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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) == Unused Reference: 'UNPv1' is defined on line 366, but no explicit reference was found in the text ** Obsolete normative reference: RFC 793 (Obsoleted by RFC 9293) ** Downref: Normative reference to an Unknown state RFC: RFC 1011 Summary: 2 errors (**), 0 flaws (~~), 4 warnings (==), 6 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 TCP Maintenance and Minor F. Gont 3 Extensions (tcpm) UTN/FRH 4 Internet-Draft A. Yourtchenko 5 Updates: 793, 1011, 1122 Cisco 6 (if approved) August 23, 2010 7 Intended status: Standards Track 8 Expires: February 24, 2011 10 On the implementation of the TCP urgent mechanism 11 draft-ietf-tcpm-urgent-data-06.txt 13 Abstract 15 This document analyzes how current TCP implementations process TCP 16 urgent indications, and how the behavior of some widely-deployed 17 middle-boxes affect how urgent indications are processed by end 18 systems. This document updates the relevant specifications such that 19 they accommodate current practice in processing TCP urgent 20 indications, provides advice to applications that make use of the 21 urgent mechanism, and raises awareness about the reliability of TCP 22 urgent indications in the current Internet. 24 Status of this Memo 26 This Internet-Draft is submitted to IETF in full conformance with the 27 provisions of BCP 78 and BCP 79. 29 Internet-Drafts are working documents of the Internet Engineering 30 Task Force (IETF). Note that other groups may also distribute 31 working documents as Internet-Drafts. The list of current Internet- 32 Drafts is at http://datatracker.ietf.org/drafts/current/. 34 Internet-Drafts are draft documents valid for a maximum of six months 35 and may be updated, replaced, or obsoleted by other documents at any 36 time. It is inappropriate to use Internet-Drafts as reference 37 material or to cite them other than as "work in progress." 39 This Internet-Draft will expire on February 24, 2011. 41 Copyright Notice 43 Copyright (c) 2010 IETF Trust and the persons identified as the 44 document authors. All rights reserved. 46 This document is subject to BCP 78 and the IETF Trust's Legal 47 Provisions Relating to IETF Documents 48 (http://trustee.ietf.org/license-info) in effect on the date of 49 publication of this document. Please review these documents 50 carefully, as they describe your rights and restrictions with respect 51 to this document. Code Components extracted from this document must 52 include Simplified BSD License text as described in Section 4.e of 53 the Trust Legal Provisions and are provided without warranty as 54 described in the Simplified BSD License. 56 Table of Contents 58 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 59 2. Specification of the TCP urgent mechanism . . . . . . . . . . 4 60 2.1. Semantics of urgent indications . . . . . . . . . . . . . 4 61 2.2. Semantics of the Urgent Pointer . . . . . . . . . . . . . 5 62 2.3. Allowed length of urgent data . . . . . . . . . . . . . . 5 63 3. Current implementation practice of TCP urgent data . . . . . . 5 64 3.1. Semantics of urgent indications . . . . . . . . . . . . . 5 65 3.2. Semantics of the Urgent Pointer . . . . . . . . . . . . . 6 66 3.3. Allowed length of urgent data . . . . . . . . . . . . . . 6 67 3.4. Interaction of middle-boxes with TCP urgent indications . 7 68 4. Updating RFC 793, RFC 1011, and RFC 1122 . . . . . . . . . . . 7 69 5. Advice to new applications employing TCP . . . . . . . . . . . 7 70 6. Advice to applications that make use of the urgent 71 mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . 8 72 7. Security Considerations . . . . . . . . . . . . . . . . . . . 8 73 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 74 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8 75 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9 76 10.1. Normative References . . . . . . . . . . . . . . . . . . . 9 77 10.2. Informative References . . . . . . . . . . . . . . . . . . 9 78 Appendix A. Survey of the processing of TCP urgent 79 indications by some popular TCP implementations . . . 