Limitations of IPv4 - Networking Stuffs - 2

[ganeshbala]

Understanding the Limitations of IPv4

Understanding IPv6 MTU

In IPv6, the link layer must support a minimum MTU size of 1,280 bytes. Any link layer that does not support a minimum MTU size of 1,280 bytes must use a link-layer fragmentation and reassembly method. The LAN or WAN technologies with their associated IPv6 MTUs are listed below:

    * Ethernet - Ethernet II encapsulation: 1,500
    * Ethernet - IEEE 802.3 SubNetwork Access Protocol [SNAP] encapsulation: 1,492
    * Token Ring: differs
    * FDDI: 4,352
    * PPP: 1,500
    * X.25: 1,280
    * Frame Relay: 1,592
    * Attached Resource Computer Network (ARCNet): 9,072
    * Asynchronous Transfer Mode (ATM) - Null or SNAP encapsulation: 9,180

Understanding Internet Control Message Protocol version 6 (ICMPv6)

IPv6 utilizes ICMP version 6 (ICMPv6), defined in RFC 2463. ICMPv6 provides the structure for:

    * Neighbor Discovery (ND): In IPv6, ND replaces ARP, ICMPv4 Redirect and Router Discovery messages. Neighbor Discovery (ND) consists of five ICMPv6 messages that administer node communication.
          o Router Solicitation
          o Router Advertisement
          o Neighbor Solicitation
          o Neighbor Advertisement
          o Redirect
    * Multicast Listener Discovery (MLD): Multicast Listener Discovery (MLD) consists of three ICMPv6 messages that are the same as Internet Group Management Protocol version 2 (IGMPv2) for IPv4. MLD manages subnet multicast membership. The three ICMPv6 messages are:
          o Multicast Listener Query
          o Multicast Listener Report
          o Multicast Listener Done

      The ICMPv6 header is identified by a value of 58 for the Next Header field in the prior header. The fields contained in the ICMPv6 header are:
          o Type: Indicates whether the message is an ICMPv6 error message or an ICMPv6 informational message.
          o Code: Distinguishes between many messages in a specified message type.
          o Checksum: Contains the checksum of the ICMPv6 message.
          o Message body: Contains the ICMPv6 message data.

      The types of ICMPv6 messages are:
          o Error messages: These messages report forwarding and delivery errors by the destination host and intermediate router, and includes the following messages:
                + Destination Unreachable: Sent when the packet cannot be forwarded to the upper-layer protocol or destination node.
                + Packet Too Big: Sent when link MTU on the forwarding interface is smaller than the IPv6 packet size.
                + Parameter Problem: Sent when an error in the IPv6 header or an extension header is preventing the packet from being processed.
                + Time Exceeded: Sent when the Hop Limit field reaches zero
          o Information messages: These messages provide additional host capabilities, including Multicast Listener Discovery (MLD and Neighbor Discovery (ND), and includes the following messages:
                + Echo Request: This message it sent to a destination, requesting an instant response.
                + Echo Reply: This message is sent as a response to an Echo Request message.
      Understanding IPv6 Neighbor Discovery (ND)

      IPv6 Neighbor Discovery (ND), defined in RFC 2461, consists of processes and messages that determine associations between neighboring nodes. In IPv6, ND replaces ARP, ICMPv4 Redirect and Router Discovery messages. Neighbor Discovery (ND) consists of five ICMPv6 messages that administer node communication.

      The Neighbor Discovery (ND) processes consist of:
          o Router discovery: This ND process is the same as the ICMPv4 router discovery. Router discovery occurs when a host discovers the local routers on a connected link.
          o Prefix discovery: The prefix discovery ND process is much the same as the sending of ICMPv4 Address Mask Request and Address Mask Reply messages. In the prefix discovery process, hosts discover the prefixes for the local link destinations.
          o Parameter discovery: This ND process is used by hosts to discover operating parameters such as link MTU, for packets.
          o Address resolution: The address resolution ND process is the same as IPv4 ARP. During address resolution, nodes resolve the IPv6 address of a neighbor, to its associated link-layer address.
          o Address autoconfiguration: This is the process whereby IP addresses are automatically assigned for interfaces.
          o Next-hop determination: This process enables a node to determine a neighbor's IPv6 addresses that the packet is being forwarded to. A next-hop address is either of the following:
                + Destination address
                + Address of default router
          o Duplicate address detection: In duplicate address detection, a node can determine whether an address is being used by another neighboring node.
          o Neighbor unreachability detection: This process enables a node to determine that a neighbor is not receiving packets.
          o Redirect function: This the process that informs a host of a more optimal first-hop IPv6 address to the specific destination.

      Nodes use Neighbor Discovery (ND) for the following purposes:
          o To ascertain when a neighboring node's link-layer address changes.
          o To ascertain if a neighboring node can be reached.
          o To resolve the link-layer address of the neighboring node that the packet must be forwarded to.

      Routers use Neighbor Discovery (ND) for the following purposes:
          o To advertise themselves
          o To advertise routes and host configuration parameters.
          o To communicate information on other more optimal next-hop addresses.

