Chaos RTAPE protocol

The Chaos RTAPE protocol is used to provide a remote tape server on a Chaos network. The protocol is structured as a sequence of messages called a record stream, and each message specifies an operation to be performed or data to transmit.

A description of the protocol—as reverse engineered from ITS' DUMP backup program and the MIT/Symbolics RTAPE.c server for BSD Unix—follows.

Record stream

Once a connection has been established, the server sends a packet with the string RECORD STREAM VERSION 1. The client responds with the same string. Presumably the parties can negotiate another version, but so far only version 1 is known.

After that, both sides will exchange messages. Each message begins with an octet which is the opcode. After that comes two octets, in big-endian order, which is the size of the data that follows. Finally come as many octets of data as specified by the size.

The data portion may be an ASCII text string, or binary data to transmit tape blocks. If text data has several fields, they are separated by a space characters; numbers are in decimal with an optional minus sign.

Operations

Client-to-server opcodes:

Opcode	Name	Data
1	Login	?
2	Mount	Text: type reel drive size density options
3	Probe	Binary: two octets with an ID
4	Read	Nothing (read all records until file mark), or text: number-of-records
5	Write	Text: block-size (decimal)
6	Rewind	Nothing
7	Rewind-sync	Nothing
8	Rewind-unload	Nothing
9	Space file	Text: number-of-files
10	Space record	Text: number-of-records
12	Write file mark	Nothing
13	Close	Nothing

Server-to-client opcodes:

Opcode	Name	Data
33	Login response	?
34	Read data	Binary: tape block data
35	Read file mark	Nothing
36	Status	See below

Status messages may be solicited by the Probe opcode, or the server can send them at any time to signal an error.

The Read operation may transmit several tape blocks, but the client can interrupt this at any time by sending a Probe.

The data in a mount message needs further explanation:

Field	Description
Type	Mode: READ, WRITE, or BOTH
Reel	?
Drive	The name of the tape drive, as understood by the server
Size	(Preferred? max?) tape block size
Density	Tape density, in bytes per inch
Options	A space-separated list. May include NOREWIND and OFFLINE

Status information

The Status message has this data:

Offset	Size	Description	Data
0	1	Version	Only 1 is known
1	2	ID	Matching the Probe ID (Solicited flag set), or 0
3	3	Number of blocks read
6	3	Number of blocks skipped
9	3	Number of blocks discarded
12	1	Last operation received
13	2	Density
15	2	Retries in last operation
17	1	Length of next string
18	n	Name of drive
18+n	2	Flags
20+n	?	Optional error message

The two-octet flags field, when taken as a little-endian number:

Bit	Description
0	Solicited, e.g. requested by Probe
1	Drive is at beginning of tape
2	Drive is past end of tape
3	Last operation reached a file mark
4	Not logged in
5	Tape is mounted
6	Error message follows
7	Hard error
8	Soft error
9	Drive is offline