
Access Control List (ACL) filters). Checking the value of the L2TPv3
Cookie is similar to any sort of ACL that inspects the contents of a
packet header, except that we give ourselves the luxury of "seeding"
the L2TPv3 header with a value that is very difficult to spoof.
MPLS over L2TPv3 may be advantageous compared to [RFC 4023], if:
Two routers are already "adjacent" over an L2TPv3 tunnel
established for some other reason, and wish to exchange 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, service integrity and resource
protection are of 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 Access Control
Lists (ACLs) and numbering methods may be used to protect the PE
routers from rogue IP sources, but may be subject to error and
cumbersome to maintain at all edge points at all times. The
L2TPv3 Cookie provides a simple 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 core network).
(The first two of the above applicability statements were adopted
from [RFC 4023].)
In summary, L2TPv3 can provide a balance between the limited security
against IP spoofing attacks offered by [RFC 4023] vs. the greater
security and associated operational and processing overhead offered
Townsley, et al. Standards Track PAGE 5
RFC 4817 MPLS over L2TPv3 March 2007
by [MPLS-IPSEC]. Further, MPLS over L2TPv3 may be faster in some
hardware, particularly if that hardware is already optimized to
classify incoming L2TPv3 packets carrying IP framed in a variety of
ways. For example, IP encapsulated by High-Level Data Link Control
(HDLC) or Frame Relay over L2TPv3 may be equivalent in complexity to
IP encapsulated by MPLS over L2TPv3.
5. Congestion Considerations
This document specifies an encapsulation method for MPLS inside the
L2TPv3 tunnel payload. MPLS can carry a number of different
protocols as payloads. When an MPLS/L2TPv3 flow carries IP-based
traffic, the aggregate traffic is assumed to be TCP friendly due to
the congestion control mechanisms used by the payload traffic.
Packet loss will trigger the necessary reduction in offered load, and
no additional congestion avoidance action is necessary.
When an MPLS/L2TPv3 flow carries payload traffic that is not known to
be TCP friendly and the flow runs across an unprovisioned path that
could potentially become congested, the application that uses the
encapsulation specified in this document MUST employ additional
mechanisms to ensure that the offered load is reduced appropriately
during periods of congestion. The MPLS/L2TPv3 flow should not exceed
the average bandwidth that a TCP flow across the same network path
and experiencing the same network conditions would achieve, measured
on a reasonable timescale. This is not necessary in the case of an
unprovisioned path through an over-provisioned network, where the
potential for congestion is avoided through the over-provisioning of
the network.
The comparison to TCP cannot be specified exactly but is intended as
an "order-of-magnitude" comparison in timescale and throughput. The
timescale on which TCP throughput is measured is the roundtrip time
of the connection. In essence, this requirement states that it is
not acceptable to deploy an application using the encapsulation
specified in this document on the best-effort Internet, which
consumes bandwidth arbitrarily and does not compete fairly with TCP
within an order of magnitude. One method of determining an
acceptable bandwidth is described in [RFC 3448]. Other methods exist,
but are outside the scope of this document.
6. Security Considerations
There are three main concerns when transporting MPLS-labeled traffic
between PEs using IP tunnels. The first is the possibility that a
third party may insert packets into a packet stream between two PEs.
The second is that a third party might view the packet stream between
two PEs. The third is that a third party may alter packets in a
Townsley, et al. Standards Track PAGE 6
RFC 4817 MPLS over L2TPv3 March 2007
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 should
consider the cost to mitigate the risk.
Security is also discussed as part of the applicability discussion in
Section 4 of this document.
6.1. 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 [RFC 3931] and the "Context Validation" considerations in
Section 6.2 of this document apply. Those two sections highlight the
benefits of the L2TPv3 Cookie.
6.2. Context Validation
The L2TPv3 Cookie does not provide protection via encryption.
However, when used with a sufficiently random, 64-bit value that is
kept secret from an off-path attacker, 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 that 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
Townsley, et al. Standards Track PAGE 7
RFC 4817 MPLS over L2TPv3 March 2007
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.
