[Docs] [txt|pdf] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

Versions: 00 01 02 03 04 05 06 07 08

INTERNET DRAFT                                              C. Huitema
<draft-ietf-ngtrans-shipworm-08.txt>                         Microsoft
Expires March 17, 2003                              September 17, 2002

Teredo: Tunneling IPv6 over UDP through NATs

Status of this memo

This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.

This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups.  Note that other groups may also distribute
working documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other documents
at any time.  It is inappropriate to use Internet-Drafts as
reference material or to cite them other than as "work in progress."

The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.

The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.

Abstract

We propose here a service that enables nodes located behind one or
several IPv4 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.

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." There are however limits to any solution that is based on

Huitema                                                      [Page  1]

INTERNET DRAFT               Teredo                 September 17, 2002

such brokers: the quality of service is not very good, since the
traffic follows a "dog leg" 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, we tend to prefer 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
several 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
can enlist the help of "Teredo servers" and "Teredo relays" to learn
their "global address" and to obtain connectivity, and how clients,
servers and relays can be organized in Teredo networks.

The specification is organized as follow. 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 models of operation and deployment. Section 5 contains
the format of the messages and the specification of the protocol.
Section 6 is a discussion of the key design choices. Section 7
presents the guideline for some further work that would be
complementary to the current approach. Section 8 contains a security
discussion, and section 9 contains IANA considerations.

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 that 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 that is used as a helper to provide IPv6
connectivity to Teredo clients.

Huitema                                                      [Page  2]

INTERNET DRAFT               Teredo                 September 17, 2002

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 which is used to construct the IPv6
address of Teredo clients.

2.5.1   Global Teredo IPv6 service prefix

An IPv6 addressing prefix whose value is XXXX:XXXX:/32.
(TBD IANA; experiments use the value 3FFE:831F::/32, taken from a
range of experimental IPv6 prefixes assigned to Microsoft.)

2.6     Teredo UDP port

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

2.7     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.

2.8     Teredo service port

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

2.9     Teredo server address

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

2.10    Teredo mapped address and Teredo mapped port

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

2.11    Teredo IPv6 client prefix

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


Huitema                                                      [Page  3]

INTERNET DRAFT               Teredo                 September 17, 2002

2.12    Teredo node identifier

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

2.13    Teredo IPv6 address

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

2.14    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,
described in section 5.

2.15    Teredo secondary port

A UDP port used to determine the appropriate value of the refresh
interval, but not used to carry any Teredo traffic.

2.16    Teredo IPv4 Discovery Address

An IPv4 multicast address used to discover other Teredo clients on
the same IPv4 subnet.

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.
Relaying packets over TCP would be possible, but would result in a
very poor quality of service; relaying 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 an UDP packet
coming through for which there is not yet an existing mapping. The

Huitema                                                      [Page  4]

INTERNET DRAFT               Teredo                 September 17, 2002

handling of UDP "sessions" by NAT devices differs by two important
parameters, the type and the duration of the mappings.

3.1.1   Types of UDP mappings

Experience shows that the implementers of NAT devices can adopt
widely different treatments of UDP mappings:

1) Some implement the simplest solution, which is to map an internal
UDP port, defined by an internal address and a port number on the
corresponding host, to an external port, defined by a global address
managed by the NAT and a port number valid for that address. In this
simple case, the mapping is retained as long as the port is active,
and is removed after an inactivity timer. As long as the mapping is
retained, any packet received by the NAT for the external port is
relayed to the internal address and port. These NATs are usually
called "cone NATs".

2) Some implement a more complex solution, in which the NAT not only
establishes a mapping for the UDP port, but also maintains a list of
external hosts to which traffic has been sent from that port. The
packets originating from third party hosts to which the local host
has not yet sent traffic are rejected. These NATs are usually called
"restricted cone NATs".

3) Instead of keeping just a list of authorized hosts, some NAT
implementations keep a list of authorized host and port pairs. UDP
packets coming from remote addresses are rejected if the internal
host has not yet sent traffic to the outside host and port pair. The
NATs are often called "port-restricted cone NATs"

4) Finally, some NATs map the same internal address and port pair to
different external address and port pairs, depending on the address
of the remote host. These NATs are usually called "symmetric NATs".

Measurement campaigns and studies of documentations have shown that
most NATs implement either option 1 or option 2, i.e. cone NATs or
restricted cone NATs. The Teredo solution ensures connectivity for
clients located behind cone NATs, restricted cone NATs, or port-
restricted cone NATs; it contains optimizations for clients located
behind a cone NAT; it does not provide connectivity for clients
located behind a symmetric NAT.

3.1.2   Lifetime of UDP mappings

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 want 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

Huitema                                                      [Page  5]

INTERNET DRAFT               Teredo                 September 17, 2002

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.

In some cases, e.g. NATs that manage ISDN or dial-up connections,
the mappings will be release when the connection is released, i.e.
when no traffic is observed 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.


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
transition schemes designed by the NGTRANS working group. 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 or if it is provided by a
6to4 router co-located with the local NAT, 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, it is however required that the client
learn the IPv4 address of a server that is willing to serve them;

Huitema                                                      [Page  6]

INTERNET DRAFT               Teredo                 September 17, 2002

some servers may also require some form of access control.

3.2.3   Minimal load on servers

During the peak of the transition, there will be a requirement to
deploy a large number of servers throughout the Internet. 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 request that servers only
enable the packet exchange between clients, but do not 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 servers and relays. In order to
facilitate the deployment of these servers, the Teredo procedures
are designed to minimize the fraction of traffic that has to be
routed through the servers.

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

Huitema                                                      [Page  7]

INTERNET DRAFT               Teredo                 September 17, 2002

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 servers 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.

4       Model of operation and deployment

A Teredo Network is composed of a set of clients, servers and
relays. In this section, we present the model of operation of a
given network, and then we present the deployment model.

4.1     Model of operation

The Teredo service requires the cooperation of three kinds of
actors: Teredo clients, who want to use IPv6 despite being located
behind a NAT, Teredo servers who will facilitate the service, and
Teredo relays that provide for the interconnection between the
Teredo service and the "native IPv6 Internet."

In order to enable the service, the Teredo servers must have IPv6
connectivity and an unencumbered IPv4 connection: they must have a
global IPv4 address; and they must have global IPv6 connectivity
independently of the Teredo service.

The Teredo relays must be connected to the IPv6 Internet and must
participate in IPv6 routing; they must be able to announce
reachability of the "Teredo service IPv6 prefix" over IPv6. They
must then be able to relay packets over IPv4 UDP towards Teredo
clients.

The primary role of the servers is to enable NAT traversal. The

Huitema                                                      [Page  8]

INTERNET DRAFT               Teredo                 September 17, 2002

service is designed in such a way that, when NAT traversal is
guaranteed, packets can flow on a direct path between source and
destination, bypassing the Teredo server.

4.1.1   Encoding of Teredo addresses

The Teredo addresses are composed of 5 components:

  +-------------+-------------+-------+------+-------------+
  | Prefix      | Server IPv4 | Flags | Port | Client IPv4 |
  +-------------+-------------+-------+------+-------------+

- Prefix: the 32 bit Teredo service prefix.
- Server IPv4: the IPv4 address of a Teredo server.
- Flags: a set of 16 bits that document type of address and NAT.
- Port: the obfuscated "mapped UDP port" of the Teredo service at
the client
- Client IPv4: the obfuscated "mapped IPv4 address" of a 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.

A third party sends IPv6 packets to a Teredo client by sending these
packets over UDP to the client IPv4 address and port if the server
address is null, or if the third party has recently received direct
traffic from the client. In the other cases, the third party will
have to first synchronize with the client, by sending an initial
bubble through the server.

