Network Working Group X. Xiao Internet Draft E. Crabbe A. Hannan Frontier Globalcenter V. Paxson ACIRI/ICSI Expiration Date: March 2000 September 1999 TCP Processing of the IP Precedence Field 1. Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. 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 document will expire in March 2000. 2. Abstract This draft describes a potential conflict between TCP [RFC793] and DiffServ [RFC2475] on the use of the three leftmost bits in the TOS octet of an IPv4 header [RFC791] or the Traffic Class octet of an IPv6 header [RFC2460]. In a network that contains DiffServ capable nodes, such a conflict can cause failures in establishing TCP connections or can cause some established TCP connections to be reset undesirably. This draft proposes a modification to TCP for resolving the conflict. Xiao et al [Page 1] ID TCP and the IP Precedence Field September 1999 3. Introduction In TCP, each connection has a set of states associated with it. Such states are reflected by a set of variables stored in the TCP Control Block (TCB) of both ends. Such variables may include the local and remote socket number, precedence of the connection, security level and compartment, etc. Both ends must agree on the setting of the precedence and security parameters in order to establish a connection and keep it open. There is no field in the TCP header that indicates the precedence of a segment. Instead, the precedence field in the header of the IP packet is used as the indication. The security level and compartment are likewise carried in the IP header, but as IP options rather than a fixed header field. Because of this difference, the problem with precedence discussed in this memo does not apply to them. TCP requires that the precedence (and security parameters) of a connection must remain unchanged during the lifetime of the connection. Therefore, for an established TCP connection with precedence, the receipt of a segment with different precedence indicates an error. The connection must be reset [RFC793, page 37]. With the advent of DiffServ, intermediate nodes may modify the Differentiated Services Codepoint (DSCP) [RFC2474] of the IP header to indicate the desired Per-hop Behavior (PHB) [RFC2475, RFC2597, RFC2598]. The DSCP includes the three bits formerly known as the precedence field. Because any modification to those three bits will be considered illegal by endpoints that are precedence-aware, they may cause failures in establishing connections, or may cause established connections to be reset. 4. Terminology Segment: the unit of data that TCP sends to IP Precedence Field: the three leftmost bits in the TOS octet of an IPv4 header or the Traffic Class octet of an IPv6 header. Note that in DiffServ, these three bits may or may not used to denote the precedence of the IP packet. TOS Field: bits 3-6 in the TOS octet of IPv4 header [RFC 1349]. MBZ field: Must Be Zero The structure of the TOS octet is depicted below: Xiao et al [Page 2] ID TCP and the IP Precedence Field September 1999 0 1 2 3 4 5 6 7 +-----+-----+-----+-----+-----+-----+-----+-----+ | PRECEDENCE | TOS | MBZ | +-----+-----+-----+-----+-----+-----+-----+-----+ Traffic Class Octet: the TOS octet's counterpart in IPv6 DS Field: the TOS octet of an IPv4 header, or the Traffic Class octet of an IPv6 header, is renamed the Differentiated Services (DS) Field by DiffServ. The structure of the DS field is depicted below: 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | DSCP | CU | +---+---+---+---+---+---+---+---+ DSCP: Differentiated Service Code Point, the leftmost 6 bits in the DS field. CU: currently unused. Per-hop Behavior (PHB): a description of the externally observable forwarding treatment applied at a differentiated services-compliant node to a behavior aggregate. 5. Problem Description The manipulation of the DSCP to achieve the desired PHB by DiffServ- capable nodes may conflict with TCP's use of the precedence field. This conflict can potentially cause problems for TCP implementations that conform to RFC 793. First, page 36 of RFC 793 states: If the connection is in any non-synchronized state (LISTEN, SYN-SENT, SYN-RECEIVED), and the incoming segment acknowledges something not yet sent (the segment carries an unacceptable ACK), or if an incoming segment has a security level or compart- ment which does not exactly match the level and compartment requested for the connection, a reset is sent. If our SYN has not been acknowledged and the precedence level of the incoming segment is higher than the precedence level requested then either raise the local precedence level (if allowed by the user and the system) or send a reset; or if the precedence level of the incoming segment is lower than the precedence level requested then continue as if the precedence matched exactly (if the remote TCP cannot raise the precedence level to match ours Xiao et al [Page 3] ID TCP and the IP Precedence Field September 1999 this will be detected in the next segment it sends, and the con- nection will be terminated then). If our SYN has been ack- nowledged (perhaps in this incoming segment) the precedence level of the incoming segment must match the local precedence level exactly, if it does not a reset must be sent. This leads to Problem #1: For a precedence-aware TCP module, if dur- ing TCP's synchronization process, the precedence fields of the SYN and/or ACK packets are modified by the intermediate nodes, resulting in the received ACK packet having a different precedence from the precedence picked by this TCP module, the TCP connection cannot be established, even if both modules actually agree on an identical pre- cedence for the connection. Then, on page 37, RFC 793 states: If the connection is in a synchronized state (ESTABLISHED, FIN- WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK, TIME-WAIT), ... [unrelated statements snipped] If an incoming segment has a security level, or compartment, or precedence which does not exactly match the level, and compartment, and precedence requested for the connection, a reset is sent and connection goes to the CLOSED state. This leads to Problem #2: For a precedence-aware TCP module, if the precedence field of a received segment from an established TCP con- nection has been changed en route by the intermediate nodes so as to be different from the precedence specified during the connection setup, the TCP connection will be reset. Each of problems #1 and #2 has a mirroring problem. They cause TCP connections that must be reset according to RFC 793 not to be reset. Problem #3: A TCP connection may be established between two TCP modules that pick different precedence, because the precedence fields of the SYN and ACK packets are modified by intermediate nodes, resulting in both modules thinking that they are in agreement for the precedence of the connection. Problem #4: A TCP connection has been established normally by two TCP modules that pick the same precedence. But in the middle of the data transmission, one of the TCP modules changes the precedence of its segments. According to RFC 793, the TCP connection must be reset. In a DiffServ-capable environment, if the precedence of the segments is altered by intermediate nodes such that it retains the expected value when arriving at the other TCP module, the connection will not be reset. Xiao et al [Page 4] ID TCP and the IP Precedence Field September 1999 6. Proposed Modification to TCP The proposed modification to TCP is that TCP must ignore the pre- cedence of all received segments. More specifically: (1) In TCP's synchronization process, the TCP modules at both ends must ignore the precedence fields of the SYN and ACK packets. The TCP connection will be established if all the conditions specified by RFC 793 are satisfied except the precedence of the connection. (2) After a connection is established, each end sends segments with its desired precedence. The two ends' precedence may be the same or may be different (because precedence is ignored during connection setup time). The precedence fields may be changed by the intermediate nodes too. They will be ignored by the other end. The TCP connection will not be reset. Problems #1 and #2 are solved by this proposed modification. Problems #3 and #4 become non-issues because TCP must ignore the precedence. In a DiffServ-capable environment, the two cases described in prob- lems #3 and #4 should be allowed. The proposed modification to TCP is in conformance with TCP's design philosophy. In RFC 793, page 36, it is stated that: As a general rule, reset (RST) must be sent whenever a segment arrives which apparently is not intended for the current connec- tion. A reset must not be sent if it is not clear that this is the case. With the deployment of DiffServ, the precedence field can be modified by intermediate network nodes. A change in the precedence of a received segment does not necessarily indicate that the segment is not intended for the connection. Therefore, a RST must not be sent based solely on the change of the precedence of a received segment, no matter whether the TCP connection is in a non-synchronized state or synchronized state. 7. Security Considerations The RST generation rules given on page 37 of RFC 793 appear to indi- cate that the reception of any segment with an incorrect precedence field terminates a connection. If this is true regardless of the correctness of the sequence numbers in the segment's header, then the RFC 793 rules present a serious denial-of-service threat, as all an attacker must do to terminate a connection is guess the port numbers and then send two segments with different precedence values; one of Xiao et al [Page 5] ID TCP and the IP Precedence Field September 1999 them is certain to terminate the connection. Accordingly, the change to TCP processing proposed in this memo would yield a significant gain in terms of TCP resilience. On the other hand, the stricter processing rules of RFC 793 in prin- ciple make TCP spoofing attacks more difficult, as the attacker must not only guess the victim TCP's initial sequence number, but also its precedence setting. Finally, the security issues of each PHB group are addressed in the PHB group's specification [RFC2597, RFC2598]. 8. References [RFC791] Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981. [RFC793] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, September 1981. [RFC1349] Almquist, P., "Type of Service in the Internet Protocol Suite", RFC 1349, July 1992. [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998. [RFC2474] Nichols, K., Blake, S., Baker, F. and D. Black, "Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers", RFC 2474, December 1998. [RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z. and W. Weiss, "An Architecture for Differentiated Services", RFC 2475, December 1998. [RFC2597] Heinanen, J., Baker, F., Weiss, W. and J. Wroclawski, "Assured Forwarding PHB Group", RFC 2587, June 1999. [RFC2598] Jacobson, V., Nichols, K. and K. Poduri, "An Expedited Xiao et al [Page 6] ID TCP and the IP Precedence Field September 1999 Forwarding PHB", RFC 2598, June 1999. 9. Authors' Addresses Xipeng Xiao Frontier Globalcenter 141 Caspian Court Sunnyvale, CA 94087 USA Phone: +1 408-543-4801 Edward Crabbe Frontier Globalcenter 141 Caspian Court, Sunnyvale, CA 94087 USA Phone: +1 408-543-4827 Alan Hannan Frontier Globalcenter 141 Caspian Court, Sunnyvale, CA 94087 USA Phone: +1 408-543-4891 Vern Paxson ACIRI/ICSI 1947 Center Street Suite 600 Berkeley, CA 94704-1198 USA Phone: +1 510-642-4274 x302 Xiao et al [Page 7]