Mobile IPv4 Message String Extension
Extracted elements (15)
The extension was created because the existing failure reason codes in Mobile IPv4 provide very limited information for end-user interpretation. A human-readable string (e.g., 'Prepaid Quota for the user is exhausted') allows meaningful feedback to be conveyed to the Mobile Node user beyond what numeric codes can express.
UTF-8 encoding was mandated (rather than allowing multiple encodings) to ensure interoperability and avoid ambiguity in text representation. This decision was reinforced by reviewers who justified the need for a single, universal encoding.
Messages received from external sources such as RADIUS servers may arrive in non-UTF-8 encodings. The Home Agent is responsible for converting such messages to UTF-8 before including them in the extension, ensuring consistent encoding for the Mobile Node.
If the Home Agent receives the message from an external source (e.g., a RADIUS server) in an encoding other than UTF-8, the Home Agent MUST convert it to UTF-8 encoded ISO-10646 characters before including it in the extension.
The Length field MUST be set to 1 plus the total length of the Text field.
The Text field MUST NOT affect the operation of the protocol; it is intended solely as a human-readable message for the user.
The text message in the Text field MUST be in UTF-8 encoded ISO-10646 (RFC 3629) characters.
The Message String Extension is a skippable extension conforming to the Short Extension format for Mobile IPv4, identified by Type 145. It carries a human-readable text string intended for the user of the Mobile Node, and can be added by the Home Agent or Foreign Agent to Registration Reply or Registration Revocation messages.
The Message String Extension is only valid for use within Mobile IPv4 Registration Reply and Registration Revocation messages. Either the Home Agent or Foreign Agent or both may add this extension.
The Sub-Type field distinguishes the source of the extension: subtype 1 indicates the extension originates from the Home Agent, and subtype 2 indicates it originates from the Foreign Agent.
Type number 145 is reserved for the Message String Extension from the skippable mobility extensions space (128-255) for Mobile IPv4. A new subtype space is created for this extension type: subtype 1 for Home Agent use and subtype 2 for Foreign Agent use. Future subtype allocations require expert review.
If the extension is added by the Foreign Agent (subtype 2) to a Registration Reply, it MUST appear after the Mobile-Home Authentication Extension (when present) and MUST appear before the Mobile-Foreign Authentication Extension (when present). If no security association exists between Mobile Node and Foreign Agent, the extension may be unauthenticated.
If the extension is added by the Home Agent (subtype 1) to a Registration Reply or Registration Revocation message, it MUST appear before the Mobile-Home Authentication Extension to ensure it is covered by the authentication.
The Message String Extension provides no confidentiality; the text is transferred unencrypted. If sensitive information is included for display purposes, it must be protected by other means outside this extension.
The Message String Extension has a 1-byte Type field (value 145), a 1-byte Length field (total extension bytes minus Type and Length fields), a 1-byte Sub-Type field (1=Home Agent, 2=Foreign Agent), and a variable-length Text field. The Length field MUST be set to 1 plus the total length of the Text field.