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 which 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:

-       The bits "UG" should be set to the value "00", indicating a non-
global unicast identifier;
-       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

Huitema                                                      [Page  9]

INTERNET DRAFT               Teredo                 September 17, 2002

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.

There are thus two valid 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.

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.

4.1.2   Obtaining an address

The first phase of Teredo operation is the acquisition of a Teredo
address prefix by the client. To do this, the client selects a
Teredo server, and sends it a Router Solicitation message. The
server replies with a router advertisement containing a Teredo
prefix, composed of the Teredo service prefix and the IPv4 address
of the server; the message also contains an "origin" indication that
specifies the IPv4 address and port number from which the server
received the router solicitation.

In order to explain how this works, we will use an hypothetical
example: a Teredo client is located at the private IP address
10.0.0.2 in a private network; the NAT connecting this network to
the public Internet responds to the local address 10.0.0.1 and is
visible as the public address 9.0.0.1. We present here a simplified
version of the procedure, in which we are only concerned with
determining the "mapping" of the client's address; in the next
section, we will explain how this procedure is in fact combined with
"cone NAT determination".
















Huitema                                                      [Page 10]


INTERNET DRAFT               Teredo                 September 17, 2002

                                          .-----------------------
                                          |  Private network
                .--.      _             .-----.     .----------.
               (IPv4)  src=9.0.0.1:4096 | NAT |     | Teredo   |
             (Internet)<--------------  | BOX | <-- |  Client  |
               (    )   (UDPv4 tunneled |     |     '----------'
                '--'         IPv6)      '-----'       10.0.0.2:1234
                 |                9.0.0.1 | 10.0.0.1
                 |                        |
                 |                        |
                 V                        |
          .--------------.                '-----------------------
          |   Teredo     |
          |   Server     |
          '--------------'
            [1.2.3.4]

When the Teredo client is turned on, it sends an IPv6 router
solicitation over UDP to the Teredo server, using the source address
and ports 10.0.0.2:1234. The NAT intercepts the packet, establishes
a mapping, and changes the source address and port to 9.0.0.1:4096.
(These values are indeed just examples.)

The Teredo server receives the packet and notes the source address.
It sends back a router advertisement that contains the server's
prefix (e.g. PREF:0102:0304::/64); the advertisement also contains
an origin indication that specifies the IPv4 address and UDP port
from which the router advertisement was received, in this case
9.0.0.1:4096. The router sends the router advertisement back over
UDP to the mapped IPv4 address 9.0.0.1:4096. The packet will have
the following format:

  +------+-----+-------------------+-------------+
  | IPv4 | UDP | Origin indication | IPv6 RA     |
  +------+-----+-------------------+-------------+

The "origin indication" field specifies the "mapped IPv4 address"
and "mapped port" of the client; the IPv6 router advertisement
specifies the prefix announced by the server.

The IPv4 packet is received by the NAT, which has remembered the
mapping between this external address and port pair and the private
address and port pair, 10.0.0.2:1234; the NAT forwards the packet to
the Teredo client.

Upon reception of this prefix, the client composes a Teredo address,
which in our example will be:

        PREF:102:304::EFFF:F6FF:FFFE

In this address, "PREF" is the Teredo IPv6 service prefix of length

Huitema                                                      [Page 11]


INTERNET DRAFT               Teredo                 September 17, 2002

32; "102:304" is the hexadecimal notation of the address of the
Teredo server, in this example 1.2.3.4 (expressed with leading zeros
as 0102:0304); "EFFF" is equal to the XOR of "FFFF" with "1000",
which is the hexadecimal notation of the mapped port "4096"; and
"F6FF:FFFE" is equal to the XOR of "FFFF:FFFF" with "0900:0001",
which is the hexadecimal notation of the IPv4 mapped address
"9.0.0.1".

In some environments, it is necessary to secure this exchange, to
avoid the risk of an attacker "spoofing" the server's return. In
these environments, the client will be provisioned with a secret key
and a key identifier; the key is shared with the server. Both the
client's solicitation and the server's response will be protected by
an authentication token, carried in the UDP message:

  +------+-----+----------------+-------------+
  | IPv4 | UDP | Authentication | IPv6 RS     |
  +------+-----+----------------+-------------+

  +------+-----+----------------+-------------------+-------------+
  | IPv4 | UDP | Authentication | Origin indication | IPv6 RA     |
  +------+-----+----------------+-------------------+-------------+

The authentication token uses the secret key and the key identifier
to guarantee the integrity and the authenticity of the packets.

4.1.3   Determining the type of NAT

The previous section presented a simplified version of the prefix
assessment procedure. For better efficiency, the client needs to
determine the type of NAT behind which it is located: clients
located behind a "cone" NAT can receive traffic directly, and we
want to take advantage of this optimization: if the client is
located behind a cone NAT, it should use Teredo addresses in which
the "cone" bit is set to 1.

To achieve this result, the client first sends a router solicitation
message from a link-local address in which the "cone" bit is set to
1; when the server observes that the cone bit is set, it sends the
router advertisement in a UDP packet from a "secondary" IPv4
address, i.e., a different IPv4 address than the "server address" to
which the solicitation was sent. Only nodes located behind a cone
NAT will be able to receive this reply; in consequence, if the
client receives this reply, from an IPv4 address different than the
server address, it determines that it is located behind a cone NAT.

If the client does not receive a response to the first solicitation,
it repeats the procedure and sends a solicitation message from a
link-local address in which the "cone" bit is set to 0; the server
will send the reply from its "nominal" IPv4 address, so the answer
can be received by a non-cone NAT. The client will assume that it is
not behind a cone NAT.

Huitema                                                      [Page 12]


INTERNET DRAFT               Teredo                 September 17, 2002

The Teredo service does not work if the client is located behind a
symmetric NAT. The client must thus do an extra step, after
receiving the router advertisement from the server nominal address:
it should repeat the procedure by sending a solicitation to a
secondary server IPv4 address, and compare the mapped addresses and
mapped port in the two replies. If they are not the same, the client
detects that it is behind a symmetric NAT and cannot use the Teredo
service.

4.1.4   First packet from an IPv6 node to a Teredo node

The transmission of packets between a regular IPv6 node and a Teredo
node is presented in the following diagram, in which "A" is a Teredo
client located behind the NAT "N", "S" the Teredo server chosen by
"A", "B" a regular IPv6 node, and "R" a Teredo Relay close to "B" in
the IPv6 Internet Topology.

              .----------------------------------
              | IPv6 Internet
              |                          +---+
              |                          | B |
              |                          +---+
              |                            !
              |   +-----+                +-!-+
              `---| S   |----------------|R! |----
  .---------------|     |----------------| ! |---
  |IPv4 Internet  |   ..|<.................* |
  |               +---#-+                +---+
  |                   #                   ^!
  |                   #                   :!
  |                   #...................:!
  | bubble+origin --> #:.-.-.-.-.-.-.-.-.-.-
  |                   #:!
  |                   v:v
  |               +----:--+
  `---------------| N #:! |--------------------------
               .--|   #:! |--------------------
               |  +---#-!-+ Natted domain
               |      #^!
               |      #:!
               |      v:v
               |  +-----+
               |  | A   |
               |  +-----+

We assume that A has already established its Teredo address through
an RA/RS exchange with S, as explained in section 4.1.2; in this
example, we will assume that the cone NAT determination failed. We
will assume that the results of these exchanges are the following:

- A's private address and port are: 10.0.0.2:1234.

