Network Working Group F. Templin Internet-Draft Nokia Expires: July
18,25, 2003 T. Gleeson Cisco Systems K.K. M. Talwar D. Thaler Microsoft Corporation January 17,24, 2003 Intra-Site Automatic Tunnel Addressing Protocol (ISATAP) draft-ietf-ngtrans-isatap-11.txtdraft-ietf-ngtrans-isatap-12.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 18,25, 2003. Copyright Notice Copyright (C) The Internet Society (2003). 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 . . . . . . . . . . . . . . . . . . . . . . . . . 3 4. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 5. Basic IPv6 Operation . . . . . . . . . . . . . . . . . . . . . 4 6. Automatic Tunneling . . . . . . . . . . . . . . . . . . . . . 5 7. Neighbor Discovery . . . . . . . . . . . . . . . . . . . . . . 7 8. Deployment Considerations . . . . . . . . . . . . . . . . . . 10 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 10. Security considerations . . . . . . . . . . . . . . . . . . . 11 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12 Normative References . . . . . . . . . . . . . . . . . . . . . 12 Informative References . . . . . . . . . . . . . . . . . . . . 13 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 1415 A. Major Changes . . . . . . . . . . . . . . . . . . . . . . . . 1516 B. Rationale for Interface Identifier Construction . . . . . . . 17 C. DynamicISATAP Interface MTU Discovery . . . . . . .Considerations . . . . . . . . . . . . . 18 Intellectual Property and Copyright Statements . . . . . . . . 2223 1. Introduction This document presents a simple approach called the Intra-Site Automatic Tunnel Addressing Protocol (ISATAP) that enables incremental deployment of IPv6  within IPv4  sites. 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 a link layer for IPv6. Specific details for the operation of IPv6 and automatic tunneling over ISATAP links are given, including an interface identifier format that embeds an IPv4 address. This format supports IPv6 address configuration and simple link-layer address mapping. Also specified is the operation of IPv6 Neighbor Discovery and deployment/security 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 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) 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, 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. advertising ISATAP interface: same meaning as "advertising interface" in (, section 6.2.2). ISATAP address: an on-link address on an ISATAP interface and with an interface identifier constructed as specified in Section 5.25.1 5. Basic IPv6 Operation ISATAP 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 as a Non-Broadcast, Multiple Access (NBMA) link layer. The following considerations for IPv6 on ISATAP links are noted: 5.1 Interface Identifiers and Unicast Addresses ISATAP interface identifiers use "modified EUI-64" format (, section 2.5.1) and are formed by appending an IPv4 address on the ISATAP link to the 32-bit string '00-00-5E-FE'. Appendix B includes non-normative rationale for this construction rule. With reference to (, sections 2.5.4, 2.5.6), 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 | +------------------------------+---------------+----------------+ 5.2 ISATAP Link/Interface Configuration AnISATAP link consistslinks consist 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. At least one link-layer address per advertisingNeighbor discovery on ISATAP interface SHOULDlinks (see: Section 7) provides the functional equivalent of unicast virtual circuits (VCs) required for other NBMA media types (, section 4.6). Neighbor state information MAY be added tokept in the Potential Routers List (see Section 7.3.1).Conceptual Neighbor Cache (, section 5.1). 5.3 Link Layer Address Options With reference to (, section 5.2), when the [NTL] and [STL] fields in an ISATAP link layer address option encode 0, the [NBMA Number] field encodes a 4-octet IPv4 address. 5.4 Multicast and Anycast As for any IPv6 interface, anISATAP interface is required tointerfaces recognize certain IPv6 multicast and anycasta node's required addresses as specified in (, section 2.8). Mechanisms for sending multicast and anycast packetsmulticast/anycast emulation on ISATAP links (e.g., )adaptations of MLD , PIM-SM , MARS , etc.) are left assubject for future work.companion document(s). 6. Automatic Tunneling The common tunneling mechanisms specified in (, sections 2 and 3) are used, with the following noted considerations for ISATAP: 6.1 Dual IP Layer Operation ISATAP uses the same specification found in (, section 2). That is, ISATAP nodes provide complete IPv4 and IPv6 implementations and are able to send and receive both IPv4 and IPv6 packets. Address configuration and DNS considerations are the same as (, sections 2.1 through 2.3). 