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Thubert, Ed. 5 Expires: 10 August 2020 Cisco Systems 6 C. Bormann 7 Universitaet Bremen TZI 8 7 February 2020 10 On Forwarding 6LoWPAN Fragments over a Multihop IPv6 Network 11 draft-ietf-6lo-minimal-fragment-11 13 Abstract 15 This document introduces the capability to forward 6LoWPAN fragments. 16 This method reduces the latency and increases end-to-end reliability 17 in route-over forwarding. It is the companion to using virtual 18 reassembly buffers which is a pure implementation technique. 20 Status of This Memo 22 This Internet-Draft is submitted in full conformance with the 23 provisions of BCP 78 and BCP 79. 25 Internet-Drafts are working documents of the Internet Engineering 26 Task Force (IETF). Note that other groups may also distribute 27 working documents as Internet-Drafts. The list of current Internet- 28 Drafts is at https://datatracker.ietf.org/drafts/current/. 30 Internet-Drafts are draft documents valid for a maximum of six months 31 and may be updated, replaced, or obsoleted by other documents at any 32 time. It is inappropriate to use Internet-Drafts as reference 33 material or to cite them other than as "work in progress." 35 This Internet-Draft will expire on 10 August 2020. 37 Copyright Notice 39 Copyright (c) 2020 IETF Trust and the persons identified as the 40 document authors. All rights reserved. 42 This document is subject to BCP 78 and the IETF Trust's Legal 43 Provisions Relating to IETF Documents (https://trustee.ietf.org/ 44 license-info) in effect on the date of publication of this document. 45 Please review these documents carefully, as they describe your rights 46 and restrictions with respect to this document. Code Components 47 extracted from this document must include Simplified BSD License text 48 as described in Section 4.e of the Trust Legal Provisions and are 49 provided without warranty as described in the Simplified BSD License. 51 Table of Contents 53 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 54 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 55 2.1. BCP 14 . . . . . . . . . . . . . . . . . . . . . . . . . 3 56 2.2. Referenced Work . . . . . . . . . . . . . . . . . . . . . 3 57 2.3. New Terms . . . . . . . . . . . . . . . . . . . . . . . . 4 58 3. Overview of 6LoWPAN Fragmentation . . . . . . . . . . . . . . 4 59 4. Limits of Per-Hop Fragmentation and Reassembly . . . . . . . 6 60 4.1. Latency . . . . . . . . . . . . . . . . . . . . . . . . . 6 61 4.2. Memory Management and Reliability . . . . . . . . . . . . 6 62 5. Forwarding Fragments . . . . . . . . . . . . . . . . . . . . 7 63 6. Virtual Reassembly Buffer (VRB) Implementation . . . . . . . 9 64 7. Security Considerations . . . . . . . . . . . . . . . . . . . 10 65 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 66 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11 67 10. Normative References . . . . . . . . . . . . . . . . . . . . 11 68 11. Informative References . . . . . . . . . . . . . . . . . . . 11 69 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12 71 1. Introduction 73 The original 6LoWPAN fragmentation is defined in [RFC4944] and it is 74 implicitly defined for use over a single IP hop through possibly 75 multiple Layer-2 (mesh-under) hops in a meshed 6LoWPAN Network. 76 Although [RFC6282] updates [RFC4944], it does not redefine 6LoWPAN 77 fragmentation. 79 This means that over a Layer-3 (route-over) network, an IP packet is 80 expected to be reassembled at every hop at the 6LoWPAN sublayer, 81 pushed to Layer-3 to be routed, and then fragmented again if the next 82 hop is another similar 6LoWPAN link. This draft introduces an 83 alternate approach called 6LoWPAN Fragment Forwarding (FF) whereby an 84 intermediate node forwards a fragment as soon as it is received if 85 the next hop is a similar 6LoWPAN link. The routing decision is made 86 on the first fragment, which has all the IPv6 routing information. 87 The first fragment is forwarded immediately and a state is stored to 88 enable forwarding the next fragments along the same path. 90 Done right, 6LoWPAN Fragment Forwarding techniques lead to more 91 streamlined operations, less buffer bloat and lower latency. It may 92 be wasteful if some fragments are missing after the first one since 93 the first fragment will still continue until the 6LoWPAN endpoint 94 that will attempt to perform the reassembly, and may be misused to 95 the point that the end-to-end latency falls behind that of per-hop 96 recomposition. 