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Bormann 7 Universitaet Bremen TZI 8 31 January 2020 10 On Forwarding 6LoWPAN Fragments over a Multihop IPv6 Network 11 draft-ietf-6lo-minimal-fragment-09 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 3 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 till the 6LoWPAN endpoint that 94 will attempt to perform the reassembly, and may be misused to the 95 point that performances fall behind that of per-hop recomposition. 97 This specification provides a generic overview of FF, discusses 98 advantages and caveats, and introduces a particular 6LoWPAN Fragment 99 Forwarding technique called Virtual Reassembly Buffer that can be 100 used while conserving the message formats defined in [RFC4944]. 102 2. Terminology 104 2.1. BCP 14 106 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 107 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 108 "OPTIONAL" in this document are to be interpreted as described in BCP 109 14 [RFC2119][RFC8174] when, and only when, they appear in all 110 capitals, as shown here. 112 2.2. Referenced Work 114 Past experience with fragmentation has shown that misassociated or 115 lost fragments can lead to poor network behavior and, occasionally, 116 trouble at application layer. The reader is encouraged to read "IPv4 117 Reassembly Errors at High Data Rates" [RFC4963] and follow the 118 references for more information. That experience led to the 119 definition of "Path MTU discovery" [RFC8201] (PMTUD) protocol that 120 limits fragmentation over the Internet. 122 "IP Fragmentation Considered Fragile" [FRAG-ILE] discusses security 123 threats that are linked to using IP fragmentation. The 6LoWPAN 124 fragmentation takes place underneath, but some issues described there 125 may still apply to 6LoWPAN fragments. 127 Readers are expected to be familiar with all the terms and concepts 128 that are discussed in "IPv6 over Low-Power Wireless Personal Area 129 Networks (6LoWPANs): Overview, Assumptions, Problem Statement, and 130 Goals" [RFC4919] and "Transmission of IPv6 Packets over IEEE 802.15.4 131 Networks" [RFC4944]. 133 Quoting the "Multiprotocol Label Switching (MPLS) Architecture" 134 [RFC3031]: with MPLS, 'packets are "labeled" before they are 135 forwarded'. At subsequent hops, there is no further analysis of the 136 packet's network layer header. Rather, the label is used as an index 137 into a table which specifies the next hop, and a new label". The 138 MPLS technique is leveraged in the present specification to forward 139 fragments that actually do not have a network layer header, since the 140 fragmentation occurs below IP. 142 2.3. New Terms 144 This specification uses the following terms: 146 6LoWPAN endpoints: The nodes in charge of generating or expanding a 147 6LoWPAN header from/to a full IPv6 packet. The 6LoWPAN endpoints 148 are the points where fragmentation and reassembly take place. 150 Compressed Form: This specification uses the generic term Compressed 151 Form to refer to the format of a datagram after the action of 152 [RFC6282] and possibly [RFC8138] for RPL [RFC6550] artifacts. 154 datagram_size: The size of the datagram in its Compressed Form 155 before it is fragmented. The datagram_size is expressed in a unit 156 that depends on the MAC layer technology, by default a byte. 158 datagram_tag: An identifier of a datagram that is locally unique to 159 the Layer-2 sender. Associated with the MAC address of the 160 sender, this becomes a globally unique identifier for the 161 datagram. 163 fragment_offset: The offset of a particular fragment of a datagram 164 in its Compressed Form. The fragment_offset is expressed in a 165 unit that depends on the MAC layer technology and is by default a 166 byte. 168 3. Overview of 6LoWPAN Fragmentation 170 We use Figure 1 to illustrate 6LoWPAN fragmentation. We assume node 171 A forwards a packet to node B, possibly as part of a multi-hop route 172 between IPv6 source and destination nodes which are neither A nor B. 174 +---+ +---+ 175 ... ---| A |-------------------->| B |--- ... 