Network Working Group Y. Nishida Internet-Draft GE Global Research Intended status: Experimental July 28, 2013 Expires: January 29, 2014 A-PAWS: Alternative Approach for PAWS draft-nishida-tcpm-apaws-00 Abstract This documents describe a technique called A-PAWS which can provide protection against old duplicates segments like PAWS. While PAWS requires TCP to set timestamp options in all segments in a TCP connection, A-PAWS supports the same feature without using timestamps. A-PAWS is designed to be used complementary with PAWS. TCP needs to use PAWS when it is necessary and activates A-PAWS only when it is safe to use. Without impairing the reliability and the robustness of TCP, A-PAWS can provide more option space to other TCP extensions. Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. 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." This Internet-Draft will expire on January 29, 2014. Copyright Notice Copyright (c) 2013 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect Nishida Expires January 29, 2014 [Page 1] Internet-Draft Alternative PAWS July 2013 to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Conventions and Terminology . . . . . . . . . . . . . . . . . 5 3. The A-PAWS Design . . . . . . . . . . . . . . . . . . . . . . 6 3.1. Signaling Method . . . . . . . . . . . . . . . . . . . . . 6 3.2. Sending Behavior . . . . . . . . . . . . . . . . . . . . . 7 3.3. Receiving Behavior . . . . . . . . . . . . . . . . . . . . 7 4. When To Use A-PAWS . . . . . . . . . . . . . . . . . . . . . . 8 5. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 9 5.1. Protection Against Early Incarnations . . . . . . . . . . 9 5.2. Protection Against Security Threats . . . . . . . . . . . 9 5.3. Middlebox Considerations . . . . . . . . . . . . . . . . . 9 5.4. Aggressive Mode in A-PAWS . . . . . . . . . . . . . . . . 10 6. Security Considerations . . . . . . . . . . . . . . . . . . . 11 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13 8.1. Normative References . . . . . . . . . . . . . . . . . . . 13 8.2. Informative References . . . . . . . . . . . . . . . . . . 13 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 15 Nishida Expires January 29, 2014 [Page 2] Internet-Draft Alternative PAWS July 2013 1. Introduction PAWS (Protect Against Wrapped Sequences) defined in [RFC1323] is a technique that can identify old duplicate segments in a TCP connection. An old duplicate segment can be generated when it has been delayed by queueing, etc. If such a segment has the sequence number which falls within the receiver's current window, the receiver will accept it without any warning or error. However, this segment can be a segment created by an old connection that has the same port and address pair, or a segments sent 2**32 bytes earlier on the same connection. Although this situation rarely happens, it impairs the reliability of TCP. PAWS utilizes timestamp option in [RFC1323] to provide protection against this. It is assumed that every received TCP segment contains a timestamp. PAWS can identify old duplicate segments by comparing the timestamp in the received segments and the timestamps from other segments received recently. If both TCP endpoints agree to use PAWS, all segments belong to this connection should have timestamp. Since PAWS is the only standardized protection against old duplicate segments, it has been implemented and used in most TCP implementations. However, as some TCP extensions such as [RFC2018], [RFC5925] and [RFC6824] also requires a certain amount of option space in non-SYN segments, using 10-12 bytes length in option space for timestamp in all segments tends to be considered expensive in recent discussions. In addition, although PAWS is necessary for connections which transmit more than 2**32 bytes, it is not very important for other connections since [RFC0793] already has protection against segments from old connections by using timers. Moreover, some research results indicates that most of TCP flows tend to transmit small amount of data, which means only small fraction of TCP connections really need PAWS [QIAN11]. Timestamp option is also used for RTTM (Round Trip Time Measurement) in [RFC1323]. Gathering many RTT samples from the timestamp in every TCP segment looks useful approach to improve RTO estimation. However, some research results shows the number of samples per RTT does not affect the effectiveness of the RTO [MALLMAN99]. Hence, we can think if PAWS is not used, sending a few timestamps per RTT will be sufficient. Based on these observations, we propose a new technique called A-PAWS which can archive similar protection against old duplicates segments. The