RFC0426: Reconnection Protocol

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Network Working Group                                         Bob Thomas
Request for Comments: 426                                      BBN-TENEX
NIC: 13011                                               23 January 1973
Categories: Protocols, TELNET
References: 36,318,333,435

                         Reconnection Protocol

   There are situations in which it is desirable to move one or both
   ends of a communication path from one host to another.  This note
   describes several situations in which the ability to reconnect is
   useful, presents a mechanism to achieve reconnection, sketches how
   the mechanism could be added to Host-Host or TELNET protocol, and
   recommends a place for the mechanism in the protocol hierarchy.

1. Some Examples:

A. Consider the case of an executive program which TIP users could use
   to get network status information, send messages, link to other
   users, etc.  Due to the TIP's limited resources the executive program
   would probably not run on the TIP itself but rather would run on one
   or more larger hosts who would be willing to share some of their
   resources with the TIP (see Figure 1).

   The TIP user could access the executive by typing a command such as
   "@ EXEC"; the TIP would then ICP to Host1's executive port.  After
   obtaining the latest network news and perhaps sending a few messages,
   the user would be ready to log into Host2 (in general not the same as
   Host1) and do some work.  At that point he would like to tell the
   executive program that he is ready to use Host2 and have executive
   hand him off to Host2.  To do this the executive program would first
   interact with Host2, telling it to expect a call from TIP, and then
   would instruct the TIP to reconnect to Host2.  When the user logs off
   Host2 he could be passed back to the executive at Host1 prepatory to
   doing more work elsewhere. The reconnection activity would be
   invisible to the TIP user.
          ______               |              ______
         |      |              |             |      |
         | EXEC |<-------------+------------>| USER |
         |______|              |  /          |______|
           Host1               V /              TIP
                 ______         /
                |      |<------/
                               Figure 1

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RFC 426                  Reconnection Protocol              January 1973

B. Imagine a scenario in which a user could use the same name and
   password (and perhaps account) to log into any server on the network.
   For reasons of security and economy it would be undesirable to have
   every name and password stored at every site.  A user wanting to use
   a Host that doesn't have his name or password locally would connect
   to it and attempt to log in as usual (See Figure 2).  The Host,
   discovering that it doesn't know the user, would hand him off to a
   network authentication service which can determine whether the user
   is who he claims to be. If the user passes the authentication test he
   can be handed back to Host which can then provide him service.  The
   idea is that the shuffling of the user back and forth between Host
   and Authenticator should invisible to the user.

   (a)   ______      for authentication     ______
        |      |            |              |      |
        |      |<-----------+------------->| User |
        |______|            | /            |______|
          Host              |/
             _______      / |
            |       |    /  v
            |       |<---

         ______                             ______
        |      |                           |      |
        |      |<--\             ^     /-->| User |
        |______|    \            |    /    |______|
          Host       \           |   /
                                 | /
                               / |
                              /  | authentication
             _______         /   | complete
            |       |       /
            |       |<------

                           Figure 2

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RFC 426                  Reconnection Protocol              January 1973

   If the user doesn't trust the Host and is afraid that it might read
   his password rather than pass him off to the authenticator he could
   connect directly to the authentication service.  After
   authentication, the Authenticator can pass him off to the Host.

C. The McROSS air traffic simulation system (see 1972 SJCC paper)
   already supports reconnection.  It permits an on-going simulation to
   reconfigure itself by allowing parts to move from computer to
   computer.  For example, in a simulation of air traffic in the
   Northeast the program fragment simulating the New York Enroute air
   space could move from Host2 to Host5 (see Figure 3).  As part of the
   reconfiguration process the New York Terminal area simulator and
   Boston Enroute area simulators break their connections with New York
   Enroute simulator at Host2 and reconnect to it at Host5.

