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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 PWE3 S. Bryant, Ed. 3 Internet-Draft C. Filsfils 4 Intended status: Standards Track Cisco Systems 5 Expires: April 27, 2010 U. Drafz 6 Deutsche Telekom 7 V. Kompella 8 J. Regan 9 Alcatel-Lucent 10 S. Amante 11 Level 3 Communications 12 October 24, 2009 14 Flow Aware Transport of Pseudowires over an MPLS PSN 15 draft-ietf-pwe3-fat-pw-02 17 Status of this Memo 19 This Internet-Draft is submitted to IETF in full conformance with the 20 provisions of BCP 78 and BCP 79. 22 Internet-Drafts are working documents of the Internet Engineering 23 Task Force (IETF), its areas, and its working groups. Note that 24 other groups may also distribute working documents as Internet- 25 Drafts. 27 Internet-Drafts are draft documents valid for a maximum of six months 28 and may be updated, replaced, or obsoleted by other documents at any 29 time. It is inappropriate to use Internet-Drafts as reference 30 material or to cite them other than as "work in progress." 32 The list of current Internet-Drafts can be accessed at 33 http://www.ietf.org/ietf/1id-abstracts.txt. 35 The list of Internet-Draft Shadow Directories can be accessed at 36 http://www.ietf.org/shadow.html. 38 This Internet-Draft will expire on April 27, 2010. 40 Copyright Notice 42 Copyright (c) 2009 IETF Trust and the persons identified as the 43 document authors. All rights reserved. 45 This document is subject to BCP 78 and the IETF Trust's Legal 46 Provisions Relating to IETF Documents in effect on the date of 47 publication of this document (http://trustee.ietf.org/license-info). 48 Please review these documents carefully, as they describe your rights 49 and restrictions with respect to this document. 51 Abstract 53 Where the payload carried over a pseudowire carries a number of 54 identifiable flows it can in some circumstances be desirable to carry 55 those flows over the equal cost multiple paths (ECMPs) that exist in 56 the packet switched network. Most forwarding engines are able to 57 hash based on label stacks and use this to balance flows over ECMPs. 58 This draft describes a method of identifying the flows, or flow 59 groups, to the label switched routers by including an additional 60 label in the label stack. 62 Requirements Language 64 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 65 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 66 document are to be interpreted as described in RFC2119 [RFC2119]. 68 Table of Contents 70 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 71 1.1. ECMP in Label Switched Routers . . . . . . . . . . . . . . 5 72 1.2. Flow Label . . . . . . . . . . . . . . . . . . . . . . . . 5 73 2. Native Service Processing Function . . . . . . . . . . . . . . 6 74 3. Pseudowire Forwarder . . . . . . . . . . . . . . . . . . . . . 6 75 3.1. Encapsulation . . . . . . . . . . . . . . . . . . . . . . 7 76 4. Signaling the Presence of the Flow Label . . . . . . . . . . . 8 77 4.1. Structure of Flow Label Sub-TLV . . . . . . . . . . . . . 9 78 5. Multi-Segment Pseudowires . . . . . . . . . . . . . . . . . . 9 79 6. OAM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 80 7. Applicability of FAT PWs . . . . . . . . . . . . . . . . . . . 11 81 7.1. Equal Cost Multiple Paths . . . . . . . . . . . . . . . . 12 82 7.2. Link Aggregation Groups . . . . . . . . . . . . . . . . . 13 83 7.3. The Single Large Flow Case . . . . . . . . . . . . . . . . 13 84 7.4. MPLS-TP . . . . . . . . . . . . . . . . . . . . . . . . . 14 85 8. Applicability to MPLS . . . . . . . . . . . . . . . . . . . . 15 86 9. Security Considerations . . . . . . . . . . . . . . . . . . . 15 87 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15 88 11. Congestion Considerations . . . . . . . . . . . . . . . . . . 16 89 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16 90 13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16 91 13.1. Normative References . . . . . . . . . . . . . . . . . . . 16 92 13.2. Informative References . . . . . . . . . . . . . . . . . . 17 93 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18 95 1. Introduction 97 A pseudowire (PW) [RFC3985] is normally transported over one single 98 network path, even if multiple Equal Cost Multiple Paths (ECMP) exit 99 between the ingress and egress PW provider edge (PE) 100 equipments[RFC4385] [RFC4928]. This is required to preserve the 101 characteristics of the emulated service (e.g. to avoid misordering 102 SAToP pseudowire packets [RFC4553] or subjecting the packets to 103 unusable inter-arrival times ). The use of a single path to preserve 104 order remains the default mode of operation of a pseudowire (PW). 105 The new capability proposed in this document is an OPTIONAL mode 106 which may be used when the use of ECMP paths for is known to be 107 beneficial (and not harmful) to the operation of the PW. 109 Some pseudowires are used to transport large volumes of IP traffic 110 between routers at two locations. One example of this is the use of 111 an Ethernet pseudowire to create a virtual direct link between a pair 112 of routers. Such pseudowire's may carry from hundred's of Mbps to 113 Gbps of traffic. Such pseudowire's do not require strict ordering to 114 be preserved between packets of the pseudowire. They only require 115 ordering to be preserved within the context of each individual 116 transported IP flow. Some operators have requested the ability to 117 explicitly configure such a pseudowire to leverage the availability 118 of multiple ECMP paths. This allows for better capacity planning as 119 the statistical multiplexing of a larger number of smaller flows is 120 more efficient than with a smaller set of larger flows. Although 121 Ethernet is used as an example above, the mechanisms described in 122 this draft are general mechanisms that may be applied to any 123 pseudowire type in which there are identifiable flows, and in which 124 there is no requirement to preserve the order between those flows. 126 Typically, forwarding hardware can deduce that an IP payload is being 127 directly carried by an MPLS label stack, and is capable of looking at 128 some fields in packets to construct hash buckets for conversations or 129 flows. However, an intermediate node has no information on the type 130 pseudowire being carried in the packet. This limits the forwarder at 131 the intermediate node to only being able to make an ECMP choice based 132 on a hash of the label stack. In the case of a pseudowire emulating 133 a high bandwidth trunk, the granularity obtained by hashing the 134 default label stack is inadequate for satisfactory load-balancing. 135 The ingress node, however, is in the special position of being able 136 to look at the un-encapsulated packet and spread flows amongst any 137 available ECMP paths, or even any Loop-Free Alternates [RFC5286] . 138 This draft proposes a method to introduce granularity on the hashing 139 of traffic running over pseudowires by introducing an additional 140 label, chosen by the ingress node, and placed at the bottom of the 141 label stack. 143 In addition to providing an indication of the flow structure for use 144 in ECMP forwarding decisions, the mechanism described in the document 145 may also be used to select flows for distribution over an 802.1ad 146 link aggregation group that has been used in an MPLS network. 148 1.1. ECMP in Label Switched Routers 150 Label switched routers commonly hash the label stack or some elements 151 of the label stack as a method of discriminating between flows, in 152 order to distribute those flows over the available equal cost 153 multiple paths that exist in the network. Since the label at the 154 bottom of stack is usually the label most closely associated with the 155 flow, this normally provides the greatest entropy, and hence is 156 usually included in the hash. This draft describes a method of 157 adding an additional label at the bottom of stack in order to 158 facilitate the load balancing of the flows within a pseudowire over 159 the available ECMPs. A similar design for general MPLS use has also 160 been proposed [I-D.kompella-mpls-entropy-label], however that is 161 outside the scope of this draft. 163 An alternative method of load balancing by creating a number of 164 pseudowires and distributing the flows amongst them was considered, 165 but was rejected because: 167 o It did not introduce as much entropy as the load balance label 168 method. 170 o It required additional pseudowires to be set up and maintained. 