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rfc:rfc5925

Internet Engineering Task Force (IETF) J. Touch Request for Comments: 5925 USC/ISI Obsoletes: 2385 A. Mankin Category: Standards Track Johns Hopkins Univ. ISSN: 2070-1721 R. Bonica

                                                      Juniper Networks
                                                             June 2010
                   The TCP Authentication Option

Abstract

 This document specifies the TCP Authentication Option (TCP-AO), which
 obsoletes the TCP MD5 Signature option of RFC 2385 (TCP MD5).  TCP-AO
 specifies the use of stronger Message Authentication Codes (MACs),
 protects against replays even for long-lived TCP connections, and
 provides more details on the association of security with TCP
 connections than TCP MD5.  TCP-AO is compatible with either a static
 Master Key Tuple (MKT) configuration or an external, out-of-band MKT
 management mechanism; in either case, TCP-AO also protects
 connections when using the same MKT across repeated instances of a
 connection, using traffic keys derived from the MKT, and coordinates
 MKT changes between endpoints.  The result is intended to support
 current infrastructure uses of TCP MD5, such as to protect long-lived
 connections (as used, e.g., in BGP and LDP), and to support a larger
 set of MACs with minimal other system and operational changes.  TCP-
 AO uses a different option identifier than TCP MD5, even though TCP-
 AO and TCP MD5 are never permitted to be used simultaneously.  TCP-AO
 supports IPv6, and is fully compatible with the proposed requirements
 for the replacement of TCP MD5.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc5925.

Touch, et al. Standards Track [Page 1] RFC 5925 The TCP Authentication Option June 2010

Copyright Notice

 Copyright (c) 2010 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Touch, et al. Standards Track [Page 2] RFC 5925 The TCP Authentication Option June 2010

Table of Contents

 1. Introduction ....................................................4
    1.1. Conventions Used in This Document ..........................4
    1.2. Applicability Statement ....................................5
    1.3. Executive Summary ..........................................6
 2. The TCP Authentication Option ...................................7
    2.1. Review of TCP MD5 Option ...................................7
    2.2. The TCP Authentication Option Format .......................8
 3. TCP-AO Keys and Their Properties ...............................10
    3.1. Master Key Tuple ..........................................10
    3.2. Traffic Keys ..............................................12
    3.3. MKT Properties ............................................13
 4. Per-Connection TCP-AO Parameters ...............................14
 5. Cryptographic Algorithms .......................................15
    5.1. MAC Algorithms ............................................15
    5.2. Traffic Key Derivation Functions ..........................18
    5.3. Traffic Key Establishment and Duration Issues .............22
         5.3.1. MKT Reuse Across Socket Pairs ......................22
         5.3.2. MKTs Use within a Long-Lived Connection ............23
 6. Additional Security Mechanisms .................................23
    6.1. Coordinating Use of New MKTs ..............................23
    6.2. Preventing Replay Attacks within Long-Lived Connections ...24
 7. TCP-AO Interaction with TCP ....................................26
    7.1. TCP User Interface ........................................27
    7.2. TCP States and Transitions ................................28
    7.3. TCP Segments ..............................................28
    7.4. Sending TCP Segments ......................................29
    7.5. Receiving TCP Segments ....................................30
    7.6. Impact on TCP Header Size .................................32
    7.7. Connectionless Resets .....................................33
    7.8. ICMP Handling .............................................34
 8. Obsoleting TCP MD5 and Legacy Interactions .....................35
 9. Interactions with Middleboxes ..................................35
    9.1. Interactions with Non-NAT/NAPT Middleboxes ................36
    9.2. Interactions with NAT/NAPT Devices ........................36
 10. Evaluation of Requirements Satisfaction .......................36
 11. Security Considerations .......................................42
 12. IANA Considerations ...........................................43
 13. References ....................................................44
    13.1. Normative References .....................................44
    13.2. Informative References ...................................45
 14. Acknowledgments ...............................................47

Touch, et al. Standards Track [Page 3] RFC 5925 The TCP Authentication Option June 2010

1. Introduction

 The TCP MD5 Signature (TCP MD5) is a TCP option that authenticates
 TCP segments, including the TCP IPv4 pseudoheader, TCP header, and
 TCP data.  It was developed to protect BGP sessions from spoofed TCP
 segments, which could affect BGP data or the robustness of the TCP
 connection itself [RFC2385][RFC4953].
 There have been many recent concerns about TCP MD5.  Its use of a
 simple keyed hash for authentication is problematic because there
 have been escalating attacks on the algorithm itself [Wa05].  TCP MD5
 also lacks both key-management and algorithm agility.  This document
 adds the latter, and provides a simple key coordination mechanism
 giving the ability to move from one key to another within the same
 connection.  It does not however provide for complete cryptographic
 key management to be handled in band of TCP, because TCP SYN segments
 lack sufficient remaining space to handle such a negotiation (see
 Section 7.6).  This document obsoletes the TCP MD5 option with a more
 general TCP Authentication Option (TCP-AO).  This new option supports
 the use of other, stronger hash functions, provides replay protection
 for long-lived connections and across repeated instances of a single
 connection, coordinates key changes between endpoints, and provides a
 more explicit recommendation for external key management.  The result
 is compatible with IPv6, and is fully compatible with proposed
 requirements for a replacement for TCP MD5 [Ed07].
 TCP-AO obsoletes TCP MD5, although a particular implementation may
 support both mechanisms for backward compatibility.  For a given
 connection, only one can be in use.  TCP MD5-protected connections
 cannot be migrated to TCP-AO because TCP MD5 does not support any
 changes to a connection's security algorithm once established.

1.1. Conventions Used in This Document

 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].
 In this document, these words will appear with that interpretation
 only when in ALL CAPS.  Lowercase uses of these words are not to be
 interpreted as carrying RFC 2119 significance.
 In this document, the characters ">>" preceeding an indented line(s)
 indicates a compliance requirement statement using the key words
 listed above.  This convention aids reviewers in quickly identifying
 or finding the explicit compliance requirements of this RFC.

Touch, et al. Standards Track [Page 4] RFC 5925 The TCP Authentication Option June 2010

1.2. Applicability Statement

 TCP-AO is intended to support current uses of TCP MD5, such as to
 protect long-lived connections for routing protocols, such as BGP and
 LDP.  It is also intended to provide similar protection to any long-
 lived TCP connection, as might be used between proxy caches, for
 example, and is not designed solely or primarily for routing protocol
 uses.
 TCP-AO is intended to replace (and thus obsolete) the use of TCP MD5.
 TCP-AO enhances the capabilities of TCP MD5 as summarized in Section
 1.3.  This document recommends overall that:
 >> TCP implementations that support TCP MD5 MUST support TCP-AO.
 >> TCP-AO SHOULD be implemented where the protection afforded by TCP
 authentication is needed, because either IPsec is not supported or
 TCP-AO's particular properties are needed (e.g., per-connection
 keys).
 >> TCP-AO MAY be implemented elsewhere.
 TCP-AO is not intended to replace the use of the IPsec suite (IPsec
 and Internet Key Exchange Protocol (IKE)) to protect TCP connections
 [RFC4301][RFC4306].  Specific differences are noted in Section 1.3.
 In fact, we recommend the use of IPsec and IKE, especially where
 IKE's level of existing support for parameter negotiation, session
 key negotiation, or rekeying are desired.  TCP-AO is intended for use
 only where the IPsec suite would not be feasible, e.g., as has been
 suggested is the case to support some routing protocols [RFC4953], or
 in cases where keys need to be tightly coordinated with individual
 transport sessions [Ed07].
 TCP-AO is not intended to replace the use of Transport Layer Security
 (TLS) [RFC5246], Secure BGP (sBGP) or Secure Origin BGP (soBGP)
 [Le09], or any other mechanisms that protect only the TCP data
 stream.  TCP-AO protects the transport layer, preventing attacks from
 disabling the TCP connection itself [RFC4953].  Data stream
 mechanisms protect only the contents of the TCP segments, and can be
 disrupted when the connection is affected.  Some of these data
 protection protocols -- notably TLS -- offer a richer set of key
 management and authentication mechanisms than TCP-AO, and thus
 protect the data stream in a different way.  TCP-AO may be used
 together with these data stream protections to complement each
 other's strengths.

Touch, et al. Standards Track [Page 5] RFC 5925 The TCP Authentication Option June 2010

1.3. Executive Summary

 This document replaces TCP MD5 as follows [RFC2385]:
 o  TCP-AO uses a separate option Kind (29).
 o  TCP-AO allows TCP MD5 to continue to be used concurrently for
    legacy connections.
 o  TCP-AO replaces TCP MD5's single MAC algorithm with MACs specified
    in a separate document and can be extended to include other MACs.
 o  TCP-AO allows rekeying during a TCP connection, assuming that an
    out-of-band protocol or manual mechanism provides the new keys.
    The option includes a 'key ID', which allows the efficient
    concurrent use of multiple keys, and a key coordination mechanism
    using a 'receive next key ID' manages the key change within a
    connection.  Note that TCP MD5 does not preclude rekeying during a
    connection, but does not require its support either.  Further,
    TCP-AO supports key changes with zero segment loss, whereas key
    changes in TCP MD5 can lose segments in transit during the
    changeover or require trying multiple keys on each received
    segment during key use overlap because it lacks an explicit key
    ID.  Although TCP recovers lost segments through retransmission,
    loss can have a substantial impact on performance.
 o  TCP-AO provides automatic replay protection for long-lived
    connections using sequence number extensions.
 o  TCP-AO ensures per-connection traffic keys as unique as the TCP
    connection itself, using TCP's Initial Sequence Numbers (ISNs) for
    differentiation, even when static master key tuples are used
    across repeated instances of connections on a single socket pair.
 o  TCP-AO specifies the details of how this option interacts with
    TCP's states, event processing, and user interface.
 o  TCP-AO is 2 bytes shorter than TCP MD5 (16 bytes overall, rather
    than 18) in the initially specified default case (using a 96-bit
    MAC).
 TCP-AO differs from an IPsec/IKE solution as follows
 [RFC4301][RFC4306]:
 o  TCP-AO does not support dynamic parameter negotiation.

Touch, et al. Standards Track [Page 6] RFC 5925 The TCP Authentication Option June 2010

 o  TCP-AO includes TCP's socket pair (source address, destination
    address, source port, destination port) as a security parameter
    index (together with the KeyID), rather than using a separate
    field as an index (IPsec's Security Parameter Index (SPI)).
 o  TCP-AO forces a change of computed MACs when a connection
    restarts, even when reusing a TCP socket pair (IP addresses and
    port numbers) [Ed07].
 o  TCP-AO does not support encryption.
 o  TCP-AO does not authenticate ICMP messages (some ICMP messages may
    be authenticated when using IPsec, depending on the
    configuration).

2. The TCP Authentication Option

 The TCP Authentication Option (TCP-AO) uses a TCP option Kind value
 of 29.  The following sections describe TCP-AO and provide a review
 of TCP MD5 for comparison.

