TCPM WG J. Touch Internet Draft USC/ISI Intended status: Informational M. Welzl Expires: April 2017 S. Islam University of Oslo J. You Huawei October 28, 2016 TCP Control Block Interdependence draft-touch-tcpm-2140bis-01.txt Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. 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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 Touch, et al. Expires April 28, 2017 [Page 1] Internet-Draft TCP Control Block Interdependence October 2016 This Internet-Draft will expire on April 28, 2016. Copyright Notice Copyright (c) 2016 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 to this document. Abstract This memo describes interdependent TCP control blocks, where part of the TCP state is shared among similar concurrent or consecutive connections. TCP state includes a combination of parameters, such as connection state, current round-trip time estimates, congestion control information, and process information. Most of this state is maintained on a per-connection basis in the TCP Control Block (TCB), but implementations can (and do) share certain TCB information across connections to the same host. Such sharing is intended to improve overall transient transport performance, while maintaining backward-compatibility with existing implementations. The sharing described herein is limited to only the TCB initialization and so has no effect on the long-term behavior of TCP after a connection has been established. Table of Contents 1. Introduction...................................................3 2. Conventions used in this document..............................3 3. Terminology....................................................4 4. The TCP Control Block (TCB)....................................4 5. TCB Interdependence............................................5 6. An Example of Temporal Sharing.................................5 7. An Example of Ensemble Sharing.................................7 8. Compatibility Issues...........................................9 9. Implications..................................................11 10. Implementation Observations..................................12 11. Security Considerations......................................13 12. IANA Considerations..........................................14 13. References...................................................15 13.1. Normative References....................................15 Touch Expires April 28, 2017 [Page 2] Internet-Draft TCP Control Block Interdependence October 2016 13.2. Informative References..................................15 14. Acknowledgments..............................................17 1. Introduction TCP is a connection-oriented reliable transport protocol layered over IP [RFC793]. Each TCP connection maintains state, usually in a data structure called the TCP Control Block (TCB). The TCB contains information about the connection state, its associated local process, and feedback parameters about the connection's transmission properties. As originally specified and usually implemented, most TCB information is maintained on a per-connection basis. Some implementations can (and now do) share certain TCB information across connections to the same host.. Such sharing is intended to lead to better overall transient performance, especially for numerous short-lived and simultaneous connections, as often used in the World-Wide Web [Be94],[Br02]. This document discusses TCB state sharing that affects only the TCB initialization, and so has no effect on the long-term behavior of TCP after a connection has been established. Path information shared across SYN destination port numbers assumes that TCP segments having the same host-pair experience the same path properties, irrespective of TCP port numbers. The observations about TCB sharing in this document apply similarly to any protocol with congestion state, including SCTP [RFC4960] and DCCP [RFC4340], as well as for individual subflows in Multipath TCP [RFC6824]. 2. Conventions used in this document The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119]. In this document, these words will appear with that interpretation only when in ALL CAPS. Lower case uses of these words are not to be interpreted as carrying significance described in RFC 2119. In this document, the characters ">>" preceding an indented line(s) indicates a statement using the key words listed above. This convention aids reviewers in quickly identifying or finding the portions of this RFC covered by these keywords. Touch Expires April 28, 2017 [Page 3] Internet-Draft TCP Control Block Interdependence October 2016 3. Terminology Host - a source or sink of TCP segments associated with a single IP address Host-pair - a pair of hosts and their corresponding IP addresses Path - an Internet