RFC 6081
Teredo Extensions
Internet Engineering Task Force (IETF) D. Thaler Request for Comments: 6081 Microsoft Updates: 4380 January 2011 Category: Standards Track ISSN: 2070-1721 Teredo ExtensionsAbstract
This document specifies a set of extensions to the Teredo protocol. These extensions provide additional capabilities to Teredo, including support for more types of Network Address Translations (NATs) and support for more efficient communication. Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in 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/rfc6081. Copyright Notice Copyright (c) 2011 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. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 3.1. Symmetric NAT Support Extension . . . . . . . . . . . . . 9 3.2. UPnP-Enabled Symmetric NAT Extension . . . . . . . . . . . 11 3.3. Port-Preserving Symmetric NAT Extension . . . . . . . . . 13 3.4. Sequential Port-Symmetric NAT Extension . . . . . . . . . 14 3.5. Hairpinning Extension . . . . . . . . . . . . . . . . . . 15 3.6. Server Load Reduction Extension . . . . . . . . . . . . . 17 4. Message Syntax . . . . . . . . . . . . . . . . . . . . . . . . 18 4.1. Trailers . . . . . . . . . . . . . . . . . . . . . . . . . 18 4.2. Nonce Trailer . . . . . . . . . . . . . . . . . . . . . . 19 4.3. Alternate Address Trailer . . . . . . . . . . . . . . . . 19 4.4. Neighbor Discovery Option Trailer . . . . . . . . . . . . 20 4.5. Random Port Trailer . . . . . . . . . . . . . . . . . . . 21 5. Protocol Details . . . . . . . . . . . . . . . . . . . . . . . 22 5.1. Common Processing . . . . . . . . . . . . . . . . . . . . 22 5.1.1. Refresh Interval . . . . . . . . . . . . . . . . . . . 22 5.1.2. Trailer Processing . . . . . . . . . . . . . . . . . . 23 5.2. Symmetric NAT Support Extension . . . . . . . . . . . . . 23 5.2.1. Abstract Data Model . . . . . . . . . . . . . . . . . 24 5.2.2. Timers . . . . . . . . . . . . . . . . . . . . . . . . 24 5.2.3. Initialization . . . . . . . . . . . . . . . . . . . . 24 5.2.4. Message Processing . . . . . . . . . . . . . . . . . . 24 5.3. UPnP-Enabled Symmetric NAT Extension . . . . . . . . . . . 25 5.3.1. Abstract Data Model . . . . . . . . . . . . . . . . . 26 5.3.2. Timers . . . . . . . . . . . . . . . . . . . . . . . . 26 5.3.3. Initialization . . . . . . . . . . . . . . . . . . . . 27 5.3.4. Message Processing . . . . . . . . . . . . . . . . . . 28 5.3.5. Shutdown . . . . . . . . . . . . . . . . . . . . . . . 29 5.4. Port-Preserving Symmetric NAT Extension . . . . . . . . . 30 5.4.1. Abstract Data Model . . . . . . . . . . . . . . . . . 30 5.4.2. Timers . . . . . . . . . . . . . . . . . . . . . . . . 31 5.4.3. Initialization . . . . . . . . . . . . . . . . . . . . 32 5.4.4. Message Processing . . . . . . . . . . . . . . . . . . 32 5.5. Sequential Port-Symmetric NAT Extension . . . . . . . . . 35 5.5.1. Abstract Data Model . . . . . . . . . . . . . . . . . 35 5.5.2. Timers . . . . . . . . . . . . . . . . . . . . . . . . 36 5.5.3. Initialization . . . . . . . . . . . . . . . . . . . . 37 5.5.4. Message Processing . . . . . . . . . . . . . . . . . . 37 5.6. Hairpinning Extension . . . . . . . . . . . . . . . . . . 39 5.6.1. Abstract Data Model . . . . . . . . . . . . . . . . . 39 5.6.2. Timers . . . . . . . . . . . . . . . . . . . . . . . . 39 5.6.3. Initialization . . . . . . . . . . . . . . . . . . . . 39 5.6.4. Message Processing . . . . . . . . . . . . . . . . . . 40
5.7. Server Load Reduction Extension . . . . . . . . . . . . . 41
5.7.1. Abstract Data Model . . . . . . . . . . . . . . . . . 41
5.7.2. Timers . . . . . . . . . . . . . . . . . . . . . . . . 41
5.7.3. Initialization . . . . . . . . . . . . . . . . . . . . 42
5.7.4. Message Processing . . . . . . . . . . . . . . . . . . 42
6. Protocol Examples . . . . . . . . . . . . . . . . . . . . . . 42
6.1. Symmetric NAT Support Extension . . . . . . . . . . . . . 42
6.2. UPnP-Enabled Symmetric NAT Extension . . . . . . . . . . . 45
6.3. Port-Preserving Symmetric NAT Extension . . . . . . . . . 47
6.4. Sequential Port-Symmetric NAT Extension . . . . . . . . . 51
6.5. Hairpinning Extension . . . . . . . . . . . . . . . . . . 54
6.6. Server Load Reduction Extension . . . . . . . . . . . . . 57
7. Security Considerations . . . . . . . . . . . . . . . . . . . 58
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 58
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 58
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 58
10.1. Normative References . . . . . . . . . . . . . . . . . . . 58
10.2. Informative References . . . . . . . . . . . . . . . . . . 59
1. Introduction
This document specifies extensions to the Teredo protocol, as
specified in [RFC4380]. These extensions provide additional
capabilities to Teredo, including support for more types of Network
Address Translations (NATs) and support for more efficient
communication.
