D
DAD
Duplicate Address
Detection. IPv6 automatically uses this process to ensure that no duplicate IP addresses
exist.
For more information, see Duplicate Address
Detection in the ExtremeXOS 21.1 User Guide.
dBm
An abbreviation for the
power ratio in decibels (dB) of the measured power referenced to one milliwatt.
DCB
Data Center Bridging is
a set of IEEE 802.1Q extensions to standard Ethernet, that provide an operational framework
for unifying Local Area Networks (LAN), Storage Area Networks (SAN) and Inter-Process
Communication (IPC) traffic between switches and endpoints onto a single transport
layer.
DCBX
The Data Center
Bridging eXchange protocol is used by DCB devices to exchange DCB configuration information
with directly connected peers.
default encapsulation
mode
In
STP, default encapsulation allows you to
specify the type of BPDU encapsulation to use for all ports added to a given STPD, not just to
one individual port. The encapsulation modes are:
- 802.1d—This mode is used for backward compatibility with previous
STP versions and for compatibility with third-party switches using IEEE standard
802.1d.
- EMISTP—Extreme Multiple Instance Spanning Tree Protocol (EMISTP)
mode is an extension of STP that allows a physical port to belong to multiple STPDs by
assigning the port to multiple VLANs.
- PVST+—This mode implements PVST+ in compatibility with third-party
switches running this version of STP.
designated port
In
STP, the designated port provides the
shortest path connection to the root bridge for the attached LAN segment. Each LAN segment has
only one designated port.
destination
address
The IP or MAC address
of the device that is to receive the packet.
Device Manager
The Device Manager is
an Extreme Networks-proprietary process that runs on every node and is responsible for
monitoring and controlling all of the devices in the system. The Device Manager is useful for
system redundancy.
device server
A specialized,
network-based hardware device designed to perform a single or specialized set of server
functions. Print servers, terminal servers, remote access servers, and network time servers
are examples of device servers.
DF
Don't fragment bit.
This is the don't fragment bit carried in the flags field of the IP header that indicates that
the packet should not be fragmented. The remote host will return ICMP notifications if the
packet had to be split anyway, and these are used in
MTU discovery.
DHCP
Dynamic Host
Configuration Protocol. DHCP allows network administrators to centrally manage and automate
the assignment of IP addresses on the corporate network. DHCP sends a new IP address when a
computer is plugged into a different place in the network. The protocol supports static or
dynamic IP addresses and can dynamically reconfigure networks in which there are more
computers than there are available IP addresses.
DiffServ
Differentiated Services. Defined in RFC 2474 and 2475, DiffServ is an
architecture for implementing scalable service differentiation in the Internet. Each IP
header has a DiffServ (DS) field, formerly known as the Type of Service (TOS) field. The
value in this field defines the QoS
priority the packet will have throughout the network by dictating the forwarding treatment
given to the packet at each node.
DiffServ is a flexible architecture that allows for either end-to-end
QoS or intra-domain QoS by implementing complex classification and mapping functions at the
network boundary or access points. In the Extreme Networks implementation, you can configure
the desired QoS by replacing or mapping the values in the DS field to egress queues that are
assigned varying priorities and bandwidths.
directory agent
(DA)
A method of organizing
and locating the resources (such as printers, disk drives, databases, e-mail directories, and
schedulers) in a network. Using SLP, networking applications can discover the existence,
location and configuration of networked devices. With Service Location Protocol, client
applications are 'User Agents' and services are advertised by 'Service Agents'.
The User Agent issues a multicast 'Service Request' (SrvRqst) on behalf
of the client application, specifying the services required. The User Agent will receive a
Service Reply (SrvRply) specifying the location of all services in the network which satisfy
the request.
For larger networks, a third entity, called a 'Directory Agent', receives
registrations from all available Service Agents. A User Agent sends a unicast request for
services to a Directory Agent (if there is one) rather than to a Service Agent.
(SLP
version 2, RFC 2608, updating RFC 2165)
diversity antenna and
receiver
The AP has two
antennae. Receive diversity refers to the ability of the AP to provide better service to a
device by receiving from the user on which ever of the two antennae is receiving the cleanest
signal. Transmit diversity refers to the ability of the AP to use its two antenna to transmit
on a specific antenna only, or on a alternate antennae. The antennae are called diversity
antennae because of this capability of the pair.
DNS
Domain Name Server.
This system is used to translate domain names to IP addresses. Although the Internet is based
on IP addresses, names are easier to remember and work with. All these names must be
translated back to the actual IP address and the DNS servers do so.
domain
In
CFM, a maintenance domain is the network, or
part of the network, that belongs to a single administration for which connectivity faults are
managed.
DoS attack
Denial of Service
attacks occur when a critical network or computing resource is overwhelmed so that legitimate
requests for service cannot succeed. In its simplest form, a DoS attack is indistinguishable
from normal heavy traffic. ExtremeXOS software has configurable parameters that allow you to
defeat DoS attacks. For more information, see DoS Protection in the
ExtremeXOS 21.1 User Guide.
DR
Designated router. In
OSPF, the DR generates an LSA for the multi-access network
and has other special responsibilities in the running of the protocol. The DR is elected by
the OSPF protocol.
DSSS
Direct-Sequence Spread
Spectrum. A transmission technology used in Local Area Wireless Network (LAWN) transmissions
where a data signal at the sending station is combined with a higher data rate bit sequence,
or chipping code, that divides the user data according to a spreading ratio. The chipping code
is a redundant bit pattern for each bit that is transmitted, which increases the signal's
resistance to interference. If one or more bits in the pattern are damaged during
transmission, the original data can be recovered due to the redundancy of the transmission.
(Compare with
FHSS.)
DTIM
DTIM delivery traffic
indication message (in 802.11 standard).
dynamic WEP
The IEEE introduced the
concept of user-based authentication using per-user encryption keys to solve the scalability
issues that surrounded static WEP. This resulted in the 802.1x standard, which makes use of
the IETF's Extensible Authentication Protocol (EAP), which was originally designed for user
authentication in dial-up networks. The 802.1x standard supplemented the EAP protocol with a
mechanism to send an encryption key to a Wireless AP. These encryption keys are used as
dynamic WEP keys, allowing traffic to each individual user to be encrypted using a separate
key.