98 This specification provides a generic overview of FF, discusses 99 advantages and caveats, and introduces a particular 6LoWPAN Fragment 100 Forwarding technique called Virtual Reassembly Buffer that can be 101 used while conserving the message formats defined in [RFC4944]. 103 2. Terminology 105 2.1. BCP 14 107 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 108 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 109 "OPTIONAL" in this document are to be interpreted as described in BCP 110 14 [RFC2119][RFC8174] when, and only when, they appear in all 111 capitals, as shown here. 113 2.2. Referenced Work 115 Past experience with fragmentation, e.g., as described in "IPv4 116 Reassembly Errors at High Data Rates" [RFC4963] and references 117 therein, has shown that mis-associated or lost fragments can lead to 118 poor network behavior and, occasionally, trouble at the application 119 layer. That experience led to the definition of the "Path MTU 120 discovery" [RFC8201] (PMTUD) protocol that limits fragmentation over 121 the Internet. 123 "IP Fragmentation Considered Fragile" [FRAG-ILE] discusses security 124 threats that are linked to using IP fragmentation. The 6LoWPAN 125 fragmentation takes place underneath, but some issues described there 126 may still apply to 6LoWPAN fragments (as discussed in further details 127 in Section 7). 129 Readers are expected to be familiar with all the terms and concepts 130 that are discussed in "IPv6 over Low-Power Wireless Personal Area 131 Networks (6LoWPANs): Overview, Assumptions, Problem Statement, and 132 Goals" [RFC4919] and "Transmission of IPv6 Packets over IEEE 802.15.4 133 Networks" [RFC4944]. 135 "Multiprotocol Label Switching (MPLS) Architecture" [RFC3031] says 136 that with MPLS, 'packets are "labeled" before they are forwarded.' 137 It goes on to say, "At subsequent hops, there is no further analysis 138 of the packet's network layer header. Rather, the label is used as 139 an index into a table which specifies the next hop, and a new label". 140 The MPLS technique is leveraged in the present specification to 141 forward fragments that actually do not have a network layer header, 142 since the fragmentation occurs below IP. 144 2.3. New Terms 146 This specification defines the following terms: 148 6LoWPAN endpoints: The 6LoWPAN endpoints are the first and last 149 nodes in an unbroken string of 6LoWPAN nodes. They are in charge 150 of generating or expanding a 6LoWPAN header from/to a full IPv6 151 packet. They are also the points where the fragmentation and 152 reassembly operations take place. 154 Compressed Form: This specification uses the generic term Compressed 155 Form to refer to the format of a datagram after the action of 156 [RFC6282] and possibly [RFC8138] for RPL [RFC6550] artifacts. 158 datagram_size: The size of the datagram in its Compressed Form 159 before it is fragmented. The datagram_size is expressed in a unit 160 that depends on the MAC layer technology, by default a byte. 162 datagram_tag: An identifier of a datagram that is locally unique to 163 the Layer-2 sender. Associated with the MAC address of the 164 sender, this becomes a globally unique identifier for the 165 datagram. 167 fragment_offset: The offset of a fragment of a datagram in its 168 Compressed Form. The fragment_offset is expressed in a unit that 169 depends on the MAC layer technology and is by default a byte. 171 3. Overview of 6LoWPAN Fragmentation 173 We use Figure 1 to illustrate 6LoWPAN fragmentation. We assume node 174 A forwards a packet to node B, possibly as part of a multi-hop route 175 between IPv6 source and destination nodes which are neither A nor B. 177 +---+ +---+ 178 ... ---| A |-------------------->| B |--- ... 