Network Working Group F. Templin Internet-Draft Nokia Expires: July 18, 2003 T. Gleeson Cisco Systems K.K. M. Talwar D. Thaler Microsoft Corporation January 17, 2003 Intra-Site Automatic Tunnel Addressing Protocol (ISATAP) draft-ietf-ngtrans-isatap-11.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, 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. Templin, et al. Expires July 18, 2003 [Page 1] Internet-Draft ISATAP January 2003 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 . . . . . . . . . . . . . . . . . . . . . . 14 A. Major Changes . . . . . . . . . . . . . . . . . . . . . . . . 15 B. Rationale for Interface Identifier Construction . . . . . . . 17 C. Dynamic MTU Discovery . . . . . . . . . . . . . . . . . . . . 18 Intellectual Property and Copyright Statements . . . . . . . . 22 Templin, et al. Expires July 18, 2003 [Page 2] Internet-Draft ISATAP January 2003 1. Introduction This document presents a simple approach called the Intra-Site Automatic Tunnel Addressing Protocol (ISATAP) that enables incremental deployment of IPv6 [1] within IPv4 [2] 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 [18] are used) o compatible with other NGTRANS mechanisms (e.g., 6to4 [19]) 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 [3]. 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 Templin, et al. Expires July 18, 2003 [Page 3] Internet-Draft ISATAP January 2003 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 [1] applies to this document. The following additional terms are defined: link, on-link, off-link: same definitions as ([4], 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 ([4], section 6.2.2). ISATAP address: an on-link address on an ISATAP interface and with an interface identifier constructed as specified in Section 5.2 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 ([5], 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. Templin, et al. Expires July 18, 2003 [Page 4] Internet-Draft ISATAP January 2003 With reference to ([5], 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 An 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. At least one link-layer address per advertising ISATAP interface SHOULD be added to the Potential Routers List (see Section 7.3.1). 5.3 Link Layer Address Options With reference to ([6], 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, an ISATAP interface is required to recognize certain IPv6 multicast and anycast addresses ([5], section 2.8). Mechanisms for sending multicast and anycast packets (e.g., [20]) are left as future work. 6. Automatic Tunneling The common tunneling mechanisms specified in ([7], 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 ([7], 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 ([7], sections 2.1 through 2.3). Templin, et al. Expires July 18, 2003 [Page 5] Internet-Draft ISATAP January 2003 6.2 Encapsulation The specification in ([7], 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 and an ICMPv6 destination unreachable indication with code 3 (Address Unreachable) [8] 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 ([1], 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, ISATAP interfaces that do not use a dynamic MTU discovery mechanism SHOULD set LinkMTU ([4], Section 6.3.2 ) to no more than 1380 bytes (1400 minus 20 bytes for IPv4 encapsulation). LinkMTU MAY be set to larger values on ISATAP interfaces that use a dynamic MTU discovery mechanism. Appendix C provides non-normative considerations for dynamic MTU discovery. The ISATAP link layer encapsulates packets of size 1380 or smaller with 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 ([7], 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 ([7], section 3.9) is used. In particular, ISATAP nodes that forward decapsulated packets MUST be configured Templin, et al. Expires July 18, 2003 [Page 6] Internet-Draft ISATAP January 2003 with a list of source IPv4 address prefixes that are acceptable. 7. Neighbor Discovery RFC 2461 [4] 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 [4]. Address resolution and the mechanisms for delivering Router Solicitations and Advertisements for ISATAP links are not specified by [4]; instead, they are specified in 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 unicast Neighbor Solicitations (NSs) and receiving a Neighbor Advertisement using the mechanisms specified in ([4], sections 7.2.2-7.2.8). Hosts SHOULD additionally perform Neighbor Unreachability Detection (NUD) as specified in ([4], 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 ([4], section 7.2.4). 7.2 Duplicate Address Detection Duplicate Address Detection ([9], 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, unicast mechanisms are required as specified Templin, et al. Expires July 18, 2003 [Page 7] Internet-Draft ISATAP January 2003 below: 7.3.1 Conceptual Data Structures ISATAP nodes use the conceptual data structures Prefix List and Default Router List exactly as in ([4], section 5.1). ISATAP links add a new conceptual data structure "Potential Router List" and the following new configuration variable: ResolveInterval Time between name service resolutions. Default and suggested minimum: 1hr A Potential Router List (PRL) is associated with every ISATAP link. Each entry in the PRL has an IPv4 address and an associated timer. The IPv4 address represents an advertising ISATAP interface, and is used to construct the link-local ISATAP address for that interface. The following sections specify the process for initializing the PRL: When a node enables an ISATAP link, it discovers IPv4 addresses for the PRL. The addresses MAY be established by a DHCPv4 [10] 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) [21] established by an out-of-band method (e.g., DHCPv4, manual configuration, etc.) MAY be used. The FQDN is resolved into IPv4 addresses 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. 