INTERNET-DRAFT E. Nordmark November 4, 2002 Sun Microsystems, Inc. Obsoletes: 2893 R. E. Gilligan Intransa, Inc. Basic Transition Mechanisms for IPv6 Hosts and Routers <draft-ietf-ngtrans-mech-v2-01.txt> Status of this Memo This document is an Internet-Draft and is subject to all provisions of Section 10 of RFC2026. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This draft expires on May 4, 2003. Abstract This document specifies IPv4 compatibility mechanisms that can be implemented by IPv6 hosts and routers. These mechanisms include providing complete implementations of both versions of the Internet Protocol (IPv4 and IPv6), and tunneling IPv6 packets over IPv4 routing infrastructures. They are designed to allow IPv6 nodes to maintain complete compatibility with IPv4, which should greatly simplify the deployment of IPv6 in the Internet, and facilitate the eventual transition of the entire Internet to IPv6. This document obsoletes RFC 2893. <draft-ietf-ngtrans-mech-v2-01.txt> [Page 1]
INTERNET DRAFT Basic IPv6 Transition Mechanisms November 2002 Contents Status of this Memo.......................................... 1 1. Introduction............................................. 3 1.1. Terminology......................................... 3 1.2. Structure of this Document.......................... 5 2. Dual IP Layer Operation.................................. 5 2.1. Address Configuration............................... 6 2.2. DNS................................................. 6 2.3. Advertising Addresses in the DNS.................... 7 3. Common Tunneling Mechanisms.............................. 9 3.1. Encapsulation....................................... 10 3.2. Tunnel MTU and Fragmentation........................ 11 3.3. Hop Limit........................................... 13 3.4. Handling IPv4 ICMP errors........................... 14 3.5. IPv4 Header Construction............................ 15 3.6. Decapsulation....................................... 17 3.7. Link-Local Addresses................................ 18 3.8. Neighbor Discovery over Tunnels..................... 19 3.9. Ingress Filtering................................... 19 4. Configured Tunneling..................................... 20 4.1. Ingress Filtering................................... 20 5. Acknowledgments.......................................... 21 6. Security Considerations.................................. 21 7. Authors' Addresses....................................... 22 8. References............................................... 22 8.1. Normative References................................ 22 8.2. Non-normative References............................ 22 9. Changes from RFC 2893.................................... 23 10. Open issues............................................. 25 11. TODO.................................................... 25 <draft-ietf-ngtrans-mech-v2-01.txt> [Page 2]
INTERNET DRAFT Basic IPv6 Transition Mechanisms November 2002 1. Introduction The key to a successful IPv6 transition is compatibility with the large installed base of IPv4 hosts and routers. Maintaining compatibility with IPv4 while deploying IPv6 will streamline the task of transitioning the Internet to IPv6. This specification defines a set of mechanisms that IPv6 hosts and routers may implement in order to be compatible with IPv4 hosts and routers. The mechanisms in this document are designed to be employed by IPv6 hosts and routers that need to interoperate with IPv4 hosts and utilize IPv4 routing infrastructures. We expect that most nodes in the Internet will need such compatibility for a long time to come, and perhaps even indefinitely. The mechanisms specified here include: - Dual IP layer (also known as Dual Stack): A technique for providing complete support for both Internet protocols -- IPv4 and IPv6 -- in hosts and routers. - Configured tunneling of IPv6 over IPv4: Point-to-point tunnels made by encapsulating IPv6 packets within IPv4 headers to carry them over IPv4 routing infrastructures. The mechanisms defined here are intended to be the core of a "transition toolbox" -- a growing collection of techniques which implementations and users may employ to ease the transition. The tools may be used as needed. Implementations and sites decide which techniques are appropriate to their specific needs. This document defines the basic set of transition mechanisms, but these are not the only tools available. Additional transition and compatibility mechanisms are specified in other documents. 1.1. Terminology The following terms are used in this document: Types of Nodes IPv4-only node: A host or router that implements only IPv4. An IPv4- only node does not understand IPv6. The installed base of IPv4 hosts and routers existing before the transition <draft-ietf-ngtrans-mech-v2-01.txt> [Page 3]