2. Terminology
Because this document extends [RFC4380], it uses the following
terminology, for consistency with [RFC4380].
Address-Restricted NAT: A restricted NAT that accepts packets from an
external host's IP address X and port Y if the internal host has sent
a packet that is destined to IP address X regardless of the
destination port. In the terminology of [RFC4787], this is a NAT
with Endpoint-Independent Mapping and Address-Dependent Filtering.
Address-Symmetric NAT: A symmetric NAT that has multiple external IP
addresses and that assigns different IP addresses and ports when
communicating with different external hosts.
Cone NAT: A NAT that maps all requests from the same internal IP
address and port to the same external IP address and port.
Furthermore, any external host can send a packet to the internal host
by sending a packet to the mapped external address and port. In the
terminology of [RFC4787], this is a NAT with Endpoint-Independent
Mapping and Endpoint-Independent Filtering.
Direct Bubble: A Teredo bubble that is sent directly to the IPv4 node whose Teredo address is contained in the Destination field of the IPv6 header, as specified in Section 2.8 of [RFC4380]. The IPv4 Destination Address and UDP Destination Port fields contain a mapped address/port. Echo Test: A mechanism to predict the mapped address/port a sequential port-symmetric NAT is using for a client behind it. Hairpinning: A feature that is available in some NATs where two or more hosts are positioned behind a NAT and each of those hosts is assigned a specific external (public) address and port by the NAT. Hairpinning support in a NAT allows these hosts to send a packet to the external address and port that is assigned to one of the other hosts, and the NAT automatically routes the packet back to the correct host. The term hairpinning is derived from the behavior of the packet, which arrives on, and is sent out to, the same NAT interface. Indirect Bubble: A Teredo bubble that is sent indirectly (via the destination's Teredo server) to another Teredo client, as specified in Section 5.2.4 of [RFC4380]. Local Address/Port: The IPv4 address and UDP port from which a Teredo client sends Teredo packets. The local port is referred to as the Teredo service port in [RFC4380]. The local address of a node may or may not be globally routable because the node can be located behind one or more NATs. Mapped Address/Port: A global IPv4 address and a UDP port that results from the translation of a node's own local address/port by one or more NATs. The node learns these values through the Teredo protocol as specified in [RFC4380]. For symmetric NATs, the mapped address/port can be different for every peer with which a node tries to communicate. Network Address Translation (NAT): The process of converting between IP addresses used within an intranet or other private network and Internet IP addresses. Nonce: A time-variant random value used in the connection setup phase to prevent message replay and other types of attacks. Peer: A Teredo client with which another Teredo client needs to communicate.
Port-Preserving NAT: A NAT that translates a local address/port to a mapped address/port such that the mapped port has the same value as the local port, as long as that same mapped address/port has not already been used for a different local address/port. Port-Restricted NAT: A restricted NAT that accepts packets from an external host's IP address X and port Y only if the internal host has sent a packet destined to IP address X and port Y. In the terminology of [RFC4787], this is a NAT with Endpoint-Independent Mapping and Address and Port-Dependent Filtering. Port-Symmetric NAT: A symmetric NAT that has only a single external IP address and hence only assigns different ports when communicating with different external hosts. Private Address: An IPv4 address that is not globally routable but is part of the private address space specified in Section 3 of [RFC1918]. Public Address: An external global address used by a NAT. Restricted NAT: A NAT where all requests from the same internal IP address and port are mapped to the same external IP address and port. Unlike the cone NAT, an external host can send packets to an internal host (by sending a packet to the external mapped address and port) only if the internal host has first sent a packet to the external host. There are two kinds of restricted NATs: address-restricted NATs and port-restricted NATs. Sequential Port-Symmetric NAT: A port-symmetric NAT that allocates external ports sequentially for every {internal IP address and port, destination IP address and port} tuple. The delta used in the sequential assignment is typically 1 or 2 for most such NATs. Symmetric NAT: A NAT where all requests from the same internal IP address and port and to the same destination IP address and port are mapped to the same external IP address and port. Requests from the same internal IP address and port to a different destination IP address and port may be mapped to a different external IP address and port. Furthermore, a symmetric NAT accepts packets received from an external host's IP address X and port Y only if some internal host has sent packets to IP address X and port Y. In the terminology of [RFC4787], this is a NAT with a mapping behavior of either Address- Dependent Mapping or Address- and Port-Dependent Mapping, and a filtering behavior of either Address-Dependent Filtering or Address- and Port-Dependent Filtering.
