GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:rfc1986

Network Working Group W. Polites Request for Comments: 1986 W. Wollman Category: Experimental D. Woo

                                                 The MITRE Corporation
                                                             R. Langan
                                                       U.S. ARMY CECOM
                                                           August 1996
  Experiments with a Simple File Transfer Protocol for Radio Links
       using Enhanced Trivial File Transfer Protocol (ETFTP)

Status of this Memo

 This memo defines an Experimental Protocol for the Internet
 community.  This memo does not specify an Internet standard of any
 kind.  Discussion and suggestions for improvement are requested.
 Distribution of this memo is unlimited.

1. INTRODUCTION SECTION

 This document is a description of the Enhanced Trivial File Transfer
 Protocol (ETFTP). This protocol is an experimental implementation of
 the NETwork BLock Transfer Protocol (NETBLT), RFC 998 [1], as a file
 transfer application program. It uses the User Datagram Protocol
 (UDP), RFC 768 [2], as its transport layer. The two protocols are
 layered to create the ETFTP client server application. The ETFTP
 program is named after Trivial File Transfer Protocol (TFTP), RFC
 1350 [3], because the source code from TFTP is used as the building
 blocks for the ETFTP program. This implementation also builds on but
 differs from the work done by the National Imagery Transmission
 Format Standard [4].
 This document is published for discussion and comment on improving
 the throughput performance of data transfer utilities over Internet
 Protocol (IP) compliant, half duplex, radio networks.
 There are many file transfer programs available for computer
 networks.  Many of these programs are designed for operations through
 high-speed, low bit error rate (BER) cabled networks. In tactical
 radio networks, traditional file transfer protocols, such as File
 Transfer Protocol (FTP) and TFTP, do not always perform well. This is
 primarily because tactical half duplex radio networks typically
 provide slow-speed, long delay, and high BER communication links.
 ETFTP is designed to allow a user to control transmission parameters
 to optimize file transfer rates through half-duplex radio links.

Polites, Wollman & Woo Experimental [Page 1] RFC 1986 ETFTP August 1996

 The tactical radio network used to test this application was
 developed by the Survivable Adaptive Systems (SAS) Advanced
 Technology Demonstration (ATD). Part of the SAS ATD program was to
 address the problems associated with extending IP networks across
 tactical radios.  Several tactical radios, such as, SINgle Channel
 Ground and Airborne Radio Systems (SINCGARS), Enhanced Position
 Location Reporting Systems (EPLRS), Motorola LST-5C, and High
 Frequency (HF) radios have been interfaced to the system.  This
 document will discuss results obtained from using ETFTP across a
 point-to-point LST-5C tactical SATellite COMmunications (SATCOM)
 link. The network includes a 25 Mhz 486 Personal Computer (PC) called
 the Army Lightweight Computer Unit (LCU), Cisco 2500 routers,
 Gracilis PackeTen Network switches, Motorola Sunburst Cryptographic
 processors, a prototype forward error correction (FEC) device, and
 Motorola LST-5C tactical Ultra High Frequency (UHF) satellite
 communications (SATC!  OM) radio. Table 1, "Network Trans fer Rates,"
 describes the equipment network connections and the bandwidth of the
 physical media interconnecting the devices.
 Table 1: Network Transfer Rates
 +-------------------------------+-------------------------------+
 | Equipment                     | Rate (bits per second)        |
 +-------------------------------+-------------------------------+
 | Host Computer (486 PC)        | 10,000,000 Ethernet           |
 +-------------------------------+-------------------------------+
 | Cisco Router                  | 10,000,000 Ethernet to        |
 |                               | 19,200 Serial Line Internet   |
 |                               | Protocol (SLIP)               |
 +-------------------------------+-------------------------------+
 | Gracilis PackeTen             | 19,200 SLIP to                |
 |                               | 16,000 Amateur Radio (AX.25)  |
 +-------------------------------+-------------------------------+
 | FEC                           | half rate or quarter rate     |
 +-------------------------------+-------------------------------+
 | Sunburst Crypto               | 16,000                        |
 +-------------------------------+-------------------------------+
 | LST-5C Radio                  | 16,000                        |
 +-------------------------------+-------------------------------+
 During 1993, the MITRE team collected data for network configurations
 that were stationary and on-the-move. This network configuration did
 not include any Forward Error Correction (FEC) at the link layer.
 Several commercially available implementations of FTP were used to
 transfer files through a 16 kbps satellite link. FTP relies upon the
 Transmission Control Protocol (TCP) for reliable communications.  For
 a variety of file sizes, throughput measurements ranged between 80
 and 400 bps. At times, TCP connections could be opened, however, data

Polites, Wollman & Woo Experimental [Page 2] RFC 1986 ETFTP August 1996

 transfers would be unsuccessful. This was most likely due to the
 smaller TCP connection synchronization packets, as compared to the
 TCP data packets.  Because of the high bit error rate of the link,
 the smaller packets were much more likely to be received without
 error. In most cases, satellite channel utilization was less than 20
 percent.  Very often a file transfer would fail because FTP
 implementations would curtail the transfer due t!  o the poor
 conditions of the commu nication link.
 The current focus is to increase the throughput and channel
 utilization over a point to point, half duplex link. Follow on
 experiments will evaluate ETFTP's ability to work with multiple hosts
 in a multicast scenario. Evaluation of the data collected helped to
 determine that several factors limited data throughput. A brief
 description of those limiting factors, as well as, solutions that can
 reduce these networking limitations is provided below.