10 80 A.1. FreeBSD . . . . . . . . . . . . . . . . . . . . . . . . . 10 81 A.2. Linux . . . . . . . . . . . . . . . . . . . . . . . . . . 10 82 A.3. NetBSD . . . . . . . . . . . . . . . . . . . . . . . . . . 10 83 A.4. OpenBSD . . . . . . . . . . . . . . . . . . . . . . . . . 11 84 A.5. Cisco IOS software . . . . . . . . . . . . . . . . . . . . 11 85 A.6. Microsoft Windows 2000, Service Pack 4 . . . . . . . . . . 11 86 A.7. Microsoft Windows 2008 . . . . . . . . . . . . . . . . . . 11 87 A.8. Microsoft Windows 95 . . . . . . . . . . . . . . . . . . . 12 88 Appendix B. Changes from previous versions of the draft (to 89 be removed by the RFC Editor before publishing 90 this document as an RFC) . . . . . . . . . . . . . . 12 91 B.1. Changes from draft-ietf-tcpm-urgent-data-05 . . . . . . . 12 92 B.2. Changes from draft-ietf-tcpm-urgent-data-04 . . . . . . . 12 93 B.3. Changes from draft-ietf-tcpm-urgent-data-03 . . . . . . . 12 94 B.4. Changes from draft-ietf-tcpm-urgent-data-02 . . . . . . . 12 95 B.5. Changes from draft-ietf-tcpm-urgent-data-01 . . . . . . . 12 96 B.6. Changes from draft-ietf-tcpm-urgent-data-00 . . . . . . . 12 97 B.7. Changes from draft-gont-tcpm-urgent-data-01 . . . . . . . 13 98 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13 100 1. Introduction 102 This document analyzes how some current TCP implementations process 103 TCP urgent indications, and how the behavior of some widely-deployed 104 middle-boxes affect the processing of urgent indications by hosts. 105 This document updates RFC 793 [RFC0793], RFC 1011 [RFC1011], and RFC 106 1122 [RFC1122] such that they accommodate current practice in 107 processing TCP urgent indications, provides advice to applications 108 using the urgent mechanism, and raises awareness about the 109 reliability of TCP urgent indications in the current Internet. 111 Section 2 describes what the current IETF specifications state with 112 respect to TCP urgent indications. Section 3 describes how some 113 current TCP implementations actually process TCP urgent indications. 114 Section 4 updates RFC 793 [RFC0793], RFC 1011 [RFC1011], and RFC 1122 115 [RFC1122], such that they accommodate current practice in processing 116 TCP urgent indications. Section 5 provides advice to to new 117 applications employing TCP, with respect to the TCP urgent mechanism. 118 Section 6 provides advice to existing applications that use or rely 119 on the TCP urgent mechanism. 121 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 122 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 123 document are to be interpreted as described in RFC 2119 [RFC2119]. 125 2. Specification of the TCP urgent mechanism 127 2.1. Semantics of urgent indications 129 TCP incorporates an "urgent mechanism" that allows the sending user 130 to stimulate the receiving user to accept some "urgent data" and to 131 permit the receiving TCP to indicate to the receiving user when all 132 the currently known urgent data have been received by the user. 134 The TCP urgent mechanism permits a point in the data stream to be 135 designated as the end of urgent information. Whenever this point is 136 in advance of the receive sequence number (RCV.NXT) at the receiving 137 TCP, that TCP must tell the user to go into "urgent mode"; when the 138 receive sequence number catches up to the urgent pointer, the TCP 139 must tell user to go into "normal mode" [RFC0793]. This means, for 140 example, that data that was received as "normal data" might become 141 "urgent data" if an urgent indication is received in some successive 142 TCP segment before that data is consumed by the TCP user. 144 The URG control flag indicates that the "Urgent Pointer" field is 145 meaningful and must be added to the segment sequence number to yield 146 the urgent pointer. The absence of this flag indicates that there is 147 no urgent data outstanding [RFC0793]. 