      Hosts use Neighbor Discovery (ND) for the following purposes:
          o To discover neighboring routers.
          o For autoconfigure configuration parameters such as addresses, address prefixes and routes.

      The messages that are used to perform the processes of IPv6 ND are:
          o Router Solicitation: These messages are used by hosts to discover IPv6 routers on the link. The fields in the Router Solicitation message are: Type, Code, Checksum, Reserved and Source Link-Layer Address option.
          o Router Advertisement: These messages are sent by IPv6 routers as a response to receiving a Router Solicitation message. The Router Advertisement message contains the following information:
                + Information to determine the link prefixes. This information is used by hosts.
                + Information on whether address autoconfiguration should be used.
                + Information on specific routes.
                + The time period for which address autoconfiguration addresses are valid.
                + Link MTU.

            The fields found in a Router Advertisement message are explained below:
                + Type; has a value of 134.
                + Code; has a value of 0
                + Checksum; has a value of the ICMPv6 checksum.
                + Current Hop Limit; defines the Hop Limit field in the IPv6 header's default value. This is for packets sent by host which have received the Router Advertisement message.
                + Managed Address Configuration flag; specifies that hosts that receive the Router Advertisement message have to utilize stateful address configuration to obtain addresses.
                + Other Stateful Configuration flag; specifies that hosts that receive the Router Advertisement message have to utilize stateful address configuration to obtain other configuration information (not addresses).
                + Home Agent flag; specifies that the router also fulfils the role of home agent.
                + Default Router Preference; specifies the preference level of the router with regard to default router.
                + Reserved; reserved for future use.
                + Router Lifetime; defines the lifetime of the router with regard to default router.
                + Reachable Time; specifies the time for which a node can regard a neighboring node as being reachable once it has received the necessary reachable confirmation.
                + Retransmission Timer; specifies the time between retransmissions of the Neighbor Solicitation messages.
                + Source Link-Layer Address option; holds the interface' link-layer address whereon the Router Advertisement message was transmitted.
                + MTU option; holds the MTU of the link.
                + Prefix Information options; holds the on-link prefixes for address autoconfiguration.
                + Advertisement Interval option; holds the interval for ensuing unsolicited multicast Router Advertisement messages which are sent from the home agent router.
                + Home Agent Information option; holds the home agent's preference and lifetime information.
                + Route Information options; holds route information for the local routing table to perform forwarding decisions.
          o Neighbor Solicitation: This message is used to discover an IPv6 node's link-layer address. The Neighbor Solicitation message is sent by IPv6 hosts. The fields in a Neighbor Solicitation message are: Type, Code, Checksum, Reserved, Target Address, and Source Link-Layer Address option.
          o Neighbor Advertisement: This message is sent as a response to a Neighbor Solicitation message. The Neighbor Advertisement message holds the following information:
                + Information to determine the type of Neighbor Advertisement message. This information is used by nodes.
                + The link-layer address of the sender.
                + The role of the sender in the network.

            The fields found in a Neighbor Advertisement message are explained below:
                + Type; has a value of 136.
                + Code; has a value of 0
                + Checksum; has a value of the ICMPv6 checksum.
                + Router flag; defines the Neighbor Advertisement message sender's role.
                + Solicited flag; specifies that the message is sent in response to a Neighbor Solicitation message.
                + Override flag; specifies that the Target Link-Layer Address option's link-layer address overrides the existing neighbor cache entry's link-layer address.
                + Reserved; is reserved for future purposes.
                + Target Address; specifies the address which is advertised.
                + Target Link-Layer Address option; holds the target's link-layer address.
          o " Redirect: This message informs the initial host of a more optimal first-hop address for the destination. Routers send these messages when unicast traffic is being transmitted.

            The fields found in a Redirect message are explained below:
                + Type; has a value of 137.
                + Code; has a value of 0
                + Checksum; has a value of the ICMPv6 checksum.
                + Reserved; is reserved for future purposes.
                + Target Address; defines the more efficient next-hop address for packets being sent to the node specified in Destination Address.
                + Destination Address; holds the destination address of the packet. This address resulted in the router sending the Redirect message.
                + Target Link-Layer Address option; holds the target's link-layer address.
                + Redirected Header option; holds the foremost segment of the initial packet which resulted in the Redirect message being sent.
      Understanding IPv6 Multicast Listener Discovery (MLD)

      Multicast Listener Discovery (MLD), defined in RFC 2710, is the same as Internet Group Management Protocol version 2 (IGMPv2) for IPv4. However, with IPv6, multicast support is required. Multicast support was optional in IPv4. Multicast Listener Discovery (MLD) consists of three ICMPv6 messages that are the same as Internet Group Management Protocol version 2 (IGMPv2) for IPv4. These messages are communicated between routers and hosts. MLD manages subnet multicast membership, and enables routers to discover the list of multicast addresses of listening hosts for every connected subnet. In IPv6, multicast traffic is transmitted to one destination address, and received by multiple IPv6 hosts. The IPv6 hosts listen for a particular IPv6 multicast address. All packets that have this particular address is received and processed by the hosts.

