Cisco's Mobile IPv4 Host Configuration Extensions
Carrying home network configuration in Mobile IPv4 registration messages.
Leung, K., Patel, A., Tsirtsis, G., and E. Klovning, "Cisco's Mobile IPv4 Host Configuration Extensions", RFC 4332, DOI 10.17487/RFC4332, December 2005, <https://www.rfc-editor.org/info/rfc4332>.
@misc{rfc4332,
series = {Request for Comments},
number = 4332,
howpublished = {RFC 4332},
publisher = {RFC Editor},
doi = {10.17487/RFC4332},
url = {https://www.rfc-editor.org/info/rfc4332},
author = {K. Leung and A. Patel and G. Tsirtsis and E. Klovning},
title = {{Cisco's Mobile IPv4 Host Configuration Extensions}},
pagetotal = 11,
year = 2005,
month = dec,
} RFC text
Contents
Network Working Group K. Leung
Request for Comments: 4332 A. Patel
Category: Informational Cisco Systems
G. Tsirtsis
Flarion Technologies
E. Klovning
Birdstep Technology ASA
December 2005
Cisco's Mobile IPv4 Host Configuration Extensions
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2005).
IESG Note
This RFC is not a candidate for any level of Internet Standard. The
IETF disclaims any knowledge of the fitness of this RFC for any
purpose and in particular notes that the decision to publish is not
based on IETF review for such things as security, congestion control,
or inappropriate interaction with deployed protocols. The RFC Editor
has chosen to publish this document at its discretion. Readers of
this document should exercise caution in evaluating its value for
implementation and deployment. See RFC 3932 for more information.
This RFC does not offer any security mechanisms to provide data
origin authentication and integrity, yet these security services are
vitally important in this context.
Abstract
An IP device requires basic host configuration to be able to
communicate. For example, it will typically require an IP address
and the address of a DNS server. This information is configured
statically or obtained dynamically using Dynamic Host Configuration
Protocol (DHCP) or Point-to-Point Protocol/IP Control Protocol
(PPP/IPCP). However, both DHCP and PPP/IPCP provide host
configuration based on the access network. In Mobile IPv4, the
registration process boots up a Mobile Node at an access network,
also known as a foreign network. The information to configure the
host needs to be based on the home network. This document describes
the Cisco vendor-specific extensions to Mobile IPv4 to provide the
base host configuration in Registration Request and Reply messages.
Table of Contents
1. Introduction ....................................................2
2. Host Configuration Extensions Summary ...........................3
3. Host Configuration Extensions ...................................4
3.1. Host Configuration Request Extension .......................5
3.2. Home Network Length Prefix Extension .......................5
3.3. DNS Server Extension .......................................6
3.4. DHCP Server Extension ......................................6
3.5. DHCP Client ID Extension ...................................7
3.6. Default Gateway Extension ..................................7
3.7. DNS Suffix Extension .......................................8
3.8. Configuration URL Extension ................................8
4. Security Considerations .........................................9
5. Acknowledgements ................................................9
6. Informative References ..........................................9
1. Introduction
An IPv4 device requires some basic configuration to communicate with
other nodes. Typically, it has an IP address for an interface and
DNS server's IP address to resolve the peer's hostname to an IP
address. DHCP [RFC2131] and PPP/IPCP [RFC1332] provide host
configuration information on the access network interface, but this
is inadequate in a Mobile IPv4 environment. In Mobile IPv4
[RFC3344], a Mobile Node has a virtual network interface on the home
network, anchored by the Home Agent. The IP address, home subnet
prefix, default gateway, and home network's DNS servers are essential
in the boot up of a network interface. In some cases, these are the
only pieces of information needed by the Mobile Node.
The Mobile IPv4 registration process provides the mechanism for a
Mobile Node to boot up on a foreign network. Upon the successful
registration, the Mobile Node can communicate with the Correspondent
Node. The need to provide an efficient method to obtain the host
configuration exists. If the Mobile Node is a DHCP client, it can
obtain configuration parameters from the DHCP server in the home
network after the initial registration.
This document introduces the Cisco vendor-specific extensions (VSEs)
[RFC3115] to provide the means for a Mobile Node to download some
fundamental configuration associated with the home network via the
Home Agent. These extensions provide information for home subnet
prefix, DNS server, DHCP server, DHCP client identifier, default
gateway, DNS suffix, and configuration URL.
2. Host Configuration Extensions Summary
The following Cisco vendor-specific extensions provide the host
configuration for a Mobile Node. The "Host Configuration Request"
extension is allowed only in the Registration Request. The rest of
the extensions are appended in the Registration Reply.
o Host Configuration Request
* Request for host configuration information from the Mobile Node
to the Home Agent.
o Home Network Prefix Length
* The length of the subnet prefix on the home network.
o Default Gateway
* The default gateway's IP address on the home network.
o DNS Server
* The DNS server's IP address in the home network.
o DNS Suffix
* The DNS suffix for hostname resolution in the home network.
o DHCP Client ID
* The DHCP Client ID used to obtain the IP address. When the
Mobile Node returns home and is responsible for managing its
own address, this information maps to the client identifier
option as defined in section 9.14 of [RFC2132] and referenced
in [RFC2131].
o DHCP Server
* The DHCP server's IP address in the home network.
o Configuration URL
* The URL for the Mobile Node to download configuration
parameters from a server.
