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TCPM Working Group R. Bonica
Internet-Draft Juniper Networks
Expires: August 17, 2007 B. Weis
S. Viswanathan
Cisco Systems
A. Lange
Alcatel
O. Wheeler
BT
February 13, 2007
Authentication for TCP-based Routing and Management Protocols
draft-bonica-tcp-auth-06
Status of this Memo
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Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
This memo describes a TCP extension that enhances security for BGP,
LDP and other TCP-based protocols. It is intended for applications
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where secure administrative access to both the end-points of the TCP
connection is normally available. TCP peers can use this extension
to authenticate messages passed between one another.
The strategy described herein improves upon current practice, which
is described in RFC 2385. Using this new strategy, TCP peers can
update authentication keys during the lifetime of a TCP connection.
TCP peers can also use stronger authentication algorithms to
authenticate routing messages.
Table of Contents
1. Conventions Used In This Document . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Proposal . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Applications . . . . . . . . . . . . . . . . . . . . . . . . . 6
6. TCP Enhanced Authentication Option . . . . . . . . . . . . . . 6
7. Key Attributes . . . . . . . . . . . . . . . . . . . . . . . . 8
8. MAC Calculation . . . . . . . . . . . . . . . . . . . . . . . 8
9. Authentication Algorithms . . . . . . . . . . . . . . . . . . 9
10. Migration Issues . . . . . . . . . . . . . . . . . . . . . . . 10
11. Future Enhancements . . . . . . . . . . . . . . . . . . . . . 10
12. Implications . . . . . . . . . . . . . . . . . . . . . . . . . 11
12.1. Connectionless Resets . . . . . . . . . . . . . . . . . . 11
12.2. Performance . . . . . . . . . . . . . . . . . . . . . . . 11
12.3. TCP Header Size . . . . . . . . . . . . . . . . . . . . . 11
12.4. Backwards Compatibility . . . . . . . . . . . . . . . . . 12
12.5. ICMP-based attacks . . . . . . . . . . . . . . . . . . . 12
12.6. Relationship With TLS . . . . . . . . . . . . . . . . . . 12
13. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 12
14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 13
15. Security Considerations . . . . . . . . . . . . . . . . . . . 13
16. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
17. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
17.1. Normative References . . . . . . . . . . . . . . . . . . 14
17.2. Informative References . . . . . . . . . . . . . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 16
Intellectual Property and Copyright Statements . . . . . . . . . . 17
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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 RFC2119 [1].
2. Terminology
The following terms are used in this document:
key - A data structure used to authenticate TCP segments. One or
more keys can be associated with a TCP connection. Each key
contains an identifier, a shared secret, an algorithm identifier,
an "active flag" and an "eligible flag".
key set - A set of keys that is associated with a TCP connection.
A single key set can be associated with multiple TCP connections.
Each key within a key set contains an identifier that is unique
within the key set.
active key - Each key set contains exactly one active key. The
sending TCP station uses the shared secret from its active key to
generate a Message Authentication Code (MAC) for outgoing TCP
segments. The "active flag" on a key indicates whether a
particular key is active.
eligible key - Each key set contains zero or more eligible keys.
The receiving TCP station uses the shared secret from a key to
authenticate an incoming TCP segment only if that key is eligible.
The "eligible flag" on a key indicates whether a particular key is
eligible.
3. Introduction
RFC 2385 [8] proposes a mechanism that authenticates TCP [2] sessions
by including a message authentication code (MAC) in each TCP header.
Authentication coverage includes the following fields:
- the TCP pseudo-header
- the TCP header, excluding options, and assuming a checksum of
zero
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- the TCP segment data (if any)
To spoof a connection using the scheme described above, an attacker
would not only have to guess TCP sequence numbers, but would also
have to obtain the key that was used to calculate the MAC. This key
never appears in the connection stream.
RFC 3562 [9] addresses key management considerations regarding the
TCP MD5 Signature Option. Based upon the strength of the MD5 [10]
hashing algorithm, RFC 3562 recommends that keys be changed at least
every 90 days.
Unfortunately, the strategy described in RFC 2385 permits keys to be
changed during the lifetime of a TCP connection only so long as the
change is synchronized at both ends. This limitation has proven to
be a significant deterrent to the effective deployment of the TCP MD5
Signature Option. This memo addresses that limitation.
Also, the MD5 algorithm does not now provide a sufficient level of
security, and recently published attacks motivate its replacement.
