Spurious Retransmission Detection (SRD) with the TCP Echo Options - - PowerPoint PPT Presentation

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Oct 02, 2023 189 likes •271 views

Spurious Retransmission Detection (SRD) with the TCP Echo Options draft-zimmermann-tcpm-spurious-rxmit Richard Scheffenegger <rs@netapp.com> Alexander Zimmermann <alexander.zimmermann@netapp.com> Problem Statement Eifel detection

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Spurious Retransmission Detection (SRD) with the TCP Echo Options

draft-zimmermann-tcpm-spurious-rxmit

Richard Scheffenegger <rs@netapp.com> Alexander Zimmermann <alexander.zimmermann@netapp.com>

SLIDE 2

Problem Statement

Eifel detection – Uses TCP Timestamp options [RFC 7323] to detect spurious retransmissions – Limited applicability due to TSecr semantics, and TSval granularity – No detection of reordering during loss recovery Idea: Make every segment – including all retransmissions – uniquely identifiable (to the sender) – Allows all functionality of Eifel, even during corner cases – Enables new capabilities (lost retransmission detection)

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Spurious Retransmission Detection (SRD)

Mechanism – Use TCP Echo Option to send a (small) counter in each segment to keep MSS equal for retransmissions – Increase counter when sending a new round of retransmissions e.g. (re-)entering loss recovery – Check counter in received ACK

Equal to current value valid retransmission Else spurious retransmissions

Property – Semantics of TCP Echo allows to determine the exact

rdering of transmissions, even in case of reordering

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Example (Semantics)

RFC7323 TSecr reflects TS of last in-sequence segment

4 Source Destination Source Destination

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Example (Eifel vs. SRD)

Granularity of TS often too coarse

5 Source Destination Source Destination

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Example (Eifel vs. SRD)

Eifel only works on first retransmitted segment

6 Source Destination Source Destination

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Example (Eifel vs. SRD)

Allows lost retransmission detection

7 Source Destination Source Destination

X X X X

SLIDE 8

Moving forward…

Less overhead than RFC7323 Timestamps Solves the retransmission ambiguity problem completely – More Complex scenarios involving Fwd Loss / Fwd Reordering / ACK Loss / ACK Reordering – Enables Lost Retransmission Detection (LRD) while strictly adhering to packet conservation principles – QUIC has similar “control sequence number” Next steps – Received initial feedback (clarifications) – Eventually asking for adoption