Link Quality

 The channel quality of a typical narrow-band UHF satellite link does
 not sufficiently support data communications without the addition of
 a forward error correction (FEC) capability.  From the data
 collected, it was determined that the UHF satellite link supports, on
 average, a 10e-3 bit error rate.
 Solution: A narrow-band UHF satellite radio FEC prototype was
 developed that improves data reliability, without excessively
 increasing synchronization requirements. The prototype FEC increased
 synchronization requirements by less than 50 milliseconds (ms). The
 FEC implementation will improve an average 10e-3 BER channel to an
 average 10e-5 BER channel.

Delays

 Including satellite propagation delays, the tactical satellite radios
 require approximately 1.25 seconds for radio synchronization prior to
 transmitting any data across the communication channel.  Therefore,
 limiting the number of channel accesses required will permit data
 throughput to increase. This can be achieved by minimizing the number
 of acknowledgments required during the file transfer.  FTP generates
 many acknowledgments which decreases throughput by increasing the
 number of satellite channel accesses required.
 To clarify, when a FTP connection request is generated, it is sent
 via Ethernet to the router and then forwarded to the radio network
 controller (RNC).  The elapsed time is less than 30 ms. The RNC keys
 the crypto unit and 950 ms later modem/crypto synchronization occurs.
 After synchronization is achieved, the FTP connection request is

Polites, Wollman & Woo Experimental [Page 3] RFC 1986 ETFTP August 1996

 transmitted. The transmitting terminal then drops the channel and the
 modem/crypto synchronization is lost. Assuming that the request was
 received successfully, the receiving host processes the request and
 sends an acknowledgment. Again the modem/crypto have to synchronize
 prior to transmitting the acknowledgment. Propagation delays over a
 UHF satellite also adds roughly 500 ms to the total round trip delay.
 Solution: When compared to FTP, NETBLT significantly reduces the
 number of acknowledgments required to complete a file transfer.
 Therefore, leveraging the features available within an implementation
 of NETBLT will significantly improve throughput across the narrow-
 band UHF satellite communication link.
 To reduce the number of channel accesses required, a number of AX.25
 parameters were modified.  These included the value of p for use
 within the p-persistence link layer protocol, the slot time, the
 transmit tail time, and the transmit delay time.  The p-persistence
 is a random number threshold between 0 and 255.  The slot time is the
 time to wait prior to attempting to access the channel.  The transmit
 tail increases the amount of time the radio carrier is held on, prior
 to dropping the channel. Transmit delay is normally equal to the
 value of the radio synchronization time.  By adjusting these
 parameters to adapt to the tactical satellite environment, improved
 communication performance can be achieved.
 First, in ETFTP, several packets within a buffer are transmitted
 within one burst. If the buffer is partitioned into ten packets, each
 of 1024 bytes, then 10,240 bytes of data is transmitted with each
 channel access. It is possible to configure ETFTP's burstsize to
 equal the number of packets per buffer. Second, the transmit tail
 time was increased to hold the key down on the transmitter long
 enough to insure all of the packets within the buffer are sent in a
 single channel access. These two features, together, allow the system
 to transmit an entire file (example, 100,000 bytes) with only a
 single channel access by adjusting buffer size. Thirdly, the ETFTP
 protocol only acknowledges each buffer, not each packet. Thus, a
 single acknowledgment is sent from the receiving terminal containing
 a request for the missing packets within each buffer, reducing the
 number of acknowledgment packets sent. Which in turn, reduced the
 number of times the channel has to be turned around.
 To reduce channel access time, p-persistence was set to the maximum
 value and slot time to a minimum value. These settings support
 operations for a point-to-point communication link between two users.
 This value of p would not be used if more users were sharing the
 satellite channel.

Polites, Wollman & Woo Experimental [Page 4] RFC 1986 ETFTP August 1996

Backoffs

 TCP's slow start and backoff algorithms implemented in most TCP
 packages assume that packet loss is due to network congestion.  When
 operating across a tactical half duplex communication channel
 dedicated to two users, packet loss is primarily due to poor channel
 quality, not network congestion. A linear backoff at the transport
 layer is recommended. In a tactical radio network there are numerous
 cases where a single host is connected to multiple radios. In a
 tactical radio network, layer two will handle channel access.
 Channel access will be adjusted through parameters like p-persistence
 and slot time. The aggregate effect of the exponential backoff from
 the transport layer added to the random backoff of the data link
 layer, will in most cases, cause the radio network to miss many
 network access opportunities. A linear backoff will reduce the number
 missed data link network access opportunities
 Solution: Tunable parameters and timers have been modified to
 resemble those suggested by NETBLT.

Packet Size

 In a tactical environment, channel conditions change rapidly.
 Continuously transmitting large packets under 10e-3 BER conditions
 reduces effective throughput.
 Solution: Packet sizes are dynamically adjusted based upon the
 success of the buffer transfers. If 99 percent of all packets within
 a buffer are received successfully, packet size can be increased to a
 negotiated value.  If 50 percent or more of all packets within a
 buffer are not successfully delivered, the packet size can be
 decreased to a negotiated value.