   NY Terminal     NY Enroute    Boston Enroute  Boston Terminal
     _____            _____            _____         _____
    |     |      /   |     |   \      |     |       |     |
    |_____|  \ /     |_____|     \ /  |_____|       |_____|
              X        move       X
             / \        |        / \
             |  \       V       /  |
             V   \    _____    /   V
      reconnect   \  |     |  /   reconnect
                    NY Enroute

                             Figure 3
2. A Reconnection Mechanism

   The mechanism proposed here could be added to the existing Host-Host
   protocol or to the TELNET protocol.  The mechanism is first described
   and then its adaptation to each of the protocols is discussed.

      The reconnection mechanism includes four commands:

         Reconnect Request: RRQ <path>
         Reconnect OK:      ROK <path>
         Reconnect No:      RNO <path>
         Reconnect Do:      RDO <path> <new destination>

   where <path> is the communication path to be redirected to <new

   Assume that H1 wants to move its end of communication path A-C from
   itself to port D at H3 (See figure 4).

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RFC 426                  Reconnection Protocol              January 1973

     (a) situation                 (b) desired situation

     H2          H3                  H2           H3
    ___         ___                 ___          ___
   |   |       |   |               |   |        |   |
   |  C|<-+    |D  |               |  C|<------>|D  |
   |___|  |    |___|               |___|        |___|
          |   ___                             ___
          |  |   |                           |   |
          +->|A  |                           |A  |
             |___|                           |___|
               H1                              H1

                          Figure 4

   The reconnection proceeds by steps:

           a. H1 arranges for the reconnection by sending RRQ to
                   H1->H2:   RRQ (path A-C)

           b. H2 agrees to reconnect and acknowledges with ROK:

                   H2->H1:   ROK (path C-A)

           c. H1 notes that H2 has agreed to reconnect and
              instructs H2 to perform the reconnection:

                   H1->H2:   RDO (path A-C) (Host H3, Port D)

           d. H1 breaks paths A-C.
              H2 breaks path C-A and initiates path C-D.

   In order for the reconnection to succeed H1 must, of course, have
   arranged for H3's cooperation.  One way H1 could do this would be to
   establish the path B-D and then proceed through the reconnection
   protocol exchange with H3 concurrently with its exchange with H2 (See
   Figure 5):

           H1->H3:  RRQ (path B-D)
           H3->H1:  ROK (path D-B)
           H1->H3:  RDO (path B-D) (Host H2, Port C)

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RFC 426                  Reconnection Protocol              January 1973

          H2                                   H3
        ______                               ______
       |      |                             |      |
       |   C  |                             |  D   |
        ---\--                               -/----
            \       /-->          <--\       /
              \- -/--- --- --- --- --- \---/
               \ /                      \ /
                X                        X
               / \                      / \
              /   \                    /   \
    reconnection   \                  /   reconnection
                    \    ________    /
                     ---|A      B|---
                        |        |

                          Figure 5

   Either of the parties may use the RNO command to refuse or abort the
   reconnection.  H2 could respond to H1's RRQ with RNO; H1 can abort
   the reconnection by responding to ROK with RNO rather than RDO.

   It is easy to insure that messages in transit are not lost during the
   reconnection.  Receipt of the ROK message by H1 is taken to mean that
   no further messages are coming from H2; similarly receipt of RDO from
   H1 by H2 is taken to mean that no further messages are coming from

   To complete the specification of the reconnection mechanism consider
   the situation in which two "adjacent" entities initiate

      (a) situation               (b) desired situation

     H1            H4                H1            H4
    ____          ____              ____          ____
   |    |        |    |            |    |        |    |
   |   C|        |E   |            |   C|--------|E   |
   |____|        |____|            |____|        |____|

     H2            H3                H2            H3
    ____          ____              ____          ____
   |    |        |    |            |    |        |    |
   |   B|--------|D   |            |   B|        |D   |
   |____|        |____|            |____|        |____|

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RFC 426                  Reconnection Protocol              January 1973

   H2 and H3 "simultaneously" try to arrange for reconnection:

           H2->H3:  RRQ (path B-D)
           H3->H2:  RRQ (path D-B)