2.1. Review of TCP MD5 Option

 For review, the TCP MD5 option is shown in Figure 1.
             +---------+---------+-------------------+
             | Kind=19 |Length=18|   MD5 digest...   |
             +---------+---------+-------------------+
             |          ...digest (con't)...         |
             +---------------------------------------+
             |                  ...                  |
             +---------------------------------------+
             |                  ...                  |
             +-------------------+-------------------+
             | ...digest (con't) |
             +-------------------+
                Figure 1: The TCP MD5 Option [RFC2385]
 In the TCP MD5 option, the length is fixed, and the MD5 digest
 occupies 16 bytes following the Kind and Length fields (each one
 byte), using the full MD5 digest of 128 bits [RFC1321].
 The TCP MD5 option specifies the use of the MD5 digest calculation
 over the following values in the following order:
 1. The IP pseudoheader (IP source and destination addresses, protocol
    number, and segment length).

Touch, et al. Standards Track [Page 7] RFC 5925 The TCP Authentication Option June 2010

 2. The TCP header excluding options and checksum.
 3. The TCP data payload.
 4. A key.

2.2. The TCP Authentication Option Format

 TCP-AO provides a superset of the capabilities of TCP MD5, and is
 minimal in the spirit of SP4 [SDNS88].  TCP-AO uses a new Kind field,
 and similar Length field to TCP MD5, a KeyID field, and a RNextKeyID
 field as shown in Figure 2.
          +------------+------------+------------+------------+
          |  Kind=29   |   Length   |   KeyID    | RNextKeyID |
          +------------+------------+------------+------------+
          |                     MAC           ...
          +-----------------------------------...
             ...-----------------+
             ...  MAC (con't)    |
             ...-----------------+
           Figure 2: The TCP Authentication Option (TCP-AO)
 TCP-AO defines these fields as follows:
 o  Kind: An unsigned 1-byte field indicating TCP-AO.  TCP-AO uses a
    new Kind value of 29.
    >> An endpoint MUST NOT use TCP-AO for the same connection in
    which TCP MD5 is used.  When both options appear, TCP MUST
    silently discard the segment.
    >> A single TCP segment MUST NOT have more than one TCP-AO in its
    options sequence.  When multiple TCP-AOs appear, TCP MUST discard
    the segment.
 o  Length: An unsigned 1-byte field indicating the length of the
    option in bytes, including the Kind, Length, KeyID, RNextKeyID,
    and MAC fields.
    >> The Length value MUST be greater than or equal to 4.  When the
    Length value is less than 4, TCP MUST discard the segment.
    >> The Length value MUST be consistent with the TCP header length.
    When the Length value is invalid, TCP MUST discard the segment.

Touch, et al. Standards Track [Page 8] RFC 5925 The TCP Authentication Option June 2010

    This Length check implies that the sum of the sizes of all
    options, when added to the size of the base TCP header (5 words),
    matches the TCP Offset field exactly.  This full verification can
    be computed because RFC 793 specifies the size of the required
    options, and RFC 1122 requires that all new options follow a
    common format with a fixed-length field location
    [RFC793][RFC1122].  A partial verification can be limited to check
    only TCP-AO, so that the TCP-AO length, when added to the TCP-AO
    offset from the start of the TCP header, does not exceed the TCP
    header size as indicated in the TCP header Offset field.
    Values of 4 and other small values larger than 4 (e.g., indicating
    MAC fields of very short length) are of dubious utility but are
    not specifically prohibited.
 o  KeyID: An unsigned 1-byte field indicating the Master Key Tuple
    (MKT, as defined in Section 3.1) used to generate the traffic keys
    that were used to generate the MAC that authenticates this
    segment.
    It supports efficient key changes during a connection and/or to
    help with key coordination during connection establishment, to be
    discussed further in Section 6.1.  Note that the KeyID has no
    cryptographic properties -- it need not be random, nor are there
    any reserved values.
    >> KeyID values MAY be the same in both directions of a
    connection, but do not have to be and there is no special meaning
    when they are.
    This allows MKTs to be installed on a set of devices without
    coordinating the KeyIDs across that entire set in advance, and
    allows new devices to be added to that set using a group of MKTs
    later without requiring renumbering of KeyIDs.  These two
    capabilities are particularly important when used with wildcards
    in the TCP socket pair of the MKT, i.e., when an MKT is used among
    a set of devices specified by a pattern (as noted in Section 3.1).
 o  RNextKeyID: An unsigned 1-byte field indicating the MKT that is
    ready at the sender to be used to authenticate received segments,
    i.e., the desired 'receive next' key ID.
    It supports efficient key change coordination, to be discussed
    further in Section 6.1.  Note that the RNextKeyID has no
    cryptographic properties -- it need not be random, nor are there
    any reserved values.

Touch, et al. Standards Track [Page 9] RFC 5925 The TCP Authentication Option June 2010

 o  MAC: Message Authentication Code.  Its contents are determined by
    the particulars of the security association.  Typical MACs are
    96-128 bits (12-16 bytes), but any length that fits in the header
    of the segment being authenticated is allowed.  The MAC
    computation is described further in Section 5.1.
    >> Required support for TCP-AO MACs is defined in [RFC5926]; other
    MACs MAY be supported.
 TCP-AO fields do not indicate the MAC algorithm either implicitly (as
 with TCP MD5) or explicitly.  The particular algorithm used is
 considered part of the configuration state of the connection's
 security and is managed separately (see Section 3).
 Please note that the use of TCP-AO does not affect TCP's advertised
 Maximum Segment Size (MSS), as is the case for all TCP options
 [Bo09].
 The remainder of this document explains how TCP-AO is handled and its
 relationship to TCP.

3. TCP-AO Keys and Their Properties

 TCP-AO relies on two sets of keys to authenticate incoming and
 outgoing segments: Master Key Tuples (MKTs) and traffic keys.  MKTs
 are used to derive unique traffic keys, and include the keying
 material used to generate those traffic keys, as well as indicating
 the associated parameters under which traffic keys are used.  Such
 parameters include whether TCP options are authenticated, and
 indicators of the algorithms used for traffic key derivation and MAC
 calculation.  Traffic keys are the keying material used to compute
 the MAC of individual TCP segments.

3.1. Master Key Tuple

 A Master Key Tuple (MKT) describes TCP-AO properties to be associated
 with one or more connections.  It is composed of the following:
 o  TCP connection identifier.  A TCP socket pair, i.e., a local IP
    address, a remote IP address, a TCP local port, and a TCP remote
    port.  Values can be partially specified using ranges (e.g.,
    2-30), masks (e.g., 0xF0), wildcards (e.g., "*"), or any other
    suitable indication.
 o  TCP option flag.  This flag indicates whether TCP options other
    than TCP-AO are included in the MAC calculation.  When options are
    included, the content of all options, in the order present, is
    included in the MAC, with TCP-AO's MAC field zeroed out.  When the

Touch, et al. Standards Track [Page 10] RFC 5925 The TCP Authentication Option June 2010

    options are not included, all options other than TCP-AO are
    excluded from all MAC calculations (skipped over, not zeroed).
    Note that TCP-AO, with its MAC field zeroed out, is always
    included in the MAC calculation, regardless of the setting of this
    flag; this protects the indication of the MAC length as well as
    the key ID fields (KeyID, RNextKeyID).  The option flag applies to
    TCP options in both directions (incoming and outgoing segments).
 o  IDs.  The values used in the KeyID or RNextKeyID of TCP-AO; used
    to differentiate MKTs in concurrent use (KeyID), as well as to
    indicate when MKTs are ready for use in the opposite direction
    (RNextKeyID).
    Each MKT has two IDs - -- a SendID and a RecvID.  The SendID is
    inserted as the KeyID of the TCP-AO option of outgoing segments,
    and the RecvID is matched against the TCP-AO KeyID of incoming
    segments.  These and other uses of these two IDs are described
    further in Sections 7.4 and 7.5.
    >> MKT IDs MUST support any value, 0-255 inclusive.  There are no
    reserved ID values.
    ID values are assigned arbitrarily, i.e., the values are not
    monotonically increasing, have no reserved values, and are
    otherwise not meaningful.  They can be assigned in sequence, or
    based on any method mutually agreed by the connection endpoints
    (e.g., using an external MKT management mechanism).
    >> IDs MUST NOT be assumed to be randomly assigned.
 o  Master key.  A byte sequence used for generating traffic keys,
    this may be derived from a separate shared key by an external
    protocol over a separate channel.  This sequence is used in the
    traffic key generation algorithm described in Section 5.2.
    Implementations are advised to keep master key values in a
    private, protected area of memory or other storage.
 o  Key Derivation Function (KDF).  Indicates the key derivation
    function and its parameters, as used to generate traffic keys from
    master keys.  It is explained further in Section 5.2 of this
    document and specified in detail in [RFC5926].
 o  Message Authentication Code (MAC) algorithm.  Indicates the MAC
    algorithm and its parameters as used for this connection.  It is
    explained further in Section 5.1 of this document and specified in
    detail in [RFC5926].

Touch, et al. Standards Track [Page 11] RFC 5925 The TCP Authentication Option June 2010

 >> Components of an MKT MUST NOT change during a connection.
 MKT component values cannot change during a connection because TCP
 state is coordinated during connection establishment.  TCP lacks a
 handshake for modifying that state after a connection has been
 established.
 >> The set of MKTs MAY change during a connection.
 MKT parameters are not changed.  Instead, new MKTs can be installed,
 and a connection can change which MKT it uses.
 >> The IDs of MKTs MUST NOT overlap where their TCP connection
 identifiers overlap.
 This document does not address how MKTs are created by users or
 processes.  It is presumed that an MKT affecting a particular
 connection cannot be destroyed during an active connection -- or,
 equivalently, that its parameters are copied to an area local to the
 connection (i.e., instantiated) and so changes would affect only new
 connections.  The MKTs can be managed by a separate application
 protocol.

3.2. Traffic Keys

 A traffic key is a key derived from the MKT and the local and remote
 IP address pairs and TCP port numbers, and, for established
 connections, the TCP Initial Sequence Numbers (ISNs) in each
 direction.  Segments exchanged before a connection is established use
 the same information, substituting zero for unknown values (e.g.,
 ISNs not yet coordinated).
 A single MKT can be used to derive any of four different traffic
 keys:
 o  Send_SYN_traffic_key
 o  Receive_SYN_traffic_key
 o  Send_other_traffic_key
 o  Receive_other_traffic_key
 Note that the keys are unidirectional.  A given connection typically
 uses only three of these keys, because only one of the SYN keys is
 typically used.  All four are used only when a connection goes
 through 'simultaneous open' [RFC793].

