d information. The process is reversed and the information is reassembled and reencapsulated into TCP/IP packet form. This information will be embedded into an Ethernet frame(s) and sent to the iSCSI initiator at the server, where it will be decapsulated and reencapsulated onto the IP LAN for subsequent transmission to the requesting client. The iSCSI protocol stack is essentially an insertion of a few things right above the traditional layer 4, as shown in Figure 11.3. The main purpose of TCP, a layer 4 protocol that runs over IP, is to ensure the reliable transmission. The iSCSI layer, which is now at layer 5, runs right over TCP and ensures that the bit packaging of the underlying transmission becomes routable by serializing the inherently parallel SCSI structure. The native SCSI command set and SCSI bus protocol run over iSCSI (now in layer 6). The respective operating system layer and the final application software are located above that layer. A whole array of industry players has adopted the IP storage realm and supports iSCSI with prod- ucts that behave in a complementary fashion when deployed in a SAN: Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STORAGE NETWORK PROCESSORS (SNPS) STORAGE NETWORK PROCESSORS (SNPs) 203 IP-based LAN Storage Device e.g. disk array tape library, etc. Client devices iSCSI target (adapter board or ASIC) IP-based commands encapsulated in TCP/IP in an Ethernet Client Server frame request Ethernet Ethernet switch switch IP-based iSCSI initiator IP-based (adapter board or ASIC) FIGURE 11.2 The iSCSI SAN principle of operation. Application software Operating system Core SCSI command set for communication between hosts and storage devices iSCSI layer TCP layer IP layer FIGURE 11.3 The iSCSI protocol stack. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright 