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56-639: A separate Internet Control Message Protocol (called ICMPv6 ) is used with IPv6 . ICMP is part of the Internet protocol suite as defined in RFC 792. ICMP messages are typically used for diagnostic or control purposes or generated in response to errors in IP operations (as specified in RFC 1122). ICMP errors are directed to the source IP address of the originating packet. For example, every device (such as an intermediate router ) forwarding an IP datagram first decrements

112-415: A CPU . More sophisticated devices use application-specific integrated circuits (ASICs) to increase performance or add advanced filtering and firewall functionality. When multiple routers are used in interconnected networks, the routers can exchange information about destination addresses using a routing protocol . Each router builds up a routing table , a list of routes, between two computer systems on

168-540: A pseudo-header of IPv6 header fields according to the IPv6 standard, which consists of the source and destination addresses, the packet length and the next header field, the latter of which is set to the value 58 . Following this pseudo header, the checksum is continued with the ICMPv6 message. The checksum computation is performed according to Internet protocol standards using 16-bit ones' complement summation, followed by

224-680: A switching node using software and an interface computer were first proposed by Donald Davies for the NPL network in 1966. The same idea was conceived by Wesley Clark the following year for use in the ARPANET , which were named Interface Message Processors (IMPs). The first interface computer was implemented at the National Physical Laboratory in the United Kingdom in early 1969, followed later that year by

280-497: A wide area network (WAN), so they may have considerable memory installed, multiple WAN interface connections, and substantial onboard data processing routines. They may also provide connectivity to groups of file servers or other external networks. In enterprises, a core router may provide a collapsed backbone interconnecting the distribution tier routers from multiple buildings of a campus, or large enterprise locations. They tend to be optimized for high bandwidth but lack some of

336-502: A data packet comes in on a line, the router reads the network address information in the packet header to determine the ultimate destination. Then, using information in its routing table or routing policy , it directs the packet to the next network on its journey. Data packets are forwarded from one router to another through an internetwork until it reaches its destination node . The most familiar type of IP routers are home and small office routers that forward IP packets between

392-534: A final ones' complement of the checksum itself and inserting it into the checksum field. Note that this differs from the way it is calculated for IPv4 in ICMP , but is similar to the calculation done in TCP . The payload of an ICMPv6 message varies according the type of message being sent. It begins at bit 32 immediately after the header described above. For some messages such as destination unreachable or time exceeded there

448-403: A fixed network maximum load. An ICMP error message must never be sent in response to another ICMP error message. Router (computing) A router is a computer and networking device that forwards data packets between computer networks , including internetworks such as the global Internet . A router is connected to two or more data lines from different IP networks . When

504-444: A framework for extensions to implement new features. Several extensions have been published, defining new ICMPv6 message types as well as new options for existing ICMPv6 message types. For example, Neighbor Discovery Protocol (NDP) is a node discovery protocol based on ICMPv6 which replaces and enhances functions of ARP . Secure Neighbor Discovery (SEND) is an extension of NDP with extra security. Multicast Listener Discovery (MLD)

560-470: A layer 3 protocol in the modern five-layer TCP/IP protocol definitions (by Kozierok, Comer, Tanenbaum, Forouzan, Kurose, Stallings). There is no TCP or UDP port number associated with ICMP packets as these numbers are associated with the transport layer above. The ICMP packet is encapsulated in an IPv4 packet. The packet consists of header and data sections. The ICMP header starts after the IPv4 header and

616-452: A part of reconnaissance attack to gather information on the target network, therefore ICMP Address Mask Reply is disabled by default on Cisco IOS. Address mask reply is used to reply to an address mask request message with an appropriate subnet mask. Where: Destination unreachable is generated by the host or its inbound gateway to inform the client that the destination is unreachable for some reason. Reasons for this message may include:

