Internet Engineering Task Force G. Montenegro, Editor INTERNET DRAFT Sun Microsystems, Inc. August 10, 1997 Reverse Tunneling for Mobile IP draft-ietf-mobileip-tunnel-reverse-03.txt Status of This Memo This document is a submission by the Mobile IP Working Group of the Internet Engineering Task Force (IETF). Comments should be submitted to the Working Group mailing list at "mobile-ip@SmallWorks.COM". 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 inappropriate to use Internet- Drafts as reference material or to cite them other than as ``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 ftp.is.co.za (Africa), nic.nordu.net (Europe), munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or ftp.isi.edu (US West Coast). Abstract Mobile IP uses tunneling from the home agent to the mobile node's care-of address, but rarely in the reverse direction. Usually, a mobile node sends its packets through a router on the foreign network, and assumes that routing is independent of source address. When this assumption is not true, it is convenient to establish a topologically correct reverse tunnel from the care-of address to the home agent. This document proposes backwards-compatible extensions to Mobile IP in order to support topologically correct reverse tunnels. This document does not attempt to solve the problems posed by firewalls located between the home agent and the mobile node's care-of address. Montenegro Expires February 10, 1998 [Page 1] INTERNET DRAFT Reverse Tunneling for Mobile IP August 1997 Table of Contents 1. Introduction ................................................... 3 1.1. Terminology .................................................. 4 1.2. Assumptions .................................................. 4 1.3. Justification ................................................ 4 2. Overview ....................................................... 5 3. New Packet Formats ............................................. 5 3.1. Mobility Agent Advertisement Extension ....................... 5 3.2. Registration Request ......................................... 6 3.3. Encapsulating Delivery Style Extension ....................... 7 3.4. Cookie Extensions ............................................ 8 3.4.1 Mobile-Foreign Cookie Extension ............................. 8 3.4.2 Foreign-Home Cookie Extension ............................... 10 3.5. New Registration Reply Codes ................................. 11 4. Changes in Protocol Behavior ................................... 12 4.1. Mobile Node Considerations ................................... 12 4.1.1. Sending Registration Requests to the Foreign Agent ......... 12 4.1.2. Receiving Registration Replies from the Foreign Agent ...... 13 4.2. Foreign Agent Considerations ................................. 14 4.2.1. Receiving Registration Requests from the Mobile Node ....... 14 4.2.2. Relaying Registration Requests to the Home Agent ........... 15 4.3. Home Agent Considerations .................................... 16 4.3.1. Receiving Registration Requests from the Foreign Agent ..... 16 4.3.2. Sending Registration Replies to the Foreign Agent .......... 17 5. Mobile Node to Foreign Agent Delivery Styles ................... 18 5.1. Direct Delivery Style ........................................ 18 5.1.1. Packet Processing .......................................... 18 5.1.2. Packet Header Format and Fields ............................ 18 5.2. Encapsulating Delivery Style ................................. 19 5.2.1 Packet Processing ........................................... 20 5.2.2. Packet Header Format and Fields ............................ 20 5.3. Support for Broadcast and Multicast Datagrams ................ 21 5.4. Selective Reverse Tunneling .................................. 22 6. Security Considerations ........................................ 22 6.1. Reverse-tunnel Hijacking and Denial-of-Service Attacks ....... 22 6.2. Ingress Filtering ............................................ 23 7. Acknowledgements ............................................... 24 References ........................................................ 24 Editor and Chair Addresses ........................................ 24 Montenegro Expires February 10, 1998 [Page 2] INTERNET DRAFT Reverse Tunneling for Mobile IP August 1997 1. Introduction Section 1.3 of the Mobile IP specification [1] lists the following assumption: It is assumed that IP unicast datagrams are routed based on the destination address in the datagram header (i.e., not by source address). Because of security concerns (e.g. IP spoofing attacks), and in accordance with the IAB [8] and CERT [3] advisories to this effect, routers that break this assumption are increasingly more common. In the presence of such routers, the source and destination IP address in a packet must be topologically correct. The forward tunnel complies with this, as its endpoints (home agent address and care-of address) are properly assigned addresses for their respective locations. On the other hand, the source IP address of a packet transmitted by the mobile node does not correspond to the network prefix from where it emanates. This document discusses topologically correct reverse tunnels. Mobile IP does dictate the use of reverse tunnels in the context of multicast datagram routing and mobile routers. However, the