TCPM WG J. Touch
Internet Draft USC/ISI
Obsoletes: 2385 A. Mankin
Intended status: Proposed Standard Johns Hopkins Univ.
Expires: May 2009 R. Bonica
Juniper Networks
November 3, 2008
The TCP Authentication Option
draft-ietf-tcpm-tcp-auth-opt-02.txt
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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 static
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keying or an external, out-of-band key management mechanism; in
either case, TCP-AO also protects connections when using the same key
across repeated instances of a connection. 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 its own option identifier, even
though used mutually exclusive of TCP MD5 on a given TCP connection.
TCP-AO supports IPv6, and is fully compatible with the requirements
for the replacement of TCP MD5.
Table of Contents
1. Introduction...................................................3
1.1. Executive Summary.........................................4
1.2. List of TBD Items.........................................5
1.3. Changes from Previous Versions............................5
1.3.1. New in draft-ietf-tcp-auth-opt-02....................5
1.3.2. New in draft-ietf-tcp-auth-opt-01....................6
1.3.3. New in draft-ietf-tcp-auth-opt-00....................7
1.3.4. New in draft-touch-tcp-simple-auth-03................8
1.3.5. New in draft-touch-tcp-simple-auth-02................8
1.3.6. New in draft-touch-tcp-simple-auth-01................8
1.4. Summary of RFC-2119 Requirements..........................8
2. Conventions used in this document..............................9
3. The TCP Authentication Option..................................9
3.1. Review of TCP MD5 Option..................................9
3.2. TCP-AO Option............................................10
4. Preventing replay attacks within long-lived connections.......13
5. Computing connection keys from TSAD entries...................14
6. Security Association Management...............................16
7. TCP-AO Interaction with TCP...................................19
7.1. User Interface...........................................19
7.2. TCP States and Transitions...............................20
7.3. TCP Segments.............................................20
7.4. Sending TCP Segments.....................................21
7.5. Receiving TCP Segments...................................21
7.6. Impact on TCP Header Size................................23
8. Key Establishment and Duration Issues.........................23
8.1. Key reuse across socket pairs............................24
8.2. Key use within a long-lived connection...................24
8.3. Implementing the TSAD as an External Database............24
9. Obsoleting TCP MD5 and Legacy Interactions....................26
10. Interactions with non-NAT/NAPT Middleboxes...................26
11. Interactions with NAT/NAPT Devices...........................27
12. Evaluation of Requirements Satisfaction......................27
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13. Security Considerations......................................29
14. IANA Considerations..........................................32
15. Acknowledgments..............................................32
16. References...................................................32
16.1. Normative References....................................32
16.2. Informative References..................................33
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, but notes that TCP does not provide a sufficient
framework for cryptographic key management. This document obsoletes
the TCP MD5 option with a more general TCP Authentication Option
(TCP-AO), to support the use of other, stronger hash functions,
provide replay protection for long-lived connections and across
repeated instances of a single connection, and to provide a more
structured recommendation on external key management. The result is
compatible with IPv6, and is fully compatible with requirements under
development for a replacement for TCP MD5 [Be07].
This document is not intended to replace the use of the IPsec suite
(IPsec and IKE) to protect TCP connections [RFC4301][RFC4306]. 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 for some routing protocols, or in cases where
keys need to be tightly coordinated with individual transport
sessions [Be07].
Note that TCP-AO obsoletes TCP MD5, although a particular
implementation may support both 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 configuration once
established.
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1.1. Executive Summary
This document replaces TCP MD5 as follows [RFC2385]:
o TCP-AO uses a separate option Kind for TCP-AO (TBD-IANA-KIND).
o TCP-AO allows TCP MD5 to continue to be used for other (legacy)
connections.
o TCP-AO replaces MD5's single MAC algorithm with two prespecified
MACs (TBD-WG-MACS), and allows extension to include other MACs.
o TCP-AO allows rekeying during a TCP connection, assuming that an
out-of-band protocol or manual mechanism coordinates the key
change. In such cases, a key ID allows the efficient concurrent
use of multiple keys. Note that TCP MD5 does not preclude rekeying
during a connection, but does not require its support either.
Further, TCP-AO supports rekeying with zero packet loss, whereas
rekeying in TCP MD5 can lose packets in transit during the
changeover or require trying multiple keys on each received
segment during key use overlap.
o TCP-AO provides automatic key rollover to provide replay
protection for long-lived connections.
o TCP-AO ensures per-connection keys as unique as the TCP connection
itself, using TCP's ISNs for differentiation, even when static
keys are used for repeated instances of a socket pair.
o This document provides more detail in how this option interacts
with TCP's states, event processing, and user interface.
o The TCP-AO option is 3 bytes shorter than TCP MD5 (15 bytes
overall, rather than 18) in the default case (assuming a 96-bit
MAC).
This document differs from an IPsec/IKE solution in that TCP-AO as
follows [RFC4301][RFC4306]:
o TCP-AO does not support dynamic parameter negotiation.
o TCP-AO uses TCP's socket pair (source address, destination
address, source port, destination port) as a security parameter
index, rather than using a separate field as a primary index
(IPsec's SPI).
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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) [Be07].
o TCP-AO does not support encryption.
o TCP-AO does not authenticate ICMP messages (some ICMP messages may
be authenticated via IPsec, depending on the configuration).
1.2. List of TBD Items
[NOTE: to be omitted upon final publication as RFC]
SAAG: The following items are to be determined (TBD) prior to
publication. Once a value is chosen, it should be replaced for the
notation below throughout this document and the item removed from
this list.
