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RFC 6720 - The Generalized TTL Security Mechanism (GTSM) for the


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Internet Engineering Task Force (IETF)                      C. Pignataro
Request for Comments: 6720                                      R. Asati
Updates: 5036                                              Cisco Systems
Category: Standards Track                                    August 2012
ISSN: 2070-1721

           The Generalized TTL Security Mechanism (GTSM) for
                 the Label Distribution Protocol (LDP)

Abstract

   The Generalized TTL Security Mechanism (GTSM) describes a generalized
   use of a packet's Time to Live (TTL) (IPv4) or Hop Limit (IPv6) to
   verify that the packet was sourced by a node on a connected link,
   thereby protecting the router's IP control plane from CPU
   utilization-based attacks.  This technique improves security and is
   used by many protocols.  This document defines the GTSM use for the
   Label Distribution Protocol (LDP).

   This specification uses a bit reserved in RFC 5036 and therefore
   updates RFC 5036.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6720.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................2
      1.1. Specification of Requirements ..............................3
      1.2. Scope ......................................................3
   2. GTSM Procedures for LDP .........................................4
      2.1. GTSM Flag in the Common Hello Parameter TLV ................4
      2.2. GTSM Sending and Receiving Procedures for LDP Link Hello ...5
      2.3. GTSM Sending and Receiving Procedures for LDP
           Initialization .............................................5
   3. LDP Peering Scenarios and GTSM Considerations ...................5
   4. Security Considerations .........................................6
   5. Acknowledgments .................................................7
   6. References ......................................................7
      6.1. Normative References .......................................7
      6.2. Informative References .....................................8

1.  Introduction

   LDP [RFC5036] specifies two peer discovery mechanisms, a Basic one
   and an Extended one, both using UDP transport.  The Basic Discovery
   mechanism is used to discover LDP peers that are directly connected
   at the link level, whereas the Extended Discovery mechanism is used
   to locate Label Switching Router (LSR) neighbors that are not
   directly connected at the link level.  Once discovered, the LSR
   neighbors can establish the LDP peering session, using the TCP
   transport connection.

   The Generalized TTL Security Mechanism (GTSM) [RFC5082] is a
   mechanism based on IPv4 Time To Live (TTL) or IPv6 Hop Limit value
   verification so as to provide a simple and reasonably robust defense
   from infrastructure attacks using forged protocol packets from
   outside the network.  GTSM can be applied to any protocol peering

   session that is established between routers that are adjacent.
   Therefore, GTSM can protect an LDP protocol peering session
   established using Basic Discovery.

   This document specifies LDP enhancements to accommodate GTSM.  In
   particular, this document specifies the enhancements in the following
   areas:

   1.  The Common Hello Parameter TLV of LDP Link Hello message

   2.  Sending and Receiving procedures for LDP Link Hello message

   3.  Sending and Receiving procedures for LDP Initialization message

   GTSM specifies that "it SHOULD NOT be enabled by default in order to
   remain backward compatible with the unmodified protocol" (see Section
   3 of [RFC5082]).  This document specifies a "built-in dynamic GTSM
   capability negotiation" for LDP to suggest the use of GTSM.  GTSM
   will be used as specified in this document provided both peers on an
   LDP session can detect each others' support for GTSM procedures and
   agree to use it.  That is, the desire to use GTSM (i.e., its
   negotiation mechanics) is enabled by default without any
   configuration.

   This specification uses a bit reserved in Section 3.5.2 of [RFC5036]
   and therefore updates [RFC5036].

1.1.  Specification of Requirements

   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 [RFC2119].

1.2.  Scope

   This document defines procedures for LDP using IPv4 routing but not
   for LDP using IPv6 routing, since the latter has GTSM built into the
   protocol definition [LDP-IPV6].

   Additionally, the GTSM for LDP specified in this document applies
   only to single-hop LDP peering sessions and not to multi-hop LDP
   peering sessions, in line with Section 5.5 of [RFC5082].
   Consequently, any LDP method or feature (such as LDP IGP
   Synchronization [RFC5443] or LDP Session Protection [LDP-SPROT]) that
   relies on multi-hop LDP peering sessions would not work with GTSM and
   will require (statically or dynamically) disabling the GTSM
   capability.  See Section 3.

2.  GTSM Procedures for LDP

2.1.  GTSM Flag in the Common Hello Parameter TLV

   A new flag in the Common Hello Parameter TLV, named G flag (for
   GTSM), is defined by this document in a previously reserved bit.  An
   LSR indicates that it is capable of applying GTSM procedures, as
   defined in this document, to the subsequent LDP peering session, by
   setting the GTSM flag to 1.  The Common Hello Parameters TLV, defined
   in Section 3.5.2 of [RFC5036], is updated as shown in Figure 1.

