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Akyol 3 Document: draft-agarwal-mpls-ttl-00.txt Pluris 4 Category Informational 5 Expires: August 2001 February 2001 7 TTL Processing in MPLS Networks 9 Status of this Memo 11 This document is an Internet-Draft and is in full conformance 12 with all provisions of Section 10 of RFC2026. 14 Internet-Drafts are working documents of the Internet Engineering 15 Task Force (IETF), its areas, and its working groups. Note that 16 other groups may also distribute working documents as Internet- 17 Drafts. 19 Internet-Drafts are draft documents valid for a maximum of six 20 months and may be updated, replaced, or obsoleted by other documents 21 at any time. It is inappropriate to use Internet-Drafts as 22 reference material or to cite them other than as "work in progress." 24 The list of current Internet-Drafts can be accessed at 25 http://www.ietf.org/ietf/1id-abstracts.txt 26 The list of Internet-Draft Shadow Directories can be accessed at 27 http://www.ietf.org/shadow.html. 29 Abstract 31 This document describes TTL processing in hierarchical MPLS 32 networks. 34 Conventions used in this document 36 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 37 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in 38 this document are to be interpreted as described in [RFC-2119]. 40 1. Introduction and Motivation 42 This document describes TTL processing in hierarchical MPLS 43 networks. We believe that this document adds details that have not 44 been addressed in [MPLS-ARCH, MPLS-ENCAPS], and that the methods 45 presented in this document complement [MPLS-DS]. 47 2. TTL Processing in MPLS Networks 48 2.1 Terminology and Background 50 As defined in [MPLS-ENCAPS], MPLS packets use a MPLS shim header 51 that indicates the following information about a packet: 53 a. MPLS Label (20 bits) 54 b. TTL (8 bits) 55 c. Bottom of stack (1 bit) 56 d. Experimental bits (3 bits) 58 The experimental bits were later redefined in [MPLS-DS] to indicate 59 the scheduling and shaping behavior that could be associated with a 60 MPLS packet. 62 [MPLS-DS] also defined two models for MPLS tunnel operation: Pipe 63 and Uniform models. In the Pipe model, a MPLS network acts like a 64 conduit when MPLS packets traverse the network such that only the 65 LSP ingress and egress points are visible to nodes that are outside 66 the tunnel. On the other hand, the Uniform model makes all the nodes 67 that a LSP traverses visible to nodes outside the tunnel. We will 68 extend the Pipe and Uniform models to include TTL processing in the 69 following sections. Furthermore, TTL processing when performing 70 Penultimate Hop Pop (PHP) is also described in this document. For a 71 detailed description of Pipe and Uniform models, please see [MPLS- 72 DS]. 74 TTL processing in MPLS networks can be broken down into two logical 75 blocks: (i) the incoming TTL determination to take into account any 76 tunnel egress due to MPLS Pop operations; (ii) packet processing of 77 (possibly) exposed packet & outgoing TTL. 79 2.2 New Terminology 81 iTTL: The TTL value to use as the incoming TTL. No checks are 82 performed on the iTTL. 84 oTTL: This is the TTL value used as the outgoing TTL value. It is 85 always (iTTL _ 1) unless otherwise stated. 87 oTTL Check: Check if oTTL is greater than 0. If the oTTL Check is 88 false, then the packet is not forwarded. Note that the oTTL check is 89 performed only if any outgoing TTL (either IP or MPLS) is set to 90 oTTL. 92 2.3 Incoming TTL (iTTL) determination 94 If the incoming packet is an IP packet, then the iTTL is the TTL 95 value of the incoming IP packet. 97 If the incoming packet is a MPLS packet and we are performing a 98 Push/Swap/PHP, then the iTTL is the TTL of the topmost incoming 99 label. 101 If the incoming packet is a MPLS packet and we are performing a Pop 102 (tunnel termination), the iTTL is based on the tunnel type (Pipe or 103 Uniform) of the LSP that was popped. If the popped label belonged to 104 a Pipe model LSP, then the iTTL is the TTL of the label/IP-packet 105 exposed after the label was popped. If the popped label belonged to 106 a Uniform model LSP, then the iTTL is equal to the TTL of the popped 107 label. If multiple Pop operations are performed sequentially, then 108 the procedure given above is repeated with one exception: the iTTL 109 computed during the previous Pop is used as the TTL of subsequent 110 label being popped; i.e. the TTL contained in the subsequent label 111 is essentially ignored and replaced with the iTTL computed during 112 the previous pop. 114 2.4 Outgoing TTL Determination and Packet Processing 116 After the iTTL computation is performed, the outgoing TTL of 117 the (labeled) packet is calculated and packet headers are 118 updated. 120 If the packet was routed as an IP packet, the TTL value of the 121 IP packet is set to oTTL (iTTL _ 1). The TTL value(s) for any 122 pushed label(s) are determined as described in section 2.5. 124 For packets that are routed as MPLS, we have three cases: 126 1) Swap-only: The routed label is swapped with another label 127 and the TTL of the outgoing label is set to oTTL. 129 2) Swap followed by a Push: The swapped operation is performed 130 as described in (1). The TTL value(s) of any pushed label(s) 131 are determined as described in section 2.5. 133 3) Penultimate Hop Pop (PHP): The routed label is popped. The 134 oTTL check should be performed irrespective of whether the oTTL 135 is used in any outgoing label/IP-header. The oTTL used for the 136 TTL check is the unmodified oTTL (iTTL _1). If the PHPed label 137 belonged to a Pipe model LSP, then the oTTL is set to the TTL 138 of the PHP exposed IP-packet/label - but the TTL of the PHP 139 exposed IP-header/label is NOT updated. If the PHPed label was 140 a Uniform model LSP, then the TTL of the PHP exposed IP- 141 header/label is set to the oTTL. The TTL values of additional 142 labels are determined as described in Section 2.5. 144 2.5 Tunnel Ingress Processing (Push) 146 For each pushed Uniform model label, the TTL is copied from the 147 label/IP-packet immediately underneath it. 149 For each pushed Pipe model label, the TTL field is set to a value 150 configured by the network operator. In most implementations, this 151 value is set to 255 by default. 153 3. Conclusion 155 This Internet Draft describes how TTL field can be processed in a 156 MPLS network. We clarified the various methods that are applied in 157 the presence of hierarchical tunnels and completed the integration 158 of Pipe and Uniform models with TTL processing. 160 4. Security Considerations 162 This document does not add any new security issues other than the 163 ones defined in [MPLS-ENCAPS, MPLS-DS]. 165 5. References 167 [MPLS-ARCH] E. Rosen, A. Viswanathan, R. Callon, "Multiprotocol 168 Label Switching Architecture," RFC 3031. 170 [MPLS-ENCAPS] E. Rosen, D. Tappan, G. Fedorkow, Y. Rekhter, D. 171 Farinacci, T. Li, A. Conta, "MPLS Label Stack Encoding," RFC3032. 173 [MPLS-DS] F. Le Faucheur, L. Wu, B. Davie, S. Davari, P. Vaananen, 174 R. Krishnan, P. Cheval, J. Heinanen, "MPLS Support of Differentiated 175 Services," draft-ietf-mpls-diff-ext-07.txt. (Work in progress) 177 Author's Addresses 179 Puneet Agarwal 180 Pluris 181 10455 Bandley Drive 182 Cupertino, CA 95014 183 Email: puneet@pluris.com 185 Bora Akyol 186 Pluris 187 10455 Bandley Drive 188 Cupertino, CA 95014 189 Email: akyol@akyol.org 191 Expiration 193 This document will expire in August 2001.