--- 1/draft-ietf-ngtrans-isatap-10.txt 2006-02-05 00:50:58.000000000 +0100 +++ 2/draft-ietf-ngtrans-isatap-11.txt 2006-02-05 00:50:58.000000000 +0100 @@ -1,22 +1,22 @@ -NGTRANS Working Group F. Templin +Network Working Group F. Templin Internet-Draft Nokia -Expires: July 4, 2002 T. Gleeson +Expires: July 18, 2003 T. Gleeson Cisco Systems K.K. M. Talwar D. Thaler Microsoft Corporation - January 03, 2002 + January 17, 2003 Intra-Site Automatic Tunnel Addressing Protocol (ISATAP) - draft-ietf-ngtrans-isatap-10.txt + 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. @@ -25,601 +25,482 @@ 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 4, 2002. + This Internet-Draft will expire on July 18, 2003. Copyright Notice - Copyright (C) The Internet Society (2002). All Rights Reserved. + 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. Non-Broadcast, Multiple Access (NBMA) Operation . . . . . . 4 - 5.1 Multicast . . . . . . . . . . . . . . . . . . . . . . . . . 5 - 5.2 Interface Identifiers and Address Construction . . . . . . . 5 - 5.3 ISATAP Link/Interface Configuration . . . . . . . . . . . . 5 - 5.4 Link Layer Address Options . . . . . . . . . . . . . . . . . 6 - 6. Automatic Tunneling . . . . . . . . . . . . . . . . . . . . 6 - 6.1 Dual IP Layer Operation . . . . . . . . . . . . . . . . . . 6 - 6.2 Encapsulation . . . . . . . . . . . . . . . . . . . . . . . 6 - 6.3 Tunnel MTU and Fragmentation . . . . . . . . . . . . . . . . 7 - 6.4 Handling IPv4 ICMP Errors . . . . . . . . . . . . . . . . . 8 - 6.5 Local-Use IPv6 Unicast Addresses . . . . . . . . . . . . . . 8 - 6.6 Ingress Filtering . . . . . . . . . . . . . . . . . . . . . 8 - 7. Neighbor Discovery for ISATAP Links . . . . . . . . . . . . 8 - 7.1 Address Resolution . . . . . . . . . . . . . . . . . . . . . 9 - 7.2 Router and Prefix Discovery . . . . . . . . . . . . . . . . 9 - 7.2.1 Conceptual Data Structures . . . . . . . . . . . . . . . . . 9 - 7.2.2 Validity Checks for Router Advertisements . . . . . . . . . 10 - 7.2.3 Router Specification . . . . . . . . . . . . . . . . . . . . 11 - 7.2.4 Host Specification . . . . . . . . . . . . . . . . . . . . . 11 - 8. ISATAP Deployment Considerations . . . . . . . . . . . . . . 12 - 8.1 Host And Router Deployment Considerations . . . . . . . . . 12 - 8.2 Site Administration Considerations . . . . . . . . . . . . . 12 - 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . 13 - 10. Security considerations . . . . . . . . . . . . . . . . . . 13 - 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 . . . . . . . . . . . . . . . . . . . . . . . . . 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 1. Introduction - This document presents a simple approach that enables incremental - deployment of IPv6 [1] within IPv4-based [2] 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 a link layer for IPv6. + 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. - This document specifies details for the operation of IPv6 over 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. Also specified in this - document is the operation of IPv6 Neighbor Discovery for ISATAP. The - document finally presents deployment and security considerations. + 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 (i.e., no configured tunnel - state) + 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 [3] are used), but does - not allow the virtual ISATAP link to span a Network Address - Translator [4] + (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 [5]. + 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 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 ([6], section 2.1). + 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 - 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. Non-Broadcast, Multiple Access (NBMA) Operation +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. RFC 2491 [7] provides a general - architecture for IPv6 over NBMA networks that forms the basis for - companion documents such as the present. The following subsections - present NBMA considerations for IPv6 on ISATAP links: - -5.1 Multicast - - ISATAP links most closely meet the description for connectionless - service found in the last paragraph of ([7], 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. + Access (NBMA) link layer. The following considerations for IPv6 on + ISATAP links are noted: -5.2 Interface Identifiers and Address Construction +5.1 Interface Identifiers and Unicast Addresses - ([7], 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 ([8], 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.) + 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. - Global and Local-use ISATAP addresses are constructed as follows: + 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 | +------------------------------+---------------+----------------+ - 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: '140.173.129.8'. The address - is alternately written as: - - 3FFE:1A05:510:1111:0:5EFE:140.173.129.8 - - Examples for local-use addresses are obvious from the above and with - reference to ([8], section 2.5.6). - -5.3 ISATAP Link/Interface Configuration - - ISATAP Link/Interface configuration is consistent with ([7], sections - 5.1.1 and 5.1.2). +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. + IPv4 for tunneling within a site. -5.4 Link Layer Address Options + ISATAP interfaces are configured over ISATAP links; each IPv4 address + assigned to an underlying link is seen as a link-layer address for + ISATAP. - ([7], 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: + At least one link-layer address per advertising ISATAP interface + SHOULD be added to the Potential Routers List (see Section 7.3.1). - o the [NTL] and [STL] fields MUST be zero +5.3 Link Layer Address Options - o the [NBMA