RFC 7059
A Comparison of IPv6-over-IPv4 Tunnel Mechanisms
Pages: 41
Informational
Informational
Part 1 of 2 – Pages 1 to 20
Independent Submission S. Steffann Request for Comments: 7059 S.J.M. Steffann Consultancy Category: Informational I. van Beijnum ISSN: 2070-1721 Institute IMDEA Networks R. van Rein OpenFortress November 2013 A Comparison of IPv6-over-IPv4 Tunnel MechanismsAbstract
This document provides an overview of various ways to tunnel IPv6 packets over IPv4 networks. It covers mechanisms in current use, touches on several mechanisms that are now only of historic interest, and discusses some newer tunnel mechanisms that are not widely used at the time of publication. The goal of the document is helping people with an IPv6-in-IPv4 tunneling need to select the mechanisms that may apply to them. Status of This Memo This document is not an Internet Standards Track specification; it is published for informational purposes. This is a contribution to the RFC Series, independently of any other RFC stream. The RFC Editor has chosen to publish this document at its discretion and makes no statement about its value for implementation or deployment. Documents approved for publication by the RFC Editor are not a candidate for any level of Internet Standard; see Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7059. Copyright Notice Copyright (c) 2013 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Tunnel Mechanisms . . . . . . . . . . . . . . . . . . . . . . 6 3.1. Configured Tunnels (Manual Tunnels / 6in4) . . . . . . . . 7 3.2. Automatic Tunneling . . . . . . . . . . . . . . . . . . . 8 3.3. IPv6 over IPv4 without Explicit Tunnels (6over4) . . . . . 9 3.4. Generic Routing Encapsulation (GRE) . . . . . . . . . . . 10 3.5. Connection of IPv6 Domains via IPv4 Clouds (6to4) . . . . 10 3.5.1. 6to4 Provider Managed Tunnels . . . . . . . . . . . . 11 3.6. Anything In Anything (AYIYA) . . . . . . . . . . . . . . . 12 3.7. Intra-Site Automatic Tunnel Addressing (ISATAP) . . . . . 13 3.8. Tunneling IPv6 over UDP through NATs (Teredo) . . . . . . 14 3.9. IPv6 Rapid Deployment (6rd) . . . . . . . . . . . . . . . 15 3.10. Native IPv6 behind NAT44 CPEs (6a44) . . . . . . . . . . . 16 3.11. Locator/ID Separation Protocol (LISP) . . . . . . . . . . 16 3.12. Subnetwork Encapsulation and Adaptation Layer (SEAL) . . . 18 3.13. Peer-to-Peer IPv6 on Any Internetwork (6bed4) . . . . . . 19 4. Related Protocols . . . . . . . . . . . . . . . . . . . . . . 20 4.1. Tunnel Setup Protocol (TSP) . . . . . . . . . . . . . . . 20 4.2. SixXS Heartbeat Protocol . . . . . . . . . . . . . . . . . 20 4.3. Tunnel Information and Control Protocol (TIC) . . . . . . 21 5. Common Aspects . . . . . . . . . . . . . . . . . . . . . . . . 22 5.1. Protocol 41 Encapsulation . . . . . . . . . . . . . . . . 22 5.2. NAT and Firewalls . . . . . . . . . . . . . . . . . . . . 22 5.3. MTU Considerations . . . . . . . . . . . . . . . . . . . . 24 5.4. IPv4 Addresses Embedded in IPv6 Addresses . . . . . . . . 25 6. Evaluation of Tunnel Mechanisms . . . . . . . . . . . . . . . 28 6.1. Efficiency of IPv4 Address Use . . . . . . . . . . . . . . 28 6.2. Supported Network Topologies . . . . . . . . . . . . . . . 29 6.3. Robustness . . . . . . . . . . . . . . . . . . . . . . . . 30 6.4. Gateway State . . . . . . . . . . . . . . . . . . . . . . 32 6.5. Performance . . . . . . . . . . . . . . . . . . . . . . . 33 7. Security Considerations . . . . . . . . . . . . . . . . . . . 34 8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 35 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 35 10. Informative References . . . . . . . . . . . . . . . . . . . . 35 Appendix A. Evaluation Criteria . . . . . . . . . . . . . . . . . 40
1. Introduction
During the transition from IPv4 to IPv6, IPv6 islands are separated by a sea of IPv4. Tunnels provide connectivity between these IPv6 islands. Tunnels work by encapsulating IPv6 packets inside IPv4 packets, as shown in the figure. +---------------+ | IPv4 | | Header | +---------------+ : Optional : : Encapsulation : : Header : +---------------+ +---------------+ | IPv6 | | IPv6 | | Header | | Header | +---------------+ +---------------+ | Transport | | Transport | | Layer | ===> | Layer | | Header | | Header | +---------------+ +---------------+ | | | | ~ Data ~ ~ Data ~ | | | | +---------------+ +---------------+ Encapsulating IPv6 in IPv4 Various tunnel mechanisms have been proposed over time. So many, in fact, that it is difficult to get an overview. Some tunnel mechanisms have been abandoned by the community, others have known problems, and yet others have shown to be reliable. Some tunnel mechanisms were designed with a particular use case in mind; others are generic. There may be documented limitations as well as limitations that have cropped up in deployment. This document provides an overview of available and/or noteworthy tunnel mechanisms, with the intention to guide selection of the best mechanism for a particular purpose. As such, the discussion of the different tunnel mechanisms is limited to the working principles of the different mechanisms and a few important details. Please use the references to learn the full details of each mechanism. For brevity, only the most relevant documents are referenced. Refer to these for additional specifications, updates, and links to older versions of protocol specifications, as well as links to more general background information.