source IP address is set to the mobile node's home address, so these tunnels are not topologically correct. Notice that there are several uses for reverse tunnels regardless of their topological correctness: - Mobile routers: reverse tunnels obviate the need for recursive tunneling [1]. - Multicast: reverse tunnels enable a mobile node away from home to (1) join multicast groups in its home network, and (2) transmit multicast packets such that they emanate from its home network [1]. - The TTL of packets sent by the mobile node (particularly when sends packets to other hosts in its home network) may be so low that they might expire before reaching their destination. A reverse tunnel solves the problem as it represents a TTL decrement of one [5]. Montenegro Expires February 10, 1998 [Page 3] INTERNET DRAFT Reverse Tunneling for Mobile IP August 1997 1.1. Terminology The discussion below uses terms defined in the Mobile IP specification. Additionally, it uses the following terms: Forward Tunnel A tunnel that shuttles packets towards the mobile node. It starts at the home agent, and ends at the mobile node's care-of address. Reverse Tunnel A tunnel that starts at the mobile node's care-of address and terminates at the home agent. 1.2. Assumptions Mobility is constrained to a common IP address space (e.g. the routing fabric between, say, the mobile node and the home agent is not partitioned into a "private" and a "public" network). This document does not attempt to solve the firewall traversal problem. Rather, it assumes one of the following is true: - There are no intervening firewalls along the path of the tunneled packets. - Any intervening firewalls share the security association necessary to process any authentication [6] or encryption [7] headers which may have been added to the tunneled packets. The reverse tunnels considered here are symmetric, that is, they use the same configuration (encapsulation method, IP address endpoints) as the forward tunnel. IP in IP encapsulation [2] is assumed unless stated otherwise. Route optimization [4] introduces forward tunnels initiated at a correspondent host. Since a mobile node may not know if the correspondent host can decapsulate packets, reverse tunnels in that context are not discussed here. 1.3. Justification Why not let the mobile node itself initiate the tunnel to the home Montenegro Expires February 10, 1998 [Page 4] INTERNET DRAFT Reverse Tunneling for Mobile IP August 1997 agent? This is indeed what it should do if it is already operating with a topologically correct co-located care-of address. However, one of the primary objectives of the Mobile IP specification is not to require this mode of operation. The mechanisms outlined in this document are primarily intended for use by mobile nodes that rely on the foreign agent for forward tunnel support. It is desirable to continue supporting these mobile nodes, even in the presence of filtering routers. 2. Overview A mobile node arrives at a foreign network, listens for agent advertisements and selects a foreign agent that supports reverse tunnels. It requests this service when it registers through the selected foreign agent. At this time, and depending on how the mobile node wishes to deliver packets to the foreign agent, it also requests either the Direct or the Encapsulating Delivery Style (section 5). In the Direct Delivery Style, the mobile node designates the foreign agent as its default router and proceeds to send packets directly to the foreign agent, i.e., without encapsulation. The foreign agent intercepts them, and tunnels them to the home agent. In the Encapsulating Delivery Style, the mobile node encapsulates all its outgoing packets to the foreign agent. The foreign agent decapsulates and re-tunnels them to the home agent, using the foreign agent's care-of address as the entry-point of this new tunnel. 3. New Packet Formats 3.1. Mobility Agent Advertisement Extension Montenegro Expires February 10, 1998 [Page 5] INTERNET DRAFT Reverse Tunneling for Mobile IP August 1997 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 | Sequence Number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Lifetime |R|B|H|F|M|G|V|T| reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | zero or more Care-of Addresses | | ... | The only change to the Mobility Agent Advertisement Extension [1] is the additional 'T' bit: T Agent offers reverse tunneling service. A foreign agent that sets the 'T' bit MUST support the two delivery styles currently supported: Direct and Encapsulating Delivery Style (section 5). Using this information, a mobile node is able to choose a foreign agent that supports reverse tunnels. Notice that if a mobile node does not understand this bit, it simply ignores it as per [1]. 