6.3. Securing the Tunnel with IPsec
L2TPv3 tunnels may also be secured using IPsec, as specified in
Section 4.1.3 of [RFC 3931]. 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 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 is set up automatically as the result of, for
example, information distributed by BGP, then the use of BGP's
Message Digest 5 (MD5)-based authentication mechanism can serve this
purpose.
The MPLS over L2TPv3 encapsulated packets should be considered as
originating at the tunnel head and being destined for the tunnel
tail; IPsec transport mode SHOULD thus be used.
Note that the tunnel tail and the tunnel head are Label Switched Path
(LSP) adjacencies (for label distribution adjacencies, see
[RFC 3031]), 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.
Townsley, et al. Standards Track PAGE 8
RFC 4817 MPLS over L2TPv3 March 2007
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 Encapsulating Security
Payload (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 Security Association (SA) is the one expected.
Key distribution may be done either manually or automatically by
means of the Internet Key Exchange (IKE) Protocol [RFC 2409] or IKEv2
[RFC 4306]. 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. If
replay protection is regarded as necessary for a particular tunnel,
automatic key distribution should be configured.
7. Acknowledgements
Thanks to Robert Raszuk, Clarence Filsfils, and Eric Rosen for their
review of this document. Some text was adopted from [RFC 4023].
Townsley, et al. Standards Track PAGE 9
RFC 4817 MPLS over L2TPv3 March 2007
8. References
8.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
Encoding", RFC 3032, January 2001.
[RFC 3931] Lau, J., Townsley, M., and I. Goyret, "Layer Two
Tunneling Protocol - Version 3 (L2TPv3)", RFC 3931,
March 2005.
[RFC 4023] Worster, T., Rekhter, Y., and E. Rosen, "Encapsulating
MPLS in IP or Generic Routing Encapsulation (GRE)", RFC
4023, March 2005.
[RFC 4086] Eastlake, D., 3rd, Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC
4086, June 2005.
8.2. Informative References
[MPLS-IPSEC] Rosen, E., De Clercq, J., Paridaens, O., T'Joens, Y.,
and C. Sargor, "Architecture for the Use of PE-PE IPsec
Tunnels in BGP/MPLS IP VPNs", Work in Progress, August
2005.
[RFC 3031] Rosen, E., Viswanathan, A., and R. Callon,
"Multiprotocol Label Switching Architecture", RFC 3031,
January 2001.
[RFC 2409] Harkins, D. and D. Carrel, "The Internet Key Exchange
(IKE)", RFC 2409, November 1998.
[RFC 4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
RFC 4306, December 2005.
[RFC 3448] Handley, M., Floyd, S., Padhye, J., and J. Widmer, "TCP
Friendly Rate Control (TFRC): Protocol Specification",
RFC 3448, January 2003.
Townsley, et al. Standards Track PAGE 10
RFC 4817 MPLS over L2TPv3 March 2007
Authors' Addresses
W. Mark Townsley
Cisco Systems
USA
Phone: +1-919-392-3741
EMail: mark@townsley.net
Carlos Pignataro
Cisco Systems
7025 Kit Creek Road
PO Box 14987
Research Triangle Park, NC 27709
USA
Phone: +1-919-392-7428
EMail: cpignata@cisco.com
Scott Wainner
Cisco Systems
13600 Dulles Technology Drive
Herndon, VA 20171
USA
EMail: swainner@cisco.com
Ted Seely
Sprint Nextel
12502 Sunrise Valley Drive
Reston, VA 20196
USA
Phone: +1-703-689-6425
EMail: tseely@sprint.net
Jeff Young
EMail: young@jsyoung.net
Townsley, et al. Standards Track PAGE 11
RFC 4817 MPLS over L2TPv3 March 2007
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Copyright © The IETF Trust (2007).
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contained in BCP 78, and except as set forth therein, the authors
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Acknowledgement
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Townsley, et al. Standards Track PAGE 12
Encapsulation of MPLS over Layer 2 Tunneling Protocol Version 3
RFC TOTAL SIZE: 26538 bytes
PUBLICATION DATE: Monday, March 26th, 2007
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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