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Network Working Group Request for Comments: 2080 Category: Standards Track

G. Malkin Xylogics R. Minnear Ipsilon Networks January 1997 RIPng for IPv6

Status of this Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Abstract This document specifies a routing protocol for an IPv6 internet. It is based on protocols and algorithms currently in wide use in the IPv4 Internet. This specification represents the minimum change to the Routing Information Protocol (RIP), as specified in RFC 1058 [1] and RFC 1723 [2], necessary for operation over IPv6 [3]. Acknowledgements This document is a modified version of RFC 1058, written by Chuck Hedrick [1]. The modifications reflect RIP-2 and IPv6 enhancements, but the original wording is his. We'd like to thank Dennis Ferguson and Thomas Narten for their input. Table of Contents 1. Introduction . . . . . . . . 1.1 Theoretical Underpinnings 1.2 Limitations of the Protocol 2. Protocol Specification . . . 2.1 Message Format . . . . . . 2.1.1 Next Hop . . . . . . . . 2.2 Addressing Considerations 2.3 Timers . . . . . . . . . . 2.4 Input Processing . . . . . 2.4.1 Request Messages . . . . 2.4.2 Response Messages . . . ... ... .. ... ... ... ... ... ... ... ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2 .3 .3 .4 .5 .7 .8 .9 . 10 . 10 . 11

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2.5 Output Processing . . 2.5.1 Triggered Updates . 2.5.2 Generating Response 2.6 Split Horizon . . . . 3. Control Functions . . . 4. Security Considerations. References . . . . . . . . . Authors' Addresses . . . . . 1. Introduction

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This memo describes one protocol in a series of routing protocols based on the Bellman-Ford (or distance vector) algorithm. This algorithm has been used for routing computations in computer networks since the early days of the ARPANET. The particular packet formats and protocol described here are based on the program "routed," which is included with the Berkeley distribution of Unix. In an international network, such as the Internet, it is very unlikely that a single routing protocol will used for the entire network. Rather, the network will be organized as a collection of Autonomous Systems (AS), each of which will, in general, be administered by a single entity. Each AS will have its own routing technology, which may differ among AS's. The routing protocol used within an AS is referred to as an Interior Gateway Protocol (IGP). A separate protocol, called an Exterior Gateway Protocol (EGP), is used to transfer routing information among the AS's. RIPng was designed to work as an IGP in moderate-size AS's. It is not intended for use in more complex environments. For information on the context into which RIP version 1 (RIP-1) is expected to fit, see Braden and Postel [6]. RIPng is one of a class of algorithms known as Distance Vector algorithms. The earliest description of this class of algorithms known to the author is in Ford and Fulkerson [8]. Because of this, they are sometimes known as Ford-Fulkerson algorithms. The term Bellman-Ford is also used, and derives from the fact that the formulation is based on Bellman's equation [4]. The presentation in this document is closely based on [5]. This document contains a protocol specification. For an introduction to the mathematics of routing algorithms, see [1]. The basic algorithms used by this protocol were used in computer routing as early as 1969 in the ARPANET. However, the specific ancestry of this protocol is within the Xerox network protocols. The PUP protocols [7] used the Gateway Information Protocol to exchange routing information. A somewhat updated version of this protocol was adopted for the Xerox Network Systems (XNS) architecture, with the name Routing Information Protocol [9]. Berkeley's routed is largely the same as the Routing

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Information Protocol, with XNS addresses replaced by a more address format capable of handling IPv4 and other types of and with routing updates limited to one every 30 seconds. this similarity, the term Routing Information Protocol (or is used to refer to both the XNS protocol and the protocol routed. 1.1 Theoretical Underpinnings

