Improve mesh media throughput (#845)

* Improve mesh media throughput

* Reserve media slots atomically

* Prioritize small fragment trains

---------

Co-authored-by: jack <jackjackbits@users.noreply.github.com>

bitchat/Features/media/ImageUtils.swift

@@ -13,10 +13,10 @@ enum ImageUtilsError: Error {
 }
 
 enum ImageUtils {
-    private static let compressionQuality: CGFloat = 0.85
+    private static let compressionQuality: CGFloat = 0.82
-    private static let targetImageBytes: Int = 60_000
+    private static let targetImageBytes: Int = 45_000
 
-    static func processImage(at url: URL, maxDimension: CGFloat = 512) throws -> URL {
+    static func processImage(at url: URL, maxDimension: CGFloat = 448) throws -> URL {
         // Security H1: Check file size BEFORE reading into memory
         let attrs = try FileManager.default.attributesOfItem(atPath: url.path)
         guard let fileSize = attrs[.size] as? Int else {
@@ -38,7 +38,7 @@ enum ImageUtils {
     }
 
     #if os(iOS)
-    static func processImage(_ image: UIImage, maxDimension: CGFloat = 512) throws -> URL {
+    static func processImage(_ image: UIImage, maxDimension: CGFloat = 448) throws -> URL {
         return try autoreleasepool {
             // Scale the image first
             let scaled = scaledImage(image, maxDimension: maxDimension)
@@ -106,7 +106,7 @@ enum ImageUtils {
         return data as Data
     }
     #else
-    static func processImage(_ image: NSImage, maxDimension: CGFloat = 512) throws -> URL {
+    static func processImage(_ image: NSImage, maxDimension: CGFloat = 448) throws -> URL {
         return try autoreleasepool {
             let scaled = scaledImage(image, maxDimension: maxDimension)
             guard let inputCG = scaled.cgImage(forProposedRect: nil, context: nil, hints: nil) else {

bitchat/Features/voice/VoiceRecorder.swift

@@ -14,6 +14,7 @@ final class VoiceRecorder: NSObject, AVAudioRecorderDelegate {
 
     private let queue = DispatchQueue(label: "com.bitchat.voice-recorder")
     private let paddingInterval: TimeInterval = 0.5
+    private let maxRecordingDuration: TimeInterval = 120
 
     private var recorder: AVAudioRecorder?
     private var currentURL: URL?
@@ -75,14 +76,14 @@ final class VoiceRecorder: NSObject, AVAudioRecorderDelegate {
                 AVFormatIDKey: kAudioFormatMPEG4AAC,
                 AVSampleRateKey: 16_000,
                 AVNumberOfChannelsKey: 1,
-                AVEncoderBitRateKey: 20_000
+                AVEncoderBitRateKey: 16_000
             ]
 
             let audioRecorder = try AVAudioRecorder(url: outputURL, settings: settings)
             audioRecorder.delegate = self
             audioRecorder.isMeteringEnabled = true
             audioRecorder.prepareToRecord()
-            audioRecorder.record()
+            audioRecorder.record(forDuration: maxRecordingDuration)
 
             recorder = audioRecorder
             currentURL = outputURL

bitchat/Services/BLE/BLEService.swift

@@ -147,7 +147,35 @@ final class BLEService: NSObject {
     private var ingressByMessageID: [String: (link: LinkID, timestamp: Date)] = [:]
 
