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rfc:rfc4380

Network Working Group C. Huitema Request for Comments: 4380 Microsoft Category: Standards Track February 2006

                  Teredo: Tunneling IPv6 over UDP
            through Network Address Translations (NATs)

Status of This Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2006).

Abstract

 We propose here a service that enables nodes located behind one or
 more IPv4 Network Address Translations (NATs) to obtain IPv6
 connectivity by tunneling packets over UDP; we call this the Teredo
 service.  Running the service requires the help of "Teredo servers"
 and "Teredo relays".  The Teredo servers are stateless, and only have
 to manage a small fraction of the traffic between Teredo clients; the
 Teredo relays act as IPv6 routers between the Teredo service and the
 "native" IPv6 Internet.  The relays can also provide interoperability
 with hosts using other transition mechanisms such as "6to4".

Table of Contents

 1. Introduction ....................................................3
 2. Definitions .....................................................4
    2.1. Teredo Service .............................................4
    2.2. Teredo Client ..............................................4
    2.3. Teredo Server ..............................................4
    2.4. Teredo Relay ...............................................4
    2.5. Teredo IPv6 Service Prefix .................................4
    2.6. Global Teredo IPv6 Service Prefix ..........................4
    2.7. Teredo UDP Port ............................................4
    2.8. Teredo Bubble ..............................................4
    2.9. Teredo Service Port ........................................5
    2.10. Teredo Server Address .....................................5
    2.11. Teredo Mapped Address and Teredo Mapped Port ..............5
    2.12. Teredo IPv6 Client Prefix .................................5

Huitema Standards Track [Page 1] RFC 4380 Teredo February 2006

    2.13. Teredo Node Identifier ....................................5
    2.14. Teredo IPv6 Address .......................................5
    2.15. Teredo Refresh Interval ...................................5
    2.16. Teredo Secondary Port .....................................6
    2.17. Teredo IPv4 Discovery Address .............................6
 3. Design Goals, Requirements, and Model of Operation ..............6
    3.1. Hypotheses about NAT Behavior ..............................6
    3.2. IPv6 Provider of Last Resort ...............................8
    3.3. Operational Requirements ...................................9
    3.4. Model of Operation ........................................10
 4. Teredo Addresses ...............................................11
 5. Specification of Clients, Servers, and Relays ..................13
    5.1. Message Formats ...........................................13
    5.2. Teredo Client Specification ...............................16
    5.3. Teredo Server Specification ...............................31
    5.4. Teredo Relay Specification ................................33
    5.5. Implementation of Automatic Sunset ........................36
 6. Further Study, Use of Teredo to Implement a Tunnel Service .....37
 7. Security Considerations ........................................38
    7.1. Opening a Hole in the NAT .................................38
    7.2. Using the Teredo Service for a Man-in-the-Middle Attack ...39
    7.3. Denial of the Teredo service ..............................42
    7.4. Denial of Service against Non-Teredo Nodes ................43
 8. IAB Considerations .............................................46
    8.1. Problem Definition ........................................46
    8.2. Exit Strategy .............................................47
    8.3. Brittleness Introduced by Teredo ..........................48
    8.4. Requirements for a Long-Term Solution .....................50
 9. IANA Considerations ............................................50
 10. Acknowledgements ..............................................50
 11. References ....................................................51
    11.1. Normative References .....................................51
    11.2. Informative References ...................................52

Huitema Standards Track [Page 2] RFC 4380 Teredo February 2006

1. Introduction

 Classic tunneling methods envisaged for IPv6 transition operate by
 sending IPv6 packets as payload of IPv4 packets; the 6to4 proposal
 [RFC3056] proposes automatic discovery in this context.  A problem
 with these methods is that they don't work when the IPv6 candidate
 node is isolated behind a Network Address Translator (NAT) device:
 NATs are typically not programmed to allow the transmission of
 arbitrary payload types; even when they are, the local address cannot
 be used in a 6to4 scheme. 6to4 will work with a NAT if the NAT and
 6to4 router functions are in the same box; we want to cover the
 relatively frequent case when the NAT cannot be readily upgraded to
 provide a 6to4 router function.
 A possible way to solve the problem is to rely on a set of "tunnel
 brokers".  However, there are limits to any solution that is based on
 such brokers: the quality of service may be limited, since the
 traffic follows a dogleg route from the source to the broker and then
 the destination; the broker has to provide sufficient transmission
 capacity to relay all packets and thus suffers a high cost.  For
 these two reasons, it may be desirable to have solutions that allow
 for "automatic tunneling", i.e., let the packets follow a direct path
 to the destination.
 The automatic tunneling requirement is indeed at odds with some of
 the specificities of NATs.  Establishing a direct path supposes that
 the IPv6 candidate node can retrieve a "globally routable" address
 that results from the translation of its local address by one or more
 NATs; it also supposes that we can find a way to bypass the various
 "per destination protections" that many NATs implement.  In this
 memo, we will explain how IPv6 candidates located behind NATs use
 "Teredo servers" to learn their "global address" and to obtain
 connectivity, how they exchange packets with native IPv6 hosts
 through "Teredo relays", and how clients, servers, and relays can be
 organized in Teredo networks.
 The specification is organized as follows.  Section 2 contains the
 definition of the terms used in the memo.  Section 3 presents the
 hypotheses on NAT behavior used in the design, as well as the
 operational requirements that the design should meet.  Section 4
 presents the IPv6 address format used by Teredo.  Section 5 contains
 the format of the messages and the specification of the protocol.
 Section 6 presents guidelines for further work on configured tunnels
 that would be complementary to the current approach.  Section 7
 contains a security discussion, section 8 contains a discussion of
 the Unilateral Self Address Fixing (UNSAF) issues, and section 9
 contains IANA considerations.

Huitema Standards Track [Page 3] RFC 4380 Teredo February 2006

2. Definitions

 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 [RFC2119].
 This specification uses the following definitions:

2.1. Teredo Service

 The transmission of IPv6 packets over UDP, as defined in this memo.

2.2. Teredo Client

 A node that has some access to the IPv4 Internet and wants to gain
 access to the IPv6 Internet.

2.3. Teredo Server

 A node that has access to the IPv4 Internet through a globally
 routable address, and is used as a helper to provide IPv6
 connectivity to Teredo clients.

2.4. Teredo Relay

 An IPv6 router that can receive traffic destined to Teredo clients
 and forward it using the Teredo service.

2.5. Teredo IPv6 Service Prefix

 An IPv6 addressing prefix that is used to construct the IPv6 address
 of Teredo clients.

2.6. Global Teredo IPv6 Service Prefix

 An IPv6 addressing prefix whose value is 2001:0000:/32.

2.7. Teredo UDP Port

 The UDP port number at which Teredo servers are waiting for packets.
 The value of this port is 3544.

2.8. Teredo Bubble

 A Teredo bubble is a minimal IPv6 packet, made of an IPv6 header and
 a null payload.  The payload type is set to 59, No Next Header, as
 per [RFC2460].  The Teredo clients and relays may send bubbles in
 order to create a mapping in a NAT.

Huitema Standards Track [Page 4] RFC 4380 Teredo February 2006

2.9. Teredo Service Port

 The port from which the Teredo client sends Teredo packets.  This
 port is attached to one of the client's IPv4 addresses.  The IPv4
 address may or may not be globally routable, as the client may be
 located behind one or more NAT.

2.10. Teredo Server Address

 The IPv4 address of the Teredo server selected by a particular
 client.

2.11. Teredo Mapped Address and Teredo Mapped Port

 A global IPv4 address and a UDP port that results from the
 translation of the IPv4 address and UDP port of a client's Teredo
 service port by one or more NATs.  The client learns these values
 through the Teredo protocol described in this memo.

2.12. Teredo IPv6 Client Prefix

 A global scope IPv6 prefix composed of the Teredo IPv6 service prefix
 and the Teredo server address.

2.13. Teredo Node Identifier

 A 64-bit identifier that contains the UDP port and IPv4 address at
 which a client can be reached through the Teredo service, as well as
 a flag indicating the type of NAT through which the client accesses
 the IPv4 Internet.

2.14. Teredo IPv6 Address

 A Teredo IPv6 address obtained by combining a Teredo IPv6 client
 prefix and a Teredo node identifier.

2.15. Teredo Refresh Interval

 The interval during which a Teredo IPv6 address is expected to remain
 valid in the absence of "refresh" traffic.  For a client located
 behind a NAT, the interval depends on configuration parameters of the
 local NAT, or the combination of NATs in the path to the Teredo
 server.  By default, clients assume an interval value of 30 seconds;
 a longer value may be determined by local tests, as described in
 section 5.

Huitema Standards Track [Page 5] RFC 4380 Teredo February 2006

2.16. Teredo Secondary Port

 A UDP port used to send or receive packets in order to determine the
 appropriate value of the refresh interval, but not used to carry any
 Teredo traffic.

2.17. Teredo IPv4 Discovery Address

 An IPv4 multicast address used to discover other Teredo clients on
 the same IPv4 subnet.  The value of this address is 224.0.0.253.

3. Design Goals, Requirements, and Model of Operation

 The proposed solution transports IPv6 packets as the payload of UDP
 packets.  This is based on the observation that TCP and UDP are the
 only protocols guaranteed to cross the majority of NAT devices.
 Tunneling packets over TCP would be possible, but would result in a
 poor quality of service; encapsulation over UDP is a better choice.
 The design of our solution is based on a set of hypotheses and
 observations on the behavior of NATs, our desire to provide an "IPv6
 provider of last resort", and a list of operational requirements.  It
 results in a model of operation in which the Teredo service is
 enabled by a set of servers and relays.

3.1. Hypotheses about NAT Behavior

 NAT devices typically incorporate some support for UDP, in order to
 enable users in the natted domain to use UDP-based applications.  The
 NAT will typically allocate a "mapping" when it sees a UDP packet
 coming through for which there is not yet an existing mapping.  The
 handling of UDP "sessions" by NAT devices differs by two important
 parameters, the type and the duration of the mappings.
 The type of mappings is analyzed in [RFC3489], which distinguishes
 between "cone NAT", "restricted cone NAT", "port restricted cone NAT"
 and "symmetric NAT".  The Teredo solution ensures connectivity for
 clients located behind cone NATs, restricted cone NATs, or port-
 restricted cone NATs.
 Transmission of regular IPv6 packets only takes place after an
 exchange of "bubbles" between the parties.  This exchange would often
 fail for clients behind symmetric NAT, because their peer cannot
 predict the UDP port number that the NAT expects.
 Clients located behind a symmetric NAT will only be able to use
 Teredo if they can somehow program the NAT and reserve a Teredo
 service port for each client, for example, using the DMZ functions of

Huitema Standards Track [Page 6] RFC 4380 Teredo February 2006

 the NAT.  This is obviously an onerous requirement, at odds with the
 design goal of an automatic solution.  However, measurement campaigns
 and studies of documentations have shown that, at least in simple
 "unmanaged" networks, symmetric NATs are a small minority; moreover,
 it seems that new NAT models or firmware upgrades avoid the
 "symmetric" design.
 Investigations on the performance of [RFC3489] have shown the
 relative frequency of a particular NAT design, which we might call
 "port conserving".  In this design, the NAT tries to keep the same
 port number inside and outside, unless the "outside" port number is
 already in use for another mapping with the same host.  Port
 conserving NAT appear as "cone" or "restricted cone NAT" most of the
 time, but they will behave as "symmetric NAT" when multiple internal
 hosts use the same port number to communicate to the same server.
 The Teredo design minimizes the risk of encountering the "symmetric"
 behavior by asking multiple hosts located behind the same NAT to use
 different Teredo service ports.
 Other investigation in the behavior of NAT also outlined the
 "probabilistic rewrite" behavior.  Some brands of NAT will examine
 all packets for "embedded addresses", IP addresses, and port numbers
 present in application payloads.  They will systematically replace
 32-bit values that match a local address by the corresponding mapped
 address.  The Teredo specification includes an "obfuscation"
 procedure in order to avoid this behavior.
 Regardless of their types, UDP mappings are not kept forever.  The
 typical algorithm is to remove the mapping if no traffic is observed
 on the specified port for a "lifetime" period.  The Teredo client
 that wants to maintain a mapping open in the NAT will have to send
 some "keep alive" traffic before the lifetime expires.  For that, it
 needs an estimate of the "lifetime" parameter used in the NAT.  We
 observed that the implementation of lifetime control can vary in
 several ways.
 Most NATs implement a "minimum lifetime", which is set as a parameter
 of the implementation.  Our observations of various boxes showed that
 this parameter can vary between about 45 seconds and several minutes.
 In many NATs, mappings can be kept for a duration that exceeds this
 minimum, even in the absence of traffic.  We suspect that many
 implementation perform "garbage collection" of unused mappings on
 special events, e.g., when the overall number of mappings exceeds
 some limit.

