| Network Working Group | F. Templin |
| Internet-Draft | Boeing Research & Technology |
| Intended status: Informational | June 01, 2011 |
| Expires: December 03, 2011 |
Operational Guidance for IPv6 Deployment in IPv4 Sites using ISATAP
draft-templin-v6ops-isops-09.txt
Many end user sites in the Internet today still have predominantly IPv4 internal infrastructures. These sites range in size from small home/office networks to large corporate enterprise networks, but share the commonality that IPv4 continues to provide satisfactory internal routing and addressing services for most applications. As more and more IPv6-only services are deployed in the Internet, however, end user devices within such sites will increasingly require at least basic IPv6 functionality for external access. It is also expected that more and more IPv6-only devices will be deployed within the site over time. This document therefore provides operational guidance for deployment of IPv6 within predominantly IPv4 sites using the Intra-Site Automatic Tunnel Addressing Protocol (ISATAP).
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This Internet-Draft will expire on December 03, 2011.
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End user sites in the Internet today currently use IPv4 routing and addressing internally for core operating functions such as web browsing, filesharing, network printing, e-mail, teleconferencing and numerous other site-internal networking services. Such sites typically have an abundance of public or private IPv4 addresses for internal networking, and are separated from the public Internet by firewalls, packet filtering gateways, proxies, address translators and other site border demarcation devices. To date, such sites have had little incentive to enable IPv6 services internally [RFC1687].
End-user sites that currently use IPv4 services internally come in endless sizes and varieties. For example, a home network behind a Network Address Translator (NAT) may consist of a single link supporting a few laptops, printers etc. As a larger example, a small business may consist of one or a few offices with several networks connecting considerably larger numbers of computers, routers, handheld devices, printers, faxes, etc. Moving further up the scale, large banks, restaurants, major retailers, large corporations, etc. may consist of hundreds or thousands of branches worldwide that are tied together in a complex global enterprise network. Additional examples include personal-area networks, mobile vehicular networks, disaster relief networks, tactical military networks, and various forms of Mobile Ad-hoc Networks (MANETs). These cases and more are discussed in RANGERS[RFC6139].
With the proliferation of IPv6 devices in the public Internet, however, existing IPv4 sites will increasingly require a means for enabling IPv6 services so that hosts within the site can communicate with IPv6-only correspondents. Such services must be deployable with minimal configuration, and in a fashion that will not cause disruptions to existing IPv4 services. The Intra-Site Automatic Tunnel Addressing Protocol (ISATAP) [RFC5214] provides a simple-to-use service that sites can deploy in the near term to meet these requirements. This document therefore provides operational guidance for using ISATAP to enable IPv6 services within predominantly IPv4 sites while causing no disruptions to existing IPv4 services.
Many existing sites within the Internet predominantly use IPv4-based services for their internal networking needs, but there is a growing requirement for enabling IPv6 services to support communications with IPv6-only correspondents. Smaller sites that wish to enable IPv6 typically arrange to obtain public IPv6 prefixes from an Internet Service Provider (ISP), where the prefixes may be either purely native, the near-native prefixes offered by 6rd [RFC5969] or the transitional prefixes offered by 6to4 [RFC3056]. Larger sites typically obtain provider independent IPv6 prefixes from an Internet registry and advertise the prefixes into the IPv6 routing system on their own behalf, i.e., they act as an ISP unto themselves. In either case, after obtaining IPv6 prefixes the site can automatically enable IPv6 services internally by configuring ISATAP.
The ISATAP service uses a Non-Broadcast, Multiple Access (NBMA) tunnel virtual interface model [RFC2491][RFC2529] based on IPv6-in-IPv4 encapsulation [RFC4213]. The encapsulation format can further use Differentiated Service (DS) [RFC2983] and Explicit Congestion Notification (ECN) [RFC3168] mapping between the inner and outer IP headers to ensure expected per-hop behavior within well-managed sites.
The ISATAP service is based on three basic node types known as advertising ISATAP routers, non-advertising ISATAP routers and ISATAP hosts. Advertising ISATAP routers configure their site-facing ISATAP interfaces as advertising router interfaces (see: [RFC4861], Section 6.2.2). Non-advertising ISATAP routers configure their site-facing ISATAP interfaces as non-advertising router interfaces and obtain IPv6 addresses/prefixes via autoconfiguration exchanges with advertising ISATAP routers. Finally, ISATAP hosts configure their site-facing ISATAP interfaces as simple host interfaces and also coordinate their autoconfiguration operations with advertising ISATAP routers. In this sense, advertising ISATAP routers are "servers" while non-advertising ISATAP routers and ISATAP hosts are "clients" in the service model.
Advertising ISATAP routers arrange to add their IPv4 address to the Potential Router List (PRL) within the site name service. The name service could be either the DNS or some other site-internal name resolution system, but the PRL should be published in such a way that ISATAP clients can resolve the name "isatap.domainname" for the "domainname" suffix associated with their attached link. For example, if the domainname suffix associated with an ISATAP client's attached link is "example.com", then the name of the PRL for that link attachment point is "isatap.example.com". Alternatively, if the site name service is operating without a domainname suffix, then the name of the PRL is simply "isatap". (In either case, however, site administrators should ensure that the name resolution returns IPv4 addresses of advertising ISATAP routers that are topologically close to each ISATAP client depending on their locations.)
After the PRL is published, ISATAP clients within the site will automatically perform a standard IPv6 Neighbor Discovery Router Solicitation (RS) / Router Advertisement (RA) exchange with advertising ISATAP routers [RFC4861][RFC5214]. Each ISATAP client can also test the round-trip delays to multiple advertising ISATAP routers listed in the PRL during an initial exchange, and select those routers with the smallest delays. Alternatively, site administrators could include an IPv4 anycast address in the PRL and assign the address to multiple advertising ISATAP routers. In that case, IPv4 routing within the site would direct the ISATAP client to the nearest advertising ISATAP router.
Following router discovery, ISATAP clients initiate Stateless Address AutoConfiguration (SLAAC) [RFC4862][RFC5214], the Dynamic Host Configuration Protocol for IPv6 (DHCPv6) [RFC3315] or both.
