TCPM WG J. Touch Internet Draft USC/ISI Expires: April 2007 A. Mankin October 22, 2006 The TCP Simple Authentication Option draft-touch-tcpm-tcp-simple-auth-02.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html This Internet-Draft will expire on April 22, 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 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. Touch & Mankin Expires April 22, 2007 [Page 1] Internet-Draft The TCP Simple Authentication Option October 2006 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. 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-02................5 1.2.2. New in draft-touch-tcp-simple-auth-01................5 1.3. Summary of RFC-2119 Requirements..........................5 2. The TCP Simple Authentication Option...........................5 2.1. Review of TCP/MD5 Option..................................6 2.2. TCP-SA Option.............................................6 3. Security Association Management................................9 4. TCP-SA Interaction with TCP...................................11 4.1. User Interface...........................................11 4.2. TCP States and Transitions...............................12 4.3. TCP Segments.............................................12 4.4. Sending TCP Segments.....................................13 4.5. Receiving TCP Segments...................................13 4.6. Impact on TCP Header Size................................14 5. Key Establishment and Duration Issues.........................14 5.1. Implementing the TSAD as an External Database............15 6. Interactions with TCP/MD5.....................................16 7. Security Considerations.......................................17 8. IANA Considerations...........................................18 9. Conclusions...................................................18 10. Acknowledgments..............................................18 11. References...................................................18 11.1. Normative References....................................18 11.2. Informative References..................................19 Author's Addresses...............................................20 Intellectual Property Statement..................................20 Disclaimer of Validity...........................................21 Copyright Statement..............................................21 Touch & Mankin Expires April 22, 2007 [Page 2] Internet-Draft The TCP Simple Authentication Option October 2006 Acknowledgment...................................................21 1. Introduction The TCP MD5 Signature (TCP/MD5) is a TCP option that authenticates TCP segments, including the TCP pseudo-header, 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][To06]. 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. 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. 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 [Be06b]. 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. 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. Touch & Mankin Expires April 22, 2007 [Page 3] Internet-Draft The TCP Simple Authentication Option October 2006 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). This document differs from currently competing proposals to update TCP/MD5 as follows [Bo06][We05][We06]: 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. 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) [Be06b]. o Does not support encryption. Touch & Mankin Expires April 22, 2007 [Page 4] Internet-Draft The TCP Simple Authentication Option October 2006 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-02 o Add reference to Bellovin's need-for-TCP-auth doc [Be06b]. 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. 1.2.2. 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. Touch & Mankin Expires April 22, 2007 [Page 5] Internet-Draft The TCP Simple Authentication Option October 2006 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] 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: Touch & Mankin Expires April 22, 2007 [Page 6] Internet-Draft The TCP Simple Authentication Option October 2006 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. 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 Touch & Mankin Expires April 22, 2007 [Page 7] Internet-Draft The TCP Simple Authentication Option October 2006 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. 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 defined over the following fields in the following order: 1. the TCP pseudoheader: IP source and destination addresses, zero- padded protocol number and segment length, all in network byte order, i.e., exactly as used for the TCP checksum [RFC793]: +--------+--------+--------+--------+ | Source Address | +--------+--------+--------+--------+ | Destination Address | +--------+--------+--------+--------+ | zero | Proto | TCP Length | +--------+--------+--------+--------+ Figure 5 TCP pseudoheader [RFC793] 2. TCP header, by default including options, and where the checksum and TCP-SA MAC fields are set to zero, all in network byte order 3. TCP data 4. Connection key Touch & Mankin Expires April 22, 2007 [Page 8] Internet-Draft The TCP Simple Authentication Option October 2006 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). Middleboxes may alter TCP options en-route are a kind of attack and would be successfully detected by default by TCP-SA. In cases where the configuration of the connection's security 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][Bo06][We05][We06]. 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 be shared 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. 2. For each of inbound (received TCP segments) and outbound (sent TCP segments) on this connection: a. TCP Option exclusion bit. A bit that, when set, indicates that TCP options are excluded from all MAC calculations. When the bit is clear, all TCP options are included. >> The TCP Option exclusion bit MUST default to "clear". >> The TCP Option exclusion bit MUST NOT change during a TCP connection. b. One or more connection key tuples. Each tuple consists of a set as follows: i. KeyID. A single byte used to differentiate overlapping Connection keys. Touch & Mankin Expires April 22, 2007 [Page 9] Internet-Draft The TCP Simple Authentication Option October 2006 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 computed (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, 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, TCP-SA MUST NOT occur in those segments. 