The intended audience for this document is everyone who needs a connection to the IPv6 Internet at large, but is not in the position to use native (untunneled) IPv6 connectivity, and thus needs to select an appropriate tunnel mechanism. However, when native IPv6 connectivity is available, this should be preferred over tunneled connectivity as per rule 7 in Section 6 of [RFC6724]. This document is also intended as a quick reference to tunnel mechanisms for the IETF community. The scope of this document is limited to tunnel mechanisms for providing IPv6 connectivity over an IPv4 infrastructure. Mechanisms for Virtual Private Networks (VPNs) and security architectures such as IPsec [RFC4301], as well as IPv4-over-IPv6 tunneling, are out of scope for this document as they serve a different purpose, even if they could technically be used to provide IPv6 connectivity.2. Terminology
Anycast: Mechanism to provide a service, in multiple locations and/or using multiple servers, by configuring each server with the same IP address. Carrier-Grade NAT (CGN): A Network Address Translation (see NAT) device used by an ISP so multiple subscribers can be served using a single IPv4 address. Dual stack: Also known as "dual IP layer". Nodes run IPv4 and IPv6 side by side, and can communicate with other dual stack nodes (using IPv4 or IPv6), as well as IPv4-only nodes (using IPv4) and IPv6-only nodes (using IPv6). Most current operating systems are set up to use IPv4 when available as well as use IPv6 when available, allowing them to run in IPv4-only, IPv6-only, or dual stack mode as circumstances permit. Except for a few things concerning the Domain Name System (DNS), there is no separate specification for dual stack beyond the specifications relevant to running IPv4 and IPv6. Dual stack is one of the three IPv4-to- IPv6 transition tools; the others are translation and tunnels. Encapsulation: Transporting a packet as data inside another packet. For instance, an IPv6 packet inside an IPv4 packet. Firewall: A device that selectively filters IP packets, allowing some protocols through but not others. A firewall may act as a switch, operating below the IP layer, or as a router. Host: A device that communicates using the Internet Protocol and only transmits packets from its own address.
ISP: Internet Service Provider; the party connecting the outside of
the local network's perimeter to the public Internet.
MTU: Maximum Transmission Unit, the maximum size of a packet that
can be transmitted over a link (or tunnel) without splitting it
into multiple fragments.
NAT: Network Address Translation or Network Address Translator. NAT
makes it possible for a number of hosts to share a single IP
address. TCP and UDP port numbers are used to distinguish the
traffic to/from different hosts served by the NAT; protocols other
than TCP and UDP may be incompatible with NAT due to lack of port
numbers. NAT also breaks protocols that depend on the IP
addresses used in some way. Typically, NAT devices behave as a
host towards the public Internet, and as a router towards the
internal network.
NBMA: Non-Broadcast Multi-Access. This is a network configuration
in which nodes can exchange packets directly by addressing them at
the desired destination. However, broadcasts or multicasts are
not supported, so autodiscovery mechanisms such as IPv6 Neighbour
Discovery must be modified to use unicast to work.
Node: A device that implements IP, either a host or a router; also
known as a system. See note at "NAT".
Path stretch: The difference between the shortest path through the
network and the path (tunneled) packets actually take.
PMTUD: Path MTU Discovery, a method to determine the smallest MTU on
the path between two nodes. There are separate specifications for
PMTUD over IPv4 [RFC1191] and IPv6 [RFC1981].
Router: A device that forwards IP packets that it didn't generate
itself. See note at "NAT".
System: A device that implements IP, either a host or a router; a
network node.
Translation: The IPv6 and IPv4 headers are similar enough that it is
possible to translate between them. This allows IPv6-only hosts
to communicate with IPv4-only hosts. The original specification
for translating between IPv6 and IPv4 was heavily criticised by
the Internet Architecture Board, but new specifications for
translating between IPv6 and IPv4 were later published [RFC6145].
Translation is one of the three IPv4-to-IPv6 transition tools; the
others are dual stack and tunnels.
Tunnel: By encapsulating IPv6 packets inside IPv4 packets, IPv6-
capable hosts and IPv6-capable networks isolated from other IPv6-
capable systems or the IPv6 Internet at large can exchange IPv6
packets over IPv4-only infrastructure. There are numerous ways to
tunnel IPv6 over IPv4. This document compares these mechanisms.
Tunneling is one of the three IPv4-to-IPv6 transition tools; the
others are translation and dual stack.