In addition, the keyed hash MAC construction used by RFC 2385 has
serious cryptographic weaknesses. An attacker who can find a
collision in the underlying hash function can forge a MAC using a
simple chosen-message attack [11]. This memo makes use of MAC
algorithms that do not have these weaknesses. It also provides a
mechanism to add additional algorithms as the state-of-the-art in
cryptography progresses.
4. Proposal
This memo proposes a TCP Enhanced Authentication Option that is used
as follows:
Network operators associate a set of keys with each protected TCP
connection. Each key contains an identifier that is unique to the
key set, a shared secret, an algorithm identifier, an "active flag"
and an "eligible flag".
Whenever TCP generates a segment, it searches the associated key set
for its active key. Each key set MUST have exactly one active key
that is identified as such by having its "active flag" set.
Having identified the active key, TCP executes the following
sequence:
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- append the TCP Enhanced Authentication Option to the TCP header.
(See Section 6 of this document for details regarding the TCP
Enhanced Authentication Option.)
- update the TCP Enhanced Authentication Option to include the
active key's unique identifier
- calculate a Message Authentication Code (MAC) using the shared
secret from the active key. (See Section 8 of this document for
MAC calculation details.)
- update the TCP Enhanced Authentication Option to include the MAC
that was calculated above
- calculate and update the TCP checksum
- forward the segment to a TCP peer.
The receiving TCP associates the inbound TCP segment with a local key
set based upon source IP address, destination IP address, source port
and destination port. It then searches the associated key set for a
key whose identifier matches that which was specified by the incoming
segment option.
If TCP finds such a key and if that key's "eligible flag" is set, TCP
continues processing. If no matching eligible key is found then TCP
MUST declare an authentication failure and discard the segment.
TCP verifies that the algorithm used to produce the MAC is correct.
The verification is done by comparing the algorithm attribute
associated with the key with the algorithm id listed in the segment
option. If the algorithm identifiers do not match, then the MAC
calculation will fail. TCP MUST declare an authentication error and
discard the segment.
TCP uses the shared secret from the key to calculate a MAC. TCP will
accept the segment if the calculated MAC matches the MAC specified by
the inbound segment. Otherwise, TCP MUST declare an authentication
failure and discard the segment.
TCP MUST also declare an authentication failure and discard a segment
if the segment is received from a connection that is associated with
a key set and the segment does not include the TCP Enhanced
Authentication Option.
To help protect against denial of service attacks it is RECOMMENDED
that the inbound TCP segment is validated against the normal TCP
criteria (e.g. that the segment sequence number is within the current
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receive window) prior to the MAC being calculated. Inbound TCP
segments on connections requiring the Enhanced Authentication Option
MUST NOT cause the current TCP state variables for that connection to
be updated unless the MAC has been verified as correct.
An authentication failure MUST NOT produce any response back to the
sender. Routers SHOULD log authentication failures.
Unlike other TCP extensions (e.g., the Window Scale option [12]), the
absence of the option in the SYN,ACK segment must not cause the
sender to disable its sending of authentication data. 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 authentication
data must be under the complete control of the application, not at
the mercy of the remote host not understanding the option.
5. Applications
The mechanisms described in this memo are intended for use with
applications that manipulate key set contents. An application might
toggle the active and eligible flags based upons system time,
connection duration or any other external event.
6. TCP Enhanced Authentication Option
Figure 1 depicts the TCP Enhanced Authentication Option format.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Kind | Length |T|K| Alg ID |Res| Key ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Authentication Data |
| // |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Option Syntax
Kind: 8 bits
The Kind field identifies the TCP Enhanced Authentication Option.
This value will be assigned by IANA.
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Length: 8 bits
The Length field specifies the length of the TCP Enhanced
Authentication Option, in octets. This count includes two octets
representing the Kind and Length fields.
The valid range for this field is from 4 to 40 octets, inclusive.
For all algorithms specified in this memo the value will be 16
octets.
T-Bit: 1 bit
The T-bit specifies whether TCP Options were omitted from the TCP
header for the purpose of MAC calculation. A value of 1 indicates
that all TCP options other than the Extended Authentication Option
were omitted. A value of 0 indicates that TCP options were included.
The default value is 0. (See Section 8 of this document for MAC
calculation details.)
K-Bit: 1 bit
This bit is reserved for future enhancement. Its value MUST be equal
to zero. See Section 11 for details.
Alg ID: 6 bits
The Alg ID field identifies the MAC algorithm. See Section 9 for
permissible values.
Res: 2 bits
These bits are reserved. They MUST be set to zero.
Key ID: 6 bits
The Key ID field identifies the key that was used to generate the
message digest.
Authentication Data: Variable length
The Authentication Data field contains data that is used to
authenticate the TCP segment. This data includes, but need not be
restricted to, a MAC. The length and format of the Authentication
Data Field can be derived from the Alg ID. See Section 9 for
details.
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7. Key Attributes
A key set is a set of keys, where each key is {L[i], S[i], A[i],
E[i]}:
i Key identifier, integer (0...63)
L[i] Authentication algorithm to use with key[i].
S[i] Shared secret to use with key[i].
A[i] Active flag to use with key[i]
E[i] Eligible flag to use with key[i]
For the purpose of this document, key[i] is defined as the key whose
identifer is equal to i.
A list of values for L[i] is provided in Section 9 of this document.
The format of S[i] depends upon L[i]. Also see Section 9 for
details.
L[i] and S[i] MUST be configured symmetrically on TCP peers. That
is, if key[i] is configured on two peer systems, L[i] and S[i] must
be configured identically on each system.
For each key set, exactly one element MUST have A[i] set. Zero or
more elements MAY have E[i] set.
In general, network operators should avoid reusing shared secrets.
The degree to which an operator can reuse keys is defined by local
security policy.
During the lifetime of a TCP connection, network operators may add
and delete keys from the key set. However, the network operator must
ensure that the active key is always configured on both TCP
endpoints.
8. MAC Calculation
The sending TCP calculates a MAC by applying the authentication
algorithm from the active key to the following items in the order
that they are listed:
- the TCP pseudo-header
- the TCP header, assuming a checksum of zero. (See below for a
discussion of TCP Options within the the TCP header.)
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- the TCP segment data (if any)
For IPv4, the pseudo-header is described in RFC 793 [2]. It includes
the 32-bit source IP address, the 32-bit destination IP address, the
zero-extended protocol number (to form 16 bits), and the 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 described in RFC 2460 [3]. It
includes the 128-bit source IPv6 address, the 128-bit destination
IPv6 address, the zero-extended next header value (to form 32 bits),
and the 32-bit segment length.
The header and pseudo-header are in network byte order.
By default, for the purpose of MAC calculation, the TCP header
includes all TCP options, including the TCP Enhanced Authentication
Option with its Authentication Data Field (i.e., MAC) set to zero.
However, TCP implementations MAY omit all other TCP options from the
MAC calculation; the TCP Enhanced Authentication Option itself must
still be included in the calculation, as described above. When
implementation do so, they MUST set the T-bit in the TCP Enhanced
Authentication Option.
The receiving TCP calculates the MAC in a manner identical to the
sending TCP. However, it MUST examine the T-bit from the incoming
TCP Enhanced Authentication Option to determine whether incoming TCP
Options should be included in the MAC calculation.
9. Authentication Algorithms
The following MAC Algorithms are suitable for use with this option.
- AES-128-CMAC-96. AES with a 128-bit key in the CMAC mode of
operation [4] [5]. When this algorithm is used, implementations
MUST specify a value of 1 (in binary, 000001) in the TCP Enhanced
Authentication Option Alg ID field. Also, the Authentication Data
field must contain exactly 96 bits representing the MAC, truncated
to that length, with the high-order bit first. The
AES-128-CMAC-96 algorithm MUST be implemented for an
implementation to conform to this specification.
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- HMAC-SHA-1-96. SHA-1 with a 160-bit key in the HMAC mode of
operation [6]. When this algorithm is used, implementations MUST
specify a value of 2 (in binary, 000010) in the TCP Enhanced
Authentication Option ALG ID field. Also, the Authentication Data
field must contain exactly 96 bits representing the MAC, truncated
to that length, with the high-order bit first. This algorithm MAY
be implemented for an implementation to conform to this
specification.
The above algorithms are expected to safe to use in this application
for many years. New algorithms may be added to this list as
necessary, but it is important that they be properly vetted by the
cryptographic community. To this end, the above algorithms are
described in a list maintained by IANA, and the algorithm identifier
associated with the algorithm is placed in the TCP Authentication
Option header. Other algorithms may also be defined by an
implementation using a Private Use identifier, but the suitability of
those algorithms when used with the TCP Extended Authentication
option is not assured.
Implementations MUST present a management interface through which the
user can specify any member of the key space. For example, if the
key contains 128 bits, the command line interface might accept this
value as a string of exactly 32 hexadecimal digits, with each
hexadecimal digit representing 4 bits of the shared secret.
Implementations MAY employ authentication algorithms not listed
above.
10. Migration Issues
Either the TCP Enhanced Authentication Option or RFC 2385 may be
applied to a TCP segment, but the two options SHOULD NOT be present
in the same TCP segment. An implementation MAY support both options
for a particular TCP session during migration from RFC 2385, but they
MUST use different keys so as not to weaken the security of the TCP
Enhanced Authentication Option (see the Security Considerations
section for details). A receiver SHOULD accept either option. A
sender MAY choose to continue sending RFC 2385 options until it has
evidence that the other TCP endpoint shows use of the TCP Enhanced
Authentication Option, in which case it migrates to the TCP Enhanced
Authentication Option.
11. Future Enhancements
In the future, the TCP Enhanced Authentication Option will be
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enhanced to support automated session key distribution. The K-bit is
reserved for the purpose of indicating that session key distribution
extensions are present. These extensions are beyond the scope of
this memo.
12. Implications
12.1. Connectionless Resets
A connectionless reset will be ignored by the receiver of the reset
if the originator of that reset does not know the key and therefore
cannot generate the proper authentication data for the segment. This
means, for example, that connection attempts by a TCP which is
generating authentication data 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 some
protocols recover quickly from peer crashes.
12.2. Performance
The performance hit in calculating digests may inhibit the use of
this option. Performance will vary depending upon processor type,
authentication algorithm, packet size and number of MAC calculations
per second.
12.3. TCP Header Size
As with other options that are added to every segment, the size of
the TCP Enhanced Authentication 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 octets) is now increased by the size of the TCP Enhanced
Authentication Option.
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 options must be less than or
equal to 60 octets. This leaves 40 octets for options.
As a concrete example, assume that an implementation 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 the TCP Enhanced Authentication Option
using AES-128-CMAC-96, the SYN packet will contain the following:
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-- 4 octets MSS option
-- 4 octets window scale option (3 octets padded to 4 in many
implementations)
-- 12 octets for timestamp
-- 16 octets for the TCP Enhanced Authentication Option
This sums to 36 octets, leaving only four octets for future
expansion.
12.4. Backwards Compatibility
On any particular TCP connection, use of the TCP Enhanced
Authentication Option precludes use of the TCP MD5 Signature Option.
However, use of the TCP Enhanced Authentication Option on one
connection does not preclude the use of the TCP MD5 Signature Option
on another connection by the same system.
12.5. ICMP-based attacks
The mechanism described in this document does not provide significant
protection against ICMP-based attacks [13].
12.6. Relationship With TLS
The Transport Layer Security protocol (TLS) [14] provides
confidentiality and message authentication to TCP connections.
However, TLS works above the TCP layer, and does not protect the TCP
connection itself. In contrast, the TCP Authentication Option
provides protection against attacks on the TCP layer, such as
connection reset attacks.
13. Contributors
The following individuals contributed to this document:
Chandrashekhar Appanna (achandra@cisco.com)
Andy Heffernan (ahh@juniper.net)
Kapil Jain (kapil@juniper.net)
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David McGrew (mcgrew@cisco.com)
Satish Mynam (mynam@cisco.com)
Anantha Ramaiah (ananth@cisco.com)
14. Acknowledgments
Thanks to Steve Bellovin, Ted Faber, Ross Callon, Joe Touch and Ran
Atkinson for their comments regarding this draft.
15. Security Considerations
This proposal describes a strong authentication method for
authenticating TCP segments. It defines the use of cryptographic MAC
algorithms, which are considered state-of-the-art. As such, their
expected lifetime of usefulness extends for several years. But
cryptographic algorithms have an effective lifetime, depending on
advancing processor speed and cryptographic research. This proposal
provides for the future addition of new MAC algorithms as they are
needed.
Management of RFC 2385 keys has been a significant operational
problem, both in terms of key synchronization and key selection.
Current guidance [9] warns against sharing RFC 2385 keys between
systems, and recommends changing keys according to a schedule. The
same general operational issues are relevant for the management of
MAC keys.
Because the TCP Authentication Option relies on manual configuration,
it is possible that misconfigurations will occur. We review the
scenarios and describe their impact on security.
