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Akyol 5 Document: draft-ietf-mpls-ttl-01.txt Cisco Systems 6 Category: Informational 7 Expires: November 2002 May 2002 9 TTL Processing in MPLS Networks 11 Status of this Memo 13 This document is an Internet-Draft and is in full conformance 14 with all provisions of Section 10 of RFC2026. 16 Internet-Drafts are working documents of the Internet Engineering 17 Task Force (IETF), its areas, and its working groups. Note that 18 other groups may also distribute working documents as Internet- 19 Drafts. 21 Internet-Drafts are draft documents valid for a maximum of six 22 months and may be updated, replaced, or obsoleted by other documents 23 at any time. It is inappropriate to use Internet-Drafts as 24 reference material or to cite them other than as "work in progress." 26 The list of current Internet-Drafts can be accessed at 27 http://www.ietf.org/ietf/1id-abstracts.txt 28 The list of Internet-Draft Shadow Directories can be accessed at 29 http://www.ietf.org/shadow.html. 31 Abstract 33 This document describes TTL processing in hierarchical MPLS 34 networks. TTL processing in both pipe and uniform model hierarchical 35 tunnels are specified with examples for both "push" and "pop" cases. 36 The document also complements [MPLS-DS] and ties together the 37 terminology introduced in that document with TTL processing in 38 hierarchical MPLS networks. 40 Conventions used in this document 42 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 43 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in 44 this document are to be interpreted as described in [RFC-2119]. 46 1. Introduction and Motivation 48 This document describes TTL processing in hierarchical MPLS 49 networks. We believe that this document adds details that have not 50 been addressed in [MPLS-ARCH, MPLS-ENCAPS], and that the methods 51 presented in this document complement [MPLS-DS]. 53 2. TTL Processing in MPLS Networks 55 2.1. Changes to RFC 3032 [MPLS-ENCAPS] 57 a) [MPLS-ENCAPS] only covers the Uniform Model and does NOT 58 address the Pipe Model or the Short Pipe Model. This draft 59 will address these 2 models and for completeness will also 60 address the Uniform Model. 61 b) [MPLS-ENCAPS] does not cover hierarchical LSPs. This draft 62 will address this issue. 63 c) [MPLS-ENCAPS] does not define TTL processing in the presence 64 of Penultimate Hop Popping (PHP). This draft will address 65 this issue. 67 2.2. Terminology and Background 69 As defined in [MPLS-ENCAPS], MPLS packets use a MPLS shim header 70 that indicates the following information about a packet: 72 a. MPLS Label (20 bits) 73 b. TTL (8 bits) 74 c. Bottom of stack (1 bit) 75 d. Experimental bits (3 bits) 77 The experimental bits were later redefined in [MPLS-DS] to indicate 78 the scheduling and shaping behavior that could be associated with a 79 MPLS packet. 81 [MPLS-DS] also defined two models for MPLS tunnel operation: Pipe 82 and Uniform models. In the Pipe model, a MPLS network acts like a 83 circuit when MPLS packets traverse the network such that only the 84 LSP ingress and egress points are visible to nodes that are outside 85 the tunnel. A Short variation of the Pipe Model is also defined in 86 [MPLS-DS] to differentiate between different egress forwarding and 87 QoS treatments. On the other hand, the Uniform model makes all the 88 nodes that a LSP traverses visible to nodes outside the tunnel. We 89 will extend the Pipe and Uniform models to include TTL processing in 90 the following sections. Furthermore, TTL processing when performing 91 Penultimate Hop Pop (PHP) is also described in this document. For a 92 detailed description of Pipe and Uniform models, please see [MPLS- 93 DS]. 95 TTL processing in MPLS networks can be broken down into two logical 96 blocks: (i) the incoming TTL determination to take into account any 97 tunnel egress due to MPLS Pop operations; (ii) packet processing of 98 (possibly) exposed packet & outgoing TTL. 100 We also note here that signaling treatment for TTL behavior using 101 either RSVP-TE or LDP is out of the scope of this document. 103 2.3. New Terminology 105 iTTL: The TTL value to use as the incoming TTL. No checks are 106 performed on the iTTL. 108 oTTL: This is the TTL value used as the outgoing TTL value (see 109 section 3.5 for exception). It is always (iTTL - 1) unless otherwise 110 stated. 