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For example, consider the coding scheme that encodes each source bit as follows is encoded as 111 is encoded as For example, consider a sender that sends 111. If there is one bit in error, the receiver could receive 011 or 101 or 110. In these three cases, the receiver will decode the received bit pattern as a 1 since it contains a majority of bits set to 1. If there are two bits in error, the receiver will not be able anymore to recover from the transmission error. This simple coding scheme forces the sender to transmit three bits for each source bit. However, it allows the receiver to correct single bit errors. More advanced coding systems that allow to recover from errors are used in several types of physical layers. Transport protocols use error detection schemes, but none of the widely used transport protocols rely on error correction schemes. To detect errors, a segment is usually divided into two parts : a header that contains the fields used by the transport protocol to ensure reliable delivery. The header contains a checksum or Cyclical Redundancy Check (CRC) [Williams1993] that is used to detect transmission errors a payload that contains the user data passed by the application layer. Some segment headers also include a length , which indicates the total length of the segment or the length of the payload. The simplest error detection scheme is the checksum. A checksum is basically an arithmetic sum of all the bytes that a segment is composed of. There are different types of checksums. For example, an eight bit checksum can be computed as the arithmetic sum of all the bytes of (both the header and trailer of) the segment. The checksum is computed by the sender before sending the segment and the receiver verifies the checksum upon reception of each segment. The receiver discards segments received with an invalid checksum. Checksums can be easily implemented in software, but their error detection capabilities are limited. Cyclical Redundancy Checks (CRC) have better error detection capabilities [SGP98], but require more CPU when implemented in software. Note: Checksums, CRCs, Most of the protocols in the TCP/IP protocol suite rely on the simple Internet checksum in order to verify that the received segment has not been affected by transmission errors. Despite its popularity and ease of implementation, the Internet checksum is not the only available checksum mechanism. Cyclical Redundancy Checks (CRC) are very powerful error detection schemes that are used notably on disks, by many datalink layer protocols and file formats such as zip or png. They can easily be implemented efficiently in hardware and have better error-detection capabilities than the Internet checksum [SGP98] . However, when the first transport protocols were designed, CRCs were considered to be too CPU-intensive for software implementations and other checksum mechanisms were used instead. The TCP/IP community chose the Internet checksum, the OSI community chose the Fletcher checksum [Sklower89] . Now, there are efficient techniques to quickly compute CRCs in software [Feldmeier95] , the SCTP protocol initially chose the Adler-32 checksum but replaced it recently with a CRC (see RFC 3309). The second imperfection of the network layer is that segments may be lost. As we will see later, the main cause of packet losses in the network layer is the lack of buffers in intermediate routers. Since the receiver sends an acknowledgement segment after having received each data segment, the simplest solution to deal with losses is to use a retransmission timer. When the sender sends a segment, it starts a retransmission timer. he value of this retransmission timer should be larger than the round-trip-time, i.e. the delay between the transmission of a data segment and the reception of the corresponding acknowledgement. When the retransmission timer expires, the sender assumes that the data segment has been lost and retransmits it. This is illustrated in the figure below. Unfortunately, retransmission timers alone are not sufficient to recover from segment losses. Let us consider, as an example, the situation depicted below where an acknowledgement is lost. In this case, the sender retransmits.