Huitema                                                      [Page 13]


INTERNET DRAFT               Teredo                 September 17, 2002

- A's mapped address and port are: 9.0.0.1:4096.
- A's IPv6 address has been set to PREF:102:304::EFFF:F6FF:FFFE
- The server's IPv4 address and port are: 1.2.3.4:3544
- The relay's IPv4 address and port are: 5.6.7.8:3544
- B's IPv6 address is: 2000::B

When B sends a packet to A, B simply follows IPv6 rules. The packet
is forwarded to the nearest relay, R, over IPv6. R examines the
destination address, and observes that the address includes the
notation of a server, 1.2.3.4. Since R has not yet received any
direct traffic from A, and since A is not located behind a cone NAT,
R cannot send the packet directly to A: it would probably be
rejected by the NAT N. Instead, R queues the packet, and formats a
bubble that it forwards towards A over UDP, to the address of S:
1.2.3.4:3544.

When S receives the bubble, it notices that the source is not a
Teredo client, but instead a regular IPv6 address. S forwards the
packet to A using a special envelope. The packet will have the
following format:

  +------+-----+-------------------+-------------+
  | IPv4 | UDP | Origin indication | IPv6 bubble |
  +------+-----+-------------------+-------------+

In the IPv4 and UDP header, the source will be set to the server's
address and port, 1.2.3.4:3544, and the destination to A's mapped
address, as indicated in the IPv6 destination address: 9.0.0.1:4096.
The origin indication element will carry the IPv4 address and UDP
port of R: 5.6.7.8:3544.

The NAT N will receive this message. It will use the existing
mapping to rewrite 9.0.0.1:4096 to 10.0.0.2:1234, and forward the
packet to A. When A receives the packet, it will immediately send a
bubble back to the origin of the bubble, i.e. towards R, at the
address 5.6.7.8:3544.

When R receives the bubble from A, it notes that direct transmission
towards A is now possible. It forwards the queued packet to the
mapped address of A, 9.0.0.1:4096. The packet is received by the
NAT; since A has recently sent a packet to R, a mapping exists and
the packet is forwarded to A.

If the cone NAT validation had been successful, A would have used
the IPv6 address PREF:102:304:8000:EFFF:F6FF:FFFE, and R would have
sent B's packet directly to A.

4.1.5   First packet from a Teredo node to a regular IPv6 node

The exchange of packets between a Teredo Node and a regular IPv6
node is presented in the following diagram, in which "A" is a Teredo
client located behind the NAT "N", "S" the Teredo server chosen by

Huitema                                                      [Page 14]


INTERNET DRAFT               Teredo                 September 17, 2002

"A", "B" a regular IPv6 node, and "R" a Teredo Relay close to "B" in
the IPv6 Internet Topology.

              .----------------------------------
              | IPv6 Internet
              |                          +---+
              |       ..................>| B |
              |       :                  +---+
              |       :                    :^
              |       :                    :!
              |   +---:--+               +-:!-+
              `---| S :  |---------------|R:! |----
  .---------------|   :  |---------------| :! |---
  |IPv4 Internet  |   :..|<.................! |
  |               +----#-+               +----+
  |                   ^#                    ^
  |                   :#                    !
  |                   :#                    !
  | bubble+origin --> :#.-.-.-.-.-.-.-.-.-.-
  |                   :#!
  |                   :v!
  |               +---:-!-+
  `---------------| N :#! |--------------------------
               .--|   :#! |--------------------
               |  +---:#--+ Natted domain
               |      ^#^
               |      :#!
               |      :v!
               |  +-------+
               |  | A     |
               |  +-------+

We assume that A has already established its Teredo address through
an RA/RS exchange with S, as explained in section 4.1.2; in this
example, we will assume that the client is not located behind a cone
NAT. We will assume that the results of these exchanges are the
following:

- A's private address and port are: 10.0.0.2:1234.
- A's mapped address and port are: 9.0.0.1:4096.
- A's IPv6 address has been set to PREF:102:304::EFFF:F6FF:FFFE
- The server's IPv4 address and port are: 1.2.3.4:3544
- The relay's IPv4 address and port are: 5.6.7.8:3544
- B's IPv6 address is: 2000::B

A has to transmit an IPv6 packet to B. A's first action is to learn
the address of the relay R closest to B. To do so, A sends an ICMPv6
"echo request" toward B. This request carries the source address
PREF:102:304::EFFF:F6FF:FFFE (A's address), and the destination
address 2000::B (B's address); the Data field of the echo request
carries a nonce value, chosen by A. The request is encapsulated by A
in a UDP datagram, from source address and port 10.0.0.2:1234, to

Huitema                                                      [Page 15]


INTERNET DRAFT               Teredo                 September 17, 2002

destination address and port 1.2.3.4:3544.

The packet is received by the NAT N. N uses the existing mapping for
10.0.0.2:1234, and replaces the UDP source address and port by the
mapped values 9.0.0.1:4096, before forwarding the packet.

The packet is received over IPv4 by the server. S discards the IPv4
and UDP header, and forwards the content of the packet over IPv6,
which will be received by B, and which will appear to come from A's
address, PREF:102:304::EFFF:F6FF:FFFE.

When the request is received, B sends the echo reply to A; B simply
follows IPv6 routing rules. The packet is forwarded to the nearest
relay, R, over IPv6. R examines the destination address, and
observes that the address includes the address of a server, 1.2.3.4.
Since R has not received any direct traffic from A, R forwards the
packet over UDP to the address of S: 1.2.3.4:3544.

When S receives the packet, it notices that the source is not a
Teredo client, but instead a regular IPv6 address. S forwards the
packet to A using a special envelope. The packet will have the
following format:

  +------+-----+-------------------+-------------+
  | IPv4 | UDP | Origin indication | IPv6 packet |
  +------+-----+-------------------+-------------+

In the IPv4 and UDP header, the source will be set to the server's
address and port, 1.2.3.4:p, and the destination to A's mapped
address, as indicated in the IPv6 destination address: 9.0.0.1:4096.
The origin indication element will carry the IPv4 address and UDP
port of R: 5.6.7.8:3544.

The NAT N will receive this message. It will use the existing
mapping to rewrite 9.0.0.1:4096 to 10.0.0.2:1234, and forward the
packet to A. When A receives the packet, it will notice the origin
indication, as reported by the server, and it will have learned that
B can possibly be reached through the relay address 5.6.7.8:p. A can
also note that the Data field of the ICMPv6 echo reply matches the
nonce that was previously sent, which provides a reasonable
assurance that the packet does in fact come from B. At that point, A
can send the original data packet to B, by encapsulating it in a UDP
datagram bound to the IPv4 address and UDP port of R: 5.6.7.8:3544;
R will then normally relay the packet to B. Once the knowledge of
R's address has been acquired, A will send all further packets
directly through R, without having to repeat the ICMP exchange.

If the cone NAT validation had been successful, A would have used
the IPv6 address PREF:102:304:8000:EFFF:F6FF:FFFE, and R would have
sent B's reply directly to A. In that case, A would have learned R's
address from the IPv4 source address and UDP source port of the
incoming packet.

Huitema                                                      [Page 16]


INTERNET DRAFT               Teredo                 September 17, 2002

Finally, we may observe that this procedure supposes that the relay
will send the ICMP echo reply to a non-cone client directly through
the server. This is a slight variation of the procedure described in
the previous section, in which the IPv6 packets are queued waiting
for completion of a bubble exchange between client and relay. The
relay may in fact choose to queue the ICMPv6 echo reply, just like
it queues normal packets, and forward the ICMPv6 echo reply when the
bubble exchange is complete. In that case, the client will learn the
relay's address from the IPv4 source address and UDP source port of
the incoming packet.