TBD-IANA-KIND new TCP option Kind for TCP-AO, assigned by IANA
TBD-WG-MACS list of default required MAC algorithms
TBD-WG-MACLEN default length of MAC used in the TCP-AO MAF
1.3. Changes from Previous Versions
[NOTE: to be omitted upon final publication as RFC]
1.3.1. New in draft-ietf-tcp-auth-opt-02
o List issue - Replay Protection: incorporated key rollover based on
extended sequence number space, not using KeyID space.
o List issue - Unique Connection Keys: ISNs are used to generate
unique connection keys even when static keys used for repeated
instances of a socket pair.
o List issue - Header Format and Alignment: Moved KeyID to front.
o List issue - Reserved KeyID Value: Suggestion to reserve a single
KeyID value for implementation optimization received no support on
the WG list, so this was not changed.
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o List issue - KeyID Randomness: KeyIDs are not assumed random; a
note was added that nonce-based filtering should be done on a
portion of the MAC (incorporated into the algorithm), and that
header fields should not be assumed to have cryptographic
properties (e.g., randomness).
o List issue - Support for NATs: preliminary rough consensus
suggests that TCP-AO should not be augmented to support NAT
traversal. Existing mechanisms for such traversal (UDP support)
can be applied, or IPsec NAT traversal is recommended in such
cases instead.
o IETF-72 topic - providing algorithm ID and T-bit (options
excluded) locations in the header: (No current consensus was
reached on this topic, so no change was made.)
o IETF-72 topic - providing additional header bits for in-band key
change signaling (draft-bonica's "K" bit): (No current consensus
was reached on this topic, so no change was made.)
o Clarified TCP-AO as obsoleting TCP MD5.
o Clarified the MAC Type as referring to the IANA registry of IKEv2
transforms, not the RFC establishing that registry.
o Added citation to the Wang/Yu paper regarding attacks on MD5 Wa05
to replace reports in Be05 and Bu06.
o Explained why option exclusion can't be changed during a
connection.
o Clarified that AO explicitly allows rekeying during a TCP
connection, without impacting packet loss.
o Described TCP-AO's interaction with reboots more clearly, and
explained the need to clear out old state that persists
indefinitely.
1.3.2. New in draft-ietf-tcp-auth-opt-01
o Require KeyID in all versions. Remove odd/even indicator of KeyID
usage.
o Relax restrictions on key reuse: requiring an algorithm for nonce
introduction based on ISNs, and suggest key rollover every 2^31
bytes (rather than using an extended sequence number, which
introduces new state to the TCP connection).
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o Clarify NAT interaction; currently does not support omitting the
IP addresses or TCP ports, both of which would be required to
support NATs without any coordination. This appears to present a
problem for key management - if the key manager knows the received
addrs and ports, it should coordinate them (as indicated in Sec
8).
o Options are included or excluded all-or-none. Excluded options are
deleted, not just zeroed, to avoid the impact of reordering or
length changes of such options.
o Augment replay discussion in security considerations.
o Revise discussion of IKEv2 MAC algorithm names.
o Remove executive summary comparison to expired documents.
o Clarified key words to exclude lower case usage.
1.3.3. New in draft-ietf-tcp-auth-opt-00
o List of TBD values, and indication of how each is determined.
o Changed TCP-SA to TCP-AO (removed 'simple' throughout).
o Removed proposed NAT mechanism; cited RFC-3947 NAT-T as
appropriate approach instead.
o Made several changes coordinated in the TCP-AUTH-DT as follow:
o Added R. Bonica as co-author.
o Use new TCP option Kind in the core doc.
o Addresses the impact of explicit declines on security.
o Add limits to TSAD size (2 <= TSAD <= 256).
o Allow 0 as a legitimate KeyID.
o Allow the WG to determine the two appropriate required MAC
algorithms.
o Add TO-DO items.
o Added discussion at end of Introduction as to why TCP MD5
connections cannot be upgraded to TCP-AO.
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1.3.4. New in draft-touch-tcp-simple-auth-03
o Added support for NAT/NAPT.
o Added support for IPv6.
o Added discussion of how this proposal satisfies requirements under
development, including those indicated in [Be07].
o Clarified the byte order of all data used in the MAC.
o Changed the TCP option exclusion bit from a bit to a list.
1.3.5. New in draft-touch-tcp-simple-auth-02
o Add reference to Bellovin's need-for-TCP-auth doc [Be07].
o Add reference to SP4 [SDNS88].
o Added notes that TSAD to be externally implemented; this was
compatible with the TSAD described in the previous version.
o Augmented the protocol to allow a KeyID, required to support
efficient overlapping keys during rekeying, and potentially useful
during connection establishment. Accommodated by redesigning the
TSAD.
o Added the odd/even indicator for the KeyID.
o Allow for the exclusion of all TCP options in the MAC calculation.
1.3.6. New in draft-touch-tcp-simple-auth-01
o Allows intra-session rekeying, assuming out-of-band coordination.
o MUST allow TSAD entries to change, enabling rekeying within a TCP
connection.
o Omits discussion of the impact of connection reestablishment on
BGP, because added support for rekeying renders this point moot.
o Adds further discussion on the need for rekeying.
1.4. Summary of RFC-2119 Requirements
[NOTE: a summary will be placed here prior to last call]
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2. 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. Lower case uses of these words are not to be
interpreted as carrying RFC-2119 significance.
3. The TCP Authentication Option
The TCP Authentication Option (TCP-AO) uses a TCP option Kind value
of TBD-IANA-KIND.
3.1. Review of TCP MD5 Option
For review, the TCP MD5 option is shown in Figure 1.