INTERNET DRAFT Basic IPv6 Transition Mechanisms November 2002 begins are IPv4-only nodes. IPv6/IPv4 node: A host or router that implements both IPv4 and IPv6. IPv6-only node: A host or router that implements IPv6, and does not implement IPv4. The operation of IPv6-only nodes is not addressed here. IPv6 node: Any host or router that implements IPv6. IPv6/IPv4 and IPv6-only nodes are both IPv6 nodes. IPv4 node: Any host or router that implements IPv4. IPv6/IPv4 and IPv4-only nodes are both IPv4 nodes. Types of IPv6 Addresses IPv4-compatible IPv6 address: An IPv6 address bearing the high-order 96-bit prefix 0:0:0:0:0:0, and an IPv4 address in the low-order 32- bits. IPv4-compatible addresses are no longer used by this specification, IPv6-native address: The remainder of the IPv6 address space. An IPv6 address that bears a prefix other than 0:0:0:0:0:0. Techniques Used in the Transition IPv6-over-IPv4 tunneling: The technique of encapsulating IPv6 packets within IPv4 so that they can be carried across IPv4 routing infrastructures. Configured tunneling: IPv6-over-IPv4 tunneling where the IPv4 tunnel endpoint address is determined by configuration information on <draft-ietf-ngtrans-mech-v2-01.txt> [Page 4]
INTERNET DRAFT Basic IPv6 Transition Mechanisms November 2002 the encapsulating node. The tunnels can be either unidirectional or bidirectional. Bidirectional configured tunnels behave as virtual point-to-point links. Other transition mechanisms, including other tunneling mechanisms, are outside the scope of this document. The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this document, are to be interpreted as described in [16]. 1.2. Structure of this Document The remainder of this document is organized as follows: - Section 2 discusses the operation of nodes with a dual IP layer, IPv6/IPv4 nodes. - Section 3 discusses the common mechanisms used in some IPv6- over-IPv4 tunneling techniques, including configured tunneling. - Section 4 discusses configured tunneling. 2. Dual IP Layer Operation The most straightforward way for IPv6 nodes to remain compatible with IPv4-only nodes is by providing a complete IPv4 implementation. IPv6 nodes that provide a complete IPv4 and IPv6 implementations are called "IPv6/IPv4 nodes." IPv6/IPv4 nodes have the ability to send and receive both IPv4 and IPv6 packets. They can directly interoperate with IPv4 nodes using IPv4 packets, and also directly interoperate with IPv6 nodes using IPv6 packets. Even though a node may be equipped to support both protocols, one or the other stack may be disabled for operational reasons. Here we use a rather loose notion of "stack". A stack being enabled has IP addresses assigned etc, but whether or not any particular application is available on the stacks is explicitly not defined. Thus IPv6/IPv4 nodes may be operated in one of three modes: - With their IPv4 stack enabled and their IPv6 stack disabled. <draft-ietf-ngtrans-mech-v2-01.txt> [Page 5]
INTERNET DRAFT Basic IPv6 Transition Mechanisms November 2002 - With their IPv6 stack enabled and their IPv4 stack disabled. - With both stacks enabled. IPv6/IPv4 nodes with their IPv6 stack disabled will operate like IPv4-only nodes. Similarly, IPv6/IPv4 nodes with their IPv4 stacks disabled will operate like IPv6-only nodes. IPv6/IPv4 nodes MAY provide a configuration switch to disable either their IPv4 or IPv6 stack. The IPv6-over-IPv4 tunneling techniques, which are described in sections 3 and 4, may or may not be used in addition to the dual IP layer operation. An IPv6/IPv4 node MAY support configured tunneling. 2.1. Address Configuration Because they support both protocols, IPv6/IPv4 nodes may be configured with both IPv4 and IPv6 addresses. IPv6/IPv4 nodes use IPv4 mechanisms (e.g., DHCP) to acquire their IPv4 addresses, and IPv6 protocol mechanisms (e.g., stateless address autoconfiguration and/or DHCPv6) to acquire their IPv6-native addresses. 2.2. DNS The Domain Naming System (DNS) is used in both IPv4 and IPv6 to map between hostnames and IP addresses. A new resource record type named "AAAA" has been defined for IPv6 addresses [6]. Since IPv6/IPv4 nodes must be able to interoperate directly with both IPv4 and IPv6 nodes, they must provide resolver libraries capable of dealing with IPv4 "A" records as well as IPv6 "AAAA" records. Note that the lookup of A versus AAAA records is independent of whether the DNS packets are carried in IPv4 or IPv6 packets. DNS resolver libraries on IPv6/IPv4 nodes MUST be capable of handling both AAAA and A records. However, when a query locates an AAAA record holding an IPv6 address, and an A record holding an IPv4 address, the resolver library MAY filter or order the results returned to the application in order to influence the version of IP packets used to communicate with that node. In terms of filtering, the resolver library has three alternatives: - Return only the IPv6 address(es) to the application. - Return only the IPv4 address(es) to the application. <draft-ietf-ngtrans-mech-v2-01.txt> [Page 6]
