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H
Hashed Message Authentication Code — See HMAC.
hashing — Cryptographic technique applied over and over (iteratively)
to a message of arbitrary length to produce a hash “message
digest” or “signature” of fixed length that is appended
to the message when it is sent. In security, used to validate that
the contents of a message have not been altered in transit. The Secure
Hash Algorithm (SHA-1) and Message Digest 5 (MD5) are commonly used
hashes. See also SHA-1, MD5.
HDLC — High-Level Data Link Control. An International Telecommunication
Union (ITU) standard for a bit-oriented data-link layer protocol on
which most other bit-oriented protocols are based.
health monitor — JUNOS software extension to the RMON alarm system that provides
predefined monitoring for file system, CPU, and memory usage. The
health monitor also supports unknown or dynamic object instances such
as JUNOS processes.
hello interval — Amount of time an OSPF router continues to send a hello packet
to each adjacent neighbor.
hello mechanism — Process used by an RSVP router to enhance the detection of network
outages in an MPLS network.
HLR — Home Location Register. Database containing information about
a subscriber and the current location of a subscriber’s mobile
station.
HMAC — Hashed Message Authentication Code. A mechanism for message
authentication that uses cryptographic hash functions. HMAC can be
used with any iterative cryptographic hash function—for example,
MD5 or SHA-1—in combination with a secret shared key. The cryptographic
strength of HMAC depends on the properties of the underlying hash
function. Defined in RFC 2104, HMAC: Keyed-Hashing for Message
Authentication.
hold down — Timer used by distance-vector protocols to prevent the propagation
of incorrect routing knowledge to other routers in the network.
hold time — Maximum number of seconds allowed to elapse between successive
keepalive or update messages that a BGP system receives from a peer.
host membership query — Internet Group Management Protocol (IGMP) packet sent by a router
to determine whether interested receivers exist on a broadcast network
for multicast traffic.
host membership report — Internet Group Management Protocol (IGMP) packet sent by an
interested receiver for a particular multicast group address. Hosts
send report messages when they first join a group or in response to
a query packet from the local router.
host module — On an M160 router, provides the routing and system management
functions of the router. Consists of the Routing Engine and Miscellaneous
Control Subsystem (MCS).
host subsystem — On a T640 routing node, provides the routing and system management
functions of the router. Consists of a Routing Engine and an adjacent
Control Board (CB).
hot standby — In JUNOS, method used with link services intelligent queuing
interfaces (LSQ) to enable rapid switchover between primary and secondary
(backup) PICs. See also warm standby.
HSCSD — High-Speed Circuit Switched Data. Circuit-switched wireless
data transmission for mobile users, at data rates up to 38.4 Kbps.
HTTP — Hypertext Transfer Protocol. Method used to publish and receive
information on the Web, such as text and graphic files.
HTTPS — Hypertext Transfer Protocol over Secure Sockets Layer. Similar
to HTTP with an added encryption layer that encrypts and decrypts
user page requests and pages that are returned by a Web server. Used
for secure communication, such as payment transactions.
Hypertext Transfer Protocol — See HTTP.
Hypertext Transfer Protocol over Secure Sockets
Layer — See HTTPS.
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