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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 303 instances of weird spacing in the document. Is it really formatted ragged-right, rather than justified? ** There are 29 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 286: '... VC merging MUST be communicated t...' RFC 2119 keyword, line 322: '... FR-LSRs SHOULD operate on loop fr...' RFC 2119 keyword, line 323: '...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...' == (298 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 (13 December 2000) is 8533 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 13 June 2000 6 Expires 13 December 2000 8 Use of Label Switching on Frame Relay Networks 9 Specification 11 draft-ietf-mpls-fr-05.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) or 23 bits, and it can span two or 187 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 239 7 6 5 4 3 2 1 0 (bit order) 240 +-----+-----+-----+-----+-----+-----+-----+-----00 241 (octet) 0 | DLCI(high order) | 0 | 0 | 242 +-----+-----+-----+-----+-----+-----+-----+----- 243 1 | DLCI | 0 | 0 | 0 | 0 | 244 +-----+-----+-----+-----+-----+-----+-----+-----+ 245 2 | DLCI | 0 | 246 +-----+-----+-----+-----+-----+-----+-----+-----+ 247 3 | DLCI (low order) | 0 | 1 | 248 +-----+-----+-----+-----+-----+-----+-----+-----+ 250 23 bits DLCI 252 The use of the frame relay null encapsulation implies that labels 253 implicitly encode the network protocol type. 255 Rules regarding the construction of the label stack, and error 256 messages returned to the frame source are also described in [STACK]. 258 The generic encapsulation contains "n" labels for a label stack of 259 depth "n" [STACK], where the top stack entry carries significant 260 values for the EXP, S , and TTL fields [STACK] but not for the label, 261 which is rather carried in the DLCI field of the Frame Relay data 262 link header encoded in Q.922 [ITU] address format. 264 5. Frame Relay Label Switching Processing 266 5.1 Use of DLCIs 268 Label switching is accomplished by associating labels with routes and 269 using the label value to forward packets, including determining the 270 value of any replacement label. See [ARCH] for further details. In a 271 FR-LSR, the top (current) MPLS label is carried in the DLCI field of 272 the Frame Relay data link layer header of the frame. The top label 273 carries implicitly information about the network protocol type. 275 For two connected FR-LSRs, a full-duplex connection must be available 276 for LDP. The DLCI for the LDP VC is assigned a value by way of 277 configuration, similar to configuring the DLCI used to run IP routing 278 protocols between the switches. 280 With the exception of this configured value, the DLCI values used for 281 MPLS in the two directions of the link may be treated as belonging to 282 two independent spaces, i.e. VCs may be half-duplex, each direction 283 with its own DLCI. 285 The allowable ranges of DLCIs, the size of DLCIs, and the support for 286 VC merging MUST be communicated through LDP messages. Note that the 287 range of DLCIs used for labels depends on the size of the DLCI field. 289 5.2 Homogeneous LSPs 291 If is an LSP, it is possible that LSR1, LSR2, and 292 LSR3 will use the same encoding of the label stack when transmitting 293 packet P from LSR1, to LSR2, and then to LSR3. Such an LSP is 294 homogeneous. 296 5.3 Heterogeneous LSPs 298 If is an LSP, it is possible that LSR1 will use 299 one encoding of the label stack when transmitting packet P to LSR2, 300 but LSR2 will use a different encoding when transmitting a packet P 301 to LSR3. In general, the MPLS architecture supports LSPs with 302 different label stack encodings on different hops. When a labeled 303 packet is received, the LSR must decode it to determine the current 304 value of the label stack, then must operate on the label stack to 305 determine the new label value of the stack, and then encode the new 306 value appropriately before transmitting the labeled packet to its 307 next hop. 309 Naturally there will be MPLS networks which contain a combination of 310 Frame Relay switches operating as LSRs, and other LSRs, which operate 311 using other MPLS encapsulations, such as the Generic (MPLS shim 312 header), or ATM encapsulation. In such networks there may be some 313 LSRs, which have Frame Relay interfaces as well as MPLS Generic 314 ("MPLS Shim") interfaces. This is one example of an LSR with 315 different label stack encodings on different hops of the same LSP. 316 Such an LSR may swap off a Frame Relay encoded label on an incoming 317 interface and replace it with a label encoded into a Generic MPLS 318 (MPLS shim) header on the outgoing interface. 