Predominantly IPv4 sites can enable SLAAC services for ISATAP clients that need to communicate with IPv6 correspondents. SLAAC services are enabled using either the "shared" or "individual" prefix model. In the shared prefix model, all advertising ISATAP routers advertise a common prefix (e.g., 2001:db8::/64) to ISATAP clients within the site. In the individual prefix model, advertising ISATAP router advertise individual prefixes (e.g., 2001:db8:0:1::/64, 2001:db8:0:2::/64, 2001:db8:0:3::/64, etc.) to ISATAP clients within the site. Note that combinations of the shared and individual prefix models are also possible, in which some of the site's ISATAP routers advertise shared prefixes and others advertise individual prefixes
The following sections discuss operational considerations for enabling ISATAP SLAAC services within predominantly IPv4 sites.
Advertising ISATAP routers that support SLAAC services send RA messages in response to RS messages received on an advertising ISATAP interface. SLAAC services are enabled when advertising ISATAP routers advertise non-link-local IPv6 prefixes in Prefix Information Options (PIOs) with the A flag set to 1[RFC4861]. When there are multiple advertising ISATAP routers, the routers can advertise a shared IPv6 prefix or individual IPv6 prefixes.
Non-advertising ISATAP routers that engage in SLAAC behave the same as for hosts (see below).
ISATAP hosts resolve the PRL and send RS messages to obtain RA messages from an advertising ISATAP router. When the host receives RA messages, it uses SLAAC to configure IPv6 addresses from any advertised prefixes with the A flag set to 1 as specified in [RFC4862][RFC5214] then assigns the addresses to the ISATAP interface. The host also assigns any of the advertised prefixes with the L flag set to 1 to the ISATAP interface.
Any IPv6 addresses configured in this fashion and assigned to an ISATAP interface are known as ISATAP addresses.
Figure 1 depicts a reference ISATAP network topology for allowing hosts within a predominantly IPv4 site to configure ISATAP services using SLAAC with the shared prefix model. The scenario shows two advertising ISATAP routers ('A', 'B'), two ISATAP hosts ('C', 'D'), and an ordinary IPv6 host ('E') outside of the site in a typical deployment configuration. In this model, routers 'A' and 'B' both advertise the same (shared) IPv6 prefix 2001:db8::/64 into the IPv6 routing system, and also advertise the prefix to ISATAP clients within the site for SLAAC purposes.
.-(::::::::) 2001:db8:1::1
.-(::: IPv6 :::)-. +-------------+
(:::: Internet ::::) | IPv6 Host E |
`-(::::::::::::)-' +-------------+
`-(::::::)-'
+------------+ +------------+
| Router A |---.---| Router B |.
,| (isatap) | | (isatap) | `\
. | 192.0.2.1 | | 192.0.2.1 | \
/ +------------+ +------------+ \
: fe80::*:192.0.2.17 fe80::*:192.0.2.33 :
\ 2001:db8::/64 2001:db8::/64 /
: :
: :
+- IPv4 Site -+
; (PRL: 192.0.2.1) :
| ;
: -+-'
`-. .)
\ _)
`-----+--------)----+'----'
fe80::*:192.0.2.18 fe80::*:192.0.2.34
2001:db8::*:192.0.2.18 2001:db8::*:192.0.2.34
+--------------+ +--------------+
| (isatap) | | (isatap) |
| Host C | | Host D |
+--------------+ +--------------+
(* == "5efe")
With reference to Figure 1, advertising ISATAP routers 'A' and 'B' within the IPv4 site connect to the IPv6 Internet either directly or via a companion gateway (e.g., as shown in Figure 3). The routers advertise the shared prefix 2001:db8::/64 into the IPv6 Internet routing system either as a singleton /64 or as part of a shorter aggregated IPv6 prefix if the routing system will not accept prefixes as long as a /64. For the purpose of this example, we also assume that the IPv4 site is configured within multiple IPv4 subnets - each with an IPv4 prefix length of /28.
Advertising ISATAP routers 'A' and 'B' both configure the IPv4 anycast address 192.0.2.1, e.g., on a loopback interface, and the site administrator places the single IPv4 address 192.0.2.1 in the PRL for the site. 'A' and 'B' then both advertise the anycast address/prefix into the site's IPv4 routing system so that ISATAP clients can locate the router that is topologically closest.
Advertising ISATAP router 'A' next configures a site-interior IPv4 interface with address 192.0.2.17 and netmask /28, then configures an advertising ISATAP router interface with link-local ISATAP address fe80::5efe:192.0.2.17 over the IPv4 interface. In the same fashion, 'B' configures a site-interior IPv4 interface with address 192.0.2.33/28, then configures its advertising ISATAP router interface with link-local ISATAP address fe80::5efe:192.0.2.33.
ISATAP host 'C' connects to the site via an IPv4 interface with address 192.0.2.18/28, and also configures an ISATAP host interface with link-local ISATAP address fe80::5efe:192.0.2.18 over the IPv4 interface. 'C' next resolves the PRL, and sends an IPv6-in-IPv4 encapsulated RS message to the IPv4 address 192.0.2.1, where IPv4 routing will direct it to the closest of either 'A' or 'B'. Assuming 'A' is closest, 'C' receives an RA from 'A' then configures a default IPv6 route with next-hop address fe80::5efe:192.0.2.17 via the ISATAP interface and processes the IPv6 prefix 2001:db8::/64 advertised in the PIO. If the A flag is set in the PIO, 'C' uses SLAAC to automatically configure the ISATAP address 2001:db8::5efe:192.0.2.18 and assigns the address to the ISATAP interface. If the L flag is set, 'C' also assigns the prefix 2001:db8::/64 to the ISATAP interface.
In the same fashion, ISATAP host 'D' configures its IPv4 interface with address 192.0.2.34/28 and configures its ISATAP interface with link-local ISATAP address fe80::5efe:192.0.2.34. 'D' next performs an anycast RS/RA exchange that is serviced by 'B', then uses SLAAC to autoconfigure the ISATAP address 2001:db8::5efe:192.0.2.34 and a default IPv6 route with next-hop address fe80::5efe:192.0.2.33. Finally, IPv6 host 'E' connects to an IPv6 network outside of the site. 'E' configures its IPv6 interface in a manner specific to its attached IPv6 link, and autoconfigures the IPv6 address 2001:db8:1::1.
