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Errata Exist
Network Working Group                                 J. Hofmueller, Ed.
Request for Comments: 4824                              A. Bachmann, Ed.
Category: Informational                                IO. zmoelnig, Ed.
                                                            1 April 2007

                   The Transmission of IP Datagrams
            over the Semaphore Flag Signaling System (SFSS)

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The IETF Trust (2007).


   This document specifies a method for encapsulating and transmitting
   IPv4/IPv6 packets over the Semaphore Flag Signal System (SFSS).

Table of Contents

1. Introduction ....................................................2
2. Definitions .....................................................2
3. Protocol Discussion .............................................3
   3.1. IP-SFS Frame Description ...................................3
   3.2. SFS Coding .................................................4
   3.3. IP-SFS Data Signals ........................................5
   3.4. IP-SFS Control Signals .....................................6
   3.5. Protocol Limitations .......................................7
   3.6. Implementation Limitations .................................7
4. Interface Discussion ............................................7
   4.1. Data Link Control ..........................................8
   4.2. Establishing a Connection ..................................8
   4.3. State Idle .................................................8
   4.4. Session Initiation .........................................8
   4.5. State Transmitting .........................................9
   4.6. State Receiving ...........................................10
   4.7. Terminating a Connection ..................................11
   4.8. Further Remarks ...........................................11
5. Security Considerations ........................................11
6. Acknowledgements ...............................................11
7. References .....................................................12

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RFC 4824                      IP over SFSS                    April 2007

1.  Introduction

   This document specifies IP-SFS, a method for the encapsulation and
   transmission of IPv4/IPv6 packets over the Semaphore Flag Signaling
   System (SFSS).  The SFSS is an internationally recognized alphabetic
   communication system based upon the waving of a pair of hand-held
   flags [JCroft, Wikipedia].  Under the SFSS, each alphabetic character
   or control signal is indicated by a particular flag pattern, called a
   Semaphore Flag Signal (SFS).

   IP-SFS provides reliable transmission of IP datagrams over a half-
   duplex channel between two interfaces.  At the physical layer, SFSS
   uses optical transmission, normally through the atmosphere using
   solar illumination and line-of-sight photonics.  A control protocol
   (Section 4) allows each interface to contend for transmission on the
   common channel.

   This specification defines only unicast transmission.  Broadcast is
   theoretically possible, but there are some physical restrictions on
   channel direction dispersion.  This is a topic for future study.

   The diagram in Figure 1 illustrates the place of the SFSS in the
   Internet protocol hierarchy.

             +-----+     +-----+       +-----+
             | TCP |     | UDP |  ...  |     |  Host Layer
             +-----+     +-----+       +-----+
                |           |             |
             |    Internet Protocol & ICMP   |  Internet Layer
             |             SFSS              |  Link Layer

                     Figure 1: Protocol Relationships

2.  Definitions

   Link:    A link consists of two (2) interfaces that share a common

   Link Partner:
            The opposite interface.

   Session: The transmission of an entire IP datagram.

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RFC 4824                      IP over SFSS                    April 2007

   SFS:     One Semaphore Flag Signal, i.e., one flag pattern (Section

   SFSS:    The Semaphore Flag Signaling System.

   IP-SFS:  IP over Semaphore Flag Signaling System.

3.  Protocol Discussion

   IP-SFS adapts the standard SFSS to encode an alphabet of 16 signals
   (flag patterns) to represent data values 0-15 (Section 3.2.1) and 9
   signals to represent control functions (Section 3.2.2).  With 16 data
   signals, IP-SFS transmission is based upon 4-bit nibbles, two per
   octet.  Each of the signal patterns defined in Section 3.2 is called
   an SFS.

3.1.  IP-SFS Frame Description

   IP datagrams are formatted into IP-SFS frames by adding IP-SFS
   headers and trailers.  Figure 2 shows the format of one IP-SFS frame.
   The frame is delimited by a control SFS called FST (Frame Start) and
   a control SFS called FEN (Frame End).  It is composed of a series of
   4-bit nibbles, one per SFS.

   An IP datagram will be fragmented into multiple successive IP-SFS
   frames if necessary.  When an IP datagram is fragmented into N
   frames, the first frame will be sent with frame number N-1, the
   second with frame number N-2, ..., and the last with frame number 0.

              0        1        2        3
          |   FST  |Protocol|CksumTyp|Frame No|Frame No|
                   |                                   |
                   //       DATA  Payload              //
                   |                                   |
                   |  CRC   |  CRC   |  CRC   |  CRC   |   FEN   |

            Note that each field represents one SFS or 4 bits.