   Thus, H2 sees an RRQ from H3 rather than an ROK or RNO in response to
   its RRQ to H3.  This "race" situation can be resolved by having the
   reconnections proceed in series rather than in parallel: first one
   entity (say H2) performs its reconnect and then the other (H3)
   performs its reconnect. There are several means that could be used to
   decide which gets to go first.  Perhaps the simplest is to base the
   decision on sockets and site addresses: the entity for which the 40
   bit number formed by concatenating the 32 bit socket number with the
   8 bit site address is largest gets to go first.  Using this mechanism
   the rule is the following:

      If H2 receives an RRQ from H3 in response to an RRQ of its own:
         (let NH2,NH3 = the 40 bit numbers corresponding to H2 and H[2])

      a. if NH2>NH3 then both H2 and H3 interpret H3's RRQ as an ROK in
         response to H2's RRQ.

      b. if NH2<NH3 then both interpret H3's RRQ as an RNO in response
         to H2's RRQ.  This would be the only case in which it makes
         sense to "ignore" the refusal and try again - of course,
         waiting until completion of the first reconnect before doing

   Once an ordering has been determined the reconnection proceeds as
   though there was no conflict.

   The following diagram describes the legal protocol command/response
   exchange sequences for a reconnection initiated by P:

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RFC 426                  Reconnection Protocol              January 1973

                        ___                 ___
                       | P |---------------| Q |
                       |___|               |___|
   | P --> Q ||  R R Q  |
   |         ||         |             |         |
   | Q --> P ||  R O K  |  R N O  ----|  R R Q  |
   |         ||         |         | E |         |
                   |                       |
      +------------+                       v
      |                      Yes   +----------+   No
      |   +------------------------| NP > NQ? |------+
      |   |                        +----------+      |
    __v___v_______________________________           |
   |         ||             |             |          |
   | P --> Q ||  R D O  ----|  R N O  ----|          |
   |         ||         | E |         | E |          |
   |_________||_________|___|_________|___|          |
   |         ||             |             |
   | Q --> P ||  R D O  ----|  R N O  ----|
   |         ||         | E |         | E |

   NP and NQ are the 40 bit numbers for P and Q; E indicates end of

3.  Adding the Reconnection Mechanism to Host-Host Protocol

        The four reconnect commands could be included directly in
        Host-Host protocol as follows:

           RRQ <my socket> <your socket>
           ROK <my socket> <your socket>
           RNO <my socket> <your socket>
           RDO <my socket> <your socket> <new host> <new socket>

   The ROK and RDO commands for a send connection should not be sent
   until there are no messages in transit over the connection.  The RDO

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RFC 426                  Reconnection Protocol              January 1973

   command is to be interpreted as a CLS.

   The reconnection:

     H2           H3                    H2           H3
    ___          ___                   ___          ___
   |   |        |   |                 |  C|--------|D  |
   |_C_|        |_D_|                 |___|        |___|
     |            |
     |            |        ===>
     |    ____    |                          ____
      ---|A  B|---                          |    |
         |____|                             |____|
           H1                                 H1

    could be accomplished as follows:

         H1->H2:  RRQ A C
         H1->H3:  RRQ B D
         H2->H1:  ROK C A
         H3->H1:  ROK D B
         H1->H2:  RDO A C H3 D
         H1->H3:  RDO B D H2 C
         H2->H1:  CLS C A
         H3->H1:  CLS D B
         H2->H3:  STR C D size
         H3->H2:  RTS D C link

   Note that it is possible for the STR from H2 to arrive at H3 before
   the RDO from H1.  H3 must be prepared to queue such an RFC until it
   gets an RDO or RNO from H1.  Stated differently, transmission of an
   ROK for a local socket causes the socket to move from an "open" state
   to a "reconnect pending" state and indicates willingness to queue
   subsequent RFC's until receipt of a "matching" RDO or RNO.

4. Reconnection in TELNET Protocol

   Independently of whether Host-Host protocol directly supports
   reconnection, the reconnection mechanism could be included in TELNET
   with the addition to the TELNET protocol of the five commands:

         RDO <host> <socket>
         RWT <host> <socket>

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RFC 426                  Reconnection Protocol              January 1973

   where RRQ, ROK, RNO, RDO, and RWT are appropriately chosen characters
   in the range 128 to 255.  The first three commands require no
   parameters since they refer to the connections they are received on.
   For RDO and RWT, <host> is an 8 bit (= 1 TELNET character) host
   address and <socket> is a 32 bit (= 4 TELNET characters) number that
   specifies a TELNET receive socket at the specified host.