149 The TCP urgent mechanism is NOT a mechanism for sending "out-of-band" 150 data: the so-called "urgent data" should be delivered "in-line" to 151 the TCP user. 153 2.2. Semantics of the Urgent Pointer 155 There is some ambiguity in RFC 793 [RFC0793] with respect to the 156 semantics of the Urgent Pointer. Section 3.1 (page 17) of RFC 793 157 [RFC0793] states that the Urgent Pointer "communicates the current 158 value of the urgent pointer as a positive offset from the sequence 159 number in this segment. The urgent pointer points to the sequence 160 number of the octet following the urgent data. This field is only be 161 interpreted in segments with the URG control bit set". However, 162 Section 3.9 (page 56) of RFC 793 [RFC0793] states, when describing 163 the processing of the SEND call in the ESTABLISHED and CLOSE-WAIT 164 states, that "If the urgent flag is set, then SND.UP <- SND.NXT-1 and 165 set the urgent pointer in the outgoing segments". 167 RFC 1011 [RFC1011] clarified this ambiguity in RFC 793 stating that 168 "Page 17 is wrong. The urgent pointer points to the last octet of 169 urgent data (not to the first octet of non-urgent data)". RFC 1122 170 [RFC1122] formally updated RFC 793 by stating, in Section 4.2.2.4 171 (page 84), that "the urgent pointer points to the sequence number of 172 the LAST octet (not LAST+1) in a sequence of urgent data." 174 2.3. Allowed length of urgent data 176 RFC 793 [RFC0793] allows TCP peers to send urgent data of any length, 177 as the TCP urgent mechanism simply provides a pointer to an 178 interesting point in the data stream. In this respect, Section 179 4.2.2.4 (page 84) of RFC 1122 explicitly states that "A TCP MUST 180 support a sequence of urgent data of any length". 182 3. Current implementation practice of TCP urgent data 184 3.1. Semantics of urgent indications 186 As discussed in Section 1, the TCP urgent mechanism simply permits a 187 point in the data stream to be designated as the end of urgent 188 information, but does NOT provide a mechanism for sending out of band 189 data. 191 Unfortunately, virtually all TCP implementations process TCP urgent 192 data differently. By default, the last byte of "urgent data" is 193 delivered "out of band" to the application. That is, it is not 194 delivered as part of the normal data stream. For example, the "out 195 of band" byte is read by an application when a recv(2) system call 196 with the MSG_OOB flag set is issued. 198 Most implementations provide a socket option (SO_OOBINLINE) that 199 allows an application to override the (broken) default processing of 200 urgent data, so that it is delivered "in band" to the application, 201 thus providing the semantics intended by the IETF specifications. 203 3.2. Semantics of the Urgent Pointer 205 All the popular implementations that the authors of this document 206 have been able to test interpret the semantics of the TCP Urgent 207 Pointer as specified in Section 3.1 of RFC 793. This means that even 208 when RFC 1122 officially updated RFC 793 to clarify the ambiguity in 209 the semantics of the Urgent Pointer, this clarification was never 210 reflected into actual implementations (i.e., virtually all 211 implementations default to the semantics of the urgent pointer 212 specified in Section 3.1 of RFC 793). 214 Some operating systems provide a system-wide toggle to override this 215 behavior, and interpret the semantics of the Urgent Pointer as 216 clarified in RFC 1122. However, this system-wide toggle has been 217 found to be inconsistent. For example, Linux provides the sysctl 218 "tcp_stdurg" (i.e., net.ivp4.tcp_stdurg) that, when set, supposedly 219 changes the system behavior to interpret the semantics of the TCP 220 Urgent Pointer as specified in RFC 1122. However, this sysctl 221 changes the semantics of the Urgent Pointer only for incoming 222 segments, but not for outgoing segments. This means that if this 223 sysctl is set, an application might be unable to interoperate with 224 itself if both the TCP sender and the TCP receiver are running on the 225 same host. 227 3.3. Allowed length of urgent data 229 While Section 4.2.2.4 (page 84) of RFC 1122 explicitly states that "A 230 TCP MUST support a sequence of urgent data of any length", in 231 practice all those implementations that interpret TCP urgent 232 indications as a mechanism for sending out-of-band data keep a buffer 233 of a single byte for storing the "last byte of urgent data". Thus, 234 if successive indications of urgent data are received before the 235 application reads the pending "out of band" byte, that pending byte 236 will be discarded (i.e., overwritten by the new byte of urgent data). 