2. PROTOCOL DESCRIPTION

 Throughout this document the term packet is used to describe a
 datagram that includes all network overhead. A block is used to
 describe information, without any network encapsulation.
 The original source files for TFTP, as downloaded from ftp.uu.net,
 were modified to implement the ETFTP/NETBLT protocol. These same
 files are listed in "UNIX Network Programming" [5].
 ETFTP was implemented for operations under the Santa Cruz Operations
 (SCO) UNIX. In the service file, "/etc/services", an addition was
 made to support "etftp" at a temporary well known port of "1818"
 using "UDP" protocol. The file, "/etc/inetd.conf", was modified so
 the "inetd" program could autostart the "etftpd" server when a

Polites, Wollman & Woo Experimental [Page 5] RFC 1986 ETFTP August 1996

 connection request came in on the well known port.
 As stated earlier, the transport layer for ETFTP is UDP, which will
 not be discussed further here. This client server application layer
 protocol is NETBLT, with four notable differences.
 The first change is that this NETBLT protocol is implemented on top
 of the UDP layer. This allowed the NETBLT concepts to be tested
 without modifying the operating system's transport or network layers.
 Table 2, "Four Layer Protocol Model," shows the protocol stack for
 FTP, TFTP and ETFTP.
 Table 2: Four Layer Protocol Model
 +---------------------------------------------------------------+
 |                         PROTOCOL STACK                        |
 +---------------+---------------+---------------+---------------+
 |APPLICATION    |FTP            |TFTP           |ETFTP/NETBLT   |
 +---------------+---------------+---------------+---------------+
 |TRANSPORT      |TCP            |UDP            |UDP            |
 +---------------+---------------+---------------+---------------+
 |NETWORK        |IP                                             |
 +---------------+---------------+---------------+---------------+
 |LINK           |Ethernet, SLIP, AX.25                          |
 +---------------+---------------+---------------+---------------+
 The second change is a carryover from TFTP, which allows files to be
 transferred in netascii or binary modes. A new T bit flag is assigned
 to the reserved field of the OPEN message type.
 The third change is to re-negotiate the DATA packet size. This change
 affects the OPEN, NULL-ACK, and CONTROL_OK message types.  A new R
 bit is assigned to the reserved field of the OPEN message type.
 The fourth change is the addition of two new fields to the OPEN
 message type. The one field is a two byte integer for radio delay in
 seconds, and the next field is two bytes of padding.
 The ETFTP data encapsulation is shown in Table 3, "ETFTP Data
 Encapsulation,". The Ethernet, SLIP, and AX.25 headers are mutually
 exclusive. They are stripped off and added by the appropriate
 hardware layer.

Polites, Wollman & Woo Experimental [Page 6] RFC 1986 ETFTP August 1996

 Table 3: ETFTP Data Encapsulation
 +------------+------------+------------+------------+-----------+
 |Ethernet(14)|            |            |ETFTP/      |           |
 |SLIP(2)     |IP(20)      |UDP(8)      |NETBLT(24)  |DATA(1448) |
 |AX.25(20)   |            |            |            |           |
 +------------+------------+------------+------------+-----------+

2.1 MESSAGE TYPES AND FORMATS

 Here are the ETFTP/NETBLT message types and formats.
 MESSAGES        VALUES
 OPEN    0  Client request to open a new connection
 RESPONSE        1  Server positive acknowledgment for OPEN
 KEEPALIVE       2  Reset the timer
 QUIT    3  Sender normal Close request
 QUITACK 4  Receiver acknowledgment of QUIT
 ABORT   5  Abnormal close
 DATA    6  Sender packet containing data
 LDATA   7  Sender last data block of a buffer
 NULL-ACK        8  Sender confirmation of CONTROL_OK changes
 CONTROL 9  Receiver request to
         GO      0 Start transmit of next buffer
         OK      1 Acknowledge complete buffer
         RESEND  2 Retransmit request
 REFUSED 10 Server negative acknowledgment of OPEN
 DONE    11 Receiver acknowledgment of QUIT.
 Packets are "longword-aligned", at four byte word boundaries.
 Variable length strings are NULL terminated, and padded to the four
 byte boundary. Fields are listed in network byte order. All the
 message types share a common 12 byte header. The common fields are:
 Checksum        IP compliant checksum
 Version Current version ID
 Type    NETBLT message type
 Length  Total byte length of packet
 Local Port      My port ID
 Foreign Port    Remote port ID
 Padding Pad as necessary to 4 byte boundary
 The OPEN and RESPONSE messages are similar and shown in Table 4,
 "OPEN and RESPONSE Message Types,". The client string field is used
 to carry the filename to be transferred.

Polites, Wollman & Woo Experimental [Page 7] RFC 1986 ETFTP August 1996

 Table 4: OPEN and RESPONSE Message Types
                    1                   2                   3
  1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
 +---------------+---------------+---------------+---------------+
 |Checksum                       |Version        |Type           |
 +---------------+---------------+---------------+---------------+
 |Length                         |Local Port                     |
 +---------------+---------------+---------------+---------------+
 |Foreign Port                   |Longword Alignment Padding     |
 +---------------+---------------+---------------+---------------+
 |Connection ID                                                  |
 +---------------+---------------+---------------+---------------+
 |Buffer size                                                     |
 +---------------+---------------+---------------+---------------+
 |Transfer size                                                   |
 +---------------+---------------+---------------+---------------+
 |DATA Packet size                |Burstsize                      |
 +---------------+---------------+---------------+---------------+
 |Burstrate                      |Death Timer Value              |
 +---------------+---------------+---------------+---------------+
 |Reserved(MBZ)          |R|T|C|M|Maximum # Outstanding Buffers  |
 +---------------+---------------+---------------+---------------+
 |*Radio Delay                   |*Padding                       |
 +---------------+---------------+---------------+---------------+
 |Client String . . .            |Longword Alignment Padding     |
 +---------------+---------------+---------------+---------------+
 Connection ID   The unique connection number
 Buffer size     Bytes per buffer
 Transfer size   The length of the file in bytes
 DATA Packet size        Bytes per ETFTP block
 Burstsize       Concatenated packets per burst
 Burstrate       Milliseconds per burst
 Death Timer     Seconds before closing idle links
 Reserved        M bit is mode: 0=read/put, 1=write/get
         C bit is checksum: 0=header, 1=all
         *T bit is transfer: 0=netascii, 1=binary
         *R bit is re-negotiate: 0=off, 1=on
 Max # Out Buffs Maximum allowed un-acknowledged buffers
 Radio Delay     *Seconds of delay from send to receive
 Padding *Unused
 Client String   Filename.
 The KEEPALIVE, QUITACK, and DONE messages are identical to the common
 header, except for the message type values. See Table 5, "KEEPALIVE,
 QUITACK, and DONE Message Types,".

Polites, Wollman & Woo Experimental [Page 8] RFC 1986 ETFTP August 1996

 Table 5: KEEPALIVE, QUITACK, and DONE Message Types
                    1                   2                   3
  1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
 +---------------+---------------+---------------+---------------+
 |Checksum                       |Version        |Type           |
 +---------------+---------------+---------------+---------------+
 |Length                         |Local Port                     |
 +---------------+---------------+---------------+---------------+
 |Foreign Port                   |Longword Alignment Padding     |
 +---------------+---------------+---------------+---------------+
 The QUIT, ABORT, and REFUSED messages allow a string field to carry
 the reason for the message. See Table 6, "QUIT, ABORT, and REFUSED
 Message Types,".
 Table 6: QUIT, ABORT, and REFUSED Message Types
                    1                   2                   3
  1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
 +---------------+---------------+---------------+---------------+
 |Checksum                       |Version        |Type           |
 +---------------+---------------+---------------+---------------+
 |Length                         |Local Port                     |
 +---------------+---------------+---------------+---------------+
 |Foreign Port                   |Longword Alignment Padding     |
 +---------------+---------------+---------------+---------------+
 |Reason for QUIT/ABORT/REFUSED . . .                            |
 +---------------+---------------+---------------+---------------+
 |. . .                          |Longword Alignment Padding     |
 +---------------+---------------+---------------+---------------+
 The DATA and LDATA messages make up the bulk of the messages
 transferred. The last packet of each buffer is flagged as an LDATA
 message. Each and every packet of the last buffer has the reserved L
 bit set. The highest consecutive sequence number is used for the
 acknowledgment of CONTROL messages. It should contain the ID number
 of the current CONTROL message being processed. Table 7, "DATA and
 LDATA Message Types,", shows the DATA and LDATA formats.

Polites, Wollman & Woo Experimental [Page 9] RFC 1986 ETFTP August 1996

 Table 7: DATA and LDATA Message Types
                    1                   2                   3
  1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
 +---------------+---------------+---------------+---------------+
 |Checksum                       |Version        |Type           |
 +---------------+---------------+---------------+---------------+
 |Length                         |Local Port                     |
 +---------------+---------------+---------------+---------------+
 |Foreign Port                   |Longword Alignment Padding     |
 +---------------+---------------+---------------+---------------+
 |Buffer Number                                                  |
 +---------------+---------------+---------------+---------------+
 |High Consecutive Seq Num Rcvd  |Packet Number                  |
 +---------------+---------------+---------------+---------------+
 |Data Area Checksum Value       |Reserved (MBZ)               |L|
 +---------------+---------------+---------------+---------------+
 Buffer Number   The first buffer number starts at 0
 Hi Con Seq Num  The acknowledgment for CONTROL messages
 Packet Number   The first packet number starts at 0
 Data Checksum   Checksum for data area only
 Reserved        L: the last buffer bit: 0=false, 1=true
 The NULL-ACK message type is sent as a response to a CONTROL_OK
 message that modifies the current packet size, burstsize, or
 burstrate. In acknowledging the CONTROL_OK message, the sender is
 confirming the change request to the new packet size, burstsize, or
 burstrate. If no modifications are requested, a NULL-ACK message is
 unnecessary. See Table 8, "NULL-ACK Message Type," for further
 details.
 Table 8: NULL-ACK Message Type
                    1                   2                   3
  1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
 +---------------+---------------+---------------+---------------+
 |Checksum                       |Version        |Type           |
 +---------------+---------------+---------------+---------------+
 |Length                         |Local Port                     |
 +---------------+---------------+---------------+---------------+
 |Foreign Port                   |Longword Alignment Padding     |
 +---------------+---------------+---------------+---------------+
 |High Consecutive Seq Num Rcvd  |New Burstsize                  |
 +---------------+---------------+---------------+---------------+
 |New Burstrate                  |*New DATA Packet size           |
 +---------------+---------------+---------------+---------------+