Huitema Standards Track [Page 7] RFC 4380 Teredo February 2006

 In some cases, e.g., NATs that manage Integrated Services Digital
 Network (ISDN) or dial-up connections, the mappings will be released
 when the connection is released, i.e., when no traffic is observed on
 the connection for a period of a few minutes.
 Any algorithm used to estimate the lifetime of mapping will have to
 be robust against these variations.
 In some cases, clients are located behind multiple NAT.  The Teredo
 procedures will ensure communications between clients between
 multiple NATs and clients "on the other side" of these NATs.  They
 will also ensure communication when clients are located in a single
 subnet behind the same NAT.
 The procedures do not make any hypothesis about the type of IPv4
 address used behind a NAT, and in particular do not assume that these
 are private addresses defined in [RFC1918].

3.2. IPv6 Provider of Last Resort

 Teredo is designed to provide an "IPv6 access of last resort" to
 nodes that need IPv6 connectivity but cannot use any of the other
 IPv6 transition schemes.  This design objective has several
 consequences on when to use Teredo, how to program clients, and what
 to expect of servers.  Another consequence is that we expect to see a
 point in time at which the Teredo technology ceases to be used.

3.2.1. When to Use Teredo

 Teredo is designed to robustly enable IPv6 traffic through NATs, and
 the price of robustness is a reasonable amount of overhead, due to
 UDP encapsulation and transmission of bubbles.  Nodes that want to
 connect to the IPv6 Internet SHOULD only use the Teredo service as a
 "last resort" option: they SHOULD prefer using direct IPv6
 connectivity if it is locally available, if it is provided by a 6to4
 router co-located with the local NAT, or if it is provided by a
 configured tunnel service; and they SHOULD prefer using the less
 onerous 6to4 encapsulation if they can use a global IPv4 address.

3.2.2. Autonomous Deployment

 In an IPv6-enabled network, the IPv6 service is configured
 automatically, by using mechanisms such as IPv6 Stateless Address
 Autoconfiguration [RFC2462] and Neighbor Discovery [RFC2461].  A
 design objective is to configure the Teredo service as automatically
 as possible.  In practice, however, it is required that the client
 learn the IPv4 address of a server that is willing to serve the
 client; some servers may also require some form of access control.

Huitema Standards Track [Page 8] RFC 4380 Teredo February 2006

3.2.3. Minimal Load on Servers

 During the peak of the transition, there will be a requirement to
 deploy Teredo servers supporting a large number of Teredo clients.
 Minimizing the load on the server is a good way to facilitate this
 deployment.  To achieve this goal, servers should be as stateless as
 possible, and they should also not be required to carry any more
 traffic than necessary.  To achieve this objective, we require only
 that servers enable the packet exchange between clients, but we don't
 require servers to carry the actual data packets: these packets will
 have to be exchanged directly between the Teredo clients, or through
 a destination-selected relay for exchanges between Teredo clients and
 other IPv6 clients.

3.2.4. Automatic Sunset

 Teredo is meant as a short-term solution to the specific problem of
 providing IPv6 service to nodes located behind a NAT.  The problem is
 expected to be resolved over time by transforming the "IPv4 NAT" into
 an "IPv6 router".  This can be done in one of two ways:  upgrading
 the NAT to provide 6to4 functions or upgrading the Internet
 connection used by the NAT to a native IPv6 service, and then adding
 IPv6 router functionality in the NAT.  In either case, the former NAT
 can present itself as an IPv6 router to the systems behind it.  These
 systems will start receiving the "router advertisements"; they will
 notice that they have IPv6 connectivity and will stop using Teredo.

3.3. Operational Requirements

3.3.1. Robustness Requirement

 The Teredo service is designed primarily for robustness: packets are
 carried over UDP in order to cross as many NAT implementations as
 possible.  The servers are designed to be stateless, which means that
 they can easily be replicated.  We expect indeed to find many such
 servers replicated at multiple Internet locations.

3.3.2. Minimal Support Cost

 The service requires the support of Teredo servers and Teredo relays.
 In order to facilitate the deployment of these servers and relays,
 the Teredo procedures are designed to minimize the amount of
 coordination required between servers and relays.
 Meeting this objective implies that the Teredo addresses will
 incorporate the IPv4 address and UDP port through which a Teredo
 client can be reached.  This creates an implicit limit on the

Huitema Standards Track [Page 9] RFC 4380 Teredo February 2006

 stability of the Teredo addresses, which can only remain valid as
 long as the underlying IPv4 address and UDP port remain valid.

3.3.3. Protection against Denial of Service Attacks

 The Teredo clients obtain mapped addresses and ports from the Teredo
 servers.  The service must be protected against denial of service
 attacks in which a third party spoofs a Teredo server and sends
 improper information to the client.

3.3.4. Protection against Distributed Denial of Service Attacks

 Teredo relays will act as a relay for IPv6 packets.  Improperly
 designed packet relays can be used by denial of service attackers to
 hide their address, making the attack untraceable.  The Teredo
 service must include adequate protection against such misuse.

3.3.5. Compatibility with Ingress Filtering

 Routers may perform ingress filtering by checking that the source
 address of the packets received on a given interface is "legitimate",
 i.e., belongs to network prefixes from which traffic is expected at a
 network interface.  Ingress filtering is a recommended practice, as
 it thwarts the use of forged source IP addresses by malfeasant
 hackers, notably to cover their tracks during denial of service
 attacks.  The Teredo specification must not force networks to disable
 ingress filtering.

3.4. Model of Operation

 The operation of Teredo involves four types of nodes: Teredo clients,
 Teredo servers, Teredo relays, and "plain" IPv6 nodes.
 Teredo clients start operation by interacting with a Teredo server,
 performing a "qualification procedure".  During this procedure, the
 client will discover whether it is behind a cone, restricted cone, or
 symmetric NAT.  If the client is not located behind a symmetric NAT,
 the procedure will be successful and the client will configure a
 "Teredo address".
 The Teredo IPv6 address embeds the "mapped address and port" through
 which the client can receive IPv4/UDP packets encapsulating IPv6
 packets.  If the client is not located behind a cone NAT,
 transmission of regular IPv6 packets must be preceded by an exchange
 of "bubbles" that will install a mapping in the NAT.  This document
 specifies how the bubbles can be exchanged between Teredo clients in
 order to enable transmission along a direct path.

Huitema Standards Track [Page 10] RFC 4380 Teredo February 2006

 Teredo clients can exchange IPv6 packets with plain IPv6 nodes (e.g.,
 native nodes or 6to4 nodes) through Teredo relays.  Teredo relays
 advertise reachability of the Teredo prefix to a certain subset of
 the IPv6 Internet: a relay set up by an ISP will typically serve only
 the IPv6 customers of this ISP; a relay set-up for a site will only
 serve the IPv6 hosts of this site.  Dual-stack hosts may implement a
 "local relay", allowing them to communicate directly with Teredo
 hosts by sending IPv6 packets over UDP and IPv4 without having to
 advertise a Teredo IPv6 address.
 Teredo clients have to discover the relay that is closest to each
 native IPv6 or 6to4 peer.  They have to perform this discovery for
 each native IPv6 or 6to4 peer with which they communicate.  In order
 to prevent spoofing, the Teredo clients perform a relay discovery
 procedure by sending an ICMP echo request to the native host.  This
 message is a regularly formatted IPv6 ICMP packet, which is
 encapsulated in UDP and sent by the client to its Teredo server; the
 server decapsulates the IPv6 message and forwards it to the intended
 IPv6 destination.  The payload of the echo request contains a large
 random number.  The echo reply is sent by the peer to the IPv6
 address of the client, and is forwarded through standard IPv6 routing
 mechanisms.  It will naturally reach the Teredo relay closest to the
 native or 6to4 peer, and will be forwarded by this relay using the
 Teredo mechanisms.  The Teredo client will discover the IPv4 address
 and UDP port used by the relay to send the echo reply, and will send
 further IPv6 packets to the peer by encapsulating them in UDP packets
 sent to this IPv4 address and port.  In order to prevent spoofing,
 the Teredo client verifies that the payload of the echo reply
 contains the proper random number.
 The procedures are designed so that the Teredo server only
 participates in the qualification procedure and in the exchange of
 bubbles and ICMP echo requests.  The Teredo server never carries
 actual data traffic.  There are two rationales for this design:
 reduce the load on the server in order to enable scaling, and avoid
 privacy issues that could occur if a Teredo server kept copies of the
 client's data packets.

4. Teredo Addresses

 The Teredo addresses are composed of 5 components:
 +-------------+-------------+-------+------+-------------+
 | Prefix      | Server IPv4 | Flags | Port | Client IPv4 |
 +-------------+-------------+-------+------+-------------+
  1. Prefix: the 32-bit Teredo service prefix.
  2. Server IPv4: the IPv4 address of a Teredo server.

Huitema Standards Track [Page 11] RFC 4380 Teredo February 2006

  1. Flags: a set of 16 bits that document type of address and NAT.
  2. Port: the obfuscated "mapped UDP port" of the Teredo service at

the client.

  1. Client IPv4: the obfuscated "mapped IPv4 address" of the client.
 In this format, both the "mapped UDP port" and "mapped IPv4 address"
 of the client are obfuscated.  Each bit in the address and port
 number is reversed; this can be done by an exclusive OR of the 16-bit
 port number with the hexadecimal value 0xFFFF, and an exclusive OR of
 the 32-bit address with the hexadecimal value 0xFFFFFFFF.
 The IPv6 addressing rules specify that "for all unicast addresses,
 except those that start with binary value 000, Interface IDs are
 required to be 64 bits long and to be constructed in Modified EUI-64
 format".  This dictates the encoding of the flags, 16 intermediate
 bits that should correspond to valid values of the most significant
 16 bits of a Modified EUI-64 ID:
        0       0 0       1
       |0       7 8       5
       +----+----+----+----+
       |Czzz|zzUG|zzzz|zzzz|
       +----+----+----+----+
 In this format:
  1. The bits "UG" should be set to the value "00", indicating a non-

global unicast identifier;

  1. The bit "C" (cone) should be set to 1 if the client believes it is

behind a cone NAT, to 0 otherwise; these values determine

    different server behavior during the qualification procedure, as
    specified in Section 5.2.1, as well as different bubble processing
    by clients and relays.
 -  The bits indicated with "z" must be set to zero and ignored on
    receipt.
 Thus, there are two currently specified values of the Flags field:
 "0x0000" (all null) if the cone bit is set to 0, and "0x8000" if the
 cone bit is set to 1.  (Further versions of this specification may
 assign new values to the reserved bits.)
 In some cases, Teredo nodes use link-local addresses.  These
 addresses contain a link-local prefix (FE80::/64) and a 64-bit
 identifier, constructed using the same format as presented above.  A
 difference between link-local addresses and global addresses is that
 the identifiers used in global addresses MUST include a global scope
 unicast IPv4 address, while the identifiers used in link-local
 addresses MAY include a private IPv4 address.

Huitema Standards Track [Page 12] RFC 4380 Teredo February 2006

5. Specification of Clients, Servers, and Relays

 The Teredo service is realized by having clients interact with Teredo
 servers through the Teredo service protocol.  The clients will also
 receive IPv6 packets through Teredo relays.  The client behavior is
 specified in Section 5.2.
 The Teredo server is designed to be stateless.  It waits for Teredo
 requests and for IPv6 packets on the Teredo UDP port; it processes
 the requests by sending a response to the appropriate address and
 port; it forwards some Teredo IPv6 packets to the appropriate IPv4
 address and UDP port, or to native IPv6 peers of Teredo clients.  The
 precise behavior of the server is specified in Section 5.3.
 The Teredo relay advertises reachability of the Teredo service prefix
 over IPv6.  The scope of advertisement may be the entire Internet or
 a smaller subset such as an ISP network or an IPv6 site; it may even
 be as small as a single host in the case of "local relays".  The
 relay forwards Teredo IPv6 packets to the appropriate IPv4 address
 and UDP port.  The relay behavior is specified in Section 5.4.
 Teredo clients, servers, and relays must implement the sunset
 procedure defined in Section 5.5.

5.1. Message Formats

5.1.1. Teredo IPv6 Packet Encapsulation

 Teredo IPv6 packets are transmitted as UDP packets [RFC768] within
 IPv4 [RFC791].  The source and destination IP addresses and UDP ports
 take values that are specified in this section.  Packets can come in
 one of two formats, simple encapsulation and encapsulation with
 origin indication.
 When simple encapsulation is used, the packet will have a simple
 format, in which the IPv6 packet is carried as the payload of a UDP
 datagram:
 +------+-----+-------------+
 | IPv4 | UDP | IPv6 packet |
 +------+-----+-------------+
 When relaying some packets received from third parties, the server
 may insert an origin indication in the first bytes of the UDP
 payload:

Huitema Standards Track [Page 13] RFC 4380 Teredo February 2006

 +------+-----+-------------------+-------------+
 | IPv4 | UDP | Origin indication | IPv6 packet |
 +------+-----+-------------------+-------------+
 The origin indication encapsulation is an 8-octet element, with the
 following content:
 +--------+--------+-----------------+
 |  0x00  | 0x00   | Origin port #   |
 +--------+--------+-----------------+
 |  Origin IPv4 address              |
 +-----------------------------------+
 The first two octets of the origin indication are set to a null
 value; this is used to discriminate between the simple encapsulation,
 in which the first 4 bits of the packet contain the indication of the
 IPv6 protocol, and the origin indication.
 The following 16 bits contain the obfuscated value of the port number
 from which the packet was received, in network byte order.  The next
 32 bits contain the obfuscated IPv4 address from which the packet was
 received, in network byte order.  In this format, both the original
 "IPv4 address" and "UDP port" of the client are obfuscated.  Each bit
 in the address and port number is reversed; this can be done by an
 exclusive OR of the 16-bit port number with the hexadecimal value
 0xFFFF, and an exclusive OR of the 32-bit address with the
 hexadecimal value 0xFFFFFFFF.
 For example, if the original UDP port number was 337 (hexadecimal
 0151) and original IPv4 address was 1.2.3.4 (hexadecimal 01020304),
 the origin indication would contain the value "0000FEAEFEFDFCFB".
 When exchanging Router Solicitation (RS) and Router Advertisement
 (RA) messages between a client and its server, the packets may
 include an authentication parameter:
 +------+-----+----------------+-------------+
 | IPv4 | UDP | Authentication | IPv6 packet |
 +------+-----+----------------+-------------+
 The authentication encapsulation is a variable-length element,
 containing a client identifier, an authentication value, a nonce
 value, and a confirmation byte.