INTERNET DRAFT Basic IPv6 Transition Mechanisms November 2002 - Return both types of addresses to the application. If it returns only the IPv6 address(es), the application will communicate with the node using IPv6. If it returns only the IPv4 address(es), the application will communicate with the node using IPv4. If it returns both types of addresses, the application will have the choice which address to use, and thus which IP protocol to employ. If it returns both, the resolver MAY elect to order the addresses -- IPv6 first, or IPv4 first. Since most applications try the addresses in the order they are returned by the resolver, this can affect the IP version "preference" of applications. The decision to filter or order DNS results is implementation specific. IPv6/IPv4 nodes MAY provide policy configuration to control filtering or ordering of addresses returned by the resolver, or leave the decision entirely up to the application. An implementation MUST allow the application to control whether or not such filtering takes place. More details on this subject are specified in [19]. 2.3. Advertising Addresses in the DNS There are some constraint placed on the use of the DNS during transition. The constraints allow nodes to prefer either IPv6 or IPv4 addresses when both types of addresses are returned by the DNS. Most of these are obvious but are stated here for completeness. The recommendation is that AAAA records for a node should not be added to the DNS until all of these are true: 1) The address is assigned to the interface on the node. 2) The address is configured on the interface. 3) The interface is on a link which is connected to the IPv6 infrastructure. If an IPv6 node is isolated from an IPv6 perspective (e.g., it is not connected to the 6bone to take a concrete example) constraint #3 would mean that it should not have an address in the DNS. This works great when other dual stack nodes try to contact the <draft-ietf-ngtrans-mech-v2-01.txt> [Page 7]
INTERNET DRAFT Basic IPv6 Transition Mechanisms November 2002 isolated dual stack node. There is no IPv6 address in the DNS thus the peer doesn't even try communicating using IPv6 but goes directly to IPv4 (we are assuming both nodes have A records in the DNS.) However, this does not work well when the isolated node is trying to establish communication. Even though it does not have an IPv6 address in the DNS it will find AAAA records in the DNS for the peer. Since the isolated node has IPv6 addresses assigned to at least one interface it will try to communicate using IPv6. If it has no IPv6 route to the 6bone (e.g., because the local router was upgraded to advertise IPv6 addresses using Neighbor Discovery but that router doesn't have any IPv6 routes) this communication will fail. Typically this means a few minutes of delay as TCP times out. The TCP specification [10] says that ICMP unreachable messages could be due to routing transients thus they should not immediately terminate the TCP connection. This means that the normal TCP timeout of a few minutes apply. Once TCP times out the application will hopefully try the IPv4 addresses based on the A records in the DNS, but this will be painfully slow. A possible implication of the recommendations above is that, if one enables IPv6 on a node on a link without IPv6 infrastructure, and choose to add AAAA records to the DNS for that node, then external IPv6 nodes that might see these AAAA records will possibly try to reach that node using IPv6 and suffer delays or communication failure due to unreachability. (A delay is incurred if the application correctly falls back to using IPv4 if it can not establish communication using IPv6 addresses. If this fallback is not done the application would fail to communicate in this case.) Thus it is suggested that either the recommendations be followed, or care be taken to only do so with nodes that will not be impacted by external accessing delays and/or communication failure. In the future, when a node discontinues its use of IPv4, analogous constraints apply with respect to the node's A records in the DNS; the removal of the A records should be tied to when the node can no longer be reached using IPv4. <draft-ietf-ngtrans-mech-v2-01.txt> [Page 8]
INTERNET DRAFT Basic IPv6 Transition Mechanisms November 2002 3. Common Tunneling Mechanisms In most deployment scenarios, the IPv6 routing infrastructure will be built up over time. While the IPv6 infrastructure is being deployed, the existing IPv4 routing infrastructure can remain functional, and can be used to carry IPv6 traffic. Tunneling provides a way to utilize an existing IPv4 routing infrastructure to carry IPv6 traffic. IPv6/IPv4 hosts and routers can tunnel IPv6 datagrams over regions of IPv4 routing topology by encapsulating them within IPv4 packets. Tunneling can be used in a variety of ways: - Router-to-Router. IPv6/IPv4 routers interconnected by an IPv4 infrastructure can tunnel IPv6 packets between themselves. In this case, the tunnel spans one segment of the end-to-end path that the IPv6 packet takes. - Host-to-Router. IPv6/IPv4 hosts can tunnel IPv6 packets to an intermediary IPv6/IPv4 router that is reachable via an IPv4 infrastructure. This type of tunnel spans the first segment of the packet's end-to-end path. - Host-to-Host. IPv6/IPv4 hosts that are interconnected by an IPv4 infrastructure can tunnel IPv6 packets between themselves. In this case, the tunnel spans the entire end-to-end path that the packet takes. - Router-to-Host. IPv6/IPv4 routers can tunnel IPv6 packets to their final destination IPv6/IPv4 host. This tunnel spans only the last segment of the end-to-end path. Tunneling techniques are usually classified according to the mechanism by which the encapsulating node determines the address of the node at the end of the tunnel. In the first two tunneling methods listed above -- router-to-router and host-to-router -- the IPv6 packet is being tunneled to a router. The endpoint of this type of tunnel is an intermediary router which must decapsulate the IPv6 packet and forward it on to its final destination. When tunneling to a router, the endpoint of the tunnel is different from the destination of the packet being tunneled. In some cases, the addresses in the IPv6 packet being tunneled can not provide the IPv4 address of the tunnel endpoint. In those cases, the tunnel endpoint address must be determined from configuration information on the node performing the encapsulation. We use the term "configured tunneling" to describe the type of tunneling where the endpoint is explicitly configured. <draft-ietf-ngtrans-mech-v2-01.txt> [Page 9]