6.2 EncapsulationEncapsulation/Decapsulation The specificationspecifications in (, section 3.1) issections 3.1 and 3.6) are used. Additionally, the IPv6 next-hop address for packets sentencapsulated on an ISATAP link MUST be an ISATAP address; other packets are discarded and an ICMPv6 destination unreachable indication with code 3 (Address Unreachable) (, section 3.1) is returned to the source. 6.3 Tunnel MTU and Fragmentation ISATAP automatic tunnel interfaces 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 for ISATAP interfaces: o Nearly all IPv4 nodes connect to physical links with MTUs of 1500 bytes or larger (e.g., Ethernet) o Sub-IPv4 layer encapsulations (e.g., VPN) may occur on some paths o Commonly-deployed VPN interfaces use an MTU of 1400 bytes To maximize efficiency and minimize IPv4 fragmentation for the predominant deployment case, the ISATAP interfaces that do not use a dynamic MTU discovery mechanism SHOULD set LinkMTU (,interface MTU, or "LinkMTU" (see: , Section 6.3.2 )), SHOULD be set to no more than 1380 bytes (1400 minus 20 bytes for IPv4 encapsulation). LinkMTU MAY be set to larger values on ISATAP interfaces that usewhen a dynamic link layer MTU discovery mechanism.mechanism is used or when a static MTU assignment is used and additional fragmentation in the site's IPv4 network is deemed acceptable. See Appendix C provides non-normative considerationsfor non-normative ISATAP interface MTU considerations. When a dynamic MTU discovery. Thediscovery mechanism is not used, the ISATAP link layer encapsulates IPv6 packets of size 1380 or smallerwith the Don't Fragment (DF) bit not set in the encapsualting IPv4 header. 6.4 Handling IPv4 ICMP Errors IPv4 ICMP errors and ARP failures are processed as link error notifications. 6.5 Local-Use IPv6 Unicast Addresses The specification in (, section 3.7) is not used. Instead, local use IPv6 unicast addresses are formed as specified in Section 5.1. 6.6 Ingress Filtering The specification in (, section 3.9) is used. In particular, ISATAP nodes that forward decapsulated packets MUST be configured with a list ofverify the tunnel source IPv4address prefixes that areis acceptable. 7. Neighbor Discovery The specification in (, section 3.8) applies only to configured tunnels. 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 for ISATAP links are not specified by ; instead, they are specified in the following sections of this document. 7.1 Address Resolution and Neighbor Unreachability Detection 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. Following static address resolution, hosts SHOULD perform an initial reachability confirmation by sending unicastNeighbor Solicitations (NSs)Solicitation (NS) message(s) and receiving a Neighbor Advertisement (NA) message using the mechanisms specified in (, sections 7.2.2-7.2.8).section 7.2.). When the ISATAP interface provides a multicast emulation mechanism (see: Section 5.4) solicitations are sent to the solicited-node multicast address corresponding to the target address. Otherwise, the solicitation is sent to the target's unicast address. Hosts SHOULD additionally perform Neighbor Unreachability Detection (NUD) as specified in (, section 7.3). 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). 7.2 Duplicate Address Detection Duplicate Address Detection (, section 5.4) is not required for ISATAP addresses, since duplicate address detection is assumed already performed for the IPv4 addresses from which they derive. 7.3 Router and Prefix Discovery Since ISATAP nodes will typically not receive unsolicited multicast Router Advertisements, unicastThe following sections describe mechanisms are required as specified below:to support the router and prefix discovery process (, section 6) on ISATAP links: 7.3.1 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 a new conceptual data structure "Potential Router List" (PRL) and the following new configuration variable: ResolveIntervalPrlRefreshInterval Time in seconds between name service resolutions. Default and suggested minimum: 1hrsuccessive refreshments of the PRL after initialization. SHOULD be no less than 3,600 seconds. Default: 3,600 seconds A Potential Router List (PRL)PRL is associated with every ISATAP link. Each entry in the PRL ("PRL(i)") has an IPv4 address and an associated timer. The IPv4 address("V4ADDR(i)") that represents an advertising ISATAP interface,interface and is used to construct the link-local ISATAP address for that interface.an associated timer ("TIMER(i)"). The following sections specify theprocess for initializing and refreshing the PRL:PRL is described below: When a node enables an ISATAP link, it discovers IPv4 addresses forinitializes the PRL.PRL with IPv4 addresses. The addresses MAY be established bydiscovered via a DHCPv4  option for ISATAP (option code TBD), manual configuration, or an unspecified alternate method (e.g., DHCPv4 vendor-specific option). When no other mechanisms are available, a DNS fully-qualified domain name (FQDN)  established by an out-of-band method (e.g., DHCPv4, manual configuration, etc.) MAY be used. The FQDN is resolved into IPv4 addresses for the PRL through a static host file, a site-specific name service, querying a DNS server within the site, or an unspecified alternate method. The following notes apply: 1. Site administrators maintain a list of IPv4 addresses representing advertising ISATAP interfaces and make them available via one or more of the mechanisms described above. 