176 +---+ +---+ 177 # (frag. 5) 179 123456789 123456789 180 +---------+ +---------+ 181 | # ###| |### # | 182 +---------+ +---------+ 183 outgoing incoming 184 fragmentation reassembly 185 buffer buffer 187 Figure 1: Fragmentation at node A, reassembly at node B. 189 Node A starts by compacting the IPv6 packet using the header 190 compression mechanism defined in [RFC6282]. If the resulting 6LoWPAN 191 packet does not fit into a single Link-Layer frame, node A's 6LoWPAN 192 sublayer cuts it into multiple 6LoWPAN fragments, which it transmits 193 as separate Link-Layer frames to node B. Node B's 6LoWPAN sublayer 194 reassembles these fragments, inflates the compressed header fields 195 back to the original IPv6 header, and hands over the full IPv6 packet 196 to its IPv6 layer. 198 In Figure 1, a packet forwarded by node A to node B is cut into nine 199 fragments, numbered 1 to 9 as follows: 201 * Each fragment is represented by the '#' symbol. 203 * Node A has sent fragments 1, 2, 3, 5, 6 to node B. 205 * Node B has received fragments 1, 2, 3, 6 from node A. 207 * Fragment 5 is still being transmitted at the link layer from node 208 A to node B. 210 The reassembly buffer for 6LoWPAN is indexed in node B by: 212 * a unique Identifier of Node A (e.g., Node A's Link-Layer address) 214 * the datagram_tag chosen by node A for this fragmented datagram 216 Because it may be hard for node B to correlate all possible Link- 217 Layer addresses that node A may use (e.g., short vs. long addresses), 218 node A must use the same Link-Layer address to send all the fragments 219 of the same datagram to node B. 221 Conceptually, the reassembly buffer in node B contains: 223 * a datagram_tag as received in the incoming fragments, associated 224 to Link-Layer address of node A for which the received 225 datagram_tag is unique, 227 * the actual packet data from the fragments received so far, in a 228 form that makes it possible to detect when the whole packet has 229 been received and can be processed or forwarded, 231 * a state indicating the fragments already received, 233 * a datagram_size, 235 * a timer that allows discarding a partially reassembled packet 236 after some timeout. 238 A fragmentation header is added to each fragment; it indicates what 239 portion of the packet that fragment corresponds to. Section 5.3 of 240 [RFC4944] defines the format of the header for the first and 241 subsequent fragments. All fragments are tagged with a 16-bit 242 "datagram_tag", used to identify which packet each fragment belongs 243 to. Each datagram can be uniquely identified by the sender Link- 244 Layer addresses of the frame that carries it and the datagram_tag 245 that the sender allocated for this datagram. [RFC4944] also mandates 246 that the first fragment is sent first and with a particular format 247 that is different than that of the next fragments. Each fragment but 248 the first one can be identified within its datagram by the datagram- 249 offset. 251 Node B's typical behavior, per [RFC4944], is as follows. Upon 252 receiving a fragment from node A with a datagram_tag previously 253 unseen from node A, node B allocates a buffer large enough to hold 254 the entire packet. The length of the packet is indicated in each 255 fragment (the datagram_size field), so node B can allocate the buffer 256 even if the first fragment it receives is not fragment 1. As 257 fragments come in, node B fills the buffer. When all fragments have 258 been received, node B inflates the compressed header fields into an 259 IPv6 header, and hands the resulting IPv6 packet to the IPv6 layer 260 which performs the route lookup. This behavior typically results in 261 per-hop fragmentation and reassembly. That is, the packet is fully 262 reassembled, then (re)fragmented, at every hop. 264 4. Limits of Per-Hop Fragmentation and Reassembly 266 There are at least 2 limits to doing per-hop fragmentation and 267 reassembly. See [ARTICLE] for detailed simulation results on both 268 limits. 270 4.1. Latency 272 When reassembling, a node needs to wait for all the fragments to be 273 received before being able to generate the IPv6 packet, and possibly 274 forward it to the next hop. This repeats at every hop. 276 This may result in increased end-to-end latency compared to a case 277 where each fragment is forwarded without per-hop reassembly. 279 4.2. Memory Management and Reliability 281 Constrained nodes have limited memory. Assuming a reassembly buffer 282 for a 6LoWPAN MTU of 1280 bytes as defined in section 4 of [RFC4944], 283 typical nodes only have enough memory for 1-3 reassembly buffers. 