TCPM WG J. Touch
Internet Draft USC/ISI
Intended status: Proposed Standard A. Mankin
Expires: January 2008 July 7, 2007
The TCP Simple Authentication Option
draft-touch-tcpm-tcp-simple-auth-03.txt
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Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
This document specifies a TCP Simple Authentication Option (TCP-SA)
which is intended to replace the TCP MD5 Signature option of RFC-2385
(TCP/MD5). TCP-SA specifies the use of stronger HMAC-based hashes and
provides more details on the association of security associations
with TCP connections. TCP-SA assumes an external, out-of-band
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mechanism (manual or via a separate protocol) for session key
establishment, parameter negotiation, and rekeying, replicating the
separation of key management and key use as in the IPsec suite. The
result is intended to be a simple modification to support current
infrastructure uses of TCP/MD5, such as to protect BGP and LDP, to
support a larger set of hashes with minimal other system and
operational changes. TCP-SA requires no new option identifier, though
it is intended to be mutually exclusive with TCP/MD5 on a given TCP
connection. It can be used in the presence of NATs/NAPTs, and
supports IPv6, and is fully compatible with requirements under
development for an update to TCP/MD5.
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].
Table of Contents
1. Introduction...................................................3
1.1. Executive Summary.........................................3
1.2. Changes from Previous Versions............................5
1.2.1. New in draft-touch-tcp-simple-auth-03................5
1.2.2. New in draft-touch-tcp-simple-auth-02................5
1.2.3. New in draft-touch-tcp-simple-auth-01................6
1.3. Summary of RFC-2119 Requirements..........................6
2. The TCP Simple Authentication Option...........................6
2.1. Review of TCP/MD5 Option..................................6
2.2. TCP-SA Option.............................................7
3. Security Association Management...............................11
4. TCP-SA Interaction with TCP...................................13
4.1. User Interface...........................................13
4.2. TCP States and Transitions...............................14
4.3. TCP Segments.............................................15
4.4. Sending TCP Segments.....................................15
4.5. Receiving TCP Segments...................................16
4.6. Impact on TCP Header Size................................17
5. Key Establishment and Duration Issues.........................17
5.1. Implementing the TSAD as an External Database............18
6. Interactions with TCP/MD5.....................................20
7. Interactions with NAT/NAPT Devices............................20
8. Evaluation of Requirements Satisfaction.......................20
9. Security Considerations.......................................23
10. IANA Considerations..........................................25
11. Acknowledgments..............................................25
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12. References...................................................25
12.1. Normative References....................................25
12.2. Informative References..................................26
Author's Addresses...............................................27
Intellectual Property Statement..................................28
Disclaimer of Validity...........................................28
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][To07].
There have been many recently-documented 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 [Be05]
[Bu06]. TCP/MD5 also lacks both key management and algorithm
agility, and cannot be used when either party is behind NATs/NAPTs.
This document proposes to add the latter, but suggests that TCP
should not be the framework for cryptographic key management. This
document updates the TCP/MD5 option to become a more general TCP
Simple Authentication Option (TCP-SA), to support the use of other,
stronger hash functions and to provide a more structured
recommendation on external key management. The result also supports
operation behind NAT/NAPT devices, is compatible with IPv6, and is
fully compatible with requirements under development for an update to
TCP/MD5 [Be06].
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-SA 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 [Be06].
1.1. Executive Summary
This document updates TCP/MD5 as follows [RFC2385]:
o Reuses TCP/MD5's option Kind (=19), but allows TCP/MD5 to continue
to be used for other connections.
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o Replaces signed MD5 with HMAC-MD5-96, and allows other MACs at the
implementer's discretion.
o Allows rekeying during a TCP connection, assuming that an out-of-
band protocol or manual mechanism coordinates the change of key
and that incorrectly keyed segments are ignored. In such cases, a
key ID may be used to make key selection more efficient.
o Provides more detail in how this option interacts with TCP's
states, event processing, and user interface.
o Proposed option is 4 bytes shorter (14 bytes overall, rather than
18) in the default case (HMAC-MD5-96).
o Proposed option supports NAT/NAPTs, and protects the resulting TCP
segment as received behind the NAT/NAPT.
