Editing Routing Information Protocol

{{Short description|Computer network protocol}}
{{IPstack}}
{{Use dmy dates|date=September 2020}}

The '''Routing Information Protocol''' ('''RIP''') is one of the oldest [[distance-vector routing protocol]]s which employs the [[Hopcount|hop count]] as a [[Metrics (networking)|routing metric]]. RIP prevents [[routing loop problem|routing loops]] by implementing a limit on the number of [[Hop (networking)|hops]] allowed in a path from source to destination. The largest number of hops allowed for RIP is 15, which limits the size of networks that RIP can support.

RIP implements the [[split horizon]], [[route poisoning]], and [[holddown]] mechanisms to prevent incorrect routing information from being propagated.

In RIPv1 routers broadcast updates with their routing table every 30 seconds. In the early deployments, [[routing table]]s were small enough that the traffic was not significant. As networks grew in size, however, it became evident there could be a massive traffic burst every 30 seconds, even if the routers had been initialized at random times.

In most networking environments, RIP is not the preferred choice of [[routing protocol]], as its [[Convergence (routing)#Convergence time|time to converge]] and [[scale (computing)|scalability]] are poor compared to [[Enhanced Interior Gateway Routing Protocol|EIGRP]], [[Open Shortest Path First|OSPF]], or [[IS-IS]]. However, it is easy to configure, because RIP does not require any parameters, unlike other protocols.

RIP uses the [[User Datagram Protocol]] (UDP) as its transport protocol, and is assigned the reserved [[port number]] 520.<ref name="IANA">{{cite web|publisher=The [[Internet Assigned Numbers Authority]] (IANA)|title=Service Name and Transport Protocol Port Number Registry|website=www.iana.org|page=10|access-date=25 February 2022|url=https://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.xhtml?&page=10}}</ref>

==Development of distance-vector routing==
Based on the [[Bellman–Ford algorithm]] and the [[Ford–Fulkerson algorithm]], [[distance-vector routing protocol]]s started to be implemented from 1969 onwards in [[data networks]] such as the [[ARPANET]] and [[CYCLADES]]. The predecessor of RIP was the Gateway Information Protocol (GWINFO) which was developed by [[Xerox]] in the mid-1970s to route its experimental network. As part of the [[Xerox Network Systems]] (XNS) protocol suite GWINFO transformed into the XNS Routing Information Protocol. This XNS RIP in turn became the basis for early routing protocols, such as [[Novell]]'s IPX RIP, [[AppleTalk]]'s Routing Table Maintenance Protocol (RTMP), and the IP RIP. The 1982 [[Berkeley Software Distribution]] of the [[UNIX]] operating system implemented RIP in the ''routed'' [[Daemon (computing)|daemon]]. The 4.2BSD release proved popular and became the basis for subsequent UNIX versions, which implemented RIP in the ''routed'' or ''gated'' daemon. Ultimately, RIP had been extensively deployed<ref>{{Cite book|title= CCIE Professional Development: Routing TCP/IP Volume I, Second Edition|author1=Jeff Doyle |author2=Jennifer Carroll |publisher= ciscopress.com|year=2005 |isbn= 9781587052026|page=169}}</ref> before the standard, written by Charles Hedrick, was passed as RIPv1 in 1988.{{Ref RFC|1058}}

==The RIP hop count==
The routing metric used by RIP counts the number of routers that need to be passed to reach a destination IP network. The hop count 0 denotes a network that is directly connected to the router. 16 hops denote a network that is unreachable, according to the RIP hop limit.<ref name="ciscopress.com">{{Cite book|title= CCIE Professional Development: Routing TCP/IP Volume I, Second Edition|author1=Jeff Doyle |author2=Jennifer Carroll |publisher= ciscopress.com|year=2005 |isbn= 9781587052026|pages=170}}</ref>

==Versions==
There are three standardized versions of the Routing Information Protocol: ''RIPv1'' and ''RIPv2'' for [[IPv4]], and ''RIPng'' for [[IPv6]].