4.1.6   Exchanges between two Teredo nodes

The following diagram shows two Teredo clients, A and B, connected
through the NATs N1 and N2. The exchanges will use the Teredo server
S2, chosen by B. We will assume that the NAT N2 is a "restricted
cone" or "port-restricted cone" NAT; in this example, we will also
assume that A is behind a restricted cone or port-restricted cone
NAT.

     .-------------------------------------------------------
     | IPv4 Internet
     |
     |           4-Data packet
     |      .-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.
     |      !                               +----+    !
     |      !     2-bubble                  | S2 |    !
     |      !..............................>|.........!
     |      !:                              +----+   :!
     |      !:     3-Bubble                          :!
     |      !:  ...................................  :!
     |      !:  :.................................:  :!
     |      !:  v:     1-Bubble                  v:  vv
     |     +!----:+                             +-:----+
     `-----|!:N1::|-----------------------------| :N2:!|-----
        .--|!:  ::|----.                   .----| :  :!|--.
        |  +-:--:-+    |                   |    +----:!+  |
        |   ^^  :^     |                   |      ^  :!   |
        |   !:  ::     |                   |      :  :!   |
        |   !:  v:     |                   |      :  vv   |
        |  +------+    |                   |     +-----+  |
        |  |  A   |    |                   |     | B   |  |
        |  +------+    |                   |     +-----+  |
        `--------------/                   `--------------/

We will assume that A and B have already obtained Teredo addresses
by RS/RA exchanges with they respective servers S1 and S2, and they
have made sure that they can receive packets from these respective
servers, for example by sending a keepalive packet every minute. In
the example, we will assume the following values:


Huitema                                                      [Page 17]


INTERNET DRAFT               Teredo                 September 17, 2002

- A's private address and port are: 10.0.0.2:1234.
- A's mapped address and port are: 9.0.0.1:4096.
- A is served by S1, whose address is: 1.2.3.4
- A's IPv6 address has been set to PREF:102:304::EFFF:F6FF:FFFE
- B's private address and port are: 10.0.0.3:3456.
- B's mapped address and port are: 8.0.0.1:1024.
- B is served by S2, whose address is: 9.10.11.12
- B's IPv6 address has been set to PREF:90A:B0C::FBFF:F7FF:FFFE

A has learned B's address, for example from the DNS, and starts UDP
or TCP communication with B. The first data packet will be sent from
A's IPv6 address (PREF:102:304::EFFF:F6FF:FFFE) to B's IPv6 address
(PREF:90A:B0C::FBFF:F7FF:FFFE). Since A has no prior knowledge of B,
and since B is not located behind a cone NAT, A cannot send the
packet directly to B; the packet is queued. Instead, A prepares two
bubbles, from IPv6 source address PREF:102:304::EFFF:F6FF:FFFE to
IPv6 destination address PREF:90A:B0C::FBFF:F7FF:FFFE.

The first bubble is sent from A to the mapped IPv4 address of B,
8.0.0.1:1024. It will probably not be received by B, since there is
no establish mapping at the NAT N2. The purpose of this bubble is
only to establish a mapping in the NAT N1.

The second bubble is sent from A to S2's address, 9.10.11.12:3544.
The bubble passes through N1. S2 receives it and transmits it to B.
Since there is an established mapping in N2 for transmission between
B and S2, the bubble is forwarded to B.

B responds to this bubble with its own bubble, from IPv6 source
address PREF:90A:B0C::FBFF:F7FF:FFFE to IPv6 destination address
PREF:102:304::EFFF:F6FF:FFFE. This response bubble is sent directly
to the mapped address of A, 9.0.0.1:4096. Since a mapping was
established by the first bubble, this third bubble is received by A.

When A receives the third bubble, it knows that direct communication
with B is now possible. The queued packet can now be directly
transmitted.

4.1.7   Exchanges between two Teredo nodes on the same link

The following diagram shows two Teredo clients, A and B, connected
to the same link, which is connected to the Internet through the NAT
N1. The exchanges will use the Teredo server S1. We are not making
any assumption about the NAT N1. This scenario explains how the
exchanges between clients on the same link can be optimized to avoid
routing all packets through the server S1.







Huitema                                                      [Page 18]


INTERNET DRAFT               Teredo                 September 17, 2002

     .------------------------------------------
     | IPv4 Internet
     |     +------+
     |     |  S1  |
     |     +------+
     |      ^!  ^!
     |      !!  !!
     |      !!  !!
     |      !!  !! Router Solicit,
     |      !!  !! Router Advertisement
     |      !!  !!
     |      !v  !v
     |     +!---!-+
     `-----|!!N1! |----------------------------
        .--|!!  !!|-------------------------.
        |  +-!--!!+                         |
        |   ^!  !!                          |
        |   !!  !`-.-.-.-.-.-.-.-.-.-.-.    |
        |   !!  `-.-.-.-.-.-.-.-.-.-.-.!    |
        |   !!      .->___   <-.      !!    |
        |   !v     / multicast  \     !v    |
        |  +------+  bubbles     +------+   |
        |  |  A   |              |  B   |   |
        |  +------+ <===========>+------+   |
        |         direct transmissions      |
        `-----------------------------------/

Both A and B discover their Teredo prefix by interacting with the
server S1, as explained in 4.1.2.

In order to enable direct transmission on the local link, both A and
B send Teredo bubbles to the Teredo IPv4 Discovery Address, an IPv4
multicast address whose scope is limited to the local link. These
bubbles enable the local hosts to discover the local IPv4 address
and the local UDP port associated with the Teredo IPv6 address of a
local host. The data packets can then be sent over UDP to this
address, avoiding the long path through the server S1.

4.2     Deployment model

The deployment model makes three assumptions:

- That all clients, servers and relays will be cognizant of the
Teredo service prefix and the Teredo port,

- That the clients will be configured with the IPv4 address of a
server,

- That there will be an adequate deployment of Teredo relays.

4.2.1   Server deployment

Huitema                                                      [Page 19]


INTERNET DRAFT               Teredo                 September 17, 2002

Servers may be deployed either as part of an ISP offer to its
subscribers, or as an enabler for an application that requires
direct IPv6 communication between client hosts. Servers are expected
to perform some amount of access control; for example, an ISP server
may refuse to serve requests that don't originate from an address
within this ISP's network; a server set up by an application
provider may require that clients provide some form of proof that
they are actually using the application.

Servers will originate IPv6 packets whose source address will be the
Teredo IPv6 address of one of their clients. In order to abide with
IPv6 ingress filtering rules, servers should only do so if, as an
IPv6 router, they advertise reachability of the Teredo service
prefix.

4.2.2   Relay deployment

The ingress filtering rule implies that all Teredo servers should be
able to act as Teredo relays; however, there is no requirement that
all Teredo relays act as Teredo servers. The only deployment
requirement for Teredo relays is IPv4 connectivity, and the capacity
to advertise a route to the Teredo service. There can be three kinds
of Teredo relays:

- Globally accessible Teredo relays announce reachability of the
Teredo service to the whole Internet, e.g. by means of BGP.

- Domain-specific Teredo relays announce reachability of the Teredo
service to a specific domain, e.g. by means of IGP.

- Host-specific Teredo relays announce reachability of the Teredo
service within a single host.

In the initial deployment of the Teredo service, we expect to find a
small number of globally accessible relays. We also expect that, if
the service is deployed to enable a specific application, all the
hosts that participate in the application and have adequate IPv4
access will implement a host-based Teredo relay.