285 To illustrate this we use the topology from Figure 2, where nodes A, 286 B, C and D all send packets through node E. We further assume that 287 node E's memory can only hold 3 reassembly buffers. 289 +---+ +---+ 290 ... --->| A |------>| B | 291 +---+ +---+\ 292 \ 293 +---+ +---+ 294 | E |--->| F | ... 295 +---+ +---+ 296 / 297 / 298 +---+ +---+ 299 ... --->| C |------>| D | 300 +---+ +---+ 302 Figure 2: Illustrating the Memory Management Issue. 304 When nodes A, B and C concurrently send fragmented packets, all 3 305 reassembly buffers in node E are occupied. If, at that moment, node 306 D also sends a fragmented packet, node E has no option but to drop 307 one of the packets, lowering end-to-end reliability. 309 5. Forwarding Fragments 311 A 6LoWPAN Fragment Forwarding technique makes the routing decision on 312 the first fragment, which is always the one with the IPv6 address of 313 the destination. Upon a first fragment, a forwarding node (e.g. node 314 B in a A->B->C sequence) that does fragment forwarding MUST attempt 315 to create a state and forward the fragment. This is an atomic 316 operation, and if the first fragment cannot be forwarded then the 317 state MUST be removed. 319 Since the datagram_tag is uniquely associated to the source Link- 320 Layer address of the fragment, the forwarding node MUST assign a new 321 datagram_tag from its own namespace for the next hop and rewrite the 322 fragment header of each fragment with that datagram_tag. 324 When a forwarding node receives a fragment other than a first 325 fragment, it MUST look up state based on the source Link-Layer 326 address and the datagram_tag in the received fragment. If no such 327 state is found, the fragment MUST be dropped; otherwise the fragment 328 MUST be forwarded using the information in the state found. 330 Compared to Section 3, the conceptual reassembly buffer in node B now 331 contains, assuming that node B is neither the source nor the final 332 destination: 334 * a datagram_tag as received in the incoming fragments, associated 335 to Link-Layer address of node A for which the received 336 datagram_tag is unique, 338 * the Link-Layer address that node B uses as source to forward the 339 fragments 341 * the Link-Layer address of the next hop C that is resolved on the 342 first fragment 344 * a datagram_tag that node B uniquely allocated for this datagram 345 and that is used when forwarding the fragments of the datagram 347 * a buffer for the remainder of a previous fragment left to be sent, 349 * a timer that allows discarding the stale FF state after some 350 timeout. The duration of the timer should be longer than that 351 which covers the reassembly at the receiving end point. 353 A node that has not received the first fragment cannot forward the 354 next fragments. This means that if node B receives a fragment, node 355 A was in possession of the first fragment at some point. In order to 356 keep the operation simple, it makes sense to be consistent with 357 [RFC4944] and enforce that the first fragment is always sent first. 358 When that is done, if node B receives a fragment that is not the 359 first and for which it has no state, then node B treats this as an 360 error and refrain from creating a state or attempting to forward. 361 This also means that node A should perform all its possible retries 362 on the first fragment before it attempts to send the next fragments, 363 and that it should abort the datagram and release its state if it 364 fails to send the first fragment. 366 One benefit of Fragment Forwarding is that the memory that is used to 367 store the packet is now distributed along the path, which limits the 368 buffer bloat effect. Multiple fragments may progress in parallel 369 along the network as long as they do not interfere. An associated 370 caveat is that on a half duplex radio, if node A sends the next 371 fragment at the same time as node B forwards the previous fragment to 372 a node C down the path then node B will miss the next fragment from 373 node A. If node C forwards the previous fragment to a node D at the 374 same time and on the same frequency as node A sends the next fragment 375 to node B, this may result in a hidden terminal problem at B whereby 376 the transmission from C interferes with that from A unbeknownst of 377 node A. It results that consecutive fragments must be reasonably 378 spaced in order to avoid the 2 forms of collision described above. A 379 node that has multiple packets or fragments to send via different 380 next-hop routers may interleave the messages in order to alleviate 381 those effects. 