path between the IP addresses of two hosts 4. The TCP Control Block (TCB) A TCB describes the data associated with each connection, i.e., with each association of a pair of applications across the network. The TCB contains at least the following information [RFC793]: Local process state pointers to send and receive buffers pointers to retransmission queue and current segment pointers to Internet Protocol (IP) PCB Per-connection shared state macro-state connection state timers flags local and remote host numbers and ports TCP option state micro-state send and receive window state (size*, current number) round-trip time and variance cong. window size (snd_cwnd)* cong. window size threshold (ssthresh)* path maximum transmission unit (PMTU)* max window size seen* MSS# round-trip time and variance# The per-connection information is shown as split into macro-state and micro-state, terminology borrowed from [Co91]. Macro-state describes the finite state machine; we include the endpoint numbers and components (timers, flags) used to help maintain that state. Macro-state describes the protocol for establishing and maintaining shared state about the connection. Micro-state describes the protocol after a connection has been established, to maintain the reliability and congestion control of the data transferred in the connection. Touch Expires April 28, 2017 [Page 4] Internet-Draft TCP Control Block Interdependence October 2016 We further distinguish two other classes of shared micro-state that are associated more with host-pairs than with application pairs. One class is clearly host-pair dependent (#, e.g., MSS, RTT), and the other is host-pair dependent in its aggregate (*, e.g., congestion window information, current window sizes, etc.). 5. TCB Interdependence There are two cases of TCB interdependence. Temporal sharing occurs when the TCB of an earlier (now CLOSED) connection to a host is used to initialize some parameters of a new connection to that same host, i.e., in sequence. Ensemble sharing occurs when a currently active connection to a host is used to initialize another (concurrent) connection to that host. 6. An Example of Temporal Sharing The TCB data cache is accessed in two ways: it is read to initialize new TCBs and written when more current per-host state is available. New TCBs are initialized using context from past connections as follows: TEMPORAL SHARING - TCB Initialization Cached TCB New TCB ---------------------------------------- old_PMTU old_PMTU old_MSS old_MSS old_RTT old_RTT old_RTTvar old_RTTvar old_option (option specific) old_ssthresh old_ssthresh old_snd_cwnd old_snd_cwnd Most cached TCB values are updated when a connection closes. Two exceptions are PMTU, which is updated after Path MTU Discovery [RFC4821], and MSS, which is updated whenever the MSS option is received in a TCP header. Touch Expires April 28, 2017 [Page 5] Internet-Draft TCP Control Block Interdependence October 2016 Sharing MSS information affects only data in the SYN of the next connection, because MSS information is typically included in most TCP segments. [TBD - complete this section with details for TFO and other options whose state may, must, or must not be shared] The way in which other TCP option state can be shared depends on the details of that option. E.g., TFO state includes the TCP Fast Open Cookie [RFC7413] or, in case TFO fails, a negative TCP Fast Open response (from [RFC 7413]: "The client MUST cache negative responses from the server in order to avoid potential connection failures. Negative responses include the server not acknowledging the data in the SYN, ICMP error messages, and (most importantly) no response (SYN-ACK) from the server at all, i.e., connection timeout."). TFOinfo is cached when a connection is established. Other TCP option state might not be as readily cached. E.g., TCP-AO [RFC5925] success or failure between a host pair for a single SYN destination port might be usefully cached. TCP-AO success or failure to other SYN destination ports on that host pair is never useful to cache because TCP-AO security parameters can vary per service. TEMPORAL SHARING - Cache Updates Cached TCB Current TCB when? New Cached TCB ---------------------------------------------------------------- old_PMTU curr_PMTU PMTUD current (cur)_PMTU old_MSS curr_MSS MSSopt cur_MSS old_RTT curr_RTT CLOSE merge(curr,old) old_RTTvar curr_RTTvar CLOSE merge(curr,old) old_option curr option ESTAB depends on option) old_ssthresh curr_ssthresh CLOSE merge(curr,old) old_snd_cwnd curr_snd_cwnd CLOSE merge(curr,old) Caching PMTU and MSS is trivial; reported values are cached, and the most recent values are used. The cache is updated when the MSS option is received or after PMTUD (i.e., when an ICMPv4 Fraqmentation Needed [RFC1191] or ICMPv6 Packet Too Big message