Touch, et al. Standards Track [Page 12] RFC 5925 The TCP Authentication Option June 2010

 The relationship between MKTs and traffic keys is shown in Figure 3.
 Traffic keys are indicated with a "*".  Note that every MKT can be
 used to derive any of the four traffic keys, but only the keys
 actually needed to handle the segments of a connection need to be
 computed.  Section 5.2 provides further details on how traffic keys
 are derived.
                   MKT-A                            MKT-B
          +---------------------+        +------------------------+
          | SendID = 1          |        | SendID = 5             |
          | RecvID = 2          |        | RecvID = 6             |
          | MAC = HMAC-SHA1     |        | MAC = AES-CMAC         |
          | KDF = KDF-HMAC-SHA1 |        | KDF = KDF-AES-128-CMAC |
          +---------------------+        +------------------------+
                     |                                |
          +----------+----------+                     |
          |                     |                     |
          v                     v                     v
     Connection 1          Connection 2          Connection 3
 +------------------+  +------------------+  +------------------+
 | * Send_SYN_key   |  | * Send_SYN_key   |  | * Send_SYN_key   |
 | * Recv_SYN_key   |  | * Recv_SYN_key   |  | * Recv_SYN_key   |
 | * Send_Other_key |  | * Send_Other_key |  | * Send_Other_key |
 | * Recv_Other_key |  | * Recv_Other_key |  | * Recv_Other_key |
 +------------------+  +------------------+  +------------------+
         Figure 3: Relationship between MKTs and Traffic Keys

3.3. MKT Properties

 TCP-AO requires that every protected TCP segment match exactly one
 MKT.  When an outgoing segment matches an MKT, TCP-AO is used.  When
 no match occurs, TCP-AO is not used.  Multiple MKTs may match a
 single outgoing segment, e.g., when MKTs are being changed.  Those
 MKTs cannot have conflicting IDs (as noted elsewhere), and some
 mechanism must determine which MKT to use for each given outgoing
 segment.
 >> An outgoing TCP segment MUST match at most one desired MKT,
 indicated by the segment's socket pair.  The segment MAY match
 multiple MKTs, provided that exactly one MKT is indicated as desired.
 Other information in the segment MAY be used to determine the desired
 MKT when multiple MKTs match; such information MUST NOT include
 values in any TCP option fields.

Touch, et al. Standards Track [Page 13] RFC 5925 The TCP Authentication Option June 2010

 We recommend that the mechanism used to select from among multiple
 MKTs use only information that TCP-AO would authenticate.  Because
 MKTs may indicate that options other than TCP-AO are ignored in the
 MAC calculation, we recommend that TCP options should not be used to
 determine MKTs.
 >> An incoming TCP segment including TCP-AO MUST match exactly one
 MKT, indicated solely by the segment's socket pair and its TCP-AO
 KeyID.
 Incoming segments include an indicator inside TCP-AO to select from
 among multiple matching MKTs -- the KeyID field.  TCP-AO requires
 that the KeyID alone be used to differentiate multiple matching MKTs,
 so that MKT changes can be coordinated using the TCP-AO key change
 coordination mechanism.
 >> When an outgoing TCP segment matches no MKTs, TCP-AO is not used.
 TCP-AO is always used when outgoing segments match an MKT, and is not
 used otherwise.

4. Per-Connection TCP-AO Parameters

 TCP-AO uses a small number of parameters associated with each
 connection that uses TCP-AO, once instantiated.  These values can be
 stored in the Transport Control Block (TCB) [RFC793].  These values
 are explained in subsequent sections of this document as noted; they
 include:
 1. Current_key - the MKT currently used to authenticate outgoing
    segments, whose SendID is inserted in outgoing segments as KeyID
    (see Section 7.4, step 2.f).  Incoming segments are authenticated
    using the MKT corresponding to the segment and its TCP-AO KeyID
    (see Section 7.5, step 2.c), as matched against the MKT TCP
    connection identifier and the MKT RecvID.  There is only one
    current_key at any given time on a particular connection.
    >> Every TCP connection in a non-IDLE state MUST have at most one
    current_key specified.
 2. Rnext_key - the MKT currently preferred for incoming (received)
    segments, whose RecvID is inserted in outgoing segments as
    RNextKeyID (see Section 7.4, step 2.d).
    >> Each TCP connection in a non-IDLE state MUST have at most one
    rnext_key specified.

Touch, et al. Standards Track [Page 14] RFC 5925 The TCP Authentication Option June 2010

 3. A pair of Sequence Number Extensions (SNEs).  SNEs are used to
    prevent replay attacks, as described in Section 6.2.  Each SNE is
    initialized to zero upon connection establishment.  Its use in the
    MAC calculation is described in Section 5.1.
 4. One or more MKTs.  These are the MKTs that match this connection's
    socket pair.
 MKTs are used, together with other parameters of a connection, to
 create traffic keys unique to each connection, as described in
 Section 5.2.  These traffic keys can be cached after computation, and
 can be stored in the TCB with the corresponding MKT information.
 They can be considered part of the per-connection parameters.

5. Cryptographic Algorithms

 TCP-AO uses cryptographic algorithms to compute the MAC (Message
 Authentication Code) that is used to authenticate a segment and its
 headers; these are called MAC algorithms and are specified in a
 separate document to facilitate updating the algorithm requirements
 independently from the protocol [RFC5926].  TCP-AO also uses
 cryptographic algorithms to convert MKTs, which can be shared across
 connections, into unique traffic keys for each connection.  These are
 called Key Derivation Functions (KDFs) and are specified [RFC5926].
 This section describes how these algorithms are used by TCP-AO.

5.1. MAC Algorithms

 MAC algorithms take a variable-length input and a key and output a
 fixed-length number.  This number is used to determine whether the
 input comes from a source with that same key, and whether the input
 has been tampered with in transit.  MACs for TCP-AO have the
 following interface:
    MAC = MAC_alg(traffic_key, message)
    INPUT: MAC_alg, traffic_key, message
    OUTPUT: MAC
 where:
 o  MAC_alg - the specific MAC algorithm used for this computation.
    The MAC algorithm specifies the output length, so no separate
    output length parameter is required.  This is specified as
    described in [RFC5926].

Touch, et al. Standards Track [Page 15] RFC 5925 The TCP Authentication Option June 2010

 o  Traffic_key - traffic key used for this computation.  This is
    computed from the connection's current MKT as described in Section
    5.2.
 o  Message - input data over which the MAC is computed.  In TCP-AO,
    this is the TCP segment prepended by the IP pseudoheader and TCP
    header options, as described in Section 5.1.
 o  MAC - the fixed-length output of the MAC algorithm, given the
    parameters provided.
 At the time of this writing, the algorithms' definitions for use in
 TCP-AO, as described in [RFC5926], are each truncated to 96 bits.
 Though the algorithms each output a larger MAC, 96 bits provides a
 reasonable trade-off between security and message size.  However,
 this could change in the future, so TCP-AO size should not be assumed
 as fixed length.
 The MAC algorithm employed for the MAC computation on a connection is
 done so by definition in the MKT, per the definition in [RFC5926].
 The mandatory-to-implement MAC algorithms for use with TCP-AO are
 described in a separate RFC [RFC5926].  This allows the TCP-AO
 specification to proceed along the IETF Standards Track even if
 changes are needed to its associated algorithms and their labels (as
 might be used in a user interface or automated MKT management
 protocol) as a result of the ever evolving world of cryptography.
 >> Additional algorithms, beyond those mandated for TCP-AO, MAY be
 supported.
 The data input to the MAC is in the following fields in the following
 sequence, interpreted in network-standard byte order:
 1. The Sequence Number Extension (SNE), in network-standard byte
    order, as follows (described further in Section 6.2):
                +--------+--------+--------+--------+
                |                SNE                |
                +--------+--------+--------+--------+
                  Figure 4: Sequence Number Extension
    The SNE for transmitted segments is maintained locally in the
    SND.SNE value; for received segments, a local RCV.SNE value is
    used.  The details of how these values are maintained and used are
    in Sections 6.2, 7.4, and 7.5.

Touch, et al. Standards Track [Page 16] RFC 5925 The TCP Authentication Option June 2010

 2. The IP pseudoheader: IP source and destination addresses, protocol
    number, and segment length, all in network byte order, prepended
    to the TCP header below.  The IP pseudoheader is exactly as used
    for the TCP checksum in either IPv4 or IPv6 [RFC793][RFC2460]:
             +--------+--------+--------+--------+
             |           Source Address          |
             +--------+--------+--------+--------+
             |         Destination Address       |
             +--------+--------+--------+--------+
             |  Zero  | Proto  |    TCP Length   |
             +--------+--------+--------+--------+
               Figure 5: TCP IPv4 Pseudoheader [RFC793]
             +--------+--------+--------+--------+
             |                                   |
             +                                   +
             |                                   |
             +           Source Address          +
             |                                   |
             +                                   +
             |                                   |
             +                                   +
             +--------+--------+--------+--------+
             |                                   |
             +                                   +
             |                                   |
             +         Destination Address       +
             |                                   |
             +                                   +
             |                                   |
             +--------+--------+--------+--------+
             |     Upper-Layer Payload Length    |
             +--------+--------+--------+--------+
             |      Zero       |   Next Header   |
             +--------+--------+--------+--------+
               Figure 6: TCP IPv6 Pseudoheader [RFC2460]
 3. The TCP header, by default including options, and where the TCP
    checksum and TCP-AO MAC fields are set to zero, all in network-
    byte order.
    The TCP option flag of the MKT indicates whether the TCP options
    are included in the MAC.  When included, only the TCP-AO MAC field
    is zeroed.

Touch, et al. Standards Track [Page 17] RFC 5925 The TCP Authentication Option June 2010

    When TCP options are not included, all TCP options except for TCP-
    AO are omitted from MAC processing.  Again, the TCP-AO MAC field
    is zeroed for the MAC processing.
 4. The TCP data, i.e., the payload of the TCP segment.
    Note that the traffic key is not included as part of the data; the
    MAC algorithm indicates how to use the traffic key, for example,
    as HMACs do [RFC2104][RFC2403].  The traffic key is derived from
    the current MKT as described in Section 5.2.

5.2. Traffic Key Derivation Functions

 TCP-AO's traffic keys are derived from the MKTs using Key Derivation
 Functions (KDFs).  The KDFs used in TCP-AO have the following
 interface:
    traffic_key = KDF_alg(master_key, context, output_length)
    INPUT: KDF_alg, master_key, context, output_length
    OUTPUT: traffic_key
 where:
 o  KDF_alg - The specific Key Derivation Function (KDF) that is the
    basic building block used in constructing the traffic key, as
    indicated in the MKT.  This is specified as described in
    [RFC5926].
 o  Master_key - The master_key string, as will be stored into the
    associated MKT.
 o  Context - The context used as input in constructing the
    traffic_key, as specified in [RFC5926].  The specific way this
    context is used, in conjunction with other information, to create
    the raw input to the KDF is also explained further in [RFC5926].
 o  Output_length - The desired output length of the KDF, i.e., the
    length to which the KDF's output will be truncated.  This is
    specified as described in [RFC5926].
 o  Traffic_key - The desired output of the KDF, of length
    output_length, to be used as input to the MAC algorithm, as
    described in Section 5.1.

