NGTRANS Working Group F. Templin Internet-Draft Nokia Expires: July
1, 20034, 2002 T. Gleeson Cisco Systems K.K. M. Talwar D. Thaler Microsoft Corporation December 31,January 03, 2002 Intra-Site Automatic Tunnel Addressing Protocol (ISATAP) draft-ietf-ngtrans-isatap-09.txtdraft-ietf-ngtrans-isatap-10.txt Status of this Memo This document is an Internet-Draft and is in full conformance with 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 Internet-Draft will expire on July 1, 2003.4, 2002. Copyright Notice Copyright (C) The Internet Society (2002). All Rights Reserved. Abstract This document specifies an Intra-Site Automatic Tunnel Addressing Protocol (ISATAP) that connects IPv6 hosts and routers within IPv4 sites. ISATAP treats the site's IPv4 infrastructure as a link layer for IPv6 with no requirement for IPv4 multicast. ISATAP enables intra-site automatic IPv6-in-IPv4 tunneling whether globally assigned or private IPv4 addresses are used. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Applicability Statement . . . . . . . . . . . . . . . . . . 3 3. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 43 4. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 4 5. Basic IPv6Non-Broadcast, Multiple Access (NBMA) Operation on ISATAP Links. . . . . . 4 5.1 Multicast . . . . . . . . . . . . . . . . . . . . . . . . . 5 5.15.2 Interface Identifiers and Address Construction . . . . . . . 5 5.25.3 ISATAP Link/Interface Configuration . . . . . . . . . . . . 5 5.3 Dual Stack Operation and5.4 Link Layer Address Configuration .Options . . . . . . 6 5.4 Tunneling Mechanisms. . . . . . . . . . . 6 6. Automatic Tunneling . . . . . . . . . 6 5.4.1 Encapsulation. . . . . . . . . . . 6 6.1 Dual IP Layer Operation . . . . . . . . . . . . 6 5.4.2 Tunnel MTU and Fragmentation. . . . . . 6 6.2 Encapsulation . . . . . . . . . . 6 5.4.3 Handling IPv4 ICMP Errors. . . . . . . . . . . . . 6 6.3 Tunnel MTU and Fragmentation . . . . 7 5.4.4 Decapsulation. . . . . . . . . . . . 7 6.4 Handling IPv4 ICMP Errors . . . . . . . . . . . 7 5.4.5 Link-Local Addresses. . . . . . 8 6.5 Local-Use IPv6 Unicast Addresses . . . . . . . . . . . . . . 7 5.4.68 6.6 Ingress Filtering . . . . . . . . . . . . . . . . . . . . . 7 6.8 7. Neighbor Discovery and Address Autoconfigurationfor ISATAP Links . . . . . . . . . . . . 8 6.17.1 Address Resolution . . . . . . . . . . . . . . . . . . . . . 8 6.2 Address Autoconfiguration and9 7.2 Router and Prefix Discovery . . . . . . . . . . . . . . . . 9 18.104.22.168.1 Conceptual Data Structures . . . . . . . . . . . . . . . . . 9 22.214.171.124.2 Validity Checks for Router Advertisements . . . . . . . . . 10 126.96.36.199.3 Router Specification . . . . . . . . . . . . . . . . . . . . 10 6.2.411 7.2.4 Host Specification . . . . . . . . . . . . . . . . . . . . . 11 7.8. ISATAP Deployment Considerations . . . . . . . . . . . . . . 12 7.18.1 Host And Router Deployment Considerations . . . . . . . . . 12 7.28.2 Site Administration Considerations . . . . . . . . . . . . . 12 8.9. IANA Considerations . . . . . . . . . . . . . . . . . . . . 13 9.10. Security considerations . . . . . . . . . . . . . . . . . . 13 10.11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 14 Normative References . . . . . . . . . . . . . . . . . . . . 14 Informative References . . . . . . . . . . . . . . . . . . . 15 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 16 A. Major Changes . . . . . . . . . . . . . . . . . . . . . . . 17 B. Rationale for Interface Identifier Construction . . . . . . 18 C. Dynamic Per-neighbor MTU Discovery . . . . . . . . . . . . . 19 Intellectual Property and Copyright Statements . . . . . . . 21 1. Introduction This document presents a simple approach that enables incremental deployment of IPv6  within IPv4-based  sites in a manner that is compatible with inter-domain tunneling mechanisms, e.g., RFC 3056 (6to4) .sites. We refer to this approach as the Intra-Site Automatic Tunnel Addressing Protocol (ISATAP). ISATAP allows dual-stack nodes that do not share a physical link with an IPv6 router to automatically tunnel packets to the IPv6 next-hop address through IPv4, i.e., the site's IPv4 infrastructure is treated as ana link layer for IPv6. This document specifies details for the transmissionoperation of IPv6 packetsover ISATAP links (i.e., automatic IPv6-in-IPv4 tunneling), including an interface identifier format that embeds an IPv4 address. This format supports IPv6 protocol mechanisms for address configuration as well as simple link-layer address mapping. Simple validity checks for received packets are given.Also specified in this document is the operation of IPv6 Neighbor Discovery for ISATAP. The document finally presents deployment and security considerations for ISATAP.considerations. 