Improve handling of maximum PDU lengths in scpproxy and lower the negotiable size

[Enet4]

Summary

• [ul] Reduce MINIMUM_PDU_SIZE to 1018 and changes its documentation. Not sure if anyone will ever need to request a PDU length smaller than this. Resolves No such thing as a "minimum PDU size, specified by the standard" #703
  • All other tools have been updated accordingly
• [scpproxy] Implement maximum PDU length clamping so that the two endings of the association are restrained by the maximum PDU length defined by the proxy. Resolves scpproxy breaks communication when its maximum PDU length is lower than the ones from the association established #694.

[pgimeno4d]

The test is failing because it is splitting 9000 bytes in several sends, and expects them to be split in three fragments (4K + 4K + about 1K). Now that the 4K have been reduced to 1K, it's no longer three fragments. So I propose to change 9000 to 2500 which is 1K + 1K + about 0.5K:

diff --git a/ul/src/association/pdata.rs b/ul/src/association/pdata.rs
index 9a770dce..cde40326 100644
--- a/ul/src/association/pdata.rs
+++ b/ul/src/association/pdata.rs
@@ -776,7 +776,7 @@ mod tests {
     fn test_write_large_pdata_and_finish() {
         let presentation_context_id = 32;
 
-        let my_data: Vec<_> = (0..9000).map(|x: u32| x as u8).collect();
+        let my_data: Vec<_> = (0..2500).map(|x: u32| x as u8).collect();
 
         let mut buf = Vec::new();
         {
@@ -822,7 +822,7 @@ mod tests {
                 );
                 assert_eq!(
                     data_3.data.len(),
-                    9000 - (MINIMUM_PDU_SIZE - PDV_HEADER_SIZE) as usize * 2
+                    2500 - (MINIMUM_PDU_SIZE - PDV_HEADER_SIZE) as usize * 2
                 );
 
                 // check data consistency
@@ -834,7 +834,7 @@ mod tests {
                 );
                 assert_eq!(
                     data_1.data.len() + data_2.data.len() + data_3.data.len(),
-                    9000
+                    2500
                 );
 
                 let data_1 = &data_1.data;
@@ -858,7 +858,7 @@ mod tests {
     async fn test_async_write_large_pdata_and_finish() {
         let presentation_context_id = 32;
 
-        let my_data: Vec<_> = (0..9000).map(|x: u32| x as u8).collect();
+        let my_data: Vec<_> = (0..2500).map(|x: u32| x as u8).collect();
 
         let mut buf = Vec::new();
         {
@@ -905,7 +905,7 @@ mod tests {
                 );
                 assert_eq!(
                     data_3.data.len(),
-                    9000 - (MINIMUM_PDU_SIZE - PDV_HEADER_SIZE) as usize * 2
+                    2500 - (MINIMUM_PDU_SIZE - PDV_HEADER_SIZE) as usize * 2
                 );
 
                 // check data consistency
@@ -917,7 +917,7 @@ mod tests {
                 );
                 assert_eq!(
                     data_1.data.len() + data_2.data.len() + data_3.data.len(),
-                    9000
+                    2500
                 );
 
                 let data_1 = &data_1.data;

(edit: full commit here: pgimeno4d@b717aee )
Or alternatively, making it calculated, but not sure it's worth it.

[Enet4]

Thanks for the input @pgimeno4d! I went with that suggestion in order to fix the tests.