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. 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 Touch & Mankin Expires April 22, 2007 [Page 10] Internet-Draft The TCP Simple Authentication Option October 2006 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 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. >> 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 (FINE- 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]. Touch & Mankin Expires April 22, 2007 [Page 11] Internet-Draft The TCP Simple Authentication Option October 2006 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. 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. Touch & Mankin Expires April 22, 2007 [Page 12] Internet-Draft The TCP Simple Authentication Option October 2006 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 and the outgoing MAC is NONE, omit the TCP-SA option. Proceed with computing the TCP checksum and transmit the segment. 4. If there is a TSAD entry 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 connection key, MAC, and data from the segment as specified in Section 2.2. c. Insert the MAC in the TCP-SA field. d. Proceed with computing the TCP checksum 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 and the incoming MAC is NONE, proceed with TCP processing. 4. If there is a TSAD entry and the incoming MAC is not NONE: a. Check that the segment's TCP-SA Length matches the length indicated by the indexed TSAD. Touch & Mankin Expires April 22, 2007 [Page 13] Internet-Draft The TCP Simple Authentication Option October 2006 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 MAC algorithm and connection key, and portions of the segment as indicated in Section 2.2. 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, or use of the TCP-SA 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-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]. TCP-SA supports rekeying in which new keys are negotiated out-of- band, either via a protocol or a manual procedure [Be06a]. New keys Touch & Mankin Expires April 22, 2007 [Page 14] Internet-Draft The TCP Simple Authentication Option October 2006 use is coordinated using the out-of-band mechanism to update the LSAD 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 LSAD, 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: o TCP connection identifier (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) Touch & Mankin Expires April 22, 2007 [Page 15] Internet-Draft The TCP Simple Authentication Option October 2006 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 Zero or more connection key tuples: 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. 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 Touch & Mankin Expires April 22, 2007 [Page 16] Internet-Draft The TCP Simple Authentication Option October 2006 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. 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 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 [Go06]. Touch & Mankin Expires April 22, 2007 [Page 17] Internet-Draft The TCP Simple Authentication Option October 2006 >> 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]. 8. 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. 9. Conclusions (to be completed) 10. Acknowledgments This document was inspired by the revisions to TCP/MD5 suggested by Brian Weis and Ron Bonica [Bo06][We05]. Russ Housley suggested L4/application layer management of the TSAD. The KeyID field was motivated by Steve Bellovin. 11. References 11.1. 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. Touch & Mankin Expires April 22, 2007 [Page 18] Internet-Draft The TCP Simple Authentication Option October 2006 [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. [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. 11.2. 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 [Be06a] Bellovin, S., "Key Change Strategies for TCP-MD5," draft- bellovin-keyroll2385-03.txt, (work in progress), Sept. 2006. [Be06b] Bellovin, S., "Towards a TCP Security Option," draft- bellovin-tcpsec-00.txt, (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/ [Bo06] Bonica, R., "Authentication for TCP-based Routing and Management Protocols," draft-bonica-tcp-auth-05, (work in progress), Jul. 2006. [Go06] Gont, F., "ICMP attacks against TCP," draft-ietf-tcpm-icmp- attacks-00.txt, (expired work in progress), Feb. 2006. [RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm," RFC-1321, Informational, April 1992. [RFC3562] Leech, M., "Key Management Considerations for the TCP MD5 Signature Option," RFC-3562, Informational, July 2003. Touch & Mankin Expires April 22, 2007 [Page 19] Internet-Draft The TCP Simple Authentication Option October 2006 [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. [SDNS88] Secure Data Network Systems, "Security Protocol 4 (SP4)," Specification SDN.401, Revision 1.2, July 12, 1988. [To06] Touch, J., "Defending TCP Against Spoofing Attacks," draft- ietf-tcpm-tcp-antispoof-05.txt, (work in progress), Oct. 2006. [We05] Weis, B., "TCP Message Authentication Code Option," draft- weis-tcp-mac-option-00.txt, (expired work in progress), Dec. 2005. [We06] Weis, B., "Automated key selection extension for the TCP Authentication Option," draft-weis-tcp-auth-auto-ks-01, (work in progress), Feb. 2006. 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/ Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to Touch & Mankin Expires April 22, 2007 [Page 20] Internet-Draft The TCP Simple Authentication Option October 2006 pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. 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Disclaimer of Validity This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Copyright Statement Copyright (C) The Internet Society (2006). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Touch & Mankin Expires April 22, 2007 [Page 21]