Connection Retry and Strategy Selection

Relevant source files

• intra/core/sigcond.go
• intra/dialers/cdial.go
• intra/dialers/direct_split.go
• intra/dialers/example/main.go
• intra/dialers/link.go
• intra/dialers/pdial.go
• intra/dialers/rdial.go
• intra/dialers/retrier.go
• intra/dialers/retrier_test.go
• intra/dialers/split_and_desync.go
• intra/dialers/tlsdial.go
• intra/dialers/types.go
• intra/rwconn.go

This document describes the connection retry mechanism and strategy selection logic in Firestack's dialers subsystem. The retrier provides transparent retry capabilities with configurable splitting strategies to bypass network censorship and improve connection reliability. For the actual splitting implementations and desynchronization attacks, see 5.2. Splitting and Desync Attacks.

Purpose and Scope

The retrier system in intra/dialers/ implements intelligent connection retry logic with multiple dial strategies. It transparently retries failed connections by:

1. Buffering the first write ("hello" packet) for replay via a tee buffer [intra/dialers/retrier.go:108].
2. Selecting appropriate splitting strategies based on configuration and attempt count [intra/dialers/retrier.go:248-312].
3. Managing multiple dialers with preference caching via ippPins [intra/dialers/retrier.go:68].
4. Coordinated retry completion across concurrent readers and writers using SigCond [intra/dialers/retrier.go:116].

The system supports both single-dialer mode (with strategy progression) and multi-dialer mode (cycling through different dialers).

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Core Components

retrier Structure

The retrier struct implements core.DuplexConn and core.RetrierConn interfaces, acting as a transparent wrapper around TCP connections with retry capabilities [intra/dialers/retrier.go:119-120].

Title: retrier Entity Relationship

Sources: intra/dialers/retrier.go73-117 intra/core/sigcond.go14-76

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Entry Points and Modes

The retrier supports two dialing modes with different entry points:

Entry Point	Mode	Purpose	Max Retries	Strategy Selection
DialWithSplitRetry	Single-dialer	Strategy-based retries	maxRetryCount (3)	Based on DialerOpts.Retry + Strat
DialAny	Multi-dialer	Dialer failover	len(dialers)	Fixed RetryWithSplit + SplitNever

Title: retrier Initialization Flow

Sources: intra/dialers/retrier.go150-167 intra/dialers/retrier.go198-222 intra/dialers/retrier.go169-175

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Retry Strategies

The settings.Retry constants control when retries occur and when splitting is applied [intra/dialers/retrier.go:248-312]:

Retry Strategy Types

Title: Retry Logic States

Strategy	Split on Attempt 0?	Split on Attempt 1+?	Max Attempts	Use Case
RetryNever	✓ (if Strat set)	N/A	1	Fast-fail, no retries
RetryWithSplit	✗	✓	maxRetries	Normal connection, split on retry
RetryAfterSplit	✓	✓ (until max)	maxRetries	Aggressive splitting upfront

Sources: intra/dialers/retrier.go248-312 intra/dialers/retrier.go50-63

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Split Strategies

Split strategies determine how the initial TCP segment is fragmented. These are settings.Split* constants:

Strategy	Description	Implementation	Auto Cycle Order
SplitNever	No splitting	Direct protect.Dial() [intra/dialers/retrier.go:405]	N/A
SplitTCP	Split TCP segment	splitter with TCP mode [intra/dialers/direct_split.go:64]	1st (Auto)
SplitTCPOrTLS	Split TLS record if port 443, else TCP	splitter with TLS mode [intra/dialers/direct_split.go:66]	2nd (Auto)
SplitDesync	TCB Desynchronization Attack	overwriteSplitter [intra/dialers/split_and_desync.go:45]	3rd (Auto)
SplitAuto	Cycle through strategies on retry	See Auto Mode Logic	Cycles 1→2→3

Sources: intra/dialers/retrier.go405-419 intra/dialers/direct_split.go61-73

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Strategy Selection Algorithm

The dialStratLocked() function implements the core strategy selection logic [intra/dialers/retrier.go:248]:

Strategy Selection Flow

Title: dialStratLocked Logic

Sources: intra/dialers/retrier.go248-312

Auto Mode Strategy Cycling

When Strat == SplitAuto, the retrier cycles through strategies based on attemptCycle = retryCount % maxRetries [intra/dialers/retrier.go:278]:

Retry Strategy	attemptCycle=0	attemptCycle=1	attemptCycle=2	attemptCycle=3+
RetryNever	SplitTCP	N/A	N/A	N/A
RetryWithSplit	SplitDesync	SplitTCP	SplitTCPOrTLS	SplitDesync
RetryAfterSplit	SplitTCP	SplitTCPOrTLS	SplitTCP (no split)	N/A

Note: For RetryAfterSplit + SplitAuto, splitting stops at retryCount >= maxRetries, so the strategy doesn't matter for later cycles [intra/dialers/retrier.go:266-269].