Number] encodes a 4-octet IPv4 address + 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. - o the [NBMA Subaddress] field is omitted +5.4 Multicast and Anycast - ([7], 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. + 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 ([9], sections 2 and 3) - are used, with the following noted specific considerations for - ISATAP: + 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 ([9], section 2). That + 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. ISATAP - nodes operate with both their IPv4 and IPv6 stacks enabled. - - Address configuration considerations are the same as for ([9], - section 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. + are able to send and receive both IPv4 and IPv6 packets. - DNS considerations are the same as ([9], sections 2.2 and 2.3). + Address configuration and DNS considerations are the same as ([7], + sections 2.1 through 2.3). 6.2 Encapsulation - The specification in ([9], section 3.1) is used. Additionally, the + 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) [10] is + unreachable indication with code 3 (Address Unreachable) [8] is returned to the source. 6.3 Tunnel MTU and Fragmentation - The specification in ([9], 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 ([1], - Section 5), but the following considerations for the MTU of ISATAP - interfaces apply: + 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 VPNs use an MTU of 1400 bytes - - Unless a dynamic per-neighbor MTU discovery mechanism is implemented, - ISATAP interfaces MUST 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 for the predominant - deployment case. ISATAP_MTU MAY be set to a larger value when the - encapsulator implements a dynamic per-neighbor MTU discovery - mechanism, but this value SHOULD NOT exceed the largest MTU of all - underlying links (minus 20 bytes for IPv4 encapsulation). Appendix C - provides non-normative considerations for dynamic per-neighbor MTU - discovery. - - 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 [11]. The ISATAP link layer encapsulates packets of - size 1380 bytes or smaller with the Don't Fragment (DF) bit NOT set - in the encapsualting IPv4 header. - - Nodes 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 [12], section 3.3.2 - specifies that the Effective MTU to Receive (EMTU_R) for IPv4 nodes: + o Commonly-deployed VPN interfaces use an MTU of 1400 bytes - "...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)". + 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. - 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. + 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 - The specification in ([9], section 3.4) MAY be used. IPv4 ICMP - errors and ARP failures are otherwise processed as link error + IPv4 ICMP errors and ARP failures are processed as link error notifications. 6.5 Local-Use IPv6 Unicast Addresses - The specification in ([9], section 3.7) is NOT used. Instead, local - use IPv6 unicast addresses are formed exactly as specified in ([8], - section 2.5.6). + 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 ([9], section 3.9) is used on ISATAP router - 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.) - - Additionally, packets 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 7.2.1) - - Packets that do not satisfy the above conditions are silently - discarded. + The specification in ([7], section 3.9) is used. In particular, + ISATAP nodes that forward decapsulated packets MUST be configured + with a list of source IPv4 address prefixes that are acceptable. -7. Neighbor Discovery for ISATAP Links +7. Neighbor Discovery - RFC 2461 [6] provides the following guidelines for non-broadcast + 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 [6]. + 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 [6]; instead, they are specified in this document. (Note that - these mechanisms MAY potentially apply to other types of NBMA links - in the future.) + by [4]; instead, they are specified in this document. -7.1 Address Resolution +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, ISATAP hosts SHOULD perform an - initial reachability confirmation by sending unicast Neighbor - Solicitations (NSs) and receiving a Neighbor Advertisement using the - mechanisms specified in ([6], sections 7.2.2-7.2.8). (Note that - implementations MAY omit the source/target link layer options in NS/ - NA messages when unicast is used.) + 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). - ISATAP hosts SHOULD additionally perform Neighbor Unreachability - Detection (NUD) as specified in ([6], section 7.3). ISATAP routers - MAY perform the above-specified reachability detection and NUD - procedures, but this might not scale in all environments. + 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 ([6], + All ISATAP nodes MUST send solicited neighbor advertisements ([4], section 7.2.4). -7.2 Router and Prefix Discovery +7.2 Duplicate Address Detection - Since NBMA multicast emulation mechanisms are not used, ISATAP nodes - will not receive unsolicited multicast Router Advertisements. Thus, - alternate mechanisms are required and specified below: + 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. -7.2.1 Conceptual Data Structures +7.3 Router and Prefix Discovery + + Since ISATAP nodes will typically not receive unsolicited multicast + Router Advertisements, unicast mechanisms are required as specified + below: + +7.3.1 Conceptual Data Structures ISATAP nodes use the conceptual data structures Prefix List and - Default Router List exactly as in ([6], section 5.1). ISATAP links - add a new conceptual data structures "Potential Router List" and the + 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. - 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. The - following sections specify the process for initializing the PRL: + 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 first discovers IPv4 addresses - for the PRL. The addresses SHOULD be established by a DHCPv4 [13] - option for ISATAP (option code TBD), by manual configuration, or by - an unspecified alternate method (e.g., DHCPv4 vendor-specific - option). + 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) [20] MAY be used. In this case, the FQDN is resolved - into IPv4 addresses for the PRL through a static host file, a - site-specific name service, or by querying an IPv4-based DNS server. - Unspecified alternate methods for domain name resolution may also be - used. The following notes apply: + 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 ISATAP router interfaces and make them available via - one or more of the mechanisms described above. + 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 - administrators are encouraged to use the convention - "isatap.domainname" (e.g., isatap.example.com). + manual configuration MUST be supported. 3. After initialization, nodes periodically re-initialize the PRL - (after ResolveInterval). When DNS is used, client DNS resolvers - use the IPv4 transport to resolve the names and follow the cache - invalidation procedures in [20] when the DNS time-to-live - expires. + (e.g., after ResolveInterval). When DNS is used, client + resolvers use the IPv4 transport. -7.2.2 Validity Checks for Router Advertisements +7.3.2 Validation of Router Advertisements Messages - A node MUST silently discard any Router Advertisement messages it - receives that do not satisfy both the validity checks in ([6], - section 6.1.2) and the following additional validity check for - ISATAP: + The specification in ([4], 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: [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 -7.2.3 Router Specification +7.3.3 Router Specification - Advertising ISATAP interfaces of routers behave the same as - advertising interfaces described in ([6], 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. + 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). - 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 ([6], section 6.2.7) is desired. +7.3.4 Host Specification -7.2.4 Host Specification +7.3.4.1 Sending Router Solicitations - 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. ISATAP nodes SHOULD use stateful - configuration to assign IPv6 prefixes and default router information. - When stateful configuration is 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: + 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 to any router. - Default and suggested minimum: 15min. + Minimum time between sending Router Solicitations. Default and + suggested minimum: 15min. - When a PRL(i) is selected, the host sets its associated timer to + When a PRL(i) is selected, the host sets TIMER(i) to MinRouterSolicitInterval and initiates solicitation following a short - delay as in ([6], 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. + 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). - 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 ([6], section - 6.3.7). +7.3.4.2 Processing Router Advertisements - Hosts process received Router Advertisements exactly as in ([6], - 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. + 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 + 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). -8. ISATAP Deployment Considerations + 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 ([6], section 5.3) and ([14], 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. + 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 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. + 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 router - 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 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 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 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 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) [21] is requested in the event - that the IESG recommends this document for standards track. + 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 - 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. + 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. - If IPv4 source address filtering is not correctly implemented, the - ISATAP validity checks will not be effective in preventing IPv6 - source address spoofing. + 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. - 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. + 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 [6]), 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 - ([9], 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. + (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). - The ISATAP address format does not support privacy extensions for - stateless address autoconfiguration [22]. 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.) + 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.) 