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672-414: A security feature by all experts. Some experts argue that open source routers are more secure and reliable than closed source routers because errors and potentially exploitable vulnerabilities are more likely to be discovered and addressed in an open-source environment. Routers are also often distinguished on the basis of the network in which they operate. A router in a local area network (LAN) of

728-576: A single organization is called an interior router . A router that is operated in the Internet backbone is described as exterior router . While a router that connects a LAN with the Internet or a wide area network (WAN) is called a border router , or gateway router . Routers intended for ISP and major enterprise connectivity usually exchange routing information using the Border Gateway Protocol (BGP). RFC   4098 defines

784-418: A variety of sources, such as a default or static routes that are configured manually, or dynamic entries from routing protocols where the router learns routes from other routers. A default route is one that is used to route all traffic whose destination does not otherwise appear in the routing table; it is common – even necessary – in small networks, such as a home or small business where

840-423: Is a mechanism for routers to convey routing information to hosts. The message informs a host to update its routing information (to send packets on an alternative route). If a host tries to send data through a router (R1) and R1 sends the data on another router (R2) and a direct path from the host to R2 is available (that is, the host and R2 are on the same subnetwork ), then R1 will send a redirect message to inform

896-532: Is carried in one or more Extention Objects, which are preceded by an ICMP Extension Header. Extension objects have the following general structure: ICMPv6 Internet Control Message Protocol version 6 ( ICMPv6 ) is the implementation of the Internet Control Message Protocol (ICMP) for Internet Protocol version 6 (IPv6). ICMPv6 is an integral part of IPv6 and performs error reporting and diagnostic functions. ICMPv6 has

952-403: Is discarded until the queue is no longer full. But as no acknowledgement mechanism is present in the network layer, the client does not know whether the data has reached the destination successfully. Hence some remedial measures should be taken by the network layer to avoid these kind of situations. These measures are referred to as source quench. In a source quench mechanism, the router sees that

1008-445: Is generated by a gateway to inform the source of a discarded datagram due to the time to live field reaching zero. A time exceeded message may also be sent by a host if it fails to reassemble a fragmented datagram within its time limit. Time exceeded messages are used by the traceroute utility to identify gateways on the path between two hosts. Where: Timestamp is used for time synchronization. The originating timestamp

1064-521: Is identified by its protocol number , 1 . All ICMP packets have an eight-byte header and variable-sized data section. The first four bytes of the header have fixed format, while the last four bytes depend on the type and code of the ICMP packet. ICMP error messages contain a data section that includes a copy of the entire IPv4 header, plus at least the first eight bytes of data from the IPv4 packet that caused

1120-399: Is implemented using the ICMP echo request and echo reply messages. ICMP uses the basic support of IP as if it were a higher-level protocol, however, ICMP is actually an integral part of IP. Although ICMP messages are contained within standard IP packets, ICMP messages are usually processed as a special case, distinguished from normal IP processing. In many cases, it is necessary to inspect

1176-738: Is in common use. Some routers can connect to Data service units for T1 connections via serial ports. The hierarchical internetworking model divides enterprise networks into three layers: core, distribution, and access. Access routers, including small office/home office (SOHO) models, are located at home and customer sites such as branch offices that do not need hierarchical routing of their own. Typically, they are optimized for low cost. Some SOHO routers are capable of running alternative free Linux-based firmware like Tomato , OpenWrt , or DD-WRT . Distribution routers aggregate traffic from multiple access routers. Distribution routers are often responsible for enforcing quality of service across

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1232-453: Is no defined message body. Others define a use only for the first four bytes of the body with no other defined content: In the case of NDP messages the first four bytes are either reserved or used for flags/hoplimit. While the reset of body has unspecified structured data: For a redirect the first bytes of the message body are reserved but not used. This is followed by a Target and destination address. Unspecified options can be attached to

1288-455: Is set to the time (in milliseconds since midnight) the sender last touched the packet. The receive and transmit timestamps are not used. Where: Timestamp Reply replies to a Timestamp message. It consists of the originating timestamp sent by the sender of the Timestamp as well as a receive timestamp indicating when the Timestamp was received and a transmit timestamp indicating when

