{"id":1812,"date":"2017-01-31T22:15:00","date_gmt":"2017-01-31T22:15:00","guid":{"rendered":"https:\/\/live-infoblox-blog.pantheonsite.io\/?p=1812"},"modified":"2022-10-19T16:07:38","modified_gmt":"2022-10-19T23:07:38","slug":"ipv6-neighbor-discovery-cache-part-1-of-2","status":"publish","type":"post","link":"https:\/\/www.infoblox.com\/blog\/ipv6-coe\/ipv6-neighbor-discovery-cache-part-1-of-2\/","title":{"rendered":"IPv6 Neighbor Discovery Cache – Part 1 (of 2)"},"content":{"rendered":"

Join me as we\u00a0probe<\/i><\/strong>\u00a0without further\u00a0delay<\/strong>\u00a0<\/i>the secrets of the IPv6 Neighbor Discovery cache. I\u2019m sure we’ll find our goal of better managing IPv6 eminently reachable<\/strong> <\/em>without incomplete<\/strong> <\/em>knowledge or stale<\/strong> <\/em>writing!<\/p>\n

Lately,\u00a0I\u2019ve been revisiting Jeff Doyle\u2019s classic\u00a0Routing TCP\/IP<\/a><\/em>\u00a0volumes<\/a>\u00a0and they are as great as I remember them. If you\u2019ve ever had the pleasure of meeting Jeff, his writing style is very much like his affect in real life: approachable and friendly. Relatedly, in his texts, he\u00a0manages both a thorough and precise review of the subject matter without\u00a0resorting to the coldly technical prose that is a better fit for the actual RFCs he\u2019s explicating for the reader.<\/p>\n

The volume 1 chapter on IPv6 is particularly good and, in spite of the decade plus that has passed since the latest edition was published, doesn\u2019t feel dated. Granted, the technical content hasn\u2019t really changed significantly: for example,\u00a0Neighbor Discovery circa 2005 is essentially the same\u00a0Neighbor Discovery we\u2019re still\u00a0dealing with in 2017. But the combination of\u00a0Mr. Doyle\u2019s affable writing\u00a0style and expert sense of what\u2019s critically important to cover make the chapter an excellent choice to visit, and revisit, often.<\/p>\n

In fact, it was Jeff\u2019s sub-chapter on IPv6 Neighbor Unreachability Detection that reminded me I probably\u00a0needed a refresher on the various possible\u00a0states for Neighbor Discovery cache entries. So with the incomparable Mr. Doyle and his writing as the inspiration, let\u2019s review the\u00a0IPv6\u00a0Neighbor Discovery cache and its various states.<\/p>\n

But before we get to that, a quick review of IPv6 Neighbor Discovery is in order.\u00a0IPv6\u00a0Neighbor Discovery is commonly referred to and most basically understood\u00a0as the replacement for IPv4\u00a0Address Resolution Protocol (ARP). At its most fundamental\u00a0IPv6 ND contains the set of mechanisms responsible for mapping Layer 3 (IPv6)\u00a0to Layer 2 (link-layer; most typically Ethernet\u00a0MAC)\u00a0addresses on a network segment or link. Also,\u00a0IPv6 nodes can use ND to discover routers on the link as well as to learn which other IPv6 nodes\u00a0are still reachable (and also\u00a0if link-layer addresses have changed).<\/p>\n

All of these functions are facilitated by the Neighbor Discovery cache. The ND cache is nearly identical in contents\u00a0and format to the ARP cache in IPv4.<\/p>\n

\"IPv4
IPv4 ARP cache from a Windows 7 VM<\/figcaption><\/figure>\n
\"IPv6
IPv6 ND cache from a Ubuntu VM<\/figcaption><\/figure>\n

Recall that IPv4 ARP relies on the use of the\u00a0Layer 2\u00a0broadcast. A host learns the IP address of the device it wishes to communicate with (via DNS, for example) but still\u00a0doesn\u2019t know the link-layer address of that device. ARP sends a frame with the target\u00a0IP address as the destination to the broadcast address. All devices connected to that link must process this broadcast frame but only the device whose IP address matches the destination will respond back to the requesting host, filling in the destination link-layer address. This becomes an ARP entry like the ones displayed above: an IPv4\u00a0address corresponding to a physical address. Future packets destined for that IP address can now be immediately (i.e., no additional\u00a0ARP process required)\u00a0encapsulated in Layer 2 frames with the proper destination link-layer address supplied by the ARP cache.<\/p>\n

An obvious liability of this mechanism is that every host must process all broadcast frames whether or not those frames\u00a0actually contain an\u00a0ARP request destined for that host. The more hosts on a LAN segment, the more compute resources are\u00a0wasted by processing unwanted ARP requests.<\/p>\n

IPv6 Neighbor Discover functions in a similar way but with some critical optimizations. The basic\u00a0scenario requiring the resolution of a\u00a0Layer 3\u00a0address to a Layer 2 address\u00a0is the same as in IPv4 ARP: An IPv6 host knows the IPv6 address of the destination but not its link-layer address. However, instead of sending the ARP request to a\u00a0broadcast destination address as in IPv4, it sends a Neighbor Solicitation message to\u00a0the solicited node multicast destination\u00a0address. This solicited node multicast address\u00a0is derived from the Layer 3 unicast address on the target host (see examples below). Only the host listening to the multicast group address corresponding to its solicited node multicast address (which itself corresponds to its Layer 3 unicast address) will\u00a0respond with a Neighbor Advertisement message,\u00a0providing its link-layer address to the requesting node which then creates a Neighbor Discovery cache entry.<\/p>\n

\"IPv6<\/p>\n

\"IPv6<\/p>\n

Because obviously\u00a0networks can and do\u00a0change, with devices that were once available going offline and new devices coming online, both\u00a0the ARP cache and ND cache must offer mechanisms to bring their entries up-to-date \u2014\u00a0as well as information reflecting the current status of the ND cache\u00a0entries.<\/p>\n

This is where the five states of IPv6 Neighbor Discovery cache come into play. They are:<\/p>\n

    \n
  1. Incomplete<\/li>\n
  2. Reachable<\/li>\n
  3. Stale<\/li>\n
  4. Delay<\/li>\n
  5. Probe<\/li>\n<\/ol>\n

    A status of\u00a0incomplete<\/i>\u00a0indicates that address resolution is in concurrently\u00a0progress and the neighbor\u2019s link-layer address has yet to be\u00a0determined. In strict terms this means that a\u00a0Neighbor Solicitation message with the solicited node address of the target\u00a0has been sent but a corresponding Neighbor Advertisement has yet to be received.<\/p>\n

    A status of\u00a0reachable<\/i>\u00a0indicates that a positive confirmation of reachability for the associated ND\u00a0cache entry\u00a0was received within a\u00a0certain time interval (defined as the ReachableTime). This positive confirmation could be the receipt of a Neighbor Advertisement following a Neighbor Solicitation or an upper-layer protocol successfully using the ND\u00a0cache entry.<\/p>\n

    A\u00a0stale<\/i>\u00a0ND\u00a0cache\u00a0entry means the ReachableTime has elapsed before a subsequent\u00a0confirmation of reachability has been received.<\/p>\n

    A status of\u00a0delay<\/i>\u00a0indicates that, though the ReachableTime has expired, a\u00a0packet was recently sent to the cache entry\u2019s address\u00a0by an upper-layer protocol. After a defined interval, a Neighbor Solicitation is sent and the ND cache entry is changed to\u00a0probe<\/i>\u00a0status. The\u00a0delay<\/i>\u00a0status merely provides time for upper-layer protocols to potentially refresh the cache entry without resorting to a Neighbor Solicitation.<\/p>\n

    As just mentioned, the\u00a0probe<\/i>\u00a0status occurs when a Neighbor Solicitation is sent, and possibly resent, until a response is received. (Ideally, an upper-layer protocol would have verified reachability during the\u00a0delay<\/i>\u00a0phase.)<\/p>\n

    So now that we know what the IPv6\u00a0Neighbor Discovery cache entry states are, how do we observe them on popular operating systems? We\u2019ll take a look at that and more in part 2!<\/p>\n","protected":false},"excerpt":{"rendered":"

    Join me as we\u00a0probe\u00a0without further\u00a0delay\u00a0the secrets of the IPv6 Neighbor Discovery cache. I\u2019m sure we’ll find our goal of better managing IPv6 eminently reachable without incomplete knowledge or stale writing! Lately,\u00a0I\u2019ve been revisiting Jeff Doyle\u2019s classic\u00a0Routing TCP\/IP\u00a0volumes\u00a0and they are as great as I remember them. If you\u2019ve ever had the pleasure of meeting Jeff, his […]<\/p>\n","protected":false},"author":319,"featured_media":1060,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"inline_featured_image":false,"_genesis_hide_title":false,"_genesis_hide_breadcrumbs":false,"_genesis_hide_singular_image":false,"_genesis_hide_footer_widgets":false,"_genesis_custom_body_class":"","_genesis_custom_post_class":"","_genesis_layout":"","footnotes":""},"categories":[17],"tags":[16,38],"class_list":{"0":"post-1812","1":"post","2":"type-post","3":"status-publish","4":"format-standard","5":"has-post-thumbnail","7":"category-ipv6-coe","8":"tag-infoblox","9":"tag-ipv6","10":"entry"},"acf":[],"yoast_head":"\nIPv6 Neighbor Discovery Cache - Part 1 (of 2)<\/title>\n<meta name=\"description\" content=\"It is important to understand what IPv6 Neighbor Discovery caches are. 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