===RIP version 1===
The original specification of RIP was published in 1988.{{Ref RFC|1058}} When starting up, and every 30 seconds thereafter, a router with RIPv1 implementation [[Broadcasting (networking)|broadcasts]] to {{IPaddr|255.255.255.255}} a request message through every RIPv1 enabled interface. Neighbouring routers receiving the request message respond with a RIPv1 segment, containing their [[routing table]]. The requesting router updates its own routing table, with the reachable IP network address, hop count and next hop, that is the router interface IP address from which the RIPv1 response was sent. As the requesting router receives updates from different neighbouring routers it will only update the reachable networks in its routing table, if it receives information about a reachable network it has not yet in its routing table or information that a network it has in its routing table is reachable with a lower hop count. Therefore, a RIPv1 router will in most cases only have one entry for a reachable network, the one with the lowest hop count. If a router receives information from two different neighbouring router that the same network is reachable with the same hop count but via two different routes, the network will be entered into the routing table two times with different next hop routers. The RIPv1 enabled router will then perform what is known as equal-cost load balancing for IP packets.<ref name="ciscopress.com"/>

RIPv1 enabled routers not only request the routing tables of other routers every 30 seconds, they also listen to incoming requests from neighbouring routers and send their own routing table in turn. RIPv1 routing tables are therefore updated every 25 to 35 seconds.<ref name="ciscopress.com"/> The RIPv1 protocol adds a small random time variable to the update time, to avoid routing tables synchronizing across a LAN.<ref>{{Cite book|title=CCIE Professional Development: Routing TCP/IP Volume I, Second Edition |author1=Jeff Doyle |author2=Jennifer Carroll |publisher=ciscopress.com|year=2005 |isbn=9781587052026 |page=171}}</ref> It was thought, as a result of random initialization, the routing updates would spread out in time, but this was not true in practice. Sally Floyd and [[Van Jacobson]] showed in 1994 that, without slight randomization of the update timer, the timers synchronized over time.<ref>[http://www.icir.org/floyd/papers/sync_94.pdf The Synchronization of Periodic Routing Messages], S. Floyd & V. Jacobson,April 1994</ref>

RIPv1 can be configured into silent mode, so that a router requests and processes neighbouring routing tables, and keeps its routing table and hop count for reachable networks up to date, but does not needlessly send its own routing table into the network. Silent mode is commonly implemented to hosts.<ref>{{Cite book|title= CCIE Professional Development: Routing TCP/IP Volume I, Second Edition|author1=Jeff Doyle |author2=Jennifer Carroll |publisher= ciscopress.com|year=2005 |isbn= 9781587052026|page=175}}</ref>

RIPv1 uses [[classful address|classful]] routing. The periodic routing updates do not carry [[subnetwork|subnet]] information, lacking support for [[VLSM|variable length subnet masks]] (VLSM). This limitation makes it impossible to have different-sized [[subnetwork|subnet]]s inside of the same [[network class]]. In other words, all subnets in a network class must have the same size. There is also no support for router authentication, making RIP vulnerable to various attacks.

===RIP version 2===
Due to the deficiencies of the original RIP specification, RIP version 2 (RIPv2) was developed in 1993,<ref name="ciscopress.com"/> published in 1994,{{Ref RFC|1723}} and declared [[Internet Standard]] 56 in 1998.{{Ref RFC|2453}} It included the ability to carry subnet information, thus supporting [[Classless Inter-Domain Routing]] (CIDR). To maintain [[backward compatibility]], the hop count limit of 15 remained. RIPv2 has facilities to fully interoperate with the earlier specification if all ''Must Be Zero'' protocol fields in the RIPv1 messages are properly specified. In addition, a ''compatibility switch'' feature{{Ref RFC|2453|repeat=yes}} allows fine-grained interoperability adjustments.

In an effort to avoid unnecessary load on hosts that do not participate in routing, RIPv2 ''[[multicast]]s'' the entire routing table to all adjacent routers at the address [[Multicast address|{{IPaddr|224.0.0.9}}]], as opposed to RIPv1 which uses [[Broadcasting (networking)|broadcast]]. [[Unicast]] addressing is still allowed for special applications.