1344-524: Is set to the value 58 . The ICMPv6 message consists of a header and the protocol payload. The header contains only three fields: Type (8 bits), Code (8 bits), and Checksum (16 bits). Control messages are identified by the value in the type field. The code field gives additional context information for the message. Some messages serve the same purpose as the correspondingly named ICMP message types. There are two subtypes of Multicast Listener Query messages: These two subtypes are differentiated by

1400-413: Is the simplest and most easily implemented: the router simply drops new incoming packets once buffer space in the router is exhausted. RED probabilistically drops datagrams early when the queue exceeds a pre-configured portion of the buffer, until reaching a pre-determined maximum, when it drops all incoming packets, thus reverting to tail drop. WRED can be configured to drop packets more readily dependent on

1456-488: Is used by IPv6 routers for discovering multicast listeners on a directly attached link, much like Internet Group Management Protocol (IGMP) is used in IPv4 . Multicast Router Discovery (MRD) allows the discovery of multicast routers. ICMPv6 messages may be classified as error messages and information messages . ICMPv6 messages are transported by IPv6 packets in which the IPv6 Next Header value for ICMPv6

1512-482: Is used to enable data packets to be forwarded from one transmission system to another. Routers may also be used to connect two or more logical groups of computer devices known as subnets , each with a unique network prefix . Routers may provide connectivity within enterprises, between enterprises and the Internet, or between internet service providers ' (ISPs') networks, they are also responsible for directing data between different networks. The largest routers (such as

1568-652: The Cisco CRS-1 or Juniper PTX) interconnect the various ISPs, or may be used in large enterprise networks. Smaller routers usually provide connectivity for typical home and office networks. All sizes of routers may be found inside enterprises. The most powerful routers are usually found in ISPs, academic and research facilities. Large businesses may also need more powerful routers to cope with ever-increasing demands of intranet data traffic. A hierarchical internetworking model for interconnecting routers in large networks

1624-520: The International Network Working Group (INWG). These gateway devices were different from most previous packet switching schemes in two ways. First, they connected dissimilar kinds of networks, such as serial lines and local area networks . Second, they were connectionless devices, which had no role in assuring that traffic was delivered reliably, leaving that function entirely to the hosts . This particular idea,

1680-651: The Timestamp reply was sent. Where: The use of Timestamp and Timestamp Reply messages to synchronize the clocks of Internet nodes has largely been replaced by the UDP-based Network Time Protocol and the Precision Time Protocol . Address mask request is normally sent by a host to a router in order to obtain an appropriate subnet mask . Recipients should reply to this message with an Address mask reply message. Where: ICMP Address Mask Request may be used as

1736-637: The end-to-end principle , was pioneered in the CYCLADES network. The idea was explored in more detail, with the intention to produce a prototype system as part of two contemporaneous programs. One was a program at Xerox PARC to explore new networking technologies, which produced the PARC Universal Packet system. Some time after early 1974, the first Xerox routers became operational. Due to corporate intellectual property concerns, it received little attention outside Xerox for years. The other

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1792-552: The time to live (TTL) field in the IP header by one. If the resulting TTL is 0, the packet is discarded and an ICMP time exceeded message is sent to the datagram's source address. Many commonly used network utilities are based on ICMP messages. The traceroute command can be implemented by transmitting IP datagrams with specially set IP TTL header fields, and looking for ICMP time exceeded in transit and destination unreachable messages generated in response. The related ping utility

1848-796: The IMPs at the University of California, Los Angeles , the Stanford Research Institute , the University of California, Santa Barbara , and the University of Utah School of Computing in the United States. All were built with the Honeywell 516 . These computers had fundamentally the same functionality as a router does today. The idea for a router (called a gateway at the time) initially came about through an international group of computer networking researchers called

