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rfc:rfc2154

Network Working Group S. Murphy Request for Comments: 2154 M. Badger Category: Experimental B. Wellington

                                           Trusted Information Systems
                                                             June 1997
                    OSPF with Digital Signatures

Status of this Memo

 This memo defines an Experimental Protocol for the Internet
 community.  This memo does not specify an Internet standard of any
 kind.  Discussion and suggestions for improvement are requested.
 Distribution of this memo is unlimited.

Abstract

 This memo describes the extensions to OSPF required to add digital
 signature authentication to Link State data, and to provide a
 certification mechanism for router data.  Added LSA processing and
 key management is detailed.  A method for migration from, or co-
 existence with, standard OSPF V2 is described.

Table of Contents

 1 Acknowledgements .............................................   2
 2 Introduction .................................................   2
 3 LSA Processing ...............................................   4
 3.1 Signed LSA .................................................   4
 3.2 Router Public Key LSA (PKLSA) ..............................   5
 3.3 MaxAge Processing ..........................................   7
 4 Key Management ...............................................   8
 4.1 Identifying Keys ...........................................   8
 4.1.1 Identifying Router Keys and PKLSAs .......................   8
 4.1.2 Identifying TE Public Keys ...............................   8
 4.1.3 Key to use for Signing ...................................   9
 4.1.4 Key to use for Verification ..............................   9
 4.2 Trusted Entity (TE) Requirements ...........................  10
 4.3 Scope for Keys and Signature Algorithms.....................  10
 4.4 Router Key Replacement .....................................  11
 4.5 Trusted Entity Key Replacement .............................  12
 4.6 Flexible Cryptographic Environments ........................  14
 4.6.1 Multiple Signature Algorithms ............................  14
 4.6.2 Multiple Trusted Entities ................................  15
 4.6.3 Multiple Keys for One Router .............................  16
 5 Compatibility with Standard OSPF V2 ..........................  16
 6 Special Considerations/Restrictions for the ABR-ASBR .........  17
 7 LSA formats ..................................................  18

Murphy, et. al. Experimental [Page 1] RFC 2154 OSPF with Digital Signatures June 1997

 7.1 Router Public Key LSA (PKLSA) ..............................  18
 7.2 Router Public Key Certificate ..............................  20
 7.3 Signed LSA .................................................  23
 8 Configuration Information ....................................  26
 9 Remaining Vulnerabilities ....................................  26
 9.1 Area Border Routers ........................................  27
 9.2 Internal Routers ...........................................  27
 9.3 Autonomous System Border Routers ...........................  28
 10 Security Considerations .....................................  28
 11 References ..................................................  29
 12 Authors' Addresses ..........................................  29

1. Acknowledgements

 The idea of signing routing information is not new.  Foremost, of
 course, there is the design that Radia Perlman reported in her thesis
 [4] and in her book [5] for signing link state information and for
 distribution of the public keys used in the signing.  IDPR [7] also
 recommends the use of public key based signatures of link state
 information.  Kumar and Crowcroft [2] discuss the use of secret and
 public key authentication of inter-domain routing protocols.  Finn [1]
 discusses the use of secret and public key authentication of several
 different routing protocols.  The design reported here is closest to
 that reported in [4] and [7].  It should be noted that [4] also
 presents techniques for protecting the forwarding of data packets, a
 topic that is not considered here, as we consider it not within the
 scope of the OSPF working group.
 The authors would also like to acknowledge many fruitful discussions
 with many members of the OSPF working group, particularly Fred Baker
 of Cisco Systems, Dennis Ferguson of MCI Telecommunications Corp.,
 John Moy of Cascade Communications Corp., Curtis Villamizar of ANS,
 Inc., and Rob Coltun of FORE Systems.

2. Introduction

 It is well recognized that there is a need for greater security in
 routing protocols. OSPF currently provides "simple password"
 authentication where the password travels "in the clear", and there
 is work in progress[11] to provide keyed MD5 authentication for OSPF
 protocol packets between neighbors.  The simple password
 authentication is vulnerable because any listener can discover and
 use the password.  Keyed MD5 authentication is very useful for
 protection of protocol packets passed between neighbors, but does not
 address authentication of routing data that is flooded from source to
 eventual destination, through routers which may themselves be faulty
 or subverted.

Murphy, et. al. Experimental [Page 2] RFC 2154 OSPF with Digital Signatures June 1997

 The basic idea of this proposal is to add digital signatures to OSPF
 LSA data, distribute certified router information and keys, and use a
 neighbor-to-neighbor authentication algorithm (like keyed MD5) to
 protect local protocol exchanges.  The content of a Hello packet,
 Link State Request, Link State Update, or Database Description will
 be protected by the neighbor-to-neighbor algorithm.  The LSAs that
 are being flooded inside the Link State Update packets are
 individually protected by a digital signature.  Each LSA will be
 signed by the originator of that information and the signature will
 stay with the data in its travels via OSPF flooding.  This will
 provide end-to-end integrity and authentication for LSA data. The
 digital signature attached to an LSA by the source router provides
 assurance that the data comes from the advertising router.  It will
 also ensure that the data has not been modified by some other router
 in the course of flooding.  In the case where incorrect routing data
 is originated by a faulty router, the signature will identify the
 source of the problem.
 Digital signatures are implemented using public key cryptography.
 There are some good books on the subject of cryptography [6], but the
 high level view of how this design uses public key cryptography is as
 follows: Each router has a pair of keys, a public key and a private
 key.  The private key is used to generate a unique signature of a
 block of data (in this case, the LSA). Each router signs its LSAs by
 first running a one-way hash algorithm (like MD5 or SHA) on the data,
 and then using its private key to sign the digest.  The signature of
 an LSA is appended to the LSA. The public key can be used by any
 other router to verify the signature.  The private key must be kept
 secret by one router and the public key must be distributed to all
 the routers that will receive link state information from the signer.
 The distribution is accomplished by creating a new LSA, the Public
 Key LSA (PKLSA), and distributing it via the standard OSPF flooding
 procedure.  Flooding will ensure that a router public key is sent
 everywhere that the router's signed LSAs are sent.
 Any router can send out a public key and claim to be a given router,
 so the public key itself provides no assurance of the actual identity
 of the sender. This assurance must be provided by a Trusted Entity.
 The Trusted Entity (TE) is a system that generates certificates for
 routers.  A certificate is a packet of information about a router
 that identifies the router and supplies a public key. Certified
 router information will include the router id, its role, the address
 ranges that the router may advertise, a timestamp and the router's
 public key. The certificate is signed by the TE.  Each router must be
 configured with a certificate and a TE public key to use in verifying
 other routers' certificates.  A router PKLSA contains the certificate
 for that router.  A router receiving a PKLSA verifies the certificate
 using the TE public key, and then verifies the whole LSA using the

Murphy, et. al. Experimental [Page 3] RFC 2154 OSPF with Digital Signatures June 1997

 router public key contained in the certificate. Successful
 verification provides assurance that the PKLSA is from the correct
 router, and that it has not been altered by any other router in the
 flood path.
 OSPF with Digital Signatures is backward compatible with standard
 OSPF V2 in a limited way.  Within an AS there may be "signed" areas
 and "unsigned" areas.  The behavior of a mixed AS is discussed in
 section 5.
 Digital signatures for OSPF LSAs can be implemented with the
 following major functions:
 (1) Support for a digital signature algorithm
 (2) Support for a signed version of all routing information LSAs
 (3) Support for a new LSA: Router Public Key LSA (PKLSA)
 (4) A mechanism for key certification and certificate distribution
 (5) Extra configuration data (detail in section 7):
       Trusted Entity (TE) information and key(s)
       Router certification data and key
       Area environment flag (signed/unsigned)
       Timing intervals
 An implementation of this design exists, based on the OSPF in Gated
 version 3.5Beta3.  This implementation is available for
 use/experimentation.  Please contact the authors for information.

