diff --git a/gencapdefs.py b/gencapdefs.py
index 985f2ac3..26011fa1 100644
--- a/gencapdefs.py
+++ b/gencapdefs.py
@@ -105,6 +105,12 @@ CAPDEFS = [
         url="https://ircv3.net/specs/extensions/channel-rename",
         standard="draft IRCv3",
     ),
+    CapDef(
+        identifier="Resume",
+        name="draft/resume-0.5",
+        url="https://github.com/DanielOaks/ircv3-specifications/blob/master+resume/extensions/resume.md",
+        standard="proposed IRCv3",
+    ),
     CapDef(
         identifier="SASL",
         name="sasl",
diff --git a/irc/caps/defs.go b/irc/caps/defs.go
index 4bfd3e29..bc165ce7 100644
--- a/irc/caps/defs.go
+++ b/irc/caps/defs.go
@@ -7,7 +7,7 @@ package caps
 
 const (
 	// number of recognized capabilities:
-	numCapabs = 27
+	numCapabs = 28
 	// length of the uint64 array that represents the bitset:
 	bitsetLen = 1
 )
@@ -65,6 +65,10 @@ const (
 	// https://github.com/ircv3/ircv3-specifications/pull/417
 	Relaymsg Capability = iota
 
+	// Resume is the proposed IRCv3 capability named "draft/resume-0.5":
+	// https://github.com/DanielOaks/ircv3-specifications/blob/master+resume/extensions/resume.md
+	Resume Capability = iota
+
 	// EchoMessage is the IRCv3 capability named "echo-message":
 	// https://ircv3.net/specs/extensions/echo-message-3.2.html
 	EchoMessage Capability = iota
@@ -138,6 +142,7 @@ var (
 		"draft/multiline",
 		"draft/register",
 		"draft/relaymsg",
+		"draft/resume-0.5",
 		"echo-message",
 		"extended-join",
 		"invite-notify",
diff --git a/irc/channel.go b/irc/channel.go
index 1fdd4332..14a54f60 100644
--- a/irc/channel.go
+++ b/irc/channel.go
@@ -1035,6 +1035,80 @@ func (channel *Channel) Part(client *Client, message string, rb *ResponseBuffer)
 	client.server.logger.Debug("channels", fmt.Sprintf("%s left channel %s", details.nick, chname))
 }
 
+// Resume is called after a successful global resume to:
+// 1. Replace the old client with the new in the channel's data structures
+// 2. Send JOIN and MODE lines to channel participants (including the new client)
+// 3. Replay missed message history to the client
+func (channel *Channel) Resume(session *Session, timestamp time.Time) {
+	channel.resumeAndAnnounce(session)
+	if !timestamp.IsZero() {
+		channel.replayHistoryForResume(session, timestamp, time.Time{})
+	}
+}
+
+func (channel *Channel) resumeAndAnnounce(session *Session) {
+	channel.stateMutex.RLock()
+	memberData, found := channel.members[session.client]
+	channel.stateMutex.RUnlock()
+	if !found {
+		return
+	}
+	oldModes := memberData.modes.String()
+	if 0 < len(oldModes) {
+		oldModes = "+" + oldModes
+	}
+
+	// send join for old clients
+	chname := channel.Name()
+	details := session.client.Details()
+	// TODO: for now, skip this entirely for auditoriums,
+	// but really we should send it to voiced clients
+	if !channel.flags.HasMode(modes.Auditorium) {
+		for _, member := range channel.Members() {
+			for _, mSes := range member.Sessions() {
+				if mSes == session || mSes.capabilities.Has(caps.Resume) {
+					continue
+				}
+
+				if mSes.capabilities.Has(caps.ExtendedJoin) {
+					mSes.Send(nil, details.nickMask, "JOIN", chname, details.accountName, details.realname)
+				} else {
+					mSes.Send(nil, details.nickMask, "JOIN", chname)
+				}
+
+				if 0 < len(oldModes) {
+					mSes.Send(nil, channel.server.name, "MODE", chname, oldModes, details.nick)
+				}
+			}
+		}
+	}
+
+	rb := NewResponseBuffer(session)
+	// use blocking i/o to synchronize with the later history replay
+	if rb.session.capabilities.Has(caps.ExtendedJoin) {
+		rb.Add(nil, details.nickMask, "JOIN", channel.name, details.accountName, details.realname)
+	} else {
+		rb.Add(nil, details.nickMask, "JOIN", channel.name)
+	}
+	channel.SendTopic(session.client, rb, false)
+	channel.Names(session.client, rb)
+	rb.Send(true)
+}
+
+func (channel *Channel) replayHistoryForResume(session *Session, after time.Time, before time.Time) {
+	var items []history.Item
+	afterS, beforeS := history.Selector{Time: after}, history.Selector{Time: before}
+	_, seq, _ := channel.server.GetHistorySequence(channel, session.client, "")
+	if seq != nil {
+		items, _ = seq.Between(afterS, beforeS, channel.server.Config().History.ZNCMax)
+	}
+	rb := NewResponseBuffer(session)
+	if len(items) != 0 {
+		channel.replayHistoryItems(rb, items, false)
+	}
+	rb.Send(true)
+}
+
 func (channel *Channel) replayHistoryItems(rb *ResponseBuffer, items []history.Item, autoreplay bool) {
 	// send an empty batch if necessary, as per the CHATHISTORY spec
 	chname := channel.Name()
diff --git a/irc/client.go b/irc/client.go
index cff4669a..80f7a723 100644
--- a/irc/client.go
+++ b/irc/client.go
@@ -56,6 +56,8 @@ const (
 	// This is how long a client gets without sending any message, including the PONG to our
 	// PING, before we disconnect them:
 	DefaultTotalTimeout = 2*time.Minute + 30*time.Second
+	// Resumeable clients (clients who have negotiated caps.Resume) get longer:
+	ResumeableTotalTimeout = 3*time.Minute + 30*time.Second
 
