feat: add helper methods to compute UDP checksum

crates/scion/src/udp/datagram.rs

@@ -1,22 +1,59 @@
 #![allow(unused)]
 use bytes::{Buf, Bytes};
-use scion_proto::{packet::ByEndpoint, wire_encoding::WireDecode};
+use scion_proto::{
+    packet::{AddressHeader, ByEndpoint, ChecksumDigest},
+    wire_encoding::WireDecode,
+};
 
+#[derive(Debug, thiserror::Error)]
+pub(super) enum UdpDecodeError {
+    #[error("datagram is empty or was truncated")]
+    DatagramEmptyOrTruncated,
+}
+
+/// Scion UDP datagram
+///
+/// The SCION UDP datagram format includes a checksum that is calculated based on
+/// the [RFC].
+///
+/// [RFC]: https://www.ietf.org/archive/id/draft-dekater-scion-dataplane-00.html
+#[derive(Debug, Default)]
 pub(super) struct UdpDatagram {
+    /// The source and destination ports
     pub port: ByEndpoint<u16>,
+    /// The length of the header and payload
     pub length: u16,
+    /// SCION checksum, computed with a pseudo-header
     pub checksum: u16,
+    /// The UDP payload
     pub payload: Bytes,
 }
 
 impl UdpDatagram {
-    const HEADER_LEN: usize = 16;
-}
+    const PROTOCOL_NUMBER: u8 = 17;
+    const HEADER_LEN: usize = 8;
 
-#[derive(Debug, thiserror::Error)]
-pub(super) enum UdpDecodeError {
-    #[error("datagram is empty or was truncated")]
-    DatagramEmptyOrTruncated,
+    /// Compute the checksum for this datagram using the provided address header.
+    pub fn calculate_checksum(&self, address_header: &AddressHeader) -> u16 {
+        ChecksumDigest::with_pseudoheader(address_header, Self::PROTOCOL_NUMBER, self.length.into())
+            .add_u16(self.port.source)
+            .add_u16(self.port.destination)
+            .add_u16(self.length)
+            .add_u16(self.checksum)
+            .add_slice(&self.payload)
+            .checksum()
+    }
+
+    /// Returns true if the checksum successfully verifies, otherwise false.
+    pub fn verify_checksum(&self, address_header: &AddressHeader) -> bool {
+        self.calculate_checksum(address_header) == 0
+    }
+
+    /// Clears then sets the checksum to the value returned by [`Self::calculate_checksum()`].
+    pub fn set_checksum(&mut self, address_header: &AddressHeader) {
+        self.checksum = 0;
+        self.checksum = self.calculate_checksum(address_header);
+    }
 }
 
 impl<T> WireDecode<T> for UdpDatagram
@@ -34,8 +71,6 @@ where
         let destination = data.get_u16();
         let length = data.get_u16();
         let checksum = data.get_u16();
-        // TODO(jsmith): Check for additional verifications to be done on these fields
-        // TODO(jsmith): Determine whether we should check the checksum
 
         let payload_length = usize::from(length) - Self::HEADER_LEN;
         if payload_length <= data.remaining() {