320 5.4 Frame Relay Label Switching Loop Prevention and Control 322 FR-LSRs SHOULD operate on loop free FR-LSPs or LSP Frame Relay 323 segments. Therefore, FR-LSRs SHOULD use loop detection and MAY use 324 loop prevention mechanisms as described in [ARCH], and [LDP]. 326 5.4.1 FR-LSRs Loop Control - MPLS TTL processing 328 The MPLS TTL encoded in the MPLS label stack is a mechanism used to: 330 (a) suppress loops; 332 (b) limit the scope of a packet. 334 When a packet travels along an LSP, it should emerge with the same 335 TTL value that it would have had if it had traversed the same 336 sequence of routers without having been label switched. If the 337 packet travels along a hierarchy of LSPs, the total number of LSR- 338 hops traversed should be reflected in its TTL value when it emerges 339 from the hierarchy of LSPs [ARCH]. 341 The initial value of the MPLS TTL is loaded into a newly pushed label 342 stack entry from the previous TTL value, whether that is from the 343 network layer header when no previous label stack existed, or from a 344 pre-existent lower level label stack entry. 346 A FR-LSR switching same level labeled packets does not decrement the 347 MPLS TTL. A sequence of such FR-LSR is a "non-TTL segment". 349 When a packet emerges from a "non-TTL LSP segment", it should however 350 reflect in the TTL the number of LSR-hops it traversed. In the 351 unicast case, this can be achieved by propagating a meaningful LSP 352 length or LSP Frame Relay segment length to the FR-LSR ingress nodes, 353 enabling the ingress to decrement the TTL value before forwarding 354 packets into a non-TTL LSP segment [ARCH]. 356 When an ingress FR-LSR determines upon decrementing the MPLS TTL that 357 a particular packet's TTL will expire before the packet reaches the 358 egress of the "non-TTL LSP segment", the FR-LSR MUST not label switch 359 the packet, but rather follow the specifications in [STACK] in an 360 attempt to return an error message to the packet's source: 362 - it treats the packet as an expired packet and return an ICMP 363 message to its source. 365 - it forwards the packet, as an unlabeled packet, with a TTL 366 that reflects the IP (network layer) forwarding. 368 If the incoming TTL is 1, only the first option applies. 370 In the multicast case, a meaningful LSP length or LSP segment length 371 is propagated to the FR-LSR egress node, enabling the egress to 372 decrement the TTL value before forwarding packets out of the non-TTL 373 LSP segment. 375 5.4.2 Performing MPLS TTL calculations 377 The calculation applied to the "input TTL" that yields the "output 378 TTL" depends on (i)the "input encapsulation", (ii)the "forwarding 379 encapsulation", and (iii)the "output encapsulation". The 380 relationship among (i),(ii), and (iii), can be defined as a function 381 "D" of "input encapsulation" (ie), "forwarding encapsulation" (fe), 382 and "output encapsulation" (oe). Subsequently the calculation applied 383 to the "input TTL" to yield the "output TTL" can be described as: 385 output TTL = input TTL - D(ie, fe, oe) 387 or in a brief notation: 389 output TTL = input TTL - d 391 where "d" has three possible values: "0","1", or "the number of hops 392 of the LSP segment": 394 For unicast transmission: 396 +================+=================+=================+=================+ 397 | | Type of | Type of | Type of | 398 | d | Input | Forwarding | Output | 399 | | Encapsulation | Encapsulation | Encapsulation | 400 +================+=================+=================+=================+ 401 | 0 | Frame Relay | Frame Relay | Frame Relay | 402 +----------------+-----------------+-----------------+-----------------+ 403 | 1 | any | Generic MPLS | Generic MPLS | 404 +----------------+-----------------+-----------------+-----------------+ 405 | number of hops | | Generic MPLS | | 406 | of | any | or | Frame Relay | 407 | LSP segment | |IP(network layer)| | 408 +================+=================+=================+=================+ 410 The "number of hops of the LSP segment" is the value of the "hop 411 count" that is attached with the label used when the packet is 412 forwarded, if LDP [LDP] has provided such a "hop count" value when it 413 distributed the label for the LSP, that is the LDP message had a "hop 414 count object". If LDP didn't provide a "hop count", or it provided an 415 "unknown" value, the default value of the "number of hops of the 416 segment" is 1. 