Internet Engineering Task Force G. Montenegro INTERNET DRAFT Sun Microsystems, Inc. January 31, 1996 Reverse Tunneling for Mobile IP draft-ietf-mobileip-tunnel-reverse-00.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 net, 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 July 31, 1997 [Page 1]
INTERNET DRAFT Reverse Tunneling for Mobile IP January 1996 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 location 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 it addresses other hosts in its home network) may be so low that they may expire before reaching their destination. A reverse tunnel solves the problem as it represents a TTL decrement of one [5]. Montenegro Expires July 31, 1997 [Page 2]
INTERNET DRAFT Reverse Tunneling for Mobile IP January 1996 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. Light-weight mobile node A mobile node that relies on a separate foreign agent for tunneling services (i.e. the care-of address belongs to the foreign agent). 1.2. Assumptions Mobility is constrained to one 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 cannot know if the correspondent host can decapsulate packets, reverse tunnels in that context are not discussed here. Montenegro Expires July 31, 1997 [Page 3]
INTERNET DRAFT Reverse Tunneling for Mobile IP January 1996 1.3. Justification Why not let the mobile node itself initiate the tunnel to the home agent? This is indeed what it should do if it is already operating with a topologically significant co-located address. However, one of the primary objectives of the Mobile IP specification is to not *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 "lightweight" mobile nodes, even in the presence of filtering routers. 2. Overview A light-weight mobile node arrives at a foreign net, listens for 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 lightweight or the encapsulating style of delivery (section 5). In the lightweight delivery style, the mobile node designates the foreign agent as its default router and proceeds to send packets as usual. 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 tunnels again, this time, directly to the home agent. 3. New Packet Formats 3.1. Agent Advertisements: Mobile Service Extension Montenegro Expires July 31, 1997 [Page 4]
INTERNET DRAFT Reverse Tunneling for Mobile IP January 1996 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 Mobile Service 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 (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. 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 3.4). 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 July 31, 1997 [Page 5]
INTERNET DRAFT Reverse Tunneling for Mobile IP January 1996 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. Lightweight mobile nodes ask the foreign agent to reverse-tunnel its packets. 3.3. Reverse Tunnel Extension The Reverse Tunnel Extension is used to further specify reverse tunneling behavior. Currently, it is only possible to request the encapsulating style of delivery, but future behavior may be defined. The Reverse Tunnel Extension MUST NOT be included if the 'T' bit is not set in the Registration Request. If this extension is absent, or if no style is explicitly requested, the Lightweight Delivery is assumed. Besides the latter, currently only the Encapsulating style is defined (section 5). Montenegro Expires July 31, 1997 [Page 6]
INTERNET DRAFT Reverse Tunneling for Mobile IP January 1996 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 |E| reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type 128 Length 2 E Encapsulating style of delivery. Encapsulation is done according to what was negotiated for the forward tunnel (i.e., IP in IP is assumed unless specified otherwise). reserved Ignored upon reception. Must be set to zero when transmitting. 3.4. New Registration Reply Codes Foreign and home agent replies must convey if the reverse tunnel request failed. Four new reply codes are defined. The use of codes 74 and 137 is preferred over code 70 for foreign agents and code 134 for home agents [1]: Service denied by the foreign agent: 74 requested reverse tunnel unavailable 75 reverse tunnel is mandatory and 'T' bit not set and Service denied by the home agent: 137 requested reverse tunnel unavailable 138 reverse tunnel is mandatory and 'T' bit not set 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: 4.1. Mobile Node Considerations A mobile node sets the 'T' bit in its Registration Request to Montenegro Expires July 31, 1997 [Page 7]
INTERNET DRAFT Reverse Tunneling for Mobile IP January 1996 petition a reverse tunnel. It may optionally also include a Reverse Tunnel Extension. Possible outcomes are: - The foreign agent returns a registration denial. Depending on the reply code and following the error checking guidelines in [1], the mobile node MAY try zeroing the 'T' bit, eliminating the Reverse Tunnel Extension (if one was present), and issuing a new registration. - The home agent returns a registration denial. Depending on the reply code and following the error checking guidelines in [1], the mobile node MAY try zeroing the 'T' bit, eliminating the Reverse Tunnel Extension (if one was present), and issuing a new registration. - 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 address, it SHOULD encapsulate all outgoing data such that the destination address of the outer header is the home agent. Not doing so does not necessarily preclude data transmission, but it defeats the purpose of the reverse tunnel. If the care-of address belongs to a separate foreign agent, the mobile node MUST employ whatever delivery style was requested (lightweight or encapsulated) and proceed as specified in section 5. 