Hashed Message Authentication Code



Cryptographic technique that is applied over and over (iteratively) to a message of arbitrary length that produces a hash "message digest" or "signature" of fixed length that is appended to the message when 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 and MD5.


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.

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.


Home Location Register. Database containing information about a subscriber and the current location of a subscriber's mobile station.


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 the time a BGP system receives successive keepalive or update messages 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).


High-Speed Circuit Switched Data. Circuit-switched wireless data transmission for mobile users, at data rates up to 38.4 Kbps.