+---------+---------+-------------------+
| Kind=19 |Length=18| MD5 digest... |
+---------+---------+-------------------+
| |
+---------------------------------------+
| |
+---------------------------------------+
| |
+-------------------+-------------------+
| |
+-------------------+
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, 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 TCP pseudoheader (IP source and destination addresses,
protocol number, and segment length).
2. The TCP header excluding options and checksum.
3. The TCP data payload.
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4. The connection key.
3.2. TCP-AO Option
The new TCP-AO option 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, as well as a KeyID
field as shown in Figure 2.
+----------+----------+----------+----------+
| Kind | Length | KeyID | MAC |
+----------+----------+----------+----------+
| MAC (con't) ...
+----------------------------------...
...-----------------+
... MAC (con't) |
...-----------------+
Figure 2 The TCP-AO Option
The TCP-AO defines the following fields:
o Kind: An unsigned 1-byte field indicating the TCP-AO Option. TCP-
AO uses a new Kind value of TBD-IANA-KIND. Because of how keys are
managed (see Section 6), an endpoint will not use TCP-AO for the
same connection in which TCP MD5 is used.
>> A single TCP segment MUST NOT have more than one TCP-AO option.
o Length: An unsigned 1-byte field indicating the length of the TCP-
AO option in bytes, including the Kind, Length, KeyID, and MAC
fields.
>> The Length value MUST be greater than or equal to 3.
>> The Length value MUST be consistent with the TCP header length;
this is a consistency check and avoids overrun/underrun abuse.
Values of 3 and other small values are of dubious utility (e.g.,
for MAC=NONE, or small values for very short MACs) but not
specifically prohibited.
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o KeyID: An unsigned 1-byte field is used to support efficient key
changes during a connection and/or to help with key coordination
during connection establishment, and will be discussed further in
Section 4. Note that the KeyID has no cryptographic properties -
it need not be random, nor are there any reserved values.
o MAC: Message Authentication Field. 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.
>> TCP-AO MUST support TBD-WG-MACS; other MACs MAY be supported
[RFC2403].
The MAC is computed over the following fields in the following order:
1. The extended sequence number (ESN), in network-standard byte
order, as follows:
+--------+--------+--------+--------+
| ESN |
+--------+--------+--------+--------+
Figure 3 Extended sequence number
The ESN for transmitted segments is locally maintained from a
locally maintained SND.ESN value, for received segments, a local
RCV.ESN value is used. The details of how these values are
maintained and used is described in Sections 4, 7.4, and 7.5.
2. The TCP pseudoheader: IP source and destination addresses,
protocol number and segment length, all in network byte order,
prepended to the TCP header below. The 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 4 TCP IPv4 pseudoheader [RFC793]
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+--------+--------+--------+--------+
| |
+ +
| |
+ Source Address +
| |
+ +
| |
+ +
+--------+--------+--------+--------+
| |
+ +
| |
+ Destination Address +
| |
+ +
| |
+--------+--------+--------+--------+
| Upper-Layer Packet Length |
+--------+--------+--------+--------+
| zero | Next Header |
+--------+--------+--------+--------+
Figure 5 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
4. TCP data, in network byte order
Note that the connection key is not included here; we expect that the
MAC algorithm will indicate how to use the key, e.g., as HMACs do in
general [RFC2104][RFC2403]. The connection key is derived from the
TSAD key entry as described in Sections 6, 7.4, and 7.5.
By default,TCP-AO includes the TCP options in the MAC calculation
because these options are intended to be end-to-end and some are
required for proper TCP operation (e.g., SACK, timestamp, large
windows). Middleboxes that alter TCP options en-route are a kind of
attack and would be successfully detected by TCP-AO. In cases where
the configuration of the connection's security association state
indicates otherwise, the TCP options can be excluded from the MAC
calculation. When options are excluded, all options - including TCP-
AO - are skipped over during the MAC calculation (rather than being
zeroed).
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The TCP-AO option does 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 association and is managed separately (see
Section 6).
4. 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
differentiate these legitimate replays from each other, and so it
adds a 32-bit extended sequence number (ESN) for transmitted and
received segments.
TCP-AO thus maintains SND.ESN for transmitted segments, and RCV.ESN
for received segments, both initialized as zero when a connection
begins. The intent of these ESNs is, together with TCP's 32-bit
sequence numbers, to provide a 64-bit overall sequence number space.
For transmitted segments SND.ESN can be implemented by extending
TCP's sequence number to 64-bits; SND.ESN 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.ESN from the
received 32-bit sequence number and the current connection context.
The implementation of ESNs is not specified in this document, but one
possible way is described here that can be used for either RCV.ESN,
SND.ESN, or both.
Consider an implementation with two ESNs as required (SND.ESN,
RCV.ESN), 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.ESN
o RCV.PREV_SEQ, needed to detect rollover of RCV.ESN
o SND.ESN_FLAG, which indicates when to increment the SND.ESN
o RCV.ESN_FLAG, which indicates when to increment the RCV.ESN
o ROLL, a temporary variable used to simplify the code
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When a segment is received, the following algorithm (written in C)
computes the ESN used in the MAC; an equivalent algorithm can be
applied to the "SND" side:
ROLL = (RCV.PREV_SEQ > 0xffff) && (SEG.SEQ < 0xffff);
if ((RCV.ESN_FLAG == 0) && (ROLL)) {
RCV.ESN = RCV.ESN + 1;
RCV.ESN_FLAG = 1;
}
# we've already incremented the RCV.ESN at this point
if (ROLL) {
ESN = RCV.ESN - 1; # use the pre-increment value
} else {
ESN = RCV.ESN; # use the current value
}
RCV.PREV_SEQ = SEG.SEQ;
if (SEG.SEQ > 0xffff) {
RCV.ESN_FLAG = 0;
}
5. Computing connection keys from TSAD entries
TSAD key entries, described in Section 6, are used in conjunction
with a TCP's connection ISNs to generate unique connection keys. This
allows a static TSAD key to be reused across different connections,
or across different instances of connections within a socket pair,
while maintaining unique connection keys. Unique connection keys are
generated without relying on external key management properties.