                       Figure 2: IP-SFS Frame Format

   FST:       Frame Start control SFS

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RFC 4824                      IP over SFSS                    April 2007

   Protocol:  4 bits -- Internetwork-layer protocol code

       0      None.

       1      For IPv4.

       2      For IPv6.

       3      For IPv4 frame gzip-compressed.

       4      For IPv6 frame gzip-compressed.

       5...15 Reserved for future use.

   CksumTyp:  4 bits (one data SFS) -- Checksum Type

       0      none.

       1      CCITT CRC 16 (polynomial: x^16 + x^12 + x^5+1).

       2...15 Reserved for future use.

   Frame No:  8 bits (2 data SFSs):
              Frame number for fragmented IP datagram.

   DATA:      0 to 510 data SFSs (Section 3.2.1) representing 0 to 255
              octets of payload.

   CRC:       16 bits as four data SFSs.
              CRC checksum.  Preset to 0xFFFF.  One's complement of
              checksum is transmitted.

   FEN:       Frame ENd control SFS.

   The number of transmitted SFSs per minute (Spm) depends on the
   experience of participating interfaces.  Resulting link speed in bits
   per second for IP-SFS is (Spm/60)*4, not counting framing overhead.

3.2.  SFS Coding

   Data signals and control signals are based upon standard SFS
   encoding, as described by [JCroft], [Wikipedia], and other sources on
   the Internet.  The 16 data signals are interpreted as 4-bit nibbles,
   while the 9 control signals are used for data link control.

   IP-SFS defines the 16 data signals by the original SFSS encodings for
   letters A to P and the 9 control signals represented by SFSS
   encodings Q to X.

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RFC 4824                      IP over SFSS                    April 2007

3.3.  IP-SFS Data Signals

   Figure 3 illustrates the 16 SFSs used to transmit data frames over
   the link.  The illustrations show each SFS as seen from the receiving

                   SFS        0     __0      \0      |0
                             /||      ||      ||      ||
                             / \     / \     / \     / \
                              A       B       C       D
                   IP-SFS    0x00    0x01    0x02    0x03


                   SFS        0/      0__     0     __0
                             ||      ||      ||\     /|
                             / \     / \     / \     / \
                              E       F       G       H
                   IP-SFS    0x04    0x05    0x06    0x07


                   SFS       \0      |0__     0|      0/
                             /|       |      /|      /|
                             / \     / \     / \     / \
                              I       J       K       L
                   IP-SFS    0x08    0x09    0x0A    0x0B


                   SFS        0__     0     _\0     __0|
                             /|      /|\      |       |
                             / \     / \     / \     / \
                              M       N       O       P
                   IP-SFS    0x0C    0x0D    0x0E    0x0F

                      Figure 3: IP-SFS Data Signals.

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RFC 4824                      IP over SFSS                    April 2007

3.4.  IP-SFS Control Signals

   Nine control signals are used to signal special IP-SFS conditions.
   Their meanings are listed in Figure 4.  The illustrations show each
   SFS as seen from the receiving side.

                   SFS      __0/    __0__   __0      \0|
                              |       |       |\      |
                             / \     / \     / \     / \
                              Q       R       S       T
                   IP-SFS    FST     FEN     SUN     FUN


                   SFS       \0/     \0__     0/_     0/
                              |       |       |       |\
                             / \     / \     / \     / \
                              U       V       W       X
                   IP-SFS    ACK     KAL     NAK     RTR


                   SFS        0__     0__
                             /|       |\
                             / \     / \
                              Y       Z
                   IP-SFS    RTT    unused


                   SFS      _\0/_
                             / \
                   IP-SFS   unused

                     Figure 4: IP-SFS Control Signals.

   FST: Frame STart.  Signals the start of a new frame.

   FEN: Frame ENd.  Signals the end of one frame.

   SUN: Signal UNdo.  Cancels the transmission of one or more individual
        SFSs within the current frame.  This signal will be
        unacknowledged by the receiver.

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   FUN: Frame UNdo.  As long as Frame ENd is not sent, the transmitter
        or the receiver may send a FUN to restart the transmission of
        the current frame.  This signal will be unacknowledged and may
        be ignored by the receiver.

   ACK: Frame ACK.  Acknowledges reception of one frame.

   KAL: KeepALive.  Keep a connection alive.  Is to be transmitted in
        State Idle at a frequency of at least KAL_FREQ (see
        Section 4.2).  This signal will be unacknowledged.

   NAK: Frame No AcK.  The frame received is incorrect.

