Routing

XGL uses 16-bit node IDs. Local source_id must not be 0 or XGL_BROADCAST_ID (0xFFFF). Received frames are rejected when the source is zero, the source is broadcast, or source equals target except for broadcast. target_id may be the local node, a remote node, or XGL_BROADCAST_ID.

TODO(xgen-link): confirm broadcast/multicast address policy beyond XGL_BROADCAST_ID.

Route Table

Each route binds:

• target_id
• PHY callbacks
• max_frame_size
• read_freq_hz
• metric

Forwarding must check route MTU. Frames larger than max_frame_size return XGL_ERR_BUFFER_TOO_SMALL and are dropped.

Routing Decision Order

1. Check whether the frame targets the local node.
2. If local, deliver to transport.
3. If remote, check TTL.
4. Lookup route for target_id.
5. Check complete frame length against route MTU.
6. Update TTL and recompute header/frame CRC.
7. Call egress PHY.

Decision Table

Input condition	Decision	Error/counter behavior	Evidence
source_id == 0	Drop	XGL_ERR_INVALID_PARAM, RX drop count	src/network/xgl_network.c, src/network/xgl_network_receive.c
source_id == XGL_BROADCAST_ID	Drop	XGL_ERR_INVALID_PARAM, RX drop count	src/network/xgl_network.c
source_id == target_id and target is not broadcast	Drop	XGL_ERR_INVALID_PARAM, RX drop count	src/network/xgl_network.c
target_id == local_id	Local delivery to transport	Increment network RX stats	src/network/xgl_network_receive.c
target_id == XGL_BROADCAST_ID	Local delivery to transport	Treated as local by xgl_network_is_local()	include/xgl/internal/xgl_network.h
Remote target and route missing	Drop	XGL_ERR_ROUTE_NOT_FOUND, RX drop count, error callback	src/network/xgl_network_receive.c, test/test_network.cpp
Remote target and ttl <= 1	Drop	XGL_ERR_TTL_EXPIRED, RX drop count, error callback	src/network/xgl_network_receive.c, test/test_network.cpp
Remote target and frame exceeds route MTU	Drop	XGL_ERR_BUFFER_TOO_SMALL, RX drop count	src/network/xgl_network_receive.c, test/test_network.cpp
Remote target, route found, TTL valid, MTU fits	Forward	Decrement TTL, recompute header CRC and frame CRC, call route PHY TX	src/network/xgl_network_receive.c, test/test_network.cpp

TTL

TTL is decremented on every forwarded hop. A remote packet with ttl <= 1 returns XGL_ERR_TTL_EXPIRED and must not be forwarded; forwarded frames therefore leave the node with ttl >= 1.

Authenticated frames keep their original end-to-end tag while TTL changes. The forwarding path only updates TTL and CRC fields; authentication remains valid because hop-mutable fields are excluded from the canonical AAD.

Delivery Path

• Local target: deliver to transport.
• Remote target: lookup route and forward.
• Broadcast/multicast: address ranges are reserved; current reliable delivery is unicast-focused.

Multi-PHY Notes

Each PHY can use a different read_freq_hz. xgl_run() polls routes and combines transport/reassembly deadlines into the next wakeup time. Low-power systems should not wake the full protocol stack at a fixed high frequency just because one PHY is slow.