3.2. Registration Request Reverse tunneling support is added directly into the Registration Request by using one of the "rsvd" bits. If a foreign or home agent that does not support reverse tunnels receives a request with the 'T' bit set, the Registration Request fails. This results in a registration denial (failure codes are specified in section 3.5). Most home agents would not object to providing reverse tunnel support, because they "SHOULD be able to decapsulate and further deliver packets addressed to themselves, sent by a mobile node" [1]. In the case of topologically correct reverse tunnels, the packets are not sent by the mobile node as distinguished by its home address. Rather, the outermost (encapsulating) IP source address on such datagrams is the care-of address of the mobile node. Nevertheless, home agents probably already support the required decapsulation and further forwarding. Montenegro Expires February 10, 1998 [Page 6] INTERNET DRAFT Reverse Tunneling for Mobile IP August 1997 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 |S|B|D|M|G|V|T|-| Lifetime | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Home Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Home Agent | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Care-of Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Identification | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Extensions ... +-+-+-+-+-+-+-+- The only change to the Registration Request packet is the additional 'T' bit: T If the 'T' bit is set, the mobile node asks its home agent to accept a reverse tunnel from the care-of address. Mobile nodes using a foreign agent care-of address ask the foreign agent to reverse-tunnel its packets. 3.3. Encapsulating Delivery Style Extension The Encapsulating Delivery Style Extension MAY be included by the mobile node in registration requests to further specify reverse tunneling behavior. It is expected to be used only by the foreign agent. Accordingly, the foreign agent MUST consume this extension (i.e. it must not relay it to the home agent or include it in replies to the mobile node). As per Section 3.6.1.3 of [1], the mobile node MUST include the Encapsulating Delivery Style Extension after the Mobile-Home Authentication Extension, and before the Mobile-Foreign Authentication Extension, if present. Encapsulating Extension MUST NOT be included if the 'T' bit is not set in the Registration Request. If this extension is absent, Direct Delivery is assumed. Encapsulation is done according to what was negotiated for the forward tunnel (i.e., IP in IP is assumed unless specified otherwise). For more details on the delivery styles, please refer to section 5. Montenegro Expires February 10, 1998 [Page 7] INTERNET DRAFT Reverse Tunneling for Mobile IP August 1997 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type 128 Length 0 3.4. Cookie Extensions In the absence of mobility security associations, Cookie Extensions are used by mobile nodes, foreign agents and home agents to provide some level of protection against reverse tunnel hijacking and denial of service attacks (see Section 6). Cookies are not needed if the mobile node operates in co-located mode since the two entities involved (mobile node and home agent) MUST have a mobility security association in place [1]. The Mobile-Foreign Cookie Extension is used only between the mobile node and the foreign agent. Similarly, use of the Foreign-Home Cookie Extension is limited to registration traffic between the foreign agent and the home agent. If a Registration Request with the 'T' bit on includes a Cookie Extension, the corresponding successful (code 0) Registration Reply MUST include one as well. 3.4.1 Mobile-Foreign Cookie Extension Mobile nodes and foreign agents MAY include a Mobile-Foreign Cookie Extension in Registration Requests and Replies, subject to the conditions detailed in Sections 4.1 and 4.2. The foreign agent MUST consume this extension (i.e. it MUST NOT relay it to the home agent). If the Mobile-Foreign Cookie Extension is present, as per Sections 3.6.1.3 and 3.7.3.2 of [1], the mobile node and the foreign agent Montenegro Expires February 10, 1998 [Page 8] INTERNET DRAFT Reverse Tunneling for Mobile IP August 1997 MUST include it after the Mobile-Home Authentication Extension. For further details about the processing of this extension, at the mobile node and the foreign agent, refer to Sections 4.1 and 4.2, respectively. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Current Cookie | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Previous Cookie | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type 130 Length 10 Reserved 0 Current Cookie In Registration Requests, it is a value generated randomly by the mobile node. In Registration Replies, the foreign agent sets it to the value sent by the mobile node as the "Current Cookie" in the corresponding Registration Request. Previous Cookie Set to 0 by the foreign agent in Registration Replies. Set to 0 by the mobile node in initial Registration Requests. In re-registrations, the mobile node sets this field to the "Current Cookie" field of the Mobile-Foreign Cookie Extension in the last successful (code 0) Registration Reply. Montenegro Expires February 10, 1998 [Page 9] INTERNET DRAFT Reverse Tunneling for Mobile IP August 1997 3.4.2 Foreign-Home Cookie Extension Foreign agents and home agents MAY include a Foreign-Home Cookie Extension in Registration Requests and Replies, subject to the conditions detailed in Sections 4.2 and 4.3. If the Foreign-Home Cookie Extension is present, as per Sections 3.7.3.2 and 3.8.3.3 of [1], the foreign agent and the home agent MUST include it after the Mobile-Home Authentication Extension. For further details about the processing of this extension, at the foreign agent and the home agent, refer to Sections 4.2 and 4.3, respectively. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Current Cookie | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Previous Cookie | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type 131 Length 10 Reserved 0 Current Cookie In Registration Requests, it is a value generated randomly by the foreign agent. In Registration Replies, the home agent sets it to the value sent by the foreign agent as the "Current Cookie" field of the Foreign-Home Cookie Extension in the corresponding Registration Request. Previous Cookie Montenegro Expires February 10, 1998 [Page 10] INTERNET DRAFT Reverse Tunneling for Mobile IP August 1997 Set to 0 by the home agent in Registration Replies. Set to 0 by the foreign node in initial Registration Requests. In re-registrations, the foreign agent sets this field to the "Current Cookie" field of the Foreign-Home Cookie Extension in the last Registration Reply. 3.5. New Registration Reply Codes Foreign and home agent registration replies MUST convey if the reverse tunnel request failed. These new reply codes are defined: Service denied by the foreign agent: 74 requested reverse tunnel unavailable 75 reverse tunnel is mandatory and 'T' bit not set 76 bad cookie 77 bad cookie from home agent 78 mobile node too distant and Service denied by the home agent: 137 requested reverse tunnel unavailable 138 reverse tunnel is mandatory and 'T' bit not set 139 requested encapsulation unavailable 140 bad cookie In response to a Registration Request with the 'T' bit set, mobile nodes may receive (and MUST accept) code 70 (poorly formed request) from foreign agents and code 134 (poorly formed request) from home agents. However, foreign and home agents that support reverse tunneling MUST use codes 74 and 137, respectively. Absence of the 'T' bit in a Registration Request MAY elicit denials with codes 75 and 138 at the foreign agent and the home agent, respectively. Forward and reverse tunnels are symmetric, i.e. both are able to use the same tunneling options negotiated at registration. This implies that the home agent MUST deny registrations if an unsupported form of tunneling is requested (code 139). Notice that Mobile IP [1] already defines the analogous failure code 72 for use by the foreign agent. Montenegro Expires February 10, 1998 [Page 11] INTERNET DRAFT Reverse Tunneling for Mobile IP August 1997 In response to a Registration Request with the 'T' bit set, mobile nodes may receive (and MUST accept) codes 76 (bad cookie), 77 (bad cookie from home agent) and 78 (mobile node too distant). from foreign agents. Similarly, foreign agents MUST accept code 140 (bad cookie) from home agents, and then relay it to mobile nodes. These codes are sent as response to the absence of an expected Cookie Extension, or if the values in the fields are unexpected. 4. Changes in Protocol Behavior Reverse tunnels must be handled appropriately by the different mobility entities. Differences in protocol behavior with respect to the Mobile IP specification are specified in the subsequent sections. 4.1. Mobile Node Considerations This section describes how the mobile node handles registrations that request a reverse tunnel. 4.1.1. Sending Registration Requests to the Foreign Agent In addition to the considerations in [1], a mobile node sets the 'T' bit in its Registration Request to petition a reverse tunnel. If registering via a separate foreign agent, it MUST use either one of the following: a. If the mobile node has a security association with the foreign agent, it MUST use it as specified in [1]. b. Otherwise, it MUST append a Mobile-Foreign Cookie Extension to the Registration Request after the Mobile-Home Authentication Extension. The "Current Cookie" field MUST be generated randomly by the mobile node using guidelines similar to those used in generating nonces. If using a Mobile-Foreign Cookie Extension, the mobile node MUST set the TTL field of the IP header to 255. This is meant to limit a denial of service attack introduced by the use of Cookie Extensions (Section 6). The mobile node MAY optionally include an Encapsulating Delivery Montenegro Expires February 10, 1998 [Page 12] INTERNET DRAFT Reverse Tunneling for Mobile IP August 1997 Style Extension. 4.1.2. Receiving Registration Replies from the Foreign Agent If the mobile node included a Mobile-Foreign Cookie Extension in its Registration Request, the Registration Reply MUST also include one. If it doesn't, the mobile node MUST ignore the reply. If such an extension is present, the mobile node MUST verify that the value of the "Current Cookie" field in the reply is equal to that in the request. If it isn't, the mobile node MUST ignore this reply. The mobile node SHOULD treat flag either of these two conditions as security exceptions. Possible valid responses are: - A registration denial issued by either the home agent or the foreign agent: a. The mobile node follows the error checking guidelines in [1], and depending on the reply code, MAY try modifying the registration request (for example by eliminating the request for alternate forms of encapsulation), and issuing a new registration. b. Depending on the reply code, the mobile node MAY try zeroing the 'T' bit, eliminating the Encapsulating Delivery Style Extension (if one was present), and issuing a new registration. c. The foreign agent returns code 76 ("bad cookie"). If reissuing the Registration Request, the mobile node MUST set the "Previous Cookie" to the