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(See Section 2.2 of https://www.ietf.org/id-info/checklist for how to handle the case when there are no actions for IANA.) ** The document seems to lack separate sections for Informative/Normative References. All references will be assumed normative when checking for downward references. ** There are 256 instances of weird spacing in the document. Is it really formatted ragged-right, rather than justified? ** The abstract seems to contain references ([ARCH], [LDP]), which it shouldn't. Please replace those with straight textual mentions of the documents in question. ** The document seems to lack a both a reference to RFC 2119 and the recommended RFC 2119 boilerplate, even if it appears to use RFC 2119 keywords. RFC 2119 keyword, line 96: '... The keywords MUST, MUST NOT, MAY, O...' RFC 2119 keyword, line 97: '... SHALL, SHALL NOT, SHOULD, SHOUL...' RFC 2119 keyword, line 299: '... FR-LSRs MUST use a mechanism that i...' RFC 2119 keyword, line 335: '...ent", the FR-LSR MUST not label switch...' RFC 2119 keyword, line 450: '... label, which MUST be associated on...' (14 more instances...) Miscellaneous warnings: ---------------------------------------------------------------------------- == Line 15 has weird spacing: '...ment is an I...' == Line 16 has weird spacing: '...cuments of t...' == Line 17 has weird spacing: '... groups may ...' == Line 21 has weird spacing: '...-Drafts may ...' == Line 26 has weird spacing: '... please check...' == (251 more instances...) == Using lowercase 'not' together with uppercase 'MUST', 'SHALL', 'SHOULD', or 'RECOMMENDED' is not an accepted usage according to RFC 2119. Please use uppercase 'NOT' together with RFC 2119 keywords (if that is what you mean). Found 'MUST not' in this paragraph: When an ingress FR-LSR determines upon decrementing the MPLS TTL that a particular packet's TTL will expire before the packet reaches the egress of the "non-TTL LSP segment", the FR-LSR MUST not label switch the packet, but rather follow the specifications in [STACK] in an attempt to return an error message to the packet's source. -- The document seems to lack a disclaimer for pre-RFC5378 work, but may have content which was first submitted before 10 November 2008. If you have contacted all the original authors and they are all willing to grant the BCP78 rights to the IETF Trust, then this is fine, and you can ignore this comment. If not, you may need to add the pre-RFC5378 disclaimer. (See the Legal Provisions document at https://trustee.ietf.org/license-info for more information.) -- The document date (August 1997) is 9751 days in the past. Is this intentional? Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) == Missing Reference: 'LDP' is mentioned on line 772, but not defined == Missing Reference: 'ATM' is mentioned on line 777, but not defined -- Possible downref: Non-RFC (?) normative reference: ref. 'MIFR' -- Possible downref: Non-RFC (?) normative reference: ref. 'ARCH' -- Possible downref: Non-RFC (?) normative reference: ref. 'STACK' Summary: 11 errors (**), 0 flaws (~~), 12 warnings (==), 5 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 MPLS Working Group A. Conta (Lucent) 3 INTERNET-DRAFT P. Doolan (Ennovate) 4 A. Malis (Ascend) 5 August 1997 7 Use of Label Switching on Frame Relay Networks 9 Specification 11 draft-ietf-mpls-fr-01.txt 13 Status of this Memo 15 This document is an Internet-Draft. Internet-Drafts are working 16 documents of the Internet Engineering Task Force (IETF), its Areas, 17 and its Working Groups. Note that other groups may also distribute 18 working documents as Internet-Drafts. 20 Internet-Drafts are draft documents valid for a maximum of six 21 months. Internet-Drafts may be updated, replaced, or obsoleted by 22 other documents at any time. It is not appropriate to use Internet- 23 Drafts as reference material or to cite them other than as a "working 24 draft" or "work in progress." 26 To learn the current status of any Internet-Draft, please check the 27 "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow 28 Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe), 29 munnari.oz.au (Pacific Rim), frp.ietf.org (US East Coast), or 30 ftp.isi.edu (US West Coast). 32 Distribution of this memo is unlimited. 34 Abstract 36 This document defines the model and generic mechanisms for 37 Multiprotocol Label Switching on Frame Relay networks. Furthermore, 38 it extends and clarifies portions of the Multiprotocol Label 39 Switching Architecture described in [ARCH] and the Label Distribution 40 Protocol described in [LDP] relativ to Frame Relay Networks. MPLS 41 enables the use of Frame Relay Switches as Label Switching Routers 42 (LSRs). 