Following this autoconfiguration, when host 'C' inside the site has an IPv6 packet to send to host 'E' outside the site, it prepares the packet with source address 2001:db8::5efe:192.0.2.18 and destination address 2001:db8:1::1. 'C' then uses IPv6-in-IPv4 encapsulation to forward the packet to the link-local address of its default router 'A' (i.e., fe80::5efe:192.0.2.17). 'A' in turn decapsulates the packet and forwards it into the public IPv6 Internet where it will be conveyed to 'E' via normal IPv6 routing. In the same fashion, host 'D' uses IPv6-in-IPv4 encapsulation via its default router 'B' to send IPv6 packets to IPv6 Internet hosts such as 'E'.
When host 'E' outside the site sends IPv6 packets to ISATAP host 'C' inside the site, the IPv6 routing system may direct the packet to either of 'A' or 'B'. If the site is not partitioned internally, the router that receives the packet can use ISATAP to statelessly forward the packet directly to 'C'. If the site may be partitioned internally, however, the packet must first be forwarded to 'C's serving router based on IPv6 routing information. This implies that, in a partitioned site, the advertising ISATAP routers must connect within a full or partial mesh of IPv6 links, and must either run a dynamic IPv6 routing protocol or configure static routes so that incoming IPv6 packets can be forwarded to the correct serving router.
In this example, 'A' can configure the IPv6 route 2001:db8::5efe:192.0.2.32/124 with the IPv6 address of the next hop toward 'B' in the mesh network as the next hop, and 'B' can configure the IPv6 route 2001:db8::5efe:192.0.2.16/124 with the IPv6 address of the next hop toward 'A' as the next hop. (Notice that the /124 prefixes properly cover the /28 prefix of the IPv4 address that is embedded within the IPv6 ISATAP address.) In that case, when 'A' receives a packet from the IPv6 Internet with destination address 2001:db8::5efe:192.0.2.34, it first forwards the packet toward 'B' over an IPv6 mesh link. 'B' in turn uses ISATAP to forward the packet into the site, where IPv4 routing will direct it to 'D'. In the same fashion, when 'B' receives a packet from the IPv6 Internet with destination address 2001:db8::5efe:192.0.2.18, it first forwards the packet toward 'A' over an IPv6 mesh link. 'A' then uses ISATAP to forward the packet into the site, where IPv4 routing will direct it to 'C'.
Finally, when host 'C' inside the site connects to host 'D' inside the site, it has the option of using the native IPv4 service or the ISATAP IPv6-in-IPv4 encapsulation service. When there is operational assurance that IPv4 services between the two hosts are available, the hosts may be better served to continue to use legacy IPv4 services in order to avoid encapsulation overhead and to avoid any IPv4 protocol-41 filtering middleboxes that may be in the path. If 'C' and 'D' may be in different IPv4 network partitions, however, IPv6-in-IPv4 encapsulation should be used with one or both of routers 'A' and 'B' serving as intermediate gateways.
Figure 2 depicts a reference ISATAP network topology for allowing hosts within a predominantly IPv4 site to configure ISATAP services using SLAAC with the individual prefix model. The scenario shows two advertising ISATAP routers ('A', 'B'), two ISATAP hosts ('C', 'D'), and an ordinary IPv6 host ('E') outside of the site in a typical deployment configuration. In the figure, ISATAP routers 'A' and 'B' both advertise different prefixes taken from the aggregated prefix 2001:db8::/48, with 'A' advertising 2001:db8:0:1::/64 and 'B' advertising 2001:db8:0:2::/64.
.-(::::::::) 2001:db8:1::1
.-(::: IPv6 :::)-. +-------------+
(:::: Internet ::::) | IPv6 Host E |
`-(::::::::::::)-' +-------------+
`-(::::::)-'
+------------+ +------------+
| Router A |---.---| Router B |.
,| (isatap) | | (isatap) | `\
. | 192.0.2.1 | | 192.0.2.1 | \
/ +------------+ +------------+ \
: fe80::*:192.0.2.17 fe80::*:192.0.2.33 :
\ 2001:db8:0:1::/64 2001:db8:0:2::/64 /
: :
: :
+- IPv4 Site -+
; (PRL: 192.0.2.1) :
| ;
: -+-'
`-. .)
\ _)
`-----+--------)----+'----'
fe80::*:192.0.2.18 fe80::*:192.0.2.34
2001:db8:0:1::*:192.0.2.18 2001:db8:0:2::*:192.0.2.34
+--------------+ +--------------+
| (isatap) | | (isatap) |
| Host C | | Host D |
+--------------+ +--------------+
(* == "5efe")
With reference to Figure 2, advertising ISATAP routers 'A' and 'B' within the IPv4 site connect to the IPv6 Internet either directly or via a companion gateway (e.g., as shown in Figure 3). Router 'A' advertises the individual prefix 2001:db8:0:1::/64 into the IPv6 Internet routing system, and router 'B' advertises the individual prefix 2001:db8:0:2::/64. The routers could instead both advertise a shorter shared prefix such as 2001:db8::/48 into the IPv6 routing system, but in that case they would need to configure a mesh of IPv6 links between themselves in the same fashion as described for the shared prefix model in Section 3.4. For the purpose of this example, we also assume that the IPv4 site is configured within multiple IPv4 subnets - each with an IPv4 prefix length of /28.
Advertising ISATAP routers 'A' and 'B' both configure the IPv4 anycast address 192.0.2.1, e.g., on a loopback interface, and the site administrator places the single IPv4 address 192.0.2.1 in the PRL for the site. 'A' and 'B' then both advertise the anycast address/prefix into the site's IPv4 routing system so that ISATAP clients can locate the router that is topologically closest.