Touch, et al. Standards Track [Page 18] RFC 5925 The TCP Authentication Option June 2010

 The context used as input to the KDF combines the TCP socket pair
 with the endpoint Initial Sequence Numbers (ISNs) of a connection.
 This data is unique to each TCP connection instance, which enables
 TCP-AO to generate unique traffic keys for that connection, even from
 an MKT used across many different connections or across repeated
 connections that share a socket pair.  Unique traffic keys are
 generated without relying on external key management properties.  The
 KDF context is defined in Figures 7 and 8.
             +--------+--------+--------+--------+
             |           Source Address          |
             +--------+--------+--------+--------+
             |         Destination Address       |
             +--------+--------+--------+--------+
             |   Source Port   |    Dest. Port   |
             +--------+--------+--------+--------+
             |            Source ISN             |
             +--------+--------+--------+--------+
             |             Dest. ISN             |
             +--------+--------+--------+--------+
             Figure 7: KDF Context for an IPv4 Connection

Touch, et al. Standards Track [Page 19] RFC 5925 The TCP Authentication Option June 2010

             +--------+--------+--------+--------+
             |                                   |
             +                                   +
             |                                   |
             +           Source Address          +
             |                                   |
             +                                   +
             |                                   |
             +                                   +
             +--------+--------+--------+--------+
             |                                   |
             +                                   +
             |                                   |
             +         Destination Address       +
             |                                   |
             +                                   +
             |                                   |
             +--------+--------+--------+--------+
             |   Source Port   |    Dest. Port   |
             +--------+--------+--------+--------+
             |            Source ISN             |
             +--------+--------+--------+--------+
             |             Dest. ISN             |
             +--------+--------+--------+--------+
             Figure 8: KDF Context for an IPv6 Connection
 Traffic keys are directional, so "source" and "destination" are
 interpreted differently for incoming and outgoing segments.  For
 incoming segments, source is the remote side; whereas for outgoing
 segments, source is the local side.  This further ensures that
 connection keys generated for each direction are unique.
 For SYN segments (segments with the SYN set, but the ACK not set),
 the destination ISN is not known.  For these segments, the connection
 key is computed using the context shown above, in which the
 destination ISN value is zero.  For all other segments, the ISN pair
 is used when known.  If the ISN pair is not known, e.g., when sending
 a reset (RST) after a reboot, the segment should be sent without
 authentication; if authentication was required, the segment cannot
 have been MAC'd properly anyway and would have been dropped on
 receipt.
 >> TCP-AO SYN segments (SYN set, no ACK set) MUST use a destination
 ISN of zero (whether sent or received); all other segments use the
 known ISN pair.

Touch, et al. Standards Track [Page 20] RFC 5925 The TCP Authentication Option June 2010

 Overall, this means that each connection will use up to four distinct
 traffic keys for each MKT:
 o  Send_SYN_traffic_key - the traffic key used to authenticate
    outgoing SYNs.  The source ISN is known (the TCP connection's
    local ISN), and the destination (remote) ISN is unknown (and so
    the value 0 is used).
 o  Receive_SYN_traffic_key - the traffic key used to authenticate
    incoming SYNs.  The source ISN is known (the TCP connection's
    remote ISN), and the destination (remote) ISN is unknown (and so
    the value 0 is used).
 o  Send_other_traffic_key - the traffic key used to authenticate all
    other outgoing TCP segments.
 o  Receive_other_traffic_key - the traffic key used to authenticate
    all other incoming TCP segments.
 The following table describes how each of these traffic keys is
 computed, where the TCP-AO algorithms refer to source (S) and
 destination (D) values of the IP address, TCP port, and ISN, and each
 segment (incoming or outgoing) has a value that refers to the local
 side of the connection (l) and remote side (r):
                             S-IP S-port S-ISN D-IP D-port D-ISN
 ----------------------------------------------------------------
  Send_SYN_traffic_key       l-IP l-port l-ISN r-IP r-port 0
  Receive_SYN_traffic_key    r-IP r-port r-ISN l-IP l-port 0
  Send_other_traffic_key     l-IP l-port l-ISN r-IP r-port r-ISN
  Receive_other_traffic_key  r-IP r-port r-ISN l-IP l-port l-ISN
 The use of both ISNs in the traffic key computations ensures that
 segments cannot be replayed across repeated connections reusing the
 same socket; their 32-bit space avoids repeated use except under
 reboot, and reuse assumes both sides repeat their use on the same
 connection.  We do expect that:
 >> Endpoints should select ISNs pseudorandomly, e.g., as in
 [RFC1948].
 A SYN is authenticated using a destination ISN of zero (whether sent
 or received), and all other segments would be authenticated using the
 ISN pair for the connection.  There are other cases in which the
 destination ISN is not known, but segments are emitted, such as after
 an endpoint reboots, when it is possible that the two endpoints would
 not have enough information to authenticate segments.  This is
 addressed further in Section 7.7.

Touch, et al. Standards Track [Page 21] RFC 5925 The TCP Authentication Option June 2010

5.3. Traffic Key Establishment and Duration Issues

 TCP-AO does not provide a mechanism for traffic key negotiation or
 parameter negotiation (MAC algorithm, length, or use of TCP-AO on a
 connection), or for coordinating rekeying during a connection.  We
 assume out-of-band mechanisms for MKT establishment, parameter
 negotiation, and rekeying.  This separation of MKT use from MKT
 management is similar to that in the IPsec suite [RFC4301][RFC4306].
 We encourage users of TCP-AO to apply known techniques for generating
 appropriate MKTs, including the use of reasonable master key lengths,
 limited traffic key sharing, and limiting the duration of MKT use
 [RFC3562].  This also includes the use of per-connection nonces, as
 suggested in Section 5.2.
 TCP-AO supports rekeying in which new MKTs are negotiated and
 coordinated out of band, either via a protocol or a manual procedure
 [RFC4808].  New MKT use is coordinated using the out-of-band
 mechanism to update both TCP endpoints.  When only a single MKT is
 used at a time, the temporary use of invalid MKTs could result in
 segments being dropped; although TCP is already robust to such drops,
 TCP-AO uses the KeyID field to avoid such drops.  A given connection
 can have multiple matching MKTs, where the KeyID field is used to
 identify the MKT that corresponds to the traffic key used for a
 segment, to avoid the need for expensive trial-and-error testing of
 MKTs in sequence.
 TCP-AO provides an explicit MKT coordination mechanism, described in
 Section 6.1.  Such a mechanism is useful when new MKTs are installed,
 or when MKTs are changed, to determine when to commence using
 installed MKTs.
 Users are advised to manage MKTs following the spirit of the advice
 for key management when using TCP MD5 [RFC3562], notably to use
 appropriate key lengths (12-24 bytes) and to avoid sharing MKTs among
 multiple BGP peering arrangements.

5.3.1. MKT Reuse Across Socket Pairs

 MKTs can be reused across different socket pairs within a host, or
 across different instances of a socket pair within a host.  In either
 case, replay protection is maintained.
 MKTs reused across different socket pairs cannot enable replay
 attacks because the TCP socket pair is included in the MAC, as well
 as in the generation of the traffic key.  MKTs reused across repeated

Touch, et al. Standards Track [Page 22] RFC 5925 The TCP Authentication Option June 2010

 instances of a given socket pair cannot enable replay attacks because
 the connection ISNs are included in the traffic key generation
 algorithm, and ISN pairs are unlikely to repeat over useful periods.

5.3.2. MKTs Use within a Long-Lived Connection

 TCP-AO uses Sequence Number Extensions (SNEs) to prevent replay
 attacks within long-lived connections.  Explicit MKT rollover,
 accomplished by external means and indexed using the KeyID field, can
 be used to change keying material for various reasons (e.g.,
 personnel turnover), but is not required to support long-lived
 connections.

6. Additional Security Mechanisms

 TCP-AO adds mechanisms to support efficient use, especially in
 environments where only manual keying is available.  These include
 the previously described mechanisms for supporting multiple
 concurrent MKTs (via the KeyID field) and for generating unique per-
 connection traffic keys (via the KDF).  This section describes
 additional mechanisms to coordinate MKT changes and to prevent replay
 attacks when a traffic key is not changed for long periods of time.

6.1. Coordinating Use of New MKTs

 At any given time, a single TCP connection may have multiple MKTs
 specified for each segment direction (incoming, outgoing).  TCP-AO
 provides a mechanism to indicate when a new MKT is ready, which
 allows the sender to commence use of that new MKT.  This mechanism
 allows new MKT use to be coordinated, to avoid unnecessary loss due
 to sender authentication using an MKT not yet ready at the receiver.
 Note that this is intended as an optimization.  Deciding when to
 start using a key is a performance issue.  Deciding when to remove an
 MKT is a security issue.  Invalid MKTs are expected to be removed.
 TCP-AO provides no mechanism to coordinate their removal, as we
 consider this a key management operation.
 New MKT use is coordinated through two ID fields in the header:
 o  KeyID
 o  RNextKeyID

Touch, et al. Standards Track [Page 23] RFC 5925 The TCP Authentication Option June 2010

 KeyID represents the outgoing MKT information used by the segment
 sender to create the segment's MAC (outgoing), and the corresponding
 incoming keying information used by the segment receiver to validate
 that MAC.  It contains the SendID of the MKT in active use in that
 direction.
 RNextKeyID represents the preferred MKT information to be used for
 subsequent received segments ('receive next').  That is, it is a way
 for the segment sender to indicate a ready incoming MKT for future
 segments it receives, so that the segment receiver can know when to
 switch MKTs (and thus their KeyIDs and associated traffic keys).  It
 indicates the RecvID of the MKT desired for incoming segments.
 There are two pointers kept by each side of a connection, as noted in
 the per-connection information (see Section 4):
 o  Currently active outgoing MKT (current_key)
 o  Current preference for incoming MKT (rnext_key)
 Current_key indicates an MKT that is used to authenticate outgoing
 segments.  Upon connection establishment, it points to the first MKT
 selected for use.
 Rnext_key points to an incoming MKT that is ready and preferred for
 use.  Upon connection establishment, this points to the currently
 active incoming MKT.  It can be changed when new MKTs are installed
 (e.g., by either automatic MKT management protocol operation or user
 manual selection).
 Rnext_key is changed only by manual user intervention or MKT
 management protocol operation.  It is not manipulated by TCP-AO.
 Current_key is updated by TCP-AO when processing received TCP
 segments as discussed in the segment processing description in
 Section 7.5.  Note that the algorithm allows the current_key to
 change to a new MKT, then change back to a previously used MKT (known
 as "backing up").  This can occur during an MKT change when segments
 are received out of order, and is considered a feature of TCP-AO,
 because reordering does not result in drops.  The only way to avoid
 reuse of previously used MKTs is to remove the MKT when it is no
 longer considered permitted.

