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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 an Authors' Addresses Section. ** The document seems to lack separate sections for Informative/Normative References. All references will be assumed normative when checking for downward references. ** There are 304 instances of weird spacing in the document. Is it really formatted ragged-right, rather than justified? ** There are 28 instances of too long lines in the document, the longest one being 1 character in excess of 72. ** 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 98: '... The keywords MUST, MUST NOT, MAY, O...' RFC 2119 keyword, line 99: '... SHALL, SHALL NOT, SHOULD, SHOUL...' RFC 2119 keyword, line 299: '... VC merging MUST be communicated t...' RFC 2119 keyword, line 335: '... FR-LSRs SHOULD operate on loop fr...' RFC 2119 keyword, line 336: '...efore, FR-LSRs SHOULD use loop detec...' (22 more instances...) Miscellaneous warnings: ---------------------------------------------------------------------------- == Line 37 has weird spacing: '... This docum...' == Line 39 has weird spacing: '... it extends...' == Line 42 has weird spacing: '...enables the ...' == Line 76 has weird spacing: '...tecture is d...' == Line 78 has weird spacing: '... Relay switc...' == (299 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 (1 November 2000) is 8549 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) -- Possible downref: Non-RFC (?) normative reference: ref. 'ARCH' -- Possible downref: Non-RFC (?) normative reference: ref. 'LDP' -- Possible downref: Non-RFC (?) normative reference: ref. 'STACK' -- Possible downref: Non-RFC (?) normative reference: ref. 'ATM' -- Possible downref: Non-RFC (?) normative reference: ref. 'ITU' -- Possible downref: Non-RFC (?) normative reference: ref. 'FRF' Summary: 8 errors (**), 0 flaws (~~), 9 warnings (==), 8 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 MPLS Working Group A. Conta (3COM) 3 INTERNET-DRAFT P. Doolan (Ennovate) 4 A. Malis (Lucent) 5 1 May 2000 6 Expires 1 November 2000 8 Use of Label Switching on Frame Relay Networks 9 Specification 11 draft-ietf-mpls-fr-04.txt 13 Status of this Memo 15 This document is an Internet-Draft and is in full conformance 16 with all provisions of Section 10 of RFC2026. 18 Internet-Drafts are working documents of the Internet Engineering 19 Task Force (IETF), its areas, and its working groups. Note that 20 other groups may also distribute working documents as 21 Internet-Drafts. 23 Internet-Drafts are draft documents valid for a maximum of six 24 months and may be updated, replaced, or obsoleted by other 25 documents at any time. It is inappropriate to use Internet- 26 Drafts as reference material or to cite them other than as 27 "work in progress." 29 The list of current Internet-Drafts can be accessed at 30 http://www.ietf.org/ietf/1id-abstracts.txt 32 The list of Internet-Draft Shadow Directories can be accessed at 33 http://www.ietf.org/shadow.html. 35 Abstract 37 This document defines the model and generic mechanisms for 38 Multiprotocol Label Switching on Frame Relay networks. Furthermore, 39 it extends and clarifies portions of the Multiprotocol Label 40 Switching Architecture described in [ARCH] and the Label Distribution 41 Protocol (LDP) described in [LDP] relative to Frame Relay Networks. 42 MPLS enables the use of Frame Relay Switches as Label Switching 43 Routers (LSRs). 45 Table of Contents 47 Status of this Memo.........................................1 48 Table of Contents...........................................2 49 1. Introduction................................................3 50 2. Terminology.................................................3 51 3. Special Characteristics of Frame Relay Switches.............5 52 4. Label Encapsulation.........................................5 53 5. Frame Relay Label Switching Processing......................7 54 5.1 Use of DLCIs..............................................7 55 5.2 Homogeneous LSPs..........................................8 56 5.3 Heterogeneous LSPs........................................8 57 5.4 Frame Relay Label Switching Loop Prevention and Control...8 58 5.4.1 FR-LSRs Loop Control - MPLS TTL Processing.............9 59 5.4.2 Performing MPLS TTL calculations......................10 60 5.5 Label Processing by Ingress FR-LSRs......................13 61 5.6 Label Processing by Core FR-LSRs.........................14 62 5.7 Label Processing by Egress FR-LSRs.......................14 63 6 Label Switching Control Component for Frame Relay..........15 64 6.1 Hybrid Switches (Ships in the Night) ...................16 65 7 Label Allocation and Maintenance Procedures ...............16 66 7.1 Edge LSR Behavior........................................16 67 7.2 Efficient use of label space-Merging FR-LSRs.............19 68 7.3 LDP message fields specific to Frame Relay...............20 69 8 Security Considerations ..................................22 70 9 Acknowledgments ..........................................23 71 10 References ...............................................23 72 11 Authors' Addresses .......................................24 73 Appendix A - changes since previous versions..................25 74 1. Introduction 76 The Multiprotocol Label Switching Architecture is described in 77 [ARCH]. It is possible to use Frame Relay switches as Label Switching 78 Routers. Such Frame Relay switches run network layer routing 79 algorithms (such as OSPF, IS-IS, etc.), and their forwarding is based 80 on the results of these routing algorithms. No specific Frame Relay 81 routing is needed. 83 When a Frame Relay switch is used for label switching, the top 84 (current) label, on which forwarding decisions are based, is carried 85 in the DLCI field of the Frame Relay data link layer header of a 86 frame. Additional information carried along with the top (current) 87 label, but not processed by Frame Relay switching, along with other 88 labels, if the packet is multiply labeled, are carried in the generic 89 MPLS encapsulation defined in [STACK]. 