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Network Working Group                                          E. Vyncke
Internet-Draft                                                S. Previdi
Intended status: Standards Track                     Cisco Systems, Inc.
Expires: April 30, 2015                                 October 27, 2014

       IPv6 Segment Routing Header (SRH) Security Considerations


   Segment Routing (SR) allows a node to steer a packet through a
   controlled set of instructions, called segments, by prepending a SR
   header to the packet.  A segment can represent any instruction,
   topological or service-based.  SR allows to enforce a flow through
   any path (topological, or application/service based) while
   maintaining per-flow state only at the ingress node to the SR domain.

   Segment Routing can be applied to the IPv6 data plane with the
   addition of a new type of Routing Extension Header.  This draft
   analyzes the security aspects of the Segment Routing Extension Header
   (SRH) and how it is used by SR capable nodes to deliver a secure

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 30, 2015.

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Copyright Notice

   Copyright (c) 2014 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
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Segment Routing Documents . . . . . . . . . . . . . . . . . .   2
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Threat model  . . . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Source routing threats  . . . . . . . . . . . . . . . . .   3
     3.2.  Applicability of RFC 5095 to SRH  . . . . . . . . . . . .   4
     3.3.  Service stealing threat . . . . . . . . . . . . . . . . .   5
     3.4.  Topology disclosure . . . . . . . . . . . . . . . . . . .   5
   4.  Security fields in SRH  . . . . . . . . . . . . . . . . . . .   5
     4.1.  Selecting a hash algorithm  . . . . . . . . . . . . . . .   6
     4.2.  Performance impact of HMAC  . . . . . . . . . . . . . . .   7
     4.3.  Pre-shared key management . . . . . . . . . . . . . . . .   7
   5.  Deployment Models . . . . . . . . . . . . . . . . . . . . . .   8
     5.1.  Nodes within the SR domain  . . . . . . . . . . . . . . .   8
     5.2.  Nodes outside of the SR domain  . . . . . . . . . . . . .   8
     5.3.  SR path exposure  . . . . . . . . . . . . . . . . . . . .   9
     5.4.  Impact of BCP-38  . . . . . . . . . . . . . . . . . . . .   9
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   7.  Manageability Considerations  . . . . . . . . . . . . . . . .  10
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  10
     10.2.  Informative References . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Segment Routing Documents

   Segment Routing terminology is defined in

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   Segment Routing use cases are described in

   Segment Routing IPv6 use cases are described in

   Segment Routing protocol extensions are defined in
   [I-D.ietf-isis-segment-routing-extensions], and

2.  Introduction

   This document analyzes the security threat model, the security issues
   and proposed solutions related to the new routing header for segment

   The SRH is simply another version of the routing header as described
   in RFC 2460 [RFC2460] and is:

   o  inserted by a SR edge router when entering the segment routing
      domain or by the source host itself.  The source host can even be
      outside the SR domain;

   o  inspected and acted upon when reaching the destination address of
      the IP header per RFC 2460 [RFC2460].

   Routers on the path that simply forward an IPv6 packet (i.e. the IPv6
   destination address is none of theirs) will never inspect and process
   the SRH.  Routers whose one interface IPv6 address equals the
   destination address field of the SRH will have to parse the SRH and,
   if supported and if the local configuration allows it, will act on
   the SRH.

3.  Threat model

3.1.  Source routing threats

   Using a SRH is similar to source routing, therefore it has some well-
   known security issues as described in [RFC4942] section 2.1.1 and

   o  amplification attacks: where a packet could be forged in such a
      way to cause looping among a set of SR-enabled routers causing
      unnecessary traffic, hence a Denial of Service (DoS) against

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   o  reflection attack: where a hacker could force an intermediate node
      to appear as the immediate attacker, hence hiding the real
      attacker from naive forensic;

   o  bypass attack: where an intermediate node could be used as a
      stepping stone (for example in a De-Militarized Zone) to attack
      another host (for example in the datacenter or any back-end

   These security issues did lead to obsoleting the routing-header type
   0, RH-0, with [RFC5095] because:

   o  it was assumed to be inspected and acted upon by default by each
      and every router on the Internet;

   o  it contained multiple segments in the payload.

   Therefore, if intermediate nodes ONLY act on valid and authorized SRH
   (such as within a single administrative domain), then there is no
   security threat similar to RH-0.