3. LSA Processing

3.1. Signed LSA

 A signed LSA contains the standard OSPF V2 header and data plus key
 identification information, a signature length and a signature.  The
 top bit of the LS type field is set to indicate the presence of a
 signature.  The signature covers the LSA header (starting with the
 options field), the LSA data, and the key identification information
 and the signature length that must be appended to the LSA data.
 There are two exceptions to this coverage: first, an LSA created with
 age=MaxAge has a signature that begins with the age field (see
 section on maxage); second, the LSA header checksum is set to zero
 for the generation of the signature.  To assist in parsing the
 message, the key id information and the signature length fields are
 placed at the end of the LSA, following the signature.  However, the

Murphy, et. al. Experimental [Page 4] RFC 2154 OSPF with Digital Signatures June 1997

 message must be signed and verified with these fields immediately
 appended to the LSA data.  This can be accomplished either by doing
 the sign and verify "in parts" (allowed by RSAREF), or by storing the
 LSA data with appended fields and the LSA signature separately in the
 link state database (LSDB).
 When a signed LSA is received, the signature can be verified using
 the public key of the advertising router contained in the advertising
 router's PKLSA.  If the signature verifies, then the signed LSA is
 stored for use in routing calculations. If the signature verification
 fails, the LSA must be discarded. If the identified key is not
 available (in a PKLSA from the advertising router), then the signed
 LSA must be stored for a period of time defined by the configurable
 MAX_TRANSIT_DELAY interval.  If the key arrives within this interval,
 the LSA will be processed then.  If the key does not arrive within
 this interval, the LSA will be discarded.  This delay period prevents
 loss of routing information due to LSAs arriving prior to their
 associated PKLSAs (which should not normally be the case, but could
 happen).
 If the LSA is a Router Links LSA, the router's advertised links must
 be checked against the allowed address ranges stored in the PKLSA for
 the advertising router.  All network links (link types 2 and 3) must
 have an IP address that fits in one of the ranges defined by the list
 of address ranges in the PKLSA (format 7.2).  If there is a link that
 does not fit into one of these ranges, then an error must be logged
 and the LSA must be discarded.  Careful subnetting and corresponding
 ranges can provide very tight control on what is advertised.  A much
 less restrictive, but still useful, level of control can be obtained
 by defining allowed address ranges for an area, so that all routers
 in an area could be configured with the same set.  To trivially
 satisfy this checking, one range with a zero address and mask can be
 defined that contains all IP addresses.
 Link State Acknowledgements must be sent for all LSAs that are
 discarded due to verification failures, that are stored waiting for
 keys, and that are discarded because they are advertising a link that
 they are not allowed to advertise.

3.2. Router Public Key LSA (PKLSA)

 A Router Public Key LSA (PKLSA) is sent in the same manner as all
 other LSAs.  This LSA contains the router's public key and
 identifying information that has been certified by a Trusted Entity.
 The router public key is used to verify signatures produced by this
 router.  There is only one PKLSA stored per router in the LSDB for an
 area, so the Router Id and LS type can be used to retrieve a given
 PKLSA.  The Router Id is stored in the PKLSA Link State Id field to

Murphy, et. al. Experimental [Page 5] RFC 2154 OSPF with Digital Signatures June 1997

 use in retrieving the PKLSA. Identification information in the
 certified data (TE Id, Rtr Key Id) can be used to uniquely identify
 the current router key (section 7.2).
 To assist in parsing the message, the router signature length and the
 certification length fields are at the end of the LSA, following the
 signature.  The message must be signed and verified with these fields
 immediately appended to the LSA data.  The router signature of the
 PKLSA is verified in the same manner as other signed LSAs.  In
 addition, the certification must be verified using the referenced TE
 public key.  If either verification fails, for any reason, the PKLSA
 is discarded.
 A successfully verified PKLSA is stored for use in verifying signed
 LSAs from the advertising router. For every router that this router
 is in contact with, there may be one PKLSA stored at any given time.
 Each PKLSA is uniquely identified by the values (TE Id, Rtr Key Id)
 in the certified data (format in 7.2).  When a PKLSA arrives for a
 given router, and there is already a PKLSA stored for that router,
 the PKLSA with the most recent "Create Time" is the one kept.
 Whenever groups of LSAs are sent by a router (as when synchronizing
 databases or sending updates), the PKLSAs must be sent/requested
 before other LSAs to minimize the time spent processing LSAs that
 arrive prior to their associated keys.  The PKLSA is sent at
 intervals like all other LSAs, and it is sent immediately if a router
 obtains a new key to distribute. A PKLSA is sent via OSPF flooding
 within an OSPF area.  PKLSAs are not flooded outside an area with the
 exception of an Autonomous System Border Router's PKLSAs which must
 be flooded wherever AS external LSAs are flooded.  The decision to
 flood or not flood can be implemented by checking the router role
 (Rtr, ABR, ASBR, ABR-ASBR) stored in the certified part of the PKLSA.
 A router may flush its keys from routing tables by flooding a PKLSA
 for that key with age=MaxAge.  This is called premature aging of the
 PKLSA.  A key can also be removed from routing tables (superseded) by
 a PKLSA from the same router, containing a valid certificate for a
 new key with a more recent Create Time.  If a key is superseded by a
 more recent key it is not necessary to flush the old key with a
 "MaxAge" PKLSA.
 When a new key is received, the LSAs stored in the LSDB that are
 signed with the old key must be replaced within MAX_TRANSIT_DELAY.
 if the sending router is working properly.  This is because a router
 distributing a new key sends all of its self-originated LSAs signed
 with the new key immediately after sending the new PKLSA.  (See
 section 4.4 on Router Key Replacement).  To ensure that data signed
 with an old (possibly subverted) key does not persist in the LSDB in

Murphy, et. al. Experimental [Page 6] RFC 2154 OSPF with Digital Signatures June 1997

 error, all LSAs signed with a flushed or superseded key are aged to
 within MAX_TRANSIT_DELAY of MaxAge.  This should allow time for the
 new LSAs signed with the new key to arrive.  If new LSAs do not
 arrive, or if the key has been flushed and not replaced, then the old
 LSA data will disappear from the LSDB in a timely fashion.
 Link State Acknowledgements must be sent for PKLSAs that are
 discarded due to verification failures or because the PKLSA was less
 recent than the one already stored.