Tunnel broker: A service that provides tunneled connectivity to the
IPv6 Internet, such as SixXS [SIXXS], tunnelbroker.net
[TUNBROKER], and gogo6 [GOGO6].
3. Tunnel Mechanisms
Automatic tunnels (Section 3.2), configured tunnels (Section 3.1),
6over4 (Section 3.3), 6to4 (Section 3.5), the Intra-Site Automatic
Tunnel Addressing Protocol (ISATAP) (Section 3.7), and 6rd
(Section 3.9) solve similar problems at different scales. They all
encapsulate IPv6 packets immediately inside an IPv4 packet, without
using additional headers. This is called "protocol 41 encapsulation"
(see Section 5.1), as the Protocol field in the IPv4 header is set to
41 to indicate that what follows is an IPv6 packet.
6to4, 6rd, ISATAP, and automatic tunneling each generate an IPv6
address or range of IPv6 addresses for the host or router running the
protocol based on the system's IPv4 address in one way or another
(see Section 5.4). This lets 6to4, 6rd, ISATAP, and automatic
tunnels determine the IPv4 destination address in the outer IPv4
header from the IPv6 address of the destination, allowing for
automatic operation without the need to administratively configure
the remote tunnel endpoint.
6over4 and ISATAP provide IPv6 connectivity between IPv6-capable
systems within a single organisation's network that is otherwise IPv4
only. 6rd allows ISPs to provide IPv6 connectivity to their
customers over IPv4-only last-mile infrastructures. 6to4 directly
provides connectivity to the global IPv6 Internet without involving
an ISP.
Configured tunnels (Section 3.1) also use protocol 41 encapsulation
but rely on manual configuration of the remote tunnel endpoint. (The
Heartbeat Protocol (Section 4.2) solves this.) Configured tunnels
can be used within an organisation's network but are typically used
by tunnel-broker services to provide connectivity to the IPv6
Internet. GRE (Section 3.4) is similar to configured tunnels, but
also supports encapsulating protocols other than IPv6.
AYIYA (Section 3.6) is similar to configured tunnels and GRE but typically uses a UDP header for better compatibility with NATs and is generally used with the Tunnel Information and Control (TIC) protocol (Section 4.3) to set up the tunnel rather than rely on manual configuration. Teredo (Section 3.8), 6a44 (Section 3.10), and 6bed4 (Section 3.13) are similar to 6to4, except that they are designed to work through NATs by running over UDP. Of these, Teredo and 6bed4 assume no ISP involvement and 6a44 does; 6bed4 is designed to work over direct IPv4 paths between peers whenever possible. The Locator/ID Separation Protocol (LISP) (Section 3.11) is a system for abstracting the identifying function from the location function of IP addresses; this allows for the use of IPv6 for the former and IPv4 for the latter. The Subnetwork Encapsulation and Adaptation Layer (SEAL) (Section 3.12) and its companion technologies (Virtual Enterprise Traversal (VET), Asymmetric Extended Route Optimization (AERO), Internet Routing Overlay Network (IRON), and Routing and Addressing in Networks with Global Enterprise Recursion (RANGER)) provide a configured tunnel system for IPv6-in-IPv4 tunneling to default routers as well as automatic tunnel endpoint discovery for optimisation of more-specific routes. Dual-Stack Lite [RFC6333] and MAP [MAP], both developed by the IETF Softwire working group, often come up in discussions about IPv6 tunneling. However, they are _not_ IPv6-in-IPv4 tunnel mechanisms. They are IPv4-in-IPv6 mechanisms for providing IPv4 connectivity over an IPv6 infrastructure. Please refer to Section 5 for more information about issues common to many tunnel mechanisms; those issues are not discussed separately for each mechanism. The mechanisms are discussed below in roughly chronological order.3.1. Configured Tunnels (Manual Tunnels / 6in4)
Configured and automatic tunnels are the two oldest tunnel mechanisms, originally published in "Transition Mechanisms for IPv6 Hosts and Routers" [RFC1933] in 1996. The latest specification of configured tunnels is "Basic Transition Mechanisms for IPv6 Hosts and Routers" [RFC4213], published in 2005. The mechanism is sometimes called "manual tunnels", "static tunnels", "protocol 41 tunnels", or "6in4". Configured tunnels connect two systems in point-to-point fashion using protocol 41 encapsulation. The configuration that the name of the mechanism alludes to consists of a remote "tunnel endpoint".