is received [RFC1981] or the equivalent is inferred, e.g. as from PLMTUD [RFC4821]), respectively, so the cache always has the most recent values from any connection. For MSS, the cache is consulted Touch Expires April 28, 2017 [Page 6] Internet-Draft TCP Control Block Interdependence October 2016 only at connection establishment and not otherwise updated, which means that MSS options do not affect current connections. The default MSS is never saved; only reported MSS values update the cache, so an explicit override is required to reduce the MSS. Other options are copied or merged depending on the details of each option. E.g., TFO state is updated when a connection is established and read before establishing a new connection. RTT values are updated by a more complicated mechanism [RFC1644][Ja86]. Dynamic RTT estimation requires a sequence of RTT measurements. As a result, the cached RTT (and its variance) is an average of its previous value with the contents of the currently active TCB for that host, when a TCB is closed. RTT values are updated only when a connection is closed. The method for merging old and current values needs to attempt to reduce the transient for new connections. [THESE MERGE FUNCTIONS NEED TO BE SPECIFIED, considering e.g. [DM16] - TBD]. The updates for RTT, RTTvar and ssthresh rely on existing information, i.e., old values. Should no such values exist, the current values are cached instead. 7. An Example of Ensemble Sharing Sharing cached TCB data across concurrent connections requires attention to the aggregate nature of some of the shared state. For example, although MSS and RTT values can be shared by copying, it may not be appropriate to copy congestion window or ssthresh information (see section 8 for a discussion of congestion window or ssthresh sharing). ENSEMBLE SHARING - TCB Initialization Cached TCB New TCB ---------------------------------- old_PMTU old_PMTU old_MSS old_MSS old_RTT old_RTT old_RTTvar old_RTTvar old_option (option-specific) Touch Expires April 28, 2017 [Page 7] Internet-Draft TCP Control Block Interdependence October 2016 ENSEMBLE SHARING - Cache Updates Cached TCB Current TCB when? New Cached TCB ----------------------------------------------------------- old_PMTU curr_PMTU PMTUD/PLPMTUD curr_PMTU old_MSS curr_MSS MSSopt curr_MSS old_RTT curr_RTT update rtt_update(old,cur) old_RTTvar curr_RTTvar update rtt_update(old,cur) old_option curr option (depends) (option specific) For ensemble sharing, TCB information should be cached as early as possible, sometimes before a connection is closed. Otherwise, opening multiple concurrent connections may not result in TCB data sharing if no connection closes before others open. The amount of work involved in updating the aggregate average should be minimized, but the resulting value should be equivalent to having all values measured within a single connection. The function "rtt_update" in the ensemble sharing table indicates this operation, which occurs whenever the RTT would have been updated in the individual TCP connection. As a result, the cache contains the shared RTT variables, which no longer need to reside in the TCB [Ja86]. Congestion window size and ssthresh aggregation are more complicated in the concurrent case. When there is an ensemble of connections, we need to decide how that ensemble would have shared these variables, in order to derive initial values for new TCBs. Any assumption of this sharing can be incorrect, including this one, because identical endpoint address pairs may not share network paths. In current implementations, new congestion windows are set at an initial value of 4-10 segments [RFC3390][RFC6928], so that the sum of the current windows is increased for any new connection. This can have detrimental consequences where several connections share a highly congested link. There are several ways to initialize the congestion window in a new TCB among an ensemble of current connections to a host, as shown below. Current TCP implementations initialize it to four segments as standard [rfc3390] and 10 segments experimentally [RFC6928] and T/TCP hinted that it should be initialized to the old window size [RFC1644]. In the former cases, the assumption is that new connections should behave as conservatively as possible. In the Touch Expires April 28, 2017 [Page 8] Internet-Draft TCP Control Block Interdependence October 2016 latter T/TCP case, no accommodation is made for concurrent aggregate behavior. In either case, the sum of window sizes can increase, rather than remain constant. A different approach is to give each pending connection its "fair share" of the available congestion window, and let the connections balance from there. The assumption we make here is that new connections are implicit requests for an equal share of