TCPM J. Touch Internet Draft Independent consultant Intended status: Informational April 12, 2021 Expires: October 2021 Sequence Number Extension for Windowed Protocols draft-touch-sne-00.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 not be modified, and derivative works of it may not be created, except to format it for publication as an RFC or to translate it into languages other than English. 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 October 12, 2021. Copyright Notice Copyright (c) 2021 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. Code Components extracted from this Touch Expires October 12, 2021 [Page 1] Internet-Draft TCP-AO Test Vectors April 2021 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. Abstract Sliding window protocols use finite sequence numbers to determine segment placement and order. These sequence number spaces wrap around and are reused during the operation of such protocols. This document describes a way to extend the size of these sequence numbers at the endpoints to avoid the impact of that wrap and reuse without transmitting additional information in the packet header. The resulting extended sequence numbers can be used at the endpoints in encryption and authentication algorithms to ensure input bit patterns do not repeat over the lifetime of a connection. Table of Contents 1. Introduction...................................................2 2. Background.....................................................3 3. Related Discussion.............................................4 4. Example Code...................................................5 5. Validation Suite...............................................8 6. Security Considerations........................................9 7. IANA Considerations............................................9 8. References.....................................................9 8.1. Normative References......................................9 8.2. Informative References....................................9 9. Acknowledgments................................................9 1. Introduction Protocols use sequence numbers to maintain ordering and, in sliding window systems, to control the amount of outstanding unacknowledged information. These sequence numbers are finite and thus commonly wrap-around during long connections, reusing past values. It can be useful for protocols to keep track of this wrap-around in a separate counter, such that the sequence number and counter together form an equivalent number space that does not wrap. This technique was introduced as "sequence number extension" in TCP-AO [RFC5925]. The example provided there was intended to introduce the concept, but the pseudocode provided is not complete. This document presents the formal requirements for sequence number extension (SNE), a code example, and a check sequence that can be Touch Expires October 12, 2021 [Page 2] Internet-Draft TCP-AO Test Vectors April 2021 used to validate this and alternate implementations. Section 2 provides background on the concept, Section 3 discusses currently known uses of SNE. Section 4 provides a framework for testing SNE implementations, including example code for the SNE function, and Section 5 provides a sequence that can be used by that code for validation. Section 6 concludes with a discussion of security issues. 2. Background Protocols use sequence numbers to maintain message order. The transmitter typically increments them either once per message or by the length of the message. The receiver uses them to reorder messages and detect gaps due to inferred loss. Sequence numbers are represented within those messages (e.g., in the headers) as values of a finite, unsigned number space. This space is typically represented in a fixed-length bit string, whose values range from [0..(2^N-1)], inclusive. The use of finite representations has repercussions on the use of these values at both the transmitter and receiver. Without additional constraints, when the number space "wraps around", it is be impossible for the receiver to distinguish between the uses of the same value. This would defeat their use for reordering. As a consequence, additional constraints are required. Transmitters are typically required to limit reuse until they can confirm that receivers would successfully differentiate the two uses of the same value. This is accomplished by defining "forward" as values that advance the values by less than half the sequence number space, and ensuring that receivers never experience a series of values that violate that rule. We define a sequence space as follows: o An unsigned integer range from 0..