Add CID based datagram routing

Adds functions to route datagrams and identify connections by DTLS 1.2
Connection IDs.

Signed-off-by: Daniel Mangum <[email protected]>

Author: hasheddan

connection_id.go

@@ -3,7 +3,14 @@
 
 package dtls
 
-import "crypto/rand"
+import (
+	"crypto/rand"
+
+	"github.com/pion/dtls/v2/pkg/protocol"
+	"github.com/pion/dtls/v2/pkg/protocol/extension"
+	"github.com/pion/dtls/v2/pkg/protocol/handshake"
+	"github.com/pion/dtls/v2/pkg/protocol/recordlayer"
+)
 
 // RandomCIDGenerator is a random Connection ID generator where CID is the
 // specified size. Specifying a size of 0 will indicate to peers that sending a
@@ -26,3 +33,69 @@ func OnlySendCIDGenerator() func() []byte {
 		return nil
 	}
 }
+
+// cidDatagramRouter extracts connection IDs from incoming datagram payloads and
+// uses them to route to the proper connection.
+// NOTE: properly routing datagrams based on connection IDs requires using
+// constant size connection IDs.
+func cidDatagramRouter(size int) func([]byte) (string, bool) {
+	return func(packet []byte) (string, bool) {
+		pkts, err := recordlayer.ContentAwareUnpackDatagram(packet, size)
+		if err != nil || len(pkts) < 1 {
+			return "", false
+		}
+		for _, pkt := range pkts {
+			h := &recordlayer.Header{
+				ConnectionID: make([]byte, size),
+			}
+			if err := h.Unmarshal(pkt); err != nil {
+				continue
+			}
+			if h.ContentType != protocol.ContentTypeConnectionID {
+				continue
+			}
+			return string(h.ConnectionID), true
+		}
+		return "", false
+	}
+}
+
+// cidConnIdentifier extracts connection IDs from outgoing ServerHello records
+// and associates them with the associated connection.
+// NOTE: a ServerHello should always be the first record in a datagram if
+// multiple are present, so we avoid iterating through all packets if the first
+// is not a ServerHello.
+func cidConnIdentifier() func([]byte) (string, bool) {
+	return func(packet []byte) (string, bool) {
+		pkts, err := recordlayer.UnpackDatagram(packet)
+		if err != nil || len(pkts) < 1 {
+			return "", false
+		}
+		var h recordlayer.Header
+		if hErr := h.Unmarshal(pkts[0]); hErr != nil {
+			return "", false
+		}
+		if h.ContentType != protocol.ContentTypeHandshake {
+			return "", false
+		}
+		var hh handshake.Header
+		var sh handshake.MessageServerHello
+		for _, pkt := range pkts {
+			if hhErr := hh.Unmarshal(pkt[recordlayer.FixedHeaderSize:]); hhErr != nil {
+				continue
+			}
+			if err = sh.Unmarshal(pkt[recordlayer.FixedHeaderSize+handshake.HeaderLength:]); err == nil {
+				break
+			}
+		}
+		if err != nil {
+			return "", false
+		}
+		for _, ext := range sh.Extensions {
+			if e, ok := ext.(*extension.ConnectionID); ok {
+				return string(e.CID), true
+			}
+		}
+		return "", false
+	}
+}

go.mod

@@ -3,6 +3,7 @@ module github.com/pion/dtls/v2
 require (
 	github.com/pion/logging v0.2.2
 	github.com/pion/transport/v2 v2.2.2-0.20230802201558-f2dffd80896b
+	github.com/stretchr/testify v1.8.4
 	golang.org/x/crypto v0.12.0
 	golang.org/x/net v0.13.0
 )