179 +---+ +---+ 180 # (frag. 5) 182 123456789 123456789 183 +---------+ +---------+ 184 | # ###| |### # | 185 +---------+ +---------+ 186 outgoing incoming 187 fragmentation reassembly 188 buffer buffer 190 Figure 1: Fragmentation at node A, reassembly at node B. 192 Node A starts by compacting the IPv6 packet using the header 193 compression mechanism defined in [RFC6282]. If the resulting 6LoWPAN 194 packet does not fit into a single Link-Layer frame, node A's 6LoWPAN 195 sublayer cuts it into multiple 6LoWPAN fragments, which it transmits 196 as separate Link-Layer frames to node B. Node B's 6LoWPAN sublayer 197 reassembles these fragments, inflates the compressed header fields 198 back to the original IPv6 header, and hands over the full IPv6 packet 199 to its IPv6 layer. 201 In Figure 1, a packet forwarded by node A to node B is cut into nine 202 fragments, numbered 1 to 9 as follows: 204 * Each fragment is represented by the '#' symbol. 206 * Node A has sent fragments 1, 2, 3, 5, 6 to node B. 208 * Node B has received fragments 1, 2, 3, 6 from node A. 210 * Fragment 5 is still being transmitted at the link layer from node 211 A to node B. 213 The reassembly buffer for 6LoWPAN is indexed in node B by: 215 * a unique Identifier of Node A (e.g., Node A's Link-Layer address) 217 * the datagram_tag chosen by node A for this fragmented datagram 219 Because it may be hard for node B to correlate all possible Link- 220 Layer addresses that node A may use (e.g., short vs. long addresses), 221 node A must use the same Link-Layer address to send all the fragments 222 of the same datagram to node B. 224 Conceptually, the reassembly buffer in node B contains: 226 * a datagram_tag as received in the incoming fragments, associated 227 to Link-Layer address of node A for which the received 228 datagram_tag is unique, 230 * the actual packet data from the fragments received so far, in a 231 form that makes it possible to detect when the whole packet has 232 been received and can be processed or forwarded, 234 * a state indicating the fragments already received, 236 * a datagram_size, 238 * a timer that allows discarding a partially reassembled packet 239 after some timeout. 241 A fragmentation header is added to each fragment; it indicates what 242 portion of the packet that fragment corresponds to. Section 5.3 of 243 [RFC4944] defines the format of the header for the first and 244 subsequent fragments. All fragments are tagged with a 16-bit 245 "datagram_tag", used to identify which packet each fragment belongs 246 to. Each datagram can be uniquely identified by the sender Link- 247 Layer addresses of the frame that carries it and the datagram_tag 248 that the sender allocated for this datagram. [RFC4944] also mandates 249 that the first fragment is sent first and with a particular format 250 that is different than that of the next fragments. Each fragment but 251 the first one can be identified within its datagram by the datagram- 252 offset. 254 Node B's typical behavior, per [RFC4944], is as follows. Upon 255 receiving a fragment from node A with a datagram_tag previously 256 unseen from node A, node B allocates a buffer large enough to hold 257 the entire packet. The length of the packet is indicated in each 258 fragment (the datagram_size field), so node B can allocate the buffer 259 even if the first fragment it receives is not fragment 1. As 260 fragments come in, node B fills the buffer. When all fragments have 261 been received, node B inflates the compressed header fields into an 262 IPv6 header, and hands the resulting IPv6 packet to the IPv6 layer 263 which performs the route lookup. This behavior typically results in 264 per-hop fragmentation and reassembly. That is, the packet is fully 265 reassembled, then (re)fragmented, at every hop. 267 4. Limits of Per-Hop Fragmentation and Reassembly 269 There are at least 2 limitations to doing per-hop fragmentation and 270 reassembly. See [ARTICLE] for detailed simulation results on both 271 limitations. 273 4.1. Latency 275 When reassembling, a node needs to wait for all the fragments to be 276 received before being able to generate the IPv6 packet, and possibly 277 forward it to the next hop. This repeats at every hop. 279 This may result in increased end-to-end latency compared to a case 280 where each fragment is forwarded without per-hop reassembly. 282 4.2. Memory Management and Reliability 284 Constrained nodes have limited memory. Assuming a reassembly buffer 285 for a 6LoWPAN MTU of 1280 bytes as defined in section 4 of [RFC4944], 286 typical nodes only have enough memory for 1-3 reassembly buffers. 