2. Applicability Statement ISATAP provides the following features: o treats site's IPv4 infrastructure as a link layer for IPv6 using automatic IPv6-in-IPv4 tunneling (i.e., no configured tunnel state) o enables incremental deployment of IPv6 hosts within IPv4 sites with no aggregation scaling issues at border gateways o requires no special IPv4 services within the site (e.g., multicast) o supports both stateless address autoconfiguration and manual configuration o supports networks that use non-globally unique IPv4 addresses (e.g., when private address allocations  are used), but does not allow the virtual ISATAP link to span a Network Address Translator  o compatible with other NGTRANS mechanisms (e.g., 6to4 )) 3. Requirements 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 . This document also makes use of internal conceptual variables to describe protocol behavior and external variables that an implementation must allow system administrators to change. The specific variable names, how their values change, and how their settings influence protocol behavior are provided to demonstrate protocol behavior. An implementation is not required to have them in the exact form described here, so long as its external behavior is consistent with that described in this document. 4. Terminology The terminology of RFC 2460  applies to this document. The following additional terms are defined: link; on-link:link, on-link, off-link: same definitions as (, section 2.1). underlying link: a link layer that supports IPv4 (for ISATAP), and MAY also support IPv6 natively. ISATAP link: one or more underlying links used for tunneling. The IPv4 network layer addresses of the underlying links are used as link-layer addresses on the ISATAP link. ISATAP interface: a node's attachment to an ISATAP link. ISATAP address: an on-link address on an ISATAP interface and with an interface identifier constructed as specified in Section 5.15.2 ISATAP router: an IPv6 node that has an ISATAP interface over which it forwards packets not explicitly addressed to itself. ISATAP host: any node that has an ISATAP interface and is not an ISATAP router. 5. Basic IPv6Non-Broadcast, Multiple Access (NBMA) Operation on ISATAP LinksISATAP links transmit IPv6 packets via automatic tunnels using the site's IPv4 infrastructure as a link layer for IPv6, i.e., IPv6 treats the site's IPv4 infrastructure is treatedas a Non-Broadcast, Multiple Access (NBMA) link layer. RFC 2491  provides a general architecture for IPv6 over NBMA networks that forms the basis for companion documents such as the present. The following subsections outline basic operational detailspresent NBMA considerations for IPv6 on ISATAP links: 5.1 Interface Identifiers and Address Construction (RFC2491 , section 5.1) requires companion documents to specifyMulticast ISATAP links most closely meet the exact mechanism for generating interface tokens (i.e., identifiers). Interface identifiersdescription for ISATAP are compatible withconnectionless service found in the EUI-64 identifier format (, section 2.5.1),last paragraph of (, section 1), i.e., ISATAP addresses provide the sender with an NBMA destination address to which it can transmit packets whenever it desires. Thus, multicast emulation mechanisms are not required to support host-side operation of the IPv6 neighbor discovery protocol. 5.2 Interface Identifiers and Address Construction (, section 5.1) requires companion documents to specify the exact mechanism for generating interface tokens (i.e., identifiers). Interface identifiers for ISATAP are compatible with the EUI-64 identifier format (, section 2.5.1), and are constructed by appending an IPv4 address on the ISATAP link to the 32-bit string '00-00-5E-FE'. (Appendix B includes non-normative text explaining the rationale for this construction rule.) Global and Local-use ISATAP addresses are constructed as follows: | 64 bits | 32 bits | 32 bits | +------------------------------+---------------+----------------+ | global or local-use unicast | 0000:5EFE | IPv4 Address | | prefix | | of ISATAP link | +------------------------------+---------------+----------------+ Figure 1 For example, the global unicast address: 3FFE:1A05:510:1111:0:5EFE:8CAD:8108 has a prefix of '3FFE:1A05:510:1111::/64' and an ISATAP interface identifier with embedded IPv4 address: '188.8.131.52'. The address is alternately written as: 3FFE:1A05:510:1111:0:5EFE:184.108.40.206 (Similar examplesExamples for local-use addresses are madeobvious byfrom the above and with reference to the IPv6 addressing architecture document.) 5.2(, section 2.5.6). 5.3 ISATAP Link/Interface Configuration ISATAP Link/Interface configuration is consistent with (RFC2491 ,(, sections 5.1.1 and 5.1.2). Using the terminology of Section 4, anAn ISATAP link consists of one or more underlying links that support IPv4 for tunneling within a site. ISATAP interfaces are configured over ISATAP links; each IPv4 address assigned to an underlying link is seen as a link-layer address for ISATAP. 5.35.4 Link Layer Address Options (, section 5.2) requires companion documents to specify the contents of the [NTL], [STL], [NBMA Number] and [NBMA Subaddress] fields for link layer address options. For ISATAP links: o the [NTL] and [STL] fields MUST be zero o the [NBMA Number] encodes a 4-octet IPv4 address o the [NBMA Subaddress] field is omitted (, section 5.2) does NOT require companion documents to specify the value for [Length], i.e., the total length of the option in 8 octets. Senders may therefore set [Length] to any value between 1 and 255; when [Length] is greater than 1, receivers treat any bytes that follow the [NBMA Number] as null-padding. 