2.There are no mandatory rules for the selection of a FQDN, but manual configuration MUST be supported. 3. After initialization, nodes periodically re-initialize the PRL (e.g., after ResolveInterval).When DNS is used, client resolvers use the IPv4 transport. After initialization, nodes periodically refresh the PRL (i.e., using one or more of the methods described above) after PrlRefreshInterval. 7.3.2 Validation of Router Advertisements Messages The specification in (, section 6.1.2) is used. Additionally, received RA messages that contain Prefix Information options and/or encode non-zero values in the Cur Hop Limit, Router Lifetime, Reachable Time, or Retrans Timer fields (see: , section 4.2) MUST satisfy the following validity check for ISATAP: o the network-layer (IPv6) source address is an ISATAP address and embeds an IPv4 address from the PRLV4ADDR(i) for some PRL(i) 7.3.3 Router Specification Routers with advertising ISATAP interfaces behave the same as described in (, section 6.2). AdvertisingAs permitted by (, section 6.2.6), advertising ISATAP interfaces SHOULD send unicast RA messages to a node's unicast address, as permitted by (, section 6.2.6).soliciting host's address when the solicitation's source address is not the unspecified address. 7.3.4 Host Specification When no unsolicited RA messages containing prefix information options and/or non-zero router lifetime values are received, hosts MAY send Router Solicitation (RS) messages using the specification in Section 18.104.22.168. RA messages (whether solicited or unsolicited) are processed using the specification in Section 22.214.171.124. 126.96.36.199 Sending Router Solicitations All entries in the PRLPRL(i)'s are assumed to represent active advertising ISATAP interfaces within the site, i.e., the PRL provides trust basis only; not reachability detection. Hosts periodically solicit information from one or more entries in the PRL ("PRL(i)")PRL(i) by sending unicastRouter Solicitation (RS) messages using PRL(i)'s IPv4 address ("V4ADDR_PRL(i)") and associated timer ("TIMER(i)").messages. The manner of selecting a PRL(i) for solicitation and/or deprecating a previously-selected PRL(i) is outside the scope of this specification. Hosts add the following variable to support the solicitation process: MinRouterSolicitInterval Minimum time in seconds between sending Router Solicitations. Default and suggested minimum: 15min. When a PRL(i) is selected,successive solicitations of the host sets TIMER(i) to MinRouterSolicitInterval and initiates solicitation following a short delay.same advertising ISATAP interface. SHOULD be no less than 900 seconds. Default: 900 seconds Solicitation consists of sending RS messages tousing the ISATAPinterface's link-local address constructed from V4ADDR_PRL(i), i.e.,unicast addresses as the source address. When the ISATAP interface provides a multicast emulation mechanism (see: Section 5.4), RS messages are sent to the All-Routers multicast address. Otherwise, they are sent to 'FE80::0:5EFE:V4ADDR_PRL(i)' instead of 'All-Routers-multicast'. Theythe link-local ISATAP address constructed from V4ADDR(i) for some PRL(i) selected for solicitation. The RS messages are otherwise sent exactly as in (, section 6.3.7). 188.8.131.52 Processing Router Advertisements Hosts process received RA messages exactly as in (, section 6.3.4) and (, section 5.5.3) except that, when an RA message contains an MTU option, hosts SHOULD NOT copy the option's value into the ISATAP interface LinkMTU. Instead, when the ISATAP link layer implements a per-neighbor path MTU cache, hosts SHOULD copy the MTU option's value into the cache entry5.5.3). (But, see Appendix C for non-normative considerations for the router that sent theRA message (see: Appendix C).messages containing MTU options.) When the network-layersource address in anof the RA message is an ISATAP address that embeds V4ADDR_PRL(i)V4ADDR(i) for some PRL(i) selected for solicitation, hosts additionally reset TIMER(i). Let "MIN_LIFETIME" be the minimum value in the router lifetime or valid lifetime of any prefixes advertisedthe lifetime(s) encoded in options included in the RA message. Then, TIMER(i) is reset to: MAX((0.5 * MIN_LIFETIME), MinRouterSolicitInterval) 8. Deployment Considerations 8.1 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 specificationHost Specification (Section 7.3.4) 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 both a native IPv6 and ISATAP interface over the same physical link. 