INTERNET DRAFT Basic IPv6 Transition Mechanisms November 2002 In the last two tunneling methods -- host-to-host and router-to-host -- the IPv6 packet is tunneled all the way to its final destination. In this case, the destination address of both the IPv6 packet and the encapsulating IPv4 header identify the same node. However, the tunneling mechanism specified in this document does not handle these cases any differently; the IPv4 addresses is still determined using configuration information using configured tunneling. The underlying mechanisms for tunneling are: - The entry node of the tunnel (the encapsulating node) creates an encapsulating IPv4 header and transmits the encapsulated packet. - The exit node of the tunnel (the decapsulating node) receives the encapsulated packet, reassembles the packet if needed, removes the IPv4 header, updates the IPv6 header, and processes the received IPv6 packet. - The encapsulating node MAY need to maintain soft state information for each tunnel recording such parameters as the MTU of the tunnel in order to process IPv6 packets forwarded into the tunnel. In cases where the number of tunnels that any one host or router is using is large, it is helpful to observe that this state information can be cached and discarded when not in use. The remainder of this section discusses the common mechanisms. A subsequent section discusses how the tunnel endpoint address is determined for configured tunneling. 3.1. Encapsulation The encapsulation of an IPv6 datagram in IPv4 is shown below: <draft-ietf-ngtrans-mech-v2-01.txt> [Page 10]
INTERNET DRAFT Basic IPv6 Transition Mechanisms November 2002 +-------------+ | IPv4 | | Header | +-------------+ +-------------+ | IPv6 | | IPv6 | | Header | | Header | +-------------+ +-------------+ | Transport | | Transport | | Layer | ===> | Layer | | Header | | Header | +-------------+ +-------------+ | | | | ~ Data ~ ~ Data ~ | | | | +-------------+ +-------------+ Encapsulating IPv6 in IPv4 In addition to adding an IPv4 header, the encapsulating node also has to handle some more complex issues: - Determine when to fragment and when to report an ICMP "packet too big" error back to the source. - How to reflect IPv4 ICMP errors from routers along the tunnel path back to the source as IPv6 ICMP errors. Those issues are discussed in the following sections. 3.2. Tunnel MTU and Fragmentation The encapsulating node could view encapsulation as IPv6 using IPv4 as a link layer with a very large MTU (65535-20 bytes to be exact; 20 bytes "extra" are needed for the encapsulating IPv4 header). The encapsulating node would need only to report IPv6 ICMP "packet too big" errors back to the source for packets that exceed this MTU. However, such a scheme would be inefficient for two reasons: 1) It would result in more fragmentation than needed. IPv4 layer fragmentation SHOULD be avoided due to the performance problems caused by the loss unit being smaller than the retransmission unit [11]. <draft-ietf-ngtrans-mech-v2-01.txt> [Page 11]
INTERNET DRAFT Basic IPv6 Transition Mechanisms November 2002 2) Any IPv4 fragmentation occurring inside the tunnel, i.e. between the encapsulating node and the decapsulating node, would have to be reassembled at the tunnel endpoint. For tunnels that terminate at a router, this would require additional memory to reassemble the IPv4 fragments into a complete IPv6 packet before that packet could be forwarded onward. Hence, the encapsulating node MUST NOT treat the tunnel as an interface with an MTU of 64 kilobytes, but use the smaller MTU specified below. The fragmentation inside the tunnel can be reduced to a minimum by having the encapsulating node track the IPv4 Path MTU across the tunnel, using the IPv4 Path MTU Discovery Protocol [8] and recording the resulting path MTU. The IPv6 layer in the encapsulating node can then view a tunnel as a link layer with an MTU equal to the IPv4 path MTU, minus the size of the encapsulating IPv4 header. Note that this does not completely eliminate IPv4 fragmentation in the case when the IPv4 path MTU would result in an IPv6 MTU less than 1280 bytes. (Any link layer used by IPv6 has to have an MTU of at least 1280 bytes [4].) In this case the IPv6 layer has to "see" a link layer with an MTU of 1280 bytes and the encapsulating node has to use IPv4 fragmentation in order to forward the 1280 byte IPv6 packets. This dynamic MTU determination is OPTIONAL. However, if it is implemented it SHOULD have the behavior described in this document and the tunnel MTU MUST be not exceed 4400 bytes. If it is not implemented instead the node MUST instead limit the size of the IPv6 packets it tunnels to 1280 bytes i.e., treat the tunnel interface as having a fixed interface MTU of 1280 bytes. An implementation MAY have a configuration knob which can be used to set a larger value of the tunnel MTU than 1280 bytes, but if so the default MUST be 1280 bytes. The encapsulating node can employ the following algorithm to determine when to forward an IPv6 packet that is larger than the tunnel's path MTU using IPv4 fragmentation, and when to return an IPv6 ICMP "packet too big" message: <draft-ietf-ngtrans-mech-v2-01.txt> [Page 12]