1904-526: The contents of the ICMP message and deliver the appropriate error message to the application responsible for transmitting the IP packet that prompted the ICMP message to be sent. ICMP is a network-layer protocol; this makes it a layer 3 protocol in the seven-layer OSI model . Based on the four-layer TCP/IP model, ICMP is an internet-layer protocol, which makes it a layer 2 protocol in the Internet Standard RFC 1122 TCP/IP four-layer model or

1960-471: The contents of the Multicast Address field, as described in section 3.6 of RFC 2710 Note that the table above is not comprehensive. The current complete list of assigned ICMPv6 types can be found at this link: IANA: ICMPv6 Parameters . ICMPv6 provides a minimal level of message integrity verification by the inclusion of a 16-bit checksum in its header. The checksum is calculated starting with

2016-508: The default route simply sends all non-local traffic to the Internet service provider . The default route can be manually configured (as a static route); learned by dynamic routing protocols; or be obtained by DHCP . A router can run more than one routing protocol at a time, particularly if it serves as an autonomous system border router between parts of a network that run different routing protocols; if it does so, then redistribution may be used (usually selectively) to share information between

2072-402: The different protocols running on the same router. Besides deciding to which interface a packet is forwarded, which is handled primarily via the routing table, a router also has to manage congestion when packets arrive at a rate higher than the router can process. Three policies commonly used are tail drop , random early detection (RED), and weighted random early detection (WRED). Tail drop

2128-405: The end: When an ICMPv6 node receives a packet, it must undertake actions that depend on the type of message. The ICMPv6 protocol must limit the number of error messages sent to the same destination to avoid network overloading. For example, if a node continues to forward erroneous packets, ICMP will signal the error to the first packet and then do so periodically, with a fixed minimum period or with

2184-618: The error message. The length of ICMP error messages should not exceed 576 bytes. This data is used by the host to match the message to the appropriate process. If a higher level protocol uses port numbers, they are assumed to be in the first eight bytes of the original datagram's data. The variable size of the ICMP packet data section has been exploited . In the " Ping of death ", large or fragmented ICMP packets are used for denial-of-service attacks . ICMP data can also be used to create covert channels for communication. These channels are known as ICMP tunnels . Control messages are identified by

2240-416: The features of edge routers. External networks must be carefully considered as part of the overall security strategy of the local network. A router may include a firewall , VPN handling, and other security functions, or they may be handled by separate devices. Routers also commonly perform network address translation which restricts connections initiated from external connections but is not recognized as

2296-414: The home computers and the Internet. More sophisticated routers, such as enterprise routers, connect large business or ISP networks to powerful core routers that forward data at high speed along the optical fiber lines of the Internet backbone . Routers can be built from standard computer parts but are mostly specialized purpose-built computers . Early routers used software -based forwarding, running on

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2352-498: The host that the best route for the destination is via R2. The host should then change its route information and send packets for that destination directly to R2. The router will still send the original datagram to the intended destination. However, if the datagram contains routing information, this message will not be sent even if a better route is available. RFC 1122 states that redirects should only be sent by gateways and should not be sent by Internet hosts. Where: Time Exceeded

2408-415: The incoming data rate is much faster than the outgoing data rate, and sends an ICMP message to the clients, informing them that they should slow down their data transfer speeds or wait for a certain amount of time before attempting to send more data. When a client receives this message, it automatically slows down the outgoing data rate or waits for a sufficient amount of time, which enables the router to empty

2464-401: The interconnected networks. The software that runs the router is composed of two functional processing units that operate simultaneously, called planes : A router may have interfaces for multiple types of physical layer connections, such as copper cables, fiber optic , or wireless transmission. It can also support multiple network layer transmission standards. Each network interface

2520-423: The layer-3 IP packet, specifically the destination IP address. When a router receives a packet, it searches its routing table to find the best match between the destination IP address of the packet and one of the addresses in the routing table. Once a match is found, the packet is encapsulated in the layer-2 data link frame for the outgoing interface indicated in the table entry. A router typically does not look into

