NetWare
is a network operating system (NOS) that provides transparent remote
file access and numerous other distributed network services, including
printer sharing and support for various applications such as electronic
mail transfer and database access. NetWare
specifies the upper five layers of the OSI reference model and as such,
runs on virtually any media-access protocol (Layer 2). Additionally,
NetWare runs on virtually any kind of computer system, from PCs to
mainframes. This chapter summarizes the principal communications
protocols that support NetWare.
NetWare was developed by Novell, Inc.,
and introduced in the early 1980swas It was derived from Xerox
Network Systems (XNS),
which was created by Xerox
Corporation in the late 1970s, and is based on a client-server
architecture. Clients (sometimes called workstations) request
services, such as file and printer access, from servers.
NetWare's
client-server architecture supports remote access
that is transparent to users through remote
procedure calls. A remote procedure call begins when the local
computer program running on the client sends a procedure call to the
remote server. The server then executes the remote procedure call and
returns the requested information to the local client.
Figure 31-1 illustrates the NetWare
protocol suite, the media-access protocols on which NetWare runs, and
the relationship between the NetWare protocols and the OSI reference
model. This chapter addresses the elements and operations of these
protocol components.
Figure 31-1: The
NetWare protocol suite maps to all OSI layers.
The NetWare suite of protocols supports
several media-access (Layer 2) protocols, including
Ethernet/IEEE 802.3, Token Ring/IEEE 802.5, Fiber Distributed Data
Interface (FDDI), and Point-to-Point Protocol (PPP). Figure 31-2
highlights NetWare's breadth of media-access support.
Figure 31-2: NetWare supports
most common media-access protocols.
Internetwork Packet
Exchange (IPX) is the original NetWare network-layer (Layer 3) protocol
used to route packets through an internetwork. IPX is a connectionless
datagram-based network protocol and, as such, is similar to the Internet
Protocol found in TCP/IP networks.
IPX
uses the services of a dynamic distance vector-routing protocol (Routing
Information Protocol [RIP]) or a link-state routing protocol (NetWare
Link-State Protocol [NLSP]). IPX RIP sends routing updates every 60
seconds. To make best-path routing decisions, IPX RIP uses a
"tick" as the metric, which in principle is the delay expected
when using a particular length. One tick is 1/18th of a second. In the
case of two paths with an equal tick count, IPX RIP uses the hop count
as a tie breaker. (A hop is the passage of a
packet through a router.) IPX's RIP
is not compatible with RIP implementations used in other networking
environments.
As with other network addresses, Novell
IPX network addresses must be unique. These addresses are represented in
hexadecimal format and consist of two parts: a network number and a node
number. The IPX network number, which is assigned by the network
administrator, is 32 bits long. The node number, which usually is the
Media Access Control (MAC) address for one of the system's network
interface cards (NICs), is 48 bits long.
IPX's use of a MAC address for the node
number enables the system to send nodes to predict what MAC address to
use on a data link. (In contrast, because the host portion of an IP
network address has no correlation to the MAC address, IP nodes must use
the Address-Resolution Protocol [ARP] to determine the destination
MAC address.)
Novell NetWare IPX supports multiple
encapsulation schemes on a single router interface, provided that
multiple network numbers are assigned. Encapsulation is the process of
packaging upper-layer protocol information and data into a frame.
NetWare supports the following four encapsulation schemes:
Figure 31-3 illustrates these
encapsulation types.
Figure 31-3: Four IPX
encapsulation types exist.
The Service
Advertisement Protocol (SAP) is an IPX protocol
through which network resources, such as file servers and print servers,
advertise their addresses and the services they provide. Advertisements
are sent via SAP every 60 seconds. Services are identified by a
hexadecimal number, which is called a SAP
identifier (for example, 4 = file server, and 7 = print server).
A SAP operation begins when routers
listen to SAPs and build a table of all known services along with their
network address. Routers then send their SAP table every 60 seconds.
Novell clients can send a query requesting a particular file, printer,
or gateway service. The local router responds to the query with the
network address of the requested service, and the client then can
contact the service directly.
SAP is pervasive in current networks
based on NetWare 3.11 and earlier but is utilized less frequently in
NetWare 4.0 networks because workstations can locate services by
consulting a NetWare Directory Services (NDS) Server. SAP, however,
still is required in NetWare 4.0 networks for workstations when they
boot up to locate
an NDS server.
Using the SAP identifier, SAP
advertisements can be filtered on a router's input or output port, or
from a specific router. SAP filters conserve network bandwidth and are
especially useful in large Novell installations where hundreds of SAP
services exist.
In general, the use of SAP filters
is recommended for services that are not required for a particular
network. Remote sites, for example, probably do not need to receive SAP
advertising print services located at a central site. A SAP output
filter at the central site (preferred) or a SAP input filter that uses
the SAP identifier for a print server at the remote site prevents the
router from including print services in SAP
updates.
The Sequenced Packet
Exchange (SPX) protocol is the most common
NetWare transport protocol at Layer 4 of the OSI model. SPX resides atop
IPX in the NetWare Protocol Suite. SPX is a reliable,
connection-oriented protocol that supplements the datagram service
provided by the IPX, NetWare's network-layer (Layer 3) protocol. SPX was
derived from the Xerox Networking Systems (XNS) Sequenced Packet
Protocol (SPP). Novell also offers Internet Protocol
support in the form of the User
Datagram Protocol (UDP). IPX datagrams are encapsulated inside UDP/IP
headers for transport across an IP-based internetwork.
NetWare supports a wide variety of
upper-layer protocols, including NetWare Shell, NetWare Remote
Procedure Call, NetWare Core Protocol, and Network Basic Input/Output
System.
The NetWare
shell runs clients (often called workstations in the NetWare community)
and intercepts application input/output (I/O) calls to determine whether
they require network access for completion. If the application request
requires network access, the NetWare shell packages the request and
sends it to lower-layer software for processing and network
transmission. If the application request does not require network
access, the request is passed to the local I/O resources. Client
applications are unaware of any network access required for completion
of application calls.
NetWare Remote
Procedure Call (NetWare RPC) is another more general redirection
mechanism similar in concept to the NetWare shell supported by Novell.
NetWare Core Protocol
(NCP) is a series of server routines designed to satisfy application
requests coming from, for example, the NetWare shell. The services
provided by NCP include file access, printer access, name management,
accounting, security, and file synchronization.
NetWare also supports the Network
Basic Input/Output System (NetBIOS) session-layer interface
specification from IBM and Microsoft. NetWare's NetBIOS emulation
software allows programs written to the industry-standard NetBIOS
interface to run within NetWare system.
NetWare application-layer
services include NetWare message-handling
service (NetWare MHS), Btrieve, NetWare loadable
modules (NLMs), and IBM Logical Unit (LU) 6.2 network-addressable
units (NAUs). NetWare MHS is a message-delivery system that
provides electronic mail transport. Btrieve is Novell's implementation
of the binary tree (btree) database-access mechanism. NLMs are add-on
modules that attach into a NetWare system. NLMs currently available from
Novell and third parties include alternate protocol stacks,
communication services, and database services. In terms of IBM LU 6.2
NAU support, NetWare allows peer-to-peer connectivity and information
exchange across IBM networks. NetWare packets are encapsulated within LU
6.2 packets for transit across an IBM network.
The IPX packet is the
basic unit of Novell NetWare internetworking. Figure 31-4
illustrates the format of a NetWare IPX packet.
Figure 31-4: A NetWare IPX
packet consists of eleven fields.
The following descriptions summarize the
IPX packet fields
illustrated in Figure 31-4:
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