Huitema Standards Track [Page 14] RFC 4380 Teredo February 2006

 +--------+--------+--------+--------+
 |  0x00  | 0x01   | ID-len | AU-len |
 +--------+--------+--------+--------+
 |  Client identifier (ID-len        |
 +-----------------+-----------------+
 |  octets)        |  Authentication |
 +-----------------+--------+--------+
 | value (AU-len octets)    | Nonce  |
 +--------------------------+--------+
 | value (8 octets)                  |
 +--------------------------+--------+
 |                          | Conf.  |
 +--------------------------+--------+
 The first octet of the authentication encapsulation is set to a null
 value, and the second octet is set to the value 1; this enables
 differentiation from IPv6 packets and from origin information
 indication encapsulation.  The third octet indicates the length in
 bytes of the client identifier; the fourth octet indicates the length
 in bytes of the authentication value.  The computation of the
 authentication value is specified in Section 5.2.2. The
 authentication value is followed by an 8-octet nonce, and by a
 confirmation byte.
 Both ID-len and AU-len can be set to null values if the server does
 not require an explicit authentication of the client.
 Authentication and origin indication encapsulations may sometimes be
 combined, for example, in the RA responses sent by the server.  In
 this case, the authentication encapsulation MUST be the first element
 in the UDP payload:
 +------+-----+----------------+--------+-------------+
 | IPv4 | UDP | Authentication | Origin | IPv6 packet |
 +------+-----+----------------+--------+-------------+

5.1.2. Maximum Transmission Unit

 Since Teredo uses UDP as an underlying transport, a Teredo Maximum
 Transmission Unit (MTU) could potentially be as large as the payload
 of the largest valid UDP datagram (65507 bytes).  However, since
 Teredo packets can travel on unpredictable paths over the Internet,
 it is best to contain this MTU to a small size, in order to minimize
 the effect of IPv4 packet fragmentation and reassembly.  The default
 link MTU assumed by a host, and the link MTU supplied by a Teredo
 server during router advertisement SHOULD normally be set to the
 minimum IPv6 MTU size of 1280 bytes [RFC2460].

Huitema Standards Track [Page 15] RFC 4380 Teredo February 2006

 Teredo implementations SHOULD NOT set the Don't Fragment (DF) bit of
 the encapsulating IPv4 header.

5.2. Teredo Client Specification

 Before using the Teredo service, the client must be configured with:
  1. the IPv4 address of a server.
  2. a secondary IPv4 address of that server.
 If secure discovery is required, the client must also be configured
 with:
  1. a client identifier,
  2. a secret value, shared with the server,
  3. an authentication algorithm, shared with the server.
 A Teredo client expects to exchange IPv6 packets through a UDP port,
 the Teredo service port.  To avoid problems when operating behind a
 "port conserving" NAT, different clients operating behind the same
 NAT should use different service port numbers.  This can be achieved
 through explicit configuration or, in the absence of configuration,
 by picking the service port number at random.
 The client will maintain the following variables that reflect the
 state of the Teredo service:
  1. Teredo connectivity status,
  2. Mapped address and port number associated with the Teredo service

port,

  1. Teredo IPv6 prefix associated with the Teredo service port,
  2. Teredo IPv6 address or addresses derived from the prefix,
  3. Link local address,
  4. Date and time of the last interaction with the Teredo server,
  5. Teredo Refresh Interval,
  6. Randomized Refresh Interval,
  7. List of recent Teredo peers.
 Before sending any packets, the client must perform the Teredo
 qualification procedure, which determines the Teredo connectivity
 status, the mapped address and port number, and the Teredo IPv6
 prefix.  It should then perform the cone NAT determination procedure,
 which determines the cone NAT status and may alter the value of the
 prefix.  If the qualification is successful, the client may use the
 Teredo service port to transmit and receive IPv6 packets, according
 to the transmission and reception procedures.  These procedures use
 the "list of recent peers".  For each peer, the list contains:

Huitema Standards Track [Page 16] RFC 4380 Teredo February 2006

  1. The IPv6 address of the peer,
  2. The mapped IPv4 address and mapped UDP port of the peer,
  3. The status of the mapped address, i.e., trusted or not,
  4. The value of the last nonce sent to the peer,
  5. The date and time of the last reception from the peer,
  6. The date and time of the last transmission to the peer,
  7. The number of bubbles transmitted to the peer.
 The list of peers is used to enable the transmission of IPv6 packets
 by using a "direct path" for the IPv6 packets.  The list of peers
 could grow over time.  Clients should implement a list management
 strategy, for example, deleting the least recently used entries.
 Clients should make sure that the list has a sufficient size, to
 avoid unnecessary exchanges of bubbles.
 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.
 In order to avoid triangle routing for stations that are located
 behind the same NAT, the Teredo clients MAY use the optional local
 client discovery procedure defined in Section 5.2.8. Using this
 procedure will also enhance connectivity when the NAT cannot do
 "hairpin" routing, i.e., cannot redirect a packet sent from one
 internal host to the mapped address and port of another internal
 host.

5.2.1. Qualification Procedure

 The purposes of the qualification procedure are to establish the
 status of the local IPv4 connection and to determine the Teredo IPv6
 client prefix of the local Teredo interface.  The procedure starts
 when the service is in the "initial" state, and it results in a
 "qualified" state if successful, and in an "off-line" state if
 unsuccessful.

Huitema Standards Track [Page 17] RFC 4380 Teredo February 2006

        /---------\
        | Initial |
        \---------/
             |
        +----+----------+
        | Set ConeBit=1 |
        +----+----------+
             |
             +<-------------------------------------------+
             |                                            |
        +----+----+                                       |
        | Start   |<------+                               |
        +----+----+       |                    +----------+----+
             |            |                    | Set ConeBit=0 |
             v            |                    +----------+----+
        /---------\ Timer | N                             ^
        |Starting |-------+ attempts /----------------\Yes|
        \---------/----------------->| ConeBit == 1 ? |---+
             | Response              \----------------/
             |                              | No
             V                              V
      /---------------\ Yes            /----------\
      | ConeBit == 1? |-----+          | Off line |
      \---------------/     |          \----------/
          No |              v
             |         /----------\
             |         | Cone NAT |
       +-----+-----+   \----------/
       | New Server|
       +-----+-----+
             |
        +----+----+
        | Start   |<------+
        +----+----+       |
             |            |
             v            |
        /---------\ Timer |
        |Starting |-------+ N attempts /----------\
        \---------/------------------->| Off line |
             | Response                \----------/
             |
             V

Huitema Standards Track [Page 18] RFC 4380 Teredo February 2006

       /------------\ No      /---------------\
       | Same port? |-------->| Symmetric NAT |
       \------------/         \---------------/
             | Yes
             V
        /----------------------\
        | Restricted Cone NAT  |
        \----------------------/
 Initially, the Teredo connectivity status is set to "Initial".
 When the interface is initialized, the system first performs the
 "start action" by sending a Router Solicitation message, as defined
 in [RFC2461].  The client picks a link-local address and uses it as
 the IPv6 source of the message; the cone bit in the address is set to
 1 (see Section 4 for the address format); the IPv6 destination of the
 RS is the all-routers multicast address; the packet will be sent over
 UDP from the service port to the Teredo server's IPv4 address and
 Teredo UDP port.  The connectivity status moves then to "Starting".
 In the starting state, the client waits for a router advertisement
 from the Teredo server.  If no response comes within a time-out T,
 the client should repeat the start action, by resending the Router
 Solicitation message.  If no response has arrived after N
 repetitions, the client concludes that it is not behind a cone NAT.
 It sets the cone bit to 0, and repeats the procedure.  If after N
 other timer expirations and retransmissions there is still no
 response, the client concludes that it cannot use UDP, and that the
 Teredo service is not available; the status is set to "Off-line".  In
 accordance with [RFC2461], the default time-out value is set to T=4
 seconds, and the maximum number of repetitions is set to N=3.
 If a response arrives, the client checks that the response contains
 an origin indication and a valid router advertisement as defined in
 [RFC2461], that the IPv6 destination address is equal to the link-
 local address used in the router solicitation, and that the router
 advertisement contains exactly one advertised Prefix Information
 option.  This prefix should be a valid Teredo IPv6 server prefix: the
 first 32 bits should contain the global Teredo IPv6 service prefix,
 and the next 32 bits should contain the server's IPv4 address.  If
 this is the case, the client learns the Teredo mapped address and
 Teredo mapped port from the origin indication.  The IPv6 source
 address of the Router Advertisement is a link-local server address of
 the Teredo server.  (Responses that are not valid advertisements are
 simply discarded.)

Huitema Standards Track [Page 19] RFC 4380 Teredo February 2006

 If the client has received an RA with the cone bit in the IPv6
 destination address set to 1, it is behind a cone NAT and is fully
 qualified.  If the RA is received with the cone bit set to 0, the
 client does not know whether the local NAT is restricted or
 symmetric.  The client selects the secondary IPv4 server address, and
 repeats the procedure, the cone bit remaining to the value zero.  If
 the client does not receive a response, it detects that the service
 is not usable.  If the client receives a response, it compares the
 mapped address and mapped port in this second response to the first
 received values.  If the values are different, the client detects a
 symmetric NAT: it cannot use the Teredo service.  If the values are
 the same, the client detects a port-restricted or restricted cone
 NAT: the client is qualified to use the service.  (Teredo operates
 the same way for restricted and port-restricted NAT.)
 If the client is qualified, it builds a Teredo IPv6 address using the
 Teredo IPv6 server prefix learned from the RA and the obfuscated
 values of the UDP port and IPv4 address learned from the origin
 indication.  The cone bit should be set to the value used to receive
 the RA, i.e., 1 if the client is behind a cone NAT, 0 otherwise.  The
 client can start using the Teredo service.

5.2.2. Secure Qualification

 The client may be required to perform secured qualification.  The
 client will perform exactly the algorithm described in Section 5.2.1,
 but it will incorporate an authentication encapsulation in the UDP
 packet carrying the router solicitation message, and it will verify
 the presence of a valid authentication parameter in the UDP message
 that carries the router advertisement provided by the sender.
 In these packets, the nonce value is chosen by the client, and is
 repeated in the response from the server; the client identifier is a
 value with which the client was configured.
 A first level of protection is provided by just checking that the
 value of the nonce in the response matches the value initially sent
 by the client.  If they don't match, the packet MUST be discarded.
 If no other protection is used, the authentication payload does not
 contain any identifier or authentication field; the ID-len and AU-len
 fields are set to a null value.  When stronger protection is
 required, the authentication payload contains the identifier and
 location fields, as explained in the following paragraphs.
 The confirmation byte is set to 0 by the client.  A null value
 returned by the server indicates that the client's key is still
 valid; a non-null value indicates that the client should obtain a new
 key.

Huitema Standards Track [Page 20] RFC 4380 Teredo February 2006

 When stronger authentication is provided, the client and the server
 are provisioned with a client identifier, a shared secret, and the
 identification of an authentication algorithm.  Before transmission,
 the authentication value is computed according to the specified
 algorithm; on reception, the same algorithm is used to compute a
 target value from the content of the receive packet.  The receiver
 deems the authentication successful if the two values match.  If they
 don't, the packet MUST be discarded.
 To maximize interoperability, this specification defines a default
 algorithm in which the authentication value is computed according the
 HMAC specification [RFC2104] and the SHA1 function [FIPS-180].
 Clients and servers may agree to use HMAC combined with a different
 function, or to use a different algorithm altogether, such as for
 example AES-XCBC-MAC-96 [RFC3566].
 The default authentication algorithm is based on the HMAC algorithm
 according to the following specifications:
  1. the hash function shall be the SHA1 function [FIPS-180].
  2. the secret value shall be the shared secret with which the client

was configured.

 The clear text to be protected includes:
  1. the nonce value,
  2. the confirmation byte,
  3. the origin indication encapsulation, if it is present,
  4. the IPv6 packet.
 The HMAC procedure is applied to the concatenation of these four
 components, without any additional padding.