general address, Because of just RIP) used by

An introduction to the theory and math behind Distance Vector protocols is provided in [1]. It has not been incorporated in this document for the sake of brevity. 1.2 Limitations of the Protocol This protocol does not solve every possible routing problem. As mentioned above, it is primarily intended for use as an IGP in networks of moderate size. In addition, the following specific limitations are be mentioned: - The protocol is limited to networks whose longest path (the network's diameter) is 15 hops. The designers believe that the basic protocol design is inappropriate for larger networks. Note that this statement of the limit assumes that a cost of 1 is used for each network. This is the way RIPng is normally configured. If the system administrator chooses to use larger costs, the upper bound of 15 can easily become a problem. - The protocol depends upon "counting to infinity" to resolve certain unusual situations (see section 2.2 in [1]). If the system of networks has several hundred networks, and a routing loop was formed involving all of them, the resolution of the loop would require either much time (if the frequency of routing updates were limited) or bandwidth (if updates were sent whenever changes were detected). Such a loop would consume a large amount of network bandwidth before the loop was corrected. We believe that in realistic cases, this will not be a problem except on slow lines. Even then, the problem will be fairly unusual, since various precautions are taken that should prevent these problems in most cases. - This protocol uses fixed "metrics" to compare alternative routes. It is not appropriate for situations where routes need to be chosen based on real-time parameters such a measured delay, reliability, or load. The obvious extensions to allow metrics of this type are likely to introduce instabilities of a sort that the protocol is not designed to handle.

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2. Protocol Specification RIPng is intended to allow routers to exchange information for computing routes through an IPv6-based network. RIPng is a distance vector protocol, as described in [1]. RIPng should be implemented only in routers; IPv6 provides other mechanisms for router discovery [10]. Any router that uses RIPng is assumed to have interfaces to one or more networks, otherwise it isn't really a router. These are referred to as its directly-connected networks. The protocol relies on access to certain information about each of these networks, the most important of which is its metric. The RIPng metric of a network is an integer between 1 and 15, inclusive. It is set in some manner not specified in this protocol; however, given the maximum path limit of 15, a value of 1 is usually used. Implementations should allow the system administrator to set the metric of each network. In addition to the metric, each network will have an IPv6 destination address prefix and prefix length associated with it. These are to be set by the system administrator in a manner not specified in this protocol. Each router that implements RIPng is assumed to have a routing table. This table has one entry for every destination that is reachable throughout the system operating RIPng. Each entry contains at least the following information: - The IPv6 prefix of the destination. - A metric, which represents the total cost of getting a datagram from the router to that destination. This metric is the sum of the costs associated with the networks that would be traversed to get to the destination. - The IPv6 address of the next router along the path to the destination (i.e., the next hop). If the destination is on one of the directly-connected networks, this item is not needed. - A flag to indicate that information about the route has changed recently. This will be referred to as the "route change flag." - Various timers associated with the route. details on timers. See section 2.3 for more

The entries for the directly-connected networks are set up by the router using information gathered by means not specified in this protocol. The metric for a directly-connected network is set to the cost of that network. As mentioned, 1 is the usual cost. In that case, the RIPng metric reduces to a simple hop-count. More complex metrics may be used when it is desirable to show preference for some

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networks over others (e.g., to indicate of differences in bandwidth or reliability). Implementors may also choose to enter additional routes. These or networks outside the scope of referred to as "static routes." these initial ones are added and in the following sections. allow the system administrator to would most likely be routes to hosts the routing system. They are Entries for destinations other than updated by the algorithms described

In order for the protocol to provide complete every router in the AS must participate in the where multiple IGPs are in use, there must be which can leak routing information between the 2.1 Message Format

information on routing, protocol. In cases at least one router protocols.

RIPng is a UDP-based protocol. Each router that uses RIPng has a routing process that sends and receives datagrams on UDP port number 521, the RIPng port. All communications intended for another router's RIPng process are sent to the RIPng port. All routing update messages are sent from the RIPng port. Unsolicited routing update messages have both the source and destination port equal to the RIPng port. Those sent in response to a request are sent to the port from which the request came. Specific queries may be sent from ports other than the RIPng port, but they must be directed to the RIPng port on the target machine. The RIPng packet format is: 0 1 2 3 01234567890123456789012345678901 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | command (1) | version (1) | must be zero (2) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Route Table Entry 1 (20) ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ ... ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Route Table Entry N (20) ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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where each Route Table Entry (RTE) has the following format: 0 1 2 3 01234567890123456789012345678901 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ IPv6 prefix (16) ~ | | +---------------------------------------------------------------+ | route tag (2) | prefix len (1)| metric (1) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The maximum number of RTEs is defined below. Field sizes are given in octets. Unless otherwise specified, fields contain binary integers, in network byte order, with the mostsignificant octet first (big-endian). Each tick mark represents one bit. Every message contains a RIPng header which consists of a version number. This document describes version 1 of (see section 2.4). The command field is used to specify of this message. The commands implemented in version 1 1 - request a command and the protocol the purpose are:

A request for the responding system to send all or part of its routing table. A message containing routing table. This to a request, or it update generated by all or part of the sender's message may be sent in response may be an unsolicited routing the sender.