 	// round off the ping interval by this much, see below:
 	PingCoalesceThreshold = time.Second
@@ -65,6 +67,15 @@ var (
 	MaxLineLen = DefaultMaxLineLen
 )
 
+// ResumeDetails is a place to stash data at various stages of
+// the resume process: when handling the RESUME command itself,
+// when completing the registration, and when rejoining channels.
+type ResumeDetails struct {
+	PresentedToken    string
+	Timestamp         time.Time
+	HistoryIncomplete bool
+}
+
 // Client is an IRC client.
 type Client struct {
 	account            string
@@ -72,6 +83,7 @@ type Client struct {
 	accountRegDate     time.Time
 	accountSettings    AccountSettings
 	awayMessage        string
+	brbTimer           BrbTimer
 	channels           ChannelSet
 	ctime              time.Time
 	destroyed          bool
@@ -101,6 +113,7 @@ type Client struct {
 	registered         bool
 	registerCmdSent    bool // already sent the draft/register command, can't send it again
 	registrationTimer  *time.Timer
+	resumeID           string
 	server             *Server
 	skeleton           string
 	sessions           []*Session
@@ -155,6 +168,7 @@ type Session struct {
 
 	fakelag              Fakelag
 	deferredFakelagCount int
+	destroyed            uint32
 
 	certfp     string
 	peerCerts  []*x509.Certificate
@@ -174,6 +188,8 @@ type Session struct {
 
 	registrationMessages int
 
+	resumeID              string
+	resumeDetails         *ResumeDetails
 	zncPlaybackTimes      *zncPlaybackTimes
 	autoreplayMissedSince time.Time
 