6. Automatic Tunneling The common tunneling mechanisms specified in (, sections 2 and 3) are used, with the following noted specific considerations for ISATAP: 6.1 Dual StackIP Layer Operation and Address ConfigurationISATAP uses the same specification found in (, section 2). That is, ISATAP nodes implement "IPv6/IPv4" or "dual-stack" configurationsprovide complete IPv4 and IPv6 implementations and are able to send and receive both IPv4 and IPv6 packets. ISATAP nodes operate with both their IPv4 and IPv6 stacks enabled. Address configuration and DNSconsiderations are the same as for (, sections 2.1 and 2.2) 5.4 Tunneling Mechanisms The common tunneling mechanisms specified in (, sections 3.1 through 3.7) are used, with the following noted specific considerations: 5.4.1 Encapsulation Thesection 2.1). Additionally, ISATAP nodes require that IPv4 address configuration take place on at least one underlying link prior to IPv6 address configuration on an ISATAP link. DNS considerations are the same as (, sections 2.2 and 2.3). 6.2 Encapsulation The specification in (, section 3.1) is used. Additionally, the IPv6 next-hop address for packets sent on an ISATAP link MUST be an ISATAP address; other packets are discarded (i.e., not encapsulated)and an ICMPv6 destination unreachable indication with code 3 (Address Unreachable)  is returned to the source. 220.127.116.11 Tunnel MTU and Fragmentation The specification in (, section 3.2) is NOT used; the specification in this section is used instead. ISATAP uses automatic tunnel interfaces that may be configured over multiple underlying links with diverse maximum transmission units (MTUs). The minimum MTU for IPv6 interfaces is 1280 bytes (, Section 5), but the following considerations apply when IPv4 is used as a link layerfor IPv6:the MTU of ISATAP interfaces apply: o nearlyNearly all IPv4 nodes accept unfragmented packets upconnect to physical links with MTUs of 1500 bytes or larger (e.g., Ethernet) o sub-IPv4Sub-IPv4 layer encapsulations (e.g., VPN) may occur on some paths o commonly-deployedCommonly-deployed VPNs use an MTU of 1400 bytes Thus,Unless a dynamic per-neighbor MTU discovery mechanism is implemented, ISATAP interfaces SHOULDMUST use an MTU (ISATAP_MTU) of no more than 1380 bytes (1400 minus 20 bytes for IPv4 encapsulation) to maximize efficiency and minimize IPv4 fragmentation.fragmentation for the predominant deployment case. ISATAP_MTU MAY be set to a larger valuesvalue when the encapsulator usesimplements a dynamic per-neighbor MTU discovery. When larger values are used, ISATAP_MTUdiscovery mechanism, but this value SHOULD NOT exceed the maximumlargest MTU of all underlying links minus(minus 20 bytes for link layer encapsulation. (AppendixIPv4 encapsulation). Appendix C provides non-normative considerations for dynamic per-neighbor MTU discovery.) As with ordinary IPv6 interfaces, thediscovery. The network layer (IPv6) forwards packets of size ISATAP_MTU or smaller to the ISATAP interface. All other packets are dropped, and an ICMPv6 "packet too big" message with MTU = ISATAP_MTU is returned to the source . The ISATAP interfaces send alllink layer encapsulates packets of size 1380 bytes or smaller with the Don't Fragment (DF) bit NOT set in the encapsualting IPv4 header. 5.4.3Nodes that configure ISATAP interfaces MUST have IPv4 reassembly buffers large enough to receive packets with the DF bit not set in the encapsulating IPv4 header. RFC 1122 , section 3.3.2 specifies that the Effective MTU to Receive (EMTU_R) for IPv4 nodes: "...MUST be greater than or equal to 576, SHOULD be either configurable or indefinite, and SHOULD be greater than or equal to the MTU of the connected network(s)". With reference to this specification, the EMTU_R for nodes that configure ISATAP interfaces MUST be greater than or equal to 1500 bytes (i.e., the predominant deployment case for connected IPv4 networks) and SHOULD be either configurable or indefinite. 6.4 Handling IPv4 ICMP Errors The specification in (, section 3.4) MAY be used. IPv4 ICMP errors and ARP failures are otherwise processed as link error notifications. 5.4.4 Decapsulation The specification in (, section 3.6) is used. 5.4.5 Link-Local6.5 Local-Use IPv6 Unicast Addresses The specification in (, section 3.7) is NOT used. Instead, link-locallocal use IPv6 unicast addresses are formed by appending an interface identifier,exactly as definedspecified in Section 5.1, to the prefix FE80::/64. 5.4.6(, section 2.5.6). 6.6 Ingress Filtering The network layer (IPv6) destination address of a packet receivedspecification in (, section 3.9) is used on anISATAP interface is either local (i.e., matchesrouter interfaces. (ISATAP host interfaces silently discard any packets received with a foreign IPv6 destination address, i.e., an address not configured on the local IPv6 stack) or foreign. The decapsulator MUST be configured with a list of IPv4 address prefixes that are acceptable, i.e., an ingress filter list (default deny all). For packets with foreign network layer (IPv6) destination addresses, the link layer (IPv4) source address MUST be explicitly allowed by ingress filtering. Packets that do not satisfy this condition are silently discarded.stack.) Additionally, allpackets (whether foreign or local)received on ISATAP host and router interfaces MUST satisfy at least one (i.e., one or both) of the following validity checks: o the network-layer (IPv6) source address is an on-link ISATAP address with an interface identifier that embeds the link-layer (IPv4) source address o the link-layer (IPv4) source address is in the Potential Routers List (see Section 6.2.1) Packets that do not satisfy the above conditions are silently discarded. 