INTERNET-DRAFT Patrick Masotta
Intended status: Standard Track Serva
Expires: Feb 16, 2015 Aug 16, 2014
TFTP Windowsize Option
draft-masotta-tftpexts-windowsize-opt-11.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
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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. Congestion and Error Control...................................4
5. Proof of Concept...............................................5
6. Error Handling.................................................6
7. Security Considerations........................................7
8. IANA Considerations............................................8
9. References.....................................................8
9.1. Normative References......................................8
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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", "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 MUST be modified to
include the windowsize option as follows. Note that all fields
except "opc" MUST be NULL-terminated.
+-------+---~~---+---+---~~---+---+-----~~-----+---+---~~---+---+
| opc |filename| 0 | mode | 0 | windowsize | 0 | #blocks| 0 |
+-------+---~~---+---+---~~---+---+-----~~-----+---+---~~---+---+
opc
The opcode field MUST contain either a 1, for Read Requests, or 2,
for Write Requests, as defined in [RFC1350].
filename
The name of the file to be read or written, as defined in
[RFC1350].
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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 number of blocks in a window MUST be specified in ASCII. 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:
+-------+--------+---+-------+---+------------+---+------+---+
| 1 | foobar | 0 | octet | 0 | windowsize | 0 | 16 | 0 |
+-------+--------+---+-------+---+------------+---+------+---+
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 MUST
send 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.
4. 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].
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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
operating conditions.
5. Proof of Concept
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.
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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^
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)
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.
Comparatively the same 180 MB transfer performed over an SMB/CIFS
mapped drive on the same scenario took 23 seconds.
6. Error Handling
In case of an error the last ACK received MUST set the beginning of
the next windowsize window to send.
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[Requester]<------------------->[Provider]
<-traffic-> transfer block# window block#
... (windowsize=4)
<- | n+01 | 1
<- | n+02 | 2
<- | n+03 | 3
<- | n+04 | 4
|ACK n+04| ->
<- | n+05 | 1
Error |<- | n+06 | 2
<- | n+07 | 3
|ACK n+05| ->
<- | n+06 | 1
<- | n+07 | 2
<- | n+08 | 3
<- | n+09 | 4
|ACK n+09| ->
<- | n+10 | 1
Error |<- | n+11 | 2
<- | n+12 | 3
|ACK n+10| ->| Error
<- | n+13 | 4
<- | n+10 | 1
<- | n+11 | 2
<- | n+12 | 3
<- | n+13 | 4
|ACK n+13| ->
...
Section of a transfer including errors and error recovery
7. Security Considerations
TFTP includes no login or access control mechanisms. Care must be
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. The use of TFTP is
appropriate on networks where the inherent protocol limitations are
not a liability.
TFTP includes no protection against an on-path attacker, care must be
taken in controlling windowsize values according to provider,
requester, and network environment capabilities. TFTP service is
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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 should 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.
[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
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