Polites, Wollman & Woo Experimental [Page 10] RFC 1986 ETFTP August 1996

 The CONTROL messages have three subtypes: GO, OK, and RESEND as shown
 in Tables 9-12. The CONTROL message common header may be followed by
 any number of longword aligned subtype messages.
 Table 9: CONTROL Message Common Header
                    1                   2                   3
  1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
 +---------------+---------------+---------------+---------------+
 |Checksum                       |Version        |Type           |
 +---------------+---------------+---------------+---------------+
 |Length                         |Local Port                     |
 +---------------+---------------+---------------+---------------+
 |Foreign Port                   |Longword Alignment Padding     |
 +---------------+---------------+---------------+---------------+
 Table 10: CONTROL_GO Message Subtype
                    1                   2                   3
  1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
 +---------------+---------------+---------------+---------------+
 |Subtype        |Padding        |Sequence Number                |
 +---------------+---------------+---------------+---------------+
 |Buffer Number                                                  |
 +---------------+---------------+---------------+---------------+
 Table 11: CONTROL_OK Message Subtype
                   1                   2                   3
  1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
 +---------------+---------------+---------------+---------------+
 |Subtype        |Padding        |Sequence Number                |
 +---------------+---------------+---------------+---------------+
 |Buffer Number                                                  |
 +---------------+---------------+---------------+---------------+
 |New Offered Burstsize          |New Offered Burstrate          |
 +---------------+---------------+---------------+---------------+
 |Current Control Timer Value    |*New DATA Packet size           |
 +---------------+---------------+---------------+---------------+

Polites, Wollman & Woo Experimental [Page 11] RFC 1986 ETFTP August 1996

 Table 12: CONTROL_RESEND Message Subtype
                    1                   2                   3
  1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
 +---------------+---------------+---------------+---------------+
 |Subtype        |Padding        |Sequence Number                |
 +---------------+---------------+---------------+---------------+
 |Buffer Number                                                  |
 +---------------+---------------+---------------+---------------+
 |Number of Missing Packets      |Longword Alignment Padding     |
 +---------------+---------------+---------------+---------------+
 |Packet Number (2 bytes)        |. . .                          |
 +---------------+---------------+---------------+---------------+
 |. . .                          |Longword Alignment Padding     |
 +---------------+---------------+---------------+---------------+

2.2 ETFTP COMMAND SET

 Being built from TFTP source code, ETFTP shares a significant portion
 of TFTP's design. Like TFTP, ETFTP does NOT support user password
 validation. The program does not support changing directories (i.e.
 cd), neither can it list directories, (i.e. ls). All filenames must
 be given in full paths, as relative paths are not supported. The
 internal finite state machine was modified to support NETBLT message
 types.
 The NETBLT protocol is implemented as closely as possible to what is
 described in RFC 998, with a few exceptions. The client string field
 in the OPEN message type is used to carry the filename of the file to
 be transferred. Netascii or binary transfers are both supported. If
 enabled, new packet sizes, burstsizes, and burstrates are re-
 negotiated downwards when half or more of the blocks in a buffer
 require retransmission. If 99% of the packets in a buffer is
 successfully transferred without any retransmissions, packet size is
 re-negotiated upwards.
 The interactive commands supported by the client process are similar
 to TFTP. Here is the ETFTP command set. Optional parameters are in
 square brackets. Presets are in parentheses.
 ?       help, displays command list
 ascii   mode ascii, appends CR-LF per line
 autoadapt       toggles backoff function (on)
 baudrate baud   baud rate (16000 bits/sec)
 binary  mode binary, image transfer
 blocksize bytes packet size in bytes (512 bytes/block)
 bufferblock blks        buffer size in blocks (128 blocks/buff)
 burstsize packets       burst size in packets (8 blocks/burst)

Polites, Wollman & Woo Experimental [Page 12] RFC 1986 ETFTP August 1996

 connect host [p]        establish connection with host at port p
 exit    ends program
 get rfile lfile copy remote file to local file
 help    same as ?
 mode choice     set transfer mode (binary)
 multibuff num   number of buffers (2 buffers)
 put lfile rfile copy local file to remote file
 quit    same as exit
 radiodelay sec  transmission delay in seconds (2 sec)
 status  display network parameters
 trace   toggles debug display (off).