Given a TSAD key, the TCP socket pair, and the connection ISNs, the
connection key used in the MAC algorithm is computed as follows,
truncated to the same length as the TSAD key, using the same MAC
algorithm as the TSAD key (TALG):
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Conn_key = TALG(TSAD_key, connblock)
The connection block (connblock) is defined as follows (IP addresses
are correspondingly longer for IPv6 addresses):
+--------+--------+--------+--------+
| Source IP |
+--------+--------+--------+--------+
| Destination IP |
+--------+--------+--------+--------+
| Source Port | Dest. Port |
+--------+--------+--------+--------+
| Source ISN |
+--------+--------+--------+--------+
| Destination ISN |
+--------+--------+--------+--------+
Figure 6 Connection block used for connection key generation
"Source" and "destination" are defined by the direction of the
segment being MAC'd; for incoming packets, source is the remote side,
whereas for outgoing packets source is the local side. This further
ensures that keys 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 key is
computed using the connection block 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 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.
>> Segments sent in response to connections for which the ISNs are
not known SHOULD NOT use TCP-AO.
Once a connection is established, a connection key would typically be
cached to avoid recomputing it on a per-segment basis. The use of
both ISNs in the connection key computation ensures that segments
cannot be replayed across repeated connections reusing the same
socket pair (provided the ISN pair does not repeat, which is
extremely unlikely).
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In general, a SYN would be MAC'd using a destination ISN of zero
(whether sent or received), and all other segments would be MAC'd
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 is possible that the two endpoints
would not have enough information to authenticate segments. In such
cases, TCP's timeout mechanism will allow old state to be cleared to
enable new connections, except where the user timeout is disabled; 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 [Je07].
6. Security Association Management
TCP-AO relies on a TCP Security Association Database (TSAD). TSAD
entries are assumed to exist at the endpoints where TCP-AO is used,
in advance of the connection:
1. TCP connection identifier (ID), i.e., socket pair - IP source
address, IP destination address, TCP source port, and TCP
destination port [RFC793]. TSAD entries are uniquely determined by
their TCP connection ID, which is used to index those entries.
>> There MUST be no more than one matching TSAD entry per
direction for a TCP connection ID.
2. For each of inbound (for received TCP segments) and outbound (for
sent TCP segments) directions for this connection (except as
noted):
a. TCP option flag. When 0, this flag allows default operation,
i.e., TCP options are included in the MAC calculation, with
TCP-AO's MAC field zeroed out. When 1, all options (including
TCP-AO) are excluded from all MAC calculations (skipped over,
not simply zeroed).
>> The TCP option flag MUST default to 0 (i.e., options not
excluded).
>> The TCP option flag MUST NOT change during a TCP
connection.
The TCP option flag 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.
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b. An extended sequence number (ESN). The ESN enables each
segment's MAC calculation to have unique input data, even when
payload data is retransmitted and the TCP sequence number
repeats due to wraparound. The ESN is initialized to zero upon
connection establishment. Its use in the MAC calculation is
described in Section 3.2, and its management is described in
Section 4.
c. An ordered list of zero or more key tuples. Each tuple is
defined as the set <KeyID, MAC type, key length, key> as
follows:
>> TSAD key tuple components MUST NOT change during a
connection.
Keeping the tuple components static ensures that the KeyID
uniquely determines the properties of a packet; this supports
use of the KeyID to determine the packet properties.
>> The set of TSAD key tuples MAY change during a connection,
but KeyIDs of those tuples MUST NOT overlap. I.e., tuple
parameter changes MUST be accompanied by key changes.
i. KeyID. A single byte used to differentiate connection
keys in concurrent use.
>> A TSAD implementation MUST support at least two KeyIDs
per connection per direction, and MAY support up to 256.
>> A KeyID MUST support any value, 0-255 inclusive. There
are no reserved KeyID values.
KeyID values are assigned arbitrarily. They can be
assigned in sequence, or based on any method mutually
agreed by the connection endpoints (e.g., using an
external key management mechanism).
>> KeyIDs MUST NOT be assumed to be randomly assigned.
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ii. MAC type. Indicates the MAC used for this connection,
referencing types registered in the IKEv2 Transform Type
3 (Integrity Algorithms) Registry of the IANA established
by [RFC4306]. This includes each MAC algorithm (e.g.,
HMAC-MD5, HMAC-SHA1, UMAC, etc.) and the length of the
MAC as truncated to (e.g., 96, 128, etc.). Note that TCP-
AO refers to the IKEv2 list of transforms, but TCP-AO is
not dependent on IKEv2 itself.
>> A MAC type of "NONE" MUST be supported, to indicate
that authentication is not used in this direction; this
allows asymmetric use of TCP-AO.
>> At least one direction (inbound/outbound) SHOULD have
a non-"NONE" MAC in practice, but this MUST NOT be
strictly required by an implementation.
>> When the outbound MAC is set to values other than
"NONE", TCP-AO MUST occur in every outbound TCP segment
for that connection; when set to NONE or when no tuple
exists, TCP-AO MUST NOT occur in those segments.
