RFC 895

A Standard for the Transmission of IP Datagrams over Experimental Ethernet Networks 🎀

Network Working Group 🌸 Request for Comments: 895 🦋 Author: Jon Postel, ISI ✨ Date: April 1984 📅 Obsoletes: nothing listed 🙈 Updates: nothing listed 💅

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Status of this Memo ✨

okay so this RFC is basically the instruction manual for sending IP datagrams 📦 over an Experimental Ethernet 🔌. it says this is a standard way to do it 💖 and that it's a standard protocol for the ARPA Internet community 🌈. the IP piece is [1] and the Experimental Ethernet piece is [2] 🫶

Introduction 🦋

this memo is for the Experimental Ethernet, which runs at 3-megabit/second and uses 8-bit addresses ⚡. if you want the regular Ethernet (10-megabit/second with 48-bit addresses) it's a whole different story, and that's covered in [3] 💜

Frame Format 🎀

IP datagrams go inside normal Experimental Ethernet frames 🧚. the type field of the Ethernet frame must contain the value 513 (1001 octal) 🔢. then the data field holds the IP header, and right after it comes the IP data 💌

if the frame is too short, the data field should be padded to hit the Experimental Ethernet minimum frame size 🍓. that padding isn't part of the IP packet, so it doesn't get counted in the total length field of the IP header 🙈

the biggest IP datagram you can send over an Experimental Ethernet is 1536 octets 📏. implementations are encouraged to support full-length packets 💅. gateway implementations MUST be prepared to accept full-length packets and fragment them if necessary 🔥 this is a non-negotiable, bestie. and if a system can't receive full-length packets, it should try to discourage everyone else from sending them, like by using the TCP Maximum Segment Size option [4] 🙅‍♀️

Note: datagrams on the Ethernet can be longer than the general Internet default maximum packet size of 576 octets 👀. hosts on an Ethernet should keep that in mind when they send datagrams to hosts that aren't on the same Ethernet 🫣. it may be smart to send smaller datagrams so you don't cause unnecessary fragmentation at intermediate gateways 🧠. check [4] for more on this 💖

Address Mappings 🗺️

there are a few ways to map a 32-bit Internet address to an 8-bit Experimental Ethernet address 🪄. the easiest thing to do is take the last eight bits of the host number part of the Internet address and use that as the host's Experimental Ethernet address 💁‍♀️. this is the recommended approach 👑

Broadcast Address 📣

the broadcast Internet address (the address on that network with a host part of all binary ones) should be mapped to the broadcast Experimental Ethernet address, which is address zero 🌟

Trailer Formats 🧁

some versions of Unix 4.2bsd use a different encapsulation method to get better network performance with the VAX virtual memory architecture 🖥️. systems on the same Ethernet can use this format between themselves if they both agree 🤝

no host is required to implement it 🙅‍♀️. and no datagrams in this format should be sent to any host unless the sender has positive knowledge that the recipient will be able to interpret them 🔒. the details of the trailer encapsulation are in [6] 📖

(note: right now Unix 4.2bsd will either always use trailers or never use them, per interface, depending on a boot-time option ⏰. this is expected to change in the future 🔮. Unix 4.2bsd also uses a non-standard Internet broadcast address with a host part of all zeroes, and that will also be changed in the future 💀)

Byte Order 🔢

as described in Appendix B of the Internet Protocol specification [1] 📚, the IP datagram gets sent over the Ethernet as a series of 8-bit bytes 🍰

References 📚

[1] Postel, J., "Internet Protocol", RFC-791, USC/Information Sciences Institute, September 1981. 🧾

[2] Metcalfe, R. and D. Boggs, "Ethernet: Distributed Packet Switching for Local Computer Networks", Communications of the ACM, V.19, N.7, pp 395-402, July 1976. 🧾

[3] Hornig, C., "A Standard for the Transmission of IP Datagrams over Ethernet Networks", RFC-894, Symbolics Cambridge Research Center, April 1984. 🧾

[4] Postel, J., "The TCP Maximum Segment Size Option and Related Topics", RFC-879, USC/Information Sciences Institute, November 1983. 🧾

[5] Plummer, D., "An Ethernet Address Resolution Protocol", RFC-826, Symbolics Cambridge Research Center, November 1982. 🧾

[6] Leffler, S., and M. Karels, "Trailer Encapsulations", RFC-893, University of California at Berkeley, April 1984. 🧾