computer science

ICMP hole punching

ICMP hole punching is a computer 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 ICMP hole punching rather than just read about it. In short: ICMP hole punching is a technique employed in network address translator (NAT) applications for maintaining Internet Control Message Protocol (ICMP) packet streams that traverse the NAT. NAT traversal techniques are typically required for client-to-client networking applications on the Internet involving hosts connected in private networks, especially in peer-to-peer and Voice over Internet Protocol (VoIP) deploymen…

Key takeaways

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

Reference excerpt

ICMP hole punching is a technique employed in network address translator (NAT) applications for maintaining Internet Control Message Protocol (ICMP) packet streams that traverse the NAT. NAT traversal techniques are typically required for client-to-client networking applications on the Internet involving hosts connected in private networks, especially in peer-to-peer and Voice over Internet Protocol (VoIP) deployments. ICMP hole punching establishes connectivity between two hosts communicating across one or more network address translators in either a peer-to-peer or client–server model. Typically, third party hosts on the public transit network are used to establish UDP or TCP port states that may be used for direct communications between the communicating hosts, however ICMP hole punching requires no third party involvement to pass information between one or more NATs by exploiting a NAT's loose acceptance of inbound ICMP Time Exceeded packets. Once an ICMP Time Exceeded packet reaches the destination NAT, arbitrary data in the packet expected by the NAT allows the packet to reach the destination server, allowing the destination server to obtain the client's public IP address and other data stored in the packet from the client.

Overview Currently the only method of ICMP hole punching or hole punching without third party involvement (autonomous NAT traversal) was developed by Samy Kamkar on January 22, 2010 and released in the open source software pwnat, and the method was later published in the IEEE. According to the paper:

The proposed technique assumes that the client has somehow learned the current external (globally routable) IP address of the server's NAT. The key idea for enabling the server to learn the client's IP address is for the server to periodically send a message to a fixed, known IP address. The simplest approach uses ICMP ECHO REQUEST messages to an unallocated IP address, such as 1.2.3.4. Since 1.2.3.4 is not allocated, the ICMP REQUEST will not be routed by routers without a default route; ICMP DESTINATION UNREACHABLE messages that may be created by those routers can just be ignored by the server. As a result of the messages sent to 1.2.3.4, the NAT will enable routing of replies in response to this request. The connecting client will then fake such a reply. Specifically, the client will transmit an ICMP message indicating TTL_EXPIRED. Such a message could legitimately be transmitted by any Internet router and the sender address would not be expected to match the server's target IP. The server listens for (fake) ICMP replies and upon receipt initiates a connection to the sender IP specified in the ICMP reply.

See also Hole punching (networking) Port Control Protocol (PCP) TCP hole punching UDP hole punching

References

Illustrations

Worked examples

Example 1 — a first encounter with ICMP hole punching

Start with the simplest possible case. Write down what ICMP hole punching claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer 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 ICMP hole punching 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 ICMP hole punching 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 ICMP hole punching

In research: ICMP hole punching appears in computer 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 ICMP hole punching 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: ICMP hole punching is common in secondary-school and first-year university syllabi. It links to neighbouring topics Computer network security, so understanding it makes those chapters shorter.
In everyday life: Look for ICMP hole punching 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.

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How to study ICMP hole punching in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what ICMP hole punching 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 ICMP hole punching out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is ICMP hole punching in simple terms?

Why does ICMP hole punching matter?

Because it connects several computer 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 ICMP hole punching?

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 ICMP hole punching.