91 Frame Relay permanent virtual circuits (PVCs) could be configured to 92 carry label switching based traffic. The DLCIs would be used as MPLS 93 Labels and the Frame Relay switches would become Frame Relay Label 94 Switching Routers, while the MPLS traffic would be encapsulated 95 according to this specification, and would be forwarded based on 96 network layer routing information. 98 The keywords MUST, MUST NOT, MAY, OPTIONAL, REQUIRED, RECOMMENDED, 99 SHALL, SHALL NOT, SHOULD, SHOULD NOT are to be interpreted as 100 defined in RFC 2119. 102 This document is a companion document to [STACK] and [ATM]. 104 2. Terminology 106 LSR 108 A Label Switching Router (LSR) is a device which implements the 109 label switching control and forwarding components described in 110 [ARCH]. 112 LC-FR 114 A label switching controlled Frame Relay (LC-FR) interface is a 115 Frame Relay interface controlled by the label switching control 116 component. Packets traversing such an interface carry labels in 117 the DLCI field. 119 FR-LSR 120 A FR-LSR is an LSR with one or more LC-FR interfaces which 121 forwards frames between two such interfaces using labels carried 122 in the DLCI field. 124 FR-LSR domain 126 A FR-LSR domain is a set of FR-LSRs, which are mutually 127 interconnected by LC-FR interfaces. 129 Edge Set 131 The Edge Set of an FR-LSR domain is the set of LSRs, which are 132 connected to the domain by LC-FR interfaces. 134 Forwarding Encapsulation 136 The Forwarding Encapsulation is the type of MPLS encapsulation 137 (Frame Relay, ATM, Generic) of a packet that determines the 138 packet's MPLS forwarding, or the network layer encapsulation if 139 that packet is forwarded based on the network layer (IP, 140 etc...)header. 142 Input Encapsulation 144 The Input Encapsulation is the type of MPLS encapsulation (Frame 145 Relay, ATM, Generic) of a packet when that packet is received on 146 an LSR's interface, or the network layer (IP, 147 etc...)encapsulation if that packet has no MPLS encapsulation. 149 Output Encapsulation 151 The Output Encapsulation is the type of MPLS encapsulation 152 (Frame Relay, ATM, Generic) of a packet when that packet is 153 transmitted on an LSR's interface, or the network layer (IP, 154 etc...)encapsulation if that packet has no MPLS encapsulation. 156 Input TTL 158 The Input TTL is the MPLS TTL of the top of the stack when a 159 labeled packet is received on an LSR interface, or the network 160 layer (IP) TTL if the packet is not labeled. 162 Output TTL 164 The Output TTL is the MPLS TTL of the top of the stack when a 165 labeled packet is transmitted on an LSR interface, or the 166 network layer (IP) TTL if the packet is not labeled. 168 Additionally, this document uses terminology from [ARCH]. 170 3. Special characteristics of Frame Relay Switches 172 While the label switching architecture permits considerable 173 flexibility in LSR implementation, a FR-LSR is constrained by the 174 capabilities of the (possibly pre-existing) hardware and the 175 restrictions on such matters as frame format imposed by the 176 Multiprotocol Interconnect over Frame Relay [MIFR], or Frame Relay 177 standards [FRF], etc.... Because of these constraints, some special 178 procedures are required for FR-LSRs. 180 Some of the key features of Frame Relay switches that affect their 181 behavior as LSRs are: 183 - the label swapping function is performed on fields (DLCI) in the 184 frame's Frame Relay data link header; this dictates the size and 185 placement of the label(s) in a packet. The size of the DLCI 186 field can be 10 (default), 17, or 23 bits, and it can span two, 187 or four bytes in the header. 189 - there is generally no capability to perform a `TTL-decrement' 190 function as is performed on IP headers in routers. 192 - congestion control is performed by each node based on parameters 193 that are passed at circuit creation. Flags in the frame headers 194 may be set as a consequence of congestion, or exceeding the 195 contractual parameters of the circuit. 197 - although in a standard switch it may be possible to configure 198 multiple input DLCIs to one output DLCI resulting in a 199 multipoint-to-point circuit, multipoint-to-multipoint VCs are 200 generally not fully supported. 202 This document describes ways of applying label switching to Frame 203 Relay switches, which work within these constraints. 205 4. Label Encapsulation 207 By default, all labeled packets should be transmitted with the 208 generic label encapsulation as defined in [STACK], using the frame 209 relay null encapsulation mechanism: 211 0 1 (Octets) 212 +-----------------------+-----------------------+ 213 (Octets)0 | | 214 / Q.922 Address / 215 / (length 'n' equals 2 or 4) / 216 | | 217 +-----------------------+-----------------------+ 218 n | . | 219 / . / 220 / MPLS packet / 221 | . | 222 +-----------------------+-----------------------+ 224 "n" is the length of the Q.922 Address which can be 2 or 4 225 octets. 227 The Q.922 [ITU] representation of a DLCI (in canonical order - 228 the first bit is stored in the least significant, i.e., the 229 right-most bit of a byte in memory) [CANON]is the following: 231 7 6 5 4 3 2 1 0 (bit order) 232 +-----+-----+-----+-----+-----+-----+-----+-----+ 233 (octet) 0 | DLCI(high order) | 0 | 0 | 234 +-----+-----+-----+-----+-----+-----+-----+-----+ 235 1 | DLCI(low order) | 0 | 0 | 0 | 1 | 236 +-----+-----+-----+-----+-----+-----+-----+-----+ 238 10 bits DLCI 240 7 6 5 4 3 2 1 0 (bit order) 241 +-----+-----+-----+-----+-----+-----+-----+-----+ 242 (octet) 0 | DLCI(high order) | 0 | 0 | 243 +-----+-----+-----+-----+-----+-----+-----+-----+ 244 1 | DLCI | 0 | 0 | 0 | 0 | 245 +-----+-----+-----+-----+-----+-----+-----+-----+ 246 2 | DLCI(low order) | 0 | 247 +-----+-----+-----+-----+-----+-----+-----+-----+ 248 3 | unused (set to 0) | 1 | 1 | 249 +-----+-----+-----+-----+-----+-----+-----+-----+ 251 17 bits DLCI 252 7 6 5 4 3 2 1 0 (bit order) 253 +-----+-----+-----+-----+-----+-----+-----+-----00 254 (octet) 0 | DLCI(high order) | 0 | 0 | 255 +-----+-----+-----+-----+-----+-----+-----+----- 256 1 | DLCI | 0 | 0 | 0 | 0 | 257 +-----+-----+-----+-----+-----+-----+-----+-----+ 258 2 | DLCI | 0 | 259 +-----+-----+-----+-----+-----+-----+-----+-----+ 260 3 | DLCI (low order) | 0 | 1 | 261 +-----+-----+-----+-----+-----+-----+-----+-----+ 263 23 bits DLCI 265 The use of the frame relay null encapsulation implies that labels 266 implicitly encode the network protocol type. 268 Rules regarding the construction of the label stack, and error 269 messages returned to the frame source are also described in [STACK]. 