ICMPv 6 Error Message Types Informational Message Types

ICMPv 6 • Error Message Types • Informational Message Types

ICMPv 6 Header Type (1 Byte) Code (1 Byte) Checksum (2 Bytes) Data (Variable) 1

ICMPv 6 Error Message • • Destination Unreachable Packet too Big Time Exceeded Parameter Problem

Destination Unreachable • A Destination Unreachable message is generated if an IP datagram cannot be delivered. • A Type field with the value 1 identifies this message. Type (1 Byte) Code (1 Byte) 1 Code: 0: no route to destination 1: communication administratively prohibited 2: address unreachable 4: port unreachable Checksum (2 Bytes) Unused (4 Bytes) Data (Variable) Must be zero ignored by receiver

Packet Too Big • If a router cannot forward a packet because it is larger than the MTU of the outgoing link, it will generate a Packet Too Big message. • This ICMPv 6 message type is used as part of the Path MTU discovery process that I discuss later in this chapter. Type (1 Byte) 2 Code (1 Byte) 0 Checksum (2 Bytes) MTU (4 Bytes) Data (Variable) The Maximum transmission unit of the next hop link

Time Exceeded • When a router forwards a packet, it always decrements the hop limit by one. • The hop limit makes sure that a packet does not endlessly travel through a network. • If a router receives a packet with a hop limit of 1 and decrements the limit to 0, it discards the packet, generates a Time Exceeded message with a code value of 0, and sends this message back to the source host

Time Exceeded Type (1 Byte) Code (1 Byte) 3 Code: 0: Hop limit exceeded in transit 1: Fragment reassembly time exceeded Checksum (2 Bytes) Unused (4 Bytes) Data (Variable) Must be zero ignored by receiver

Parameter Problem • If an IPv 6 node cannot complete the processing of a packet because it has a problem identifying a field in the IPv 6 header or in an Extension header. • it must discard the packet, and it should send an ICMP Parameter Problem message back to the source of the problem packet.

Parameter Problem Type (1 Byte) Code (1 Byte) 4 Code: 0: Erroneous header field encountered 1: Unrecognized next header type encountered 2: Unrecognized IPv 6 option encountered Checksum (2 Bytes) Pointer (4 Bytes) Data (Variable) Identify the offset where error was decided

ICMP Informational Messages • In RFC 4443, two types of informational messages are defined: the Echo Request and the Echo Reply messages. • Other ICMP informational messages are used for Path MTU Discovery and Neighbor Discovery. • The Echo Request and Echo Reply messages are used for one of the most common TCP/IP utilities: Packet INternet Groper (ping).

Echo Request Message Type (1 Byte) 128 Code (1 Byte) 0 Checksum (2 Bytes) Identifier (2 Bytes) Sequence No (2 Bytes) Data (Variable) Used to Match echo reply to this request may be zero Zero or more bytes of arbitrary data • The Identifier and Sequence Number fields are used to match requests with replies. • The reply must always contain the same numbers as the request.

Echo Reply Message Type (1 Byte) 129 Code (1 Byte) 0 Checksum (2 Bytes) Identifier (2 Bytes) Sequence No (2 Bytes) Data (Variable) The identifier from the invoking echo request The SN from the invoking echo request Data from the invoking echo request • The Identifier and Sequence Number fields must match the fields in the request • The reply must always contain the same numbers as the request.

Neighbor Discovery (ND) • combines Address Resolution Protocol (ARP) and ICMP Router Discovery and Redirect • With IPv 4, we have no means to detect whether a neighbor is reachable • With the Neighbor Discovery protocol, a Neighbor Unreachability Detection (NUD) mechanism has been defined • Duplicate IP address detection (DAD) has also been implemented

Neighbor Discovery (ND) • For autoconfiguration of IPv 6 addresses • To determine network prefixes, routes and other configuration information • For Duplicate IP address detection (DAD) • To determine layer two addresses of nodes on the same link • To find neighboring routers that can forward their packets • To keep track of which neighbors are reachable and which are not • To detect changed link-layer addresses

Router Solicitation and Router Advertisement • Routers send out Router Advertisement messages at regular intervals • Hosts can request Router Advertisements by issuing a Router Solicitation message • This will trigger routers to issue Router Advertisements immediately, outside of the regular interval

Neighbor Solicitation and Neighbor Advertisement • This pair of messages fulfills two functions: the link-layer address resolution that is handled by ARP in IPv 4, and the Neighbor Unreachability Detection mechanism

Path MTU Discovery • With IPv 6, routers do not fragment packets anymore • Path MTU discovery tries to ensure that a packet is sent using the largest possible size that is supported on a certain route • The Path MTU is the smallest link MTU of all links from a source to a destination • a host assumes that the Path MTU is the same as the MTU of the first hop link and it uses that size. • If the packet is too big for a certain router along the path to deliver the packet to the next link, the router discards the packet and sends back an ICMPv 6 Packet Too Big message