science

Teredo tunneling

Teredo tunneling is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Teredo tunneling rather than just read about it. In short: In computer networking, Teredo is a Microsoft transition technology that gives full IPv6 connectivity for IPv6-capable hosts that are on the IPv4 Internet but have no native connection to an IPv6 network. Unlike similar protocols such as 6to4, it can perform its function even from behind network address translation (NAT) devices such as home routers.

Key takeaways

Teredo tunneling belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Teredo tunneling to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Teredo tunneling from memory before moving on to harder problems.

Reference excerpt

In computer networking, Teredo is a Microsoft transition technology that gives full IPv6 connectivity for IPv6-capable hosts that are on the IPv4 Internet but have no native connection to an IPv6 network. Unlike similar protocols such as 6to4, it can perform its function even from behind network address translation (NAT) devices such as home routers. Teredo operates using a platform independent tunneling protocol that provides IPv6 (Internet Protocol version 6) connectivity by encapsulating IPv6 datagram packets within IPv4 User Datagram Protocol (UDP) packets. Teredo routes these datagrams on the IPv4 Internet and through NAT devices. Teredo nodes elsewhere on the IPv6 network (called Teredo relays) receive the packets, un-encapsulate them, and pass them on. Teredo is a temporary measure. In the long term, all IPv6 hosts should use native IPv6 connectivity. Teredo should be disabled when native IPv6 connectivity becomes available. Christian Huitema developed Teredo at Microsoft, and the IETF standardized it as RFC 4380. The Teredo server listens on UDP port 3544.

Purpose

For 6to4, the most common IPv6 over IPv4 tunneling protocol, requires that the tunnel endpoint have a public IPv4 address. However, many hosts currently attach to the IPv4 Internet through one or several NAT devices, usually because of IPv4 address shortage. In such a situation, the only available public IPv4 address is assigned to the NAT device, and the 6to4 tunnel endpoint must be implemented on the NAT device itself. The problem is that many NAT devices currently deployed cannot be upgraded to implement 6to4, for technical or economic reasons. Teredo alleviates this problem by encapsulating IPv6 packets within UDP/IPv4 datagrams, which most NATs can forward properly. Thus, IPv6-aware hosts behind NATs can serve as Teredo tunnel endpoints even when they don't have a dedicated public IPv4 address. In effect, a host that implements Teredo can gain IPv6 connectivity with no cooperation from the local network environment. In the long term, all IPv6 hosts should use native IPv6 connectivity. The temporary Teredo protocol includes provisions for a sunset procedure: Teredo implementation should provide a way to stop using Teredo connectivity when IPv6 matures and connectivity becomes available using a less brittle mechanism. As of IETF89, Microsoft plans to deactivate their Teredo servers for Windows clients in the first half of 2014 (exact date TBD), and encourage the deactivation of publicly operated Teredo relays.

Overview The Teredo protocol performs several functions:

Diagnoses UDP over IPv4 (UDPv4) connectivity and discovers the kind of NAT present (using a simplified replacement to the STUN protocol) Assigns a globally routable unique IPv6 address to each host using it Encapsulates IPv6 packets inside UDPv4 datagrams for transmission over an IPv4 network (this includes NAT traversal) Routes traffic between Teredo hosts and native (or otherwise non-Teredo) IPv6 hosts

Node types Teredo defines several different kinds of nodes:

Teredo client A host that has IPv4 connectivity to the Internet from behind a NAT and uses the Teredo tunneling protocol to access the IPv6 Internet. Teredo clients are assigned an IPv6 address that starts with the Teredo prefix (2001::/32). Teredo server A well-known host used for initial configuration of a Teredo tunnel. A Teredo server never forwards any traffic for the client (apart from IPv6 pings), and has therefore modest bandwidth requirements (a few hundred bits per second per client at most), which means a single server can support many clients. Additionally, a Teredo server can be implemented in a fully stateless manner, thus using the same amount of memory regardless of how many clients it supports. Teredo relay The remote end of a Teredo tunnel. A Teredo relay must forward all of the data on behalf of the Teredo clients it serves, with the exception of direct Teredo client to Teredo client exchanges. Therefore, a relay requires a lot of bandwidth and can only support a limited number of simultaneous clients. Each Teredo relay serves a range of IPv6 hosts (e.g. a single campus or company, an ISP or a whole operator network, or even the whole IPv6 Internet); it forwards traffic between any Teredo clients and any host within said range. Teredo host-specific relay A Teredo relay whose range of service is limited to the very host it runs on. As such, it has no particular bandwidth or routing requirements. A computer with a host-specific relay uses Teredo to communicate with Teredo clients, but sticks to its main IPv6 connectivity provider to reach the rest of the IPv6 Internet.

IPv6 addressing Each Teredo client is assigned a public IPv6 address, which is constructed as follows (the higher order bit is numbered 0):

Bits 0 to 31 hold the Teredo prefix (2001::/32). Bits 32 to 63 embed the primary IPv4 address of the Teredo server that is used. Bits 64 to 79 hold some flags and other bits; the format for these 16 bits, MSB first, is "CRAAAAUG AAAAAAAA". The "C" bit was set to 1 if the Teredo client is located behind a cone NAT, 0 otherwise, but RFC 5991 changed it to always be 0 to avoid revealing this fact to strangers. The "R" bit is currently unassigned and should be sent as 0. The "U" and "G" bits are set to 0 to emulate the "Universal/local" and "Group/individual" bits in MAC addresses. The 12 "A" bits were 0 in the original RFC 4380 specification, but were changed to random bits chosen by the Teredo client in RFC 5991 to provide the Teredo node with additional protection against IPv6-based scanning attacks. Bits 80 to 95 contain the obfuscated UDP port number. This is the port number that the NAT maps to the Teredo client, with all bits inverted. Bits 96 to 127 contain the obfuscated IPv4 address. This is the public IPv4 address of the NAT with all bits inverted.

As an example, the IPv6 address 2001:0000:4136:e378:8000:63bf:3fff:fdd2 refers to a Teredo client that:

Uses Teredo server at address 65.54.227.120 (4136e378 in hexadecimal) Is behind a cone NAT and client is not fully compliant with RFC 5991 (bit 64 is set) Is probably (99.98%) not compliant with RFC 5991 (the 12 random bits are all 0, which happens less than 0.025% of the time) Uses UDP mapped port 40000 on its NAT (in hexadecimal not 63bf equals 9c40, or decimal number 40000) Has a NAT public IPv4 address of 192.0.2.45 (not 3ffffdd2 equals c000022d, which is to say, 192.0.2.45)

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with Teredo tunneling

Start with the simplest possible case. Write down what Teredo tunneling claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Teredo tunneling before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Teredo tunneling ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Teredo tunneling

In research: Teredo tunneling appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Teredo tunneling in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Teredo tunneling is common in secondary-school and first-year university syllabi. It links to neighbouring topics IPv6 transition technologies, Internet architecture, Tunneling protocols, so understanding it makes those chapters shorter.
In everyday life: Look for Teredo tunneling outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study Teredo tunneling in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Teredo tunneling means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Teredo tunneling out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Teredo tunneling in simple terms?

Why does Teredo tunneling matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Teredo tunneling?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Teredo tunneling.