2 - response

For each of contains a destination the cost to

these message types, the remainder of the datagram list of RTEs. Each RTE in this list contains a prefix, the number of significant bits in the prefix, and reach that destination (metric).

The destination prefix is the usual 128-bit, IPv6 address prefix stored as 16 octets in network byte order. The route tag field is an attribute assigned to a route which must be preserved and readvertised with a route. The intended use of the route tag is to provide a method of separating "internal" RIPng routes (routes for networks within the RIPng routing domain) from "external" RIPng routes, which may have been imported from an EGP or another IGP.

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Routers supporting protocols other than RIPng should be configurable to allow the route tag to be configured for routes imported from different sources. For example, routes imported from an EGP should be able to have their route tag either set to an arbitrary value, or at least to the number of the Autonomous System from which the routes were learned. Other uses of the route tag are valid, as long as all routers in the RIPng domain use it consistently. The prefix length field is the length in bits of the significant part of the prefix (a value between 0 and 128 inclusive) starting from the left of the prefix. The metric field contains a value between 1 and 15 inclusive, specifying the current metric for the destination; or the value 16 (infinity), which indicates that the destination is not reachable. The maximum datagram size is limited by the MTU of the medium over which the protocol is being used. Since an unsolicited RIPng update is never propagated across a router, there is no danger of an MTU mismatch. The determination of the number of RTEs which may be put into a given message is a function of the medium's MTU, the number of octets of header information preceeding the RIPng message, the size of the RIPng header, and the size of an RTE. The formula is: +| MTU - sizeof(IPv6_hdrs) - UDP_hdrlen - RIPng_hdrlen #RTEs = INT | --------------------------------------------------| RTE_size +2.1.1 Next Hop -+ | | | -+

RIPng provides the ability to specify the immediate next hop IPv6 address to which packets to a destination specified by a route table entry (RTE) should be forwarded in much the same way as RIP-2 [2]. In RIP-2, each route table entry has a next hop field. Including a next hop field for each RTE in RIPng would nearly double the size of the RTE. Therefore, in RIPng, the next hop is specified by a special RTE and applies to all of the address RTEs following the next hop RTE until the end of the message or until another next hop RTE is encountered. A next hop RTE is identified by a value of 0xFF in the metric field of an RTE. The prefix field specifies the IPv6 address of the next hop. The route tag and prefix length in the next hop RTE must be set to zero on sending and ignored on receiption.

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The next hop Route Table Entry (RTE) has the following format: 0 1 2 3 01234567890123456789012345678901 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ IPv6 next hop address (16) ~ | | +---------------------------------------------------------------+ | must be zero (2) |must be zero(1)| 0xFF | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Specifying a value of 0:0:0:0:0:0:0:0 in the prefix field of a next hop RTE indicates that the next hop address should be the originator of the RIPng advertisement. An address specified as a next hop must be a link-local address. The purpose of the next hop RTE is to eliminate packets being routed through extra hops in the system. It is particularly useful when RIPng is not being run on all of the routers on a network. Note that next hop RTE is "advisory". That is, if the provided information is ignored, a possibly sub-optimal, but absolutely valid, route may be taken. If the received next hop address is not a link-local address, it should be treated as 0:0:0:0:0:0:0:0. 2.2 Addressing Considerations The distinction between network, subnet and host routes does not need to be made for RIPng because an IPv6 address prefix is unambiguous. Any prefix with a prefix length of zero is used to designate a default route. It is suggested that the prefix 0:0:0:0:0:0:0:0 be used when specifying the default route, though the prefix is essentially ignored. A default route is used when it is not convenient to list every possible network in the RIPng updates, and when one or more routers in the system are prepared to handle traffic to the networks that are not explicitly listed. These "default routers" use the default route as a path for all datagrams for which they have no explicit route. The decision as to how a router becomes a default router (i.e., how a default route entry is created) is left to the implementor. In general, the system administrator will be provided with a way to specify which routers should create and advertise default route entries. If this mechanism is used, the implementation should allow the network administrator to choose the metric associated with the default route advertisement. This will make it possible to establish a precedence amoung multiple default routers. The default route entries are handled by RIPng in exactly the same manner as any other destination prefix. System

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administrators not propagate own preferred generally not enforcing this 2.3 Timers

should take care to make sure that default routes do further than is intended. Generally, each AS has its default router. Therefore, default routes should leave the boundary of an AS. The mechanisms for restriction are not specified in this document.