INTERNET DRAFT Basic IPv6 Transition Mechanisms November 2002 if (IPv4 path MTU - 20) is less than or equal to 1280 if packet is larger than 1280 bytes Send IPv6 ICMP "packet too big" with MTU = 1280. Drop packet. else Encapsulate but do not set the Don't Fragment flag in the IPv4 header. The resulting IPv4 packet might be fragmented by the IPv4 layer on the encapsulating node or by some router along the IPv4 path. endif else if packet is larger than (IPv4 path MTU - 20) or packet is larger than 4400 Send IPv6 ICMP "packet too big" with MTU = MIN(IPv4 path MTU - 20, 4400). Drop packet. else Encapsulate and set the Don't Fragment flag in the IPv4 header. endif endif Encapsulating nodes that have a large number of tunnels might not be able to store the IPv4 Path MTU for all tunnels. Such nodes can, at the expense of additional fragmentation in the network, avoid using the IPv4 Path MTU algorithm across the tunnel and instead use the MTU of the link layer (under IPv4) in the above algorithm instead of the IPv4 path MTU. In this case the Don't Fragment bit MUST NOT be set in the encapsulating IPv4 header. 3.3. Hop Limit IPv6-over-IPv4 tunnels are modeled as "single-hop". That is, the IPv6 hop limit is decremented by 1 when an IPv6 packet traverses the tunnel. The single-hop model serves to hide the existence of a tunnel. The tunnel is opaque to users of the network, and is not detectable by network diagnostic tools such as traceroute. The single-hop model is implemented by having the encapsulating and decapsulating nodes process the IPv6 hop limit field as they would if they were forwarding a packet on to any other datalink. That is, <draft-ietf-ngtrans-mech-v2-01.txt> [Page 13]
INTERNET DRAFT Basic IPv6 Transition Mechanisms November 2002 they decrement the hop limit by 1 when forwarding an IPv6 packet. (The originating node and final destination do not decrement the hop limit.) The TTL of the encapsulating IPv4 header is selected in an implementation dependent manner. The current suggested value is published in the "Assigned Numbers" RFC. Implementations MAY provide a mechanism to allow the administrator to configure the IPv4 TTL such as the one specified in the IP Tunnel MIB [17]. 3.4. Handling IPv4 ICMP errors In response to encapsulated packets it has sent into the tunnel, the encapsulating node might receive IPv4 ICMP error messages from IPv4 routers inside the tunnel. These packets are addressed to the encapsulating node because it is the IPv4 source of the encapsulated packet. The ICMP "packet too big" error messages are handled according to IPv4 Path MTU Discovery [8] and the resulting path MTU is recorded in the IPv4 layer. The recorded path MTU is used by IPv6 to determine if an IPv6 ICMP "packet too big" error has to be generated as described in section 3.2. The handling of other types of ICMP error messages depends on how much information is included in the "packet in error" field, which holds the encapsulated packet that caused the error. Many older IPv4 routers return only 8 bytes of data beyond the IPv4 header of the packet in error, which is not enough to include the address fields of the IPv6 header. More modern IPv4 routers are likely to return enough data beyond the IPv4 header to include the entire IPv6 header and possibly even the data beyond that. If the offending packet includes enough data, the encapsulating node MAY extract the encapsulated IPv6 packet and use it to generate an IPv6 ICMP message directed back to the originating IPv6 node, as shown below: <draft-ietf-ngtrans-mech-v2-01.txt> [Page 14]
INTERNET DRAFT Basic IPv6 Transition Mechanisms November 2002 +--------------+ | IPv4 Header | | dst = encaps | | node | +--------------+ | ICMP | | Header | - - +--------------+ | IPv4 Header | | src = encaps | IPv4 | node | +--------------+ - - Packet | IPv6 | | Header | Original IPv6 in +--------------+ Packet - | Transport | Can be used to Error | Header | generate an +--------------+ IPv6 ICMP | | error message ~ Data ~ back to the source. | | - - +--------------+ - - IPv4 ICMP Error Message Returned to Encapsulating Node 3.5. IPv4 Header Construction When encapsulating an IPv6 packet in an IPv4 datagram, the IPv4 header fields are set as follows: Version: 4 IP Header Length in 32-bit words: 5 (There are no IPv4 options in the encapsulating header.) Type of Service: 0 unless otherwise specified. (See [20] and [21] for issues relating to the ToS byte and tunneling.) <draft-ietf-ngtrans-mech-v2-01.txt> [Page 15]