available link bandwidth, which should be granted at the expense of current connections. [TBD - a new method for safe congestion sharing will be described] 8. Compatibility Issues For the congestion and current window information, the initial values computed by TCB interdependence may not be consistent with the long-term aggregate behavior of a set of concurrent connections between the same endpoints. Under conventional TCP congestion control, if a single existing connection has converged to a congestion window of 40 segments, two newly joining concurrent connections assume initial windows of 10 segments [RFC6928], and the current connection's window doesn't decrease to accommodate this additional load and connections can mutually interfere. One example of this is seen on low-bandwidth, high-delay links, where concurrent connections supporting Web traffic can collide because their initial windows were too large, even when set at one segment. [TBD - this paragraph needs to be revised based on new recommendations] Under TCB interdependence, all three connections could change to use a congestion window of 12 (rounded down to an even number from 13.33, i.e., 40/3). This would include both increasing the initial window of the new connections (vs. current recommendations [RFC6928]) and decreasing the congestion window of the current connection (from 40 down to 12). This gives the new connections a larger initial window than allowed by [RFC6928], but maintains the aggregate. Depending on whether the previous connections were in steady-state, this can result in more bursty behavior, e.g., when previous connections are idle and new connections commence with a large amount of available data to transmit. Additionally, reducing the congestion window of an existing connection needs to account for the number of packets that are already in flight. Because this proposal attempts to anticipate the aggregate steady- state values of TCB state among a group or over time, it should avoid the transient effects of new connections. In addition, because it considers the ensemble and temporal properties of those Touch Expires April 28, 2017 [Page 9] Internet-Draft TCP Control Block Interdependence October 2016 aggregates, it should also prevent the transients of short-lived or multiple concurrent connections from adversely affecting the overall network performance. There have been ongoing analysis and experiments to validate these assumptions. For example, [Ph12] recommends to only cache ssthresh for temporal sharing when flows are long. Sharing ssthresh between short flows can deteriorate the overall performance of individual connections[Ph12, Nd16], although this may benefit overall network performance. [TBD - the details of this issue need to be summarized and clarified herein]. [TBD - placeholder for corresponding RTT discussion] Due to mechanisms like ECMP and LAG [RFC7424], TCP connections sharing the same host-pair may not always share the same path. This does not matter for host-specific information such as RWIN and TCP option state, such as TFOinfo. When TCB information is shared across different SYN destination ports, path-related information can be incorrect; however, the impact of this error is potentially diminished if (as discussed here) TCB sharing affects only the transient event of a connection start or if TCB information is shared only within connections to the same SYN destination port. In case of Temporal Sharing, TCB information could also become invalid over time. Because this is similar to the case when a connection becomes idle, mechanisms that address idle TCP connections (e.g., [RFC7661]) could also be applied to TCB cache management. There may be additional considerations to the way in which TCB interdependence rebalances congestion feedback among the current connections, e.g., it may be appropriate to consider the impact of a connection being in Fast Recovery [RFC5861] or some other similar unusual feedback state, e.g., as inhibiting or affecting the calculations described herein. TCP is sometimes used in situations where packets of the same host- pair always take the same path. Because ECMP and LAG examine TCP port numbers, they may not be supported when TCP segments are encapsulated, encrypted, or altered - for example, some Virtual Private Networks (VPNs) are known to use proprietary UDP encapsulation methods. Similarly, they cannot operate when the TCP header is encrypted, e.g., when using IPsec ESP. TCB interdependence among the entire set sharing the same endpoint IP addresses should work without problems under these circumstances. Moreover, measures to increase the probability that connections use the same path could be applied: e.g., the connections could be given the same IPv6 flow label. TCB interdependence can also be extended to sets of host IP address pairs that share the same network path conditions, such as when a group of addresses is on the same LAN (see Section 9). Touch Expires April 28, 2017 [Page 10] Internet-Draft TCP Control Block Interdependence October 2016 9. Implications There are several implications to incorporating TCB interdependence in TCP implementations. First, it may reduce the need for application-layer multiplexing for performance enhancement [RFC7231]. Protocols like HTTP/2 [RFC7540] avoid connection reestablishment costs by serializing or multiplexing a set of per- host connections across a single TCP connection. This avoids TCP's per-connection OPEN handshake and also avoids recomputing MSS, RTT, and congestion windows. By avoiding the so-called, "slow-start restart," performance can be optimized. TCB interdependece can provide the "slow-start restart avoidance" of multiplexing, without requiring a multiplexing mechanism at the application layer. TCB interdependence pushes some of the TCP implementation from the traditional transport layer (in the ISO model), to the network layer. This acknowledges that some state is in fact per-host-pair or can be per-path as indicated solely by that host-pair. Transport protocols typically manage per-application-pair associations (per stream), and network protocols manage per-host-pair and path associations (routing). Round-trip time, MSS, and congestion information could be more appropriately handled in a network-layer fashion, aggregated among concurrent connections, and shared across connection instances [RFC3124]. An earlier version of RTT sharing suggested implementing RTT state at the IP layer, rather than at the TCP layer [Ja86]. Our observations are for sharing state among TCP connections, which avoids some of the difficulties in an IP-layer solution. One such problem is determining the associated prior outgoing packet for an incoming packet, to infer RTT from the exchange. Because RTTs are still determined inside the TCP layer, this is simpler than at the IP layer. This is a case where information should be computed at the transport layer, but could be shared at the network layer. Per-host-pair associations are not the limit of these techniques. It is possible that TCBs could be similarly shared between hosts on a subnet or within a cluster, because the predominant path can be subnet-subnet, rather than host-host. Additionally, TCB interdependence can be applied to any protocol with congestion state, including SCTP [RFC4960] and DCCP [RFC4340], as well as for individual subflows in Multipath TCP [RFC6824]. There may be other information that can be shared between concurrent connections. For example, knowing that another connection has just tried to expand its window size and failed, a connection may not attempt to do the same for some period. The idea is that existing Touch Expires April 28, 2017 [Page 11] Internet-Draft TCP Control Block Interdependence October 2016 TCP implementations infer the behavior of all competing connections, including those within the same host or subnet. One possible optimization is to make that implicit feedback explicit, via extended information associated with the endpoint IP address and its TCP implementation, rather than per-connection state in the TCB. Like its initial version in 1997, this document's approach to TCB interdependence focuses on sharing a set of TCBs by updating the TCB state to reduce the impact of transients when connections begin or end. Other mechanisms have since been proposed to continuously share information between all ongoing communication (including connectionless protocols), updating the congestion state during any congestion-related event (e.g., timeout, loss confirmation, etc.) [RFC3124]. By dealing exclusively with transients, TCB interdependence is more likely to exhibit the same behavior as unmodified, independent TCP connections. 10. Implementation Observations The observation that some TCB state is host-pair specific rather than application-pair dependent is not new and is a common engineering decision in layered protocol implementations. A discussion of sharing RTT information among protocols layered over IP, including UDP and TCP, occurred in [Ja86]. Although now deprecated, T/TCP was the first to propose using caches in order to maintain TCB states (see Appendix A for more information). Touch Expires April 28, 2017 [Page 12] Internet-Draft TCP Control Block Interdependence October 2016 The table below describes the current implementation status for some TCB information in Linux kernel version 4.6, FreeBSD 10 and Windows (as of October 2016). TCB data Status ----------------------------------------------------------- old_MSS Cached and shared in Linux old_RTT Cached and shared in FreeBSD old_RTTvar Cached and shared in FreeBSD old