rfc6081.txt

rfc6081.txt

Internet Engineering Task Force (IETF) D. Thaler

Request for Comments: 6081 Microsoft

Updates: 4380 January 2011

Category: Standards Track

ISSN: 2070-1721

Teredo Extensions

Abstract

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.

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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

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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

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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

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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

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RFC 6081 Teredo Extensions January 2011

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.

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RFC 6081 Teredo Extensions January 2011

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:

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RFC 6081 Teredo Extensions January 2011

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.

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RFC 6081 Teredo Extensions January 2011

--------------------------------------------

/ \

< 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

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RFC 6081 Teredo Extensions January 2011

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].

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RFC 6081 Teredo Extensions January 2011

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:

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RFC 6081 Teredo Extensions January 2011

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) |

| |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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RFC 6081 Teredo Extensions January 2011

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.

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RFC 6081 Teredo Extensions January 2011

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.

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RFC 6081 Teredo Extensions January 2011

5. Protocol Details

5.1. Common Processing

The behavior in this section applies to multiple extensions.

Packets equivalent to those sent for a peer the first time a

connection is being established MAY be generated at other

implementation-specific times. (For example, an implementation might

choose to do so when its Neighbor Cache Entry for the peer is in the

PROBE state.)

5.1.1. Refresh Interval

Section 5.2 of [RFC4380] states:

The client must regularly perform the maintenance procedure in

order to guarantee that the Teredo service port remains usable.

The need to use this procedure or not depends on the delay since

the last interaction with the Teredo server. The refresh

procedure takes as a parameter the "Teredo refresh interval".

This parameter is initially set to 30 seconds; it can be updated

as a result of the optional "interval determination procedure".

The randomized refresh interval is set to a value randomly chosen

between 75% and 100% of the refresh interval.

This requirement can be problematic when the client is behind a NAT

that expires state in less than 30 seconds. The optional interval

determination procedure (Section 5.2.7 of [RFC4380]) also does not

provide for intervals under 30 seconds. Hence, this document refines

the behavior by saying the initial parameter SHOULD be configurable

and the default MUST be 30 seconds. An implementation MAY set the

randomized refresh interval to a value randomly chosen within an

implementation-specific range. Such a range MUST fall within 50% to

150% of the refresh interval.

Section 5.2.5 of [RFC4380] states that:

At regular intervals, the client MUST check the "date and time of

the last interaction with the Teredo server" to ensure that at

least one packet has been received in the last Randomized Teredo

Refresh Interval. If this is not the case, the client SHOULD send

a router solicitation message to the server, as specified in

Section 5.2.1;

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RFC 6081 Teredo Extensions January 2011

This document refines the behavior as follows. A Teredo client MAY

choose to send additional router solicitation messages to the server

at other implementation-specific times. (For example, an

implementation might choose to do so when its Neighbor Cache Entry

for the router is in the PROBE state.)

5.1.2. Trailer Processing

A Teredo client MUST process the sequence of trailers in the same

order as they appear in the packet. If the Teredo client does not

recognize the trailer Type while processing the trailers in the

Teredo packet, the client MUST discard the packet if the highest-

order bits of the trailer Type contain 01, or else the Teredo client

MUST skip past the trailer. A Teredo client MUST stop processing the

trailers as soon as a malformed trailer appears in the sequence of

trailers in the packet. A trailer is defined as malformed if it has

any of the following properties:

o The length in bytes of the remainder of the UDP datagram is less

than 2 (the size of the Type and Length fields of a trailer).

o The length in bytes of the remainder of the UDP datagram is less

than 2 + the value of the Length field of the trailer.

5.2. Symmetric NAT Support Extension

Section 5.2.1 of [RFC4380] advises that no Teredo IPv6 address be

configured if the Teredo client is positioned behind a symmetric NAT.

For Teredo clients positioned behind symmetric NATs, the mapped

address/port used by its NAT when communicating with a Teredo peer is

different from the mapped address/port embedded in the Teredo

client’s Teredo IPv6 address. The Symmetric NAT Support Extension

provides a solution to this problem.

In addition, Section 5.2.9 of [RFC4380] specifies a direct IPv6

connectivity test to determine that the mapped address/port in the

Teredo IPv6 address of a peer is not spoofed. It does this through

the use of a nonce in ICMPv6 Echo Request and Response messages

(which are defined in Section 4 of [RFC4443]). However, the direct

IPv6 connectivity test is limited only to communication between

Teredo IPv6 addresses and non-Teredo IPv6 addresses. In the

following extension, we introduce the use of a nonce in direct and

indirect bubbles and provide a mechanism to verify that the mapped

address/port are not spoofed.

This extension is optional; an implementation SHOULD support it.

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RFC 6081 Teredo Extensions January 2011

5.2.1. Abstract Data Model

This section describes a conceptual model of possible data

organization that an implementation maintains to participate in this

protocol. The described organization is provided to facilitate the

explanation of how the protocol behaves. This document does not

mandate that implementations adhere to this model as long as their

external behavior is consistent with that described in this document.

In addition to the state specified in Section 5.2 of [RFC4380], the

following are also required.

Peer Entry: The following additional state is required on a per-peer

basis:

o Nonce Sent: The value of the nonce sent in the last indirect

bubble sent to the Teredo peer.

o Nonce Received: The value of the nonce received in the last

indirect bubble received from the Teredo peer.

5.2.2. Timers

No timers are necessary other than those in [RFC4380].

5.2.3. Initialization

No initialization is necessary other than that specified in

[RFC4380].

5.2.4. Message Processing

Except as specified in the following sections, the rules for message

processing are as specified in [RFC4380].

5.2.4.1. Sending an Indirect Bubble

The rules for when indirect bubbles are sent to a Teredo peer are

specified in Section 5.2.6 of [RFC4380]. When a Teredo client sends

an indirect bubble, it MUST generate a random 4-byte value and

include it in the Nonce field of a Nonce Trailer (Section 4.2)

appended to the indirect bubble, and also store it in the Nonce Sent

field of its Peer Entry for that Teredo peer.

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RFC 6081 Teredo Extensions January 2011

5.2.4.2. Sending a Direct Bubble

The rules for when direct bubbles are sent to a Teredo peer are

specified in Section 5.2.6 of [RFC4380]. When a Teredo client sends

a direct bubble to a peer after receiving an indirect bubble with a

Nonce Trailer, it MUST include in the direct bubble a Nonce Trailer

with the same nonce value.

If the Teredo client is about to send a direct bubble before it has

received an indirect bubble from the Teredo peer, the Teredo client

MUST NOT include a Nonce Trailer.

5.2.4.3. Receiving an Indirect Bubble

The rules for processing an indirect bubble are specified in Section

5.2.3 of [RFC4380]. In addition, when a Teredo client receives an

indirect bubble containing a Nonce Trailer, the Teredo client MUST

store the nonce in the Nonce Received field of its Peer Entry for

that Teredo peer. If an indirect bubble is received without a Nonce

Trailer, and the Nonce Received field in the Peer Entry is non-zero,

the Nonce Received field SHOULD be set to zero.

5.2.4.4. Receiving a Direct Bubble

If the mapped address/port of the direct bubble matches the mapped

address/port embedded in the source Teredo IPv6 address, the direct

bubble MUST be accepted, as specified in Section 5.2.3 of [RFC4380].

In addition, if the mapped address/port does not match the embedded

address/port but the direct bubble contains a Nonce Trailer with a

nonce that matches the Nonce Sent field of the Teredo peer, the

direct bubble MUST be accepted.

If neither of the above conditions is true, the direct bubble MUST be

dropped.

If the direct bubble is accepted, the Teredo client MUST record the

mapped address/port from which the direct bubble is received in the

mapped address/port fields of the Teredo peer, as specified in

Section 5.2 of [RFC4380].

5.3. UPnP-Enabled Symmetric NAT Extension

The UPnP-enabled Symmetric NAT Extension is optional; an

implementation SHOULD support it. This extension has the Symmetric

NAT Support Extension (Section 5.2) as a dependency. Any node that

implements this extension MUST also implement the Symmetric NAT

Support Extension.

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RFC 6081 Teredo Extensions January 2011

5.3.1. Abstract Data Model

This section describes a conceptual model of possible data

organization that an implementation maintains to participate in this

protocol. The described organization is provided to facilitate the

explanation of how the protocol behaves. This document does not

mandate that implementations adhere to this model as long as their

external behavior is consistent with that described in this document.

This extension extends the abstract data model in Section 5.2.1 by

adding the following additional fields.

