File Transfer and Error Recovery
Suggestions for handshaking, file access, and error recovery in an early transfer protocol.
Sunberg, R.L., "File Transfer and Error Recovery", RFC 133, DOI 10.17487/RFC0133, April 1971, <https://www.rfc-editor.org/info/rfc133>.
@misc{rfc133,
series = {Request for Comments},
number = 133,
howpublished = {RFC 133},
publisher = {RFC Editor},
doi = {10.17487/RFC0133},
url = {https://www.rfc-editor.org/info/rfc133},
author = {R.L. Sunberg},
title = {{File Transfer and Error Recovery}},
pagetotal = 4,
year = 1971,
month = apr,
} RFC text
Contents
Network Working Group R. L. Sunberg
Request for Comments: 133 Harvard University
NIC 6710 27 April 1971
[Categories C.4, C.5, C.6, D.4, D.7, D.7]
FILE TRANSFER AND ERROR RECOVERY
1 FILE TRANSFER PROTOCOL
1A Handshaking
I think that Mr Bhushan(RFC #114, NIC 5823) is not strict enough in
his concept of a transaction sequence. Every transaction should
prompt a response from its recipient (recall Kalin's crates --
RFC #60, NIC 4762). Control should pass back and forth until the
server terminates. The server _always_ gets the last word (more on
error recovery later).
Some sample interchanges are given.
User Server Comments
<...> ==> Establish a connection
<== <...>
<I><...> ==> Identify self
<== <+> Ok, ready
<R><...> ==> Retrieval request
<== <rs> I've got your file
<rr> ==> Send it
<== <,><...> Here's the first part
<rr> ==> Got it
<== <+> All done
<S><...> ==> Store request
<== <rr> Ok, go ahead
<#><...> ==> Here's some protection stuff
<== <rr> Ok
<*><...> ==> Here's the file
<== <+> Got it. All done.
See section 2B, below, for examples of error recovery.
1B Extensions to the file transfer protocol
The file transfer protocol needs a mechanism for accessing individual
records of a file. This will be particularly useful when very large
data bases appear on the network. The following definitions should
be added to the protocol:
The store(S) and retrieve(R) requests have the data field format
<key>, where <key> has the syntax:
<key>::=<devicename>RS<filename>US<keyname> | <filename>US<keyname>.
-- -- --
The <pathname> syntax is changed to:
<pathname>::=<devicename> | <filename> | <pathname>RS<filename>.
--
If a retrieve(R) request is given with a data field with <key>
syntax rather than <pathname> syntax, then the returned data will
consist of the record following the matching <key>. If a store(S)
request is given with a data field of <key> syntax, then the
supplied data will replace the record following the matching
<keyname>. If the keyname does not exist, the record will be
appended to the named file. The individual installation must
provide the linkage between the <keyname> and the record it
references.
In addition, the lookup(L) request will provide a list of keynames
into a file (or the name of a file which contains the keynames).
Transaction code F (request File directory) requests a listing of
available files. The data field of the F transaction is of the
form: <pathname>GS<pathname>GS... All files in the server system
-- --
which match one or more of the given <pathname> specifiers are
listed in a return file. The format of the data fields of this
file is: <pathname>GS<pathname>GS... If a <pathname> field in
-- --
the request transaction does not include a <name> field, the
default is all files on the given device. Some examples are given:
<F><DC1 DSK[62,50]] GS JOE>
--- --
This example requests a list of all files on the disk specified by
[62,50] plus all files named JOE. The response could contain in
the data field:
<DC1 DSK[62,50] RS ALPHA RS BETA RS JOE GS DC1 DSK[10,50] RS JOE>
--- -- -- -- -- --- --
This message states that in the [62,50] area of the disk there are
files ALPHA, BETA, and JOE, and that JOE is also a file in the
[10,50] area of the disk.
2 ERROR RECOVERY
2A Error recovery procedures have been noticeably lacking to date.
The usual approach has been to close the connection and start from
scratch. Mr Bhushan proposes a third level abort but doesn't
really detail the implementation. I propose a multilevel error
recovery procedure as follows.