     0                   1                   2                   3
     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |0|0| Common Hello Parms(0x0400)|      Length                   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |      Hold Time                |T|R|G|   Reserved              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    T, Targeted Hello
       As specified in [RFC5036].

    R, Request Send Targeted Hellos
       As specified in [RFC5036].

    G, GTSM
       A value of 1 specifies that this LSR supports GTSM procedures,
       where a value of 0 specifies that this LSR does not support GTSM.

    Reserved
       This field is reserved.  It MUST be set to zero on transmission
       and ignored on receipt.

           Figure 1: GTSM Flag in the Common Hello Parameter TLV

   The G flag is meaningful only if the T flag is set to 0 (which must
   be the case for Basic Discovery); otherwise, the value of the G flag
   is ignored on receipt.

   Any LSR not supporting GTSM for LDP as defined in this document
   (i.e., an LSR that does not recognize the G flag) would continue to
   ignore the G flag, independent of the values of the T and R flags, as
   per Section 3.5.2 of [RFC5036].  Similarly, an LSR that does
   recognize the G flag but that does not support GTSM (either because
   it is not implemented or because it is so configured) would not set
   the G flag (i.e., G=0) when sending LDP Link Hellos and would
   effectively ignore the G flag when receiving LDP Link Hello messages.

2.2.  GTSM Sending and Receiving Procedures for LDP Link Hello

   First, LSRs using LDP Basic Discovery [RFC5036] send LDP Hello
   messages to link-level multicast address (224.0.0.2 or "all
   routers").  Such messages are never forwarded beyond one hop and are
   RECOMMENDED to have their IP TTL or Hop Count = 1.

   Unless configured otherwise, an LSR that supports GTSM procedures
   MUST set the G flag (for GTSM) to 1 in the Common Hello Parameter TLV
   in the LDP Link Hello message [RFC5036].

   If an LSR that supports GTSM and is configured to use it recognizes
   the presence of the G flag (in the Common Hello Parameter TLV) with
   the value = 1 in the received LDP Link Hello message, then it MUST
   enforce GTSM for LDP in the subsequent TCP/LDP peering session with
   the neighbor that sent the Hello message, as specified in Section 2.3
   of this document.

   If an LSR does not recognize the presence of the G flag (in the
   Common Hello Parameter TLV of Link Hello message), or recognizes the
   presence of G flag with the value = 0, then the LSR MUST NOT enforce
   GTSM for LDP in the subsequent TCP/LDP peering session with the
   neighbor that sent the Hello message.  This ensures backward
   compatibility as well as automatic GTSM deactivation.

2.3.  GTSM Sending and Receiving Procedures for LDP Initialization

   If an LSR that has sent and received LDP Link Hello with G flag = 1
   from the directly connected neighbor, then the LSR MUST enforce GTSM
   procedures, as defined in Section 3 of [RFC5082], in the forthcoming
   TCP Transport Connection with that neighbor.  This means that the LSR
   MUST check for the incoming unicast packets' TTL or Hop Count to be
   255 for the particular LDP/TCP peering session and decide the further
   processing as per [RFC5082].

   If an LSR that has sent LDP Link Hello with G flag = 1, but received
   LDP Link Hello with G flag = 0 from the directly connected neighbor,
   then the LSR MUST NOT enforce GTSM procedures, as defined in Section
   3 of [RFC5082], in the forthcoming TCP Transport Connection with that
   neighbor.

3.  LDP Peering Scenarios and GTSM Considerations

   This section discusses GTSM considerations arising from the LDP
   peering scenarios used, including single-hop versus multi-hop LDP
   neighbors, as well as the use of LDP Basic Discovery versus Extended
   Discovery.

   The reason that the GTSM capability negotiation is enabled for Basic
   Discovery by default (i.e., G=1) but not for Extended Discovery is
   that the usage of Basic Discovery typically relates to a single-hop
   LDP peering session, whereas the usage of Extended Discovery
   typically relates to a multi-hop LDP peering session.  GTSM
   protection for multi-hop LDP sessions is outside the scope of this
   specification (see Section 1.2).  However, it is worth clarifying the
   following exceptions that may occur with Basic or Extended Discovery
   usage:

   a.  Two adjacent LSRs (i.e., back-to-back PE routers) forming a
       single-hop LDP peering session after doing an Extended Discovery
       (e.g., for Pseudowire signaling)

   b.  Two adjacent LSRs forming a multi-hop LDP peering session after
       doing a Basic Discovery, due to the way IP routing is set up
       between them (either temporarily or permanently)

   c.  Two adjacent LSRs (i.e., back-to-back PE routers) forming a
       single-hop LDP peering session after doing both Basic and
       Extended Discovery