UPnP-Enabled NAT flag: This is a Boolean value, set to TRUE if the

NAT positioned in front of the Teredo client is UPnP enabled. The

default value of this flag is FALSE.

UPnP-Mapped Address/Port: The mapped address/port assigned via UPnP

to the Teredo client by the UPnP-enabled NAT behind which the Teredo

client is positioned. Note that this field has a valid value only if

the NAT to which the Teredo client is connected is UPnP enabled.

Also, note that if the Teredo client is positioned behind a single

NAT only (as opposed to a series of nested NATs), this value is the

same as the mapped address/port embedded in its Teredo IPv6 address.

Symmetric NAT flag: This is a Boolean value, set to TRUE if the

Teredo client is positioned behind a symmetric NAT.

Peer Entry: The following state needs to be added on a per-peer

basis:

o Symmetric Peer flag: This is a Boolean value and is TRUE if the

Teredo peer is positioned behind a symmetric NAT.

A Teredo client SHOULD also maintain the following state that is

persisted across reboots:

o Persisted UPnP-Mapped Port: The mapped port assigned via UPnP to

the Teredo client by the UPnP-enabled NAT behind which the Teredo

client is positioned. Note that this value is the same as the

UPnP-Mapped Port value when both are non-zero. The default value

is all zero bytes.

5.3.2. Timers

No timers are necessary other than those in [RFC4380].

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RFC 6081 Teredo Extensions January 2011

5.3.3. Initialization

Prior to beginning the qualification procedure, the Teredo client

MUST first perform the uninitialization procedure specified in

Section 5.3.5.1 if the Persisted UPnP-Mapped Port is supported and

non-zero.

The Teredo client MUST then invoke the AddPortMapping function, as

specified in Section 2.4.16 of [UPNPWANIP], with the following

parameters:

o NewRemoteHost: "" (empty string)

o NewExternalPort: Local Port value

o NewProtocol: UDP

o NewInternalPort: Local Port value

o NewInternalClient: Local Address value

o NewEnabled: TRUE

o NewPortMappingDescription: "TEREDO"

o NewLeaseDuration: 0

The successful completion of the AddPortMapping function indicates

that the NAT has created a port mapping from the external port of the

NAT to the internal port of the Teredo client node. The parameters

are specified so that any external host should be able to send

packets to the Teredo client by sending packets to the mapped

address/port. If the AddPortMapping function fails, the Teredo

client MUST continue without using this extension. Otherwise, it

MUST proceed as follows.

The Teredo client MUST set the UPnP-Mapped Port (and Persisted UPnP-

Mapped Port, if supported) to the Local Port value specified in

AddPortMapping. The Teredo client MUST then call the

GetExternalIPAddress function specified in Section 2.4.18 of

[UPNPWANIP]. If the GetExternalIPAddress function fails, the Teredo

client SHOULD perform the uninitialization procedure specified in

Section 5.3.5.1 and continue without using this extension. If the

GetExternalIPAddress function succeeds, the Teredo client MUST

proceed as follows.

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The Teredo client MUST set the UPnP-Mapped Address to the address

returned from the GetExternalIPAddress function, and set the UPnP-

Enabled NAT flag to TRUE.

During the qualification procedure (as specified in Section 5.2.1 of

[RFC4380]) when the Teredo client receives a response from the

secondary Teredo server, the Teredo client MUST compare the mapped

address/port learned from the secondary Teredo server with the mapped

address/port associated with the Teredo server. If either the mapped

address or the mapped port value is different, the Symmetric NAT flag

MUST be set to TRUE.

After the qualification procedure, the mapped address/port learned

from the Teredo server MUST be compared to the UPnP-Mapped Address/

Port. If both are the same, the Teredo client is positioned behind a

single NAT and the UPnP-Mapped Address/Port MUST be zeroed out.

5.3.4. Message Processing

Except as specified in the following sections, the rules for message

processing are as specified in Section 5.2.3 of [RFC4380].

5.3.4.1. Receiving a Direct Bubble

Except as indicated below, the rules for handling a direct bubble are

as specified in Section 5.2.4.4.

A Teredo client positioned behind a UPnP-enabled NAT (port-restricted

NAT as well as symmetric NAT) will receive all packets sent to the

mapped address/port embedded in its Teredo IPv6 address. Thus, when

a Teredo client receives a direct bubble, it MUST compare the mapped

address/port from which the packet was received with the mapped

address/port embedded in the Teredo IPv6 address in the source

address field of the IPv6 header. If the two are not the same, it

indicates that the Teredo peer is positioned behind a symmetric NAT,

and it MUST set the Symmetric Peer flag in its Peer Entry.

5.3.4.2. Sending a Direct Bubble

The rules for sending a direct bubble are specified in Section 5.2.6

of [RFC4380] and Section 5.2.4.2 of this document. These rules are

further refined as follows.

If the Teredo client sending the direct bubble meets all of the

following criteria:

o The Symmetric NAT flag is set to TRUE.

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o The UPnP-Enabled NAT flag is set to TRUE.

o The UPnP-Mapped Address/Port are set to zero.

o The peer’s Symmetric Peer flag is set to TRUE.

then the Teredo client MUST send the direct bubble to the mapped

address/port embedded in the peer’s Teredo IPv6 address.

This is because Symmetric-to-Symmetric and Port-Restricted-to-

Symmetric NAT communication between the Teredo client and the peer

would have failed anyway. However, by taking a chance that the peer

might also be positioned behind a UPnP-enabled NAT just like the

Teredo client itself, the Teredo client can try sending the direct

bubble to the mapped address/port in the peer’s Teredo IPv6 address.

If the packet does go through, communication is established.

5.3.4.3. Sending a Data Packet

The rules for sending a data packet are specified in Section 5.2.4 of

[RFC4380]. These rules are further refined as follows.

If the Teredo client sending the data packet meets all of the

following criteria:

o The Symmetric NAT flag is set to TRUE.

o The UPnP-Enabled NAT flag is set to TRUE.

o The UPnP-Mapped Address/Port are set to zero.

o The peer’s Symmetric Peer flag is set to TRUE.

then the Teredo client MUST send the data packet to the mapped

address/port embedded in the peer’s Teredo IPv6 address.

5.3.5. Shutdown

When Teredo client functionality is being shut down, uninitialization

MUST be performed as specified in Section 5.3.5.1.

5.3.5.1. Uninitialization

First determine the mapped port as follows. If Persisted UPnP-Mapped

Port is supported, use it as the mapped port. Otherwise, use the

UPnP-Mapped Port.

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If the mapped port is non-zero, the Teredo client MUST call the

DeletePortMapping function, as specified in Section 2.4.17 of

[UPNPWANIP], with the following parameters:

o NewRemoteHost: "" (empty string)

o NewExternalPort: the mapped port

o NewProtocol: UDP

5.4. Port-Preserving Symmetric NAT Extension

The Port-Preserving Symmetric NAT Extension is optional; an

implementation SHOULD support it. This extension has the Symmetric

NAT Support Extension (as specified in Section 5.2) as a dependency.

Any node that implements this extension MUST also implement the

Symmetric NAT Support Extension.

5.4.1. Abstract Data Model

This section describes a conceptual model of possible data

organization that an implementation maintains to participate in this

protocol. The described organization is provided to facilitate the

explanation of how the protocol behaves. This document does not

mandate that implementations adhere to this model as long as their

external behavior is consistent with that described in this document.

The Port-Preserving Symmetric NAT Extension extends the abstract data

model in Section 5.2.1 by adding the following additional fields.

Port-Preserving NAT flag: This is a Boolean value, set to TRUE if the

Teredo client is positioned behind a port-preserving NAT.

Symmetric NAT flag: This is a Boolean value, set to TRUE if the

Teredo client is positioned behind a symmetric NAT.

Peer Entry: The following fields need to be added on a per-peer

basis:

o Random Port: This field contains the value of the external port

that the Teredo client predicts that its NAT has assigned it for

communication with the peer. Set to zero by default.

o Peer Random Port: This field contains the value of the random port

that the peer is using for communication with this Teredo client.

Set to zero by default.

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o Direct Receive on Primary Port: This is a Boolean value, set to

TRUE if a packet is received from the Teredo peer on the primary

local port. Set to FALSE by default.

o Direct Receive on Random Port: This is a Boolean value, set to

TRUE if a packet is received from the Teredo peer on the Random

Port. Set to FALSE by default.

o Connection Refresh Count: This field contains the number of direct

bubbles that have been sent to the peer since the last time data

was sent to the peer.

o Last Data Packet Sent Timestamp: This field contains the timestamp

of the last data packet sent to the peer. This timestamp is

different from the field that stores the data and time of last

transmission to the peer (as specified in Section 5.2 of

[RFC4380]) because the RFC-defined field is also updated every

time a direct bubble is sent.

