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Versions: 00 01 02 03 04 05 draft-ietf-mpls-opportunistic-encrypt

Network Working Group                                          A. Farrel
Internet-Draft                                          Juniper Networks
Intended status: Experimental
Expires: April 26, 2015                                       S. Farrell
                                                 Trinity College, Dublin
                                                        October 26, 2014


              Opportunistic Security in MPLS Networks

          draft-farrelll-mpls-opportunistic-encrypt-03.txt

Abstract

   This document describes a way to apply opportunistic security
   between adjacent nodes on an MPLS Label Switched Path (LSP) or
   between end points of an LSP.  It explains how keys may be agreed
   to enable encryption, and how key identifiers are exchanged in
   encrypted MPLS packets.  Finally, this document describes the
   applicability of this approach to opportunistic security in MPLS
   networks with an indication of the level of improved security as
   well as the continued vulnerabilities.

   This document does not describe security for MPLS control plane
   protocols.

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."

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
   publication of this document.  Please review these documents


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   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.

Notation

   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 [RFC2119].

Table of Contents

   1. Introduction ................................................... 3
   1.1. Experimental Status .......................................... 4
   2. Principles of Opportunistic Security ........................... 5
   2.1. Why Do We Need Opportunistic Security? ....................... 5
   2.2. Opportunistic Security at 10,000ft ........................... 6
   2.3. What about a Man-in-the-Middle? .............................. 8
   2.4. OS in MPLS Overview .......................................... 9
   3. MPLS Packet Encryption ........................................ 11
   3.1. MPLS Encryption Label ....................................... 13
   3.2. Control Word ................................................ 14
   3.3. Considerations for ECMP ..................................... 15
   3.4. Backward Compatibility ...................................... 16
   3.5. MTU Considerations .......................................... 17
   3.6. Recursive Encryption ........................................ 17
   4. Key Exchange For Opportunistic Security in MPLS ............... 17
   4.1. MPLS G-ACh Advertisement Protocol for Key Exchange .......... 18
   4.2. Key Exchange Protocol ....................................... 18
   4.3. Indicating the Return Path .................................. 23
   4.4. Protecting the Key Exchange Protocol Messages ............... 24
   5. Applicability of MPLS Opportunistic Security  ................. 24
   6. Security Considerations ....................................... 26
   6.1. Security Improvements ....................................... 26
   6.2. Continued Vulnerabilities ................................... 26
   6.3. New Security Considerations ................................. 26
   7. Manageability Considerations .................................. 27
   7.1. MITM Detection .............................................. 27
   8.  IANA Considerations .......................................... 27
   8.1. GAP Key Exchange TLV ........................................ 27
   8.2. Key Derivation Functions and Symmetric Algorithms ........... 27
   9.  Acknowledgements ............................................. 28
   10.  References .................................................. 25
   10.1.  Normative References ...................................... 28
   10.2.  Informative References .................................... 28
   Authors' Addresses ............................................... 30


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1. Introduction

   MPLS is an established data plane protocol in the Internet.  It is
   found in the majority of core service provider networks and most end-
   to-end traffic in the Internet will be carried over MPLS at some
   point in its path.  The MPLS data plane is defined by [RFC3031] and
   [RFC3032].

   Data security (e.g., confidentiality) in MPLS has previously relied
   on just two features:

   - Physical isolation of MPLS networks has been used to ensure that
     interception of MPLS traffic was not possible.

   - Higher-layer protocol security (such as IPsec [RFC4302], [RFC4303])
     has been used whenever a particular flow has determined that
     security was desirable.

   These features have a number of significant vulnerabilities:

   - Networks are increasingly easily compromised physically such that
     "taps" may be inserted in links between routers [RFC7258].

   - Routers may be compromised either in their entirety or through
     the management/control plane (or misconfiguration).  This may
     result in packets being diverted to transit inspection points on
     their way to their destination.

   - The increased support for point-to-multipoint (P2MP) MPLS means
     that routers can easily be configured (or misconfigured) to make a
     copy of data and to send it to an additional destination.

   - End-to-end payload security may be hard to manage and operate and
     is not turned on by default by many users.  While this form of
     security is desirable, the network should also improve the security
     of data transfer that it offers.

   The concept of Opportunistic Security (OS) is introduced in
   [I-D.dukhovni-opportunistic-security].  This document describes an OS
   design pattern for the MPLS data plane.  It shows what part of an
   MPLS packet may be encrypted and provides a way to indicate that
   the packet is encrypted as well as to carry a key identifier
   with each packet.

   MPLS opportunistic security can be achieved between adjacent Label
   Switching Routers (LSRs) on an MPLS Label Switched Path (LSP), and
   also between end points of an LSP.



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   This document also provides a mechanism for keys to be exchanged to
   facilitate encryption.  Finally, this document describes the
   applicability of OS in MPLS networks with an indication of the level
   of improved security as well as the continued vulnerabilities.

   This document does not describe security for MPLS control plane
   protocols.

   Please note that a discussion of the applicability of MPLS
   opportunistic security is provided in Section 5.

1.1. Experimental Status

   This document is presented as experimental.  Before advancing this
   work on the IETF's Standards Track, it is important to get experience
   of the practicality of the mechanisms described.  In particular
   whether it is practical to achieve these mechanisms in existing
   hardware, and whether the imposition of additional MPLS labels is
   acceptable in the MPLS data plane.  Additionally, the consequences
   of the reduced MTU caused by inserting the additional MPLS label and
   control word as well as the fact that the encrypted packet will be
   larger than the unencrypted packet need to be investigated.

   It is currently believed that MPLS OS can be deployed progressively
   without the need to negotiate capabilities outside the key exchange
   mechanisms described here.  This means that no specific walled garden
   needs to be described in this documenmt.

   Experimentation and further investigation of the security aspects of
   these mechanisms are encouraged especially with regard to mitigation
   of man-in-the-middle attacks.  Consideration of the impact of MPLS OS
   on MPLS Operations, Administration, and Management (OAM) and other
   MPLS management techniques also needs further exploration.

   The key functions of MPLS OS described in Section 2.4 are based on
   an initial set of choices that may be adequate for MPLS OS.  However,
   security knowledge is evolving and if may be advisable to "upgrade"
   for example to Elliptic Curve Diffie-Hellman (ECDH) [RFC6239], using
   NIST curves or new curves (such as 25519).  Furthermore, an
   alternative key derivation functions could be chosen, or symmetric
   cipher mode could be used.  Note that changing to a symmetric cipher
   that is faster in software, but less likely to be available in
   hardware would not be a good change.

   Section 2.4 also describes the frequency with which keys should be
   changed.  The values described here should be subject to more
   research and experimentation since key change is fundamental to the
   actual security of the encryption.


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   Section 4.2.3 defines the input parameters to the key definition
   function and includes the LSP identifier.  This identifier is only
   needed if the scope of the key is per LSP.  This document is written
   on that assumption because of the need to rotate the key after a
   certain number of packets have been transmitted.  However, this could
   be the subject of some investigation since dropping the LSP
   identifier would simplify the TLV and the computation.  It would also
   address the issue of identifying the LSP in the case of LDP.