112 oTTL Check: Check if oTTL is greater than 0. If the oTTL Check is 113 false, then the packet is not forwarded. Note that the oTTL check is 114 performed only if any outgoing TTL (either IP or MPLS) is set to 115 oTTL (see section 3.5 for exception). 117 3. TTL Processing in different Models 119 This sections describes the TTL processing for LSPs conforming to 120 each of the 3 models (Uniform, Short Pipe and Pipe) in the 121 presence/absence of PHP (where applicable). 123 3.1. TTL Processing for Uniform Model LSPs (with or without PHP) 125 (consistent with [MPLS-ENCAPS]): 127 ========== LSP =============================> 129 +--Swap--(n-2)-...-swap--(n-i)---+ 130 / (outer header) \ 131 (n-1) (n-i) 132 / \ 133 >--(n)--Push...............(x).....................Pop--(n-i-1)-> 134 (I) (inner header) (E or P) 136 (n) represents the TTL value in the corresponding header 137 (x) represents non-meaningful TLL information 138 (I) represents the LSP ingress node 139 (P) represents the LSP penultimate node 140 (E) represents the LSP Egress node 142 This picture shows TTL processing for a uniform model MPLS LSP. Note 143 that the inner and outer TTLs of the packets are synchronized at 144 tunnel ingress and egress. 146 3.2. TTL Processing for Short Pipe Model LSPs 148 3.2.1. TTL Processing for Short Pipe Model LSPs without PHP 150 ========== LSP =============================> 152 +--Swap--(N-1)-...-swap--(N-i)-----+ 153 / (outer header) \ 154 (N) (N-i) 155 / \ 156 >--(n)--Push...............(n-1).....................Pop--(n-2)-> 157 (I) (inner header) (E) 159 (N) represents the TTL value (may have no relationship to n) 160 (n) represents the tunneled TTL value in the encapsulated header 161 (I) represents the LSP ingress node 162 (E) represents the LSP Egress node 164 Short Pipe Model was introduced in [MPLS-DS]. In the short pipe 165 model, the forwarding treatment at the egress LSR is based on the 166 tunneled packet as opposed to the encapsulating packet. 168 3.2.2. TTL Processing for Short Pipe Model with PHP: 170 ========== LSP =====================================> 171 +-Swap-(N-1)-...-swap-(N-i)-+ 172 / (outer header) \ 173 (N) (N-i) 174 / \ 175 >--(n)--Push.............(n-1)............Pop-(n-1)-Decr.-(n-2)-> 177 (I) (inner header) (P) (E) 179 (N) represents the TTL value (may have no relationship to n) 180 (n) represents the tunneled TTL value in the encapsulated header 181 (I) represents the LSP ingress node 182 (P) represents the LSP penultimate node 183 (E) represents the LSP egress node. 185 Since the label has already the popped by the LSP penultimate node, 186 the LSP egress node just decrements the header TTL. 188 Also note that at the end of short pipe model LSP, the TTL of the 189 tunneled packet has been decremented by two either with or without 190 PHP. 192 3.3. TTL Processing for Pipe Model LSPs (without PHP only): 194 ========== LSP =============================> 196 +--Swap--(N-1)-...-swap--(N-i)-----+ 197 / (outer header) \ 198 (N) (N-i) 199 / \ 200 >--(n)--Push...............(n-1)....................Pop--(n-2)-> 201 (I) (inner header) (E) 203 (N) represents the TTL value (may have no relationship to n) 204 (n) represents the tunneled TTL value in the encapsulated header 205 (I) represents the LSP ingress node 206 (E) represents the LSP Egress node 208 From the TTL perspective, the treatment for a Pipe Model LSP is 209 identical to the Short Pipe Model without PHP. 211 3.4. Incoming TTL (iTTL) determination 213 If the incoming packet is an IP packet, then the iTTL is the TTL 214 value of the incoming IP packet. 216 If the incoming packet is a MPLS packet and we are performing a 217 Push/Swap/PHP, then the iTTL is the TTL of the topmost incoming 218 label. 