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662 INTRODUCTION TO PACKET NETWORKS AND VoIPFigure 20.4-2 SCTP chunk format. (Source: RFC 2960 [18].)bundled into a packet, but if a message is longer than the MTU of the path, it can befragmented into multiple packets. Some control chunks cannot be bundled and forthe details we refer the reader to [23]. There are 13 currently defined chunk types(Table 20.4-1); one (DATA) is for user payloads, and the others are for control.The format of the DATA chunk is shown in Fig. 20.4-3. Reliable delivery andsequencing are handled separately: every chunk, even when out of sequence, isacknowledged based on the value of TSN, while SSN is used to sequence chunkswithin a stream. Acknowledgments and retransmission are managed via controlchunks. The option of providing reliability with no sequencing is controlled bythe U (Unsequenced) bit in the chunk header. Bits B (Beginning) and E (End) areused for fragmentation.Associations are established with a four-way handshake (Fig. 20.4-4), similar tothe three-way handshake of TCP, but with the addition of the cookie exchange. Thecookie is a data package that is generated by the recipient of the INIT request and isincluded in the INIT ACK chunk.TABLE 20.4-1 Chunk TypesChunk Type Acronym CodePayload data DATA 0Initiation INIT 1Initiation acknowledgment INIT ACK 2Selective acknowledgment SACK 3Shutdown association SHUTDOWN 7Shutdown acknowledgment SHUTDOWN ACK 8Cookie echo COOKIE ECHO 10Cookie acknowledgment COOKIE ACK 11Shutdown complete SHUTDOWN COMPLETE 14Figure 20.4-3 DATA chunk format. (Source: RFC 2960 [18].)