basic idea of A-PAWS is not using option in non-SYN segments, while it can attain the same protection against old all duplicate segments as PAWS. A-PAWS is designed to be used complementary with PAWS. This means an implementation that supports A-PAWS is still required to supports PAWS. A-PAWS is activated only when it is safe Nishida Expires January 29, 2014 [Page 3] Internet-Draft Alternative PAWS July 2013 to use. This sounds the applicability of A-PAWS is limited, however, we believe TCP will have a lot of chances to save the option space if it uses A-PAWS. There are some discussions that PAWS can also be used to enhance security, however, we still believe that A-PAWS can maintain the same level of security as PAWS. Detailed discussions on this point are provided in Section 5. A-PAWS is an experimental idea yet, but we hope it will contribute to facilitating the use of TCP option space. Nishida Expires January 29, 2014 [Page 4] Internet-Draft Alternative PAWS July 2013 2. Conventions and Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. Nishida Expires January 29, 2014 [Page 5] Internet-Draft Alternative PAWS July 2013 3. The A-PAWS Design A-PAWS assumes PAWS as it is designed to be used complementary with PAWS. Hence, a node which supports A-PAWS MUST support PAWS. The following mechanisms are required in TCP in order to perform A-PAWS. 3.1. Signaling Method A-PAWS requires a signaling mechanism to enable this fature. This is because A-PAWS presumes the use of timestamp, while it allows TCP to transmit segments without timestamp. In [RFC1323], once timestamp option has been negotiated to be used in the connection, TCP stacks are required to set timestamp options in all TCP segments. If TCP receives segments without TS option after timestamp negotiation, the segments will be discarded. Hence, if one endpoint uses A-PAWS and the other endpoint uses PAWS, the segments from the A-PAWS endpoint will be discarded. The signaling mechanism for A-PAWS should address this point. The followings are possible approach. 1) Synchronous Option Exchange This method requires using new codepoints for TCP option and exchange them during SYN negotiation. The use of [I-D.ietf-tcpm-experimental-options] is recommended. Since A-PAWS negotiation is 1 bit information (on or off), new option does not need to have any content. Hence, if 16-bit EXID is used, the option size for A-PAWS can be kept in 4 bytes length. We currently recommend the use of different codepoints for SYN and SYN-ACK. This is because it is known that some TCP implementations blindly echo-back their unknown options in SYN segments. Also, it is recommended to set the option in the third ACK: ACK for SYN-ACK for robust signaling in case of the lost of the option in SYN-ACK. 2) Asynchronous Option Exchange This method also uses new codepoints for TCP option and exchange them during SYN negotiation. However, the signaling of this method is treated as unidirectional. This means when TCP set A-PAWS option, it indicates that this endpoint is willing to accept segments without timestamps by A-PAWS logic. However, it does not indicate this endpoint will transmit segments without timestamp. TCP is allowed to transmit segments with A-PAWS logic only when it receives the segments with A-PAWS option. 3) Option Exchange in non-SYN Segments This method is similar to the synchronous or asynchronous option exchange in SYN segments, however, it uses A-PAWS option in non- SYN segments for signaling. This approach does not need to utilize option space in SYN segments which is highly competitive Nishida Expires January 29, 2014 [Page 6] Internet-Draft Alternative PAWS July 2013 resource. 4) Timestamp Field Negotiation This method is based on timestamp field negotiation proposed in [I-D.scheffenegger-tcpm-timestamp-negotiation]. The draft proposes to utilize timestamp values in SYN segments in order to negotiate additional capabilities for timestamps. Although the detailed mechanisms are still under discussion, it does not require option space. 3.2. Sending Behavior A-PAWS enabled TCP transmits segments, it needs to follow the rules below. 1. TCP needs to check how many bytes has been transmitted in a connection. If the transmitted bytes exceeds 2**32 - 'Sender.Offset', TCP migrates PAWS mode and MUST set timestamp option in all segments to be transmitted. The value for 'Sender.Offset' is discussed in Section 5. 2. If the number of bytes transmitted in a TCP connection does not exceeds 2**32 - 'Sender.Offset', TCP MAY omit timestamp option in segments as long as it does not affect RTTM. This draft does not define how much TCP can omit timestamps because it should be determined by RTTM. 3.3. Receiving Behavior A-PAWS enabled TCP receives segments, it needs to follow the rules below. 