Advertising ISATAP router 'A' next configures a site-interior IPv4 interface with address 192.0.2.17/28, then configures an advertising ISATAP router interface with link-local ISATAP address fe80::5efe:192.0.2.17 over the IPv4 interface. In the same fashion, 'B' configures the IPv4 interface address 192.0.2.33/28, then configures its advertising ISATAP router interface with link-local ISATAP address fe80::5efe:192.0.2.33.
ISATAP host 'C' connects to the site via an IPv4 interface with address 192.0.2.18/28, and also configures an ISATAP host interface with link-local ISATAP address fe80::5efe:192.0.2.18 over the IPv4 interface. 'C' next resolves the PRL, and sends an IPv6-in-IPv4 encapsulated RS message to the IPv4 address 192.0.2.1, where IPv4 routing will direct it to the closest of either 'A' or 'B'. Assuming 'A' is closest, 'C' receives an RA from 'A' then configures a default IPv6 route with next-hop address fe80::5efe:192.0.2.17 via the ISATAP interface and processes the IPv6 prefix 2001:db8:0:1::/64 advertised in the PIO. If the A flag is set in the PIO, 'C' uses SLAAC to automatically configure the ISATAP address 2001:db8:0:1::5efe:192.0.2.18 and assigns the address to the ISATAP interface. If the L flag is set, 'C' also assigns the prefix 2001:db8:0:1::/64 to the ISATAP interface.
In the same fashion, ISATAP host 'D' configures its IPv4 interface with address 192.0.2.34/28 and configures its ISATAP interface with link-local ISATAP address fe80::5efe:192.0.2.34. 'D' next performs an anycast RS/RA exchange that is serviced by 'B', then uses SLAAC to autoconfigure the ISATAP address 2001:db8:0:2::5efe:192.0.2.34 and a default IPv6 route with next-hop address fe80::5efe:192.0.2.33. Finally, IPv6 host 'E' connects to an IPv6 network outside of the site. 'E' configures its IPv6 interface in a manner specific to its attached IPv6 link, and autoconfigures the IPv6 address 2001:db8:1::1.
Following this autoconfiguration, when host 'C' inside the site has an IPv6 packet to send to host 'E' outside the site, it prepares the packet with source address 2001:db8:0:1::5efe:192.0.2.18 and destination address 2001:db8:1::1. 'C' then uses IPv6-in-IPv4 encapsulation to forward the packet to the link-local ISATAP address of 'A' (fe80::5efe:192.0.2.17), where 'A' in turn decapsulates the packet and forwards it into the public IPv6 Internet where it will be conveyed to 'E' via normal IPv6 routing. In the same fashion, host 'D' uses IPv6-in-IPv4 encapsulation via its default router 'B' to send IPv6 packets to IPv6 Internet hosts such as 'E'.
When host 'E' outside the site sends IPv6 packets to ISATAP host 'C' inside the site, the IPv6 routing system will direct the packet to 'A' since 'A' advertises the individual prefix that matches 'C's destination address. 'A' can then use ISATAP to statelessly forward the packet directly to 'C'. If 'A' and 'B' both advertise the shared shorter prefix 2001:db8::/48 into the IPv6 routing system, however packets coming from 'E' may be directed to either 'A' or 'B'. In that case, the advertising ISATAP routers must connect within a full or partial mesh of IPv6 links the same as for the shared prefix model, and must either run a dynamic IPv6 routing protocol or configure static routes so that incoming IPv6 packets can be forwarded to the correct serving router.
In this example, 'A' can configure the IPv6 route 2001:db8:0:2::/64 with the IPv6 address of the next hop toward 'B' in the mesh network as the next hop, and 'B' can configure the IPv6 route 2001:db8:0.1::/64 with the IPv6 address of the next hop toward 'A' as the next hop. Then, when 'A' receives a packet from the IPv6 Internet with destination address 2001:db8:0:2::5efe:192.0.2.34, it first forwards the packet toward 'B' over an IPv6 mesh link. 'B' in turn uses ISATAP to forward the packet into the site, where IPv4 routing will direct it to 'D'. In the same fashion, when 'B' receives a packet from the IPv6 Internet with destination address 2001:db8:0:1::5efe:192.0.2.18, it first forwards the packet toward 'A' over an IPv6 mesh link. 'A' then uses ISATAP to forward the packet into the site, where IPv4 routing will direct it to 'C'.
Finally, when host 'C' inside the site connects to host 'D' inside the site, it has the option of using the native IPv4 service or the ISATAP IPv6-in-IPv4 encapsulation service. When there is operational assurance that IPv4 services between the two hosts are available, the hosts may be better served to continue to use legacy IPv4 services in order to avoid encapsulation overhead and to avoid any IPv4 protocol-41 filtering middleboxes that may be in the path. If 'C' and 'D' may be in different IPv4 network partitions, however, IPv6-in-IPv4 encapsulation should be used with one or both of routers 'A' and 'B' serving as intermediate gateways.
In common practice, firewalls, gateways and packet filtering devices of various forms are often deployed in order to divide the site into separate partitions. In both the shared and individual prefix models described above, the entire site can be represented by the aggregate IPv6 prefix assigned to the site, while each site partition can be represented by "sliver" IPv6 prefixes taken from the aggregate. In order to provide a simple service that does not interact poorly with the site topology, site administrators should therefore institute an address plan to align IPv6 sliver prefixes with IPv4 site partition boundaries.
For example, in the shared prefix model in Section 3.4, the aggregate prefix is 2001:db8::/64, and the sliver prefixes are 2001:db8::5efe:192.0.2.0/124, 2001:db8::5efe:192.0.2.16/124, 2001:db8::5efe:192.0.2.32/124, etc. In the individual prefix model in Section 3.5, the aggregate prefix is 2001:db8::/48 and the sliver prefixes are 2001:db8:0:0::/64, 2001:db8:0:1::/64, 2001:db8:0:2::/64, etc.
When individual prefixes are used, site administrators can configure advertising ISATAP routers to advertise different individual (sliver) prefixes to different sets of clients, e.g., based on the client's IPv4 subnet prefix. When a shared prefix is used, the site administrator could instead configure the ISATAP routers to advertise the shared (aggregate) prefix to all clients.
