NETWORK WORKING GROUP                                   R. L. Sunberg
Request for Comments #133                               Harvard University

IC 6710                                                 27 April 1971

FILE TRANSFER AND ERROR RECOVERY

[Categories C.4, C.5, C.6, D.4, D.7, D.7]

1 FILE TRANSFER PROTOCOL

1A Handshaking

1A1 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).

1A2 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.

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See section 2B, below, for examples of error recovery.

1B Extensions to the file transfer protocol

1B1 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).

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1B2 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 move of the given <pathname> specifiers are

listed in a return file. The format of the data fiels 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 DC1DSK[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

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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 meanst 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

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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 assumed 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 ]

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