Mobile-IPv4 Configuration Option for PPP IPCP
updated by
- rfc-2794 — Mobile IP Network Access Identifier Extension for IPv4
updates
- rfc-2002 — IP Mobility Support
Extracted elements (26)
A unique IP address MUST NOT be assigned to a mobile node unless absolutely necessary: no unique address is required when the mobile node connects to its home link or when it uses a foreign agent's care-of address. This preserves the shared-address efficiency of foreign agent care-of addresses.
Configure-Nak of the Mobile-IPv4 option is explicitly left undefined (and prohibited) in this version of the specification to reserve semantic space for a future useful meaning, avoiding premature commitment that would cause interoperability problems.
The Mobile-IPv4 option is intentionally designed not to duplicate any functionality of the IP-Address option; the two options work in conjunction, with Mobile-IPv4 carrying the permanent home address and IP-Address carrying any dynamically assigned care-of address.
Without this option, a mobile node connecting via PPP to a foreign link cannot use a foreign agent care-of address without first being assigned a unique IP address. The ambiguity in IPCP negotiation (the peer cannot distinguish a mobile node proposing its home address from a conventional node proposing a non-routable address) defeats the shared-address advantage of foreign agents.
The Mobile-IPv4 option is designed to be backwards compatible with conventional nodes that do not implement it. When a peer sends Configure-Reject for the Mobile-IPv4 option, the mobile node falls back to standard IP-Address negotiation as if the option were never proposed.
This specification updates RFC 2002 Appendix B behavior: a mobile node connecting to a foreign link SHOULD send Request(IP=0) rather than Request(IP=home) when falling back to IP-Address-only negotiation, since the peer might have no means for assigning addresses other than IPCP.
Wireless links may employ handoff and proxying mechanisms that do not require tearing down the PPP link but still require registration with a new foreign agent. Mobile nodes MUST employ move detection algorithms regardless of PPP link continuity.
A mobile node implementing move detection based on the Prefix-Length Extension MUST compare the prefix of any advertising agent with that of its current foreign agent and, upon receiving an advertisement specifying a different prefix, MUST register with the new foreign agent.
A mobile node in the IPCP Opened state MUST renegotiate IPCP upon receiving an IPCP Configure-Request from its peer, as specified in RFC 1661.
A mobile node MUST NOT include the deprecated IP-Addresses option (plural) in any Configure-Request that also contains a Mobile-IPv4 option or an IP-Address option.
A mobile node that fails to receive an Agent Advertisement within the Lifetime advertised by its current foreign agent MUST assume that it has lost contact with that foreign agent and SHOULD immediately register with any newly advertising foreign agent.
A mobile node that receives a Configure-Ack containing a Mobile-IPv4 Configuration Option MUST receive an Agent Advertisement (possibly in response to an Agent Solicitation) before sending a Registration Request if connecting to a foreign link, because the foreign agent may enforce a mandatory registration policy.
A mobile node that receives a Configure-Reject containing a Mobile-IPv4 Configuration Option SHOULD fall back to IPCP negotiation using the IP-Address option alone, beginning with Request(IP=home) if on the home link or Request(IP=0) otherwise.
A node MUST NOT send a Configure-Nak containing a Mobile-IPv4 Configuration Option, as doing so is currently undefined and may cause interoperability problems when a meaning is ultimately defined.
A node not performing mobile node functionality (including pure home agent or foreign agent nodes) MUST NOT include a Mobile-IPv4 Configuration Option within any Configure-Request message.
A node that sends a Configure-Ack containing a Mobile-IPv4 Configuration Option SHOULD send an Agent Advertisement immediately upon IPCP for that link entering the Opened state.
A peer that is performing neither home agent nor foreign agent functionality SHOULD send a Configure-Reject in response to any Configure-Request that contains a Mobile-IPv4 Configuration Option.
Ack(IP=0) MUST NOT be sent by a peer. An implementation MUST NOT use 0 as its IP address upon receiving Ack(IP=0), but MAY use some other non-zero interface address for packets sent on its PPP interface.
Foreign agents and home agents SHOULD send an Agent Advertisement over a PPP link immediately after IPCP for that link enters the Opened state, to enable timely move detection.
If a mobile node and a foreign or home agent agree in IPCP to use Van Jacobson Header Compression, the mobile node MUST NOT set the 'V' bit in its Mobile IP Registration Request, since enabling VJ compression at both PPP and Mobile IP layers is redundant and likely to cause confusion.
Mobile nodes that connect via PPP MUST correctly implement PPP's IPCP and MUST be prepared to renegotiate IPCP at any time, including renegotiating the IP-Address and Mobile-IPv4 options, because movement will likely change the PPP peer.
The Mobile Node's Home Address field in the Mobile-IPv4 Configuration Option MUST NOT be zero. Configure-Nak'ing this option is undefined and MUST NOT be sent by compliant implementations.
The Mobile-IPv4 Configuration Option for IPCP has Type=4 and carries the mobile node's home IP address. It is used alongside the IP-Address option to negotiate appropriate Mobile IP behavior during the IPCP phase of PPP.
This document introduces no known security threats beyond those facing any PPP node or Mobile IP node. Service providers SHOULD use cryptographically strong authentication (e.g., CHAP, RFC 1994) to prevent theft-of-service. Users requiring confidentiality should use PPP link encryption, IP-layer encryption, or application-layer encryption. Mobile IP authentication (RFC 2002) protects against trivial denial-of-service attacks against a mobile node and its home agent.
IPCP negotiation with Mobile-IPv4 option defines seven numbered request states and lettered peer responses. Mobile nodes SHOULD start with Request(IP=0,MIPv4=home) to prefer co-located CoA, or Request(MIPv4=home) to prefer foreign agent CoA. Responses include Ack (Opened), Nak (redirect to new state), or Reject (fall back). Nak(MIPv4=any) MUST NOT be sent in any state.
The Mobile-IPv4 IPCP option encoding: Type (1 octet, value=4), Length (1 octet, value=6), Mobile Node's Home Address (4 octets, MUST NOT be zero). Total length is 6 bytes.