A Conceptual Overview of iSCSI

There is a note of interest here for users of network protocol analyzers. Because there is no

requirement that PDUs begin on a TCP Data Segment boundary, it is possible that simply grabbing

an Ethernet frame with a protocol analyzer may result in an incorrect interpretation. To determine

the start of a PDU, the end of the previous PDU must be known. While analyzers do use various

heuristics to determine the start of a PDU within the first few TCP Data Segments seen, the

heuristics are not foolproof, and there may be “false positives” possibly interpreting real data as the

start of a PDU for a specific protocol. The potential existence of “false positives” should always be

considered when debugging network problems.

5. Network Entities and iSCSI nodes

A network entity is an abstraction for any black-box network host. iSCSI uses the concept of

network entity to define a context for initiators and targets. A network entity can be viewed as a

container with one or more network portals for connectivity (see Figure 5). A network entity is not a

concept specific to iSCSI and may be used to describe network hosts in general.

Figure 5. iSCSI Network Entity

Within the network entity, iSCSI defines one or more iSCSI nodes. An iSCSI initiator or target is

typically defined by a single iSCSI node within a network entity. Multiple iSCSI nodes may be

defined within a network entity. A network entity may contain both initiator and target iSCSI nodes,

as well as unrelated network protocols such as NFS.

Each iSCSI node is identified by a worldwide unique iSCSI name (an ASCII string). iSCSI uses

target node names to uniquely identify available SCSI storage on a TCP/IP based network.

Because of the vastness of the internet, and the possibility that a small isolated LAN may someday

be attached to larger LANs, the uniqueness of all iSCSI node names must be maintained. iSCSI

vendors typically pre-define iSCSI node names to maintain their uniqueness, but this is not required.

Accessibility of an iSCSI node from all network portals of a network entity is not mandatory. The

number of network portals actually providing access to an iSCSI node is a function of the network

iSCSI

Node

Network Entity

iSCSI

Node 1

iSCSI

Node 2

iSCSI

Node n

Network

Portal 1

Network

Portal 2

Network

Portal m