TCPM Working Group R. Bonica
Internet-Draft A. Heffernan
Expires: March 20, 2006 Juniper Networks
September 16, 2005
Authentication for TCP-based Routing and Management Protocols
draft-bonica-tcp-auth-00
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Copyright (C) The Internet Society (2005).
Abstract
This memo extends RFC 2385 to support time-based key rollover and
multiple hashing algorithms. Operators can use the time-based key
rollover feature to in order to periodically update the key that is
used to create a message digest for each TCP segment. Operators may
also wish to select the hashing algorithm used to create the message
digest depending upon the perceived threat level and the
computational capabilities of their hardware platforms.
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Table of Contents
1. Conventions Used In This Document . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Proposal . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Implications . . . . . . . . . . . . . . . . . . . . . . . . . 7
5.1. Connectionless Resets . . . . . . . . . . . . . . . . . . 7
5.2. Performance . . . . . . . . . . . . . . . . . . . . . . . 7
5.3. TCP Header Size . . . . . . . . . . . . . . . . . . . . . 7
5.4. Key Configuration . . . . . . . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6.1. Signature Coverage . . . . . . . . . . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
8. Normative References . . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
Intellectual Property and Copyright Statements . . . . . . . . . . 11
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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. Introduction
RFC 2385 [2] proposes a mechanism that secures BGP [3] sessions using
MD5 [4] authentication. Specifically, RFC 2385 proposes a TCP MD5
Signature Option that can be appended to each TCP header. The MD5
Signature Option contains a 16-byte MD5 digest that serves as a
signature for the TCP segment. The MD5 digest is calculated over the
following fields:
- the TCP pseudo-header
- the TCP header, excluding options, and assuming a checksum of
zero
- the TCP segment data (if any)
- an independently-specified key or password, known to both TCPs
and presumably connection-specific
To spoof a connection using the scheme described above, 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 determined by the application.
RFC 3562 [5] addresses key management considerations regarding the
TCP MD5 Signature Option. Specifically, based upon the strength of
the MD5 hashing algorithm, RFC 3562 recommends that keys SHOULD be
changed at least every 90 days.
Unfortunately, the strategy described in RFC 2385 permits keys to be
changed during the lifetime of a particular TCP connection only so
long as the change is synchronized at both ends. This limitation has
proven to be a significant deterrent to the deployment of the TCP MD5
Signature Option for BGP.
This document addresses the above mentioned limitation. It also
extends the strategy proposed in RFC 2385 to allow for other hashing
algorithms besides MD5.
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3. Proposal
This document proposes a new TCP option that is used as follows.
Operators configure a list of authentication elements for each
protected TCP connection. Each authentication element includes the
following data items:
- an authentication element identifier (integer [0..255])
- a key
- a hash algorithm
- a start time
Each authentication element in the list must include a unique element
identifier and a unique start time. Whenever TCP generates a
segment, it selects an authentication element from the list based on
the following criteria:
- the start time is less than or equal to the current time
- the start time is greater than that of all other elements in the
list whose start time is less than the current time.
TCP then inserts the new option and calculates a message digest. It
calculates a message digest by applying the hash algorithm from the
selected authentication element to the following items in the order
that they are listed:
- the TCP pseudo-header
- the TCP header, excluding options (but with a correct Data
Offset field), and assuming a checksum of zero
- the TCP segment data (if any)
- the key specified by the selected authentication element
For IPv4, the pseudo-header is described in RFC 793 [6]. 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.
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For IPv6, the pseudo-header is described in RFC 2460 [7]. 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.
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.
Having calculated the message digest, TCP updates the new TCP option
to include the message digest and the identifier of the
authentication element that was used to create the message digest.
TCP then calculates a checksum and forwards the segment to its TCP
peer.
The TCP peer is also configured with a list of authentication
elements for the connection. Having received a TCP segment, the TCP
peer scans its list of authentication elements, searching for an
element whose identifier matches that which was specified by the
incoming TCP option. If such an authentication element is found, TCP
uses the key from that authentication element to calculate a message
digest. If the calculated message digest matches the message digest
received in the incoming TCP segment, the segment is accepted.