When the Mobile Node needs to obtain its host configuration, the Host
Configuration Request VSE is appended to the Registration Request.
This VSE indicates to the Home Agent that either all or selected host
configuration VSEs need to be appended to the Registration Reply. If
the Home Agent retrieved the information from a DHCP server (in Proxy
DHCP mode), then the DHCP Client ID and DHCP Server extensions are
appended in the Registration Reply. These DHCP-related extensions
are populated with values that had been used in the DHCP messages
exchanged between the Home Agent and the DHCP server.
The VSEs are authenticated as part of the registration message using
any of the authentication mechanism defined for Mobile IP ([RFC3344],
[RFC3012]).
This message MAY contain extensions defined in Mobile IP, including
vendor-specific extensions [RFC3115].
3. Host Configuration Extensions
Cisco's host configuration extensions to Mobile IPv4 are based on the
vendor-specific extensions defined in [RFC3115]. The format of the
VSE TLV (Type-Length-Value) 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vendor/Org-ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vendor-NVSE-Type | Vendor-NVSE-Value ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: 134
Length:
Indicates the length (in bytes) of the data field within this
extension, excluding the Type and Length fields.
Reserved:
Reserved for future use. To be set to 0 while sending, ignored
on reception.
Vendor/Org-ID:
9 (Cisco Systems)
Vendor-NVSE-Type:
14 (Host Configuration)
Vendor-NVSE-Value:
Format is shown below for each subtype. The Sub-Type field is
an integer from 0 to 255.
3.1. Host Configuration Request Extension
This format of the Host Configuration Request extension is shown
below.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-Type | Selector |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Sub-Type:
0
Selector:
0 indicates all host configuration available to the Home
Agent (HA) is requested by the Mobile Node.
3.2. Home Network Length Prefix Extension
This format of the Home Network Prefix Length extension is shown
below.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-Type | Prefix Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Sub-Type:
1
Prefix Length:
The number of bits in the home subnet prefix.
3.3. DNS Server Extension
This format of the DNS Server extension is shown below.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-Type | Primary DNS Server
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. . . | Secondary DNS Server
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. . . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Sub-Type:
2
Primary DNS Server:
The IP address of the primary DNS server.
Secondary DNS Server:
The IP address of the secondary DNS server.
3.4. DHCP Server Extension
This format of the DHCP Server extension is shown below.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-Type | DHCP Server
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. . . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Sub-Type:
3
DHCP Server:
The IP address of the DHCP server.
3.5. DHCP Client ID Extension
This format of the DHCP Client ID extension is shown below.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-Type | Client ID . . .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Sub-Type:
4
Client ID:
DHCP servers use this value to index their database of address
bindings. This value is expected to be unique for all clients
in an administrative domain. The size of field is between 2
and 255 octets.
3.6. Default Gateway Extension
This format of the Default Gateway extension is shown below.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-Type | Default Gateway
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. . . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Sub-Type:
5
Default Gateway:
The IP address of the default gateway for the Mobile Node on
the home network.
3.7. DNS Suffix Extension
This format of the DNS Suffix extension is shown below.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-Type | DNS Suffix . . .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Sub-Type:
6
DNS Suffix:
The DNS suffix to be appended to the name of Mobile Node when
completing its fully qualified domain name (FQDN). The size of
field is between 1 and 246 octets.
3.8. Configuration URL Extension
This format of the Configuration URL extension is shown below.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-Type | URL String . . .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Sub-Type:
7
URL String:
The Mobile Node can retrieve configuration parameters via the
URL. The URL is at most 246 bytes in length.
4. Security Considerations
The host configuration extensions follow the same rules for Mobile IP
extensions in registration messages. See the Security Considerations
section in RFC 3344.
The Configuration URL extension may trigger the Mobile Node to
download the configuration parameters from a server. The protection
of the data transfer is outside the scope of this document. Possible
options include encryption of data before transfer or using HTTPS.
5. Acknowledgements
The authors would like to acknowledge Jayshree Bharatia, Kuntal
Chowdhury, Avi Lior, and Lila Madour for their contributions to the
work in progress titled "Mobile IPv4 Extension for Configuration
Options Exchange".
6. Informative References
[RFC1332] McGregor, G., "The PPP Internet Protocol Control Protocol
(IPCP)", RFC 1332, May 1992.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, March 1997.
[RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, March 1997.
[RFC3012] Perkins, C. and P. Calhoun, "Mobile IPv4 Challenge/
Response Extensions", RFC 3012, November 2000.