2.3 DATA TRANSFER AND FLOW CONTROL

 This is the scenario between client and server transfers:
 Client sends OPEN for connection, blocksize, buffersize, burstsize,
 burstrate, transfer mode, and get or put. See M bit of reserved
 field.
 Server sends a RESPONSE with the agreed parameters.
 Receiver sends a CONTROL_GO request sending of first buffer.
 Sender starts transfer by reading the file into multiple memory
 buffers. See Figure 1, "File Segmentation,". Each buffer is divided
 according to the number of bytes/block. Each block becomes a DATA
 packet, which is concatenated according to the blocks/burst.  Bursts
 are transmitted according to the burstrate. Last data block is
 flagged as LDATA type.
 +---+     +---+      +---+ +---+ +---+      +---+ +---+ +---+
 |   |     | 0 |      | L | | 4 | | 3 | ---- | 2 | | 1 | | 0 |
 |   |     | +---+    +---+ +---+ +---+      +---+ +---+ +---+
 |   |     +-|   | -->      +---+ +---+      +---+ +---+ +---+
 |   | -->   | 1 |          | L | | 3 | ---- | 2 | | 1 | | 0 |
 +---+       +---+          +---+ +---+      +---+ +---+ +---+
 File   Multi Buffers  Blocks per Burst
 Figure 1. File Segmentation
 Receiver acknowledges buffer as CONTROL_OK or CONTROL_RESEND.
 If blocks are missing, a CONTROL_RESEND packet is transmitted. If
 half or more of the blocks in a buffer are missing, an adaptive
 algorithm is used for the next buffer transfer. If no blocks are
 missing, a CONTROL_OK packet is transmitted.

Polites, Wollman & Woo Experimental [Page 13] RFC 1986 ETFTP August 1996

 Sender re-transmits blocks until receipt of a CONTROL_OK. If the
 adaptive algorithm is set, then new parameters are offered, in the
 CONTROL_OK message. The priority of the adaptive algorithm is:
  1. Reduce packetsize by half (MIN = 16 bytes/packet)
  2. Reduce burstsize by one (MIN = 1 packet/burst)
  3. Reduce burstrate to actual tighttimer rate
 If new parameters are valid, the sender transmits a NULL-ACK packet,
 to confirm the changes.
 Receiver sends a CONTROL_GO to request sending next buffer.
 At end of transfer, sender sends a QUIT to close the connection.
 Receiver acknowledges the close request with a DONE packet.

2.4 TUNABLE PARAMETERS

 These parameters directly affect the throughput rate of ETFTP.
 Packetsize      The packetsize is the number of 8 bit bytes per
 packet. This number refers to the user data bytes in a block, (frame),
 exclusive of any network overhead. The packet size has a valid range
 from 16 to 1,448 bytes. The Maximum Transfer Unit (MTU) implemented in
 most commercial network devices is 1,500 bytes. The de-facto industry
 standard is 576 byte packets.
 Bufferblock     The bufferblock is the number of blocks per buffer.
 Each implementation may have restrictions on available memory, so the
 buffersize is calculated by multiplying the packetsize times the
 bufferblocks.
 Baudrate        The baudrate is the bits per second transfer rate of
 the slowest link (i.e., the radios). The baudrate sets the speed of
 the sending process. The sending process cannot detect the actual
 speed of the network, so the user must set the correct baudrate.
 Burstsize       The burstsize in packets per burst sets how many
 packets are concatenated and burst for transmission efficiency. The
 burstsize times the packetsize must not exceed the available memory of
 any intervening network devices. On the Ethernet portion of the
 network, all the packets are sent almost instantaneously. It is
 necessary to wait for the network to drain down its memory buffers,
 before the next burst is sent. The sending process needs to regulate
 the rate used to place packets into the network.

Polites, Wollman & Woo Experimental [Page 14] RFC 1986 ETFTP August 1996

 Radiodelay      The radiodelay is the time in seconds per burst it
 takes to synchronize with the radio controllers. Any additional
 hardware delays should be set here. It is the aggregate delay of the
 link layer, such as transmitter key-up, FEC, crypto synchronization,
 and propagation delays.
 These parameters above are used to calculate a burstrate, which is the
 length of time it takes to transmit one burst. The ov is the overhead
 of 72 bytes per packet of network encapsulation. A byte is defined as
 8 bits. The burstrate value is:
   burstrate = (packetsize+ov)*burstsize*8/baudrate
 In a effort to calculate the round trip time, when data is flowing in
 one direction for most of the transfer, the OPEN and RESPONSE message
 types are timed, and the tactical radio delays are estimated. Using
 only one packet in each direction to estimate the rate of the link is
 statistically inaccurate. It was decided that the radio delay should
 be a constant provided by the user interface.  However, a default
 value of 2 seconds is used. The granularity of this value is in
 seconds because of two reasons. The first reason is that the UNIX
 supports a sleep function in seconds only. The second reason is that
 in certain applications, such as deep space probes, a 16-bit integer
 maximum of 32,767 seconds would suffice.

2.5 DELAYS AND TIMERS

 From these parameters, several timers are derived. The control timer
 is responsible for measuring the per buffer rate of transfer. The
 SENDER copy is nicknamed the loosetimer.
   loosetimer = (burstrate+radiodelay)*bufferblock/burstsize
 The RECEIVER copy of the timer is nicknamed the tighttimer, which
 measures the elapsed time between CONTROL_GO and CONTROL_OK packets.
 The tighttimer is returned to the SENDER to allow the protocol to
 adjust for the speed of the network.
 The retransmit timer is responsible for measuring the network receive
 data function. It is used to set an alarm signal (SIGALRM) to
 interrupt the network read. The retransmit timer (wait) is initially
 set to be the greater of twice the round trip or 4 times the
 radiodelay, plus a constant 5 seconds.

