HTTP/1.1 200 OK Date: Tue, 09 Apr 2002 05:28:58 GMT Server: Apache/1.3.20 (Unix) Last-Modified: Thu, 10 Nov 1994 23:00:00 GMT ETag: "3049a8-16045-2ec2a5f0" Accept-Ranges: bytes Content-Length: 90181 Connection: close Content-Type: text/plain Internet Engineering Task Force C. Perkins, editor INTERNET DRAFT IBM 21 October 1994 IP Mobility Support draft-ietf-mobileip-protocol-07.txt Abstract This document specifies protocol enhancements that allow transparent routing of IP datagrams to mobile node in the Internet. The mobile node is always identified by its home address, regardless of its current point of attachment to the Internet. While situated away from its home, a mobile node is also associated with a care-of address, which provides information about its current point of attachment to the Internet. The protocol provides for registering the care-of address with a home agent. The home agent sends traffic destined for the mobile node through a tunnel to the care-of address. Status of This Memo This document is a submission to the Mobile-IP Working Group of the Internet Engineering Task Force (IETF). Comments should be submitted to the mobile-ip@sunroof.eng.sun.com mailing list. Distribution of this memo is unlimited. This document is an Internet-Draft. Internet Drafts are working documents of the Internet Engineering Task Force (IETF), its Areas, and its Working Groups. Note that other groups may also distribute working documents as Internet Drafts. Internet Drafts are draft documents valid for a maximum of six months, and may be updated, replaced, or obsoleted by other documents at any time. It is not appropriate to use Internet Drafts as reference material, or to cite them other than as a ``working draft'' or ``work in progress.'' To learn the current status of any Internet-Draft, please check the ``1id-abstracts.txt'' listing contained in the internet-drafts Shadow Directories on ds.internic.net (US East Coast), nic.nordu.net (Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim). Perkins, editor Expires 21 April 1994 [Page i] Internet Draft IP Mobility Support 21 October 1994 Contents Abstract i Status of This Memo i 1. Introduction 1 1.1. Requirements . . . . . . . . . . . . . . . . . . . . . . 2 1.2. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1.3. Assumptions . . . . . . . . . . . . . . . . . . . . . . . 3 1.4. Specification Language . . . . . . . . . . . . . . . . . 3 1.5. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4 2. Agent Discovery 6 2.1. Authentication . . . . . . . . . . . . . . . . . . . . . 6 2.2. Agent Solicitation . . . . . . . . . . . . . . . . . . . 6 2.3. Agent Advertisement . . . . . . . . . . . . . . . . . . . 7 3. Registration 9 3.1. Authentication . . . . . . . . . . . . . . . . . . . . . 10 3.2. Registration Request . . . . . . . . . . . . . . . . . . 10 3.3. Registration Reply . . . . . . . . . . . . . . . . . . . 12 4. Mobility Message Extensions 14 4.1. Mobility Extension . . . . . . . . . . . . . . . . . . . 15 4.2. Home Address Extension . . . . . . . . . . . . . . . . . 16 4.3. Key Identifier . . . . . . . . . . . . . . . . . . . . . 17 4.4. Mobile-Home Authentication Extension . . . . . . . . . . 17 4.5. Minimal Encapsulation Extension . . . . . . . . . . . . . 18 5. Forwarding Datagrams to the Mobile Node 19 5.1. IP in IP Encapsulation . . . . . . . . . . . . . . . . . 19 5.2. Minimal Encapsulation . . . . . . . . . . . . . . . . . . 19 6. Mobile Node Considerations 22 6.1. Configuration and Registration Tables . . . . . . . . . . 22 6.2. Registration When Away From Home . . . . . . . . . . . . 23 6.3. Registration without a foreign agent . . . . . . . . . . 23 6.4. De-registration When At Home . . . . . . . . . . . . . . 24 6.5. Registration Replies . . . . . . . . . . . . . . . . . . 25 6.6. Registration Retransmission . . . . . . . . . . . . . . . 25 6.7. Simultaneous Registrations . . . . . . . . . . . . . . . 26 6.8. Mobile Routers . . . . . . . . . . . . . . . . . . . . . 26 Perkins, editor Expires 21 April 1994 [Page ii] Internet Draft IP Mobility Support 21 October 1994 7. Foreign Agent Considerations 27 7.1. Configuration and Registration Tables . . . . . . . . . . 28 7.2. Receiving Registration Requests . . . . . . . . . . . . . 28 7.3. Receiving Registration Replies . . . . . . . . . . . . . 28 7.4. Decapsulation . . . . . . . . . . . . . . . . . . . . . . 29 7.5. Mobility . . . . . . . . . . . . . . . . . . . . . . . . 29 8. Home Agent Considerations 30 8.1. Configuration and Registration Tables . . . . . . . . . . 30 8.2. Receiving Registration Requests . . . . . . . . . . . . . 30 8.3. Simultaneous Registrations . . . . . . . . . . . . . . . 31 8.4. Registration Expiration . . . . . . . . . . . . . . . . . 31 8.5. Encapsulation . . . . . . . . . . . . . . . . . . . . . . 32 8.6. Mobility . . . . . . . . . . . . . . . . . . . . . . . . 32 9. Security Considerations 33 9.1. Message Authentication Codes . . . . . . . . . . . . . . 33 9.2. Tunneling to Care-of Addresses . . . . . . . . . . . . . 33 9.3. Key management . . . . . . . . . . . . . . . . . . . . . 34 9.4. Picking good keys . . . . . . . . . . . . . . . . . . . . 34 9.5. Privacy . . . . . . . . . . . . . . . . . . . . . . . . . 34 9.6. Replay Protection for Registration Requests . . . . . . . 34 10. Acknowledgements 35 A. Gratuitous and Proxy ARP 36 B. Link-Layer considerations 37 B.1. Point-to-Point Link-Layers . . . . . . . . . . . . . . . 37 B.2. Multi-Point Link-Layers . . . . . . . . . . . . . . . . . 38 C. TCP Considerations 38 C.1. TCP Timers . . . . . . . . . . . . . . . . . . . . . . . 38 C.2. TCP Congestion Management . . . . . . . . . . . . . . . . 38 D. Tunnel Management 39 Chair's Address 42 Editor's Address 42 Perkins, editor Expires 21 April 1994 [Page iii] Internet Draft IP Mobility Support 21 October 1994 1. Introduction Current versions of the Internet Protocol make an implicit assumption that a node's point of attachment remains fixed. Datagrams are sent to a node based on the location information contained in the node's IP address. If a node moves while keeping its IP address unchanged, its network number will not reflect its new point of attachment. The routing protocols will not be able to route datagrams to it correctly. This document defines new functions that allow a node to roam on the Internet, without changing its IP address. The following entities are defined: Mobile Node A host or router that changes its point of attachment from one network or subnetwork to another. Home Agent A router that maintains a registry of the current mobility bindings for that mobile node, and encapsulates datagrams for delivery to the mobile node while it is away from home. Foreign Agent A router that assists a locally reachable mobile node that is away from its home network. Care-of Address The care-of address terminates the end of a tunnel toward a mobile node. Depending on the foreign network configuration, the care-of address may be either dynamically assigned to the mobile node or associated with a foreign agent. The following support services are defined: Agent Discovery Home agents and foreign agents advertise their availability on each link for which they provide service. A newly arrived mobile node can send a solicitation on the link to learn if any prospective agents are present. Perkins, editor Expires 21 April 1994 [Page 1] Internet Draft IP Mobility Support 21 October 1994 Registration When the mobile node is away from home, it registers a care-of address with a home agent. Depending on its method of attachment, the mobile node will register either directly with a home agent, or through a foreign agent which forwards the registration to the home agent. Encapsulation Once a mobile node has registered a care-of address with its home agent, that home agent intercepts datagrams destined for the mobile node, builds another datagram with the intercepted datagram enclosed within, and forwards the resulting datagram to the entity at the care-of address. Decapsulation At the care-of address, the enclosed datagram is extracted. When the mobile node receives packets sent to its own care-of address, it decapsulates its own datagrams. When the care-of address is associated with a foreign agent, the foreign agent decapsulates the datagrams. If the datagram is addressed to a mobile node which the foreign agent is currently serving, it will deliver the datagram to the mobile node. 1.1. Requirements A mobile node using its home address shall be able to communicate with other nodes after having been disconnected from the Internet, and then reconnected at a different point of attachment. A mobile node shall continue to be capable of communicating directly with existing nodes which do not implement the mobility functions described in this document. A mobile node shall provide authentication in its registration messages. 1.2. Goals The mobile node's directly attached link is likely to be bandwidth limited. Only a few administrative messages should be sent between a mobile node and an agent. The size of these messages should be kept as short as possible. Perkins, editor Expires 21 April 1994 [Page 2] Internet Draft IP Mobility Support 21 October 1994 As few messages as possible which duplicate functionality are sent on mobile links. This is particularly important on wireless and congested links. 