271 The generic encapsulation contains "n" labels for a label stack of 272 depth "n" [STACK], where the top stack entry carries significant 273 values for the EXP, S , and TTL fields [STACK] but not for the label, 274 which is rather carried in the DLCI field of the Frame Relay data 275 link header encoded in Q.922 [ITU] address format. 277 5. Frame Relay Label Switching Processing 279 5.1 Use of DLCIs 281 Label switching is accomplished by associating labels with routes and 282 using the label value to forward packets, including determining the 283 value of any replacement label. See [ARCH] for further details. In a 284 FR-LSR, the top (current) MPLS label is carried in the DLCI field of 285 the Frame Relay data link layer header of the frame. The top label 286 carries implicitly information about the network protocol type. 288 For two connected FR-LSRs, a full-duplex connection must be available 289 for LDP. The DLCI for the LDP VC is assigned a value by way of 290 configuration, similar to configuring the DLCI used to run IP routing 291 protocols between the switches. 293 With the exception of this configured value, the DLCI values used for 294 MPLS in the two directions of the link may be treated as belonging to 295 two independent spaces, i.e. VCs may be half-duplex, each direction 296 with its own DLCI. 298 The allowable ranges of DLCIs, the size of DLCIs, and the support for 299 VC merging MUST be communicated through LDP messages. Note that the 300 range of DLCIs used for labels depends on the size of the DLCI field. 302 5.2 Homogeneous LSPs 304 If is an LSP, it is possible that LSR1, LSR2, and 305 LSR3 will use the same encoding of the label stack when transmitting 306 packet P from LSR1, to LSR2, and then to LSR3. Such an LSP is 307 homogeneous. 309 5.3 Heterogeneous LSPs 311 If is an LSP, it is possible that LSR1 will use 312 one encoding of the label stack when transmitting packet P to LSR2, 313 but LSR2 will use a different encoding when transmitting a packet P 314 to LSR3. In general, the MPLS architecture supports LSPs with 315 different label stack encodings on different hops. When a labeled 316 packet is received, the LSR must decode it to determine the current 317 value of the label stack, then must operate on the label stack to 318 determine the new label value of the stack, and then encode the new 319 value appropriately before transmitting the labeled packet to its 320 next hop. 322 Naturally there will be MPLS networks which contain a combination of 323 Frame Relay switches operating as LSRs, and other LSRs, which operate 324 using other MPLS encapsulations, such as the Generic (MPLS shim 325 header), or ATM encapsulation. In such networks there may be some 326 LSRs, which have Frame Relay interfaces as well as MPLS Generic 327 ("MPLS Shim") interfaces. This is one example of an LSR with 328 different label stack encodings on different hops of the same LSP. 329 Such an LSR may swap off a Frame Relay encoded label on an incoming 330 interface and replace it with a label encoded into a Generic MPLS 331 (MPLS shim) header on the outgoing interface. 333 5.4 Frame Relay Label Switching Loop Prevention and Control 335 FR-LSRs SHOULD operate on loop free FR-LSPs or LSP Frame Relay 336 segments. Therefore, FR-LSRs SHOULD use loop detection and MAY use 337 loop prevention mechanisms as described in [ARCH], and [LDP]. 339 5.4.1 FR-LSRs Loop Control - MPLS TTL processing 341 The MPLS TTL encoded in the MPLS label stack is a mechanism used to: 343 (a) suppress loops; 345 (b) limit the scope of a packet. 347 When a packet travels along an LSP, it should emerge with the same 348 TTL value that it would have had if it had traversed the same 349 sequence of routers without having been label switched. If the 350 packet travels along a hierarchy of LSPs, the total number of LSR- 351 hops traversed should be reflected in its TTL value when it emerges 352 from the hierarchy of LSPs [ARCH]. 354 The initial value of the MPLS TTL is loaded into a newly pushed label 355 stack entry from the previous TTL value, whether that is from the 356 network layer header when no previous label stack existed, or from a 357 pre-existent lower level label stack entry. 359 A FR-LSR switching same level labeled packets does not decrement the 360 MPLS TTL. A sequence of such FR-LSR is a "non-TTL segment". 362 When a packet emerges from a "non-TTL LSP segment", it should however 363 reflect in the TTL the number of LSR-hops it traversed. In the 364 unicast case, this can be achieved by propagating a meaningful LSP 365 length or LSP Frame Relay segment length to the FR-LSR ingress nodes, 366 enabling the ingress to decrement the TTL value before forwarding 367 packets into a non-TTL LSP segment [ARCH]. 369 When an ingress FR-LSR determines upon decrementing the MPLS TTL that 370 a particular packet's TTL will expire before the packet reaches the 371 egress of the "non-TTL LSP segment", the FR-LSR MUST not label switch 372 the packet, but rather follow the specifications in [STACK] in an 373 attempt to return an error message to the packet's source: 375 - it treats the packet as an expired packet and return an ICMP 376 message to its source. 