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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]



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.

Sunberg [Page 1] RFC 133 File Transfer and Error Recovery April 1971

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
 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>
         ---             --

Sunberg [Page 2] RFC 133 File Transfer and Error Recovery April 1971

 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:
    ---            --       --      --     -- ---            --
 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.


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.

Sunberg [Page 3] RFC 133 File Transfer and Error Recovery April 1971

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.


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 ]

Sunberg [Page 4]

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