6.2. Preventing Replay Attacks within Long-Lived Connections

 TCP uses a 32-bit sequence number, which may, for long-lived
 connections, roll over and repeat.  This could result in TCP segments
 being intentionally and legitimately replayed within a connection.
 TCP-AO prevents replay attacks, and thus requires a way to

Touch, et al. Standards Track [Page 24] RFC 5925 The TCP Authentication Option June 2010

 differentiate these legitimate replays from each other, and so it
 adds a 32-bit Sequence Number Extension (SNE) for transmitted and
 received segments.
 The SNE extends the TCP sequence number so that segments within a
 single connection are always unique.  When the TCP's sequence number
 rolls over, there is a chance that a segment could be repeated in
 total; using an SNE differentiates even identical segments sent with
 identical sequence numbers at different times in a connection.  TCP-
 AO emulates a 64-bit sequence number space by inferring when to
 increment the high-order 32-bit portion (the SNE) based on
 transitions in the low-order portion (the TCP sequence number).
 TCP-AO thus maintains SND.SNE for transmitted segments, and RCV.SNE
 for received segments, both initialized as zero when a connection
 begins.  The intent of these SNEs is, together with TCP's 32-bit
 sequence numbers, to provide a 64-bit overall sequence number space.
 For transmitted segments, SND.SNE can be implemented by extending
 TCP's sequence number to 64 bits; SND.SNE would be the top (high-
 order) 32 bits of that number.  For received segments, TCP-AO needs
 to emulate the use of a 64-bit number space and correctly infer the
 appropriate high-order 32-bits of that number as RCV.SNE from the
 received 32-bit sequence number and the current connection context.
 The implementation of SNEs is not specified in this document, but one
 possible way is described here that can be used for either RCV.SNE,
 SND.SNE, or both.
 Consider an implementation with two SNEs as required (SND.SNE, RCV.
 SNE), and additional variables as listed below, all initialized to
 zero, as well as a current TCP segment field (SEG.SEQ):
 o  SND.PREV_SEQ, needed to detect rollover of SND.SEQ
 o  RCV.PREV_SEQ, needed to detect rollover of RCV.SEQ
 o  SND.SNE_FLAG, which indicates when to increment the SND.SNE
 o  RCV.SNE_FLAG, which indicates when to increment the RCV.SNE
 When a segment is received, the following algorithm (in C-like
 pseudocode) computes the SNE used in the MAC; this is the "RCV" side,
 and an equivalent algorithm can be applied to the "SND" side:

Touch, et al. Standards Track [Page 25] RFC 5925 The TCP Authentication Option June 2010

    /* set the flag when the SEG.SEQ first rolls over */
    if ((RCV.SNE_FLAG == 0)
       && (RCV.PREV_SEQ > 0x7fff) && (SEG.SEQ < 0x7fff)) {
          RCV.SNE = RCV.SNE + 1;
          RCV.SNE_FLAG = 1;
    }
    /* decide which SNE to use after incremented */
    if ((RCV.SNE_FLAG == 1) && (SEG.SEQ > 0x7fff)) {
       SNE = RCV.SNE - 1; # use the pre-increment value
    } else {
       SNE = RCV.SNE; # use the current value
    }
    /* reset the flag in the *middle* of the window */
    if ((RCV.PREV_SEQ < 0x7fff) && (SEG.SEQ > 0x7fff)) {
       RCV.SNE_FLAG = 0;
    }
    /* save the current SEQ for the next time through the code */
    RCV.PREV_SEQ = SEG.SEQ;
 In the above code, the first time the sequence number rolls over,
 i.e., when the new number is low (in the bottom half of the number
 space) and the old number is high (in the top half of the number
 space), the SNE is incremented and a flag is set.
 If the flag is set and a high number is seen, it must be a reordered
 segment, so use the pre-increment SNE; otherwise, use the current
 SNE.
 The flag will be cleared by the time the number rolls all the way
 around.
 The flag prevents the SNE from being incremented again until the flag
 is reset, which happens in the middle of the window (when the old
 number is in the bottom half and the new is in the top half).
 Because the receive window is never larger than half of the number
 space, it is impossible to both set and reset the flag at the same
 time -- outstanding segments, regardless of reordering, cannot
 straddle both regions simultaneously.

7. TCP-AO Interaction with TCP

 The following is a description of how TCP-AO affects various TCP
 states, segments, events, and interfaces.  This description is
 intended to augment the description of TCP as provided in RFC 793,
 and its presentation mirrors that of RFC 793 as a result [RFC793].

Touch, et al. Standards Track [Page 26] RFC 5925 The TCP Authentication Option June 2010

7.1. TCP User Interface

 The TCP user interface supports active and passive OPEN, SEND,
 RECEIVE, CLOSE, STATUS, and ABORT commands.  TCP-AO does not alter
 this interface as it applies to TCP, but some commands or command
 sequences of the interface need to be modified to support TCP-AO.
 TCP-AO does not specify the details of how this is achieved.
 TCP-AO requires that the TCP user interface be extended to allow the
 MKTs to be configured, as well as to allow an ongoing connection to
 manage which MKTs are active.  The MKTs need to be configured prior
 to connection establishment, and the set of MKTs may change during a
 connection:
 >> TCP OPEN, or the sequence of commands that configure a connection
 to be in the active or passive OPEN state, MUST be augmented so that
 an MKT can be configured.
 >> A TCP-AO implementation MUST allow the set of MKTs for ongoing TCP
 connections (i.e., not in the CLOSED state) to be modified.
 The MKTs associated with a connection need to be available for
 confirmation; this includes the ability to read the MKTs:
 >> TCP STATUS SHOULD be augmented to allow the MKTs of a current or
 pending connection to be read (for confirmation).
 Senders may need to be able to determine when the outgoing MKT
 changes (KeyID) or when a new preferred MKT (RNextKeyID) is
 indicated; these changes immediately affect all subsequent outgoing
 segments:
 >> TCP SEND, or a sequence of commands resulting in a SEND, MUST be
 augmented so that the preferred outgoing MKT (current_key) and/or the
 preferred incoming MKT (rnext_key) of a connection can be indicated.
 It may be useful to change the outgoing active MKT (current_key) even
 when no data is being sent, which can be achieved by sending a zero-
 length buffer or by using a non-send interface (e.g., socket options
 in Unix), depending on the implementation.
 It is also useful to indicate recent segment KeyID and RNextKeyID
 values received; although there could be a number of such values,
 they are not expected to change quickly, so any recent sample should
 be sufficient:

Touch, et al. Standards Track [Page 27] RFC 5925 The TCP Authentication Option June 2010

 >> TCP RECEIVE, or the sequence of commands resulting in a RECEIVE,
 MUST be augmented so that the KeyID and RNextKeyID of a recently
 received segment is available to the user out of band (e.g., as an
 additional parameter to RECEIVE or via a STATUS call).

7.2. TCP States and Transitions

 TCP includes the states LISTEN, SYN-SENT, SYN-RECEIVED, ESTABLISHED,
 FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK, TIME-WAIT, and
 CLOSED.
 >> An MKT MAY be associated with any TCP state.

7.3. TCP Segments

 TCP includes control (at least one of SYN, FIN, RST flags set) and
 data (none of SYN, FIN, or RST flags set) segments.  Note that some
 control segments can include data (e.g., SYN).
 >> All TCP segments MUST be checked against the set of MKTs for
 matching TCP connection identifiers.
 >> TCP segments whose TCP-AO does not validate MUST be silently
 discarded.
 >> A TCP-AO implementation MUST allow for configuration of the
 behavior of segments with TCP-AO but that do not match an MKT.  The
 initial default of this configuration SHOULD be to silently accept
 such connections.  If this is not the desired case, an MKT can be
 included to match such connections, or the connection can indicate
 that TCP-AO is required.  Alternately, the configuration can be
 changed to discard segments with the AO option not matching an MKT.
 >> Silent discard events SHOULD be signaled to the user as a warning,
 and silent accept events MAY be signaled to the user as a warning.
 Both warnings, if available, MUST be accessible via the STATUS
 interface.  Either signal MAY be asynchronous, but if so, they MUST
 be rate-limited.  Either signal MAY be logged; logging SHOULD allow
 rate-limiting as well.
 All TCP-AO processing occurs between the interface of TCP and IP; for
 incoming segments, this occurs after validation of the TCP checksum.
 For outgoing segments, this occurs before computation of the TCP
 checksum.

Touch, et al. Standards Track [Page 28] RFC 5925 The TCP Authentication Option June 2010

 Note that use of TCP-AO on a connection is not negotiated within TCP.
 It is the responsibility of the receiver to determine when TCP-AO is
 required via other means (e.g., out of band, manually or with a key
 management protocol) and to enforce that requirement.

7.4. Sending TCP Segments

 The following procedure describes the modifications to TCP to support
 inserting TCP-AO when a segment departs.
 >> Note that TCP-AO MUST be the last TCP option processed on outgoing
 segments, because its MAC calculation may include the values of other
 TCP options.
 1. Find the per-connection parameters for the segment:
     a. If the segment is a SYN, then this is the first segment of a
        new connection.  Find the matching MKT for this segment based
        on the segment's socket pair.
        i. If there is no matching MKT, omit TCP-AO.  Proceed with
           transmitting the segment.
       ii. If there is a matching MKT, then set the per-connection
           parameters as needed (see Section 4).  Proceed with the
           step 2.
     b. If the segment is not a SYN, then determine whether TCP-AO is
        being used for the connection and use the MKT as indicated by
        the current_key value from the per-connection parameters (see
        Section 4) and proceed with the step 2.
 2. Using the per-connection parameters:
     a. Augment the TCP header with TCP-AO, inserting the appropriate
        Length and KeyID based on the MKT indicated by current_key
        (using the current_key MKT's SendID as the TCP-AO KeyID).
        Update the TCP header length accordingly.
     b. Determine SND.SNE as described in Section 6.2.
     c. Determine the appropriate traffic key, i.e., as pointed to by
        the current_key (as noted in Section 6.1, and as probably
        cached in the TCB).  That is, use the send_SYN_traffic_key for
        SYN segments and the send_other_traffic_key for other
        segments.

Touch, et al. Standards Track [Page 29] RFC 5925 The TCP Authentication Option June 2010

     d. Determine the RNextKeyID as indicated by the rnext_key
        pointer, and insert it in the TCP-AO RNextKeyID field (using
        the rnext_key MKT's RecvID as the TCP-AO KeyID) (as noted in
        Section 6.1).
     e. Compute the MAC using the MKT (and cached traffic key) and
        data from the segment as specified in Section 5.1.
     f. Insert the MAC in the TCP-AO MAC field.
     g. Proceed with transmitting the segment.

7.5. Receiving TCP Segments

 The following procedure describes the modifications to TCP to support
 TCP-AO when a segment arrives.
 >> Note that TCP-AO MUST be the first TCP option processed on
 incoming segments, because its MAC calculation may include the values
 of other TCP options that could change during TCP option processing.
 This also protects the behavior of all other TCP options from the
 impact of spoofed segments or modified header information.
 >> Note that TCP-AO checks MUST be performed for all incoming SYNs to
 avoid accepting SYNs lacking TCP-AO where required.  Other segments
 can cache whether TCP-AO is needed in the TCB.
 1. Find the per-connection parameters for the segment:
     a. If the segment is a SYN, then this is the first segment of a
        new connection.  Find the matching MKT for this segment, using
        the segment's socket pair and its TCP-AO KeyID, matched
        against the MKT's TCP connection identifier and the MKT's
        RecvID.
        i. If there is no matching MKT, remove TCP-AO from the
           segment.  Proceed with further TCP handling of the segment.
           NOTE: this presumes that connections that do not match any
           MKT should be silently accepted, as noted in Section 7.3.
       ii. If there is a matching MKT, then set the per-connection
           parameters as needed (see Section 4).  Proceed with step 2.