172 1.2. Flow Label 174 An additional label is interposed between the pseudowire label and 175 the control word, or if the control word is not present, between the 176 pseudowire label and the pseudowire payload. This additional label 177 is called the flow label. Indivisible flows within the pseudowire 178 MUST be mapped to the same flow label by the ingress PE. The flow 179 label stimulates the correct ECMP load balancing behaviour in the 180 packet switched network (PSN). On receipt of the pseudowire packet 181 at the egress PE (which knows this additional label is present) the 182 flow label is discarded without processing. 184 Note that the flow label MUST NOT be an MPLS reserved label (values 185 in the range 0..15) [RFC3032], but is otherwise unconstrained by the 186 protocol. 188 Considerations of the TTL value are described in the Security section 189 of this document. The flow label can never become the top label in 190 normal operation, and hence the TTL in the flow label is never used 191 to determine whether the packet should be discarded due to TTL 192 expiry. Therefore there are no lower restrictions on the TTL value. 194 2. Native Service Processing Function 196 The Native Service Processing (NSP) function [RFC3985] is a component 197 of a PE that has knowledge of the structure of the emulated service 198 and is able to take action on the service outside the scope of the 199 pseudowire. In this case it is required that the NSP in the ingress 200 PE identify flows, or groups of flows within the service, and 201 indicate the flow (group) identity of each packet as it is passed to 202 the pseudowire forwarder. As an example, where the PW type is an 203 Ethernet, the NSP might parse the ingress Ethernet traffic and 204 consider all of the IP traffic. This traffic could then be 205 categorised into flows by considering all traffic with the same 206 source and destination address pair to be a single indivisible flow. 207 Since this is an NSP function, by definition, the method used to 208 identify a flow is outside the scope of the pseudowire design. 209 Similarly, since the NSP is internal to the PE, the method of flow 210 indication to the pseudowire forwarder is outside the scope of this 211 document. 213 3. Pseudowire Forwarder 215 The pseudowire forwarder must be provided with a method of mapping 216 flows to load balanced paths. 218 The forwarder must generate a label for the flow or group of flows. 219 How the load balance label values are determined is outside the scope 220 of this document, however the load balance label allocated to a flow 221 MUST NOT be an MPLS reserved label and SHOULD remain constant for the 222 life of the flow. It is recommended that the method chosen to 223 generate the load balancing labels introduces a high degree of 224 entropy in their values, to maximise the entropy presented to the 225 ECMP path selection mechanism in the LSRs in the PSN, and hence 226 distribute the flows as evenly as possible over the available PSN 227 ECMP paths. The forwarder at the ingress PE prepends the pseudowire 228 control word (if applicable), and then pushes the flow label, 229 followed by the pseudowire label. 231 The forwarder at the egress PE uses the pseudowire label to identify 232 the pseudowire. From the context associated with the pseudowire 233 label, the egress PE can determine whether a flow label is present. 234 If a flow label is present, the label is discarded. 236 All other pseudowire forwarding operations are unmodified by the 237 inclusion of the flow label. 239 3.1. Encapsulation 241 The PWE3 Protocol Stack Reference Model modified to include flow 242 label is shown in Figure 1 below 244 +-------------+ +-------------+ 245 | Emulated | | Emulated | 246 | Ethernet | | Ethernet | 247 | (including | Emulated Service | (including | 248 | VLAN) |<==============================>| VLAN) | 249 | Services | | Services | 250 +-------------+ +-------------+ 251 | Flow | | Flow | 252 +-------------+ Pseudowire +-------------+ 253 |Demultiplexer|<==============================>|Demultiplexer| 254 +-------------+ +-------------+ 255 | PSN | PSN Tunnel | PSN | 256 | MPLS |<==============================>| MPLS | 257 +-------------+ +-------------+ 258 | Physical | | Physical | 259 +-----+-------+ +-----+-------+ 261 Figure 1: PWE3 Protocol Stack Reference Model 263 The encapsulation of a pseudowire with a flow label is shown in 264 Figure 2 below 265 +-------------------------------+ 266 | | 267 | Payload | 268 | | n octets 269 | | 270 +-------------------------------+ 271 | Optional Control Word | 4 octets 272 +-------------------------------+ 273 | Flow label | 4 octets 274 +-------------------------------+ 275 | PW label | 4 octets 276 +-------------------------------+ 277 | MPLS Tunnel label(s) | n*4 octets (four octets per label) 278 +-------------------------------+ 280 Figure 2: Encapsulation of a pseudowire with a pseudowire load 281 balancing label 283 4. Signaling the Presence of the Flow Label 285 When using the signalling procedures in [RFC4447], a Pseudowire 286 Interface Parameter Sub-TLV type is used to synchronise the flow 287 label states between the ingress and egress PEs. 289 A PE that wishes to use a flow label includes in its label mapping 290 message a Flow Label Sub-TLV (FL Sub-TLV) with F = 1 (see 291 Section 4.1). A PE that can correctly process a flow label, and is 292 willing to receive one, but does not wish to send a flow label, 293 includes an FL Sub-TLV with F = 0. 295 If a PE has sent an FL Sub-TLV with F = 1, and has received an FL 296 Sub-TLV it MUST include a flow lablel in the label stack. 298 If a PE has sent an FL Sub-TLV with F = 1 and does not receive an FL 299 Sub-TLV it MUST send a new label mapping using an FL Sub-TLV with F = 300 0. 302 A PE that has sent an FL Sub-TLV with F = 0 MUST NOT include a flow 303 lablel in the label stack. 305 If a PE that previously did not received a label binding without a FL 306 Sub-TLV receives a new a label mapping with one included, it MAY send 307 a new label mapping including an FL Sub-TLV with F = 1. 309 The signalling procedures in [RFC4447] state that "Processing of the 310 interface parameters should continue when unknown interface 311 parameters are encountered, and they MUST be silently ignored." The 312 signalling proceedure described here is therefore backwards 313 compatible with existing implementations. 315 If PWE3 signalling [RFC4447] is not in use for a pseudowire, then 316 whether the flow label is used MUST be identically provisioned in 317 both PEs at the pseudowire endpoints. If there is no provisioning 318 support for this option, the default behaviour is not to include the 319 flow label. 321 Note that what is signalled is the desire to include the flow label 322 in the label stack. The value of the label is a local matter for the 323 ingress PE, and the label value itself is not signalled. 325 4.1. Structure of Flow Label Sub-TLV 327 The structure of the flow label TLV is shown in Figure 3. 329 0 1 2 3 330 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 331 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 332 | FL | Length |F| must be zero | 333 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 335 Figure 3: Flow Label Sub-TLV 337 Where: 339 o FL is the flow label sub-TLV identifier assigned by IANA. 341 o Length is the length of the TLV in octets and is 4. 343 o When F=1 a flow label will be pushed. When F=0 a flow label will 344 not be pushed. 346 5. Multi-Segment Pseudowires 348 The flow label mechanism described in this document works on multi- 349 segment PWs without requiring modification to the Switching PEs 350 (S-PEs). This is because the flow label is transparent to the label 351 swap operation, and because interface parameter Sub-TLV signalling is 352 transitive. 354 6. OAM 356 The following OAM considerations apply to this method of load 357 balancing. 359 Where the OAM is only to be used to perform a basic test that the 360 pseudowires have been configured at the PEs, VCCV [RFC5085] messages 361 may be sent using any load balance pseudowire path, i.e. using any 362 value for the flow label. 364 Where it is required to verify that a pseudowire is fully functional 365 for all flows, VCCV [RFC5085] connection verification message MUST be 366 sent over each ECMP path to the pseudowire egress PE. This problem 367 is difficult to solve and scales poorly. We believe that this 368 problem is addressed by the following two methods: 370 1. If a failure occurs within the PSN, this failure will normally be 371 detected by the PSN's Interior Gateway protocol (IGP) link/node 372 failure detection mechanism (loss of light, bidirectional 373 forwarding detection [I-D.ietf-bfd-base] or IGP hello detection), 374 and the IGP convergence will naturally modify the ECMP set of 375 network paths between the Ingress and Egress PE's. Hence the PW 376 is only impacted during the normal IGP convergence time. 378 2. If the failure is related to the individual corruption of an 379 Label Forwarding Information dataBase (LFIB) entry in a router, 380 then only the network path using that specific entry is impacted. 