Polites, Wollman & Woo Experimental [Page 15] RFC 1986 ETFTP August 1996

    wait = MAX ( 2*roundtriptime,  4*radiodelay ) + 5 seconds
 and
    alarm timeout = wait.
 Each time the same read times out, a five second backoff is added to
 the next wait. The backoff is necessary because the initial user
 supplied radiodelay, or the initial measured round trip time may be
 incorrect.
 The retransmit timer is set differently for the RECEIVER during a
 buffer transfer. Before the arrival of the first DATA packet, the
 original alarm time out is used. Once the DATA packets start
 arriving, and for the duration of each buffer transfer, the RECEIVER
 alarm time out is reset to the expected arrival time of the last DATA
 packet (blockstogo) plus the delay (wait). As each DATA packet is
 received, the alarm is decremented by one packet interval. This same
 algorithm is used for receiving missing packets, during a RESEND.
   alarmtimeout = blockstogo*burstrate/burstsize + wait
 The death timer is responsible for measuring the idle time of a
 connection. In the ETFTP program, the death timer is set to be equal
 to the accumulated time of ten re-transmissions plus their associated
 backoffs. As such, the death timer value in the OPEN and RESPONSE
 message types is un-necessary. In the ETFTP program, this field could
 be used to transfer the radio delay value instead of creating the two
 new fields.
 The keepalive timer is responsible for resetting the death timer.
 This timer will trigger the sending of a KEEPALIVE packet to prevent
 the remote host from closing a connection due to the expiration of
 its death timer. Due to the nature of the ETFTP server process, a
 keepalive timer was not necessary, although it is implemented.

2.6 TEST RESULTS

 The NETBLT protocol has been tested on other high speed networks
 before, see RFC 1030 [6]. These test results in Tables 13 and 14,
 "ETFTP Performance," were gathered from files transferred across the
 network and LST-5C TACSAT radios.  The radios were connected together
 via a coaxial cable to provide a "clean" link. A clean link is
 defined to a BER of 10e-5. The throughput rates are defined to be the
 file size divided by the elapsed time resulting in bits per second
 (bps).  The elapsed time is measured from the time of the "get" or
 "put" command to the completion of the transfer. This is an all
 inclusive time measurement based on user perspective. It includes the

Polites, Wollman & Woo Experimental [Page 16] RFC 1986 ETFTP August 1996

 connection time, transfer time, error recovery time, and disconnect
 time. The user concept of elapsed time is the length of time it takes
 to copy a file from disk to disk. These results show only the average
 performances, including the occasional packet re-transmissions. The
 network configuration was set as:
 ETFTP Parameters:
 Filesize                101,306 bytes
 Radiodelay      2 seconds
 Buffersize      16,384-131,072 bytes
 Packetsize      512-2048 bytes
 Burstsize               8-16 packets/burst
 Gracilis PackeTen Parameters:
 0 TX Delay      400 milliseconds
 1 P Persist     255 [range 1-255]
 2 Slot Time     30 milliseconds
 3 TX Tail               300 milliseconds
 4 Rcv Buffers   8 2048 bytes/buffer
 5 Idle Code     Flag
 Radio Parameters:
 Baudrate                16,000 bps
 Encryption      on
 Table 13: ETFTP Performance at 8 Packets/Burst in Bits/Second
 +-----------+-----------+-----------+-----------+-----------+
 |buffersize |packetsize |packetsize |packetsize |packetsize |
 |(bytes)    |2,048 bytes|1,448 bytes|1,024 bytes|512 bytes  |
 +-----------+-----------+-----------+-----------+-----------+
 |    16,384 |     7,153 |     6,952 |     6,648 |     5,248 |
 +-----------+-----------+-----------+-----------+-----------+
 |    32,768 |     7,652 |     7,438 |     7,152 |     4,926 |
 +-----------+-----------+-----------+-----------+-----------+
 |    65,536 |     8,072 |     8,752 |     8,416 |     5,368 |
 +-----------+-----------+-----------+-----------+-----------+
 |   131,072 |     8,828 |     9,112 |     7,888 |     5,728 |
 +-----------+-----------+-----------+-----------+-----------+

Polites, Wollman & Woo Experimental [Page 17] RFC 1986 ETFTP August 1996

 Table 14: ETFTP Performance at 16 Packets/Burst in Bits/Second
 +-----------+-----------+-----------+-----------+-----------+
 |buffersize |packetsize |packetsize |packetsize |packetsize |
 |(bytes)    |2,048 bytes|1,448 bytes|1,024 bytes|512 bytes  |
 +-----------+-----------+-----------+-----------+-----------+
 |    16,384 |     5,544 |     5,045 |     4,801 |     4,570 |
 +-----------+-----------+-----------+-----------+-----------+
 |    32,768 |     8,861 |     8,230 |     8,016 |     7,645 |
 +-----------+-----------+-----------+-----------+-----------+
 |    65,536 |     9,672 |     9,424 |     9,376 |     8,920 |
 +-----------+-----------+-----------+-----------+-----------+
 |   131,072 |    10,432 |    10,168 |     9,578 |     9,124 |
 +-----------+-----------+-----------+-----------+-----------+