418 When sending a label binding upstream, the "hop count" associated 419 with the corresponding binding from downstream, if different than the 420 "unknown" value, MUST be incremented by 1, and the result transmitted 421 upstream as the hop count associated with the new binding (the 422 "unknown" value is transmitted unchanged). If the new "hop count" 423 value exceeds the "maximum" value, the FR-LSR MUST NOT pass the 424 binding upstream, but instead MUST send an error upstream 425 [LDP][ARCH]. 427 For multicast transmission: 429 +================+=================+=================+=================+ 430 | | Type of | Type of | Type of | 431 | d | Input | Forwarding | Output | 432 | | Encapsulation | Encapsulation | Encapsulation | 433 +================+=================+=================+=================+ 434 | 0 | Frame Relay | Frame Relay | Frame Relay | 435 +----------------+-----------------+-----------------+-----------------+ 436 | | | Generic MPLS | | 437 | 1 | any | or | Frame Relay | 438 | | |IP(network layer)| | 439 +----------------+-----------------+-----------------+-----------------+ 440 | number of hops | | Generic MPLS | | 441 | of | Frame Relay | or | any | 442 | LSP segment | |IP(network layer)| | 443 +================+=================+=================+=================+ 445 Referring to the "forwarding encapsulation" with the abbreviation "I" 446 for IP (network layer), "G" for Generic MPLS, and "F" for Frame 447 Relay MPLS, referring to an LSR interface with the abbreviation "i" 448 if the input or output encapsulation is IP and no MPLS encapsulation, 449 "g" when the input or output MPLS encapsulation is Generic MPLS, "f" 450 when it is Frame Relay, "a" when it is ATM, and furthermore 451 considering the symbols "iIf", "gGf", "fFf", etc... as LSRs with 452 input, forwarding and output encapsulations as referred above, the 453 following describes examples of TTL calculations for the Homogeneous 454 and Heterogeneous LSPs discussed in previous sections: 456 Homogeneous LSP 457 --------------- 458 IP_ttl = n IP_ttl=mpls_ttl-1 = n-6 459 --------->iIf fIi---------> 460 | mpls_ttl = n-5 ^ 461 | | 462 number of hops 1| Frame Relay |5 463 | | 464 V 2 3 4 | 465 fFf--->fFf--->fFf--->fFf 466 "iIf" is "ingress LSR" in Frame Relay LSP and 467 calculates: mpls_ttl = IP_TTL - number of hops = n-5 468 "fIi" is "egress LSR" from Frame Relay LSP, and 469 calculates: IP_ttl = mpls_ttl-1 = n-6 471 Heterogeneous LSP 472 ----------------- 473 ingress LSR egress LSR 474 IP_ttl = n IP_ttl = n - 15 475 links LAN PPP FR ATM PPP FR LAN 476 --->iIg-->gGg-->gGf fGa aGg-->gGf fGg-->gIi---> 477 hops 1 2 | 6 | | 9 | 10 | 13 ^ 14 15 478 |1 4| |1 3| |1 3| 479 V 2 3 | V 2 | V 2 | 480 fFf-->fFf-->fFf aAa-->aAa fFf-->fFf 481 mpls_ttl 482 n-1 n-2 (n-2)-4=n-6 (n-6)-3=n-9 n-10 n-13 n-14 484 "iIg" is "ingress LSR" in LSP; it calculates: mpls_ttl=n-1 485 "gGf" is "egress LSR" from Generic MPLS segment, and 486 "ingress LSR" in Frame Relay segment and calculates: mpls_ttl=n-6 487 "fGa" "egress LSR" from Frame Relay segment, and 488 "ingress LSR" in ATM segment and calculates: mpls_ttl=n-9 489 "gGf" is "egress LSR" from Generic MPLS segment, and 490 "ingress LSR" in Frame Relay segment and calculates: mpls_ttl=n-13 491 "fGg" is "egress LSR" from Frame Relay segment, and 492 ingress LSR" in Generic MPLS segment and calculates: mpls_ttl=n-14 493 "gIi" is "egress LSR" from LSP and calculates: IP_ttl=n-15 495 And further examples: 497 Frame Relay Unicast -- TTL calculated at ingress 499 (ingress LSR) 1 2 3 4 500 x--->---+--->---+--->>--+-->>---x (egress LSR) 501 o.ttl=i.ttl-4 | 2 3 502 ^ 503 hops 1| 504 | 505 x (ingress LSR) 506 o.ttl=i.ttl-3 507 Frame Relay Multicast -- TTL calculated at egress 509 (egress LSR)x o.ttl=i.ttl-3 510 hops | 511 ^3 512 (ingress LSR) | o.ttl=i.ttl-4 513 x--->---+--->---+--->---+--->---x (egress LSR) 514 1 2 3 4 516 5.5 Label Processing by Ingress FR-LSRs 518 When a packet first enters an MPLS domain, the packet is forwarded by 519 normal network layer forwarding operations with the exception that 520 the outgoing encapsulation will include an MPLS label stack [STACK] 521 with at least one entry. The frame relay null encapsulation will 522 carry information about the network layer protocol implicitly in the 523 label, which MUST be associated only with that network protocol. The 524 TTL field in the top label stack entry is filled with the network 525 layer TTL (or hop limit) resulted after network layer forwarding 526 [STACK]. The further FR-LSR processing is similar in both possible 527 cases: 529 (a) the LSP is homogeneous -- Frame Relay only -- and the FR-LSR is 530 the ingress. 