238 In order to avoid urgent data from being discarded, some 239 implementations queue each of the received "urgent bytes", so that 240 even if another urgent indication is received before the pending 241 urgent data are consumed by the application, those bytes do not need 242 to be discarded. Some of these implementations have been known to 243 fail to enforce any limits on the amount of urgent data that they 244 queue, thus resulting vulnerable to trivial resource exhaustion 245 attacks [CPNI-TCP]. 247 It should be reinforced that the aforementioned implementations are 248 broken. The TCP urgent mechanism is not a mechanism for delivering 249 out-of-band data. 251 3.4. Interaction of middle-boxes with TCP urgent indications 253 As a result of the publication of Network Intrusion Detection (NIDs) 254 evasion techniques based on TCP urgent indications [phrack], some 255 middle-boxes clear the urgent indications by clearing the URG flag 256 and setting the Urgent Pointer to zero. This causes the "urgent 257 data" to become "in line" (that is, accessible by the read(2) call or 258 the recv(2) call without the MSG_OOB flag) in the case of those TCP 259 implementations that implement the urgent mechanism as out-of-band 260 data (as described in Section 3.1). An example of such a middle-box 261 is the Cisco PIX firewall [Cisco-PIX]. This should discourage 262 applications from depending on urgent indications for their correct 263 operation, as urgent indications may not be not reliable in the 264 current Internet. 266 4. Updating RFC 793, RFC 1011, and RFC 1122 268 Considering that as long as both the TCP sender and the TCP receiver 269 implement the same semantics for the Urgent Pointer there is no 270 functional difference in having the Urgent Pointer point to "the 271 sequence number of the octet following the urgent data" vs. "the last 272 octet of urgent data", and since all known implementations interpret 273 the semantics of the Urgent Pointer as pointing to "the sequence 274 number of the octet following the urgent data", we hereby update RFC 275 793 [RFC0793], RFC 1011 [RFC1011], and RFC 1122 [RFC1122], such that 276 "the urgent pointer points to the sequence number of the octet 277 following the urgent data" (in segments with the URG control bit 278 set), thus accommodating virtually all existing TCP implementations. 280 5. Advice to new applications employing TCP 282 As a result of the issues discussed in Section 3.2 and Section 3.4, 283 new applications SHOULD NOT employ the TCP urgent mechanism. 284 However, TCP implementations MUST still include support for the 285 urgent mechanism such that existing applications can still use it. 287 6. Advice to applications that make use of the urgent mechanism 289 Even though applications SHOULD NOT employ the urgent mechanism, 290 applications that still decide to employ it MUST set the SO_OOBINLINE 291 socket option, such that "urgent data" is delivered inline, as 292 intended by the IETF specifications. 294 7. Security Considerations 296 Given that there are two different interpretations of the semantics 297 of the Urgent Pointer in current implementations (e.g., depending on 298 the value of the tcp_stdurg sysctl), and that middle-boxes (such as 299 packet scrubbers) or the end-systems themselves could cause the 300 urgent data to be processed "in band", there exists ambiguity in how 301 "urgent data" sent by a TCP will be processed by the intended 302 recipient. This might make it difficult for a Network Intrusion 303 Detection System (NIDS) to track the application-layer data 304 transferred to the destination system, and thus lead to false 305 negatives or false positives in the NIDS [CPNI-TCP]. 