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 advertising ISATAP interface(s) SHOULD be added (either automatically or manually) to the site's address records for advertising ISATAP interfaces. 8.2 Site Administration Considerations The following considerations are noted for sites that deploy ISATAP: o ISATAP links are administratively defined by a set of advertising ISATAP interfaces and set of nodes which discover those interface addresses. Thus, ISATAP links are defined by administrative (not physical) boundaries. o Hosts and routers that use ISATAP can be deployed in an ad-hoc fashion. In particular, hosts can be deployed with little/no advanced knowledge of existing routers, and routers can be deployed with no reconfiguration requirements for hosts. o Site administrators maintain a list of IPv4 addresses representing advertising ISATAP interfaces and make them available via one or more of the mechanisms described in Section 7.3.1. ISATAP nodes use this list to initialize and periodically refresh the entries on thePRL. Responsible site administration can reduce the control traffic. At a minimum, administrators SHOULD ensure that dynamically advertised information for the site's PRL is well maintained. 9. IANA Considerations A DHCPv4 option code for ISATAP (TBD)  may be requested in the event that this document (oror a derivative thereof is moved to standards track. Modifications to the IANA "ethernet-numbers" registry (e.g., based on text in Appendix B) may be requested in the event that this document or a derivative thereof)thereof is moved to standards track. 10. Security considerations ISATAP site border routers and firewalls MUST implement IPv6 ingress filtering and MUST NOT allowforward packets with site-local source and/or destination addresses (i.e., addresses with prefix FEC0::/10) to enter or leaveoutside of the site.site . In addition to possible attacks against IPv6, security attacks against IPv4 must also be considered. In particular, border routers and firewalls MUST implement IPv4 ingress filtering and ip-protocol-41 filtering. Even with IPv4 and IPv6 ingress filtering, reflection attacks can originate from compromised nodes within an ISATAP site that spoof IPv6 source addresses. Security mechanisms for reflection attack mitigation (e.g., , ,, , etc.) SHOULD be used in routers with advertising ISATAP interfaces. At a minimum, ISATAP site border gateways MUST log potential source address spoofing cases. (RFC 2461 ,IPv6 Neighbor Discovery trust models and threats  apply also to ISATAP. However, (, section 6.1.2) implies4.4.) shows that nodes trust received Router Advertisement (RA) messages from on-link routers, as indicated by a valuemost of 255these threats are mitigated in corporate networks that implement site security mechanisms, i.e., the IPv6 'hop-limit' field. ISATAP links require an additional validation checkapplicability space for received RA messages (see: Section 7.3.2).ISATAP. ISATAP addresses do 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 is especially true when private address allocations  are used.) 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.  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.  Hinden, R. and S. Deering, "IP Version 6 Addressing Architecture", draft-ietf-ipngwg-addr-arch-v3-11 (work in progress), October 2002.  Armitage, G., Schulter, P., Jork, M. and G. Harter, "IPv6 over Non-Broadcast Multiple Access (NBMA) networks", RFC 2491, January 1999.  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.  Thomson, S. and T. Narten, "IPv6 Stateless Address Autoconfiguration", RFC 2462, December 1998. Informative References  Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, March 1997.  Savola, P., "Security Considerations for 6to4", draft-savola-ngtrans-6to4-security-01 (work in progress), March 2002.  Bellovin, S., Leech, M. and T. Taylor, "ICMP Traceback Messages", draft-ietf-itrace-03 (work in progress), January 2003.  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.  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. Informative References Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G. and E. Lear, "Address Allocation for Private Internets", BCP 5, RFC 1918, February 1996.  Carpenter, B. and K. Moore, "Connection of IPv6 Domains via IPv4 Clouds", RFC 3056, February 2001.  Deering, S., Fenner, W. and B. Haberman, "Multicast Listener Discovery (MLD) for IPv6", RFC 2710, October 1999.  Estrin, D., Farinacci, D., Helmy, A., Thaler, D., Deering, S., Handley, M. and V. Jacobson, "Protocol Independent Multicast-Sparse Mode (PIM-SM): Protocol Specification", RFC 2362, June 1998.  