[RFC3115] Dommety, G. and K. Leung, "Mobile IP Vendor/
Organization-Specific Extensions", RFC 3115, April 2001.
[RFC3344] Perkins, C., "IP Mobility Support for IPv4", RFC 3344,
August 2002.
Authors' Addresses
Kent Leung
Cisco Systems
170 W. Tasman Drive
San Jose, CA 95134
US
Phone: +1 408-526-5030
EMail: kleung@cisco.com
Alpesh Patel
Cisco Systems
170 W. Tasman Drive
San Jose, CA 95134
US
Phone: +1 408-853-9580
EMail: alpesh@cisco.com
George Tsirtsis
Flarion Technologies
Bedminster One
135 Route 202/206 South
Bedminster, NJ 07921
US
Phone: +1 908-947-7059
EMail: g.tsirtsis@flarion.com
Espen Klovning
Birdstep Technology ASA
Bryggegata 7
Oslo, 0250
Norway
Phone: +47 95 20 26 29
EMail: espen@birdstep.com
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AI ELI5
On this page
§1Mobile IPv4 Configuration Problem
In Mobile IPv4, a mobile device (called a Mobile Node) travels between networks. When it arrives at a foreign network, it must register with its Home Agent to maintain its home IP address and stay reachable. However, the mobile node still needs basic host configuration information such as DNS servers, a default gateway, and the home network's subnet prefix. These are typically delivered by DHCP or PPP/IPCP during connection to the access network, but those protocols deliver configuration for the visited network, not the home network.
Without home network configuration, the mobile node cannot resolve names using the home network's DNS infrastructure, cannot reach the home network's subnet directly, and may not find the correct gateway. This RFC describes how Cisco's Mobile IPv4 extensions solve this by carrying configuration in the registration messages themselves.
When you arrive at a coffee shop and connect, that shop can tell you their WiFi settings, but not your home phone's settings. Your phone calls home and asks for the right settings to use while traveling.
§2Extension Types and Request Model
These Cisco vendor-specific extensions use the Mobile IP vendor-specific extension (VSE) format defined in RFC 3115, with Vendor-ID 9 (Cisco Systems) and Vendor-NVSE-Type 14 (Host Configuration). They provide seven configuration parameters: home network prefix length (number of bits in the subnet mask), primary and secondary DNS server addresses, DHCP server address, DHCP client identifier, default gateway address, DNS suffix (for FQDN resolution), and a configuration URL.
The mobile node sends a Host Configuration Request extension in the Mobile IPv4 Registration Request to ask the Home Agent for these parameters. A selector value of 0 means the node is requesting all available host configuration. The Home Agent responds in the Registration Reply with the requested extensions. If the Home Agent acts in Proxy DHCP mode (retrieving configuration from a DHCP server on the home network), it includes the DHCP Client ID and DHCP Server extensions with the values used in those DHCP exchanges.
When you arrive, you send a message saying 'can you send me my home settings?' The Home Agent replies with a list of settings: your phone book, gateway, and where to call if you need more setup.
§3Extension Message Formats
All extensions follow a consistent structure: a Sub-Type field identifying the configuration parameter, followed by the parameter value. The Home Network Prefix Length (Sub-Type 1) carries the number of bits in the home subnet mask. The DNS Server extension (Sub-Type 2) holds both primary and secondary DNS server IPv4 addresses. The DHCP Server extension (Sub-Type 3) provides the DHCP server's address, and the DHCP Client ID extension (Sub-Type 4) carries a client identifier (2 to 255 octets) matching an entry in the DHCP server's database.
The Default Gateway extension (Sub-Type 5) provides the IPv4 address of the default gateway on the home network. The DNS Suffix extension (Sub-Type 6) is a string (1 to 246 octets) that is appended to the mobile node's name during DNS resolution to form a fully qualified domain name. The Configuration URL extension (Sub-Type 7) is a URL (at most 246 bytes) where the mobile node can download additional configuration parameters, allowing for dynamic or centralized configuration management.
Each setting is a small labeled box: which DNS to use, how big the home network is, who the gateway is, and where to download more settings if needed.
§4Security and Authentication
The extensions are authenticated as part of the Mobile IPv4 registration message using existing Mobile IPv4 authentication mechanisms (RFC 3344 or RFC 3012). This means the entire Registration Reply, including the host configuration extensions, is protected by the same authentication method that secures the registration itself. A mobile node can trust the configuration because it came in an authenticated message from the Home Agent.
However, the RFC notes that the Configuration URL extension may trigger a separate download of parameters from a web server, and the protection of that data transfer is outside this RFC's scope. Implementations should use HTTPS or another encryption method to protect configuration downloads. Additionally, the RFC states that the extensions do not provide data origin authentication or integrity services beyond what Mobile IPv4 already provides, and administrators should be aware of this limitation when deploying the extensions.
The home office signs the message so you know it really came from them. But if you download more settings from a link it gives you, you need to check that link is safe yourself.