6. Neighbor Discovery and Address Autoconfiguration RFC 2491  provides a general architecture for IPv6 over NBMA networks, including multicast mechanisms to support host-side operation of the IPv6 neighbor discovery protocol. ISATAP links most closely meet the description for connectionless service found in the last paragraph of (, section 1), i.e., ISATAP addresses provide the sender with an NBMA destination address to which it can transmit packets whenever it desires. Thus, the RFC 2491 multicast mechanisms are not required for address resolution and not otherwise implemented on ISATAP links due to traffic scaling considerations (i.e., ISATAP links7.2.1) Packets that do not satisfy the above conditions are unicast-only).silently discarded. 7. Neighbor Discovery for ISATAP Links RFC 2461  provides the following guidelines for non-broadcast multiple access (NBMA) link support: "Redirect, Neighbor Unreachability Detection and next-hop determination should be implemented as described in this document. Address resolution and the mechanism for delivering Router Solicitations and Advertisements on NBMA links is not specified in this document." ISATAP links SHOULD implement Redirect, Neighbor Unreachability Detection, and next-hop determination exactly as specified in . Address resolution and the mechanisms for delivering Router Solicitations and Advertisements onfor ISATAP links use the specifications foundare not specified by ; instead, they are specified in this document. 6.1(Note that these mechanisms MAY potentially apply to other types of NBMA links in the future.) 7.1 Address Resolution ISATAP addresses are resolved to link-layer addresses (IPv4) by a static computation, i.e., the last four octets are treated as an IPv4 address. (, section 5.2) requires companion documents to specify the format for link layer address options, however, link layer address options are not needed for address resolution in ISATAP. Thus, no format is specified and the following specification from (, section 3.8) applies: "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."Following static address resolution, ISATAP hosts SHOULD implement theperform an initial reachability confirmation specificationsby sending unicast Neighbor Solicitations (NSs) and receiving a Neighbor Advertisement using the mechanisms specified in ,(, sections 7.2.2-18.104.22.168.2-7.2.8). (Note that applyimplementations MAY omit the source/target link layer options in NS/ NA messages when unicast Neighbor Solicitations (NS) are used.is used.) ISATAP hosts SHOULD additionally perform Neighbor Unreachability Detection (NUD) as specified in (RFC 2461 ,(, section 7.3). ISATAP routers MAY perform the above-specified reachability detection and NUD procedures, but this might not scale in all environments. All ISATAP nodes MUST send solicited neighbor advertisements (, section 7.2.4). ISATAP links disable Duplicate Address Detection, as permitted by (, section 4). 6.2 Address Autoconfiguration and7.2 Router and Prefix Discovery Since NBMA multicast emulation mechanisms are not used onused, ISATAP links,nodes will not receive unsolicited multicast Router Advertisements. (RFC 2462 , section 5.5.2) requires that hosts use stateful autoconfiguration (i.e., DHCPv6 ) in the absence of Router Advertisements. When statelful autoconfiguration is not available, nodes useThus, alternate mechanisms (described below) for routerare required and prefix discovery. 6.2.1specified below: 7.2.1 Conceptual Data Structures ISATAP nodes use the conceptual data structures Prefix List and Default Router List exactly as in (, section 5.1). ISATAP links add twoa new conceptual data structures "Potential Router List" and "Stateful Autoconfiguration Server List".the following new configuration variable: ResolveInterval Time between name service resolutions. Default and suggested minimum: 1hr A Potential Router List (PRL) and Stateful Autoconfiguration Server List (SASL)is associated with every ISATAP link. The PRL provides a trust basis for router validation (see security considerations). Each entry in the PRL has an IPv4 address and an associated timer. The IPv4 address represents a router's ISATAP interface (likely to be an "advertising interface"), and is used to construct the ISATAP link-local address for that interface. Similarly, each entry in the SASL has an IPv4 address and associated timer.The IPv4 address represents a DHCPv6 server attached to the ISATAP link, and is used to constructfollowing sections specify the ISATAP link-local addressprocess for that DHCPv6 server.initializing the PRL: When a node enables an ISATAP link, it first discovers IPv4 addresses for the PRL and SASL.PRL. The addresses MAYSHOULD be established by a DHCPv4  option for ISATAP (option code TBD), by manual configuration, or by an unspecified alternativealternate method (e.g., DHCPv4 vendor-specific option; DNS () fully-qualified domain names).option). When no other mechanisms are available, a DNS