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Open Shortest Path First. A link-state intradomain routing protocol that is often used in enterprise and ISP networks. OSPF is defined in and :rfc:`2328` and :rfc:`5340`
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packet
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a packet is the unit of information transfer in the network layer
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PBL
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Problem-based learning is a teaching approach that relies on problems.
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POP
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The Post Office Protocol (POP), defined :rfc:`1939`, is an application-level protocol that allows a client to download email messages stored on a server.
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remote login
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A service that enables a user to connect to a distant server over the network. Telnet, defined in :rfc:`854` and the BSD rlogin services defined in :rfc:`1282` were popular in the past. They have been deprecated for security reasons and are now replaced by :term:`ssh`.
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resolver
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A server that implements the DNS protocol and can resolve queries. A resolver usually serves a set of clients (e.g. all hosts in campus or all clients of a given ISP). It sends DNS queries to nameservers everywhere on behalf of its clients and stores the received answers in its cache. A resolver must know the IP addresses of the root nameservers.
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Routing Information Protocol. An intradomain routing protocol based on distance vectors that is sometimes used in enterprise networks. RIP is defined in :rfc:`2453`.
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RIR
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Regional Internet Registry. An organization that manages IP addresses and AS numbers on behalf of :term:`IANA`.
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root nameserver
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A name server that is responsible for the root of the domain names hierarchy. There are currently a dozen root nameservers and each DNS resolver See http://www.root-servers.org/ for more information about the operation of these root servers.
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round-trip-time
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The round-trip-time (RTT) is the delay between the transmission of a segment and the reception of the corresponding acknowledgment in a transport protocol.
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router
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A relay operating in the network layer.
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