3.2.  Applicability of RFC 5095 to SRH

   In the segment routing architecture described in
   [I-D.filsfils-spring-segment-routing] there are basically two kinds
   of nodes (routers and hosts):

   o  nodes within the segment routing domain, which is within one
      single administrative domain, i.e., where all nodes are trusted
      anyway else the damage caused by those nodes could be worse than
      amplification attacks: traffic interception, man-in-the-middle
      attacks, more server DoS by dropping packets, and so on.

   o  nodes outside of the segment routing domain, which is outside of
      the administrative segment routing domain hence they cannot be
      trusted because there is no physical security for those nodes,
      i.e., they can be replaced by hostile nodes or can be coerced in
      wrong behaviors.

   The use case for segment routing consists of the single
   administratuve domain where all non-trusted nodes will not
   participate in segment routing and where all segment routing nodes
   ignore SRH created by outsides.  Hence, the RFC 5095 [RFC5095]
   attacks are not applicable as all participating nodes can be trusted.

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3.3.  Service stealing threat

   Segment routing is used for added value services, there is also a
   need to prevent non-participating nodes to use those services; this
   is called 'service stealing prevention'.

3.4.  Topology disclosure

   The SRH also contains all IPv6 addresses of intermediate SR-nodes,
   this obviously reveals those addresses to the potentially hostile
   attackers if those attackers are able to intercept packets containing

4.  Security fields in SRH

   This section summarizes the use of specific fields in the SRH; they
   are integral part of [I-D.previdi-6man-segment-routing-header] and
   they are again described here for reader's sake.  They are based on a
   key-hashed message authentication code (HMAC).

   The security-related fields in SRH are:

   o  HMAC Key-id, 8 bits wide;

   o  HMAC, 256 bits wide (optional, exists only if HMAC Key-id is not

   The HMAC field is the output of the HMAC computation (per RFC 2104
   [RFC2104]) using a pre-shared key identified by HMAC Key-id and of
   the text which consists of the concatenation of:

   o  the source IPv6 address;

   o  last segment field;

   o  an octet whose bit-0 is the clean-up bit flag and others are 0;

   o  HMAC Key-id;

   o  all addresses in the Segment List;

   The purpose of the HMAC field is to verify the validity, the
   integrity and the authorization of the SRH itself.  If an outsider of
   the SR domain does not have access to a current pre-shared secret,
   then it cannot compute the right HMAC field and the first SR router
   on the path processing the SRH and configured to check the validity
   of the HMAC will simply reject the packet.

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   The HMAC field is located at the end of the SRH simply because only
   the router on the ingress of the SR domain needs to process it, then
   all other SR nodes can ignore it (based on local policy) because they
   can trust the upstream router.  This is to speed up forwarding
   operations because SR routers which do not validate the SRH do not
   need to parse the SRH until the end.

   The HMAC Key-id field allows for the simultaneous existence of
   several hash algorithms (SHA-256, SHA3-256 ... or future ones) as
   well as pre-shared keys.  This allows for pre-shared key roll-over
   when two pre-shared keys are supported for a while when all SR nodes
   converged to a fresher pre-shared key.  The HMAC Key-id field is
   opaque, i.e., it has neither syntax not semantic except as an index
   to the right combination of pre-shared key and hash algorithm and
   except that a value of 0 means that there is no HMAC field.  It could
   also allow for interoperation among different SR domains if allowed
   by local policy.

   When a specific SRH is linked to a time-related service (such as
   turbo-QoS for a 1-hour period) where the DA, Segment ID (SID) are
   identical, then it is important to refresh the shared-secret
   frequently as the HMAC validity period expires only when the HMAC
   Key-id and its associated shared-secret expires.  How HMAC Key-ids
   and pre-shared secrets are synchronized between participating nodes
   in the SR domain is outside of the scope of this document (RFC 6407
   [RFC6407] GDOI could be a basis).

4.1.  Selecting a hash algorithm

   The HMAC field in the SRH is 256 bit wide.  Therefore, the HMAC MUST
   be based on a hash function whose output is at least 256 bits.  If
   the output of the hash function is 256, then this output is simply
   inserted in the HMAC field.  If the output of the hash function is
   larger than 256 bits, then the output value is truncated to 256 by
   taking the least-significant 256 bits and inserting them in the HMAC

   SRH implementations can support multiple hash functions but MUST
   implement SHA-2 [FIPS180-4] in its SHA-256 variant.

   NOTE: SHA-1 is currently used by some early implementations used for
   quick interoperations testing, the 160-bit hash value must then be
   right-hand padded with 96 bits set to 0.  The authors understand that
   this is not secure but is ok for limited tests.