307 Probably the best way to avoid the security implications of TCP 308 urgent data is to avoid having applications use the TCP urgent 309 mechanism altogether. Packet scrubbers could probably be configured 310 to clear the URG bit, and set the Urgent Pointer to zero. This would 311 basically cause the urgent data to be put "in band". However, this 312 might cause interoperability problems or undesired behavior in the 313 applications running on top of TCP. 315 8. IANA Considerations 317 This document has no actions for IANA. 319 9. Acknowledgements 321 The authors of this document would like to thank (in alphabetical 322 order) David Borman, Wesley Eddy, John Heffner, Alfred Hoenes, Carlos 323 Pignataro, Anantha Ramaiah, Joe Touch, Michael Welzl, Dan Wing, and 324 Alexander Zimmermann for providing valuable feedback on earlier 325 versions of this document. 327 Additionally, Fernando would like to thank David Borman and Joe Touch 328 for a fruitful discussion about TCP urgent mode at IETF 73 329 (Minneapolis). 331 10. References 333 10.1. Normative References 335 [RFC0793] Postel, J., "Transmission Control Protocol", STD 7, 336 RFC 793, September 1981. 338 [RFC1011] Reynolds, J. and J. Postel, "Official Internet protocols", 339 RFC 1011, May 1987. 341 [RFC1122] Braden, R., "Requirements for Internet Hosts - 342 Communication Layers", STD 3, RFC 1122, October 1989. 344 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 345 Requirement Levels", BCP 14, RFC 2119, March 1997. 347 10.2. Informative References 349 [CPNI-TCP] 350 Gont, F., "Security Assessment of the Transmission Control 351 Protocol (TCP)", http://www.cpni.gov.uk/Docs/ 352 tn-03-09-security-assessment-TCP.pdf, 2009. 354 [Cisco-PIX] 355 Cisco PIX, "http://www.cisco.com/en/US/docs/security/asa/ 356 asa70/command/reference/tz.html#wp1288756". 358 [FreeBSD] The FreeBSD project, "http://www.freebsd.org". 360 [Linux] The Linux Project, "http://www.kernel.org". 362 [NetBSD] The NetBSD project, "http://www.netbsd.org". 364 [OpenBSD] The OpenBSD project, "http://www.openbsd.org". 366 [UNPv1] Stevens, W., "UNIX Network Programming, Volume 1. 367 Networking APIs: Sockets and XTI", Prentice Hall PTR , 368 1997. 370 [Windows2000] 371 Microsoft Windows 2000, "http://technet.microsoft.com/ 372 en-us/library/bb726981(printer).aspx". 374 [Windows95] 375 Microsoft Windows 95, 376 "ftp://ftp.demon.co.uk/pub/mirrors/win95netfaq/ 377 faq-c.html". 379 [phrack] Ko, Y., Ko, S., and M. Ko, "NIDS Evasion Method named 380 "SeolMa"", Phrack Magazine, Volume 0x0b, Issue 0x39, Phile 381 #0x03 of 0x12 http://www.phrack.org/ 382 issues.html?issue=57&id=3#article, 2001. 384 Appendix A. Survey of the processing of TCP urgent indications by some 385 popular TCP implementations 387 A.1. FreeBSD 389 FreeBSD 8.0 [FreeBSD] interprets the semantics of the urgent pointer 390 as specified in Section 4 of this document. It does not provide any 391 sysctl to override this behavior. 393 FreeBSD provides the SO_OOBINLINE socket option that, when set, 394 causes TCP "urgent data" to remain "in band". That is, it will be 395 accessible by the read(2) call or the recv(2) call without the 396 MSG_OOB flag. 398 FreeBSD supports only one byte of urgent data. That is, only the 399 byte preceding the Urgent Pointer is considered as "urgent data". 401 A.2. Linux 403 Linux 2.6.15-53-386 [Linux] interprets the semantics of the urgent 404 pointer as specified in Section 4 of this document. It provides the 405 net.ipv4.tcp_stdurg sysctl to override this behavior to interpret the 406 Urgent Pointer as specified in RFC 1122 [RFC1122]. However, this 407 sysctl only affects the processing of incoming segments (the Urgent 408 Pointer in outgoing segments will still be set as specified in 409 Section 4 of this document). 411 Linux provides the SO_OOBINLINE socket option that, when set, causes 412 TCP "urgent data" to remain "in band". That is, it will be 413 accessible by the read(2) call or the recv(2) call without the 414 MSG_OOB flag. 416 Linux supports only one byte of urgent data. That is, only the byte 417 preceding the Urgent Pointer is considered as "urgent data". 419 A.3. NetBSD 421 NetBSD 5.0.1 [NetBSD] interprets the semantics of the urgent pointer 422 as specified in Section 4 of this document. It does not provide any 423 sysctl to override this behavior. 