PMTU Cached and shared in FreeBSD and Windows old TFOinfo Cached and shared in Linux and Windows old_snd_cwnd Not shared old_ssthresh Cached and shared in FreeBSD and Linux: FreeBSD: arithmetic mean of ssthresh and previous value if a previous value exists; Linux: depending on state, max(cwnd/2, ssthresh) in most cases 11. Security Considerations These suggested implementation enhancements do not have additional ramifications for explicit attacks. These enhancements may be susceptible to denial-of-service attacks if not otherwise secured. For example, an application can open a connection and set its window size to zero, denying service to any other subsequent connection between those hosts. TCB sharing may be susceptible to denial-of-service attacks, wherever the TCB is shared, between connections in a single host, or between hosts if TCB sharing is implemented within a subnet (see Implications section). Some shared TCB parameters are used only to create new TCBs, others are shared among the TCBs of ongoing connections. New connections can join the ongoing set, e.g., to optimize send window size among a set of connections to the same host. Attacks on parameters used only for initialization affect only the transient performance of a TCP connection. For short connections, the performance ramification can approach that of a denial-of- service attack. E.g., if an application changes its TCB to have a Touch Expires April 28, 2017 [Page 13] Internet-Draft TCP Control Block Interdependence October 2016 false and small window size, subsequent connections would experience performance degradation until their window grew appropriately. The solution is to limit the effect of compromised TCB values. TCBs are compromised when they are modified directly by an application or transmitted between hosts via unauthenticated means (e.g., by using a dirty flag). TCBs that are not compromised by application modification do not have any unique security ramifications. Note that the proposed parameters for TCB sharing are not currently modifiable by an application. All shared TCBs MUST be validated against default minimum parameters before used for new connections. This validation would not impact performance, because it occurs only at TCB initialization. This limits the effect of attacks on new connections to reducing the benefit of TCB sharing, resulting in the current default TCP performance. For ongoing connections, the effect of incoming packets on shared information should be both limited and validated against constraints before use. This is a beneficial precaution for existing TCP implementations as well. TCBs modified by an application SHOULD NOT be shared, unless the new connection sharing the compromised information has been given explicit permission to use such information by the connection API. No mechanism for that indication currently exists, but it could be supported by an augmented API. This sharing restriction SHOULD be implemented in both the host and the subnet. Sharing on a subnet SHOULD utilize authentication to prevent undetected tampering of shared TCB parameters. These restrictions limit the security impact of modified TCBs both for connection initialization and for ongoing connections. Finally, shared values MUST be limited to performance factors only. Other information, such as TCP sequence numbers, when shared, are already known to compromise security. 12. IANA Considerations There are no IANA implications or requests in this document. This section should be removed upon final publication as an RFC. Touch Expires April 28, 2017 [Page 14] Internet-Draft TCP Control Block Interdependence October 2016 13. References 13.1. Normative References [RFC793] Postel, Jon, "Transmission Control Protocol," Network Working Group RFC-793/STD-7, ISI, Sept. 1981. [RFC1191] Mogul, J., Deering, S., "Path MTU Discovery," RFC 1191, Nov. 1990. [RFC1981] McCann, J., Deering. S., Mogul, J., "Path MTU Discovery for IP version 6," RFC 1981, Aug. 1996. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC4821] Mathis, M., Heffner, J., "Packetization Layer Path MTU Discovery," RFC 4821, Mar. 2007. [RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S., Jain, A., "TCP Fast Open", RFC 7413, Dec. 2014. 