532 (b) the LSP is heterogeneous -- Frame Relay, PPP, Ethernet, ATM, 533 etc... segments form the LSP -- and the FR-LSR is the ingress into a 534 Frame Relay 535 segment. 537 For unicast packets, the MPLS TTL SHOULD be decremented with the 538 number of hops of the Frame Relay LSP (homogeneous), or Frame Relay 539 segment of the LSP (heterogeneous). An LDP constructing the LSP 540 SHOULD pass meaningful information to the ingress FR-LSR regarding 541 the number of hops of the "non-TTL segment". 543 For multicast packets, the MPLS TTL SHOULD be decremented by 1. An 544 LDP constructing the LSP SHOULD pass meaningful information to the 545 egress FR-LSR regarding the number of hops of the "non-TTL segment". 547 Next, the MPLS encapsulated packet is passed down to the Frame Relay 548 data link driver with the top label as output DLCI. The Frame Relay 549 frame carrying the MPLS encapsulated packet is forwarded onto the 550 Frame Relay VC to the next LSR. 552 5.6 Label Processing by Core FR-LSRs 554 In a FR-LSR, the current (top) MPLS label is carried in the DLCI 555 field of the Frame Relay data link layer header of the frame. Just as 556 in conventional Frame Relay, for a frame arriving at an interface, 557 the DLCI carried by the Frame Relay data link header is looked up in 558 the DLCI Information Base, replaced with the correspondent output 559 DLCI, and transmitted on the outgoing interface (forwarded to the 560 next hop node). 562 The current label information is also carried in the top of the label 563 stack. In the top-level entry, all fields except the label 564 information, which is carried and switched in the Frame Relay frame 565 data link-layer header, are of current significance. 567 5.7 Label Processing by Egress FR-LSRs 569 When reaching the end of a Frame Relay LSP, the FR-LSR pops the label 570 stack [ARCH]. If the label popped is the last label, it is necessary 571 to determine the particular network layer protocol which is being 572 carried. The label stack carries no explicit information to identify 573 the network layer protocol. This must be inferred from the value of 574 the label which is popped from the stack. 576 If the label popped is not the last label, the previous top level 577 MPLS TTL is propagated to the new top label stack entry. 579 If the FR-LSR is the egress switch of a Frame Relay segment of a 580 hybrid LSP, and the end of the Frame Relay segment is not the end of 581 the LSP, the MPLS packet will be processed for forwarding onto the 582 next segment of the LSP based on the information held in the Next Hop 583 Label Forwarding Entry (NHLFE) [ARCH]. The output label is set to the 584 value from the NHLFE, and the MPLS TTL is decremented by the 585 appropriate value depending the type of the output interface and the 586 type of transmit operation (see section 6.3). Further, the MPLS 587 packet is forwarded according to the MPLS specifications for the 588 particular link of the next segment of the LSP. 590 For unicast packets, the MPLS TTL SHOULD be decremented by one if the 591 output interface is a generic one, or with the number of hops of the 592 next ATM segment of the LSP (heterogeneous), if the output interface 593 is an ATM (non-TTL) interface. 595 For multicast packets, the MPLS TTL SHOULD be decremented by the 596 number of hops of the FR segment being exited. An LDP constructing 597 the LSP SHOULD pass meaningful information to the egress FR-LSR 598 regarding the number of hops of the FR "non-TTL segment". 600 6. Label Switching Control Component for Frame Relay 602 To support label switching a Frame Relay Switch MUST implement the 603 control component of label switching, which consists primarily of 604 label allocation and maintenance procedures. Label binding 605 information MAY be communicated by several mechanisms, one of which 606 is the Label Distribution Protocol (LDP) [LDP]. 608 Since the label switching control component uses information learned 609 directly from network layer routing protocols, this implies that the 610 switch MUST participate as a peer in these protocols (e.g., OSPF, 611 IS-IS). 613 In some cases, LSRs may use other protocols (e.g. RSVP, PIM, BGP) to 614 distribute label bindings. In these cases, a Frame Relay LSR should 615 participate in these protocols. 