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 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 Teredo IPv6 packets to the appropriate IPv4
address and UDP port.


Huitema                                                      [Page 20]


INTERNET DRAFT               Teredo                 September 17, 2002

The Teredo relay advertises reachability of the Teredo service
prefix over IPv6. It forwards Teredo IPv6 packets to the appropriate
IPv4 address and UDP port.

Teredo clients, servers and relays must implement the sunset
procedure defined in section 5.5.

5.1     Message formats

5.1.1   Teredo IPv6 packets 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 packets received from third parties, the server may
insert an origin indication in the first bytes of the UDP payload:

  +------+-----+-------------------+-------------+
  | 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

Huitema                                                      [Page 21]


INTERNET DRAFT               Teredo                 September 17, 2002

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 and Router Advertisement
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.

  +--------+--------+--------+--------+
  |  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 of
the client identifier; the fourth octet indicates the length 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.

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:




Huitema                                                      [Page 22]


INTERNET DRAFT               Teredo                 September 17, 2002

  +------+-----+----------------+--------+-------------+
  | 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].

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:

-       the IPv4 address of a server.

If secure discovery is required, the client must also be configured
with:

- a client identifier,
- a secret value, shared with the server.

A Teredo client expects to exchange IPv6 packets through an UDP
port, the Teredo service port. The client will maintain the
following variables that reflect the state of the Teredo service:

- Teredo connectivity status,
- Mapped address and port number associated with the Teredo service
port,
- Teredo IPv6 prefix associated with the Teredo service port,
- Teredo IPv6 address or addresses derived from the prefix,
- Random link local address,
- Date and time of the last interaction with the Teredo server,
- Teredo Refresh Interval,
- Randomized Refresh Interval,
- 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,

Huitema                                                      [Page 23]


INTERNET DRAFT               Teredo                 September 17, 2002

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:

- The IPv6 address of the peer,
- The mapped IPv4 address and mapped UDP port of the peer,
- The status of the mapped address, i.e. trusted or not,
- The value of the last "nonce" sent to the peer,
- The date and time of the last reception from the peer,
- The date and time of the last transmission to the peer,
- 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.

5.2.1   Qualification procedure

The purpose of the qualification procedure is 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 results in a
"qualified" state if successful, and in an "off-line" state if
unsuccessful.










Huitema                                                      [Page 24]


INTERNET DRAFT               Teredo                 September 17, 2002

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


Huitema                                                      [Page 25]


INTERNET DRAFT               Teredo                 September 17, 2002

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; 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 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.)

If the client has received an RA with the "Cone" bit 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 a 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 is detects a
restricted cone NAT: the client is qualified to use the service.

If the client is qualified, it builds a Teredo IPv6 address using

Huitema                                                      [Page 26]


INTERNET DRAFT               Teredo                 September 17, 2002

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 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. 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.

The authentication value is computed according to the HMAC
specification [RFC2104] using the following specifications:

-       the hash function shall be the MD5 function [RFC1321].
-       the secret value shall be the shared secret with which the client
was configured

The clear text to be protected includes:

-       the nonce value,
-       the confirmation byte,
-       the origin indication encapsulation, if it is present,
-       the IPv6 packet.

If the HMAC verification fails, the packet is silently discarded.

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 5.1.1, and that it contains either a valid
IPv6 packet as specified in [RFC2460], 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                                                      [Page 27]


INTERNET DRAFT               Teredo                 September 17, 2002

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 values are different, 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
should be accepted; the date and time of the last reception from the
peer should be 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
content 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 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 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. The client SHOULD create a new peer entry
for the IPv6 source address; the mapped IPv4 address and mapped port
in the entry should be set to the value from which the packet was

Huitema                                                      [Page 28]


INTERNET DRAFT               Teredo                 September 17, 2002

received, and the status should be set to "trusted". However,
clients MAY decide to only accept local discovery bubbles if the
mapped IPv4 address included in the IPv6 source address is the same
as the mapped IPv4 address of the client.

5) 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 600 seconds from
the 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 mapped IPv4 address and mapped UDP
port of 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 5.2.8. 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 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.

4) 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

Huitema                                                      [Page 29]


INTERNET DRAFT               Teredo                 September 17, 2002

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 3 and 4, 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 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.)

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 select
a new randomized link-local address and use this address to send a
router solicitation message to the server, as specified in 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.

When the router advertisement is received, the client SHOULD check
its validity as specified in 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.

When a Teredo client attempts to send a 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.

Bubbles may be lost in transit, and it is reasonable to enhance the
reliability of the Teredo service by allowing multiple

Huitema                                                      [Page 30]


INTERNET DRAFT               Teredo                 September 17, 2002

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:

- If the client implements the local discovery procedure it SHOULD
NOT send a bubble to a local peer;

- 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;

- 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.

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:

- Teredo secondary connectivity status,
- Mapped address and port number of the Teredo secondary port,
- Teredo secondary IPv6 prefix associated with the secondary port,
- Teredo secondary IPv6 address derived from this prefix,
- Date and time of the last interaction on the secondary port,
- Maximum Teredo Refresh Interval.
- 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

Huitema                                                      [Page 31]


INTERNET DRAFT               Teredo                 September 17, 2002

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.

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. If a spurious packet is received on
the secondary port, the client SHOULD repeat the maintenance
procedure on this port and reset the date and time of the last
interaction on the secondary port.

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 wait for
discovery bubbles to be received on the Teredo IPv4 Discovery
Address, and 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:

- IPv4 source address: the IPv4 address of the sender

- IPv4 destination address: the Teredo IPv4 Discovery Address

- IPv4 ttl: 1

Huitema                                                      [Page 32]


INTERNET DRAFT               Teredo                 September 17, 2002

- UDP source port: the Teredo service port of the sender

- UDP destination port: the Teredo UDP port

- UDP payload: a minimal IPv6 packet, as follows

- IPv6 source: the Teredo IPv6 address of the sender

- IPv6 destination: the all-nodes on-link multicast address

- IPv6 payload type: 59 (No Next Header, as per [RFC2460])

- IPv6 payload length: 0

- 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.

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

Huitema                                                      [Page 33]


INTERNET DRAFT               Teredo                 September 17, 2002

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 be
spoofed 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 it wants to send a packet to an IPv6 node on the IPv6 Internet,
the client 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. 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 bound to the local server IPv4 address, and to the Teredo
port. The rules of section 5.2.3 specify how the reception of this
packet will be processed.


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.

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, which are
processed according to the IPv6 specification.

5.3.1   Processing of Teredo IPv6 packets

Upon reception of a packet on the Teredo port, the Teredo server
will first check that the UDP payload contains a valid IPv6 packet;
if this is not the case, the packet will be silently discarded.

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 IPv6 packet, the packet
MUST be silently discarded.


Huitema                                                      [Page 34]


INTERNET DRAFT               Teredo                 September 17, 2002

2)      If the UDP packet is not a bubble or an ICMPv6 message, it should
be discarded.

3)      If the IPv4 source address is not in the format of a global
unicast address, the packet MUST be silently discarded.

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 SHOULD be accepted; it
MUST be discarded if the server requires secure qualification and
the authentication encapsulation is absent or cannot be verified.

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, 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:

- The destination IPv4 address is derived from the IPv6 destination.

- The source IPv4 address is the server's IPv4 address.

- The destination UDP port is derived from the IPv6 destination.


Huitema                                                      [Page 35]


INTERNET DRAFT               Teredo                 September 17, 2002

-       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.

5.3.2   Processing of router solicitations

When the Teredo server receives a Router Solicitation message (RS,
[RFC2641]), 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 [RFC2641]. 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. 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.

