# Overview¶

The score library implements the Simple COded REliability (SCORE) protocol for reliable unicast and multicast communication over UDP. The protocol is designed for distribution of data from a sender to one or more receivers. It is primarily intended for reliable transmission, but it also supports best-effort delivery, e.g. for cases where no feedback channel is available. Reliability is ensured using an erasure correcting code, provided by Kodo.

## Libraries¶

The score libraries are implemented in several projects and each project has its own git repository. It is recommended to choose a single project and work with the API that is exposed by that project.

score-cpp
The score-cpp library defines a simple, high-level C++ API to conveniently access the basic functionality of Score, such as sending and receiving data. It is very easy to integrate score-cpp into your C++ project, so it is the recommended option for most users.
score-c
The score-c library provides a simple C API that allows the programmer to use Score in a C program. The C API also enables interoperability with other programming languages that are not directly supported.
score
The score repository contains the low-level C++ implementation of the SCORE protocol. This is intended for experienced C++ developers who would like to know more about the inner workings of the library.

## Features¶

Proactive Redundancy
Redundancy generated using an erasure correcting code can be scheduled proactively e.g. to overcome some know link erasure probability and/or to reduce delay due to reduced feedback and scheduling additional redundancy.
Reactive Redundancy
When too little data is received to ensure reliability, additional redundancy can be generated using and erasure correcting code and transmitted from the sender.
Data Flush
When a file transfer is completed or a pause in a stream occurs, the outgoing data at the sender can be flushed to ensure reliability and avoid adding delay.
Sender Profiles
Different profiles catering to different scnearios and use cases are provided. The stream profile sender should be chosen for scenarios where timely delivery is more important than reliability, e.g. audio or video streaming. The object profile should be used in scenarios where data integrity (reliable transmission) is important, e.g. file delivery.
The receivers can provide feedback to the sender over the link where data is transmitted or over some independent link, e.g. if a one-way link is used for transmitting the data. The feedback is can be used by the sender to report user statistics and adapt to changing network conditions.
Serializing and Deseserializing (Optional)
Application data can be send as atomic units (messages) or as a stream.

## Parameters¶

The score protocol provides a set of parameters that can be adpated to accommodate the needs of different applications. Should the default stream and object senders not provide the needed performance, a set of parameters can be adjusted depending on the requirements:

• Symbol size
• Generation size
• Data redundancy
• Feedback probability
• Generation window size
• Send rate

All of these parameters can be controlled by a custom controller which can adjust the parameter based on available state information and the priority of the specific application.

### Symbol and Generation Size¶

The symbol size defines the size (in bytes) of the data chunks that the score protocol operates on. In general, this symbol size should be as big as possible, while staying below the network MTU. E.g. for WiFi networks with an MTU of 1500 bytes, a symbol size of roughly 1400 bytes is recommended. For low bandwidth streams that require a low delay, it may be beneficial to lower the symbol size. This lowers the data delivery delay, as data packets are sent more frequently from the sender, as well as the repair process is triggered more often.

The generation size defines the number of symbols grouped together by score and processed for repair. A larger generation size generally means more efficient operation (slightly less data pushed to the network), while a smaller generation size generally means lower latency between transmission and delivery of data.

### Data Redundancy¶

The data redundancy parameter is used to pro-actively introduce repair data to counteract packet loss on the network. A parameter value of 0.0 means that only the original data is transmitted initially, and then the repair phase must handle any loss that may occur. Increasing the value to e.g. 0.25 will add 25% repair data in addition to the original data. The stream will then be able to pro-actively repair packet loss of up to 20% without the need for a repair phase. As packet loss on a link (especially wireless multicast) is difficult to predict, this approach will generally decrease the efficiency of the transmission. However, the data delivery delay will generally be reduced, as data is readily available without the need for repair phases.

### Feedback Probability¶

Internally, score schedules repair data based on feedback received from receivers. Feedback messages are generated at specific feedback events during transmission. For multicast scenarios with a lot of receivers (e.g. more than 20), feedback from every receiver is not needed. In fact, it may cause problems as these feedback messages take up wireless capacity. To reduce the feedback to a sensible amount, the feedback probability parameter can be adjusted. This parameter describes the probability that a receiver generates feedback at a feedback event. Setting the value to 1.0 (default) causes a receiver to send feedback at every feedback event. Reducing the value to e.g. 0.5 causes the receiver to generate feedback with a 50% chance. When reducing this parameter, only a subset of all receivers will generate feedback for each feedback event, reducing the network load induced by feedback. As the information in the feedback messages mostly overlap between receivers, the transmission performance is not reduced as long as enough feedback messages are generated. Generally the original performance should be preserved if roughly 10 feedback messages are generated at each feedback event. For example in a multicast scenario with approximately 100 receivers, a feedback probability of 0.1 should not reduce score transmission performance. The feedback probability can also be set to 0.0 to obtain a “best effort” mode of operation, where no feedback will be sent at all, and the recovery of packet loss fully relies on the data redundancy parameter.

### Generation Window Size¶

The sender temporarily stores previously transmitted data in an internal buffer in case a receiver needs repair data at a later point in time. The size of this internal buffer denotes how far back in the data stream repair data can be requested. When a repair is needed for a segment in the data stream, all successfully received subsequent segments will be held back until this segment is repaired. The receivers are aware of the internal buffer size of the sender. If the segment is not repaired before it is discarded (overwritten) at the sender, the receiver will discard this data as well, and proceed to the following data segments. That is, the maximum period of time that subsequent data is held back is implicitly defined by the size of this internal buffer. If the application requires a low transmission latency (e.g. live video streaming), it is recommended to reduce this buffer size.

Consider for example a live video stream with an average bitrate of 4 Mbps. If a target latency between transmission and presentation should be below one second, the maximum internal buffer size should not exceed 0.5 MB, as this (on average) corresponds to one second worth of video. The buffer size is defined in ‘number of generations’, and the resulting size in bytes is thus implicitly declared through parameters symbol size and generation size. For a symbol size of 1000 and a generation size of 10, the generation window size parameter should be set to 50 in order to get a resulting internal buffer size of 500 kB (= 0.5 MB).

### Send Rate¶

It is possible to limit the send rate to a desired max rate, either to make room for other network traffic or to prevent packet loss due to congestion. Since score uses UDP traffic, it does not implement any sort of congestion control algorithm. The send rate does not force the data stream to be sent out with a certain rate. That is, sending a stream that is generated by the application at rate X will also be sent at roughly rate X, as long as the specified send rate Y is higher than X. The send rate is a limit, and only guarantees that this limit is not exceeded. The actual send rate may be lower than the specified limit.

## Platform Support¶

Score is portable to a wide range of platforms (both hardware platforms and operating systems).

We ensure compatibility with the supported platforms through a suite of unit tests, the current status can be checked at the Steinwurf Buildbot page. At the bottom of the main page, you can find detailed information about which platforms and compilers are currently tested by Steinwurf.

Note

The Buildbot is used for several different libraries. The score-c and score-cpp libraries can be found in the overview on the main page.