44 Table of Contents 46 Status of this Memo.........................................1 47 Table of Contents...........................................2 48 1. Introduction................................................3 49 2. Terminology.................................................3 50 3. Special Characteristics of Frame Relay Switches.............4 51 4. Label Encapsulation.........................................5 52 5. Frame Relay Label Switching Processing......................6 53 5.1 Use of DLCIs..............................................6 54 5.2 Homogenous LSPs...........................................7 55 5.3 Heterogenous LSPs.........................................7 56 5.4 Frame Relay Label Switching Loop Prevention and Control...8 57 5.4.1 FR-LSRs Loop Control - MPLS TTL Processing.............8 58 5.4.2 Performing MPLS TTL calculations.......................9 59 5.5 Label Processing by Ingress FR-LSRs......................11 60 5.6 Label Processing by Core FR-LSRs.........................12 61 5.7 Label Processing by Egress FR-LSRs.......................12 62 6 Label Switching Control Component for Frame Relay..........13 63 6.1 Hybrid Switches (Ships in the Night) ...................14 64 7 Label Allocation and Maintenance Procedures ...............14 65 7.1 Edge LSR Behavior........................................14 66 7.2 Efficient use of label space-Merging FR-LSRs.............17 67 8 Data Encapsulation over Frame Relay........................17 68 9 Security Considerations ..................................17 69 10 Acknowledgments ..........................................18 70 11 References ...............................................18 71 12 Authors' Addresses .......................................19 72 1. Introduction 74 The Multiprotocol Label Switching Architecture is described in 75 [ARCH]. It is possible to use Frame Relay switches as Label Switching 76 Routers. Such Frame Relay switches run network layer routing 77 algorithms (such as OSPF, IS-IS, etc.), and their forwarding is based 78 on the results of these routing algorithms. No specific Frame Relay 79 routing is needed. 81 When a Frame Relay switch is used for label switching, the current 82 label, on which forwarding decisions are based, is carried in the 83 DLCI field of the Frame Relay data link layer header of a frame. 84 Additional information carried along with the current label, but not 85 processed by Frame Relay switching, along with other labels, if the 86 packet is multiply labeled, are carried in the generic MPLS 87 encapsulation defined in [STACK]. 89 Frame Relay permanent virtual circuits (PVCs) could be configured to 90 carry label switching based traffic. The DLCIs would be used as MPLS 91 Labels and the Frame Relay switches would become MPLS switches while 92 the MPLS traffic would be encapsulated according to this 93 specification, and would be forwarded based on network layer routing 94 information. 96 The keywords MUST, MUST NOT, MAY, OPTIONAL, REQUIRED, RECOMMENDED, 97 SHALL, SHALL NOT, SHOULD, SHOULD NOT are to be interpreted as 98 defined in RFC 2119. 100 2. Terminology 102 LSR 104 A Label Switching Router (LSR) is a device which implements the 105 label switching control and forwarding components described in 106 [ARCH]. 108 LC-FR 110 A label switching controlled Frame Relay (LC-FR) interface is a 111 Frame Relay interface controlled by the label switching control 112 component. Packets traversing such an interface carry labels in 113 the DLCI field. 115 FR-LSR 117 A FR-LSR is an LSR with one or more LC-FR interfaces which 118 forwards frames onto these interfaces using labels carried in 119 the DLCI field. 121 FR-LSR cloud 123 A FR-LSR cloud is a set of FR-LSRs which are mutually 124 interconnected by LC-FR interfaces. 126 Edge Set 128 The Edge Set of an FR-LSR cloud is the set of LSRs which are 129 connected to the cloud by LC-FR interfaces. 131 Additionally, this document uses terminology from [ARCH]. 133 3. Special characteristics of Frame Relay Switches 135 While the label switching architecture permits considerable 136 flexibility in LSR implementation, a FR-LSR is constrained by the 137 capabilities of the (possibly pre-existing) hardware and the 138 restrictions on such matters as frame format imposed by the 139 Multiprotocol Interconnect over Frame Relay [MIFR], or Frame Relay 140 standards (Q.922, etc). Because of these constraints, some special 141 procedures are required for FR-LSRs. 143 Some of the key features of Frame Relay switches that affects their 144 behavior as LSRs are: 146 - the label swapping function is performed on fields (DLCI) in the 147 frame's Frame Relay data link header; this dictates the size and 148 placement of the label(s) in a packet. The size of the DLCI 149 field can be 10 (default), 17, or 23 bits, and it can span two, 150 or four bytes in the header. 152 - there is generally no capability to perform a `TTL-decrement' 153 function as is performed on IP headers in routers. 