Advertising ISATAP routers can set the L flag in each advertised prefix as an indication to clients as to when ISATAP IPv6 services should be preferred or de-preferred with respect to native IPv4 services. For example, if an advertsing router advertises a prefix to multiple clients which might not be able to send IPv6-in-IPv4 encapsulated packets to each other directly within the site, the router should set the L flag to 0 as an indication that the client should prefer IPv4 over IPv6 destinations that configure addresses from the same prefix. (Otherwise, the clients would be obliged to use the advertising ISATAP router as an IPv6 first-hop toward the destination even though the destination could be reached directly via IPv4.)
Site administrators can instead (or in addition) implement address selection policy rules [RFC3484] through explicit configurations in each ISATAP client. Site administrators implement this policy by configuring address selection policy rules [RFC3484] in each ISATAP client in order to give preference to IPv4 destination addresses over destination addresses derived from one of the client's IPv6 sliver prefixes.
For example, site administrators can configure each ISATAP client associated with a sliver prefix such as 2001:db8::5efe:192.0.2.64/124 to add the prefix to its address selection policy table with a lower precedence than the prefix ::ffff:0:0/96. In this way, IPv4 addresses are preferred over IPv6 addresses from within the same sliver. The prefix could be added to each ISATAP client either manually, or through an automated service such as a DHCP option [I-D.ietf-6man-addr-select-opt]. In this way, clients will use IPv4 communications to reach correspondents within the same IPv4 site partition, and will use IPv6 communications to reach correspondents in other partitions and/or outside of the site.
It should be noted that sliver prefixes longer than /64 cannot be advertised for SLAAC purposes. Also, sliver prefixes longer than /64 do not allow for interface identifier rewriting by address translators. These factors may favor the individual prefix model in some deployment scenarios, while the flexibility afforded by the shared prefix model may be more desirable in others.
Finally, site administrators should configure ISATAP routers to not send ICMPv6 Redirect messages to inform a source client of a better next hop toward the destination unless there is strong assurance that the client and the next hop are within the same IPv4 site partition.
In sites that provide IPv6 services through ISATAP with SLAAC as described in this section, advertising ISATAP routers must take operational precautions to avoid routing loops. For example, with reference to Figure 2 an IPv6 packet that enters the site via advertising ISATAP router 'A' must not be allowed to exit the site via advertising ISATAP router 'B' based on an invalid SLAAC address.
As a simple mitigation, each advertising ISATAP router should drop any packets coming from the IPv6 Internet that would be forwarded back to the Internet via another advertising router. Additionally, each advertising ISATAP router should drop any encapsulated packets received from another advertising router that would be forwarded to the IPv6 Internet. (Note that IPv6 packets with link-local ISATAP addresses are excluded from these checks, since they cannot be forwarded by an IPv6 router and may be necessary for router-to-router coordinations.) This corresponds to the mitigation documented in Section 3.2.3 of [I-D.ietf-v6ops-tunnel-loops], but other mitigations specified in that document can also be employed.
Again with reference to Figure 2, when 'A' receives a packet coming from the IPv6 Internet with destination address 2001:db8:1::5efe:192.0.2.2, it drops the packet since the IPv4 address 192.0.2.2 corresponds to advertising ISATAP router 'B'. Similarly, when 'B' receives a packet coming from the tunnel with an IPv6 destination address that would cause the packet to be forwarded back out to the IPv6 Internet and with an IPv4 source address 192.0.2.1, it drops the packet since 192.0.2.1 corresponds to advertising ISATAP router 'A'.
Whether or not advertising ISATAP routers make stateless IPv6 services available using SLAAC, they can also provide managed IPv6 services to ISATAP clients (i.e., both hosts and non-advertising ISATAP routers) using the Dynamic Host Configuration Protocol for IPv6 (DHCPv6). Any addresses/prefixes obtained via DHCPv6 are distinct from any IPv6 prefixes advertised on the ISATAP interface for SLAAC purposes, however. In this way, DHCPv6 addresses/prefixes are reached by viewing the ISATAP tunnel interface as a "transit" rather than viewing it as an ordinary IPv6 host interface. We refer to this as the "no prefix" model.
ISATAP nodes employ the source address verification checks specified in Section 7.3 of [RFC5214] as a prerequisite for decapsulation of packets received on an ISATAP interface. In order to accommodate direct communications with hosts and non-advertising ISATAP routers that use DHCPv6, ISATAP nodes that support route optimization must employ an additional source address verification check. Namely, the node also considers the outer IPv4 source address correct for the inner IPv6 source address if:
The following sections discuss operational considerations for enabling ISATAP DHCPv6 services within predominantly IPv4 sites.
Advertising ISATAP routers that support DHCPv6 services send RA messages in response to RS messages received on an advertising ISATAP interface. Advertising ISATAP routers also configure either a DHCPv6 relay or server function to service DHCPv6 requests received from ISATAP clients.
Non-advertising ISATAP routers can acquire IPv6 prefixes, e.g., through the use of DHCPv6 Prefix Delegation [RFC3633] via an advertising router in the same fashion as described for host-based DHCPv6 stateful address autoconfiguration in Section 4.3. The advertising router in turn maintains IPv6 forwarding table entries that list the IPv4 address of the non-advertising router as the link-layer address of the next hop toward the delegated IPv6 prefixes.
In many use case scenarios (e.g., small enterprise networks, MANETs, etc.), advertising and non-advertising ISATAP routers can engage in a proactive dynamic IPv6 routing protocol (e.g., OSPFv3, RIPng, etc.) over their ISATAP interfaces so that IPv6 routing/forwarding tables can be populated and standard IPv6 forwarding between ISATAP routers can be used. In other scenarios (e.g., large enterprise networks, highly mobile MANETs, etc.), this might be impractical dues to scaling issues. When a proactive dynamic routing protocol cannot be used, non-advertising ISATAP routers send RS messages to obtain RA messages from an advertising ISATAP router, i.e., they act as "hosts" on their non-advertising ISATAP interfaces.
