TFTP Windowsize Option
The information below is for an old version of the document that is already published as an RFC.
This is an older version of an Internet-Draft that was ultimately published as RFC 7440.
|Last updated||2018-12-20 (Latest revision 2014-10-16)|
|RFC stream||Internet Engineering Task Force (IETF)|
|IETF conflict review||conflict-review-masotta-tftpexts-windowsize-opt, conflict-review-masotta-tftpexts-windowsize-opt, conflict-review-masotta-tftpexts-windowsize-opt, conflict-review-masotta-tftpexts-windowsize-opt, conflict-review-masotta-tftpexts-windowsize-opt, conflict-review-masotta-tftpexts-windowsize-opt, conflict-review-masotta-tftpexts-windowsize-opt|
|Stream||WG state||Submitted to IESG for Publication|
|Document shepherd||Joel Jaeggli|
|Shepherd write-up||Show Last changed 2014-09-01|
|IESG||IESG state||RFC 7440 (Proposed Standard)|
|Responsible AD||Joel Jaeggli|
|Send notices to||(None)|
|IANA||IANA review state||IANA OK - No Actions Needed|
|IANA action state||No IANA Actions|
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 Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html This Internet-Draft will expire on Feb 16, 2014. Copyright Notice Copyright (c) 2014 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Patrick Masotta Expires Apr 16 2015 [Page 1] INTERNET-DRAFT TFTP Windowsize Option Oct 2014 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 Patrick Masotta Expires Mar 16, 2015 [Page 2] INTERNET-DRAFT TFTP Windowsize Option Oct 2014 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]. Patrick Masotta Expires Mar 16, 2015 [Page 3] INTERNET-DRAFT TFTP Windowsize Option Oct 2014 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. Patrick Masotta Expires Mar 16, 2015 [Page 4] INTERNET-DRAFT TFTP Windowsize Option Oct 2014 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. Patrick Masotta Expires Mar 16, 2015 [Page 5] INTERNET-DRAFT TFTP Windowsize Option Oct 2014 [ 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. Patrick Masotta Expires Mar 16, 2015 [Page 6] INTERNET-DRAFT TFTP Windowsize Option Oct 2014 ^ | 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% Patrick Masotta Expires Mar 16, 2015 [Page 7] INTERNET-DRAFT TFTP Windowsize Option Oct 2014 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. Patrick Masotta Expires Mar 16, 2015 [Page 8] INTERNET-DRAFT TFTP Windowsize Option Oct 2014 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. Patrick Masotta Expires Mar 16, 2015 [Page 9] INTERNET-DRAFT TFTP Windowsize Option Oct 2014 [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. Patrick Masotta Expires Mar 16, 2015 [Page 10] INTERNET-DRAFT TFTP Windowsize Option Oct 2014 Authors' Addresses Patrick Masotta Serva 300 W 11th Avenue, Denver, CO 80204 Email: masotta[-at-]vercot[-dot-]com URI: http://www.vercot.com/~serva/ Patrick Masotta Expires Mar 16, 2015 [Page 11]