computer science

Source routing

Source routing 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 Source routing rather than just read about it. In short: In computer networking, source routing, also called path addressing, allows a sender of a data packet to partially or completely specify the route the packet takes through the network. In contrast, in conventional routing, routers in the network determine the path incrementally based on the packet's destination.

Key takeaways

Source routing 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 Source routing to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Source routing from memory before moving on to harder problems.

Reference excerpt

In computer networking, source routing, also called path addressing, allows a sender of a data packet to partially or completely specify the route the packet takes through the network. In contrast, in conventional routing, routers in the network determine the path incrementally based on the packet's destination. Another routing alternative, label switching, is used in connection-oriented networks such as X.25, Frame Relay, Asynchronous Transfer Mode and Multiprotocol Label Switching. Source routing allows easier troubleshooting, improved traceroute, and enables a node to discover all the possible routes to a host. It does not allow a source to directly manage network performance by forcing packets to travel over one path to prevent congestion on another. Many high-performance interconnects including Myrinet, Quadrics, IEEE 1355, and SpaceWire support source routing.

Internet Protocol In the Internet Protocol, two header options are available which are rarely used: "strict source and record route" (SSRR) and "loose source and record route" (LSRR). Because of security concerns, packets marked LSRR are frequently blocked on the Internet. If not blocked, LSRR can allow an attacker to spoof an address but still successfully receive response packets by forcing return traffic for spoofed packets to return through the attacker's device. In IPv6, two forms of source routing have been developed. The first approach was the Type 0 Routing header. This routing header was designed to support the same use cases as the IPv4 header options. As there were several significant attacks against this routing header, its utilisation was deprecated. A more secure form of source routing was being developed within the IETF as of 2017 to support the IPv6 version of segment routing.

Software-defined networking Software-defined networking can also be enhanced when source routing is used in the forwarding plane. Studies have shown significant improvements in convergence times as a result of the reduced state that must be distributed by the controller into the network.

Myrinet When using source routing with Myrinet, the sender of the packet prepends the complete route, one byte for every crossbar, to each packet header. Each crossbar examines the first routing byte of the packet. When using source routing, that byte indicates a particular port of that crossbar; when that port becomes available, the router discards that routing byte and sends the rest of the packet out that port.

SpaceWire Each packet traveling through a SpaceWire network can use path addressing (source routing) or logical addressing or some combination. The router examines the first data character of the packet; when it indicates some specific port of the router, the router discards that routing character and sends the rest of the packet out that port.

See also Bang path Dynamic Source Routing Policy-based routing can also be used to route packets using their source addresses. Scalable Source Routing

References

External links "Source Routing". IBM Internet Security Systems. Archived from the original on February 24, 2008. Source Routing Not Considered Harmful

Worked examples

Example 1 — a first encounter with Source routing

Start with the simplest possible case. Write down what Source routing 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 Source routing 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 Source routing 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 Source routing

In research: Source routing 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 Source routing 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: Source routing is common in secondary-school and first-year university syllabi. It links to neighbouring topics Routing algorithms, so understanding it makes those chapters shorter.
In everyday life: Look for Source routing 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 Source routing in 20 minutes

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

Frequently asked questions

What is Source routing in simple terms?

Why does Source routing 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 Source routing?

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 Source routing.