After the non-advertising ISATAP router acquires IPv6 prefixes, it can sub-delegate them to routers and links within its attached IPv6 edge networks, then can forward any outbound IPv6 packets coming from its edge networks via other ISATAP nodes on the link.
ISATAP hosts resolve the PRL and send RS messages to obtain RA messages from an advertising ISATAP router. Whether or not IPv6 prefixes for SLAAC are advertised, the host can acquire IPv6 addresses, e.g., through the use of DHCPv6 stateful address autoconfiguration [RFC3315]. To acquire addresses, the host performs standard DHCPv6 exchanges while mapping the IPv6 "All_DHCP_Relay_Agents_and_Servers" link-scoped multicast address to the IPv4 address of an advertising ISATAP router.
After the host receives IPv6 addresses, it assigns them to its ISATAP interface and forwards any of its outbound IPv6 packets via the advertising router as a default router. The advertising router in turn maintains IPv6 forwarding table entries that list the IPv4 address of the host as the link-layer address of the delegated IPv6 addresses. Note that IPv6 addresses acquired from DHCPv6 therefore need not be ISATAP addresses, i.e., even though the addresses are assigned to the ISATAP interface.
Figure 3 depicts a reference ISATAP network topology that uses DHCPv6. The scenario shows two advertising ISATAP routers ('A', 'B'), two non-advertising ISATAP routers ('C', 'E'), an ISATAP host ('G'), and three ordinary IPv6 hosts ('D', 'F', 'H') in a typical deployment configuration:
.-(::::::::) 2001:db8:3::1
.-(::: IPv6 :::)-. +-------------+
(:::: Internet ::::) | IPv6 Host H |
`-(::::::::::::)-' +-------------+
`-(::::::)-'
,~~~~~~~~~~~~~~~~~,
,----|companion gateway|--.
/ '~~~~~~~~~~~~~~~~~' :
/ |.
,-' `.
; +------------+ +------------+ )
: | Router A | | Router B | /
: | (isatap) | | (isatap) | : fe80::*192.0.2.6
: | 192.0.2.1 | | 192.0.2.1 | ; 2001:db8:2::1
+ +------------+ +------------+ \ +--------------+
fe80::*:192.0.2.2 fe80::*:192.0.2.3 | (isatap) |
| ; | Host G |
: IPv4 Site -+-' +--------------+
`-. (PRL: 192.0.2.1) .)
\ _)
`-----+--------)----+'----'
fe80::*:192.0.2.4 fe80::*:192.0.2.5 .-.
+--------------+ +--------------+ ,-( _)-.
| (isatap) | | (isatap) | .-(_ IPv6 )-.
| Router C | | Router E |--(__Edge Network )
+--------------+ +--------------+ `-(______)-'
2001:db8:0::/48 2001:db8:1::/48 |
| 2001:db8:1::1
.-. +-------------+
,-( _)-. 2001:db8::1 | IPv6 Host F |
.-(_ IPv6 )-. +-------------+ +-------------+
(__Edge Network )--| IPv6 Host D |
`-(______)-' +-------------+
(* == "5efe")
In Figure 3, advertising ISATAP routers 'A' and 'B' within the IPv4 site connect to the IPv6 Internet via a companion gateway. (Note that the routers may instead connect to the IPv6 Internet directly as shown in Figure 1. For the purpose of this example, we also assume that the IPv4 site is configured within a single IPv4 subnet.
Advertising ISATAP routers 'A' and 'B' both configure the IPv4 anycast address 192.0.2.1, e.g., on a loopback interface, and the site administrator places the single IPv4 address 192.0.2.1 in the PRL for the site. 'A' and 'B' then both advertise the anycast address/prefix into the site's IPv4 routing system so that ISATAP clients can locate the router that is topologically closest.
Advertising ISATAP router 'A' next configures a site-interior IPv4 interface with address 192.0.2.2, then configures an advertising ISATAP router interface with link-local ISATAP address fe80::5efe:192.0.2.2 over the IPv4 interface. In the same fashion, 'B' configures the IPv4 interface address 192.0.2.3, then configures its advertising ISATAP router interface with link-local ISATAP address fe80::5efe:192.0.2.3.
Non-advertising ISATAP router 'C' connects to one or more IPv6 edge networks and also connects to the site via an IPv4 interface with address 192.0.2.4, but it does not advertise the site's IPv4 anycast address/prefix. 'C' next configures a non-advertising ISATAP router interface with link-local ISATAP address fe80::5efe:192.0.2.4, then discovers router 'A' via an IPv6-in-IPv4 encapsulated RS/RA exchange. 'C' next receives the IPv6 prefix 2001:db8::/48 through a DHCPv6 prefix delegation exchange via 'A', then engages in an IPv6 routing protocol over its ISATAP interface and announces the delegated IPv6 prefix. 'C' finally sub-delegates the prefix to its attached edge networks, where IPv6 host 'D' autoconfigures the address 2001:db8::1.
Non-advertising ISATAP router 'E' connects to the site, configures its ISATAP interface, performs an RS/RA exchange, receives a DHCPv6 prefix delegation, and engages in the IPv6 routing protocol the same as for 'C'. In particular, 'E' configures the IPv4 address 192.0.2.5 and the link-local ISATAP address fe80::5efe:192.0.2.5. 'E' then receives the delegated IPv6 prefix 2001:db8:1::/48 and sub-delegates the prefix to its attached edge networks, where IPv6 host 'F' autoconfigures IPv6 address 2001:db8:1::1.
ISATAP host 'G' connects to the site via an IPv4 interface with address 192.0.2.6, and also configures an ISATAP host interface with link-local ISATAP address fe80::5efe:192.0.2.6 over the IPv4 interface. 'G' next performs an anycast RS/RA exchange to discover 'B" and configure a default IPv6 route with next-hop address fe80::5efe:192.0.2.3. 'G' then receives the IPv6 address 2001:db8:2::1 from a DHCPv6 address configuration exchange via 'B'; it then assigns the address to the ISATAP interface but does not assign a non-link-local IPv6 prefix to the interface.