378 - it forwards the packet, as an unlabeled packet, with a TTL 379 that reflects the IP (network layer) forwarding. 381 If the incoming TTL is 1, only the first option applies. 383 In the multicast case, a meaningful LSP length or LSP segment length 384 is propagated to the FR-LSR egress node, enabling the egress to 385 decrement the TTL value before forwarding packets out of the non-TTL 386 LSP segment. 388 5.4.2 Performing MPLS TTL calculations 390 The calculation applied to the "input TTL" that yields the "output 391 TTL" depends on (i)the "input encapsulation", (ii)the "forwarding 392 encapsulation", and (iii)the "output encapsulation". The 393 relationship among (i),(ii), and (iii), can be defined as a function 394 "D" of "input encapsulation" (ie), "forwarding encapsulation" (fe), 395 and "output encapsulation" (oe). Subsequently the calculation applied 396 to the "input TTL" to yield the "output TTL" can be described as: 398 output TTL = input TTL - D(ie, fe, oe) 400 or in a brief notation: 402 output TTL = input TTL - d 404 where "d" has three possible values: "0","1", or "the number of hops 405 of the LSP segment": 407 For unicast transmission: 409 +================+=================+=================+=================+ 410 | | Type of | Type of | Type of | 411 | d | Input | Forwarding | Output | 412 | | Encapsulation | Encapsulation | Encapsulation | 413 +================+=================+=================+=================+ 414 | 0 | Frame Relay | Frame Relay | Frame Relay | 415 +----------------+-----------------+-----------------+-----------------+ 416 | 1 | any | Generic MPLS | Generic MPLS | 417 +----------------+-----------------+-----------------+-----------------+ 418 | number of hops | | Generic MPLS | | 419 | of | any | or | Frame Relay | 420 | LSP segment | |IP(network layer)| | 421 +================+=================+=================+=================+ 423 The "number of hops of the LSP segment" is the value of the "hop 424 count" that is attached with the label used when the packet is 425 forwarded, if LDP [LDP] has provided such a "hop count" value when it 426 distributed the label for the LSP, that is the LDP message had a "hop 427 count object". If LDP didn't provide a "hop count", or it provided an 428 "unknown" value, the default value of the "number of hops of the 429 segment" is 1. 431 When sending a label binding upstream, the "hop count" associated 432 with the corresponding binding from downstream, if different than the 433 "unknown" value, MUST be incremented by 1, and the result transmitted 434 upstream as the hop count associated with the new binding (the 435 "unknown" value is transmitted unchanged). If the new "hop count" 436 value exceeds the "maximum" value, the FR-LSR MUST NOT pass the 437 binding upstream, but instead MUST send an error upstream 438 [LDP][ARCH]. 440 For multicast transmission: 442 +================+=================+=================+=================+ 443 | | Type of | Type of | Type of | 444 | d | Input | Forwarding | Output | 445 | | Encapsulation | Encapsulation | Encapsulation | 446 +================+=================+=================+=================+ 447 | 0 | Frame Relay | Frame Relay | Frame Relay | 448 +----------------+-----------------+-----------------+-----------------+ 449 | | | Generic MPLS | | 450 | 1 | any | or | Frame Relay | 451 | | |IP(network layer)| | 452 +----------------+-----------------+-----------------+-----------------+ 453 | number of hops | | Generic MPLS | | 454 | of | Frame Relay | or | any | 455 | LSP segment | |IP(network layer)| | 456 +================+=================+=================+=================+ 458 Referring to the "forwarding encapsulation" with the abbreviation "I" 459 for IP (network layer), "G" for Generic MPLS, and "F" for Frame 460 Relay MPLS, referring to an LSR interface with the abbreviation "i" 461 if the input or output encapsulation is IP and no MPLS encapsulation, 462 "g" when the input or output MPLS encapsulation is Generic MPLS, "f" 463 when it is Frame Relay, "a" when it is ATM, and furthermore 464 considering the symbols "iIf", "gGf", "fFf", etc... as LSRs with 465 input, forwarding and output encapsulations as referred above, the 466 following describes examples of TTL calculations for the Homogeneous 467 and Heterogeneous LSPs discussed in previous sections: 469 Homogeneous LSP 470 --------------- 471 IP_ttl = n IP_ttl=mpls_ttl-1 = n-6 472 --------->iIf fIi---------> 473 | mpls_ttl = n-5 ^ 474 | | 475 number of hops 1| Frame Relay |5 476 | | 477 V 2 3 4 | 478 fFf--->fFf--->fFf--->fFf 479 "iIf" is "ingress LSR" in Frame Relay LSP and 480 calculates: mpls_ttl = IP_TTL - number of hops = n-5 481 "fIi" is "egress LSR" from Frame Relay LSP, and 482 calculates: IP_ttl = mpls_ttl-1 = n-6 484 Heterogeneous LSP 485 ----------------- 486 ingress LSR egress LSR 487 IP_ttl = n IP_ttl = n - 15 488 links LAN PPP FR ATM PPP FR LAN 489 --->iIg-->gGg-->gGf fGa aGg-->gGf fGg-->gIi---> 490 hops 1 2 | 6 | | 9 | 10 | 13 ^ 14 15 491 |1 4| |1 3| |1 3| 492 V 2 3 | V 2 | V 2 | 493 fFf-->fFf-->fFf aAa-->aAa fFf-->fFf 494 mpls_ttl 495 n-1 n-2 (n-2)-4=n-6 (n-6)-3=n-9 n-10 n-13 n-14 497 "iIg" is "ingress LSR" in LSP; it calculates: mpls_ttl=n-1 498 "gGf" is "egress LSR" from Generic MPLS segment, and 499 "ingress LSR" in Frame Relay segment and calculates: mpls_ttl=n-6 500 "fGa" "egress LSR" from Frame Relay segment, and 501 "ingress LSR" in ATM segment and calculates: mpls_ttl=n-9 502 "gGf" is "egress LSR" from Generic MPLS segment, and 503 "ingress LSR" in Frame Relay segment and calculates: mpls_ttl=n-13 504 "fGg" is "egress LSR" from Frame Relay segment, and 505 ingress LSR" in Generic MPLS segment and calculates: mpls_ttl=n-14 506 "gIi" is "egress LSR" from LSP and calculates: IP_ttl=n-15 508 And further examples: 510 Frame Relay Unicast -- TTL calculated at ingress 512 (ingress LSR) 1 2 3 4 513 x--->---+--->---+--->>--+-->>---x (egress LSR) 514 o.ttl=i.ttl-4 | 2 3 515 ^ 516 hops 1| 517 | 518 x (ingress LSR) 519 o.ttl=i.ttl-3 520 Frame Relay Multicast -- TTL calculated at egress 522 (egress LSR)x o.ttl=i.ttl-3 523 hops | 524 ^3 525 (ingress LSR) | o.ttl=i.ttl-4 526 x--->---+--->---+--->---+--->---x (egress LSR) 527 1 2 3 4 529 5.5 Label Processing by Ingress FR-LSRs 531 When a packet first enters an MPLS domain, the packet is forwarded by 532 normal network layer forwarding operations with the exception that 533 the outgoing encapsulation will include an MPLS label stack [STACK] 534 with at least one entry. The frame relay null encapsulation will 535 carry information about the network layer protocol implicitly in the 536 label, which MUST be associated only with that network protocol. The 537 TTL field in the top label stack entry is filled with the network 538 layer TTL (or hop limit) resulted after network layer forwarding 539 [STACK]. The further FR-LSR processing is similar in both possible 540 cases: 542 (a) the LSP is homogeneous -- Frame Relay only -- and the FR-LSR is 543 the ingress. 545 (b) the LSP is heterogeneous -- Frame Relay, PPP, Ethernet, ATM, 546 etc... segments form the LSP -- and the FR-LSR is the ingress into a 547 Frame Relay 548 segment. 550 For unicast packets, the MPLS TTL SHOULD be decremented with the 551 number of hops of the Frame Relay LSP (homogeneous), or Frame Relay 552 segment of the LSP (heterogeneous). An LDP constructing the LSP 553 SHOULD pass meaningful information to the ingress FR-LSR regarding 554 the number of hops of the "non-TTL segment". 556 For multicast packets, the MPLS TTL SHOULD be decremented by 1. An 557 LDP constructing the LSP SHOULD pass meaningful information to the 558 egress FR-LSR regarding the number of hops of the "non-TTL segment". 560 Next, the MPLS encapsulated packet is passed down to the Frame Relay 561 data link driver with the top label as output DLCI. The Frame Relay 562 frame carrying the MPLS encapsulated packet is forwarded onto the 563 Frame Relay VC to the next LSR. 565 5.6 Label Processing by Core FR-LSRs 567 In a FR-LSR, the current (top) MPLS label is carried in the DLCI 568 field of the Frame Relay data link layer header of the frame. Just as 569 in conventional Frame Relay, for a frame arriving at an interface, 570 the DLCI carried by the Frame Relay data link header is looked up in 571 the DLCI Information Base, replaced with the correspondent output 572 DLCI, and transmitted on the outgoing interface (forwarded to the 573 next hop node). 575 The current label information is also carried in the top of the label 576 stack. In the top-level entry, all fields except the label 577 information, which is carried and switched in the Frame Relay frame 578 data link-layer header, are of current significance. 580 5.7 Label Processing by Egress FR-LSRs 582 When reaching the end of a Frame Relay LSP, the FR-LSR pops the label 583 stack [ARCH]. If the label popped is the last label, it is necessary 584 to determine the particular network layer protocol which is being 585 carried. The label stack carries no explicit information to identify 586 the network layer protocol. This must be inferred from the value of 587 the label which is popped from the stack. 589 If the label popped is not the last label, the previous top level 590 MPLS TTL is propagated to the new top label stack entry. 592 If the FR-LSR is the egress switch of a Frame Relay segment of a 593 hybrid LSP, and the end of the Frame Relay segment is not the end of 594 the LSP, the MPLS packet will be processed for forwarding onto the 595 next segment of the LSP based on the information held in the Next Hop 596 Label Forwarding Entry (NHLFE) [ARCH]. The output label is set to the 597 value from the NHLFE, and the MPLS TTL is decremented by the 598 appropriate value depending the type of the output interface and the 599 type of transmit operation (see section 6.3). Further, the MPLS 600 packet is forwarded according to the MPLS specifications for the 601 particular link of the next segment of the LSP. 603 For unicast packets, the MPLS TTL SHOULD be decremented by one if the 604 output interface is a generic one, or with the number of hops of the 605 next ATM segment of the LSP (heterogeneous), if the output interface 606 is an ATM (non-TTL) interface. 608 For multicast packets, the MPLS TTL SHOULD be decremented by the 609 number of hops of the FR segment being exited. An LDP constructing 610 the LSP SHOULD pass meaningful information to the egress FR-LSR 611 regarding the number of hops of the FR "non-TTL segment". 613 6. Label Switching Control Component for Frame Relay 615 To support label switching a Frame Relay Switch MUST implement the 616 control component of label switching, which consists primarily of 617 label allocation and maintenance procedures. Label binding 618 information MAY be communicated by several mechanisms, one of which 619 is the Label Distribution Protocol (LDP) [LDP]. 621 Since the label switching control component uses information learned 622 directly from network layer routing protocols, this implies that the 623 switch MUST participate as a peer in these protocols (e.g., OSPF, 624 IS-IS). 626 In some cases, LSRs may use other protocols (e.g. RSVP, PIM, BGP) to 627 distribute label bindings. In these cases, a Frame Relay LSR should 628 participate in these protocols. 630 In the case where Frame Relay circuits are established via LDP, or 631 RSVP, or others, with no involvement from traditional Frame Relay 632 mechanisms, it is assumed that circuit establishing contractual 633 information such as input/output maximum frame size, 634 incoming/outgoing requested/agreed throughput, incoming/outgoing 635 acceptable throughput, incoming/outgoing burst size, 636 incoming/outgoing frame rate, used in transmitting, and congestion 637 control MAY be passed to the FR-LSRs through RSVP, or can be 638 statically configured. It is also assumed that congestion control and 639 frame header flagging as a consequence of congestion, would be done 640 by the FR-LSRs in a similar fashion as for traditional Frame Relay 641 circuits. With the goal of emulating a best-effort router as default, 642 the default VC parameters, in the absence of LDP, RSVP, or other 643 mechanisms participation to setting such parameters, should be zero 644 CIR, so that input policing will set the DE bit in incoming frames, 645 but no frames are dropped. 