diff --git a/downstream.go b/downstream.go
index a713bef..5e57cb3 100644
--- a/downstream.go
+++ b/downstream.go
@@ -81,6 +81,33 @@ var needAllDownstreamCaps = map[string]string{
 	"multi-prefix":  "",
 }
 
+// passthroughIsupport is the set of ISUPPORT tokens that are directly passed
+// through from the upstream server to downstream clients.
+//
+// This is only effective in single-upstream mode.
+var passthroughIsupport = map[string]bool{
+	"AWAYLEN":    true,
+	"CHANLIMIT":  true,
+	"CHANMODES":  true,
+	"CHANNELLEN": true,
+	"CHANTYPES":  true,
+	"EXCEPTS":    true,
+	"EXTBAN":     true,
+	"HOSTLEN":    true,
+	"INVEX":      true,
+	"KICKLEN":    true,
+	"MAXLIST":    true,
+	"MAXTARGETS": true,
+	"MODES":      true,
+	"NETWORK":    true,
+	"NICKLEN":    true,
+	"PREFIX":     true,
+	"SAFELIST":   true,
+	"TARGMAX":    true,
+	"TOPICLEN":   true,
+	"USERLEN":    true,
+}
+
 type downstreamConn struct {
 	conn
 
@@ -880,8 +907,18 @@ func (dc *downstreamConn) welcome() error {
 		fmt.Sprintf("CHATHISTORY=%v", dc.srv.HistoryLimit),
 	}
 
-	if uc := dc.upstream(); uc != nil && uc.isupport["NETWORK"] != nil {
-		isupport = append(isupport, fmt.Sprintf("NETWORK=%v", *uc.isupport["NETWORK"]))
+	if uc := dc.upstream(); uc != nil {
+		for k := range passthroughIsupport {
+			v, ok := uc.isupport[k]
+			if !ok {
+				continue
+			}
+			if v != nil {
+				isupport = append(isupport, fmt.Sprintf("%v=%v", k, *v))
+			} else {
+				isupport = append(isupport, k)
+			}
+		}
 	}
 
 	dc.SendMessage(&irc.Message{
@@ -904,12 +941,9 @@ func (dc *downstreamConn) welcome() error {
 		Command: irc.RPL_MYINFO,
 		Params:  []string{dc.nick, dc.srv.Hostname, "soju", "aiwroO", "OovaimnqpsrtklbeI"},
 	})
-	// TODO: other RPL_ISUPPORT tokens
-	dc.SendMessage(&irc.Message{
-		Prefix:  dc.srv.prefix(),
-		Command: irc.RPL_ISUPPORT,
-		Params:  append(append([]string{dc.nick}, isupport...), "are supported"),
-	})
+	for _, msg := range generateIsupport(dc.srv.prefix(), dc.nick, isupport) {
+		dc.SendMessage(msg)
+	}
 	dc.SendMessage(&irc.Message{
 		Prefix:  dc.srv.prefix(),
 		Command: irc.ERR_NOMOTD,
diff --git a/irc.go b/irc.go
index 9a09da4..94fdc7f 100644
--- a/irc.go
+++ b/irc.go
@@ -17,7 +17,10 @@ const (
 	err_invalidcapcmd = "410"
 )
 
-const maxMessageLength = 512
+const (
+	maxMessageLength = 512
+	maxMessageParams = 15
+)
 
 // The server-time layout, as defined in the IRCv3 spec.
 const serverTimeLayout = "2006-01-02T15:04:05.000Z"
@@ -348,6 +351,30 @@ func join(channels, keys []string) []*irc.Message {
 	return msgs
 }
 
+func generateIsupport(prefix *irc.Prefix, nick string, tokens []string) []*irc.Message {
+	maxTokens := maxMessageParams - 2 // 2 reserved params: nick + text
+
+	var msgs []*irc.Message
+	for len(tokens) > 0 {
+		var msgTokens []string
+		if len(tokens) > maxTokens {
+			msgTokens = tokens[:maxTokens]
+			tokens = tokens[maxTokens:]
+		} else {
+			msgTokens = tokens
+			tokens = nil
+		}
+
+		msgs = append(msgs, &irc.Message{
+			Prefix:  prefix,
+			Command: irc.RPL_ISUPPORT,
+			Params:  append(append([]string{nick}, msgTokens...), "are supported"),
+		})
+	}
+
+	return msgs
+}
+
 type joinSorter struct {
 	channels []string
 	keys     []string
diff --git a/upstream.go b/upstream.go
index c0c86b8..91071de 100644
--- a/upstream.go
+++ b/upstream.go
@@ -613,6 +613,8 @@ func (uc *upstreamConn) handleMessage(msg *irc.Message) error {
 		if err := parseMessageParams(msg, nil, nil); err != nil {
 			return err
 		}
+
+		var downstreamIsupport []string
 		for _, token := range msg.Params[1 : len(msg.Params)-1] {
 			parameter := token
 			var negate, hasValue bool
@@ -658,7 +660,21 @@ func (uc *upstreamConn) handleMessage(msg *irc.Message) error {
 			if err != nil {
 				return err
 			}
+
+			if passthroughIsupport[parameter] {
+				downstreamIsupport = append(downstreamIsupport, token)
+			}
 		}
+
+		uc.forEachDownstream(func(dc *downstreamConn) {
+			if dc.network == nil {
+				return
+			}
+			msgs := generateIsupport(dc.srv.prefix(), dc.nick, downstreamIsupport)
+			for _, msg := range msgs {
+				dc.SendMessage(msg)
+			}
+		})
 	case "BATCH":
 		var tag string
 		if err := parseMessageParams(msg, &tag); err != nil {