383 6. Virtual Reassembly Buffer (VRB) Implementation 385 Virtual Reassembly Buffer (VRB) is the implementation technique 386 described in [LWIG-VRB] in which a forwarder does not reassemble each 387 packet in its entirety before forwarding it. 389 VRB overcomes the limits listed in Section 4. Nodes do not wait for 390 the last fragment before forwarding, reducing end-to-end latency. 391 Similarly, the memory footprint of VRB is just the VRB table, 392 reducing the packet drop probability significantly. 394 There are, however, limits: 396 Non-zero Packet Drop Probability: The abstract data in a VRB table 397 entry contains at a minimum the Link-Layer address of the 398 predecessor and that of the successor, the datagram_tag used by 399 the predecessor and the local datagram_tag that this node will 400 swap with it. The VRB may need to store a few octets from the 401 last fragment that may not have fit within MTU and that will be 402 prepended to the next fragment. This yields a small footprint 403 that is 2 orders of magnitude smaller compared to needing a 404 1280-byte reassembly buffer for each packet. Yet, the size of the 405 VRB table necessarily remains finite. In the extreme case where a 406 node is required to concurrently forward more packets that it has 407 entries in its VRB table, packets are dropped. 409 No Fragment Recovery: There is no mechanism in VRB for the node that 410 reassembles a packet to request a single missing fragment. 411 Dropping a fragment requires the whole packet to be resent. This 412 causes unnecessary traffic, as fragments are forwarded even when 413 the destination node can never construct the original IPv6 packet. 415 No Per-Fragment Routing: All subsequent fragments follow the same 416 sequence of hops from the source to the destination node as the 417 first fragment, because the IP header is required to route the 418 fragment and is only present in the first fragment. A side effect 419 is that the first fragment must always be forwarded first. 421 The severity and occurrence of these limits depends on the Link-Layer 422 used. Whether these limits are acceptable depends entirely on the 423 requirements the application places on the network. 425 If the limits are present and not acceptable for the application, 426 future specifications may define new protocols to overcome these 427 limits. One example is [FRAG-RECOV] which defines a protocol which 428 allows fragment recovery. 430 7. Security Considerations 432 Secure joining and the Link-Layer security that it sets up protects 433 against those attacks from network outsiders. 435 "IP Fragmentation Considered Fragile" [FRAG-ILE] discusses security 436 threats that are linked to using IP fragmentation. The 6LoWPAN 437 fragmentation takes place underneath, but some issues described there 438 may still apply to 6LoWPAN fragments. 440 * Overlapping fragment attacks are possible with 6LoWPAN fragments 441 but there is no known firewall operation that would work on 442 6LoWPAN fragments at the time of this writing, so the exposure is 443 limited. An implementation of a firewall SHOULD NOT forward 444 fragments but recompose the IP packet, check it in the 445 uncompressed form, and then forward it again as fragments if 446 necessary. 448 * Resource exhaustion attacks are certainly possible and a sensitive 449 issue in a constrained network. An attacker can perform a Denial- 450 of-Service (DoS) attack on a node implementing VRB by generating a 451 large number of bogus first fragments without sending subsequent 452 fragments. This causes the VRB table to fill up. When hop-by-hop 453 reassembly is used, the same attack can be more damaging if the 454 node allocates a full datagram_size for each bogus first fragment. 455 With the VRB, the attack can be performed remotely on all nodes 456 along a path, but each node suffers a lesser hit. this is because 457 the VRB does not need to remember the full datagram as received so 458 far but only possibly a few octets from the last fragment that 459 could not fit in it. An implementation MUST protect itself to 460 keep the number of VRBs within capacity, and that old VRBs are 461 protected by a timer of a reasonable duration for the technology 462 and destroyed upon timeout. 