fully-qualified domain names are used,name (FQDN)  MAY be used. In this case, the FQDN is resolved into IPv4 addresses for the PRL and SASL are discoveredthrough a static host filefile, a site-specific name service, or by querying an IPv4-based DNS server to resolve the domain names into address records (e.g., DNS 'A' resource records) containing IPv4 addresses.server. Unspecified alternativealternate methods for domain name resolution may also be used. The following notes apply when DNS fully-qualified domain names are used:apply: 1. Site administrators maintain domain names anda list of IPv4 addresses for the PRL and SASL for the site'srepresenting ISATAP service, e.g., as address records in the site's name service. Administrators may also advertiserouter interfaces and make them available via one or more of the domain names in a DHCPv4 option for ISATAP.mechanisms described above. 2. There are no mandatory rules for the selection of domain names,a FQDN, but administrators are encouraged to use the convention "(list_name).isatap.domainname""isatap.domainname" (e.g., prl.isatap.example.com).isatap.example.com). 3. After initialization, nodes periodically re-initialize the PRL and SASL, e.g., once per hour.(after ResolveInterval). When DNS is used, client DNS resolvers use the IPv4 transport to resolve the names and follow the cache invalidation procedures in  when the DNS time-to-live expires. 22.214.171.124.2 Validity Checks for Router Advertisements A node MUST silently discard any Router Advertisement messages it receives that do not satisfy both the validity checks in (, section 6.1.2) and the following additional validity check for ISATAP: o the network-layer (IPv6) source address is an ISATAP address and embeds an IPv4 address from the PRL 126.96.36.199.3 Router Specification Advertising ISATAP interfaces of routers behave the same as advertising interfaces described in (, section 6.2). However, periodic unsolicited multicast Router Advertisements are not used, thus the "interval timer" associated with advertising interfaces is not used for that purpose. When an ISATAP router receives a valid Router Solicitation on an advertising ISATAP interface, it replies with a unicast Router Advertisement to the address of the node which sent the Router Solicitation. The source address of the Router Advertisement is a link-local unicast address associated with the interface. This MAY be the same as the destination address of the Router Solicitation. ISATAP routers MAY engage in the solicitation process described under Host Specification below, e.g., if Router Advertisement consistency verification (, section 6.2.7) is desired. 188.8.131.52.4 Host Specification All entries in the PRL are assumed to represent active ISATAP routers within the site, i.e., the PRL provides trust basis only; not reachability detection. When stateful autoconfiguration is available (i.e., when the SASL is non-null and at least one DHCPv6 server is reachable), hosts may send unicast messages directly to the DHCPv6 server as specified in (, section 1.1). HostsISATAP nodes SHOULD attempt stateful autoconfiguration for each entry in the SASL (i.e., until an attempt succeeds) before concluding thatuse stateful autoconfiguration is unavailable.configuration to assign IPv6 prefixes and default router information. When stateful autoconfigurationconfiguration is unavailable,not available, hosts MAY periodically solicit information from one or more entries in the PRL ("PRL(i)") by sending unicast Router Solicitation messages using the IPv4 address ("V4ADDR_PRL(i)") and associated timer in the entry. Hosts add the following variable to support the solicitation process: MinRouterSolicitInterval Minimum time between sending Router Solicitations to any router. Default and suggested minimum 800,000 milliseconds (15min).minimum: 15min. When a PRL(i) is selected, the host sets its associated timer to MinRouterSolicitInterval and initiates solicitation following a short delay as in (, section 6.3.7). The manner of choosing particular routers in the PRL for solicitation is outside the scope of this specification. The solicitation process repeats when the associated timer expires. Solicitation consists of sending Router Solicitations to the ISATAP link-local address constructed from the entry's IPv4 address, i.e., they are sent to 'FE80::0:5EFE:V4ADDR_PRL(i)' instead of 'All-Routers multicast'. They are otherwise sent exactly as in (, section 6.3.7). Hosts process received Router Advertisements exactly as in (, section 6.3.4). Hosts additionally reset the timer associated with the V4ADDR_PRL(i) embedded in the network-layer source address in each solicited Router Advertisement received. The timer is reset to either 0.5 * (the minimum value in the router lifetime or valid lifetime of any on-link prefixes received in the advertisement) or MinRouterSolicitInterval; whichever is longer. 