5.4.2. Timers

Other than those in [RFC4380], the Port-Preserving Symmetric NAT

Extension requires the following additional timer.

Peer Refresh Timer: A timer to refresh peer connections through the

random port, on which no data has been sent for a while.

5.4.2.1. Peer Refresh Timer Expiry

When the Peer Refresh Timer expires, the Teredo client MUST go

through its list of peers and for each peer to which the Teredo

client is communicating through the random port, the Teredo client

MUST check the Last Data Packet Sent Timestamp to determine if data

has been sent to the peer in the last 30 seconds, and check the

Connection Refresh Count field to determine if the count has reached

the maximum allowed value of 20. If both checks are FALSE, the

Teredo client MUST send a direct bubble (as specified in

Section 5.4.4.3) to the peer and increment the Connection Refresh

Count. This direct bubble is sent as an attempt to keep the port

mappings on all the intermediate NATs alive while the application/

user may be temporarily inactive. If on the other hand, data has

been sent to the peer in the last 30 seconds, the Connection Refresh

Count MUST be reset to zero.

The Peer Refresh Timer MUST then be rescheduled to expire in 30

seconds.

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5.4.3. Initialization

In addition to the behavior specified in [RFC4380], the Port-

Preserving NAT flag and Symmetric NAT flag MUST be set to FALSE when

the Teredo client is started. The Peer Refresh Timer MUST be started

and scheduled to expire in 30 seconds.

During the qualification procedure (as specified in Section 5.2.1 of

[RFC4380]), when the Teredo client receives a response from the

Teredo server address, the Teredo client MUST compare the Port value

in the origin indication, as specified in Section 5.1.1 of [RFC4380],

with the Local Port value. If both values match, the client MUST set

the Port-Preserving NAT flag to TRUE.

5.4.4. Message Processing

5.4.4.1. Sending a Data Packet

On receiving a data packet to be transmitted to the Teredo peer (in

addition to the rules specified in Section 5.2.4 of [RFC4380]), the

Teredo client MUST update the Last Data Packet Sent Timestamp when

the packet is actually sent.

5.4.4.2. Sending an Indirect Bubble

The rules for sending an indirect bubble are as specified in

Section 5.2.4.1 of this document and Section 5.2.6 of [RFC4380]. In

addition to those rules, if the Port-Preserving NAT flag is TRUE, the

Teredo client MUST do the following:

o If the Symmetric NAT flag is set, the Teredo peer is not marked as

"trusted" (as specified in Section 5.2 of [RFC4380]), and the

Random Port is zero, the Teredo client MUST first select a random

port number to use, and then begin listening on that port. Since

the NAT is port-preserving, the Teredo client can predict that the

external port assigned will be equal to the random port chosen,

and hence the Teredo client MUST store the random port chosen in

the Random Port field of the Peer Entry.

o If the Random Port value is non-zero, the Teredo client MUST

append a Random Port Trailer to the indirect bubble.

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5.4.4.3. Sending a Direct Bubble

The rules for when direct bubbles are sent to a Teredo peer are as

specified in Section 5.2.6 of [RFC4380]. In addition,

Section 5.2.4.2 defines rules for enabling communication for clients

positioned behind a symmetric NAT. In addition to the rules defined

in both the aforementioned sections, if the Port-Preserving NAT flag

is TRUE, the following rules apply also.

If the Symmetric NAT flag is set, and the Teredo peer is not marked

as "trusted" (as specified in Section 5.2 of [RFC4380]) the Teredo

client MUST send a direct bubble destined to the mapped address/port

embedded in the Teredo IPv6 address of the Teredo peer. If the peer

Random Port field is non-zero, the Teredo client MUST send another

direct bubble from its own random port, destined to the peer random

port. The IPv4 destination address MUST be the mapped address

embedded in the Teredo IPv6 address. In addition, the Teredo client

MUST include the Random Port Trailer (Section 4.5).

5.4.4.4. Receiving an Indirect Bubble

The rules for processing an indirect bubble are as specified in

Section 5.2.4.3 of this document and Section 5.2.3 of [RFC4380]. In

addition to these rules, if the incoming indirect bubble has a Random

Port Trailer, the following additional processing MUST be done.

If the Peer Random Port field of the Peer Entry is zero, the Teredo

client MUST store the port from the Random Port Trailer in the Peer

Random Port field of the Peer Entry.

If the Peer Random Port field is non-zero and if either the Peer

Random Port field and the new advertised port have the same value, or

if active data has been exchanged between the two Teredo clients in

the last 30 seconds (that is, "time of last transmission" or "time of

last reception", as specified in Section 5.2 of [RFC4380], is set to

a time that is less than 30 seconds ago), the new advertised port

value MUST be ignored.

If the Peer Random Port field is non-zero and the new advertised port

value is different from the Peer Random Port value, and it has been

more than 30 seconds since the last exchange of data packets between

the two Teredo clients, (that is, "time of last transmission" and

"time of last reception" are set to a time that is more than 30

seconds ago), the Teredo client SHOULD store the new advertised port

value in the Peer Random Port field and, if the Port-Preserving NAT

flag is TRUE, then clear the Random Port field, and stop listening on

the old random port. This allows communication to be re-established

if either side changes the random port that it is using.

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5.4.4.5. Receiving a Direct Bubble

The rules for handling direct bubbles are specified in

Section 5.2.4.4 of this document and Section 5.2.3 of [RFC4380]. The

rules for whether to accept a direct bubble are extended as follows,

when the Port-Preserving NAT flag is TRUE:

o If the direct bubble is received on the primary port and the

Teredo peer is not "trusted", the status field of the Teredo

client MUST be changed to "trusted" and the Direct Receive on

Primary Port flag MUST be set to TRUE. The mapped address/port

from which the direct bubble was received MUST be recorded in the

mapped address/port fields of the Teredo peer, as specified in

Section 5.2 of [RFC4380]. The Teredo client MUST then set the

Random Port field in the Peer Entry to zero and stop listening on

the old random port.

o If the direct bubble is received on the primary port, the Teredo

peer is "trusted", and the Direct Receive on Primary flag is set

to TRUE, the Teredo client MUST compare the mapped address/port of

the direct bubble with the mapped address/port of the Peer Entry.

If both mappings are the same, the direct bubble MUST be accepted.

If the mappings are different and it has been more than 30 seconds

since the last packet exchange with the Teredo peer (that is,

"time of last transmission" and "time of last reception", as

defined in Section 5.2 of [RFC4380], are set to a time that is

more than 30 seconds ago), the mapping on the Teredo peer’s NAT

has changed and communication needs to be re-established. This

MUST be done by changing the status of the peer to "not-trusted",

setting the Direct Receive on Primary Port flag to FALSE, and

sending an indirect bubble to the Teredo peer via its Teredo

server.

o If the direct bubble is received on the primary port, the Teredo

peer is "trusted", the Direct Receive on Primary Port flag is set

to FALSE, and the Direct Receive on Random Port flag is set to

TRUE, the mapped address/port from which the direct bubble is

received MUST be stored in the mapped address/port fields of the

Peer Entry. The Direct Receive on Primary Port flag MUST be set

to TRUE. The Teredo client MUST then set the Random Port field in

the Peer Entry to zero and stop listening on the old random port.

Finally, the Direct Receive on Random Port flag MUST be set to

FALSE.

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o If the direct bubble is received on the random port and the Teredo

peer is not "trusted", the status field of the Teredo client MUST

be changed to "trusted" and the Direct Receive on Random Port flag

MUST be set to TRUE. The mapped address/port from which the

direct bubble was received MUST be recorded in the mapped address/

port fields of the Teredo Peer Entry, as specified in Section 5.2

of [RFC4380].

o If the direct bubble is received on the random port, the Teredo

peer is "trusted", and the Direct Receive on Primary Port flag is

FALSE, the Teredo client MUST compare the mapped address/port in

the direct bubble with the mapped address/port in the Peer Entry.

If the two mappings are the same, the direct bubble MUST be

accepted. If the mappings are different, it implies that the NAT

had deleted the mapping and when it reassigned the mapping, a

different external port was chosen. In this instance, the Teredo

client SHOULD set the Random Port field to zero, stop listening on

the old random port, and send an indirect bubble to the Teredo

peer as specified in Section 5.4.4.2.

