TCP Packet Trimming Extension#

Flavius Mazilu, Valentin Bobaru, Costin Raiciu (University Politehnica of Bucharest).

• Why: Modern datacenter switch silicon (Arista, Broadcom Tomahawk 5, Nvidia, Juniper are all cited) already supports packet trimming — discard payload, keep the header, mark it — but TCP has no way to consume the signal. This is the host-side counterpart to a shipping hardware feature, and it mirrors the trimming used in the MRC/SRv6 AI-backend design above.
• What: A TCP extension: a "Trimming Permitted" option negotiated in the 3WHS; data packets then carry DSCP_TRIMMABLE; when a switch trims, the receiver gets the header (with DSCP_TRIMMED and updated IP checksum — TCP checksum is intentionally invalid) and returns a NACK option carrying the trimmed packet's sequence number. The sender treats the NACK as an explicit loss/congestion signal and enters fast recovery — deterministic loss recovery instead of SACK/RACK inference.
• How: Optional two-queues-per-port scheme puts trimmed packets on a higher-priority queue so NACKs return quickly; NACKs could get their own DSCP (Ultra Ethernet's UET proposes DSCP_CONTROL for exactly this).
• Where applicable: Data centers and controlled administrative domains only — explicitly not for the open Internet. Draft -00 with open questions (checksum handling, congestion-control response) posed to TCPM; watch whether the WG picks it up.