Mobile IPv4 Message String Extension
A way for Mobile IPv4 agents to attach a human-readable text message to registration replies.
Sastry, V., Leung, K., and A. Patel, "Mobile IPv4 Message String Extension", RFC 4917, DOI 10.17487/RFC4917, June 2007, <https://www.rfc-editor.org/info/rfc4917>.
@misc{rfc4917,
series = {Request for Comments},
number = 4917,
howpublished = {RFC 4917},
publisher = {RFC Editor},
doi = {10.17487/RFC4917},
url = {https://www.rfc-editor.org/info/rfc4917},
author = {V. Sastry and K. Leung and A. Patel},
title = {{Mobile IPv4 Message String Extension}},
pagetotal = 7,
year = 2007,
month = jun,
} RFC text
Contents
Network Working Group V. Sastry
Request for Comments: 4917 Samsung Electronics
Category: Standards Track K. Leung
A. Patel
Cisco Systems
June 2007
Mobile IPv4 Message String Extension
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 (C) The IETF Trust (2007).
Abstract
This document specifies a new extension for use in Mobile IPv4. This
extension can be added by the Home Agent and the Foreign Agent to
Registration Reply messages. This extension carries a text string
that is intended for the user of the Mobile Node.
Table of Contents
1. Introduction ....................................................2
2. Terminology .....................................................2
3. Mobile IPv4 Message String Extension Format .....................2
4. Operation and Use of the Message String Extension ...............3
5. Security Considerations .........................................4
6. IANA Considerations .............................................4
7. Acknowledgements ................................................5
8. Normative References ............................................5
1. Introduction
This document specifies a new skippable extension that can be added
by the Foreign Agent and Home Agent in any registration message
targeted for the Mobile Node. Such a message may be either a
Registration Reply or Registration Revocation (i.e., co-located
Care-of Address mode). For the Registration Reply message, this
extension can be added regardless of whether the registration has
succeeded or failed.
The content of the text string in this extension and its usage by the
Mobile Node is implementation specific. The text string in this
extension is intended for the user of the Mobile Node. For example,
this message can be displayed on the Mobile Node's user interface,
logged, or handled in any other implementation dependent way,
depending on the form of the Mobile Node.
Typical contents of the text string will indicate a registration
failure reason, or give a welcome message on successful registration.
This is important, as the failure reason code gives very limited
information for interpretation by the user of the Mobile Node. For
example, a string like "registration failed : Prepaid Quota for the
user is exhausted" can give a human readable description of the
result of Mobile IP registration.
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 [RFC2119].
3. Mobile IPv4 Message String Extension Format
The Message String Extension conforms to the Short Extension format
specified for Mobile IPv4 [RFC3344]. The Message String Extension is
a skippable extension.
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 | Sub-Type | Text ....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type:
145: An 8-bit identifier of the type mobility option.
Length:
An 8-bit unsigned integer. Length of the extension, in bytes,
excluding the extension Type and the extension Length fields.
This field MUST be set to 1 plus the total length of the Text
field.
Sub-Type:
1: Extension comes from the Home Agent
2: Extension comes from the Foreign Agent
Text:
The Text field is one or more octets, and its contents are
implementation dependent. It is intended to be human readable,
and MUST NOT affect the operation of the protocol. The message
MUST be in UTF-8 encoded ISO-10646 [RFC3629] characters. The
number of octets in the encoded representation of the message is
always exactly the value of the Length field minus one. (The
number of unicode characters represented by this octet sequence
may be smaller than the number of octets.)
4. Operation and Use of the Message String Extension
The Message String Extension is only valid for use within Mobile IPv4
Registration Reply and Registration Revocation messages. The Message
String Extension is a skippable extension. Either the Home Agent or
Foreign Agent or both can add the Message String Extension to
registration messages. The usage of Text field of the Message String
Extension is implementation dependent. For example, the message can
be displayed on the Mobile Node's user interface, logged, or handled
in an implementation dependent way, depending on the form of the
Mobile Node. The Mobile Node may throttle how often the user is
notified of the message.
As an example, the Home Agent may reject the first Registration
Request because the prepaid quota for the user is reached and may
attach a Message String Extension with the text "Prepaid quota
reached. Please contact www.paymore.example.com to update balance".
The Mobile Node could display this on the user interface. As a
response, the user of the Mobile Node may take the required action to
update the prepaid account and retry the registration process. The
Home Agent may accept this Registration Request and attach a Message
String Extension with the text "Welcome to
www.serviceprovider.example.com". The Mobile Node could display this
on the user interface, thus confirming a successful creation of
binding on Home Agent.
In the case that the message is not originated by the Home Agent
itself, but for instance, is received from a RADIUS server [RFC2865],
it could be received in some other encoding than UTF-8. If so, the
Home Agent MUST convert the message to UTF-8 encoded ISO-10646
[RFC3629] characters.
5. Security Considerations
The Message String Extension can be added by the Home Agent or
Foreign Agent or both. The protection of the extension is based on
the ordering method specified for message authentication in RFC 3344
[RFC3344] and emphasized below.
If the extension is added by the Home Agent (extension with subtype
1) to a Registration Reply or Registration Revocation message, it
MUST appear before Mobile-Home Authentication Extension [RFC3344].
If the extension is added by the Foreign Agent (extension with
subtype 2) to a Registration Reply message, it MUST appear after
Mobile-Home Authentication Extension [RFC3344] whenever present.
Also the extension MUST appear before the Mobile-Foreign
Authentication Extension whenever present. However, since security
association between the Mobile Node and Foreign Agent is optional, it
is possible that the extension is not authenticated in this case.