This document differs from currently competing proposals to update
TCP/MD5 as follows [Bo07][We05][We07]:
o Fully compatible with requirements currently under development.
o Does not require a new TCP option Kind value.
o Does not support dynamic parameter negotiation.
o Does not support in-band session key negotiation.
o Does not support in-band session rekeying.
o Does not require additional timers.
o Always authenticates the TCP options as well as the segment
pseudoheader, header, and data.
o Provides more detail in how this option interacts with TCP's
states, event processing, and user interface.
o Proposed option is 2 bytes shorter (14 bytes overall, rather than
16) in the default case (HMAC-MD5-96)
o Does not expose the MAC algorithm in the header.
o Does not require a key ID; it allows for one where key overlap is
desired to support efficient rekeying.
o Supports NAT/NAPT while still protecting the segment as received.
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o Supports TCP over either IPv4 or IPv6.
This document differs from an IPsec/IKE solution as follows
[RFC4301][RFC4306]
o Does not support dynamic parameter negotiation.
o Does not require a key ID (SPI), but does allow one.
o Does not protect from replay attacks.
o Forces a change of connection key when a connection restarts, even
when reusing a TCP socket pair (IP addresses and port numbers)
[Be06].
o Does not support encryption.
o Does not authenticate ICMP messages (some may be authenticated in
IPsec, depending on the configuration).
1.2. Changes from Previous Versions
[NOTE: to be omitted upon final publication as RFC]
1.2.1. 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 [Be06].
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.
1.2.2. New in draft-touch-tcp-simple-auth-02
o Add reference to Bellovin's need-for-TCP-auth doc [Be06].
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.
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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.
1.2.3. 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.
1.3. Summary of RFC-2119 Requirements
[NOTE: a summary will be placed here prior to last call]
2. The TCP Simple Authentication Option
The TCP Simple Authentication Option (TCP-SA) re-uses the Kind value
currently assigned to TCP/MD5.
2.1. Review of TCP/MD5 Option
For review, the TCP/MD5 option is shown in Figure 1.
+---------+---------+-------------------+
| Kind=19 |Length=18| MD5 digest... |
+---------+---------+-------------------+
| |
+---------------------------------------+
| |
+---------------------------------------+
| |
+-------------------+-------------------+
| |
+-------------------+
Figure 1 Current TCP MD5 Option [RFC2385]
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In the current 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 current 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. TCP header excluding options and checksum
3. TCP data
4. connection key
2.2. TCP-SA Option
The new TCP-SA option is intended to be a superset of the TCP/MD5
option, and minimal in the spirit of SP4 [SDNS88]. TCP-SA reuses the
same Kind and Length fields, and is shown in Figure 2.
+---------+---------+-----------------...
| Kind=19 | Len=var | MAF... ...
+---------+---------+-----------------...
Figure 2 Proposed TCP-SA Option
The TCP-SA defines the following fields:
o Kind: An unsigned field indicating the TCP-SA Option. TCP-SA
reuses the Kind value=19. Because of how keys are managed (see
Section 3), an endpoint will not use TCP-SA for the same
connection where TCP/MD5 is used, and so there would be no
confusion as to how to interpret incoming Kind=19 segments.
o Length: An unsigned 8-bit field indicating the length of the TCP-
SA option in bytes, including the Kind and Length fields.
>> The Length MUST be greater than or equal to 2.
>> The Length value MUST be consistent with the TCP header length.
Values of 2 and other small values are of dubious utility but not
specifically prohibited. See the MAF description for implications
of odd/even lengths.
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o MAF: The MAF is a Message Authentication Field. Its contents are
determined by the particulars of the security association, where
there are two possible variants. When the Length is even, the
option appears as in Figure 3. When the Length is odd, the option
appears as in Figure 4.
+---------+---------+-------------------+
| Kind=19 | Len=var | MAC |
+---------+---------+-------------------+
| MAC (con't)... ...
+-------------------------------------...
Figure 3 TCP-SA MAF without key identifier
+---------+---------+-------------------+
| Kind=19 | Len=var | MAC |
+---------+---------+-------------------+
| MAC (con't)... ...
+-------------------+---------+-------...