Note that once the Direct Receive on Primary Port flag is TRUE, the

client will stop listening on the random port and hence a direct

bubble cannot be received on the random port. As a result, this case

is intentionally omitted above.

5.5. Sequential Port-Symmetric NAT Extension

The Sequential Port-Symmetric NAT Extension is optional; an

implementation SHOULD support it. This extension has the Symmetric

NAT Support Extension (Section 5.2) as a dependency. Any node that

implements this extension MUST also implement the Symmetric NAT

Support Extension, as well as the Port-Preserving NAT Extension

(Section 5.4).

5.5.1. Abstract Data Model

This section describes a conceptual model of possible data

organization that an implementation maintains to participate in this

protocol. The described organization is provided to facilitate the

explanation of how the protocol behaves. This document does not

mandate that implementations adhere to this model as long as their

external behavior is consistent with that described in this document.

The Sequential Port-Symmetric NAT Extension extends the abstract data

model in Section 5.4.1 by adding the following additional state.

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Peer Entry: The following fields need to be added on a per-peer

basis:

o EchoTestNonce1: The value of the nonce sent as part of the

authentication encapsulation, as specified in Section 5.1.1 of

[RFC4380], in the router solicitation packet sent to the Teredo

server address as part of the Echo Test.

o EchoTestNonce2: The value of the nonce sent as part of the

authentication encapsulation in the router solicitation packet

sent to the secondary Teredo server address as part of the Echo

Test.

o EchoTestLowerPort: The value of the external port mapping

extracted from the origin indication of the router advertisement

received from the Teredo server address as part of the Echo Test.

A value of 0 indicates that no such router advertisement has been

received.

o EchoTestUpperPort: The value of the external port mapping

extracted from the origin indication of the router advertisement

received from the secondary Teredo server address as part of the

Echo Test. A value of 0 indicates that no such router

advertisement has been received.

o EchoTestRetryCounter: The number of times an Echo Test has been

attempted.

5.5.2. Timers

In addition to the timers specified in Section 5.4.2, the following

additional timer is required per Peer Entry.

Echo Test Failover Timer: A one-shot timer that runs whenever an Echo

Test is in progress.

5.5.2.1. Peer Refresh Timer Expiry

The processing of the Peer Refresh Timer Expiry MUST be completed as

specified in Section 5.4.2.1. In addition to those rules, the Teredo

client MUST set the EchoTestLowerPort, EchoTestUpperPort, and

EchoTestRetryCounter to zero.

5.5.2.2. Echo Test Failover Timer Expiry

If the Echo Test Failover Timer expires, the Teredo client MUST do

the following.

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If the value of the EchoTestRetryCounter is two, then the Teredo

client MUST send an indirect bubble as specified in Section 5.2.4.1.

If the value of the EchoTestRetryCounter is one, then the Teredo

client MUST start another Echo Test as specified in

Section 5.5.4.1.1.

5.5.3. Initialization

No behavior changes are required beyond what is specified in

Section 5.4.3.

5.5.4. Message Processing

Except as specified in the following sections, the rules for message

processing are as specified in Section 5.4.4.

5.5.4.1. Handling a Request to Send an Indirect Bubble

Whenever [RFC4380] or other extensions specified in this document

specify that an indirect bubble is to be sent, the following actions

apply at that time instead if the Symmetric NAT flag is TRUE and the

Port-Preserving NAT flag is FALSE. Note that any behavior specified

by [RFC4380] or other extensions in this document still applies to

how indirect bubbles are constructed, but such behavior is done at a

later time as specified in Section 5.5.4.4.

If the Symmetric NAT flag is TRUE, and the Port-Preserving NAT flag

is FALSE, and the Teredo peer is not marked as "trusted" (as

specified in Section 5.2 of [RFC4380]), and the Random Port is zero,

then the Teredo client MUST select a random port number to use, begin

listening on that port, and start an Echo Test as specified below.

5.5.4.1.1. Starting an Echo Test

To start an Echo Test, the Teredo client MUST send the following

three packets from this port:

o First, a router solicitation (as specified in Section 5.2.1 of

[RFC4380]) MUST be sent to the Teredo server address. The router

solicitation MUST include an authentication encapsulation with a

randomly generated Nonce field, as specified in Section 5.1.1 of

[RFC4380]. The nonce included in the authentication encapsulation

MUST then be stored in the EchoTestNonce1 field of the Peer Entry.

o Second, a direct bubble MUST be sent to the peer.

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o Third, a router solicitation MUST be sent to the secondary Teredo

server address. The router solicitation MUST include an

authentication encapsulation with a randomly generated Nonce

field, as specified in Section 5.1.1 of [RFC4380]. The nonce

included in the authentication encapsulation MUST then be stored

in the EchoTestNonce2 field of the Peer Entry.

The Teredo client MUST then increment the EchoTestRetryCounter and

set the Echo Test Failover Timer to expire in a number of seconds

equal to EchoTestRetryCounter.

5.5.4.2. Sending an Indirect Bubble

The rules for sending an indirect bubble are as specified in

Section 5.2.4.1 of this document and Section 5.2.6 of [RFC4380]. In

addition to those rules, if the Symmetric NAT flag is TRUE, and the

Port-Preserving NAT flag is FALSE, and the Random Port value is non-

zero, then the Teredo client MUST append a Random Port Trailer to the

indirect bubble.

5.5.4.3. Receiving a Direct Bubble

The processing of the direct bubble MUST be completed as specified in

Section 5.4.4.5, as if the Port-Preserving NAT flag were TRUE. After

the processing is complete, if the Direct Bubble Received on Primary

flag is TRUE, and the Echo Test Failover Timer is running, then the

Echo Test Failover Timer MUST be canceled and EchoTestLowerPort,

EchoTestUpperPort, and EchoTestRetryCounter MUST be set to zero.

5.5.4.4. Receiving a Router Advertisement

The rules for processing a router advertisement are as specified in

Section 5.2.1 of [RFC4380]. In addition to those rules, if the

router advertisement contains an authentication encapsulation, the

Teredo client MUST look for a Peer Entry whose EchoTestNonce1 or

EchoTestNonce2 field matches the nonce in the authentication

encapsulation. If a Peer Entry is found, the Teredo client MUST do

the following.

If the received nonce is equal to EchoTestNonce1 and

EchoTestLowerPort is zero, then EchoTestLowerPort MUST be set to the

external port mapping extracted from the origin indication of this

router advertisement.

If the received nonce is equal to EchoTestNonce2 and

EchoTestUpperPort is zero, then EchoTestUpperPort MUST be set to the

external port mapping extracted from the origin indication of this

router advertisement.

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If the EchoTestUpperPort and EchoTestLowerPort are now both non-zero,

the Teredo client MUST then set the Random Port field of the Peer

Entry to (EchoTestUpperPort + EchoTestUpperPort)/2, rounded down, and

send an indirect bubble as specified in Section 5.5.4.2.

5.6. Hairpinning Extension

This extension is optional; an implementation SHOULD support it.

5.6.1. Abstract Data Model

This section describes a conceptual model of possible data

organization that an implementation maintains to participate in this

protocol. The described organization is provided to facilitate the

explanation of how the protocol behaves. This document does not

mandate that implementations adhere to this model as long as their

external behavior is consistent with that described in this document.

In addition to the state specified in Section 5.2 of [RFC4380], the

following are also required:

UPnP Mapped Address/Port: The mapped address/port assigned via UPnP

to the Teredo client by the UPnP-enabled NAT behind which the Teredo

client is positioned. This field has a valid value only if the NAT

to which the Teredo client is connected is UPnP enabled. In

addition, if the Teredo client is positioned behind a single NAT only

(as opposed to a series of nested NATs), this value will be the same

as the mapped address/port embedded in its Teredo IPv6 address.

Peer Entry: Per-peer state is extended beyond what is described in

[RFC4380] by including the following:

o Alternate Address/Port list: The list of alternate address/port

pairs advertised by the peer.

5.6.2. Timers

No timers are necessary other than those in [RFC4380].

5.6.3. Initialization

Behavior is as specified in [RFC4380], with the following additions.

Prior to beginning the qualification procedure, the Teredo client

MUST invoke the AddPortMapping function (as specified in Section

2.4.16 of [UPNPWANIP]) with the parameters specified in

Section 5.3.3. If successful, it indicates that the NAT has created

a port mapping from the external port of the NAT to the internal port

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of the Teredo client node. If the AddPortMapping function is

successful, the Teredo client MUST store the mapping assigned by the

NAT in its UPnP Mapped Address/Port state.