1.3. Assumptions The protocols defined in this document place no additional requirements on assignment of IP addresses. That is, a mobile node will be assigned an IP address by the organization that owns the machine, and will be able to use that IP address regardless of the current point of attachment. It is assumed that mobile nodes are able to change their point of attachment to the Internet no more frequently than once per second. No protocol enhancements are required in hosts or routers that are not serving any of the mobility functions. Similarly, no additional protocols are needed by a router (that is not acting as a home agent or a foreign agent) to route datagrams to or from a mobile node. It is assumed that IP datagrams are routed to a destination without regard to the source of the datagram. 1.4. Specification Language In this document, several words are used to signify the requirements of the specification. These words are often capitalized. MUST This word, or the adjective "required", means that the definition is an absolute requirement of the specification. MUST NOT This phrase means that the definition is an absolute prohibition of the specification. SHOULD This word, or the adjective "recommended", means that there may exist valid reasons in particular circumstances to ignore this item, but the full implications must be understood and carefully weighed before choosing a different course. MAY This word, or the adjective "optional", means that this item is one of an allowed set of alternatives. An implementation which does not include this option MUST be prepared to Perkins, editor Expires 21 April 1994 [Page 3] Internet Draft IP Mobility Support 21 October 1994 interoperate with another implementation which does include the option. silently discard The implementation discards the packet without further processing, and without indicating an error to the sender. The implementation SHOULD provide the capability of logging the error, including the contents of the discarded packet, and SHOULD record the event in a statistics counter. 1.5. Terminology This document frequently uses the following terms: Agent Advertisement A periodic advertisement constructed by attaching a special extension to a Router Advertisement [5] message. Authentication Type This includes the algorithm and algorithm mode. Note that a single algorithm (such as DES) might have several modes (for example, CBC and ECB)(see [16], [12]). Correspondent A peer with which a mobile node is communicating. The correspondent may be either mobile or stationary. Home Address A long-term IP address that is assigned to a mobile node. It remains unchanged regardless of where the node is attached to the Internet. Datagrams addressed to the home address are intercepted by the home agent while the mobile node is registered with that home agent. Link A communication facility or medium over which nodes can communicate at the link layer; a link underlies the network layer. Perkins, editor Expires 21 April 1994 [Page 4] Internet Draft IP Mobility Support 21 October 1994 Mobile Agent Either a home agent or a foreign agent. Mobility Binding The association of a home address with a care-of address, and the remaining lifetime of the association. Mobility Security Association The security relationship between two nodes that is used with Mobile IP protocol messages. This relationship includes the authentication type (including algorithm and algorithm mode) and the secret (such as a shared key, or appropriate public/private key pair). Routing Prefix The high-order bits in an address, which are used by routers to locate a link for delivery of a datagram. Source Address An IP address belonging to the interface on which this message is sent. Perkins, editor Expires 21 April 1994 [Page 5] Internet Draft IP Mobility Support 21 October 1994 2. Agent Discovery To communicate with a foreign or home agent, a mobile node must learn either the IP address or the link address of that agent. It is assumed that a link-layer connection has been established between the agent and the mobile node. The method used to establish such a link-layer connection is not specified in this document. After establishing a link-layer connection, the mobile node learns whether there are any agents available. If the address of any agent matches the mobile node's stored address for its home agent, the mobile node is at home. An agent which is not identified by a link-layer protocol MUST implement ICMP Router Discovery [5]. The Router Advertisements indicate whether the router is also a home agent or a foreign agent. When multiple methods of agent identification are in use, the mobile node SHOULD first attempt registration with routers sending Router Advertisements in preference to those sending link-layer advertisements. This ordering maximizes the likelihood that the registration will be recognized, thereby minimizing the number of registration attempts. An administrative domain MAY require registration with a foreign agent even when another registration method is in use. This facility (see subsection 4.1) is envisioned for service providers with packet filtering fire-walls, or visiting policies (such as accounting) which require exchanges of authorization. 2.1. Authentication No authentication is required for the advertisement and solicitation process. These messages MAY be authenticated using the IP Authentication Header [1], which is external to the messages described here. Further work on authentication of advertisement and solicitation is outside of the scope of this document. Whenever an externally authenticated message fails authentication, the message is silently discarded. 2.2. Agent Solicitation Every mobile node MUST implement ICMP Router Solicitation if it needs to obtain a care-of address in an agent advertisement. However, the solicitation is only sent when no care-of address has been determined through a link-layer protocol or prior Router Advertisement. Any Perkins, editor Expires 21 April 1994 [Page 6] Internet Draft IP Mobility Support 21 October 1994 foreign agent or home agent which is not identified by a link- layer protocol MUST respond to ICMP Router Solicitation. The same procedures, defaults, and constants are used as described in "ICMP Router Discovery Messages" [5], except that the mobile node may solicit more often than once every three seconds and MAX_SOLICITATIONS does not apply for mobile nodes that are currently unconnected to any foreign agent. A mobile node MAY send a solicitation once each MOBILE_SOLICITATION_INTERVAL (1 second?) until the solicitation is answered by a mobile agent, and the mobile node can finally issue a registration request. 2.3. Agent Advertisement Every mobile node MUST correctly process ICMP Router Advertisements. Any foreign agent or home agent which is not identified by a link- layer protocol MUST send ICMP Router Advertisements. An agent which is identified by a link-layer protocol SHOULD also implement Router Advertisements. However, the advertisements need not be sent, except when the site policy requires registration with the agent, or as a response to a specific solicitation. The same procedures, defaults, and constants are used as described in "ICMP Router Discovery Messages" [5], except as specified herein; a foreign agent MUST NOT send Router Advertisements more often than once per second. The Router Advertisements are extended by examining the number of advertised addresses. When the IP total length indicates that the ICMP message is longer than needed for the number of addresses present, the remainder is interpreted as extensions. The extensions are described in section 4. The Mobility Extension (subsection 4.1) is required, and indicates that the router is an mobile agent. Other extensions indicate optionally supported features (see, e.g., subsection 5.2). Perkins, editor Expires 21 April 1994 [Page 7] Internet Draft IP Mobility Support 21 October 1994 The Code field of the ICMP Router Advertisement is interpreted as follows: 0 If the Mobility Extension is present, the router supports mobility registration. The router also handles common traffic -- that is, IP data packets not necessarily related to mobile nodes. 