7.8. ISATAP Deployment Considerations 7.18.1 Host And Router Deployment Considerations For hosts, if an underlying link supports both IPv4 (over which ISATAP is implemented) and also supports IPv6 natively, then ISATAP MAY be enabled if the native IPv6 layer does not receive Router Advertisements (i.e., does not have connection with an IPv6 router). After a non-link-local address has been configured and a default router acquired on the native link, the host SHOULD discontinue the router solicitation process described in the host specification and allow existing ISATAP address configurations to expire as specified in (, section 5.3) and (,(, section 5.5.4). Any ISATAP addresses added to the DNS for this host should also be removed. In this way, ISATAP use will gradually diminish as IPv6 routers are widely deployed throughout the site. Routers MAY configure an interface to simultaneously support both native IPv6, and also ISATAP (over IPv4). Routing will operate as usual between these two domains. Note that the prefixes used on the ISATAP and native IPv6 interfaces will be distinct. The IPv4 address(es) configured on a router's ISATAP interface(s) SHOULD be added (either automatically or manually) to the site's address records for ISATAP router interfaces. 7.28.2 Site Administration Considerations The following considerations are noted for sites that deploy ISATAP: o ISATAP links are administratively defined by a set of router interfaces, a set of stateful autoconfiguration servers,interfaces and set of nodes which discover those interface and server addresses Thus, ISATAP links are defined by administrative (not physical) boundaries. o ISATAP hosts and routers can be deployed in an ad-hoc and independent fashion. In particular, ISATAP hosts can be deployed with little/no advanced knowledge of existing ISATAP routers, and ISATAP routers can deployed with no reconfiguration requirements for hosts. o When stateful autoconfiguration is not available, ISATAP nodes MAY periodically send unicast Router Solicitations to and receive unicast Router Advertisements from to one or more members of the potential router list. A well-deployed stateful autoconfiguration service within the site can minimize and/or eliminate the need for periodic solicitation. o ISATAP nodes periodically refresh the entries on the PRL and SASL.PRL. Responsible site administration can reduce the control traffic. At a minimum, administrators SHOULD ensure that dynamically advertised information for the site's PRL and SASL areis well maintained. 8.9. IANA Considerations A DHCPv4 option code for ISATAP (TBD)  is requested in the event that the IESG recommends this document for standards track. 9.10. Security considerations Site administrators are advised that, in addition to possible attacks against IPv6, security attacks against IPv4 MUST also be considered. Responsible IPv4 site security management is strongly encouraged. In particular, border gateways SHOULD implement filtering to detect spoofed IPv4 source addresses at a minimum; ip-protocol-41 filtering SHOULD also be implemented. If IPv4 source address filtering is not correctly implemented, the ISATAP validity checks will not be effective in preventing IPv6 source address spoofing. If filtering for ip-protocol-41 is not correctly implemented, IPv6 source address spoofing is clearly possible, but this can be eliminated if both IPv4 source address filtering, and the ISATAP validity checks are implemented. (RFC 2461 ), section 6.1.2 implies that nodes trust Router Advertisements they receive from on-link routers, as indicated by a value of 255 in the IPv6 'hop-limit' field. Since this field is not decremented when ip-protocol-41 packets traverse multiple IPv4 hops (, section 3), ISATAP links require a different trust model. In particular, ONLY those Router Advertisements received from a member of the Potential Routers List are trusted; all others are silently discarded. This trust model is predicated on IPv4 source address filtering, as described above. The ISATAP address format does not support privacy extensions for stateless address autoconfiguration .. However, since the ISATAP interface identifier is derived from the node's IPv4 address, ISATAP addresses do not have the same level of privacy concerns as IPv6 addresses that use an interface identifier derived from the MAC address. (This issue is the same for NAT'd addresses.) 10.11. Acknowledgements Some of the ideas presented in this draft were derived from work at SRI with internal funds and contractual support. Government sponsors who supported the work include Monica Farah-Stapleton and Russell Langan from U.S. Army CECOM ASEO, and Dr. Allen Moshfegh from U.S. Office of Naval Research. Within SRI, Dr. Mike Frankel, J. Peter Marcotullio, Lou Rodriguez, and Dr. Ambatipudi Sastry supported the work and helped foster early interest. The following peer reviewers are acknowledged for taking the time to review a pre-release of this document and provide input: Jim Bound, Rich Draves, Cyndi Jung, Ambatipudi Sastry, Aaron Schrader, Ole Troan, Vlad Yasevich. The authors acknowledge members of the NGTRANS community who have made significant contributions to this effort, including Rich Draves, Alain Durand, Nathan Lutchansky, Karen Nielsen, Art Shelest, Margaret Wasserman, and Brian Zill. The authors also wish to acknowledge the work of Quang Nguyen  under the guidance of Dr. Lixia Zhang that proposed very similar ideas to those that appear in this document. This work was first brought to the authors' attention on September 20, 2002. Normative References  Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998.  Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981.  Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G. and E. Lear, "Address Allocation for Private Internets", BCP 5, RFC 1918, February 1996.  Egevang, K. and P. Francis, "The IP Network Address Translator (NAT)", RFC 1631, May 1994.  Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.  Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery for IP Version 6 (IPv6)", RFC 2461, December 1998.  Armitage, G., Schulter, P., Jork, M. and G. Harter, "IPv6 over Non-Broadcast Multiple Access (NBMA) networks", RFC 2491, January 1999.  Hinden, R. and S. Deering, "IP Version 6 Addressing Architecture", draft-ietf-ipngwg-addr-arch-v3-11 (work in progress), October 2002.  Gilligan, R. and E. Nordmark, "Basic Transition Mechanisms for IPv6 Hosts and Routers", draft-ietf-ngtrans-mech-v2-01 (work in progress), November 2002.  Conta, A. and S. Deering, "Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification", RFC 2463, December 1998.  McCann, J., Deering, S. and J. Mogul, "Path MTU Discovery for IP version 6", RFC 1981, August 1996.  Braden, R., "Requirements for Internet Hosts - Communication Layers", STD 3, RFC 1122, October 1989.  Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, March 1997.  Thomson, S. and T. Narten, "IPv6 Stateless Address Autoconfiguration", RFC 2462, December 1998.  Droms, R., "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", draft-ietf-dhc-dhcpv6-28 (work in progress), November 2002.  Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, March 1997. Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, November 1990.  Postel, J., "Internet Control Message Protocol", STD 5, RFC 792, September 1981.  Baker, F., "Requirements for IP Version 4 Routers", RFC 1812, June 1995.  Droms, R., "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", draft-ietf-dhc-dhcpv6-28 (work in progress), November 2002. Informative References  Carpenter, B. and K. Moore, "Connection of IPv6 Domains via IPv4 Clouds", RFC 3056, February 2001.  Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, November 1987.  Droms, R., "Procedures and IANA Guidelines for Definition of New DHCP Options and Message Types", BCP 43, RFC 2939, September 2000.  Narten, T. and R. Draves, "Privacy Extensions for Stateless Address Autoconfiguration in IPv6", RFC 3041, January 2001.  Nguyen, Q., "http://irl.cs.ucla.edu/vet/report.ps", spring 1998.  Lahey, K., "TCP Problems with Path MTU Discovery", RFC 2923, September 2000. Authors' Addresses Fred L. Templin Nokia 313 Fairchild Drive Mountain View, CA 94110 US Phone: +1 650 625 2331 EMail: email@example.com Tim Gleeson Cisco Systems K.K. Shinjuku Mitsu Building 2-1-1 Nishishinjuku, Shinjuku-ku Tokyo 163-0409 Japan EMail: firstname.lastname@example.org Mohit Talwar Microsoft Corporation One Microsoft Way Redmond, WA> 98052-6399 US Phone: +1 425 705 3131 EMail: email@example.com Dave Thaler Microsoft Corporation One Microsoft Way Redmond, WA 98052-6399 US Phone: +1 425 703 8835 EMail: firstname.lastname@example.org Appendix A. Major Changes changes from version 09 to version 10: o Rearranged/revised sections 5, 6, 7 o updated MTU section changes from version 08 to version 09: o Added stateful autoconfiguration mechanism o Normative references to RFC 2491, RFC 2462 o Moved non-normative MTU text to appendix C changes from version 07 to version 08: o updated MTU section changes from version 06 to version 07: o clarified address resolution, Neighbor Unreachability Detection o specified MTU/MRU requirements changes from earlier versions to version 06: o Addressed operational issues identified in 05 based on discussion between co-authors o Clarified ambiguous text per comments from Hannu Flinck; Jason Goldschmidt o Moved historical text in section 4.1 to Appendix B in response to comments from Pekka Savola o Identified operational issues for anticipated deployment scenarios o Included reference to Quang Nguyen work Appendix B. Rationale for Interface Identifier Construction ISATAP specifies an EUI64-format address construction for the Organizationally-Unique Identifier (OUI) owned by the Internet Assigned Numbers Authority (IANA). This format (given below) is used to construct both native EUI64 addresses for general use and modified EUI-64 format interface identifiers for IPv6 unicast addresses: |0 2|2 3|3 3|4 6| |0 3|4 1|2 9|0 3| +------------------------+--------+--------+------------------------+ | OUI ("00-00-5E"+u+g) | TYPE | TSE | TSD | +------------------------+--------+--------+------------------------+ Where the fields are: OUI IANA's OUI: 00-00-5E with 'u' and 'g' bits (3 octets) TYPE Type field; specifies use of (TSE, TSD) (1 octet) TSE Type-Specific Extension (1 octet) TSD Type-Specific Data (3 octets) And the following interpretations are specified based on TYPE: TYPE (TSE, TSD) Interpretation ---- ------------------------- 0x00-0xFD RESERVED for future IANA use 0xFE (TSE, TSD) together contain an embedded IPv4 address 0xFF TSD is interpreted based on TSE as follows: TSE TSD Interpretation --- ------------------ 0x00-0xFD RESERVED for future IANA use 0xFE TSD contains 24-bit EUI-48 intf id 0xFF RESERVED by IEEE/RAC Figure 2 Thus, if TYPE=0xFE, TSE is an extension of TSD. If TYPE=0xFF, TSE is an extension of