Armitage, G., "Support for Multicast over 6to4UNI 3.0/3.1 based ATM Networks", draft-ietf-ngtrans-6to4-multicast-01 (work in progress), July 2002. RFC 2022, November 1996.  Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, March 1997.  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.  Hinden, R., "IPv6 Globally Unique Site-Local Addresses", draft-hinden-ipv6-global-site-local-00 (work in progress), December 2002.  Savola, P., "Security Considerations for 6to4", draft-savola-ngtrans-6to4-security-01 (work in progress), March 2002.  Bellovin, S., Leech, M. and T. Taylor, "ICMP Traceback Messages", draft-ietf-itrace-03 (work in progress), January 2003.  Nikander, P., "IPv6 Neighbor Discovery trust models and threats", draft-ietf-send-psreq-01 (work in progress), January 2003.  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.  Jacobson, V., Braden, B. and D. Borman, "TCP ExtensionsR., "Requirements for High Performance", RFC 1323,Internet Hosts - Communication Layers", STD 3, RFC 1122, October 1989.  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.  Lahey, K., "TCP Problems with Path MTU Discovery", RFC 2923, September 2000.  McCann, J., Deering, S. and J. Mogul, "Path MTU Discovery for IP version 6", RFC 1981, August 1996.  Jacobson, V., Braden, B. and D. Borman, "TCP Extensions for High Performance", RFC 1323, May 1992.  Templin, F., "Neighbor Affiliation Protocol for IPv6-over-(foo)-over-IPv4", draft-templin-v6v4-ndisc-01 (work in progress), November 2002. Authors' Addresses Fred L. Templin Nokia 313 Fairchild Drive Mountain View, CA 94110 US Phone: +1 650 625 2331 EMail: firstname.lastname@example.org Tim Gleeson Cisco Systems K.K. Shinjuku Mitsu Building 2-1-1 Nishishinjuku, Shinjuku-ku Tokyo 163-0409 Japan EMail: email@example.com Mohit Talwar Microsoft Corporation One Microsoft Way Redmond, WA> 98052-6399 US Phone: +1 425 705 3131 EMail: firstname.lastname@example.org Dave Thaler Microsoft Corporation One Microsoft Way Redmond, WA 98052-6399 US Phone: +1 425 703 8835 EMail: email@example.com Appendix A. Major Changes changes from version 11 to version 12: o Added comments from co-authors o Revised PRL initialization o Updated MTU section changes from version 10 to version 11: o Added multicast/anycast subsection o Revised PRL initialization o Updated neighbor discovery, security consideration sections o Updated MTU section 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 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" andin "ISATAP format" and the next four octets embed an IPv4 address encoded in network byte order. Appendix C. ISATAP Interface MTU Considerations ISATAP encapsulators and decapsulators are IPv6 neighbors that may be separated by multiple link layer (IPv4) forwarding hops. Thus, the path MTU of the underlying IPv4 network may determine the uni- directional IPv6 per-neighbor MTU from the encapsulator to the decapsulator. (Note that this constitutes the MTU of only one hop in what may be a multiple-hop IPv6 path.) When the encapsulator's ISATAP interface configures a large LinkMTU value (see: Section 6.3), special considerations apply as described in the following non-normative sections: C.1 Stateless (Static) MTU Assignment Nodes that connect to the Internet should be able to reassemble and/ or discard IPv4 packets up to 64KB in length when the DF bit is not set in the encapsulating IPv4 header. Nodes that cannot reassemble/ discard maximum-length IPv4 packets are vulnerable to buffer overrun attacks. This issue may be obviated for nodes that are accessed only within a site (i.e., do not connect directly to the Internet) since site border gateways, etc. can filter and discard fragments of large packets before they reach constrained node(s). When the ISATAP encapsulator does not implement a dynamic link layer mechanism to determine per-neighbor MTUs, all IPv6 packets are encapsulated with the DF bit not set in the IPv4 header. Additionally, LinkMTU may be set to a value that is no more than the smallest Effective MTU to Receive (EMTU_R) (see: RFC 1122 , section 3.3.2) for all potential decapsulators in the site. The value chosen for LinkMTU must be at least 1280 bytes (the minimum IPv6 MTU) and such that the potential worst-case level of fragmentation in the underlying IPv4 network is deemed "acceptable" by the site's standards. For example, when all decapsulators in the site are known to have an EMTU_R of 10KB and the site's IPv4 routers are optimized for IPv4 fragmentation, encapsulators may be able to use LinkMTU values as large as 10KB (minus 20 bytes for IPv4 encapsulation). Conversely, when IPv4 fragmentation causes performance degradation along some paths, LinkMTU should be set to a smaller value. Nodes that use a static MTU