16 A home or foreign agent which supports registration, but is not routing common traffic. A foreign agent includes the care-of address as a router address. Upon receipt of an agent advertisement, a mobile node compares the route address to that of the home agent(s) in its list. If there is an exact match, the mobile node is at home. Otherwise, the care-of address may be chosen from among advertising agents in the same fashion as the mobile node would choose a first hop router. The highest preference router address which falls within a subnet that the mobile node has configured on its mobile interface(s) is used for the care-of address. It is very likely that no advertised routing prefix matches when the mobile node is not at home. In this case, the highest preference non-matching router address is used for the care-of address. A home agent which does not provide foreign agent services will have preference values less than the highest foreign agent preference. DISCUSSION: What is this value? Perkins, editor Expires 21 April 1994 [Page 8] Internet Draft IP Mobility Support 21 October 1994 3. Registration The registration function exchanges information between mobile nodes and home agents. Registration creates a mobility binding, associating the mobile node's home address with a care-of address which can be used to reach the mobile node. When assigned a transient care-of address, a mobile node can act without a foreign agent, and register or de-register directly with a home agent. This registration process involves the exchange of only 2 messages: a) The mobile node sends a registration request to a home agent, to ask that home agent to provide the requested service. b) The home agent sends a registration reply to the mobile node to grant or deny service. An administrative domain MAY require registration through a foreign agent (see the description of the "F" bit, in subsection 4.1). When the care-of address is associated with a foreign agent, the foreign agent acts as a relay between the mobile node and home agent. This extended registration process involves the exchange of 4 messages: a) The mobile node sends a registration request to the prospective foreign agent to begin the registration process. b) The foreign agent relays the request to the home agent, asking that home agent to provide the requested service. c) The home agent sends a registration reply to the foreign agent to grant or deny service. d) The foreign agent sends a copy of the registration reply to the mobile node to inform it of the disposition of its request. The registration messages defined in this section(3.2, 3.3) use the User Datagram Protocol header [18]. The UDP checksum is required. Any mobility message with an incorrect or zero UDP checksum is silently discarded. Perkins, editor Expires 21 April 1994 [Page 9] Internet Draft IP Mobility Support 21 October 1994 3.1. Authentication Each mobile node, foreign agent, and home agent MUST support the maintenance of an internal table holding a list of security associations for mobile entities, indexed by their IP address. Mobile node to home agent registration messages are required to be authenticated with the Mobile-Home Authentication Extension (see subsection 4.4). See section 9.1 for support requirements for authentication algorithms. Mobile-Foreign and Foreign-Home Authentications use the IP Authentication Header [1]. Only one mobility security association at a time is in effect between any given pair of participating nodes. Whenever a mobility security association exists between a pair of nodes, all registration messages between these nodes MUST be authenticated. 3.2. Registration Request The registration request message is sent by a mobile node to its home agent, so that the home agent can create a new mobility binding for the mobile node (with a new lifetime). The registration may be relayed to the home agent by the foreign agent from which the mobile node is accepting service, or it may be sent directly in case the mobile node has received a temporary care-of address by some other means (e.g, DHCP [7]). IP fields: Source An IP address belonging to the interface on which this message is sent. A mobile node MUST use the transient care-of address when assigned; otherwise, the home address is used. Destination The IP address of the agent, when known. When the IP address is unknown (the agent was discovered via a link-layer protocol), the "All Mobile Agents" multicast address (224.0.0.11). The link-layer unicast address is used to deliver the datagram to the correct agent. Perkins, editor Expires 21 April 1994 [Page 10] Internet Draft IP Mobility Support 21 October 1994 UDP fields: Source Port variable Destination Port 434 The UDP Header is followed by the Mobile-IP fields 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Code | Lifetime | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Home Agent | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Care-of Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + Identification + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Extensions ... +-+-+-+-+-+-+-+- Type 1 Code Optional capabilities: 0 remove prior registrations 1 retain prior registrations Lifetime The number of seconds remaining before the registration is considered expired. A value of zero indicates a request for deregistration. A value of all ones indicates infinity. Home Agent The IP address of a home agent. Care-of Address The IP address for the decapsulation end of a tunnel. Identification A 64-bit sequence number, assigned by the mobile node, used to assist in matching requests with replies, and in protecting against replay attacks (see section 9.6). The Home Address Extension (subsection 4.2) is required. The Mobile-Home Authentication Extension (subsection 4.4) is required, Perkins, editor Expires 21 April 1994 [Page 11] Internet Draft IP Mobility Support 21 October 1994 and immediately follows all non-authentication extensions, except those foreign agent specific extensions which may be added to the packet after the mobile node computes the authentication. 3.3. Registration Reply The registration reply message is returned by a home agent to a mobile node which has sent a registration request (subsection 3.2) message. If the mobile node is accepting service from a foreign agent, that foreign agent will receive the reply from the home agent and subsequently relay it to the mobile node. The reply message contains the necessary codes to inform the mobile node about the status of its request, along with the lifetime granted by the home agent, which MAY be smaller than the original request. When the lifetime of the reply is greater than the original request, the excess time SHOULD be ignored. When the lifetime of the reply is smaller than the original request, another registration SHOULD occur before the lifetime expires. IP fields: The source and destination IP addresses of the request message are swapped for the reply message. UDP fields: The source port and destination port of the request message are swapped for the reply message. Note that the source IP address and the source UDP port of the original registration request must be saved in order for the foreign agent to return the reply to the correct mobile node UDP port. DISCUSSION: I think this means that well-known port 435 will go unused. Perkins, editor Expires 21 April 1994 [Page 12] Internet Draft IP Mobility Support 21 October 1994 The UDP Header is followed by the Mobile-IP fields 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Code | Lifetime | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + Identification + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Extensions ... +-+-+-+-+-+-+-+- Type 3 Code One of the following codes: 0 service will be provided. Service denied by the foreign agent: 16 reason unspecified. 17 administratively prohibited. 18 insufficient resources. 19 mobile node failed authentication. 20 home agent failed authentication. 21 requested lifetime too long. Service denied by the home agent: 32 reason unspecified. 33 administratively prohibited. 34 insufficient resources. 35 mobile node failed authentication. 36 foreign agent failed authentication. Up-to-date values of the Code field are specified in the most recent "Assigned Numbers" [20]. Lifetime The seconds remaining before the registration is considered expired. A value of zero confirms a request for de-registration. A value of all ones indicates infinity. Identification The registration identification is copied from the request message. Perkins, editor Expires 21 April 1994 [Page 13] Internet Draft IP Mobility Support 21 October 1994 4. Mobility Message Extensions Each message begins with a short fixed part, followed by one or more mobility message extensions in type-length-value format. These extensions may apply to agent advertisement messages (subsection 2.3) and registration messages (section 3). The Home Address Extension (subsection 4.2) is required. The Mobile-Home Authentication Extension (subsection 4.4) is required, and immediately follows all non-authentication extensions, except those foreign agent specific extensions which may be added to the packet when it is being relayed through a foreign agent. When forwarded by a foreign agent, extensions which are specific to the foreign agent are removed. All others are copied without modification. 0 1 2 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- | Extension | Length | Data ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- Extension Current values are assigned as follows: 16 Mobility 17 Home Address 18 Key Identifier 32 Mobile-Home Authentication 64 Minimal Encapsulation 65 GRE Encapsulation (see [10]) Up-to-date values are specified in the most recent "Assigned Numbers" [20]. Length Indicates the length (in bytes) of the data field. The length does not include the Extension and Length bytes. Data This field is zero or more bytes in length and contains the value(s) for this extension. The format and length of the data field is determined by the extension and length fields. Perkins, editor Expires 21 April 1994 [Page 14] Internet Draft IP Mobility Support 21 October 1994 Extensions allow variable amounts of information to be carried within each datagram. The end of the list of extensions is indicated by the total length of the IP datagram. When an extension is encountered which is not recognized, it is ignored. The length field of the extension is used to skip the data field in searching for the next extension. 