This is the IPv4 address of the system on the other side of the tunnel. When a system (potentially) has multiple IPv4 addresses, the local tunnel endpoint address may also need to be configured. The need to explicitly set up configured tunnels makes them more difficult to deploy than automatic mechanisms. However, because there is a fixed, single remote tunnel endpoint, performance is predictable and the tunnel is easy to debug. In the early days, it was not unheard of for a small network to get IPv6 connectivity from another continent. This excessive path stretch makes communication over short geographic distances much less efficient because the distance travelled by packets may be larger than the geographic distance by an order of magnitude or more. Configured tunnels are widely implemented. Common operating systems can terminate configured tunnels, as well as IPv6-capable routers and home gateways. The mechanism is versatile but is mostly used between isolated smaller IPv6-capable networks and the IPv6 Internet, often through a "tunnel broker" such as tunnelbroker.net [TUNBROKER], SixXS [SIXXS], or gogo6 [GOGO6]. [RFC4891] discusses the use of IPsec to protect the confidentiality and integrity of IPv6 traffic exchanged over configured tunnels.3.2. Automatic Tunneling
Automatic tunneling is described in [RFC2893], "Transition Mechanisms for IPv6 Hosts and Routers", but removed in [RFC4213], which is a replacement of RFC 2893. Configured tunnels (Section 3.1) are closely related to automatic tunnels and are specified in RFCs 2893 and 4213, too. Both use protocol 41 encapsulation. Hosts that are capable of automatic tunneling use special IPv6 addresses: IPv4-compatible addresses. An IPv4-compatible IPv6 address consists of 96 zero bits followed by the system's IPv4 address. When sending packets to destinations within the IPv4- compatible ::/96 prefix, the IPv4 destination address in the outer IPv4 header is taken from the IPv4 address in the IPv4-compatible IPv6 destination address. Automatic tunneling has a big limitation: it only allows for communication between IPv6-capable systems that both support automatic tunneling. There are no provisions for communicating with the native IPv6 Internet. As such, the mechanism is of almost no practical use and is not implemented in current operating systems, as 6to4 (Section 3.5) does what automatic tunneling was supposed to do, but it also provides connectivity to the rest of the IPv6 Internet.
3.3. IPv6 over IPv4 without Explicit Tunnels (6over4)
"Transmission of IPv6 over IPv4 Domains without Explicit Tunnels" [RFC2529] was published in 1999. The mechanism is commonly known as "6over4". 6over4 is designed to work within a single organisation's IPv4 network, where IPv6-capable hosts and routers are separated by IPv4- only routers. 6over4 treats the IPv4 network as a "virtual Ethernet" for the purpose of IPv6 communication. It uses IPv4 multicast to tunnel IPv6 multicast packets. A node's IPv4 address is included in the Interface Identifier used on the virtual 6over4 interface, allowing the exchange of protocol 41 encapsulated packets between 6over4 nodes without prior administrative configuration. Because multicast is supported, standard IPv6 Neighbour Discovery and Stateless Address Autoconfiguration [RFC4862] can be used. Although, like automatic tunnels (Section 3.2) and other mechanisms, 6over4 embeds the IPv4 address of the host in the IPv6 address, the destination IPv4 address in the outer IPv4 header is *not* derived from the IPv6 address embedded in the inner IPv6 header, but learnt through Neighbour Discovery [RFC4861]. In effect, the IPv4 addresses of the hosts are used as link-layer addresses in the same way that Media Access Control (MAC) addresses are used on Ethernet networks. One or more routers with connectivity to the global IPv6 Internet send out Router Advertisements to provide 6over4 hosts with connectivity to the rest of the IPv6 Internet. 6over4 has the minimal overhead for protocol 41 encapsulation and doesn't require manual configuration. Hosts can only take advantage of 6over4 if they run the mechanism themselves. 6over4 packets can't pass through a NAT successfully, as the IPv4 address exchanged through Neighbour Discovery will be different from the one needed to reach the host in question, and because, without port numbers, protocol 41 doesn't allow for multiplexing multiple hosts using this encapsulation behind a single IPv4 address. However, 6over4 works within IPv4 domains that reside behind a NAT in their entirety and use RFC 1918 addressing. Because of its reliance on IPv4 multicast and because local IPv6 communication is relatively easy to facilitate using IPv6 routers, 6over4 is not supported in current operating systems. ISATAP (Section 3.7) provides very similar functionality without requiring IPv4 multicast capability and is implemented more widely.