>> When the inbound MAC is set to values other than
"NONE", TCP-AO MUST occur in every inbound TCP segment
for that connection; when set to "NONE" or when no tuple
exists, TCP-AO SHOULD NOT be added to those segments, but
MAY occur and MUST be ignored.
iii. Key length. A byte indicating the length of the key in
bytes.
iv. Key. A byte sequence used for generating connection keys,
this may be derived from a separate shared key by an
external protocol over a separate channel. This sequence
is used in network-standard byte order in the key
generation algorithm described in Section 5.
It is anticipated that TSAD entries for TCP connections in states
other than CLOSED can be stored in the TCP Control Block (TCB) or in
a separate database (see Section 8.1 for notes on the latter); TSAD
entries for pending connections (in passive or active OPEN) may be
stored in a separate database. This means that in a single host there
should be only a single database that is consulted by all pending
connections, the same way that there is only one set of TCBs.
Multiple databases could be used to support virtual hosts, i.e.,
groups of interfaces.
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Note that the TCP-AO fields omit an explicit algorithm ID; that
algorithm is already specified by the TCP connection ID and stored in
the TSAD.
Also note that this document does not address how TSAD entries are
created by users/processes; it specifies how they must be destroyed
corresponding to connection states, but users/processes may destroy
entries as well. It is presumed that a TSAD entry affecting a
particular connection cannot be destroyed during an active connection
- or, equivalently, that its parameters are copied to TSAD entries
local to the connection (i.e., instantiated) and so changes would
affect only new connections. The TSAD could be managed by a separate
application protocol, and can be stored in a separate database if
desired.
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 RFC793
[RFC793].
7.1. User Interface
The TCP user interface supports active and passive OPEN, SEND,
RECEIVE, CLOSE, STATUS and ABORT commands.
>> 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
a TSAD entry can be configured.
Users are advised to not inappropriately reuse keys [RFC3562]. As
noted in Section 3.2, this is accomplished in TCP-AO by the use of
unique per-connection nonces in conjunction with conventional keys.
>> TCP STATUS SHOULD be augmented to allow the TSAD entry of a
current or pending connection to be read (for confirmation).
>> A TCP-AO implmentation MUST allow TSAD entries for ongoing TCP
connections (i.e., not in the CLOSED state) to be modified.
Parameters not used to index a connection MAY be modified; parameters
used to index a connection MUST NOT be modified.
TSAD entries for TCP connections not in the CLOSED state are deleted
indirectly using the CLOSE or ABORT commands.
TCP SEND and RECEIVE are not affected by TCP-AO.
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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.
>> A TSAD entry MAY be associated with any TCP state.
>> A TSAD entry MAY underspecify the TCP connection for the LISTEN
state. Such an entry MUST NOT be used for more than one connection
progressing out of the LISTEN 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 TSAD for matching TCP
connection IDs.
>> TCP segments matching TSAD entries with non-NULL MACs without TCP-
AO, or with TCP-AO and whose MACs and KeyIDs do not validate MUST be
silently discarded.
>> TCP segments with TCP-AO but not matching TSAD entries MUST be
silently accepted; this is required for equivalent function with TCPs
not implementing TCP-AO.
>> 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.
Note that the TCP-AO option is not negotiated. It is the
responsibility of the receiver to determine when TCP-AO is required
and to enforce that requirement.
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7.4. Sending TCP Segments
The following procedure describes the modifications to TCP to support
TCP-AO when a segment departs.
1. Check the segment's TCP connection ID against the TSAD
2. If there is NO TSAD entry, omit the TCP-AO option. Proceed with
computing the TCP checksum and transmit the segment.
3. If there is a TSAD entry with zero key tuples, omit the TCP-AO
option. Proceed with computing the TCP checksum and transmit the
segment.
4. If there is a TSAD entry and a key tuple and the outgoing MAC is
NONE, omit the TCP-AO option. Proceed with computing the TCP
checksum and transmit the segment.
5. If there is a TSAD entry and a key tuple and the outgoing MAC is
not NONE:
a. Augment the TCP header with the TCP-AO, inserting the
appropriate Length and KeyID based on the indexed TSAD entry.
Update the TCP header length accordingly.
b. Determine SND.ESN as described in Section 4.
c. Determine the connection key from the indexed TSAD entry as
described in Section 5.
d. Compute the MAC using the indexed TSAD entry and data from the
segment as specified in Section 3.2, including the TCP
pseudoheader and TCP header. Include or exclude the options as
indicated by the TSAD entry's TCP option exclusion flag.
e. Insert the MAC in the TCP-AO field.
f. Proceed with computing the TCP checksum on the outgoing packet
and transmit the segment.
7.5. Receiving TCP Segments
The following procedure describes the modifications to TCP to support
TCP-AO when a segment arrives.
1. Check the segment's TCP connection ID against the TSAD.
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2. If there is NO TSAD entry, proceed with TCP processing.
3. If there is a TSAD entry with zero key tuples, proceed with TCP
processing.
4. If there is a TSAD entry with a key tuple and the incoming MAC is
NONE, proceed with TCP processing.
5. If there is a TSAD entry with a key tuple and the incoming MAC is
not NONE:
a. Check that the segment's TCP-AO Length matches the length
indicated by the indexed TSAD.
i. If Lengths differ, silently discard the segment. Log
and/or signal the event as indicated in Section 7.3.
b. Use the KeyID value to index the appropriate key for this
connection.
i. If the TSAD has no entry corresponding to the segment's
KeyID, silently discard the segment.
c. Determine the segment's RCV.ESN as described in Section 4.
d. Determine the segment's connection key from the indexed TSAD
entry as described in Section 5.
e. Compute the segment's MAC using the indexed TSAD entry and
portions of the segment as indicated in Section 3.2.