647 Control and state information for the circuits based on MPLS MAY be 648 communicated through LDP. 650 Support of label switching on a Frame Relay switch requires 651 conformance only to [FRF] (framing, bit-stuffing, headers, FCS) 652 except for section 2.3 (PVC control signaling procedures, aka LMI). 653 Q.933 signaling for PVCs and/or SVCs is not required. PVC and/or SVC 654 signaling may be used for non-MPLS (standard Frame Relay) PVCs and/or 655 SVCs when both are running on the same interface as MPLS, as 656 discussed in the next section. 658 6.1 Hybrid Switches (Ships in the Night) 660 The existence of the label switching control component on a Frame 661 Relay switch does not preclude the ability to support the Frame Relay 662 control component defined by the ITU and Frame Relay Forum on the 663 same switch and the same interfaces (NICs). The two control 664 components, label switching and those defined by ITU/Frame Relay 665 Forum, would operate independently. 667 Definition of how such a device operates is beyond the scope of this 668 document. However, only a small amount of information needs to be 669 consistent between the two control components, such as the portions 670 of the DLCI space which are available to each component. 672 7. Label Allocation and Maintenance Procedures 674 The mechanisms and message formats of a Label Distribution Protocol 675 are documented in [ARCH] and [LDP]. The "downstream-on-demand" label 676 allocation and maintenance mechanism discussed in this section MUST 677 be used by FR-LSRs that do not support VC merging, and it MAY also be 678 used by FR-LSRs that do support VC merging (note that this mechanism 679 applies to hop-by-hop routed traffic): 681 7.1 Edge LSR Behavior 683 Consider a member of the Edge Set of a FR-LSR domain. Assume that, as 684 a result of its routing calculations, it selects a FR-LSR as the next 685 hop of a certain route (FEC), and that the next hop is reachable via 686 a LC-Frame Relay interface. Assume that the next-hop FR-LSR is an 687 "LDP-peer" [ARCH][LDP]. The Edge LSR sends an LDP "request" message 688 for a label binding from the next hop, downstream LSR. When the Edge 689 LSR receives in response from the downstream LSR the label binding 690 information in an LDP "mapping" message, the label is stored in the 691 Label Information Base (LIB) as an outgoing label for that FEC. The 692 "mapping" message may contain the "hop count" object, which 693 represents the number of hops a packet will take to cross the FR-LSR 694 domain to the Egress FR-LSR when using this label. This information 695 may be stored for TTL calculation. Once this is done, the LSR may use 696 MPLS forwarding to transmit packets in that FEC. 698 When a member of the Edge Set of the FR-LSR domain receives an LDP 699 "request" message from a FR-LSR for a FEC, it means it is the 700 Egress-FR-LSR. It allocates a label, creates a new entry in its Label 701 Information Base (LIB), places that label in the incoming label 702 component of the entry, and returns (via LDP) a "mapping" message 703 containing the allocated label back upstream to the LDP peer that 704 originated the request. The "mapping" message contains the "hop 705 count" object value set to 1. 707 When a routing calculation causes an Edge LSR to change the next hop 708 for a route, and the former next hop was in the FR-LSR domain, the 709 Edge LSR should notify the former next hop (via an LDP "release" 710 message) that the label binding associated with the route is no 711 longer needed. 713 When a Frame Relay-LSR receives an LDP "request" message for a 714 certain route (FEC) from an LDP peer connected to the FR-LSR over a 715 LC-FR interface, the FR-LSR takes the following actions: 717 - it allocates a label, creates a new entry in its Label 718 Information Base (LIB), and places that label in the incoming 719 label component of the entry; 721 - it propagates the "request", by sending an LDP "request" 722 message to the next hop LSR, downstream for that route (FEC); 724 In the "ordered control" mode [ARCH], the FR-LSR will wait for its 725 "request" to be responded from downstream with a "mapping" message 726 before returning the "mapping" upstream in response to a "request" 727 ("ordered control" approach [ARCH]). In this case, the FR-LSR 728 increments the hop count it received from downstream and uses this 729 value in the "mapping" it returns upstream. 731 Alternatively, the FR-LSR may return the binding upstream without 732 waiting for a binding from downstream ("independent control" approach 733 [ARCH]). In this case, it uses a reserved value for hop count in the 734 "mapping", indicating that it is 'unknown'. The correct value for hop 735 count will be returned later, as described below. 737 Since both the "ordered" and "independent" control has advantages and 738 disadvantages, this is left as an implementation, or configuration 739 choice. 741 Once the FR-LSR receives in response the label binding in an LDP 742 "mapping" message from the next hop, it places the label into the 743 outgoing label component of the LIB entry. 745 Note that a FR-LSR, or a member of the edge set of a FR-LSR domain, 746 may receive multiple binding requests for the same route (FEC) from 747 the same FR-LSR. It must generate a new "mapping" for each "request" 748 (assuming adequate resources to do so), and retain any existing 749 mapping(s). For each "request" received, a FR-LSR should also 750 generate a new binding "request" toward the next hop for the route 751 (FEC). 