# Compute the TCP retransmission timeout from measured round-trip time > Use RFC 6298 — Computing TCP's Retransmission Timer to review this narrow operational decision without extending the source beyond its stated scope. - Canonical URL: https://update.dsesecurity.com/updates/compute-the-tcp-retransmission-timeout-from-measured-round-trip-time/ - Publisher: Detection Systems & Engineering (DSE Security) - Author: DSE Security Editorial Team - Published: 2026-08-27T12:16:33+00:00 - Modified: 2026-08-27T12:36:16+00:00 - Last reviewed by DSE: 2026-08-26 - Resource type: Explainer - DSE priority: Advisory - Topics: IT, Networks & Infrastructure - Reading time: 3 minutes ## What you need to know Use RFC 6298 — Computing TCP's Retransmission Timer to review this narrow operational decision without extending the source beyond its stated scope. ## Potentially affected Teams, systems, services, or facilities within the stated scope of RFC 6298 — Computing TCP's Retransmission Timer ## DSE recommendation Compare the observed state with the cited official source, document applicability and exceptions, and test any approved change with rollback safeguards. ## Article Frame this document as a source-led configuration and assurance check: Compute the TCP retransmission timeout from measured round-trip time. Only the official source and traced locations below supply facts. Confirm applicability before acting. ## Source fact: The official [RFC 6298 — Computing TCP’s Retransmission Timer](https://www.rfc-editor.org/rfc/rfc6298.html) from RFC Editor / Internet Engineering Task Force supports the following bounded statements: - Before measuring RTT, a TCP sender should use a one-second RTO; its first sample sets SRTT to the sample, RTTVAR to half the sample, and RTO to SRTT plus the larger of clock granularity or four times RTTVAR. The research record locates this support at Section 2 (The Basic Algorithm), rules 2.1 and 2.2. - Karn’s algorithm forbids RTT samples from retransmitted segments unless timestamps remove the ambiguity, and TCP must obtain at least one RTT sample per round trip when possible. The research record locates this support at Section 3 (Taking RTT Samples). - When the retransmission timer expires, TCP retransmits the earliest unacknowledged segment, doubles the RTO, and restarts the timer with that backed-off value. The research record locates this support at Section 5 (Managing the RTO Timer), rules 5.4-5.6. The source support ends with the statements listed above. Use them to examine address plans, interfaces, routes, peers, protocol roles, timers, middleboxes, and intended failure domains in the applicable environment, not to imply a wider guarantee. ## What the source does not establish This RFC evidence supports only the named network-protocol decision; it does not select vendor settings, topology, capacity, or an acceptable failure mode. Do not read the source as proof of implementation or permission to change production. Its guidance remains conditional on DNS, Active Directory authentication, PKI, time, routing policy, transport reachability, and monitoring and the environment’s recorded constraints. ## Applicability questions - For source statement 1 at Section 2 (The Basic Algorithm), rules 2.1 and 2.2, which observable configuration, record, or test can confirm applicability here? - For source statement 2 at Section 3 (Taking RTT Samples), which observable configuration, record, or test can confirm applicability here? - For source statement 3 at Section 5 (Managing the RTO Timer), rules 5.4-5.6, which observable configuration, record, or test can confirm applicability here? - Within address plans, interfaces, routes, peers, protocol roles, timers, middleboxes, and intended failure domains, which versions, roles, and configuration states define the review population? - Could DNS, Active Directory authentication, PKI, time, routing policy, transport reachability, and monitoring invalidate the test, hide a failure, or change applicability? - Who owns the decision, and which observation requires stopping, escalation, or rollback? ## DSE recommendation: DSE recommends using the cited source as the evidence anchor for this decision. Make the source, asset scope, owner, and expected outcome explicit in the review record. Record the source location, examined part of address plans, interfaces, routes, peers, protocol roles, timers, middleboxes, and intended failure domains, observed and expected states, owner, and reason for deviation. For an approved change, define prerequisites, a limited test path, success and stop conditions, monitoring, and rollback. Check DNS, Active Directory authentication, PKI, time, routing policy, transport reachability, and monitoring in design order. Protect credentials, keys, recovery material, personal data, and sensitive topology in evidence. ## Verification and evidence A reviewer should be able to retrace the decision from Section 2 (The Basic Algorithm), rules 2.1 and 2.2; Section 3 (Taking RTT Samples); Section 5 (Managing the RTO Timer), rules 5.4-5.6 through configuration snapshots, route or neighbor state, packet captures, counters, topology records, and controlled failover results. Record what was collected, where, when, by whom, and which system or role it represents. Close the review only when the evidence, exception handling, resulting action, and after-state are linked. Schedule a new review after material technical, organizational, incident, or source changes; today’s observation is not a continuing guarantee. ## Official references - [RFC 6298 — Computing TCP’s Retransmission Timer](https://www.rfc-editor.org/rfc/rfc6298.html) — RFC Editor / Internet Engineering Task Force ## Primary reference - Name: RFC 6298 — Computing TCP's Retransmission Timer - Authority: www.rfc-editor.org - URL: https://www.rfc-editor.org/rfc/rfc6298.html - Source publication date: Not stated by the source ## Citation and use Preferred citation: “Compute the TCP retransmission timeout from measured round-trip time,” DSE Security, https://update.dsesecurity.com/updates/compute-the-tcp-retransmission-timeout-from-measured-round-trip-time/ Publishing principles: https://update.dsesecurity.com/updates/dse-updates-editorial-methodology/ Usage and citation policy: https://update.dsesecurity.com/usage/ Copyright © 2026 Detection Systems & Engineering. All rights reserved.