381 If the PW is load balanced over multiple network paths, then this 382 failure can only be detected if, by chance, the transported OAM 383 flow is mapped onto the impacted network path, or all paths are 384 tested. This type of error may be better solved be solved by 385 other means such as LSP self test [I-D.ietf-mpls-lsr-self-test]. 387 To troubleshoot the MPLS PSN, including multiple paths, the 388 techniques described in [RFC4378] and [RFC4379] can be used. 390 Where the pseudowire OAM is carried out of band (VCCV Type 2) 391 [RFC5085] it is necessary to insert an "MPLS Router Alert Label" in 392 the label stack. The resultant label stack is a follows: 394 +-------------------------------+ 395 | | 396 | Payload | 397 | | n octets 398 | | 399 +-------------------------------+ 400 | Optional Control Word | 4 octets 401 +-------------------------------+ 402 | Flow label | 4 octets 403 +-------------------------------+ 404 | PW label | 4 octets 405 +-------------------------------+ 406 | Router Alert label | 4 octets 407 +-------------------------------+ 408 | MPLS Tunnel label(s) | n*4 octets (four octets per label) 409 +-------------------------------+ 411 Figure 4: Use of Router Alert LAbel 413 7. Applicability of FAT PWs 415 A node within the PSN is not able to perform deep-packet-inspection 416 (DPI) of the PW as the PW technology is not self-describing: the 417 structure of the PW payload is only known to the ingress and egress 418 PE devices. The method proposed in this document provides a 419 statistical mitigation of the problem of load balance in those cased 420 where a PE is able to discern flows embedded in the traffic received 421 on the attachment circuit. 423 The methods describe in this document are transparent to the PSN and 424 as such do not require any new capability from the PSN. 426 The requirement to load-balance over multiple PSN paths occurs when 427 the ratio between the PW access speed and the PSN's core link 428 bandwidth is large (e.g. >= 10%). ATM and FR are unlikely to meet 429 this property. Ethernet may have this property, and for that reason 430 this document focuses on Ethernet. Applications for other high- 431 access-bandwidth PW's (e.g. Fibre Channel) may be defined in the 432 future. 434 This design applies to MPLS pseudowires where it is meaningful to de- 435 construct the packets presented to the ingress PE into flows. The 436 mechanism described in this document promotes the distribution of 437 flows within the pseudowire over different network paths. This in 438 turn means that whilst packets within a flow are delivered in order 439 (subject to normal IP delivery perturbations due to topology 440 variation), order is not maintained amongst packets of different 441 flows. It is not proposed to associate a different sequence number 442 with each flow. If sequence number support is required this 443 mechanism is not applicable. 445 Where it is known that the traffic carried by the Ethernet pseudowire 446 is IP the method of identifying the flows are well known and can be 447 applied. Such methods typically include hashing on the source and 448 destination addresses, the protocol ID and higher-layer flow- 449 dependent fields such as TCP/UDP ports, L2TPv3 Session ID's etc. 451 Where it is known that the traffic carried by the Ethernet pseudowire 452 is non-IP, techniques used for link bundling between Ethernet 453 switches may be reused. In this case however the latency 454 distribution would be larger than is found in the link bundle case. 455 The acceptability of the increased latency is for further study. Of 456 particular importance the Ethernet control frames SHOULD always be 457 mapped to the same PSN path to ensure in-order delivery. 