After the qualification procedure, the mapped address/port learned

from the Teredo server MUST be compared to the UPnP Mapped Address/

Port. If both are the same, the Teredo client is positioned behind a

single NAT and the UPnP Mapped Address/Port MUST be zeroed out.

5.6.4. Message Processing

5.6.4.1. Sending an Indirect Bubble

The rules for when indirect bubbles are sent to a Teredo peer are as

specified in Section 5.2.6 of [RFC4380]. If communication between a

Teredo client and a Teredo peer has not been established, the Teredo

client MUST include the Alternate Address Trailer in the indirect

bubble. The Alternate Address Trailer MUST include the node’s local

address/port in the Alternate Address/Port list. If the UPnP Mapped

Address/Port is non-zero, the Alternate Address Trailer MUST also

include it in the list.

Hairpinning requires "direct IPv6 connectivity tests" (as specified

in Section 5.2.9 of [RFC4380]) to succeed before it can accept

packets from an IPv4 address and port not embedded in the Teredo IPv6

address. Hence, the indirect bubble MUST also include a Nonce

Trailer.

5.6.4.2. Receiving an Indirect Bubble

The rules for processing indirect bubbles are as specified in Section

5.2.3 of [RFC4380]. In addition to those rules, when a Teredo client

receives an indirect bubble with the Alternate Address Trailer, it

SHOULD first verify that the Alternate Address Trailer is correctly

formed (as specified in Section 4.3), and drop the bubble if not.

Otherwise, it MUST set the Alternate Address/Port list in its Peer

Entry to the list in the trailer. The Teredo client, besides sending

direct bubbles to the mapped address/port embedded in the Teredo IPv6

address (as specified in Section 5.2.6 of [RFC4380]), MUST also send

a direct bubble to each mapped address/port advertised in the

Alternate Address Trailer.

In each of the direct bubbles, the Teredo client MUST include a Nonce

Trailer with the nonce value received in the indirect bubble.

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5.6.4.3. Receiving a Direct Bubble

If the mapped address/port of the direct bubble matches the mapped

address/port embedded in the source Teredo IPv6 address, the direct

bubble MUST be accepted, as specified in Section 5.2.3 of [RFC4380].

If the mapped address/port does not match the embedded address/port,

but the direct bubble contains a Nonce Trailer with a nonce that

matches the Nonce Sent field of the Teredo peer, the direct bubble

MUST be accepted.

If neither of the above rules match, the direct bubble MUST be

dropped.

5.7. Server Load Reduction Extension

This extension is optional; an implementation SHOULD support it.

5.7.1. Abstract Data Model

This section describes a conceptual model of possible data

organization that an implementation maintains to participate in this

protocol. The described organization is provided to facilitate the

explanation of how the protocol behaves. This document does not

mandate that implementations adhere to this model as long as their

external behavior is consistent with that described in this document.

In addition to the state specified in Section 5.2 of [RFC4380], the

following are also required.

Peer Entry: The following state needs to be added on a per-peer

basis:

o Count of Solicitations Transmitted: The number of Solicitation

packets sent.

5.7.2. Timers

Retransmission Timer: A timer used to retransmit Teredo Neighbor

Solicitation packets.

When the retransmission timer expires, the Teredo client MUST

retransmit a direct bubble with a Neighbor Discovery Option Trailer,

and increment the Count of Solicitations Transmitted. If the count

is less than three, it MUST then reset the timer to expire in two

seconds. Otherwise (if the count is now three), it MUST send an

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RFC 6081 Teredo Extensions January 2011

indirect bubble to the Teredo peer to re-establish connectivity as if

no communication between the Teredo client and the Teredo peer had

been established.

5.7.3. Initialization

No initialization is necessary other than that specified in

[RFC4380].

5.7.4. Message Processing

Except as specified below, processing is the same as specified in

[RFC4380].

5.7.4.1. Sending a Data Packet

Upon receiving a data packet to be transmitted to the Teredo peer,

the Teredo client MUST determine whether data has been exchanged

between the Teredo client and peer in either direction in the last 30

seconds (using the state as specified in Section 5.2 of [RFC4380]).

If not, the Teredo client MUST send a direct bubble with a Neighbor

Discovery Option Trailer having the DiscoveryType field set to

TeredoDiscoverySolicitation. The Count of Solicitations Transmitted

field MUST be set to 1. The retransmission timer MUST be set to

expire in two seconds.

5.7.4.2. Receiving a Direct Bubble

The rules for processing direct bubbles are as specified in Section

5.2.3 of [RFC4380]. In addition to those rules, upon receiving a

direct bubble containing a Neighbor Discovery Option Trailer with

DiscoveryType field set to TeredoDiscoverySolicitation, the Teredo

client MUST respond with a direct bubble with the Neighbor Discovery

Option Trailer having the DiscoveryType field set to

TeredoDiscoveryAdvertisement.

6. Protocol Examples

The following sections describe several operations as used in common

scenarios to illustrate the function of Teredo Extensions.

6.1. Symmetric NAT Support Extension

The following protocol example illustrates the use of the Symmetric

NAT Support Extension.

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In Figure 2 (Section 3.1), assume that Teredo Client A, which is

positioned behind a port-symmetric NAT, wants to communicate with

Teredo Client B, which is positioned behind an address-restricted

NAT.

The qualification procedure where the Teredo client determines that

it is positioned behind a symmetric NAT is exactly the same as that

specified in Section 5.2.1 of [RFC4380]. Because of the Symmetric

NAT Extension, Client A continues to configure a Teredo IPv6 address

even after determining that the Teredo client is positioned behind a

symmetric NAT.

Next the following packet exchange helps Teredo Client A (A)

establish communication with Teredo Client B (B).

Teredo Client A’s Client B’s Teredo

Client Teredo Teredo Client

A NAT Server Server NAT B

| | | | | |

| | | Direct Bubble to B | | |

1 |--------------------------------------------------->| |

| | | | | |

|Indirect Bubble to B via B’s Teredo Server| | |

2 |----------------------------------------->|----------------->|

| | | | | |

| | | Direct Bubble to A | | |

| |<--------------------------------------------------| 3

| | | | | |

| | |Indirect Bubble to A via A’s Teredo Server|

|<-----------------|<-----------------------------------------| 4

| | | | | |

| | | Direct Bubble to B | | |

5 |------------------------------------------------------------>|

| | | | | |

|Indirect Bubble to B via B’s Teredo Server| | |

6 |----------------------------------------->|----------------->|

| | | | | |

| | | Direct Bubble to A | | |

|<------------------------------------------------------------| 7

| | | | | |

Port-Symmetric NAT to Address-Restricted NAT Packet

Exchange

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1. A sends a direct bubble (Packet 1) destined to the mapped

address/port embedded in B’s Teredo IPv6 address. The mapped

port in the source field of the packet assigned by Client A’s

NAT is different from the mapped port embedded in A’s Teredo

IPv6 address. This is characteristic of the port-symmetric NAT

positioned in front of A. The mapped address in the source

field of the packet is the same as the mapped address embedded

in the Teredo IPv6 address of A.

2. The aforementioned direct bubble is dropped by B’s NAT because

it has not seen an outgoing packet destined to A’s mapped IPv4

address.

3. A sends an indirect bubble (Packet 2) destined to B via Client

B’s Teredo server.

4. The above-mentioned indirect bubble is received by B. B then

responds with the following packets. The first packet sent by B

is a direct bubble (Packet 3) destined to the mapped address/

port embedded in A’s Teredo IPv6 address.

5. The above-mentioned direct bubble is dropped by A’s NAT because

the NAT has not seen any outgoing packet sourced from the mapped

address/port embedded in A’s Teredo IPv6 address and destined to

the mapped address/port embedded in B’s Teredo IPv6 address.

6. B also sends an indirect bubble (Packet 4) destined to A via A’s

Teredo Server.

7. The aforementioned indirect bubble is successfully received by

A. A responds to the indirect bubble with its own direct bubble

(Packet 5). This direct bubble is exactly the same as the first

direct bubble (Packet 1) sent by A.