appropriate value. If the mobile node ignores this value, it can (a) wait for the current binding at the foreign agent to expire, (b) attempt registering using another foreign agent. d. The foreign agent returns codes 77 (bad cookie from home agent) or 78 (mobile node too distant). The mobile node MAY try issuing a new registration via another foreign agent or registering with another home agent. It SHOULD also flag this event as a security exception. e. The foreign agent relays code 140 ("bad cookie") from the home agent. The mobile node MAY try issuing a new registration via another foreign agent. It SHOULD also flag this event as a security exception. Montenegro Expires February 10, 1998 [Page 13] INTERNET DRAFT Reverse Tunneling for Mobile IP August 1997 - The home agent returns a Registration Reply indicating that the service will be provided. In this last case, the mobile node has succeeded in establishing a reverse tunnel between its care-of address and its home agent. If the mobile node is operating with a co-located care-of address, it MAY encapsulate outgoing data such that the destination address of the outer header is the home agent. This ability to selectively reverse-tunnel packets is discussed further in section 5.4. If the care-of address belongs to a separate foreign agent, the mobile node MUST employ whatever delivery style was requested (Direct or Encapsulating) and proceed as specified in section 5. A successful registration reply is an assurance that both the foreign agent and the home agent support whatever alternate forms of encapsulation (other than IP in IP) were requested. Accordingly, the mobile node MAY use them at its discretion. If a valid Mobile-Foreign Cookie Extension is present in the Registration Reply, the mobile node MUST record the value reported by the foreign agent in the "Current Cookie" field. This value MUST be used to set the "Previous Cookie" field of the Mobile-Foreign Cookie Extension in a subsequent re-registration. Failure to do so will result in a registration denial. Of course, after the registration expires at the foreign agent the mobile node may register with a "Previous Cookie" set to 0. In order to re-register after rebooting, a mobile node MAY choose to record a Registration Reply's "Current Cookie" field in non-volatile storage. Alternatively, it MAY request a sufficiently short lifetime in its registration (e.g. comparable to the time the mobile node takes to reboot). 4.2. Foreign Agent Considerations This section describes how the foreign agent handles registrations that request a reverse tunnel. 4.2.1. Receiving Registration Requests from the Mobile Node A foreign agent that receives a Registration Request with the 'T' bit set processes the packet as specified in the Mobile IP specification [1], and determines whether it can accomodate the Montenegro Expires February 10, 1998 [Page 14] INTERNET DRAFT Reverse Tunneling for Mobile IP August 1997 forward tunnel request. If it cannot, it returns an appropriate code. In particular, if the foreign agent is unable to support the requested form of encapsulation it MUST return code 72. The foreign agent MAY reject Registration Requests without the 'T' bit set by denying them with code 75 (reverse tunnel is mandatory and 'T' bit not set). A foreign agent expects a Registration Request with 'T' bit on to be secured by either of these: a. Mobile-Foreign Authentication Extension (preferred), or b. Mobile-Foreign Cookie Extension. If neither of the above is present, the foreign agent MUST reject the registration with code 76 (bad cookie). If the Registration Request includes a Mobile-Foreign Extension, the foreign agent MUST verify that the TTL field of the IP header is set to 255. Otherwise, it MUST reject the registration with code 78 (mobile node too distant). The foreign agent MUST record the value of the "Current Cookie" field. This value MUST match the "Previous Cookie" field in subsequent re-registrations issued by the same mobile node. A mismatch MUST result in the foreign agent's denying the registration with code 76 (bad cookie). As a last check, the foreign agent verifies that it can support a reverse tunnel with the same configuration. If it cannot, it MUST return a Registration Reply denying the request with code 74 (requested reverse tunnel unavailable). 4.2.2. Relaying Registration Requests to the Home Agent Otherwise, the foreign agent MUST relay the Registration Request to the home agent. The foreign agent MUST use either of these: a. Foreign-Home Authentication Extension (preferred), or b. Foreign-Home Cookie Extension. If the foreign agent receives a denial with code 140 (bad cookie) from the home agent, it MUST relay it to the mobile node. If the foreign agent included a Foreign-Home Cookie Extension when it relayed the Registration Request, the Registration Reply MUST Montenegro Expires February 10, 1998 [Page 15] INTERNET DRAFT Reverse Tunneling for Mobile IP August 1997 also include one. If it doesn't, the foreign agent MUST send a denial to the mobile node with code 77 (bad cookie from home agent). If such an extension is present, the foreign agent MUST verify that the value of the "Current Cookie" field in the reply is equal to that in the request. If it isn't, the foreign agent MUST send a denial to the mobile node with code 77 (bad cookie from home agent). The foreign