4.2. Foreign Agent Considerations A foreign agent that receives a Registration Request with the 'T' bit set MAY either: - Return a Registration Reply denying the request. Valid return codes are 74 (requested reverse tunnel unavailable) or 70 (poorly formed request). Code 74 is preferred. - Verify the packet according to [1] and then relay it to the home agent. Upon receipt of a Registration Reply that satisfies validity checks, it MUST update its visitor list, including indication that this mobile node has been granted a reverse tunnel and the delivery style expected (section 5). Montenegro Expires July 31, 1997 [Page 8]
INTERNET DRAFT Reverse Tunneling for Mobile IP January 1996 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 A home agent that receives a Registration Request with the 'T' bit set processes the packet as specified in the Mobile IP specification [1]. As a result, it determines if it can accomodate the forward tunnel request. As a last check, the home agent verifies that it can support a reverse tunnel with the same configuration. If it can, the home agent sends back a Registration Reply with code 0 or 1. A registration denial should send back code 137 (requested reverse tunnel unavailable) or 134 (poorly formed Request). Code 137 is preferred. After a successful registration, the home agent will 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. The home agent MUST decapsulate, recover the original packet, and then forward it on behalf of its sender (the mobile node) to the destination address (the correspondent host). 5. Mobile Node to Foreign Agent Delivery Styles 5.1. Lightweight Delivery Style This delivery mechanism is very simple to implement, 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. It is achieved by the mobile node's designating 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 modify its forwarding function to detect packets sent by the mobile node, and re-encapsulate before forwarding. Montenegro Expires July 31, 1997 [Page 9]
INTERNET DRAFT Reverse Tunneling for Mobile IP January 1996 Packet format received by the foreign agent (lightweight delivery): Data Link fields: Source Address = mobile node's MAC address Destination Address = foreign agent's MAC address 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 (lightweight delivery): Data Link fields: Source Address = foreign agent's MAC address Destination Address = next hop router's MAC address IP fields (encapsulating header): Source Address = foreign agent's 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 5.2. Encapsulating Delivery Style This mechanism requires that the mobile node implement encapsulation. The mobile node 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 encapsulating delivery. The foreign agent does not have to modify its forwarding function. Rather, it receives the encapsulated packets and after verifying that they were sent by the mobile node, recovers the inner packets, re-encapsulates them and sends them to the home agent. Montenegro Expires July 31, 1997 [Page 10]
INTERNET DRAFT Reverse Tunneling for Mobile IP January 1996 Packet format received by the foreign agent (encapsulated delivery): Data Link fields: Source Address = mobile node's MAC address Destination Address = foreign agent's MAC address IP fields (encapsulating header): Source Address = mobile node's 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 Packet format forwarded by the foreign agent (encapsulated delivery): Data Link fields: Source Address = foreign agent's MAC address Destination Address = next hop router's MAC address IP fields (encapsulating header): Source Address = foreign agent's 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 5.3. Support for Broadcast and Multicast Datagrams If a mobile node is operating with a co-located address, broadcast and multicast datagrams are handled according to Sections 4.3 and 4.4 of the Mobile IP specification [1]. Light-weight mobile nodes MAY have their broadcast and multicast datagrams reverse-tunneled by the foreign agent. However, this requires the use of the the encapulating 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 mobile node, namely, by reverse tunneling it 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 Montenegro Expires July 31, 1997 [Page 11]
INTERNET DRAFT Reverse Tunneling for Mobile IP January 1996 authentication procedure, which reduces the risk for attack. However, once the tunnel is set up, a malicious user 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. 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." 7. Acknowledgements The encapsulating style of delivery was proposed by Charlie Perkins. 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-05.txt, November 1996. [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. Montenegro Expires July 31, 1997 [Page 12]
INTERNET DRAFT Reverse Tunneling for Mobile IP January 1996 [8] P. Ferguson and D. Senie. Network Ingress Filtering: Defending Against IP Source Address Spoofing -- work in progress, draft-ferguson-ingress-filtering-01.txt, February 1996 Author's Address Gabriel E. Montenegro Sun Microsystems, Inc. 2550 Garcia Avenue Mailstop UMPK 15-214 Mountain View, California 94043-1100 Tel: (415)786-6288 Fax: (415)786-6445 gab@eng.sun.com Montenegro Expires July 31, 1997 [Page 13]