This section describes all events that are triggered by timers. Every 30 seconds, the RIPng process is awakened to send an unsolicited Response message, containing the complete routing table (see section 2.6 on Split Horizon), to every neighboring router. When there are many routers on a single network, there is a tendency for them to synchronize with each other such that they all issue updates at the same time. This can happen whenever the 30 second timer is affected by the processing load on the system. It is undesirable for the update messages to become synchronized, since it can lead to unnecessary collisions on broadcast networks (see [13] for more details). Therefore, implementations are required to take one of two precautions: - The 30-second updates are triggered by a clock whose rate is not affected by system load or the time required to service the previous update timer. - The 30-second timer is offset by a small random time (+/- 0 to 15 seconds) each time it is set. The offset is derived from: 0.5 * the update period (i.e. 30). There are two timers associated with each route, a "timeout" and a "garbage-collection time." Upon expiration of the timeout, the route is no longer valid; however, it is retained in the routing table for a short time so that neighbors can be notified that the route has been dropped. Upon expiration of the garbage-collection timer, the route is finally removed from the routing table. The timeout is initialized when a route is established, and any time an update message is received for the route. If 180 seconds elapse from the last time the timeout was initialized, the route is considered to have expired, and the deletion process described below begins for that route.

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Deletions can occur for one of two reasons: the timeout expires, or the metric is set to 16 because of an update received from the current router (see section 2.4.2 for a discussion of processing updates from other routers). In either case, the following events happen: - The garbage-collection timer is set for 120 seconds. - The metric for the route is set to 16 (infinity). route to be removed from service. This causes the

- The route change flag is to indicate that this entry has been changed. - The output process is signalled to trigger a response. Until the garbage-collection timer expires, the route is included in all updates sent by this router. When the garbage-collection timer expires, the route is deleted from the routing table. Should a new route to this network be established while the garbagecollection timer is running, the new route will replace the one that is about to be deleted. In this case the garbage-collection timer must be cleared. Triggered updates also use a small timer; however, this is best described in section 2.5.1. 2.4 Input Processing This section will describe the handling of datagrams received on the RIPng port. Processing will depend upon the value in the command field. Version 1 supports only two commands: Request and Response. 2.4.1 Request Messages

A Request is used to ask for a response containing all or part of a router's routing table. Normally, Requests are sent as multicasts, from the RIPng port, by routers which have just come up and are seeking to fill in their routing tables as quickly as possible. However, there may be situations (e.g., router monitoring) where the routing table of only a single router is needed. In this case, the Request should be sent directly to that router from a UDP port other than the RIPng port. If such a Request is received, the router responds directly to the requestor's address and port with a globally valid source address since the requestor may not reside on the directly attached network.

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The Request is processed entry by entry. If there are no entries, no response is given. There is one special case. If there is exactly one entry in the request, and it has a destination prefix of zero, a prefix length of zero, and a metric of infinity (i.e., 16), then this is a request to send the entire routing table. In that case, a call is made to the output process to send the routing table to the requesting address/port. Except for this special case, processing is quite simple. Examine the list of RTEs in the Request one by one. For each entry, look up the destination in the router's routing database and, if there is a route, put that route's metric in the metric field of the RTE. If there is no explicit route to the specified destination, put infinity in the metric field. Once all the entries have been filled in, change the command from Request to Response and send the datagram back to the requestor. Note that there is a difference in metric handling for specific and whole-table requests. If the request is for a complete routing table, normal output processing is done, including Split Horizon (see section 2.6 on Split Horizon). If the request is for specific entries, they are looked up in the routing table and the information is returned as is; no Split Horizon processing is done. The reason for this distinction is the expectation that these requests are likely to be used for different purposes. When a router first comes up, it multicasts a Request on every connected network asking for a complete routing table. It is assumed that these complete routing tables are to be used to update the requestor's routing table. For this reason, Split Horizon must be done. It is further assumed that a Request for specific networks is made only by diagnostic software, and is not used for routing. In this case, the requester would want to know the exact contents of the routing table and would not want any information hidden or modified. 2.4.2 Response Messages