Finally, IPv6 host 'H' connects to an IPv6 network outside of the ISATAP domain. 'H' configures its IPv6 interface in a manner specific to its attached IPv6 link, and autoconfigures the IPv6 address 2001:db8:3::1.
Following this autoconfiguration, when host 'D' has an IPv6 packet to send to host 'F', it prepares the packet with source address 2001:db8::1 and destination address 2001:db8:1::1, then sends the packet into the edge network where IPv6 forwarding will eventually convey it to router 'C'. 'C' then uses IPv6-in-IPv4 encapsulation to forward the packet to router 'E', since it has discovered a route to 2001:db8:1::/48 with next hop 'E' via dynamic routing over the ISATAP interface. Router 'E' finally sends the packet into the edge network where IPv6 forwarding will eventually convey it to host 'F'.
In a second scenario, when 'D' has a packet to send to ISATAP host 'G', it prepares the packet with source address 2001:db8::1 and destination address 2001:db8:2::1, then sends the packet into the edge network where it will eventually be forwarded to router 'C' the same as above. 'C' then uses IPv6-in-IPv4 encapsulation to forward the packet to router 'A' (i.e., 'C's default router), which in turn forwards the packet to 'G'. Note that this operation entails two hops across the ISATAP link (i.e., one from 'C' to 'A', and a second from 'A' to 'G'). If 'G' also participates in the dynamic IPv6 routing protocol, however, 'C' could instead forward the packet directly to 'G' without involving 'A'.
In a third scenario, when 'D' has a packet to send to host 'H' in the IPv6 Internet, the packet is forwarded to 'C' the same as above. 'C' then forwards the packet to 'A', which forwards the packet into the IPv6 Internet.
In a final scenario, when 'G' has a packet to send to host 'H' in the IPv6 Internet, the packet is forwarded directly to 'B', which forwards the packet into the IPv6 Internet.
Site administrators configure advertising ISATAP routers that also support the DHCPv6 relay/server function to send RA messages with the M flag set to 1 as an indication to clients that the stateful DHCPv6 address autoconfiguration services area available. If stateless DHCPv6 services are also available, the RA messages also set the O flag to 1.
As discussed in Section 3.5, gateways and packet filtering devices of various forms are often deployed in order to divide the site into separate partitions. Although the purely DHCPv6 model does not involve the advertisement of non-link-local IPv6 prefixes on ISATAP interfaces, alignment of IPv6 prefixes used for DHCPv6 address assignment with IPv4 site partitions is still recommended so that ISATAP clients can prefer native IPv4 communications over ISATAP IPv6 services for correspondents within their contiguous IPv4 partition.
For example, if the site is assigned the aggregate prefix 2001:db8::/48, then the site administrators can assign the sliver prefixes 2001:db8:0:0::/64, 2001:db8:0:1::/64, 2001:db8:0:2::/64, etc. to the different IPv4 partitions within the site. The administrators can then institute a policy that prefers native IPv4 addresses for communications between clients covered by the same IPv6 sliver prefix.
Site administrators can implement this policy implicitly by configuring advertising ISATAP routers to advertise each sliver prefix with the L flag set to 0 as an indication that IPv4 should be preferred over IPv6 destinations that configure addresses from the same prefix. Site administrators can instead (or in addition) implement address selection policy rules [RFC3484] through explicit configurations in each ISATAP client.
For example, each ISATAP client associated with the sliver prefix 2001:db8:0:0::/64 can add the prefix to its address selection policy table with a lower precedence than the prefix ::ffff:0:0/96. In this way, IPv4 addresses are preferred over IPv6 addresses from within the same sliver. The prefix could be added to each ISATAP client either manually, or through an automated service such as a DHCP option [I-D.ietf-6man-addr-select-opt]. In this way, clients will use IPv4 communications to reach correspondents within the same IPv4 site partition, and will use IPv6 communications to reach correspondents in other partitions and/or outside of the site.
Finally, site administrators should configure ISATAP routers to not send ICMPv6 Redirect messages to inform a source client of a better next hop toward the destination unless there is strong assurance that the client and the next hop are within the same IPv4 site partition (see Section 4.6 for further considerations).
With respect to the reference operational scenarios depicted in Figure 3, there may be use cases in which a proactive dynamic IPv6 routing protocol cannot be used. For example, in large enterprise network deployments it would be impractical for all ISATAP routers to engage in a common routing protocol instance due to scaling considerations.
In those cases, an on-demand routing capability can be enabled in which ISATAP nodes send initial packets via an advertising ISATAP router and receive redirection messages back. For example, when a non-advertising ISATAP router 'C' has a packet to send to a host located behind non-advertising ISATAP router 'E', it can send the initial packets via advertising router 'A' which will return redirection messages to inform 'C' that 'E' is a better first hop. Protocol details for this redirection procedure (including a means for detecting whether the direct path is usable) are specified in [I-D.templin-aero].
In a purely DHCPv6-based ISATAP deployment, no non-link-local IPv6 prefixes are assigned to ISATAP router interfaces. Therefore, an ISATAP router cannot mistake another router for an ISATAP host due to an address that matches an on-link prefix. This corresponds to the mitigation documented in Section 3.2.4 of [I-D.ietf-v6ops-tunnel-loops].
Any routing loops introduced in the DHCPv6 scenario would therefore be due to a misconfiguration in IPv6 routing the same as for any IPv6 router, and hence are out of scope for this document.
Sections 3 and 4 depict ISATAP network topologies with only two advertising ISATAP routers within the site. In order to support larger numbers of ISATAP clients (and/or multiple site partitions), the site can deploy more advertising ISATAP routers to support load balancing and generally shortest-path routing.
Such an arrangement requires that the advertising ISATAP routers participate in an IPv6 routing protocol instance so that IPv6 addresses/prefixes can be mapped to the correct ISATAP router. The routing protocol instance can be configured as either a full mesh topology involving all advertising ISATAP routers, or as a partial mesh topology with each advertising ISATAP router associating with one or more companion gateways. Each such companion gateway would in turn participate in a full mesh between all companion gateways.