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. @@ -627,108 +508,120 @@ 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 [23] + 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] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G. and E. - Lear, "Address Allocation for Private Internets", BCP 5, RFC - 1918, February 1996. - - [4] Egevang, K. and P. Francis, "The IP Network Address Translator - (NAT)", RFC 1631, May 1994. - - [5] Bradner, S., "Key words for use in RFCs to Indicate Requirement + [3] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. - [6] Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery + [4] Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery for IP Version 6 (IPv6)", RFC 2461, December 1998. - [7] Armitage, G., Schulter, P., Jork, M. and G. Harter, "IPv6 over - Non-Broadcast Multiple Access (NBMA) networks", RFC 2491, - January 1999. - - [8] Hinden, R. and S. Deering, "IP Version 6 Addressing + [5] Hinden, R. and S. Deering, "IP Version 6 Addressing Architecture", draft-ietf-ipngwg-addr-arch-v3-11 (work in progress), October 2002. - [9] Gilligan, R. and E. Nordmark, "Basic Transition Mechanisms for + [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 IPv6 Hosts and Routers", draft-ietf-ngtrans-mech-v2-01 (work in progress), November 2002. - [10] Conta, A. and S. Deering, "Internet Control Message Protocol + [8] Conta, A. and S. Deering, "Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification", RFC 2463, December 1998. - [11] McCann, J., Deering, S. and J. Mogul, "Path MTU Discovery for - IP version 6", RFC 1981, August 1996. - - [12] Braden, R., "Requirements for Internet Hosts - Communication - Layers", STD 3, RFC 1122, October 1989. + [9] Thomson, S. and T. Narten, "IPv6 Stateless Address + Autoconfiguration", RFC 2462, December 1998. - [13] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, + [10] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, March 1997. - [14] Thomson, S. and T. Narten, "IPv6 Stateless Address - Autoconfiguration", RFC 2462, December 1998. + [11] Savola, P., "Security Considerations for 6to4", + draft-savola-ngtrans-6to4-security-01 (work in progress), March + 2002. - [15] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, + [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. - [16] Postel, J., "Internet Control Message Protocol", STD 5, RFC + [14] Postel, J., "Internet Control Message Protocol", STD 5, RFC 792, September 1981. - [17] Baker, F., "Requirements for IP Version 4 Routers", RFC 1812, + [15] Baker, F., "Requirements for IP Version 4 Routers", RFC 1812, June 1995. - [18] Droms, R., "Dynamic Host Configuration Protocol for IPv6 - (DHCPv6)", draft-ietf-dhc-dhcpv6-28 (work in progress), - November 2002. + [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] Mockapetris, P., "Domain names - implementation and + [20] Thaler, D., "Support for Multicast over 6to4 Networks", + draft-ietf-ngtrans-6to4-multicast-01 (work in progress), July + 2002. + + [21] Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, November 1987. - [21] Droms, R., "Procedures and IANA Guidelines for Definition of + [22] Droms, R., "Procedures and IANA Guidelines for Definition of New DHCP Options and Message Types", BCP 43, RFC 2939, September 2000. - [22] Narten, T. and R. Draves, "Privacy Extensions for Stateless + [23] Narten, T. and R. Draves, "Privacy Extensions for Stateless Address Autoconfiguration in IPv6", RFC 3041, January 2001. - [23] Nguyen, Q., "http://irl.cs.ucla.edu/vet/report.ps", spring + [24] Nguyen, Q., "http://irl.cs.ucla.edu/vet/report.ps", spring 1998. - [24] Lahey, K., "TCP Problems with Path MTU Discovery", RFC 2923, + [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 @@ -754,20 +647,30 @@ 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 @@ -830,124 +733,193 @@ 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 +Appendix C. Dynamic 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. + 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 [15] 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 [16] - and RFC 1812 [17], section 4.3.2.3). Additionally, ICMPv4 + 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 is inadequate and can result in black holes that are difficult - to diagnose [24]. + alone may be inadequate and can result in black holes that are + difficult to diagnose [25]. - 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 to the neighbor and receiving positive - confirmation of receipt. Two methods are possible: + 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: - In the first method, the encapsulator does NOT set the DF bit in the - IPv4 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: + 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 - o special mechanisms required on both encapsulator and decapsulator + 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) - o extra state required on both encapsulator and decapsulator + 3. Hybrid - the encapsulator and decapsulator engage in a dialogue + and use "intelligent" probing to monitor the path MTU - o complex protocol signalling between encapsulator and decapsulator + These methods are discussed in detail in the following subsections: - 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. Setting the DF bit prevents the network - from fragmenting the packets and protects decapsulators from - receiving packets that might overrun the IPv4 reassembly buffer. - Additionally, special mechanisms and state are needed only on the - encapsulator, and no complex protocol signalling between the - encapsulator and decapsulator is required. +C.1 Encapsulator-driven Method - In either method, implementations may choose to couple the probing - process with neighbor cache management procedures ([6], 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. + 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. - Additional notes: + 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]. - 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. + 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]). - 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. +C.2 Decapsulator-driven Method - 3. ICMPv4 "fragmentation needed" messages may result when a link + 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 + (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 + 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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