INTERNET DRAFT Basic IPv6 Transition Mechanisms November 2002 Total Length: Payload length from IPv6 header plus length of IPv6 and IPv4 headers (i.e., IPv6 payload length plus a constant 60 bytes). Identification: Generated uniquely as for any IPv4 packet transmitted by the system. Flags: Set the Don't Fragment (DF) flag as specified in section 3.2. Set the More Fragments (MF) bit as necessary if fragmenting. Fragment offset: Set as necessary if fragmenting. Time to Live: Set in implementation-specific manner. Protocol: 41 (Assigned payload type number for IPv6) Header Checksum: Calculate the checksum of the IPv4 header. Source Address: IPv4 address of outgoing interface of the encapsulating node. The source address MAY alternatively be administratively specified to be a specific IPv4 address assigned to the encapsulating node. Destination Address: IPv4 address of tunnel endpoint. Any IPv6 options are preserved in the packet (after the IPv6 header). <draft-ietf-ngtrans-mech-v2-01.txt> [Page 16]
INTERNET DRAFT Basic IPv6 Transition Mechanisms November 2002 3.6. Decapsulation When an IPv6/IPv4 host or a router receives an IPv4 datagram that is addressed to one of its own IPv4 address, and the value of the protocol field is 41, it reassembles if the packet if it is fragmented at the IPv4 level, then it removes the IPv4 header and submits the IPv6 datagram to its IPv6 layer code. The decapsulating node MUST be capable of reassembling an IPv4 packet that is 4420 bytes (4400 bytes plus IPv4 header). This limit allows dynamic tunnel MTU determination by the encapsulator to take advantage of larger IPv4 path MTUs. An implementation MAY have a configuration knob which can be used to set a larger value of the tunnel MRU than 4420 bytes, but the value MUST not be set below 4420. The decapsulation is shown below: +-------------+ | IPv4 | | Header | +-------------+ +-------------+ | IPv6 | | IPv6 | | Header | | Header | +-------------+ +-------------+ | Transport | | Transport | | Layer | ===> | Layer | | Header | | Header | +-------------+ +-------------+ | | | | ~ Data ~ ~ Data ~ | | | | +-------------+ +-------------+ Decapsulating IPv6 from IPv4 When decapsulating the packet, the IPv6 header is not modified. (See [20] and [21] for issues relating to the Type of Service byte and tunneling.) If the packet is subsequently forwarded, its hop limit is decremented by one. As part of the decapsulation the node SHOULD silently discard a packet with an invalid IPv4 source address such as a multicast address, a broadcast address, 0.0.0.0, and 127.0.0.1. In general it SHOULD apply the rules for martian filtering in [18] and ingress filtering [13] on the IPv4 source address. The decapsulating node performs IPv4 reassembly before decapsulating <draft-ietf-ngtrans-mech-v2-01.txt> [Page 17]
INTERNET DRAFT Basic IPv6 Transition Mechanisms November 2002 the IPv6 packet. All IPv6 options are preserved even if the encapsulating IPv4 packet is fragmented. The encapsulating IPv4 header is discarded. After the decapsulation the node SHOULD silently discard a packet with an invalid IPv6 source address. This includes IPv6 multicast addresses, the unspecified address, and the loopback address but also IPv4-compatible IPv6 source addresses where the IPv4 part of the address is an (IPv4) multicast address, broadcast address, 0.0.0.0, or 127.0.0.1. In general it SHOULD apply the rules for martian filtering in [18] and ingress filtering [13] on the IPv4-compatible source address. After the IPv6 packet is decapsulated, it is processed almost the same as any received IPv6 packet. The only difference being that a decapsulated packet MUST NOT be forwarded unless the node has been explicitly configured to forward such packets for the given IPv4 source address. This configuration can be implicit in e.g., having a configured tunnel which matches the IPv4 source address. This restriction is needed to prevent tunneling to be used as a tool to circumvent ingress filtering [13]. 3.7. Link-Local Addresses The configured tunnels are IPv6 interfaces (over the IPv4 "link layer") thus MUST have link-local addresses. The link-local addresses are used by routing protocols operating over the tunnels. The Interface Identifier [14] for such an Interface SHOULD be the 32-bit IPv4 address of that interface, with the bytes in the same order in which they would appear in the header of an IPv4 packet, padded at the left with zeros to a total of 64 bits. Note that the "Universal/Local" bit is zero, indicating that the Interface Identifier is not globally unique. When the host has more than one IPv4 address in use on the physical interface concerned, an administrative choice of one of these IPv4 addresses is made. The IPv6 Link-local address [14] for an IPv4 virtual interface is formed by appending the Interface Identifier, as defined above, to the prefix FE80::/64. <draft-ietf-ngtrans-mech-v2-01.txt> [Page 18]