288 To illustrate this we use the topology from Figure 2, where nodes A, 289 B, C and D all send packets through node E. We further assume that 290 node E's memory can only hold 3 reassembly buffers. 292 +---+ +---+ 293 ... --->| A |------>| B | 294 +---+ +---+\ 295 \ 296 +---+ +---+ 297 | E |--->| F | ... 298 +---+ +---+ 299 / 300 / 301 +---+ +---+ 302 ... --->| C |------>| D | 303 +---+ +---+ 305 Figure 2: Illustrating the Memory Management Issue. 307 When nodes A, B and C concurrently send fragmented packets, all 3 308 reassembly buffers in node E are occupied. If, at that moment, node 309 D also sends a fragmented packet, node E has no option but to drop 310 one of the packets, lowering end-to-end reliability. 312 5. Forwarding Fragments 314 A 6LoWPAN Fragment Forwarding technique makes the routing decision on 315 the first fragment, which is always the one with the IPv6 address of 316 the destination. Upon a first fragment, a forwarding node (e.g. node 317 B in a A->B->C sequence) that does fragment forwarding MUST attempt 318 to create a state and forward the fragment. This is an atomic 319 operation, and if the first fragment cannot be forwarded then the 320 state MUST be removed. 322 Since the datagram_tag is uniquely associated to the source Link- 323 Layer address of the fragment, the forwarding node MUST assign a new 324 datagram_tag from its own namespace for the next hop and rewrite the 325 fragment header of each fragment with that datagram_tag. 327 When a forwarding node receives a fragment other than a first 328 fragment, it MUST look up state based on the source Link-Layer 329 address and the datagram_tag in the received fragment. If no such 330 state is found, the fragment MUST be dropped; otherwise the fragment 331 MUST be forwarded using the information in the state found. 333 Compared to Section 3, the conceptual reassembly buffer in node B now 334 contains, assuming that node B is neither the source nor the final 335 destination: 337 * a datagram_tag as received in the incoming fragments, associated 338 to Link-Layer address of node A for which the received 339 datagram_tag is unique, 341 * the Link-Layer address that node B uses as source to forward the 342 fragments 344 * the Link-Layer address of the next hop C that is resolved on the 345 first fragment 347 * a datagram_tag that node B uniquely allocated for this datagram 348 and that is used when forwarding the fragments of the datagram 350 * a buffer for the remainder of a previous fragment left to be sent, 352 * a timer that allows discarding the stale FF state after some 353 timeout. The duration of the timer should be longer than that 354 which covers the reassembly at the receiving end point. 356 A node that has not received the first fragment cannot forward the 357 next fragments. This means that if node B receives a fragment, node 358 A was in possession of the first fragment at some point. To keep the 359 operation simple, it makes sense to be consistent with [RFC4944] and 360 enforce that the first fragment is always sent first. When that is 361 done, if node B receives a fragment that is not the first and for 362 which it has no state, then node B treats this as an error and 363 refrains from creating a state or attempting to forward. This also 364 means that node A should perform all its possible retries on the 365 first fragment before it attempts to send the next fragments, and 366 that it should abort the datagram and release its state if it fails 367 to send the first fragment. 369 One benefit of Fragment Forwarding is that the memory that is used to 370 store the packet is now distributed along the path, which limits the 371 buffer bloat effect. Multiple fragments may progress in parallel 372 along the network as long as they do not interfere. An associated 373 caveat is that on a half duplex radio, if node A sends the next 374 fragment at the same time as node B forwards the previous fragment to 375 a node C down the path then node B will miss the next fragment from 376 node A. If node C forwards the previous fragment to a node D at the 377 same time and on the same frequency as node A sends the next fragment 378 to node B, this may result in a hidden terminal problem at B whereby 379 the transmission from C interferes with that from A unbeknownst of 380 node A. It results that consecutive fragments must be reasonably 381 spaced to avoid the 2 forms of collision described above. A node 382 that has multiple packets or fragments to send via different next-hop 383 routers may interleave the messages in order to alleviate those 384 effects. 