There is no confidentiality provided by the extension; the message is
transferred unencrypted, and if sensitive information is sent for
display purposes, it may need to be protected by other means.
6. IANA Considerations
This specification reserves number 145 for the Message String
Extension in Section 3 from the space of numbers for skippable
mobility extensions (i.e., 128-255) defined for Mobile IPv4 [RFC3344]
at http://www.iana.org/assignments/mobileip-numbers.
This specification also creates a new subtype space for the type
number of this extension. The subtype values 1 and 2 are defined in
this specification. The subtype value 1 is reserved for use by the
Home Agent and subtype value 2 is reserved for use by the Foreign
Agent. Similar to the procedures specified for Mobile IPv4 [RFC3344]
number spaces, future allocations from this number space require
expert review [RFC2434].
7. Acknowledgements
The authors would like to thank Avi Lior, Curtis Provost, and Henrik
Levkowetz for their useful comments on an earlier version of this
document. Also, Russ Housley, Vidya Narayanan, Blake Ramsdell, Paul
Hoffman, and Jeff Hutzelman provided justifications to mandate the
need for only UTF-8 encoding in the message and solicited better
clarifications in the security considerations section.
8. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
[RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson,
"Remote Authentication Dial In User Service (RADIUS)", RFC
2865, June 2000.
[RFC3344] Perkins, C., Ed., "IP Mobility Support for IPv4", RFC
3344, August 2002.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
Authors' Addresses
Venkateshwara Sastry
Samsung Electronics
124/C 5th D Cross
Girinagar I Phase
Bangalore 560085
India
Phone: +91-80-26725942
EMail: venkat.sastry@gmail.com
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
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Copyright (C) The IETF Trust (2007).
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AI ELI5
On this page
§1Why a Message String
RFC 4917 defines a new skippable extension that a Foreign Agent or Home Agent can add to a registration message aimed at a Mobile Node. That message may be a Registration Reply or a Registration Revocation. On a Registration Reply the extension can be added whether the registration succeeded or failed.
The content of the string and what the Mobile Node does with it are implementation specific. It is meant for the user of the Mobile Node, so it can be displayed, logged or handled in some other way. Typical text gives a failure reason or a welcome message. The RFC says this matters because the failure reason code gives very limited information to a user, for example a string like "registration failed : Prepaid Quota for the user is exhausted".
A number code like "error 3" does not help most people. A short sentence in plain words does.
§3The Extension Format
The extension uses the Short Extension format from RFC 3344. It has a Type, a Length, a Sub-Type and then the Text.
- Type: 145, an 8-bit identifier.
- Length: an 8-bit unsigned integer giving the extension length in bytes, not counting the Type and Length fields. It MUST be 1 plus the total length of the Text field.
- Sub-Type: 1 means the extension comes from the Home Agent, 2 means it comes from the Foreign Agent.
- Text: one or more octets, implementation dependent, intended to be human readable, and it MUST NOT affect the operation of the protocol. It MUST be UTF-8 encoded ISO-10646 (RFC 3629).
The number of octets in the encoded text is always exactly Length minus one. The number of Unicode characters can be smaller than the number of octets.
It is a small labeled box: a tag saying what kind of box it is, how big it is, who sent the note, and then the note itself written in a standard alphabet.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
TYPE | Type | 1 byte | 145 | Identifies the extension type. |
LENGTH | Length | 1 byte | Extension length in bytes excluding Type and Length; 1 plus the Text length. | |
SUB-TYPE | Sub-Type | 1 byte | 1 for Home Agent, 2 for Foreign Agent. |
§4How the Extension Is Used
The extension is valid only inside Mobile IPv4 Registration Reply and Registration Revocation messages. The Home Agent, the Foreign Agent or both may add it. The Mobile Node may throttle how often it notifies the user of the message.
The RFC gives an example. A Home Agent rejects a first Registration Request because the user's prepaid quota is reached, and attaches the text "Prepaid quota reached. Please contact www.paymore.example.com to update balance". The user tops up and retries. The Home Agent then accepts and attaches a welcome text, which confirms to the user that the binding was created.
If the Home Agent gets the message from elsewhere, such as a RADIUS server (RFC 2865), in another encoding, the Home Agent MUST convert it to UTF-8.
If your account is out of credit, the network can say so in words. After you pay, it can say "welcome". Your phone decides how to show it, and will not nag you too often.
§5Ordering and Protection
Protection of the extension depends on the ordering rules for message authentication in RFC 3344.
- If added by the Home Agent (sub-type 1) to a Registration Reply or Registration Revocation, it MUST appear before the Mobile-Home Authentication Extension.
- If added by the Foreign Agent (sub-type 2) to a Registration Reply, it MUST appear after the Mobile-Home Authentication Extension when that is present, and before the Mobile-Foreign Authentication Extension when that is present.
A security association between the Mobile Node and Foreign Agent is optional, so the Foreign Agent's extension may be unauthenticated. The extension gives no confidentiality: the message travels unencrypted, so sensitive information may need protection by other means.
The note must sit in the right spot in the stack so the official stamp covers it. Some notes might not be stamped at all, and anyone could read them on the way.
§6Number Assignments
The RFC reserves number 145 for the Message String Extension from the space of skippable mobility extensions, which is 128-255 and was defined for Mobile IPv4 in RFC 3344.
It also creates a new sub-type space for this extension type. Value 1 is reserved for use by the Home Agent and value 2 for the Foreign Agent. Future allocations from this space require expert review, following the procedures for the Mobile IPv4 number spaces.
The RFC claims one number on the official list for this note type, and starts a small list of its own with two entries: one for each sender.