TYPE. Other values for TYPE (thus, other interpretations of TSE, TSD) are reserved for future IANA use. The above specification is compatible with all aspects of EUI64, including support for encapsulating legacy EUI-48 interface identifiers (e.g., an IANA EUI-48 format multicast address such as: '01-00-5E-01-02-03' is encapsulated as: '01-00-5E-FF-FE-01-02-03'). But, the specification also provides a special TYPE (0xFE) to indicate an IPv4 address is embedded. Thus, when the first four octets of an IPv6 interface identifier are: '00-00-5E-FE' (note: the 'u/l' bit MUST be 0) the interface identifier is said to be in "ISATAP format" and the next four octets embed an IPv4 address encoded in network byte order. Appendix C. Dynamic Per-neighbor MTU Discovery ISATAP encapsulators and decapsulators are IPv6 neighbors that may be separated by multiple link layer (IPv4) forwarding hops. When ISATAP_MTU is larger than 1380 bytes, the encapsulator must implement a dynamic link layer mechanism to discover per-neighbor MTUs. IPv4 path MTU discovery  relies on ICMPv4 "fragmentation needed" messages, but these do not provide enough information for stateless translation into ICMPv6 "packet too big" messages (see: RFC 792  and RFC 1812 , section 184.108.40.206). Additionally, ICMPv4 "fragmentation needed" messages can be spoofed, filtered, or not sent at all by some forwarding nodes. Thus, IPv4 Path MTU discovery used alone is inadequate and can result in black holes that are difficult to diagnose .. The ISATAP encapsulator may implement an alternate per-neighbor MTU discovery mechanism, e.g., periodic and/or on-demand probing of the IPv4 path to the decapsulator. Probing consists of sending packets larger than 1380 bytes withto the neighbor and receiving positive confirmation of receipt. Two methods are possible: In the first method, the encapsulator does NOT set the DF bit setin the IPv4 header.header of probe packets. In this case, the encapsulator must have a priori knowledge of the decapsulator's reassembly buffer size and should have a priori knowledge of the decapsulator's link MTU. This method has the advantage that probe packets will be delivered even if the network performs fragmentation, thus ordinary data packets may be used for probing resulting in greater efficiency. Disadvantages for this method include: o special mechanisms required on both encapsulator and decapsulator o extra state required on both encapsulator and decapsulator o complex protocol signalling between encapsulator and decapsulator o possible extended periods of network fragmentation In the second (and preferred) method, the encapsulator sets the DF bit in the IPv4 header of probe packets. Neighbor Solicitation (NS) packets with padding bytes added should be used for this purpose, since successful delivery results in a positive acknowledgement that the probe succeeded, i.e., in the form of a Neighbor Advertisement (NA) from the decapsulator. (NB:Setting the DF bit prevents the network from fragmenting the packets and protects decapsulators from receiving probepackets that wouldmight overrun the receive bufferIPv4 reassembly buffer. Additionally, special mechanisms and state are needed only on an underlying link, thusthe encapsulator, and no maximum receive unit (MRU)complex protocol signalling between the encapsulator and decapsulator is required.) Implementationsrequired. In either method, implementations may choose to couple the probing process with neighbor cache management procedures (, section 7), e.g. to maintain timers, state variables and/or a queue of packets waiting for probes to complete. Packets retained on the queue are forwarded when probes succeed, and provide state for sending ICMPv6 "packet too big" messages to the source when probes fail. Implementations may choose to store per-neighbor MTU information in the IPv4 path MTU discovery cache, in the ISATAP link layer's private data structures, etc. Additional notes: 1. Per-neighbor MTUs may vary dynamically due to fluctuations in the IPv4 forwarding path and/or multipath routing (e.g., when QoS routing is used in the IPv4 network). For such neighbors, encapsulators should detect a "losing battle" and reduce the per-neighbor MTU size to no more than 1380 bytes. 2. When not probing, encapsulators may send packets to a neighbor with MTU greater than 1380 bytes either with the DF bit set or not set. When the DF bit is set, undetected packet loss may occur in the network if the neighbor's MTU decreases. When the DF bit is NOT set, undetected packet loss is less likely but may occur either in the network or at the neighbor's reassembly buffer. 3. ICMPv4 "fragmentation needed" messages may result when a link restriction is encountered but may also come from denial of service attacks. Implementations should treat ICMPv4 "fragmentation needed" messages as "tentative" negative acknowledgments and apply heuristics to determine when to suspect an actual link restriction and when to ignore the messages. IPv6 packets lost due actual link restrictions are perceived as lost due to congestion by the original source, but robust implementations minimize instances of such packet loss without ICMPv6 "packet too big" messages returned to the sender. 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