Teredo Bubble: A Teredo control message (specified in Section 2.8 of [RFC4380]) that is used to create a mapping in a NAT. There are two types of Teredo bubbles: direct bubbles and indirect bubbles. Teredo Client: A node that has access to the IPv4 Internet and wants to gain access to the IPv6 Internet using the Teredo protocol. Teredo IPv6 Address: An IPv6 address of a Teredo client, as specified in Section 2.14 of [RFC4380]. Teredo Secondary Server Address: A secondary IPv4 address of a Teredo server with which a Teredo client is configured, as specified in Section 5.2 of [RFC4380]. Teredo Server: A node that has a globally routable address on the IPv4 Internet, and is used as a helper to provide IPv6 connectivity to Teredo clients. Teredo Server Address: A (primary) IPv4 address of a Teredo server with which a Teredo client is configured, as specified in Section 5.2 of [RFC4380]. UPnP-enabled NAT: A NAT that has the UPnP device control protocol enabled, as specified in [UPNPWANIP]. (Note that today, by default, most UPnP-capable NATs have the UPnP device control protocol disabled.) The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].3. Overview
The Teredo protocol (as specified in [RFC4380]) enables nodes located behind one or more IPv4 NATs to obtain IPv6 connectivity by tunneling packets over UDP. When a node behind a NAT needs to communicate with a peer (i.e., another node) that is behind a NAT, there are four sets of IPv4 address/port pairs of interest: o The node's own IPv4 address/port. o The external IPv4 address/port to which the node's NAT translates. o The peer's own IPv4 address/port. o The external IPv4 address/port to which the peer's NAT translates.
When the node sends a packet to a peer, the node needs to send it from the node's own IPv4 address/port, destined to the peer's external IPv4 address/port. By the time it arrives at the peer (i.e., after passing through both NATs), the peer will see the same packet as coming from the node's external IPv4 address/port, destined to the peer's own IPv4 address/port. In this document, the term local address/port refers to a Teredo client's own IPv4 address/port, and mapped address/port refers to the external IPv4 address/port to which its NAT translates the local address/port. That is, the mapped address/port is what the IPv4 Internet sees the Teredo client as. A Teredo client running on a node communicates with a Teredo server to discover its mapped address/port. The mapped address/port, along with the Teredo server address, is used to generate an IPv6 address known as a Teredo IPv6 address. This allows any peer that gets the node's IPv6 address to easily determine the external IPv4 address/ port to which to send IPv6 packets encapsulated in IPv4 UDP messages. This document specifies extensions to the Teredo protocol. These Teredo extensions are independent of each other and can be implemented in isolation, except that the UPnP-Symmetric NAT Extension and the Port-Preserving Symmetric NAT Extension both require the Symmetric NAT Support Extension to be implemented. An implementation of this specification can support any combination of the Teredo extensions, subject to the above-mentioned restriction. The following matrix outlines the connectivity improvements of some of the extensions outlined in this document.
Destination NAT
| | | | | | Port-| | |
| | | | UPnP | UPnP | pres.| Seq. | |
| | Addr.| Port | Port | Port | Port-| Port-| Port-| Addr
Source NAT| Cone | rest.| rest.| rest.| symm.| symm.| symm.| symm.| symm
----------+------+------+------+------+------+------+------+------+-----
Cone | Yes | Yes | Yes | Yes | SNS | SNS | SNS | SNS | SNS
----------+------+------+------+------+------+------+------+------+-----
Address | Yes | Yes | Yes | Yes | SNS | SNS | SNS | SNS | No
restricted| | | | | | | | |
----------+------+------+------+------+------+------+------+------+-----
Port | Yes | Yes | Yes | Yes | No | SNS+ | SNS+ | No | No
restricted| | | | | | PP | SS | |
----------+------+------+------+------+------+------+------+------+-----
UPnP Port-| Yes | Yes | Yes | Yes | SNS+ | No | No | No | No
restricted| | | | | UPnP | | | |
----------+------+------+------+------+------+------+------+------+-----
UPnP Port | SNS | SNS | No | SNS+ | SNS+ | No | No | No | No
symmetric | | | | UPnP | UPnP | | | |
----------+------+------+------+------+------+------+------+------+-----
Port- | | | SNS | | | SNS | SNS | |
preserving| SNS | SNS | + | No | No | + | + | No | No
Port- | | | PP | | | PP | SS | |
symmetric | | | | | | | | |
----------+------+------+------+------+------+------+------+------+-----
Sequential| | | SNS | | | | | |
Port- | SNS | SNS | + | No | No | No | No | No | No
symmetric | | | SS | | | | | |
----------+------+------+------+------+------+------+------+------+-----
Port- | SNS | SNS | No | No | No | No | No | No | No
symmetric | | | | | | | | |
----------+------+------+------+------+------+------+------+------+-----
Address- | SNS | No | No | No | No | No | No | No | No
symmetric | | | | | | | | |
----------+------+------+------+------+------+------+------+------+-----
Yes = Supported by [RFC4380].
