RFC 1972

RFC 1972: A Method for the Transmission of IPv6 Packets over Ethernet Networks 🦋

• Network Working Group, Request for Comments 1972 🎀
• Author: Matt Crawford, from Fermilab 💖
• Category: Standards Track 👑
• Date: August 1996 🌸
• Obsoletes/Updates: nothing listed in the header, so this one doesn't obsolete or update any other RFC 🧚

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Status of this Memo ✨

This document specifies an Internet standards track protocol for the Internet community, and it's asking for discussion and suggestions for improvements 🫶. For the standardization state and status of this protocol, check the current edition of the "Internet Official Protocol Standards" (STD 1). Distribution of this memo is unlimited 💅.

Introduction 🦋

Okay bestie, this one is about how IPv6 packets ride inside Ethernet frames 🎀. It also covers how IPv6 link-local addresses get formed on Ethernet, plus the contents of the Source/Target Link-layer Address option. That option shows up in the Router Solicitation, Router Advertisement, Neighbor Solicitation, and Neighbor Advertisement messages from [DISC], whenever those messages get sent over Ethernet 🧚. The IPv6 packets themselves are described in [IPV6] ✨.

Maximum Transmission Unit 💅

The default MTU for IPv6 on Ethernet is 1500 octets 🌈. A Router Advertisement [DISC] can shrink that by carrying an MTU option with a smaller value, or you can configure each node by hand 🙈. But if a Router Advertisement comes in with an MTU option bigger than 1500, or bigger than a manually configured value that's under 1500, that MTU option must be ignored 💀.

Frame Format 🎀

IPv6 packets go into standard Ethernet frames, no cap 🫶. The ethernet header holds the Destination and Source ethernet addresses, plus the ethernet type code. That type code must contain the value 86DD hexadecimal 💖. The data field has the IPv6 header, then the payload right after it, and possibly some padding octets so the frame hits the minimum frame size for Ethernet 🧚.

    +-------+-------+-------+-------+-------+-------+       ^
    |          Destination Ethernet address         |       |
    +-------+-------+-------+-------+-------+-------+    ethernet
    |            Source Ethernet address            |     header
    +-------+-------+-------+-------+-------+-------+       |
    |  86      DD   |                                       v
    +-------+-------+-------+-------+-------+------+------+
    |            IPv6 header and payload ...              /
    +-------+-------+-------+-------+-------+------+------+

Stateless Autoconfiguration and Link-Local Addresses 👑

The address token [CONF] for an Ethernet interface is the interface's built-in 48-bit IEEE 802 address 🦋. It's in canonical bit order, with the octets in the same order they'd appear in an ethernet frame header. (The individual/group bit sits in the first octet, and the OUI is in the first three octets.) A different MAC address that's set manually or by software should not be used as the address token 🙈.

An IPv6 address prefix used for stateless autoconfiguration of an ethernet interface must be 80 bits in length 💜.

The IPv6 Link-local address [AARCH] for an Ethernet interface is formed by appending the interface's IEEE 802 address to the 80-bit prefix FE80:: 🌈.

   +-------+-------+-------+-------+-------+-------+------+------+
   |  FE      80      00      00      00      00      00     00  |
   +-------+-------+-------+-------+-------+-------+------+------+
   |  00      00   |               Ethernet Address              |
   +-------+-------+-------+-------+-------+-------+------+------+

Address Mapping -- Unicast 🌸

The way you map IPv6 addresses to Ethernet link-layer addresses is explained in [DISC] 🧚. When the link layer is Ethernet, the Source/Target Link-layer Address option has this form 💖:

  +-------+-------+-------+-------+-------+-------+-------+-------+
  | Type  |Length |               Ethernet Address                |
  +-------+-------+-------+-------+-------+-------+-------+-------+

Option fields:

• Type: 1 for Source Link-layer address. 2 for Target Link-layer address.
• Length: 1 (in units of 8 octets).
• Ethernet Address: The 48 bit Ethernet IEEE 802 address, in canonical bit order. This is the address the interface currently responds to, and it may be different from the built-in address used as the address token ✨.

Address Mapping -- Multicast 🌈

An IPv6 packet with a multicast destination address DST gets sent to an Ethernet multicast address 🦋. The first two octets of that address are 3333 hexadecimal, and the last four octets are the last four octets of DST, ordered from most to least significant 💅.

             +-------+-------+-------+-------+-------+-------+
             |   33  |   33  | DST13 | DST14 | DST15 | DST16 |
             +-------+-------+-------+-------+-------+-------+

Security Considerations 💀

🚨 Bestie, this section is serious. The whole thing says: Security issues are not discussed in this memo. Nothing more is covered here, so keep that in mind 🙈.

References 📚

[AARCH] Hinden, R., and S. Deering, "IP Version 6 Addressing
        Architecture", RFC 1884, December 1995.

[CONF] Thomson, S., and T. Narten, "IPv6 Stateless Address
       Autoconfiguration", RFC 1971, August 1996.

[DISC] Narten, T., Nordmark, E., and W. Simpson, "Neighbor Discovery
       for IP Version 6 (IPv6)", RFC 1970, August 1996.

[IPV6] Deering, S., and R. Hinden, "Internet Protocol, Version 6
       (IPv6) Specification", RFC 1883, December 1995.

Author's Address 🎀

Matt Crawford
Fermilab MS 368
PO Box 500
Batavia, IL 60510
USA

Phone: +1 708 840-3461
EMail: crawdad@fnal.gov