1. TCP needs to check how many bytes has been received in a TCP connection. If it exceeds 2**32 bytes, A-PAWS nodes SHOULD discard the received segments which does not have timestamp option. TCP MUST perform PAWS check when received bytes exceeds 2**32 bytes. 2. If the number of bytes received in a TCP connection does not exceeds 2**32 bytes, A-PAWS nodes SHOULD accept the segments even if it does not have timestamp option. A-PAWS nodes MAY skip PAWS check until the received bytes exceeds 2**32 bytes. Nishida Expires January 29, 2014 [Page 7] Internet-Draft Alternative PAWS July 2013 4. When To Use A-PAWS In basic principal, A-PAWS capable nodes can always use A-PAWS as long as the peers agree with them. However, the following cases require special considerations to enable A-PAWS. 1. As "When To Keep Quiet" section in [RFC0793] suggests, it is recommended that TCP keeps quiet for a MSL upon starting up or recovering from a crash where memory of sequence numbers has been lost. However, if timestamps are being used and if the timestamp clock can be guaranteed to be increased monotonically, this quiet time may be unnecessary. Because TCP can identify the segments from old connections by checking the timestamp. We think some TCP implementations may disable the quiet time because of using timestamps from this reason. However, since A-PAWS nodes does not set timestamp options in all segments, TCP cannot rely on this approach. To avoid decreasing the robustness of TCP connection, TCP MUST NOT use A-PAWS for a MSL upon starting up or recovering from a crash. 2. Various TCP implementations provide APIs such as setsockopt() that can set SO_REUSEADDR flag on TCP connections. If this flag is set, the TCP connection allows to reuse the same local port without waiting for 2 MSL period. While this option is useful when users want to relaunch applications immediately, it makes the TCP connection a little vulnerable as TCP stack might receive duplicate segments from earlier incarnations. It has been said that PAWS can contribute to mitigate this risk by checking the timestamps in segments. In order to keep the same level of protection, TCP SHOULD NOT enable A-PAWS when SO_REUSEADDR flag is set. Nishida Expires January 29, 2014 [Page 8] Internet-Draft Alternative PAWS July 2013 5. Discussion As A-PAWS is an experimental logic, the following points need to be considered and discussed. 5.1. Protection Against Early Incarnations There are some discussions that timestamp can enhance the robustness against early incarnations. Since A-PAWS does not set timestamps in all segments, some may say that it degrades the robustness of TCP. We believe that the degradation caused by A-PAWS on this point is negligible. As long as TCP limits the usage of A-PAWS as described in Section 4, duplicate segments from early incarnations should not be received by TCP. 5.2. Protection Against Security Threats A TCP connection can be identified by a 5-tuple: source address, destination address, source port number, destination port number and protocol. Crackers need to guess all these parameters when they try malicious attacks on the connection. PAWS can enhance the protection for this as it additionally requires timestamp checking. However, we think the effect of PAWS against malicious attacks is limited due to the simplicity of PAWS check. In PAWS, a segment can be considered as an old duplicate if the timestamp in the segment less than some timestamps recently received on the connection. The "less than" in this context is determined by processing timestamp values as 32 bit unsigned integers in a modular 32-bit space. For example, if t1 and t2 are timestamp values, t1 < t2 is verified when 0 < (t2 - t1) < 2**31 computed in unsigned 32-bit arithmetic. Hence, if crackers set a random value in the timestamp option, there will be 50% chance for them to trick PAWS check. Moreover, there will be more chances if they send multiple segments with different timestamps, which will not be difficult to perform. In addition, we think there might be a case where using PAWS increases security risks. PAWS recommends to increase timestamp over a system when TCP waives the "quiet time" described in [RFC0793]. However, if timestamps are generated from a global counter, it may leak some information such as system uptime as discussed in [SILBERSACK05]. A-PAWS might be able to allows TCP to use random timestamp values per connections. 