RFC 877

RFC 877: A Standard for the Transmission of IP Datagrams Over Public Data Networks 🎀

okay bestie, the title block is giving: RFC 877, written by J. T. Korb from Purdue University in September 1983 🌸 It doesn't list a category, and it doesn't say it obsoletes or updates anything. Now let's get into it 💅

This one is a standard that CSNET, the VAN gateway, and other orgs adopted for sending IP datagrams over X.25-based public data networks. Iconic energy 🦋

An X.25 virtual circuit gets opened on demand, the moment a datagram shows up at the network interface for transmission ✨ It gets closed after some period of inactivity, and the length of that period depends on how much an open virtual circuit costs 💸 It can also get closed if the interface runs out of virtual circuits. For managing circuits during peak demand, check out [1] 🧚

STANDARDS ✨

1.1 The first octet in the Call User Data Field (the first data octet in the Call Request packet) is used for protocol demultiplexing 🔥 The value CC (binary 11001100, decimal 204) means INTERNET PROTOCOL. Main character energy 👑

1.2 IP datagrams are sent as X.25 "complete packet sequences" 📦 So datagrams start on packet boundaries, and the M bit ("more data") is used for any datagram bigger than one packet. There are no additional headers or other data in the packets. Clean, no extras 💖

1.3 Unless a larger packet size gets negotiated, the max size of an IP datagram over X.25 is 576 octets 🌈 If two sites negotiate a big packet size (like 1024 octets), a datagram that big is allowed. Iykyk 🙈

1.4 Either site may close a virtual circuit 💀 If the circuit gets closed or reset while a datagram is mid-transmission, that datagram is lost. Not me losing data 😭

GENERAL REMARKS ✨

2.1 Protocols above IP, like TCP, don't affect this standard 🧚 Specifically, no attempt is made to open X.25 virtual circuits that match up with TCP connections. Separate lanes, bestie 🛣️

2.2 Features of X.25 that this document doesn't discuss aren't used 💅 For example, interrupt packets and the D bit (indicating end-to-end significance) are not used. Pass ✨

2.3 Negotiable features (facilities) of X.25 are allowed 🫶 For example, sites are free to negotiate larger packet and window sizes. Go big or go home 🔥

2.4 Some sites, like CSNET sites, may try to open multiple virtual circuits to a single site 👀 Sites should handle those incoming calls gracefully. That means transmitting on the extra circuits if possible and accepting incoming datagrams from them. But don't accept the CALL REQUEST just to immediately close the connection or ignore datagrams sent on those circuits. That's so messy, periodt 😭

REFERENCE 📚

[1] Comer, D.E. and Korb, J.T., "CSNET Protocol Software: The IP-to-X.25 Interface", SIGCOMM Symposium on Communications Architectures and Protocols, March 1983. 🦋