4.1. Mobility Extension The Mobility Extension is used to indicate that a Router Advertisement message is actually an agent advertisement being sent by a home agent or foreign agent (see subsection 2.3). All agents which send agent advertisements are presumed to be foreign agents. When agents cannot accept new requests for service from mobile clients, they will set the Busy bit; if the Busy bit is turned off, the agent may attract new mobile clients. An agent which wishes to serve only as a home agent, MUST set the Busy bit in addition to the "H" bit in the mobility 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Extension | Length | Sequence Number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |R|B|H|F|reservd| +-+-+-+-+-+-+-+-+ Extension 16 Length 3 Sequence Number Contains the number of advertisement messages sent since the node was initialized. F Foreign agent registration required bit. When this bit is set to 1, the mobile node SHOULD register through the foreign agent, even when the mobile node has acquired a transient care-of address. B Busy bit. The agent is not willing to accept any more registrations, even though it continues to send advertisements with a positive preference. H Agent is offering service as a home agent. Perkins, editor Expires 21 April 1994 [Page 15] Internet Draft IP Mobility Support 21 October 1994 reservd Sent as zero; ignored on reception. The sequence number MUST count the current advertisement; that is, it begins with one (1) and wraps to zero (0). When this value decreases, or the value one (1) follows any other value than zero (0), the mobile node MUST assume that any current registration has been lost. This field cannot roll over in less than MIN_ADVERTISEMENT_INTERVAL*(2**16) seconds (more than 18 hours), and rollover is unambiguously indicated by the value zero (0) and followed by the value one (1). 4.2. Home Address Extension The home address extension is be found in registration requests (see subsection 3.2). This extension requests that packets for entire subnets be tunneled and delivered to the mobile node, not just packets for the mobile node's particular IP address. More than one home address extension MAY be present. If any have nonzero prefix size, a prefix is inferred by retaining the most significant bits (specified by the prefix size) of the home address. Then the home agent will intercept packets for any destinations that match that prefix and tunnel them to the mobile node. After decapsulation, the foreign agent will then deliver the packets to that mobile node in its visitor list, which has been associated with the inferred prefix matching the tunneled packet. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Extension | Length | reserved | Prefix Size | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Home Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Extension 17 Length 6 reserved Sent as zero; ignored on reception. Prefix Size The size of the left-justified bit-mask that is applied to the home address to determine the IP routing prefix. Ranges from 0 to 30. Set to zero by mobile nodes which are not routers. Home Address The IP address of the mobile node. Perkins, editor Expires 21 April 1994 [Page 16] Internet Draft IP Mobility Support 21 October 1994 4.3. Key Identifier The key identifier extension is be found in registration requests (see subsection 3.2). This extension informs the home agent that authentication is performed using a cryptographic key or algorithm different than the home agent would use by default. If a home agent receives a registration request which does not contain this extension, the home agent assumes that the mobile node used the default Message Authentication Code (see subsection 9.1) to authenticate the registration. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Extension | Length | Key Identifier | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Extension 18 Length 4 reserved Sent as zero; ignored on reception. Key Identifier The key identifier may be chosen from a list which is privately configured between the home agent and the mobile node. In this case, the identifier is completely opaque; no information about which cryptographic algorithm to be used is obvious from the specific value of the key identifier. However, it is also possible for the home agent and mobile node to agree that the value of the key identifier corresponds to the identifier of the cryptographic algorithm used, according to a table to be established and kept by IANA, the Internet Assigned Numbers Authority. So, if there have not been any specific values for key identifier configured between the home agent and the the mobile host, the home agent will understand the key identifier to select a particular cryptographic algorithm and mode. See subsection 9.1 for a primitive list of algorithms and modes. 4.4. Mobile-Home Authentication Extension This extension is found in all registration requests and replies, and is intended to eliminate problems which are well known to result from the uncontrolled propagation of remote redirects in the Internet Perkins, editor Expires 21 April 1994 [Page 17] Internet Draft IP Mobility Support 21 October 1994 (section 9). See subsection 9.1 for information about support requirements for message authentication codes, etc. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Extension | Length | Authenticator ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Extension 32 Length The number of data bytes in the extension. Authenticator (variable length) A hash value taken over a stream of bytes including the shared secret, all prior extensions in their entirety, and the type and length of this extension, but not including the Authenticator field itself. 4.5. Minimal Encapsulation Extension The Minimal Encapsulation Extension is found in agent advertisements (subsection 2.3) and registration requests (subsection 5.2). 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Extension | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Extension 64 Length 0 Perkins, editor Expires 21 April 1994 [Page 18] Internet Draft IP Mobility Support 21 October 1994 5. Forwarding Datagrams to the Mobile Node 5.1. IP in IP Encapsulation Support for IP in IP encapsulated datagrams is required. An outer, full-sized IP fragmentation header is inserted before the datagram's IP header: +---------------------------+ | Outer IP Header | +---------------------------+ +---------------------------+ | IP Header | | IP Header | +---------------------------+ ====> +---------------------------+ | | | | | IP Payload | | IP Payload | | | | | +---------------------------+ +---------------------------+ The format of the IP header is described in [19]. The outer IP header source and destination addresses identify the "endpoints" of the tunnel. The inner IP header source and destination addresses identify the sender and recipient of the datagram. The protocol field in the outer IP header is set to protocol number 4 for the encapsulation protocol. The destination field in the outer IP header set to the care-of address of the mobile node. The source field in the outer IP header is set to the IP address of the encapsulating agent. When the datagram is encapsulated, the Time To Live (TTL) field in the outer IP header is set to be the same as the original datagram. When decapsulating, the outer IP TTL minus one is inserted into the inner IP TTL. Thus, IP hops are counted, but the actual routers interior to the tunnel are not identified. 5.2. Minimal Encapsulation A minimal forwarding header is defined for datagrams which are not fragmented prior to encapsulating. When a datagram is already fragmented prior to encapsulating, IP in IP is used. Use of this encapsulating method is optional. Perkins, editor Expires 21 April 1994 [Page 19] Internet Draft IP Mobility Support 21 October 1994 The minimal header is inserted between the datagram's IP header and the rest of the datagram: +---------------------------+ +---------------------------+ | IP Header | | Modified IP Header | +---------------------------+ ====> +---------------------------+ | | | Forwarding Header | | IP Payload | +---------------------------+ | | | | +---------------------------+ | IP Payload | | | +---------------------------+ A foreign agent which is capable of decapsulating the minimal header will include the Minimal Encapsulation Extension (subsection 4.5) in its Router Advertisements. A mobile node indicates the capability of decapsulating the minimal header at the care-of address by the inclusion of the Minimal Encapsulation Extension in its registration request. The Minimal Encapsulation Extension is not included in the registration reply. The use of the minimal header is entirely at the discretion of the home agent. The format of the minimal forwarding header 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Protocol |S| reserved | Header Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Home Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Correspondent Source Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Protocol Copied from the protocol field in the original IP header. Perkins, editor Expires 21 April 1994 [Page 20] Internet Draft IP Mobility Support 21 October 1994 S Source field present bit, which indicates that the Correspondent Source Address field is present. 0 not present. 