2576-505: The mid-1970s and in the 1980s, general-purpose minicomputers served as routers. Modern high-speed routers are network processors or highly specialized computers with extra hardware acceleration added to speed both common routing functions, such as packet forwarding, and specialized functions such as IPsec encryption. There is substantial use of Linux and Unix software-based machines, running open source routing code, for research and other applications. The Cisco IOS operating system

2632-471: The packet payload, but only at the layer-3 addresses to make a forwarding decision, plus optionally other information in the header for hints on, for example, quality of service (QoS). For pure IP forwarding, a router is designed to minimize the state information associated with individual packets. Once a packet is forwarded, the router does not retain any historical information about the packet. The routing table itself can contain information derived from

2688-496: The physical connection to the host does not exist (distance is infinite); the indicated protocol or port is not active; the data must be fragmented but the 'don't fragment' flag is on. Unreachable TCP ports notably respond with TCP RST rather than a destination unreachable type 3 as might be expected. Destination unreachable is never reported for IP multicast transmissions. With the following field contents: ICMP messages can be extended with extra information. This information

2744-511: The queue. Thus the source quench ICMP message acts as flow control in the network layer. Since research suggested that "ICMP Source Quench [was] an ineffective (and unfair) antidote for congestion", routers' creation of source quench messages was deprecated in 1995 by RFC 1812. Furthermore, forwarding of and any kind of reaction to (flow control actions) source quench messages was deprecated from 2012 by RFC 6633. Where: Redirect requests data packets be sent on an alternative route. ICMP Redirect

2800-411: The router or host buffer is approaching its limit. Data is sent at a very high speed from a host or from several hosts at the same time to a particular router on a network. Although a router has buffering capabilities, the buffering is limited to within a specified range. The router cannot queue any more data than the capacity of the limited buffering space. Thus if the queue gets filled up, incoming data

2856-411: The type of traffic. Another function a router performs is traffic classification and deciding which packet should be processed first. This is managed through QoS , which is critical when Voice over IP is deployed, so as not to introduce excessive latency . Yet another function a router performs is called policy-based routing where special rules are constructed to override the rules derived from

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2912-440: The types of BGP routers according to their functions: Wi-Fi routers combine the functions of a router with those of a wireless access point . They are typically devices with a small form factor, operating on the standard electric power supply for residential use. Connected to the Internet as offered by an Internet service provider , they provide Internet access through a wireless network for home or office use. The concepts of

2968-414: The value in the type field. The code field gives additional context information for the message. Some control messages have been deprecated since the protocol was first introduced. Source Quench requests that the sender decrease the rate of messages sent to a router or host. This message may be generated if a router or host does not have sufficient buffer space to process the request, or may occur if

3024-408: Was independently designed. Major router operating systems, such as Junos and NX-OS , are extensively modified versions of Unix software. The main purpose of a router is to connect multiple networks and forward packets destined either for directly attached networks or more remote networks. A router is considered a layer-3 device because its primary forwarding decision is based on the information in

3080-436: Was led by William Yeager and MIT's by Noel Chiappa . Virtually all networking now uses TCP/IP, but multiprotocol routers are still manufactured. They were important in the early stages of the growth of computer networking when protocols other than TCP/IP were in use. Modern routers that handle both IPv4 and IPv6 are multiprotocol but are simpler devices than ones processing AppleTalk, DECnet, IPX, and Xerox protocols. From

3136-782: Was the DARPA -initiated program, which created the TCP/IP architecture in use today. The first true IP router was developed by Ginny Travers at BBN , as part of that DARPA-initiated effort, during 1975–1976. By the end of 1976, three PDP-11 -based routers were in service in the experimental prototype Internet. Mike Brecia, Ginny Travers, and Bob Hinden received the IEEE Internet Award for early IP routers in 2008. The first multiprotocol routers were independently created by staff researchers at MIT and Stanford in 1981 and both were also based on PDP-11s. Stanford's router program

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