753 When a routing calculation causes a FR-LSR to change the next hop for 754 a route (FEC), the FR-LSR should notify the former next hop (via an 755 LDP "release" message) that the label binding associated with the 756 route is no longer needed. 758 When a LSR receives a notification that a particular label binding is 759 no longer needed, the LSR may deallocate the label associated with 760 the binding, and destroy the binding. This mode is the "conservative 761 label retention mode" [ARCH]. In the case where a FR-LSR receives 762 such notification and destroys the binding, it should notify the next 763 hop for the route that the label binding is no longer needed. If a 764 LSR does not destroy the binding (the FR-LSR is configured in 765 "liberal label retention mode" [ARCH]), it may re-use the binding 766 only if it receives a request for the same route with the same hop 767 count as the request that originally caused the binding to be 768 created. 770 When a route changes, the label bindings are re-established from the 771 point where the route diverges from the previous route. LSRs 772 upstream of that point are (with one exception, noted below) 773 oblivious to the change. Whenever a LSR changes its next hop for a 774 particular route, if the new next hop is a FR-LSR or a member of the 775 edge set reachable via a LC-FR interface, then for each entry in its 776 LIB associated with the route the LSR should request (via LDP) a 777 binding from the new next hop. 779 When a FR-LSR receives a label binding from a downstream neighbor, it 780 may already have provided a corresponding label binding for this 781 route to an upstream neighbor, either because it is using 782 "independent control" or because the new binding from downstream is 783 the result of a routing change. In this case, it should extract the 784 hop count from the new binding and increment it by one. If the new 785 hop count is different from that which was previously conveyed to the 786 upstream neighbor (including the case where the upstream neighbor was 787 given the value 'unknown') the FR-LSR must notify the upstream 788 neighbor of the change. Each FR-LSR in turn increments the hop count 789 and passes it upstream until it reaches the ingress Edge LSR. 791 Whenever a FR-LSR originates a label binding request to its next hop 792 LSR as a result of receiving a label binding request from another 793 (upstream) LSR, and the request to the next hop LSR is not satisfied, 794 the FR-LSR should destroy the binding created in response to the 795 received request, and notify the requester (via an LDP "withdraw" 796 message). 798 When an LSR determines that it has lost its LDP session with another 799 LSR, the following actions are taken: 801 - MUST discard any binding information learned via this 802 connection; 804 - For any label bindings that were created as a result of 805 receiving label binding requests from the peer, the LSR may 806 destroy these bindings (and deallocate labels associated 807 with these binding). 809 7.2 Efficient use of label space - Merging FR-LSRs 811 The above discussion assumes that an edge LSR will request one label 812 for each prefix in its routing table that has a next hop in the FR- 813 LSR domain. In fact, it is possible to significantly reduce the 814 number of labels needed by having the edge LSR request instead one 815 label for several routes. Use of many-to-one mappings between routes 816 (address prefixes) and labels using the notion of Forwarding 817 Equivalence Classes (as described in [ARCH]) provides a mechanism to 818 conserve the number of labels. 820 Note that conserving label space (VC merging) may be restricted in 821 case the frame traffic requires Frame Relay fragmentation. The issue 822 is that Frame Relay fragments must be transmitted in sequence, i.e. 823 fragments of distinct frames must not be interleaved. If the 824 fragmenting FR-LSR ensures the transmission in sequence of all 825 fragments of a frame, without interleaving with fragments of other 826 frames, then label conservation (VC merging) can be performed. 828 When label conservation is used, when a FR-LSR receives a binding 829 request from an upstream LSR for a certain FEC, and it does already 830 have an outgoing label binding for that FEC, it does not need to 831 issue a downstream binding request. Instead, it may allocate an 832 incoming label, and return that label in a binding to the upstream 833 requester. Packets received from the requester, with that label as 834 top label, will be forwarded after replacing the label with the 835 existing outgoing label for that FEC. If the FR-LSR does not have an 836 outgoing label binding for that FEC, but does have an outstanding 837 request for one, it need not issue another request. This means that 838 in a label conservation case, a FR-LSR must respond with a new 839 binding for every upstream request, but it may need to send one 840 binding request downstream. 842 In case of label conservation, if a change in the routing table 843 causes a FR-LSR to select a new next hop for one of its FECs, it MAY 844 release the binding for that route from the former next hop. If it 845 doesn't already have a corresponding binding for the new next hop, it 846 must request one (note that the choice depends on the label retention 847 mode [ARCH]). 