RFC 489 UNKNOWN PROTOCOLS · CONNECTION

Comment on resynchronization of connection status proposal

Postel suggests better names for connection recovery commands.

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PUBLISHED
March 1973
AUTHORS
J. Postel
STREAM
Legacy
COMMANDS PROPOSED
RAS and RAR
WHAT IS RESET
Allocation quantities only
DOI
10.17487/RFC0489

RFC text

Original text · RFC Editor View on rfc-editor.org ↗
Network Working Group                                     Jon Postel
Request for Comments: #489                                UCLA-NMC
NIC #15298                                                March 26, 1973


       Comment on Resynchronization of Connection Status Proposal


This is a comment on the proposal by Burchfiel and Tomlinson in RFC 467
for a procedure in the host-to host protocol for resynchronization of
connection status.  I endorse their proposal with the following trivial
change.  The commands proposed might be more appropriately be called
"reset connection allocation sender" and "reset connection allocation
receiver" since the only aspect of the connection which is reset is the
allocation.  I therefore use the names RAS and RAR respectively.

The table below shows in overly concise notation my interpretation of
the resynchronizing procedure proposed by Burchfiel and Tomlinson, this
presentation is not intended to supersede their document but to clarify
the procedure.  The sequence shown here can be initiated by either the
sender or receiver either for internally generated reasons or upon the
receipt of a RAS or RAR, if this latter is the case then sender step 5
or receiver step 4 is satisfied.

SENDER                                 RECEIVER

1. Set state to "wait-for-RAR"         1. Set state to "wait-for RAS"
2. Wait till no RFNM outstanding       2. Send RAR
3. Send RAS                            3. Process messages until
4. Process allocates until             4. RAS received then
5. RAR received then                   5. Zero allocation quantities
6. Zero allocation quantities          6. Set state to "open"
7. Set state to "open"                 7. Send a new allocate










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PROBLEMRFC 467 proposed a procedure to recover when the Host-Host connection got out of synchronization and lost track of how much data each side could send.
MECHANISMJon Postel endorsed the proposal and renamed the two commands RAS (reset allocation sender) and RAR (reset allocation receiver) to clarify that only the sending allowances, not the entire connection, were being reset. He included a table showing the synchronization sequence.
CLARIFYING RECOVERYThe new names made the purpose of the recovery procedure clearer and were adopted in later protocol specifications.

§1Connection Recovery

This RFC is Jon Postel's brief comment on a proposal by Burchfiel and Tomlinson in RFC 467. The Host-Host protocol needs a way to recover when both communicating hosts lose synchronization about how much data each side is allowed to send. The allocation is a critical part of the flow control mechanism, and when it gets out of sync, the connection becomes stuck.

Computers talking over the network need a way to fix the situation when they forget who said what and get confused.

§2The RAS and RAR Commands

Postel proposes renaming the recovery commands to RAS (reset allocation sender) and RAR (reset allocation receiver). These names are more precise than the original proposal, because they emphasize that only the allocation quantities are being reset, not the whole connection. The sender and receiver follow a sequence: the sender sets a wait state, waits for outstanding acknowledgments, sends RAS, and processes responses until receiving RAR. The receiver sends RAR, processes incoming messages, receives RAS, zeros its allocation, and returns to open state.

Two new commands, RAS and RAR, let the sender and receiver fix how much data each one can send, step by step, until they agree again.

§3Synchronization Procedure

The procedure can be initiated by either the sender or receiver, and either for reasons of its own or after receiving a RAS or RAR command from the other side. The RFC provides a concise side-by-side table showing what each side does at each step. This recovery mechanism is critical to ensuring reliable connection management in the early Host-Host protocol.

Either side of the conversation can start the fix, and there is a clear sequence of steps that brings them back into sync.

§4References