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Versions: (RFC 2385) 00 01

 INTERNET-DRAFT                                            Andy Heffernan
 <draft-ietf-idr-rfc2385bis-00.txt>                      Juniper Networks
                                                             January 2002
 
 
       Protection of BGP Sessions via the TCP MD5 Signature Option
 
 Status of this Memo
 
 
    This document is an Internet Draft.  Internet Drafts are working
    documents of the Internet Engineering Task Force (IETF), its Areas,
    and its Working Groups.  Note that other groups may also distribute
    working documents as Internet Drafts.
 
    This document is an Internet-Draft and is subject to all
    provisions of Section 10 of RFC2026.
 
    Internet Drafts are draft documents valid for a maximum of six
    months.  Internet Drafts may be updated, replaced, or obsoleted by
    other documents at any time.  It is not appropriate to use Internet
    Drafts as reference material or to cite them other than as a "working
    draft" or "work in progress."
 
    Please check the I-D abstract listing contained in each Internet
    Draft directory to learn the current status of this or any Internet
    Draft.
 
 
 Abstract
 
    This memo describes a TCP extension to enhance security for BGP.  It
    defines a new TCP option for carrying an MD5 [RFC1321] digest in a
    TCP segment.  This digest acts like a signature for that segment,
    incorporating information known only to the connection end points.
    Since BGP uses TCP as its transport, using this option in the way
    described in this paper significantly reduces the danger from certain
    security attacks on BGP.
 
 
 1.0  Introduction
 
    The primary motivation for this option is to allow BGP to protect
    itself against the introduction of spoofed TCP segments into the
    connection stream.  Of particular concern are TCP resets.
 
    To spoof a connection using the scheme described in this paper, an
    attacker would not only have to guess TCP sequence numbers, but would
    also have had to obtain the password included in the MD5 digest.
    This password never appears in the connection stream, and the actual
    form of the password is up to the application.  It could even change
 
 
 
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    during the lifetime of a particular connection so long as this change
    was synchronized on both ends (although retransmission can become
    problematical in some TCP implementations with changing passwords).
 
    Finally, there is no negotiation for the use of this option in a
    connection, rather it is purely a matter of site policy whether or
    not its connections use the option.
 
 
 2.0  Proposal
 
    Every segment sent on a TCP connection to be protected against
    spoofing will contain the 16-byte MD5 digest produced by applying the
    MD5 algorithm to these items in the following order:
 
        1. the TCP pseudo-header
        2. the TCP header, excluding options (but with a correct Data Offset
           field), and assuming a checksum of zero
        3. the TCP segment data (if any)
        4. an independently-specified key or password, known to both TCPs
           and presumably connection-specific
 
    For IPv4, the pseudo-header is as described in RFC 793, namely the
    32-bit source IP address, 32-bit destination IP address, zero-
    extended protocol number (to form 16 bits), and 16-bit segment
    length.  Note that this includes use of IPv4 via IPv4-mapped IPv6
    addresses, in which case the source and destination IP addresses are
    from the IPv4 portions of the IPv6 source and destination addresses,
    respectively.
 
    For IPv6, the pseudo-header is as described in RFC 2460, namely the
    128-bit source IPv6 address, 128-bit destination IPv6 address, zero-
    extended next header value (to form 32 bits), and 32-bit segment
    length.
 
    For any other network protocol, the pseudo-header is as described in
    the document that defines how upper-level protocols like TCP compute
    their checksums.
 
    The header and pseudo-header are in network byte order.  The nature
    of the key is deliberately left unspecified, but it must be known by
    both ends of the connection.  A particular TCP implementation will
    determine what the application may specify as the key.
 
    Upon receiving a signed segment, the receiver must validate it by
    calculating its own digest from the same data (using its own key) and
    comparing the two digest.  A failing comparison must result in the
    segment being dropped and must not produce any response back to the
 
 
 
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    sender.  Logging the failure is probably advisable.
 
    Unlike other TCP extensions (e.g., the Window Scale option
    [RFC1323]), the absence of the option in the SYN,ACK segment must not
    cause the sender to disable its sending of signatures.  This
    negotiation is typically done to prevent some TCP implementations
    from misbehaving upon receiving options in non-SYN segments.  This is
    not a problem for this option, since the SYN,ACK sent during
    connection negotiation will not be signed and will thus be ignored.
    The connection will never be made, and non-SYN segments with options
    will never be sent.  More importantly, the sending of signatures must
    be under the complete control of the application, not at the mercy of
    the remote host not understanding the option.
 
 
 3.0  Syntax
 
    The proposed option has the following format:
 
              +---------+---------+-------------------+
              | Kind=19 |Length=18|   MD5 digest...   |
              +---------+---------+-------------------+
              |                                       |
              +---------------------------------------+
              |                                       |
              +---------------------------------------+
              |                                       |
              +-------------------+-------------------+
              |                   |
              +-------------------+
 
    The MD5 digest is always 16 bytes in length, and the option would
    appear in every segment of a connection.
 