155 - congestion control is performed by each node based on parameters 156 that are passed at circuit creation. Flags in the frame headers 157 may be set as a consequence of congestion, or exceeding the 158 contractual parameters of the circuit. 160 - although in a standard switch it may be possible to configure 161 multiple input DLCIs to one output DLCI resulting in a 162 multipoint-to-point circuit, multipoint-to-multipoint VCs are 163 generally not fully supported. 165 This document describes ways of applying label switching to Frame 166 Relay switches which work within these constraints. 168 4. Label Encapsulation 170 By default, all labeled packets should be transmitted with the 171 generic label encapsulation as defined in [STACK], using the frame 172 relay null encapsulation mechanism. The labels implicitly encode the 173 network protocol type, consequently those particular labels cannot be 174 used with other network protocols. Rules regarding the construction 175 of the label stack, and error messages returned to the frame source 176 are also described in [STACK]. 178 0 1 (Octets) 179 +-----------------------+-----------------------+ 180 (Octets)0 | | 181 / Q.922 Address / 182 / (length 'n' equals 2 or 4) / 183 | | 184 +-----------------------+-----------------------+ 185 n | . | 186 / . / 187 / MPLS packet / 188 | . | 189 +-----------------------+-----------------------+ 191 "n" is the length of the Q.922 Address which can be 2 or 4 192 octets. 194 The Q.922 representation of a DLCI (in canonical order - the 195 first bit is stored in the least significant, i.e., the right- 196 most bit of a byte in memory) [CANON]is the following: 198 7 6 5 4 3 2 1 0 (bit order) 199 +-----+-----+-----+-----+-----+-----+-----+-----+ 200 (octet) 0 | DLCI(high order) | 0 | 0 | 201 +-----+-----+-----+-----+-----+-----+-----+-----+ 202 1 | DLCI(low order) | 0 | 0 | 0 | 1 | 203 +-----+-----+-----+-----+-----+-----+-----+-----+ 205 10 bits DLCI 207 7 6 5 4 3 2 1 0 (bit order) 208 +-----+-----+-----+-----+-----+-----+-----+-----+ 209 (octet) 0 | DLCI(high order) | 0 | 0 | 210 +-----+-----+-----+-----+-----+-----+-----+-----+ 211 1 | DLCI | 0 | 0 | 0 | 0 | 212 +-----+-----+-----+-----+-----+-----+-----+-----+ 213 2 | DLCI(low order) | 0 | 214 +-----+-----+-----+-----+-----+-----+-----+-----+ 215 3 | unused (set to 0) | 1 | 1 | 216 +-----+-----+-----+-----+-----+-----+-----+-----+ 218 17 bits DLCI 220 7 6 5 4 3 2 1 0 (bit order) 221 +-----+-----+-----+-----+-----+-----+-----+-----00 222 (octet) 0 | DLCI(high order) | 0 | 0 | 223 +-----+-----+-----+-----+-----+-----+-----+----- 224 1 | DLCI | 0 | 0 | 0 | 0 | 225 +-----+-----+-----+-----+-----+-----+-----+-----+ 226 2 | DLCI | 0 | 227 +-----+-----+-----+-----+-----+-----+-----+-----+ 228 3 | DLCI (low order) | 0 | 1 | 229 +-----+-----+-----+-----+-----+-----+-----+-----+ 231 23 bits DLCI 233 The generic encapsulation contains "n" labels for a label stack of depth 234 "n" [STACK], where the top stack entry carries significant values for 235 the COS, S , and TTL fields [STACK] but not for the label, which is 236 rather carried in the DLCI field of the Frame Relay data link header 237 encoded in Q.922 address format. 239 5. Frame Relay Label Switching Processing 241 5.1 Use of DLCIs 243 Label switching is accomplished by associating labels with routes and 244 using the label value to forward packets, including determining the 245 value of any replacement label. See [ARCH] for further details. In a 246 FR-LSR, the current (top) MPLS label is carried in the DLCI field of 247 the Frame Relay data link layer header of the frame. The top label 248 carries implicitly information about the network protocol type. 250 For two connected FR-LSRs, a full-duplex connection must be available 251 for LDP. The DLCI for the LDP VC is assigned a value by way of 252 configuration, similar to configuring the DLCI used to run IP routing 253 protocols between the switches. 255 With the exception of this configured value, the DLCI values used for 256 MPLS in the two directions of the link may be treated as belonging to 257 two independent spaces, i.e. VCs may be half-duplex, each direction 258 with its own DLCI. In case of DLCI aggregation (DLCI space 259 conservation), half-duplex (unidirectional) VCs are desired, since a 260 "many to few" aggregation is possible in one direction but not in 261 reverse. 263 The allowable ranges of DLCIs are always communicated through LDP. 264 Note that the range of DLCIs used for labels depends on the size of 265 the DLCI field. 267 5.2 Homogenous LSPs 269 If is an LSP, it is possible that LSR1, LSR2, and 270 LSR3 will use the same encoding of the label stack when transmitting 271 packet P from LSR1, to LSR2, and then to LSR3. Such an LSP is 272 homogenous. 