5.2.3. Packet Reception

 The Teredo client receives packets over the Teredo interface.  The
 role of the packet reception procedure, besides receiving packets, is
 to maintain the date and time of the last interaction with the Teredo
 server and the "list of recent peers".
 When a UDP packet is received over the Teredo service port, the
 Teredo client checks that it is encoded according to the packet
 encoding rules defined in Section 5.1.1, and that it contains either
 a valid IPv6 packet or the combination of a valid origin indication
 encapsulation and a valid IPv6 packet, possibly protected by a valid
 authentication encapsulation.  If this is not the case, the packet is
 silently discarded.

Huitema Standards Track [Page 21] RFC 4380 Teredo February 2006

 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 to its own Teredo
 address.
 Then, the Teredo client examines the IPv4 source address and UDP port
 number from which the packet is received.  If these values match the
 IPv4 address of the server and the Teredo port, the client updates
 the "date and time of the last interaction with the Teredo server" to
 the current date and time; if an origin indication is present, the
 client should perform the "direct IPv6 connectivity test" described
 in Section 5.2.9.
 If the IPv4 source address and UDP port number are different from the
 IPv4 address of the server and the Teredo port, the client examines
 the IPv6 source address of the packet:
 1) If there is an entry for the source IPv6 address in the list of
 peers whose status is trusted, the client compares the mapped IPv4
 address and mapped port in the entry with the source IPv4 address and
 source port of the packet.  If the values match, the packet is
 accepted; the date and time of the last reception from the peer is
 updated.
 2) If there is an entry for the source IPv6 address in the list of
 peers whose status is not trusted, the client checks whether the
 packet is an ICMPv6 echo reply.  If this is the case, and if the
 ICMPv6 data of the reply matches the nonce stored in the peer entry,
 the packet should be accepted; the status of the entry should be
 changed to "trusted", the mapped IPv4 and mapped port in the entry
 should be set to the source IPv4 address and source port from which
 the packet was received, and the date and time of the last reception
 from the peer should be updated.  Any packet queued for this IPv6
 peer (as specified in Section 5.2.4) should be de-queued and
 forwarded to the newly learned IPv4 address and UDP port.
 3) If the source IPv6 address is a Teredo address, the client
 compares the mapped IPv4 address and mapped port in the source
 address with the source IPv4 address and source port of the packet.
 If the values match, the client MUST create a peer entry for the IPv6
 source address in the list of peers; it should update the entry if
 one already existed; the mapped IPv4 address and mapped port in the
 entry should be set to the value from which the packet was received,
 and the status should be set to "trusted".  If a new entry is
 created, the last transmission date is set to 30 seconds before the
 current date, and the number of bubbles to zero.  If the packet is a

Huitema Standards Track [Page 22] RFC 4380 Teredo February 2006

 bubble, it should be discarded after this processing; otherwise, the
 packet should be accepted.  In all cases, the client must de-queue
 and forward any packet queued for that destination.
 4) If the IPv4 destination address through which the packet was
 received is the Teredo IPv4 Discovery Address, the source address is
 a valid Teredo address, and the destination address is the "all nodes
 on link" multicast address, the packet should be treated as a local
 discovery bubble.  If no local entry already existed for the source
 address, a new one is created, but its status is set to "not
 trusted".  The client SHOULD reply with a unicast Teredo bubble, sent
 to the source IPv4 address and source port of the local discovery
 bubble; the IPv6 source address of the bubble will be set to local
 Teredo IPv6 address; the IPv6 destination address of the bubble
 should be set to the IPv6 source address of the local discovery
 bubble.  (Clients that do not implement the optional local discovery
 procedure will not process local discovery bubbles.)
 5) If the source IPv6 address is a Teredo address, and the mapped
 IPv4 address and mapped port in the source address do not match the
 source IPv4 address and source port of the packet, the client checks
 whether there is an existing "local" entry for that IPv6 address.  If
 there is such an entry, and if the local IPv4 address and local port
 indicated in that entry match the source IPv4 address and source
 port of the packet, the client updates the "local" entry, whose
 status should be set to "trusted".  If the packet is a bubble, it
 should be discarded after this processing; otherwise, the packet
 should be accepted.  In all cases, the client must de-queue and
 forward any packet queued for that destination.
 6) In the other cases, the packet may be accepted, but the client
 should be conscious that the source address may be spoofed; before
 processing the packet, the client should perform the "direct IPv6
 connectivity test" described in Section 5.2.9.
 Whatever the IPv4 source address and UDP source port, the client that
 receives an IPv6 packet MAY send a Teredo bubble towards that target,
 as specified in Section 5.2.6.

5.2.4. Packet Transmission

 When a Teredo client has to transmit a packet over a Teredo
 interface, it examines the destination IPv6 address.  The client
 checks first if there is an entry for this IPv6 address in the list
 of recent Teredo peers, and if the entry is still valid: an entry
 associated with a local peer is valid if the last reception date and
 time associated with that list entry is less that 30 seconds from the

Huitema Standards Track [Page 23] RFC 4380 Teredo February 2006

 current time; an entry associated with a non-local peer is valid if
 the last reception date and time associated with that list entry is
 less that 30 seconds from the current time.  (Local peer entries can
 only be present if the client uses the local discovery procedure
 discussed in Section 5.2.8.)
 The client then performs the following:
 1) If there is an entry for that IPv6 address in the list of peers,
 and if the status of the entry is set to "trusted", the IPv6 packet
 should be sent over UDP to the IPv4 address and UDP port specified in
 the entry.  The client updates the date of last transmission in the
 peer entry.
 2) If the destination is not a Teredo IPv6 address, the packet is
 queued, and the client performs the "direct IPv6 connectivity test"
 described in Section 5.2.9. The packet will be de-queued and
 forwarded if this procedure completes successfully.  If the direct
 IPv6 connectivity test fails to complete within a 2-second time-out,
 it should be repeated up to 3 times.
 3) If the destination is the Teredo IPv6 address of a local peer
 (i.e., a Teredo address from which a local discovery bubble has been
 received in the last 600 seconds), the packet is queued.  The client
 sends a unicast Teredo bubble to the local IPv4 address and local
 port specified in the entry, and a local Teredo bubble to the Teredo
 IPv4 discovery address.
 4) If the destination is a Teredo IPv6 address in which the cone bit
 is set to 1, the packet is sent over UDP to the mapped IPv4 address
 and mapped UDP port extracted from that IPv6 address.
 5) If the destination is a Teredo IPv6 address in which the cone bit
 is set to 0, the packet is queued.  If the client is not located
 behind a cone NAT, it sends a direct bubble to the Teredo
 destination, i.e., to the mapped IP address and mapped port of the
 destination.  In all cases, the client sends an indirect bubble to
 the Teredo destination, sending it over UDP to the server address and
 to the Teredo port.  The packet will be de-queued and forwarded when
 the client receives a bubble or another packet directly from this
 Teredo peer.  If no bubble is received within a 2-second time-out,
 the bubble transmission should be repeated up to 3 times.
 In cases 4 and 5, before sending a packet over UDP, the client MUST
 check that the IPv4 destination address is in the format of a global
 unicast address; if this is not the case, the packet MUST be silently

Huitema Standards Track [Page 24] RFC 4380 Teredo February 2006

 discarded.  (Note that a packet can legitimately be sent to a non-
 global unicast address in case 1, as a result of the local discovery
 procedure.)
 The global unicast address check is designed to thwart a number of
 possible attacks in which an attacker tries to use a Teredo host to
 attack either a single local IPv4 target or a set of such targets.
 For the purpose of this specification, and IPv4 address is deemed to
 be a global unicast address if it does not belong to or match:
  1. the "local" subnet 0.0.0.0/8,
  2. the "loopback" subnet 127.0.0.0/8,
  3. the local addressing ranges 10.0.0.0/8,
  4. the local addressing ranges 172.16.0.0/12,
  5. the local addressing ranges 192.168.0.0/16,
  6. the link local block 169.254.0.0/16,
  7. the block reserved for 6to4 anycast addresses 192.88.99.0/24,
  8. the multicast address block 224.0.0.0/4,
  9. the "limited broadcast" destination address 255.255.255.255,
  10. the directed broadcast addresses corresponding to the subnets to

which the host is attached.

 A list of special-use IPv4 addresses is provided in [RFC3330].
 For reliability reasons, clients MAY decide to ignore the value of
 the cone bit in the flag, skip the "case 4" test and always perform
 the "case 5", i.e., treat all Teredo peers as if they were located
 behind non-cone NAT.  This will result in some increase in traffic,
 but may avoid reliability issues if the determination of the NAT
 status was for some reason erroneous.  For the same reason, clients
 MAY also decide to always send a direct bubble in case 5, even if
 they do not believe that they are located behind a non-cone NAT.

5.2.5. Maintenance

 The Teredo client must ensure that the mappings that it uses remain
 valid.  It does so by checking that packets are regularly received
 from the Teredo server.
 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;
 the client should use the same value of the cone bit that resulted in
 the reception of an RA during the qualification procedure.

Huitema Standards Track [Page 25] RFC 4380 Teredo February 2006

 When the router advertisement is received, the client SHOULD check
 its validity as specified in Section 5.2.1; invalid advertisements
 are silently discarded.  If the advertisement is valid, the client
 MUST check that the mapped address and port correspond to the current
 Teredo address.  If this is not the case, the mapping has changed;
 the client must mark the old address as invalid and start using the
 new address.

5.2.6. Sending Teredo Bubbles

 The Teredo client may have to send a bubble towards another Teredo
 client, either after a packet reception or after a transmission
 attempt, as explained in Sections 5.2.3 and 5.2.4. There are two
 kinds of bubbles: direct bubbles, which are sent directly to the
 mapped IPv4 address and mapped UDP port of the peer, and indirect
 bubbles, which are sent through the Teredo server of the peer.
 When a Teredo client attempts to send a direct bubble, it extracts
 the mapped IPv4 address and mapped UDP port from the Teredo IPv6
 address of the target.  It then checks whether there is already an
 entry for this IPv6 address in the current list of peers.  If there
 is no entry, the client MUST create a new list entry for the address,
 setting the last reception date and the last transmission date to 30
 seconds before the current date, and the number of bubbles to zero.
 When a Teredo client attempts to send an indirect bubble, it extracts
 the Teredo server IPv4 address from the Teredo prefix of the IPv6
 address of the target (different clients may be using different
 servers); the bubble will be sent to that IPv4 address and the Teredo
 UDP port.
 Bubbles may be lost in transit, and it is reasonable to enhance the
 reliability of the Teredo service by allowing multiple transmissions;
 however, bubbles will also be lost systematically in certain NAT
 configurations.  In order to strike a balance between reliability and
 unnecessary retransmissions, we specify the following:
  1. The client MUST NOT send a bubble if the last transmission date

and time is less than 2 seconds before the current date and time;

  1. The client MUST NOT send a bubble if it has already sent 4 bubbles

to the peer in the last 300 seconds without receiving a direct

   response.
 In the other cases, the client MAY proceed with the transmission of
 the bubble.  When transmitting the bubble, the client MUST update the
 last transmission date and time to that peer, and must also increment
 the number of transmitted bubbles.

Huitema Standards Track [Page 26] RFC 4380 Teredo February 2006

5.2.7. Optional Refresh Interval Determination Procedure

 In addition to the regular client resources described in the
 beginning of this section, the refresh interval determination
 procedure uses an additional UDP port, the Teredo secondary port, and
 the following variables:
  1. Teredo secondary connectivity status,
  2. Mapped address and port number of the Teredo secondary port,
  3. Teredo secondary IPv6 prefix associated with the secondary port,
  4. Teredo secondary IPv6 address derived from this prefix,
  5. Date and time of the last interaction on the secondary port,
  6. Maximum Teredo Refresh Interval.
  7. Candidate Teredo Refresh Interval.
 The secondary connectivity status, mapped address and prefix are
 determined by running the qualification procedure on the secondary
 port.  When the client uses the interval determination procedure, the
 qualification procedure MUST be run for the secondary port
 immediately after running it on the service port.  If the secondary
 qualification fails, the interval determination procedure will not be
 used, and the interval value will remain to the default value, 30
 seconds.  If the secondary qualification succeeds, the maximum
 refresh interval is set to 120 seconds, and the candidate Teredo
 refresh interval is set to 60 seconds, i.e., twice the Teredo refresh
 interval.  The procedure is then performed at regular intervals,
 until it concludes:
 1) wait until the candidate refresh interval is elapsed after the
    last interaction on the secondary port.
 2) send a Teredo bubble to the Teredo secondary IPv6 address, through
    the service port.
 3) wait for reception of the bubble on the secondary port.  If a
    timer of 2 seconds elapses without reception, repeat step 2 at
    most three times.  If there is still no reception, the candidate
    has failed; if there is a reception, the candidate has succeeded.
 4) if the candidate has succeeded, set the Teredo refresh interval to
    the candidate value, and set a new candidate value to the minimum
    of twice the new refresh interval, or the average of the refresh
    interval and the maximum refresh interval.