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4.2.  Performance impact of HMAC

   While adding a HMAC to each and every SR packet increases the
   security, it has a performance impact.  Nevertheless, it must be
   noted that:

   o  the HMAC field is used only when SRH is inserted by a device (such
      as a home set-up box) which is outside of the segment routing
      domain.  If the SRH is added by a router in the trusted segment
      routing domain, then, there is no need for a HMAC field, hence no
      performance impact.

   o  when present, the HMAC field MUST only be checked and validated by
      the first router of the segment routing domain, this router is
      named 'validating SR router'.  Downstream routers MAY NOT inspect
      the HMAC field.

   o  this validating router can also have a cache of <IPv6 header +
      SRH, HMAC field value> to improve the performance.  It is not the
      same use case as in IPsec where HMAC value was unique per packet,
      in SRH, the HMAC value is unique per flow.

   o  Last point, hash functions such as SHA-2 have been optmized for
      security and performance and there are multiple implementations
      with good performance.

   With the above points in mind, the performance impact of using HMAC
   is minimized.

4.3.  Pre-shared key management

   The field HMAC Key-id allows for:

   o  key roll-over: when there is a need to change the key (the hash
      pre-shared secret), then multiple pre-shared keys can be used
      simultaneously.  The validating routing can have a table of <HMAC
      Key-id, pre-shared secret> for the currently active and future

   o  different algorithm: by extending the previous table to <HMAC Key-
      id, hash function, pre-shared secret>, the validating router can
      also support simultaneously several hash algorithms (see section
      Section 4.1)

   The pre-shared secret distribution can be done:

   o  in the configuration of the validating routers, either by static
      configuration or any SDN oriented approach;

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   o  dynamically using a trusted key distribution such as [RFC6407]

   NOTE: this section needs more work but the intent is NOT to define

5.  Deployment Models

5.1.  Nodes within the SR domain

   A SR domain is defined as a set of interconnected routers where all
   routers at the perimeter are configured to insert and act on SRH.
   Some routers inside the SR domain can also act on SRH or simply
   forward IPv6 packets.

   The routers inside a SR domain can be trusted to generate SRH and to
   process SRH received on interfaces that are part of the SR domain.
   These nodes MUST drop all packets received on an interface that is
   not part of the SR domain and containing a SRH whose HMAC field
   cannot be validated by local policies.  This includes obviously
   packet with a SRH generated by a non-cooperative SR domain.

   If the validation fails, then these packets MUST be dropped, ICMP
   error messages (parameter problem) SHOULD be generated (but rate
   limited) and SHOULD be logged.

5.2.  Nodes outside of the SR domain

   Nodes outside of the SR domain cannot be trusted for physical
   security; hence, they need to request by some trusted means (outside
   of the scope of this document) a complete SRH for each new connection
   (i.e. new destination address).  The received SRH MUST include a HMAC
   Key-id and HMAC field which is computed correctly (see Section 4).

   When an outside node sends a packet with an SRH and towards a SR
   domain ingress node, the packet MUST contain the HMAC Key-id and HMAC
   field and the the destination address MUST be an address of a SR
   domain ingress node .

   The ingress SR router, i.e., the router with an interface address
   equals to the destination address, MUST verify the HMAC field with
   respect to the HMAC Key-id.

   If the validation is successful, then the packet is simply forwarded
   as usual for a SR packet.  As long as the packet travels within the
   SR domain, no further HMAC check needs to be done.  Subsequent
   routers in the SR domain MAY verify the HMAC field when they process
   the SRH (i.e. when they are the destination).

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   If the validation fails, then this packet MUST be dropped, an ICMP
   error message (parameter problem) SHOULD be generated (but rate
   limited) and SHOULD be logged.

5.3.  SR path exposure

   As the intermediate SR nodes addresses appears in the SRH, if this
   SRH is visible to an outsider then he/she could reuse this knowledge
   to launch an attack on the intermediate SR nodes or get some insider
   knowledge on the topology.  This is especially applicable when the
   path between the source node and the first SR domain ingress router
   is on the public Internet.

   The first remark is to state that 'security by obscurity' is never
   enough; in other words, the security policy of the SR domain MUST
   assume that the internal topology and addressing is known by the
   attacker.  A simple traceroute will also give the same information
   (with even more information as all intermediate nodes between SID
   will also be exposed).  IPsec Encapsulating Security Payload
   [RFC4303] cannot be use to protect the SRH as per RFC4303 the ESP
   header must appear after any routing header (including SRH).

