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IETF RFC 6245
Generic Routing Encapsulation (GRE) Key Extension for Mobile IPv4
Last modified on Saturday, May 28th, 2011
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Internet Engineering Task Force (IETF) P. Yegani
Request for Comments: 6245 Juniper Networks
Category: Standards Track K. Leung
ISSN: 2070-1721 Cisco Systems
A. Lior
Bridgewater Systems
K. Chowdhury
J. Navali
Cisco Systems
May 2011
Generic Routing Encapsulation (GRE) Key Extension for Mobile IPv4
 Abstract
The Generic Routing Encapsulation (GRE) specification contains a Key
field, which MAY contain a value that is used to identify a
particular GRE data stream. This specification defines a new Mobile
IP extension that is used to exchange the value to be used in the GRE
Key field. This extension further allows the Mobility Agents to set
up the necessary protocol interfaces prior to receiving the mobile
node traffic. The new extension allows a Foreign Agent to request
GRE tunneling without disturbing the Home Agent behavior specified
for Mobile IPv4. GRE tunneling with the Key field allows the
operators to have home networks that consist of multiple Virtual
Private Networks (VPNs), which may have overlapping home addresses.
When the tuple <Care of Address, Home Address, and Home Agent
Address> is the same across multiple subscriber sessions, GRE
tunneling will provide a means for the Foreign Agent and Home Agent
to identify data streams for the individual sessions based on the GRE
key. In the absence of this key identifier, the data streams cannot
be distinguished from each other -- a significant drawback when using
IP-in-IP tunneling.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/RFC 6245.
Yegani, et al. Standards Track PAGE 1 
RFC 6245 GRE Key Ext. for MIP4 May 2011
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................2
2. Terminology .....................................................3
3. GRE Key Extension ...............................................3
4. Operation and Use of the GRE Key Extension ......................3
4.1. Foreign Agent Requirements for GRE Tunneling Support .......3
4.2. Home Agent Requirements for GRE Tunneling Support ..........4
4.3. Mobile Node Requirements for GRE Tunneling Support .........5
5. GRE Key Extension and Tunneling Procedures ......................5
6. IANA Considerations .............................................6
7. Security Considerations .........................................6
8. Acknowledgements ................................................7
9. Normative References ............................................7
1. Introduction
This document specifies a new extension for use by a Foreign Agent
(FA) operating Mobile IP for IPv4. The new extension allows a
Foreign Agent to request Generic Routing Encapsulation (GRE)
[RFC 2784] tunneling without disturbing the Home Agent (HA) behavior
specified for Mobile IPv4 [RFC 5944]. This extension contains the GRE
key [RFC 2890] required for establishing a GRE tunnel between the FA
and the HA.
GRE tunneling with the Key field allows the operators to have home
networks that consist of multiple Virtual Private Networks (VPNs),
which may have overlapping home addresses. When the tuple <Care of
Address, Home Address, and Home Agent Address> is the same across
Yegani, et al. Standards Track PAGE 2
RFC 6245 GRE Key Ext. for MIP4 May 2011
multiple subscriber sessions, GRE tunneling will provide a means for
the FA and the HA to identify data streams for the individual
sessions based on the GRE key. In the absence of this key
identifier, the data streams cannot be distinguished from each other
-- a significant drawback when using IP-in-IP tunneling.
2. Terminology
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]. Other
terminology is used as already defined in [RFC 5944].
3. GRE Key Extension
The format of the GRE Key Extension conforms to the extension format
specified for Mobile IPv4 [RFC 5944]. This extension option is used
by the Foreign Agent to supply GRE key and other necessary
information to the Home Agent to establish a GRE tunnel between the
FA and the HA.
4. Operation and Use of the GRE Key Extension
4.1. Foreign Agent Requirements for GRE Tunneling Support
The FA MUST support IP-in-IP tunneling of datagrams for Mobile IPv4
[RFC 5944]. The FA may support GRE tunneling that can be used, for
example, to allow for overlapping private home IP addresses
(Section 4.2.2.5 of [X.S0011-E]). If the FA is capable of supporting
GRE encapsulation, it should set the 'G' (GRE encapsulation) bit in
the Flags field in the Agent Advertisement message sent to the Mobile
Node (MN) during the Mobile IP session establishment.