Huitema Standards Track [Page 27] RFC 4380 Teredo February 2006

 5) if the candidate has failed, set the maximum refresh interval to
    the candidate value.  If the current refresh interval is larger
    than or equal to 75% of the maximum, the determination procedure
    has concluded; otherwise, set a new candidate value to the average
    of the refresh interval and the maximum refresh interval.
 6) if the procedure has not concluded, perform the maintenance
    procedure on the secondary port, which will reset the date and
    time of the last interaction on the secondary port, and may result
    in the allocation of a new Teredo secondary IPv6 address; this
    would not affect the values of the refresh interval, candidate
    interval, or maximum refresh interval.
 The secondary port MUST NOT be used for any other purpose than the
 interval determination procedure.  It should be closed when the
 procedure ends.

5.2.8. Optional Local Client Discovery Procedure

 It is desirable to enable direct communication between Teredo clients
 that are located behind the same NAT, without forcing a systematic
 relay through a Teredo server.  It is hard to design a general
 solution to this problem, but we can design a partial solution when
 the Teredo clients are connected through IPv4 to the same link.
 A Teredo client who wishes to enable local discovery SHOULD join the
 IPv4 multicast group identified by Teredo IPv4 Discovery Address.
 The client SHOULD wait for discovery bubbles to be received on the
 Teredo IPv4 Discovery Address.  The client SHOULD send local
 discovery bubbles to the Teredo IPv4 Discovery Address at random
 intervals, uniformly distributed between 200 and 300 seconds.  A
 local Teredo bubble has the following characteristics:
  1. IPv4 source address: the IPv4 address of the sender
  1. IPv4 destination address: the Teredo IPv4 Discovery Address
  1. IPv4 ttl: 1
  1. UDP source port: the Teredo service port of the sender
  1. UDP destination port: the Teredo UDP port
  1. UDP payload: a minimal IPv6 packet, as follows
  1. IPv6 source: the global Teredo IPv6 address of the sender
  1. IPv6 destination: the all-nodes on-link multicast address

Huitema Standards Track [Page 28] RFC 4380 Teredo February 2006

  1. IPv6 payload type: 59 (No Next Header, as per [RFC2460])
  1. IPv6 payload length: 0
  1. IPv6 hop limit: 1
 The local discovery procedure carries a denial of service risk, as
 malevolent nodes could send fake bubbles to unsuspecting parties, and
 thus capture the traffic originating from these parties.  The risk is
 mitigated by the filtering rules described in Section 5.2.5, and also
 by "link only" multicast scope of the Teredo IPv4 Discovery Address,
 which implies that packets sent to this address will not be forwarded
 across routers.
 To benefit from the "link only multicast" protection, the clients
 should silently discard all local discovery bubbles that are received
 over a unicast address.  To further mitigate the denial of service
 risk, the client MUST silently discard all local discovery bubbles
 whose IPv6 source address is not a well-formed Teredo IPv6 address,
 or whose IPv4 source address does not belong to the local IPv4
 subnet; the client MAY decide to silently discard all local discovery
 bubbles whose Teredo IPv6 address do not include the same mapped IPv4
 address as its own.
 If the bubble is accepted, the client checks whether there is an
 entry in the list of recent peers that correspond to the mapped IPv4
 address and mapped UDP port associated with the source IPv6 address
 of the bubble.  If there is such an entry, the client MUST update the
 local peer address and local peer port parameters to reflect the IPv4
 source address and UDP source port of the bubble.  If there is no
 entry, the client MUST create one, setting the local peer address and
 local peer port parameters to reflect the IPv4 source address and UDP
 source port of the bubble, the last reception date to the current
 date and time, the last transmission date to 30 seconds before the
 current date, and the number of bubbles to zero.  The state of the
 entry is set to "not trusted".
 Upon reception of a discovery bubble, clients reply with a unicast
 bubble as specified in Section 5.2.3.

5.2.9. Direct IPv6 Connectivity Test

 The Teredo procedures are designed to enable direct connections
 between a Teredo host and a Teredo relay.  Teredo hosts located
 behind a cone NAT will receive packets directly from relays; other
 Teredo hosts will learn the original addresses and UDP ports of third
 parties through the local Teredo server.  In all of these cases,
 there is a risk that the IPv6 address of the source will be spoofed

Huitema Standards Track [Page 29] RFC 4380 Teredo February 2006

 by a malevolent party.  Teredo hosts must make two decisions, whether
 to accept the packet for local processing and whether to transmit
 further packets to the IPv6 address through the newly
 learned IPv4 address and UDP port.  The basic rule is that the hosts
 should be generous in what they accept and careful in what they send.
 Refusing to accept packets due to spoofing concerns would compromise
 connectivity and should only be done when there is a near certainty
 that the source address is spoofed.  On the other hand, sending
 packets to the wrong address should be avoided.
 When the client wants to send a packet to a native IPv6 node or a
 6to4 node, it should check whether a valid peer entry already exists
 for the IPv6 address of the destination.  If this is not the case,
 the client will pick a random number (a nonce) and format an ICMPv6
 Echo Request message whose source is the local Teredo address, whose
 destination is the address of the IPv6 node, and whose Data field is
 set to the nonce.  (It is recommended to use a random number at least
 64 bits long.)  The nonce value and the date at which the packet was
 sent will be documented in a provisional peer entry for the IPV6
 destination.  The ICMPv6 packet will then be sent encapsulated in a
 UDP packet destined to the Teredo server IPv4 address and to the
 Teredo port.  The rules of Section 5.2.3 specify how the reply to
 this packet will be processed.

5.2.10. Working around symmetric NAT

 The client procedures are designed to enable IPv6 connectivity
 through the most common types of NAT, which are commonly called "cone
 NAT" and "restricted cone NAT" [RFC3489].  Some NATs employ a
 different design; they are often called "symmetric NAT".  The
 qualification algorithm in Section 5.2.1 will not succeed when the
 local NAT is a symmetric NAT.
 In many cases, it is possible to work around the limitations of these
 NATs by explicitly reserving a UDP port for Teredo service on a
 client, using a function often called "DMZ" in the NAT's manual.
 This port will become the "service port" used by the Teredo hosts.
 The implementers of Teredo functions in hosts must make sure that the
 value of the service port can be explicitly provisioned, so that the
 user can provision the same value in the host and in the NAT.
 The reservation procedure guarantees that the port mapping will
 remain the same for all destinations.  After the explicit
 reservation, the qualification algorithm in Section 5.2.1 will
 succeed, and the Teredo client will behave as if behind a "cone NAT".

Huitema Standards Track [Page 30] RFC 4380 Teredo February 2006

 When different clients use Teredo behind a single symmetric NAT, each
 of these clients must reserve and use a different service port.

5.3. Teredo Server Specification

 The Teredo server is designed to be stateless.  The Teredo server
 waits for incoming UDP packets at the Teredo Port, using the IPv4
 address that has been selected for the service.  In addition, the
 server is able to receive and transmit some packets using a different
 IPv4 address and a different port number.
 The Teredo server acts as an IPv6 router.  As such, it will receive
 Router Solicitation messages, to which it will respond with Router
 Advertisement messages as explained in Section 5.3.2.  It may also
 receive other packets, for example, ICMPv6 messages and Teredo
 bubbles, which are processed according to the IPv6 specification.
 By default, the routing functions of the Teredo server are limited.
 Teredo servers are expected to relay Teredo bubbles, ICMPv6 Echo
 requests, and ICMPv6 Echo replies, but they are not expected to relay
 other types of IPv6 packets.  Operators may, however, decide to
 combine the functions of "Teredo server" and "Teredo relay", as
 explained in Section 5.4.

5.3.1. Processing of Teredo IPv6 Packets

 Before processing the packet, the Teredo server MUST check the
 validity of the encapsulated IPv6 source address, the IPv4 source
 address, and the UDP source port:
 1)  If the UDP content is not a well-formed Teredo IPv6 packet, as
 defined in Section 5.1.1, the packet MUST be silently discarded.
 2)  If the UDP packet is not a Teredo bubble or an ICMPv6 message, it
 SHOULD be discarded.  (The packet may be processed if the Teredo
 server also operates as a Teredo relay, as explained in Section 5.4.)
 3)  If the IPv4 source address is not in the format of a global
 unicast address, the packet MUST be silently discarded (see Section
 5.2.4 for a definition of global unicast addresses).
 4)  If the IPv6 source address is an IPv6 link-local address, the
 IPv6 destination address is the link-local scope all routers
 multicast address (FF02::2), and the packet contains an ICMPv6 Router
 Solicitation message, the packet MUST be accepted.  It MUST be
 discarded if the server requires secure qualification and the
 authentication encapsulation is absent or verification fails.

Huitema Standards Track [Page 31] RFC 4380 Teredo February 2006

 5)  If the IPv6 source address is a Teredo IPv6 address, and if the
 IPv4 address and UDP port embedded in that address match the IPv4
 source address and UDP source port, the packet SHOULD be accepted.
 6)  If the IPv6 source address is not a Teredo IPv6 address, and if
 the IPv6 destination address is a Teredo address allocated through
 this server, the packet SHOULD be accepted.
 7)  In all other cases, the packet MUST be silently discarded.
 The Teredo server will then check the IPv6 destination address of the
 encapsulated IPv6 packet:
 If the IPv6 destination address is the link-local scope all routers
 multicast address (FF02::2), or the link-local address of the server,
 the Teredo server processes the packet; it may have to process Router
 Solicitation messages and ICMPv6 Echo Request messages.
 If the destination IPv6 address is not a global scope IPv6 address,
 the packet MUST NOT be forwarded.
 If the destination address is not a Teredo IPv6 address, the packet
 should be relayed to the IPv6 Internet using regular IPv6 routing.
 If the IPv6 destination address is a valid Teredo IPv6 address as
 defined in Section 2.13, the Teredo Server MUST check that the IPv4
 address derived from this IPv6 address is in the format of a global
 unicast address; if this is not the case, the packet MUST be silently
 discarded.
 If the address is valid, the Teredo server encapsulates the IPv6
 packet in a new UDP datagram, in which the following parameters are
 set:
  1. The destination IPv4 address is derived from the IPv6 destination.
  1. The source IPv4 address is the Teredo server IPv4 address.
  1. The destination UDP port is derived from the IPv6 destination.
  1. The source UDP port is set to the Teredo UDP Port.
 If the destination IPv6 address is a Teredo client whose address is
 serviced by this specific server, the server should insert an origin
 indication in the first bytes of the UDP payload, as specified in
 Section 5.1.1.  (To verify that the client is served by this server,
 the server compares bits 32-63 of the client's Teredo IPv6 address to
 the server's IPv4 address.)

Huitema Standards Track [Page 32] RFC 4380 Teredo February 2006

5.3.2. Processing of Router Solicitations

 When the Teredo server receives a Router Solicitation message (RS,
 [RFC2461]), it retains the IPv4 address and UDP port from which the
 solicitation was received; these become the Teredo mapped address and
 Teredo mapped port of the client.  The router uses these values to
 compose the origin indication encapsulation that will be sent with
 the response to the solicitation.
 The Teredo server responds to the router solicitation by sending a
 Router Advertisement message [RFC2461].  The router advertisement
 MUST advertise the Teredo IPv6 prefix composed from the service
 prefix and the server's IPv4 address.  The IPv6 source address should
 be set to a Teredo link-local server address associated to the local
 interface; this address is derived from the IPv4 address of the
 server and from the Teredo port, as specified in Section 4; the cone
 bit is set to 1.  The IPv6 destination address is set to the IPv6
 source address of the RS.  The Router Advertisement message must be
 sent over UDP to the Teredo mapped address and Teredo mapped port of
 the client; the IPv4 source address and UDP source port should be set
 to the server's IPv4 address and Teredo Port.  If the cone bit of the
 client's IPv6 address is set to 1, the RA must be sent from a
 different IPv4 source address than the server address over which the
 RS was received; if the cone bit is set to zero, the response must be
 sent back from the same address.
 Before sending the packet, the Teredo server MUST check that the IPv4
 destination address is in the format of a global unicast address; if
 this is not the case, the packet MUST be silently discarded (see
 Section 5.2.4 for a definition of global unicast addresses).
 If secure qualification is required, the server MUST insert a valid
 authentication parameter in the UDP packet carrying the router
 advertisement.  The client identifier and the nonce value used in the
 authentication parameter MUST be the same identifier and nonce as
 received in the router solicitation.  The confirmation byte MUST be
 set to zero if the client identifier is still valid, and a non-null
 value otherwise; the authentication value SHOULD be computed using
 the secret that corresponds to the client identifier.

5.4. Teredo Relay Specification

 Teredo relays are IPv6 routers that advertise reachability of the
 Teredo service IPv6 prefix through the IPv6 routing protocols.  (A
 minimal Teredo relay may serve just a local host, and would not
 advertise the prefix beyond this host.)  Teredo relays will receive
 IPv6 packets bound to Teredo clients.  Teredo relays should be able

Huitema Standards Track [Page 33] RFC 4380 Teredo February 2006

 to receive packets sent over IPv4 and UDP by Teredo clients; they may
 apply filtering rules, e.g., only accept packets from Teredo clients
 if they have previously sent traffic to these Teredo clients.
 The receiving and sending rules used by Teredo relays are very
 similar to those of Teredo clients.  Teredo relays must use a Teredo
 service port to transmit packets to Teredo clients; they must
 maintain a "list of peers", identical to the list of peers maintained
 by Teredo clients.