   Section 4.2.3 also specifies that the alt is not used.  Further
   investigation is needed to see whether this input parameter would add
   value.

   Note that this experiment uses a special-purpose MPLS label.  Since
   this document is experimental it makes use of an extended special-
   purpose label from the experimental range.  If this work is moved to
   be published on the standards track, it will be possible to achieve
   the same function using a simple special-purpose label rather than an
   extended special-purpose label.

2. Principles of Opportunistic Security

   This section provides an overview of opportunistic security in the
   context of MPLS.  Readers are advised to familiarise themselves with
   some of the attck vecotrs discussed in [RFC7258] and with the more
   general descirption of opportunistic security as described in
   [I-D.dukhovni-opportunistic-security].  The text here is intended for
   the consumption of MPLS experts who may not have a background in
   security: it is, therefore, tutorial and simplistic in nature.

2.1. Why Do We Need Opportunistic Security?

   To introduce this discussion we start from a basic view of how
   encryption is typically used in IETF protocols.

   Say we have two protocol entities, Alice and Bob, and they would like
   some message "M" sent from Alice to Bob to have confidentiality.
   Alice needs to send M encrypted with algorithm "E" under some
   symmetric (e.g., AES) key, "k".  Thus Alice wants to send Bob
   "E(k,M)", but for Bob to be able to understand (i.e., decrypt) the
   message Alice and Bob both need to agree on the key that will be
   used: this is called their shared secret.

   In many IETF protocols, such as the common usage of Transport Layer
   Security (TLS) S/MIME Cryptographic Message Syntax (CMS) or Pretty
   Good Privacy (PGP), Alice simply invents a random key "k" and then
   encrypts that under Bob's public key "Pub-b" and sends Bob both
   E(Pub-b,k) and E(k,M).  (There are lots of other details and other


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   options for how this can be handled, but we ignore those for now.)
   In such cases, before Alice can send "E(k,M)", she needs to acquire
   Bob's public key and she needs to be certain that it really is Bob's
   public key and not Charlie's.  That knowledge requires some long-term
   key management, which is often done using a Public Key Infrastructure
   (PKI) so that Alice actually stores the public key (Pub-ca) of a
   Certification Authority (CA), and Bob gets his public key (Pub-b)
   "certified" by the CA, which means the CA creates a digitally signed
   data structure "Cert(Pub-ca,Pub-b)".  The crucial thing is that
   Alice, Bob, and a CA need to co-ordinate before Alice and Bob can
   agree on a key "k", and that process imposes a key-management burden.

   Doing such key management is clearly quite possible, since TLS and
   IPsec and other well-deployed technologies depend on it.  But, in
   the case of HTTP/TLS on the public web, we see that only roughly 30%
   of web sites actually take on this burden, even though the software
   required is ubiquitous and, at least for 2nd level DNS domains in
   .com for example, there are CAs who offer free domain-validated
   certificates.  While some of the 70% who don't set up certificates
   might not actually want confidentiality, there are certainly some who
   would and arguably many that would benefit from confidentiality, if
   it just happened out of the box, without an administrator having to
   do anything.  And there are also arguably many other protocols where
   the same is true.

   An alternative to the PKI is manual configuration of keys at Alice
   and Bob.  Manual configuration is used in a large number of cases in
   deployments, however it has a set of issues that make it problematic.
   These issues include:
   - the scale of configuration that is needed for a full set of SAs
     between all communicating parties
   - the likelihood of configuration errors
   - the security vulnerabilities associated with manual keying and
     unsecured exchange of keys.

   Opportunistic Security (OS) is a protocol design pattern to achieve
   encryption between Alice and Bob without requiring key-management
   through CAs and without relying on manual configuration of keys.

2.2. Opportunistic Security at 10,000ft

   Instead of the "key transport" mechanisms described in Section 2.1,
   OS aims to use "key agreement".  With key management, Alice invents
   "k" and safely transports it to Bob encrypted with Bob's public key
   as "E(Pub-b,k)".  With key agreement, both Alice and Bob contribute
   to calculating "k" as follows.

   Assume that Alice and Bob are using some protocol where they can


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   exchange a few messages in order to agree on the key "k" to use.
   With a Diffie-Hellman key agreement ("D-H") both Alice and Bob have
   public and private values, where the private value can be randomly
   generated, perhaps even once per message "M".  They swap the public
   values, and can then, thanks to the "magic" of Diffie-Hellman, derive
   a key "k" that nobody else can know.

   In this way Alice sends Bob "Pub-a" and Bob sends Alice "Pub-b" and
   at that point both of them can safely calculate a shared secret "k"
   from those values.  And after that Alice can send Bob "E(k,M)".

   From here on, we change the terminology slightly and refer to Alice
   as the initiator, with private key "i" and Bob as the recipient, with
   private key "r" so that our notation is closer to that used in
   IPsec's Internet Key Exchange Protocol (IKE) on which we model our
   use of OS.

   D-H works as follows:  Let "p" be well-known large prime number that
   we use for all modular arithmetic (meaning that "a^b" is actually
   "(a^b) mod p"), and let "g" be another well-known value (called a
   generator for the group determined by "p").  Also let Alice and Bob's
   private values be "i" and "r" respectively.  Now, if Alice sends Bob
   "g^i" as her public value, and Bob similarly sends Alice "g^r" then
   both of them can easily calculate "g^(i*r)" or "g^ir" but nobody else
   can, since calculating "x" when only given "g^x" is a computationally
   hard problem for any "x".  Once both Alice and Bob have the value
   "g^ir" in hand, they can easily derive a value "k" from that using
   any of a number of well-known key derivation functions (KDFs) such
   that k = f(g^ir) for a KDF "f".

   As you can see from the above, Alice and Bob do not need to pre-
   arrange anything other than "g", "p" and "f", and those can be public
   information that is used by everyone everywhere (or at least by all
   participants in a particular deployment).  Yet, Alice and Bob have
   managed to derive a common and private value for a key "k" that they
   can use to encrypt (and decrypt) "M".

   This method of using the OS pattern provides strong confidentiality
   and can be built into any protocol that allows Alice and Bob to
   occasionally exchange public values.

   There are also additional advantages to key agreement when compared
   to key transport.  The most important of those is that with key
   agreement we can easily ensure that k has a property called Perfect
   Forward Secrecy (PFS).  That means that an attacker has to separately
   attack each key k.  In contrast, if we use the key transport
   approach, then an attacker who somehow accesses Bob's private key
   "Priv-b" can record lots of traffic and later go back and decrypt all


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   the "E(Pub-b,k)" values that all Alices have ever sent to Bob.  With
   key agreement as described, since both Alice and Bob contribute to
   the value k, and since Alice and Bob will typically periodically
   generate new private values i and r (perhaps even for every single
   M), compromise of one party is far less catastrophic, and an attacker
   who gets access to one private value gets far less benefit.

