An IPv6 packet is the smallest message entity exchanged via the Internet Protocol across an Internet Protocol version 6 (IPv6) network.
Contents
- Fixed header
- Extension headers
- Hop by hop options and destination options
- Routing
- Routing types
- Fragment
- Authentication Header AH and Encapsulating Security Payload ESP
- Payload
- Standard payload length
- Jumbogram
- Fragmentation
- Fragmenting
- Reassembly
- Security
- References
Packets consist of control information for addressing and routing, and a payload consisting of user data. The control information in IPv6 packets is subdivided into a mandatory fixed header and optional extension headers. The payload of an IPv6 packet is typically a datagram or segment of the higher-level Transport Layer protocol, but may be data for an Internet Layer (e.g., ICMPv6) or Link Layer (e.g., OSPF) instead.
IPv6 packets are typically transmitted over a Link Layer protocol, such as Ethernet which encapsulates each packet in a frame, but this may also be a higher layer tunneling protocol, such as IPv4 when using 6to4 or Teredo transition technologies.
Routers do not fragment IPv6 packets, as they do for IPv4. Hosts are "strongly recommended" to implement Path MTU Discovery to take advantage of MTUs greater than the smallest MTU of 1280 octets. A node may use the IPv6 Fragment header to fragment the packet at the source and have it reassembled at the destination(s).
Fixed header
The fixed header of an IPv6 packet consists of its first 40 octets (320 bits). It has the following format:
In order to increase performance, and since current link layer technology and transport or application layer protocols are assumed to provide sufficient error detection, the header has no checksum to protect it.
Extension headers
Extension headers carry optional Internet Layer information, and are placed between the fixed header and the upper-layer protocol header. The headers form a chain, using the Next Header fields. The Next Header field in the fixed header indicates the type of the first extension header; the Next Header field of the last extension header indicates the type of the upper-layer protocol header in the payload of the packet.
All extension headers are a multiple of 8 octets in size; some extension headers require internal padding to meet this requirement.
There are several extension headers defined, and new extension headers may be defined in the future. Extension headers are to be examined and processed at the packet's destination only, except for Hop-by-Hop Options, which need to be processed at every intermediate node on the packet's path, including sending and receiving node. The defined extension headers below are listed in the preferred order, should there be more than one extension header following the fixed header. Note that all extension headers are optional and should only appear at most once, except for the Destination Options header, which may appear twice.
If a node does not recognize a specific extension header, it should discard the packet and send a Parameter Problem message (ICMPv6 type 4, code 1). When a Next Header value 0 appears in a header other than the fixed header a node should do the same.
Value 59 (No Next Header) in the Next Header field indicates that there is no next header whatsoever following this one, not even a header of an upper-layer protocol. It means that, from the header's point of view, the IPv6 packet ends right after it: the payload should be empty. There could, however, still be data in the payload if the payload length in the first header of the packet is greater than the length of all extension headers in the packet. This data should be ignored by hosts, but passed unaltered by routers.
Hop-by-hop options and destination options
The Hop-by-Hop Options extension header needs to be examined by all nodes on the packet's path, including sending and receiving nodes. The Destination Options extension header need to be examined by the destination node(s) only. The extension headers are both at least 8 octets in size; if more options are present than will fit in that space, blocks of 8 octets are added to the header repeatedly—containing options and padding—until all options are represented.
Routing
The Routing extension header is used to direct a packet to one or more intermediate nodes before being sent to its destination. The header is at least 8 octets in size; if more Type-specific Data is needed than will fit in 4 octets, blocks of 8 octets are added to the header repeatedly, until all Type-specific Data is placed.
Routing types
Due to the fact that with Routing Header type 0 a simple but effective denial-of-service attack could be launched, this header is deprecated and host and routers are required to ignore these headers.
Routing Header type 1 is used for the Nimrod project funded by DARPA.
Routing Header type 2 is a limited version of type 0 and is used for Mobile IPv6, where it can hold the Home Address of the Mobile Node.
Fragment
In order to send a packet that is larger than the path MTU, the sending node splits the packet into fragments. The Fragment extension header carries the information necessary to reassemble the original (unfragmented) packet.
Authentication Header (AH) and Encapsulating Security Payload (ESP)
The Authentication Header and the Encapsulating Security Payload are part of IPsec and are used identically in IPv6 and in IPv4.