     // Backpressure-aware write queue per peripheral
-    private var pendingPeripheralWrites: [String: [Data]] = [:]
+    private struct OutboundPriority: Comparable {
+        let level: Int
+        let suborder: Int
+
+        static let high = OutboundPriority(level: 0, suborder: 0)
+        static func fragment(totalFragments: Int) -> OutboundPriority {
+            OutboundPriority(level: 1, suborder: max(1, min(totalFragments, Int(UInt16.max))))
+        }
+        static let fileTransfer = OutboundPriority(level: 2, suborder: Int.max - 1)
+        static let low = OutboundPriority(level: 2, suborder: Int.max)
+
+        static func < (lhs: OutboundPriority, rhs: OutboundPriority) -> Bool {
+            if lhs.level != rhs.level { return lhs.level < rhs.level }
+            return lhs.suborder < rhs.suborder
+        }
+    }
+    private struct PendingWrite {
+        let priority: OutboundPriority
+        let data: Data
+    }
+    private struct PendingFragmentTransfer {
+        let packet: BitchatPacket
+        let pad: Bool
+        let maxChunk: Int?
+        let directedPeer: PeerID?
+        let transferId: String?
+    }
+    private var pendingPeripheralWrites: [String: [PendingWrite]] = [:]
+    private var pendingFragmentTransfers: [PendingFragmentTransfer] = []
     // Debounce duplicate disconnect notifies
     private var recentDisconnectNotifies: [PeerID: Date] = [:]
     // Store-and-forward for directed messages when we have no links
@@ -303,6 +331,7 @@ final class BLEService: NSObject {
             recentAnnounceBySender.removeAll()
             recentAnnounceOrder.removeAll()
             pendingPeripheralWrites.removeAll()
+            pendingFragmentTransfers.removeAll()
             pendingNotifications.removeAll()
             pendingDirectedRelays.removeAll()
             ingressByMessageID.removeAll()
@@ -454,10 +483,11 @@ final class BLEService: NSObject {
         
         // Send immediately to all connected peers
         if let data = leavePacket.toBinaryData(padding: false) {
+            let leavePriority = priority(for: leavePacket, data: data)
             // Send to peripherals we're connected to as central
             for state in peripherals.values where state.isConnected {
                 if let characteristic = state.characteristic {
-                    writeOrEnqueue(data, to: state.peripheral, characteristic: characteristic)
+                    writeOrEnqueue(data, to: state.peripheral, characteristic: characteristic, priority: leavePriority)
                 }
             }
             
@@ -591,10 +621,19 @@ final class BLEService: NSObject {
 
     func cancelTransfer(_ transferId: String) {
         collectionsQueue.async(flags: .barrier) { [weak self] in
-            guard let self = self, let state = self.activeTransfers.removeValue(forKey: transferId) else { return }
+            guard let self = self else { return }
+            if let state = self.activeTransfers.removeValue(forKey: transferId) {
                 state.workItems.forEach { $0.cancel() }
                 TransferProgressManager.shared.cancel(id: transferId)
                 SecureLogger.debug("🛑 Cancelled transfer \(transferId.prefix(8))…", category: .session)
+                self.messageQueue.async { [weak self] in
+                    self?.startNextPendingTransferIfNeeded()
+                }
+            } else if let pendingIndex = self.pendingFragmentTransfers.firstIndex(where: { $0.transferId == transferId }) {
+                self.pendingFragmentTransfers.remove(at: pendingIndex)
+                TransferProgressManager.shared.cancel(id: transferId)
+                SecureLogger.debug("🛑 Removed pending transfer \(transferId.prefix(8))… before start", category: .session)
+            }
         }
     }
     
@@ -737,6 +776,8 @@ final class BLEService: NSObject {
         guard let recipientPeerID = PeerID(hexData: packet.recipientID) else { return }
         var sentEncrypted = false
 
+        let outboundPriority = priority(for: packet, data: data)
+
         // Per-link limits for the specific peer
         var peripheralMaxLen: Int?
         if let perUUID = (DispatchQueue.getSpecific(key: bleQueueKey) != nil) ? peerToPeripheralUUID[recipientPeerID] : bleQueue.sync(execute: { peerToPeripheralUUID[recipientPeerID] }) {
@@ -769,7 +810,7 @@ final class BLEService: NSObject {
            let state = (DispatchQueue.getSpecific(key: bleQueueKey) != nil) ? peripherals[peripheralUUID] : bleQueue.sync(execute: { peripherals[peripheralUUID] }),
            state.isConnected,
            let characteristic = state.characteristic {
-            writeOrEnqueue(data, to: state.peripheral, characteristic: characteristic)
+            writeOrEnqueue(data, to: state.peripheral, characteristic: characteristic, priority: outboundPriority)
             sentEncrypted = true
         }
 
@@ -826,6 +867,7 @@ final class BLEService: NSObject {
             }
             return nil
         }()
+        let outboundPriority = priority(for: packet, data: data)
 
         let states = snapshotPeripheralStates()
         var minCentralWriteLen: Int?
@@ -901,7 +943,7 @@ final class BLEService: NSObject {
             let pid = s.peripheral.identifier.uuidString
             guard selectedPeripheralIDs.contains(pid) else { continue }
             if let ch = s.characteristic {
-                writeOrEnqueue(data, to: s.peripheral, characteristic: ch)
+                writeOrEnqueue(data, to: s.peripheral, characteristic: ch, priority: outboundPriority)
             }
         }
         // Notify selected subscribed centrals
@@ -980,7 +1022,7 @@ final class BLEService: NSObject {
         
