Internet Draft                                        Puneet Agarwal
                                                                 Pluris
                                                          Bora A. Akyol
   Document: draft-ietf-mpls-ttl-01.txt draft-ietf-mpls-ttl-02.txt                   Cisco Systems
   Category: Informational
   Expires: November 2002                                      May 2002

                      TTL

              Time to Live (TTL) Processing in MPLS Networks

   Status of this Memo

   This document is an Internet-Draft and is in full conformance
   with all provisions of Section 10 of RFC2026.

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   Abstract

   This document describes TTL processing in hierarchical MPLS
   networks. TTL processing in both pipe and uniform model hierarchical
   tunnels are specified with examples for both "push" and "pop" cases.
   The document also complements [MPLS-DS] rfc-3270 "MPLS Support of
   Differentiated Services" and ties together the terminology
   introduced in that document with TTL processing in hierarchical MPLS
   networks.

   Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in
   this document are to be interpreted as described in [RFC-2119].

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1. Introduction and Motivation

   This document describes TTL Time to Live (TTL) processing in
   hierarchical MPLS networks. We believe that this document adds
   details that have not been addressed in [MPLS-ARCH, MPLS-ENCAPS],
   and that the methods presented in this document complement [MPLS-DS]. [MPLS-
   DS].

2. TTL Processing in MPLS Networks

  2.1. Changes to RFC 3032 [MPLS-ENCAPS]

        a) [MPLS-ENCAPS] only covers the Uniform Model and does NOT
           address the Pipe Model or the Short Pipe Model. This draft
           will address these 2 models and for completeness will also
           address the Uniform Model.
        b) [MPLS-ENCAPS] does not cover hierarchical LSPs. This draft
           will address this issue.
        c) [MPLS-ENCAPS] does not define TTL processing in the presence
           of Penultimate Hop Popping (PHP). This draft will address
           this issue.

  2.2. Terminology and Background

   As defined in [MPLS-ENCAPS], MPLS packets use a MPLS shim header
   that indicates the following information about a packet:

   a. MPLS Label (20 bits)
   b. TTL (8 bits)
   c. Bottom of stack (1 bit)
   d. Experimental bits (3 bits)

   The experimental bits were later redefined in [MPLS-DS] to indicate
   the scheduling and shaping behavior that could be associated with a
   MPLS packet.

   [MPLS-DS] also defined two models for MPLS tunnel operation: Pipe
   and Uniform models. In the Pipe model, a MPLS network acts like a
   circuit when MPLS packets traverse the network such that only the
   LSP ingress and egress points are visible to nodes that are outside
   the tunnel. A Short variation of the Pipe Model is also defined in
   [MPLS-DS] to differentiate between different egress forwarding and
   QoS treatments. On the other hand, the Uniform model makes all the
   nodes that a LSP traverses visible to nodes outside the tunnel. We
   will extend the Pipe and Uniform models to include TTL processing in
   the following sections. Furthermore, TTL processing when performing
   Penultimate Hop Pop (PHP)
   PHP is also described in this document. For a detailed description
   of Pipe and Uniform models, please see [MPLS-
   DS]. [MPLS-DS].

   TTL processing in MPLS networks can be broken down into two logical
   blocks: (i) the incoming TTL determination to take into account any

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                   TTL Processing in MPLS Networks           May 2002

   tunnel egress due to MPLS Pop operations; (ii) packet processing of
   (possibly) exposed packet & outgoing TTL.

   We also note here that signaling treatment for TTL behavior using
   either RSVP-TE the LSP type (pipe, short pipe or LDP
   uniform model) is out of the scope of this document. document, and that is
   also not addressed in the current versions of the label distribution
   protocols, e.g. LDP [MPLS-LDP] and RSVP-TE [MPLS-RSVP].

  2.3. New Terminology

   iTTL: The TTL value to use as the incoming TTL. No checks are
   performed on the iTTL.

   oTTL: This is the TTL value used as the outgoing TTL value (see
   section 3.5 for exception). It is always (iTTL - 1) unless otherwise
   stated.

   oTTL Check: Check if oTTL is greater than 0. If the oTTL Check is
   false, then the packet is not forwarded. Note that the oTTL check is
   performed only if any outgoing TTL (either IP or MPLS) is set to
   oTTL (see section 3.5 for exception).