SNS = Supported with the Symmetric NAT Support Extension.
SNS+UPnP = Supported with the Symmetric NAT Support Extension and UPnP
Symmetric NAT Extension.
SNS+PP = Supported with the Symmetric NAT Support Extension and Port-
Preserving Symmetric NAT Extension.
SNS+SS = Supported with the Symmetric NAT Support Extension and
Sequential Port-Symmetric NAT Extension.
No = No connectivity. Figure 1: Matrix of Connectivity Improvements for Teredo Extensions Note that as with [RFC4380], if the qualification process is not successful, Teredo will not be configured with an IPv6 address, and connectivity will function as if Teredo were not present. Similarly, for any combination of NAT types that are not supported by Teredo and the extensions defined herein, the connectivity tests between a client and a peer will fail within a finite period of time, allowing the client to handle this case as with any other type of unreachable destination address (e.g., by trying another address of the destination such as a native IPv4 address).3.1. Symmetric NAT Support Extension
The qualification procedure (as specified in Section 5.2.1 of [RFC4380]) is a process that allows a Teredo client to determine the type of NAT that it is behind, in addition to its mapped address/port as seen by its Teredo server. However, Section 5.2.1 of [RFC4380] suggests that if the client learns it is behind a symmetric NAT, the Teredo client should go into an "offline state" where it is not able to use Teredo. The primary reason for doing so is that it is not easy for Teredo clients to connect to each other if either or both of them are positioned behind a symmetric NAT. Because of the way a symmetric NAT works, a peer sees a different mapped address/port in the IPv4/UDP headers of packets coming from a Teredo client than the node's Teredo server sees (and hence appears in the node's Teredo IPv6 address). Consequently, a symmetric NAT does not allow incoming packets from a peer that are addressed to the mapped address/port embedded in the node's Teredo IPv6 address. Thus, the incoming packets are dropped and communication with Teredo clients behind symmetric NATs is not established. With the Symmetric NAT Support Extension, Teredo clients begin to use Teredo even after they detect that they are positioned behind a symmetric NAT. Consider the topology shown in Figure 2. Teredo Client B uses Teredo Server 2 to learn that its mapped address/port is 192.0.2.10:8192, and constructs a Teredo IPv6 address, as specified in Section 4 of [RFC4380]. Hence, c633:6476 is the hexadecimal value of the address of Teredo Server 2 (198.51.100.118), the mapped port is exclusive- OR'ed with 0xffff to form dfff, and the Mapped Address is exclusive- OR'ed with 0xffffffff to form 3fff:fdf5.