@@ -247,6 +263,20 @@ func (s *Session) IP() net.IP {
 	return s.realIP
 }
 
+// returns whether the session was actively destroyed (for example, by ping
+// timeout or NS GHOST).
+// avoids a race condition between asynchronous idle-timing-out of sessions,
+// and a condition that allows implicit BRB on connection errors (since
+// destroy()'s socket.Close() appears to socket.Read() as a connection error)
+func (session *Session) Destroyed() bool {
+	return atomic.LoadUint32(&session.destroyed) == 1
+}
+
+// sets the timed-out flag
+func (session *Session) SetDestroyed() {
+	atomic.StoreUint32(&session.destroyed, 1)
+}
+
 // returns whether the client supports a smart history replay cap,
 // and therefore autoreplay-on-join and similar should be suppressed
 func (session *Session) HasHistoryCaps() bool {
@@ -345,6 +375,7 @@ func (server *Server) RunClient(conn IRCConn) {
 		client.requireSASLMessage = banMsg
 	}
 	client.history.Initialize(config.History.ClientLength, time.Duration(config.History.AutoresizeWindow))
+	client.brbTimer.Initialize(client)
 	session := &Session{
 		client:     client,
 		socket:     socket,
@@ -432,6 +463,7 @@ func (server *Server) AddAlwaysOnClient(account ClientAccount, channelToStatus m
 		client.SetMode(m, true)
 	}
 	client.history.Initialize(0, 0)
+	client.brbTimer.Initialize(client)
 
 	server.accounts.Login(client, account)
 
@@ -525,7 +557,7 @@ func (client *Client) lookupHostname(session *Session, overwrite bool) {
 	cloakedHostname := config.Server.Cloaks.ComputeCloak(ip)
 	client.stateMutex.Lock()
 	defer client.stateMutex.Unlock()
-	// update the hostname if this is a new connection, but not if it's a reattach
+	// update the hostname if this is a new connection or a resume, but not if it's a reattach
 	if overwrite || client.rawHostname == "" {
 		client.rawHostname = hostname
 		client.cloakedHostname = cloakedHostname
@@ -643,7 +675,14 @@ func (client *Client) run(session *Session) {
 	isReattach := client.Registered()
 	if isReattach {
 		client.Touch(session)
-		client.playReattachMessages(session)
+		if session.resumeDetails != nil {
+			session.playResume()
+			session.resumeDetails = nil
+			client.brbTimer.Disable()
+			session.SetAway("") // clear BRB message if any
+		} else {
+			client.playReattachMessages(session)
+		}
 	}
 
 	firstLine := !isReattach
@@ -662,6 +701,11 @@ func (client *Client) run(session *Session) {
 				quitMessage = "connection closed"
 			}
 			client.Quit(quitMessage, session)
+			// since the client did not actually send us a QUIT,
+			// give them a chance to resume if applicable:
+			if !session.Destroyed() {
+				client.brbTimer.Enable()
+			}
 			break
 		}
 
@@ -812,6 +856,9 @@ func (client *Client) updateIdleTimer(session *Session, now time.Time) {
 
 func (session *Session) handleIdleTimeout() {
 	totalTimeout := DefaultTotalTimeout
+	if session.capabilities.Has(caps.Resume) {
+		totalTimeout = ResumeableTotalTimeout
+	}
 	pingTimeout := DefaultIdleTimeout
 	if session.isTor {
 		pingTimeout = TorIdleTimeout
@@ -868,6 +915,151 @@ func (session *Session) Ping() {
 	session.Send(nil, "", "PING", session.client.Nick())
 }
 