Otherwise, TCP declares an authentication failure and discard the
datagram. An authentication failure MUST NOT produce any response
back to the sender. Logging the failure is highly advisable.
Unlike other TCP extensions (e.g., the Window Scale option [8]), 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.
4. Syntax
The proposed TCP Enhanced Authentication Option has the following
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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 | Auth ID | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
| Message Digest |
| // |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Option Syntax
Kind: 8 bits
The Kind field identifies the TCP Enhanced Authentication Option.
This value will be assigned by IANA.
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.
Auth ID: 8 bits
The Auth ID field identifies the authentication element that was used
to generate the message digest.
Reserved: 8 bits
Must be equal to zero.
Message Digest: Variable length
A Message Digest that serves as a signature for the TCP segment. The
length of the Message Digest, and therefore, the length of the entire
option, is determined by the hash algorithm.
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The following table maps hash algorithms to the size of the digests
that the produce:
+----------------+--------+
| Hash Algorithm | Octets |
+----------------+--------+
| MD5 [4] | 16 |
| SHA-1 [9] | 20 |
| SHA-224 [10] | 28 |
+----------------+--------+
Permissible hash algorithms are not restricted to those listed above.
Table 1
5. Implications
5.1. 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
therefore 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 some protocols
recover quickly from peer crashes.
5.2. Performance
The performance hit in calculating digests may inhibit the use of
this option. Performance will vary depending upon processor type,
hash algorithm and the number of digest calculations that are
performed per incoming TCP segment.
5.3. 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
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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 octets. This leaves 40 octets for options.
As a concrete example, assume that a TCP implementation defaults to
sending window-scaling for connections it initiates. The most loaded
segment will be the initial SYN packet to start the connection. With
a TCP Enhanced Authentication object using SHA1 authentication, the
SYN packet will contain the following:
-- 4 octets MSS option
-- 4 octets window scale option (3 octets padded to 4 in this
implementation)
-- 24 octets for the TCP Enhanced Authentication Option
-- 2 octets for end-of-option-list, to pad to a 32-bit boundary.
This sums to exactly 34 octets. This leaves only 6 octets for
additional TCP options. Some longer options (e.g. Timestamp) would
not fit in that space.
5.4. 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 octets or
less, as this is current practice.
6. Security Considerations
This document defines a weak but easily deployed security mechanism
for TCP-based routing protocols. It is anticipated that future work
will provide different stronger mechanisms for dealing with these
issues.
6.1. Signature Coverage
A further weakness exists due to the exclusion of option data from
the signature. This decision was made to simplify the protocol
definition and implementation, but might possibly leave a connection
vulnerable since option data can be rewritten without detection.
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7. IANA Considerations
IANA will assign a codepoint for the TCP Enhanced Authentication
Option.
8. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Heffernan, A., "Protection of BGP Sessions via the TCP MD5
Signature Option", RFC 2385, August 1998.
[3] Rekhter, Y. and T. Li, "A Border Gateway Protocol 4 (BGP-4)",
RFC 1771, March 1995.
[4] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992.
[5] Leech, M., "Key Management Considerations for the TCP MD5
Signature Option", RFC 3562, July 2003.
[6] Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
September 1981.
[7] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
Specification", RFC 2460, December 1998.
[8] Jacobson, V., Braden, B., and D. Borman, "TCP Extensions for
High Performance", RFC 1323, May 1992.
[9] Eastlake, D. and P. Jones, "US Secure Hash Algorithm 1 (SHA1)",
RFC 3174, September 2001.
[10] Housley, R., "A 224-bit One-way Hash Function: SHA-224",
RFC 3874, September 2004.
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Authors' Addresses
Ronald P. Bonica
Juniper Networks
2251 Corporate Park Drive
Herndon, VA 20171
US
Phone: +1 571 203 1704
Email: rbonica@juniper.net
Andy Heffernan
Juniper Networks
1194 N. Mathilda Ave.
Sunnyvale, CA 94089
US
Phone: +1 408 745 2037
Email: ahh@juniper.net
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