         // Fire-and-forget principle: always use .withoutResponse for speed
         // CoreBluetooth will handle fragmentation at L2CAP layer
-        writeOrEnqueue(data, to: peripheral, characteristic: characteristic)
+        writeOrEnqueue(data, to: peripheral, characteristic: characteristic, priority: .high)
     }
 
     private func handleFileTransfer(_ packet: BitchatPacket, from peerID: PeerID) {
@@ -2347,7 +2389,28 @@ extension BLEService {
         return Set(scored.prefix(k).map { $0.id })
     }
 
-    private func writeOrEnqueue(_ data: Data, to peripheral: CBPeripheral, characteristic: CBCharacteristic) {
+    private func priority(for packet: BitchatPacket, data: Data) -> OutboundPriority {
+        guard let messageType = MessageType(rawValue: packet.type) else { return .low }
+        switch messageType {
+        case .fragment:
+            let total = fragmentTotalCount(from: packet.payload)
+            return OutboundPriority.fragment(totalFragments: total)
+        case .fileTransfer:
+            return .fileTransfer
+        default:
+            return .high
+        }
+    }
+
+    private func fragmentTotalCount(from payload: Data) -> Int {
+        guard payload.count >= 12 else { return Int(UInt16.max) }
+        let totalHigh = Int(payload[10])
+        let totalLow = Int(payload[11])
+        let total = (totalHigh << 8) | totalLow
+        return max(total, 1)
+    }
+
+    private func writeOrEnqueue(_ data: Data, to peripheral: CBPeripheral, characteristic: CBCharacteristic, priority: OutboundPriority) {
         // BLE operations run on bleQueue; keep queue affinity
         bleQueue.async { [weak self] in
             guard let self = self else { return }
@@ -2363,19 +2426,21 @@ extension BLEService {
                     if newSize > capBytes {
                         SecureLogger.warning("⚠️ Dropping oversized write chunk (\(newSize)B) for peripheral \(uuid)", category: .session)
                     } else {
-                        // Append and trim from the front to respect cap
+                        let item = PendingWrite(priority: priority, data: data)
-                        var total = queue.reduce(0) { $0 + $1.count }
+                        var total = queue.reduce(0) { $0 + $1.data.count } + newSize
-                        queue.append(data)
+                        let insertIndex = queue.firstIndex { item.priority < $0.priority } ?? queue.count
-                        total += newSize
+                        queue.insert(item, at: insertIndex)
                         if total > capBytes {
                             var removedBytes = 0
                             while total > capBytes && !queue.isEmpty {
-                                let removed = queue.removeFirst()
+                                let removed = queue.removeLast()
-                                removedBytes += removed.count
+                                removedBytes += removed.data.count
-                                total -= removed.count
+                                total -= removed.data.count
                             }
+                            if removedBytes > 0 {
                                 SecureLogger.warning("📉 Trimmed pending write buffer for \(uuid) by \(removedBytes)B to \(total)B", category: .session)
                             }
+                        }
                         self.pendingPeripheralWrites[uuid] = queue.isEmpty ? nil : queue
                     }
                 }
@@ -2388,7 +2453,7 @@ extension BLEService {
         bleQueue.async { [weak self] in
             guard let self = self else { return }
             guard let state = self.peripherals[uuid], let ch = state.characteristic else { return }
-            var queueCopy: [Data] = []
+            var queueCopy: [PendingWrite] = []
             self.collectionsQueue.sync {
                 queueCopy = self.pendingPeripheralWrites[uuid] ?? []
             }
@@ -2396,7 +2461,7 @@ extension BLEService {
             var sent = 0
             for item in queueCopy {
                 if peripheral.canSendWriteWithoutResponse {
-                    peripheral.writeValue(item, for: ch, type: .withoutResponse)
+                    peripheral.writeValue(item.data, for: ch, type: .withoutResponse)
                     sent += 1
                 } else {
                     break
@@ -2405,16 +2470,17 @@ extension BLEService {
             if sent > 0 {
                 self.collectionsQueue.async(flags: .barrier) {
                     var q = self.pendingPeripheralWrites[uuid] ?? []
-                    if sent <= q.count {
+                    if sent > 0 {
-                        q.removeFirst(sent)
+                        let toRemove = min(sent, q.count)
-                    } else {
+                        if toRemove > 0 {
-                        q.removeAll()
+                            q.removeFirst(toRemove)
                         }
                         self.pendingPeripheralWrites[uuid] = q.isEmpty ? nil : q
                     }
                 }
             }
         }
+    }
     