459 7.1. Equal Cost Multiple Paths 461 ECMP in packet switched networks is statistical in nature. The 462 mapping of flows to a particular path does not take into account the 463 bandwidth of the flow being mapped or the current bandwidth usage of 464 the members of the ECMP set. This simplification works well when the 465 distribution of flows is evenly spread over the ECMP set and there 466 are a large number of flows that have low bandwidth relative to the 467 paths. The random allocation of a flow to a path provides a good 468 approximation to an even spread of flows, provided that polarisation 469 effects are avoided. The method proposed in this document has the 470 same statistical properties as an IP PSN. 472 ECMP is a load-sharing mechanism that is based on sharing the load 473 over a number of layer 3 paths through the PSN. Often however 474 multiple links exist between a pair of LSRs that are considered by 475 the IGP to be a single link. These are known as link bundles. The 476 mechanism described in this document can also be used to distribute 477 the flows within a pseudowire over the members of the link bundle by 478 using the flow label value to identify candidate flows. How that 479 mapping takes place is outside the scope of this specification. 480 Similar considerations apply to link aggregation groups. 482 In the ECMP case and the link bundling case the NSP may attempt to 483 take bandwidth into consideration when allocating groups of flows to 484 a common path. That is permitted, but it must be borne in mind that 485 the semantics of a label stack entry (LSE) as defined by [RFC3032] 486 cannot be modified, the value of the flow label cannot be modified at 487 any point on the LSP, and the interpretation of bit patterns in, or 488 values of, the flow label by an LSR are undefined. 490 A different type of load balancing is the desire to carry a 491 pseudowire over a set of PSN links in which the bandwidth of members 492 of the link set is less than the bandwidth of the pseudowire. This 493 problem is addressed in [I-D.stein-pwe3-pwbonding]. Such a mechanism 494 can be considered complementary to this mechanism. 496 7.2. Link Aggregation Groups 498 A Link Aggregation Group (LAG) is used to bond together several 499 physical circuits between two adjacent nodes so they appear to 500 higher-layer protocols as a single, higher bandwidth "virtual" pipe. 501 These may co-exist in various parts of a given network. An advantage 502 of LAGs is that they reduce the number of routing and signalling 503 protocol adjacencies between devices, reducing control plane 504 processing overhead. As with ECMP, the key problem related to LAGs 505 is that due to inefficiencies in LAG load-distribution algorithms, a 506 particular component of a LAG may experience congestion. The 507 mechanism proposed here may be able to assist in producing a more 508 uniform flow distribution. 510 The same considerations requiring a flow to go over a single member 511 of an ECMP path set apply to a member of a LAG. 513 7.3. The Single Large Flow Case 515 Clearly the operator should make sure that the service offered using 516 PW technology and the method described in this document does not 517 exceed the maximum planned link capacity, unless it can be guaranteed 518 that it conforms to the Internet traffic profile of a very large 519 number of small flows. 521 If the payload on a PW is made of a single inner flow (i.e. an 522 encrypted connection between two routers), or the flow identifiers 523 are too deeply buried in the packet, then the functionality described 524 in this document does not give any benefits, though neither does it 525 cause harm relative to the existing situation. The most common case 526 where a single flow dominated the traffic on a PW is when it is used 527 to transport enterprise traffic. Enterprise traffic may well consist 528 of a large single TCP flows, or encrypted flows that cannot be 529 handled by the methods described in this document. 531 An operator has six options under these circumstances: 533 1. The operator can do nothing and the system will work as it does 534 without the flow label. 536 2. The operator can make the customer aware that the service 537 offering has a restriction on flow bandwidth and police flows to 538 that restriction. This would allow customers offering multiple 539 flows to use a larger fraction their access bandwidth, whilst 540 preventing an single flow from consuming a fraction of internal 541 link bandwidth that the operator considered excessive. 