617 In the case where Frame Relay circuits are established via LDP, or 618 RSVP, or others, with no involvement from traditional Frame Relay 619 mechanisms, it is assumed that circuit establishing contractual 620 information such as input/output maximum frame size, 621 incoming/outgoing requested/agreed throughput, incoming/outgoing 622 acceptable throughput, incoming/outgoing burst size, 623 incoming/outgoing frame rate, used in transmitting, and congestion 624 control MAY be passed to the FR-LSRs through RSVP, or can be 625 statically configured. It is also assumed that congestion control and 626 frame header flagging as a consequence of congestion, would be done 627 by the FR-LSRs in a similar fashion as for traditional Frame Relay 628 circuits. With the goal of emulating a best-effort router as default, 629 the default VC parameters, in the absence of LDP, RSVP, or other 630 mechanisms participation to setting such parameters, should be zero 631 CIR, so that input policing will set the DE bit in incoming frames, 632 but no frames are dropped. 634 Control and state information for the circuits based on MPLS MAY be 635 communicated through LDP. 637 Support of label switching on a Frame Relay switch requires 638 conformance only to [FRF] (framing, bit-stuffing, headers, FCS) 639 except for section 2.3 (PVC control signaling procedures, aka LMI). 640 Q.933 signaling for PVCs and/or SVCs is not required. PVC and/or SVC 641 signaling may be used for non-MPLS (standard Frame Relay) PVCs and/or 642 SVCs when both are running on the same interface as MPLS, as 643 discussed in the next section. 645 6.1 Hybrid Switches (Ships in the Night) 647 The existence of the label switching control component on a Frame 648 Relay switch does not preclude the ability to support the Frame Relay 649 control component defined by the ITU and Frame Relay Forum on the 650 same switch and the same interfaces (NICs). The two control 651 components, label switching and those defined by ITU/Frame Relay 652 Forum, would operate independently. 654 Definition of how such a device operates is beyond the scope of this 655 document. However, only a small amount of information needs to be 656 consistent between the two control components, such as the portions 657 of the DLCI space which are available to each component. 659 7. Label Allocation and Maintenance Procedures 661 The mechanisms and message formats of a Label Distribution Protocol 662 are documented in [ARCH] and [LDP]. The "downstream-on-demand" label 663 allocation and maintenance mechanism discussed in this section MUST 664 be used by FR-LSRs that do not support VC merging, and it MAY also be 665 used by FR-LSRs that do support VC merging (note that this mechanism 666 applies to hop-by-hop routed traffic): 668 7.1 Edge LSR Behavior 670 Consider a member of the Edge Set of a FR-LSR domain. Assume that, as 671 a result of its routing calculations, it selects a FR-LSR as the next 672 hop of a certain route (FEC), and that the next hop is reachable via 673 a LC-Frame Relay interface. Assume that the next-hop FR-LSR is an 674 "LDP-peer" [ARCH][LDP]. The Edge LSR sends an LDP "request" message 675 for a label binding from the next hop, downstream LSR. When the Edge 676 LSR receives in response from the downstream LSR the label binding 677 information in an LDP "mapping" message, the label is stored in the 678 Label Information Base (LIB) as an outgoing label for that FEC. The 679 "mapping" message may contain the "hop count" object, which 680 represents the number of hops a packet will take to cross the FR-LSR 681 domain to the Egress FR-LSR when using this label. This information 682 may be stored for TTL calculation. Once this is done, the LSR may use 683 MPLS forwarding to transmit packets in that FEC. 685 When a member of the Edge Set of the FR-LSR domain receives an LDP 686 "request" message from a FR-LSR for a FEC, it means it is the 687 Egress-FR-LSR. It allocates a label, creates a new entry in its Label 688 Information Base (LIB), places that label in the incoming label 689 component of the entry, and returns (via LDP) a "mapping" message 690 containing the allocated label back upstream to the LDP peer that 691 originated the request. The "mapping" message contains the "hop 692 count" object value set to 1. 694 When a routing calculation causes an Edge LSR to change the next hop 695 for a route, and the former next hop was in the FR-LSR domain, the 696 Edge LSR should notify the former next hop (via an LDP "release" 697 message) that the label binding associated with the route is no 698 longer needed. 