The Common Address Redundancy Protocol or CARP is a computer

networking protocol A communication protocol is a system of rules that allows two or more entities of a communications system to transmit information via any kind of variation of a physical quantity. The protocol defines the rules, syntax, semantics and synchroniza ...

which allows multiple

hosts A host is a person responsible for guests at an event or for providing hospitality during it. Host may also refer to: Places * Host, Pennsylvania, a village in Berks County People *Jim Host (born 1937), American businessman *Michel Host ...

on the same

local area network A local area network (LAN) is a computer network that interconnects computers within a limited area such as a residence, school, laboratory, university campus or office building. By contrast, a wide area network (WAN) not only covers a large ...

to share a set of

IP addresses An Internet Protocol address (IP address) is a numerical label such as that is connected to a computer network that uses the Internet Protocol for communication.. Updated by . An IP address serves two main functions: network interface ident ...

. Its primary purpose is to provide

failover Failover is switching to a redundant or standby computer server, system, hardware component or network upon the failure or abnormal termination of the previously active application, server, system, hardware component, or network in a computer net ...

redundancy, especially when used with firewalls and routers. In some configurations, CARP can also provide load balancing functionality. CARP provides functionality similar to Virtual Router Redundancy Protocol (VRRP) and to

Cisco Systems Cisco Systems, Inc., commonly known as Cisco, is an American-based multinational digital communications technology conglomerate corporation headquartered in San Jose, California. Cisco develops, manufactures, and sells networking hardware, ...

Hot Standby Router Protocol In computer networking, the Hot Standby Router Protocol (HSRP) is a Cisco proprietary redundancy protocol for establishing a fault-tolerant default gateway. Version 1 of the protocol was described in in 1998. Version 2 of the protocol includes ...

(HSRP). It is implemented in several BSD-based

operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ef ...

s and has been

ported In software engineering, porting is the process of adapting software for the purpose of achieving some form of execution in a computing environment that is different from the one that a given program (meant for such execution) was originally desi ...

Linux Linux ( or ) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution, whi ...

(ucarp).ucarp manpage
/ref>

Example

If there is a single computer running a

packet filter In computing, a firewall is a network security system that monitors and controls incoming and outgoing network traffic based on predetermined security rules. A firewall typically establishes a barrier between a trusted network and an untrusted n ...

, and it goes down, the networks on either side of the packet filter can no longer communicate with each other, or they communicate without any packet filtering. If, however, there are two computers running a packet filter, running CARP, then if one fails, the other will take over, and computers on either side of the packet filter will not be aware of the failure, so operation will continue as normal. In order to make sure the new active/primary operates the same as the old one, the packet filter used must support synchronization of state between the two computers.

Principle of redundancy

A group of hosts using CARP is called a "group of redundancy". The group of redundancy allocates itself an IP address which is shared or divided among the members of the group. Within this group, a host is designated as "active/primary". The other members are "standby". The main host is that which "takes" the IP address. It answers any traffic or ARP request brought to the attention of this address. Each host can belong to several groups of redundancy. Each host must have a second unique IP address. A common use of CARP is the creation of a group of redundant firewalls. The

virtual IP address A virtual IP address (VIP or VIPA) is an IP address that does not correspond to a physical network interface. Uses for VIPs include network address translation (especially, one-to-many NAT), fault-tolerance, and mobility. Usage For one-to-man ...

allotted to the group of redundancy is indicated as the address of the default router on the computers behind this group of firewalls. If the main firewall breaks down or is disconnected from the network, the virtual IP address will be taken by one of the firewall slaves and the service availability will not be interrupted.

History

In the late 1990s the

Internet Engineering Task Force The Internet Engineering Task Force (IETF) is a standards organization for the Internet and is responsible for the technical standards that make up the Internet protocol suite (TCP/IP). It has no formal membership roster or requirements an ...

(IETF) began work on a protocol for router redundancy. In 1997, Cisco informed the IETF that it had patents in this area and, in 1998, pointed out its patent on

HSRP In computer networking, the Hot Standby Router Protocol (HSRP) is a Cisco proprietary redundancy protocol for establishing a fault-tolerant default gateway. Version 1 of the protocol was described in in 1998. Version 2 of the protocol includes ...