5.3. Middlebox Considerations There might be a case where the signaling for A-PAWS is confused by middleboxes. For example, in case of synchronous option exchange, if the option for A-PAWS is removed from SYN-ACK, we will have a Nishida Expires January 29, 2014 [Page 9] Internet-Draft Alternative PAWS July 2013 situation where one node enables A-PAWS and the other node disables A-PAWS. When an A-PAWS endpoint sends segments without timestamp and a PAWS endpoint receives them, the segments will be discarded. Hence, if synchronous option exchange is used, we recommend to set A-PAWS option in the third ACK for robust signaling. If the third ACK does not contain A-PAWS option, the receiver should disable A-PAWS. In case of asynchronous option exchange signaling, endpoints will not be messed up even if middleboxes discard A-PAWS option. This is because A-PAWS sender logic is activated only when TCP receives a segment with A-PAWS options. A-PAWS receiver logic does not need to know whether the sender is using PAWS or A-PAWS. On the other hand, as an A-PAWS endpoint always needs to activate A-PAWS receiver logic even if the peer uses PAWS, it will require extra overhead compared to the synchronous signaling. However, we believe the overhead will be acceptable in most cases because of the simplicity of A-PAWS logic. Another concern on middleboxes is that they can insert or delete some bytes in TCP connections. If a middlebox inserts extra bytes into a TCP connections, there might be a situation where an A-PAWS sender can transmit segments without timestamp, while an A-PAWS receiver perform PAWS check on them as it already has received 2**32 bytes. In order to avoid discarding segments unnecessarily, we recommend that A-PAWS sender should have a certain amount of offset bytes in order to migrate PAWS mode before the receiver receives 2**32 bytes. We call this protocol parameter 'Sender.Offset'. The proper value for 'Sender.Offset' needs to be discussed. 5.4. Aggressive Mode in A-PAWS The current A-PAWS requires TCP to migrate PAWS mode after sending/ receiving 2**32 bytes. However, if both nodes check if 2 MSL has already passed during sending/receiving 2**32 bytes, they might be able to continue using A-PAWS. We call this "Aggressive mode". We currently do not recommend this mode as it requires additional complexities in A-PAWS. Nishida Expires January 29, 2014 [Page 10] Internet-Draft Alternative PAWS July 2013 6. Security Considerations We believe A-PAWS can maintain the same level of security as PAWS does, but further discussions will be needed. Some security aspects of A-PAWS are discussed in Section 5. Nishida Expires January 29, 2014 [Page 11] Internet-Draft Alternative PAWS July 2013 7. IANA Considerations This document may request new TCP option codepoints or other resources. Nishida Expires January 29, 2014 [Page 12] Internet-Draft Alternative PAWS July 2013 8. References 8.1. Normative References [RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, September 1981. [RFC1323] Jacobson, V., Braden, B., and D. Borman, "TCP Extensions for High Performance", RFC 1323, May 1992. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. 8.2. Informative References [I-D.ietf-tcpm-experimental-options] Touch, J., "Shared Use of Experimental TCP Options", draft-ietf-tcpm-experimental-options-06 (work in progress), June 2013. [I-D.scheffenegger-tcpm-timestamp-negotiation] Scheffenegger, R., Kuehlewind, M., and B. Trammell, "Additional negotiation in the TCP Timestamp Option field during the TCP handshake", draft-scheffenegger-tcpm-timestamp-negotiation-05 (work in progress), October 2012. [MALLMAN99] Allman, M. and V. Paxson, "On Estimating End-to-End Network Path Properties", Proceedings of the ACM SIGCOMM , September 1999. [QIAN11] Qian, L. and B. Carpenter, "A Flow-Based Performance Analysis of TCP and TCP Applications", 3rd International Conference on Computer and Network Technology (ICCNT 2011) . [RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP Selective Acknowledgment Options", RFC 2018, October 1996. [RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP Authentication Option", RFC 5925, June 2010. [RFC6824] Ford, A., Raiciu, C., Handley, M., and O. Bonaventure, "TCP Extensions for Multipath Operation with Multiple Addresses", RFC 6824, January 2013. [SILBERSACK05] Nishida Expires January 29, 2014 [Page 13] Internet-Draft Alternative PAWS July 2013 Silbersack, M., "Improving TCP/IP security through randomization without sacrificing interoperability.", EuroBSDCon 2005 , November 2005. Nishida Expires January 29, 2014 [Page 14] Internet-Draft Alternative PAWS July 2013 Author's Address Yoshifumi Nishida GE Global Research 2623 Camino Ramon San Ramon, CA 94583 USA Email: nishida@wide.ad.jp Nishida Expires January 29, 2014 [Page 15]