5.4.1. Transmission by Relays to Teredo Clients

 When a Teredo relay has to transmit a packet to a Teredo client, it
 examines the destination IPv6 address.  By definition, the Teredo
 relays will only send over UDP IPv6 packets whose IPv6 destination
 address is a valid Teredo IPv6 address.
 Before processing these packets, the Teredo Relay MUST check that the
 IPv4 destination address embedded in the Teredo IPv6 address is in
 the format of a global unicast address; if this is not the case, the
 packet MUST be silently discarded (see Section 5.2.4 for a definition
 of global unicast addresses).
 The relay then checks if there is an entry for this IPv6 address in
 the list of recent Teredo peers, and if the entry is still valid.
 The relay then performs the following:
 1) If there is an entry for that IPv6 address in the list of peers,
 and if the status of the entry is set to "trusted", the IPv6 packet
 should be sent over UDP to the mapped IPv4 address and mapped UDP
 port of the entry.  The relay updates the date of last transmission
 in the peer entry.
 2) If there is no trusted entry in the list of peers, and if the
 destination is a Teredo IPv6 address in which the cone bit is set to
 1, the packet is sent over UDP to the mapped IPv4 address and mapped
 UDP port extracted from that IPv6 address.
 3) If there is no trusted entry in the list of peers, and if the
 destination is a Teredo IPv6 address in which the cone bit is set to
 0, the Teredo relay creates a bubble whose source address is set to a
 local IPv6 address, and whose destination address is set to the
 Teredo IPv6 address of the packet's destination.  The bubble is sent
 to the server address corresponding to the Teredo destination.  The
 entry becomes trusted when a bubble or another packet is received
 from this IPv6 address; if no such packet is received before a time-
 out of 2 seconds, the Teredo relay may repeat the bubble, up to three
 times.  If the relay fails to receive a bubble after these

Huitema Standards Track [Page 34] RFC 4380 Teredo February 2006

 repetitions, the entry is removed from the list of peers.  The relay
 MAY queue packets bound to untrusted entries; the queued packets
 SHOULD be de-queued and forwarded when the entry becomes trusted;
 they SHOULD be deleted if the entry is deleted.  To avoid denial of
 service attacks, the relays SHOULD limit the number of packets in
 such queues.
 In cases 2 and 3, the Teredo relay should create a peer entry for the
 IPv6 address; the entry status is marked as trusted in case 2 (cone
 NAT) and not trusted in case 3.  In case 3, if the Teredo relay
 happens to be located behind a non-cone NAT, it should also send a
 bubble directly to the mapped IPv4 address and mapped port number of
 the Teredo destination.  This will "open the path" for the return
 bubble from the Teredo client.
 For reliability reasons, relays MAY decide to ignore the value of the
 cone bit in the flag, and always perform the "case 3", i.e., treat
 all Teredo peers as if they were located behind a non-cone NAT.  This
 will result in some increase in traffic, but may avoid
 reliability issues if the determination of the NAT status was for
 some reason erroneous.  For the same reason, relays MAY also decide
 to always send a direct bubble to the mapped IPv4 address and mapped
 port number of the Teredo destination, even if they do not believe
 that they are located behind a non-cone NAT.

5.4.2. Reception from Teredo Clients

 The Teredo relay may receive packets from Teredo clients; the packets
 should normally only be sent by clients to which the relay previously
 transmitted packets, i.e., clients whose IPv6 address is present in
 the list of peers.  Relays, like clients, use the packet reception
 procedure to maintain the date and time of the last interaction with
 the Teredo server and the "list of recent peers".
 When a UDP packet is received over the Teredo service port, the
 Teredo relay checks that it contains a valid IPv6 packet as specified
 in [RFC2460].  If this is not the case, the packet is silently
 discarded.
 Then, the Teredo relay examines whether the IPv6 source address is a
 valid Teredo address, and if the mapped IPv4 address and mapped port
 match the IPv4 source address and port number from which the packet
 is received.  If this is not the case, the packet is silently
 discarded.
 The Teredo relay then examines whether there is an entry for the IPv6
 source address in the list of recent peers.  If this is not the case,

Huitema Standards Track [Page 35] RFC 4380 Teredo February 2006

 the packet may be silently discarded.  If this is the case, the entry
 status is set to "trusted"; the relay updates the "date and time of
 the last interaction" to the current date and time.
 Finally, the relay examines the destination IPv6 address.  If the
 destination belongs to a range of IPv6 addresses served by the relay,
 the packet SHOULD be accepted and forwarded to the destination.  In
 the other cases, the packet SHOULD be silently discarded.

5.4.3. Difference between Teredo Relays and Teredo Servers

 Because Teredo servers can relay Teredo packets over IPv6, all Teredo
 servers must be capable of behaving as Teredo relays.  There is,
 however, no requirement that Teredo relays behave as Teredo servers.
 The dual role of server and relays implies an additional complexity
 for the programming of servers: the processing of incoming packets
 should be a combination of the server processing rules defined in
 Section 5.3.1, and the relay processing rules defined in Section
 5.4.2.  (Section 5.3 only specifies the rules implemented by a pure
 server, not a combination relay+server.)

5.5. Implementation of Automatic Sunset

 Teredo is designed as an interim transition mechanism, and it is
 important that it should not be used any longer than necessary.  The
 "sunset" procedure will be implemented by Teredo clients, servers,
 and relays, as specified in this section.
 The Teredo-capable nodes MUST NOT behave as Teredo clients if they
 already have IPv6 connectivity through any other means, such as
 native IPv6 connectivity.  In particular, nodes that have a global
 IPv4 address SHOULD obtain connectivity through the 6to4 service
 rather than through the Teredo service.  The classic reason why a
 node that does not need connectivity would still enable the Teredo
 service is to guarantee good performance when interacting with Teredo
 clients; however, a Teredo-capable node that has IPv4 connectivity
 and that has obtained IPv6 connectivity outside the Teredo service
 MAY decide to behave as a Teredo relay, and still obtain good
 performance when interacting with Teredo clients.
 The Teredo servers are expected to participate in the sunset
 procedure by announcing a date at which they will stop providing the
 service.  This date depends on the availability of alternative
 solutions to their clients, such as "dual-mode" gateways that behave
 simultaneously as IPv4 NATs and IPv6 routers.  Most Teredo servers
 will not be expected to operate more than a few years.  Teredo relays
 are expected to have the same life span as Teredo servers.

Huitema Standards Track [Page 36] RFC 4380 Teredo February 2006

6. Further Study, Use of Teredo to Implement a Tunnel Service

 Teredo defines a NAT traversal solution that can be provided using
 very little resource at the server.  Ongoing IETF discussions have
 outlined the need for both a solution like Teredo and a more
 controlled NAT traversal solution, using configured tunnels to a
 service provider [RFC3904].  This section provides a tentative
 analysis of how Teredo could be extended to also support a configured
 tunnel service.
 It may be possible to design a tunnel server protocol that is
 compatible with Teredo, in the sense that the same client could be
 used either in the Teredo service or with a tunnel service.  In fact,
 this could be done by configuring the client with:
  1. The IPv4 address of a Teredo server that acts as a tunnel broker
  2. A client identifier
  3. A shared secret with that server
  4. An agreed-upon authentication algorithm.
 The Teredo client would use the secure qualification procedure, as
 specified in Section 5.2.2. Instead of returning a Teredo prefix in
 the router advertisement, the server would return a globally routable
 IPv6 prefix; this prefix could be permanently assigned to the client,
 which would provide the client with a stable address.  The server
 would have to keep state, i.e., memorize the association between the
 prefix assigned to the client and the mapped IPv4 address and mapped
 UDP port of the client.
 The Teredo server would advertise reachability of the client prefix
 to the IPv6 Internet.  Any packet bound to that prefix would be
 transmitted to the mapped IPv4 address and mapped UDP port of the
 client.
 The Teredo client, when it receives the prefix, would notice that
 this prefix is a global IPv6 prefix, not in the form of a Teredo
 prefix.  The client would at that point recognize that it should
 operate in tunnel mode.  A client that operates in tunnel mode would
 execute a much simpler transmission procedure: it would forward any
 packet sent to the Teredo interface to the IPv4 address and Teredo
 UDP port of the server.
 The Teredo client would have to perform the maintenance procedure
 described in Section 5.2.5. The server would receive the router
 solicitation, and could notice a possible change of mapped IPv4
 address and mapped UDP port that could result from the
 reconfiguration of the mappings inside the NAT.  The server should
 continue advertising the same IPv6 prefix to the client, and should

Huitema Standards Track [Page 37] RFC 4380 Teredo February 2006

 update the mapped IPv4 address and mapped UDP port associated to this
 prefix, if necessary.
 There is as yet no consensus that a tunnel-mode extension to Teredo
 should be developed.  This section is only intended to provide
 suggestions to the future developers of such services.  Many details
 would probably have to be worked out before a tunnel-mode extension
 would be agreed upon.

7. Security Considerations

 The main objective of Teredo is to provide nodes located behind a NAT
 with a globally routable IPv6 address.  The Teredo nodes can use IP
 security (IPsec) services such as Internet Key Exchange (IKE),
 Authentication Header (AH), or Encapsulation Security Payload (ESP)
 [RFC4306, RFC4302, RFC4303], without the configuration restrictions
 still present in "Negotiation of NAT-Traversal in the IKE" [RFC3947].
 As such, we can argue that the service has a positive effect on
 network security.  However, the security analysis must also envisage
 the negative effects of the Teredo services, which we can group in
 four categories: security risks of directly connecting a node to the
 IPv6 Internet, spoofing of Teredo servers to enable a man-in-the-
 middle attack, potential attacks aimed at denying the Teredo service
 to a Teredo client, and denial of service attacks against non-Teredo
 participating nodes that would be enabled by the Teredo service.
 In the following, we review in detail these four types of issues, and
 we present mitigating strategies for each of them.

7.1. Opening a Hole in the NAT

 The very purpose of the Teredo service is to make a machine reachable
 through IPv6.  By definition, the machine using the service will give
 up whatever firewall service was available in the NAT box, however
 limited this service may be [RFC2993].  The services that listen to
 the Teredo IPv6 address will become the potential target of attacks
 from the entire IPv6 Internet.  This may sound scary, but there are
 three mitigating factors.
 The first mitigating factor is the possibility to restrict some
 services to only accept traffic from local neighbors, e.g., using
 link-local addresses.  Teredo does not support communication using
 link-local addresses.  This implies that link-local services will not
 be accessed through Teredo, and will be restricted to whatever other
 IPv6 connectivity may be available, e.g., direct traffic with
 neighbors on the local link, behind the NAT.

Huitema Standards Track [Page 38] RFC 4380 Teredo February 2006

 The second mitigating factor is the possible use of a "local
 firewall" solution, i.e., a piece of software that performs locally
 the kind of inspection and filtering that is otherwise performed in a
 perimeter firewall.  Using such software is recommended.
 The third mitigating factor is the availability of IP security
 (IPsec) services such as IKE, AH, or ESP [RFC4306, RFC4302, RFC4303].
 Using these services in conjunction with Teredo is a good policy, as
 it will protect the client from possible attacks in intermediate
 servers such as the NAT, the Teredo server, or the Teredo relay.
 (However, these services can be used only if the parties in the
 communication can negotiate a key, which requires agreeing on some
 credentials; this is known to be a hard problem.)

7.2. Using the Teredo Service for a Man-in-the-Middle Attack

 The goal of the Teredo service is to provide hosts located behind a
 NAT with a globally reachable IPv6 address.  There is a possible
 class of attacks against this service in which an attacker somehow
 intercepts the router solicitation, responds with a spoofed router
 advertisement, and provides a Teredo client with an incorrect
 address.  The attacker may have one of two objectives: it may try to
 deny service to the Teredo client by providing it with an address
 that is in fact unreachable, or it may try to insert itself as a
 relay for all client communications, effectively enabling a variety
 of "man-in-the-middle" attack.