When multiple devices are configured with the same key, it is
possible that one or more of the devices is configured to use the
wrong MAC algorithm. If the misconfigured device is using a MAC that
is significantly weaker than that used by the correctly configured
devices, where the weakness allows an attacker to recover the MAC
key, the misconfiguration reduces the security of the properly
configured devices. An attacker who can recover the key through
cryptanalysis of the weaker algorithm can use that information to
attack the stronger algorithm. For this reason, implementations
SHOULD verify the length of the keys entered into the system, and
reject keys that are too short. The extent of vulnerability will
also be reduced when the receiver discards TCP segments due to a MAC
Algorithm ID mismatch (i.e., the MAC Algorithm ID field in the TCP
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segment and the MAC Algorithm ID associated with the key do not
match). When this event is detected, it SHOULD provide that
information to an administrator, e.g. through logging or a management
interface. This attack is not applicable to AES-CMAC or HMAC, since
neither of those MACs is vulnerable to a key recovery attack.
When one or more devices are configured with a particular key, it is
possible that another device is configured with a slightly different
key, due to a typographical error. For example, the two keys might
differ only in a single hexadecimal digit. Message authentication
codes that are vulnerable whenever two related keys are used could be
vulnerable in this scenario. In order to protect against this
potential vulnerability, it is RECOMMENDED that no MACs with such
vulnerabilities be used. Neither AES-CMAC nor HMAC have such a
vulnerability.
16. IANA Considerations
The terms "Standards Action" and "Private Use" in this section
indicate the polices described for these terms in [7].
A new TCP Option Kind value must be defined in the IANA TCP
Parameters registry.
The option header contains an 8-bit ALG ID, for which IANA is to
create and maintain a registry entitled "MAC Algorithm IDs". This
document defines the following message authentication code types:
MAC Algorithm ID Value
---------------- -----
RESERVED 0
AES-128-CMAC-96 1
HMAC-SHA-1-96 2
Standards Action 3-47
Private Use 48-63
Note to RFC Editor: this section may be removed on publication as an
RFC.
17. References
17.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
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[2] Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
September 1981.
[3] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
Specification", RFC 2460, December 1998.
[4] National Institute of Standards and Technology, "Recommendation
for Block Cipher Modes of Operation: The CMAC Mode for
Authentication", FIPS PUB 800-38B, May 2005, <http://
csrc.nist.gov/publications/nistpubs/800-38B/SP_800-38B.pdf>.
[5] Song, JH., Poovendran, R., Lee, J., and T. Iwata, "The AES-CMAC
Algorithm", RFC 4493, June 2006.
[6] National Institute of Standards and Technology, "The Keyed-Hash
Message Authentication Code (HMAC)", FIPS PUB 198, March 2002,
<http://csrc.nist.gov/publications/fips/fips198/fips-198a.pdf>.
[7] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
17.2. Informative References
[8] Heffernan, A., "Protection of BGP Sessions via the TCP MD5
Signature Option", RFC 2385, August 1998.
[9] Leech, M., "Key Management Considerations for the TCP MD5
Signature Option", RFC 3562, July 2003.
[10] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992.
[11] Bellare, M., Canetti, R., and H. Krawczyk, "Keying Hash
Functions for Message Authentication", Proceedings of
Crypto'96 , LNCS 1109, pp. 1-15., June 1996, <An extended
version of this paper is available at
http://www.research.ibm.com/security/bck2.ps>.
[12] Jacobson, V., Braden, B., and D. Borman, "TCP Extensions for
High Performance", RFC 1323, May 1992.
[13] Gont, F., "ICMP attacks against TCP",
draft-ietf-tcpm-icmp-attacks-01 (work in progress),
October 2006.
[14] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999.
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Authors' Addresses
Ronald P. Bonica
Juniper Networks
2251 Corporate Park Drive
Herndon, VA 20171
US
Email: rbonica@juniper.net
Brian Weis
Cisco Systems
170 W. Tasman Drive
San Jose, CA 95134-1706
US
Email: bew@cisco.com
Sriram Viswanathan
Cisco Systems
170 W. Tasman Drive
San Jose, CA 95134
US
Email: sriram_v@cisco.com
Andrew Lange
Alcatel
710 E. Middlefield Road
Mountain View, CA 94043
US
Email: andrew.lange@alcatel.com
Owen N. Wheeler
British Telecommunications plc
Adastral Park
Martlesham Heath
IPSWICH, Suffolk IP5 3RE
GB
Email: owen.wheeler@bt.com
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Internet-Draft TCP Enhanced Authentication Option February 2007
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