Advertising ISATAP routers distribute IPv6 prefixes to ISATAP clients within the site via DHCPv6 and/or SLAAC. If the site subsequently reconnects to a different ISP, however, the site must renumber to use addresses derived from the new IPv6 prefixes [RFC1900][RFC4192][RFC5887].
For IPv6 services provided by SLAAC, site renumbering in the event of a change in an ISP-served IPv6 prefix entails a simple renumbering of IPv6 addresses and/or prefixes that are assigned to the ISATAP interfaces of clients within the site. In some cases, filtering rules (e.g., within site border firewall filtering tables) may also require renumbering, but this operation can be automated and limited to only one or a few administrative "touch points".
In order to renumber the ISATAP interfaces of clients within the site using SLAAC, advertising ISATAP routers need only schedule the services offered by the old ISP for deprecation and begin to advertise the IPv6 prefixes provided by the new ISP. ISATAP client interface address lifetimes will eventually expire, and the host will renumber its interfaces with addresses derived from the new prefixes. ISATAP clients should also eventually remove any deprecated SLAAC prefixes from their address selection policy tables, but this action is not time-critical.
Finally, site renumbering in the event of a change in an ISP-served IPv6 prefix further entails locating and rewriting all IPv6 addresses in naming services, databases, configuration files, packet filtering rules, documentation, etc. If the site has published the IPv6 addresses of any site-internal nodes within the public Internet DNS system, then the corresponding resource records will also need to be updated during the renumbering operation. This can be accomplished via secure dynamic updates to the DNS.
IPv6-in-IPv4 encapsulation overhead effectively reduces the size of IPv6 packets that can traverse the tunnel in relation to the actual Maximum Transmission Unit (MTU) of the underlying IPv4 network path between the encapsulator and decapsulator. Two methods for accommodating IPv6 path MTU discovery over IPv6-in-IPv4 tunnels (i.e., the static and dynamic methods) are documented in Section 3.2 of [RFC4213].
The static method places a "safe" upper bound on the size of IPv6 packets permitted to enter the tunnel, however the method can be overly conservative when larger IPv4 path MTUs are available. The dynamic method can accommodate much larger IPv6 packet sizes in some cases, but can fail silently if the underlying IPv4 network path does not return the necessary error messages.
This document notes that sites that include well-managed IPv4 links, routers and other network middleboxes are candidates for use of the dynamic MTU determination method, which may provide for a better operational IPv6 experience in the presence of IPv6-in-IPv4 tunnels. The dynamic MTU determination method can potentially also present a larger MTU to IPv6 correspondents outside of the site, since IPv6 path MTU discovery is considered robust even over the wide area in the public IPv6 Internet.
When an advertising ISATAP router configures an IPv4 anycast address, and site administrators place the address in the PRL, the router uses the anycast address as the IPv4 source address for all IPv6-in-IPv4 encapsulated packets it sends. However, the router must also derive its ISATAP link-local addresses from an IPv4 unicast address assigned to an underlying IPv4 interface instead of from the anycast address.
For example, if an advertising ISATAP router configures the IPv4 anycast address 192.0.2.1 and also configures an ordinary IPv4 interface with IPv4 unicast address 192.0.2.91, the router must configure the ISATAP link-local address fe80::5efe:192.0.2.91 and use this address as the IPv6 source / destination address in link-local messages it exchanges with other ISATAP nodes.
This arrangement is necessary so that ISATAP clients can unambiguously differentiate advertising ISATAP routers. Furthermore, since the IPv4 anycast source address is a member of the PRL, ISATAP clients will accept any messages coming from the advertising router even though the IPv4 source address does not match the IPv4 address embedded in the IPv6 source address.
[RFC4554] proposes a use of VLANs for IPv4-IPv6 coexistence in enterprise networks. The ISATAP approach provides a more flexible and broadly-applicable alternative, and with fewer administrative touch points.
The tunnel broker service [RFC3053] uses point-to-point tunnels that require end users to establish an explicit administrative configuration of the tunnel far end, which may be outside of the administrative boundaries of the site.
6to4 [RFC3056] and Teredo [RFC4380] provide "last resort" unmanaged automatic tunneling services when no other means for IPv6 connectivity is available. These services are given lower priority when the ISATAP managed service and/or native IPv6 services are enabled.
6rd [RFC5969] enables a stateless prefix delegation capability based on IPv4-embedded IPv6 prefixes, whereas ISATAP enables a stateful prefix delegation capability based on native IPv6 prefixes.
IRON [RFC6179], RANGER [RFC5720], VET [RFC5558] and SEAL [RFC5320] were developed as the "next-generation" of ISATAP and extend to a wide variety of use cases [RFC6139]. However, these technologies are not yet widely implemented or deployed.
This document has no IANA considerations.
In addition to the security considerations documented in [RFC5214], sites that use ISATAP should take care to ensure that no routing loops are enabled [I-D.ietf-v6ops-tunnel-loops]. Additional security concerns with IP tunneling are documented in [RFC6169].
The following are acknowledged for their insights that helped shape this work: Fred Baker, Brian Carpenter, Remi Despres, Thomas Henderson, Philip Homburg, Lee Howard, Ray Hunter, Joel Jaeggli, John Mann, Gabi Nakibly, Hemant Singh, Mark Smith, Ole Troan, Gunter Van de Velde, ...
| [RFC5214] | Templin, F., Gleeson, T. and D. Thaler, "Intra-Site Automatic Tunnel Addressing Protocol (ISATAP)", RFC 5214, March 2008. |
| [RFC1918] | Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G. and E. Lear, "Address Allocation for Private Internets", BCP 5, RFC 1918, February 1996. |
| [RFC4861] | Narten, T., Nordmark, E., Simpson, W. and H. Soliman, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, September 2007. |
| [RFC4862] | Thomson, S., Narten, T. and T. Jinmei, "IPv6 Stateless Address Autoconfiguration", RFC 4862, September 2007. |
| [RFC4213] | Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms for IPv6 Hosts and Routers", RFC 4213, October 2005. |
| [RFC3315] | Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C. and M. Carney, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3315, July 2003. |
| [RFC3633] | Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic Host Configuration Protocol (DHCP) version 6", RFC 3633, December 2003. |