386 6. Virtual Reassembly Buffer (VRB) Implementation 388 Virtual Reassembly Buffer (VRB) is the implementation technique 389 described in [LWIG-VRB] in which a forwarder does not reassemble each 390 packet in its entirety before forwarding it. 392 VRB overcomes the limitations listed in Section 4. Nodes do not wait 393 for the last fragment before forwarding, reducing end-to-end latency. 394 Similarly, the memory footprint of VRB is just the VRB table, 395 reducing the packet drop probability significantly. 397 There are other caveats, however: 399 Non-zero Packet Drop Probability: The abstract data in a VRB table 400 entry contains at a minimum the Link-Layer address of the 401 predecessor and that of the successor, the datagram_tag used by 402 the predecessor and the local datagram_tag that this node will 403 swap with it. The VRB may need to store a few octets from the 404 last fragment that may not have fit within MTU and that will be 405 prepended to the next fragment. This yields a small footprint 406 that is 2 orders of magnitude smaller compared to needing a 407 1280-byte reassembly buffer for each packet. Yet, the size of the 408 VRB table necessarily remains finite. In the extreme case where a 409 node is required to concurrently forward more packets that it has 410 entries in its VRB table, packets are dropped. 412 No Fragment Recovery: There is no mechanism in VRB for the node that 413 reassembles a packet to request a single missing fragment. 414 Dropping a fragment requires the whole packet to be resent. This 415 causes unnecessary traffic, as fragments are forwarded even when 416 the destination node can never construct the original IPv6 packet. 418 No Per-Fragment Routing: All subsequent fragments follow the same 419 sequence of hops from the source to the destination node as the 420 first fragment, because the IP header is required in order to 421 route the fragment and is only present in the first fragment. A 422 side effect is that the first fragment must always be forwarded 423 first. 425 The severity and occurrence of these caveats depends on the Link- 426 Layer used. Whether they are acceptable depends entirely on the 427 requirements the application places on the network. 429 If the caveats are present and not acceptable for the application, 430 future specifications may define new protocols to overcome them. One 431 example is [FRAG-RECOV] which defines a protocol which allows 432 fragment recovery. 434 7. Security Considerations 436 Secure joining and the Link-Layer security that it sets up protects 437 against those attacks from network outsiders. 439 "IP Fragmentation Considered Fragile" [FRAG-ILE] discusses security 440 threats that are linked to using IP fragmentation. The 6LoWPAN 441 fragmentation takes place underneath, but some issues described there 442 may still apply to 6LoWPAN fragments. 444 * Overlapping fragment attacks are possible with 6LoWPAN fragments 445 but there is no known firewall operation that would work on 446 6LoWPAN fragments at the time of this writing, so the exposure is 447 limited. An implementation of a firewall SHOULD NOT forward 448 fragments but recompose the IP packet, check it in the 449 uncompressed form, and then forward it again as fragments if 450 necessary. 452 * Resource exhaustion attacks are certainly possible and a sensitive 453 issue in a constrained network. An attacker can perform a Denial- 454 of-Service (DoS) attack on a node implementing VRB by generating a 455 large number of bogus first fragments without sending subsequent 456 fragments. This causes the VRB table to fill up. When hop-by-hop 457 reassembly is used, the same attack can be more damaging if the 458 node allocates a full datagram_size for each bogus first fragment. 459 With the VRB, the attack can be performed remotely on all nodes 460 along a path, but each node suffers a lesser hit. This is because 461 the VRB does not need to remember the full datagram as received so 462 far but only possibly a few octets from the last fragment that 463 could not fit in it. An implementation MUST protect itself to 464 keep the number of VRBs within capacity, and ensure that old VRBs 465 are protected by a timer of a reasonable duration for the 466 technology and destroyed upon timeout. 