Touch, et al. Standards Track [Page 30] RFC 5925 The TCP Authentication Option June 2010

 2. Using the per-connection parameters:
     a. Check that the segment's TCP-AO Length matches the length
        indicated by the MKT.
        i. If the lengths differ, silently discard the segment.  Log
           and/or signal the event as indicated in Section 7.3.
     b. Determine the segment's RCV.SNE as described in Section 6.2.
     c. Determine the segment's traffic key from the MKT as described
        in Section 5.1 (and as likely cached in the TCB).  That is,
        use the receive_SYN_traffic_key for SYN segments and the
        receive_other_traffic_key for other segments.
     d. Compute the segment's MAC using the MKT (and its derived
        traffic key) and portions of the segment as indicated in
        Section 5.1.
        i. If the computed MAC differs from the TCP-AO MAC field
           value, silently discard the segment.  Log and/or signal the
           event as indicated in Section 7.3.
     e. Compare the received RNextKeyID value to the currently active
        outgoing KeyID value (current_key MKT's SendID).
        i. If they match, no further action is required.
       ii. If they differ, determine whether the RNextKeyID MKT is
           ready.
           1. If the MKT corresponding to the segment's socket pair
              and RNextKeyID is not available, no action is required
              (RNextKeyID of a received segment needs to match the
              MKT's SendID).
           2. If the matching MKT corresponding to the segment's
              socket pair and RNextKeyID is available:
              a. Set current_key to the RNextKeyID MKT.
     f. Proceed with TCP processing of the segment.
 It is suggested that TCP-AO implementations validate a segment's
 Length field before computing a MAC to reduce the overhead incurred
 by spoofed segments with invalid TCP-AO fields.

Touch, et al. Standards Track [Page 31] RFC 5925 The TCP Authentication Option June 2010

 Additional reductions in MAC validation overhead can be supported in
 the MAC algorithms, e.g., by using a computation algorithm that
 prepends a fixed value to the computed portion and a corresponding
 validation algorithm that verifies the fixed value before investing
 in the computed portion.  Such optimizations would be contained in
 the MAC algorithm specification, and thus are not specified in TCP-AO
 explicitly.  Note that the KeyID cannot be used for connection
 validation per se, because it is not assumed random.

7.6. Impact on TCP Header Size

 TCP-AO, using the initially required 96-bit MACs, uses a total of 16
 bytes of TCP header space [RFC5926].  TCP-AO is thus 2 bytes smaller
 than the TCP MD5 option (18 bytes).
 Note that the TCP option space is most critical in SYN segments,
 because flags in those segments could potentially increase the option
 space area in other segments.  Because TCP ignores unknown segments,
 however, it is not possible to extend the option space of SYNs
 without breaking backward compatibility.
 TCP's 4-bit data offset requires that the options end 60 bytes (15
 32-bit words) after the header begins, including the 20-byte header.
 This leaves 40 bytes for options, of which 15 are expected in current
 implementations (listed below), leaving at most 25 for other uses.
 TCP-AO consumes 16 bytes, leaving 9 bytes for additional SYN options
 (depending on implementation dependant alignment padding, which could
 consume another 2 bytes at most).
 o  SACK permitted (2 bytes) [RFC2018][RFC3517]
 o  Timestamps (10 bytes) [RFC1323]
 o  Window scale (3 bytes) [RFC1323]
 After a SYN, the following options are expected in current
 implementations of TCP:
 o  SACK (10bytes) [RFC2018][RFC3517] (18 bytes if D-SACK [RFC2883])
 o  Timestamps (10 bytes) [RFC1323]
 TCP-AO continues to consume 16 bytes in non-SYN segments, leaving a
 total of 24 bytes for other options, of which the timestamp consumes
 10.  This leaves 14 bytes, of which 10 are used for a single SACK
 block.  When two SACK blocks are used, such as to handle D-SACK, a
 smaller TCP-AO MAC would be required to make room for the additional
 SACK block (i.e., to leave 18 bytes for the D-SACK variant of the

Touch, et al. Standards Track [Page 32] RFC 5925 The TCP Authentication Option June 2010

 SACK option) [RFC2883].  Note that D-SACK is not supportable in TCP
 MD5 in the presence of timestamps, because TCP MD5's MAC length is
 fixed and too large to leave sufficient option space.
 Although TCP option space is limited, we believe TCP-AO is consistent
 with the desire to authenticate TCP at the connection level for
 similar uses as were intended by TCP MD5.

7.7. Connectionless Resets

 TCP-AO allows TCP resets (RSTs) to be exchanged provided both sides
 have established valid connection state.  After such state is
 established, if one side reboots, TCP-AO prevents TCP's RST mechanism
 from clearing out old state on the side that did not reboot.  This
 happens because the rebooting side has lost its connection state, and
 thus its traffic keys.
 It is important that implementations are capable of detecting
 excesses of TCP connections in such a configuration and can clear
 them out if needed to protect its memory usage [Ba10].  To protect
 against such state from accumulating and not being cleared out, a
 number of recommendations are made:
 >> Connections using TCP-AO SHOULD also use TCP keepalives [RFC1122].
 The use of TCP keepalives ensures that connections whose keys are
 lost are terminated after a finite time; a similar effect can be
 achieved at the application layer, e.g., with BGP keepalives
 [RFC4271].  Either kind of keepalive helps ensure the TCP state is
 cleared out in such a case; the alternative, of allowing
 unauthenticated RSTs to be received, would allow one of the primary
 vulnerabilities that TCP-AO is intended to prevent.
 Keepalives ensure that connections are dropped across reboots, but
 this can have a detrimental effect on some protocols.  Specifically,
 BGP reacts poorly to such connection drops, even if caused by the use
 of BGP keepalives; "graceful restart" was introduced to address this
 effect [RFC4724], and extended to support BGP with MPLS [RFC4781].
 As a result:
 >> BGP connections SHOULD require support for graceful restart when
 using TCP-AO.

Touch, et al. Standards Track [Page 33] RFC 5925 The TCP Authentication Option June 2010

 We recognize that support for graceful restart is not always
 feasible.  As a result:
 >> When BGP without graceful restart is used with TCP-AO, both sides
 of the connection SHOULD save traffic keys in storage that persists
 across reboots and restore them after a reboot, and SHOULD limit any
 performance impacts that result from this storage/restoration.

7.8. ICMP Handling

 TCP can be attacked both in band, using TCP segments, or out of band
 using ICMP.  ICMP packets cannot be protected using TCP-AO
 mechanisms; however, in this way, both TCP-AO and IPsec do not
 directly solve the need for protected ICMP signaling.  TCP-AO does
 make specific recommendations on how to handle certain ICMPs, beyond
 what IPsec requires, and these are made possible because TCP-AO
 operates inside the context of a TCP connection.
 IPsec makes recommendations regarding dropping ICMPs in certain
 contexts or requiring that they are endpoint authenticated in others
 [RFC4301].  There are other mechanisms proposed to reduce the impact
 of ICMP attacks by further validating ICMP contents and changing the
 effect of some messages based on TCP state, but these do not provide
 the level of authentication for ICMP that TCP-AO provides for TCP
 [Go10].  As a result, we recommend a conservative approach to
 accepting ICMP messages as summarized in [Go10]:
 >> A TCP-AO implementation MUST default to ignore incoming ICMPv4
 messages of Type 3 (destination unreachable), Codes 2-4 (protocol
 unreachable, port unreachable, and fragmentation needed -- 'hard
 errors'), and ICMPv6 Type 1 (destination unreachable), Code 1
 (administratively prohibited) and Code 4 (port unreachable) intended
 for connections in synchronized states (ESTABLISHED, FIN-WAIT-1, FIN-
 WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK, TIME-WAIT) that match MKTs.
 >> A TCP-AO implementation SHOULD allow whether such ICMPs are
 ignored to be configured on a per-connection basis.
 >> A TCP-AO implementation SHOULD implement measures to protect ICMP
 "packet too big" messages, some examples of which are discussed in
 [Go10].
 >> An implementation SHOULD allow ignored ICMPs to be logged.
 This control affects only ICMPs that currently require 'hard errors',
 which would abort the TCP connection [RFC1122].  This recommendation
 is intended to be similar to how IPsec would handle those messages,
 with an additional default assumed [RFC4301].

Touch, et al. Standards Track [Page 34] RFC 5925 The TCP Authentication Option June 2010

8. Obsoleting TCP MD5 and Legacy Interactions

 TCP-AO obsoletes TCP MD5.  As we have noted earlier:
 >> TCP implementations that support TCP MD5 MUST support TCP-AO.
 Systems implementing TCP MD5 only are considered legacy, and ought to
 be upgraded when possible.  In order to support interoperation with
 such legacy systems until upgrades are available:
 >> TCP MD5 SHOULD be supported where interactions with legacy systems
 are needed.
 >> A system that supports both TCP-AO and TCP MD5 MUST use TCP-AO for
 connections unless not supported by its peer, at which point it MAY
 use TCP MD5 instead.
 >> A TCP implementation MUST NOT use both TCP-AO and TCP MD5 for a
 particular TCP connection, but MAY support TCP-AO and TCP MD5
 simultaneously for different connections (notably to support legacy
 use of TCP MD5).
 The Kind value explicitly indicates whether TCP-AO or TCP MD5 is used
 for a particular connection in TCP segments.
 It is possible that MKTs could be augmented to support TCP MD5,
 although use of MKTs is not described in RFC 2385.
 It is possible to require TCP-AO for a connection or TCP MD5, but it
 is not possible to require 'either'.  When an endpoint is configured
 to require TCP MD5 for a connection, it must be added to all outgoing
 segments and validated on all incoming segments [RFC2385].  TCP MD5's
 requirements prohibit the speculative use of both options for a given
 connection, e.g., to be decided by the other end of the connection.

9. Interactions with Middleboxes

 TCP-AO may interact with middleboxes, depending on their behavior
 [RFC3234].  Some middleboxes either alter TCP options (such as TCP-
 AO) directly or alter the information TCP-AO includes in its MAC
 calculation.  TCP-AO may interfere with these devices, exactly where
 the device modifies information TCP-AO is designed to protect.