3.4. Generic Routing Encapsulation (GRE)
Generic Routing Encapsulation (GRE) [RFC2784] is a generic point-to- point tunnel mechanism that allows many other protocols to be encapsulated in IP. GRE is a simple protocol that is similar to configured tunnels (Section 3.1) when used for IPv6-in-IPv4 tunneling. The main benefit of GRE is that it can encapsulate any protocol's packets, not only IPv6 packets. The GRE header causes an extra overhead of 8 to 16 bytes depending on which options are used. GRE sets the Protocol field in the IP header to 47. The GRE header can optionally contain a checksum, a key to separate different traffic flows (for example, different tunnels) between the same endpoints, and a sequence number that can be used to prevent packets from being processed out of order. GRE is implemented in many routers but not in most consumer-level home gateways or desktop operating systems.3.5. Connection of IPv6 Domains via IPv4 Clouds (6to4)
6to4 is specified in "Connection of IPv6 Domains via IPv4 Clouds" [RFC3056]. It creates a block of IPv6 addresses from a locally configured IPv4 address by concatenating that IPv4 address to the prefix 2002::/16, resulting in a /48 IPv6 prefix. Addresses in 2002::/16 are considered reachable through the tunnel interface, so the 6to4 network functions as a Non-Broadcast Multi-Access (NBMA) network through which 6to4 users can communicate. IPv6 packets are encapsulated by adding an IPv4 header with the Protocol field set to 41. The /48 prefix allows a single system running 6to4 to act as a gateway or router for a large number of IPv6 hosts. Alternatively, an individual host may run 6to4 and not act as a gateway or router. The system running 6to4 must have a globally reachable IPv4 address. Using 6to4 with a private IPv4 address [RFC1918] is not possible. "An Anycast Prefix for 6to4 Relay Routers" [RFC3068] specifies an anycast mechanism for 6to4 relays that provide connectivity between the 6to4 network and the regular IPv6 Internet. All public relays share the IPv4 address 192.88.99.1, which corresponds to 2002:c058: 6301::. Relays advertise reachability towards 2002::/16 to the native IPv6 Internet, so packets addressed to systems using 6to4 addresses are routed to the closest gateway. The gateway encapsulates these packets and forwards them to the IPv4 address included in the IPv6 address. Systems running 6to4 have a default
route pointing to 2002:c058:6301::, so they tunnel packets addressed to non-6to4 IPv6 destinations to the closest relay, which decapsulates the packet and forwards them as IPv6 packets. The 6to4 protocol adds minimal overhead for protocol 41 encapsulation and requires no manual configuration from users. The biggest problem specific to 6to4 is that it is unpredictable which 6to4 anycast relay is used. These relays are often provided by third parties on a best- effort basis. In practice, this has caused unpredictable performance. Traffic from the 6to4 network to the regular IPv6 Internet will likely use a different 6to4 relay than the traffic in the opposite direction. If either of those relays is not reliable, then the communication between those networks is affected. Especially the lack of control over the relay used for return traffic is considered to be a problem with 6to4. To avoid problems with 6to4, the IPv6 Default Address Selection algorithm [RFC6724] gives IPv4 addresses a higher preference than 6to4 addresses. When making a connection, a system will prefer native IPv6 over IPv4, and IPv4 over 6to4 IPv6. This causes 6to4 to be used only when a destination is not reachable over IPv4 and no other IPv6 connectivity is available. For more information about 6to4, see "Advisory Guidelines for 6to4 Deployment" [RFC6343]. *Warning*: Although many, if not all, 6to4 implementations disable the mechanism when the system only has an RFC 1918 address, recently a block of IPv4 addresses has been set aside for use in service-provider- operated Network Address Translators, also known as Carrier-Grade NATs (CGNs). [RFC6598] sets aside the block 100.64.0.0/10 for the use between CGNs and subscriber devices. As 100.64.0.0/10 is not an RFC 1918 address block, systems implementing 6to4 may fail to disable the mechanism, but due to the shared nature of the 100.64.0.0/10 prefix, 6to4 cannot work using these addresses. The same issue is present if an ISP decides to use regular global unicast IPv4 address space behind a CGN.3.5.1. 6to4 Provider Managed Tunnels
[RFC6732] describes "6to4 provider managed tunnels", which are a way to make 6to4 work behind a CGN. This is accomplished by running a 6to4 gateway at the 6to4 gateway anycast address, and then translating the IPv6 addresses used by 6to4 users behind the CGN to IPv6 addresses from the ISP's range. Unlike IPv4 NAT, where multiple
internal hosts share a single public IPv4 address, prefix translation maps entire prefixes, so each host has its own public IPv6 address and can receive incoming packets as usual. However, if IPv6 applications are not aware that translation is happening (and they have no reason to expect that it is), they may not use their externally visible address in referrals, so applications that use referrals are likely to fail. Additionally, the translation is only specified for packets that flow through the 6to4 gateway, not for packets sent directly to other 6to4 users. So, communication with other 6to4 users is not possible. As such, the use of 6to4 provider managed tunnels is discouraged except as a very last resort.3.6. Anything In Anything (AYIYA)
AYIYA [AYIYA] is designed for use by the SixXS [SIXXS] tunnel-broker service. For more information, see the specification [MASSAR]. The AYIYA protocol defines a method for encapsulating any protocol in any other protocol. The most common way of deploying AYIYA is to use the following sequence of headers: IPv4-UDP-AYIYA-IPv6, although other combinations like IPv4-AYIYA-IPv6 or IPv6-SCTP-AYIYA-IPv4 are also possible. That document does not limit the contents nor the protocol that carries the AYIYA packets. In this document, we only look at the most common usage (IPv4-UDP-AYIYA-IPv6) that is deployed on the SixXS tunnel brokers to provide IPv6 access to clients behind NAT devices. AYIYA specifies the encapsulation, identification, checksum, security, and certain management operations that can be used once the tunnel is established. It does not specify how the tunnel configuration parameters can be negotiated. Typically, the TIC protocol described in Section 4.3 is used for that part of the tunnel setup, although the Tunnel Setup Protocol (TSP) (Section 4.1) could just as well be used. AYIYA provides a point-to-point tunnel, over which the endpoints can route traffic for any source and destination. When using SHA-1 hashing for authentication, as is common when using the Automatic IPv6 Connectivity Client Utility (AICCU) client with a SixXS tunnel server, the total packet overhead is 72 bytes (20 for the IPv4 header, 8 for UDP, and 44 for AYIYA).