assignment SHOULD copy the next four octets embed an IPv4 address encodedvalue in network byte order. Appendix C. Dynamican MTU Discoveryoption received in any Router Advertisement message into LinkMTU for the ISATAP encapsulators and decapsulators are IPv6 neighbors that may be separated by multiple link layer (IPv4) forwarding hops. When an encapsulator'sinterface configures a LinkMTUas specified in (, Section 6.3.2) value larger than 1380 bytes,section 6.3.4). C.2 Stateful (Dynamic) MTU Determination When the encapsulator implements a dynamic MTU determination mechanism it keeps a link layer (IPv4) mechanism is required to discovercache of per-neighbor MTU values (e.g., as ancillary data in the IPv6 neighbor cache, in the IPv4 path MTUs. The following text gives non-normative considerations for dynamicMTU discovery.discovery cache, etc.). IPv4 path MTU discovery  uses ICMPv4 "fragmentation needed" messages, but these generally do not provide enough information for stateless translation to 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 may be inadequate and can result in black holes that are difficult to diagnose .. Alternate methods for determining per-neighbor MTUs should be used when RFC 1191 path MTU discovery is deemed inadequate. In any method,these methods, the encapsulator uses periodic and/or on-demand probing of the IPv4 path to the decapsulator.decapsulator to initialize and update cache entries. The following three probing methods (among others) are possible: 1. Encapsulator-driven - the encapsulator periodically sends probe packets with the DF bit set in the IPv4 header and waits for a positive acknowledgement from the decapsulator that the probe was received 2. Decapsulator-driven - the encapsulator sends all packets with the DF bit NOT set in the IPv4 header unless and until the decapsulator sends a "Fragmentation Experienced" indication(s) 3. Hybrid - the encapsulator and decapsulator engage in a dialogue and use "intelligent" probing to monitor the path MTU These methods are discussed in detail in the following subsections: C.1C.2.1 Encapsulator-driven Method In this method, the encapsulator sets the DF bit in the IPv4 header of probe packets. Probe packets may be sent either when the encapsulator's link layer forwards a large data packet to the decapsulator (i.e., on-demand) or when the path MTU for the decapsulator has not been verified for some time (i.e., periodic). IPv6 Neighbor Solicitation (NS) or ICMPv6 ECHO_REQUEST packets with padding bytes added could be used for this purpose, since successful delivery results in a positive acknowledgement that the probe succeeded vis-a-vis a response from the decapsulator. While the decapsulator is being probed,probing, the encapsulator maintains a queue of packets that have the decapsulator as the IPv6 next-hop address. The queue should be large enough to buffer the (delay*bandwidth) product for the round-trip time (RTT) to the decapsulator.If the probe succeeds, packets in the queue that are no larger than the probe size are sent to the decapsulator. If the probe fails, packets that are larger than the last known successful probe are dropped and an ICMPv6 "packet too big" message returned to the sender .. The queue should be large enough to buffer the (delay*bandwidth) product for the round-trip time to the decapsulator. When smaller queues are used, loss of packets that are too big for the yet-to-be-determined path MTU may occur with no ICMPv6 "packet too big" message returned. Such loss may occur only in rare instances, but may result in unpredictable behavior in senders that base their adaptation solely on ICMPv6 "packet too big" messages. This method has the advantage that the decapsulator need not implement any special mechanisms, since standard IPv6 request/ response mechanisms are used. Additionally, the encapsulator is assured that any packets that are too large for the decapsulator to receive will be dropped by the network. Disadvantages for this method include the fact that probe packets do not carry data and thus consume network resources. Additionally, packetqueues may become large on Long, Fat Networks (LFNs) (see: RFC 1323 ). C.2). C.2.2 Decapsulator-driven Method In this method, the encapsulator sends all packets with the DF bit NOT set in the IPv4 header with the expectation that the decapsulator will send a "Fragmentation Experienced" indication if the IPv4 network fragments packets. In other words, the decapsulator simply sends all packets that are no larger than LinkMTU unless and until it receives "Fragmentation Experienced" messages from the decapsulator. The decapsulator can use IPv6 Router Advertisement (RA) messages with an MTU option as the means for both reporting fragmentation and informing the encapsulator of a new MTU value to use. This method has the distinct