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 as received in
the router solicitation; the confirmation byte should 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.
Teredo relays will receive IPv6 packets bound to Teredo clients.
Teredo relays should be able to receive packets sent over IPv4 and
UDP by Teredo clients; they may apply filtering rules, e.g. only

Huitema                                                      [Page 36]


INTERNET DRAFT               Teredo                 September 17, 2002

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. However, Teredo relays do not have to
perform the qualification procedure.

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 Server 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.

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 client updates the date of last transmission
in the peer entry.

2) 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 the destination is a Teredo IPv6 address in which the cone bit
is set to 0, the packet is queued. 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 non-null server
address corresponding to the Teredo destination. The packet will be
de-queued and forwarded when a bubble or another packet will be
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.

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), 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

Huitema                                                      [Page 37]


INTERNET DRAFT               Teredo                 September 17, 2002

bubble from the Teredo client.

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, 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 is the "all nodes multicast address", the packet should
be processed locally. 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
5.3.1, and the relay processing rules defined in 5.4.2.

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

Huitema                                                      [Page 38]


INTERNET DRAFT               Teredo                 September 17, 2002

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, perhaps until
at most 2006.

Teredo relays are expected to have the same life span as Teredo
servers.

6       Discussion of the solution

This section is an attempt at answering various questions about the
design choices.

6.1     Why do we require address obfuscation?

The Teredo address, as specified in section 4.1.1, include an
obfuscated copy of the mapped IPv4 address and UDP port of the
client. This is done to prevent abusive NAT "smartness." We have
experimental evidence that some NATs, probably in a desire to help
applications operate more transparently across NATs, are programmed
to look for occurrence of a 32-bit value that matches their local
address, and to replace any such value by the local IP address
allocated to the client; some may also attempt to translate the port
values; this treatment is performed by some NATs even if they don't
know the details of the application protocol. By obfuscating the
address and port, we prevent the NAT from recognizing their own IPv4
address in the UDP packets exchanged between client and server, and
avoid the errors caused by a possible rewriting.

6.2     Why do we have bubbles and lists of peers?

Our algorithm is designed to provide robustness: the client will
always wait for a successful bubble or packet reception before
transmitting data packets over UDP. This ensures that data packets
will always be transmitted on a direct path to another Teredo

Huitema                                                      [Page 39]


INTERNET DRAFT               Teredo                 September 17, 2002

client, or on a direct path to the Teredo relay nearest from an IPv6
peer.

6.3     Why do servers only process bubbles and ICMPv6 messages?

In this specification, Teredo servers are requested to only forward
some minimal packets: initial bubbles between Teredo clients, ICMPv6
messages between Teredo clients and IPv6 peers. This has two
advantages: it greatly reduces the transmission load of servers, and
it also helps in solving some the security issues.

Restricting the traffic to a few bubbles means that the server will
only have to carry a few hundred bits of data for any exchange
between clients and peers. Previous designs allowed transmission of
data through the server, which placed the server at risk: a lazily
programmed client could skip sending bubbles, and send all its
traffic through the server.

Restricting the server to only carry bubbles and ICMPv6 packets
removes a privacy risk: if servers were allowed to carry data, a
client could be convinced to send all its data through a rogue
server, where it could easily be observed. With our design, a server
does not see any actual data, and thus poses a much reduced privacy
risk to its clients.

Restricting the type of packets that a server can relay also reduces
another security risk, the use of the server as a reflection point
in a denial of service attack. An attacker can induce a server to
reflect packets towards a third party, but the structure of these
packets is very limited, which prevents the use of the server in a
"magic packet" attack.

6.4     What if two clients are behind the same NAT?

Our design choice implies some restrictions in the Teredo service.
The first restriction concerns two clients connected to the Internet
through the same NAT.
















Huitema                                                      [Page 40]


INTERNET DRAFT               Teredo                 September 17, 2002

                                          .-----------------------
                                          |  Private network
                .--.   src=9.0.0.1:4096 .-----.     .----------.
               (IPv4)  src=9.0.0.1:4097;| NAT |     | Teredo   |
             (Internet)<--------------  | BOX | <-- | Client-1 |
               (    )   (UDP tunneled   |     | <.  '----------'
                '--'         IPv6)      '-----'  |    10.0.0.2:1234
                 |                9.0.0.1 |      |  .----------.
                 |                        |      |  | Teredo   |
                 |                        |      '- | Client-2 |
                 V                        |         '----------'
          .--------------.                |           10.0.0.3:1234
          |   Teredo     |                '-----------------------
          |   Server     |
          '--------------'

The first client uses the private address and port 10.0.0.2:1234,
which is mapped to 9.0.0.1:4096 by the NAT; the second client uses
10.0.0.3:1234, mapped to 9.0.0.1:4097. If the first client tries to
send a direct packet to the second client, that packet will be
routed to the address 9.0.0.1:4097, i.e. to the external IP address
of the local NAT. There is no guarantee that the NAT will be able to
correctly process these packets; there is indeed some possibility
that they may be lost, and that the two clients will not be able to
communicate using Teredo.

We deliberately accept this restriction, as the alternative would be
to relay the traffic between the two internal clients through the
Teredo server. Since the clients are located behind the same NAT, in
the same domain, there is a risk that the clients might exchange
sensitive data without necessarily using proper protection; sending
this data over the Internet to the Teredo server would expose the
clients to a significant risk of information disclosure.

6.5     What about symmetric NAT?

The exchange of bubbles will fail if one of the Teredo clients is
located behind a symmetric NAT; in practice, this means that clients
located behind a symmetric NAT should not use Teredo.

6.6     Do we need the Refresh Interval Determination Procedure?

When the client is initialized, the Teredo Refresh Interval is set
to 30 seconds. This value is lower than the minimum interval found
necessary in a measurement campaign conducted by a Microsoft team in
January 2001; the measured values ranged between 45 seconds and more
than 15 minutes. There is always a risk that some NAT manufacturers
program some ever smaller time to live intervals for their mappings,
but doing so would break many applications and would probably
generate an uproar from Internet users. By picking a conservatively
small value, we guarantee that the service will work with most NATs.

Huitema                                                      [Page 41]


INTERNET DRAFT               Teredo                 September 17, 2002

On the other hand, picking a conservative value increases the
maintenance traffic and the load on the Teredo servers. We know that
in many cases interval as large as 5 or 10 minutes would be
adequate; however, we also know that there is a high risk of false
positives, e.g. when a NAT is connected by an ISDN "on demand" link.
The determination procedure is designed to quickly find whether a
value larger than 30 seconds is adequate, while not trying to
achieve a value larger than 2 minutes. The parameters have been
chosen for rapid convergence, i.e. at most 3 iterations between the
initial value of 30 seconds and the maximum value of 120 seconds or
2 minutes.

6.7     Why do we use a Randomized Refresh Interval?

We specify in the maintenance procedure that the interval between
successive refresh must be a random value chosen between 75% and
100% of the Teredo Refresh Interval. This randomization procedure is
meant to avoid the possible risk of synchronization that is inherent
to any periodic refresh mechanism; if synchronization occurred, all
Teredo clients would send their router solicitation messages quasi
simultaneously to the Teredo server, which would overwhelm the
server. A synchronization phenomenon caused by periodic messages is
studied in [SYNCHRO]; the 75%-100% interval is meant to meet the
guidelines developed in this reference publication.

6.8     Scaling, failover and access control

The Teredo service is designed to impose minimal requirements on
servers and relays: capability to send packets over IPv4 using a
regular IPv4 address; capability to send and receive packets over
IPv6; capability to advertise reachability of the Teredo service
prefix in at least some limited scope. These minimal requirements
make it easy to deploy a large number of servers and relays, thus
ensuring scalability of the service.