If the MN does not set the 'G' bit, the FA MAY fall back to using
IP-in-IP encapsulation for the session per [RFC 5944].
If the MN does not set the 'G' bit and does not set the 'D'
(Decapsulation by mobile node) bit (i.e., the mobile node does not
request GRE tunneling and is not using a co-located care-of address),
and the local policy allows the FA to override the 'G' bit setting
received from the MN, the FA MUST include the GRE Key Extension as
defined in this memo in the Registration Request (RRQ) that it
propagates to the HA. The presence of this extension is a request
for GRE encapsulation that takes precedence over the setting of the
'G' bit in the Registration Request. The FA MUST NOT modify the 'G'
bit in the Registration Request because it is protected by the
Mobile-Home Authentication extension.
Yegani, et al. Standards Track PAGE 3
RFC 6245 GRE Key Ext. for MIP4 May 2011
If the FA does not support GRE encapsulation, the FA MUST reset the
'G' bit in the Agent Advertisement message. In this case, if the MN
sets the 'G' bit in the Registration Request message, the FA returns
a Registration Reply (RRP) message to the MN with code 'requested
encapsulation unavailable' (72) per [RFC 5944].
If the FA allows GRE encapsulation, and either the MN requested GRE
encapsulation or local policy dictates using GRE encapsulation for
the session, and the 'D' bit is not set (i.e., the mobile node is not
using a co-located care-of address), the FA MUST include the GRE Key
in the GRE Key Extension in all Mobile IP Registration Requests
(including initial, renewal, and de-registration requests) before
forwarding the request to the HA. The FA may include a GRE key of
value zero in the GRE Key Extension to signal that the HA assigns GRE
keys in both directions. The GRE key assignment in the FA and the HA
is outside the scope of this memo.
The GRE Key Extension SHALL follow the format defined in [RFC 5944].
This extension SHALL be added after the MN-HA and MN-FA Challenge and
MN-AAA (Mobile Node - Authentication, Authorization, and Accounting)
extensions (if any) and before the FA-HA Auth extension (if any).
4.2. Home Agent Requirements for GRE Tunneling Support
The HA MUST follow the procedures specified in [RFC 5944] in
processing this extension in Registration Request messages.
If the HA receives the GRE Key Extension in a Registration Request
and does not recognize this non-skippable extension, it MUST silently
discard the message. The HA MUST use other alternative forms of
encapsulation (e.g., IP-in-IP tunneling), when requested by the
mobile node per [RFC 5944].
If the HA receives the GRE Key Extension in a Registration Request
and recognizes the GRE Key Extension but is not configured to support
GRE encapsulation, it MUST send an RRP with code 'requested
encapsulation unavailable (139)' [RFC 3024].
If the HA receives a Registration Request with a GRE Key Extension
but without the 'G' bit set, the HA SHOULD treat this as if the 'G'
bit is set in the Registration Request; i.e., the presence of a GRE
Key Extension indicates a request for GRE encapsulation.
If the HA receives the GRE Key Extension in a Registration Request,
and it recognizes the GRE Key Extension as well as supports GRE
encapsulation, the following procedures should apply:
Yegani, et al. Standards Track PAGE 4
RFC 6245 GRE Key Ext. for MIP4 May 2011
o The HA SHOULD accept the RRQ and send an RRP with code
'registration accepted (0)'.
o The HA MUST assign a GRE key and include the GRE Key Extension in
the RRP before sending it to the FA.
o The HA MUST include the GRE Key Extension in all RRPs in response
to any RRQ that included the GRE Key Extension, when a GRE key is
available for the registration.
If the HA receives the GRE Key Extension in the initial Registration
Request and recognizes the GRE Key Extension carrying a GRE key value
of zero, it SHOULD accept the RRQ and send an RRP with code
'registration accepted (0)', and the following procedures apply:
o The HA MUST assign GRE keys for both directions and include these
keys in the GRE Key Extension in the RRP before sending it to
the FA.
o The HA MUST include the GRE Key Extension in the RRP in response
to the initial RRQ that included the GRE Key Extension, when a GRE
key is available for the registration.
4.3. Mobile Node Requirements for GRE Tunneling Support
If the MN is capable of supporting GRE encapsulation, it SHOULD set
the 'G' bit in the Flags field in the Registration Request per
[RFC 5944].