7.3.2 Validation of Router Advertisements Messages The specification in ([4], section 6.1.2) is used. Additionally, received RA messages that contain Prefix Information Templin, et al. Expires July 18, 2003 [Page 8] Internet-Draft ISATAP January 2003 options and/or encode non-zero values in the Cur Hop Limit, Router Lifetime, Reachable Time, or Retrans Timer fields (see: [4], 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 PRL 7.3.3 Router Specification Routers with advertising ISATAP interfaces behave the same as described in ([4], section 6.2). Advertising ISATAP interfaces send RA messages to a node's unicast address, as permitted by ([4], section 6.2.6). 7.3.4 Host Specification 7.3.4.1 Sending Router Solicitations All entries in the PRL 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)") by sending unicast Router Solicitation (RS) messages using PRL(i)'s IPv4 address ("V4ADDR_PRL(i)") and associated timer ("TIMER(i)"). 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 between sending Router Solicitations. Default and suggested minimum: 15min. When a PRL(i) is selected, the host sets TIMER(i) to MinRouterSolicitInterval and initiates solicitation following a short delay. Solicitation consists of sending RS messages to the ISATAP link-local address constructed from V4ADDR_PRL(i), i.e., they are sent to 'FE80::0:5EFE:V4ADDR_PRL(i)' instead of 'All-Routers-multicast'. They are otherwise sent exactly as in ([4], section 6.3.7). 7.3.4.2 Processing Router Advertisements Hosts process received RA messages exactly as in ([4], section 6.3.4) and ([9], 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 Templin, et al. Expires July 18, 2003 [Page 9] Internet-Draft ISATAP January 2003 per-neighbor path MTU cache, hosts SHOULD copy the MTU option's value into the cache entry for the router that sent the RA message (see: Appendix C). When the network-layer source address in an RA message is an ISATAP address that embeds V4ADDR_PRL(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 advertised 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 specification and allow existing ISATAP address configurations to expire as specified in ([4], section 5.3) and ([9], 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 Templin, et al. Expires July 18, 2003 [Page 10] Internet-Draft ISATAP January 2003 fashion. In particular, hosts can be deployed with little/no advanced knowledge of existing routers, and routers can deployed with no reconfiguration requirements for hosts. o ISATAP nodes periodically refresh the entries on the PRL. 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) [22] may be requested in the event that this document (or a derivative thereof) is moved to standards track. 10. Security considerations ISATAP site border routers and firewalls MUST implement IPv6 ingress filtering and MUST NOT allow packets with site-local source and/or destination addresses (i.e., addresses with prefix FEC0::/10) to enter or leave the 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 nodes within an ISATAP site that spoof IPv6 source addresses. Security mechanisms for reflection attack mitigation (e.g., [11], [12], 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 [4], section 6.1.2) implies that nodes trust received Router Advertisement (RA) messages from on-link routers, as indicated by a value of 255 in the IPv6 'hop-limit' field. ISATAP links require an additional validation check for received RA messages (see: Section 7.3.2). ISATAP addresses do not support privacy extensions for stateless address autoconfiguration [23]. 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 [18] are used.) Templin, et al. Expires July 18, 2003 [Page 11] Internet-Draft ISATAP January 2003 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 [24] 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 [1] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998. [2] Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981. [3] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [4] Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery for IP Version 6 (IPv6)", RFC 2461, December 1998. [5] Hinden, R. and S. Deering, "IP Version 6 Addressing Architecture", draft-ietf-ipngwg-addr-arch-v3-11 (work in progress), October 2002. [6] Armitage, G., Schulter, P., Jork, M. and G. Harter, "IPv6 over Non-Broadcast Multiple Access (NBMA) networks", RFC 2491, January 1999. [7] Gilligan, R. and E. Nordmark, "Basic Transition Mechanisms for Templin, et al. Expires July 18, 2003 [Page 12] Internet-Draft ISATAP January 2003 IPv6 Hosts and Routers", draft-ietf-ngtrans-mech-v2-01 (work in progress), November 2002. [8] Conta, A. and S. Deering, "Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification", RFC 2463, December 1998. [9] Thomson, S. and T. Narten, "IPv6 Stateless Address Autoconfiguration", RFC 2462, December 1998. [10] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, March 1997. [11] Savola, P., "Security Considerations for 6to4", draft-savola-ngtrans-6to4-security-01 (work in progress), March 2002. [12] Bellovin, S., Leech, M. and T. Taylor, "ICMP Traceback Messages", draft-ietf-itrace-03 (work in progress), January 2003. [13] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, November 1990. [14] Postel, J., "Internet Control Message Protocol", STD 5, RFC 792, September 1981. [15] Baker, F., "Requirements for IP Version 4 Routers", RFC 1812, June 1995. [16] McCann, J., Deering, S. and J. Mogul, "Path MTU Discovery for IP version 6", RFC 1981, August 1996. [17] Braden, R., "Requirements for Internet Hosts - Communication Layers", STD 3, RFC 1122, October 1989. Informative References [18] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G. and E. Lear, "Address Allocation for Private Internets", BCP 5, RFC 1918, February 1996. [19] Carpenter, B. and K. Moore, "Connection of IPv6 Domains via IPv4 Clouds", RFC 3056, February 2001. [20] Thaler, D., "Support for Multicast over 6to4 Networks", draft-ietf-ngtrans-6to4-multicast-01 (work in progress), July 2002. Templin, et al. Expires July 18, 2003 [Page 13] Internet-Draft ISATAP January 2003 [21] Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, November 1987. [22] Droms, R., "Procedures and IANA Guidelines for Definition of New DHCP Options and Message Types", BCP 43, RFC 2939, September 2000. [23] Narten, T. and R. Draves, "Privacy Extensions for Stateless Address Autoconfiguration in IPv6", RFC 3041, January 2001. [24] Nguyen, Q., "http://irl.cs.ucla.edu/vet/report.ps", spring 1998. [25] Lahey, K., "TCP Problems with Path MTU Discovery", RFC 2923, September 2000. [26] Jacobson, V., Braden, B. and D. Borman, "TCP Extensions for High Performance", RFC 1323, May 1992. [27] 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: ftemplin@iprg.nokia.com Tim Gleeson Cisco Systems K.K. Shinjuku Mitsu Building 2-1-1 Nishishinjuku, Shinjuku-ku Tokyo 163-0409 Japan EMail: tgleeson@cisco.com Templin, et al. Expires July 18, 2003 [Page 14] Internet-Draft ISATAP January 2003 Mohit Talwar Microsoft Corporation One Microsoft Way Redmond, WA> 98052-6399 US Phone: +1 425 705 3131 EMail: mohitt@microsoft.com Dave Thaler Microsoft Corporation One Microsoft Way Redmond, WA 98052-6399 US Phone: +1 425 703 8835 EMail: dthaler@microsoft.com Appendix A. Major Changes 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 Templin, et al. Expires July 18, 2003 [Page 15] Internet-Draft ISATAP January 2003 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 Templin, et al. Expires July 18, 2003 [Page 16] Internet-Draft ISATAP January 2003 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 Templin, et al. Expires July 18, 2003 [Page 17] Internet-Draft ISATAP January 2003 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 MTU Discovery ISATAP encapsulators and decapsulators are IPv6 neighbors that may be separated by multiple link layer (IPv4) forwarding hops. When an encapsulator's interface configures a LinkMTU ([4], Section 6.3.2) value larger than 1380 bytes, a dynamic link layer (IPv4) mechanism is required to discover per-neighbor path MTUs. The following text gives non-normative considerations for dynamic MTU discovery. IPv4 path MTU discovery [13] 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 [14] and RFC 1812 [15], section 4.3.2.3). 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 [25]. Alternate methods for determining per-neighbor MTUs should be used when RFC 1191 path MTU discovery is deemed inadequate. In any method, the encapsulator uses periodic and/or on-demand probing of the IPv4 path to the decapsulator. The following three methods 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.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 Templin, et al. Expires July 18, 2003 [Page 18] Internet-Draft ISATAP January 2003 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, 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 larger than the last known successful probe are dropped and an ICMPv6 "packet too big" message returned to the sender [16]. 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, packet queues may become large on Long, Fat Networks (LFNs) (see: RFC 1323 [26]). C.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 advantages that the data packets themselves are used as probes and no queueing on the encapsulator is necessary. Additionally, fewer packets will be lost since the decapsulator will quite often be able to reassemble packets fragmented by the network. The primary disadvantage is that, using the current specifications, the encapsulator has no way of knowing whether a particular decapsulator implements the "fragmentation experienced" signalling capability. However, the "fragmentation experienced" indication can be trivially implemented in an application on the decapsulator that uses the Berkeley Packet Filter Templin, et al. Expires July 18, 2003 [Page 19] Internet-Draft ISATAP January 2003 (aka, libpcap) to listen for fragmented packets from encapsulators. When fragmented packets arrive, the application 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 application 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 RA message header ([4], section 4.2) would provide a means for the decapsulator to inform the encapsulator that dynamic MTU discovery is supported. C.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: [27] 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 signalling 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 result in a recommendation for proposed standard. C.5 Additional Notes o In all 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 loss due to link/buffer restrictions and immediately reduce their MTU estimate. o To avoid denial-of-service attacks that would cause superfluous probing based on counting down/up by small increments, plateau tables (e.g., [13], section 7) should be used when the actual MTU Templin, et al. Expires July 18, 2003 [Page 20] Internet-Draft ISATAP January 2003 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". Templin, et al. Expires July 18, 2003 [Page 21] Internet-Draft ISATAP January 2003 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. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. Copies of claims of rights made available for publication and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementors or users of this specification can be obtained from the IETF Secretariat. 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