2.3. What about a Man-in-the-Middle?

   OS as described so far is vulnerbale to Man-in-the-Middle (MITM)
   attacks.  The problem is that Alice does not know that it was
   really Bob's public value that she received; it could have been
   Charlie's public value sent by Charlie.  And Charlie could also
   send Bob his public value pretending to be Alice.  Now Charlie
   can share a key with Alice and a key with Bob so that Charlie
   can sit between Alice and Bob decrypting what he gets from Alice
   and then re-encrypting it to send to Bob.  Neither Alice nor
   Bob can tell that Charlie is present as a "Man-in-the-Middle"
   and both Alice and Bob think they are safely exchanging encrypted
   messages.

   A MITM attack like that is bad and making a protocol proof against
   such attacks comes at the cost of the key-management burden described
   in Section 2.1.  Most IETF protocols to date require that such MITM
   attacks not be feasible.

   However, despite its potential vulnerability to MITM attacks, OS
   still has value.  This value arises because of the difficulty of
   inserting a MITM actor, and the cost of processing for the MITM
   in the case of a very large number of relationships.  In
   particular, where the choice is between no encryption (as has been
   the case for MPLS to date) and OS, it is clear that using OS offers
   better (although not the best) security.

   Consider the case where an attacker taps a link on the path between
   Alice and Bob.  In this case, the attacker can capture every packet
   between the two parties, and if there is no encryption, can read
   every message.  Furthermore, consider that the attacker could tap a
   fiber in the core of the network and so capture every packet between
   a large number of Alices and their corresponding Bobs.  In these
   cases, Charlie can operate as a "passive MITM" since all he has to do
   is watch the packets.

   With OS in use, Charlie is forced to be an "active MITM".  That is he
   must engage in the D-H exchange between each pair of Alices and Bobs,
   and he must must decrypt and encrypt each packet he wants to inspect.
   This imposes a higher cost and is especially burdensome if he is
   attempting to do it in parallel for lots of Alice/Bob pairs using


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   lots of different keys and communication sessions.

   Furthermore, when D-H is in use for OS, management tools can be used
   to detect the presence of Charlie as a MITM.  This is because
   Charlie has to agree one key "kA" with Alice, and a different key
   "kB" with Bob.  As far as we know, Charlie cannot arrange that kA
   equals kB because both sides contribute to the key value in the D-H
   key agreement.  That means that if Alice and Bob can check with each
   other what value of "k" they are using and the values do not match,
   then they know that Charlie is present.  What is more, Alice and Bob
   can make this check on the value of "k" for any of the "E(k,M)" they
   ever exchanged.

   Thus, in the case of a fiber tap where many Alice/Bob pairs are
   being monitored, it only takes one Alice and Bob to detect the MITM
   attack for all Alice/Bob pairs to be alerted to the problem.  In
   such cases the cost of detection for Charlie may be even greater than
   the cost of performing the MITM attack.

   Hence we conclude that OS can have considerable value when used in
   MPLS networks.

2.4. OS in MPLS Overview

   The basic requirement for MPLS-OS is that we want to provide a way
   for two MPLS nodes to do a key exchange and to derive a session key
   from that to use in MPLS packet encryption.

   To do that we use a Diffie-Hellman key exchange as outlined in
   Section 2.2.  We model this on IKE [RFC7296] using essentially the
   same parameters.  We feed the shared Diffie-Hellman value, which is
   g^ir, into a standard KDF that also takes as input an LSP identifier
   (LSP ID) together with the sending and receiving LSR IDs - where the
   sending LSR is the point of encryption and the receiving LSR is the
   point of decryption such that the pair of LSRs define the Security
   Association (SA).  These additional inputs are used to ensure that we
   end up with different keys on an LSP even if the same g^i and g^r
   values are re-used.  The KDF to be used here is as defined in
   [RFC5869].

   The D-H values used MUST be of at least 2048-bits.  Implementations
   MUST support the 2048-bit modular exponentiation (MODP) group from
   Section 3 of [RFC3526] and SHOULD support the larger MODP groups from
   [RFC3526].

   This document also defines the mechanism used to derive an identifier
   for a key (the key-id) from the shared Diffie-Hellman value, which
   is also based on the KDF output.  The key will be used with a


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   symmetric encryption algorithm, such as AEAD_AES_GCM_128 (the
   default, following [RFC5116]).

   As with any symmetric block cipher, one should not use the same key
   for too long.  The nonce defined for these keys is derived using
   a 96 bit counter incremented by one for each encrypted packet.
   It is critical for security that nonce values MUST NOT be re-used
   with a given key.  (This is an inherent issue with how AES-GCM or any
   counter mode achieves high performance.)

   Accordingly, implementations MUST support mechanisms for key change.

   To support key change, this document defines a way for two LSRs using
   a key on an LSP to agree a new key and to switch over to using that
   key when desired.  That means that implementations MUST be able to
   handle at least two keys (old and new) for a given LSP.  Once a new
   key has been agreed then it should be used for sending packets; once
   encrypted data packets protected with the new key have been
   successfully received, the old key SHOULD be discarded.  Section 4
   describes how two LSRs agree keys: to agree a new key two LSRs simply
   run the same key agreement exchange, but this time protected with the
   old session key as described in Section 4.4.  This process can, of
   course, be repeated any number of times for the same LSP.  It is
   RECOMMENDED that the key on an LSP be changed at least once every
   day or every 10^6 packets whichever is sooner, and MUST change keys
   before encrypting 2^64 packets.  For an LSP running over a fully-
   busy 100Gbe interface, we might assume that means roughly 160
   million packets per second, or roughly 2^44 packets per day.  The
   2^64 limit therefore means changing keys daily in the busiest cases
   of some of the largest current links capacities.

   In the event of a key agreement exchange or decryption failure, an
   alarm MUST be raised to the operator.  Default (i.e., node-wide) and
   per-LSP behavior SHOULD be configurable in this case: actions may
   include reverting to non-encrypted traffic, re-attempting key
   exchange, or tearing down the LSP.  Note that a simple attack on OS
   is to tamper with key agreement exchange messages or encrypted
   packets so that OS fails.  Such attacks may be intended to cause the
   LSP to operate without encryption, so an operator should consider
   this when setting the behavior in this case.

   Section 7.1 also discusses a mechanism that allows a pair of LSRs
   using OS on an LSP to detect that a MITM attack has happened.  For
   this, we simply define a function of the shared secret, which can be
   logged and later compared.  Note that logging a sample of these
   "witness" values will likely be sufficient to detect pervasive MITM
   attacks [RFC7258].  As with the key-id, we base this on the same
   KDF output.


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   We might want to consider deriving the witness value from a separate
   invocation of the KDF that does not depend on the LSP-specific
   inputs.  The benefit from that would be that the same MITM-detection
   infrastructure could be used for many protocols.  However, that would
   require standardizing a generic D-H MITM-detection protocol, or at
   least formats, in order to be useful.  We also need to consider what
   additional information needs to be logged with the witness value so
   that comparisons can easily be made at scale but without creating new
   privacy-invasive meta-data.  That last is not much of an issue for
   MPLS-OS, but could be elsewhere.  At present we do not intend to go
   for the generic MITM-detection approach, but it is worth considering.

   An additional discussion of the applicability of MPLS-OS is found in
   Section 5.

3. MPLS Packet Encryption

   MPLS packets are encrypted according to the mechanisms described in
   this section.