8. This time around the aforementioned direct bubble is accepted by

B’s NAT because the NAT has seen an outgoing packet (Packet 3)

sourced from the mapped address/port embedded in B’s Teredo IPv6

address and destined to the mapped address/port embedded in A’s

Teredo IPv6 address. It is important to remember that A’s NAT

is port-symmetric and therefore varies only the mapped port

while the mapped address remains the same. B’s NAT is address-

restricted and cares only about prior communication with the

IPv4 address, not the specific port. At this point,

communication in one direction is now possible (B to A, but not

vice versa).

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9. After receiving the direct bubble, B remembers the new mapped

address/port that was in the source fields of the direct bubble

and uses those for future communication with A instead of the

mapped address/port embedded in A’s Teredo IPv6 address.

10. A then times out and resends an indirect bubble (Packet 6) and

in response, B sends a direct bubble (Packet 7). This direct

bubble is destined to the new learned mapped address/port and

hence A’s NAT permits the direct bubble through. Communication

is now possible in the other direction (client A to B).

6.2. UPnP-Enabled Symmetric NAT Extension

The following protocol example illustrates the use of the UPnP-

Enabled Symmetric NAT Extension in addition to the Symmetric NAT

Support Extension.

Assume that Teredo Client A, which is positioned behind a UPnP-

enabled port-symmetric NAT, wants to communicate with Teredo Client

B, which is also positioned behind a UPnP-Enabled port-symmetric NAT.

Before both clients start their qualification procedure, they use

UPnP to reserve port mappings on their respective NATs. The UPnP

operations succeed for both the clients and the clients hence know

that they are positioned behind UPnP-enabled NATs. After the

qualification procedure, both clients have valid Teredo IPv6

addresses because they both support the Symmetric NAT Support

Extension. Also, after the qualification procedure both clients will

compare their mapped address/port determined through UPnP with the

mapped address/port determined through the qualification procedure.

Because both will be the same, the clients will zero out their UPnP

mapped address/port values and conclude that they are each located

behind a single UPnP-enabled NAT.

The following packet exchange shows Teredo client A (A) establishing

communication with Teredo client B (B).

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Teredo Client A’s Client B’s Teredo

Client Teredo Teredo Client

A NAT Server Server NAT B

| | | | | |

| | | Direct Bubble to B | | |

1 |------------------------------------------------------------>|

| | | | | |

|Indirect Bubble to B via B’s Teredo Server| | |

2 |----------------------------------------->|----------------->|

| | | | | |

| | | Direct Bubble to A | | |

|<------------------------------------------------------------| 3

| | | | | |

UPnP-enabled Symmetric NAT Packet Exchange

1. A sends a direct bubble (Packet 1) to the mapped address/port

embedded in B’s Teredo IPv6 address. Because A’s NAT is a

symmetric NAT, the UDP source port field in the packet assigned

by A’s NAT is different from the mapped port embedded in A’s

Teredo IPv6 address, but the IPv4 source address of the packet is

the same as the mapped address embedded in A’s Teredo IPv6

address.

2. The above-mentioned direct bubble is received by B because it is

destined for the UPnP mapped address/port of B and hence is let

through by the NAT. At this point, B deduces that A is

positioned behind a symmetric NAT because the mapped address/port

from which the direct bubble is received is different from the

mapped address/port that is embedded in A’s Teredo IPv6 address.

Hence, it remembers that the peer is positioned behind a

symmetric NAT so that data packets will be sent to the mapped

address/port embedded in A’s Teredo IPv6 address, rather than the

mapped address/port from which the direct bubble was received.

At this point, communication in one direction is now possible (B

to A, but not vice versa).

3. A also sends an indirect bubble (Packet 2) destined to B via B’s

Teredo Server.

4. The above indirect bubble is received by B. B then responds with

a direct bubble (Packet 3) destined to the mapped address/port

embedded in A’s Teredo IPv6 address, as in step 2.

5. Because A’s NAT is also UPnP enabled, the above-mentioned direct

bubble is received by A. A also notices that B is positioned

behind a Symmetric NAT because the mapped address/port from which

the packet is received is different from the mapped address/port

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embedded in B’s Teredo IPv6 address. Hence, it remembers that

the peer is positioned behind a symmetric NAT so that data

packets will be sent to the mapped address/port embedded in B’s

Teredo IPv6 address, rather than the mapped address/port from

which the direct bubble was received. At this point,

communication is now possible in the other direction (A to B).

6.3. Port-Preserving Symmetric NAT Extension

The following protocol example illustrates the use of the Port-

Preserving Symmetric NAT Extension.

Assume that Teredo Client A (A), which is positioned behind a port-

preserving symmetric NAT, wants to communicate with Teredo Client B

(B), which is also positioned behind a port-preserving symmetric NAT.

The following packet exchange explains the configuration setup and

communication setup between the two clients.

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Teredo Client A’s Client B’s Teredo

Client Teredo Teredo Client

A NAT Server Server NAT B

| | | | | |

| | | Direct Bubble to B | | |

1 |--------------------------------------------------->| |

| | | | | |

|Indirect Bubble to B via B’s Teredo Server| | |

2 |----------------------------------------->|----------------->|

| | | | | |

| | | Direct Bubble to A | | |

| |<--------------------------------------------------| 3

| | | | | |

| | | Direct Bubble to A | | |

| |<--------------------------------------------------| 4

| | | | | |

| | |Indirect Bubble to A via A’s Teredo Server|

|<-----------------|<-----------------------------------------| 5

| | | | | |

| | | Direct Bubble to B | | |

6 |--------------------------------------------------->| |

| | | | | |

| | | Direct Bubble to B | | |

7 |------------------------------------------------------------>|

| | | | | |

|Indirect Bubble to B via B’s Teredo Server| | |

8 |----------------------------------------->|----------------->|

| | | | | |

| | | Direct Bubble to A | | |

|<------------------------------------------------------------| 9

| | | | | |

Port-Preserving Symmetric NAT Packet Exchange

1. During the qualification procedure, when the clients receive a

response from the Teredo server, they compare the Port value in

the Origin indication with the Local Port value. If both values

match, the clients set the Port-Preserving NAT flag to TRUE.

2. When the response is received from the secondary Teredo server,

the mapped address/port value in the Origin indication is

compared with the mapped address/port value learned from the

response received from the primary server. If the mappings are

different, the Symmetric NAT flag is set to TRUE.

3. It is assumed that for both Clients A and B, the Port-Preserving

NAT flag and the Symmetric NAT flag are set to TRUE at the end

of the qualification procedure.

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4. Before A sends packets to B, A checks to see if it is positioned

behind a port-preserving NAT and a symmetric NAT, which in the

example, it is. A also checks to see if the peer is "trusted",

but it currently is not. Next, A checks if the Random Port is

set to non-zero. Since it is still zero, A allocates a new

random port, begins listening on it, and stores the value in the

Random Port field.

5. A sends a direct bubble (Packet 1) from the primary port to the

mapped address/port embedded in B’s Teredo IPv6 address. This

direct bubble does not have a Nonce Trailer or a Random Port

Trailer attached to the end.

6. The aforementioned direct bubble is dropped by B’s NAT because

the NAT has not seen an outgoing packet destined to A’s mapped

address.

7. A sends an indirect bubble (Packet 2) destined to B via client

B’s Teredo server. This indirect bubble contains two trailers:

the Nonce Trailer containing a random nonce, and the Random Port

Trailer containing the random port value from the Peer Entry.

The nonce used in the Nonce Trailer is also stored in the Nonce

Sent field of the Peer Entry.

8. The aforementioned indirect bubble is received by B. B adds the

Teredo peer to its peer list. B saves the nonce value from the

Nonce Trailer in the Nonce Advertised field of the Peer Entry.

B stores the port value from the Random Port Trailer in the Peer

Random Port field in the Peer Entry.

9. B responds by sending the following packets. The first packet

sent by B is a direct bubble (Packet 3) destined to the mapped

address/port embedded in A’s Teredo IPv6 address. This packet

is sent from the primary port. It includes the Nonce Trailer

with the nonce from the Nonce Advertised field of the Peer

Entry.

10. The aforementioned direct bubble is dropped by A’s NAT because

the NAT has not seen any outgoing packet sourced from the mapped

address/port embedded in A’s Teredo IPv6 address and destined to

the mapped address/port embedded in B’s Teredo IPv6 address.

11. B then checks if it is positioned behind a port-restricted NAT

or a symmetric NAT. It also checks if the peer has already

advertised a random port. In this case, B is positioned behind

a port-preserving symmetric NAT and the peer has advertised a

random port; hence, it needs to use a random port. It checks if

its Random Port field is set to non-zero. Since it is still

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zero, B allocates a new random port, begins listening on it, and

stores it in the Random Port entry of the Peer Entry. B then

sends a direct bubble (Packet 4) destined to the mapped address

embedded in A’s Teredo IPv6 address and the port stored in the

Peer Random Port field of the Peer Entry. The direct bubble is

sent from its own random port.