Touch, et al. Standards Track [Page 35] RFC 5925 The TCP Authentication Option June 2010

9.1. Interactions with Non-NAT/NAPT Middleboxes

 TCP-AO supports middleboxes that do not change the IP addresses or
 ports of segments.  Such middleboxes may modify some TCP options, in
 which case TCP-AO would need to be configured to ignore all options
 in the MAC calculation on connections traversing that element.
 Note that ignoring TCP options may provide less protection, i.e., TCP
 options could be modified in transit, and such modifications could be
 used by an attacker.  Depending on the modifications, TCP could have
 compromised efficiency (e.g., timestamp changes), or could cease
 correct operation (e.g., window scale changes).  These
 vulnerabilities affect only the TCP connections for which TCP-AO is
 configured to ignore TCP options.

9.2. Interactions with NAT/NAPT Devices

 TCP-AO cannot interoperate natively across NAT/NAPT (Network Address
 Port Translation) devices, which modify the IP addresses and/or port
 numbers.  We anticipate that traversing such devices may require
 variants of existing NAT/NAPT traversal mechanisms, e.g.,
 encapsulation of the TCP-AO-protected segment in another transport
 segment (e.g., UDP), as is done in IPsec [RFC2663][RFC3947].  Such
 variants can be adapted for use with TCP-AO, or IPsec with NAT
 traversal can be used instead of TCP-AO in such cases [RFC3947].
 An alternate proposal for accommodating NATs extends TCP-AO
 independently of this specification [To10].

10. Evaluation of Requirements Satisfaction

 TCP-AO satisfies all the current requirements for a revision to TCP
 MD5, as summarized below [Ed07].
 1. Protected Elements
    A solution to revising TCP MD5 should protect (authenticate) the
    following elements.
    This is supported -- see Section 5.1.
    a. IP pseudoheader, including IPv4 and IPv6 versions.
       Note that optional coverage is not allowed because IP addresses
       define a connection.  If they can be coordinated across a
       NAT/NAPT, the sender can compute the MAC based on the received
       values; if not, a tunnel is required, as noted in Section 9.2.

Touch, et al. Standards Track [Page 36] RFC 5925 The TCP Authentication Option June 2010

    b. TCP header.
       Note that optional port coverage is not allowed because ports
       define a connection.  If they can be coordinated across a
       NAT/NAPT, the sender can compute the MAC based on the received
       values; if not, a tunnel is required, as noted in Section 9.2.
    c. TCP options.
       Note that TCP-AO allows the exclusion of TCP options from
       coverage, to enable use with middleboxes that modify options
       (except when they modify TCP-AO itself).  See Section 9.
    d. TCP payload data.
 2. Option Structure Requirements
    A solution to revising TCP MD5 should use an option with the
    following structural requirements.
    This is supported -- see Section 5.1.
    a. Privacy.
       The option should not unnecessarily expose information about
       the TCP-AO mechanism.  The additional protection afforded by
       keeping this information private may be of little value, but
       also helps keep the option size small.
       TCP-AO exposes only the MKT IDs, MAC, and overall option length
       on the wire.  Note that short MACs could be obscured by using
       longer option lengths but specifying a short MAC length (this
       is equivalent to a different MAC algorithm, and is specified in
       the MKT).  See Section 2.2.
    b. Allow optional per connection.
       The option should not be required on every connection; it
       should be optional on a per-connection basis.
       This is supported because the set of MKTs can be installed to
       match some connections and not others.  Connections not
       matching any MKT do not require TCP-AO.  Further, incoming
       segments with TCP-AO are not discarded solely because they
       include the option, provided they do not match any MKT.

Touch, et al. Standards Track [Page 37] RFC 5925 The TCP Authentication Option June 2010

    c. Require non-optional.
       The option should be able to be specified as required for a
       given connection.
       This is supported because the set of MKTs can be installed to
       match some connections and not others.  Connections matching
       any MKT require TCP-AO.
    d. Standard parsing.
       The option should be easily parseable, i.e., without
       conditional parsing, and follow the standard RFC 793 option
       format.
       This is supported -- see Section 2.2.
    e. Compatible with Large Windows and SACK.
       The option should be compatible with the use of the Large
       Windows and SACK options.
       This is supported -- see Section 7.6.  The size of the option
       is intended to allow use with Large Windows and SACK.  See also
       Section 1.3, which indicates that TCP-AO is 2 bytes shorter
       than TCP MD5 in the default case, assuming a 96-bit MAC.
 3. Cryptography requirements
    A solution to revising TCP MD5 should support modern cryptography
    capabilities.
    a. Baseline defaults.
       The option should have a default that is required in all
       implementations.
       TCP-AO uses a default required algorithm as specified in
       [RFC5926] and as noted in Section 5.1 of this document.
    b. Good algorithms.
       The option should use algorithms considered accepted by the
       security community, which are considered appropriately safe.
       The use of non-standard or unpublished algorithms should be
       avoided.

Touch, et al. Standards Track [Page 38] RFC 5925 The TCP Authentication Option June 2010

       TCP-AO uses MACs as indicated in [RFC5926].  The KDF is also
       specified in [RFC5926].  The KDF input string follows the
       typical design (see [RFC5926]).
    c. Algorithm agility.
       The option should support algorithms other than the default, to
       allow agility over time.
       TCP-AO allows any desired algorithm, subject to TCP option
       space limitations, as noted in Section 2.2.  The use of a set
       of MKTs allows separate connections to use different
       algorithms, both for the MAC and the KDF.
    d. Order-independent processing.
       The option should be processed independently of the proper
       order, i.e., they should allow processing of TCP segments in
       the order received, without requiring reordering.  This avoids
       the need for reordering prior to processing, and avoids the
       impact of misordered segments on the option.
       This is supported -- see Sections 7.3, 7.4, and 7.5.  Note that
       pre-TCP processing is further required, because TCP segments
       cannot be discarded solely based on a combination of connection
       state and out-of-window checks; many such segments, although
       discarded, cause a host to respond with a replay of the last
       valid ACK, e.g., [RFC793].  See also the derivation of the SNE,
       which is reconstituted at the receiver using a demonstration
       algorithm that avoids the need for reordering (in Section 6.2).
    e. Security parameter changes require key changes.
       The option should require that the MKT change whenever the
       security parameters change.  This avoids the need for
       coordinating option state during a connection, which is typical
       for TCP options.  This also helps allow "bump in the stack"
       implementations that are not integrated with endpoint TCP
       implementations.
       Parameters change only when a new MKT is used.  See Section 3.
 4. Keying requirements.
    A solution to revising TCP MD5 should support manual keying, and
    should support the use of an external automated key management
    system (e.g., a protocol or other mechanism).

Touch, et al. Standards Track [Page 39] RFC 5925 The TCP Authentication Option June 2010

    Note that TCP-AO does not specify an MKT management system.
    a. Intraconnection rekeying.
       The option should support rekeying during a connection, to
       avoid the impact of long-duration connections.
       This is supported by the use of IDs and multiple MKTs; see
       Section 3.
    b. Efficient rekeying.
       The option should support rekeying during a connection without
       the need to expend undue computational resources.  In
       particular, the options should avoid the need to try multiple
       keys on a given segment.
       This is supported by the use of the KeyID.  See Section 6.1.
    c. Automated and manual keying.
       The option should support both automated and manual keying.
       The use of MKTs allows external automated and manual keying.
       See Section 3.  This capability is enhanced by the generation
       of unique per-connection keys, which enables use of manual MKTs
       with automatically generated traffic keys as noted in Section
       5.2.
    d. Key management agnostic.
       The option should not assume or require a particular key
       management solution.
       This is supported by use of a set of MKTs.  See Section 3.
 5. Expected Constraints
    A solution to revising TCP MD5 should also abide by typical safe
    security practices.
    a. Silent failure.
       Receipt of segments failing authentication must result in no
       visible external action and must not modify internal state, and
       those events should be logged.
       This is supported - see Sections 7.3, 7.4, and 7.5.

Touch, et al. Standards Track [Page 40] RFC 5925 The TCP Authentication Option June 2010

    b. At most one such option per segment.
       Only one authentication option can be permitted per segment.
       This is supported by the protocol requirements - see Section
       2.2.
    c. Outgoing all or none.
       Segments out of a TCP connection are either all authenticated
       or all not authenticated.
       This is supported - see Section 7.4.
    d. Incoming all checked.
       Segments into a TCP connection are always checked to determine
       whether their authentication should be present and valid.
       This is supported - see Section 7.5.
    e. Non-interaction with TCP MD5.
       The use of this option for a given connection should not
       preclude the use of TCP MD5, e.g., for legacy use, for other
       connections.
       This is supported - see Section 8.
    f. "Hard" ICMP discard.
       The option should allow certain ICMPs to be discarded, notably
       Type 3 (destination unreachable), Codes 2-4 (transport protocol
       unreachable, port unreachable, or fragmentation needed and IP
       DF field set), i.e., the ones indicating the failure of the
       endpoint to communicate.
       This is supported - see Section 7.8.
    g. Maintain TCP connection semantics, in which the socket pair
       alone defines a TCP association and all its security
       parameters.
       This is supported - see Sections 3 and 9.

Touch, et al. Standards Track [Page 41] RFC 5925 The TCP Authentication Option June 2010

11. Security Considerations

 Use of TCP-AO, like the use of TCP MD5 or IPsec, will impact host
 performance.  Connections that are known to use TCP-AO can be
 attacked by transmitting segments with invalid MACs.  Attackers would
 need to know only the TCP connection ID and TCP-AO Length value to
 substantially impact the host's processing capacity.  This is similar
 to the susceptibility of IPsec to on-path attacks, where the IP
 addresses and SPI would be visible.  For IPsec, the entire SPI space
 (32 bits) is arbitrary, whereas for routing protocols typically only
 the source port (16 bits) is arbitrary (typically with less than 16
 bits of randomness [La10]).  As a result, it would be easier for an
 off-path attacker to spoof a TCP-AO segment that could cause receiver
 validation effort.  However, we note that between Internet routers,
 both ports could be arbitrary (i.e., determined a priori out of
 band), which would constitute roughly the same off-path antispoofing
 protection of an arbitrary SPI.
 TCP-AO, like TCP MD5, may inhibit connectionless resets.  Such resets
 typically occur after peer crashes, either in response to new
 connection attempts or when data is sent on stale connections; in
 either case, the recovering endpoint may lack the connection key
 required (e.g., if lost during the crash).  This may result in
 timeouts, rather than a more responsive recovery after such a crash.
 Recommendations for mitigating this effect are discussed in Section
 7.7.
 TCP-AO does not include a fast decline capability, e.g., where a SYN-
 ACK is received without an expected TCP-AO and the connection is
 quickly reset or aborted.  Normal TCP operation will retry and
 timeout, which is what should be expected when the intended receiver
 is not capable of the TCP variant required anyway.  Backoff is not
 optimized because it would present an opportunity for attackers on
 the wire to abort authenticated connection attempts by sending
 spoofed SYN-ACKs without TCP-AO.
 TCP-AO is intended to provide similar protections to IPsec, but is
 not intended to replace the use of IPsec or IKE either for more
 robust security or more sophisticated security management.  TCP-AO is
 intended to protect the TCP protocol itself from attacks that TLS,
 sBGP/soBGP, and other data stream protection mechanisms cannot.  Like
 IPsec, TCP-AO does not address the overall issue of ICMP attacks on
 TCP, but does limit the impact of ICMPs, as noted in Section 7.8.
 TCP-AO includes the TCP connection ID (the socket pair) in the MAC
 calculation.  This prevents different concurrent connections using
 the same MKT (for whatever reason) from potentially enabling a
 traffic-crossing attack, in which segments to one socket pair are