INTERNET DRAFT Basic IPv6 Transition Mechanisms November 2002 +-------+-------+-------+-------+-------+-------+------+------+ | FE 80 00 00 00 00 00 00 | +-------+-------+-------+-------+-------+-------+------+------+ | 00 00 | 00 | 00 | IPv4 Address | +-------+-------+-------+-------+-------+-------+------+------+ 3.8. Neighbor Discovery over Tunnels For unidirectional configured tunnels most of Neighbor Discovery [17] and Stateless Address Autoconfiguration [5] does not apply; only the formation of the link-local address applies. If an implementation provides bidirectional configured tunnels it MUST at least accept and respond to the probe packets used by Neighbor Unreachability Detection [7]. Such implementations SHOULD also send NUD probe packets to detect when the configured tunnel fails at which point the implementation can use an alternate path to reach the destination. Note that Neighbor Discovery allows that the sending of NUD probes be omitted for router to router links if the routing protocol tracks bidirectional reachability. For the purposes of Neighbor Discovery the configured tunnels specified in this document are assumed to NOT have a link-layer address, even though the link-layer (IPv4) does have address. This means that a sender of Neighbor Discovery packets - SHOULD NOT include Source Link Layer Address options or Target Link Layer Address options on the tunnel link. - MUST silently ignore any received SLLA or TLLA options on the tunnel link. Not using alink layer address options is consistent with how neighbor discovery is used on other point-to-point links. 3.9. Ingress Filtering The specification above contains rules that apply ingress filtering to packets before they are decapsulated. The purpose of ingress filtering in general is specified in [13]. When IP-in-IP tunneling (independent of IP versions) is used it is important that this not be a tool to bypass ingress filtering for non-tunneled packets. For instance, without specific ingress filtering checks in the decapsulating node, it would be possible for an attacker to inject a <draft-ietf-ngtrans-mech-v2-01.txt> [Page 19]
INTERNET DRAFT Basic IPv6 Transition Mechanisms November 2002 packet with: - Outer IPv4 source: real IPv4 address of attacker - Outer IPv4 destination: IPv4 address of decapsulating node - Inner IPv6 source: Alice which is either the decapsulating node or a node close to it. - Inner IPv6 destination: Bob Even if all IPv4 routers between the attacker and the decapsulating node implement IPv4 ingress filtering, and all IPv6 routers between the decapsulating node and Bob implement IPv6 ingress filter, the above spoofed packets will not be filtered out. The solution to this is to have the decapsulating node perform ingress filtering checks as part of the decapsulation as specified in section 4.1. 4. Configured Tunneling In configured tunneling, the tunnel endpoint address is determined from configuration information in the encapsulating node. For each tunnel, the encapsulating node must store the tunnel endpoint address. When an IPv6 packet is transmitted over a tunnel, the tunnel endpoint address configured for that tunnel is used as the destination address for the encapsulating IPv4 header. The determination of which packets to tunnel is usually made by routing information on the encapsulating node. This is usually done via a routing table, which directs packets based on their destination address using the prefix mask and match technique. 4.1. Ingress Filtering The decapsulating node MUST verify that the tunnel source address is acceptable before forwarding decapsulated packets to avoid circumventing ingress filtering [13]. Note that packets which are delivered to transport protocols on the decapsulating node SHOULD NOT be subject to these checks. For bidirectional configured tunnels this is done by verifying that the source address is the IPv4 address of the other end of the tunnel. For unidirectional configured <draft-ietf-ngtrans-mech-v2-01.txt> [Page 20]
INTERNET DRAFT Basic IPv6 Transition Mechanisms November 2002 tunnels the decapsulating node MUST be configured with a list of source IPv4 address prefixes that are acceptable. Such a list MUST default to not having any entries i.e., the node has to be explicitly configured to forward decapsulated packets received over unidirectional configured tunnels. 5. Acknowledgments We would like to thank the members of the IPng working group and the Next Generation Transition (ngtrans) working group for their many contributions and extensive review of this document. Special thanks are due to Jim Bound, Ross Callon, Bob Hinden, John Moy, and Pekka Savola for many helpful suggestions. 6. Security Considerations Tunneling is not known to introduce any security holes except for the possibility to circumvent ingress filtering [13]. This is prevented by requiring that decapsulating routers only forward packets if they have been configured to accept encapsulated packets from the IPv4 source address in the receive packet. An implementation of tunneling needs to be aware that while a tunnel is a link (as defined in [4]), the threat model for a tunnel might be rather different than for other links, since the tunnel potentially includes all of the Internet. The recommendations to verify that the IPv4 addresses in the encapsulated packet matches what has been configured for the tunnel, coupled with use of ingress filtering in IPv4, ameliorate some of this. In addition, an implementation must treat interfaces to different links as separate e.g. to ensure that Neighbor Discovery packets arriving on one link does not effect other links. This is especially important for tunnel links. <draft-ietf-ngtrans-mech-v2-01.txt> [Page 21]