274 5.3 Heterogenous LSPs 276 If is an LSP, it is possible that LSR1 will use 277 one encoding of the label stack when transmitting packet P to LSR2, 278 but LSR2 will use a different encoding when transmitting a packet P 279 to LSR3. In general, the MPLS architecture supports LSPs with 280 different label stack encodings on different hops. When a labeled 281 packet is received, the LSR must decode it to determine the current 282 value of the label stack, then must operate on the label stack to 283 determine the new label value of the stack, and then encode the new 284 value appropriately before transmitting the labeled packet to its 285 next hop. 287 Naturally there will be MPLS networks which contain a combination of 288 Frame Relay switches operating as LSRs, and other LSRs which operate 289 using other MPLS encapsulations, such as the MPLS shim header, or ATM 290 encapsulation. In such networks there may be some LSRs which have 291 Frame Relay interfaces as well as "MPLS Shim" interfaces. This is one 292 example of an LSR with different label stack encodings on different 293 hops of the same LSP. Such an LSR may swap off a Frame Relay encoded 294 label on an incoming interface and replace it with a label encoded 295 into an MPLS shim header on the outgoing interface. 297 5.4 Frame Relay Label Switching Loop Prevention and Control 299 FR-LSRs MUST use a mechanism that insures loop free FR- LSPs or LSP 300 FR segments. One such mechanism is the diffusion computation for loop 301 prevention [ARCH]. 303 5.4.1 FR-LSRs Loop Control - MPLS TTL processing 305 The MPLS TTL encoded in the MPLS label stack is a mechanism used to: 307 (a) suppress loops; 309 (b) limit the scope of a packet. 311 When a packet travels along an LSP, it should emerge with the same 312 TTL value that it would have had if it had traversed the same 313 sequence of routers without having been label switched. If the 314 packet travels along a hierarchy of LSPs, the total number of LSR- 315 hops traversed should be reflected in its TTL value when it emerges 316 from the hierarchy of LSPs [ARCH]. 318 The initial value of the MPLS TTL is loaded into a newly pushed label 319 stack entry from the previous TTL value, whether that is from the 320 network layer header when no previous label stack existed, or from a 321 pre-existent lower level label stack entry. 323 A FR-LSR switching same level labeled packets does not decrement the 324 MPLS TTL. A sequence of such FR-LSR is a "non-TTL segment". 326 When a packet emerges from a "non-TTL LSP segment", it should however 327 reflect in the TTL the number of LSR-hops it traversed. In the 328 unicast case, this can be achieved by propagating a meaningful LSP 329 length or LSP segment length to the FR-LSR ingress nodes, enabling 330 the ingress to decrement the TTL value before forwarding packets into 331 a non-TTL LSP segment [ARCH]. 333 When an ingress FR-LSR determines upon decrementing the MPLS TTL that 334 a particular packet's TTL will expire before the packet reaches the 335 egress of the "non-TTL LSP segment", the FR-LSR MUST not label switch 336 the packet, but rather follow the specifications in [STACK] in an 337 attempt to return an error message to the packet's source. 339 In the multicast case, a meaningful LSP length or LSP segment length 340 is propagated to the FR-LSR egress node, enabling the egress to 341 decrement the TTL value before forwarding packets out of the non-TTL 342 LSP segment. 344 5.4.2 Performing MPLS TTL calculations 346 Considering the "incoming TTL" the MPLS TTL of the top of the stack 347 when a labeled packet is received, and the "output TTL" the MPLS TTL 348 of the top of the stack when a packet leaves a node, the relationship 349 between the two is defined as a function of the type of the output 350 interface, and the type of transmit operation done on the output 351 interface (unicast or multicast): 353 output TTL = function (input TTL, output interface type, type of 354 transmit)= 356 = input TTL - funct (output interface type, type of transmit) 358 Considering the symbol"I" for an IP interface, the symbol "G" for a 359 generic MPLS ncapsulating interface, the symbol "A" for a MPLS ATM 360 encapsulating 361 interface, the symbol "F" for a MPLS FR encapsulating interface, and 362 "G_G", "F_G", etc... LSRs with specific input and output interfaces, 363 and also the symbols "O.TTL" and "I.TTL" for the "output" and "input" 364 TTL, the following describes the possible combinations: 366 input,output Unicast 368 ->G_G-> O.TTL = I.TTL - 1 370 ->F_G-> O.TTL = I.TTL - nr. of hops of starting segment (ingress 371 F) 372 ->G_F-> O.TTL = I.TTL - 1 (egress 373 F) 375 ->A_F-> O.TTL = I.TTL - nr. of hops of starting segment (ingress 376 F) 377 ->F_A-> O.TTL = I.TTL - 1 (egress 378 F) 380 ->F_F-> similar to ->A A-> no TTL processing 382 input,output Multicast 384 ->G_G-> O.TTL = I.TTL - 1 386 ->G_F-> O.TTL = I.TTL - 1 (ingress 387 F) 388 ->F_G-> O.TTL = I.TTL - nr. of hops of ending segment (egress 389 F) 391 ->A_F-> O.TTL = I.TTL - 1 (ingress 392 F) 393 ->F_A-> O.TTL = I.TTL - nr. of hops of ending segment (egress 394 F) 396 ->F_F-> similar to ->A A-> no TTL processing 397 Homogenous LSP 399 --->I_F Frame Relay F_I---> 400 hops = 5 | | 401 F_F--->F_F--->F_F--->F_F 402 loop free 403 ip_ttl = n ip_ttl=n-6 404 mpls_ttl = n-5 n-5 406 Heterogenous LSP 408 LSP LSP 409 ingress egress 411 LAN PPP FR ATM PPP FR LAN 413 --->I_G-->G_G-->G_F F_A A_G-->G_F F_G-->G_I---> 414 | / | | | | 415 hops 1 1 | 4 / | 3 | 1 | 3 | 1 1 416 F_F--F_F--F_F A_A--A_A F_F--F_F 418 loop free loop free loop free 419 ip_ttl 420 n n-15 421 mpls_ttl 422 n-1 n-2 n-6 n-9 n-10 n-13 n-14 424 Unicast -- TTL calculated at ingress 426 1 2 3 4 427 o-------o-------o-------o-------o 428 ttl=n-4 / 2 3 429 / 430 hops 1/ 431 / 432 o ttl=n-3 434 Multicast -- TTL calculated at egress 436 o ttl=n-3 437 hops / 438 3/ 439 / ttl=n-4 440 o-------o-------o-------o-------o 441 1 2 3 4 443 5.5 Label Processing by Ingress FR-LSRs 445 When a packet first enters an MPLS domain, the packet is forwarded by 446 normal network layer forwarding operations with the exception that 447 the outgoing encapsulation will include an MPLS label stack [STACK] 448 with at least one entry. The frame relay null encapsulation will 449 carry information about the network layer protocol implicitly in the 450 label, which MUST be associated only with that network protocol. The 451 TTL field in the top label stack entry is filled with the network 452 layer TTL (or hop limit) resulted after network layer forwarding 453 [STACK]. The further FR-LSR processing is similar in both possible 454 cases: 456 (a) the LSP is homogenous -- Frame Relay only -- and the FR-LSR is 457 the ingress. 459 (b) the LSP is heterogeneous -- Frame Relay, PPP, Ethernet, ATM, 460 etc... segments form the LSP -- and the FR-LSR is the ingress into a 461 Frame Relay 462 segment. 464 For unicast packets, the MPLS TTL SHOULD be decremented with the 465 number of hops of the Frame Relay LSP (homogenous), or Frame Relay 466 segment of the LSP (heterogeneous). An LDP constructing the LSP 467 SHOULD pass meaningful information to the ingress FR-LSR regarding 468 the number of hops of the "non-TTL segment". 470 For multicast packets, the MPLS TTL SHOULD be decremented by 1. An 471 LDP constructing the LSP SHOULD pass meaningful information to the 472 egress FR-LSR regarding the number of hops of the "non-TTL segment". 474 Next, the MPLS encapsulated packet is passed down to the Frame Relay 475 data link driver with the top label as output DLCI. The Frame Relay 476 frame carrying the MPLS encapsulated packet is forwarded onto the 477 Frame Relay VC to the next LSR. 479 5.6 Label Processing by Core FR-LSRs 481 In a FR-LSR, the current (top) MPLS label is carried in the DLCI 482 field of the Frame Relay data link layer header of the frame. Just as 483 in conventional Frame Relay, for a frame arriving at an interface, 484 the DLCI carried by the Frame Relay data link header is looked up in 485 the DLCI Information Base, replaced with the correspondent output 486 DLCI, and transmitted on the outgoing interface (forwarded to the 487 next hop node). 489 The current label information is also carried in the top of the label 490 stack. In the top level entry, all fields except the label 491 information, which is carried and switched in the Frame Relay frame 492 data link-layer header, are of current significance. 494 5.7 Label Processing by Egress FR-LSRs 496 When reaching the end of a Frame Relay LSP, the FR-LSR pops the label 497 stack [ARCH]. If the label popped is the last label, it is necessary 498 to determine the particular network layer protocol which is being 499 carried. The label stack carries no explicit information to identify 500 the network layer protocol. This must be inferred from the value of 501 the label which is popped from the stack. 503 If the label popped is not the last label, the previous top level 504 MPLS TTL is propagated to the new top label stack entry. 506 If the FR-LSR is the egress switch of a Frame Relay segment of a 507 hybrid LSP, and the end of the Frame Relay segment is not the end of 508 the LSP, the MPLS packet will be processed for forwarding onto the 509 next segment of the LSP based on the information held in the Next Hop 510 Label Forwarding Entry (NHLFE) [ARCH]. The output label is set to the 511 value from the NHLFE, and the MPLS TTL is decremented by the 512 appropriate value depending the type of the output interface and the 513 type of transmit operation (see secion 6.3). Further, the MPLS packet 514 is forwarded according to the MPLS specifications for the particular 515 link of the next segment of the LSP. 517 For unicast packets, the MPLS TTL SHOULD be decremented by one if the 518 output interface is a generic one, or with the number of hops of the 519 next ATM segment of the LSP (heterogeneous), if the output interface 520 is an ATM (non-TTL) interface. 