543 3. The operator could configure the ingress PE to assign a constant 544 flow label to all high bandwidth flows so that only one path was 545 affected by these flows, 547 4. The operator could configure the ingress PE to assign a random 548 flow label to all high bandwidth flows so as to minimise the 549 disruption to the network as a cost of out of order traffic to 550 the user. 552 5. The operator could configure the ingress to assign a label of 553 special significance (such as a reserved label) to all high 554 bandwidth flows so that some other action (not specified in this 555 document) could be taken on the flow. 557 The issues described above are mitigated by the following two 558 factors: 560 o Firstly, the customer of a high-bandwidth PW service has an 561 incentive to get the best transport service because an inefficient 562 use of the PSN leads to jitter and eventually to loss to the PW's 563 payload. 565 o Secondly, the customer is usually able to tailor their 566 applications to generate many flows in the PSN. A well-known 567 example is massive data transport between servers which use many 568 parallel TCP sessions. This same technique can be used by any 569 transport protocol: multiple UDP ports, multiple L2TPv3 Session 570 ID's, multiple GRE keys may be used to decompose a large flow into 571 smaller components. This approach may be applied to IPsec 572 [RFC4301] where multiple Security Parameters Indexes (SPI's) may 573 be allocated to the same security association. 575 7.4. MPLS-TP 577 The MPLS Transport Profile (MPLS-TP) [I-D.ietf-mpls-tp-requirements] 578 requirement 44 states that "MPLS-TP SHOULD support mechanisms to 579 enable the reserved bandwidth of a transport path to be decreased 580 without impacting the existing traffic on that transport path, 581 provided that the level of existing traffic is smaller than the 582 reserved bandwidth following the decrease." The flow aware transport 583 of a PW reorders packets (albeit in an application friendly way), 584 therefore SHOULD NOT be deployed in a network conforming to the 585 MPLS-TP. 587 8. Applicability to MPLS 589 A further application of this technique would be to create a basis 590 for hash diversity without having to peek below the label stack for 591 IP traffic carried over LDP LSPs. Work on the generalisation of this 592 to MPLS has been described in [I-D.kompella-mpls-entropy-label]. 593 This is can be regarded as a complementary, but distinct, approach 594 since although similar consideration may apply to the identification 595 of flows and the allocation of flow label values, the flow labels are 596 imposed by different network components, and the associated 597 signalling mechanisms are different. 599 9. Security Considerations 601 The pseudowire generic security considerations described in [RFC3985] 602 and the security considerations applicable to a specific pseudowire 603 type (for example, in the case of an Ethernet pseudowire [RFC4448] 604 apply. 606 The ingress PE SHOULD take steps to ensure that the load-balance 607 label is not used as a covert channel. 609 It is useful to give consideration to the choice of TTL value in the 610 flow label stack entry [RFC3032]. The flow label is at the bottom of 611 label stack. Therefore, even when penultimate hop popping is 612 employed, it will always be will preceded by the PW label on arrival 613 at the PE. The flow label TTL should therefore never be considered 614 by the forwarder, and hence SHOULD be set to a value of 1. This will 615 prevent the packet being inadvertently forwarded based on the value 616 of the flow label. Note that this may be a departure from 617 considerations that apply to the general MPLS case. 619 10. IANA Considerations 621 IANA is requested to allocate the next available values from the IETF 622 Consensus range in the Pseudowire Interface Parameters Sub-TLV type 623 Registry as a Flow Label indicator. 625 Parameter Length Description 626 ID 628 TBD 4 Flow Label 630 11. Congestion Considerations 632 The congestion considerations applicable to pseudowires as described 633 in [RFC3985] and any additional congestion considerations developed 634 at the time of publication apply to this design. 