...-----------------+---------+
... MAC (con't) | KeyID |
...-----------------+---------+
Figure 4 TCP-SA MAF with key identifier
Typical MACs are 96-128 bits (12-16 bytes), but any length that
fits in the header of the segment being authenticated is allowed.
Because typical MACs are even-length, TCP-SA assumes so [RFC4306].
If a particular MAC is odd-length, it is padded.
>> An odd-length MAC MUST be padded with a single 0x00 byte on
transmit. Setting this pad byte is considered part of the
authentication algorithm.
>> An odd-length MAC MUST have a trailing 0x00 pad byte on
receipt. Checking this pad byte is considered part of the
authentication algorithm.
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When the Length is odd, a key identifier (KeyID) is included in
the last byte of the option. The KeyID is used to support
efficient key rollover during a connection and/or to help with key
coordination during connection establishment, and will be
discussed further in Sections 3.
>> TCP-SA MUST support HMAC-MD5-96; other MACs MAY be supported
[RFC2403].
>> A single TCP segment MUST NOT have more than one TCP-SA option.
The MAC is computed over the following fields in the following order;
segment fields are as present in segments at the receiver (this
impacts operation of this option in the presence of NATs, as
discussed in detail in Section 7):
1. 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 5 TCP IPv4 pseudoheader [RFC793]
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+--------+--------+--------+--------+
| |
+ +
| |
+ Source Address +
| |
+ +
| |
+ +
+--------+--------+--------+--------+
| |
+ +
| |
+ Destination Address +
| |
+ +
| |
+--------+--------+--------+--------+
| Upper-Layer Packet Length |
+--------+--------+--------+--------+
| zero | Next Header |
+--------+--------+--------+--------+
Figure 6 TCP IPv6 pseudoheader [RFC2460]
2. TCP header, by default including options, and where the TCP
checksum and TCP-SA MAC fields are set to zero, all in network
byte order
3. 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, and HMAC-MD5-96 does in particular [RFC2104][RFC2403].
TCP-SA by default includes the TCP options 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-SA in its default configuration. In
cases where the configuration of the connection's security
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association state indicates otherwise, the TCP options can be
excluded from the MAC calculation.
The TCP-SA option does not indicate the MAC algorithm either
implicitly (as with TCP/MD5) or explicitly (as with some proposed
alternatives) [RFC2385][Bo07][We05][We07]. The particular algorithm
used is considered part of the configuration state of the
connection's security association and is managed separately (see
Section 3).
3. Security Association Management
TCP-SA relies on a TCP Security Association Database (TSAD). TSAD
entries are assumed to exist at the endpoints where TCP-SA 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 address [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 for TCP
connection ID.
2. For each of inbound (for received TCP segments) and outbound (for
sent TCP segments) for this connection (except as noted):
a. TCP Option exclusion list. A list of indicators that, when
set, indicates that TCP options are excluded from all MAC
calculations. When the empty, all TCP options are included.
The special value 0x00, normally indicating the end of the TCP
options list, indicates that all options are excluded (rather
than indicating them all).
>> The TCP Option list MUST default to empty.
>> The TCP Option list MUST NOT change during a TCP
connection.
>> The TCP Option list MUST ignore settings for the TCP-SA
option, i.e., the TCP-SA option MUST NOT be excluded from MAC
calculation.
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b. Receiver TCP connection ID (RID) (outbound only). Normally,
the RID would match the ID above (#1 above). Devices behind
NAT/NAPTs would have nonmatching RIDs. This value is used for
calculating the MAC at the sender.
>> RIDs MUST NOT change during a TCP connection.
c. An ordered list of zero or more connection key tuples. Each
tuple consists of a set <KeyID, MAC type, key length,
connection key> as follows:
>> TSAD key tuple components MUST NOT change during a
connection.
>> 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 (optional). A single byte used to differentiate
overlapping Connection keys. The KeyID value can be 0x00
or not stored when not included.