   When an MPLS packet is encrypted, this is indicated by the insertion
   of a new extended special-purpose label [RFC7274] in the label stack.
   This is referred to as the MPLS Encryption Label (MEL).  The format
   of the MEL is described in Section 3.1.

   The MEL MUST have the bottom of stack bit (the S bit) set and MUST be
   followed by a pseudowire control word [RFC4385].  The format of the
   control word is described in Section 3.2.

   The remainder of the MPLS packet is encrypted and cannot be parsed
   without decryption.  Implementations MUST support the
   AEAD_AES_GCM_128 encryption algorithm, as specified in Section 5.1
   of [RFC5116], which is the default as described in Section 4.2 of
   this document.

   Note that it is critical that a new nonce is used for every
   encryption.  The nonce is an implicit packet counter.  The initial
   nonce value is derived from the HMAC-based Key Derivation Function
   (HKDF) output (see Section 4.2.2) at key agreement time and the
   counter is incremented by one for each packet encrypted on the
   sending side and by one for each packet successfully decrypted on the
   receiver side.

   Although the nonce is not transmitted with the packets, a 16-bit
   counter carried in the control Word indicates the nonce value modulo
   65536.  This feature allows a receiving node to quickly spot that a
   packet has been dropped and resynch its own counter in order to be
   able to continue to decrypt received packets.  In the event that the


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   counter cannot be resynchronized or that more than 65536 packet are
   lost in one batch the receiver will encounter a decryption error.  In
   this case the receiver may report a general decryption error or may
   attempt to resynchronize by advancing its own counter in units of
   65536 according to the modulo value in the received packet.  Note
   that incrementing the counter in order to test for decryption failure
   does generate a potential DoS if, e.g., an attacker decrements the
   nonce-mod-65536 value.  Implementations that do such tests SHOULD
   maintain a small maximum window size beyond which they will cease
   attempting to decrypt.  It could be that throwing an error might be
   the more effective response if the packet loss rates are expected to
   be low enough.

   It should also be noted that the output from encryption will be 16
   octets longer than the input.

   The bottom of stack bit is set in the MEL to stop implementations
   continuing to search down the label stack (which is encrypted) and
   attempting to use the data as though it was a valid label stack.  The
   control word is needed because many implementations that find the
   bottom of stack expect the next bytes to be a control word or
   protocol indicator.

   The position of the MEL and control word depend on whether hop-by-hop
   or end-to-end encryption is being applied.

   Figure 1 illustrates the format of an example MPLS packet before and
   after hop-by-hop encryption.  The left hand part of the
   figure shows a normal MPLS packet with a label stack and payload.
   The bottom label in the stack has the S bit set.  The payload is the
   data carried by the MPLS packet (such as IP) and may be prefixed by a
   control word.

   The right hand part of Figure 1 shows the same packet after it has
   been encrypted.  The top of stack is a label with value 15 that
   indicates that an extended special-purpose label follows.  Next comes
   the MEL with the S bit set.  The label value of the MEL is from the
   experimental range 240-255 and is selected according to the scope of
   the MPLS OS experiment being run.  The MEL is followed by a control
   word.  Everything that follows the control word is the entire
   original MPLS packet encrypted.









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                    ----------- .      -----------
                   | Top Label | .    | Label 15  |
                   +-----------+  .   +-----------+
                   |   Label   |   .  | MEL   S=1 |
                   +-----------+    . +-----------+
                   | Label S=1 |     .| Ctrl Word |
                   +-----------+      +-----------+
                   |           |      |           |
                   ~  Payload  ~      ~ Encrypted ~
                   |           |      |           |
                    -----------........-----------

        Figure 1 : The Use of the MEL for Hop-by-Hop Encryption


   Figure 2 illustrates the format of an example MPLS packet before and
   after end-to-end encryption.  The left hand part of the figure shows
   a normal MPLS packet with a label stack and payload.  The bottom
   label in the stack has the S bit set and the payload may be prefixed
   by a control word.  The right hand part of the figure shows how the
   top two labels (or however many labels are needed for end-to-end
   delivery) remain at the top of the label stack.  Then follows label
   15 to indicate that an extended special-purpose label follows, then
   comes the MEL with S bit set, and a control word.  The remainder of
   the packet is encrypted and contains the rest of the label stack and
   the payload.

                    -----------        -----------
                   | Top Label |      | Top Label |
                   +-----------+      +-----------+
                   |   Label   |      |   Label   |
                   +-----------+.     +-----------+
                   |   Label   | .    | Label 15  |
                   +-----------+  .   +-----------+
                   |   Label   |   .  | MEL   S=1 |
                   +-----------+    . +-----------+
                   | Label S=1 |     .| Ctrl Word |
                   +-----------+      +-----------+
                   |           |      |           |
                   ~  Payload  ~      ~ Encrypted ~
                   |           |      |           |
                    -----------........-----------

        Figure 2 : The Use of the MEL for End-to-End Encryption

3.1. MPLS Encryption Label

   The MPLS Encryption Label (MEL) is a normal label stack entry


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   carrying an extended special-purpose label with a value from the
   experimental range 240-255.  The format of the label stack entry is
   defined in [RFC3032] and shown in Figure 3.


    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                Label                  | TC  |S|       TTL     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Figure 3 : Format of the MEL Label Stack Entry

   Label: The value of MEL for this experiment
   TC:    SHOULD be copied from the TC of the first encrypted label.
   S:     MUST be set to one.
   TTL:   SHOULD be set to zero to prevent encrypted packets being
          accidentally forwarded beyond the point of intended
          decryption.

   The sending LSR MAY choose different values for the TTL and TC fields
   if it is known that label 15 or the MEL will not be exposed as the
   top label at any point along the LSP (for example, by penultimate hop
   popping).

3.2. Control Word

   The control word is inserted after the MEL as described in Section 3.
   The S bit set to one in the MEL and the presence of the control word
   helps protect against transit nodes that may perform hashing or
   inspection of the label stack and payload packet headers when
   forwarding MPLS packets (for example, to enable ECMP).  The control
   word indicates that the payload is not a protocol that can be
   meaningfully hashed or inspected.

   The format of the control word is defined in [RFC4385] and shown in
   Figure 4.

    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 0 0| Flags |FRG|  Length   | Sequence Number               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Figure 4: Control Word for Encrypted MPLS

   Flags:           The Flags field is treated as a four-bit number.  It
                    contains the key-id that identifies the algorithm


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                    and key as established through configuration or
                    dynamic key exchange as described in Section 4.
   FRG:             Must be sent as 0, and ignored on receipt.
                    Fragmentation is not used.
   Length:          MUST be sent as 0, and ignored on receipt.
   Sequence Number: This field contains the packet counter (nonce) for
                    the encryption algorithm and key currently in use
                    modulo 65536.  It can be used by a receiver to
                    quickly check that the value of the nonce being used
                    for decryption is likely to be correct as described
                    in Section 3.

3.3. Considerations for ECMP

   As previously stated, the S bit set in the MEL and the presence of
   the control word prevent implementations from attempting to use the
   encrypted MPLS packet and its payload to determine a hash value for
   uses such as ECMP.  However, the resultant label stack shown in
   Figure 2 will probably not provide sufficient entropy for ECMP
   purposes.

   In order to increase the entropy, an implementation that inserts an
   MEL and MEL MAY also insert an Entropy Label Indicator (ELI) and
   Entropy Label (EL) as defined in [RFC6790] ELI and EL are positioned
   in the label stack before the MEL as shown in Figure 5.  The setting
   of the fields in the ELI and EL label stack entries are as described
   in [RFC6790].

   The ELI and EL will normally occur immediately before the label 15
   and MEL pair, but they MAY be placed higher up the label stack.




















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                              -----------
                             | Top Label |
                             +-----------+
                             |   Label   |
           -----------       +-----------+
          | Top Label |      |    ELI    |
          +-----------+      +-----------+
          |   Label   |      |    EL     |
          +-----------+.     +-----------+
          |   Label   | .    | Label 15  |
          +-----------+  .   +-----------+
          |   Label   |   .  | MEL   S=1 |
          +-----------+    . +-----------+
          | Label S=1 |     .| Ctrl Word |
          +-----------+      +-----------+
          |           |      |           |
          ~  Payload  ~      ~ Encrypted ~
          |           |      |           |
           -----------........-----------

       Figure 5 : The Use of ELI and EL with MEL

3.4. Backward Compatibility

   Keys and encryption algorithms may be configured manually or
   exchanged dynamically as described in Section 4.  These mechanisms
   provide a preliminary way to protect against a sender encrypting data
   that the receiver cannot decrypt, however, misconfiguration may lead
   to a sender using the MEL when the receiver does not support
   encryption.

   When a node finds an unknown label at the top of the label stack it
   must discard the packet as described in [RFC3031].  Therefore, when a
   receiver discovers label 15 and does not support extended special-
   purpose labels it will discard the packet.  Similarly when a receiver
   that supports extended special-purpose labels, but does not support
   the MEL (i.e., does not support encryption) it will discard the
   packet.  (Note that care must be taken if multiple experiments are
   being carried out in the same network since a different extended
   special-purpose label must be used for each experiment.)  The net
   result is that when a sender uses encryption in error, all packets
   that it sends on the LSP will be discarded by the receiver.  Note
   that in this discussion, "the receiver" may be the next hop if single
   hop encryption is used, or may be the end of the LSP if end-to-end
   encryption is used.

   Transit nodes that are not actively participating in the encryption
   will not inspect the MEL except potentially as part of an ECMP hash,


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   and it should be noted that the use of Special Purpose Labels in
   hashing is strongly discouraged (see Section 2.4.5.1 of [RFC7325]).
   This means that transit nodes will not encounter the MEL during
   normal packet processing and will not discard packets.

3.5. MTU Considerations

   Adding label 15, the MEL, and the Control Word as described above
   will reduce the available data size by 12 octets.  Furthermore, as
   described in Section 3, the output of the encryption algorithm is
   16 octets longer than the input.  Therefore, the use of encryption
   reduces the available MTU by 28 octets.

   When end-to-end encryption is in use this can be considered by the
   ingress LSR, however, when single-hop encryption is in use the
   participating LSRs need to advertise this reduction in link MTU
   so that the packets do not overflow.  MPLS packets MUST NOT be
   fragmented as a result of encryption.

3.6. Recursive Encryption

   The use of MEL and control word described in Section 3 may be applied
   recursively.  That is, the payload of an encrypted MPLS packet may,
   itself be an encrypted MPLS packet.  This may be particularly useful
   in the case where an MPLS VPN has native MPLS traffic.

   There are no special considerations except to note that encryption
   and decryption processing may be burdensome if an LSP and its payload
   LSP have encryption applied at the same LSR.  Additionally, it
   should be noted that, as described in Section 3.6, each recursive
   encryption reduces the MTU by 28 octets.

4. Key Exchange For Opportunistic Security in MPLS

   For encryption to be useful both ends of an encrypted session must
   know which algorithm is in use and which key to use.  The mechanism
   described in Section 3 provides a way to indicate an index into a
   table of algorithms and keys that can be used to decrypt an encrypted
   MPLS packet.

   It is possible that this table has been manually configured or set up
   using a key exchange protocol such as Internet Key Exchange version 2
   (IKEv2) [RFC7296].  However, such a process implies a stable security
   association between encrypter and decrypter of MPLS packets.  While
   such a stable association is entirely consistent with the concept of
   OS, OS nonetheless calls for a more dynamic key agreement method.

   This section provides a mechanism for adjacent MPLS LSRs, or for a


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   pair of LSRs at opposite ends of an MPLS LSP, to dynamically
   exchange keys and algorithm identifiers so that encryption may be
   applied opportunistically.

   The mechanism uses message exchanges in the MPLS Generic Associated
   Channel (G-ACh) [RFC5586] as part of the MPLS Generic Associated
   Channel (G-ACh) Advertisement Protocol (GAP) [RFC7212].  This channel
   is in-band with an LSP and may be used to carry messages between
   neighbors or between the end points of the LSP.  A type field within
   the common message header, the Associated Channel Header (ACH), is
   used to indicate the type of message carried.

   Nodes that receive G-ACh messages and do not understand them, or
   nodes that understand the G-ACh but do not recognize the ACH message
   type drop the packets as described in [RFC5586].

   Note that this mechanism may benefit from encryption that is already
   in use on an LSP.  Thus key changes using this mechanism can be made
   using encrypted messages.

4.1. MPLS G-ACh Advertisement Protocol for Key Exchange

   GAP defines messages exchanged in G-ACh on a common Associated
   Channel Type code point (0x0059) [RFC7212].  The application for
   which the messages are exchanged is defined by the Application ID
   field carried in the Applications Data Block (ADB).  MPLS OS
   capability notification and key exchange uses the GAP Application ID
   (0x0000) defined by [RFC7212] and a new ADB TLV for MPLS OS.

   Implementations that do not support GAP will discard received packets
   with this Associated Channel Type as described in [RFC5586].
   Implementations that support GAP but that do not support key exchange
   will discard received packets with this ADB TLV as described in
   [RFC7212].  Either of these discards will result in no dynamic key
   exchange, but other key definitions are still supported (such as
   manual configuration) and may be used to construct a table of
   algorithms and keys that can be used to achieve MPLS encryption using
   the mechanisms described in Section 3.

4.2. Key Exchange Protocol

4.2.1. Communication Channels

   The key exchange protocol described in this document uses a D-H
   exchange that assumes a bidirectional communication channel.  GAP is
   designed to run over a unidirectional channel and uses normal IP
   forwarding for return path messages with an optimization to use the
   return path of a bidirectional LSP.  However, LSPs in packet networks


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   are usually unidirectional.  That means that, while the key exchange
   messages can be sent on the LSP in one direction, a channel needs to
   be established for the return messages.

   This document uses a process similar to that defined for MPLS LSP
   Ping [RFC4379] in [RFC7110], and that described to indicate a return
   path for MPLS performance measurement in [I-D.bryant-mpls-oam-udp-
   return].  That is, the forward message is sent on the LSP and
   includes the identity of the return path communication channel.  The
   return path may be indicated as a UDP communication over IP, as an
   LSP running in the opostie direction, or as the reverse direction of
   a bidirecitonal LSP.

   Note that the GAP messages defined in [RFC7212] include a TLV that
   enables authentication.  This feature SHOULD be used if possible, but
   it is in the nature of opportunistic security that the necessary
   security association might not exist.  In this case the ability to
   tamper with the instructions that select a return path may provide a
   mechanism that makes MITM attacks easier.

4.2.2. Key Exchange Messages

   The format of a GAP message is described in [RFC7212].  When used for
   key exchange the GAP message includes an ADB with the fields set as
   follows.

      Application ID is set to 0x0000.

      Element Length is set to the total length in octets of this ADB
      including the Application ID and this field.

      Lifetime field SHOULD be set to zero and MUST be ignored.

   A key exchange ADB MUST include a Key Exchange TLV as shown in
   Section 4.2.3.  The ADB and MAY also include an Authentication TLV as
   described in [RFC7212] to provide authentication and integrity
   validation for a GAP message (see Section 4.4).  Additionally, the
   ADB MAY include a Source TLV as described in [RFC7212] and discussed
   in Section 4.3.

4.2.3. Key Exchange TLV

   A session key is to be established between an initiator (Alice) and
   a recipient (Bob).  The D-H public value for Alice is g^i and for
   Bob, g^r.  The shared Diffie-Hellman value is g^ir.  g^ir is
   represented as a string of octets in big endian order padded with
   zeros if necessary to make it the length of the modulus.  Both g^i
   and g^r will be 2048 bits long, if the Diffie-Hellman modulus is 2048


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   bits long.

   The Key Exchange TLV is modelled on that from Section 3.4 of
   [RFC7296] with the addition of information to identify the LSP and
   its return path, and is encoded as shown in Figure 6.

                        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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Type     |    Reserved   |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |D|Rsvd | Return|              Path Identifer                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           LSP-ID                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Algorithm   |  Group Num    |                                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                                 |
   |                                                               |
   ~                       D-H Public Value                        ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Figure 6 - Key Exchange Message TLV

   Type is set to TBD1 to indicate that this is a Key Exchange TLV

   The Reserved and Length fields are defined in [RFC7212].

   The flag D denotes the direction of the message, '0' indicates a
   message from initiator (Alice) to recipient. '1' indicates the
   reverse direction.

   The Rsvd bits are reserved.  They SHOULD be set to zero and ignored
   on receipt.

   The Return field is used on a message from the initiator to indicate
   the type of return path to be used for messages from the responder.
   The Path Identifier field is interpretted in this context.  Possible
   values are as follows:

     0  The reverse path of a bidirectional LSP is to be used for the
        response.  Used on a message from an initiator.
     1  The reverse path messages are to be sent encapsulated in UDP.
        Used on a message from an initiator.
     2  Any LSP between the recipient and the initiator may be used.
     3  Any LSP between the recipient and the initiator that is already
        using MPLS-OS may be used.
     4  The reverse path messages are to be sent on a specific LSP.


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     All other values are undefined and MUST be processed as an
     encoding error as described in Section 4.2.3.  Similarly, if the
     value zero is used on a unidirectional LSP then it MUST be handled
     as an encoding error.

   The Path Identifier is interpreted in the context of the Return
   field.  The field only has meaning on messages from the initiator and
   SHOULD be ignored on responses.  If the Return is set to the
   following values, the Path Identifier has the following meaning:

     0 In this case the Path Identifier field has no meaning and SHOULD
       be ignored.
     1 The Path Identifier field contains a UDP port number from the
       dynamic port range that the initiator will listen on for a
       response.
     2 In this case the Path Identifier field has no meaning and SHOULD
       be ignored.
     3 In this case the Path Identifier field has no meaning and SHOULD
       be ignored.
     4 The Path Identifier field contains an LSP-ID that must be used
       for reverse path messages.

   See Section 4.3 for more discussion of return paths.

   The LSP-ID parameter indicates the LSP to which this key exchange
   applies.  On messages from initiator to recipient this field MUST be
   set to the LSP on which the message flows and any mismatch MUST be
   treated as an encoding error (Section 4.2.3).  On messages from
   recipient to initiator, this value MUST be copied from the received
   message and an initiator that cannot match the message and LSP-ID to
   a message that it previously sent MUST treat the situation as an
   encoding error.

   The Algorithm field is a one octet field that specifies both the KDF
   to use and the symmetric algorithm to be used for data packet
   encryption.  A registry for values of this field is defined in
   Section 8.2.  The value 0 is used to indicate the default KDF and
   symmetric encryption mode.  An implementation receiving a value for
   an Algorithm it does not support MUST treat the case as an econding
   error as described in Section 4.2.3.  All implementations MUST
   support the default KDF.

   The Diffie-Hellman Group Num is from [RFC3526], so the group number
   for 2048 MODP is decimal 14.  Note that this is a one octet field,
   but is two octets in the [RFC7296] equivalent.  This is not an issue
   because there are only 30 MODP groups defined at present and new
   groups are not added frequently.



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   The D-H public value will contain g^i or g^r depending on the
   direction (i.e., the setting of the D flag) and is in big endian
   order.

   The length of the Diffie-Hellman public value for MODP groups MUST be
   equal to the length of the prime modulus over which the
   exponentiation was performed, prepending zero bits to the value if
   necessary.

   Once both sides have derived g^ir they need to feed that and the
   other inputs described in Section 2.4 into the KDF indicated by the
   algorithm field.  With the default algorithm (value zero), the KDF to
   be used is HKDF as specified in [RFC5869].

   The parameters for the use of HKDF are:

     Hash: SHA-256

     Salt: Not used

     Skip: Do not skip

     Info: The catenation of a fixed string indicating use of MPLS-OS,
           with the value "MPLS-OS", the first 32 bits of the key
           exchange message, with the D flag set to 0, plus the LSP
           ID and the sender and receiver LSR IDs in that order. That
           is:

      MPLS-OS||0||payloadLen||alg||group Num||LSP-ID||i-LSR-ID||r-LSR-ID

     L:    The output length in bits is 272.

   The fixed string "MPLS-OS" is used as an input here to prevent
   potential cross-protocol attacks.  Those might otherwise be
   possible if this mechanism were to be copied in other protocols.
   (If copying this mechanism for any reason, then a different
   fixed string value should be used.)

   LSP-ID is a unique identifier shared between the initiator and
   receiver (Alice and Bob) that uniquely identifies the LSP.

   [[If RSVP-TE is used for signaling, the LSP-ID is known along the LSP
    and at the two end points.  Similarly, the LSP-ID is known if the
    LSP is manually configured.  It is not so clear how the LSP-ID is
    known for LSPs established using LDP, although possibly we could use
    the FEC as defined for RFC 4379 and its extensions.]]

   i-LSR-ID and r-LSR-ID are the LSR-IDs of the initiator and receiver


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   respectively (Alice and Bob), where an LSR-ID is the 32 bit, globally
   unique identifier of the LSR as described in [RFC5036] and [RFC4990].

   The default encryption algorithm, AEAD_AES_GCM_128, specified in
   Section 3, requires a 128 bit session key.