   In the first case (a), GTSM is not enabled for the LDP peering
   session by default.  In the second case (b), GTSM is actually enabled
   by default and enforced for the LDP peering session; hence, it would
   prohibit the LDP peering session from getting established (note that
   this may impact features such as LDP IGP Synchronization [RFC5443] or
   LDP Session Protection [LDP-SPROT]).  In the third case (c), GTSM is
   enabled by default for Basic Discovery and enforced on the subsequent
   LDP peering, and is not for Extended Discovery.  However, if each LSR
   uses the same IPv4 transport address object value in both Basic and
   Extended Discoveries, then it would result in a single LDP peering
   session that would be enabled with GTSM.  Otherwise, GTSM would not
   be enforced on the second LDP peering session corresponding to the
   Extended Discovery.

   This document allows for the implementation to provide an option to
   statically (e.g., via configuration) and/or dynamically override the
   default behavior and enable/disable GTSM on a per-peer basis.  This
   would address all the exceptions listed above.

4.  Security Considerations

   This document increases the security for LDP, making it more
   resilient to off-link attacks.  Security considerations for GTSM are
   detailed in Section 5 of [RFC5082].

   As discussed in Section 3, it is possible that

   o  GTSM for LDP may not always be enforced on a single-hop LDP
      peering session, and LDP may still be susceptible to forged/
      spoofed protocol packets, if a single-hop LDP peering session is
      set up using Extended Discovery.

   o  GTSM for LDP may cause the LDP peering session to not get
      established (or may be torn down), if IP routing ever declares
      that the directly connected peer is more than one IP hop away.
      Suffice to say, use of cryptographic integrity (e.g., [RFC5925])
      is recommended as an alternate solution for detecting forged
      protocol packets (especially for the multi-hop case).

   The GTSM specification [RFC5082] says that protocol messages used for
   dynamic negotiation of GTSM support MUST be authenticated.  However,
   LDP discovery [RFC5036] uses UDP transport and does not have an
   authentication mechanism.  The GTSM specification further elaborates
   by saying that GTSM is not a substitute for authentication and does
   not secure against insider on-the-wire attacks.  LDP Basic Discovery
   uses link-level multicast address (224.0.0.2 or "all routers") that
   are never forwarded beyond the link, and this acts as a basic
   protection against off-the-wire attacks.

5.  Acknowledgments

   The authors of this document do not make any claims on the
   originality of the ideas described.  The concept of GTSM for LDP has
   been proposed a number of times and is documented in both the
   Experimental and Standards Track specifications of GTSM.  Among other
   people, we would like to acknowledge Enke Chen and Albert Tian for
   their document "TTL-Based Security Option for the LDP Hello Message".

   The authors would like to thank Loa Andersson, Bin Mo, Mach Chen,
   Vero Zheng, Adrian Farrel, Eric Rosen, Eric Gray, and Brian Weis for
   their thorough reviews and useful comments and suggestions.

6.  References

6.1.  Normative References

   [RFC2119]    Bradner, S., "Key words for use in RFCs to Indicate
                Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5036]    Andersson, L., Minei, I., and B. Thomas, "LDP
                Specification", RFC 5036, October 2007.

   [RFC5082]    Gill, V., Heasley, J., Meyer, D., Savola, P., and C.
                Pignataro, "The Generalized TTL Security Mechanism
                (GTSM)", RFC 5082, October 2007.

6.2.  Informative References

   [LDP-IPV6]   Asati, R., Manral, V., Papneja, R., and C. Pignataro,
                "Updates to LDP for IPv6", Work in Progress, June 2012.

   [LDP-SPROT]  Cisco Systems, Inc., "MPLS LDP Session Protection",
                <http://www.cisco.com/en/US/docs/ios-xml/ios/mp_ldp/
                configuration/12-4m/mp-ldp-sessn-prot.html>.

   [RFC5443]    Jork, M., Atlas, A., and L. Fang, "LDP IGP
                Synchronization", RFC 5443, March 2009.

   [RFC5925]    Touch, J., Mankin, A., and R. Bonica, "The TCP
                Authentication Option", RFC 5925, June 2010.

Authors' Addresses

   Carlos Pignataro
   Cisco Systems
   7200-12 Kit Creek Road
   Research Triangle Park, NC  27709
   USA

   EMail: cpignata@cisco.com

   Rajiv Asati
   Cisco Systems
   7025-6 Kit Creek Road
   Research Triangle Park, NC  27709
   USA

   EMail: rajiva@cisco.com

 

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