1 present. reserved Sent as zero; ignored on reception. Header Checksum The 16-bit one's complement of the one's complement sum of the encapsulation header. For computing the checksum, the checksum field is set to 0. Home Address Copied from the destination field in the original IP header. Correspondent Source Address Copied from the source field in the original IP header. Present only if the S-bit is set. The protocol field in the IP header is replaced by protocol number 55 for the minimal encapsulation protocol. The destination field in the IP header is replaced by the care-of address of the mobile node. If the encapsulating agent is not the original source of the datagram, the source field in the IP header is replaced by the IP address of the encapsulating agent. The Don't Fragment bit is set in the IP header. When decapsulating a datagram, the fields in the forwarding header are restored to the IP header, and the forwarding header is removed from the datagram. Perkins, editor Expires 21 April 1994 [Page 21] Internet Draft IP Mobility Support 21 October 1994 6. Mobile Node Considerations A mobile node listens for agent advertisements at all times that it has a link connection. In this manner, it can learn that its foreign agent has changed, or that it has arrived home. Whenever a mobile node detects a change in its point of attachment, it MUST initiate the registration process. If it is away from home, it MUST create a mobility binding by registering with its home agent (see subsections 3.2, 2.3). If the mobile node is returning to its home network, it MUST deregister with its home agent. A mobile node will operate without the support of mobility functions when it is at home. See appendix B for some considerations regarding the interaction of this mobility specification with features commonly found in link layer implementations for media which may be used with mobile nodes. Receipt of an ICMP Redirect from a registered agent MUST NOT affect the choice of agent for re-registrations. ICMP Redirect only affects the choice of preferred router for forwarding decisions. If desired, the Mobile Node can create a tunnel to its Home Agent. The definition of such a mechanism is outside the scope of this document. DISCUSSION: Why are we keeping this here? Any node can do encapsulation! Does anyone remember the motivation for it? 6.1. Configuration and Registration Tables Each mobile node will need: - home address - prefix size - one or more home agents - mobility security association for each home agent For each pending registration: - MAC address of agent - care-of address - registration identification - lifetime Perkins, editor Expires 21 April 1994 [Page 22] Internet Draft IP Mobility Support 21 October 1994 6.2. Registration When Away From Home In the absence of link-layer indications of changes in point of attachment, agent advertisements from new agents do not necessarily affect a current registration. In the absence of link-layer indications, a mobile node must not attempt to register more often than once per second. A mobile node may register with a different agent when: - transport-layer protocols indicate excessive retransmissions. - its current registration lifetime has expired. The mobile node MUST NOT register with a new agent simply because a higher preference agent has appeared, or the preference values change for the agent with which it is currently registered. If a mobile node detects a reduction in the Sequence Number of an agent advertisement from a foreign agent through which it has registered, the mobile node MUST register again. Such a reduction does not include the wrap of the Sequence Number to zero. A mobile node SHOULD NOT request a lifetime for its registration that exceeds the lifetime learned in an agent advertisement. When the method by which the care-of address is learned does not include a lifetime, the default Router Advertisement lifetime (1800 seconds) may be used. The lifetime MAY be modified by the home agent in its reply. A mobile node SHOULD register again before the lifetime of its registration expires. The mobile node MAY register again at any time. A mobile node MAY ask a home agent to terminate forwarding service to a particular care-of address, by sending a registration with a lifetime of zero. See also subsection 8.2. 6.3. Registration without a foreign agent In cases where a mobile node away from home is able to dynamically acquire a transient IP address (e.g, DHCP [7]), the mobile node can serve without a foreign agent, using the transient address as the care-of address. Then all communication between the mobile node and its home agent can proceed without the intervention of foreign agents. This eliminates the need to deploy foreign agents as separate entities. Perkins, editor Expires 21 April 1994 [Page 23] Internet Draft IP Mobility Support 21 October 1994 This feature MUST NOT be used unless the mobile node has mechanisms to detect changes in its link-layer connectivity, and can initiate acquisition of a new transient address each time such a change occurs. The lifetime of such a registration is chosen by the mobile node. The lifetime MAY be modified by the home agent in its reply. However, on those links where the mobile node detects an agent advertisement that has the "F" bit set in the Mobility Extension (see subsection 2.3), the mobile node SHOULD register through an appropriate foreign agent, even when it might otherwise be able to register directly with a home agent. 6.4. De-registration When At Home At times, a mobile node might become attached to its home link, and will no longer need any forwarding service from its home agent. A deregistration procedure MUST be used between the mobile node and its home agent. The deregistration process involves the exchange of only two messages: a) The mobile node sends a registration request directly to its home agent, with the lifetime set to zero, and the Code field set to 0, to indicate that the home agent remove all related entries. b) The home agent sends a registration reply to the mobile node to grant or deny service. In this special case, the care-of address is set to the home address. The mobile node is not required to register with its home agent. It MAY deregister each foreign agent, or it MAY allow its mobility bindings to simply expire. DISCUSSION: This appears to contradict the combination of the previous MUST, and the specification of the deregistration procedure. It is not necessary to register again with a home agent when a change of Sequence Number occurs, or the Advertisement lifetime expires, since the mobile node is not seeking encapsulating service. procedure MUST be used between the mobile node and its home agent. Perkins, editor Expires 21 April 1994 [Page 24] Internet Draft IP Mobility Support 21 October 1994 6.5. Registration Replies In order for a mobile node to accept a registration reply, the reply must have the same registration identification of its most recent registration request to the sender; otherwise, the message is silently discarded. When a reply is received which has a code indicating information from the foreign agent, the Mobile-Home Authenticator will be missing or invalid. However, if no other reply has as yet been received, the reason for denial SHOULD be accepted, and result in an appropriate action. If a later authenticated reply is received, that reply supersedes the unauthenticated reply. When a reply is received which has a code indicating that authentication failed with the home agent, the reason for denial SHOULD result in an appropriate action. DISCUSSION: Is this a reference to the need for resynchronization? Otherwise, when a reply is received with an invalid Authenticator, the message is silently discarded. The mobile node is not required to issue any message in response to a registration reply. 6.6. Registration Retransmission When no reply has been received within a reasonable time, the registration request is retransmitted. A new registration identification is chosen for each retransmission. The minimum retransmission time SHOULD be related to the speed of the link. The minimum value SHOULD be large enough to account for the size of the packets, twice the round trip time for transmission at the link speed, and at least an additional 100 milliseconds to allow for processing the packets before responding. Some circuits add another 200 milliseconds of satellite delay. The initial time MUST NOT be less than 1 second. At 9,600 bps or less, the recommended initial time is 3 seconds. At 1,200 bps or less, the recommended initial time is 5 seconds. Each successive value less than the maximum value SHOULD be at least twice the previous value. Perkins, editor Expires 21 April 1994 [Page 25] Internet Draft IP Mobility Support 21 October 1994 The maximum retransmission time SHOULD be no greater than the lifetime of the registration request. 6.7. Simultaneous Registrations Multiple simultaneous registrations are useful in several situations, for example when a mobile node is on a border between multiple cellular systems. IP explicitly allows duplication of datagrams. When the home agent allows simultaneous registrations, it will encapsulate a separate copy of each arriving datagram to each care-of address, and the mobile node will receive multiple copies of its datagrams. In order to request this optional capability, the mobile node sends the registration request with the Code set to 1. The return code in the registration reply is the same. No error occurs if the home agent is unable to fulfill the request. When the need for multiple registrations has passed, the mobile node SHOULD register again with the Code set to 0, to remove the other registrations. 