([[MD5]]) authentication for RIP was introduced in 1997.{{Ref RFC|2082}}{{Ref RFC|4822}}

Route tags were also added in RIP version 2. This functionality allows a distinction between routes learned from the RIP protocol and routes learned from other protocols.

===RIPng===
RIPng (RIP next generation) is an extension of RIPv2 for support of [[IPv6]], the next generation [[Internet Protocol]].{{Ref RFC|2080}} The main differences between RIPv2 and RIPng are:
* Support of IPv6 networking.
* While RIPv2 supports RIPv1 updates authentication, RIPng does not. IPv6 routers were, at the time, supposed to use [[IPsec]] for authentication.{{cn|date=June 2023}}
* RIPv2 encodes the next-hop into each route entry, RIPng requires specific encoding of the next hop for a set of route entries.

RIPng sends updates on UDP port 521 using the multicast group {{IPaddr|FF02::9}}.

==RIP messages between routers==
RIP messages use the [[User Datagram Protocol]] on port 520 and all RIP messages exchanged between routers are encapsulated in a UDP datagram.<ref name="ciscopress.com"/>

===RIPv1 Messages===
RIP defined two types of messages:
;Request Message
:Asking a neighbouring RIPv1 enabled router to send its routing table.
;Response Message
:Carries the routing table of a router.

==Timers==
The routing information protocol uses the following timers as part of its operation:<ref name="Aaron Balchunas">{{cite web|last=Balchunas|first=Aaron|title=Routing Information Protocol (RIP v1.03)|url=http://www.routeralley.com/guides/rip.pdf |archive-url=https://ghostarchive.org/archive/20221010/http://www.routeralley.com/guides/rip.pdf |archive-date=2022-10-10 |url-status=live|publisher=routeralley.com.|access-date=25 April 2014}}</ref>

;Update Timer
:Controls the interval between two gratuitous Response Messages. By default the value is 30 seconds. The response message is broadcast to all its RIP enabled interface.<ref name="Aaron Balchunas"/>
;Invalid Timer
:The invalid timer specifies how long a routing entry can be in the routing table without being updated. This is also called as expiration Timer. By default, the value is 180 seconds. After the timer expires the hop count of the routing entry will be set to 16, marking the destination as unreachable.<ref name="Aaron Balchunas"/>
;Flush Timer
:The flush timer controls the time between the route is invalidated or marked as unreachable and removal of entry from the routing table. By default the value is 240 seconds. This is 60 seconds longer than Invalid timer.  So for 60 seconds the router will be advertising about this unreachable route to all its neighbours. This timer must be set to a higher value than the ''invalid timer.''<ref name="Aaron Balchunas"/>
;Holddown Timer
:The hold-down timer is started per route entry, when the hop count is changing from lower value to higher value. This allows the route to get stabilized. During this time no update can be done to that routing entry.  This is not part of the RFC 1058. This is [[Cisco Systems|Cisco's]] implementation.  The default value of this timer is 180 seconds.<ref name="Aaron Balchunas"/>

==Limitations==
* The hop count cannot exceed 15, or routes will be dropped.
* Variable Length Subnet Masks are not supported by RIP version 1 (which is obsolete).
* RIP has slow convergence and [[count to infinity]] problems.<ref>{{Cite journal|url=https://tools.ietf.org/html/rfc1058|title=RFC 1058 Section 2.2|last=C. Hendrik|website=Routing Information Protocol|date=June 1988|publisher=The Internet Society|doi=10.17487/RFC1058 |doi-access=free}}</ref>