Teredo Client A uses Teredo Server 1 to learn that its mapped address/port is 192.0.2.1:4096 and, with this extension, constructs a Teredo IPv6 address (as specified in Section 4 of [RFC4380]) even though it learns that it is behind a symmetric NAT. Hence, cb00:7178 is the hexadecimal value of the address of Teredo Server 1 (203.0.113.120), the mapped port is exclusive-OR'ed with 0xffff to form efff, and the Mapped Address is exclusive-OR'ed with 0xffffffff to form 3fff:fdfe. The Symmetric NAT Support Extension enables a Teredo client positioned behind a symmetric NAT to communicate with Teredo peers positioned behind a cone or address-restricted NATs as follows, depending on what side initiates the communication. -------------------------------------------- / \ < IPv6 Internet > \ / -|----------------------------------------|- | | +----------+ +----------+ | Teredo | | Teredo | | Server 1 | | Server 2 | +----------+ +----------+ 203.0.113.120| 198.51.100.118| -|----------------------------------------|- / \ < IPv4 Internet > \ / -|----------------------------------------|- 192.0.2.1| 192.0.2.10| UDP port 4096| UDP port 8192| +---------+ +----------+ |Symmetric| |Other type| | NAT | | of NAT | +---------+ +----------+ | | +-----------------+ +-----------------+ | Teredo client A | | Teredo client B | +-----------------+ +-----------------+ 2001:0:cb00:7178:0:efff:3fff:fdfe 2001:0:c633:6476:0:dfff:3fff:fdf5 Teredo Address Teredo Address Figure 2: Symmetric NAT Example In the first case, assume that a Teredo Client B (B) positioned behind a cone or address-restricted NATs initiates communication with Teredo Client A (A) positioned behind a symmetric NAT. B sends an
indirect bubble via A's server (Teredo Server 1) to A, and A responds with a direct bubble. This direct bubble reaches B, because it is positioned behind a cone or address-restricted NAT. However, the mapped address/port in the IPv4/UDP headers of the direct bubble are different from the mapped address/port embedded in A's Teredo IPv6 address. B therefore remembers the mapped address/port of the direct bubble and uses them for future communication with A, and thus communication is established. In the second case, assume that A, positioned behind a symmetric NAT, initiates communication with B, positioned behind a cone or address- restricted NAT. A sends an indirect bubble to B via B's server (Teredo Server 2), and B responds with a direct bubble. This direct bubble is dropped by A's symmetric NAT because the direct bubble is addressed to the mapped address/port embedded in A's Teredo IPv6 address. However, communication can be established by having B respond with an indirect bubble via A's server (Teredo Server 1). Now the scenario is similar to the first case and communication will be established.3.2. UPnP-Enabled Symmetric NAT Extension
The UPnP-enabled Symmetric NAT Extension is dependent on the Symmetric NAT Support Extension. Only if Teredo clients have been enabled to acquire a Teredo IPv6 address in spite of being behind a symmetric NAT will this extension help in traversing UPnP-enabled Symmetric NATs. The Symmetric NAT Support Extension enables communication between Teredo clients behind symmetric NATs with Teredo clients behind cone NATs or address-restricted NATs. However, clients behind symmetric NATs can still not communicate with clients behind port-restricted NATs or symmetric NATs. Referring again to Figure 2 (see Section 3.1), assume that Teredo Client A is positioned behind a symmetric NAT and initiates communication with Client B, which is positioned behind a port- restricted NAT. Client A sends a direct bubble and an indirect bubble to Client B via Client B's server (Teredo Server 2). As per the characteristics of the symmetric NAT, the IPv4 source of the direct bubble contains a different mapped address and/or port than the one embedded in the Teredo server. This direct bubble is dropped because Client B's NAT does not have state to let it pass through, and Client B does not learn the mapped address/port used in the IPv4/ UDP headers. In response to the indirect bubble from Client A, Client B sends a direct bubble destined to the mapped address/port embedded in Client A's Teredo IPv6 address. This direct bubble is dropped because Client A's NAT does not have state to accept packets
destined to that mapped address/port. The direct bubble does, however, cause Client B's NAT to set up outgoing state for the mapped address/port embedded in Client A's Teredo IPv6 address. As described in Section 3.1, Client B also sends an indirect bubble that elicits a direct bubble from Client A. Unlike the case in Section 3.1, however, the direct bubble from Client A is dropped as Client B's NAT does not have state for the mapped address/port that Client A's NAT uses. Note that Client B's NAT is port-restricted and hence requires both the mapped address and port to be the same as in its outgoing state, whereas in Section 3.1, Client A's NAT was a cone or address-restricted NAT which only required the mapped address (but not port) to be the same. Thus, communication between Client A and Client B fails. If Client B were behind a symmetric NAT, the problem is further complicated by Client B's NAT using a different outgoing mapped address/port than the one embedded in Client B's Teredo IPv6 address. If a Teredo client is separated from the global Internet by a single UPnP-enabled symmetric or port-restricted NAT, it can communicate with other Teredo clients that are positioned behind a single UPnP- enabled symmetric or port-restricted NAT as follows. Teredo clients, before communicating with the Teredo server during the qualification procedure, use UPnP to reserve a translation from a local address/port to a mapped-address/port. Therefore, during the qualification procedure, the Teredo server reflects back the reserved mapped address/port, which then is included in the Teredo IPv6 address. The mapping created by UPnP allows the NAT to forward packets destined for the mapped address/port to the local address/ port, independent of the source of the packets. It typically does not, however, cause packets sourced from the local address/port to be translated to have the mapped address/port as the external source and hence continues to function as a symmetric NAT in this respect. Thus, a Teredo client, positioned behind a UPnP-enabled symmetric NAT, can receive a direct bubble sent by any Teredo peer. The Teredo client compares the peer's mapped address/port as seen in the IPv4/ UDP headers with the mapped address/port in the peer's Teredo IPv6 address. If the two mappings are different, the packet was sent by another Teredo client positioned behind a symmetric NAT. The Symmetric NAT Support Extension suggested that the Teredo client use the peer's mapped address/port seen in the IPv4/UDP headers for future communication. However, because symmetric NAT-to-symmetric NAT communication would not have been possible anyway, the Teredo client sends back a direct bubble to the mapped port/address embedded