>> When an KeyID not used, the sender MUST use the key
from the first key tuple returned, and the receiver MUST
try all key tuples in order until one succeeds.
ii. MAC type. Indicates the MAC used for this connection, as
per IKEv2 Transform Type 3 [RFC4306]. This includes the
MAC algorithm (e.g., HMAC-MD5, HMAC-SHA1, UMAC, etc.) and
the length of the MAC as truncated to (e.g., 96, 128,
etc.). Also, a setting of NONE must be supported, to
indicate that authentication is not used in this
direction; this allows asymmetric use of TCP-SA. At least
one direction (inbound/outbound) SHOULD have a non-NONE
MAC in practice, but this is not strictly required.
>> When the outbound MAC is set to values other than
NONE, TCP-SA MUST occur in every outbound TCP segment for
that connection; when set to NONE or when no tuple
exists, TCP-SA MUST NOT occur in those segments.
>> When the inbound MAC is set to values other than NONE,
TCP-SA MUST occur in every inbound TCP segment for that
connection; when set to NONE or when no tuple exists,
TCP-SA MUST NOT occur in those segments.
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iii. Key length. A byte indicating the length of the
connection key in bytes.
iv. Connection key. A byte sequence used for connection
keying, 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 MAC
calculations.
It is anticipated that TSAD entries for active or opening TCP
connections can be stored in the TCP Control Block (TCB) or in a
separate database (see Section 5.1 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 which 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.
Note that TSAD and the TCP-SA fields may omit the KeyID; the TCP
connection ID already uniquely specifies the TSAD entry, so a
separate field is not needed to specify a key unless key overlay
during rekeying is supported or is needed for key coordination during
connection establishment (see Section 5). The TCP-SA 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 or destroyed. 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 local to the
connection 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.
4. TCP-SA Interaction with TCP
The following is a description of how TCP-SA affects various TCP
states, segments, events, and interfaces. This description is
intended to augment the description of TCP as provided in RFC793
[RFC793].
4.1. User Interface
The TCP user interface supports active and passive OPEN, SEND,
RECEIVE, CLOSE, STATUS and ABORT.
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>> 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.
>> New TSAD entries MUST be checked against a cache of previously
used TSAD entries, and key reuse MUST be prohibited.
Users are advised to not inappropriately reuse keys [RFC3562].
>> TCP STATUS SHOULD be augmented to allow the TSAD entry of a
current or pending connection to be read (for confirmation).
>> TCP STATUS 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.
>> Use of CLOSE or ABORT MUST retain the TSAD entry in a cache to
assist with checking for key reuse.
This entry may correspond to one of the wait states of TCP (FIN-WAIT-
1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK, or TIME-WAIT), or may
be stored separately (for connections proceeding rapidly to CLOSED).
The size of this cache and duration of retained entries is up to the
user, where we again advise the application of known key management
principles [RFC3562].
TCP SEND and RECEIVE are not affected by TCP-SA.
4.2. 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.
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4.3. 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.
>> 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-
SA, or with TCP-SA and whose MACs and/or KeyIDs (the latter when in
use) do not validate MUST be silently discarded.
>> TCP segments with TCP-SA but not matching TSAD entries MUST be
silently accepted.
>> 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-SA 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-SA option is not negotiated. It is the
responsibility of the receiver to determine when TCP-SA is required
and to enforce that requirement.
>> Receivers MAY silently accept TCP segments with the TCP-SA option.
4.4. Sending TCP Segments
The following procedure describes the modifications to TCP to support
TCP-SA 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-SA 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-SA
option. Proceed with computing the TCP checksum and transmit the
segment.
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4. If there is a TSAD entry and a key tuple and the outgoing MAC is
NONE, omit the TCP-SA 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-SA, inserting the
appropriate Length and KeyID (the latter only if Length is
odd) based on the indexed TSAD entry. Update the TCP header
length accordingly.
b. Compute the MAC using the indexed TSAD entry and data from the
segment as specified in Section 2.2. Use the TCP pseudoheader
and TCP header as it would be received, i.e., use the RID
values rather than the actual outgoing pseudoheader and header
(see Section 7 for more information).
Note that excluded options are zeroed when used as input to
the MAC calculation.
c. Insert the MAC in the TCP-SA field.
d. Proceed with computing the TCP checksum on the outgoing packet
and transmit the segment.
4.5. Receiving TCP Segments
The following procedure describes the modifications to TCP to support
TCP-SA when a segment arrives.
