rfc426.txt
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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.
Reconnection
______ | ______
| | | | |
| EXEC |<-------------+------------>| USER |
|______| | / |______|
Host1 V / TIP
______ /
| |<------/
|______|
Host2
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 |/
X
/|
_______ / |
| | / v
| |<---
|_______|
Authenticator
(b)
______ ______
| | | |
| |<--\ ^ /-->| User |
|______| \ | / |______|
Host \ | /
------------+--/
| /
|/
|
/|
/ |
/ | authentication
_______ / | complete
| | /
| |<------
|_______|
Authenticator
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
_____ _____ _____ _____
| | / | | \ | | | |
|Host1|<----/--->|Host2|<---\---->|Host3|<----->|Host4|
|_____| \ / |_____| \ / |_____| |_____|
X move X
/ \ | / \
| \ V / |
V \ _____ / V
reconnect \ | | / reconnect
->|Host5|<-
|_____|
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
destination>.
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
H2:
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|---
| |
|________|
H1
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
H1.
To complete the specification of the reconnection mechanism consider
the situation in which two "adjacent" entities initiate
reconnections:
(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
so.
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]
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