6.8. Mobile Routers A mobile node can be a router, which is responsible for the mobility of an entire network moving together, such as on an airplane, a ship, a train, an automobile, a bicycle, or a kayak. Provision for a routing prefix in registration messages is needed when a mobile router registers through a foreign agent. This allows a foreign agent to recognize all addresses attached to the mobile node when they are decapsulated at the care-of address (see subsection 4.2). When a transient IP address has been assigned, the mobile node can register directly with the home agent, as described previously. Such a mobile node MAY advertise to other routers in the foreign routing domain. The mobile node MAY register multiple times with different home addresses and routing prefixes. This permits multiple prefixes to be routed through the mobile node. When the mobile node returns home, and deregisters with the home agent, it participates directly in routing with other routers in its home routing domain. Perkins, editor Expires 21 April 1994 [Page 26] Internet Draft IP Mobility Support 21 October 1994 7. Foreign Agent Considerations The role of the foreign agent is passive and minimal, relaying registration requests between the home agent and the mobile node, and decapsulating datagrams for delivery to the mobile node. When no mobility security association exists, this also reduces the risks resulting from absence of authentication from foreign agent messages. DISCUSSION: Does anyone know why this is here? The foreign agent MUST NOT originate a request or reply that has not been prompted by the mobile node. No request or reply is generated to indicate that the service lifetime has expired. A foreign agent MUST NOT originate a message which revokes the registration of a different foreign agent. A foreign agent SHOULD forward such revocations without modification when such revocation messages originated from an appropriate mobile node. The foreign agent SHOULD NOT advertise the presence of the mobile node which is a router to other routers in its routing domain. The preference is used to regulate the number of mobile nodes which register with the foreign agent. When the foreign agent would otherwise need to reject new registrations because of insufficient resources, the foreign agent SHOULD reduce its preference values until resources become available. Perkins, editor Expires 21 April 1994 [Page 27] Internet Draft IP Mobility Support 21 October 1994 7.1. Configuration and Registration Tables Each foreign agent will need: - care-of address For each pending or current registration, the foreign agent will need a visitor list entry containing: - Media address of mobile node - home address - prefix size - home agent - registration identification - lifetime A foreign agent that has implemented and is using authentication will also need to have the mobility security association information for each pending or current authenticated registration. Even if a foreign agent implements authentication, it might not use authentication with each registration, because of the key management difficulties. 7.2. Receiving Registration Requests Upon receipt of a registration request, if the foreign agent is unable to satisfy the request for some reason, then the foreign agent sends a registration reply to the mobile node with an appropriate code, and does not forward the request to the home agent. Otherwise, the foreign agent forwards the request to the home agent. The foreign agent must maintain a list of pending requests, which includes the IP source address and UDP source port, in order that a correctly addressed reply can be returned to the mobile node. 7.3. Receiving Registration Replies The fields of the registration reply MUST be examined for validity. A registration reply which does not relate to a pending Registration Request, or to a currently registered mobile node, is silently discarded. If the registration reply granted permission to provide service to the mobile node, then the foreign agent updates its visitor list accordingly. Perkins, editor Expires 21 April 1994 [Page 28] Internet Draft IP Mobility Support 21 October 1994 7.4. Decapsulation Every foreign agent MUST examine all arriving encapsulated traffic and compare the destination address to those entries in its visitor list, considering both the home address and routing prefix for the entries (see subsection 4.2), in order to forward to the correct mobile node. When the destination does not match any node currently in the visitor list, the datagram SHOULD be silently discarded. The datagram MUST NOT be further forwarded without modifications to the original IP header, because otherwise a routing loop is likely to result. ICMP Destination Unreachable MUST NOT be sent when a foreign agent is unable to forward a datagram. 7.5. Mobility The foreign agent can be mobile, if the link identified by the care-of address is mobile. The foreign agent could be either a node on a mobile network, or another mobile node itself. Perkins, editor Expires 21 April 1994 [Page 29] Internet Draft IP Mobility Support 21 October 1994 8. Home Agent Considerations It is the intent that the home agent have primary responsibility for processing and coordinating mobility services. The home agent for a given mobile node SHOULD be located on the link identified by the home address, if the home network is not merely a virtual network. The home agent SHOULD advertise the presence of the mobile node which is a router to other routers in its routing domain. 8.1. Configuration and Registration Tables Each home agent will need: - an IP address - prefix size for the home network, if any DISCUSSION: If the home agent controls a virtual home network, the home agent does NOT need an IP address on the virtual home network. For each authorized mobile node, the home agent will need: - home address - mobility security association - prefix size(s) for the mobile network(s), if any For each registered mobile node, the home agent will need a forwarding list entry containing: - care-of address - registration identification - lifetime 8.2. Receiving Registration Requests Upon receipt of a registration request (subsection 3.2), the home agent grants or denies the service requested by sending a registration reply (subsection 3.2) to the sender of the request, with the appropriate code set. If service permission is granted, the home agent will update its forwarding list with the care-of address of the tunnel. Perkins, editor Expires 21 April 1994 [Page 30] Internet Draft IP Mobility Support 21 October 1994 The request is validated by checking that the registration identification is not the same as a preceding request, and the Mobile-Home Authentication Extension (subsection 4.4) is correct. Other Authentication Extensions are also validated when present. When a registration request is invalid, a registration reply is sent with the appropriate error code. This reply will be used by a foreign agent to clear its pending request list, if a foreign agent was involved in relaying the registration request. The home agent MAY shorten the lifetime of the request. A mobile node requests termination of service by indicating a lifetime of zero. If the Code field set to 1, the home agent removes the mobility binding for that care-of address from its forwarding list. Otherwise, if the Code field is set to 0, the home agent removes the mobility bindings for all foreign agents associated with that mobile node from its forwarding list. On termination, no special reply is sent to additional associated foreign agents. The entries in their visitor lists are allowed to expire naturally. 8.3. Simultaneous Registrations When a home agent supports the optional capability of multiple simultaneous registrations, any datagrams forwarded are simply duplicated, and a copy is sent to each care-of address. The return code in the registration reply (subsection 3.3) is the same. No error occurs if the home agent is unable to fulfill the request, and earlier entries in the forwarding list are removed. DISCUSSION: Does anyone know why no status indication can be returned? 8.4. Registration Expiration If the lifetime for a given mobile node expires before the home agent has received a another registration request, then the associated mobility binding is erased from the forwarding list. No special registration reply is sent to the foreign agents. The entries in the visitor lists will expire naturally, and probably at the same time. Perkins, editor Expires 21 April 1994 [Page 31] Internet Draft IP Mobility Support 21 October 1994 8.5. Encapsulation Every home agent MUST examine all arriving traffic for both the home address and routing prefix in order to forward to the correct mobile node. When previously encapsulated datagrams arrive that are associated with the routing prefix of the mobile node, the home agent simply alters the destination to the care-of address. This avoids recursive encapsulation. Other previously encapsulated datagrams, which are not associated with the routing prefix, are recursively encapsulated. 