isis: implement hello PDU padding

Implement padding for Hello PDUs in accordance with the base IS-IS
specification. Hello PDUs are padded using the maximum of the link MTU
or the LSP MTU. This ensures that the maximum-sized LSP can traverse the
link, and also serves as an implicit MTU check between neighbors.

Conditional padding as implemented by other vendors isn't supported
yet.

Signed-off-by: Renato Westphal <[email protected]>

holo-isis/src/interface.rs

@@ -493,7 +493,8 @@ impl Interface {
             );
         }
 
-        Pdu::Hello(Hello::new(
+        // Generate Hello PDU.
+        let mut hello = Hello::new(
             level,
             circuit_type,
             source,
@@ -506,7 +507,13 @@ impl Interface {
                 ipv4_addrs,
                 ipv6_addrs,
             ),
-        ))
+        );
+        if self.config.hello_padding {
+            let max_size =
+                std::cmp::max(self.iso_mtu() as u16, instance.config.lsp_mtu);
+            hello.add_padding(max_size);
+        }
+        Pdu::Hello(hello)
     }
 
     pub(crate) fn hello_interval_start(

holo-isis/src/northbound/configuration.rs

@@ -562,6 +562,10 @@ fn load_callbacks() -> Callbacks<Instance> {
 
             let hello_padding = args.dnode.get_bool();
             iface.config.hello_padding = hello_padding;
+
+            let event_queue = args.event_queue;
+            event_queue.insert(Event::InterfaceUpdateHelloInterval(iface_idx, LevelNumber::L1));
+            event_queue.insert(Event::InterfaceUpdateHelloInterval(iface_idx, LevelNumber::L2));
         })
         .path(isis::interfaces::interface::interface_type::PATH)
         .modify_apply(|instance, args| {

holo-isis/src/packet/pdu.rs

@@ -27,7 +27,7 @@ use crate::packet::tlv::{
     Ipv4Reach, Ipv4ReachTlv, Ipv6AddressesTlv, Ipv6Reach, Ipv6ReachTlv,
     IsReach, IsReachTlv, LspBufferSizeTlv, LspEntriesTlv, LspEntry,
     NeighborsTlv, PaddingTlv, ProtocolsSupportedTlv, Tlv, UnknownTlv,
-    TLV_HDR_SIZE,
+    TLV_HDR_SIZE, TLV_MAX_LEN,
 };
 use crate::packet::{AreaAddr, LanId, LevelNumber, LevelType, LspId, SystemId};
 
@@ -480,6 +480,41 @@ impl Hello {
             pdu_encode_end(buf, len_pos)
         })
     }
+
+    pub(crate) fn add_padding(&mut self, max_size: u16) {
+        // Compute the total length of existing TLVs.
+        let mut total_tlv_len = 0;
+        if let Some(tlv) = &self.tlvs.protocols_supported {
+            total_tlv_len += tlv.len();
+        }
+        for tlv in &self.tlvs.area_addrs {
+            total_tlv_len += tlv.len();
+        }
+        for tlv in &self.tlvs.neighbors {
+            total_tlv_len += tlv.len();
+        }
+        for tlv in &self.tlvs.ipv4_addrs {
+            total_tlv_len += tlv.len();
+        }
+        for tlv in &self.tlvs.ipv6_addrs {
+            total_tlv_len += tlv.len();
+        }
+
+        // Calculate the total padding required.
+        let mut rem_padding = max_size as usize
+            - Header::fixed_header_length(self.hdr.pdu_type) as usize
+            - total_tlv_len;
+
+        // Add as many Padding TLVs as necessary.
+        while rem_padding >= 2 {
+            let padding_len =
+                std::cmp::min(rem_padding - TLV_HDR_SIZE, TLV_MAX_LEN);
+            self.tlvs.padding.push(PaddingTlv {
+                length: padding_len as u8,
+            });
+            rem_padding -= TLV_HDR_SIZE + padding_len;
+        }
+    }
 }
 
 impl HelloTlvs {

holo-isis/src/southbound/rx.rs

@@ -40,6 +40,7 @@ pub(crate) fn process_iface_update(
     let iface_idx = iface.index;
 
     // Update interface data.
+    let old_mtu = iface.system.mtu;
     iface.system.flags = msg.flags;
     iface.system.mtu = Some(msg.mtu);
     iface.system.mac_addr = Some(msg.mac_address);
@@ -48,9 +49,18 @@ pub(crate) fn process_iface_update(
             .interfaces
             .update_ifindex(iface_idx, Some(msg.ifindex));
     }
+    let iface = &mut arenas.interfaces[iface_idx];
+
+    // Update the padding used in Hello PDUs if the MTU has changed.
+    if iface.config.hello_padding
+        && iface.system.mtu != old_mtu
+        && iface.state.active
+        && !iface.is_passive()
+    {
+        iface.hello_interval_start(&instance, LevelType::All);
+    }
 
     // Check if IS-IS needs to be activated or deactivated on this interface.
-    let iface = &mut arenas.interfaces[iface_idx];
     iface.update(&mut instance, &mut arenas.adjacencies)?;
 
     Ok(())