AYIYA provides operational commands for querying the hostname, address, contact information, software version, and last error message. An operational command to ask the other side of the tunnel to shut down is also available. These commands in the protocol can make debugging of AYIYA tunnels easier if the tools support them. The main advantage of AYIYA is that it can provide a stable tunnel through an IPv4 NAT. The UDP port numbers allow multiple AYIYA users to share a single IPv4 address behind a NAT. The client will contact the tunnel server at regular intervals, and the tunnel server will automatically adapt to changing IPv4 addresses and/or UDP port numbers. To prevent a third party from injecting rogue packets into the tunnel, the client can optionally be authenticated by using the identity and signature fields. A timestamp is included in the AYIYA header to guard against replay attacks. There is currently a single implementation of this protocol: the AICCU [AICCU] client software used with the SixXS [SIXXS] tunnel- broker service.3.7. Intra-Site Automatic Tunnel Addressing (ISATAP)
ISATAP [RFC5214] uses protocol 41 encapsulation to provide connectivity between isolated IPv6-capable nodes within an organisation's internal network. It is similar to 6over4 (Section 3.3), but without the requirement that the IPv4 network supports multicast; unlike 6over4, ISATAP uses a Non-Broadcast Multi- Access (NBMA) communication model and thus doesn't support multicasts. The mechanism assigns IPv6 addresses whose Interface Identifier is solely defined by a node's IPv4 address, which is assumed to be unique. In order to obtain a /64 prefix, an ISATAP host needs to send a unicast Router Solicitation to receive a unicast Router Advertisement from an ISATAP router. Without the ability to send and receive IPv6 multicasts, an ISATAP host must be configured with a Potential Router List through an all-IPv4 mechanism, such as manual setup, DHCP, or the DNS. Site administrators are encouraged to use a DNS Fully Qualified Domain Name using the convention "isatap.domainname" (e.g., isatap.example.com). Hosts will accept packets with IPv4 sender addresses that are either on the Potential Router List or embedded in the IPv6 sender address. The router's prefix and the IPv4 address together define the IPv6 address for the ISATAP interface. This means that precisely one ISATAP address is available for each IPv4 address. As such, each
host needs to run ISATAP itself in order to enjoy ISATAP IPv6 connectivity. The IPv4 address in the destination IPv6 address is used to bootstrap Neighbour Discovery. [RFC5214] doesn't explicitly address the use of ISATAP using private RFC 1918 addresses. Despite that, the mechanism seems compatible with private addresses. NAT, however, breaks the relationship between the IPv4 address embedded in the IPv6 address and would therefore make communication between ISATAP hosts impossible. Any device that can communicate with the ISATAP hosts over IPv4 using protocol 41 can participate in the IPv6 subnet. ISATAP is available in Windows, Linux, and Cisco IOS.3.8. Tunneling IPv6 over UDP through NATs (Teredo)
Teredo is specified in [RFC4380] and a few updates; it is designed as an automatic tunnel mechanism of last resort. It can configure an IPv6 address behind most NAT devices, but not all. Because Teredo uses encapsulation in UDP, multiple Teredo clients can be simultaneously active behind the same NAT. For each Teredo client, a single IPv6 address is then created at the expense of a single external UDP port. The operation of Teredo is based on a classification of NAT [RFC3489] as established during an interaction with a Teredo server. This classification has since been obsoleted (by [RFC5389]) because it assigns more properties to NAT than achieved in reality. Teredo is present in Windows XP and later and is enabled by default in Windows Vista and later. However, if IPv6 connectivity is only possible through Teredo, then Windows will not look up AAAA records when resolving domain names. This means that Teredo is only used to connect to explicit IPv6 addresses obtained through another mechanism than DNS. An open-source implementation named Miredo exists for other platforms. The performance of Teredo falls noticeably short of that of IPv4. The setup time of a connection involves finding a Teredo relay nearby the native address to encapsulate and decapsulate traffic, and finding this relay can take in the order of seconds. This process is not sufficiently reliable; Teredo fails in about 37% [TERTST] of its attempts to connect to native IPv6 destinations. The roundtrip time of traffic can add tenths of a second, and jitter generally worsens if it is dependent on a public relay.