8.6. Mobility The home agent can be mobile, if the link identified by the home address it serves is mobile. The home agent could be either a node on a mobile network, or another mobile node itself. A datagram would be encapsulated on its way to the mobile network, decapsulated for delivery to the mobile node, intercepted by the home agent, and re-encapsulated to the mobile node. Perkins, editor Expires 21 April 1994 [Page 32] Internet Draft IP Mobility Support 21 October 1994 9. Security Considerations The mobile computing environment is potentially very different from the ordinary computing environment. In many cases, mobile computers will be connected to the network via wireless links. Such links are particularly vulnerable to passive eavesdropping, active replay attacks, and other active attacks. 9.1. Message Authentication Codes Home agents and mobile nodes MUST be able to perform keyed MD5 [21], with a key size of 128 bits, for authentication, with the key prefixed and suffixed to the data to be hashed. In addition, the foreign agent SHOULD support authentication using keyed MD5 and key sizes of 128 bits or greater, with manual key distribution. Additional authentication algorithms, algorithm modes, key distribution methods, and key sizes MAY also be supported. Here is a primitive list of algorithms and modes which may be used by home agents and mobile nodes. 0 128+ Keyed-MD5 with both suffix and prefix 1 128+ Keyed-MD5 with suffix 3 160 Keyed-SHA with both suffix and prefix 4 160 Keyed-SHA with suffix 6 160 Keyed-SHA1 with both suffix and prefix 7 160 Keyed-SHA1 with suffix 16 56 DES (Block cipher in MAC mode) 17 168 Triple-DES (Block cipher in MAC mode) 24 ? RC2 Variable Key Size symmetric block cipher in MAC mode DISCUSSION: If anyone knows of a better list described in an existing RFC, please let me know 9.2. Tunneling to Care-of Addresses The registration protocol described in this document will result in a mobile node's traffic being tunneled to its care-of address. This tunneling feature could be a significant vulnerability if the registration were not authentic. Such remote redirection, for instance as performed by the mobile registration protocol, is widely Perkins, editor Expires 21 April 1994 [Page 33] Internet Draft IP Mobility Support 21 October 1994 understood to be a security problem in the current Internet [2]. Moreover, the Address Resolution Protocol (ARP) is not authenticated, and can potentially be used to steal another host's traffic. The use of "Gratuitous ARP"(see Appendix A) brings with it all of the risks associated with the use of ARP. 9.3. Key management This specification requires a strong authentication mechanism (keyed MD5) which precludes many potential attacks based on the Mobile IP registration protocol. However, because key distribution is difficult in the absence of a network key management protocol, not all messages with the foreign agent are authenticated. Vulnerabilities remain in the registration protocol whenever a registration message is not authenticated. For example, in a commercial environment it might be important to authenticate all messages between the foreign agent and the home agent, so that billing is possible, and service providers don't provide service to users that are not legitimate customers of that service provider. 9.4. Picking good keys The strength of any authentication mechanism is dependent on several factors, including the innate strength of the authentication algorithm, the secrecy of the key used, the strength of the key used, and the quality of the particular implementation. This specification requires implementation of keyed MD5 for authentication, but does not preclude the use of other authentication algorithms and modes. For keyed MD5 authentication to be useful, the 128-bit key must be both secret (that is, known only to authorized parties) and pseudo-random. Eastlake, et.al. [8] provides more information on generating pseudo-random numbers. 9.5. Privacy Users who have sensitive data that they do not wish others to see should use mechanisms outside the scope of this document (such as encryption) to provide appropriate protection. Users concerned about traffic analysis should consider appropriate use of link encryption. 9.6. Replay Protection for Registration Requests A Network Time Protocol [15] formatted value is preferred. The low-order 32 bits of the NTP format represent fractional seconds, Perkins, editor Expires 21 April 1994 [Page 34] Internet Draft IP Mobility Support 21 October 1994 and those bits which are not available from a time source SHOULD be generated from a good source of randomness. A battery-backed clock is the usual source of this value. In more robust implementations, Global Positioning System or authenticated NTP values MAY be used. The elapsed time since system startup or another such monotonically increasing counter MAY be used, but is considered less secure, as it could repeat each time the machine is restarted, or when a poor source of randomness is used for the low-order bits. See Eastlake, et.al. [8] for a discussion of sources of randomness. The value MUST NOT be the same as an immediately preceding request, and SHOULD NOT repeat during the lifetime of the mobility security association between the mobile node and the home agent. It is possible to use an entirely random "nonce" in this field, or to generate nonces from previous registration exchanges. DISCUSSION: The use of nonces for replay protection may depend partially on the resolution of a patent issue. Moreover, there is a problem with interoperability; a mobile node and its home agent must agree on the use of nonces, because if a home agent expects only a nonce, it is unlikely that the mobile node's time value will be accepted. 10. Acknowledgements Special thanks to Steve Deering (Xerox PARC), along with Dan Duchamp and John Ioannidis (Columbia), for forming the working group, chairing it, and putting so much effort into its early development. Thanks also to Greg Minshall for his contributions to the group while performing the duties of chairperson. Perkins, editor Expires 21 April 1994 [Page 35] Internet Draft IP Mobility Support 21 October 1994 Thanks to the active members of the Working Group, particularly those who contributed text, including (in alphabetical order) - Ran Atkinson (Naval Research Lab), - Dave Johnson (Carnegie Mellon University), - Andrew Myles (Macquarie University), - John Penners (US West), - Al Quirt (Bell Northern Research), - Yakov Rekhter (IBM), and - Fumio Teraoka (Sony). Thanks to Charlie Kunzinger, the editor who produced the first drafts for the Working Group, and to Bill Simpson, who has produced most of the text of this draft, reflecting the discussions of the Working Group. Thanks to Greg Minshall (Novell) and Phil Karn (Qualcomm) for their generous support in hosting interim Working Group meetings. A. Gratuitous and Proxy ARP Many people will use their computers for extended periods of time on a single link, whether or not it is at their home network. When doing so, they will expect the same level of service from their infrastructure as they receive today on the home network. A separate "virtual" IP address block is not required for mobile nodes. This would require a small network to have an extra router between the mobile and non-mobile nodes, which is an unacceptable expense. This section details the special care to be taken when using ARP [17] with nodes on the same link as a mobile node. A problem can arise if a mobile node which has previously answered an ARP Request moves away from the link, leaving behind a stale entry in another node's ARP cache. For example, if a router which forwards datagrams into the home network has a stale ARP cache entry for the mobile node, any datagrams arriving through that router for the mobile node will be lost. Thus, it is important that ARP caches of nodes populating the link be updated as soon as possible. A gratuitous ARP is an ARP Reply that is broadcast to all nodes on a link, which is not in response to any ARP Request. When an ARP Reply is broadcast, all hosts are required to update their local ARP caches, whether or not the ARP Reply was in response to an ARP Request they had issued. Perkins, editor Expires 21 April 1994 [Page 36] Internet Draft IP Mobility Support 21 October 1994 When there is a physical link which corresponds to the home network, a gratuitous proxy ARP is issued by the home agent on behalf of a mobile node whenever the home agent receives a valid registration. The gratuitous proxy ARP will indicate that all remaining nodes should associate the home address of the mobile node with the link-layer address of the home agent which is now serving the mobile node. In the gratuitous ARP, the source IP address is the home address, the MAC address is the source link-layer address for the interface used, the target IP address would be the all-systems multicast address, and the target link-layer address would be the general broadcast address. While the mobile node is away from its home network, the home agent performs proxy ARP Replies for the mobile node. When a mobile node returns to its home network, it SHOULD issue a gratuitous ARP on its own behalf, immediately before sending the de- registration request to the home agent. Although the gratuitous ARP can be lost, this is not different from the usual ARP Reply problems, which are outside the scope of this document. A home