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fix: retransmit SYN-ACK if it goes missing #277

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Merged
pgallino merged 3 commits into from
Jun 8, 2025
Merged

fix: retransmit SYN-ACK if it goes missing #277

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0a039d6
fix: send an ACK upon invalid sequence number
MegaRedHand Jun 7, 2025
a1930c1
fix: retransmit SYN-ACK if it goes missing
MegaRedHand Jun 8, 2025
a999702
Merge branch 'main' into retransmit-syn-ack
pgallino Jun 8, 2025
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28 changes: 23 additions & 5 deletions src/types/network-modules/tcp/tcpState.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -184,6 +184,9 @@ export class TcpState {

// Move to SYN_SENT state
this.state = TcpStateEnum.SYN_SENT;
// NOTE: according to the RFC, we should return immediately,
// but here we wait for the connection to be established.
// This is done to make visualizations easier.
return await this.connectionQueue.pop();
}

Expand All @@ -202,10 +205,7 @@ export class TcpState {
this.initialRecvSeqNum = synSegment.sequenceNumber;

// Send a SYN-ACK
const flags = new Flags().withSyn().withAck();
const segment = this.newSegment(this.initialSendSeqNum, this.recvNext);

if (!sendIpPacket(this.srcHost, this.dstHost, segment.withFlags(flags))) {
if (!this.sendSynSegment()) {
return false;
}
this.rttEstimator.startMeasurement(this.initialSendSeqNum);
Expand Down Expand Up @@ -340,6 +340,12 @@ export class TcpState {
const segLen = segment.data.length;
if (!this.isSeqNumValid(segSeq, segLen)) {
console.debug("Sequence number not valid");

// RFC 5961 suggests sending a challenge ACK, but here we keep things simple
// If RST is not set, send an ACK in response
if (!flags.rst) {
this.notifySendPackets();
}
return ProcessingResult.DISCARD;
}

Expand Down Expand Up @@ -775,9 +781,21 @@ export class TcpState {
this.tcpQueue.close();
}

/**
* Sends a SYN or SYN-ACK segment
*/
private sendSynSegment() {
let ack = 0;
const flags = new Flags().withSyn();
const segment = this.newSegment(this.initialSendSeqNum, 0).withFlags(flags);

if (this.recvNext) {
// It's a SYN-ACK
ack = this.recvNext;
flags.withAck();
}
const segment = this.newSegment(this.initialSendSeqNum, ack);
segment.withFlags(flags);

if (!sendIpPacket(this.srcHost, this.dstHost, segment)) {
console.warn(
`Device ${this.srcHost.id} couldn't send SYN to device ${this.dstHost.id}.`,
Expand Down