RADIUS
Remote Authentication Dial-in User Service 
A combination of IETF RFCs 2138 and 2139 that provides authentication, authorization, and accounting for remote dial access users. 

RADSL
Rate Adaptive Asymmetric Digital Subscriber Line 
A version of ADSL where modems test the - telephone - line and adapt their operating speed to the fastest line speed possible. 
See ADSL

RARP
Reverse Address Resolution Protocol 
- The logical reverse of ARP that provides a method of finding IP addresses based on media addresses.
- Internet protocol used by a diskless machine at startup to find its Internet address. The machine broadcasts a request containing its physical hardware address, and a server responds by sending the machine its Internet address.

RAS
Remote Access Server 
A server residing at an ISP network to provide tunneling capability from a remote user to the corporate network.

RBS
Robbed-Bit Signaling

RCP
Real-Time Control Program

RDNIS
Redirected Dial Number Id Service

RDP
Reliable Datagram Protocol 
A protocol that provides reliable datagram service on top of the standard unreliable datagram service provided by the Internet Protocol.

RED
Random Early Detection - Cisco ¨

REJ
Reject
A link protocol command

REP
Reply

REV
Reverse - calling

RF
Radio Frequency

RFC
Request For Comments 

RI
Ring In 
The input port of a ring in/ring out concentrator module in a token ring network

RI
Routing Information

RIF
Routing Information Field 

RIP
Routing Information Protocol 
Routing protocol used by TCP/IP and Netware used to identify the connected networks
and to identify the hop count.

RIP version 2 message format
¥ If the entry 1 has the address family field = to 1 and the tag route = to 2, the 16 
   following bytes will have the password in clear.
¥ If next-hop = 0, the transmit address will be used.

RISC
Reduced Instruction Set Computer

RJ-11/12/45
Registered Jacks

RM CELLS
Rate Management cells - ATM

RMI
Remote Method Invocation

RMON
Remote Monitor

RMON2
Remote Monitor2 
Advanced RMON standard that adds broader vendor compliance and enterprise-wide traffic monitoring at the network through application layers of the OSI reference model, and additional information about network traffic at these layers.

RMT
Ring Management

RNR
Receive Not Ready

RO
Ring Out
The output port of a ring in/ring out concentrator module in a token ring network.

ROUTER
- Device which interconnects networks which do not have necessarily the same kind of support. 
- Only transmit the data which are required for the end-node
- Check and rebuild the frames without re-transmitting the error to the next network. 
- A device that forwards traffic between networks, based on network layer information and routing tables. It decides which path network traffic will follow, using routing protocols to gain information about the network and algorithms to choose the best route based on a "routing matrix." In OSI terminology, a router is a network layer intermediate system. See also bridge. 
- A protocol-specific internetworking component configured to know which LAN segments it can communicate with and that takes responsibility for getting packets from one segment to another via the best way. A router must be able to recognize all of the different network layer protocols that may be used on the networks they link together. A multi-protocol router can route many different protocols. 


RSRB
Remote Source Route Bridging - Source route bridging over WAN links. Cisco ¨

RSVP
Resource reSerVation Protocol 
RFC 2205 

A standard which manages the traffic, and assigns the priorities dynamically.
RSVP is a receiver-based protocol, which means that it treats networks much as cable television does: the selectivity is the receiving application, much as a cable TV viewer might tune in to a specific channel. The router that is connected to the receiver of particular data flow is responsible for initiating and maintaining the resources used for that data flow.
Protocol that provides a method for real-time applications to communicate requirements to network elements along the data path. RSVP conveys Quality of Service information between network elements and the application. In addition to the data used to directly invoke QoS control services, RSVP carries authentication and policy information needed to manage the use of these services.

RS-232
Physical layer interface. Virtually identical to the V.24 specification.

RS-422
A balanced electrical implementation of RS-449 for high-speed data transmission.

RS-449
Physical layer interface. Faster version of RS-232 - up to 2 Mbps - capable of longer cable runs.

RTCP
Real-Time Transport Control Protocol

RTP
Real Time Protocol - RFC 1889