==Implementations==
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* [[Cisco IOS]], software used in Cisco routers (supports version 1, version 2 and RIPng)
* Cisco NX-OS software used in Cisco Nexus data center switches (supports RIPv2 only<ref>{{Cite web|url=https://www.cisco.com/c/en/us/td/docs/switches/datacenter/nexus9000/sw/6-x/unicast/configuration/guide/l3_cli_nxos/l3_rip.html|title=Cisco Nexus 9000 Series NX-OS Unicast Routing Configuration Guide, Release 6.x - Configuring RIP &#91;Cisco Nexus 9000 Series Switches&#93;}}</ref>)
* [[Junos]] software used in Juniper routers, switches, and firewalls (supports RIPv1 and RIPv2)
* Routing and Remote Access, a [[Windows Server 2003|Windows Server]] feature, contains RIP support
* [[Quagga (software)|Quagga]], a [[free software|free]] [[open source software]] routing suite based on [[GNU Zebra]]
* [[Bird Internet routing daemon|BIRD]], a [[free software|free]] [[open source software]] routing suite
* [[Zeroshell]], a [[free software|free]] [[open source software]] routing suite
* A RIP implementation first introduced in [[Berkeley Software Distribution#4.2BSD|4.2BSD]], routed, survives in several of its descendants, including [[FreeBSD]]<ref>{{cite web|url=http://www.freebsd.org/cgi/man.cgi?query=routed&sektion=8|title=routed, rdisc – network RIP and router discovery routing daemon|website=FreeBSD manual pages}}</ref> and [[NetBSD]].<ref>{{cite web|url=http://netbsd.gw.com/cgi-bin/man-cgi?routed+8+NetBSD-current|title=routed, rdisc – network RIP and router discovery routing daemon|website=NetBSD manual pages}}</ref>
* [[OpenBSD]] introduced a new implementation, ripd, in version 4.1<ref>{{cite web|url=https://man.openbsd.org/ripd.8|title=ripd – Routing Information Protocol daemon|website=OpenBSD manual pages}}</ref> and retired routed in version 4.4.
* [[Netgear]] routers commonly offer a choice of two implementations of RIPv2;<ref>{{cite web|url=http://kb.netgear.com/app/answers/detail/a_id/24088|title=How do I change the LAN TCP/IP settings on my Nighthawk router?|website=Netgear Support pages}}</ref> these are labelled RIP_2M and RIP_2B. RIP_2M is the standard RIPv2 implementation using multicasting - which requires all routers on the network to support RIPv2 and multicasting, whereas RIP_2B sends RIPv2 packets using subnet broadcasting - making it more compatible with routers that do not support multicasting, including RIPv1 routers.
* [[Huawei]] HG633 ADSL/VDSL routers support passive and active routing with RIP v1 & v2 on the LAN and WAN side.

==Similar protocols==
[[Cisco]]'s proprietary [[Interior Gateway Routing Protocol]] (IGRP) was a somewhat more capable protocol than RIP. It belongs to the same basic family of [[distance-vector routing protocol]]s. 

Cisco has ceased support and distribution of IGRP in their router software. It was replaced by the [[Enhanced Interior Gateway Routing Protocol]] (EIGRP) which is a completely new design. While EIGRP still uses a distance-vector model, it relates to IGRP only in using the same composite routing metric. Both IGRP and EIGRP calculated a single composite metric for each route, from a formula of five variables: [[bandwidth (computing)|bandwidth]], [[Network delay|delay]], [[Reliability (computer networking)|reliability]], [[Load (computing)|load]], and [[Maximum transmission unit|MTU]]; though on Cisco routers, by default, only bandwidth and delay are used in this calculation.

==See also==
* [[Convergence (routing)]]

==References==
{{Reflist}}

==Further reading==
* Malkin, Gary Scott (2000). ''RIP: An Intra-Domain Routing Protocol''. Addison-Wesley Longman. {{ISBN|0-201-43320-6}}.
* Edward A. Taft, ''Gateway Information Protocol (revised)'' (Xerox Parc, Palo Alto, May, 1979)
* ''Xerox System Integration Standard - Internet Transport Protocols'' (Xerox, Stamford, 1981)

[[Category:Internet Standards]]
[[Category:Internet protocols]]
[[Category:Routing protocols]]