+// tryResume tries to resume if the client asked us to.
+func (session *Session) tryResume() (success bool) {
+	var oldResumeID string
+
+	defer func() {
+		if success {
+			// "On a successful request, the server [...] terminates the old client's connection"
+			oldSession := session.client.GetSessionByResumeID(oldResumeID)
+			if oldSession != nil {
+				session.client.destroy(oldSession)
+			}
+		} else {
+			session.resumeDetails = nil
+		}
+	}()
+
+	client := session.client
+	server := client.server
+	config := server.Config()
+
+	oldClient, oldResumeID := server.resumeManager.VerifyToken(client, session.resumeDetails.PresentedToken)
+	if oldClient == nil {
+		session.Send(nil, server.name, "FAIL", "RESUME", "INVALID_TOKEN", client.t("Cannot resume connection, token is not valid"))
+		return
+	}
+
+	resumeAllowed := config.Server.AllowPlaintextResume || (oldClient.HasMode(modes.TLS) && client.HasMode(modes.TLS))
+	if !resumeAllowed {
+		session.Send(nil, server.name, "FAIL", "RESUME", "INSECURE_SESSION", client.t("Cannot resume connection, old and new clients must have TLS"))
+		return
+	}
+
+	err := server.clients.Resume(oldClient, session)
+	if err != nil {
+		session.Send(nil, server.name, "FAIL", "RESUME", "CANNOT_RESUME", client.t("Cannot resume connection"))
+		return
+	}
+
+	success = true
+	client.server.logger.Debug("quit", fmt.Sprintf("%s is being resumed", oldClient.Nick()))
+
+	return
+}
+
+// playResume is called from the session's fresh goroutine after a resume;
+// it sends notifications to friends, then plays the registration burst and replays
+// stored history to the session
+func (session *Session) playResume() {
+	client := session.client
+	server := client.server
+	config := server.Config()
+
+	friends := make(ClientSet)
+	var oldestLostMessage time.Time
+
+	// work out how much time, if any, is not covered by history buffers
+	// assume that a persistent buffer covers the whole resume period
+	for _, channel := range client.Channels() {
+		for _, member := range channel.auditoriumFriends(client) {
+			friends.Add(member)
+		}
+		status, _, _ := channel.historyStatus(config)
+		if status == HistoryEphemeral {
+			lastDiscarded := channel.history.LastDiscarded()
+			if oldestLostMessage.Before(lastDiscarded) {
+				oldestLostMessage = lastDiscarded
+			}
+		}
+	}
+	cHistoryStatus, _ := client.historyStatus(config)
+	if cHistoryStatus == HistoryEphemeral {
+		lastDiscarded := client.history.LastDiscarded()
+		if oldestLostMessage.Before(lastDiscarded) {
+			oldestLostMessage = lastDiscarded
+		}
+	}
+
+	timestamp := session.resumeDetails.Timestamp
+	gap := oldestLostMessage.Sub(timestamp)
+	session.resumeDetails.HistoryIncomplete = gap > 0 || timestamp.IsZero()
+	gapSeconds := int(gap.Seconds()) + 1 // round up to avoid confusion
+
+	details := client.Details()
+	oldNickmask := details.nickMask
+	client.lookupHostname(session, true)
+	hostname := client.Hostname() // may be a vhost
+	timestampString := timestamp.Format(IRCv3TimestampFormat)
+
+	// send quit/resume messages to friends
+	for friend := range friends {
+		if friend == client {
+			continue
+		}
+		for _, fSession := range friend.Sessions() {
+			if fSession.capabilities.Has(caps.Resume) {
+				if !session.resumeDetails.HistoryIncomplete {
+					fSession.Send(nil, oldNickmask, "RESUMED", hostname, "ok")
+				} else if session.resumeDetails.HistoryIncomplete && !timestamp.IsZero() {
+					fSession.Send(nil, oldNickmask, "RESUMED", hostname, timestampString)
+				} else {
+					fSession.Send(nil, oldNickmask, "RESUMED", hostname)
+				}
+			} else {
+				if !session.resumeDetails.HistoryIncomplete {
+					fSession.Send(nil, oldNickmask, "QUIT", friend.t("Client reconnected"))
+				} else if session.resumeDetails.HistoryIncomplete && !timestamp.IsZero() {
+					fSession.Send(nil, oldNickmask, "QUIT", fmt.Sprintf(friend.t("Client reconnected (up to %d seconds of message history lost)"), gapSeconds))
+				} else {
+					fSession.Send(nil, oldNickmask, "QUIT", friend.t("Client reconnected (message history may have been lost)"))
+				}
+			}
+		}
+	}
+
+	if session.resumeDetails.HistoryIncomplete {
+		if !timestamp.IsZero() {
+			session.Send(nil, client.server.name, "WARN", "RESUME", "HISTORY_LOST", fmt.Sprintf(client.t("Resume may have lost up to %d seconds of history"), gapSeconds))
+		} else {
+			session.Send(nil, client.server.name, "WARN", "RESUME", "HISTORY_LOST", client.t("Resume may have lost some message history"))
+		}
+	}
+
+	session.Send(nil, client.server.name, "RESUME", "SUCCESS", details.nick)
+
+	server.playRegistrationBurst(session)
+
+	for _, channel := range client.Channels() {
+		channel.Resume(session, timestamp)
+	}
+
+	// replay direct PRIVSMG history
+	_, privmsgSeq, err := server.GetHistorySequence(nil, client, "")
+	if !timestamp.IsZero() && err == nil && privmsgSeq != nil {
+		after := history.Selector{Time: timestamp}
+		items, _ := privmsgSeq.Between(after, history.Selector{}, config.History.ZNCMax)
+		if len(items) != 0 {
+			rb := NewResponseBuffer(session)
+			client.replayPrivmsgHistory(rb, items, "")
+			rb.Send(true)
+		}
+	}
+
+	session.resumeDetails = nil
+}
+
 func (client *Client) replayPrivmsgHistory(rb *ResponseBuffer, items []history.Item, target string) {
 	var batchID string
 	details := client.Details()
@@ -1200,6 +1392,8 @@ func (client *Client) destroy(session *Session) {
 	client.stateMutex.Lock()
 