   A pending reconnection can be activated with either RDO or RWT.  The
   response to either is to first break the TELNET connection with the
   sender and then reopen the TELNET connection to the host and sockets
   specified.  For RDO, the connection is to be reopened by sending two
   RFC's; for RWT, by waiting for two RFC's.

   The RWT command is introduced to avoid races such as the one between
   the STR and CLS (RDO) discussed above.  In Host-Host protocol the
   race is avoided by putting a connection into "reconnect pending"
   state upon transmission of ROK.  For TELNET the race can be avoided
   by the initiator of the reconnection by judicious use of RWT and RDO.
   For example the reconnection:

     H2                           H3          H2           H3
   +---+                        +---+       +---+   M    +---+
   |   |----+             +---->|   |       |   |------->|   |
   | Y | N  |             |  Q  | Z |   ==> | Y |   N    | Z |
   |   |<-+ |      H1     | +---|   |       |   |<-------|   |
   +---+  | | M  +---+  P | |   +---+       +---+        +---+
          | +--->|   |----+ |
          |      | X |      |                        H1
          +------|   |<-----+                      +---+
                 +---+                             |   |
                   H1                              | X |
                                                   |   |
   could be accomplished as follows:

           X->Y:  RRQ
           X->Z:  RRQ
           Y->X:  ROK
           Z->X:  ROK
           X->Y:  RWT  H3 P
           X closes connections to Y
           Y closes connections to X
           Y waits for STR and RTS from H3
           X->Z: RDO H2 N
           X closes connections to Z
           Z closes connections to X
           Z sends STR and RTS to H2 which Y answers with
             matching RTS and STR to complete reconnection

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RFC 426                  Reconnection Protocol              January 1973

   The reconnection mechanism for TELNET can be made to fit nicely into
   the command format suggested by Cosell and Walden in RFC #435.
   Consider the addition of three new commands to TELNET:

        Prepare for Reconnect:                 RCP
        Begin Reconnect by sending RFC's:      RCS
        Begin Reconnect by waiting for RFC's:  RCW

   Using these three command and the DO, DON'T, WILL, WON'T commands of
   RFC #435, the commands proposed earlier become:

        RRQ => DO RCP
        ROK => WILL RCP
        RNO => WON'T RCP  ;for responses to DO RCP
               DON'T RCP  ;for responses to WILL RCP
                          ;i.e. used to cancel an RCP.
        RDO <host> <socket> => DO RCS <host> <socket>
        RWT <host> <socket> => DO RCW <host> <socket>

   As RFC #435 notes the nice thing about this structure is that a host
   choosing not to implement reconnection does not even have to know
   what RCP means.  All that it need do in response to DO RCP is to
   transmit WON'T RCP.

5. Recommendation

   I feel that reconnection is a basic notion and that its proper place
   within the protocol hierarchy is at the Host-Host level where it
   would be available for use in all higher level protocols.  The
   difficulty is that placing it there would, of course, require NCP
   rewrites.  Reluctance to make NCP modifications would probably be
   sufficient to kill interest in the proposal.

   Therefore, for pragmatic reasons, I recommend that the reconnection
   mechanism be included in TELNET as an "option" in the spirit of RFC
   #435.  This can be accomplished with the addition to the TELNET
   protocol of the RCP, RCS, RCW commands as described in Section 4.
   Modification of user- and server-TELNET programs to handle these new
   commands should be straightforward.  If a reconnection option is made
   part of TELNET protocol the TENEX hosts will support it.  In
   addition, the TIP guys (Walden and Cosell) have said that they like
   the reconnection mechanism and have agreed, in principle, to
   implement it for TIPs if it is accepted as part of TELNET protocol.

