science

Robust Header Compression

Robust Header Compression 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 Robust Header Compression rather than just read about it. In short: Robust Header Compression (ROHC) is a standardized method to compress the IP, UDP, UDP-Lite, RTP, and TCP headers of Internet packets. The need for header compression In streaming applications, the overhead of IP, UDP, and RTP is 40 bytes for IPv4, or 60 bytes for IPv6.

Key takeaways

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

Reference excerpt

Robust Header Compression (ROHC) is a standardized method to compress the IP, UDP, UDP-Lite, RTP, and TCP headers of Internet packets.

The need for header compression In streaming applications, the overhead of IP, UDP, and RTP is 40 bytes for IPv4, or 60 bytes for IPv6. For VoIP, this corresponds to around 60% of the total amount of data sent. Such large overheads may be tolerable in local wired links where capacity is often not an issue, but are excessive for wide area networks and wireless systems where bandwidth is scarce. ROHC compresses these 40 bytes or 60 bytes of overhead typically into only one or three bytes, by placing a compressor before the link that has limited capacity, and a decompressor after that link. The compressor converts the large overhead to only a few bytes, while the decompressor does the opposite. The ROHC compression scheme differs from other compression schemes, such as IETF RFC 1144 and RFC 2508, by the fact that it performs well over links where the packet loss rate is high, such as wireless links.

Main ROHC compression principles The ROHC protocol takes advantage of information redundancy in the headers of the following:

one single network packet (e.g. the payload lengths in IP and UDP headers) several network packets that belong to one single stream (e.g. the IP addresses) Redundant information is transmitted in the first packets only. The next packets contain variable information, e.g. identifiers or sequence numbers. These fields are transmitted in a compressed form to save more bits. For better performance, the packets are classified into streams before being compressed. This classification takes advantage of inter-packet redundancy. The classification algorithm is not defined by the ROHC protocol itself but left to the equipment vendor's implementation. Once a stream of packets is classified, it is compressed according to the compression profile that fits best. A compression profile defines the way to compress the different fields in the network headers. Several compression profiles are available, including the following:

Uncompressed IP-only UDP/IP UDP-Lite/IP ESP/IP RTP/UDP/IP RTP/UDP-Lite/IP TCP/IP

Modes of operation According to RFC 3095, the ROHC scheme has three modes of operation, as follows:

the Unidirectional mode (U-mode) the Bidirectional Optimistic mode (O-mode) the Bidirectional Reliable mode (R-mode) Both the compressor and the decompressor start in U-mode. They may then transition to O-mode if a usable return link is available, and the decompressor sends a positive acknowledgement, with O-mode specified, to the compressor. The transition to R-mode is achieved in the same way.

Unidirectional Mode (U-Mode) In the Unidirectional mode of operation, packets are only sent in one direction: from compressor to decompressor. This mode therefore makes ROHC usable over links where a return path from decompressor to compressor is unavailable or undesirable. In order to handle potential decompression errors, the compressor sends periodic refreshes of the stream context to the decompressor.

Bidirectional Optimistic Mode (O-Mode) The Bidirectional Optimistic mode is similar to the Unidirectional mode, except that a feedback channel is used to send error recovery requests and (optionally) acknowledgments of significant context updates from the decompressor to compressor. The O-mode aims to maximize compression efficiency and aims for sparse usage of the feedback channel.

Bidirectional Reliable Mode (R-Mode) The Bidirectional Reliable mode differs in many ways from the previous two modes. The most important differences are a more intensive usage of the feedback channel, and a stricter logic at both the compressor and the decompressor that prevents loss of context synchronization between compressor and decompressor, except for very high residual bit error rates.

Compressor/decompressor states The notion of compressor/decompressor states is orthogonal to the operational modes. Whatever the mode is, both the compressor and the decompressor work in one of their three states. They are basically finite state machines. Every incoming packet may cause the compressor/decompressor to change its internal state. Every state refers to a defined behaviour and compression level. The ROHC algorithm is similar to video compression, in that a base frame and then several difference frames are sent to represent an IP packet flow. This has the advantage of allowing ROHC to survive many packet losses in its highest compression state, as long as the base frames are not lost.

Compressor states The compressor's state machine defines the following three states:

Initialization and Refresh (IR) state First Order (FO) state Second Order (SO) state

Operations in the different compressor states In Initialization and Refresh (IR) state, the compressor has just been created or reset, and full packet headers are sent. In First-Order (FO) state, the compressor has detected and stored the static fields (such as IP addresses and port numbers) on both sides of the connection. The compressor is also sending dynamic packet field differences in FO state. Thus, FO state is essentially static and pseudo-dynamic compression. In Second-Order (SO) state, the compressor is suppressing all dynamic fields such as RTP sequence numbers, and sending only a logical sequence number and partial checksum to cause the other side to predictively generate and verify the headers of the next expected packet. In general, FO state compresses all static fields and most dynamic fields. SO state is compressing all dynamic fields predictively using a sequence number and checksum.

Transitions between compressor states Transitions between the above states occur when the compressor:

compresses a packet that contains too many variations receives a positive/negative feedback from the decompressor periodically refreshes the context

Second-Order ROHC headers – 1-byte headers A typical ROHC implementation will aim to get the terminal into Second-Order state, where a 1-byte ROHC header can be substituted for the 40-byte IPv4/UDP/RTP or the 60-byte IPv6/UDP/RTP (i.e. VoIP) header. In this state, the 8-bit ROHC header contains three fields:

a 1-bit packet-type flag (set to '1' only for longer ROHC headers) a 4-bit sequence number (with a range of −1 ... +14 packets from the base frame) a 3-bit CRC

Decompressor states The decompressor's state machine defines the following three states:

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with Robust Header Compression

Start with the simplest possible case. Write down what Robust Header Compression 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 Robust Header Compression 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 Robust Header Compression 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 Robust Header Compression

In research: Robust Header Compression 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 Robust Header Compression 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: Robust Header Compression is common in secondary-school and first-year university syllabi. It links to neighbouring topics Data compression, Internet Standards, so understanding it makes those chapters shorter.
In everyday life: Look for Robust Header Compression 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 Robust Header Compression in 20 minutes

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

Frequently asked questions

What is Robust Header Compression in simple terms?

Robust Header Compression (ROHC) is a standardized method to compress the IP, UDP, UDP-Lite, RTP, and TCP headers of Internet packets. The need for header compression In streaming applications, the overhead of IP, UDP, and RTP is 40 bytes for IPv4, or 60 bytes for IPv6.

Why does Robust Header Compression 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 Robust Header Compression?

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 Robust Header Compression.