 	details := client.detailsNoMutex()
+	brbState := client.brbTimer.state
+	brbAt := client.brbTimer.brbAt
 	wasReattach := session != nil && session.client != client
 	sessionRemoved := false
 	registered := client.registered
@@ -1241,7 +1435,9 @@ func (client *Client) destroy(session *Session) {
 	}
 
 	// should we destroy the whole client this time?
-	shouldDestroy := !client.destroyed && remainingSessions == 0 && !alwaysOn
+	// BRB is not respected if this is a destroy of the whole client (i.e., session == nil)
+	brbEligible := session != nil && brbState == BrbEnabled
+	shouldDestroy := !client.destroyed && remainingSessions == 0 && !brbEligible && !alwaysOn
 	// decrement stats on a true destroy, or for the removal of the last connected session
 	// of an always-on client
 	shouldDecrement := shouldDestroy || (alwaysOn && len(sessionsToDestroy) != 0 && len(client.sessions) == 0)
@@ -1287,6 +1483,7 @@ func (client *Client) destroy(session *Session) {
 		// send quit/error message to client if they haven't been sent already
 		client.Quit("", session)
 		quitMessage = session.quitMessage // doesn't need synch, we already detached
+		session.SetDestroyed()
 		session.socket.Close()
 
 		// clean up monitor state
@@ -1345,6 +1542,8 @@ func (client *Client) destroy(session *Session) {
 		client.server.whoWas.Append(client.WhoWas())
 	}
 
+	client.server.resumeManager.Delete(client)
+
 	// alert monitors
 	if registered {
 		client.server.monitorManager.AlertAbout(details.nick, details.nickCasefolded, false)
@@ -1366,8 +1565,20 @@ func (client *Client) destroy(session *Session) {
 	client.server.clients.Remove(client)
 
 	// clean up self
+	client.brbTimer.Disable()
+
 	client.server.accounts.Logout(client)
 
+	// this happens under failure to return from BRB
+	if quitMessage == "" {
+		if brbState == BrbDead && !brbAt.IsZero() {
+			awayMessage := client.AwayMessage()
+			if awayMessage == "" {
+				awayMessage = "Disconnected" // auto-BRB
+			}
+			quitMessage = fmt.Sprintf("%s [%s ago]", awayMessage, time.Since(brbAt).Truncate(time.Second).String())
+		}
+	}
 	if quitMessage == "" {
 		quitMessage = "Exited"
 	}
diff --git a/irc/client_lookup_set.go b/irc/client_lookup_set.go
index 8d96e021..883b367a 100644
--- a/irc/client_lookup_set.go
+++ b/irc/client_lookup_set.go
@@ -81,6 +81,26 @@ func (clients *ClientManager) Remove(client *Client) error {
 	return clients.removeInternal(client, oldcfnick, oldskeleton)
 }
 
