





Network Working Group                                          R. Bonica
Request for Comments: 3609                                           MCI
Category: Informational                                      K. Kompella
                                                        Juniper Networks
                                                                D. Meyer
                                                                  Sprint
                                                          September 2003


                Tracing Requirements for Generic Tunnels

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

Abstract

   This document specifies requirements for a generic route-tracing
   application.  It also specifies requirements for a protocol that will
   support that application.  Network operators will use the generic
   route-tracing application to verify proper operation of the IP
   forwarding plane.  They will also use the application to discover
   details regarding tunnels that support IP forwarding.

   The generic route-tracing application, specified herein, supports a
   superset of the functionality that "traceroute" currently offers.
   Like traceroute, the generic route-tracing application can discover
   the forwarding path between two interfaces that are contained by an
   IP network.  Unlike traceroute, this application can reveal details
   regarding tunnels that support the IP forwarding path.

1.  Introduction

   IP networks utilize several tunneling technologies.  Although these
   tunneling technologies provide operators with many useful features,
   they also present management challenges.  Network operators require a
   generic route-tracing application that they can use to verify the
   correct operation of the IP forwarding plane.  The generic
   route-tracing application must be capable of detecting tunnels and
   revealing tunnel details.  The application also must be useful in
   diagnosing tunnel faults.




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   Implementors also require a new protocol that will support the
   generic-route tracing application.  This document specifies
   requirements for that protocol.  It specifies requirements,
   primarily, by detailing the desired capabilities of the generic
   route-tracing application.  A particular version of generic
   route-tracing application may implement some subset of the desired
   capabilities.  It may also implement a superset of those
   capabilities.  However, protocol designers are not required to
   consider the additional capabilities when designing the new protocol.

   This document also specifies a few protocol requirements, stated as
   such.  These requirements are driven by desired characteristics of
   the generic route-tracing application.  Whenever a protocol
   requirement is stated, it is mapped to the desired characteristic of
   the route-tracing application.

2.  Review of Existing Functionality

   Currently, network operators use "traceroute" to trace through the
   forwarding path of an IP network.  Section 3.4 of [RFC-2151] provides
   a thorough description of traceroute.  Although traceroute is very
   reliable and very widely deployed, it is deficient with regard to
   tunnel tracing.

   Depending upon tunnel type, traceroute may display an entire tunnel
   as a single IP hop, or it may display the tunnel as a collection of
   IP hops, without indicating that they are part of a tunnel.

   For example, assume that engineers deploy an IP tunnel in an IP
   network.  Assume also that they configure the tunnel so that the
   ingress router does not copy the TTL value from the inner IP header
   to outer IP header.  Instead, the ingress router always sets the
   outer TTL value to its maximum permitted value.  When engineers trace
   through the network, traceroute will always display the tunnel as a
   single IP hop, hiding all components except the egress interface.

   Now assume that engineers deploy an MPLS LSP in an IP network.
   Assume also that engineers configure the MPLS LSP so that the ingress
   router propagates the TTL value from the IP header to the MPLS
   header.  When engineers trace through the network, traceroute will
   display the LSP as a series of IP hops, without indicating that they
   are part of a tunnel.









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3.  Application Requirements

   Network operators require a new route-tracing application.  The new
   application must support all functionality that traceroute currently
   offers.  It also must provide enhanced tunnel tracing capabilities.

   The following list provides specific requirements for the new
   route-tracing application:

      1) Support the notion of a security token as part of the tunnel
      trace request.  The security token identifies the tracer's
      privileges in tracing tunnels.  Network elements will use this
      security token to determine whether or not to return the requested
      information to the tracer.  In particular, appropriate privileges
      are required for items (2), (3), (6), (8), (10), (13), and (14).

      Justification: Operators may need to discover network forwarding
      details, while concealing those details from unauthorized parties.

      2) Support in-line traces.  An in-line trace reveals the path
      between the host upon which the route-tracing application executes
      and any interface in an IP network.

      Justification: Operators need to discover how the network would
      forward a datagram between any two IP interfaces.

      3) Support third-party traces.  A third-party trace reveals the
      path between any two points in an IP network.  The application
      that initiates a third-party trace need not execute upon a host or
      router that is part of the traced path.  Unlike existing solutions
      [RFC-2151] [RFC-2925], the application will not rely upon IP
      options or require access to the SNMP agent in order to support
      third-party traces.

      Justification: Operators need to discover how the network would
      forward a datagram between any two IP interfaces.

      4) Support partial traces through broken paths or tunnels.

      Justification: Operators need to identify the root cause of
      forwarding plane failures.

      5) When tracing through a tunnel, either as part of an in-line
      trace or a third-party trace, display the tunnel either as a
      single IP hop or in detail.  The user's request determines how the
      application displays tunnels, subject to the user having
      permission to do this.




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      Justification: As they discover IP forwarding details, operators
      may need to reveal or mask tunneling details.

      6) When displaying a tunnel in detail, include the tunnel type
      (e.g., GRE, MPLS), the tunnel name (if applicable), the tunnel
      identifier (if applicable) and tunnel endpoint addresses.  Also,
      include tunnel components and round trip delay across each
      component.

      Justification: As they discover IP forwarding details, operators
      may need to reveal tunneling details.

      7) Support the following tunneling technologies: GRE, MPLS, IPSEC,
      GMPLS, IP-in-IP, L2TP.  Be easily extensible to support new tunnel
      technologies.

      Justification: Operators will use the generic route-tracing
      application to discover how an IP network forwards datagrams.  As
      many tunnel types may support the IP network, the generic
      route-tracing application must detect and reveal details
      concerning multiple tunnel types.

      8) Trace through nested, heterogeneous tunnels (e.g., IP-in-IP
      over MPLS).

      Justification: Operators will use the generic route-tracing
      application to discover how an IP network forwards datagrams.  As
      nested, heterogeneous tunnels may support the IP network, the
      generic route-tracing application must detect and reveal details
      concerning nested, heterogeneous tunnels.

      9) At the users request, trace through the forwarding plane, the
      control plane or both.

      Justification: Operators need to identify the root cause of
      forwarding plane failures.  Control plane information is sometimes
      useful in determining the cause of forwarding plane failure.

      10) Support control plane tracing for all tunnel types.  When
      tracing through the control plane, the hop ingress device reports
      hop details.  The hop ingress device is the device that originates
      the hop.

      Justification: Control plane information is available regarding
      all tunnel types.






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      11) Support tracing through forwarding plane for all tunnel types
      that implement TTL decrement (or some similar mechanism).  When
      tracing through the forwarding plane, the hop egress device
      reports hop details.  The hop egress device is the device that
      terminates the hop.

      Justification: Forwarding plane information may not be available
      for tunnels that do not support TTL decrement.

      12) Support tracing through the forwarding plane for all tunnel
      types that implement TTL decrement, regardless of whether the
      tunnel engages in TTL propagation.  (That is, support tunnel
      tracing regardless of whether the TTL value is copied from an
      inner header to an outer header at tunnel ingress.)

      Justification: Forwarding plane information is always available,
      regardless of whether the tunnel engages in TTL propagation.

      13) When tracing through the control plane, display the MTU
      associated with each interface that forwards datagrams through the
      traced path.

      Justification: MTU information is sometimes useful in identifying
      the root cause of forwarding and control plane failures.

      14) When tracing through the forwarding plane, display the MTU
      associated with each interface that receives datagrams along the
      traced path.

      Justification: MTU information is sometimes useful in identifying
      the root cause of forwarding and control plane failures.

      15) Support partial traces through paths containing devices that
      do not provide protocol support for generic route tracing.  When
      the application encounters such a device, it should inform the
      user and attempt to discover details regarding the next interface
      downstream.

      Justification: The application must provide useful information
      even if the supporting protocol is not universally deployed.

4.  Protocol Requirements

   Implementors require a new protocol that supports the generic
   route-tracing application.  This protocol reveals the path between
   two points in an IP network.  When access policy permits, the
   protocol also reveals tunnel details.




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4.1.  Information Requirements

   The protocol consists of probes and probe responses.  Each probe
   elicits exactly one response.  Each response represents a hop that
   contributes to the path between two interfaces.  A hop can be either
   a top-level IP hop or lower-level hop that is contained by a tunnel.

   Justification: Because the generic route-tracing application must
   trace through broken paths, the required protocol must use a separate
   response message to deliver details regarding each hop.  The protocol
   must use a separate probe to elicit each response because the
   alternative approach, using the single probe with the IP Router Alert
   Option, is unacceptable.  Many networks forward datagrams that
   specify IP options differently than they would forward datagrams that
   do not specify IP options.  Therefore, the introduction of IP options
   would cause the application to trace a forwarding path other than the
   path that its user intended to trace.

4.2.  Transport Layer Requirements

   UDP should carry all protocol messages to their destinations.  Other
   transport mechanisms may be considered when protocol details are
   specified.

   Justification: Because the probe/response scheme described above is
   stateless, a stateless transport is required.  Candidate transports
   included UDP over IP, IP and ICMP.  ICMP was disqualified because
   carrying MPLS information in an ICMP datagram would constitute a
   layer violation.  IP was disqualified in order to conserve protocol
   identifiers.

4.3.  Stateless Protocol

   The protocol must be stateless.  That is, nodes should not have to
   maintain state between successive traceroute messages.

   Justification: Statelessness is required to support scaling and to
   prevent denial of service attacks.

4.4.  Routing Requirements

   The device that hosts the route-tracing application must maintain an
   IP route to the ingress of the traced path.  It must also maintain an
   IP route to the ingress of each tunnel for which it is requesting
   tunnel details.  The device that hosts the tunnel tracing application
   need not maintain a route to any other device that supports the
   traced path.




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   All of the devices to which the route-tracing application must
   maintain a route must maintain a route back to the route-tracing
   application.

   In order for the protocol to provide tunnel details, all devices
   contained by a tunnel must maintain an IP route to the tunnel
   ingress.

   Justification: The protocol must be sufficiently robust to operate
   when tunnel interior devices do not maintain a route back to the
   device that hosts the route tracing application.

5.  Security Considerations

   A configurable access control policy determines the degree to which
   features described herein are delivered.  The access control policy
   requires user identification and authorization.

   The new protocol must not introduce security holes nor consume
   excessive resources (e.g., CPU, bandwidth).  It also must not be
   exploitable by those launching DoS attacks or replaying messages.

6.  Informative References

   [RFC-2151]  Kessler, G. and S. Shepard, "A Primer On Internet and
               TCP/IP Tools and Utilities", FYI 30, RFC 2151, June 1997.

   [RFC-2925]  White, K., "Definitions of Managed Objects for Remote
               Ping, Traceroute, and Lookup Operations", RFC 2925,
               September 2000.

7.  Acknowledgements

   Thanks to Randy Bush and Steve Bellovin for their comments.

















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8.  Authors' Addresses

   Ronald P. Bonica
   MCI
   22001 Loudoun County Pkwy
   Ashburn, Virginia, 20147

   EMail: ronald.p.bonica@mci.com


   Kireeti Kompella
   Juniper Networks, Inc.
   1194 N. Mathilda Ave.
   Sunnyvale, California 94089

   EMail: kireeti@juniper.net


   David Meyer

   EMail: dmm@maoz.com






























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9.  Full Copyright Statement

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assignees.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.



















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