700 When a Frame Relay-LSR receives an LDP "request" message for a 701 certain route (FEC) from an LDP peer connected to the FR-LSR over a 702 LC-FR interface, the FR-LSR takes the following actions: 704 - it allocates a label, creates a new entry in its Label 705 Information Base (LIB), and places that label in the incoming 706 label component of the entry; 708 - it propagates the "request", by sending an LDP "request" 709 message to the next hop LSR, downstream for that route (FEC); 711 In the "ordered control" mode [ARCH], the FR-LSR will wait for its 712 "request" to be responded from downstream with a "mapping" message 713 before returning the "mapping" upstream in response to a "request" 714 ("ordered control" approach [ARCH]). In this case, the FR-LSR 715 increments the hop count it received from downstream and uses this 716 value in the "mapping" it returns upstream. 718 Alternatively, the FR-LSR may return the binding upstream without 719 waiting for a binding from downstream ("independent control" approach 720 [ARCH]). In this case, it uses a reserved value for hop count in the 721 "mapping", indicating that it is 'unknown'. The correct value for hop 722 count will be returned later, as described below. 724 Since both the "ordered" and "independent" control has advantages and 725 disadvantages, this is left as an implementation, or configuration 726 choice. 728 Once the FR-LSR receives in response the label binding in an LDP 729 "mapping" message from the next hop, it places the label into the 730 outgoing label component of the LIB entry. 732 Note that a FR-LSR, or a member of the edge set of a FR-LSR domain, 733 may receive multiple binding requests for the same route (FEC) from 734 the same FR-LSR. It must generate a new "mapping" for each "request" 735 (assuming adequate resources to do so), and retain any existing 736 mapping(s). For each "request" received, a FR-LSR should also 737 generate a new binding "request" toward the next hop for the route 738 (FEC). 740 When a routing calculation causes a FR-LSR to change the next hop for 741 a route (FEC), the FR-LSR should notify the former next hop (via an 742 LDP "release" message) that the label binding associated with the 743 route is no longer needed. 745 When a LSR receives a notification that a particular label binding is 746 no longer needed, the LSR may deallocate the label associated with 747 the binding, and destroy the binding. This mode is the "conservative 748 label retention mode" [ARCH]. In the case where a FR-LSR receives 749 such notification and destroys the binding, it should notify the next 750 hop for the route that the label binding is no longer needed. If a 751 LSR does not destroy the binding (the FR-LSR is configured in 752 "liberal label retention mode" [ARCH]), it may re-use the binding 753 only if it receives a request for the same route with the same hop 754 count as the request that originally caused the binding to be 755 created. 757 When a route changes, the label bindings are re-established from the 758 point where the route diverges from the previous route. LSRs 759 upstream of that point are (with one exception, noted below) 760 oblivious to the change. Whenever a LSR changes its next hop for a 761 particular route, if the new next hop is a FR-LSR or a member of the 762 edge set reachable via a LC-FR interface, then for each entry in its 763 LIB associated with the route the LSR should request (via LDP) a 764 binding from the new next hop. 766 When a FR-LSR receives a label binding from a downstream neighbor, it 767 may already have provided a corresponding label binding for this 768 route to an upstream neighbor, either because it is using 769 "independent control" or because the new binding from downstream is 770 the result of a routing change. In this case, it should extract the 771 hop count from the new binding and increment it by one. If the new 772 hop count is different from that which was previously conveyed to the 773 upstream neighbor (including the case where the upstream neighbor was 774 given the value 'unknown') the FR-LSR must notify the upstream 775 neighbor of the change. Each FR-LSR in turn increments the hop count 776 and passes it upstream until it reaches the ingress Edge LSR. 778 Whenever a FR-LSR originates a label binding request to its next hop 779 LSR as a result of receiving a label binding request from another 780 (upstream) LSR, and the request to the next hop LSR is not satisfied, 781 the FR-LSR should destroy the binding created in response to the 782 received request, and notify the requester (via an LDP "withdraw" 783 message). 