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RFC 426                  Reconnection Protocol              January 1973

   Addition of reconnection at the TELNET level rather than the Host-
   Host level is admittedly a compromise.  However, with it, activity of
   the sort described in Examples A and B of Section 1 will be possible.

6. Additional Remarks

A. Reconnection is not a new notion.  An early proposal for Host-Host
   protocol (RFC #36) included facilities to support reconnection.  The
   reconnection mechanism in RFC #36 supposes a configuration in which
   entities are "daisy-chained" together by connections:

          __      __      __      __      __
      ___|  |____|  |____|  |____|  |____|  |___
         |__|    |__|    |__|    |__|    |__|

   and specifies how one or more entities can break out of the chain.
   As suggested above (Figure 5) the mechanism proposed in this note
   supports that kind of reconnection.

B. In practice one would expect simultaneous initiation of reconnects by
   adjacent entities to be relatively rare.

C. The approach taken in RFC  #36 to handle simultaneous reconnection
   attempts by entities adjacent in the chain is to accomplish the
   reconnect as a single, carefully coordinated, global reconnect.  I
   feel that the sequence of locally coordinated reconnects as proposed
   above is preferable.  When N adjacent entities simultaneously attempt
   reconnection the single, globally coordinated reconnect as outlined
   in RFC #36 requires ~N^2 control messages whereas the sequential
   locally coordinated reconnect requires ~N.

D. A word about security is in order.  It should be clear that the
   decision to accept or reject a particular reconnection request is the
   responsibility of the entity (person at a terminal or process) using
   the connection. In many cases the entity may choose to delegate that
   responsibility to its NCP or TELNET (e.g., Example A, Section 1).
   However, the interface a Host provides to the reconnection mechanism
   should include means for local entities to exercise control over
   response to remotely initiated reconnection requests.  For example, a
   user-TELNET might support several modes of operation with respect to
   remotely initiated reconnections:

   1. transparent: all requested reconnections are to be performed in a
      way that is invisible to the user;

   2. visible: all requested reconnections are to be performed and the
      user is to be informed whenever a reconnection occurs;

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RFC 426                  Reconnection Protocol              January 1973

   3. confirmation: the user is to be informed of each reconnection
      request which he may accept or reject;

   4. rejection: all requested reconnects are to be rejected.

E. Reconnection can be achieved almost trivially within the Message
   Switched Protocol (MSP) proposed by Bressler, Murphy and Walden in
   RFC #333  (within MSP, "reconnection" is probably not the correct
   term).  For example use of the following conventions with that MSP
   (expressed in the terminology of RFC #333) support reconnection:

   1. unless a reconnection is in progress, rendezvous is to occur at
      the sending site;

   2. the receiving end of a communication path can be moved by passing
      the names of the rendezvous site and the ports to the new

   3. receipt of an OUT message for which the source site differs from
      the rendezvous site signals that the sending end is being moved;
      the source site should be used as the rendezvous site for
      subsequent IN messages;

   4. the sending end of a communication path can be moved by passing
      the names of the ports to the new sender; to complete the move the
      new sender uses the previous sender's site as rendezvous site for
      its first OUT message and its own site as rendezvous for
      subsequent OUT messages.

   As simple and appealing as this procedure seems, I doubt that it
   would be used in practice if MSP were to be implemented as a
   replacement for or alternative to existing Host-Host protocol.  The
   reason is that the ability to pass ports from Host to Host
   (needlessly) complicates port name allocation by requiring
   cooperation among Hosts to manage the allocation (e.g., before a Host
   can safely allocate a port name for use by a local process it must
   not only insure that the port is not in use locally but also that no
   process out in the network is using it.)  The inter-Host cooperation
   required to support the passage of ports among Hosts can probably not
   be reliably achieved in practice.  Therefore port passage of the sort
   described in RFC #333 should not be supported at the Host-Host
   protocol level.  For this reason, I feel that within an MSP
   "reconnection" would be best handled by a mechanism such as the one
   proposed in this note.

        [ This RFC was put into machine readable form for entry  ]
        [ into the online RFC archives by Anthony Anderberg 4/99 ]

Thomas                                                         [Page 12]