in the peer's Teredo IPv6 address. If the peer is also situated behind a UPnP-enabled NAT, the direct bubble will make it through and communication will be established. Even though communication is established between the two Teredo IPv6 addresses, the mappings will be asymmetric in the two directions of data transfer. Specifically, incoming packets will be destined to the reserved mapped address/port that is embedded in the Teredo IPv6 address. Outgoing packets will instead appear to come from a different mapped address/port due to the symmetric NAT behavior.3.3. Port-Preserving Symmetric NAT Extension
The Port-Preserving Symmetric NAT Extension is dependent on the Symmetric NAT Support Extension (Section 3.1). Only if Teredo clients have been enabled to acquire a Teredo IPv6 address in spite of being behind a symmetric NAT will this extension help in traversing port-preserving symmetric NATs. The Symmetric NAT Support Extension enables communication between Teredo clients behind symmetric NATs with Teredo clients behind cone NATs or address-restricted NATs. However, clients behind symmetric NATs can still not communicate with clients behind port-restricted or symmetric NATs, as described in Section 3.2. Note that the Port- Preserving Symmetric NAT Extension described here is independent of the UPnP-enabled Symmetric NAT Extension, described in Section 3.2. If a Teredo client is positioned behind a port-preserving symmetric NAT, the client can communicate with other Teredo clients positioned behind a port-restricted NAT or a port-preserving symmetric NAT as follows. Teredo clients compare the mapped port learned during the qualification procedure with their local port to determine if they are positioned behind a port-preserving NAT. If both the mapped port and the local port have the same value, the Teredo client is positioned behind a port-preserving NAT. At the end of the qualification procedure, the Teredo client also knows if it is positioned behind a symmetric NAT, as described in Section 3.1. Teredo clients positioned behind port-preserving symmetric NATs can also listen on randomly chosen local ports. If the randomly chosen local port has not been used by the symmetric NAT as a mapped port in a prior port-mapping, the NAT uses the same port number as the mapped port. Thus, the challenge is to get the first direct bubble sent out from the random port to be destined to a valid destination address and port. When the mapped address/port is embedded in the destination's Teredo IPv6 address, this is easy.
The communication setup is more complicated when the destination Teredo client is also positioned behind a port-preserving symmetric NAT. In such a case, both Teredo clients need to send their first direct bubbles to the correct destination mapped address/port. Thus, the protocol messages, which communicate one Teredo client's random port number to the other Teredo client, must be exchanged indirectly (via Teredo servers). When one Teredo client has access to the other Teredo client's random port number, it can send a direct bubble destined to the mapped address embedded in the destination's Teredo IPv6 address, and the mapped port can be the same as the destination's random port number. If both NATs are port-preserving, port-preserved mappings are created on both NATs and the second direct bubble succeeds in reaching the destination.3.4. Sequential Port-Symmetric NAT Extension
The Sequential Port-Symmetric NAT Extension is dependent on the Symmetric NAT Support Extension (Section 3.1). This extension helps in traversing a sequential port-symmetric NAT only if Teredo clients are enabled to acquire a Teredo IPv6 address even when behind a symmetric NAT. When the Sequential Port-Symmetric NAT Extension is used, if a Teredo client is positioned behind a sequential port-symmetric NAT, the client can communicate with other Teredo clients that are positioned behind a port-restricted NAT as follows. During qualification, if the client discovers it is behind a symmetric NAT that is not port-preserving, the client assumes by default that it is behind a sequential port-symmetric NAT. This assumption is proactive for the following reasons: o There is no perfect method of discovering whether the client is behind a sequential port-symmetric NAT. o These kinds of NATs are notorious for changing their behavior. At times, they could be sequential port-symmetric and at other times not. o There is no other solution for symmetric NAT traversal so this is a last resort. Teredo clients positioned behind sequential port-symmetric NATs can also listen on a randomly chosen local port when communicating with a peer. To predict the external port being used for a given peer, the client sends three packets:
o Packet 1 is a router solicitation (as specified in Section 5.2.1 of [RFC4380]) sent to the Teredo server address. o Packet 2 is a direct bubble sent to the peer. o Packet 3 is a router solicitation sent to the secondary Teredo server address. As part of the normal Teredo protocol, the Teredo server responds to packets 1 and 3. Based on the information in the responses, the client now knows that Packet 1 was seen as coming from one external port, and Packet 3 was seen as coming from another external port. Assuming the NAT is a sequential port-symmetric NAT, the external port for Packet 2 is estimated (or predicted) to be midway between the external ports for Packets 1 and 3. Note that because other applications might also have been using the NAT between packets 1 and 3, the actual port might not be exactly the midpoint. The Teredo client then communicates the predicted port to its peer, which sends a direct bubble to the communicated port. If the communicated port is indeed the external port for Packet 2, the direct bubble will reach the Teredo client.3.5. Hairpinning Extension
Hairpinning support in a NAT routes packets that are sent from a private (local) address destined to a public (mapped) address of the NAT, back to another private (local) destination address behind the same NAT. If hairpinning support is not available in a NAT, two Teredo clients behind the same NAT are not able to communicate with each other, as specified in Section 8.3 of [RFC4380]. The Hairpinning Extension enables two clients behind the same NAT to talk to each other when the NAT does not support hairpinning. This process is illustrated in the following diagram.