Again, if options are excluded (as per the TCP option
exclusion flag), they are skipped over (rather than zeroed)
when used as input to the MAC calculation.
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.
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.
Additional reductions in MAC validation can be supported by using a
MAC algorithm that partitions the MAC field into fixed and computed
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portions, where the fixed value is validated before investing in the
computed portion. This optimization would be contained in the MAC
algorithm specification. 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
The TCP-AO option typically uses a total of 17-19 bytes of TCP header
space. TCP-AO is no larger than and typically 3 bytes smaller than
the TCP MD5 option (assuming a 96-bit MAC). 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.
8. Key Establishment and Duration Issues
The TCP-AO option does not provide a mechanism for connection key
negotiation or parameter negotiation (MAC algorithm, length, or use
of the TCP-AO option) or rekeying during a connection. We assume out-
of-band mechanisms for key establishment, parameter negotiation, and
rekeying. This separation of key use from key management is similar
to that in the IPsec security suite [RFC4301][RFC4306].
We encourage users of TCP-AO to apply known techniques for generating
appropriate keys, including the use of reasonable connection key
lengths, limited connection key sharing, and limiting the duration of
connection key use [RFC3562]. This also includes the use of per-
connection nonces, as suggested in Section 3.2.
TCP-AO supports rekeying in which new keys are negotiated out-of-
band, either via a protocol or a manual procedure [RFC4808]. New keys
use is coordinated using the out-of-band mechanism to update the TSAD
at both TCP endpoints. In the default case, where only a single key
is used at a time, the temporary use of invalid keys would result in
packets being dropped; TCP is already robust to such drops. Such
drops may affect TCP's throughput temporarily, as a result TCP-AO
benefits from the use of congestion control support for temporary
path outages.
>> TCP-AO SHOULD be deployed in conjunction with support for
selective acknowledgement (SACK), including support for multiple lost
segments in the same round trip [RFC2018][RFC3517].
Note that TCP-AO's support for rekeying is designed to be minimal in
the default case. Segments carry only enough context to identify the
security association [RFC4301][RFC4306]. In TCP-AO, this context is
provided by the socket pair (IP addresses and ports for source and
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destination). The TSAD can contain multiple concurrent keys, where
the KeyID field is used to identify the key that corresponds to a
segment, to avoid the need for expensive trial-and-error testing of
keys in sequence.
The KeyID field is also useful in coordinating keys for new
connections. A TSAD entry may be configured that matches the unbound
source port, which would return a set of possible keys. The KeyID
would then indicate the specific key, allowing more efficient
connection establishment; otherwise, the keys could have been tried
in sequence. See also Section 8.1.
Implementations are encouraged to keep keys in a suitably private
area.
8.1. Key reuse across socket pairs
Keys 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.
Keys 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 connection key. Keys reused across
repeated instances of a given socket pair cannot enable replay
attacks because the connection ISNs are included in the connection
key generation algorithm, and ISN pairs are unlikely to repeat over
useful periods.
Keys should not be shared across different hosts, because this could
compromise the keying material itself.
8.2. Key use within a long-lived connection
TCP-AO uses extended sequence numbers (ESNs) to prevent replay
attacks within long-lived connections. Key rollover can be used to
change keying material for various reasons (e.g., personnel
turnover), but is not required to support long-lived connections.
8.3. Implementing the TSAD as an External Database
The TSAD implementation is considered external to TCP-AO. When an
external database is used, it would be useful to consider the
interface between TCP-AO and the TSAD. The following is largely a
restatement of information in Section 6.
The TSAD API is accessed during a connection as follows:
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o TCP connection identifier (ID) (The socket pair, sent as 4 byte IP
source address, 4 byte IP destination address, 2 byte TCP source
port, 2 byte TCP destination port).
o Direction indicator (sent as a single byte, 0x00 = inbound, 0x01 =
outbound)
o Number of bytes to be sent/received (two bytes); this is used on
the send side to trigger bytecount-based KeyID changes, and on the
receive side only for statistics or length-sensitive KeyID
selection.
o KeyID (single byte); this is provided only by a receiver (i.e.,
matching the KeyID of the received segment), where a sender would
leave this unspecified (and the call would return the appropriate
KeyID to use).
The call passes the number of bytes sent/received, and an indication
of the direction (send/receive), to enable traffic-based key
rollover.
The source port can be 'unbound', indicated by the value 0x0000. In
this case, the source port is considered a wildcard, and all
corresponding TSAD entries (indexed by the KeyID) are returned as a
list. This feature is used during connection establishment.
TSAD calls return the following parameters:
o TCP option exclusion flag (one byte, with 0x00 having the meaning
"exclude none" and 0x01 meaning "exclude all").
o An ordered list of zero or more connection key tuples:
<KeyID, MAC type, MAC length, key length, key>
o KeyID (one byte)
o MAC type (four bytes, an IKEv2 Transform Type 3 ID [RFC4306])
o MAC length (one byte)
o Key length (one byte)
o Key (byte sequence, indicating the key value)
When the TSAD returns zero keys, it is indicating that there are no
currently valid keys for the connection.
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9. Obsoleting TCP MD5 and Legacy Interactions
TCP-AO obsoletes TCP MD5. As we have noted earlier:
>> TCP implementations 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
is 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 the TSAD could be augmented to support TCP MD5,
although use of a TSAD-like system is not described in RFC2385.
It is possible to require TCP-AO for a connection or TCP MD5, but it
is not possible to require 'either'. Note that when TCP MD5 is
required on for a connection, it must be used [RFC2385]. This
prevents combined use of the two options for a given connection, to
be determined by the other end of the connection.