     // MARK: Application State Handlers (iOS)
     
@@ -2601,16 +2667,86 @@ extension BLEService {
     // MARK: Fragmentation (Required for messages > BLE MTU)
     
     private func sendFragmentedPacket(_ packet: BitchatPacket, pad: Bool, maxChunk: Int? = nil, directedOnlyPeer: PeerID? = nil, transferId: String? = nil) {
-        guard let fullData = packet.toBinaryData(padding: pad) else { return }
+        let context = PendingFragmentTransfer(packet: packet, pad: pad, maxChunk: maxChunk, directedPeer: directedOnlyPeer, transferId: transferId)
-        // Fragment the unpadded frame; each fragment will be encoded independently
+        if packet.type == MessageType.fileTransfer.rawValue {
+            let shouldQueue = collectionsQueue.sync {
+                self.activeTransfers.count >= TransportConfig.bleMaxConcurrentTransfers
+            }
+            if shouldQueue {
+                queueFragmentTransfer(context, prioritizeFront: false)
+                return
+            }
+        }
+        startFragmentedPacket(context)
+    }
 
+    private func queueFragmentTransfer(_ context: PendingFragmentTransfer, prioritizeFront: Bool) {
+        collectionsQueue.async(flags: .barrier) { [weak self] in
+            guard let self = self else { return }
+            if prioritizeFront {
+                self.pendingFragmentTransfers.insert(context, at: 0)
+            } else {
+                self.pendingFragmentTransfers.append(context)
+            }
+        }
+        if let transferId = context.transferId {
+            SecureLogger.debug("🚦 Queued media transfer \(transferId.prefix(8))… waiting for slot", category: .session)
+        } else {
+            SecureLogger.debug("🚦 Queued fragment transfer waiting for slot", category: .session)
+        }
+    }
+
+    private func startFragmentedPacket(_ context: PendingFragmentTransfer) {
+        let packet = context.packet
+        let isFileTransfer = packet.type == MessageType.fileTransfer.rawValue
+        var reservedTransferId: String?
+
+        let releaseReservedSlot: (String) -> Void = { id in
+            TransferProgressManager.shared.cancel(id: id)
+            self.collectionsQueue.async(flags: .barrier) { [weak self] in
+                self?.activeTransfers.removeValue(forKey: id)
+            }
+            self.messageQueue.async { [weak self] in
+                self?.startNextPendingTransferIfNeeded()
+            }
+        }
+
+        if isFileTransfer {
+            let candidateId = context.transferId ?? packet.payload.sha256Hex()
+            var didReserve = false
+            collectionsQueue.sync(flags: .barrier) {
+                if self.activeTransfers.count < TransportConfig.bleMaxConcurrentTransfers,
+                   self.activeTransfers[candidateId] == nil {
+                    self.activeTransfers[candidateId] = ActiveTransferState(totalFragments: 0, sentFragments: 0, workItems: [])
+                    didReserve = true
+                }
+            }
+            guard didReserve else {
+                queueFragmentTransfer(context, prioritizeFront: true)
+                return
+            }
+            reservedTransferId = candidateId
+        }
+
+        guard let fullData = packet.toBinaryData(padding: context.pad) else {
+            if let id = reservedTransferId {
+                releaseReservedSlot(id)
+            }
+            return
+        }
+        // Fragment the unpadded frame; each fragment will be encoded independently
         let fragmentID = Data((0..<8).map { _ in UInt8.random(in: 0...255) })
-        let chunk = maxChunk ?? defaultFragmentSize
+        let chunk = context.maxChunk ?? defaultFragmentSize
         let safeChunk = max(64, chunk)
         let fragments = stride(from: 0, to: fullData.count, by: safeChunk).map { offset in
             Data(fullData[offset..<min(offset + safeChunk, fullData.count)])
         }
-        guard !fragments.isEmpty else { return }
+        guard !fragments.isEmpty else {
+            if let id = reservedTransferId {
+                releaseReservedSlot(id)
+            }
+            return
+        }
 