7.2.1. Attacker Spoofing the Teredo Server

 The simple nonce verification procedure described in Section 5.2.2
 provides a first level of protection against attacks in which a third
 party tries to spoof the server.  In practice, the nonce procedure
 can be defeated only if the attacker is "on path".
 If client and server share a secret and agree on an authentication
 algorithm, the secure qualification procedure described in Section
 5.2.2 provides further protection.  To defeat this protection, the
 attacker could try to obtain a copy of the secret shared between
 client and server.  The most likely way to obtain the shared secret
 is to listen to the traffic and mount an offline dictionary attack;
 to protect against this attack, the secret shared between client and
 server should contain sufficient entropy.  (This probably requires
 some automated procedure for provisioning the shared secret and the
 algorithm.)
 If the shared secret contains sufficient entropy, the attacker would
 have to defeat the one-way function used to compute the
 authentication value.  This specification suggests a default

Huitema Standards Track [Page 39] RFC 4380 Teredo February 2006

 algorithm combining HMAC and MD5.  If the protection afforded by MD5
 was not deemed sufficient, clients and servers can agree to use a
 different algorithm, e.g., SHA1.
 Another way to defeat the protection afforded by the authentication
 procedure is to mount a complex attack, as follows:
 1) Client prepares router solicitation, including authentication
 encapsulation.
 2) Attacker intercepts the solicitation, and somehow manages to
 prevent it from reaching the server, for example, by mounting a
 short-duration DoS attack against the server.
 3) Attacker replaces the source IPv4 address and source UDP port of
 the request by one of its own addresses and port, and forwards the
 modified request to the server.
 4) Server dutifully notes the IPv4 address from which the packet is
 received, verifies that the Authentication encapsulation is correct,
 prepares a router advertisement, signs it, and sends it back to the
 incoming address, i.e., the attacker.
 5) Attacker receives the advertisement, takes note of the mapping,
 replaces the IPv4 address and UDP port by the original values in the
 intercepted message, and sends the response to the client.
 6) Client receives the advertisement, notes that the authentication
 header is present and is correct, and uses the proposed prefix and
 the mapped addresses in the origin indication encapsulation.
 The root cause of the problem is that the NAT is, in itself, a man-
 in-the-middle attack.  The Authentication encapsulation covers the
 encapsulated IPv6 packet, but does not cover the encapsulating IPv4
 header and UDP header.  It is very hard to devise an effective
 authentication scheme, since the attacker does not do anything else
 than what the NAT legally does!
 However, there are several mitigating factors that lead us to avoid
 worrying too much about this attack.  In practice, the gain from the
 attack is either to deny service to the client or to obtain a "man-
 in-the-middle" position.  However, in order to mount the attack, the
 attacker must be able to suppress traffic originating from the
 client, i.e., have denial of service capability; the attacker must
 also be able to observe the traffic exchanged between client and
 inject its own traffic in the mix, i.e., have man-in-the-middle
 capacity.  In summary, the attack is very hard to mount, and the gain
 for the attacker in terms of "elevation of privilege" is minimal.

Huitema Standards Track [Page 40] RFC 4380 Teredo February 2006

 A similar attack is described in detail in the security section of
 [RFC3489].

7.2.2. Attacker Spoofing a Teredo Relay

 An attacker may try to use Teredo either to pass itself for another
 IPv6 host or to place itself as a man-in-the-middle between a Teredo
 host and a native IPv6 host.  The attacker will mount such attacks by
 spoofing a Teredo relay, i.e., by convincing the Teredo host that
 packets bound to the native IPv6 host should be relayed to the IPv4
 address of the attacker.
 The possibility of the attack derives from the lack of any
 algorithmic relation between the IPv4 address of a relay and the
 native IPv6 addresses served by these relay.  A Teredo host cannot
 decide just by looking at the encapsulating IPv4 and UDP header
 whether or not a relay is legitimate.  If a Teredo host decided to
 simply trust the incoming traffic, it would easily fall prey to a
 relay-spoofing attack.
 The attack is mitigated by the "direct IPv6 connectivity test"
 specified in Section 5.2.9. The test specifies a relay discovery
 procedure secured by a nonce.  The nonce is transmitted from the
 Teredo host to the destination through Teredo server, which the
 client normally trusts.  The response arrives through the "natural"
 relay, i.e., the relay closest to the IPv6 destination.  Sending
 traffic to this relay will place it out of reach of attackers that
 are not on the direct path between the Teredo host and its IPv6 peer.
 End-to-end security protections are required to defend against
 spoofing attacks if the attacker is on the direct path between the
 Teredo host and its peer.

7.2.3. End-to-End Security

 The most effective line of defense of a Teredo client is probably not
 to try to secure the Teredo service itself: even if the mapping can
 be securely obtained, the attacker would still be able to listen to
 the traffic and send spoofed packets.  Rather, the Teredo client
 should realize that, because it is located behind a NAT, it is in a
 situation of vulnerability; it should systematically try to encrypt
 its IPv6 traffic, using IPsec.  Even if the IPv4 and UDP headers are
 vulnerable, the use of IPsec will effectively prevent spoofing and
 listening of the IPv6 packets by third parties.  By providing each
 client with a global IPv6 address, Teredo enables the use of IPsec

Huitema Standards Track [Page 41] RFC 4380 Teredo February 2006

 without the configuration restrictions still present in "Negotiation
 of NAT-Traversal in the IKE" [RFC3947] and ultimately enhances the
 security of these clients.

7.3. Denial of the Teredo service

 Our analysis outlines five ways to attack the Teredo service.  There
 are countermeasures for each of these attacks.

7.3.1. Denial of Service by a Rogue Relay

 An attack can be mounted on the IPv6 side of the service by setting
 up a rogue relay and letting that relay advertise a route to the
 Teredo IPv6 prefix.  This is an attack against IPv6 routing, which
 can also be mitigated by the same kind of procedures used to
 eliminate spurious route advertisements.  Dual-stack nodes that
 implement "host local" Teredo relays are impervious to this attack.

7.3.2. Denial of Service by Server Spoofing

 In Section 7.2, we discussed the use of spoofed router advertisements
 to insert an attacker in the middle of a Teredo conversation.  The
 spoofed router advertisements can also be used to provision a client
 with an incorrect address, pointing to either a non-existing IPv4
 address or the IPv4 address of a third party.
 The Teredo client will detect the attack when it fails to receive
 traffic through the newly acquired IPv6 address.  The attack can be
 mitigated by using the authentication encapsulation.

7.3.3. Denial of Service by Exceeding the Number of Peers

 A Teredo client manages a cache of recently used peers, which makes
 it stateful.  It is possible to mount an attack against the client by
 provoking it to respond to packets that appear to come from a large
 number of Teredo peers, thus trashing the cache and effectively
 denying the use of direct communication between peers.  The effect
 will last only as long as the attack is sustained.

7.3.4. Attacks against the Local Discovery Procedure

 There is a possible denial of service attack against the local peer
 discovery procedure, if attackers can manage to send spoofed local
 discovery bubbles to a Teredo client.  The checks described in
 Section 5.2.8 mitigate this attack.  Clients who are more interested
 in security than in performance could decide to disable the local
 discovery procedure; however, if local discovery is disabled, traffic
 between local nodes will end up being relayed through a server

Huitema Standards Track [Page 42] RFC 4380 Teredo February 2006

 external to the local network, which has questionable security
 implications.

7.3.5. Attacking the Teredo Servers and Relays

 It is possible to mount a brute force denial of service attack
 against the Teredo servers by sending them a very large number of
 packets.  This attack will have to be brute force, since the servers
 are stateless, and can be designed to process all the packets that
 are sent on their access line.
 The brute force attack against the Teredo servers is mitigated if
 clients are ready to "failover" to another server.  Bringing down the
 servers will, however, force the clients that change servers to
 renumber their Teredo address.
 It is also possible to mount a brute force attack against a Teredo
 relay.  This attack is mitigated if the relay under attack stops
 announcing the reachability of the Teredo service prefix to the IPv6
 network: the traffic will be picked up by the next relay.
 An attack similar to that described in Section 7.3.2 can be mounted
 against a relay.  An IPv6 host can send IPv6 packets to a large
 number of Teredo destinations, forcing the relay to establish state
 for each of these destinations.  Teredo relays can obtain some
 protection by limiting the range of IPv6 clients that they serve, and
 by limiting the amount of state used for "new" peers.

7.4. Denial of Service against Non-Teredo Nodes

 There is a widely expressed concern that transition mechanisms such
 as Teredo can be used to mount denial of service attacks, by
 injecting traffic at locations where it is not expected.  These
 attacks fall in three categories: using the Teredo servers as a
 reflector in a denial of service attack, using the Teredo server to
 carry a denial of service attack against IPv6 nodes, and using the
 Teredo relays to carry a denial of service attack against IPv4 nodes.
 The analysis of these attacks follows.  A common mitigating factor in
 all cases is the "regularity" of the Teredo traffic, which contains
 highly specific patterns such as the Teredo UDP port, or the Teredo
 IPv6 prefix.  In case of attacks, these patterns can be used to
 quickly install filters and remove the offending traffic.
 We should also note that none of the listed possibilities offer any
 noticeable amplification.

Huitema Standards Track [Page 43] RFC 4380 Teredo February 2006

7.4.1. Laundering DoS attacks from IPv4 to IPv4

 An attacker can use the Teredo servers as reflectors in a denial of
 service attack aimed at an IPv4 target.  The attacker can do this in
 one of two ways.  The first way is to construct a Router Solicitation
 message and post it to a Teredo server, using as IPv4 source address
 the spoofed address of the target; the Teredo server will then send a
 Router advertisement message to the target.  The second way is to
 construct a Teredo IPv6 address using the Teredo prefix, the address
 of a selected server, the IPv4 of the target, and an arbitrary UDP
 port, and to then send packets bound to that address to the selected
 Teredo server.
 Reflector attacks are discussed in [REFLECT], which outlines various
 mitigating techniques against such attacks.  One of these mitigations
 is to observe that "the traffic generated by the reflectors [has]
 sufficient regularity and semantics that it can be filtered out near
 the victim without the filtering itself constituting a denial-of-
 service to the victim ('collateral damage')".  The traffic reflected
 by the Teredo servers meets this condition: it is clearly
 recognizable, since it originates from the Teredo UDP port; it can be
 filtered out safely if the target itself is not a Teredo user.  In
 addition, the packets relayed by servers will carry an Origin
 indication encapsulation, which will help determine the source of the
 attack.

7.4.2. DoS Attacks from IPv4 to IPv6

 An attacker may use the Teredo servers to launch a denial of service
 attack against an arbitrary IPv6 destination.  The attacker will
 build an IPv6 packet bound for the target and will send that packet
 to the IPv4 address and UDP port of a Teredo server, to be relayed
 from there to the target over IPv6.
 The address checks specified in Section 5.3.1 provide some protection
 against this attack, as they ensure that the IPv6 source address will
 be consistent with the IPv4 source address and UDP source port used
 by the attacker: if the attacker cannot spoof the IPv4 source
 address, the target will be able to determine the origin of the
 attack.
 The address checks ensure that the IPv6 source address of packets
 forwarded by servers will start with the IPv6 Teredo prefix.  This is
 a mitigating factor, as sites under attack could use this to filter
 out all packets sourced from that prefix during an attack.  This will
 result in a partial loss of service, as the target will not be able
 to communicate with legitimate Teredo hosts that use the same prefix.

Huitema Standards Track [Page 44] RFC 4380 Teredo February 2006

 However, the communication with other IPv6 hosts will remain
 unaffected, and the communication with Teredo hosts will be able to
 resume when the attack has ceased.

7.4.3. DoS Attacks from IPv6 to IPv4

 An attacker with IPv6 connectivity may use the Teredo relays to
 launch a denial of service attack against an arbitrary IPv4
 destination.  The attacker will compose a Teredo IPv6 address using
 the Teredo prefix, a "cone" flag set to 1, the IPv4 address of the
 target, and an arbitrary UDP port.
 In the simplest variation of this attack, the attacker sends IPv6
 packets to the Teredo destination using regular IPv6 routing.  The
 packets are picked by the nearest relay, which will forward them to
 the IPv4 address of the target.  In a more elaborate variant, the
 attacker tricks a Teredo into sending packets to the target, either
 by sending a first packet with a spoofed IPv6 address and letting the
 Teredo host reply or by publishing a spoofed IPv6 address in a name
 service.
 There are three types of IPv4 addresses that an attacker may embed in
 the spoofed Teredo address.  It may embed a multicast or broadcast
 address, an local unicast address, or a global unicast address.
 With multicast or broadcast addresses, the attacker can use the
 multiplying effect of multicast routing.  By sending a single packet,
 it can affect a large number of hosts, in a way reminiscent of the
 "smurf" attack.
 By using local addresses, the attacker can reach hosts that are not
 normally reachable from the Internet, for example, hosts connected to
 the a Teredo relay by a private subnet.  This creates an exposure
 for, at a minimum, a denial of service attack against these otherwise
 protected hosts.  This is similar to attack variants using source
 routing to breach a perimeter.
 The address checks specified in Section 5.2.4, 5.3.1, and 5.4.1
 verify that packets are relayed only to a global IPv4 address.  They
 are designed to eliminate the possibility of using broadcast,
 multicast or local addresses in denial of service or other attacks.
 In what follows, we will only consider attacks targeting globally
 reachable unicast addresses.