10. 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
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affect only the TCP connections for which TCP-AO is configured to
ignore TCP options.
11. Interactions with NAT/NAPT Devices
TCP-AO cannot interoperate natively across NAT/NAPT devices, which
modify the IP addresses and/or port numbers. We anticipate that
traversing such devices will 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
[RFC2766][RFC3947]. Such variants can be adapted for use with TCP-AO,
or IPsec NAT traversal can be used instead in such cases [RFC3947].
12. Evaluation of Requirements Satisfaction
TCP-AO satisfies all the current requirements for a revision to TCP
MD5, as indicated in [Be07] and under current development. This
should not be a surprise, as the majority of the evolving
requirements are derived from its design. The following is a summary
of those requirements and notes where relevant.
1. Protected Elements - see Section 3.2.
a. TCP pseudoheader, including IPv4 and IPv6 versions. Note that
we do not allow optional coverage 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.
b. TCP header. Note that we do not allow optional port coverage
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.
c. TCP options. Allows exclusion of TCP options from coverage, as
required.
d. TCP data. Done.
2. Option structure requirements
a. Privacy. TCP-AO exposes only the key index, MAC, and overall
option length. 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 TSAD entry). See Section 3.2.
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b. Allow optional per connection. Done - see Sections 7.3, 7.4,
and 7.5.
c. Require non-optional. Done - see Sections 7.3, 7.4, and 7.5.
d. Standard parsing. Done - see Section 3.2.
e. Compatible with Large Windows. Done - see Section 3.2. The
size of the option is intended to allow use with Large Windows
and SACK. See also Section 1.1, which indicates that TCP-AO is
3 bytes shorter than TCP MD5 in the default case, assuming a
96-bit MAC.
f. Compatible with SACK. Done - see Section 3.2. The size of the
option is intended to allow use with Large Windows and SACK.
See also Section 8 regarding key management. See also Section
1.1, which indicates that TCP-AO is 3 bytes shorter than TCP
MD5 in the default case.
3. Cryptography requirements
a. Baseline defaults. TCP-AO uses TBD-WG-MACS as the default, as
noted in Section 3.2.
b. Good algorithms. TCP-AO uses TBD-WG-MACS as the default, but
does not otherwise specify the algorithms used. That would be
specified in the key management protocol, and should be
limited there.
c. Algorithm agility. TCP-AO allows any desired algorithm,
subject to TCP option space limitations, as noted in Section
3.2. The TSAD allows separate connections to use different
algorithms.
d. Pre-TCP processing. Done - see Sections 7.3, 7.4, and 7.5.
Note that pre-TCP processing is 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].
e. Parameter changes require key changes. TSAD parameters that
should not change during a connection (TCP connection ID,
receiver TCP connection ID, TCP option exclusion list) cannot
change. Other parameters change only when a key is changed,
using the key tuple mechanism in the TSAD. See Section 6.
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4. Keying requirements. TCP-AO does not specify a key management
system, but does indicate a proposed interface to the TSAD,
allowing a completely separate key system.
a. Intraconnection rekeying. Supported by the KeyID and multiple
key tuples in a TSAD entry; see Section 6.
b. Efficient rekeying. Supported by the KeyID. See Section 8.
c. Automated and manual keying. Supported by the TSAD interface.
See Section 8. Enhanced by the generation of unique per-
connection keys as noted in Section 5.
d. Key management agnostic. Supported by the TSAD interface. See
Section 8.1.
5. Expected constraints
a. Silent failure. Done - see Sections 7.3, 7.4, and 7.5.
b. At most one such option per segment. Done - see Section 3.2.
c. Outgoing all or none. Done - see Section 7.4.
d. Incoming all checked. Done - see Section 7.5.
e. Non-interaction with TCP MD5. Done - see Section 9.
f. Optional ICMP discard. Done - see Section 13.
g. Allows use of NAT/NAPT devices. Done - see Section 10.
h. Maintain TCP connection semantics, in which the socket pair
alone defines a TCP association and all its security
parameters. Done - see Sections 6 and 10.
i. Try to avoid creating a CPU DOS attack opportunity. Done - see
Section 13.
13. Security Considerations
Use of TCP-AO, like 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
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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. 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 time-
outs, rather than more responsive recovery after such a crash. As
noted in Section 5, such cases may also result in persistent TCP
state for old connections that cannot be cleared, and so
implementations should be capable of detecting an excess of such
connections and clearing their state if needed to protect memory
utilization [Je07].
TCP-AO does not include a fast decline capability, e.g., where a SYN-
ACK is received without an expected TCP-AO option 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 the TCP-AO option.
TCP-AO does not expose the MAC algorithm used to authenticate a
particular connection; that information is kept in the TSAD at the
endpoints, and is not indicated in the header.
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 does not address the issue of ICMP attacks on TCP. 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 [Go07].
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>> A TCP-AO implementation MUST allow the system administrator to
configure whether TCP will ignore incoming ICMP messages of Type 3
Codes 2-4 intended for connections that match TSAD entries with non-
NONE inbound MACs. 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. This recommendation is intended
to be similar to how IPsec would handle those messages [RFC4301].
TCP-AO includes the TCP connection ID in the MAC calculation. This
prevents connections using the same key (for whatever reason) from
potentially enabling a traffic-crossing attack, in which segments to
one socket pair are diverted to attack a different socket pair. When
multiple connections use the same key, it would be useful to know
that packets intended for one ID could not be (maliciously or
otherwise) modified in transit and end up being authenticated for the
other ID. The ID cannot be zeroed, because to do so would require
that the TSAD index was unique in both directions (ID->key and key-
>ID). That requirement would place an additional burden of uniqueness
on keys 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 key
management system.