1. Check the segments TCP connection ID against the TSAD
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-SA Length matches the length
indicated by the indexed TSAD.
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i. If Lengths differ, silently discard the segment. Log
and/or signal the event as indicated in Section 4.3.
b. If the Length is odd, 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. Compute the segment's MAC using the indexed TSAD entry and
portions of the segment as indicated in Section 2.2.
Again, note that excluded options are zeroed when used as
input to the MAC calculation.
i. If the computed MAC differs from the TCP-SA MAC field
value, silently discard the segment. Log and/or signal
the event as indicated in Section 4.3.
d. Proceed with TCP processing of the segment.
It is suggested that TCP-SA implementations validate a segment's
Length field before computing a MAC, to reduce the overhead incurred
by spoofed segments with invalid TCP-SA fields.
4.6. Impact on TCP Header Size
The TCP-SA option typically uses a total of 16-18 bytes of TCP header
space. TCP-SA is no larger than and typically 2 bytes smaller than
the TCP/MD5 option. Although TCP option space is limited, we believe
TCP-SA is consistent with the desire to authenticate TCP at the
connection level for similar uses as were intended by TCP/MD5.
5. Key Establishment and Duration Issues
The TCP-SA option does not provide a mechanism for connection key
negotiation or parameter negotiation (MAC algorithm, length, use of
the TCP-SA option, or RID) 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-SA 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].
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TCP-SA 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-SA
benefits from the use of congestion control support for temporary
path outages.
>> TCP-SA 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-SA'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-SA, this context is
provided by the socket pair (IP addresses and ports for source and
destination). In the default case, the key is identified only in the
TSAD, and coordinated by a separate mechanism not specified in TCP-
SA. In cases where such coordination is difficult, or where loss
during rekeying is inappropriate, the TSAD can contain multiple
concurrent keys. Where multiple keys are used, 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 may also be useful in coordinating keys for new
connections. A TSAD may be configured that matches the unbound source
port, which would return a set of possible keys. The KeyID would then
indicate which key, allowing more efficient connection establishment;
otherwise, the keys could be tried in sequence. See also Section 5.1.
Implementations are encouraged to keep keys in a suitably private
area. Users of TCP-SA are encouraged to use different keys for
inbound and outbound MACs on a given TCP connection.
5.1. Implementing the TSAD as an External Database
The TSAD implementation is considered external to TCP-SA. When an
external database is used, it would be useful to consider the
interface between TCP-SA and the TSAD. The following is largely a
restatement of information in Section 3.
TSAD entries are indexed 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)
o KeyID (single byte, optional, 0x00 when not present)
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 (typically also indexed by the KeyID in
that case) are returned as a list. This feature is used during
connection establishment.
TSAD calls return the following parameters:
o TCP Option exclusion indicator bit (one bit, passed as a byte with
value 0x00 or 0x01).
o Receiver TCP connection ID (RID), i.e., the socket pair expected
on received segments. This differs from the ID above when one
party is behind a NAT/NAPT device; otherwise, it matches the ID.
o An ordered list of zero or more connection key tuples:
<KeyID, MAC type, MAC length, connection key>
o KeyID (one byte, ignored if the KeyID is not present or 0x00)
o MAC type (two bytes)
o Key length (one byte)
o Connection 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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6. Interactions with TCP/MD5
TCP-SA is intended to be deployed without regard for existing TCP/MD5
option support.
>> A TCP implementation MUST NOT use both TCP-SA and TCP/MD5 for a
particular TCP connection, but MAY support TCP-SA and TCP/MD5
simultaneously for different connections.
There is no need to explicitly indicate which of TCP-SA or TCP/MD5 is
used for a particular connection in the TCP segments. Even where the
two used the same hash (e.g., if TCP-SA were to use MD5 rather than
HMAC-MD5) and the same length (128 bits), TCP-SA computes its MAC
over different data (including the TCP-SA option, notably, with the
MAC zeroed) than TCP/MD5. The probability of a TCP-SA segment being
validated by TCP/MD5 or the converse is roughly equivalent to that of
a random party guessing a valid MAC.