   RTR: Ready To Receive.  Receiver acknowledges it is ready to receive.

   RTT: Ready To Transmit.  Sender requests permission to initiate

3.5.  Protocol Limitations

   Due to the physical characteristics of the transfer channel, bit
   error rates are expected to be in the range of 1e-3 (boy scout) to
   1e-4 (professional sailor), and also depend a number of physical
   factors.  Poor visibility due to weather conditions or lack of
   illumination (e.g., night time) can drastically increase the error

   IP-SFS provides no means to handle frame reordering or dual
   (multiple) frame reception.  Thus, the protocol is not suitable in
   environments where interfaces are moving fast and/or when the path of
   light is long.

3.6.  Implementation Limitations

   Maximum payload per frame: 510 SFS (0...510) nibbles (0 to 255

   Maximum SFS per frame: 518

   Maximum frames per session: 255 (0...254)

4.  Interface Discussion

   An interface is constructed through the participation of one or more
   humans.  A knowledge of the SFSS is recommended, but its absence can
   be compensated by a well-designed GUI.

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RFC 4824                      IP over SFSS                    April 2007

4.1.  Data Link Control

   This section describes the control protocol used to allocate the
   half-duplex data link to either interface.

   Interfaces know three States: Idle, Transmitting (TX), and Receiving

4.2.  Establishing a Connection

   IP-SFS is strictly point-to-point.  Unless interface members are
   acquainted with each other, a brief introduction of both sides is
   suggested prior to setting up a link to reduce the likelihood of
   interface-spoofing attacks.

   Interfaces must agree on two different IP addresses on the same

   Interfaces are free to negotiate any period of time as TIMEOUT.
   Possible values for TIMEOUT are the time it takes to smoke a
   cigarette or the time it takes to drink a glass of water.  If
   negotiation fails, TIMEOUT defaults to 60 seconds.

   The same procedure may be applied for the KeepALive FReQuency
   (KAL_FRQ).  The period of KAL_FRQ (1/KAL_FRQ) should be at least

4.3.   State Idle

   Interfaces in State Idle must be ready to send an IP datagram
   provided by a local higher-level protocol or to receive a datagram
   from the link-partner.  Interfaces in State Idle must send keep-alive
   signals KAL at a frequency of at least KAL_FRQ.

   There are no further definitions for State Idle, but we recommend
   staying away from alcoholic beverages or other types of drugs that
   could lead to an increased number of FUN and/or SUN signals, a
   decrease in bandwidth, or an increase of line latency.

   If the number of IP datagrams in the transmission queue is >0, the
   interface may try to initiate a session by sending an RTT to the link
   partner.  If the link partner ready to receive, it returns an RTR

4.4.  Session Initiation

   An interface receiving a datagram from an Internet layer protocol
   level may start signaling RTT.

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RFC 4824                      IP over SFSS                    April 2007

   If the link partner does not respond with RTR within TIMEOUT, or the
   link partner responds with RTT, the interface switches to State Idle
   for a random period of time between 2 seconds and TIMEOUT, before
   resending the RTT.

   If the link partner transmits RTR, the interface transmits the number
   of IP-SFS frames to be transmitted in this session as two SFSs
   followed by another RTT.  If the link partner does not transmit the
   same number of IP-SFS frames followed by RTR within 3*TIMEOUT, the
   interface switches to State Idle.

   If the link partner transmits the same number of IP-SFS frames
   followed by RTR, the interface switches to State Transmitting.

   Unless obstructed through environmental noise or great distance,
   hollering can be used to request line discipline from the link
   partner in State Idle.  The use of cellphones is also an option,
   whereas throwing objects or using guns is not recommended, since it
   could result in interface damage or destruction.  This would be

4.5.  State Transmitting

   Transmission of one IP-SFS frame starts with FST.  After FST and
   before FEN have been transmitted, the interface may acknowledge a
   received FUN and restart the transmission of the active frame or
   discard the signal and continue transmission of the active IP-SFS

   If an error occurs by transmitting a wrong data SFS, the interface
   may invalidate the last data SFS by transmitting SUN followed by the
   correct signal.  A series of incorrectly transmitted data SFSs may be
   invalidated by sending a SUN for each invalid SFS, effectively back-
   spacing the sequence.

   Control SFSs cannot be invalidated.

   If an error occurs, the interface may also transmit FUN and restart
   transmission of the active IP-SFS frame.

   Whether the interfaces choose SUN or FUN for error correction is up
   to the interface, but it is suggested to use SUN for a single invalid
   SFS, and FUN whenever the interface failed to transmit several SFSs
   in a row.