The use of any security algorithm can present an opportunity for a
CPU 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 in all cases due to TCP's Sequence Number,
which is used to reorder received segments. 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 such attacks 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
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retransmission) [RFC793]. It is similarly not useful to add a
separate Sequence Number field to the TCP-AO option, because doing so
could cause a change in TCP's behavior even when segments are valid.
14. IANA Considerations
The TCP-AO option defines no new namespaces.
The TCP-AO option uses the TCP option Kind value TCP-IANA-KIND,
allocated by IANA from the TCP option Kind namespace.
To specify MAC algorithms, TCP-AO uses the 4-byte namespace of IKEv2
Transform Type 3 IDs, because that database of names already exists
(not because of any reliance on IKEv2) [RFC4306].
[NOTE: The following to be removed prior to publication as an RFC]
The TCP-AO option requires that IANA allocate a value from the TCP
option Kind namespace, to be replaced for TCP-IANA-KIND throughout
this document.
15. Acknowledgments
This document was inspired by the revisions to TCP MD5 suggested by
Brian Weis and Ron Bonica [Bo07][We05][We07]. Russ Housley suggested
L4/application layer management of the TSAD. The KeyID field was
motivated by Steve Bellovin. Eric Rescorla suggested the use of ISNs
in the connection key computation and ESNs to avoid replay attacks,
and Brian Weis extended the computation to incorporate the entire
connection ID. Alfred Hoenes, Charlie Kaufman, and Adam Langley
provided substantial feedback. The document is the result of
collaboration with the TCP Authentication Design team (tcp-auth-dt).
This document was prepared using 2-Word-v2.0.template.dot.
16. References
16.1. Normative References
[RFC793] Postel, J., "Transmission Control Protocol," STD 007, RFC
793, Standard, Sept. 1981.
[RFC2018] Mathis, M., Mahdavi, J., Floyd, S. and A. Romanow, "TCP
Selective Acknowledgement Options", RFC 2018, Proposed
Standard, April 1996.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Best Current
Practice, March 1997.
[RFC2385] Heffernan, A., "Protection of BGP Sessions via the TCP MD5
Signature Option," RFC 2385, Proposed Standard, Aug. 1998.
[RFC2403] Madson, C., R. Glenn, "The Use of HMAC-MD5-96 within ESP
and AH," RFC 2403, Proposed Standard, Nov. 1998.
[RFC2460] Deering, S., Hinden, R., "Internet Protocol, Version 6
(IPv6) Specification," RFC 2460, Proposed Standard, Dec.
1998.
[RFC3517] Blanton, E., Allman, M., Fall, K., and L. Wang, "A
Conservative Selective Acknowledgment (SACK)-based Loss
Recovery Algorithm for TCP", RFC 3517, Proposed Standard,
April 2003.
[RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol," RFC
4306, Proposed Standard, Dec. 2005.
16.2. Informative References
[Be07] Eddy, W., (ed), S. Bellovin, J. Touch, R. Bonica, "Problem
Statement and Requirements for a TCP Authentication
Option," draft-bellovin-tcpsec-01, (work in progress), Jul.
2007.
[Bo07] Bonica, R., et. al, "Authentication for TCP-based Routing
and Management Protocols," draft-bonica-tcp-auth-06, (work
in progress), Feb. 2007.
[Go07] Gont, F., "ICMP attacks against TCP," draft-ietf-tcpm-icmp-
attacks-04, (work in progress), Oct. 2008.
[Je07] Jethanandani, M., and M. Bashyam, "TCP Robustness in
Persist Condition," draft-mahesh-persist-timeout-02, (work
in progress), Oct. 2007.
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm," RFC-1321,
Informational, April 1992.
[RFC2104] Krawczyk, H., Bellare, M., Canetti, R., "HMAC: Keyed-
Hashing for Message Authentication," RFC 2104,
Informational, Feb. 1997.
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[RFC2766] Tsirtsis, G., Srisuresh, P., "Network Address Translation -
Protocol Translation (NAT-PT)," RFC 2766, Proposed
Standard, Feb. 2000.
[RFC3562] Leech, M., "Key Management Considerations for the TCP MD5
Signature Option," RFC 3562, Informational, July 2003.
[RFC3947] Kivinen, T., B. Swander, A. Huttunen, V. Volpe,
"Negotiation of NAT-Traversal in the IKE," RFC 3947, Jan.
2005.
[RFC4301] Kent, S., K. Seo, "Security Architecture for the Internet
Protocol," RFC 4301, Proposed Standard, Dec. 2005.
[RFC4808] Bellovin, S., "Key Change Strategies for TCP-MD5," RFC
4808, Informational, Mar. 2007.
[RFC4953] Touch, J., "Defending TCP Against Spoofing Attacks,"
RFC4953, Jul. 2007.
[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.
[To??] Touch, J., A. Mankin, "The TCP Simple Authentication
Option," draft-touch-tcpm-tcp-simple-auth-03, (expired work
in progress), Oct. 2006.
[Wa05] Wang, X., H. Yu, "How to break MD5 and other hash
functions," Proc. IACR Eurocrypt 2005, Denmark, pp.19-35.
[We05] Weis, B., "TCP Message Authentication Code Option," draft-
weis-tcp-mac-option-00, (expired work in progress), Dec.
2005.
[We07] Weis, B., et al., "Automated key selection extension for
the TCP Authentication Option," draft-weis-tcp-auth-auto-
ks-03, (work in progress), Oct. 2007.
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Author's 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.
Washington, DC
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
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