. Nonetheless, IETF continued work on VRRP. After some debate, the IETF VRRP working group decided to approve the standard, despite its reliance on patented techniques, as long as Cisco made the patent available to third parties under

reasonable and non-discriminatory licensing Reasonable and non-discriminatory (RAND) terms, also known as fair, reasonable, and non-discriminatory (FRAND) terms, denote a voluntary licensing commitment that standards organizations often request from the owner of an intellectual property r ...

terms. Because VRRP fixed problems with the HSRP protocol, Cisco began using VRRP instead, while still claiming it as its own. Cisco informed the

OpenBSD OpenBSD is a security-focused, free and open-source, Unix-like operating system based on the Berkeley Software Distribution (BSD). Theo de Raadt created OpenBSD in 1995 by forking NetBSD 1.0. According to the website, the OpenBSD project e ...

developers that it would enforce its patent on HSRP. Cisco's position may have been due to their lawsuit with Alcatel. As Cisco's licensing terms prevented an open-source VRRP implementation, the OpenBSD developers begun developing CARP instead. OpenBSD focuses on security. They designed CARP to use

cryptography Cryptography, or cryptology (from grc, , translit=kryptós "hidden, secret"; and ''graphein'', "to write", or '' -logia'', "study", respectively), is the practice and study of techniques for secure communication in the presence of adv ...

. This made CARP fundamentally different from VRRP and ensured that CARP did not infringe on Cisco's patent. CARP became available in October 2003. Later, it was integrated into

FreeBSD FreeBSD is a free and open-source Unix-like operating system descended from the Berkeley Software Distribution (BSD), which was based on Research Unix. The first version of FreeBSD was released in 1993. In 2005, FreeBSD was the most popular ...

(first released in May 2005 with FreeBSD 5.4),

NetBSD NetBSD is a free and open-source Unix operating system based on the Berkeley Software Distribution (BSD). It was the first open-source BSD descendant officially released after 386BSD was forked. It continues to be actively developed and is ava ...

and

(ucarp). While Cisco's US patent expired in 2014, the two incompatible protocols continue to coexist.

Incompatibility with IETF standards

OpenBSD uses VRRP's protocol number and MAC addresses. The OpenBSD project requested unique numbers from the

Internet Assigned Numbers Authority The Internet Assigned Numbers Authority (IANA) is a standards organization that oversees global IP address allocation, autonomous system number allocation, root zone management in the Domain Name System (DNS), media types, and other Inte ...

(IANA) but was denied. To allocate numbers, IANA has several requirements. At the time, these were specified i
RFC 2780
Requirements include participating in a collaborative, lengthy discussion process within the

IETF The Internet Engineering Task Force (IETF) is a standards organization for the Internet and is responsible for the technical standards that make up the Internet protocol suite (TCP/IP). It has no formal membership roster or requirements an ...

and producing a detailed textual specification of the protocol. The OpenBSD developers met neither requirement. OpenBSD's website states the following: IANA had assigned protocol number 112 to VRRP (in 1998, via RFC 2338). Protocol number 112 remains in use by VRRP. CARP also uses a range of

Ethernet Ethernet () is a family of wired computer networking technologies commonly used in local area networks (LAN), metropolitan area networks (MAN) and wide area networks (WAN). It was commercially introduced in 1980 and first standardized in 1 ...

MAC address A media access control address (MAC address) is a unique identifier assigned to a network interface controller (NIC) for use as a network address in communications within a network segment. This use is common in most IEEE 802 networking te ...

es which

IEEE The Institute of Electrical and Electronics Engineers (IEEE) is a 501(c)(3) professional association for electronic engineering and electrical engineering (and associated disciplines) with its corporate office in New York City and its operati ...

had assigned to IANA/IETF for the VRRP protocol. In spite of the overlap, it is still possible to use VRRP and CARP in the same

broadcast domain A broadcast domain is a logical division of a computer network, in which all nodes can reach each other by broadcast at the data link layer. A broadcast domain can be within the same LAN segment or it can be bridged to other LAN segments. In ...

, as long as the VRRP group ID and the CARP virtual host ID are different.

References

External links