849 If a new binding is obtained, which contain a hop count that differs 850 from that of the old binding, the FR-LSR must process the new hop 851 count: increment by 1, if different than "unknown", and notify the 852 upstream neighbors who have label bindings for this FEC of the new 853 value. To ensure that loops will be detected, if the new hop count 854 exceeds the "maximum" value, the label values for this FEC must be 855 withdrawn from all upstream neighbors to whom a binding was 856 previously sent. 858 7.3 LDP messages specific to Frame Relay 860 The Label Distribution Protocol [LDP] messages exchanged between two 861 Frame Relay "LDP-peer" LSRs may contain Frame Relay specific 862 information such as: 864 "Frame Relay Label Range": 866 0 1 2 3 867 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 1 868 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 869 | Reserved |Len| Minimum DLCI | 870 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 871 | Reserved | Maximum DLCI | 872 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 874 with the following fields: 876 Reserved 877 This fields are reserved. They must be set to zero on transmission 878 and must be ignored on receipt. 880 Len 881 This field specifies the number of bits of the DLCI. The following 882 values are supported: 884 Len DLCI bits 886 0 10 887 1 17 888 2 23 889 Minimum DLCI 890 This 23 bit field is the binary value of the lower bound of a block 891 of Data Link Connection Identifiers (DLCIs) that is supported by 892 the originating FR-LSR. The Minimum DLCI should be right justified 893 in this field and the preceding bits should be set to 0. 895 Maximum DLCI 896 This 23 bit field is the binary value of the upper bound of a block 897 of Data Link Connection Identifiers (DLCIs) that is supported by 898 the originating FR-LSR. The Maximum DLCI should be right justified 899 in this field and the preceding bits should be set to 0. 901 "Frame Relay Merge": 903 0 1 2 3 4 5 6 7 904 +-+-+-+-+-+-+-+-+ 905 | Reserved |M| 906 +-+-+-+-+-+-+-+-+ 908 with the following fields: 910 Merge 911 One bit field that specifies the merge capabilities of the FR-LSR: 913 Value Meaning 915 0 Merge NOT supported 916 1 Merge supported 918 A FR-LSR that supports VC merging MUST ensure that fragmented 919 frames from distinct incoming DLCIs are not interleaved on the 920 outgoing DLCI. 922 Reserved 923 This field is reserved. It must be set to zero on transmission and 924 must be ignored on receipt. 926 and "Frame Relay Label": 928 0 1 2 3 929 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 1 930 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 931 | Reserved |Len| DLCI | 932 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 934 with the following fields: 936 Reserved 937 This field is reserved. It must be set to zero on transmission and 938 must be ignored on receipt. 940 Len 941 This field specifies the number of bits of the DLCI. The following 942 values are supported: 944 Len DLCI bits 946 0 10 947 1 17 948 2 23 950 DLCI 951 The binary value of the Frame Relay Label. The significant number 952 of bits (10, 17, or 23) of the label value are to be encoded into 953 the Data Link Connection Identifier (DLCI) field when part of the 954 Frame Relay data link header (see Section 4.). 956 8. Security Considerations 958 This section looks at the security aspects of: 960 (a) frame traffic, 962 (b) label distribution. 964 MPLS encapsulation has no effect on authenticated or encrypted 965 network layer packets, that is IP packets that are authenticated or 966 encrypted will incur no change. 968 The MPLS protocol has no mechanisms of its own to protect against 969 misdirection of packets or the impersonation of an LSR by accident or 970 malicious intent. 972 Altering by accident or forgery an existent label in the DLCI field 973 of the Frame Relay data link layer header of a frame or one or more 974 fields in a potentially following label stack affects the forwarding 975 of that frame. 977 The label distribution mechanism can be secured by applying the 978 appropriate level of security to the underlying protocol carrying 979 label information - authentication or encryption - see [LDP]. 981 9. Acknowledgments 983 The initial version of this document was derived from the Label 984 Switching over ATM document [ATM]. 986 Thanks for the extensive reviewing and constructive comments from (in 987 alphabetical order) Dan Harrington, Milan Merhar, Martin Mueller, 988 Eric Rosen. Also thanks to George Swallow for the suggestion to use 989 null encapsulation, and to Eric Gray for his reviewing. 991 Also thanks to Nancy Feldman and Bob Thomas for their collaboration 992 in including the LDP messages specific to Frame Relay LSRs. 994 10. References 996 [MIFR] T. Bradley, C. Brown, A. Malis "Multiprotocol Interconnect 997 over Frame Relay" RFC 2427, September 1998. 999 [ARCH] E. Rosen, R. Callon, A. Vishwanathan, "Multi-Protocol Label 1000 Switching Architecture", Work in Progress, July 1998. 1002 [LDP] L. Anderson, P. Doolan, N. Feldman, A. Fredette, R. Thomas, 1003 "Label Distribution Protocol", Work in Progress, August 1998. 1005 [STACK] E. Rosen et al, "Label Switching: Label Stack Encodings", 1006 Work in Progress, September 1998 1008 [ATM] B. Davie et al. "Use of Label Switching with ATM", Work in 1009 Progress, July 1998. 1011 [ITU] International Telecommunications Union, "ISDN Data Link Layer 1012 Specification for Frame Mode Bearer Services", ITU-T Recommendation 1013 Q.922, 1992. 1015 [FRF] Frame Relay Forum, User-to-Network Implementation Agreement 1016 (UNI), FRF 1.1, January 19, 1996 1017 11.Authors' Addresses 1019 Alex Conta 1020 3COM 1021 100 3COM Drive 1022 Marlborough, MA 01752 1023 +1 508 323-2297 1024 E-mail: Alex_Conta@ne.3com.com 1026 Paul Doolan 1027 Ennovate Networks 1028 60 Codman Hill Rd 1029 Boxborough MA 01719 1030 +1 978 263-2002 1031 E-mail: pdoolan@ennovatenetworks.com 1033 Andrew Malis 1034 Lucent Technologies 1035 1 Robbins Rd 1036 Westford, MA 01886 1037 +1 978 952-7414 1038 E-mail: amalis@lucent.com 1039 Appendix A - Changes since previous versions 1041 From "version 02 to 03" 1042 - Replace "cloud" with "domain", 1043 - Update references to other documents, 1044 - Change definitions in "Terminology" section, 1045 - Add more definitions to "Terminology" section, 1046 - Make editorial changes to text and figures, 1047 - Change "Performing TTL calculations" section, 1048 - Add more reviewers in "Acknowledgments" section, 1049 - Add Appendix A - changes.