agent SHOULD flag either of these two conditions as security exceptions. If the reply includes a Foreign-Home Authentication Extension, the foreign agent MUST record the value of the "Current Cookie" field. This value must be used to set the "Previous Cookie" field when relaying a subsequent re-registration from the same mobile node. Upon receipt of a Registration Reply that satisfies validity checks, the foreign agent MUST update its visitor list, including indication that this mobile node has been granted a reverse tunnel and the delivery style expected (section 5). The foreign agent MUST then relay the Registration Reply to the mobile node, including either a Mobile-Foreign Authentication Extension or Mobile-Foreign Cookie Extension, whichever was present in the corresponding Registration Request. In the latter case, the value of the "Current Cookie" MUST be equal to that in the same field of the corresponding request. While this visitor list entry is in effect, the foreign agent MUST process incoming traffic according to the delivery style, encapsulate it and tunnel it from the care-of address to the home agent's address. 4.3. Home Agent Considerations This section describes how the home agent handles registrations that request a reverse tunnel. 4.3.1. Receiving Registration Requests from the Foreign Agent A home agent that receives a Registration Request with the 'T' bit set processes the packet as specified in the Mobile IP specification [1] and determines whether it can accomodate the forward tunnel request. If it cannot, it returns an appropriate code. In particular, if the home agent is unable to support the requested form of encapsulation it MUST return code 139. Montenegro Expires February 10, 1998 [Page 16] INTERNET DRAFT Reverse Tunneling for Mobile IP August 1997 The home agent MAY reject registration requests without the 'T' bit set by denying them with code 138 (reverse tunnel is mandatory and 'T' bit not set). A home agent expects a Registration Request with 'T' bit on to be secured by either of these: a. Foreign-Home Authentication Extension (preferred), or b. Foreign-Home Cookie Extension. If neither of the above is present, the home agent MUST reject the registration with code 140 (bad cookie). If the Registration Request includes a Foreign-Home Extension, the home agent MUST record the value of the "Current Cookie" field. This value MUST match the "Previous Cookie" field in subsequent re-registrations relayed by the foreign agent on behalf of the same mobile node. A mismatch MUST result in the home agent's denying the registration with code 140 (bad cookie). As a last check, the home agent determines whether it can support a reverse tunnel with the same configuration as the forward tunnel. If it cannot, it MUST send back a registration denial with code 137. Upon receipt of a Registration Reply that satisfies validity checks, the home agent MUST update its mobility bindings list to indicate that this mobile node has been granted a reverse tunnel and the type of encapsulation expected. 4.3.2. Sending Registration Replies to the Foreign Agent In response to a valid Registration Request, a home agent MUST issue a Registration Reply to the mobile node, including either a Foreign-Home Authentication Extension or a Foreign-Home Cookie Extension, whichever was present in the corresponding Registration Request. In the latter case, the value of the "Current Cookie" MUST be equal to that in the same field of the corresponding request. After a successful registration, the home agent may receive encapsulated packets addressed to it. For each such packet it MAY search for a mobility binding whose care-of address is the source of the outer header, and whose mobile node address is the source of the inner header. If no such binding is found, or if the packet uses an encapsulation mechanism that was not negotiated at registration the home agent MUST silently discard the packet and SHOULD log the event Montenegro Expires February 10, 1998 [Page 17] INTERNET DRAFT Reverse Tunneling for Mobile IP August 1997 as a security exception. While the registration is in effect, a home agent MUST process each valid reverse tunneled packet (as determined by checks like the above) by decapsulating it, recovering the original packet, and then forwarding it on behalf of its sender (the mobile node) to the destination address (the correspondent host). 5. Mobile Node to Foreign Agent Delivery Styles This section specifies how the mobile node sends its data traffic via the foreign agent. In all cases, the mobile node learns the foreign agent's link-layer address from the link-layer header in the agent advertisement. 5.1. Direct Delivery Style This delivery mechanism is very simple to implement at the mobile node, and uses small (non-encapsulated) packets on the link between the mobile node and the foreign agent (potentially a very slow link). However, it only supports reverse-tunneling of unicast packets, and does not allow selective reverse tunneling (section 5.4). 5.1.1. Packet Processing The mobile node MUST designate the foreign agent as its default router. Not doing so will not guarantee encapsulation of all the mobile node's outgoing traffic, and defeats the purpose of the reverse tunnel. The foreign agent MUST: - detect packets sent by the mobile node, and - modify its forwarding function to encapsulate them before forwarding. 