3. TTL Processing in different Models

   This sections section describes the TTL processing for LSPs conforming to
   each of the 3 models  (Uniform, Short Pipe and Pipe) in the
   presence/absence of PHP (where applicable).

  3.1. TTL Processing for Uniform Model LSPs (with or without PHP)

        (consistent with [MPLS-ENCAPS]):

                ========== LSP =============================>

                    +--Swap--(n-2)-...-swap--(n-i)---+
                   /        (outer header)            \
                 (n-1)                                (n-i)
                 /                                       \
      >--(n)--Push...............(x).....................Pop--(n-i-1)->
               (I)           (inner header)            (E or P)

      (n) represents the TTL value in the corresponding header
      (x) represents non-meaningful TLL TTL information
      (I) represents the LSP ingress node
      (P) represents the LSP penultimate node
      (E) represents the LSP Egress node

   This picture shows TTL processing for a uniform model MPLS LSP. Note
   that the inner and outer TTLs of the packets are synchronized at
   tunnel ingress and egress.

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  3.2. TTL Processing for Short Pipe Model LSPs

     3.2.1.     TTL Processing for Short Pipe Model LSPs without PHP

                ========== LSP =============================>

                    +--Swap--(N-1)-...-swap--(N-i)-----+
                   /        (outer header)              \
                 (N)                                  (N-i)
                 /                                         \
      >--(n)--Push...............(n-1).....................Pop--(n-2)->
               (I)           (inner header)                (E)

      (N) represents the TTL value (may have no relationship to n)
      (n) represents the tunneled TTL value in the encapsulated header
      (I) represents the LSP ingress node
      (E) represents the LSP Egress node

   Short Pipe Model was introduced in [MPLS-DS]. In the short pipe
   model, the forwarding treatment at the egress LSR is based on the
   tunneled packet as opposed to the encapsulating packet.

     3.2.2.     TTL Processing for Short Pipe Model with PHP:

                ========== LSP =====================================>
                    +-Swap-(N-1)-...-swap-(N-i)-+
                   /       (outer header)        \
                 (N)                            (N-i)
                 /                                \
      >--(n)--Push.............(n-1)............Pop-(n-1)-Decr.-(n-2)->
               (I)           (inner header)       (P)      (E)

      (N) represents the TTL value (may have no relationship to n)
      (n) represents the tunneled TTL value in the encapsulated header
      (I) represents the LSP ingress node
      (P) represents the LSP penultimate node
      (E) represents the LSP egress node.

   Since the label has already the been popped by the LSP LSPÆs penultimate
   node, the LSP egress node just decrements the header TTL.

   Also note that at the end of short pipe model LSP, the TTL of the
   tunneled packet has been decremented by two either with or without
   PHP.

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  3.3. TTL Processing for Pipe Model LSPs (without PHP only):

                ========== LSP =============================>

                    +--Swap--(N-1)-...-swap--(N-i)-----+
                   /        (outer header)              \
                 (N)                                  (N-i)
                 /                                        \
      >--(n)--Push...............(n-1)....................Pop--(n-2)->
               (I)           (inner header)               (E)

      (N) represents the TTL value (may have no relationship to n)
      (n) represents the tunneled TTL value in the encapsulated header
      (I) represents the LSP ingress node
      (E) represents the LSP Egress node

   From the TTL perspective, the treatment for a Pipe Model LSP is
   identical to the Short Pipe Model without PHP.

  3.4. Incoming TTL (iTTL) determination

   If the incoming packet is an IP packet, then the iTTL is the TTL
   value of the incoming IP packet.

   If the incoming packet is a MPLS packet and we are performing a
   Push/Swap/PHP, then the iTTL is the TTL of the topmost incoming
   label.

   If the incoming packet is a MPLS packet and we are performing a Pop
   (tunnel termination), the iTTL is based on the tunnel type (Pipe or
   Uniform) of the LSP that was popped. If the popped label belonged to
   a Pipe model LSP, then the iTTL is the value of the TTL field of the
   header exposed after the label was popped (note that for the purpose
   of this draft, the exposed header may be either an IP header or an
   MPLS label). If the popped label belonged to a Uniform model LSP,
   then the iTTL is equal to the TTL of the popped label. If multiple
   Pop operations are performed sequentially, then the procedure given
   above is repeated with one exception: the iTTL computed during the
   previous Pop is used as the TTL of subsequent label being popped;
   i.e. the TTL contained in the subsequent label is essentially
   ignored and replaced with the iTTL computed during the previous pop.