13.2. Informative References [Br02] Brownlee, N. and K. Claffy, "Understanding Internet Traffic Streams: Dragonflies and Tortoises", IEEE Communications Magazine p110-117, 2002. [Be94] Berners-Lee, T., et al., "The World-Wide Web," Communications of the ACM, V37, Aug. 1994, pp. 76-82. [Br94] Braden, B., "T/TCP -- Transaction TCP: Source Changes for Sun OS 4.1.3,", Release 1.0, USC/ISI, September 14, 1994. [Co91] Comer, D., Stevens, D., Internetworking with TCP/IP, V2, Prentice-Hall, NJ, 1991. [FreeBSD] FreeBSD source code, Release 2.10, http://www.freebsd.org/ [Ja86] Jacobson, V., (mail to public list "tcp-ip", no archive found), 1986. [Nd16] Dukkipati, N., Yuchung C., and Amin V., "Research Impacting the Practice of Congestion Control." ACM SIGCOMM CCR (editorial). Touch Expires April 28, 2017 [Page 15] Internet-Draft TCP Control Block Interdependence October 2016 [DM16] Matz, D., "Optimize TCP's Minimum Retransmission Timeout for Low Latency Environments", Master's thesis, Technical University Munich, 2016. [Ph12] Hurtig, P., Brunstrom, A., "Enhanced metric caching for short TCP flows," 2012 IEEE International Conference on Communications (ICC), Ottawa, ON, 2012, pp. 1209-1213. [RFC1644] Braden, R., "T/TCP -- TCP Extensions for Transactions Functional Specification," RFC-1644, July 1994. [RFC1379] Braden, R., "Transaction TCP -- Concepts," RFC-1379, September 1992. [RFC3390] Allman, M., Floyd, S., Partridge, C., "Increasing TCP's Initial Window," RFC 3390, Oct. 2002. [RFC7231] Fielding, R., J. Reshke, Eds., "HTTP/1.1 Semantics and Content," RFC-7231, June 2014. [RFC3124] Balakrishnan, H., Seshan, S., "The Congestion Manager," RFC 3124, June 2001. [RFC4340] Kohler, E., Handley, M., Floyd, S., "Datagram Congestion Control Protocol (DCCP)," RFC 4340, Mar. 2006. [RFC4960] Stewart, R., (Ed.), "Stream Control Transmission Protocol," RFC4960, Sept. 2007. [RFC5861] Allman, M., Paxson, V., Blanton, E., "TCP Congestion Control," RFC 5861, Sept. 2009. [RFC5925] Touch, J., Mankin, A., Bonica, R., "The TCP Authentication Option," RFC 5925, June 2010. [RFC6824] Ford, A., Raiciu, C., Handley, M., Bonaventure, O., "TCP Extensions for Multipath Operation with Multiple Addresses," RFC 6824, Jan. 2013. [RFC6928] Chu, J., Dukkipati, N., Cheng, Y., Mathis, M., "Increasing TCP's Initial Window," RFC 6928, Apr. 2013. [RFC7424] Krishnan, R., Yong, L., Ghanwani, A., So, N., Khasnabish, B., "Mechanisms for Optimizing Link Aggregation Group (LAG) and Equal-Cost Multipath (ECMP) Component Link Utilization in Networks", RFC 7424, Jan. 2015 Touch Expires April 28, 2017 [Page 16] Internet-Draft TCP Control Block Interdependence October 2016 [RFC7540] Belshe, M., Peon, R., Thomson, M., "Hypertext Transfer Protocol Version 2 (HTTP/2)", RFC 7540, May 2015. [RFC7661] Fairhurst, G., Sathiaseelan, A., Secchi, R., "Updating TCP to Support Rate-Limited Traffic", RFC 7661, Oct. 2015 14. Acknowledgments The authors would like to thank for Praveen Balasubramanian for information regarding TCB sharing in Windows, and Yuchung Cheng and Michael Scharf for comments on earlier versions of the draft. This work has received funding from a collaborative research project between the University of Oslo and Huawei Technologies Co., Ltd., and is partly supported by USC/ISI's Postel Center. This document was prepared using 2-Word-v2.0.template.dot. Authors' Addresses Joe Touch USC/ISI 4676 Admiralty Way Marina del Rey, CA 90292-6695 USA Phone: +1 (310) 448-9151 Email: touch@isi.edu Michael Welzl University of Oslo PO Box 1080 Blindern Oslo N-0316 Norway Phone: +47 22 85 24 20 Email: michawe@ifi.uio.no Touch Expires April 28, 2017 [Page 17] Internet-Draft TCP Control Block Interdependence October 2016 Safiqul Islam University of Oslo PO Box 1080 Blindern Oslo N-0316 Norway Phone: +47 22 84 08 37 Email: safiquli@ifi.uio.no Jianjie You Huawei 101 Software Avenue, Yuhua District Nanjing 210012 China Email: youjianjie@huawei.com 15. Appendix A: TCB sharing history T/TCP proposed using caches to maintain TCB information across instances (temporal sharing), e.g., smoothed RTT, RTT variance, congestion avoidance threshold, and MSS [RFC1644]. These values were in addition to connection counts used by T/TCP to accelerate data delivery prior to the full three-way handshake during an OPEN. The goal was to aggregate TCB components where they reflect one association - that of the host-pair, rather than artificially separating those components by connection. At least one T/TCP implementation saved the MSS and aggregated the RTT parameters across multiple connections, but omitted caching the congestion window information [Br94], as originally specified in [RFC1379]. Some T/TCP implementations immediately updated MSS when the TCP MSS header option was received [Br94], although this was not addressed specifically in the concepts or functional specification [RFC1379][RFC1644]. In later T/TCP implementations, RTT values were updated only after a CLOSE, which does not benefit concurrent sessions. Temporal sharing of cached TCB data was originally implemented in the SunOS 4.1.3 T/TCP extensions [Br94] and the FreeBSD port of same [FreeBSD]. As mentioned before, only the MSS and RTT parameters were cached, as originally specified in [RFC1379]. Later discussion of T/TCP suggested including congestion control parameters in this cache [RFC1644]. Touch Expires April 28, 2017 [Page 18]