+// Handles a RESUME by attaching a session to a designated client. It is the
+// caller's responsibility to verify that the resume is allowed (checking tokens,
+// TLS status, etc.) before calling this.
+func (clients *ClientManager) Resume(oldClient *Client, session *Session) (err error) {
+	clients.Lock()
+	defer clients.Unlock()
+
+	cfnick := oldClient.NickCasefolded()
+	if _, ok := clients.byNick[cfnick]; !ok {
+		return errNickMissing
+	}
+
+	success, _, _, _ := oldClient.AddSession(session)
+	if !success {
+		return errNickMissing
+	}
+
+	return nil
+}
+
 // SetNick sets a client's nickname, validating it against nicknames in use
 // XXX: dryRun validates a client's ability to claim a nick, without
 // actually claiming it
diff --git a/irc/commands.go b/irc/commands.go
index 59a46839..36cae878 100644
--- a/irc/commands.go
+++ b/irc/commands.go
@@ -93,6 +93,10 @@ func init() {
 			minParams:      1,
 			allowedInBatch: true,
 		},
+		"BRB": {
+			handler:   brbHandler,
+			minParams: 0,
+		},
 		"CAP": {
 			handler:      capHandler,
 			usablePreReg: true,
@@ -253,6 +257,11 @@ func init() {
 			handler:   renameHandler,
 			minParams: 2,
 		},
+		"RESUME": {
+			handler:      resumeHandler,
+			usablePreReg: true,
+			minParams:    1,
+		},
 		"SAJOIN": {
 			handler:   sajoinHandler,
 			minParams: 1,
diff --git a/irc/config.go b/irc/config.go
index e88ff285..29cd02bb 100644
--- a/irc/config.go
+++ b/irc/config.go
@@ -570,6 +570,7 @@ type Config struct {
 		WebIRC               []webircConfig `yaml:"webirc"`
 		MaxSendQString       string         `yaml:"max-sendq"`
 		MaxSendQBytes        int
+		AllowPlaintextResume bool `yaml:"allow-plaintext-resume"`
 		Compatibility        struct {
 			ForceTrailing      *bool `yaml:"force-trailing"`
 			forceTrailing      bool
diff --git a/irc/getters.go b/irc/getters.go
index 3000c1e4..ac3da567 100644
--- a/irc/getters.go
+++ b/irc/getters.go
@@ -54,6 +54,18 @@ func (client *Client) Sessions() (sessions []*Session) {
 	return
 }
 
+func (client *Client) GetSessionByResumeID(resumeID string) (result *Session) {
+	client.stateMutex.RLock()
+	defer client.stateMutex.RUnlock()
+
+	for _, session := range client.sessions {
+		if session.resumeID == resumeID {
+			return session
+		}
+	}
+	return
+}
+
 type SessionData struct {
 	ctime     time.Time
 	atime     time.Time
@@ -145,6 +157,12 @@ func (client *Client) removeSession(session *Session) (success bool, length int)
 	return
 }
 
+func (session *Session) SetResumeID(resumeID string) {
+	session.client.stateMutex.Lock()
+	session.resumeID = resumeID
+	session.client.stateMutex.Unlock()
+}
+
 func (client *Client) Nick() string {
 	client.stateMutex.RLock()
 	defer client.stateMutex.RUnlock()
@@ -247,6 +265,18 @@ func (client *Client) uniqueIdentifiers() (nickCasefolded string, skeleton strin
 	return client.nickCasefolded, client.skeleton
 }
 
+func (client *Client) ResumeID() string {
+	client.stateMutex.RLock()
+	defer client.stateMutex.RUnlock()
+	return client.resumeID
+}
+
+func (client *Client) SetResumeID(id string) {
+	client.stateMutex.Lock()
+	defer client.stateMutex.Unlock()
+	client.resumeID = id
+}
+
 func (client *Client) Oper() *Oper {
 	client.stateMutex.RLock()
 	defer client.stateMutex.RUnlock()
diff --git a/irc/handlers.go b/irc/handlers.go
index 7f27519b..3daedfe3 100644
--- a/irc/handlers.go
+++ b/irc/handlers.go
@@ -420,6 +420,31 @@ func batchHandler(server *Server, client *Client, msg ircmsg.Message, rb *Respon
 	return false
 }
 
+// BRB [message]
+func brbHandler(server *Server, client *Client, msg ircmsg.Message, rb *ResponseBuffer) bool {
+	success, duration := client.brbTimer.Enable()
+	if !success {
+		rb.Add(nil, server.name, "FAIL", "BRB", "CANNOT_BRB", client.t("Your client does not support BRB"))
+		return false
+	} else {
+		rb.Add(nil, server.name, "BRB", strconv.Itoa(int(duration.Seconds())))
+	}
+
+	var message string
+	if 0 < len(msg.Params) {
+		message = msg.Params[0]
+	} else {
+		message = client.t("I'll be right back")
+	}
+
+	if len(client.Sessions()) == 1 {
+		// true BRB
+		rb.session.SetAway(message)
+	}
+
+	return true
+}
+
 // CAP <subcmd> [<caps>]
 func capHandler(server *Server, client *Client, msg ircmsg.Message, rb *ResponseBuffer) bool {
 	details := client.Details()
@@ -515,6 +540,15 @@ func capHandler(server *Server, client *Client, msg ircmsg.Message, rb *Response
 		rb.session.capabilities.Subtract(toRemove)
 		rb.Add(nil, server.name, "CAP", details.nick, "ACK", capString)
 
