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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

Thomas [Page 1] 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

Thomas [Page 2] 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).

Thomas [Page 3] 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)

Thomas [Page 4] 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   |
 |____|        |____|            |____|        |____|

Thomas [Page 5] 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:

Thomas [Page 6] 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

Thomas [Page 7] 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>

Thomas [Page 8] 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

Thomas [Page 9] 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.

Thomas [Page 10] 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;

Thomas [Page 11] 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]

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