Networking on z/OS
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Network layers and protocols review

Networking on z/OS

The first step in discussing network technology is to ensure that you understand the terms and acronyms. Starting from the physical layer, progressing to the data link layer (Ethernet), and moving up through the network layer (IP and routing) on to the transport layer (TCP and UDP), there are a large number of terms to be understood. These terms need to be clearly understood when z/OS systems programmers communicate with network administrators in an organization.

As a network administrator, you must have a general knowledge of network layers, the protocols at each layer, and the hardware that facilitates the transport of data. This section functions as a overview for readers already familiar with IP-related layers and protocols.

  • Networking terminology
    It goes without saying that, between two endpoints on a network, there must be an agreement on the protocol, or language, that is in use. There is some irony in the fact that the same requirement is sometimes ignored when the communications endpoints are the network administrator and the z/OS system programmers.
  • Physical media, layer 1
    The physical network (also called the physical layer) begins at the network interface card (NIC). The NIC is effectively a method of connecting the internal data bus of a computer to the external media (cables) of the network.
  • Data link layer, layer 2
    In the TCP/IP-based layered network, layer 2 is the data link layer. This layer is also called simply the link layer. The actual protocols encompassed in the link layer are numerous, and the implementation details can be found in various documents throughout the Internet and in trade texts. For the purpose of this discussion, we'll limit the scope to aspects of the link layer that a network administrator would need to know. The foremost data link layer protocol is the Ethernet protocol.
  • Network layer, layer 3
    The most significant protocol at layer 3 (also called the network layer) is the Internet Protocol, or IP. IP is the standard for routing packets across interconnected networks--hence, the name internet. It is an encapsulating protocol similar to the way Ethernet is an encapsulating protocol. If we view the original check as a unit of data needed to be sent, we now have two envelopes required to do the transmission--the check first goes into an IP envelope, and then the entire IP envelope (known as a packet) is placed into an Ethernet frame.
  • Transport layer, layer 4
    Unlike layer 3, there are really only two protocols of note found in layer 4: Transmission Control Protocol (TCP) and User Datagram Protocol (UDP). Returning to our postal mail protocol analogy, layer 3 is preoccupied with ensuring that the address on the envelope could be located and that the envelope could ultimately be delivered. Layer 4 shifts the focus to the process of the actual delivery of the envelope.
  • Network applications
    Sitting above layer 4 are the applications. Applications can use either TCP or UDP to communicate. Because of its inherent reliability, TCP tends to be used more often.
  • Network security
    Security is never far from the mind of any administrator of a network. Although security is covered extensively in a z/OS context in the section on network security, it is useful to clarify some security terminology here.
  • Systems Network Architecture (SNA)
    "Classic" SNA, based on subarea nodes, is the original networking architecture used by mainframe computers. However, with the popularity and growth of TCP/IP, SNA is changing from being a true network architecture to being what could be termed an "application and application access architecture." In other words, there are many applications that still need to communicate in SNA, but the required SNA protocols are carried over the network by IP.
Parent topic: Introduction to networking on the mainframe

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