         // Lightweight pacing to reduce floods and allow BLE buffers to drain
         // Also briefly pause scanning during long fragment trains to save battery
@@ -2619,18 +2755,16 @@ extension BLEService {
             bleQueue.async { [weak self] in
                 guard let self = self, let c = self.centralManager, c.state == .poweredOn else { return }
                 if c.isScanning { c.stopScan() }
-                // Resume scanning after we expect last fragment to be sent
+                let expectedMs = min(TransportConfig.bleExpectedWriteMaxMs, totalFragments * TransportConfig.bleExpectedWritePerFragmentMs)
-            let expectedMs = min(TransportConfig.bleExpectedWriteMaxMs, totalFragments * TransportConfig.bleExpectedWritePerFragmentMs) // ~8ms per fragment
                 self.bleQueue.asyncAfter(deadline: .now() + .milliseconds(expectedMs)) { [weak self] in
                     self?.startScanning()
                 }
             }
         }
-        let perFragMs = (directedOnlyPeer != nil || packet.recipientID != nil) ? TransportConfig.bleFragmentSpacingDirectedMs : TransportConfig.bleFragmentSpacingMs
+        let perFragMs = (context.directedPeer != nil || packet.recipientID != nil) ? TransportConfig.bleFragmentSpacingDirectedMs : TransportConfig.bleFragmentSpacingMs
 
         let transferIdentifier: String? = {
-            guard packet.type == MessageType.fileTransfer.rawValue else { return nil }
+            guard let id = reservedTransferId else { return nil }
-            let id = transferId ?? packet.payload.sha256Hex()
             collectionsQueue.sync(flags: .barrier) {
                 self.activeTransfers[id] = ActiveTransferState(totalFragments: totalFragments, sentFragments: 0, workItems: [])
             }
@@ -2648,9 +2782,8 @@ extension BLEService {
             payload.append(packet.type)
             payload.append(fragment)
 
-            // Choose recipient for the fragment: directed override if provided
             let fragmentRecipient: Data? = {
-                if let only = directedOnlyPeer { return Data(hexString: only.id) }
+                if let only = context.directedPeer { return Data(hexString: only.id) }
                 return packet.recipientID
             }()
 
@@ -2700,10 +2833,36 @@ extension BLEService {
         collectionsQueue.async(flags: .barrier) { [weak self] in
             guard let self = self, var state = self.activeTransfers[transferId] else { return }
             state.sentFragments = min(state.sentFragments + 1, state.totalFragments)
-            self.activeTransfers[transferId] = state
+            let isComplete = state.sentFragments >= state.totalFragments
-            TransferProgressManager.shared.recordFragmentSent(id: transferId)
+            if isComplete {
-            if state.sentFragments >= state.totalFragments {
                 self.activeTransfers.removeValue(forKey: transferId)
+            } else {
+                self.activeTransfers[transferId] = state
+            }
+            TransferProgressManager.shared.recordFragmentSent(id: transferId)
+            if isComplete {
+                self.messageQueue.async { [weak self] in
+                    self?.startNextPendingTransferIfNeeded()
+                }
+            }
+        }
+    }
+
+    private func startNextPendingTransferIfNeeded() {
+        collectionsQueue.async(flags: .barrier) { [weak self] in
+            guard let self = self else { return }
+            let limit = TransportConfig.bleMaxConcurrentTransfers
+            var availableSlots = max(0, limit - self.activeTransfers.count)
+            guard availableSlots > 0, !self.pendingFragmentTransfers.isEmpty else { return }
+            var toStart: [PendingFragmentTransfer] = []
+            while availableSlots > 0, !self.pendingFragmentTransfers.isEmpty {
+                toStart.append(self.pendingFragmentTransfers.removeFirst())
+                availableSlots -= 1
+            }
+            for context in toStart {
+                self.messageQueue.async { [weak self] in
+                    self?.startFragmentedPacket(context)
+                }
             }
         }
     }
@@ -2814,8 +2973,9 @@ extension BLEService {
         }
 