Huitema Standards Track [Page 45] RFC 4380 Teredo February 2006

 The attacks can be targeted at arbitrary UDP ports, such as, for
 example, the DNS port of a server.  The UDP payload must be a well-
 formed IPv6 packet, and is thus unlikely to be accepted by any well-
 written UDP service; in most case, the only effect of the attack will
 be to overload the target with random traffic.
 A special case occurs if the attack is directed to an echo service.
 The service will echo the packets.  Since the echo service sees the
 request coming from the IPv4 address of the relay, the echo replies
 will be sent back to the same relay.  According to the rules
 specified in Section 5.4, these packets will be discarded by the
 Teredo relay.  This is not a very efficient attack against the Teredo
 relays -- establishing a legitimate session with an actual Teredo
 host would create more traffic.
 The IPv6 packets sent to the target contain the IPv6 address used by
 the attacker.  If ingress filtering is used in the IPv6 network, this
 address will be hard to spoof.  If ingress filtering is not used, the
 attacker can be traced if the IPv6 routers use a mechanism similar to
 ICMP Traceback.  The ICMP messages will normally be collected by the
 same relays that forward the traffic from the attacker; the relays
 can use these messages to identify the source of an ongoing attack.
 The details of this solution will have to be developed in further
 research.

8. IAB Considerations

 The IAB has studied the problem of "Unilateral Self Address Fixing"
 (UNSAF), which is the general process by which a client attempts to
 determine its address in another realm on the other side of a NAT
 through a collaborative protocol reflection mechanism [RFC3424].
 Teredo is an example of a protocol that performs this type of
 function.  The IAB has mandated that any protocols developed for this
 purpose document a specific set of considerations.  This section
 meets those requirements.

8.1. Problem Definition

 From [RFC3424], any UNSAF proposal must provide a precise definition
 of a specific, limited-scope problem that is to be solved with the
 UNSAF proposal.  A short-term fix should not be generalized to solve
 other problems; this is why "short term fixes usually aren't".
 The specific problem being solved by Teredo is the provision of IPv6
 connectivity for hosts that cannot obtain IPv6 connectivity natively
 and cannot make use of 6to4 because of the presence of a NAT between
 them and the 6to4 relays.

Huitema Standards Track [Page 46] RFC 4380 Teredo February 2006

8.2. Exit Strategy

 From [RFC3424], any UNSAF proposal must provide the description of an
 exit strategy/transition plan.  The better short term fixes are the
 ones that will naturally see less and less use as the appropriate
 technology is deployed.
 Teredo comes with its own built-in exit strategy: as soon as a client
 obtains IPv6 connectivity by other means, either 6to4 or native IPv6,
 it can cease using the Teredo service.  In particular, we expect that
 the next generation of home routers will provide an IPv6 service in
 complement to the current IPv4 NAT service, e.g., by relaying
 connectivity obtained from the ISP, or by using a configured or
 automatic tunnel service.
 As long as Teredo is used, there will be a need to support Teredo
 relays so that the remaining Teredo hosts can communicate with native
 IPv6 hosts.  As Teredo usage declines, the traffic load on the relays
 will decline.  Over time, managers will observe a reduced traffic
 load on their relays and will turn them off, effectively increasing
 the pressure on the remaining Teredo hosts to upgrade to another form
 of connectivity.
 The exit strategy is facilitated by the nature of Teredo, which
 provides an IP-level solution.  IPv6-aware applications do not have
 to be updated to use or not use Teredo.  The absence of impact on the
 applications makes it easier to migrate out of Teredo: network
 connectivity suffices.
 There would appear to be reasons why a Teredo implementation might
 decide to continue usage of the Teredo service even if it already has
 obtained connectivity by some other means, for example:
 1. When a client is dual homed, and it wishes to improve the service
 when communicating with other Teredo hosts that are "nearby" on the
 IPv4 network.  If the client only used its native IPv6 service, the
 Teredo hosts would be reached only through the relay.  By maintaining
 Teredo, the Teredo hosts can be reached by direct transmission over
 IPv4.
 2. If, for some reason, the Teredo link is providing the client with
 better service than the native IPv6 link, in terms of bandwidth,
 packet loss, etc.
 The design of Teredo mitigates the dual-homing reason.  A host that
 wishes to communicate with Teredo peers can implement a "host-based
 relay", which is effectively an unnumbered Teredo interface.  As
 such, the dual-homed host will obtain Teredo connectivity with those

Huitema Standards Track [Page 47] RFC 4380 Teredo February 2006

 hosts that must use Teredo, but will not inadvertently encourage
 other dual-homed hosts to keep using the Teredo service.
 The bubbles and the UDP encapsulation used by Teredo introduce a
 significant overhead.  It would take exceptional circumstances for
 native technologies to provide a lesser service than Teredo.  These
 exceptional circumstances, or other unforeseen reasons, may induce
 hosts to keep using the Teredo service despite the availability of
 native IPv6 connectivity.  However, these circumstances are likely to
 be rare and transient.  Moreover, if the primary reason to use Teredo
 fades away, one can expect that Teredo relays will be progressively
 turned off and that the quality of the Teredo service will
 progressively degrade, reducing the motivation to use the Teredo
 service.

8.3. Brittleness Introduced by Teredo

 From [RFC3424], any UNSAF proposal must provide a discussion of
 specific issues that may render systems more "brittle".  For example,
 approaches that involve using data at multiple network layers create
 more dependencies, increase debugging challenges, and make it harder
 to transition.
 Teredo introduces brittleness into the system in several ways: the
 discovery process assumes a certain classification of devices based
 on their treatment of UDP; the mappings need to be continuously
 refreshed; and addressing structure may cause some hosts located
 behind a common NAT to be unreachable from each other.
 There are many similarities between these points and those introduced
 by Simple Traversal of the UDP Protocol through NAT (STUN) [RFC3489];
 however, Teredo is probably somewhat less brittle than STUN.  The
 reason is that all Teredo packets are sent from the local IPv4 Teredo
 service port, including discovery, bubbles, and actual encapsulated
 packets.  This is different from STUN, where NAT type detection and
 binding allocation use different local ports (ephemeral, in both
 cases).
 Teredo assumes a certain classification of devices based on their
 treatment of UDP (e.g., cone, protected cone and symmetric).  There
 could be devices that would not fit into one of these molds, and
 hence would be improperly classified by Teredo.
 The bindings allocated from the NAT need to be continuously
 refreshed.  Since the timeouts for these bindings are very
 implementation specific, the refresh interval cannot easily be

Huitema Standards Track [Page 48] RFC 4380 Teredo February 2006

 determined.  When the binding is not being actively used to receive
 traffic, but to wait for an incoming message, the binding refresh
 will needlessly consume network bandwidth.
 The use of the Teredo server as an additional network element
 introduces another point of potential security attack.  These attacks
 are largely prevented by the security measures provided by Teredo,
 but not entirely.
 The use of the Teredo server as an additional network element
 introduces another point of failure.  If the client cannot locate a
 Teredo server, or if the server should be unavailable due to failure,
 the Teredo client will not be able to obtain IPv6 connectivity.
 The communication with non-Teredo hosts relies on the availability of
 Teredo relays.  The Teredo design assumes that there are multiple
 Teredo relays; the Teredo service will discover the relay closest to
 the non-Teredo peer.  If that relay becomes unavailable, or is
 misbehaving, communication between the Teredo hosts and the peers
 close to that relay will fail.  This reliability issue is somewhat
 mitigated by the possibility to deploy many relays, arbitrarily close
 from the native IPv6 hosts that require connectivity with Teredo
 peers.
 Teredo imposes some restrictions on the network topologies for proper
 operation.  In particular, if the same NAT is on the path between two
 clients and the Teredo server, these clients will only be able to
 interoperate if they are connected to the same link, or if the common
 NAT is capable of "hairpinning", i.e., "looping" packets sent by one
 client to another.
 There are also additional points of brittleness that are worth
 mentioning:
  1. Teredo service will not work through NATs of the symmetric variety.
  1. Teredo service depends on the Teredo server running on a network

that is a common ancestor to all Teredo clients; typically, this is

   the public Internet.  If the Teredo server is itself behind a NAT,
   Teredo service will not work to certain peers.
  1. Teredo introduces jitter into the IPv6 service it provides, due to

the queuing of packets while bubble exchanges take place. This

   jitter can negatively impact applications, particularly latency
   sensitive ones, such as Voice over IP (VoIP).

Huitema Standards Track [Page 49] RFC 4380 Teredo February 2006

8.4. Requirements for a Long-Term Solution

 From [RFC3424], any UNSAF proposal must identify requirements for
 longer-term, sound technical solutions -- contribute to the process
 of finding the right longer-term solution.
 Our experience with Teredo has led to the following requirements for
 a long-term solution to the NAT problem: the devices that implement
 the IPv4 NAT services should in the future also become IPv6 routers.

9. IANA Considerations

 This memo documents a request to IANA to allocate a 32-bit Teredo
 IPv6 service prefix, as specified in Section 2.6, and a Teredo IPv4
 multicast address, as specified in Section 2.17.

10. Acknowledgements

 Many of the ideas in this memo are the result of discussions between
 the author and Microsoft colleagues, notably Brian Zill, John Miller,
 Mohit Talwar, Joseph Davies, and Rick Rashid.  Several encapsulation
 details are inspired from earlier work by Keith Moore.  The example
 in Section 5.1 and a number of security precautions were suggested by
 Pekka Savola.  The local discovery procedure was suggested by Richard
 Draves and Dave Thaler.  The document was reviewed by members of the
 NGTRANS and V6OPS working groups, including Brian Carpenter, Cyndi
 Jung, Keith Moore, Thomas Narten, Anssi Porttikivi, Pekka Savola, Eng
 Soo Guan, and Eiffel Wu.  Eric Klein, Karen Nielsen, Francis Dupont,
 Markku Ala-Vannesluoma, Henrik Levkowetz, and Jonathan Rosenberg
 provided detailed reviews during the IETF last call.

Huitema Standards Track [Page 50] RFC 4380 Teredo February 2006

11. References

11.1. Normative References

 [RFC768]   Postel, J., "User Datagram Protocol", STD 6, RFC 768,
            August 1980.
 [RFC791]   Postel, J., "Internet Protocol", STD 5, RFC 791, September
            1981.
 [RFC1918]  Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.,
            and E. Lear, "Address Allocation for Private Internets",
            BCP 5, RFC 1918, February 1996.
 [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
            Hashing for Message Authentication", RFC 2104, February
            1997.
 [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.
 [RFC2461]  Narten, T., Nordmark, E., and W. Simpson, "Neighbor
            Discovery for IP Version 6 (IPv6)", RFC 2461, December
            1998.
 [RFC2462]  Thomson, S. and T. Narten, "IPv6 Stateless Address
            Autoconfiguration", RFC 2462, December 1998.
 [RFC3056]  Carpenter, B. and K. Moore, "Connection of IPv6 Domains
            via IPv4 Clouds", RFC 3056, February 2001.
 [RFC3424]  Daigle, L. and IAB, "IAB Considerations for UNilateral
            Self-Address Fixing (UNSAF) Across Network Address
            Translation", RFC 3424, November 2002.
 [RFC3566]  Frankel, S. and H. Herbert, "The AES-XCBC-MAC-96 Algorithm
            and Its Use With IPsec", RFC 3566, September 2003.
 [FIPS-180] "Secure Hash Standard", Computer Systems Laboratory,
            National Institute of Standards and Technology, U.S.
            Department Of Commerce, May 1993.

Huitema Standards Track [Page 51] RFC 4380 Teredo February 2006

11.2. Informative References

 [RFC2993]  Hain, T., "Architectural Implications of NAT", RFC 2993,
            November 2000.
 [RFC3330]  IANA, "Special-Use IPv4 Addresses", RFC 3330, September
            2002.
 [RFC3489]  Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy.
            "STUN - Simple Traversal of User Datagram Protocol (UDP)
            Through Network Address Translators (NATs)", RFC 3489,
            March 2003.
 [RFC3904]  Huitema, C., Austein, R., Satapati, S., and R. van der
            Pol, "Evaluation of IPv6 Transition Mechanisms for
            Unmanaged Networks", RFC 3904, September 2004.
 [RFC3947]  Kivinen, T., Swander, B., Huttunen, A., and V. Volpe,
            "Negotiation of NAT-Traversal in the IKE", RFC 3947,
            January 2005.
 [RFC4302]  Kent, S., "IP Authentication Header", RFC 4302, December
            2005.
 [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)", RFC
            4303, December 2005.
 [RFC4306]  Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", RFC
            4306, December 2005.
 [REFLECT]  V. Paxson, "An analysis of using reflectors for
            distributed denial of service attacks", Computer
            Communication Review, ACM SIGCOMM, Volume 31, Number 3,
            July 2001, pp 38-47.

Author's Address

 Christian Huitema
 Microsoft Corporation
 One Microsoft Way
 Redmond, WA 98052-6399
 EMail: huitema@microsoft.com

Huitema Standards Track [Page 52] RFC 4380 Teredo February 2006

Full Copyright Statement

 Copyright (C) The Internet Society (2006).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
 This document and the information contained herein are provided on an
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 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

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 Intellectual Property Rights or other rights that might be claimed to
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 might or might not be available; nor does it represent that it has
 made any independent effort to identify any such rights.  Information
 on the procedures with respect to rights in RFC documents can be
 found in BCP 78 and BCP 79.
 Copies of IPR disclosures made to the IETF Secretariat and any
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 The IETF invites any interested party to bring to its attention any
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 rights that may cover technology that may be required to implement
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Acknowledgement

 Funding for the RFC Editor function is provided by the IETF
 Administrative Support Activity (IASA).

Huitema Standards Track [Page 53]

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