2.7 PERFORMANCE CONSIDERATIONS

 These tests were performed across a tactical radio link with a
 maximum data rate of 16000 bps. In testing ETFTP, it was found that
 the delay associated with the half duplex channel turnaround time was
 the biggest factor in throughput performance. Therefore, every
 attempt was made to minimize the number of times the channel needed
 to be turned around. Obviously, the easiest thing to do is to use as
 big a buffer as necessary to read in a file, as acknowledgments
 occurred only at the buffer boundaries. This is not always feasible,
 as available storage on disk could easily exceed available memory.
 However, the current ETFTP buffersize is set at a maximum of 524,288
 bytes.
 The larger packetsizes also improved performance. The limit on
 packetsize is based on the 1500 byte MTU of network store and forward
 devices. In a high BER environment, a large packetsize could be
 detrimental to success. By reducing the packetsize, even though it
 negatively impacts performance, reliability is sustained. When used
 in conjunction with FEC, both performance and reliability can be
 maintained at an acceptable level.
 The burstsize translates into how long the radio transmitters are
 keyed to transmit. In ETFTP, the ideal situation is to have the first
 packet of a burst arrive in the radio transmit buffer, as the last
 packet of the previous burst is just finished being sent. In this
 way, the radio transmitter would never be dropped for the duration of
 one buffer. In a multi-user radio network, a full buffer transmission
 would be inconsiderate, as the transmit cycle could last for several
 minutes, instead of seconds. In measuring voice communications,
 typical transmit durations are on the order of five to twenty
 seconds.  This means that the buffersize and burstsize could be
 adjusted to have similar transmission durations.

Polites, Wollman & Woo Experimental [Page 18] RFC 1986 ETFTP August 1996

3. REFERENCE SECTION

 [1] Clark, D., Lambert, M., and L. Zhang,
     "NETBLT: A Bulk Data Transfer Protocol", RFC 998, MIT,
     March 1987.
 [2] Postel, J., "User Datagram Protocol" STD 6, RFC 768,
     USC/Information Sciences Institute, August 1980.
 [3] Sollins, K., "Trivial File Transfer Protocol", STD 33,
     RFC 1350, MIT, July 1992.
 [4] MIL-STD-2045-44500, 18 June 1993, "Military Standard Tactical
     Communications Protocol 2 (TACO 2) fot the National Imagery
     Transmission Format Standard", Ft. Monmouth, New Jersey.
 [5] Stevens, W. Richard, 1990, "UNIX Network Programming",
     Prentice-Hall Inc., Englewood, New Jersey, Chapter 12.
 [6] Lambert, M., "On Testing the NETBLT Protocol over
     Divers Networks", RFC 1030, MIT, November 1987.

4. SECURITY CONSIDERATIONS

 The ETFTP program is a security loophole in any UNIX environment.
 There is no user/password validation. All the problems associated to
 TFTP are repeated in ETFTP. The server program must be owned by root
 and setuid to root in order to work. As an experimental prototype
 program, the security issue was overlooked. Since this protocol has
 proven too be a viable solution in tactical radio networks, the
 security issues will have to be addressed, and corrected.

Polites, Wollman & Woo Experimental [Page 19] RFC 1986 ETFTP August 1996

5. AUTHORS' ADDRESSES

 William J. Polites
 The Mitre Corporation
 145 Wyckoff Rd.
 Eatontown, NJ 07724
 Phone: (908) 544-1414
 EMail:wpolites@mitre.org
 William Wollman
 The Mitre Corporation
 145 Wyckoff Rd.
 Eatontown, NJ 07724
 Phone: (908) 544-1414
 EMail:wwollman@mitre.org
 David Woo
 The Mitre Corporation
 145 Wyckoff Rd.
 Eatontown, NJ 07724
 Phone: (908) 544-1414
 EMail: dwoo@mitre.org
 Russ Langan
 U.S. Army Communications Electronics Command (CECOM)
 AMSEL-RD-ST-SP
 ATTN: Russell Langan
 Fort Monmouth, NJ 07703
 Phone: (908) 427-2064
 Fax: (908) 427-2822
 EMail: langanr@doim6.monmouth.army.mil

Polites, Wollman & Woo Experimental [Page 20] RFC 1986 ETFTP August 1996

6. GLOSSARY

 ATD             Advanced Technology Demonstration
 AX.25           Amateur Radio X.25 Protocol
 BER             Bit Error Rate
 EPLRS           Enhanced Position Location Reporting Systems
 ETFTP           Enhanced Trivial File Transfer Protocol
 FEC             Forward Error Correction
 FTP             File Transfer Protocol
 HF              High Frequency
 LCU             Lightweight Computer Unit
 ms              milliseconds
 MTU             Maximum Transfer Unit
 NETBLT  NETwork Block Transfer protocol
 NITFS           National Imagery Transmission Format Standard
 PC              Personal Computer
 RNC             Radio Network Controller
 SAS             Survivable Adaptive Systems
 SATCOM  SATellite COMmunications
 SCO             Santa Cruz Operations
 SINCGARS        SINgle Channel Ground and Airborne Radio Systems
 SLIP            Serial Line Internet Protocol
 TACO2           Tactical Communications Protocol 2
 TCP             Transmission Control Protocol
 TFTP            Trivial File Transfer Protocol
 UDP             User Datagram Protocol
 UHF             Ultra High Frequency
  • Modification from NETBLT RFC 998.
  • The new packet size is a modification to the NETBLT RFC 998.
  • The new packet size is a modification to the NETBLT RFC 998.

Polites, Wollman & Woo Experimental [Page 21]

/data/webs/external/dokuwiki/data/pages/rfc/rfc1986.txt · Last modified: 1996/08/14 15:12 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki