Internet DRAFT - draft-masotta-tftpexts-windowsize-opt
draft-masotta-tftpexts-windowsize-opt
INTERNET-DRAFT Patrick Masotta
Intended status: Standard Track Serva
Expires: Apr 16, 2015 Oct 16, 2014
TFTP Windowsize Option
draft-masotta-tftpexts-windowsize-opt-13.txt
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Abstract
The Trivial File Transfer Protocol (RFC1350) is a simple, lock-step,
file transfer protocol which allows a client to get or put a file
onto a remote host. One of its primary uses is in the early stages of
nodes booting from a Local Area Network. TFTP has been used for this
application because it is very simple to implement. The Employment of
a lock-step scheme limits throughput when used on a LAN.
This document describes a TFTP option which allows the client and
server to negotiate a window size of consecutive blocks to send as an
alternative for replacing the single block lock-step schema. The TFTP
option mechanism employed is described in TFTP Option Extension
(RFC2347).
Legal
This documents and the information contained therein are provided on
an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE
IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL
WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY
WARRANTY THAT THE USE OF THE INFORMATION THEREIN WILL NOT INFRINGE
ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS
FOR A PARTICULAR PURPOSE.
Table of Contents
1. Introduction...................................................3
2. Conventions used in this document..............................3
3. Windowsize Option Specification................................3
4. Traffic Flow and Error Handling................................5
5. Proof of Concept and Windowsize Evaluation.....................6
6. Congestion and Error Control...................................8
7. Security Considerations........................................9
8. IANA Considerations............................................9
9. References.....................................................9
9.1. Normative References......................................9
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1. Introduction
TFTP is virtually unused for internet transfers today, TFTP is still
massively used in network boot/installation scenarios including EFI
(Extensible Firmware Interface). The TFTP protocol's inherently low
transfer rate has been so far partially mitigated by the use of the
blocksize negotiated extension [RFC2348]. This way the original
limitation of 512 byte blocks are in practice replaced in Ethernet
environments by blocks no larger than 1468 Bytes to avoid IP block
fragmentation. This strategy produces insufficient results when
transferring big files, for example the initial ramdisk of Linux
distributions or the PE images used in network installations by
Microsoft WDS/MDT/SCCM. Considering TFTP looks today far from
extinction this draft presents a negotiated extension, under the
terms of the TFTP Option Extension [RFC2347], that produces TFTP
transfer rates comparable to those achieved today by modern file
transfer protocols.
2. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
RFC-2119 [RFC2119].
In this document, these words will appear with that interpretation
only when in ALL CAPS. Lower case uses of these words are not to be
interpreted as carrying RFC-2119 significance.
3. Windowsize Option Specification
The TFTP Read Request or Write Request packet is modified to include
the windowsize option as follows. Note that all fields except "opc"
MUST be ASCII strings followed by a single-byte NULL character.
2B string 1B string 1B string 1B string 1B
+-------+---~~---+----+---~~---+----+-----~~-----+----+---~~---+----+
| opc |filename| 0 | mode | 0 | windowsize | 0 | #blocks| 0 |
+-------+---~~---+----+---~~---+----+-----~~-----+----+---~~---+----+
opc
The opcode field either contains a 1, for Read Requests, or 2,
for Write Requests, as defined in [RFC1350].
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filename
The name of the file to be read or written, as defined in
[RFC1350].
mode
The mode of the file transfer: "netascii", "octet", or "mail", as
defined in [RFC1350].
windowsize
The Windowsize option, "windowsize" (case in-sensitive).
#blocks
The base-10 ASCII string representation of the number of blocks in
a window. Valid values range MUST be between "1" and "65535" blocks,
inclusive. The windowsize refers to the number of consecutives blocks
transmitted before stop and wait for the reception of the
acknowledgment of the last block transmitted.
For example:
+------+--------+----+-------+----+------------+----+----+----+
|0x0001| foobar |0x00| octet |0x00| windowsize |0x00| 16 |0x00|
+------+--------+----+-------+----+------------+----+----+----+
is a Read Request, for the file named "foobar", in octet transfer
mode, with a window-size of 16 blocks (option blocksize is not
negotiated in this example, the 512 Bytes per block default applies).
If the server is willing to accept the windowsize option, it sends an
Option Acknowledgment (OACK) to the client. The specified value MUST
be less than or equal to the value specified by the client.
The client MUST then either use the size specified in the OACK, or
send an ERROR packet, with error code 8, to terminate the transfer.
The rules for determining the final packet are unchanged from
[RFC1350] and [RFC2348].
The reception of a data window with a number of blocks less than the
negotiated windowsize is the final window. If the windowsize is
greater than the amount of data to be transferred, the first window
is the final window.
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4. Traffic Flow and Error Handling
The next diagram depicts a section of the traffic flow between the
Data Sender (DSND) and the Data Receiver (DRCV) parties on a generic
windowsize TFTP file transfer.
The DSND MUST cyclically send to the DRCV the agreed windowsize
consecutives data blocks before to normally stop and wait for the ACK
of the transferred window. The DRCV MUST send to the DSND the ACK of
the last data block of the window in order to confirm a successful
data block window reception.
In case of an expected ACK not timely reaching the DSND (timeout) the
last received ACK SHALL set the beginning of the next windowsize data
block window to send.
In case of data block sequence error the DRCV SHOULD notify the DSND
by sending an ACK corresponding to the last data block correctly
received. The notified DSND SHOULD send a new data block window which
beginning MUST be set based on the received out of sequence ACK.
Traffic with windowsize = 1 MUST be equivalent to traffic specified
by RFC1350 [RFC1350].
For traffic normative not specifically addressed in this section
please refer to RFC1350 [RFC1350] and its updates.
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[ DRCV ] <---traffic---> [ DSND ]
ACK# -> <- Data Block# window block#
...
<- |DB n+01| 1
<- |DB n+02| 2
<- |DB n+03| 3
<- |DB n+04| 4
|ACK n+04| ->
<- |DB n+05| 1
Error |<- |DB n+06| 2
<- |DB n+07| 3
|ACK n+05| ->
<- |DB n+06| 1
<- |DB n+07| 2
<- |DB n+08| 3
<- |DB n+09| 4
|ACK n+09| ->
<- |DB n+10| 1
Error |<- |DB n+11| 2
<- |DB n+12| 3
|ACK n+10| ->| Error
<- |DB n+13| 4
- timeout -
<- |DB n+10| 1
<- |DB n+11| 2
<- |DB n+12| 3
<- |DB n+13| 4
|ACK n+13| ->
...
Section of a windowsize = 4 TFTP transfer including
errors and error recovery
5. Proof of Concept and Windowsize Evaluation
Performance tests were run on the prototype implementation using a
variety of windowsizes and a fixed blocksize of 1456 bytes. The
tests were run on a lightly loaded Gigabit Ethernet, between two
Toshiba Tecra Core 2 Duo 2.2 Ghz laptops, in "octet" mode,
transferring a 180 MByte file.
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^
|
300 +
Seconds | windowsize | time (s)
| ---------- ------
| x 1 257
250 + 2 131
| 4 76
| 8 54
| 16 42
200 + 32 38
| 64 35
|
|
150 +
|
| x
|
100 +
|
| x
|
50 + x
| x
| x x
|
0 +-//--+-----+-----+-----+-----+-----+-----+-->
1 2 4 8 16 32 64
windowsize (in blocks of 1456 bytes)
Comparatively the same 180 MB transfer performed over an SMB/CIFS
mapped drive on the same scenario took 23 seconds.
The comparison of transfer times (without a gateway) between the
standard lock-step schema and the negotiated windowsizes are:
Windowsize | Time Reduction (%)
---------- -----------------
1 -0%
2 -49%
4 -70%
8 -79%
16 -84%
32 -85%
64 -86%
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The transfer time decreases with the use of a windowed schema. The
reason for the reduction in time is the reduction in the number of
the required synchronous acknowledgements exchanged.
The choice of appropriate windowsize values on a particular scenario
depends on the underlying networking technology and topology, and
likely other factors as well. Operators SHOULD test various values
and SHOULD be conservative when selecting a windowsize value because
as the former table and chart shows, there is a point where the
benefit of continuing to increase the windowsize is subject to
diminishing returns.
6. Congestion and Error Control
From a congestion control (CC) standpoint the number of blocks in a
window does not pose an intrinsic threat to the ability of
intermediate devices to signal congestion through drops. The rate at
which TFTP UDP datagrams are sent SHOULD follow the CC guidelines in
Section 3.1 of RFC 5405 [RFC5405].
From an error control standpoint while RFC 1350 [RFC1350] and
subsequent updates do not specify a circuit breaker (CB), existing
implementations have always chosen to fail under certain
circumstances. Implementations SHOULD always set a maximum number of
retries for datagram retransmissions, imposing an appropriate
threshold on error recovery attempts, after which a transfer SHOULD
always be aborted to prevent pathological retransmission conditions.
An Implementation example scaled for an Ethernet environment
(1 Gb/s, MTU=1500) would be to set:
windowsize = 8
blksize = 1456
maximum retransmission attempts per block/window = 6
timeout between retransmissions = 1 S
minimum inter-packet delay = 80 uS
Implementations might well choose other values based on expected
and/or tested operating conditions.
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7. Security Considerations
TFTP includes no login or access control mechanisms. Care must be
taken when using TFTP for file transfers where authentication, access
control, confidentiality, or integrity checking are needed. Note that
those security services could be supplied above or below the layer at
which TFTP runs. Care must be also taken in the rights granted to a
TFTP server process so as not to violate the security of the server's
file system. TFTP is often installed with controls such that only
files that have public read access are available via TFTP. Also
listing, deleting, renaming, and writing files via TFTP are typically
disallowed. TFTP file transfers are NOT RECOMMENDED where the
inherent protocol limitations could raise insurmountable liability
concerns.
TFTP includes no protection against an on-path attacker, care must be
taken in controlling windowsize values according to data sender, data
receiver, and network environment capabilities. TFTP service is
frequently associated with bootstrap and initial provisioning
activities, servers in such an environment are in a position to
impose device or network specific throughput limitations as
appropriate.
This document does not add any security controls to TFTP; however,
the specified extension does not pose additional security risks
either.
8. IANA Considerations
This document has no actions for IANA.
9. References
9.1. Normative References
[RFC1350] Sollins, K., "The TFTP Protocol (Revision 2)", RFC 1350
(STD 33), October 1992.
[RFC2347] Malkin, G., Harkin, A., "TFTP Option Extension", RFC 2347
May 1998.
[RFC2348] Malkin, G., Harkin, A., "TFTP Blocksize option", RFC 2348
May 1998.
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[RFC5405] Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines
for Application Designers", BCP 145, RFC 5405, November
2008.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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Authors' Addresses
Patrick Masotta
Serva
300 W 11th Avenue,
Denver, CO 80204
Email: masotta[-at-]vercot[-dot-]com
URI: http://www.vercot.com/~serva/
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