220 If the incoming packet is a MPLS packet and we are performing a Pop 221 (tunnel termination), the iTTL is based on the tunnel type (Pipe or 222 Uniform) of the LSP that was popped. If the popped label belonged to 223 a Pipe model LSP, then the iTTL is the value of the TTL field of the 224 header exposed after the label was popped (note that for the purpose 225 of this draft, the exposed header may be either an IP header or an 226 MPLS label). If the popped label belonged to a Uniform model LSP, 227 then the iTTL is equal to the TTL of the popped label. If multiple 228 Pop operations are performed sequentially, then the procedure given 229 above is repeated with one exception: the iTTL computed during the 230 previous Pop is used as the TTL of subsequent label being popped; 231 i.e. the TTL contained in the subsequent label is essentially 232 ignored and replaced with the iTTL computed during the previous pop. 234 3.5. Outgoing TTL Determination and Packet Processing 236 After the iTTL computation is performed, the oTTL check is performed. 237 If the oTTL check succeeds, then the outgoing TTL of the 238 (labeled/unlabeled) packet is calculated and packet headers are 239 updated as defined below. 241 If the packet was routed as an IP packet, the TTL value of the IP 242 packet is set to oTTL (iTTL - 1). The TTL value(s) for any pushed 243 label(s) are determined as described in section 3.6. 245 For packets that are routed as MPLS, we have four cases: 247 1) Swap-only: The routed label is swapped with another label 248 and the TTL field of the outgoing label is set to oTTL. 250 2) Swap followed by a Push: The swapped operation is performed 251 as described in (1). The TTL value(s) of any pushed label(s) 252 are determined as described in section 3.6. 254 3) Penultimate Hop Pop (PHP): The routed label is popped. The 255 oTTL check should be performed irrespective of whether the 256 oTTL is used to update the TTL field of the outgoing header. 257 If the PHPed label belonged to a short Pipe model LSP, then 258 the TTL field of the PHP exposed header is neither checked 259 nor updated. If the PHPed label was a Uniform model LSP, 260 then the TTL field of the PHP exposed header is set to the 261 oTTL. The TTL value(s) of additional labels are determined 262 as described in section 3.6 264 4) Pop: The pop operation happens before routing and hence it 265 is not considered here. 267 3.6. Tunnel Ingress Processing (Push) 269 For each pushed Uniform model label, the TTL is copied from the 270 label/IP-packet immediately underneath it. 272 For each pushed Pipe model or Short Pipe model label, the TTL field 273 is set to a value configured by the network operator. In most 274 implementations, this value is set to 255 by default. 276 3.7. Implementation Remarks 278 1) Although iTTL can be decremented by a value larger than 1 279 while it is being updated or oTTL is being determined, this 280 feature should be only used for compensating for network 281 nodes that are not capable of decrementing TTL values. 283 2) Whenever iTTL is decremented, the implementor must make sure 284 that the value does not go negative. 286 3) In the short pipe model with PHP enabled, the TTL of the 287 tunneled packet is unchanged after the PHP operation. 289 4. Conclusion 291 This Internet Draft describes how TTL field can be processed in a 292 MPLS network. We clarified the various methods that are applied in 293 the presence of hierarchical tunnels and completed the integration 294 of Pipe and Uniform models with TTL processing. 296 5. Security Considerations 298 This document does not add any new security issues other than the 299 ones defined in [MPLS-ENCAPS, MPLS-DS]. In particular, the document 300 does not define a new protocol or expand an existing one and does 301 not introduce security problems into the existing protocols. The 302 authors believe that clarification of TTL handling in MPLS networks 303 benefits service providers and their customers since troubleshooting 304 is simplified. 306 6. References 308 [MPLS-ARCH] E. Rosen, A. Viswanathan, R. Callon, "Multiprotocol 309 Label Switching Architecture," RFC 3031. 311 [MPLS-ENCAPS] E. Rosen, D. Tappan, G. Fedorkow, Y. Rekhter, D. 312 Farinacci, T. Li, A. Conta, "MPLS Label Stack Encoding," RFC3032. 314 [MPLS-DS] F. Le Faucheur, L. Wu, B. Davie, S. Davari, P. Vaananen, 315 R. Krishnan, P. Cheval, J. Heinanen, "MPLS Support of Differentiated 316 Services," draft-ietf-mpls-diff-ext-09.txt. (Work in progress) 318 7. Author's Addresses 320 Puneet Agarwal 321 Pluris 322 10455 Bandley Drive 323 Cupertino, CA 95014 324 Email: puneet@pluris.com 326 Bora Akyol 327 Cisco Systems 328 170 W. Tasman Drive 329 San Jose, CA 95134 330 Email: bora@cisco.com