   The 272-bit HKDF output is the catenation of the session key, the
   key-id, the witness value and the high-order 16 bits of the initial
   nonce value in that order.  That is the session key is the leftmost
   128 bits of the HKDF output.  The key-id is the next 4 bits, the
   witness value is the next 124 bits and the last 16 bits are the 16
   most significant bits of the initial nonce value.  The low order 64
   bits of the initial nonce value are set to zero before the first
   call to the AES-GCM encryption function.  The key-id is carried in
   encrypted packets as described in Section 3.2.

   Note that a 4 bit key-id is adequate in a system where, for any one
   LSP there is one active key and one new or replaced key.  There might
   also be more than one algorithm, and it is possible that new keys
   need to be pipelined if roll-over is frequent.

4.2.3. Encoding Errors

   Unknown values in received key Exchange TLVs MUST be treated as
   encoding errors.  All messages that constitute encoding errors MUST
   be silently discarded.  That is, such errors MUST NOT cause response
   messages to be sent since those messages could be used as part of an
   attack to determine the capabilities of an LSR.

   An LSR SHOULD log such errors and notify the operator.  However, care
   is needed even in these actions since they may be externally visible.

4.3. Indicating the Return Path

   The key exchange for MPLS-OS requires a two-way exchange of messages.
   The Return field of the Key Exchange TLV indicates the reverse path
   to use for key exchange messages relevant to a particular LSP.

   Whenever the LSP being secured is bidirectional, the same LSP SHOULD
   be used for reverse path messages.  Otherwise, the initiator selects
   the communication channel as described in Section 4.2.3.

   If UDP is being used and it may be unclear to what address the
   messages should be sent, the initiator MUST include a Source Address
   TLV [RFC7212] to provide this information.

   Operators should consider the security implications of the return
   path.  The use of an already-secured LSP (Return type 3) may provide


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   superior security.

   Implementations MUST make the choice of return path request sent by
   an initiator available as a configuration option.

4.4. Protecting the Key Exchange Protocol Messages

   GAP includes an Authentication TLV that can be used to protect GAP
   messages as described in [RFC7212].  If there is already an SA
   between the initiator and recipient this TLV SHOULD be used.
   However, it is probable with MPLS-OS that no such SA exists and the
   point of the mechanisms described in this document is to exchange
   keys in that case, therefore, it is quite likely that the
   Authentication TLV cannot be used on the first GAP exchanges.

   As described in Section 2.4, once one key exchange has been
   successfully completed, further key exchanges should be protected
   using a previous key.  This is simply achieved since key exchange
   messages are, themselves, carried in MPLS packets on the LSP and may
   be subject to encryption exactly as any other packet.

   Furthermore, once keys have been established, they may also be used
   in the GAP Authentiation TLV.

5. Applicability of MPLS Opportunistic Security

   MPLS-OS provides another tool in the security and privacy toolkit.
   It is not a panacea and does not solve (nor is it intended to solve)
   all security or privacy problems.  In particular, the use of MPLS-OS
   does not protect user-data end-to-end that might be better secured
   using encryption at the IP layer or at higher layers.

   As noted throughout this document, the intention of OS in MPLS is to
   allow one LSR to enable encryption between itself and its neighbor,
   or between itself and the other end of an LSP, in a dynamic and un-
   planned way.  This can have benefits in a number of scenarios where
   the network that generates MPLS traffic transmits it over another
   network (for example, carrier's carrier, or some deployments of
   enterprise network).  Additionally, the use of MPLS-OS might allow a
   service provider to offer a secure edge-to-edge service for a variety
   of applications ranging from VPNs through pseudowires and where the
   payload traffic might not always be IP.  Lastly, in some non-
   traditional carriers the user data belongs to the operator or is the
   direct responsibility of the operator (for example, in data centers,
   or in large-scale private networks).

   As with all security mechanisms, there is a trade-off between a
   number of factors.  On one side is the completeness of the security


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   of the user-data, and on the other side is the complexity of
   configuring and managing the necessary security associations.
   Furthermore, while mechanisms closer to the end-user than MPLS-OS
   (for example, TLS and IPsec in tunnel mode) provide better security
   for user-data by virtue of not transmitting the data across any
   network hops without it being encrypted, such mechanisms often
   expose more metadata for inspection by snoopers within the network.

   Additionally, while a variety of per-link encryption mechanisms exist
   and could be used to guard against attacks such as fiber taps, those
   approaches do not protect against subverted nodes (i.e., routers) on
   the path since, by definition, per-link encryption does not protect
   packets once they come off the link.  MPLS-OS in the end-to-end LSP
   mode protects packets on the links and as they cross transit routers.

   Nevertheless, it is not the purpose of this document to recommend the
   use of MPLS-OS to the exclusion of all other encryption techniques.
   As already mentioned, MPLS-OS is offered as another tool in the tool
   kit and users as well as network operators are strongly advised to
   consider using a variety of tools to achieve the level of security
   and privacy that they desire.

   Note that, in order that OS can be used, one end of a peering
   (neighbor or LSP end) must decide to attempt OS and the other end
   must support it.  This can be determined by the message exchanges
   described in Section 4.2 since if one peer does not send a key
   exchange message then encryption will not be used, and if the other
   peer does not respond then it is unwilling or unable to decrypt
   messages.

   MPLS-OS should be applicable to all forms of MPLS. That is, it should
   be possible to use it in RSVP-TE systems, in LDP systems, and in
   MPLS-TP systems (by which we mean those that have manually configured
   LSPs). Equally, it should work for point-to-point (P2P) and
   multipoint-to-point (MP2P) uses of MPLS because there is a simple
   relationship between the sender (encrypter) and the receiver
   (decrypter) in both cases. In the MP2P case, the sender's identity
   can be extracted from the key identifier and there are considered to
   be enough key identifiers to allow an arbitrary number of senders on
   the LSP. There will, however, be the need for the receiver to hold OE
   state (keys, packet counters) for each sender which may be a
   significant amount of data for an MP2P LSP (although no more than if
   the same LSP were replaced by multiple P2P LSPs). Additionally, it
   should be noted that not only will each sender on an MP2P LSP have a
   different key, but each may separately decide whether to encrypt data
   or not.

   At this time it is not certain whether MPLS-OS can be applied to a


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   point-to-multipoint (P2MP) or a multipoint-to-multipoint LSP in its
   entirety because packet replication cannot handle the necessary key
   conversions for each receiver. However, MPLS-OS can certainly be
   applied to individual hops on these LSPs. Further work is needed to
   determine whether non-branching multi-hop segments of P2MP and MP2P
   LSPs can also be protected using MPLS-OS.

6. Security Considerations

6.1. Security Improvements

   See section 2.1.

6.2. Continued Vulnerabilities

   The mechanisms described in this document do not provide protection
   against certain types of MITM attacks.  For example, the key exchange
   protocol in Section 4.2 will not detect if key exchange messages or
   their responses are intercepted and discarded such that the
   initiating peer believes that encryption is not supported.
   Similarly, those messages may be tampered with such that a receiver
   cannot determine the correct mapping of table index to algorithm and
   key when an encrypted packet is received.  Furthermore, the MEL in an
   MPLS packet is not protected and may be overwritten such that a
   receiver is unable to decrypt the packet.