3.3. MaxAge Processing

 The age field in the OSPF LSA header is used to keep track of how
 long a given LSA has been in the system.  When the age field reaches
 MaxAge, a router stops using the LSA for routing, and it floods the
 MaxAge LSA to make sure that all routers stop using this LSA.  In the
 normal course of the OSPF protocol, an LSA is always replaced by an
 updated version before the age reaches MaxAge, unless the advertising
 router fails, or changes in the AS have made the routing information
 in the LSA inaccurate.  An LSA with age=MaxAge is either:
 (1) being intentionally flushed from the AS by the advertising router
     because the information in it is no longer accurate, or
 (2) an orphan LSA that has aged to MaxAge because its originating
     router has not refreshed it at the normal refresh intervals.
 The age field cannot generally be included in the signature, because
 it must be updated by routers other than the originating router.  For
 the same reason, the age field is not included in the checksum
 computation.  The age field must be protected, because if a faulty
 router started to age out other router's LSAs, it would effectively
 deny service to those other routers.
 To protect the age field, the signature must include the age field if
 and only if the originating router creates an LSA with age=MaxAge.
 Verification of the signature on a signed LSA must include the age
 field if and only if the age field value is MaxAge.  In this manner,
 the originating router can flush an LSA, but other routers cannot.
 An LSA that ages to MaxAge in the LSDB of any router is still
 discarded by that router, but it is not synchronously flushed from
 the AS.

Murphy, et. al. Experimental [Page 7] RFC 2154 OSPF with Digital Signatures June 1997

 An LSA will be removed from a router's Link State Database in one of
 two ways: 1) the router receives a version of the LSA with the age
 field set to MaxAge and a valid signature that covers the age field,
 or 2) the LSA incrementally reaches MaxAge while it is stored by the
 router.
 If a standard OSPF V2 router goes down, an LSA from that router will
 age in the LSDBs of each remaining router until it reaches MaxAge
 somewhere.  As soon as it reaches MaxAge in some router's LSDB it is
 flooded, and this causes it to be flushed from the AS in a
 synchronized fashion.  If router running OSPF with digital signatures
 goes down, its signed LSAs will be aged out by each remaining router
 individually.  This will slow database convergence but the databases
 will still converge, and a fairly obvious security hole will be
 closed.

4. Key Management

4.1. Identifying Keys

4.1.1. Identifying Router Keys and PKLSAs

 A router key is identified by the Router Id, and the identifiers
 associated with the particular key in its certificate: TE Id and
 Router Key Id.  All three of these values are stored in a PKLSA
 (format in 7.1).  The Router Id is the standard LSA header
 Advertising Router.  The (TE Id, Rtr Key Id) are stored in the PKLSA
 certified data.  The TE Id is a number assigned to a Trusted Entity
 that must uniquely identify one TE in the AS.  The TE Id in a
 certificate identifies the TE that produced the certificate.  The Rtr
 Key Id is associated with a key by the Trusted Entity that produced
 the certificate.  The Trusted Entity must produce a stream of Rtr Key
 Ids for one router such that the router will not re-use a key id
 until all references to the last key having that id are gone from the
 AS.  If a key is re-played, or re-used too soon, the Create Time in
 the key certification will determine which key is current.  Rtr Key
 Ids do not have to be sequential.

4.1.2. Identifying TE Public Keys

 Each TE public key has an associated TE Id, TE Key Id.  The
 combination of (TE Id, TE Key Id) uniquely identifies one TE public
 key in the AS.  The TE Id is a number assigned to a Trusted Entity

Murphy, et. al. Experimental [Page 8] RFC 2154 OSPF with Digital Signatures June 1997

 that uniquely identifies one TE in the AS.  The TE Key Id must
 identify one particular key for a TE at any given time.  The TE Key
 Id distinguishes between a new key and an old key for the same TE.
 The TE Key Id also differentiates between keys for different
 signature algorithms if one TE serves multiple algorithms.  Each TE
 can have at most one current key per signature algorithm.
 There can be multiple TE keys stored on each router.  A TE public key
 is used to verify the certificates issued by other routers, and in an
 AS with several TEs, any given router may need several TE public
 keys.  TE Key Ids do not have to be used sequentially, and they can
 be re-used.  There is no timestamp for TE keys because these are not
 certified.
 It is the responsibility of Configuration Management to ensure that
 TE Key Ids are not re-used before all references to a previously used
 key with the same (TE Id, TE Key Id) are gone from the AS, that a
 given (TE Id, TE Key Id) on one router identifies the same key as it
 does on any other router, and that the rules for TE Key Replacement
 (section 4.5) are followed.

4.1.3. Key to use for Signing

 A router is configured with a pair of keys.  The private key is
 protected from disclosure and is used for signing.  The public key is
 flooded in a PKLSA and is used for verifying signatures.  A router
 may have one key per area to use for signing at any given time.  A
 router may use the same key for several or all areas.

4.1.4. Key to use for Verification

 There are three uses of signature verification in this design:
 (1) The signature in a signed LSA (format in 7.3) can be verified
     with the public key distributed by the advertising router in a
     Public Key LSA.  A signed LSA contains the (TE Id, Rtr Key Id) of
     the key used to sign it.  The signed LSA's Advertising Router Id
     is used to retrieve the router's PKLSA , and the (TE Id, Rtr Key
     Id) indicates if the router key in the PKLSA is the same as the
     one used to generate the signature.
 (2) The router's signature in a PKLSA (format in 7.1) is verified
     with the public key contained in that PKLSA.

Murphy, et. al. Experimental [Page 9] RFC 2154 OSPF with Digital Signatures June 1997

 (3) The PKLSA contains data certified with a signature generated
     by a TE.  The PKLSA certified data contains the (TE Id, TE Key
     Id) for the TE key that can be used to verify the certificate
     (format in 7.2).  TE public keys must be configured on each
     router.

4.2. Trusted Entity (TE) Requirements

 This design does not specify how the Trusted Entity (TE) must be
 implemented, where it must reside, or how it must communicate with
 routers.  There are several very different possible approaches to the
 implementation of a Trusted Entity (e.g., an offline system with
 distribution of keys by floppy or secure e-mail, an online automated
 key distribution center, etc.) This design does mandate certain
 requirements for what a Trusted Entity must do.  A Trusted Entity
 must generate a certificate for each signing router that contains
 individualized information about that router (format in 7.2) and is
 signed with the Trusted Entity private key.  The Trusted Entity must
 have a unique TE Id for itself, it must create a Rtr Key Id for each
 router key that is unique for the given Router for this TE at this
 time, and it must timestamp certificates with a Create Time that is
 consistent for itself and for any other Trusted Entities operating in
 the AS.  Note: routers do not have to be time-synched, but TEs do.
 Create Time is used by routers as a relative measure to determine
 which key is more recent.
 The TE Public key, TE Id, TE Key Id and Signature Algorithm must be
 made available to each router processing certificates from this TE.
 A TE can theoretically create certificates for more than one
 signature algorithm.  The TE key and the router public key certified
 do not have to be of the same signature algorithm.
 There can be more than one TE in an AS but the TE Id must identify a
 unique TE.