2B If an error occurs which does not cause a loss of third level
transaction boundaries and only affects one side of a duplex
connection, a third level recovery is possible via a transaction
sequence abort. An example is given:
User Server Comments
<R><...> ==> Send me this file
<== <rs> Ok, I've got it
<rr> ==> Ready
<== <*><...error> Here it is (with an error)
<-><D> ==> No. (data) error
<== <-><D> Sorry, forget it
<R><...> ==> Send the file (again)
|<== <rs> Ready (doesn't get there)
... (waiting)
<-><0> ==> Error, timeout
<== <-><0> Sorry, forget it
<R><...> ==> Send the file (third time)
<== <rs> Got it
<rr> ==> Ready
<== <*><...> There it is
<rr> ==> Got it
<== <+> Done (finally>
Note that the server always gets the last word in error situations
as well as normal transmission.
2C Although the above examples are given in terms of Bhushan's
transaction codes, this form of error recovery is implementable in
any protocol which uses flagged blocking and duplex connections.
2D If errors cannot be recovered as above, then some means must be
available to clear the link completely and resynchronize. I
suggest that an 8-bit argument be appended to the interrupt-on-link
NCP message (INR, INS). The receiver would send <INR><error> to
indicate that the block boundaries were lost and all incoming data
is being discarded. The sender, upon receiving the INR, would
flush all queued output and wait for the link to clear. The NCP
would then send a <INS><newsync> message and, when it was received
(RFNM returned), a negative termination would be sent on the link.
The receiver begins accepting data again when the INS is received.
This assumes that any process can flush untransmitted data and
detect a clear link. Note that this method is useable on any
simplex connection.
2E If all else fails, one can resort to closing the faulty socket.
3 NCP VERSION NUMBERS
3A I suggest that the NCP be given a version number and the next
version include two new message types: <WRU> ('Who aRe yoU?')
requests a version number from the receiving host and <IAM><version>
('I AM') supplies that number.
3B The messages would probably be initially used in a 'can I talk to
you?' sense or not at all. Eventually, it would take on a 'what
can you do?' meaning. Accordingly, the <version> field should be
large (32 bits?) for expansion.
[ This RFC was put into machine readable form for entry ]
[ into the online RFC archives by Jose Tamayo 4/97 ]
ISO Plain Language
On this page
§1A Reply for Each Transaction
Sunberg argues that each transaction should prompt a response from its recipient. Control should pass back and forth until the server ends the exchange, so the server always has the last word.
The RFC gives sample retrieve and store exchanges. In a retrieve, the user asks for a file, receives acknowledgment that the server has it, requests the data, and acknowledges the returned parts before the server reports completion.
The proposal has the sender and receiver acknowledge each step. The server ends the exchange with the final response.
§2Record Access and File Listings
The proposal adds a key to store and retrieve requests so they can access an individual record in a file. A retrieve with a key returns the record after the matching key; a store replaces that record or appends one if the key is not present.
A lookup request can list key names in a file. A new transaction code F requests a file-directory listing for one or more pathnames.
Instead of transferring an entire file, a request can identify a record by key. Another request can list files that match given pathnames.
§3Recovering from Errors in Stages
If an error preserves transaction boundaries and affects only one side of a duplex connection, the parties can abort the current transaction sequence and try again. The example includes retrying after a data error or a timeout.
If that recovery fails, the proposal clears and resynchronizes the link. It suggests adding an 8-bit argument to the NCP interrupt-on-link messages INR and INS. Closing the faulty socket is the last resort.
The first response to an error is to stop the current exchange and retry. If the link cannot be resynchronized, the parties can close the socket.
§4NCP Version Inquiry
Sunberg suggests adding a version number to the Network Control Program (NCP). The next version would include WRU, which asks the receiving host for its version, and IAM, which supplies it.
The author suggests that the version field be 32 bits to leave room for expansion. At first, the exchange could indicate whether the hosts can communicate; later, it might indicate what each host can do.
Hosts could ask which NCP version the other host has. The author suggests a large field so it could later describe capabilities too.