636 The ability to explicitly configure a PW to leverage the availability 637 of multiple ECMP paths is beneficial to capacity planning as, all 638 other parameters being constant, the statistical multiplexing of a 639 larger number of smaller flows is more efficient than with a smaller 640 number of larger flows. 642 Note that if the classification into flows is only performed on IP 643 packets the behaviour of those flows in the face of congestion will 644 be as already defined by the IETF for packets of that type and no 645 additional congestion processing is required. 647 Where flows that are not IP are classified pseudowire congestion 648 avoidance must be applied to each non-IP load balance group. 650 12. Acknowledgements 652 The authors wish to thank Eric Grey, Kireeti Kompella, Joerg 653 Kuechemann, Wilfried Maas, Luca Martini, Mark Townsley, and Lucy Yong 654 for valuable comments on this document. 656 13. References 658 13.1. Normative References 660 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 661 Requirement Levels", BCP 14, RFC 2119, March 1997. 663 [RFC3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y., 664 Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack 665 Encoding", RFC 3032, January 2001. 667 [RFC4379] Kompella, K. and G. Swallow, "Detecting Multi-Protocol 668 Label Switched (MPLS) Data Plane Failures", RFC 4379, 669 February 2006. 671 [RFC4385] Bryant, S., Swallow, G., Martini, L., and D. McPherson, 672 "Pseudowire Emulation Edge-to-Edge (PWE3) Control Word for 673 Use over an MPLS PSN", RFC 4385, February 2006. 675 [RFC4447] Martini, L., Rosen, E., El-Aawar, N., Smith, T., and G. 676 Heron, "Pseudowire Setup and Maintenance Using the Label 677 Distribution Protocol (LDP)", RFC 4447, April 2006. 679 [RFC4448] Martini, L., Rosen, E., El-Aawar, N., and G. Heron, 680 "Encapsulation Methods for Transport of Ethernet over MPLS 681 Networks", RFC 4448, April 2006. 683 [RFC4553] Vainshtein, A. and YJ. Stein, "Structure-Agnostic Time 684 Division Multiplexing (TDM) over Packet (SAToP)", 685 RFC 4553, June 2006. 687 [RFC4928] Swallow, G., Bryant, S., and L. Andersson, "Avoiding Equal 688 Cost Multipath Treatment in MPLS Networks", BCP 128, 689 RFC 4928, June 2007. 691 [RFC5085] Nadeau, T. and C. Pignataro, "Pseudowire Virtual Circuit 692 Connectivity Verification (VCCV): A Control Channel for 693 Pseudowires", RFC 5085, December 2007. 695 13.2. Informative References 697 [I-D.ietf-bfd-base] 698 Katz, D. and D. Ward, "Bidirectional Forwarding 699 Detection", draft-ietf-bfd-base-09 (work in progress), 700 February 2009. 702 [I-D.ietf-mpls-lsr-self-test] 703 Swallow, G., "Label Switching Router Self-Test", 704 draft-ietf-mpls-lsr-self-test-07 (work in progress), 705 May 2007. 707 [I-D.ietf-mpls-tp-requirements] 708 Niven-Jenkins, B., Brungard, D., Betts, M., Sprecher, N., 709 and S. Ueno, "MPLS-TP Requirements", 710 draft-ietf-mpls-tp-requirements-10 (work in progress), 711 August 2009. 713 [I-D.kompella-mpls-entropy-label] 714 Kompella, K. and S. Amante, "The Use of Entropy Labels in 715 MPLS Forwarding", draft-kompella-mpls-entropy-label-00 716 (work in progress), July 2008. 718 [I-D.stein-pwe3-pwbonding] 719 Stein, Y., Mendelsohn, I., and R. Insler, "PW Bonding", 720 draft-stein-pwe3-pwbonding-01 (work in progress), 721 November 2008. 723 [RFC3985] Bryant, S. and P. Pate, "Pseudo Wire Emulation Edge-to- 724 Edge (PWE3) Architecture", RFC 3985, March 2005. 726 [RFC4301] Kent, S. and K. Seo, "Security Architecture for the 727 Internet Protocol", RFC 4301, December 2005. 729 [RFC4378] Allan, D. and T. Nadeau, "A Framework for Multi-Protocol 730 Label Switching (MPLS) Operations and Management (OAM)", 731 RFC 4378, February 2006. 733 [RFC5286] Atlas, A. and A. Zinin, "Basic Specification for IP Fast 734 Reroute: Loop-Free Alternates", RFC 5286, September 2008. 736 Authors' Addresses 738 Stewart Bryant (editor) 739 Cisco Systems 740 250 Longwater Ave 741 Reading RG2 6GB 742 United Kingdom 744 Phone: +44-208-824-8828 745 Email: stbryant@cisco.com 747 Clarence Filsfils 748 Cisco Systems 749 Brussels 750 Belgium 752 Email: cfilsfil@cisco.com 754 Ulrich Drafz 755 Deutsche Telekom 756 Muenster 757 Germany 759 Email: Ulrich.Drafz@t-com.net 760 Vach Kompella 761 Alcatel-Lucent 763 Email: Alcatel-Lucent vach.kompella@alcatel-lucent.com 765 Joe Regan 766 Alcatel-Lucent 768 Email: joe.regan@alcatel-lucent.comRegan 770 Shane Amante 771 Level 3 Communications 773 Email: shane@castlepoint.net