irmd: Pass MTU from IPCP to process for FRCT

FRCT needs to know the MTU for fragmentation. The MTU is now passed
from the layer serving the flow to the process as part of flow
allocation.

Signed-off-by: Dimitri Staessens <dimitri@ouroboros.rocks>
Signed-off-by: Sander Vrijders <sander@ouroboros.rocks>

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")