5.1.2. Packet Header Format and Fields This section shows the format of the packet headers used by the Direct Delivery style. The formats shown assume IP in IP encapsulation [2]. Montenegro Expires February 10, 1998 [Page 18] INTERNET DRAFT Reverse Tunneling for Mobile IP August 1997 Packet format received by the foreign agent (Direct Delivery Style): IP fields: Source Address = mobile node's home address Destination Address = correspondent host's address Upper Layer Protocol Packet format forwarded by the foreign agent (Direct Delivery Style): IP fields (encapsulating header): Source Address = foreign agent's care-of address Destination Address = home agent's address Protocol field: 4 (IP in IP) IP fields (original header): Source Address = mobile node's home address Destination Address = correspondent host's address Upper Layer Protocol These fields of the encapsulating header MUST be chosen as follows: IP Source Address Copied from the Care-of Address field within the Registration Request. IP Destination Address Copied from the Home Agent field within the Registration Request. IP Protocol Field Default is 4 (IP in IP [2]), but other methods of encapsulation MAY be used as negotiated at registration time. 5.2. Encapsulating Delivery Style This mechanism requires that the mobile node implement encapsulation, and explicitly directs packets at the foreign agent by designating it as the destination address in a new outermost header. Mobile nodes that wish to send either broadcast or multicast packets MUST use the Encapsulating Delivery Style. Montenegro Expires February 10, 1998 [Page 19] INTERNET DRAFT Reverse Tunneling for Mobile IP August 1997 5.2.1 Packet Processing The foreign agent does not modify its forwarding function. Rather, it receives an encapsulated packet and after verifying that it was sent by the mobile node, it: - decapsulates to recover the inner packet, - re-encapsulates, and sends it to the home agent. If a foreign agent receives an un-encapsulated packet from a mobile node which had explicitly requested the Encapsulated Delivery Style, then the foreign agent MUST NOT reverse tunnel such a packet and rather MUST forward it using standard, IP routing mechanisms. 5.2.2. Packet Header Format and Fields This section shows the format of the packet headers used by the Encapsulating Delivery style. The formats shown assume IP in IP encapsulation [2]. Packet format received by the foreign agent (Encapsulating Delivery Style): IP fields (encapsulating header): Source Address = mobile node's home address Destination Address = foreign agent's address Protocol field: 4 (IP in IP) IP fields (original header): Source Address = mobile node's home address Destination Address = correspondent host's address Upper Layer Protocol The fields of the encapsulating IP header MUST be chosen as follows: IP Source Address The mobile node's home address. IP Destination Address The address of the agent as learned from the IP source address of the agent's most recent registration reply. Montenegro Expires February 10, 1998 [Page 20] INTERNET DRAFT Reverse Tunneling for Mobile IP August 1997 IP Protocol Field Default is 4 (IP in IP [2]), but other methods of encapsulation MAY be used as negotiated at registration time. Packet format forwarded by the foreign agent (Encapsulating Delivery Style): IP fields (encapsulating header): Source Address = foreign agent's care-of address Destination Address = home agent's address Protocol field: 4 (IP in IP) IP fields (original header): Source Address = mobile node's home address Destination Address = correspondent host's address Upper Layer Protocol These fields of the encapsulating IP header MUST be chosen as follows: IP Source Address Copied from the Care-of Address field within the Registration Request. IP Destination Address Copied from the Home Agent field within the Registration Request. IP Protocol Field Default is 4 (IP in IP [2]), but other methods of encapsulation MAY be used as negotiated at registration time. 5.3. Support for Broadcast and Multicast Datagrams If a mobile node is operating with a co-located care-of address, broadcast and multicast datagrams are handled according to Sections 4.3 and 4.4 of the Mobile IP specification [1]. Mobile nodes using a foreign agent care-of address MAY have their broadcast and multicast datagrams reverse-tunneled by the foreign agent. However, any mobile nodes doing so MUST use the encapsulating delivery style. This delivers the datagram only to the foreign agent. The latter decapsulates it and then processes it as any other packet from the Montenegro Expires February 10, 1998 [Page 21] INTERNET DRAFT Reverse Tunneling for Mobile IP August 1997 mobile node, namely, by reverse tunneling it to the home agent. 5.4. Selective Reverse Tunneling Packets destined to local resources (e.g. a nearby printer) might be unaffected by ingress filtering. A mobile node with a co-located care-of address MAY optimize delivery of these packets by not reverse tunneling them. On the other hand, a mobile node using a foreign agent care-of address MAY use this selective reverse tunneling capability by requesting the Encapsulating Delivery Style, and following these guidelines: Packets NOT meant to be reversed tunneled: Sent using the Direct Delivery style. The foreign agent MUST process these packets as regular traffic: they MAY be forwarded but MUST NOT be reverse tunneled to the home agent. Packets meant to be reverse tunneled: Sent using the Encapsulating Delivery style. The foreign agent MUST process these packets as specified in section 5.2: they MUST be reverse tunneled to the home agent. 