   To prevent a user to leverage the gained knowledge by intercepting
   SRH, it it recommended to apply an infrastructure Access Control List
   (iACL) at the edge of the SR domain.  This iACL will drop all packets
   from outside the SR-domain whose destination is any address of any
   router inside the domain.  This security policy should be tuned for
   local operations.

5.4.  Impact of BCP-38

   BCP-38 [RFC2827], also known as "Network Ingress Filtering", checks
   whether the source address of packets received on an interface is
   valid for this interface.  The use of loose source routing such as
   SRH forces packets to follow a path which differs from the expected
   routing.  Therefore, if BCP-38 was implemented in all routers inside
   the SR domain, then packets with a SRH could be received by an
   interface where packets with the source address are not expected and
   the packets could be dropped.

   As a SR domain is usually a subset of an administrative domain, and
   as BCP-38 is only deployed at the ingress routers of this
   administrative domain and as packets arriving at those ingress
   routers have been normally forwarded using the normal routing
   information, then there is no reason why this ingress router should
   drop the SRH packet based on BCP-38

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6.  IANA Considerations

   There are no IANA request or impact in this document.

7.  Manageability Considerations


8.  Security Considerations

   Security mechanisms applied to Segment Routing over IPv6 networks are
   detailed in Section 4.

9.  Acknowledgements

   The authors would like to thank Dave Barach and David Lebrun for
   their contributions to this document.

10.  References

10.1.  Normative References

              National Institute of Standards and Technology, "FIPS
              180-4 Secure Hash Standard (SHS)", March 2012,

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

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)", RFC
              4303, December 2005.

   [RFC5095]  Abley, J., Savola, P., and G. Neville-Neil, "Deprecation
              of Type 0 Routing Headers in IPv6", RFC 5095, December

   [RFC6407]  Weis, B., Rowles, S., and T. Hardjono, "The Group Domain
              of Interpretation", RFC 6407, October 2011.

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10.2.  Informative References

              Filsfils, C., Previdi, S., Bashandy, A., Decraene, B.,
              Litkowski, S., Horneffer, M., Milojevic, I., Shakir, R.,
              Ytti, S., Henderickx, W., Tantsura, J., and E. Crabbe,
              "Segment Routing Architecture", draft-filsfils-spring-
              segment-routing-04 (work in progress), July 2014.

              Filsfils, C., Francois, P., Previdi, S., Decraene, B.,
              Litkowski, S., Horneffer, M., Milojevic, I., Shakir, R.,
              Ytti, S., Henderickx, W., Tantsura, J., Kini, S., and E.
              Crabbe, "Segment Routing Use Cases", draft-filsfils-
              spring-segment-routing-use-cases-01 (work in progress),
              October 2014.

              Previdi, S., Filsfils, C., Bashandy, A., Gredler, H.,
              Litkowski, S., Decraene, B., and J. Tantsura, "IS-IS
              Extensions for Segment Routing", draft-ietf-isis-segment-
              routing-extensions-02 (work in progress), June 2014.

              Brzozowski, J., Leddy, J., Leung, I., Previdi, S.,
              Townsley, W., Martin, C., Filsfils, C., and R. Maglione,
              "IPv6 SPRING Use Cases", draft-ietf-spring-ipv6-use-
              cases-01 (work in progress), July 2014.

              Previdi, S., Filsfils, C., Field, B., and I. Leung, "IPv6
              Segment Routing Header (SRH)", draft-previdi-6man-segment-
              routing-header-02 (work in progress), July 2014.

              Psenak, P., Previdi, S., Filsfils, C., Gredler, H.,
              Shakir, R., Henderickx, W., and J. Tantsura, "OSPFv3
              Extensions for Segment Routing", draft-psenak-ospf-
              segment-routing-ospfv3-extension-02 (work in progress),
              July 2014.

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104, February

   [RFC2827]  Ferguson, P. and D. Senie, "Network Ingress Filtering:
              Defeating Denial of Service Attacks which employ IP Source
              Address Spoofing", BCP 38, RFC 2827, May 2000.

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   [RFC4942]  Davies, E., Krishnan, S., and P. Savola, "IPv6 Transition/
              Co-existence Security Considerations", RFC 4942, September

Authors' Addresses

   Eric Vyncke
   Cisco Systems, Inc.
   De Kleetlaann 6A
   Diegem  1831

   Email: evyncke@cisco.com

   Stefano Previdi
   Cisco Systems, Inc.
   Via Del Serafico, 200
   Rome  00142

   Email: sprevidi@cisco.com

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