Teredo clients may obtain a more resilient service if they can use
several different servers. Teredo clients will detect that a server
is failing through the failure of the qualification procedure; they
may try at that point to obtain services from a different server.

6.9     What about firewalls?

The Teredo service is not designed to "transparently traverse
firewalls." A local administrator can decide to allow or disallow
the service, by programming the local firewall to authorize or deny
traffic on the Teredo UDP port.

Implementations of Teredo should include an administrative control
that explicitly enables use of the Teredo service; the service
should not start if not explicitly authorized.


Huitema                                                      [Page 42]


INTERNET DRAFT               Teredo                 September 17, 2002

Implementations of Teredo should be configured to shut down the
Teredo service when the Teredo client is connected within a "managed
network", such as an enterprise network. For example, the
implementation of Teredo in Microsoft Windows is configured to shut
down the Teredo service if the client is a member of a Windows
domain.

6.10    Why do we use the name Teredo?

"Teredo navalis" is the Latin name for a little saltwater critter
that is common in the harbors of warm seas and that digs worm holes
in immersed wood pieces, such as boat hulls or pilings. The animal
is not an actual worm - it is a mollusk. The Teredo service also
digs holes, albeit in NATs, not in wood.

On one hand, one may think that the Teredo is a pretty nasty animal.
On the other hand, the animal only survives in relatively clean and
unpolluted water; its recent comeback in several North American
harbors is a testimony to their newly retrieved cleanliness. The
Teredo service should, in turn, contribute to a newly retrieved
transparency of the Internet.

7       Use of Teredo to implement a tunnel service

It may be desirable in some cases to deploy stateful tunnel servers
instead of the stateless Teredo servers. Tunnels servers generally
require more resources, but an advantage is that they can
potentially provide the users with "permanent" IPv6 addresses.

It is 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 can be
done by configuring the client with:

-       The IPv4 address of a Teredo server that acts as a tunnel broker
-       A client identifier
-       A shared secret with that server.

The Teredo client will use the secure qualification procedure, as
specified in section 5.2.2. Instead of returning a Teredo prefix in
the router advertisement, the server will return a globally routable
IPv6 prefix; this prefix may be permanently assigned to the client,
which would provide the client with a stable address. The server
will 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 will advertise reachability of the client prefix
to the IPv6 Internet. Any packet bound to that prefix will be
transmitted to the mapped IPv4 address and mapped UDP port of the
client.


Huitema                                                      [Page 43]


INTERNET DRAFT               Teredo                 September 17, 2002

The Teredo client, when it receives the prefix, will notice that
this prefix is a global IPv6 prefix, not in the form of a Teredo
prefix. The client will at that point recognize that it should
operate in tunnel mode. A client that operates in tunnel mode will
execute a much simpler transmission procedure: it will forward any
packet sent to the Teredo interface to the IPv4 address and Teredo
UDP port of the server.

The Teredo client will have to perform the maintenance procedure
described in section 5.2.5. The server will receive the router
solicitation, and may 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
update the mapped IPv4 address and mapped UDP port associated to
this prefix, if necessary.

8       Security Considerations

The main objective of Teredo is to provide nodes located behind a
NAT with a globally routable IPv6 address. This enables such nodes
to use IP security services such as IKE, AH or ESP. 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.

8.1     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; all services declared locally will become 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 one of the limited address
scopes defined in IPv6, e.g. link-local or site-local. There is no
support for such scopes in Teredo, which implies that limited-scope
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.

The second mitigating factor is the possible use of a "local

Huitema                                                      [Page 44]


INTERNET DRAFT               Teredo                 September 17, 2002

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, already noted, is the availability of
end-to-end connectivity, which allows for deployment of IP security
services such as IKE, AH or ESP. 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.

8.2     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.

The secure qualification procedure described in section 5.2.2
enables a good protection against attacks in which a third party
tries to spoof the server. 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 be provisioned by an automatic procedure and contain
sufficient entropy.

Another way to defeat the protection afforded by the signature
procedure is to mount a complex attack, as follows:

1) Client prepares router solicitation, including authentication
header.

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.

Huitema                                                      [Page 45]


INTERNET DRAFT               Teredo                 September 17, 2002

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
signature scheme, since the attacker does not do anything else than
what the NAT legally does!

There are however several mitigating factors that lead us to avoid
worrying too much about this attack. In practice, the gain from the
attack is to either deny service to the client, or 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 is minimal.

8.2.1   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
and ultimately enhances the security of these clients.

8.3     Denial of the Teredo service

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

8.3.1   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

Huitema                                                      [Page 46]


INTERNET DRAFT               Teredo                 September 17, 2002

eliminate spurious route advertisements. Dual stack nodes that
implement a "host local" Teredo relays are impervious to this
attack.

8.3.1   Denial of service by server spoofing

In section 8.2, 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 to 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.

8.3.2   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 only last as long as the attack is sustained.

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

8.3.4   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

Huitema                                                      [Page 47]


INTERNET DRAFT               Teredo                 September 17, 2002

announcing the reachability of the Teredo service prefix to the IPv6
network: the traffic will be picked up by the next relay.

8.4     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.

8.4.1   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 determining the source of
the attack.

8.4.2   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.


Huitema                                                      [Page 48]


INTERNET DRAFT               Teredo                 September 17, 2002

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; 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.

The ICMP Traceback (ITRACE) working group is considering systems for
"tracing" the source of DOS attacks. According to the proposal, when
forwarding packets, routers can, with a low probability, generate a
Traceback message that is sent along to the destination; with enough
Traceback messages from enough routers along the path, the traffic
source and path can be determined. This set up assumes that the
source and destination are both using the same version of IP. In the
Teredo case, the ICMP Traceback packets will be sent to the Teredo
server, not the final destination. It is conceivable to "map" the
IPv4 traceback to an IPv6 traceback sent by the Teredo server; the
details of the solution should be specified by the ITRACE working
group.

8.4.3   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 null server address, the IPv4 address of the
target, an arbitrary UDP port, and an arbitrary node identifier. The
attacker will send IPv6 packets to that address; the packets will be
routed to the nearest Teredo relay, and forwarded from there to the
target.

The address checks specified in 5.4 are limited to verifying that
packets are only relayed to a global IPv4 address. This rules out a
class of attack in which the packets are bound to a broadcast or
multicast address. It also rules out another class of attack in
which the packets are bound for a private IPv4 address that would be
reachable by the relay.

The attack 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

Huitema                                                      [Page 49]


INTERNET DRAFT               Teredo                 September 17, 2002

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 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 should be specified by the
ITRACE working group.

9       IANA Considerations

This memo documents a request to IANA to allocate a Teredo IPv6
service prefix.

10      Copyright

The following copyright notice is copied from RFC 2026 [Bradner,
1996], Section 10.4, and describes the applicable copyright for this
document.

Copyright (C) The Internet Society September 17, 2002. All Rights
Reserved.

This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph
are included on all such copies and derivative works.  However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.

The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assignees.


Huitema                                                      [Page 50]


INTERNET DRAFT               Teredo                 September 17, 2002

This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS 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.

11      Intellectual Property

The following notice is copied from RFC 2026 [Bradner, 1996],
Section 10.4, and describes the position of the IETF concerning
intellectual property claims made against this document.

The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use other technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights.  Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11.  Copies of
claims of rights made available for publication and any assurances
of licenses to be made available, or the result of an attempt made
to obtain a general license or permission for the use of such
proprietary rights by implementers or users of this specification
can be obtained from the IETF Secretariat.