Teredo clients need to be configured with a Teredo server when setting up their local IPv6 address and when initiating a connection to a native IPv6 destination. The hostnames of the Teredo servers are usually preconfigured by the vendor of the Teredo implementation. All Microsoft Windows implementations use Teredo servers provided by Microsoft by default.3.9. IPv6 Rapid Deployment (6rd)
6rd [RFC5969] is used by service providers to connect customer networks behind a CPE (Customer Premises Equipment) to the IPv6 Internet. The structure of the 6rd protocol is based on 6to4, and it has the same minimal overhead as all protocols that use protocol 41 encapsulation. The main differences between 6rd and 6to4 are that 6rd is meant to be used inside a service provider's network and does not use a special IPv6 prefix but one or more prefixes routed to the service provider. As such, 6rd users aren't immediately recognisable by their IPv6 address the way 6to4 users are. Where 6to4 uses relays based on global anycast routing, 6rd uses relays provided and maintained by the service provider. Because of this architecture, the tunnel does not traverse unknown networks; this makes any debugging much easier. 6rd is completely stateless once it is configured. The tunnel endpoints can therefore be deployed using anycast. This is commonly done for the 6rd border relays deployed by the service provider to provide redundancy. Because of the different prefix, the device used as the 6rd client cannot use the hard-coded IPv6 prefix calculation and relay addresses of 6to4. Instead, the 6rd client needs to receive configuration information to work. In principle, 6rd nodes may be configured in a variety of ways, the most common one being through DHCP. If the client receives its IPv4 address from a DHCPv4 server, then the 6rd configuration can be included in the DHCP message exchange using the 6rd DHCPv4 Option defined in [RFC5969]. Manual configuration of 6rd options and configuration using [TR-069] is also possible. The main advantage of using 6rd is that it allows service providers to deploy IPv6 on last-mile access networks that for some reason cannot provide native IPv6 connectivity. It does not share the lack of predictable routing that 6to4 suffers from because all routing, encapsulation, and decapsulation are done by the service provider.
6rd is intended to be a service managed by an ISP or enterprise IT department that must explicitly make 6rd available for clients to be able to use it.3.10. Native IPv6 behind NAT44 CPEs (6a44)
Inspired by Teredo, the 6a44 tunnel is described in "Native IPv6 behind IPv4-to-IPv4 NAT Customer Premises Equipment (6a44)" [RFC6751]. Its purpose is to enable Internet Service Providers to establish IPv6 connectivity for their customers, in spite of the use of a CPE or home gateway that is not prepared for IPv6. The infrastructure required for this is a 6a44 relay in the ISP's network and a 6a44 client in the customer's internal network. 6a44 was explicitly designed to overcome the noted problems with Teredo. Where Teredo was designed as a global solution without dependency on ISP cooperation, the 6a44 tunnel explicitly assumes ISP cooperation. Instead of using Teredo's well-known prefix, a /48 prefix out of the ISP's address space is used. A well-known (anycast) IPv4 address has been assigned for the 6a44 relay to be run inside the ISP network without client configuration. This well-known address is allocated from the same IPv4 /24 as 6to4. As part of its bootstrapping, a 6a44 client requests an address from the 6a44 relay, and a regular keepalive sent by the 6a44 client to the 6a44 relay keeps mapping state in NATs and firewalls on the path alive. Traffic passed from the native IPv6 Internet to 6a44 is encapsulated in UDP and IPv4 by the relay and decapsulated by the 6a44 client; the opposite is done in the other direction.3.11. Locator/ID Separation Protocol (LISP)
The Locator/ID Separation Protocol (LISP) [RFC6830] is a protocol to separate the identity of systems from their location on the Internet and/or internal network. The addresses of the systems are called Endpoint Identifiers (EIDs), and the addresses of the gateways are called Routing Locators (RLOCs). It is possible to use IPv6 EIDs with IPv4 RLOCs and thereby use LISP for tunneling IPv6 over IPv4. LISP defines its own packet formats for encapsulation of data packets and for control messages. All such packets are then encapsulated in UDP. Data packets use port 4341, and control packets use port 4342. The LISP specification consists of several RFCs. The relevant ones for IPv6-in-IPv4 tunneling are the base specification [RFC6830], "Interworking between Locator/ID Separation Protocol (LISP) and Non- LISP Sites" [RFC6832], and "Locator/ID Separation Protocol (LISP) Map-Server Interface" [RFC6833].
+----+ +----+
| MS | | MR |
+----+ +----+ +-----+ /-----------\
| | /---| xTR |---| LISP site |
+------+ /------------\---/ +-----+ \-----------/
| PxTR |---| IP network |
+------+ \------------/---\ +-----+ /-----------\
| \---| xTR |---| LISP site |
/---------------\ +-----+ \-----------/
| Non-LISP site |
\---------------/
An Example of a LISP Deployment
LISP introduces new terminology and new concepts. The relevant ones
for this document are:
ITR: Ingress Tunnel Router, a router encapsulating data packets at
the border of a LISP site
ETR: Egress Tunnel Router, a router decapsulating data packets at
the border of a LISP site
xTR: A router performing both the ITR and the ETR functions
PITR: Proxy ITR, a router accepting traffic from non-LISP sites,
encapsulating it, and tunneling it to the LISP sites
PETR: Proxy ETR, a router accepting traffic from LISP sites to send
it to non-LISP sites
PxTR: A router performing both the PITR and the PETR functions
MS: Map Server, a server accepting RLOC registrations from ETRs
MR: Map Resolver, a server that can resolve queries for RLOCs from
ITRs
LISP ETRs register the EID prefixes for which they can handle traffic
with one or more Map Servers. ITRs and PITRs can then query Map
Resolvers to determine which RLOCs to use when sending traffic to a
LISP site. PITRs advertise aggregates of EID prefixes to the global
routing table and provide tunneling services for them so that non-
LISP sites can reach LISP sites. PETRs provide a way for LISP sites
to send traffic to non-LISP sites.