6. Security Considerations The extensions outlined in this document are subject to the security considerations outlined in the Mobile IP specification [1]. Essentialy, creation of both forward and reverse tunnels involves an authentication procedure, which reduces the risk for attack. 6.1. Reverse-tunnel Hijacking and Denial-of-Service Attacks Once the tunnel is set up, a malicious node could hijack it to inject packets into the network. Reverse tunnels might exacerbate this problem, because upon reaching the tunnel exit point packets are forwarded beyond the local network. This concern is also present in the Mobile IP specification, as it already dictates the use of reverse tunnels for certain applications. Unauthenticated exchanges involving the foreign agent allow a malicious node to pose as a valid mobile node and re-direct an existing reverse tunnel to another home agent, perhaps another Montenegro Expires February 10, 1998 [Page 22] INTERNET DRAFT Reverse Tunneling for Mobile IP August 1997 malicious node. The best way to protect against these attacks is by employing the Mobile-Foreign and Foreign-Home Authentication Extensions defined in [1]. If the necessary mobility security associations are not available, Cookie Extensions (Section 3.4) MUST be used to protect against off-the-path attacks. The Mobile-Foreign Cookie Extension enables a foreign agent to distinguish between a mobile node for which it has a current visitor list entry and an off-the-path attacker. The Foreign-Home Cookie Extension enables a foreign agent to distinguish between the home agent for a mobile node currently in a visitor list entry and an off-the-path attacker. However, cookies are unauthenticated state, and as such, may be used to launch denial-of-service attacks. For example, a malicious node may register via a foreign agent using another malicious node as its home agent. The existence of this visitor list entry at the foreign agent effectively blocks the real mobile node from registering. As a consequence, the mobile node SHOULD cache the cookies in permanent storage, otherwise it will be unable to register in the event of a reboot as long as it has a valid visitor list entry at the foreign agent. Alternatively, the mobile node MAY request appropriately short lifetimes in its Registration Requests. This document also introduces other mechanisms to reduce the range and effectiveness of the attacks. Requiring a TTL value of 255 in the IP headers of Registration Requests received at the foreign agent prevents malicious nodes more than one hop away from posing as valid mobile nodes. Additional codes for use in registration denials make those attacks that do occur easier to track. 6.2. Ingress Filtering There has been some concern regarding the long-term effectiveness of reverse-tunneling in the presence of ingress filtering. The conjecture is that network administrators will target reverse-tunneled packets (IP in IP encapsulated packets) for filtering. The ingress filtering recommendation spells out why this is not the case [8]: Tracking the source of an attack is simplified when the source is more likely to be "valid." Montenegro Expires February 10, 1998 [Page 23] INTERNET DRAFT Reverse Tunneling for Mobile IP August 1997 7. Acknowledgements The encapsulating style of delivery was proposed by Charlie Perkins. Jim Solomon has been instrumental in shaping this document into its present form. References [1] C. Perkins. IP Mobility Support. RFC 2002, October 1996. [2] C. Perkins. IP Encapsulation within IP. RFC 2003, October 1996. [3] Computer Emergency Response Team (CERT), "IP Spoofing Attacks and Hijacked Terminal Connections", CA-95:01, January 1995. Available via anonymous ftp from info.cert.org in /pub/cert_advisories. [4] D. Johnson and C. Perkins. Route Optimization in Mobile IP -- work in progress, draft-ietf-mobileip-optim-06.txt, July 1997. [5] Manuel Rodriguez, private communication, August 1995. [6] R. Atkinson. IP Authentication Header. RFC 1826, August 1995. [7] R. Atkinson. IP Encapsulating Security Payload. RFC 1827, August 1995. [8] P. Ferguson and D. Senie. Network Ingress Filtering: Defending Against IP Source Address Spoofing -- work in progress, draft-ferguson-ingress-filtering-02.txt, July 1997. Editor and Chair Addresses Montenegro Expires February 10, 1998 [Page 24] INTERNET DRAFT Reverse Tunneling for Mobile IP August 1997 Questions about this document may be directed at: Gabriel E. Montenegro Sun Microsystems, Inc. an Antonio Road Mailstop UMPK 15-214 Mountain View, California 94303 Voice: +1-415-786-6288 Fax: +1-415-786-6445 E-Mail: gabriel.montenegro@eng.sun.com The working group can be contacted via the current chairs: Jim Solomon Motorola, Inc. 1301 E. Algonquin Rd. - Rm 2240 Schaumburg, IL 60196 Voice: +1-847-576-2753 Fax: +1-847-576-3240 E-Mail: solomon@comm.mot.com Erik Nordmark Sun Microsystems, Inc. 901 San Antonio Road Mailstop UMPK17-202 Mountain View, California 94303 Voice: +1-415-786-5166 E-Mail: erik.nordmark@eng.sun.com Montenegro Expires February 10, 1998 [Page 25]