The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard.  Please address the information to the IETF Executive
Director.

12      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 early 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 the NGTRANS working group; Brian Carpenter, Cyndi Jung,
Keith Moore, Thomas Narten, Anssi Porttikivi, Pekka Savola, and Eng
Soo Guan.

13      References

[RFC768] J. Postel, "User Datagram Protocol", RFC 768, August 1980.

[RFC791] J. Postel, "Internet Protocol", RFC 791, September 1981.


Huitema                                                      [Page 51]


INTERNET DRAFT               Teredo                 September 17, 2002

[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992.

[RFC1918] Y. Rekhter, B. Moskowitz, D. Karrenberg, G. J. de Groot,
E. Lear, "Address Allocation for Private Internets", RFC 1918,
February 1996.

[RFC2119] S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.

[RFC2460] S. Deering, R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.

[RFC2461] T. Narten, E. Nordmark, W. Simpson, "Neighbor Discovery
for IP Version 6 (IPv6)", RFC 2461, December 1998.

[RFC2462] T. Narten, S. Thomson, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.

[RFC3056] B. Carpenter, K. Moore, "Connection of IPv6 Domains via
IPv4 Clouds", RFC 3056, February 2001.

[RFC3068] C. Huitema, "An Anycast Prefix for 6to4 Relay Routers",
RFC 3068, June 2001.

[RFC1750] D. Eastlake, S. Crocker, J. Schiller, "Randomness
Recommendations for Security", RFC 1750, December 1994.

[SYNCHRO] S. Floyd, V. Jacobson, "The synchronization of periodic
routing messages", ACM SIGCOMM'93 Symposium, September 1993.

[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.

14      Authors' Addresses

Christian Huitema
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052-6399

Email: huitema@microsoft.com










Huitema                                                      [Page 52]


INTERNET DRAFT               Teredo                 September 17, 2002

Table of Contents:

1 Introduction ....................................................   1
2 Definitions .....................................................   2
2.1 Teredo service ................................................   2
2.2 Teredo Client .................................................   2
2.3 Teredo Server .................................................   2
2.4 Teredo Relay ..................................................   3
2.5 Teredo IPv6 service prefix ....................................   3
2.5.1 Global Teredo IPv6 service prefix ...........................   3
2.6 Teredo UDP port ...............................................   3
2.7 Teredo bubble .................................................   3
2.8 Teredo service port ...........................................   3
2.9 Teredo server address .........................................   3
2.10 Teredo mapped address and Teredo mapped port .................   3
2.11 Teredo IPv6 client prefix ....................................   3
2.12 Teredo node identifier .......................................   4
2.13 Teredo IPv6 address ..........................................   4
2.14 Teredo Refresh Interval ......................................   4
2.15 Teredo secondary port ........................................   4
2.16 Teredo IPv4 Discovery Address ................................   4
3 Design goals, requirements, and model of operation ..............   4
3.1 Hypotheses about NAT behavior .................................   4
3.1.1 Types of UDP mappings .......................................   5
3.1.2 Lifetime of UDP mappings ....................................   5
3.2 IPv6 provider of last resort ..................................   6
3.2.1 When to use Teredo? .........................................   6
3.2.2 Autonomous deployment .......................................   6
3.2.3 Minimal load on servers .....................................   7
3.2.4 Automatic sunset ............................................   7
3.3 Operational Requirements ......................................   7
3.3.1 Robustness requirement ......................................   7
3.3.2 Minimal support cost ........................................   7
3.3.3 Protection against denial of service attacks ................   8
3.3.4 Protection against distributed denial of service attacks ....   8
3.3.5 Compatibility with ingress filtering ........................   8
4 Model of operation and deployment ...............................   8
4.1 Model of operation ............................................   8
4.1.1 Encoding of Teredo addresses ................................   9
4.1.2 Obtaining an address ........................................  10
4.1.3 Determining the type of NAT .................................  12
4.1.4 First packet from an IPv6 node to a Teredo node .............  13
4.1.5 First packet from a Teredo node to a regular IPv6 node ......  14
4.1.6 Exchanges between two Teredo nodes ..........................  17
4.1.7 Exchanges between two Teredo nodes on the same link .........  18
4.2 Deployment model ..............................................  19
4.2.1 Server deployment ...........................................  19
4.2.2 Relay deployment ............................................  20
5 Specification of clients, servers and relays ....................  20
5.1 Message formats ...............................................  21
5.1.1 Teredo IPv6 packets encapsulation ...........................  21

Huitema                                                      [Page 53]


INTERNET DRAFT               Teredo                 September 17, 2002

5.1.2 Maximum Transmission Unit ...................................  23
5.2 Teredo Client specification ...................................  23
5.2.1 Qualification procedure .....................................  24
5.2.2 Secure qualification ........................................  27
5.2.3 Packet reception ............................................  27
5.2.4 Packet transmission .........................................  29
5.2.5 Maintenance .................................................  30
5.2.6 Sending Teredo Bubbles ......................................  30
5.2.7 Optional Refresh Interval Determination Procedure ...........  31
5.2.8 Optional local client discovery procedure ...................  32
5.2.9 Direct IPv6 connectivity test ...............................  33
5.3 Teredo Server specification ...................................  34
5.3.1 Processing of Teredo IPv6 packets ...........................  34
5.3.2 Processing of router solicitations ..........................  36
5.4 Teredo Relay specification ....................................  36
5.4.1 Transmission by relays to Teredo clients ....................  37
5.4.2 Reception from Teredo clients ...............................  38
5.4.3 Difference between Teredo Relays and Teredo Servers .........  38
5.5 Implementation of automatic sunset ............................  38
6 Discussion of the solution ......................................  39
6.1 Why do we require address obfuscation? ........................  39
6.2 Why do we have bubbles and lists of peers? ....................  39
6.3 Why do servers only process bubbles and ICMPv6 messages? ......  40
6.4 What if two clients are behind the same NAT? ..................  40
6.5 What about symmetric NAT? .....................................  41
6.6 Do we need the Refresh Interval Determination Procedure? ......  41
6.7 Why do we use a Randomized Refresh Interval? ..................  42
6.8 Scaling, failover and access control ..........................  42
6.9 What about firewalls? .........................................  42
6.10 Why do we use the name Teredo? ...............................  43
7 Use of Teredo to implement a tunnel service .....................  43
8 Security Considerations .........................................  44
8.1 Opening a hole in the NAT .....................................  44
8.2 Using the Teredo service for a man-in-the-middle attack .......  45
8.2.1 End-to-end security .........................................  46
8.3 Denial of the Teredo service ..................................  46
8.3.1 Denial of service by a rogue relay ..........................  46
8.3.1 Denial of service by server spoofing ........................  47
8.3.2 Denial of service by exceeding the number of peers ..........  47
8.3.3 Attacks against the local discovery procedure ...............  47
8.3.4 Attacking the Teredo servers and relays .....................  47
8.4 Denial of service against non-Teredo nodes ....................  48
8.4.1 Laundering DOS attacks from IPv4 to IPv4 ....................  48
8.4.2 DOS attacks from IPv4 to IPv6 ...............................  48
8.4.3 DOS attacks from IPv6 to IPv4 ...............................  49
9 IANA Considerations .............................................  50
10 Copyright ......................................................  50
11 Intellectual Property ..........................................  51
12 Acknowledgements ...............................................  51
13 References .....................................................  51
14 Authors' Addresses .............................................  52


Huitema                                                      [Page 54]


Html markup produced by rfcmarkup 1.129d, available from https://tools.ietf.org/tools/rfcmarkup/