+		// if this is the first time the client is requesting a resume token,
+		// send it to them
+		if toAdd.Has(caps.Resume) {
+			token, id := server.resumeManager.GenerateToken(client)
+			if token != "" {
+				rb.Add(nil, server.name, "RESUME", "TOKEN", token)
+				rb.session.SetResumeID(id)
+			}
+		}
 	case "END":
 		if !client.registered {
 			rb.session.capState = caps.NegotiatedState
@@ -2803,6 +2837,30 @@ func renameHandler(server *Server, client *Client, msg ircmsg.Message, rb *Respo
 	return false
 }
 
+// RESUME <token> [timestamp]
+func resumeHandler(server *Server, client *Client, msg ircmsg.Message, rb *ResponseBuffer) bool {
+	details := ResumeDetails{
+		PresentedToken: msg.Params[0],
+	}
+
+	if client.registered {
+		rb.Add(nil, server.name, "FAIL", "RESUME", "REGISTRATION_IS_COMPLETED", client.t("Cannot resume connection, connection registration has already been completed"))
+		return false
+	}
+
+	if 1 < len(msg.Params) {
+		ts, err := time.Parse(IRCv3TimestampFormat, msg.Params[1])
+		if err == nil {
+			details.Timestamp = ts
+		} else {
+			rb.Add(nil, server.name, "WARN", "RESUME", "HISTORY_LOST", client.t("Timestamp is not in 2006-01-02T15:04:05.999Z format, ignoring it"))
+		}
+	}
+
+	rb.session.resumeDetails = &details
+	return false
+}
+
 // SANICK <oldnick> <nickname>
 func sanickHandler(server *Server, client *Client, msg ircmsg.Message, rb *ResponseBuffer) bool {
 	targetNick := msg.Params[0]
diff --git a/irc/help.go b/irc/help.go
index 347f981d..c631ddb4 100644
--- a/irc/help.go
+++ b/irc/help.go
@@ -129,6 +129,14 @@ longer away.`,
 
 BATCH initiates an IRCv3 client-to-server batch. You should never need to
 issue this command manually.`,
+	},
+	"brb": {
+		text: `BRB [message]
+
+Disconnects you from the server, while instructing the server to keep you
+present for a short time window. During this window, you can either resume
+or reattach to your nickname. If [message] is sent, it is used as your away
+message (and as your quit message if you don't return in time).`,
 	},
 	"cap": {
 		text: `CAP <subcommand> [:<capabilities>]
@@ -487,6 +495,12 @@ Registers an account in accordance with the draft/register capability.`,
 		text: `REHASH
 