522 For multicast packets, the MPLS TTL SHOULD be decremented by the 523 number of hops of the FR segment being exited. An LDP constructing 524 the LSP SHOULD pass meaningful information to the egress FR-LSR 525 regarding the number of hops of the FR "non-TTL segment". 527 6. Label Switching Control Component for Frame Relay 529 To support label switching a Frame Relay Switch MUST implement the 530 control component of label switching, which consists primarily of 531 label allocation and maintenance procedures. Label binding 532 information MAY be communicated by several mechanisms, one of which 533 is the Label Distribution Protocol (LDP) [LDP]. 535 Since the label switching control component uses information learned 536 directly from network layer routing protocols, this implies that the 537 switch MUST participate as a peer in these protocols (e.g., OSPF, 538 IS-IS). 540 In some cases, LSRs may use other protocols (e.g. RSVP, PIM, BGP) to 541 distribute label bindings. In these cases, a Frame Relay LSR should 542 participate in these protocols. 544 In the case where Frame Relay circuits are established via LDP, or 545 RSVP, or others, with no involvement from traditional Frame Relay 546 mechanisms, it is assumed that circuit establishing contractual 547 information such as input/output maximum frame size, 548 incoming/outgoing requested/agreed throughput, incoming/outgoing 549 acceptable throughput, incoming/outgoing burst size, 550 incoming/outgoing frame rate, used in transmitting, and congestion 551 control MAY be passed to the FR-LSRs through RSVP, or can be 552 statically configured. It is also assumed that congestion control and 553 frame header flagging as a consequence of congestion, would be done 554 by the FR-LSRs in a similar fashion as for traditional Frame Relay 555 circuits. With the goal of emulating a best-effort router as default, 556 the default VC parameters, in the absence of LDP, RSVP, or other 557 mechanisms participation to setting such parameters, should be zero 558 CIR, so that input policing will set the DE bit in incoming frames, 559 but no frames are dropped.. 561 Control and state information for the circuits based on MPLS MAY be 562 communicated through LDP. 564 Support of label switching on a Frame Relay switch requires 565 conformance only to FRF 1.1 (framing, bit-stuffing, headers, FCS) 566 except for section 2.3 (PVC control signaling procedures, aka LMI). 567 Q.933 signaling for PVCs and/or SVCs is not required. PVC and/or SVC 568 signaling may be used for non-MPLS (standard Frame Relay) PVCs and/or 569 SVCs when both are running on the same interface as MPLS, as 570 discussed in the next section. 572 6.1 Hybrid Switches (Ships in the Night) 574 The existence of the label switching control component on a Frame 575 Relay switch does not preclude the ability to support the Frame Relay 576 control component defined by the ITU and Frame Relay Forum on the 577 same switch and the same interfaces (NICs). The two control 578 components, label switching and those defined by ITU/Frame Relay 579 Forum, would operate independently. 581 Definition of how such a device operates is beyond the scope of this 582 document. However, only a small amount of information needs to be 583 consistent between the two control components, such as the portions 584 of the DLCI space which are available to each component. 586 7. Label Allocation and Maintenance Procedures 588 A possible scenario for the label allocation and maintenance for FR- 589 LSRs is "downstream-on-demand" [ARCH] as it follows (note that this 590 applies to hop-by-hop routed traffic): 592 7.1 Edge LSR Behavior 594 Consider a member of the Edge Set of a FR-LSR cloud. Assume that, as 595 a result of its routing calculations, it selects a FR-LSR as the next 596 hop of a certain route (FEC), and that the next hop is reachable via 597 a LC-Frame Relay interface. Assume that the next-hop FR-LSR is an 598 "LDP-peer" [ARCH][LDP]. The Edge LSR sends an LDP "request" message 599 for a label binding from the next hop, downstream LSR. When the Edge 600 LSR receives in response from the downstream LSR the label binding 601 information in an LDP "mapping" message, the label is stored in the 602 Label Information Base (LIB) as an outgoing label for that FEC. The 603 "mapping" message may contain the "hop count" object, which 604 represents the number of hops a packet will take to cross the FR-LSR 605 cloud to the Egress FR-LSR when using this label. This information 606 may be stored for TTL calculation. Once this is done, the LSR may use 607 MPLS forwarding to transmit packets in that FEC. 609 When a member of the Edge Set of the FR-LSR cloud receives an LDP 610 "request" message from a FR-LSR for a FEC, it means it is the 611 Egress-FR-LSR. It allocates a label, creates a new entry in its Label 612 Information Base (LIB), places that label in the incoming label 613 component of the entry, and returns (via LDP) a "mapping" message 614 containing the allocated label back upstream to the LDP peer that 615 originated the request. The "mapping" message contains the "hop 616 count" object value set to 1. 