advantagesadvantage that the data packets themselves are used as probes and no queueing ondata packets themselves are used as probes and no queuing on the encapsulator is necessary. (When large data packets for probing are not available, smaller data packets can be null-padded to the desired probe size by artificially inflating the length field in the IPv4 header; leaving the encapsulatorIPv6 length unchanged.) An additional advantage is necessary. Additionally,that fewer packets will be lost since the decapsulator will quite often be able to reassemble packets fragmented by the network. The primary disadvantage for this method is that, using the current specifications, the encapsulator has no way of knowing whether a particular decapsulator implements the "fragmentation experienced" signallingsignaling capability. However, the "fragmentation experienced" indication can be trivially implemented in an application on the decapsulator that uses the Berkeley Packet Filter (aka, libpcap) to listen for fragmented packets from encapsulators. When fragmented packets arrive, the applicationdecapsulator sends IPv6 RA messages with an MTU option to inform the encapsulator that fragmentation has been experienced and a new value for the neighbor's MTU should be used. The applicationdecapsulator additionally sends ICMPv6 "packet too big" messages to the original source when a fragmented packet is not correctly reassembled. This function need not be built into the decapsulator's operating system and can be added as an after-market feature. Finally, simply adding an extra bit in the RAa neighbor discovery message header (, section 4.2)would provide a means for the decapsulator to inform the encapsulator that dynamic MTU discovery is supported. C.3C.2.3 Hybrid Method In this method, the encapsulator and decapsulator engage in a "neighbor affiliation" protocol to negotiate link-layer parameters such as MTU. (See:  for an example of such an approach.) This approach has the advantage that bi-directional links are used and both ends of the link have unambiguous knowledge that the other end implements the protocol. However, the signallingsignaling protocol between the endpoints is complicated and additional state is required in both the encapsulator and decapsultor. C.4 Summary In summary, the decapsulator-based approach in Appendix C.2 has distinct efficiency advantages over methods that engage the encapsulator. Additionally, probing methods which use IPv4 encapsulation with the DF bit NOT set may use LinkMTU values for the ISATAP link that exceed the underlying link MTU size. Experimental verification is called for which may eventually resultThe hybrid method seems best suited to implementation in a recommendation for proposed standard. C.5reliable transport-layer protocol rather than at the network/link layer. C.2.4 Additional Notes o In all dynamic methods, some packet loss due to link/buffer restrictions may occur with no ICMPv6 "packet too big" message returned to the sender. Unenlightened senders will interpret such loss as loss due to congestion, which may result in longer convergence to the actual path MTU. Enlightened senders will interpret the loss as lossdue to link/buffer restrictions and immediately reduce their MTU estimate. o In all dynamic methods, when a Router Advertisement (RA) message includes an MTU option hosts SHOULD NOT copy the option's value into LinkMTU for the ISATAP interface. Instead, when the ISATAP interface uses a per-neighbor path MTU cache, hosts SHOULD copy the MTU option's value into the cache entry for the neighbor that sent the RA message. This leaves an ambiguous interpretation for processing received RA messages which could be eliminated if  were modified to allow Neighbor Advertisement (NA) messages to carry MTU options. o In all methods, a "minimum MTU" must be supported by all nodes for multicast (i.e., even when multicast is emulated on the NBMA IPv4 network.) The mechanisms described above speak only to the unicast case for MTU determination. o To avoid denial-of-service attacks that would cause superfluous probing based on counting down/up by small increments, plateau tables (e.g., ,, section 7) should be used when the actual MTU value is indeterminant. o 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. o Nodes that connect to the Internet are expected to be able to reassemble or discard IPv4 packets up to 64KB in length when the DF bit is not set in the encapsulating IPv4 header. Nodes that cannot reassemble or discard maximum-length IPv4 packets are vulnerable to attacks such as the "ping-of-death".Intellectual Property Statement The IETF takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; neither does it represent that it has made any effort to identify any such rights. 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