 
 4.0  Some Implications
 
 4.1  Connectionless Resets
 
    A connectionless reset will be ignored by the receiver of the reset,
    since the originator of that reset does not know the key, and so
    cannot generate the proper signature for the segment.  This means,
    for example, that connection attempts by a TCP which is generating
    signatures to a port with no listener will time out instead of being
    refused.  Similarly, resets generated by a TCP in response to
    segments sent on a stale connection will also be ignored.
    Operationally this can be a problem since resets help BGP recover
    quickly from peer crashes.
 
 
 
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 4.2  Performance
 
    The performance hit in calculating digests may inhibit the use of
    this option.  Some measurements of a sample implementation showed
    that on a 100 MHz R4600, generating a signature for simple ACK
    segment took an average of 0.0268 ms, while generating a signature
    for a data segment carrying 4096 bytes of data took 0.8776 ms on
    average.  These times would be applied to both the input and output
    paths, with the input path also bearing the cost of a 16-byte
    compare.
 
 
 4.3  TCP Header Size
 
    As with other options that are added to every segment, the size of
    the MD5 option must be factored into the MSS offered to the other
    side during connection negotiation.  Specifically, the size of the
    header to subtract from the MTU (whether it is the MTU of the
    outgoing interface or IP's minimal MTU of 576 bytes) is now at least
    18 bytes larger.
 
    The total header size is also an issue.  The TCP header specifies
    where segment data starts with a 4-bit field which gives the total
    size of the header (including options) in 32-byte words.  This means
    that the total size of the header plus option must be less than or
    equal to 60 bytes -- this leaves 40 bytes for options.
 
    As a concrete example, 4.4BSD defaults to sending window-scaling and
    timestamp information for connections it initiates.  The most loaded
    segment will be the initial SYN packet to start the connection.  With
    MD5 signatures, the SYN packet will contain the following:
 
        -- 4 bytes MSS option
        -- 4 bytes window scale option (3 bytes padded to 4 in 4.4BSD)
        -- 12 bytes for timestamp (4.4BSD pads the option as recommended
           in RFC 1323 Appendix A)
        -- 18 bytes for MD5 digest
        -- 2 bytes for end-of-option-list, to pad to a 32-bit boundary.
 
    This sums to 40 bytes, which just makes it.  (Note that other padding
    schemes are possible which would reduce the aggregate size.)
 
 4.4 Key configuration
 
    It should be noted that the key configuration mechanism of routers
    may restrict the possible keys that may be used between peers.  It is
    strongly recommended that an implementation be able to support at
    minimum a key composed of a string of printable ASCII of 80 bytes or
 
 
 
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    less, as this is current practice.
 
 
 5.0 Security Considerations
 
    This document defines a weak but currently practiced security
    mechanism for BGP.  It is anticipated that future work will provide
    different stronger mechanisms for dealing with these issues.
 
 5.1 MD5 as a hashing algorithm
 
    The MD5 algorithm has been found to be vulnerable to collision search
    attacks [Dobb], and is considered by some to be insufficiently strong
    for this type of application.
 
    This document specifies the MD5 algorithm, however, since the option
    has already been deployed operationally, and there is no "algorithm
    type" field defined to allow an upgrade using the same option number.
    The original draft did not specify a type field since this would
    require at least one more byte, and it was felt at the time that
    taking 19 bytes for the complete option (which would probably be
    padded to 20 bytes in TCP implementations) would be too much of a
    waste of the already limited option space.
 
    This does not prevent the deployment of another similar option which
    uses another hashing algorithm (like SHA-1).  Also, if most
    implementations pad the 18 byte option as defined to 20 bytes anyway,
    it would be just as well to define a new option which contains an
    algorithm type field.  This would need to be addressed in another
    draft, however.
 
 5.2 Signature coverage
 
    A further weakness exists due to the exclusion of option data from
    the signature.  This decision was made (as best as can be recalled)
    to simplify the protocol definition and implementation, but serves to
    leave a connection vulnerable since option data can be rewritten
    without detection.
 
 
 
 6.0  References
 
    [RFC793] Postel, J., "Transmission Control Protocol," RFC 793,
    September 1981.
 
    [RFC2460] Deering, S., and Hinden, R., "Internet Protocol,
     Version 6 (IPv6) Specification," RFC 2460, December 1998.
 
 
 
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    [RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm," RFC 1321,
    MIT Laboratory for Computer Science, April 1992.
 
    [RFC1323] Jacobson, V., Braden, R., and Borman, D., "TCP Extensions
    for High Performance", RFC 1323, LBL, USC/Information Sciences
    Institute, Cray Research, May 1992.
 
    [Dobb] Dobbertin, H., "The Status of MD5 After a Recent Attack", RSA
    Labs' CryptoBytes, Vol. 2 No. 2, Summer 1996.
    http://www.rsa.com/rsalabs/pubs/cryptobytes.html
 
 
 Author's Address
 
    Andy Heffernan
    Juniper Networks
    1194 N. Mathilda Avenue
    Sunnyvale, CA  94089  USA
 
    Phone:  +1 408 745-2037
    Email:  ahh@juniper.net
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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