785 When an LSR determines that it has lost its LDP session with another 786 LSR, the following actions are taken: 788 - MUST discard any binding information learned via this 789 connection; 791 - For any label bindings that were created as a result of 792 receiving label binding requests from the peer, the LSR may 793 destroy these bindings (and deallocate labels associated 794 with these binding). 796 7.2 Efficient use of label space - Merging FR-LSRs 798 The above discussion assumes that an edge LSR will request one label 799 for each prefix in its routing table that has a next hop in the FR- 800 LSR domain. In fact, it is possible to significantly reduce the 801 number of labels needed by having the edge LSR request instead one 802 label for several routes. Use of many-to-one mappings between routes 803 (address prefixes) and labels using the notion of Forwarding 804 Equivalence Classes (as described in [ARCH]) provides a mechanism to 805 conserve the number of labels. 807 Note that conserving label space (VC merging) may be restricted in 808 case the frame traffic requires Frame Relay fragmentation. The issue 809 is that Frame Relay fragments must be transmitted in sequence, i.e. 810 fragments of distinct frames must not be interleaved. If the 811 fragmenting FR-LSR ensures the transmission in sequence of all 812 fragments of a frame, without interleaving with fragments of other 813 frames, then label conservation (VC merging) can be performed. 815 When label conservation is used, when a FR-LSR receives a binding 816 request from an upstream LSR for a certain FEC, and it does already 817 have an outgoing label binding for that FEC, it does not need to 818 issue a downstream binding request. Instead, it may allocate an 819 incoming label, and return that label in a binding to the upstream 820 requester. Packets received from the requester, with that label as 821 top label, will be forwarded after replacing the label with the 822 existing outgoing label for that FEC. If the FR-LSR does not have an 823 outgoing label binding for that FEC, but does have an outstanding 824 request for one, it need not issue another request. This means that 825 in a label conservation case, a FR-LSR must respond with a new 826 binding for every upstream request, but it may need to send one 827 binding request downstream. 829 In case of label conservation, if a change in the routing table 830 causes a FR-LSR to select a new next hop for one of its FECs, it MAY 831 release the binding for that route from the former next hop. If it 832 doesn't already have a corresponding binding for the new next hop, it 833 must request one (note that the choice depends on the label retention 834 mode [ARCH]). 836 If a new binding is obtained, which contain a hop count that differs 837 from that of the old binding, the FR-LSR must process the new hop 838 count: increment by 1, if different than "unknown", and notify the 839 upstream neighbors who have label bindings for this FEC of the new 840 value. To ensure that loops will be detected, if the new hop count 841 exceeds the "maximum" value, the label values for this FEC must be 842 withdrawn from all upstream neighbors to whom a binding was 843 previously sent. 845 7.3 LDP messages specific to Frame Relay 847 The Label Distribution Protocol [LDP] messages exchanged between two 848 Frame Relay "LDP-peer" LSRs may contain Frame Relay specific 849 information such as: 851 "Frame Relay Label Range": 853 0 1 2 3 854 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 1 855 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 856 | Reserved |Len| Minimum DLCI | 857 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 858 | Reserved | Maximum DLCI | 859 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 861 with the following fields: 863 Reserved 864 This fields are reserved. They must be set to zero on transmission 865 and must be ignored on receipt. 867 Len 868 This field specifies the number of bits of the DLCI. The following 869 values are supported: 871 Len DLCI bits 873 0 10 874 1 23 875 Minimum DLCI 876 This 23 bit field is the binary value of the lower bound of a block 877 of Data Link Connection Identifiers (DLCIs) that is supported by 878 the originating FR-LSR. The Minimum DLCI should be right justified 879 in this field and the preceding bits should be set to 0. 881 Maximum DLCI 882 This 23 bit field is the binary value of the upper bound of a block 883 of Data Link Connection Identifiers (DLCIs) that is supported by 884 the originating FR-LSR. The Maximum DLCI should be right justified 885 in this field and the preceding bits should be set to 0. 