Touch, et al. Standards Track [Page 42] RFC 5925 The TCP Authentication Option June 2010

 diverted to attack a different socket pair.  When multiple
 connections use the same MKT, it would be useful to know that
 segments intended for one ID could not be (maliciously or otherwise)
 modified in transit and end up being authenticated for the other ID.
 That requirement would place an additional burden of uniqueness on
 MKTs within endsystems, and potentially across endsystems.  Although
 the resulting attack is low probability, the protection afforded by
 including the received ID warrants its inclusion in the MAC, and does
 not unduly increase the MAC calculation or MKT management.
 The use of any security algorithm can present an opportunity for a
 CPU Denial-of-Service (DoS) attack, where the attacker sends false,
 random segments that the receiver under attack expends substantial
 CPU effort to reject.  In IPsec, such attacks are reduced by the use
 of a large Security Parameter Index (SPI) and Sequence Number fields
 to partly validate segments before CPU cycles are invested validated
 the Integrity Check Value (ICV).  In TCP-AO, the socket pair performs
 most of the function of IPsec's SPI, and IPsec's Sequence Number,
 used to avoid replay attacks, isn't needed due to TCP's Sequence
 Number, which is used to reorder received segments (provided the
 sequence number doesn't wrap around, which is why TCP-AO adds the SNE
 in Section 6.2).  TCP already protects itself from replays of
 authentic segment data as well as authentic explicit TCP control
 (e.g., SYN, FIN, ACK bits) but even authentic replays could affect
 TCP congestion control [Sa99].  TCP-AO does not protect TCP
 congestion control from this last form of attack due to the
 cumbersome nature of layering a windowed security sequence number
 within TCP in addition to TCP's own sequence number; when such
 protection is desired, users are encouraged to apply IPsec instead.
 Further, it is not useful to validate TCP's Sequence Number before
 performing a TCP-AO authentication calculation, because out-of-window
 segments can still cause valid TCP protocol actions (e.g., ACK
 retransmission) [RFC793].  It is similarly not useful to add a
 separate Sequence Number field to TCP-AO, because doing so could
 cause a change in TCP's behavior even when segments are valid.

12. IANA Considerations

 The TCP Authentication Option (TCP-AO) was assigned TCP option 29 by
 IANA action.
 This document defines no new namespaces.
 To specify MAC and KDF algorithms, TCP-AO refers to a separate
 document [RFC5926].

Touch, et al. Standards Track [Page 43] RFC 5925 The TCP Authentication Option June 2010

13. References

13.1. Normative References

 [RFC793]  Postel, J., "Transmission Control Protocol", STD 7, RFC
           793, September 1981.
 [RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
           Communication Layers", STD 3, RFC 1122, October 1989.
 [RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
           Selective Acknowledgment Options", RFC 2018, October 1996.
 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
           Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2385] Heffernan, A., "Protection of BGP Sessions via the TCP MD5
           Signature Option", RFC 2385, August 1998.
 [RFC2403] Madson, C. and R. Glenn, "The Use of HMAC-MD5-96 within ESP
           and AH", RFC 2403, November 1998.
 [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
           (IPv6) Specification", RFC 2460, December 1998.
 [RFC2883] Floyd, S., Mahdavi, J., Mathis, M., and M. Podolsky, "An
           Extension to the Selective Acknowledgement (SACK) Option
           for TCP", RFC 2883, July 2000.
 [RFC3517] Blanton, E., Allman, M., Fall, K., and L. Wang, "A
           Conservative Selective Acknowledgment (SACK)-based Loss
           Recovery Algorithm for TCP", RFC 3517, April 2003.
 [RFC4306] Kaufman, C., Ed., "Internet Key Exchange (IKEv2) Protocol",
           RFC 4306, December 2005.
 [RFC4724] Sangli, S., Chen, E., Fernando, R., Scudder, J., and Y.
           Rekhter, "Graceful Restart Mechanism for BGP", RFC 4724,
           January 2007.
 [RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A Border
           Gateway Protocol 4 (BGP-4)", RFC 4271, January 2006.
 [RFC4781] Rekhter, Y. and R. Aggarwal, "Graceful Restart Mechanism
           for BGP with MPLS", RFC 4781, January 2007.

Touch, et al. Standards Track [Page 44] RFC 5925 The TCP Authentication Option June 2010

 [RFC5926] Lebovitz, G. and E. Rescorla, "Cryptographic Algorithms for
           the TCP Authentication Option (TCP-AO)", RFC 5926, June
           2010.

13.2. Informative References

 [Ba10]    Bashyam, M., Jethanandani, M., and A. Ramaiah
           "Clarification of sender behaviour in persist condition",
           Work in Progress, January 2010.
 [Bo07]    Bonica, R., Weis, B., Viswanathan, S., Lange, A., and O.
           Wheeler, "Authentication for TCP-based Routing and
           Management Protocols", Work in Progress, February 2007.
 [Bo09]    Borman, D., "TCP Options and MSS", Work in Progress, July
           2009.
 [Ed07]    Eddy, W., Ed., Bellovin, S., Touch, J., and R. Bonica,
           "Problem Statement and Requirements for a TCP
           Authentication Option", Work in Progress, July 2007.
 [Go10]    Gont, F., "ICMP Attacks against TCP", Work in Progress,
           March 2010.
 [La10]    Larsen, M. and F. Gont, "Transport Protocol Port
           Randomization Recommendations", Work in Progress, April
           2010.
 [Le09]    Lepinski, M. and S. Kent, "An Infrastructure to Support
           Secure Internet Routing", Work in Progress, October 2009.
 [RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
           April 1992.
 [RFC1323] Jacobson, V., Braden, R., and D. Borman, "TCP Extensions
           for High Performance", RFC 1323, May 1992.
 [RFC1948] Bellovin, S., "Defending Against Sequence Number Attacks",
           RFC 1948, May 1996.
 [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
           Hashing for Message Authentication", RFC 2104, February
           1997.
 [RFC2663] Srisuresh, P. and M. Holdrege, "IP Network Address
           Translator (NAT) Terminology and Considerations", RFC 2663,
           August 1999.

Touch, et al. Standards Track [Page 45] RFC 5925 The TCP Authentication Option June 2010

 [RFC3234] Carpenter, B. and S. Brim, "Middleboxes: Taxonomy and
           Issues", RFC 3234, February 2002.
 [RFC3562] Leech, M., "Key Management Considerations for the TCP MD5
           Signature Option", RFC 3562, July 2003.
 [RFC3947] Kivinen, T., Swander, B., Huttunen, A., and V. Volpe,
           "Negotiation of NAT-Traversal in the IKE", RFC 3947,
           January 2005.
 [RFC4301] Kent, S. and K. Seo, "Security Architecture for the
           Internet Protocol", RFC 4301, December 2005.
 [RFC4808] Bellovin, S., "Key Change Strategies for TCP-MD5", RFC
           4808, March 2007.
 [RFC4953] Touch, J., "Defending TCP Against Spoofing Attacks", RFC
           4953, July 2007.
 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
           (TLS) Protocol Version 1.2", RFC 5246, August 2008.
 [Sa99]    Savage, S., N. Cardwell, D. Wetherall, T. Anderson, "TCP
           Congestion Control with a Misbehaving Receiver", ACM
           Computer Communications Review, V29, N5, pp71-78, October
           1999.
 [SDNS88]  Secure Data Network Systems, "Security Protocol 4 (SP4)",
           Specification SDN.401, Revision 1.2, July 12, 1988.
 [To07]    Touch, J. and A. Mankin, "The TCP Simple Authentication
           Option", Work in Progress, July 2007.
 [To10]    Touch, J., "A TCP Authentication Option NAT Extension",
           Work in Progress, January 2010.
 [Wa05]    Wang, X., H. Yu, "How to break MD5 and other hash
           functions", Proc. IACR Eurocrypt 2005, Denmark, pp.19-35.
 [We05]    Weis, B., Appanna, C., McGrew, D., and A. Ramaiah, "TCP
           Message Authentication Code Option", Work in Progress,
           December 2005.

Touch, et al. Standards Track [Page 46] RFC 5925 The TCP Authentication Option June 2010

14. Acknowledgments

 This document evolved as the result of collaboration of the TCP
 Authentication Design team (tcp-auth-dt), whose members were
 (alphabetically): Mark Allman, Steve Bellovin, Ron Bonica, Wes Eddy,
 Lars Eggert, Charlie Kaufman, Andrew Lange, Allison Mankin, Sandy
 Murphy, Joe Touch, Sriram Viswanathan, Brian Weis, and Magnus
 Westerlund.  The text of this document is derived from a proposal by
 Joe Touch and Allison Mankin [To07] (originally from June 2006),
 which was both inspired by and intended as a counterproposal to the
 revisions to TCP MD5 suggested in a document by Ron Bonica, Brian
 Weis, Sriran Viswanathan, Andrew Lange, and Owen Wheeler [Bo07]
 (originally from September 2005) and in a document by Brian Weis
 [We05].
 Russ Housley suggested L4/application layer management of the master
 key tuples.  Steve Bellovin motivated the KeyID field.  Eric Rescorla
 suggested the use of TCP's Initial Sequence Numbers (ISNs) in the
 traffic key computation and SNEs to avoid replay attacks, and Brian
 Weis extended the computation to incorporate the entire connection ID
 and provided the details of the traffic key computation.  Mark
 Allman, Wes Eddy, Lars Eggert, Ted Faber, Russ Housley, Gregory
 Lebovitz, Tim Polk, Eric Rescorla, Joe Touch, and Brian Weis
 developed the master key coordination mechanism.
 Alfred Hoenes, Charlie Kaufman, Adam Langley, and numerous other
 members of the TCPM WG also provided substantial feedback on this
 document.
 This document was originally prepared using 2-Word-v2.0.template.dot.

Touch, et al. Standards Track [Page 47] RFC 5925 The TCP Authentication Option June 2010

Authors' Addresses

 Joe Touch
 USC/ISI
 4676 Admiralty Way
 Marina del Rey, CA 90292-6695
 U.S.A.
 Phone: +1 (310) 448-9151
 EMail: touch@isi.edu
 URL:   http://www.isi.edu/touch
 Allison Mankin
 Johns Hopkins Univ.
 Baltimore, MD
 U.S.A.
 Phone: 1 301 728 7199
 EMail: mankin@psg.com
 URL:   http://www.psg.com/~mankin/
 Ronald P. Bonica
 Juniper Networks
 2251 Corporate Park Drive
 Herndon, VA  20171
 U.S.A.
 EMail: rbonica@juniper.net

Touch, et al. Standards Track [Page 48]

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