carlos-jenkins · October 22, 2025 18:15 · Oct 22, 2025
diff --git a/predict.py b/predict.py
@@ -0,0 +1,76 @@
+import socket
+import struct
+
+
+def _ipv4_to_u32(ip_str: str) -> int:
+    # network-order (big-endian) to host int
+    return struct.unpack("!I", socket.inet_aton(ip_str))[0]
+
+
+def bond_layer3_4_hash_tcp(
+    src_ip: str,
+    dst_ip: str,
+    src_port: int,
+    dst_port: int,
+    slave_count: int
+) -> int:
+    """
+    Predict the slave index for Linux bonding xmit_hash_policy=layer3+4 for
+    unfragmented TCP.
+
+    :param str src_ip: Source IPv4 address, e.g. "192.168.1.10"
+    :param str dst_ip: Destination IPv4 address, e.g. "10.0.0.5"
+    :param int src_port: TCP source port (0..65535)
+    :param int dst_port: TCP destination port (0..65535)
+    :param int slave_count: Number of slaves (>0)
+
+    :return: Slave index in [0, slave_count-1]
+    :rtype: int
+    """
+    if not (0 <= src_port <= 0xFFFF and 0 <= dst_port <= 0xFFFF):
+        raise ValueError("Ports must be 0..65535")
+    if slave_count <= 0:
+        raise ValueError("slave_count must be > 0")
+
+    sip = _ipv4_to_u32(src_ip)
+    dip = _ipv4_to_u32(dst_ip)
+
+    # Start with ports "as in the header".
+    # A common interpretation is to pack them as 16|16 bits:
+    h = ((src_port & 0xFFFF) << 16) | (dst_port & 0xFFFF)
+
+    # Then XOR in the IPs (L3)
+    h ^= sip ^ dip
+
+    # Fold/mix as per docs
+    h ^= (h >> 16)
+    h ^= (h >> 8)
+
+    # Bonding special-case for LAYER34: discard lowest bit
+    h >>= 1
+
+    # Reduce to slave index
+    return h % slave_count
+
+
+# Example usage
+ip_src = '192.168.1.10'
+ip_dst = '10.0.0.5'
+slave_count = 2
+port_dst = 80
+
+# Show distribution of source ports to slave indices
+distribution = [0] * slave_count
+for port_src in range(1, 65536):
+    slave_index = bond_layer3_4_hash_tcp(
+        ip_src,
+        ip_dst,
+        port_src,
+        port_dst,
+        slave_count,
+    )
+    distribution[slave_index] += 1
+
+print(f'Distribution of source ports to {slave_count} slaves:')
+for idx, count in enumerate(distribution):
+    print(f' Slave {idx}: {count} ports')
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