618 When a routing calculation causes an Edge LSR to change the next hop 619 for a route, and the former next hop was in the FR-LSR cloud, the 620 Edge LSR should notify the former next hop (via an LDP "release" 621 message) that the label binding associated with the route is no 622 longer needed. 624 When a Frame Relay-LSR receives an LDP "request" message for a 625 certain route (FEC) from an LDP peer connected to the FR-LSR over a 626 LC-FR interface, the FR-LSR takes the following actions: 628 - it allocates a label, creates a new entry in its Label 629 Information Base (LIB), and places that label in the incoming 630 label component of the entry; 632 - it propagates the "request", by sending an LDP "request" 633 message to the next hop LSR, dowsnstream for that route 634 (FEC); 636 In the "ordered control" mode [ARCH], the FR-LSR will wait for its 637 "request" to be responded from downstream with a "mapping" message 638 before returning the "mapping" upstream in response to a "request" 639 ("ordered control" approach [ARCH]). In this case, the FR-LSR 640 increments the hop count it received from downstream and uses this 641 value in the "mapping" it returns upstream. 643 Alternatively, the FR-LSR may return the binding upstream without 644 waiting for a binding from downstream ("independent control" approach 645 [ARCH]). In this case, it uses a reserved value for hop count in the 646 "mapping", indicating that it is 'unknown'. The correct value for hop 647 count will be returned later, as described below. 649 Since both the "ordered" and "independent" control has advantages and 650 disadvantages, this is left as an implementation, or configuration 651 choice. 653 Once the FR-LSR receives in response the label binding in an LDP 654 "mapping" message from the next hop, it places the label into the 655 outgoing label component of the LIB entry. 657 Note that a FR-LSR, or a member of the edge set of a FR-LSR cloud, 658 may receive multiple binding requests for the same route (FEC) from 659 the same FR-LSR. It must generate a new "mapping" for each "request" 660 (assuming adequate resources to do so), and retain any existing 661 mapping(s). For each "request" received, a FR-LSR should also 662 generate a new binding "request" toward the next hop for the route 663 (FEC). 665 When a routing calculation causes a FR-LSR to change the next hop for 666 a route (FEC), the FR-LSR should notify the former next hop (via an 667 LDP "release" message) that the label binding associated with the 668 route is no longer needed. 670 When a LSR receives a notification that a particular label binding is 671 no longer needed, the LSR may deallocate the label associated with 672 the binding, and destroy the binding. This mode is the "conservative 673 label retention mode" [ARCH]. In the case where a FR-LSR receives 674 such notification and destroys the binding, it should notify the next 675 hop for the route that the label binding is no longer needed. If a 676 LSR does not destroy the binding (the FR-LSR is configured in 677 "liberal label retention mode" [ARCH]), it may re-use the binding 678 only if it receives a request for the same route with the same hop 679 count as the request that originally caused the binding to be 680 created. 682 When a route changes, the label bindings are re-established from the 683 point where the route diverges from the previous route. LSRs 684 upstream of that point are (with one exception, noted below) 685 oblivious to the change. Whenever a LSR changes its next hop for a 686 particular route, if the new next hop is a FR-LSR or a member of the 687 edge set reachable via a LC-FR interface, then for each entry in its 688 LIB associated with the route the LSR should request (via LDP) a 689 binding from the new next hop. 691 When a FR-LSR receives a label binding from a downstream neighbor, it 692 may already have provided a corresponding label binding for this 693 route to an upstream neighbor, either because it is using 694 "independent control" or because the new binding from downstream is 695 the result of a routing change. In this case, it should extract the 696 hop count from the new binding and increment it by one. If the new 697 hop count is different from that which was previously conveyed to the 698 upstream neighbor (including the case where the upstream neighbor was 699 given the value 'unknown') the FR-LSR must notify the upstream 700 neighbor of the change. Each FR-LSR in turn increments the hop count 701 and passes it upstream until it reaches the ingress Edge LSR. 703 Whenever a FR-LSR originates a label binding request to its next hop 704 LSR as a result of receiving a label binding request from another 705 (upstream) LSR, and the request to the next hop LSR is not satisfied, 706 the FR-LSR should destroy the binding created in response to the 707 received request, and notify the requester (via an LDP "withdraw" 708 message). 