Sources: intra/dialers/retrier.go278-306

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Multi-Dialer Support

The DialAny() function enables connection attempts across multiple dialers with intelligent prioritization [intra/dialers/retrier.go:198].

Multi-Dialer Architecture

Title: Multi-Dialer Execution and Pinning

Sources: intra/dialers/retrier.go198-222 intra/dialers/retrier.go177-196 intra/dialers/retrier.go372-395 intra/dialers/retrier.go798-807

ippPins Cache

The ippPins global variable is a core.Sieve[netip.AddrPort, string] that maintains a limited-time mapping between IP:port destinations and dialer IDs [intra/dialers/retrier.go:68]. It uses a TTL of desync_cache_ttl (30 seconds) [intra/dialers/split_and_desync.go:35].

Title: ippPins Cache Lifecycle

Sources: intra/dialers/retrier.go68 intra/dialers/retrier.go177-196 intra/dialers/retrier.go798-807 intra/dialers/split_and_desync.go35

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Tee Buffer and Write Replay

The retrier buffers the first write (typically the TLS ClientHello or HTTP request) in the tee field for replay on retry [intra/dialers/retrier.go:108].

Write Flow with Tee Buffer

Title: Tee Buffer Write Sequence

Sources: intra/dialers/retrier.go604-686 intra/dialers/retrier.go560-590

Retry Write-Read Sequence

Title: Retry Replay Flow

Sources: intra/dialers/retrier.go473-558 intra/dialers/retrier.go425-461

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Timeout Calculation

The calcTimeout() function computes an adaptive timeout for the first read after a write, based on connection RTT [intra/dialers/retrier.go:134]:

Timeout Formula

timeout = max(rtt * 2, ciel) + min(2 * rtt, floor)

where:
  ciel  = max(1, (cielRetryReadTimeoutSec / spread)) * time.Second
  floor = min(300, (floorRetryReadTimeoutMillis / spread)) * time.Millisecond
  spread = max(1, spread) (distributes timeout budget)

Constant	Value	Purpose
cielRetryReadTimeoutSec	9s	Maximum base timeout [intra/dialers/retrier.go:58]
floorRetryReadTimeoutMillis	1000ms	Minimum additional timeout [intra/dialers/retrier.go:60]

Sources: intra/dialers/retrier.go134-142 intra/dialers/retrier.go50-63 intra/dialers/retrier.go324-361

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Synchronization with SigCond

The retryDone field is a core.SigCond that coordinates retry completion across concurrent readers and writers [intra/dialers/retrier.go:116].

SigCond State Machine

Title: core.SigCond States

Sources: intra/core/sigcond.go14-76

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Interaction with Splitters

The retrier delegates to splitter implementations when dialStrat != SplitNever:

Strategy	Implementation	Created By
SplitTCP, SplitTCPOrTLS	splitter	intra/dialers/retrier.go414-419
SplitDesync	overwriteSplitter	intra/dialers/retrier.go409

The retrier's conn field holds either:

• protect.Conn (underlying connection)
• *splitter (when SplitTCP or SplitTCPOrTLS)
• *overwriteSplitter (when SplitDesync)

Sources: intra/dialers/retrier.go372-420 intra/dialers/direct_split.go37-42 intra/dialers/split_and_desync.go45-55

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Error Handling and Edge Cases

Connection State Management

The retrier handles several edge cases:

1. Nil connection: Functions check r.conn != nil before use [intra/dialers/retrier.go:474].
2. CloseRead/CloseWrite during retry: State is tracked via readDone and writeDone atomic flags [intra/dialers/retrier.go:85-86].
3. Empty reads: Up to maxEmptyReads (3) empty reads are tolerated before returning io.ErrNoProgress [intra/dialers/retrier.go:52].
4. Short TLS reads: For port 443 + split strategies, reads < minExpectedTLSRead (16 bytes) trigger retry with errTLSHandshake [intra/dialers/retrier.go:54].

Multi-Dialer Reset

When all dialers fail in multi-dialer mode, the nextDialerIdx resets to 0 if retries remain [intra/dialers/retrier.go:375-379]:

Sources: intra/dialers/retrier.go375-379

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Code Entity Summary

Entity	Type	File	Purpose
retrier	struct	intra/dialers/retrier.go73	Main retry orchestrator.
DialWithSplitRetry()	func	intra/dialers/retrier.go150	Single-dialer entry point.
DialAny()	func	intra/dialers/retrier.go198	Multi-dialer entry point.
dialStratLocked()	method	intra/dialers/retrier.go248	Strategy selection algorithm.
retryWriteReadLocked()	method	intra/dialers/retrier.go425	Retry dial + write + read.
calcTimeout()	func	intra/dialers/retrier.go134	Adaptive timeout calculation.
reprioritize()	func	intra/dialers/retrier.go177	Dialer reordering by cache.
ippPins	var	intra/dialers/retrier.go68	IP:port → dialer ID cache.
SigCond	struct	intra/core/sigcond.go29	Signalable boolean (Ref in retrier.go).

Sources: intra/dialers/retrier.go intra/dialers/split_and_desync.go intra/dialers/direct_split.go