-------------------------------------------- / \ < IPv6 Internet > \ / --------------------|----------------------- | +----------+ | Teredo | | Server | +----------+ 203.0.113.120| --------------------|----------------------- / \ < IPv4 Internet > \ / --------------------|----------------------- 198.51.100.118| NAT +-------+ without | NAT | hairpinning | E | support +-------+ | +------------------+--------------------+ 192.168.1.0| 192.168.1.1| UDP port 4095| UDP port 4096| +---------+ +----------+ | NAT | | NAT | | F | | G | +---------+ +----------+ | | +-----------------+ +-----------------+ | Teredo client A | | Teredo client B | +-----------------+ +-----------------+ 2001:0:cb00:7178:0:f000:39cc:9b89 2001:0:cb00:7178:0:efff:39cc:9b89 Teredo Address Teredo Address Figure 3: Hairpinning Example The Teredo Client A (A) includes, as part of its indirect bubble sent to Teredo Client B (B), its local address/port. B, upon receiving the indirect bubble, tries to establish communication by sending direct bubbles to the mapped address/port of A, and also to the local address/port of B. If a Teredo client is part of a multi-NAT hierarchy and the NAT to which the Teredo client is connected supports the UPnP protocol (as specified in [UPNPWANIP]), the Teredo client can use UPnP to determine the mapped address/port assigned to it by the NAT. This
information can be included along with the local address/port when sending the indirect bubble. The destination Teredo client now tries to establish a connection by sending direct bubbles to the mapped address/port in the Teredo IPv6 address, to the local address/port included in the bubble, and also to the mapped address/port included in the bubble. Note that UPnP support is only required if the Teredo clients are behind different NATs in a multi-NAT hierarchy. Without UPnP support, the Hairpinning Extension still allows two hosts behind the same non-hairpinning NAT to communicate using their Teredo IPv6 addresses.3.6. Server Load Reduction Extension
If communication between a Teredo client and a Teredo peer was successfully established but at a later stage was silent for a while, for efficiency, it is best to refresh the mapping state in the NATs that are positioned between them. To refresh the communication between itself and a Teredo peer, a Teredo client needs to solicit a direct bubble response from the Teredo peer. An indirect bubble is sent to solicit a direct bubble response from a Teredo peer, as specified in Section 5.2.4 of [RFC4380]. However, these indirect bubbles increase the load on the Teredo server. The Server Load Reduction Extension allows Teredo clients to send direct bubbles most of the time instead of sending indirect bubbles all of the time in the following way: 1. When a Teredo client tries to refresh its communication with a Teredo peer, it uses a direct bubble instead of an indirect bubble. However, because direct bubbles do not normally solicit a response, the direct bubble format is extended to be able to solicit a response. 2. When a Teredo client receives a direct bubble that is soliciting a response, the Teredo client responds with a direct bubble. 3. If attempts to re-establish communication with the help of direct bubbles fail, the Teredo client starts over the process of establishing communication with the Teredo peer, as specified in Section 5.2.4 of [RFC4380].