887 "Frame Relay Merge": 889 0 1 2 3 4 5 6 7 890 +-+-+-+-+-+-+-+-+ 891 | Reserved |M| 892 +-+-+-+-+-+-+-+-+ 894 with the following fields: 896 Merge 897 One bit field that specifies the merge capabilities of the FR-LSR: 899 Value Meaning 901 0 Merge NOT supported 902 1 Merge supported 904 A FR-LSR that supports VC merging MUST ensure that fragmented 905 frames from distinct incoming DLCIs are not interleaved on the 906 outgoing DLCI. 908 Reserved 909 This field is reserved. It must be set to zero on transmission and 910 must be ignored on receipt. 912 and "Frame Relay Label": 914 0 1 2 3 915 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 1 916 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 917 | Reserved |Len| DLCI | 918 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 920 with the following fields: 922 Reserved 923 This field is reserved. It must be set to zero on transmission and 924 must be ignored on receipt. 926 Len 927 This field specifies the number of bits of the DLCI. The following 928 values are supported: 930 Len DLCI bits 932 0 10 933 1 23 935 DLCI 936 The binary value of the Frame Relay Label. The significant number 937 of bits (10 or 23) of the label value are to be encoded into the 938 Data Link Connection Identifier (DLCI) field when part of the Frame 939 Relay data link header (see Section 4.). 941 8. Security Considerations 943 This section looks at the security aspects of: 945 (a) frame traffic, 947 (b) label distribution. 949 MPLS encapsulation has no effect on authenticated or encrypted 950 network layer packets, that is IP packets that are authenticated or 951 encrypted will incur no change. 953 The MPLS protocol has no mechanisms of its own to protect against 954 misdirection of packets or the impersonation of an LSR by accident or 955 malicious intent. 957 Altering by accident or forgery an existent label in the DLCI field 958 of the Frame Relay data link layer header of a frame or one or more 959 fields in a potentially following label stack affects the forwarding 960 of that frame. 962 The label distribution mechanism can be secured by applying the 963 appropriate level of security to the underlying protocol carrying 964 label information - authentication or encryption - see [LDP]. 966 9. Acknowledgments 968 The initial version of this document was derived from the Label 969 Switching over ATM document [ATM]. 971 Thanks for the extensive reviewing and constructive comments from (in 972 alphabetical order) Dan Harrington, Milan Merhar, Martin Mueller, 973 Eric Rosen. Also thanks to George Swallow for the suggestion to use 974 null encapsulation, and to Eric Gray for his reviewing. 976 Also thanks to Nancy Feldman and Bob Thomas for their collaboration 977 in including the LDP messages specific to Frame Relay LSRs. 979 10. References 981 [MIFR] T. Bradley, C. Brown, A. Malis "Multiprotocol Interconnect 982 over Frame Relay" RFC 2427, September 1998. 984 [ARCH] E. Rosen, R. Callon, A. Vishwanathan, "Multi-Protocol Label 985 Switching Architecture", Work in Progress, July 1998. 987 [LDP] L. Anderson, P. Doolan, N. Feldman, A. Fredette, R. Thomas, 988 "Label Distribution Protocol", Work in Progress, August 1998. 990 [STACK] E. Rosen et al, "Label Switching: Label Stack Encodings", 991 Work in Progress, September 1998 993 [ATM] B. Davie et al. "Use of Label Switching with ATM", Work in 994 Progress, July 1998. 996 [ITU] International Telecommunications Union, "ISDN Data Link Layer 997 Specification for Frame Mode Bearer Services", ITU-T Recommendation 998 Q.922, 1992. 1000 [FRF] Frame Relay Forum, User-to-Network Implementation Agreement 1001 (UNI), FRF 1.1, January 19, 1996 1002 11.Authors' Addresses 1004 Alex Conta 1005 3COM 1006 100 3COM Drive 1007 Marlborough, MA 01752 1008 +1 508 323-2297 1009 E-mail: Alex_Conta@ne.3com.com 1011 Paul Doolan 1012 Ennovate Networks 1013 60 Codman Hill Rd 1014 Boxborough MA 01719 1015 +1 978 263-2002 1016 E-mail: pdoolan@ennovatenetworks.com 1018 Andrew Malis 1019 Lucent Technologies 1020 1 Robbins Rd 1021 Westford, MA 01886 1022 +1 978 952-7414 1023 E-mail: amalis@lucent.com 1024 Appendix A - Changes since previous versions 1026 From "version 02 to 03" 1027 - Replace "cloud" with "domain", 1028 - Update references to other documents, 1029 - Change definitions in "Terminology" section, 1030 - Add more definitions to "Terminology" section, 1031 - Make editorial changes to text and figures, 1032 - Change "Performing TTL calculations" section, 1033 - Add more reviewers in "Acknowledgments" section, 1034 - Add Appendix A - changes.