710 When a LSR determines that it has lost its LDP session with another 711 LSR, the following actions are taken: 713 - MUST discard any binding information learned via this 714 connection; 716 - For any label bindings that were created as a result of 717 receiving label binding requests from the peer, the LSR may 718 destroy these bindings (and deallocate labels associated 719 with these binding). 721 7.2 Efficient use of label space - Merging FR-LSRs 723 The above discussion assumes that an edge LSR will request one label 724 for each prefix in its routing table that has a next hop in the FR- 725 LSR cloud. In fact, it is possible to significantly reduce the number 726 of labels needed by having the edge LSR request instead one label for 727 several routes. Use of many-to-one mappings between routes (address 728 prefixes) and labels using the notion of Forwarding Equivalence 729 Classes (as described in [ARCH]) provides a mechanism to conserve the 730 number of labels. 732 Note that conserving label space may be restricted in case the frame 733 traffic requires Frame Relay fragmentation. The issue is that Frame 734 Relay fragments must be transmitted in sequence, i.e. fragments of 735 distinct frames must not be interleaved. If the fragmenting FR-LSR 736 ensures the transmission in sequence of all fragments of a frame, 737 without interleaving with fragments of other frames, then label 738 conservation (aggregation) can be performed. 740 In the case where the label space is to be conserved, it is desirable 741 to use half-duplex (unidirectional) VCs, since a "many to few" 742 aggregation is possible in one direction but not in reverse. 744 8. Data Encapsulation over Frame Relay 746 The IP packets transmitted over VCs set up by LDP (or other 747 mechanisms) MUST be encapsulated according to section 4.1 of [MIFR]. 749 9. Security Considerations 751 This section looks at the security aspects of: 753 (a) frame traffic 755 (b) label distribution. 757 MPLS encapsulation has no effect on authenticated or encrypted 758 network layer packets, that is IP packets that are authenticated or 759 encrypted will incur no change. 761 The MPLS protocol has no mechanisms of its own to protect against 762 misdirection of packets or the impersonation of an LSR by accident or 763 malicious intent. 765 Altering by accident or forgery an existent label in the DLCI field 766 of the Frame Relay data link layer header of a frame or one or more 767 fields in a potentially following label stack affects the forwarding 768 of that frame. 770 The label distribution mechanism can be secured by applying the 771 appropriate level of security to the underlying protocol carrying 772 label information - authentication or encryption - see [LDP]. 774 10. Acknowledgments 776 The initial version of this document was derived from the Label 777 Switching over ATM document [ATM]. 779 Thanks for the extensive reviewing and constructive comments from (in 780 alphabetical order) Dan Harrington, Milan Merhar, Martin Mueller. 781 Also thanks to George Swallow for the suggestion to use null 782 encapsulation, and to Eric Gray for his reviewing. 784 11. References 786 [MIFR] T. Bradley, C. Brown, A. Malis "Multiprotocol Interconnect 787 over Frame Relay" 789 [ARCH] "Multi-Protocol Label Switching Architecture", Internet-Draft, 790 "draft-ietf-mpls-02.txt" by E. Rosen, R. Callon, A. Vishwanathan. 792 [LDP]"Label Distribution Protocol", Internet-Draft, "draft-ietf- 793 mpls-ldp-00.txt" by Anderson, Doolan, Feldman, Fredette, Thomas. 795 [STACK] "Label Switching: Label Stack Encodings", Internet-Draft, 796 "draft-mpls-label-encaps-02.txt" by Rosen et al. 798 [ATM]"Use of Label Switching with ATM", Internet-Draft, "draft- 799 davie-mpls-atm-01.txt" by Davie et al. 801 12.Authors' Addresses 803 Alex Conta 804 Lucent Technologies Inc. 805 300 Baker Ave, Suite 100 806 Concord, MA 01742 807 +1-978-287-2842 808 E-mail: aconta@lucent.com 810 Paul Doolan 811 Ennovate Networks 812 330 Codman Hill Rd 813 Boxborough MA 01719 814 +1-978-263-2002 815 E-mail: pdoolan@ennovatenetworks.com 817 Andrew Malis 818 Ascend Communications, Inc 819 1 Robbins Rd 820 Westford, MA 01886 821 +1-978-952-7414 822 E-mail: malis@ascend.com