(2^N)-1, i.e., for N bits o An operation that increments values in that space by K, where K < 2^(N-1), i.e., less than half the range. This operation is used exclusively by the transmitter. o An operation that compares two values in that space to determine their order, e.g., where X < Y implies that X comes before Y We assume that both sides begin with the same initial value, which can be anywhere in the space. That value is either assumed (e.g., 0) Touch Expires October 12, 2021 [Page 3] Internet-Draft TCP-AO Test Vectors April 2021 before the protocol begins, or coordinated before other messages are exchanged (as with TCP Initial Sequence Numbers, i.e., ISNs [RFC793]). The receiver is assumed to always receive values that are always within 2^(N-1), i.e., half the range. No other operations are supported. The transmitter is not permitted to "backup", such that values are always used in "increment" order. The receiver cannot experience loss or gaps larger than 2^(N-1)-1, which is typically enforced either by assumption or by explicit endpoint coordination. A sequence number extension (SNE) is a separate number space that can be combined with the sequence number to create a larger number space that does not wrap around during a connection. On the transmit side, SNE is trivially accomplished by incrementing a local counter once each time the sequence number increment "wraps" around, or by keeping a larger local sequence number whose least- significant part is the message sequence number and most-significant part can be considered the SNE. The transmitter typically does not need to maintain a SNE except when used in local computations, such as for HMACs in TCP-AO [RFC5925]. The goal of this document is to demonstrate that SNE can be accomplished on the receiver side without transmitting additional information in messages. It defines the stateful function compute_sne() as follows: SNE = compute_sne(seqno) Compute_sne() accepts the sequence number seen in a received message and computes the corresponding SNE. The function includes persistent local state that tracks the largest currently received SNE, seqno combination. The concatenation of SNE and seqno emulates the equivalent larger sequence number space that can avoid wrap around. Note that the function defined here is capable of receiving any series of seqno values and computing their correct corresponding SNE, as long as the series never "jumps" more than half the number space "backward" from the largest value seen "forward". 3. Related Discussion The DNS uses sequence numbers to determine when a SOE serial number is more recent than a previous one, even considering sequence space wrap [RFC1034][RFC1035]. The use of wrapped sequence numbers for Touch Expires October 12, 2021 [Page 4] Internet-Draft TCP-AO Test Vectors April 2021 sliding windows in network protocols was first described as a sequence number space [IEN74]. A more recent discussion describes this as "serial number arithmetic" and defines a comparison operator it claimed was missing in IEN74 [RFC1982]. That document defines two operations: addition (presumably shifting the window forward) and comparison (defining the order of two values). Addition is defined in that document as limited to value of 0..windowsize/2-1. Comparison is defined in that document by a set of equations therein, but that document does not provide a way for a receiver to compute the correct equivalent SNE, especially including the potential for sequence number reordering, as is demonstrated in this document. 4. Example Code The following C code is provided as a verified example of sequence number extension. The code includes both the framework used for validation and the compute_sne() function, the latter of which can be used operationally. A correct test will indicate "OK" for each test. An incorrect test will indicate "ERROR" where applicable. #include #include #define distance(x,y) (((x)<(y))?((y)-(x)):((x)-(y))) #define SNEDEBUG 1 // This is the core code, stand-alone, to compute SNE from seqno // >> replace this function with your own code to test alternates unsigned long compute_sne(unsigned long seqno) { // INPUT: 32-bit unsigned sequence number (low bits) // OUTPUT: 32-bit unsigned SNE (high bits) // variables used in this code example to compute SNE: static unsigned long RCV_SNE = 0; // high-watermark SNE static int RCV_SNE_FLAG = 1; // set during first half rollover // (prevents re-rollover) static unsigned long Touch Expires October 12, 2021 [Page 5] Internet-Draft TCP-AO Test Vectors April 2021 RCV_PREV_SEQ = 0; // high-watermark SEQ unsigned long holdSNE; // temp copy of output holdSNE = RCV_SNE; // use current SNE to start if (distance(seqno,RCV_PREV_SEQ) < 0x80000000) { // both in same SNE range? if ((seqno >= 0x80000000) && (RCV_PREV_SEQ < 0x80000000)) { // jumps fwd over N/2? RCV_SNE_FLAG = 0; // reset wrap increment flag } RCV_PREV_SEQ = MAX(seqno,RCV_PREV_SEQ); // move prev forward if needed } else { // both in diff SNE ranges if (seqno < 0x80000000) { // jumps forward over zero? RCV_PREV_SEQ = seqno; // update prev if (RCV_SNE_FLAG == 0) { // first jump over zero? (wrap) RCV_SNE_FLAG = 1; // set flag so we increment once RCV_SNE = RCV_SNE + 1; // increment window holdSNE = RCV_SNE; // use updated SNE value } } else { // jump backward over zero holdSNE = RCV_SNE - 1; // use pre-rollover SNE value } } #ifdef SNEDEBUG fprintf(stderr,"state RCV_SNE_FLAG = %1d\n", RCV_SNE_FLAG); fprintf(stderr,"state RCV_SNE = %08lx\n", RCV_SNE); fprintf(stderr,"state RCV_PREV_SEQ = %08lx\n", RCV_PREV_SEQ); #endif return holdSNE; } Touch Expires October 12, 2021 [Page 6] Internet-Draft TCP-AO Test Vectors April 2021 int main() { // variables used as input and output: unsigned long SEG_SEQ; // input - received SEQ unsigned long SNE; // output - SNE corresponding // to received SEQ // variables used to validate the computed SNE: unsigned long SEG_HIGH; // input - xmitter side SNE // -> SNE should match this value unsigned long long BIG_PREV; // prev 64-bit total seqno unsigned long long BIG_THIS = 0; // current 64-bit total seqno // -> THIS, PREV should never jump // more than half the SEQ space char *prompt = "Hex input (2 groups of 8 hex chars with a space): "; fprintf(stderr,"Input hex numbers only (0x is optional)\n\n"); fprintf(stderr,"%s\n",prompt); while (scanf("%lx %lx",&SEG_HIGH,&SEG_SEQ) == 2) { BIG_PREV = BIG_THIS; BIG_THIS = (((unsigned long long)SEG_HIGH) << 32) | ((unsigned long long)SEG_SEQ); // given SEG_SEQ, compute SNE SNE = compute_sne(SEG_SEQ); fprintf(stderr," SEG_SEQ = %08lx\n", SEG_SEQ); fprintf(stderr," SNE = %08lx\n", SNE); fprintf(stderr," SEG_HIGH = %08lx %s\n",SEG_HIGH, (SEG_HIGH == SNE)? " - OK" : " - ERROR !!!!!!!"); fprintf(stderr,"\t\tthe jump was %16llx %s %s\n", distance(BIG_PREV,BIG_THIS), ((BIG_PREV < BIG_THIS)?"+":"-"), (((distance(BIG_PREV,BIG_THIS)) > 0x7FFFFFFF) ? "ILLEGAL JUMP" : ".")); fprintf(stderr,"\n"); fprintf(stderr,"\n"); Touch Expires October 12, 2021 [Page 7] Internet-Draft TCP-AO Test Vectors April 2021 fprintf(stderr,"%s\n",prompt); } } 5. Validation Suite The following numbers are used to validate sequence number extension variants, and are shown in the order they legitimately could be received. Each line represents a single 64-bit number, represented as two hexadecimal 32-bit numbers with a space between. The numbers are formatted for use in the example code provided in Section 4. A correctly operating extended sequence number system can receive the least-significant half (the right side) and compute the correct most-significant half (the left side) correctly. It specifically tests both forward and backward jumps in received values that represent legitimate reordering. 00000000 00000000 00000000 30000000 00000000 90000000 00000000 70000000 00000000 a0000000 00000001 00000001 00000000 e0000000 00000001 00000000 00000001 7fffffff 00000001 00000000 00000001 50000000 00000001 80000000 00000001 00000001 00000001 40000000 00000001 90000000 00000001 b0000000 00000002 0fffffff 00000002 20000000 00000002 90000000 00000002 70000000 00000002 A0000000 00000003 00004000 00000002 D0000000 00000003 20000000 00000003 90000000 00000003 70000000 Touch Expires October 12, 2021 [Page 8] Internet-Draft TCP-AO Test Vectors April 2021 00000003 A0000000 00000004 00004000 00000003 D0000000 6. Security Considerations Sequence numbers and their extensions can be useful in a variety of security contexts. Because the extension part (most significant half) is determined by the previously exchanged sequence values (least significant half), the extension should not be considered as adding entropy for the purposes of message authentication or encryption. 7. IANA Considerations This document contains no IANA issues. This section should be removed upon publication as an RFC. 8. References 8.1. Normative References 8.2. Informative References [IEN74] Plummmer, W., "Sequence Number Arithmetic," IEN 74, Sept. 1978. [RFC793] Postel, J., "Transmission Control Protocol," RFC 793, September 1981. [RFC1034] Mockapetris, P., "Domain Names - Concepts and Facilities," RFC 1034, Nov. 1987. [RFC1035] Mockapetris, P., "Domain Names - Implementation and Specification," Nov. 1987. [RFC1982] Elz, R., Bush, R., "Serial Number Arithmetic," RFC 1982, Aug. 1996. [RFC5925] Touch, J., A. Mankin, R. Bonica, "The TCP Authentication Option," RFC 5925, June 2010. 9. Acknowledgments The need for a fix to the pseudocode in RFC5825 was first noted by Juhamatti Kuusisaari in April 2020. Touch Expires October 12, 2021 [Page 9] Internet-Draft TCP-AO Test Vectors April 2021 This document was prepared using 2-Word-v2.0.template.dot. Authors' Addresses Joe Touch Manhattan Beach, CA 90266 USA Phone: +1 (310) 560-0334 Email: touch@strayalpha.com Touch Expires October 12, 2021 [Page 10]