7. Interactions with NAT/NAPT Devices
TCP-SA can work in the presence of one or more NAT/NAPT devices on
the path between endpoints [RFC2766]. This solution assumes that the
key configuration - either in an automated protocol, or via manual
discovery - knows the mapping between TCP connection IDs as sent from
the source host and TCP connection IDs as received at the destination
host. A key management protocol could determine the RID by including
a copy of their view of the ID in the key exchange; this serves as an
automatic NAT/NAPT discovery mechanism.
TCP-SA requires the source compute the MAC based on TCP connection ID
as received (received ID, or RID). The MAC is consistent with the
packet as received, rather than assuming a translation step at the
receiver. We note that this translation could occur at the receiver,
but want to reduce the complexity and potential additional effort
there.
8. Evaluation of Requirements Satisfaction
TCP-SA satisfies all the current requirements for a revision to
TCP/MD5, as indicated in [Be06] and under current developemt. 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 2.2.
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a. TCP pseudoheader, including IPv4 and IPv6 versions. Note that
we do not allow optional coverage because we present a
solution that does not require such to interoperate with
NAT/NAPT devices.
b. TCP header. Note that we do not allow port coverage to be
optional because we present a solution that does not require
this to interoperate with NAT/NAPT devices.
c. TCP options. Allows exclusion of any option desired except
TCP-SA, as required.
d. TCP data. Done.
2. Option structure requirements
a. Privacy. TCP-SA 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 longer option (this is
equivalent to a different MAC algorithm, and is specified in
the TSAD entry). See Section 2.2.
b. Allow optional per connection. Done - see Sections 4.3, 4.4,
and 4.5.
c. Require non-optional. Done - see Sections 4.3, 4.4, and 4.5.
d. Standard parsing. Done - see Section 2.2.
e. Compatible with Large Windows. Done - see Section 2.2. The
size of the option is intended to allow use with Large Windows
and SACK. See also Section 1.1, which indicates that TCP-SA is
4 bytes shorter than TCP/MD5 in the default case.
f. Compatible with SACK. Done - see Section 2.2. The size of the
option is intended to allow use with Large Windows and SACK.
See also Section 5 regarding key management. See also Section
1.1, which indicates that TCP-SA is 4 bytes shorter than
TCP/MD5 in the default case.
3. Cryptography requirements
a. Baseline defaults. TCP-SA uses HMAC-MD5-96 as the default, as
noted in Section 2.2.
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b. Good algorithms. TCP-SA uses HMAC-MD5-96 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-SA allows any desired algorithm,
subject to TCP option space limitations, as noted in Section
2.2. The TSAD allows separate connections to use different
algorithms.
d. Pre-TCP processing. Done - see Sections 4.3, 4.4, and 4.5.
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 3.
4. Keying requirements. TCP-SA 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 3.
b. Efficient rekeying. Supported by the KeyID. See Section 5.
c. Automated and manual keying. Supported by the TSAD interface.
See Section 5.
d. Key management agnostic. Supported by the TSAD interface. See
Section 5.1.
5. Expected constraints
a. Silent failure. Done - see Sections 4.3, 4.4, and 4.5.
b. At most one such option per segment. Done - see Section 2.2.
c. Outgoing all or none. Done - see Section 4.4.
d. Incoming all checked. Done - see Section 4.5.
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e. Non-interaction with TCP/MD5. Done - see Section 6.
f. Optional ICMP discard. Done - see Section 9.
g. Allows use of NAT/NAPT devices. Done - see Section 7.
h. Maintain TCP connection semantics, in which only the socket
pair defines a TCP association and all its security
parameters. Done - see Sections 3 and 7.
i. Try to avoid creating a CPU DOS attack opportunity. Done - see
Section 9.
9. Security Considerations
Use of TCP-SA, like use of TCP/MD5 or IPsec, will impact host
performance. Connections that are known to use TCP-SA can be attacked
by transmitting segments with invalid MACs. Attackers would need to
know only the TCP connection ID and TCP-SA Length value to
substantially impact the host's processing capacity. This is similar
to the susceptibility of IPsec to on-path attacks, where the IP
addresses and SPI would be visible. For IPsec, the entire SPI space
(32 bits) is arbitrary, whereas for routing protocols typically only
the source port (16 bits) is arbitrary. As a result, it would be
easier for an off-path attacker to spoof a TCP-SA 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-SA, 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.
