When working with mobile network programming, Maximum Transmission Unit (MTU) values are critical for avoiding packet fragmentation and ensuring optimal data transmission. Unlike wired networks where standard MTUs are well-documented (e.g., 1500 bytes for Ethernet), mobile networks show greater variability.

Based on field testing and carrier specifications:

| Technology | Typical MTU | Notes                      |
|------------|-------------|----------------------------|
| GPRS       | 1500 bytes  | May reduce to 576 in some implementations |
| EDGE       | 1500 bytes  | Similar to GPRS            |
| UMTS       | 1500 bytes  | Default, but may vary      |
| HSPA       | 1420-1500   | Often needs tuning         |
| LTE        | 1420-1500   | Carrier-dependent          |

Here's how to programmatically handle MTU discovery in Android:

ConnectivityManager cm = (ConnectivityManager) getSystemService(CONNECTIVITY_SERVICE);
Network network = cm.getActiveNetwork();
NetworkCapabilities nc = cm.getNetworkCapabilities(network);

if (nc != null) {
    int mtu = nc.getLinkDownstreamBandwidthKbps();
    // Fallback logic if MTU detection fails
    if (mtu <= 0) {
        mtu = (nc.hasTransport(NetworkCapabilities.TRANSPORT_CELLULAR)) 
              ? 1420 : 1500;
    }
    Log.d("NETWORK_MTU", "Detected MTU: " + mtu);
}

For VoIP applications, we found these adjustments crucial:

// Set custom MTU for RTP packets
int optimalMtu = 1360; // LTE with VPN overhead
DatagramSocket socket = new DatagramSocket();
try {
    socket.setSendBufferSize(optimalMtu);
    socket.setReceiveBufferSize(optimalMtu * 2);
} catch (SocketException e) {
    // Fallback to system default
}

• VPN Overhead: Reduce MTU by 60-100 bytes when using IPsec
• TCP MSS Clamping: Essential for HSPA networks: iptables -t mangle -A FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --set-mss 1360
• IPv6 Considerations: Requires 1280 byte minimum (RFC 8200)

When developing applications for mobile networks, understanding Maximum Transmission Unit (MTU) sizes is crucial for preventing packet fragmentation and optimizing performance. Unlike wired networks with standardized 1500-byte MTUs, mobile networks exhibit significant variation.

• GPRS (2G): 576 bytes (often reduced to 512 for compatibility)
• EDGE: 576-1500 bytes (network-dependent)
• UMTS (3G): 1500 bytes (theoretical maximum, often lower)
• HSPA: 1500 bytes (with possible overhead reduction)
• LTE (4G): 1428-1500 bytes (accounting for 72-byte GTP/UDP/IP overhead)

Here's a Python example using socket programming to detect path MTU:

import socket

def detect_path_mtu(host="8.8.8.8"):
    s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
    s.setsockopt(socket.IPPROTO_IP, socket.IP_MTU_DISCOVER, socket.IP_PMTUDISC_DO)
    try:
        s.connect((host, 53))  # DNS port
        return s.getsockopt(socket.IPPROTO_IP, socket.IP_MTU)
    except socket.error:
        return 576  # Fallback to minimum
    finally:
        s.close()

print(f"Detected Path MTU: {detect_path_mtu()} bytes")

LTE networks often require special handling due to GTP tunneling overhead. The effective MTU becomes:

LTE_MTU = 1500 - 8(GTP) - 8(UDP) - 20(IP) - 8(GTP extension) = 1456 bytes

For Android developers, here's how to check network type and adjust MTU accordingly:

ConnectivityManager cm = (ConnectivityManager)getSystemService(CONNECTIVITY_SERVICE);
NetworkInfo info = cm.getActiveNetworkInfo();

int mtu = 1500; // Default
if (info != null) {
    switch(info.getType()) {
        case ConnectivityManager.TYPE_MOBILE:
            switch(info.getSubtype()) {
                case TelephonyManager.NETWORK_TYPE_GPRS:
                case TelephonyManager.NETWORK_TYPE_EDGE:
                    mtu = 576;
                    break;
                case TelephonyManager.NETWORK_TYPE_UMTS:
                case TelephonyManager.NETWORK_TYPE_HSPA:
                    mtu = 1500;
                    break;
                case TelephonyManager.NETWORK_TYPE_LTE:
                    mtu = 1428;
                    break;
            }
            break;
    }
}

• Implement automatic MTU discovery during network handoffs
• Use TCP MSS clamping (especially for LTE)
• Consider header compression (ROHC) for low-bandwidth networks
• Implement fallback mechanisms when ICMP fragmentation-needed messages are blocked