  3.5. Outgoing TTL Determination and Packet Processing

  After the iTTL computation is performed, the oTTL check is performed.
  If the oTTL check succeeds, then the outgoing TTL of the
  (labeled/unlabeled) packet is calculated and packet headers are
  updated as defined below.

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  If the packet was routed as an IP packet, the TTL value of the IP
  packet is set to oTTL (iTTL - 1). The TTL value(s) for any pushed
  label(s) are determined as described in section 3.6.

  For packets that are routed as MPLS, we have four cases:

        1) Swap-only: The routed label is swapped with another label
           and the TTL field of the outgoing label is set to oTTL.

        2) Swap followed by a Push: The swapped operation is performed
           as described in (1). The TTL value(s) of any pushed label(s)
           are determined as described in section 3.6.

        3) Penultimate Hop Pop (PHP): The routed label is popped. The
           oTTL check should be performed irrespective of whether the
           oTTL is used to update the TTL field of the outgoing header.
           If the PHPed label belonged to a short Pipe model LSP, then
           the TTL field of the PHP exposed header is neither checked
           nor  updated. If the PHPed label was a Uniform model LSP,
           then the TTL field of the PHP exposed header is set to the
           oTTL. The TTL value(s) of additional labels are determined
           as described in section 3.6

        4) Pop: The pop operation happens before routing and hence it
           is not considered here.

  3.6. Tunnel Ingress Processing (Push)

   For each pushed Uniform model label, the TTL is copied from the
   label/IP-packet immediately underneath it.

   For each pushed Pipe model or Short Pipe model label, the TTL field
   is set to a value configured by the network operator. In most
   implementations, this value is set to 255 by default.

  3.7. Implementation Remarks

        1) Although iTTL can be decremented by a value larger than 1
           while it is being updated or oTTL is being determined, this
           feature should be only used for compensating for network
           nodes that are not capable of decrementing TTL values.

        2) Whenever iTTL is decremented, the implementor implementer must make sure
           that the value does not go negative.

        3) In the short pipe model with PHP enabled, the TTL of the
           tunneled packet is unchanged after the PHP operation.

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4. Conclusion

   This Internet Draft describes how TTL field can be processed in a
   MPLS network. We clarified the various methods that are applied in
   the presence of hierarchical tunnels and completed the integration
   of Pipe and Uniform models with TTL processing.

5. Security Considerations

   This document does not add any new security issues other than the
   ones defined in [MPLS-ENCAPS, MPLS-DS]. In particular, the document
   does not define a new protocol or expand an existing one and does
   not introduce security problems into the existing protocols. The
   authors believe that clarification of TTL handling in MPLS networks
   benefits service providers and their customers since troubleshooting
   is simplified.

6. References

   [MPLS-ARCH] E. Rosen, A. Viswanathan, R. Callon, "Multiprotocol
   Label Switching Architecture," Architecture", RFC 3031.

   [MPLS-ENCAPS] E. Rosen, D. Tappan, G. Fedorkow, Y. Rekhter, D.
   Farinacci, T. Li, A. Conta, "MPLS Label Stack Encoding," Encoding", RFC3032.

   [MPLS-DS] F. Le Faucheur, L. Wu, B. Davie, S. Davari, P. Vaananen,
   R. Krishnan, P. Cheval, J. Heinanen, "MPLS Support of Differentiated
   Services," draft-ietf-mpls-diff-ext-09.txt. (Work in progress)
   Services", RFC3270.

   [MPLS-LDP] L. Andersson, P. Doolan, N. Feldman, A. Fredette, B.
   Thomas, "LDP Specification", RFC 3036.

   [MPLS-RSVP] D. Awduche, L. Berger, D. Gan, T. Li, V. Srinivasan, G.
   Swallow, "RSVP-TE: Extensions to RSVP for LSP Tunnels", RFC 3209.

7. Acknowledgements

   The authors would like to thank the members of the MPLS working
   group for their feedback. We would especially like to thank Shahram
   Davari and Loa Andersson for their careful review of the document
   and their comments.

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8. Author's Addresses

   Puneet Agarwal
   Pluris
   10455 Bandley Drive
   Cupertino, CA 95014
   Email: puneet@pluris.com

   Bora Akyol
   Cisco Systems
   170 W. Tasman Drive
   San Jose, CA 95134
   Email: bora@cisco.com

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