12. The above direct bubble is dropped by A’s NAT because the NAT

has not seen any outgoing packet sourced from the mapped address

embedded in A’s Teredo IPv6 address and random port advertised

by A.

13. B also sends an indirect bubble (Packet 5) destined to A via A’s

Teredo Server. This indirect bubble includes a Nonce Trailer

and a Random Port Trailer. The Nonce Trailer includes a new

randomly generated nonce that is also stored in the Nonce Sent

field of the Peer Entry. The Random Port Trailer includes the

value in the Random Port field of the Peer Entry.

14. The aforementioned indirect bubble is successfully received by

A. A parses the trailers and stores the nonce contained in the

Nonce Trailer in the Nonce Received field of the Peer Entry. A

stores the port advertised in the Random Port Trailer in the

Random Port field of the Peer Entry.

15. A responds with the following packets in response to the

indirect bubble received. The first packet is a direct bubble

(Packet 6) sent from the primary port and is destined to the

mapped address/port embedded in B’s Teredo IPv6 address.

16. The aforementioned direct bubble again is dropped by B’s NAT

because the NAT has not seen an outgoing packet with the same

4-tuple as the incoming packet.

17. The next packet is also a direct bubble (Packet 7) and this one

is sent from A’s random port. The packet is destined to the

mapped address embedded in B’s Teredo IPv6 address and the Peer

Random Port stored in the Peer Entry.

18. Because both NATs are port-preserving NATs and the random ports

have not been used for any other mapping, the aforementioned

direct bubble is received by B because B’s NAT has seen an

outgoing packet (Packet 4) with the same address/port pairs. B

stores the address/port from which the direct bubble was

received in the mapped address/port fields of the Peer Entry.

It changes the status of the peer to "trusted" and sets the

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Direct Receive on Random Port field to TRUE. At this point,

communication in one direction is now possible (B to A, but not

vice versa).

19. Because A still considers B to be "not-trusted", it times out

and retransmits an indirect bubble (Packet 8). This packet

contains a new nonce as part of the Nonce Trailer and also

contains the value of the random port as part of the Random Port

Trailer.

20. B receives the aforementioned indirect bubble. The processing

of this indirect bubble is similar to the processing of Packet

2. Since B received a direct bubble on its random port, it does

not respond with a direct bubble from its primary port.

Instead, it responds with a direct bubble (Packet 9) sent from

its random port, which is similar to Packet 4 mentioned above.

21. A receives the direct bubble sent by B. A stores the mapped

address/port from which the direct bubble was received in mapped

address/port fields in the Peer Entry. A changes the status of

B to "trusted" and sets the Direct Receive on Random Port field

to TRUE. At this point, the communication is now possible in

the other direction (A to B).

6.4. Sequential Port-Symmetric NAT Extension

The following protocol example illustrates the use of the Sequential

Port-Symmetric NAT Extension.

Assume that Teredo Client A (A), which is positioned behind a

sequential port-symmetric NAT and implements the Sequential Port-

Symmetric NAT Extension, wants to communicate with Teredo Client B

(B), which is positioned behind a port-restricted NAT that supports

the Port-Preserving Port-Symmetric NAT Extension. The following

packet exchange explains the configuration setup and communication

setup between the two clients.

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Teredo A’s A’s B’s

Client Primary Secondary Teredo Client

A NAT Server Server Server NAT B

| | | | | | |

| Direct Bubble to B | | | | |

1 |-------------------------------------------------->| |

| | | | | | |

|Router Solicitation | | | | |

2 |------------------->| | | | |

| | | | | | |

|Router Advertisement| | | | |

|<-------------------| 3 | | | |

| | | | | | |

4 | Direct Bubble to B | | | | |

|-------------------------------------------------->| |

| | | | | | |

| Router Solicitation | | | |

5 |---------------------------->| | | |

| | | | | | |

| Router Advertisement | | | |

|<----------------------------| 6 | | |

| | | | | | |

| Indirect Bubble to B via B’s Teredo Server | | |

7 |------------------------------------------->|-------------->|

| | | | | | |

| | | | Direct Bubble to A |

| |<-------------------------------------------------| 8

| | | | | | |

| | | | Indirect Bubble to A |

|<-------------------|<--------------------------------------| 9

| | | | | | |

| | | | Direct Bubble to A |

|<-----------------------------------------------------------| 10

| | | | | | |

| Direct Bubble to B | | | |

11 |----------------------------------------------------------->|

Sequential Port-Symmetric NAT Packet Exchange

1. During the qualification procedure, when Client A receives a

response from the Teredo Server, it compares the Port value in

the Origin indication with the Local Port value. Since they are

different, it concludes that it is not behind a port-preserving

NAT, and so assumes it is behind a sequential port-symmetric NAT.

2. When A wants to communicate with B, A starts by sending a direct

bubble (Packet 1) from its primary port. This occurs because

Client A does not know Client B’s NAT type, which could be a cone

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or address restricted NAT or UPnP-enabled NAT. Because Client A

is behind a symmetric NAT, the external port used by A’s NAT is a

new port. This direct bubble will be dropped by B’s NAT since

Client B is behind a port-restricted NAT.

3. Because Client A does not know if B is behind a port restricted

NAT or some other kind of NAT, Client A proactively opens a new

random internal port, say, port 1100.

4. Client A then performs its Echo Test as follows:

A. Client A sends a router solicitation (Packet 2) to its Teredo

Server address from port 1100. The server responds with a

router advertisement (Packet 3).

B. Client A sends a direct bubble (Packet 4) to the peer from

port 1100 destined to the port advertised in Client B’s

Teredo address, say, port 2100. This direct bubble is

dropped by Client B’s port-restricted NAT.

C. Client A sends a router solicitation (Packet 5) to its

secondary Teredo server address from port 1100. The server

responds with a router advertisement (Packet 6).

D. On receiving the corresponding router advertisements for

Packet 2 and Packet 4, Client A knows that port 1100 maps to,

say, port 1200 for Packet 2 and port 1202 for Packet 4.

E. Client A then calculates its predicted port used for Packet 2

as the average (rounded down) of 1200 and 1202, i.e., 1201.

5. Client A then sends out an indirect bubble (Packet 7). This

indirect bubble contains a random port trailer that contains the

predicted port, port 1201. This indirect bubble makes it to

Client B.

6. Client B sends out the following bubbles in response to the

indirect bubble:

A. The first direct bubble (Packet 8) is destined for the port

mapping embedded in Client A’s Teredo Address. (It has been

observed that some NATs display symmetric NAT behavior for

outgoing packets but cone NAT behavior for incoming packets.

The direct bubble described is likely to succeed if Client

A’s NAT displays such a behavior.) Since in this example,

A’s NAT is a normal sequential port-symmetric NAT, this

packet is dropped.

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B. The second packet is an indirect bubble (Packet 9) sent to

Client A without any trailers since Client B is behind a

port-restricted NAT.

C. The next packet will be a direct bubble (Packet 10) sent to

port 1201. This packet will make it in to Client A since

Client A previously sent an outgoing packet (Packet 4) with

the same four tuple. At this point, communication in one

direction is now possible (A to B, but not vice versa).

7. Client A then sends a direct bubble (Packet 11) to Client B when

it receives Packet 10. This time, the bubble makes it through to

B because it previously sent an outgoing packet (Packet 10) with

the same four tuple. At this point, communication is now

possible in the other direction (B to A).

6.5. Hairpinning Extension

The following protocol example illustrates the use of the Hairpinning

Extension.

In Figure 3 (Section 3.5), Teredo Client A (A) and Teredo Client B

(B) are positioned behind different immediate NATs in a two-layer NAT

topology; that is, the outermost NAT (NAT E) is common to both A and

B but the immediate NATs that they are connected to are different (A

is connected to NAT F while B is connected to NAT G). Further assume

that the immediate NATs that A and B are connected to are UPnP-

enabled (NAT F and NAT G are UPnP-enabled). We assume that NAT E

does not support hairpinning; that is, the NAT does not relay packets

originating from the private address space and destined for the

public address of the NAT, back to the private address of the NAT.

Before starting the qualification procedure, both A and B use UPnP to

reserve port mappings on their respective NATs. They observe that

the UPnP operation succeeds and both clients obtain valid UPnP Mapped

Address/Port values.