425 NetBSD provides the SO_OOBINLINE socket option that, when set, causes 426 TCP "urgent data" to remain "in band". That is, it will be 427 accessible by the read(2) call or the recv(2) call without the 428 MSG_OOB flag. 430 NetBSD supports only one byte of urgent data. That is, only the byte 431 preceding the Urgent Pointer is considered as "urgent data". 433 A.4. OpenBSD 435 OpenBSD 4.2 [OpenBSD] interprets the semantics of the urgent pointer 436 as specified in Section 4 of this document. It does not provide any 437 sysctl to override this behavior. 439 OpenBSD provides the SO_OOBINLINE socket option that, when set, 440 causes TCP urgent data to remain "in band". That is, it will be 441 accessible by the read(2) or recv(2) calls without the MSG_OOB flag. 443 OpenBSD supports only one byte of urgent data. That is, only the 444 byte preceding the Urgent Pointer is considered as "urgent data". 446 A.5. Cisco IOS software 448 Cisco IOS Software Releases 12.2(18)SXF7, 12.4(15)T7 interpret the 449 semantics of the urgent pointer as specified in Section 4 of this 450 document. 452 The behavior is consistent with having the SO_OOBINLINE socket option 453 turned on, i.e. the data is processed "in band". 455 A.6. Microsoft Windows 2000, Service Pack 4 457 Microsoft Windows 2000 [Windows2000] interprets the semantics of the 458 urgent pointer as specified in Section 4 of this document. It 459 provides the TcpUseRFC1122UrgentPointer system-wide variable to 460 override this behavior, interpreting the Urgent Pointer as specified 461 in RFC 1122 [RFC1122]. 463 Tests performed with a sample server application compiled using the 464 cygwin environment has shown that the default behavior is to return 465 the urgent data "in band". 467 A.7. Microsoft Windows 2008 469 Microsoft Windows 2008 interprets the semantics of the urgent pointer 470 as specified in Section 4 of this document. 472 A.8. Microsoft Windows 95 474 Microsoft Windows 95 interprets the semantics of the urgent pointer 475 as specified in Section 4 of this document. It provides the 476 BSDUrgent system-wide variable to override this behavior, 477 interpreting the Urgent Pointer as specified in RFC 1122 [RFC1122]. 478 Windows 95 supports only one byte of urgent data. That is, only the 479 byte preceding the Urgent Pointer is considered as "urgent data". 480 [Windows95] 482 Appendix B. Changes from previous versions of the draft (to be removed 483 by the RFC Editor before publishing this document as an 484 RFC) 486 B.1. Changes from draft-ietf-tcpm-urgent-data-05 488 o Draft resubmitted (with no changes) because it was close to the 489 expiration day. 491 B.2. Changes from draft-ietf-tcpm-urgent-data-04 493 o Fixes grammar errors wrt the term "data" (thanks to David Borman, 494 once again ;-) ) 496 B.3. Changes from draft-ietf-tcpm-urgent-data-03 498 o Addresses feedback sent by David Borman, and nit pointed out by 499 John Heffner. 501 B.4. Changes from draft-ietf-tcpm-urgent-data-02 503 o Addresses WGLC feedback submitted by Michael Welzl, Anantha 504 Ramaiah, and Wesley Eddy. 506 B.5. Changes from draft-ietf-tcpm-urgent-data-01 508 o Fixes reference to Cisco IOS Software (layer 8+ stuff ;-) ). 510 o Cleaned-up Appendix A.5. 512 B.6. Changes from draft-ietf-tcpm-urgent-data-00 514 o Minor editorial changes. 516 o Incorporated the specific changes/advice stated in 517 http://www.ietf.org/mail-archive/web/tcpm/current/msg04548.html in 518 different sections (Section 4, Section 5, Section 6). 520 B.7. Changes from draft-gont-tcpm-urgent-data-01 522 o Draft resubmitted as draft-ietf, as a result of wg consensus on 523 adopting the document as a tcpm wg item. 525 Authors' Addresses 527 Fernando Gont 528 Universidad Tecnologica Nacional / Facultad Regional Haedo 529 Evaristo Carriego 2644 530 Haedo, Provincia de Buenos Aires 1706 531 Argentina 533 Phone: +54 11 4650 8472 534 Email: fernando@gont.com.ar 535 URI: http://www.gont.com.ar 537 Andrew Yourtchenko 538 Cisco 539 De Kleetlaan, 7 540 Diegem B-1831 541 Belgium 543 Phone: +32 2 704 5494 544 Email: ayourtch@cisco.com