Payload
The fixed and optional IPv6 headers are followed with the upper-layer payload, the data provided by the transport layer, for example a TCP segment or a UDP datagram. The Next Header field of the last IPv6 header indicates what type of payload is contained in this packet.
Standard payload length
The payload length field of IPv6 (and IPv4) has a size of 16 bits, capable of specifying a maximum size of 65535 octets for the payload. Most Link Layer protocols cannot process packets larger than 65535 octets.
Jumbogram
An optional feature of IPv6, the jumbo payload option in a Hop-By-Hop Options extension header, allows the exchange of packets with payloads of up to one byte less than 4 GB (232 − 1 = 4294967295 bytes), by making use of a 32-bit length field. Packets with such payloads are called jumbograms.
Since both TCP and UDP include fields limited to 16 bits (length, urgent data pointer), support for IPv6 jumbograms requires modifications to the Transport Layer protocol implementation. Jumbograms are only relevant for links that have a MTU larger than 65583 octets (more than 65535 octets for the payload, plus 40 octets for the fixed header, plus 8 octets for the Hop-by-Hop extension header).
Fragmentation
Unlike in IPv4, IPv6 routers never fragment IPv6 packets. Packets exceeding the size of the maximum transmission unit of the destination link are dropped and this condition is signaled by a Packet too Big ICMPv6 type 2 message to the originating node, similarly to the IPv4 method when the Don't Fragment bit is set.
End nodes in IPv6 are expected to perform path MTU discovery to determine the maximum size of packets to send, and the upper-layer protocol is expected to limit the payload size. However, if the upper-layer protocol is unable to do so, the sending host may use the Fragment extension header in order to perform end-to-end fragmentation of IPv6 packets. Any data link layer conveying IPv6 data must be capable of delivering an IP packet containing 1280 bytes without the need to invoke end-to-end fragmentation at the IP layer.
Fragmenting
A packet containing a fragment of an original (larger) packet consists of two parts: the unfragmentable part of the original packet (which is the same for all fragments), and a piece of the fragmentable part of the original packet, identified by a Fragment Offset. The Fragment Offset of the first ("leftmost") fragment is 0.
The unfragmentable part of a packet consists of the fixed header and some of the extension headers of the original packet (if present): all extension headers up to and including the Routing extension header, or else the Hop-by-Hop extension header. If neither extension headers are present, the unfragmentable part is just the fixed header.
The Next Header value of the last (extension) header of the unfragmentable part is set to 44 to indicate that a Fragment extension header follows. After the Fragment extension header a fragment of the rest of the original packet follows.
The first fragment(s) hold the rest of the extension headers (if present). After that the rest of the payload follows. Each fragment is a multiple of 8 octets in length, except the last fragment.
Each Fragment extension header has its M flag set to 1 (indicating more fragments follow), except the last, whose flag is set to 0.
Reassembly
The original packet is reassembled by the receiving node by collecting all fragments and placing each fragment at the right offset and discarding the Fragment extension headers of the packets that carried them. Packets containing fragments need not arrive in sequence; they will be rearranged by the receiving node.
If not all fragments are received within 60 seconds after receiving the first packet with a fragment, reassembly of the original packet is abandoned and all fragments are discarded. If the first fragment was received (which contains the fixed header), a Time Exceeded message (ICMPv6 type 3, code 1) is returned to the node originating the fragmented packet, if the packet was discarded for this reason.
Receiving hosts must make a best-effort attempt to reassemble fragmented IP datagrams that, after reassembly, contain up to 1500 bytes. Hosts are permitted to make an attempt to reassemble fragmented datagrams larger than 1500 bytes, but they are also permitted to silently discard any datagram after it becomes apparent that the reassembled packet would be larger than 1500 bytes. Therefore, senders should avoid sending fragmented IP datagrams with a total reassembled size larger than 1500 bytes, unless they have previous assurance that the receiver is capable of reassembling such large datagrams.
Security
Research has shown that the use of fragmentation can be leveraged to evade network security controls. As a result, RFC 7112 requires that the first fragment of an IPv6 packet contains the entire IPv6 header chain, such that some very pathological fragmentation cases are forbidden. Additionally, as a result of research on the evasion of RA-Guard in RFC 7113, RFC 6980 has deprecated the use of fragmentation with Neighbor Discovery, and discouraged the use of fragmentation with Secure Neighbor Discovery (SEND).