5. GRE Key Extension and Tunneling Procedures
GRE tunneling support for Mobile IP will permit asymmetric GRE
keying; i.e., the FA assigns a GRE key for use in encapsulated
traffic, and the HA can assign its own GRE key. Once the GRE keys
have been exchanged, the FA uses the HA-assigned key in the
encapsulating GRE header for reverse tunneling, and the HA uses the
FA-assigned key in the encapsulating GRE header.
The format of the GRE Key Extension is as shown below.
The GRE Key Extension MAY be included in Registration Requests or
Registration Replies [RFC 5944]. The GRE Key Extension is used to
inform the recipient of the Mobile IP request of the value to be used
in the GRE Key field.
Yegani, et al. Standards Track PAGE 5
RFC 6245 GRE Key Ext. for MIP4 May 2011
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Sub-Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Key Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: GRE Key Extension
Type
48 - An 8-bit identifier of the GRE Key Extension type
(non-skippable)
Sub-Type
0
Length
4
Key Identifier
This is a four-octet value assigned during registration and
inserted in every GRE packet of the user traffic.
6. IANA Considerations
The GRE Key Extension defined in this memo is a Mobile IP extension
as defined in [RFC 5944]. IANA has assigned a Type value (48) for
this extension from the non-skippable range (0-127).
The GRE Key Extension introduces a new sub-type numbering space,
where the value 0 has been assigned from the range 0 to 255.
Approval of new GRE Key Extension sub-type values is to be made
through Expert Review with Specification Required.
7. Security Considerations
This specification does not introduce any new security
considerations, beyond those described in [RFC 5944].
Yegani, et al. Standards Track PAGE 6
RFC 6245 GRE Key Ext. for MIP4 May 2011
Despite its name, the GRE Key Extension has little to do with
security. The word "Key" here is not used in the cryptographic sense
of a shared secret that must be protected but rather in the sense of
an "index" or demultiplexing value that can be used to distinguish
packets belonging to a given flow within a GRE tunnel.
8. Acknowledgements
Thanks to Jun Wang, Gopal Dommety, and Sri Gundavelli for their
helpful comments, offline discussions, and review of the initial
draft version of this document. Also, Pete McCann and Simon
Mizikovsky provided valuable review comments.
9. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 2784] Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.
Traina, "Generic Routing Encapsulation (GRE)", RFC 2784,
March 2000.
[RFC 2890] Dommety, G., "Key and Sequence Number Extensions to
GRE", RFC 2890, September 2000.
[RFC 3024] Montenegro, G., Ed., "Reverse Tunneling for Mobile IP,
revised", RFC 3024, January 2001.
[RFC 5944] Perkins, C., Ed., "IP Mobility Support for IPv4,
Revised", RFC 5944, November 2010.
[X.S0011-E] 3rd Generation Partnership Project 2, "cdma2000 Wireless
IP Network Standard: Simple IP and Mobile IP Access
Services", 3GPP2 X.S0011-002-E Version 1.0,
November 2009, <http://www.3gpp2.org/Public_html/specs/
X.S0011-002-E_v1.0_091116.pdf>.
Yegani, et al. Standards Track PAGE 7
RFC 6245 GRE Key Ext. for MIP4 May 2011
Authors' Addresses
Parviz Yegani
Juniper Networks
1194 North Mathilda Ave.
Sunnyvale, California 94089
USA
Phone: +1 408-759-1973
EMail: pyegani@juniper.net
Kent Leung
Cisco Systems Incorporated
170 West Tasman Drive
San Jose, California 95134
USA
Phone: +1 408 526 5030
EMail: kleung@cisco.com
Avi Lior
Bridgewater Systems Corporation
303 Terry Fox Drive
Ottawa, Ontario K2K 3J1
Canada
Phone: +1 613-591-6655
EMail: avi@bridgewatersystems.com
Kuntal Chowdhury
Cisco Systems Incorporated
170 West Tasman Drive
San Jose, California 95134
USA
EMail: kchowdhu@cisco.com
Jay Navali
Cisco Systems Incorporated
170 West Tasman Drive
San Jose, California 95134
USA
EMail: jnavali@cisco.com
Yegani, et al. Standards Track PAGE 8
Generic Routing Encapsulation (GRE) Key Extension for Mobile IPv4
RFC TOTAL SIZE: 16823 bytes
PUBLICATION DATE: Saturday, May 28th, 2011
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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