A Response can be received for one of several different reasons: - response to a specific query - regular update (unsolicited response) - triggered update caused by a route change Processing is the same no matter why the Response was generated. Because processing of a Response may update the router's routing table, the Response must be checked carefully for validity. The Response must be ignored if it is not from the RIPng port. The datagram's IPv6 source address should be checked to see whether the datagram is from a valid neighbor; the source of the datagram must be a link-local address. It is also worth checking to see whether the

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response is from one of the router's own addresses. Interfaces on broadcast networks may receive copies of their own multicasts immediately. If a router processes its own output as new input, confusion is likely, and such datagrams must be ignored. As an additional check, periodic advertisements must have their hop counts set to 255, and inbound, multicast packets sent from the RIPng port (i.e. periodic advertisement or triggered update packets) must be examined to ensure that the hop count is 255. This absolutely guarantees that a packet is from a neighbor, because any intermediate node would have decremented the hop count. Queries and their responses may still cross intermediate nodes and therefore do not require the hop count test to be done. Once the datagram as a whole has been validated, process the RTEs in the Response one by one. Again, start by doing validation. Incorrect metrics and other format errors usually indicate misbehaving neighbors and should probably be brought to the administrator's attention. For example, if the metric is greater than infinity, ignore the entry but log the event. The basic validation tests are: - is the destination prefix not a link-local address) present in an RTE. - is the prefix length valid - is the metric valid (i.e., valid (e.g., not a multicast prefix and A link-local address should never be (i.e., between 0 and 128, inclusive) between 1 and 16, inclusive)

If any check fails, ignore that entry and proceed to the next. Again, logging the error is probably a good idea. Once the entry has been validated, update the metric by adding the cost of the network on which the message arrived. If the result is greater than infinity, use infinity. That is, metric = MIN (metric + cost, infinity) Now, check to see whether there destination prefix. If there is routing table, unless the metric adding a route which unusable). consists of: is already an explicit route for the no such route, add this route to the is infinity (there is no point in Adding a route to the routing table

- Setting the destination prefix and length to those in the RTE. - Setting the metric to the newly calculated metric (as described above).

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- Set the next hop address to be the address of the router from which the datagram came or the next hop address specified by a next hop RTE. - Initialize the timeout for the route. If the garbage-collection timer is running for this route, stop it (see section 2.3 for a discussion of the timers). - Set the route change flag. - Signal the output process to trigger an update (see section 2.5). If there is an existing route, compare the next hop address to the address of the router from which the datagram came. If this datagram is from the same router as the existing route, reinitialize the timeout. Next, compare the metrics. If the datagram is from the same router as the existing route, and the new metric is different than the old one; or, if the new metric is lower than the old one; do the following actions: - Adopt the route from the datagram. That is, put the new metric in, and adjust the next hop address (if necessary). - Set the route change flag and signal the output process to trigger an update. - If the new metric is infinity, start the deletion process (described above); otherwise, re-initialize the timeout. If the new metric is infinity, the deletion process begins for the route, which is no longer used for routing packets. Note that the deletion process is started only when the metric is first set to infinity. If the metric was already infinity, then a new deletion process is not started. If the new metric is the same as the old one, it is simplest to do nothing further (beyond reinitializing the timeout, as specified above); but, there is a heuristic which could be applied. Normally, it is senseless to replace a route if the new route has the same metric as the existing route; this would cause the route to bounce back and forth, which would generate an intolerable number of triggered updates. However, if the existing route is showing signs of timing out, it may be better to switch to an equally-good alternative route immediately, rather than waiting for the timeout to happen. Therefore, if the new metric is the same as the old one, examine the timeout for the existing route. If it is at least halfway to the expiration point, switch to the new route. This heuristic is optional, but highly recommended.

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Any entry that fails these tests is ignored, as it is no better than the current route. 2.5 Output Processing This section describes the processing used to create response messages that contain all or part of the routing table. This processing may be triggered in any of the following ways: - By input processing, when a Request is received. In this case, the Response is sent to only one destination (i.e. the unicast address of the requestor). - By the regular routing update. Every 30 seconds, a Response containing the whole routing table is sent to every neighboring router. - By triggered updates. Whenever the metric for a route is changed, an update is triggered. The special processing required for a Request is described