   After FEN has been transmitted, the transmitting interface waits for
   the link partner to transmit ACK or NAK.

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   If ACK has been received, the transmitting interface removes the
   active frame and starts transmitting the next IP-SFS frame.  If no
   frames are left, the interface switches to State Idle.

   If NAK has been received, the transmission failed, and the interface
   must start transmitting the active frame again.

   If the link partner does not transmit ACK or NAK within TIMEOUT, the
   transmission failed, and the interface must start retransmitting the
   active IP-SFS frame.

   If transmission of the same IP-SFS frame fails 5 times, the interface
   leaves the IP datagram in the transmission queue and switches to
   State Idle.

4.6.  State Receiving

   In State Receiving, the interface stores each SFS received from the
   link partner in the receiving queue in the order of appearance.

   After FST and before FEN have been received, the interface may
   transmit FUN at any time to request a retransmission of the entire
   IP-SFS frame.

   If the time between two received SFSs exceeds TIMEOUT, the receiving
   interface must discard all data from the active IP-SFS frame and may
   additionally transmit FUN.  If the link partner does not continue
   transmitting within a second TIMEOUT period, the interface must clear
   the receiving queue and switch to State Idle.

   If the interface receives SUN from the link partner, it must discard
   the last received data SFS (if any).  If N SUNs are received in a
   row, then the last N data SFS must be discarded, unless there are no
   more data SFS in the frame.  If there are no more data SFS in the
   frame to be discarded, the SUN signal must be ignored by the

   If the receiving interface receives FUN from the link partner, it is
   free to discard the frame received thus far.  We suggest honoring FUN
   since discarding the signal will decrease bandwidth.

   After FEN has been received, the receiving interface evaluates the
   checksum.  If the checksum is good, the interface transmits ACK.  If
   the Frame Number of the active frame is 0, the interface passes the
   entire data from the receiving queue to the higher level protocol,
   clears the receiving queue, and switches to State Idle.

   If the checksum is incorrect, the interface transmits NAK.

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4.7.  Terminating a Connection

   If the interface is in State Idle and the link partner did not
   transmit any kind of SFS for at least five times 1/KAL_FRQ, the
   connection is terminated and the interfaces are free to disband.

4.8.  Further Remarks

   Interfaces are connected to computer hardware by means of a suitable
   input device (RX) and a suitable output device (TX).  Possible
   devices include keyboard, mouse, and monitor.  Other means of
   connection are subject to availability and budget.

   Although it is theoretically possible to combine the TX and RX parts
   of an interface in one human being, we suggest dividing the
   operations among at least two people per interface.  For longer
   transmissions (multimedia streaming, video conferencing, etc.),
   consider rotating and providing substitutes.

   Bandwidth tends to vary.  Typically TX starts at about 2-4 bits/s and
   will decrease over time due to exhaustion and lack of concentration.
   When an interface in TX State signals at a rate higher than the RX
   interface is able to receive, signal loss occurs.

5.  Security Considerations

   By its nature of line-of-sight signaling, IP-SFS is considered
   insecure.  The transmission of sensitive data over IP-SFS is strongly
   discouraged unless security is provided by higher level protocols.

   Interfaces tend to keep data in their memory.  There is no way to
   determine the lifetime of data in memory.  As a side effect, data
   might show up in unwanted locations at undesired times.

   We are currently not aware of a technique to reliably delete data
   from interfaces' memory without permanent interface destruction.

6.  Acknowledgements

   We thank Eva Ursprung and Doris Jauk-Hinz from Womyn's Art Support
   (WAS), Anita Hofer, Reni Hofmueller, Ulla Klopf, Nicole Pruckermayr,
   Manfred Rittler, Martin Schitter, and Bob Braden for all their
   contributions and support of this project.

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RFC 4824                      IP over SFSS                    April 2007

7.  References

   [JCroft]     Croft, J., "Semaphore Flag Signalling System",

   [Wikipedia]  Wikipedia, "Modern semaphore", <http://

Authors' Addresses

   Jogi Hofmueller (editor)
   Brockmanngasse 65
   Graz  8010

   EMail: ip-sfs@mur.at

   Aaron Bachmann (editor)
   Ulmgasse 14 C
   Graz  8053

   EMail: ip-sfs@mur.at

   IOhannes zmoelnig (editor)
   Goethestrasse 9
   Graz  8010

   EMail: ip-sfs@mur.at

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RFC 4824                      IP over SFSS                    April 2007

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