         // Decode the original packet bytes we reassembled, so flags/compression are preserved
-        if let originalPacket = BinaryProtocol.decode(reassembled) {
+        if var originalPacket = BinaryProtocol.decode(reassembled) {
             SecureLogger.debug("✅ Reassembled packet id=\(String(format: "%016llx", fragU64)) type=\(originalPacket.type) bytes=\(reassembled.count)", category: .session)
+            originalPacket.ttl = 0
             handleReceivedPacket(originalPacket, from: peerID)
         } else {
             SecureLogger.error("❌ Failed to decode reassembled packet (type=\(originalType), total=\(total))", category: .session)
@@ -2931,6 +3091,7 @@ extension BLEService {
                 senderIsSelf: senderID == myPeerID,
                 isEncrypted: packet.type == MessageType.noiseEncrypted.rawValue,
                 isDirectedEncrypted: (packet.type == MessageType.noiseEncrypted.rawValue) && (packet.recipientID != nil),
+                isFragment: packet.type == MessageType.fragment.rawValue,
                 isDirectedFragment: packet.type == MessageType.fragment.rawValue && packet.recipientID != nil,
                 isHandshake: packet.type == MessageType.noiseHandshake.rawValue,
                 isAnnounce: packet.type == MessageType.announce.rawValue,

bitchat/Services/RelayController.swift

@@ -13,6 +13,7 @@ struct RelayController {
                        senderIsSelf: Bool,
                        isEncrypted: Bool,
                        isDirectedEncrypted: Bool,
+                       isFragment: Bool,
                        isDirectedFragment: Bool,
                        isHandshake: Bool,
                        isAnnounce: Bool,
@@ -36,6 +37,16 @@ struct RelayController {
             return RelayDecision(shouldRelay: true, newTTL: newTTL, delayMs: delayMs)
         }
 
+        if isFragment {
+            let ttlLimit = min(ttlCap, TransportConfig.bleFragmentRelayTtlCap)
+            guard ttlLimit > 1 else {
+                return RelayDecision(shouldRelay: false, newTTL: ttlLimit, delayMs: 0)
+            }
+            let newTTL = ttlLimit &- 1
+            let delayMs = Int.random(in: TransportConfig.bleFragmentRelayMinDelayMs...TransportConfig.bleFragmentRelayMaxDelayMs)
+            return RelayDecision(shouldRelay: true, newTTL: newTTL, delayMs: delayMs)
+        }
+
         // TTL clamping for broadcast
         // - Dense graphs: keep lower but still allow multi-hop bridging
         // - Announces get a bit more headroom

bitchat/Services/TransportConfig.swift

@@ -8,6 +8,10 @@ enum TransportConfig {
     static let messageTTLDefault: UInt8 = 7                 // Default TTL for mesh flooding
     static let bleMaxInFlightAssemblies: Int = 128          // Cap concurrent fragment assemblies
     static let bleHighDegreeThreshold: Int = 6              // For adaptive TTL/probabilistic relays
+    static let bleMaxConcurrentTransfers: Int = 2           // Limit simultaneous large media sends
+    static let bleFragmentRelayMinDelayMs: Int = 8          // Faster forwarding for media fragments
+    static let bleFragmentRelayMaxDelayMs: Int = 25         // Upper jitter bound for fragment relays
+    static let bleFragmentRelayTtlCap: UInt8 = 5            // Clamp fragment TTL to contain floods
 
     // UI / Storage Caps
     static let privateChatCap: Int = 1337

bitchat/Utils/FileTransferLimits.swift

@@ -5,9 +5,9 @@ enum FileTransferLimits {
     /// Absolute ceiling enforced for any file payload (voice, image, other).
     static let maxPayloadBytes: Int = 1 * 1024 * 1024 // 1 MiB
     /// Voice notes stay small for low-latency relays.
-    static let maxVoiceNoteBytes: Int = 1 * 1024 * 1024 // 1 MiB
+    static let maxVoiceNoteBytes: Int = 512 * 1024 // 512 KiB
     /// Compressed images after downscaling should comfortably fit under this budget.
-    static let maxImageBytes: Int = 1 * 1024 * 1024 // 1 MiB
+    static let maxImageBytes: Int = 512 * 1024 // 512 KiB
     /// Worst-case size once TLV metadata and binary packet framing are included for the largest payloads.
     static let maxFramedFileBytes: Int = {
         let maxMetadataBytes = Int(UInt16.max) * 2 // fileName + mimeType TLVs