   See Section 7.1 for a discussion of how active MITM attacks can be
   detected.

6.3. New Security Considerations

   If a pair of LSRs do not do the key exchange before sending any data
   packets on the LSP then those first packets will not be protected by
   OS and hence will be available to a monitor.

   If a MITM can prevent the OS key exchange from completing, e.g.
   via deleting messages or changing bits in messages, and if the LSRs
   continue to send data regardless then those data packets will be
   available to a monitor.  That is, in simple terms, a MITM attacker is
   able to prevent OS from being used through a very simple attack, and
   the only options for the end points in this situation are to send no
   data or to send data in the clear.  Again, it should be pointed out
   that this occurrence is not worse than not running OS at all, but has
   the benefit of being detectable by end points.  See Section 2.4 and
   Section 7 for a description of how alarms should be raised in these
   circumstances.

   Thus, as been previously noted, OS is not a cure for all ills or a


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   prevention against all attacks, but it does offer a way to increase
   security in some circumstances.

7. Manageability Considerations

   As described in Section 2.4 node-wide and per-LSP behavior SHOULD be
   configurable to describe the action where key agreement exchange or
   packet decryption fails.  In any case, such events MUST trigger
   alarms to the operator.

7.1. MITM Detection

   Section 2.4 introduces the concept of a function of the shared
   secret that can be compared by two LSRs that are using OS to see
   whether they are victims of an active MITM attack.

   Section 4.2 describes how a witness value is derived for the
   default KDF, HKDF.

   The participating LSRs can simply log this value plus the LSP
   and LSR IDs from time to time and a management application can
   compare the values.  If they are different for the same LSP ID,
   then an active MITM attack has taken place.

   It needs to be carefully noted that the management channel used to
   log or otherwise compare the witness values from the two LSRs MUST be
   secure.  It is likely that routers use relatively high security
   management channels for configuration and other management
   operations.

8.  IANA Considerations

8.1. GAP Key Exchange TLV

   IANA maintains a registry called "Generic Associated Channel (G-ACh)
   Parameters" with a sub-registry called "G-ACh Advertisement Protocol
   Application Registry" from which new assignments may be made through
   the "IETF review" allocation policy [RFC5226].  IANA is requested to
   make a new allocation as follows:

   Value | Description                                     | Reference
   ------+-------------------------------------------------+-----------
   TBD1  | Opportunistic Key Exchange Protocol for MPLS    | [This.ID]


8.2. Key Derivation Functions and Symmetric Algorithms

   IANA maintains a registry called "Generic Associated Channel (G-ACh)


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   Parameters".  IANA is requested to create a new sub-registry called
   "G-ACh Advertisement Protocol: MPLS Encryption Algorithms Registry"
   with new values to be assigned through "IETF Review" as defined in
   [RFC5226].

   The available range is 0 - 255.

   IANA is requested to record the following information and create an
   initial entry as follows:

   Value | Key Derivation Function | Symmetric Algorithm |  Reference
   ------+-------------------------+---------------------+-----------
   0     | HKDF                    | AEAD_AES_GCM_128    | [This.I-D]
   1-255 | Unassigned              |                     |

9.  Acknowledgements

   Many thanks to Alia Atlas for detailed discussion of the implications
   and mechanisms of MPLS opportunistic security.  Thanks also to Ron
   Bonica for encouraging this work, to Sean Turner and Stewart Bryant
   for early review, and to Jeff Haas and Ross Callon for discussions.
   Thanks to Andy Malis and Danny McPherson for advice about the use of
   the Control Word.

10.  References

10.1.  Normative References

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

   [RFC3526]  Kivinen, T., and M. Kojo, "More Modular Exponential (MODP)
              Diffie-Hellman groups for Internet Key Exchange (IKE)",
              RFC 3526, May 2003.

   [RFC4385]  Bryant, S., Swallow, G., Martini, L., and D. McPherson,
              "Pseudowire Emulation Edge-to-Edge (PWE3) Control Word for
              Use over an MPLS PSN", RFC 4385, February 2006.

   [RFC5116]  D. McGrew, "An Interface and Algorithms for Authenticated
              Encryption", RFC 5116, January 2008.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

   [RFC5586]  Bocci, M., Vigoureux, M., and S. Bryant, "MPLS Generic
              Associated Channel", RFC 5586, June 2009.


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   [RFC5869]  Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
              Key Derivation Function (HKDF)", RFC 5869, May 2010.

   [RFC6790]  Kompella, K., Drake, J., Amante, S., Henderickx, W., and
              L. Yong, "The Use of Entropy Labels in MPLS Forwarding",
              RFC 6790, November 2012.

   [RFC7212]  Frost, D., Bryant, S., and M. Bocci, "MPLS Generic
              Associated Channel (G-ACh) Advertisement Protocol", RFC
              7212, June 2014.

   [RFC7274]  Kompella, K., Andersson, L., and A. Farrel, "Allocating
              and Retiring Special-Purpose MPLS Labels" RFC 7274,
              June 2014.

10.2.  Informative References

   [I-D.bryant-mpls-oam-udp-return]
              Bryant, S., Sivabalan, S., and Soni, S., "MPLS Performance
              Measurement UDP Return Path", draft-bryant-mpls-oam-udp-
              return, work in progress.

   [I-D.dukhovni-opportunistic-security]
              V. Dukhovni, "Opportunistic Security: some protection most
              of the time", draft-dukhovni-opportunistic-security, work
              in progress.

   [RFC3031]  Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
              Label Switching Architecture", RFC 3031, January 2001.

   [RFC3032]  Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
              Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
              Encoding", RFC 3032, January 2001.

   [RFC4302]  Kent, S., "IP Authentication Header", RFC 4302,
              December 2005.

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

   [RFC4379]  Kompella, K. and G. Swallow, "Detecting Multi-Protocol
              Label Switched (MPLS) Data Plane Failures" RFC 4379,
              February 2006.

   [RFC4990]  Shiomoto, K., Papneja, R., and R. Rabbat, "Use of
              Addresses in Generalized Multiprotocol Label Switching
              (GMPLS) Networks", RFC 4990, September 2007.


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

   [RFC6239]  K. Igoe, "Suite B Cryptographic Suites for Secure Shell
              (SSH)", RFC 6239, May 2011.

   [RFC7110]  Chen, M., Cao, W., Ning, S., Jounay, F., and Delord, S.,
              "Return Path Specified Label Switched Path (LSP) Ping",
              RFC 7110, January 2014.

   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring
              Is an Attack", BCP 188, RFC 7258, May 2014.

   [RFC7296]  Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and
              T. Kivinen, "Internet Key Exchange Protocol Version 2
              (IKEv2)", STD 79, RFC 7296, October 2014.

   [RFC7325]  C. Villamizar, Ed., "MPLS Forwarding Compliance and
              Performance Requirements", RFC 7325, August 2014.

Authors' Addresses

   Adrian Farrel
   Juniper Networks

   EMail: adrian@olddog.co.uk

   Stephen Farrell
   Trinity College Dublin
   Dublin, 2
   Ireland

   Phone: +353-1-896-2354
   Email: stephen.farrell@cs.tcd.ie
















Farrel and Farrell                                             [Page 30]


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