464 * Attacks based on predictable fragment identification values are 465 also possible but can be avoided. The datagram_tag SHOULD be 466 assigned pseudo-randomly in order to defeat such attacks. 468 * Evasion of Network Intrusion Detection Systems (NIDS) leverages 469 ambiguity in the reassembly of the fragment. This sounds 470 difficult and mostly useless in a 6LoWPAN network since the 471 fragmentation is not end-to-end. 473 8. IANA Considerations 475 No requests to IANA are made by this document. 477 9. Acknowledgments 479 The authors would like to thank Carles Gomez Montenegro, Yasuyuki 480 Tanaka, Ines Robles and Dave Thaler for their in-depth review of this 481 document and improvement suggestions. Also many thanks to Georgies 482 Papadopoulos and Dominique Barthel for their own reviews, and to 483 Joerg Ott and Francesca Palombini For their constructive reviews 484 through the IESG process. 486 10. Normative References 488 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 489 Requirement Levels", BCP 14, RFC 2119, 490 DOI 10.17487/RFC2119, March 1997, 491 . 493 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 494 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 495 May 2017, . 497 [RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler, 498 "Transmission of IPv6 Packets over IEEE 802.15.4 499 Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007, 500 . 502 11. Informative References 504 [RFC4919] Kushalnagar, N., Montenegro, G., and C. Schumacher, "IPv6 505 over Low-Power Wireless Personal Area Networks (6LoWPANs): 506 Overview, Assumptions, Problem Statement, and Goals", 507 RFC 4919, DOI 10.17487/RFC4919, August 2007, 508 . 510 [RFC4963] Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly 511 Errors at High Data Rates", RFC 4963, 512 DOI 10.17487/RFC4963, July 2007, 513 . 515 [RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol 516 Label Switching Architecture", RFC 3031, 517 DOI 10.17487/RFC3031, January 2001, 518 . 520 [RFC6282] Hui, J., Ed. and P. Thubert, "Compression Format for IPv6 521 Datagrams over IEEE 802.15.4-Based Networks", RFC 6282, 522 DOI 10.17487/RFC6282, September 2011, 523 . 525 [RFC8138] Thubert, P., Ed., Bormann, C., Toutain, L., and R. Cragie, 526 "IPv6 over Low-Power Wireless Personal Area Network 527 (6LoWPAN) Routing Header", RFC 8138, DOI 10.17487/RFC8138, 528 April 2017, . 530 [RFC8201] McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed., 531 "Path MTU Discovery for IP version 6", STD 87, RFC 8201, 532 DOI 10.17487/RFC8201, July 2017, 533 . 535 [RFC6550] Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J., 536 Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur, 537 JP., and R. Alexander, "RPL: IPv6 Routing Protocol for 538 Low-Power and Lossy Networks", RFC 6550, 539 DOI 10.17487/RFC6550, March 2012, 540 . 542 [FRAG-ILE] Bonica, R., Baker, F., Huston, G., Hinden, R., Troan, O., 543 and F. Gont, "IP Fragmentation Considered Fragile", Work 544 in Progress, Internet-Draft, draft-ietf-intarea-frag- 545 fragile-17, 30 September 2019, 546 . 549 [LWIG-VRB] Bormann, C. and T. Watteyne, "Virtual reassembly buffers 550 in 6LoWPAN", Work in Progress, Internet-Draft, draft-ietf- 551 lwig-6lowpan-virtual-reassembly-01, 11 March 2019, 552 . 555 [FRAG-RECOV] 556 Thubert, P., "6LoWPAN Selective Fragment Recovery", Work 557 in Progress, Internet-Draft, draft-ietf-6lo-fragment- 558 recovery-08, 28 November 2019, 559 . 562 [ARTICLE] Tanaka, Y., Minet, P., and T. Watteyne, "6LoWPAN Fragment 563 Forwarding", IEEE Communications Standards Magazine , 564 2019. 566 Authors' Addresses 568 Thomas Watteyne (editor) 569 Analog Devices 570 32990 Alvarado-Niles Road, Suite 910 571 Union City, CA 94587 572 United States of America 574 Email: thomas.watteyne@analog.com 576 Pascal Thubert (editor) 577 Cisco Systems, Inc 578 Building D 579 45 Allee des Ormes - BP1200 580 06254 Mougins - Sophia Antipolis 581 France 583 Phone: +33 497 23 26 34 584 Email: pthubert@cisco.com 586 Carsten Bormann 587 Universitaet Bremen TZI 588 Postfach 330440 589 D-28359 Bremen 590 Germany 592 Email: cabo@tzi.org