4.3. Scope for Keys and Signature Algorithms

 The concept of "scope" relates to Router Keys, TE Keys, and Signature
 Algorithms.
 (1) The scope of a PKLSA and therefore a router key, is defined to
     be the set of routers that will receive and store that PKLSA in
     the course of OSPF flooding.  A router produces a PKLSA for each
     attached area.  In a router with more than one area, the PKLSAs
     for each area may match, or each may contain a different key.
     The scope of PKLSA for an internal router is all the routers in
     that area.  An ABR has multiple PKLSAs, each having a scope of

Murphy, et. al. Experimental [Page 10] RFC 2154 OSPF with Digital Signatures June 1997

     one attached area.  The scope of an ASBR's PKLSA is the same as
     the scope of the ASBRs ASEs - all the routers in all the non-stub
     areas in the AS.  An ASBR that is an ABR produces multiple PKLSAs
     that each have a scope of all the routers in all the non-stub
     areas in the AS.  (This last case results in some situations that
     require special management - section 6)
 (2) The scope of a TE key is defined to be the set of routers that are
     configured with this key.  If a system is configured properly,
     then a TE public key will be configured on all the routers that
     will receive PKLSAs certified by that TE key.  The minimum scope
     for a TE key is an area.  If one router distributes a key
     certified with a given TE key, then all the routers in the area
     must be able toverify the certificate.  A TE Key certifying an
     ASBRs key must have a scope of all non-stub areas in the AS.  If
     the TE key is not on some router that receives PKLSAs certified by
     that TE key, then those PKLSAs and all the LSAs that require them
     will be discarded. A TE key gets to all the routers in its scope
     via out-of-band configuration.
 (3) The scope of a signature algorithm is defined to be the set of
     routers that are capable of verifying the given algorithm's
     signatures.  The minimum scope for a signature algorithm is an
     area.  All routers in an area must be able to verify any signature
     algorithm used for signing by any router in the area.  The
     algorithm used to certify an ASBRs key must have a scope of all
     non-stub areas in the AS if the ASEs are to be accessible
     everywhere (see section 6).  If a signature algorithm is not
     available to verify an LSA, then the LSA must be discarded.  If a
     signature algorithm is not available to verify the certification
     in a PKLSA, then the PKLSA must be discarded.

4.4. Router Key Replacement

 Router keys should be changed periodically, and immediately if a key
 is found to be compromised.  The regular period for changing a key is
 some locally determined function of the size of the key and the level
 of security needed.
 Each router can have ONE valid key per area at any given time.
 Restricting the number of keys at a given time to one key per router
 per area allows key replacement to also serve the purpose of key
 revocation, without having a revocation list and without routers
 having synchronized time.  Each key for the router/area revokes the
 last key, provided the "new" key has a more recent Create Time than
 the last key.  The Create Time in each certificate is used to prevent
 an old key from being reused, but this Create Time is used only for
 comparing the relative ages of certificates, and does not require the

Murphy, et. al. Experimental [Page 11] RFC 2154 OSPF with Digital Signatures June 1997

 router to run a time synchronization protocol itself.  An ABR can use
 the same key for all it's attached areas, or it can have a unique key
 for each area.  This allows an AS to be managed by area with each
 area potentially having a different TE, signature algorithm, key
 size, and/or key.
 When a new key replaces an old key, the router must quickly replace
 LSAs signed with the old key with LSAs signed with the new key. To
 change a router key the following steps must be followed:
 (1) A valid certificate for the new key must be obtained for the
     router.
 (2) The router builds and sends a new PKLSA with the new certificate.
 (3) The router signs each self-originated LSA with the new key and
     sends them.
 When a PKLSA is received:
 (1) If the PKLSA's age = MaxAge, remove the PKLSA from the LSDB and
     age LSAs signed with this key to be MaxAge - MAX_TRANSIT_DELAY,
     if they were not already older than this.  This is a way to get
     rid of a key that should no longer be used.
 (2) If the PKLSA is a refresh LSA for an existing key, update the
     LSDB.
 (3) If the PKLSA contains a different key than the one currently
     stored for this router, compare the certificate Create Time.  If
     the PKLSA key is less recent, discard it.  If the PKLSA key is
     more recent, install it in the LSDB and remove the old key from
     the LSDB.  If an old key was deleted from the LSDB, age LSAs
     signed with this key to be MaxAge - MAX_TRANSIT_DELAY, if they
     were not already older than this.

4.5. Trusted Entity Key Replacement

 It is necessary to change a TE public key periodically.  It is
 recommended that the TE public key be relatively large, so that it
 does not frequently require replacement.  A router may store multiple
 TE public keys.  Each key is uniquely identified by TE Id and TE Key
 Id.  TE keys are used to verify certificates received from other
 routers in their PKLSAs.  When a router sends a new certificate
 signed with a new TE Key, all the routers that receive the PKLSA
 containing the certificate must have that new TE Key in order to
 verify, store, and use that PKLSA.  Management of TE public keys is
 done outside the OSPF protocol, and a method is suggested, but not

Murphy, et. al. Experimental [Page 12] RFC 2154 OSPF with Digital Signatures June 1997

 mandated by this design.  Initially all routers must be configured
 with the TE Keys they will need to verify the certificates they will
 receive.  To prevent use of a (possibly compromised) TE Key, that key
 must be replaced by a new (possibly null) TE Key having the same TE
 Id and signature algorithm.  A compromised or faulty router can
 continue using certificates signed with the old TE key, but none of
 the properly configured routers will be able to verify them.
 Changing a TE public key presents a design challenge.  When a TE
 Public Key is changed, all the certificates depending on that key
 must also change.  The router keys in the certificates may or may not
 be changed at the same time.  When the TE key and certificates
 change, all PKLSAs depending on these must be reissued. In order to
 verify these new certificates, all routers receiving the new PKLSAs
 must have the new TE Public Key.  So, the TE key replacement must be
 a synchronized event.  Routers are not required to have synchronized
 clocks.  The TE public key may well be distributed to the routers via
 an out-of-band mechanism (like a smart-card reader or other sneaker-
 net method).  It is not reasonable to require that all the routers
 obtain the TE public key at the same time.  There are probably
 several methods for meeting these requirements.  The method tested in
 our implementation is as follows:
 (1) Define a period of time needed to get the new TE key on all
     routers.  This could be minutes, hours, even days depending on
     how the distribution is accomplished.  This time period is a
     configuration value for each router (TE_KEY_DIST_INT) and must be
     the same for all routers sharing a TE.
 (2) Install a new TE key and associated certificates (if there are
     any) on each router.  Signal the router code when the new TE key
     is available to be accessed.
 (3) The router sets a timer for the TE_KEY_DIST_INT.  The router
     sets a flag indicating the presence of a new TE key.
 (4) For each router, if the timer goes off:
       Access the new TE key.
       If there are new certificates, build and send a new PKLSA.
       Age all PKLSAs in the LSDB certified by the old TE Key
               to MaxAge - MAX_TRANSIT_DELAY.

Murphy, et. al. Experimental [Page 13] RFC 2154 OSPF with Digital Signatures June 1997

 (5) For each router, if a PKLSA certified by a new TE key comes in
     before the timer goes off:
       If the new TE key cannot be accessed, discard the PKLSA and
               log an ERROR.
       Access the new TE key.
       Process the received PKLSA.
       If there are new certificates, build and send a new PKLSA.
       Age all PKLSAs in the LSDB certified by the old TE key
               to MaxAge - MAX_TRANSIT_DELAY.
 The effect of this method is that it takes a predetermined interval
 of time to change the TE public key.  That interval is the amount of
 time from the installation of the new TE key on the FIRST router
 installed, until the time that router reads the key in.  By the time
 the first router reads the key in, all other routers should have the
 new key.  If some router does not get the new TE key in time, it will
 be unable to verify all the new PKLSAs that are received.  It will
 log error messages and route data based on it's old database until
 those LSAs time out.  The simple way to fix a router in this error
 condition is to load the new TE key and restart the router.  If this
 error is expected to occur, and restarting the router is not
 acceptable, then some special purpose code will be needed to read in
 the TE key after it has been otherwise distributed, and do database
 synchronization to catch up with the other routers.
 The group of routers that need the new TE key are all the routers in
 the scope of that Trusted Entity.

4.6. Flexible Cryptographic Environments

 It is likely that an AS will have one cryptographic environment in
 use throughout the AS, with one trusted entity, one signature
 algorithm in use, and one key in use per router.  To allow those
 cases where this is not true, multiple signature algorithms, multiple
 trusted entities, and multiple keys per router are allowed.

4.6.1. Multiple Signature Algorithms

 It is possible to support multiple signature algorithms.  Each router
 and TE key has a signature algorithm associated with it.  All routers
 sending a key with a given algorithm must be capable of generating
 signatures of that kind, and all routers receiving keys with a given
 algorithm must be able to verify the signatures.  If a router
 receives an LSA signed with a signature algorithm that it does not
 support, the LSA must be discarded.  LSAs that cannot be verified by
 a router are not flooded by that router.  When using multiple
 signature algorithms, the scope of each algorithm must be determined

Murphy, et. al. Experimental [Page 14] RFC 2154 OSPF with Digital Signatures June 1997

 (see section 4.3), and routers must be configured with support for
 these algorithms accordingly.
 If an Area supports two signature algorithms and is to have full
 connectivity, some routers may sign with algorithm A and others with
 algorithm B, but all routers in the area must be able to verify
 signatures for A and B.  In an AS that is divided into areas, it is
 possible for each area to have a different signature algorithm.  The
 ABR connecting two areas would have to support both algorithms, but
 the internal routers in a given area would only have to know one
 algorithm.
 ASBRs present a problem for this sort of division.  ASEs flood
 throughout the non-stub areas of an AS.  Any router that cannot
 verify an ASE will discard it without flooding.  So, to have access
 to an ASE, a router, and all the routers in the flooding path, must
 support the algorithm used by the ASBR.  One way around these
 difficulties is to have a lowest-common-denominator algorithm that is
 used for signing by all ASBRs and is supported for verification
 throughout the AS in addition to other algorithms used.  Another
 approach is to place ASBRs on the backbone, and configure all areas
 using a signature algorithm different from the ASBR to have a default
 route to the backbone.  A combined approach will allow an ASBR to be
 in a non-backbone area if it uses a signature algorithm supported on
 the backbone, and the areas using different signature algorithms are
 configured with a default to the backbone.  There are special
 limitations in the case of a router that is an ABR and also an ASBR:
 see section 6.
 There is currently only one signature algorithm (RSA_MD5) defined for
 use by this design.  The RSA algorithm is defined in PKCS #1 [9] and
 the signature and key formats used by this design are defined in
 RFC2065 [10].

4.6.2. Multiple Trusted Entities

 It is possible to have multiple Trusted Entities in an AS.  Each TE
 has a unique TE identifier.  Every router receiving PKLSAs certified
 by a given TE must have that TE's public key.  If a router receives a
 PKLSA certified by a TE for which it does not have a public key, the
 PKLSA must be discarded.  When using multiple TEs, the scope of each
 TE must be determined (see section 4.3), and routers in this scope
 must be configured with the TE key.

Murphy, et. al. Experimental [Page 15] RFC 2154 OSPF with Digital Signatures June 1997

4.6.3. Multiple Keys for One Router

 An ABR may have one key for each attached area.  These keys may
 differ in size, algorithm and/or certifying TE.  Generally, each key
 will have a "scope" of the attached area, and there will be no
 conflict between keys.
 There are special limitations in the case of a router that is an ABR
 and also an ASBR: see section 6.

5. Compatibility with Standard OSPF V2

 OSPF with Digital Signatures is compatible with standard OSPF V2 in
 an autonomous system.  Within an AS, there may be "signed" areas and
 "unsigned" areas.  There will never be both signed and unsigned LSAs
 used in any one area.  Each area will have an environment flag
 indicating whether it is "signed" or "unsigned".  The environment
 flag is a per area configuration value for the router.  The signed
 areas must contain all routers running OSPF with Digital Signatures,
 and the unsigned areas contain routers running standard OSPF V2 code
 (or OSPF with Digital Signatures with all areas set to be unsigned).
 An area border router connecting a signed to an unsigned area must be
 running OSPF with Digital Signatures with one area set to be
 unsigned.
 In order to arrange this limited compatibility, a router running OSPF
 with Digital Signatures must be able to process both signed and
 unsigned LSAs.  The only router that will actually be processing both
 kinds of LSAs is an Area Border Router connecting a signed area to an
 unsigned area.  An ABR connecting a signed to an unsigned area will
 generate signed summaries for one area and unsigned summaries for the
 other.  An ABR must not flood signed LSAs into unsigned areas.  An
 ABR must not flood unsigned LSAs into signed areas.  This will result
 in AS External LSAs being dropped if they reach an area that has a
 different environment from the one in which they were created.  There
 are special limitations in the case of a router that is an ABR and
 also an ASBR: see section 6.
 Complete connectivity is provided within the AS, because of the
 summarization provided by ABRs connecting signed and unsigned areas.
 There are limitations on connectivity to AS external routes in an AS
 with a mixture of signed and unsigned areas, depending on the
 location of AS border routers.  An ASBR in a signed area will
 generate signed ASE LSAs.  These LSAs will be flooded to every
 contiguously connected signed area.  The connected signed areas are
 the "scope" of these ASEs.  A host located in an area that is not in
 this scope, will not have connectivity to these external routes.  An
 ASBR in an unsigned area will generate unsigned ASE LSAs.  These LSAs

Murphy, et. al. Experimental [Page 16] RFC 2154 OSPF with Digital Signatures June 1997

 will have a scope of all the contiguously connected unsigned areas,
 and will be available to hosts in this scope.  To arrange complete
 connectivity to an ASE route in an AS with signed and unsigned areas:
 (1) Place the ASBR on the backbone.
 (2) Signed Backbone: have some ABR in each unsigned area advertise a
     default route to the backbone.
 (3) Unsigned Backbone: have some ABR in each signed area advertise a
     default route to the backbone.
 Given this design for a mixed AS, routing is available throughout the
 AS, but the authentication and integrity provided by this design will
 be effective only for routes that are inside a signed area, or
 traverse only signed areas.  There is no mechanism for a data packet
 to state a preference for signed routes.  The basic rules of the OSPF
 protocol ensure that intra-area routes are preferred to inter-area
 routes, that routes within the AS are preferred to AS external
 routes, and that inter-area routes go from area1->backbone->area2.
 OSPF does not allow looping, or routes of the form area1->area2-
 >area3.  Because of these properties of OSFP routing, an AS can
 contain signed and unsigned areas, and achieve a predictable level of
 authentication.

6. Special Considerations/Restrictions for the ABR-ASBR

 There are special restrictions and configuration considerations for a
 router running OSPF with Digital Signatures that is both an Area
 Border Router and an Autonomous System Border Router.  An ASBR
 produces AS external LSAs that are flooded throughout the non-stub
 areas of the AS.  An ABR that is generating digital signatures may be
 using a different key, certifying Trusted Entity, or signature
 algorithm for each of its attached areas, or it might be signing in
 some areas and not in others.
 An ABR/ASBR with no restrictions on its configuration could produce
 multiple versions of an ASE that would all be flooded throughout the
 non-stub areas of the AS.  The results of this production of multiple
 versions of LSAs would be detrimental to performance, and could
 produce unpredictable routing behavior.

Murphy, et. al. Experimental [Page 17] RFC 2154 OSPF with Digital Signatures June 1997

 The PKLSA of an ASBR is also flooded throughout the non-stub areas of
 the AS, and in the case of an ABR/ASBR there could be multiple,
 distinct PKLSAs for a given router, one per attached area, all being
 flooded throughout the AS.  If two distinct PKLSAs from one ABR/ASBR
 router were present in one area, the key with the most recent create
 time would be stored, and all LSAs signed with a less recent key
 would be unverifiable.
 The simplest way to deal with this problem, and the method
 recommended by this document, is the following:
 If an ASBR must also be an ABR, then the security configuration (key,
 signature algorithm, certifying Trusted Entity, environment =
 signed/unsigned) for all attached areas must be the same.  This way
 the PKLSA and the ASEs produced for each area match, and there is no
 proliferation of versions of LSAs.

7. LSA formats

7.1. Router Public Key LSA (PKLSA)

 This LSA is the vehicle for distribution of a router public key.  The
 PKLSA is sent by one router, and stored by all the other routers in
 the flooding scope.  The PKLSA contains the public key that other
 routers will use to verify the signatures created by this router.  A
 Router PKLSA will be communicated in the usual database exchange and
 via flooding mechanisms. The regular period for sending this LSA is
 LSRefreshTime.  The Router PKLSA will also be sent when there is a
 new key, or a key to be flushed from the system.
 The flooding scope of a PKLSA is the area, except in the case of
 ASBRs.  The flooding scope of an ASBR's PKLSA is the same as that of
 the ASEs.  The "role" of the router (RTR, ABR, ASBR, ABR-ASBR) is
 stored in the PKLSA inside the certificate, and can be checked during
 flooding.

Murphy, et. al. Experimental [Page 18] RFC 2154 OSPF with Digital Signatures June 1997

 ROUTER PUBLIC KEY LSA
                         1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 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
    +-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-+
    |            LS Age             |   Options     |    LS Type    |
    +-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-+
    |                        Link State ID                          |
    +-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-+
    |                     Advertising Router                        |
    +-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-+
    |                     LS Sequence Number                        |
    +-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-+
    |         LS Checksum           |            Length             |
    +-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-+
    |                  Certificate (format in 7.2)                  /
    +-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-+
    |                           Signature                           /
    +-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-+
    |         Cert Length           |         Sign Length           |
    +-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-+
 LS AGE          Defined in OSPF RFC [3].
 OPTIONS         Defined in OSPF RFC [3].
 LS TYPE         16 for Router Public Key LSA.
                 First bit set to indicate a signed LSA.
 LINK STATE ID   Contains the Advertising Router Id (see next field).
 ADVERTISING ROUTER  Defined in OSPF RFC [3].
 LS SEQUENCE NUMBER  Defined in OSPF RFC [3].
 LS CHECKSUM     Defined in OSPF RFC [3].
                 Checksum does not cover the signature.
 LENGTH          Defined in OSPF RFC [3].  Length does include the
                 Signature field, Cert Length and Sign Length.
 CERTIFICATE     Format in section 7.2.

Murphy, et. al. Experimental [Page 19] RFC 2154 OSPF with Digital Signatures June 1997

 SIGNATURE       The advertising router's signature of this LSA.  This
                 can be verified using the enclosed Router Public Key.
                 The signature covers the LSA header and message
                 starting with the LSA header options field and ending
                 with the Trusted Entity certification field.  For
                 sign and verify, the last two fields (Cert Length and
                 Sign Length) are appended immediately after the
                 Certificate.  When complete, the signature is
                 inserted between the Certification and the Cert
                 Length.  There are two exceptions to this coverage:
                 1) If the LSA was generated with an age=MaxAge, then
                 the signature begins with the age field (see section
                 3.3).
                 2) The checksum in the LSA Header is set to zero for
                 the computation of the signature.
                 A pad is added to the end of the signature field to
                 allow the next field to begin on a (4 byte) word
                 boundary.
                 The format used for an RSA-MD5 signature is defined
                 in section 4.1.2 of RFC2065 [10].
 CERT LENGTH     The length in bytes of the Certification inside the
                 Certificate.
                 Does not include pad that may follow Certification.
 SIGN LENGTH     The length in bytes of the Signature.
                 Does not include pad that may follow Signature.

7.2. Router Public Key Certificate

 A router public key certificate is a package of data signed by a
 Trusted Entity.  This certificate is included in the router PKLSA and
 in the router configuration information.  To change any of the values
 in the certificate, a new certificate must be obtained from a TE.

Murphy, et. al. Experimental [Page 20] RFC 2154 OSPF with Digital Signatures June 1997

                         1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 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
    +-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-+
    |                          Router Id                            |
    +-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-+
    |     TE Id     |   TE Key Id   |   Rtr Key Id  |    Sig Alg    |
    +-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-+
    |                          Create Time                          |
    +-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-+
    |        Key Field Length       |  Router Role  |  #Net Ranges  |
    +-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-+
    |                          IP Address                           |
    +-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-+
    |                         Address Mask                          |
    +-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-+
    |           IP Address/Address Mask for each Net Range ...      /
    | ...                                                           /
    +-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-+
    |                       Router Public Key                       |
    +-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-+
    |                         Certification                         /
    +-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-+
 ROUTER ID       Advertising Router.
 TE ID           TE Id must uniquely identify one TE in the AS.
                 A number between 1-250.  0 reserved for null.
                 251-255 reserved for future needs.
 TE KEY ID       Must uniquely identify a particular key for a given
                 TE at any given time.  A TE Key Id may be re-used
                 after all references to it are gone from the AS.  A
                 number between 1-250.  0 reserved for null.  251-255
                 reserved for future needs.
 RTR KEY ID      Must be unique for the TE and Router at any given
                 time. The combination of (TE Id, Rtr Id, Rtr Key Id)
                 uniquely identifies a particular router key at a
                 given time.  A Rtr Key Id may be re-used after all
                 references to it are gone from the AS.  Create Time
                 resolves any conflict that could be caused by
                 replaying old keys.  A number between 1-250.  0
                 reserved for null.  251-255 reserved for future
                 needs.

Murphy, et. al. Experimental [Page 21] RFC 2154 OSPF with Digital Signatures June 1997

 SIG ALG         The signature algorithm for the Router Public Key.
                 The signature algorithm encompasses the hash
                 algorithm used as well.  Currently defined value =
                 RSA-MD5(1).  Values 2-252 are available for future
                 definition.  Values 0 and 253-255 are reserved.  The
                 Sig Alg value is registered with IANA.  Future
                 signature algorithms will have to be defined or
                 referenced in this document, and registered with
                 IANA.
 CREATE TIME     Timestamp set by the TE.  An unsigned number of
                 seconds since the start of January 1, 1970, GMT,
                 ignoring leap seconds.  Used to compare two
                 certificates and determine which is more recent.
                 Requires that time synchronization for TEs, but not
                 for routers.
 KEY FIELD LENGTH    The length in bytes of the Router Public Key.
                 Does not include pad that may follow Router Public
                 Key field.
 ROUTER ROLE     Router (R=1), Area Border Router (ABR=2), Autonomous
                 System Border Router (ASBR=4), ABR and ASBR (ABR-
                 ASBR=6).
 #NET RANGES     The number of network ranges that follow.  A network
                 range is defined to be an IP Address and an Address
                 Mask.  This list of ranges defines the addresses that
                 the Router is permitted to advertise in its Router
                 Links LSA.  Valid values are 0-255. If there are 0
                 ranges the router cannot advertise anything.  This is
                 not generally useful.  One range with address=0 and
                 mask=0 will allow a router to advertise any address.
 IP ADDRESS & ADDRESS MASK
                 Define a range of addresses that this router may
                 advertise.  Each is a 32 bit value.  One range with
                 address=0 and mask=0 will allow a router to advertise
                 any address.

Murphy, et. al. Experimental [Page 22] RFC 2154 OSPF with Digital Signatures June 1997

 ROUTER PUBLIC KEY    A key that can be used to verify the signatures
                 produced by this router.  The internal format for the
                 Router Public Key is signature algorithm dependent.
                 A pad is added to the end of the Router Public Key
                 field to allow the next field to begin on a (4 byte)
                 word boundary.
                 The format used for an RSA-MD5 public key is defined
                 in section 3.5 of RFC2065 [10].
 CERTIFICATION   The Trusted Entity's signature of the certified data.
                 This signature can be verified with the TE public key
                 identified by TE Id and TE Key Id given in this
                 packet.  The length of the certification depends on
                 the key size, and is stored in the PKLSA Cert Length
                 field.  A pad is added to the end of the
                 Certification to allow the next field to begin on a
                 (4 byte) word boundary.
                 The format used for an RSA-MD5 signature is defined
                 in section 4.1.2 of RFC2065 [10].

7.3 Signed LSA

 A signed LSA is an OSPF LSA with signature data and a digital
 signature attached.  The first bit of the LSA Type field is set to
 indicate the presence of a signature.  The signature follows the LSA
 Data.  Signature length and id fields are positioned at the end of
 the signed LSA.

Murphy, et. al. Experimental [Page 23] RFC 2154 OSPF with Digital Signatures June 1997

 ANY SIGNED LSA
                         1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 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
    +-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-+
    |            LS Age             |   Options     |    LS Type    |
    +-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-+
    |                        Link State ID                          |
    +-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-+
    |                     Advertising Router                        |
    +-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-+
    |                     LS Sequence Number                        |
    +-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-+
    |         LS Checksum           |            Length             |
    +-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-+
    |                            LSA Data                           /
    / ...                                                           /
    +-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-+
    |                            Signature                          /
    +-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-+
    |  Rtr Key Id   |     TE Id     |         Sign Length           |
    +-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-*-+-+-+-+-+-+-+-+
 LS AGE          Defined in OSPF RFC [3].
 OPTIONS         Defined in OSPF RFC [3].
 LS TYPE         Standard LSA Type with the first bit set to indicate
                 the presence of security data and a signature. This
                 creates a new signed LSA type for each existing type.
 LINK STATE ID   Defined in OSPF RFC [3].
 ADVERTISING ROUTER  Defined in OSPF RFC [3].
 LS SEQUENCE NUMBER  Defined in OSPF RFC [3].
 LS CHECKSUM     Defined in OSPF RFC [3].
                 Checksum does not cover the signature.
 LENGTH          Defined in OSPF RFC [3].
                 Length does include the Signature and security
                 related fields at the end of the LSA.

Murphy, et. al. Experimental [Page 24] RFC 2154 OSPF with Digital Signatures June 1997

 SIGNATURE       The advertising router's signature of this LSA.  The
                 signature covers the LSA header and data starting
                 with the LSA header options field and ending with the
                 Trusted Entity certification field.  For sign and
                 verify, the last three fields (Rtr Key Id, TE Id,
                 Sign Length) are appended to the Certificate.  When
                 complete, the signature is inserted between the
                 Certification and the Rtr Key Id.  There are two
                 exceptions to this coverage:
                 1) If the LSA was generated with an age=MaxAge, then
                 the signature begins with the age field (see section
                 3.3).
                 2) The checksum in the LSA Header is set to zero for
                 the computation  & verification of the signature.
                 A pad is added to the end of the signature to allow
                 the next field to begin on a (4 byte) word boundary.
                 The format used for an RSA-MD5 signature is defined
                 in section 4.1.2 of RFC2065 [10].
 RTR KEY ID      Used to identify the router key used to sign this
                 LSA. The combination of (TE Id, Rtr Id, Rtr Key Id)
                 uniquely identifies a particular router key at a
                 given time, and can be used to look up the PKLSA for
                 the router key needed to verify this Signed LSA.  A
                 number between 1-250.  0 reserved for null.  251-255
                 reserved for future needs.
 TE ID           The id of the Trusted Entity that produced the
                 certificate.  TE Id must uniquely identify one TE in
                 the AS.  A number between 1-250.  0 reserved for
                 null. 251-255 reserved for future needs.
 SIGN LENGTH     The length in bytes of the Signature.
                 Does not include pad that may follow Signature.