      A few IPv6 multicast-specific factors are listed below:
          o A multicast group refers to a group of hosts listening on a particular IPv6 multicast address.
          o A multicast group can span IPv6 routers over more than one subnet.
          o Membership to a multicast group is dynamic.
          o A host does not need to belong to a multicast group to send packets to a multicast address.

      A host can send IPv6 multicast packets if it can:
          o Establish which destination IPv6 multicast address should be utilized.
          o Structure an IPv6 packet with the correct destination IPv6 multicast address; and then place it on the medium.

      A host can receive IPv6 multicast packets if it can:
          o Inform IPv6 to receive multicast packets for a specific multicast address.
          o Register the network adapter with the multicast MAC address.
          o Inform local subnet routers when it is listening, at a particular multicast address, for multicast traffic.

      To ensure that IPv6 multicast packets are forwarded to subnets that have multicast group members, the IPv6 multicast router has to perform the following functions:
          o The IPv6 multicast router must be able to receive all IPv6 multicast traffic.
          o The IPv6 multicast router must be able to forward all IPv6 multicast traffic.
          o The IPv6 multicast router must be able to both receive and process the MLD Multicast Listener Report message and the Multicast Listener Done message.
          o The IPv6 multicast router must be able to query subnets for host membership information.
          o The IPv6 multicast router must be able to communicate group membership information to the other IPv6 multicast routers.

      The MLD message contains the following components:
          o IPv6 header
          o Hop-by-Hop Options extension header
          o The actual MLD message.

      The messages that are used in IPv6 Multicast Listener Discovery are:
          o Multicast Listener Query - ICMPv6 Type 130: This message is used by IPv6 multicast routers to query for multicast group membership information. There are two types of Multicast Listener Query messages which are identified by the Destination Address field of the IPv6 header and the Multicast Address field of the Multicast Listener Query message:
                + General query: This message is used to query all hosts on the subnet for multicast group members for multicast addresses.
                + Multicast-address-specific query: This message is used to query hosts of a particular multicast group on a subnet.

            The fields found in a Multicast Listener Query message are explained below:
                + Type; has a value of 130.
                + Code; has a value of 0
                + Checksum; has a value of the ICMPv6 checksum.
                + Maximum Response Delay; specifies the time allowed for a multicast group member to specify its membership. This information is sent via a MLD Multicast Listener Report message.
                + Reserved; is reserved for future purposes.
                + Multicast Address; the field is defined as an unspecified address when the message type is General query. It includes the multicast address which is being queried when the message type is Multicast-address-specific query.
          o Multicast Listener Report - ICMPv6 Type 131: This message is the same as the IGMPv2 Host Membership Report message. The Multicast Listener Report message is used by listening nodes for the following purposes:
                + Reply to a Multicast Listener Query message.
                + Indicate that it can receive multicast traffic at a multicast address

            The fields found in a Multicast Listener Report message are explained below:
                + Type; has a value of 131.
                + Code; has a value of 0
                + Checksum; has a value of the ICMPv6 checksum.
                + Maximum Response Delay; set to 0 for the Multicast Listener Report message.
                + Reserved; is reserved for future usage.
                + Multicast Address; includes the multicast address which is being reported on.
          o Multicast Listener Done - ICMPv6 Type 132: This message is the same as the IGMPv2 Leave Group message. The Multicast Listener Done message informs routers that there could possibly be no further group members of a particular multicast address.

            The fields found in a Multicast Listener Done message are explained below:
                + Type; has a value of 132.
                + Code; has a value of 0
                + Checksum; has a value of the ICMPv6 checksum.
                + Maximum Response Delay; set to 0 for the Multicast Listener Done message.
                + Reserved; is reserved for future usage.
                + Multicast Address; includes the multicast address which has no other group members on the subnet.
      Understanding Address Autoconfiguration in IPv6

      With IPv6, an IPv6 host can configure a link-local address for each interface without using a protocol such as the DHCP protocol.

      The different types of autoconfiguration are listed below. The link-local address is configured automatically with each type of autoconfiguration.
          o Stateless: Addresses are configured when Router Advertisement messages are received. The Managed Address Configuration and Other Stateful Configuration flags have to be set to 0, and must contain at least one Prefix Information options.
          o Stateful: Addresses are configured when a host receives a Router Advertisement message that has the Managed Address Configuration flag or the Other Stateful Configuration flag set to 1, with no Prefix Information options. A stateful address autoconfiguration protocol (DHCP) is used to assign addresses.
          o Both: Addresses are configured when Router Advertisement messages are received that have Prefix Information options that have the Autonomous flag set to 1, and the Managed Address Configuration flag or the Other Stateful Configuration flag set to 1.

      An autoconfigured address can be in one or more states:
          o Valid; indicates that the specific address can be used to send and receive unicast traffic. The valid state has the following states:
                + Preferred; indicates that the address is unique and has been verified.
                + Deprecated; indicates that the address is unique and has been verified, but should not be used for new traffic.
          o Tentative; indicates that the specific address is going through the verification process.
          o Valid; indicates that the specific address cannot be used to send and receive unicast traffic.
      Understanding IPv6 Routing