468 * Attacks based on predictable fragment identification values are 469 also possible but can be avoided. The datagram_tag SHOULD be 470 assigned pseudo-randomly in order to defeat such attacks. 472 * Evasion of Network Intrusion Detection Systems (NIDS) leverages 473 ambiguity in the reassembly of the fragment. This is difficult 474 and mostly useless in a 6LoWPAN network since the fragmentation is 475 not end-to-end. 477 8. IANA Considerations 479 No requests to IANA are made by this document. 481 9. Acknowledgments 483 The authors would like to thank Carles Gomez Montenegro, Yasuyuki 484 Tanaka, Ines Robles and Dave Thaler for their in-depth review of this 485 document and improvement suggestions. Also many thanks to Georgios 486 Papadopoulos and Dominique Barthel for their own reviews, and to 487 Barry Leiba, Derrell Piper, Sarah Banks, Joerg Ott and Francesca 488 Palombini for their constructive reviews through the IETF last call 489 and IESG process. 491 10. Normative References 493 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 494 Requirement Levels", BCP 14, RFC 2119, 495 DOI 10.17487/RFC2119, March 1997, 496 . 498 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 499 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 500 May 2017, . 502 [RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler, 503 "Transmission of IPv6 Packets over IEEE 802.15.4 504 Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007, 505 . 507 [RFC4919] Kushalnagar, N., Montenegro, G., and C. Schumacher, "IPv6 508 over Low-Power Wireless Personal Area Networks (6LoWPANs): 509 Overview, Assumptions, Problem Statement, and Goals", 510 RFC 4919, DOI 10.17487/RFC4919, August 2007, 511 . 513 11. Informative References 515 [RFC4963] Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly 516 Errors at High Data Rates", RFC 4963, 517 DOI 10.17487/RFC4963, July 2007, 518 . 520 [RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol 521 Label Switching Architecture", RFC 3031, 522 DOI 10.17487/RFC3031, January 2001, 523 . 525 [RFC6282] Hui, J., Ed. and P. Thubert, "Compression Format for IPv6 526 Datagrams over IEEE 802.15.4-Based Networks", RFC 6282, 527 DOI 10.17487/RFC6282, September 2011, 528 . 530 [RFC8138] Thubert, P., Ed., Bormann, C., Toutain, L., and R. Cragie, 531 "IPv6 over Low-Power Wireless Personal Area Network 532 (6LoWPAN) Routing Header", RFC 8138, DOI 10.17487/RFC8138, 533 April 2017, . 535 [RFC8201] McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed., 536 "Path MTU Discovery for IP version 6", STD 87, RFC 8201, 537 DOI 10.17487/RFC8201, July 2017, 538 . 540 [RFC6550] Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J., 541 Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur, 542 JP., and R. Alexander, "RPL: IPv6 Routing Protocol for 543 Low-Power and Lossy Networks", RFC 6550, 544 DOI 10.17487/RFC6550, March 2012, 545 . 547 [FRAG-ILE] Bonica, R., Baker, F., Huston, G., Hinden, R., Troan, O., 548 and F. Gont, "IP Fragmentation Considered Fragile", Work 549 in Progress, Internet-Draft, draft-ietf-intarea-frag- 550 fragile-17, 30 September 2019, 551 . 554 [LWIG-VRB] Bormann, C. and T. Watteyne, "Virtual reassembly buffers 555 in 6LoWPAN", Work in Progress, Internet-Draft, draft-ietf- 556 lwig-6lowpan-virtual-reassembly-01, 11 March 2019, 557 . 560 [FRAG-RECOV] 561 Thubert, P., "6LoWPAN Selective Fragment Recovery", Work 562 in Progress, Internet-Draft, draft-ietf-6lo-fragment- 563 recovery-08, 28 November 2019, 564 . 567 [ARTICLE] Tanaka, Y., Minet, P., and T. Watteyne, "6LoWPAN Fragment 568 Forwarding", IEEE Communications Standards Magazine , 569 2019. 571 Authors' Addresses 573 Thomas Watteyne (editor) 574 Analog Devices 575 32990 Alvarado-Niles Road, Suite 910 576 Union City, CA 94587 577 United States of America 579 Email: thomas.watteyne@analog.com 581 Pascal Thubert (editor) 582 Cisco Systems, Inc 583 Building D 584 45 Allee des Ormes - BP1200 585 06254 Mougins - Sophia Antipolis 586 France 588 Phone: +33 497 23 26 34 589 Email: pthubert@cisco.com 591 Carsten Bormann 592 Universitaet Bremen TZI 593 Postfach 330440 594 D-28359 Bremen 595 Germany 597 Email: cabo@tzi.org