INTERNET DRAFT Basic IPv6 Transition Mechanisms November 2002 7. Authors' Addresses Erik Nordmark Sun Microsystems Laboratories 180, avenue de l'Europe 38334 SAINT ISMIER Cedex, France Tel : +33 (0)4 76 18 88 03 Fax : +33 (0)4 76 18 88 88 EMail : erik.nordmark@sun.com Robert E. Gilligan Intransa, Inc. 2870 Zanker Rd., Suite 100 San Jose, CA 95134 Tel : +1 408 678 8600 Fax : +1 408 678 8800 Email : gilligan@intransa.com, gilligan@leaf.com 8. References 8.1. Normative References [4] Deering, S., and Hinden, R. "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998. [8] Mogul, J., and Deering, S., "Path MTU Discovery", RFC 1191, November 1990. [14] Hinden, R., and S. Deering, "IP Version 6 Addressing Architecture", RFC 2373, July 1998. 8.2. Non-normative References [5] Thomson, S., and Narten, T. "IPv6 Stateless Address Autoconfiguration," RFC 2462, December 1998. [6] Thomson, S., and Huitema C. "DNS Extensions to support IP version 6", RFC 1886, December 1995. <draft-ietf-ngtrans-mech-v2-01.txt> [Page 22]
INTERNET DRAFT Basic IPv6 Transition Mechanisms November 2002 [7] Narten, T., Nordmark, E., and Simpson, W. "Neighbor Discovery for IP Version 6 (IPv6)", RFC 2461, December 1998. [10] Braden, R., "Requirements for Internet Hosts - Communication Layers", STD 3, RFC 1122, October 1989. [11] Kent, C., and J. Mogul, "Fragmentation Considered Harmful". In Proc. SIGCOMM '87 Workshop on Frontiers in Computer Communications Technology. August 1987. [13] Ferguson, P., and Senie, D., "Network Ingress Filtering: Defeating Denial of Service Attacks which employ IP Source Address Spoofing", RFC 2267, January 1998. [16] S. Bradner, "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, March 1997. [17] D. Thaler, "IP Tunnel MIB", RFC 2667, August 1999. [18] F. Baker, "Requirements for IP Version 4 Routers", RFC 1812, June 1995. [19] R. Draves, "Default Address Selection for IPv6", Work in progress, draft-ietf-ipv6-default-addr-select-09.txt, June 2002. [20] D. Black, "Differentiated Services and Tunnels", RFC 2893, October 2000. [21] K. Ramakrishnan, S. Floyd, D. Black, "The Addition of Explicit Congestion Notification (ECN) to IP", RFC 3168, September 2001. 9. Changes from RFC 2893 The motivation for the bulk of these changes are to simplify the document to only contain the mechanisms of wide-spread use. RFC 2893 contains a mechanism called automatic tunneling. But a much more general mechanism is specified in RFC 3056 which gives each node with a (global) IPv4 address a /48 IPv6 prefix i.e., enough for a whole site. - Removed references to A6 and retained AAAA. <draft-ietf-ngtrans-mech-v2-01.txt> [Page 23]
INTERNET DRAFT Basic IPv6 Transition Mechanisms November 2002 - Removed automatic tunneling and IPv4-compatible addresses. - Removed default Configured Tunnel using IPv4 "Anycast Address" - Removed Source Address Selection section since this is now covered by another document ([19]). - Removed brief mention of 6over4. - Split into normative and non-normative references. - Drop "or equal" in if (IPv4 path MTU - 20) is less than or equal to 1280 - Dropped this: However, IPv6 may be used in some environments where interoperability with IPv4 is not required. IPv6 nodes that are designed to be used in such environments need not use or even implement these mechanisms. - Increased the minimum MRU from 1300 to 4420. - Clarified that the dynamic path MTU mechanism in section 3.2 is OPTIONAL but if it is implemented it should follow the rules in section 3.2. - Stated that when the dynamic PMTU is not implemented the sender MUST NOT by default send IPv6 packets larger than 1280 into the tunnel. - Stated that implementations MAY have a knob by which the "min MRU" and the max MTU" can be set to larger values on a tunnel by tunnel basis, but that the defaults MUST be the above numbers. - Restated the "currently underway" language about ToS to loosely point at [20] and [21]. - Stated that IPv4 source MAY also be administratively specified. (This is especially useful on multi-interface nodes and with configured tunneling) <draft-ietf-ngtrans-mech-v2-01.txt> [Page 24]
INTERNET DRAFT Basic IPv6 Transition Mechanisms November 2002 10. Open issues - Should all of section 2.2 (DNS stuff) be removed? It duplicates [19]. - Should we drop the discussion about unidirectional tunneling? - Is 1280 a reasonable MTU for encapsulators that do not implement dynamic tunnel MTU discovery? - Is 4400 a reasonable cap on the MTU for encapsulators that implement dynamic tunnel MTU discovery? - Is 4420 is reasonable minimum MRU for decapsulating nodes? - IPv6 native addresses include IPv4-mapped as currently defined. Is this an issue that needs to be dealt with? The term "IPv6- native" is only used once in the document. 11. TODO - Section 4.1: Ingress filtering as described does not guard against the attack described in 3.9. Checking must be done on the v6 address - attacker can spoof the v4 source address, unless tunnel is secured. Make it more clear that the assumption is that *if* IPv4 and IPv6 ingress filtering is deployed, the decapsulating node can't blindly decapsulate since it would circumvent the ingress filtering. - Section 3.3 reference to Assigned Numbers needs to be to online version (with proper pointer to "Assigned Numbers is obsolete" RFC) <draft-ietf-ngtrans-mech-v2-01.txt> [Page 25]