4. Message Syntax
All Teredo messages are transported over the User Datagram Protocol (UDP), as specified in Section 3 of [RFC4380]. In addition, Section 5.2.3 of [RFC4380] states: An IPv6 packet is deemed valid if it conforms to [RFC2460]: the protocol identifier should indicate an IPv6 packet and the payload length should be consistent with the length of the UDP datagram in which the packet is encapsulated. In addition, the client should check that the IPv6 destination address correspond [sic] to its own Teredo address. This document updates the word "consistent" above as follows. The IPv6 payload length is "consistent" with the length of the UDP datagram if the IPv6 packet length (i.e., the Payload Length value in the IPv6 header plus the IPv6 header size) is less than or equal to the UDP payload length (i.e., the Length value in the UDP header minus the UDP header size). This allows the use of trailers after the IPv6 packet, which are defined in the following sections.4.1. Trailers
Teredo packets can carry a variable number of type-length-value (TLV) encoded trailers, of the following format (intended to be similar to the use of IPv6 options defined in [RFC2460] section 4.2): 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | Value (variable) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type (1 byte): 8-bit identifier of the type of trailer. Length (1 byte): 8-bit unsigned integer. Length of the Value field of this trailer, in octets. Value (variable): Trailer-Type-specific data. The trailer Type identifiers are internally encoded such that their highest-order two bits specify the action that is to be taken if the host does not recognize the trailer Type:
00, 10, 11 - skip over this trailer and continue processing the
packet.
01 - discard the packet.
4.2. Nonce Trailer
The Nonce Trailer is used by the Symmetric NAT Support Extension (and
therefore the UPnP-enabled Symmetric NAT Extension and Port-
Preserving Symmetric NAT Extension also) and the Hairpinning
Extension. The Nonce Trailer can be present in both indirect and
direct bubbles. The nonce in the Nonce Trailer helps authenticate a
Teredo client positioned behind a Symmetric NAT.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type (1 byte): The Trailer Option type. This field MUST be set to
0x01.
Length (1 byte): The length in bytes of the rest of the option. This
field MUST be set to 0x04.
Nonce (4 bytes): The nonce value.
4.3. Alternate Address Trailer
The Alternate Address Trailer is used by the Hairpinning Extension.
The Alternate Address Trailer MUST NOT be present in any packets
other than indirect bubbles sent by a Teredo client. The Alternate
Address Trailer provides another Teredo client positioned behind the
same NAT with more address options that it can use to connect.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Alternate Address/Port List (variable) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type (1 byte): The Trailer Option type. This field MUST be set to
0x03.
Length (1 byte): The length in bytes of the rest of the option. The
value of this field MUST be in the range 8 to 26 (i.e., 2 bytes for
the Reserved field, and 6 bytes for each entry in the Alternate
Address/Port List). This allows for a minimum of one address/port
mapping and a maximum of four address/port mappings to be advertised.
It SHOULD be at most 14 as a maximum of two address/port mappings can
be determined by Teredo: one local address/port and one obtained
using UPnP. Because the length of the alternate address/port is 6
bytes, the valid range of values is only 8, 14, 20, and 26.
Reserved (2 bytes): This field MUST be set to 0x0000 and ignored on
receipt.
Alternate Address/Port List (variable): An array of additional
address/port pairs that can be used by other Teredo clients to
communicate with the sender. Each alternate address/port entry MUST
be formatted as follows:
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
IPv4 Address (4 bytes): An IPv4 address in network byte order. This
field MUST contain a valid unicast address.
Port (2 bytes): A port number in network byte order. This field MUST
NOT be zero.
4.4. Neighbor Discovery Option Trailer
The Neighbor Discovery Option Trailer is used by the Server Load
Reduction Extension because it allows direct bubbles to encode an
IPv6 Neighbor Solicitation (Section 4.3 of [RFC4861]), in addition to
an IPv6 Neighbor Advertisement (Section 4.4 of [RFC4861]). This
allows packets to be sent without having to relay them through a
Teredo server. The Neighbor Discovery Option Trailer allows the
receiver to differentiate between a direct bubble that is soliciting
a response versus a regular direct bubble. This allows Teredo
clients to use direct bubbles to refresh inactive connections instead
of using indirect bubbles.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | DiscoveryType | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type (1 byte): The Trailer Option type. This field MUST be set to
0x04.
Length (1 byte): The length in bytes of the rest of the option. This
field MUST be set to 0x04.
DiscoveryType (1 byte): This field MUST be set to one of the
following values:
TeredoDiscoverySolicitation (0x00): The receiver is requested to
respond with a direct bubble of DiscoveryType
TeredoDiscoveryAdvertisement.
TeredoDiscoveryAdvertisement (0x01): The direct bubble is in
response to a direct bubble or an indirect bubbles containing
DiscoveryType TeredoDiscoverySolicitation.
Reserved (3 bytes): This field MUST be set to 0x000000 on
transmission and ignored on receipt.
4.5. Random Port Trailer
The Random Port Trailer is used by the Port-Preserving Symmetric NAT
Extension in both indirect and direct bubbles.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Random Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type (1 byte): The Trailer Option type. This field MUST be set to
0x05.
Length (1 byte): The length in bytes of the rest of the option. This
field MUST be set to 0x02.
Random Port (2 bytes): The external port that the sender predicts
that its NAT has assigned it for communication with the destination.
This field MUST be specified in network byte order.
(next page on part 2)
