4 files changed, 39 insertions, 0 deletions
diff --git a/cmake/config/ipcp/broadcast.cmake b/cmake/config/ipcp/broadcast.cmake
index 79f41d10..f521ed8e 100644
--- a/cmake/config/ipcp/broadcast.cmake
+++ b/cmake/config/ipcp/broadcast.cmake
@@ -4,3 +4,6 @@ set(IPCP_BROADCAST_TARGET ipcpd-broadcast)
 
 set(IPCP_BROADCAST_MPL 100 CACHE STRING
   "Default maximum packet lifetime for the Broadcast IPCP, in ms")
+
+set(IPCP_BROADCAST_MTU 1400 CACHE STRING
+  "Layer MTU advertised by the Broadcast IPCP, in bytes")
diff --git a/cmake/config/ipcp/local.cmake b/cmake/config/ipcp/local.cmake
index 88ee8998..60ff23ee 100644
--- a/cmake/config/ipcp/local.cmake
+++ b/cmake/config/ipcp/local.cmake
@@ -5,5 +5,35 @@ set(IPCP_LOCAL_TARGET ipcpd-local)
 set(IPCP_LOCAL_MPL 100 CACHE STRING
   "Default maximum packet lifetime for the Local IPCP, in ms")
 
+set(IPCP_LOCAL_MTU 65000 CACHE STRING
+  "Layer MTU advertised by the Local IPCP, in bytes")
+
 set(IPCP_LOCAL_POLLING FALSE CACHE BOOL
   "Enable active polling in the Local IPCP for low-latency mode")
+
+# IPCP_LOCAL_MTU must fit in the largest enabled GSPP and PUP class
+# (sender-side allocation: daemons use GSPP, apps use PUP). Reserve a
+# margin for sizeof(struct ssm_pk_buff) + HEADSPACE + TAILSPACE.
+math(EXPR _ssm_pk_overhead
+  "${SSM_PK_BUFF_HEADSPACE} + ${SSM_PK_BUFF_TAILSPACE} + 64")
+
+foreach(_pool GSPP PUP)
+    set(_largest 0)
+    foreach(_pair "256;256" "512;512" "1K;1024" "2K;2048" "4K;4096"
+                  "16K;16384" "64K;65536" "256K;262144" "1M;1048576")
+        list(GET _pair 0 _name)
+        list(GET _pair 1 _bytes)
+        if(SSM_${_pool}_${_name}_BLOCKS GREATER 0
+           AND _bytes GREATER _largest)
+            set(_largest ${_bytes})
+        endif()
+    endforeach()
+    math(EXPR _avail "${_largest} - ${_ssm_pk_overhead}")
+    if(IPCP_LOCAL_MTU GREATER _avail)
+        message(FATAL_ERROR
+            "IPCP_LOCAL_MTU (${IPCP_LOCAL_MTU}) exceeds largest enabled "
+            "SSM_${_pool} class minus per-block overhead "
+            "(${_largest} - ${_ssm_pk_overhead} = ${_avail} bytes). "
+            "Lower IPCP_LOCAL_MTU or enable a larger SSM_${_pool}_*_BLOCKS.")
+    endif()
+endforeach()
diff --git a/cmake/config/ipcp/udp.cmake b/cmake/config/ipcp/udp.cmake
index 0124c261..af84a844 100644
--- a/cmake/config/ipcp/udp.cmake
+++ b/cmake/config/ipcp/udp.cmake
@@ -10,3 +10,7 @@ set(IPCP_UDP_WR_THR 3 CACHE STRING
   "Number of writer threads in UDP IPCPs")
 set(IPCP_UDP_MPL 5000 CACHE STRING
   "Default maximum packet lifetime for the UDP IPCPs, in ms")
+set(IPCP_UDP4_MTU 1472 CACHE STRING
+  "Fallback UDP4 layer MTU when getsockopt(IP_MTU) is unavailable, in bytes")
+set(IPCP_UDP6_MTU 1452 CACHE STRING
+  "Fallback UDP6 layer MTU when getsockopt(IPV6_MTU) is unavailable, in bytes")
diff --git a/cmake/config/ipcp/unicast.cmake b/cmake/config/ipcp/unicast.cmake
index 3b5b0ce7..b8d4d516 100644
--- a/cmake/config/ipcp/unicast.cmake
+++ b/cmake/config/ipcp/unicast.cmake
@@ -4,6 +4,8 @@ set(IPCP_UNICAST_TARGET ipcpd-unicast)
 
 set(IPCP_UNICAST_MPL 100 CACHE STRING
   "Default maximum packet lifetime for the Unicast IPCP, in ms")
+set(IPCP_UNICAST_MTU 1400 CACHE STRING
+  "Layer MTU advertised by the Unicast IPCP, in bytes (TODO: derive per-flow from n-1 path MTU minus DT PCI)")
 set(PFT_SIZE 256 CACHE STRING
   "Prefix forwarding table size for the Unicast IPCP")