agent may repeat the gratuitous ARP a small number of times to allow for the possibility of the first packet getting lost. B. Link-Layer considerations The mobile node primarily uses link-layer mechanisms to decide that its point of attachment has changed. Such indications include the Down/Testing/Up interface status [13], and changes in cell or administration. The mechanisms will be specific to the particular link-layer technology, and are outside the scope of this document. B.1. Point-to-Point Link-Layers The Point-to-Point-Protocol (PPP) [22] and its Internet Protocol Control Protocol (IPCP) [14], negotiates the use of IP addresses. The mobile node SHOULD first attempt to specify its home address. This allows an unrouted link to function correctly. When the home address is not accepted by the peer, but a transient IP address is dynamically assigned, that address MAY be used as the care-of address for registration. Perkins, editor Expires 21 April 1994 [Page 37] Internet Draft IP Mobility Support 21 October 1994 When the peer specifies its own IP address, that address MUST NOT be assumed to be the care-of address of a foreign agent or the IP address of a home agent. DISCUSSION: Can anyone explain better what's going on here? When Router Advertisements are received which contain the Mobility Extension, registration with the agent SHOULD take place as usual. If the link is bandwidth limited, this method is preferred over use of the transient care-of address, The encapsulation will be removed by the peer, allowing header compression techniques to function correctly [11]. B.2. Multi-Point Link-Layers Another link establishment protocol, IEEE 802.11 [6], might yield the link address of an agent. This link-layer address SHOULD be used to attempt registration. The receipt of a Router Advertisement supersedes the link-layer address, and a new registration MUST occur. C. TCP Considerations C.1. TCP Timers Most hosts and routers which implement TCP/IP do not permit easy configuration of the TCP timer values. When high-delay (e.g. SATCOM) or low-bandwidth (e.g. High-Frequency Radio) links are in use, the default TCP timer values in many systems will cause retransmissions or timeouts when the link and network is actually operating properly, though with greater than usual delays because of the medium in use. This can cause an inability to create or maintain connections over such links, and can also cause unneeded retransmissions which consume already scarce bandwidth. Vendors are encouraged to make TCP timers more configurable. Vendors of systems designed for the mobile computing markets should pick default timer values more suited to low-bandwidth, high-delay links. Users of mobile nodes should be sensitive to the possibility of timer-related difficulties. C.2. TCP Congestion Management Mobility nodes are likely to use media which have low bandwidth and are more likely to introduce errors, effectively causing more packets Perkins, editor Expires 21 April 1994 [Page 38] Internet Draft IP Mobility Support 21 October 1994 to be dropped. This introduces a conflict with the mechanisms for congestion management found in modern versions of TCP. Now, when a packet is dropped, the correspondent's TCP implementation is likely to react as if there were a source of network congestion, and initiate the slow-start mechanisms [4] designed for controlling that problem. However, those mechanisms are inappropriate for overcoming errors introduced by the links themselves, and have the effect of magnifying the discontinuity introduced by the dropped packet. This problem has been analyzed by Caceres, et. al. ( [3]); there is no easy solution available, and certainly no solution likely to be installed soon on all correspondents. While this problem has nothing to do with any of the specifications in this document, it does illustrate that providing performance transparency to mobile nodes involves understanding mechanisms outside the network layer. It also indicates the need to avoid designs which systematically drop packets; such designs might otherwise be considered favorably when making engineering tradeoffs. D. Tunnel Management It is possible that one of the routers along the tunnel interior might encounter an error while processing the datagram, causing it to return an IP ICMP error message to the source end of the tunnel. The three types of ICMP errors that can occur in this circumstance are: - Datagram Too Big - Time Exceeded - Destination Unreachable Unfortunately, ICMP only requires IP routers to return 8 bytes (64 bits) of the datagram beyond the IP header. This is not enough to include the encapsulated header, so it is not generally possible for the home agent to immediately reflect the ICMP message from the interior of a tunnel back to the source host. However, by carefully maintaining "soft state" about its tunnels, the encapsulating router can return accurate ICMP messages in most cases. The router SHOULD maintain at least the following soft state information about each tunnel: - MTU of the tunnel - TTL (path length) of the tunnel - Reachability of the end of the tunnel The router uses the ICMP messages it receives from the interior of a tunnel to update the soft state information for that tunnel. When subsequent datagrams arrive that would transit the tunnel, the router Perkins, editor Expires 21 April 1994 [Page 39] Internet Draft IP Mobility Support 21 October 1994 checks the soft state for the tunnel. If the datagram would violate the state of the tunnel (such as, the TTL is less than the tunnel TTL) the router sends an ICMP error message back to the source, but also forwards the datagram into the tunnel. Using this technique, the ICMP error messages sent by encapsulating routers will not always match up one-to-one with errors encountered within the tunnel, but they will accurately reflect the state of the network. The Don't Fragment bit is always set within the tunnel. This enables the proper MTU of the tunnel to be determined. Fragmentation which occurs because of the size of the encapsulation header is done before encapsulation, preventing more than one layer of fragmentation in a single datagram. DISCUSSION: Would anyone like to provide more explanation? Or, should we just delete most of it and be satisfied with a reference in the section about Home Agent Considerations? Tunnel soft state was originally developed for the IP address encapsulation (IPAE) specification [9]. References [1] R. Atkinson. SIPP Authentication Header. Internet Draft -- work in progress, April 1994. [2] S.M. Bellovin. Security Problems in the TCP/IP Protocol Suite. ACM Computer Communications Review, 19(2), March 1989. [3] Ramon Caceres and Liviu Iftode. The Effects of Mobility on Reliable Transport Protocols. In Proceedings of the 14th International Conference on Distributed Computing Systems, June 1994. [4] Douglas E. Comer. Internetworking with TCP/IP, volume 1. Prentice Hall, 1991. [5] S. Deering. Router Discovery. RFC 1256, September 1991. [6] Wim Diepstraten, Greg Ennis, and Phil Belanger. DFWMAC - Distributed Foundation Wireless Medium Access Control. IEEE Document P802.11-93/190, Nov 1993. Perkins, editor Expires 21 April 1994 [Page 40] Internet Draft IP Mobility Support 21 October 1994 [7] R. Droms. Dynamic Host Configuration Protocol. RFC 1541, October 1993. [8] D.E. Eastlake, S.D. Crocker, and J.I. Schiller. Randomness Requirements for Security. Internet Draft -- work in progress, October 1994. [9] R. Gilligan, E. Nordmark, and B. Hinden. IPAE: The SIPP Interoperability and Transition Mechanism. Internet Draft -- work in progress, March 1994. [10] S. Hanks, T. Li, D. Farinacci, and P. Traina. Generic routing encapsulation (gre). draft-hanks-gre-00.txt -- work in progress, October 1994. [11] V. Jacobson. Compressing TCP/IP Headers for Low-Speed Serial Links. RFC 1144, February 1990. [12] J. Kohl and C. Newman. The Kerberos Network Authentication Service (V5). RFC 1510, September 1993. [13] K. McCloghrie and F. Kastenholz. Evolution of the Interfaces Group MIP-II. RFC 1573, January 1994. [14] G. McGregor. The PPP Internet Procotol Control Protocol (IPCP). RFC 1332, May 1992. [15] D. Mills. Network Time Protocol (Version 3). RFC 1305, March 1992. [16] National Bureau of Standards. Data Encryption Standard. Federal Information Processing Standards, 1977. [17] D. Plummer. An Ethernet Address Resolution Protocol. RFC 826, November 1982. [18] J. Postel. User Datagram Protocol. RFC 768, August 1980. [19] J. Postel. Internet Protocol. RFC 791, September 1981. [20] J. Reynolds and J. Postel. Assigned Numbers. RFC 1700, October 1994. [21] R. Rivest. The MD5 Message-Digest Algorithm. RFC 1321, April 1992. [22] W. Simpson (Editor). The Point-to-Point Protocol (PPP). RFC 1661, July 1994. Perkins, editor Expires 21 April 1994 [Page 41] Internet Draft IP Mobility Support 21 October 1994 Chair's Addresses The working group can be contacted via the current chairs: Kannan Alagappan Tony Li 170 W. Tasman Dr. San Jose CA 95134 Work: +1 222 3334444 Work: +1 408 5268186 E-mail: kannan@emc.com E-mail: tli@cisco.com Editor's Address Questions about this memo can also be directed to: Charles Perkins Room J1-A25 T. J. Watson Research Center IBM Corporation P. O. Box 218 Yorktown Heights, NY 10598 Work: +1 914 7847350 Fax: +1 914 7847007 E-mail: perk@watson.ibm.com Perkins, editor Expires 21 April 1994 [Page 42]