TCP-SA 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-SA 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-SA does not address the issue of ICMP attacks on TCP. IPsec makes
recommendations regarding dropping ICMPs in certain contexts, or
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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-SA provides for TCP [Go07].
>> A TCP-SA 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-SA includes the TCP connection ID as received (i.e., the socket
pair in the headers of the received segment) in the MAC calculation,
to enable interoperability in the presence of NAT/NAPT devices. 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-SA, the socket pair performs most of the function
of IPsec's SPI, and IPsec's Sequence Number, used to avoid replay
attacks, isn't needed due to TCP's Sequence Number, which is used to
reorder received segments. Unfortunately, it is not useful to
validate TCP's Sequence Number before performing a TCP-SA
authentication calculation, because many out-of-window segments still
cause TCP protocol actions (e.g., ACK retransmission) [RFC793]. It is
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similarly not useful to add a separate Sequence Number field to the
TCP-SA option, because doing so could cause a change in TCP's
behavior even when segments are valid.
10. IANA Considerations
The TCP-SA option reuses the TCP MD5 Signature option (TCP/MD5),
where Kind=19. This document augments that use of this Kind value,
but there is no need to deprecate or override the use of TCP/MD5.
This document suggests that only one key algorithm would be
applicable in either case, and so there would be no confusion for a
given Length and key value as used for authenticating segments of a
given TCP connection.
If this document is approved as an IETF Standard, IANA is requested
to add a registration for TCP-SA to Kind=19, along with the existing
registration for TCP/MD5, and add a pointer to this document.
11. Acknowledgments
This document was inspired by the revisions to TCP/MD5 suggested by
Brian Weis and Ron Bonica [Bo07][We05]. Russ Housley suggested
L4/application layer management of the TSAD. The KeyID field was
motivated by Steve Bellovin. Alfred Hoenes provided substantial
feedback on this document.
This document was prepared using 2-Word-v2.0.template.dot.
12. References
12.1. 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.
[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.
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[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.
[RFC2403] Madson, C., R. Glenn, "The Use of HMAC-MD5-96 within ESP
and AH," RFC 2403, Proposed Standard, Nov. 1998.
12.2. Informative References
[Be05] Bellovin, S., E. Rescorla, "Deploying a New Hash
Algorithm," presented at the First NIST Cryptographic Hash
Workshop, Oct. 2005.
http://csrc.nist.gov/pki/HashWorkshop/2005/program.htm
[Be06] Bellovin, S. "Towards a TCP Security Option," draft-
bellovin-tcpsec-00, (work in progress), Oct. 2006.
[Bu06] Burr, B., "NIST Cryptographic Standards Status Report,"
Invited talk at Internet 2 5th Annual PKI R&D Workshop,
April 2006.
http://middleware.internet2.edu/pki06/proceedings/
[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-02, (work in progress), May 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.
[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.
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[RFC4301] Kent, S., K. Seo, "Security Architecture for the Internet
Protocol," RFC 4301, Proposed Standard, Dec. 2005.
[RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol," RFC
4306, Proposed Standard, Dec. 2005.
[RFC4808] Bellovin, S., "Key Change Strategies for TCP-MD5," RFC
4808, Informational, Mar. 2007.
[SDNS88] Secure Data Network Systems, "Security Protocol 4 (SP4),"
Specification SDN.401, Revision 1.2, July 12, 1988.
[To07] Touch, J., "Defending TCP Against Spoofing Attacks," draft-
ietf-tcpm-tcp-antispoof-06, (work in progress), Feb. 2007.
[We05] Weis, B., "TCP Message Authentication Code Option," draft-
weis-tcp-mac-option-00, (expired work in progress), Dec.
2005.
[We07] Weis, B., "Automated key selection extension for the TCP
Authentication Option," draft-weis-tcp-auth-auto-ks-02,
(work in progress), Mar. 2007.
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
Washington, DC
U.S.A.
Phone: 1 301 728 7199
Email: mankin@psg.com
URL: http://www.psg.com/~mankin/
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