LISP is a complex protocol if only used for tunneling. Features of LISP are that ETRs can advertise their own RLOC addresses, that one site can have multiple xTRs with independent RLOCs, and that the LISP site administrator can specify priorities and weights for those RLOCs. This provides redundancy and explicit load balancing between RLOCs. It also allows for automatic tunneling between different sites without using a PxTR if both sites use Map Servers and Map Resolvers that are interconnected, for example, by participating in the LISP Beta Network [LISPBETA]. To facilitate these interconnections, the LISP Delegated Database Tree (DDT) system is available. LISP is implemented on most Cisco devices. There are implementations available for FreeBSD and Linux, as well as a platform-independent implementation in the Python programming language. Note that for LISP to work, a mapping service not unlike the DNS must be in place.3.12. Subnetwork Encapsulation and Adaptation Layer (SEAL)
The Subnetwork Encapsulation and Adaptation Layer (SEAL) [SEAL] (along with its companion technologies cited therein) provides a hybrid configured/automatic tunneling system. SEAL itself provides a mid-layer of encapsulation between the inner IPv6 header and the outer IPv4 header, i.e., as IPv4-SEAL-IPv6. SEAL can also be used in conjunction with an outer UDP encapsulation header, e.g., if NAT traversal is necessary. The SEAL tunnel endpoint creates bidirectional configured tunnels to reach default IPv6 routers, and it discovers unidirectional automatic tunnels. SEAL tunnels can be configured over multiple underlying IPv4 links whether the addresses are provisioned from public or private IPv4 addressing domains. In that case, multihoming and traffic engineering are naturally supported. SEAL provides an optional 32-bit identifier and variable-length Integrity Check Vector that can be used for packet identification, message origin authentication, anti-replay, and a mid-layer segmentation and reassembly capability. SEAL also provides a SEAL Control Message Protocol (SCMP) used for neighbour coordinations between tunnel endpoints. These coordinations are used for functions such as tunnel MTU signalling, route optimisations, neighbour reachability testing, and so on. SEAL ensures that packets that are no larger than 1500 bytes can be transported through the tunnel by using a tunnel segmentation function. IPv6 packets that are too large to transport through the tunnel whole are split into segments. The segments are encapsulated in IPv4 and reassembled into the original IPv6 packets at the remote
tunnel endpoint. SEAL also admits packets larger than 1500 bytes into the tunnel on a best-effort basis in case the path between the tunnel endpoints can support the larger size. When SEAL is used alone without its companion technologies, it can be used in the same scenarios as for GRE. However, SEAL provides advanced capabilities that make it better suited than GRE for many use cases. There is currently an experimental open-source implementation of SEAL.3.13. Peer-to-Peer IPv6 on Any Internetwork (6bed4)
The 6bed4 tunnel is specified in "6bed4: Peer-to-Peer IPv6 on Any Internetwork" [6BED4]. A specific goal of 6bed4 is to achieve direct communication between peers when the intermediate infrastructure does not prohibit it. The advantage of direct communication is to get a performance level similar to IPv4. The address of a 6bed4 peer is formed from the publicly visible IPv4 address and UDP port. The tunnel service used for fallback connectivity can run anywhere -- perhaps at the local ISP or with a third-party service provider for 6bed4, or even on a well-known address. It is currently an NBMA protocol; there are openings for expansion with multicast. The setup of 6bed4 is somewhat similar to 6to4, except that it employs UDP so it can be used behind NAT. It also has elements found in Teredo but without a need to classify NATs and induce behaviour from that. The 6bed4 tunnel makes no assumptions about the capabilities of NAT devices beyond being able to do straightforward NAT on UDP packets. Given this, 6bed4 can create reliable IPv6 tunnels. In environments where direct connections between 6bed4 peers are possible, additional path stretch compared to IPv4 communication is avoided, so 6bed4 performance comes close to IPv4 performance. In situations where it is not possible to run over the direct path between two peers because a NAT that does not conform to [RFC4787] is on the path, a fallback to a tunnel server is used. This increases path stretch and affects scalability through its impact on roundtrip times and jitter. Another area where the tunnel server is needed is for connectivity between 6bed4 peers and native IPv6 hosts. For reasons of performance and scalability, connections between 6bed4 peers are preferred over connections between a 6bed4 peer and a native IPv6 host. A default address exists to support zero-config operation, but it is possible to locally configure a tunnel server as a fallback route, which then also defines the tunnel server for the return path.
6bed4 has been specifically designed to support real-time interactive traffic streams, such as SIP calls, between 6bed4-supporting end points, assuming that each prefers 6bed4-to-6bed4 traffic over 6bed4- to-native traffic. Under that premise, the only hosts that need to go through a tunnel server are those that are behind a NAT with Address-Dependent Mapping or Address and Port-Dependent Mapping. A number of different implementations of 6bed4 have been constructed [6BED4] during the ongoing development of its specification.
(page 20 continued on part 2)