Murphy, et. al. Experimental [Page 25] RFC 2154 OSPF with Digital Signatures June 1997

8. Configuration Information

 Trusted Entity Information Set: (one per Trusted Entity used by this
 router)
    Trusted Entity ID - TE Id
         Identifies the Trusted Entity within the AS (defined in 7.2).
    Trusted Entity Key Id - TE Key Id
         Identifies the particular key for this Trusted Entity
         (defined in 7.2).
    Trusted Entity Public Key
         A public key for this Trusted Entity.
         The format used for an RSA-MD5 public key is defined in
         section 3.5 of RFC2065 [10].
    Signature Algorithm < and optional parameters >
         The signature algorithm for the public key (defined in 7.2).
 Router Information Set: (at least one for the router)
    Router Private Key
         The router's private key that goes with the public key in the
         certificate following. The format used for the private key
         depends on the crypto package used by your implementation.
         This key is not transmitted as part of this design.  Our
         implementation uses the private key format compatible with
         RSAREF [9].
    Router Certificate (format in 7.2).
 Timing Intervals:
    Trusted Entity Key Distribution Interval (TE_KEY_DIST_INT)
         The period of time, in seconds, needed to get a TE public key
         installed on all the routers in the TE's scope.
    Maximum Transit Delay (MAX_TRANSIT_DELAY)
         The maximum period of time, in seconds, that it should take
         for an LSA to reach all the routers in the AS.
 Router Information per attached Area:
    Environment flag
         Signed=1, Unsigned=0
 9.  Remaining Vulnerabilities
 Note that with this mechanism, one router can still distribute
 incorrect data in the information for which it itself is responsible.
 Consequently, an autonomous system employing digital signatures with
 this mechanism will not be completely invulnerable to routing

Murphy, et. al. Experimental [Page 26] RFC 2154 OSPF with Digital Signatures June 1997

 disruptions from a single router.  For example, the area border
 routers and autonomous system border routers will still be able to
 inject incorrect routing information.  Also, any single internal
 router can be incorrect in the routing information it originates
 about its own links.

9.1. Area Border Routers

 Even with the design proposed here, the area border routers can
 inject incorrect routing information into their attached areas about
 the backbone and the other areas in Summary LSA's.  They can also
 inject incorrect routing information into the backbone about their
 attached area.
 Because all the area border routers in one area work from the same
 database of LSA's received in their common area, it would be possible
 for the area border routers to corroborate each other.  Any area
 border router for an area could double check the Summary LSA's
 received over the backbone from other ABR's from the area, and could
 double check the Summary LSA's flooded through the area from the
 other area border routers.  The other routers in the area or backbone
 should be warned of a failure of this check.  The warning could be a
 signed message from the area border router detecting the failure,
 flooded in the usual mechanism.
 Another possibility would be that the area border routers in an area
 could originate multiple sets of Summary LSA's -- one for itself
 containing its own information and one for each of the area border
 routers in the area containing the information each of them should
 originate.  Each router in the area or backbone could then determine
 for itself whether the area border routers agreed.  This distribution
 of information but coordination of processing is in keeping with the
 paradigm of link state protocols, where information and processing is
 duplicated in each router.
 Both alternatives mean much additional processing and additional
 message transmission, over and above the additional processing
 required for signature generation and verification.  Because the
 vulnerability is isolated to a few points in each area, because the
 source of incorrect information is detectable (in those situations
 where the incorrect information is spotted) and because the
 protection is costly, we have not added this protection to this
 design.

9.2. Internal Routers

 The internal routers can be incorrect about information they
 themselves originate.

Murphy, et. al. Experimental [Page 27] RFC 2154 OSPF with Digital Signatures June 1997

 A router could announce an incorrect metric for a valid link.  There
 is no way to guard against this, but the damage would be small and
 localized even if the router is announcing that the link is up when
 it is down or vice versa.
 A router could announce a connection that does not in fact exist.  If
 a router announces a non-existent connection to a transit network,
 the OSPF Dijkstra computation will not consider the connection
 without a similar announcement from another router at the other
 "end".  Therefore, no damage would result (above network impact to
 transmit and store the incorrect information) without the cooperation
 of another router.  A router could also announce a connection to a
 stub network or a host route that does not exist.  The Dijkstra
 computation can not perform the same check for a similar announcement
 from the other "end", because no other end exists.  This is a
 vulnerability.
 A faulty router announcing a nonexistent connection to a stub network
 or host could result in the faulty router receiving IP packets bound
 for that network or host.  Unless the faulty router then forwarded
 the packets to the correct destination by source routing, the failure
 of packet delivery could expose the incorrect routing.  To exploit
 the vulnerability deliberately, the faulty router would have to be
 able to handle and pass on the received traffic for the incorrectly
 announced destination.  Furthermore, if the incorrect routing were
 discovered, the signatures on the routing information would identify
 the faulty router as the source of the incorrect information.
 Finally, this design checks router advertisements against allowed
 address ranges certified by a trusted entity.  A faulty router could
 announce nonexistent host or stub network routes, but only to
 addresses within its allowed ranges.

9.3. Autonomous System Border Routers

 The autonomous system boundary routers can produce incorrect routing
 information in the external routes information they originate.  There
 is no way to double check or corroborate this information, as there
 is with area border routers.  No authority within an autonomous
 system exists to authorize the networks an autonomous system boundary
 router could announce, as is the case for the internal networks an
 internal router could announce.  Consequently, the autonomous system
 boundary routers remain a unprotected vulnerability.  With this in
 mind, special care should be taken to protect the autonomous system
 boundary routers with other means.

10. Security Considerations

 This entire memo is about security considerations.

Murphy, et. al. Experimental [Page 28] RFC 2154 OSPF with Digital Signatures June 1997

11. References

 [1] Finn, Gregory G., "Reducing the Vulnerability of Dynamic Computer
     Networks," ISI Research Report ISI/RR-88-201, University of
     Southern California Information Sciences Institute,
     Marina del Rey, California, June 1988.
 [2] Kumar,B and Crowcroft,J., "Integrating Security in Inter-Domain
     Routing Protocols", Computer Communications Review, Vol 23,
     No. 5, October 1993.
 [3] Moy, J., "OSPF Version 2," RFC 1583, Proteon, Inc., March 1994.
 [4] Perlman, R., "Network Layer Protocols with Byzantine Robustness",
     Ph.D. Thesis, Department of Electrical Engineering and Computer
     Science, MIT, August 1988.
 [5] Perlman, R., "Interconnections: Bridges and Routers",
     Addison-Wesley, Reading, Mass., 1992.
 [6] Schneier, B., "Applied Cryptography: Protocols, Algorithms, and
     Source Code in C," John Wiley & Sons, Inc., New York, 1994.
 [7] Steenstrup, M., "Inter-Domain Policy Routing Protocol
     Specification: Version 1", RFC 1479, BBN Systems and
     Technologies, July 1993.
 [9] PKCS #1: RSA Encryption Standard, RSA Data Security, Inc., June
     1991, Version 1.4.
 [10] Eastlake D. & Kaufman C., "Domain Name System Security
      Extensions", RFC2065, January 1997.
 [11] Moy J., "OSPF Version 2", Cascade Communications Corp,
      Work In Progress.

12. Authors' Addresses

 Sandra Murphy  murphy@tis.com
 Madelyn Badger  mrb@tis.com
 Brian Wellington  bwelling@tis.com
 Trusted Information Systems
 3060 Washington Road
 Glenwood, MD  21738

Murphy, et. al. Experimental [Page 29]

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