Next, both client A and client B implement the qualification

procedure where they determine their mapped address/port values, as

specified in Section 5.2.1 of [RFC4380].

A and B both compare their UPnP Mapped Address/Port values with the

mapped address/port values obtained through the qualification

procedure. Because both A and B are part of a two-layer NAT

topology, these values will be different. Hence, both A and B

continue to hold on to their UPnP Mapped Address/Port.

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The following packet exchange shows client A establishing

communication with client B.

Teredo Teredo Client A’s Client B’s

Client NAT Client NAT NAT Teredo Teredo

A F B G E Server Server

| | | | | | |

| | Direct Bubble to B | | | |

1 |-------------------------------------->| | |

| | | | | | |

| Indirect Bubble to B via B’s Teredo Server |

2 |----------------------------------------------------------->|

| | |<----------------------------------------|

| | | | | | |

| | | Direct Bubble to A | | |

3 | | |------------------->| | |

| | | | | | |

| | | Direct | | | |

| | |Bubble to A| | | |

4 | | |---------->| | | |

| | | | | | |

| | | Direct | | | |

| | |Bubble to A| | | |

5 | | |---------->| | | |

|<-----------------------------| | | |

| | | | | | |

| | | Indirect Bubble to A | |

6 | | |---------------------------->| |

|<-----------------------------------------------| |

| | | | | | |

|Direct Bubble to B| | | | |

7 |----------------->| | | | |

| | | | | | |

Hairpinning-Based Packet Exchange

1. A sends a direct bubble (Packet 1) to the mapped address/port

embedded in B’s Teredo IPv6 address.

2. The aforementioned direct bubble is dropped by NAT E, because it

does not support Hairpinning.

3. A sends out an indirect bubble (Packet 2) destined to B via B’s

Teredo Server. In this indirect bubble, A includes an Alternate

Address Trailer that includes both the local address/port and

the UPnP mapped address/port.

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RFC 6081 Teredo Extensions January 2011

4. The aforementioned indirect bubble is received by B. After

parsing the Alternate Address Trailer, B has a total of three

addresses to communicate with: two from the Alternate Address

Trailer and one from the mapped address/port embedded in A’s

Teredo IPv6 address. B then responds with the following

packets. The first packet sent by B is a direct bubble (Packet

3) destined to the mapped address/port embedded in A’s Teredo

IPv6 address.

5. The aforementioned direct bubble will be dropped by the NAT E

because it does not support Hairpinning.

6. Because the local address/port was the first mapping in the

Alternate Address Trailer, the second direct bubble (Packet 4)

sent by B is destined to the local address/port.

7. The aforementioned direct bubble is dropped because A and B are

positioned behind different NATs and hence have their own

private address space. A’s local address is not reachable from

B.

8. The next direct bubble (Packet 5) is sent by B destined to A’s

UPnP mapped address/port, which is the second mapping in the

Alternate Address Trailer sent by A.

9. The aforementioned direct bubble is received by A because A’s

UPnP-mapped address is reachable from B. A stores the source

address from which the direct bubble was received in the mapped

address/port fields of the Peer Entry, as defined in Section 5.2

of [RFC4380]. Also, the mapped address status field (as

specified in Section 5.2.3 of [RFC4380]) is changed to

"trusted". At this point, communication in one direction (A to

B) is now possible, but not vice versa because B has not yet

marked A as trusted.

10. B also sends an indirect bubble (Packet 6) to A via A’s Teredo

server. As part of the indirect bubble, B also includes an

Alternate Address Trailer, which contains the local address/port

and the UPnP mapped address/port of B.

11. The aforementioned indirect bubble is received by A. After

parsing the Alternate Address Trailer, A adds the two addresses

in the Alternate Address Trailer to the Alternate Address List

in the Peer Entry. Because the peer’s mapping is "trusted"

(point 9), A responds with only one direct bubble (Packet 7)

that is sent to the mapped address/port stored in the Peer

Entry.

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12. The aforementioned direct bubble is received by B. B records

the mapped address/port from which the direct bubble was

received in the mapped address/port field in its Peer Entry, and

changes the status of the mapped address to "trusted". At this

point, communication is now possible in the other direction (B

to A).

6.6. Server Load Reduction Extension

The following protocol example illustrates the use of the Server Load

Reduction Extension.

Assume that Teredo Client A (A) has established communication with

Teredo Client B (B). Also, assume that at some later point when no

data packets have been exchanged between both clients for more than

30 seconds, the communication needs to be re-established because A

wants to send a data packet to B.

The following packet exchange helps A re-establish communication with

B.

Teredo Client A’s Client B’s Teredo

Client Teredo Teredo Client

A NAT Server Server NAT B

| | | | | |

| | | Direct Bubble to B | | |

1 |------------------------------------------------------------>|

| | | | | |

| | | Direct Bubble to A | | |

|<------------------------------------------------------------| 2

| | | | | |

Server Load Reduction Packet Exchange

1. A sends a direct bubble (Packet 1) with the Neighbor Discovery

Option Trailer, with the DiscoveryType field set to

TeredoDiscoverySolicitation.

2. If the mapping on either of the NATs has not expired, the direct

bubble is received by B. B parses the Neighbor Discovery Option

and because the DiscoveryType was set to

TeredoDiscoverySolicitation, B responds with a direct bubble

(Packet 2). B’s direct bubble also contains the Neighbor

Discovery Option and the DiscoveryType is set to

TeredoDiscoveryAdvertisement.

Thaler Standards Track [Page 57]

RFC 6081 Teredo Extensions January 2011

3. The aforementioned direct bubble is received by A and at this

point, communication between the Teredo clients is re-

established.

7. Security Considerations

Security considerations are the same as those specified in Section 7

of [RFC4380].

In addition, the Hairpinning Extension introduces the possibility of

an amplification attack if a malicious user could advertise a large

number of port mappings in the Alternate Address Trailer, resulting

in a large number of direct bubbles sent in response. Because of

this, Section 4.3 explicitly limits the number of addresses that a

Teredo client will accept.

Because the nonce in the Nonce Trailer is used (as specified in

Section 5.2.4.4) to prevent spoofing of bubbles that would result in

directing traffic to the wrong place, it is important that the nonce

be random so that attackers cannot predict its value. See [RFC4086]

for further discussion of randomness requirements.

8. Acknowledgements

Thanks to Gurpreet Virdi and Poorna Gaddehosur for technical

contributions to this document, and to the V6OPS WG and Jari Arkko

for their helpful reviews.

9. IANA Considerations

IANA has created a new trailer Type registry. Requests for new

trailer Type values are made through Specification Required

[RFC5226]. Initial values are listed below.

Trailer Type Usage Reference

------------ --------------------------------- ---------

0x01 Nonce Trailer RFC 6081

0x02 Random Port Trailer RFC 6081

0x03 Alternate Address Trailer RFC 6081

0x04 Neighbor Discovery Option Trailer RFC 6081

10. References

10.1. Normative References

[RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G.,

and E. Lear, "Address Allocation for Private Internets",

BCP 5, RFC 1918, February 1996.

Thaler Standards Track [Page 58]

RFC 6081 Teredo Extensions January 2011

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate

Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6

(IPv6) Specification", RFC 2460, December 1998.

[RFC4380] Huitema, C., "Teredo: Tunneling IPv6 over UDP through

Network Address Translations (NATs)", RFC 4380,

February 2006.

[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,

"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,

September 2007.

[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an

IANA Considerations Section in RFCs", BCP 26, RFC 5226,

May 2008.

[UPNPWANIP] UPnP Forum, "WANIPConnection:1", November 2001,

<http://www.upnp.org/standardizeddcps/documents/

UPnP_IGD_WANIPConnection%201.0.pdf>.

10.2. Informative References

[RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness

Requirements for Security", BCP 106, RFC 4086,

June 2005.

[RFC4443] Conta, A., Deering, S., and M. Gupta, "Internet Control

Message Protocol (ICMPv6) for the Internet Protocol

Version 6 (IPv6) Specification", RFC 4443, March 2006.

[RFC4787] Audet, F. and C. Jennings, "Network Address Translation

(NAT) Behavioral Requirements for Unicast UDP", BCP 127,

RFC 4787, January 2007.

Author’s Address

Dave Thaler

Microsoft Corporation

One Microsoft Way

Redmond, WA 98052

USA

Phone: +1 425 703 8835

EMail: [email protected]

Thaler Standards Track [Page 59]

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