The IPv6 Payload Length field is a 16-bit field that indicates the length in bytes of just the payload following the main IPv6 header or, in other words, the data portion of the packet. It does not include the main IPv6 header. If the IPv6 packet has one or more extension headers, they are included in the number of bytes contained in the Payload Length field. Extension headers are considered part of the payload.
| IPv6 Header | IPv6 Extension Header (Optional) | Data |
The IPv6 Payload Length field is similar to the Total Length field in the IPv4 header, except for one important difference: IPv4's Total Length field includes both the IPv4 hLength field specifies only the number of bytes of data or payload; it does not include the main IPv6 header. The IPv4 header can vary in length because of the Padding and Options fields, whereas the IPv6 header is fixed at 40 bytes.
Note: The calculation to determine the number of bytes in the IPv4 payload (data) following the IPv4 header is IPv4 Total Length - IPv4 IHL = number of bytes in the IPv4 payload (data). Remember that the IPv4 IHL field is in 32-bit words, whereas the IPv4 Total Length field is in bytes.
As mentioned earlier, IPv4 Total Length is a 16-bit field, therefore allowing for IPv4 packet sizes up to 65,355 bytes. In reality, these packet sizes are much smaller due to the maximum transmission unit (MTU) of the links. IPv4 has no options for exceeding this theoretical limit. However, IPv6 does have the ability to carry larger payloads. This type of packet is known as a jumbogram, as described in RFC 2675, IPv6 Jumbograms. A jumbogram is an IPv6 packet that contains a payload greater than 65,535 bytes, the maximum allowed with the 16-bit IPv6 Payload Length field.
Jumbograms use the Jumbo Payload option in the Hop-by-Hop extension header, which is described later in this chapter. The Jumbo Payload option uses a 32-bit-length field to allow transmission of IPv6 packets with payloads between 65,536 and 4,294,967,295 (232-1) bytes (4 GB-1).
As with IPv4, the size of IPv6 packets is still limited by today's link layer MTUs. However, IPv6 jumbograms are being used for fast links inside or between supercomputers.
About the Author
Rick Graziani has been an instructor of computer networking and computer science courses at Cabrillo College in Aptos, California since 1994. Rick also teaches networking courses in the Computer Engineering department at the University of California, Santa Cruz and is on the Curriculum Engineering team for Cisco Networking Academy.
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