 Reloads the config file and updates TLS certificates on listeners`,
+	},
+	"resume": {
+		text: `RESUME <oldnick> [timestamp]
+
+Sent before registration has completed, this indicates that the client wants to
+resume their old connection <oldnick>.`,
 	},
 	"time": {
 		text: `TIME [server]
diff --git a/irc/idletimer.go b/irc/idletimer.go
new file mode 100644
index 00000000..a9f07f91
--- /dev/null
+++ b/irc/idletimer.go
@@ -0,0 +1,133 @@
+// Copyright (c) 2017 Shivaram Lingamneni <slingamn@cs.stanford.edu>
+// released under the MIT license
+
+package irc
+
+import (
+	"time"
+)
+
+// BrbTimer is a timer on the client as a whole (not an individual session) for implementing
+// the BRB command and related functionality (where a client can remain online without
+// having any connected sessions).
+
+type BrbState uint
+
+const (
+	// BrbDisabled is the default state; the client will be disconnected if it has no sessions
+	BrbDisabled BrbState = iota
+	// BrbEnabled allows the client to remain online without sessions; if a timeout is
+	// reached, it will be removed
+	BrbEnabled
+	// BrbDead is the state of a client after its timeout has expired; it will be removed
+	// and therefore new sessions cannot be attached to it
+	BrbDead
+)
+
+type BrbTimer struct {
+	// XXX we use client.stateMutex for synchronization, so we can atomically test
+	// conditions that use both brbTimer.state and client.sessions. This code
+	// is tightly coupled with the rest of Client.
+	client *Client
+
+	state    BrbState
+	brbAt    time.Time
+	duration time.Duration
+	timer    *time.Timer
+}
+
+func (bt *BrbTimer) Initialize(client *Client) {
+	bt.client = client
+}
+
+// attempts to enable BRB for a client, returns whether it succeeded
+func (bt *BrbTimer) Enable() (success bool, duration time.Duration) {
+	// TODO make this configurable
+	duration = ResumeableTotalTimeout
+
+	bt.client.stateMutex.Lock()
+	defer bt.client.stateMutex.Unlock()
+
+	if !bt.client.registered || bt.client.alwaysOn || bt.client.resumeID == "" {
+		return
+	}
+
+	switch bt.state {
+	case BrbDisabled, BrbEnabled:
+		bt.state = BrbEnabled
+		bt.duration = duration
+		bt.resetTimeout()
+		// only track the earliest BRB, if multiple sessions are BRB'ing at once
+		// TODO(#524) this is inaccurate in case of an auto-BRB
+		if bt.brbAt.IsZero() {
+			bt.brbAt = time.Now().UTC()
+		}
+		success = true
+	default:
+		// BrbDead
+		success = false
+	}
+	return
+}
+
+// turns off BRB for a client and stops the timer; used on resume and during
+// client teardown
+func (bt *BrbTimer) Disable() (brbAt time.Time) {
+	bt.client.stateMutex.Lock()
+	defer bt.client.stateMutex.Unlock()
+
+	if bt.state == BrbEnabled {
+		bt.state = BrbDisabled
+		brbAt = bt.brbAt
+		bt.brbAt = time.Time{}
+	}
+	bt.resetTimeout()
+	return
+}
+
+func (bt *BrbTimer) resetTimeout() {
+	if bt.timer != nil {
+		bt.timer.Stop()
+	}
+	if bt.state != BrbEnabled {
+		return
+	}
+	if bt.timer == nil {
+		bt.timer = time.AfterFunc(bt.duration, bt.processTimeout)
+	} else {
+		bt.timer.Reset(bt.duration)
+	}
+}
+
+func (bt *BrbTimer) processTimeout() {
+	dead := false
+	defer func() {
+		if dead {
+			bt.client.Quit(bt.client.AwayMessage(), nil)
+			bt.client.destroy(nil)
+		}
+	}()
+
+	bt.client.stateMutex.Lock()
+	defer bt.client.stateMutex.Unlock()
+
+	if bt.client.alwaysOn {
+		return
+	}
+
+	switch bt.state {
+	case BrbDisabled, BrbEnabled:
+		if len(bt.client.sessions) == 0 {
+			// client never returned, quit them
+			bt.state = BrbDead
+			dead = true
+		} else {
+			// client resumed, reattached, or has another active session
+			bt.state = BrbDisabled
+			bt.brbAt = time.Time{}
+		}
+	case BrbDead:
+		dead = true // shouldn't be possible but whatever
+	}
+	bt.resetTimeout()
+}
diff --git a/irc/numerics.go b/irc/numerics.go
index 6e4cffd3..01b22499 100644
--- a/irc/numerics.go
+++ b/irc/numerics.go
@@ -196,4 +196,10 @@ const (
 	RPL_REG_VERIFICATION_REQUIRED = "927"
 	ERR_TOOMANYLANGUAGES          = "981"
 	ERR_NOLANGUAGE                = "982"
+
+	// draft numerics
+	// these haven't been assigned actual codes, so we use RPL_NONE's code (300),
+	// since RPL_NONE is intended to be used when testing / debugging / etc features.
+
+	ERR_CANNOT_RESUME = "300"
 )
diff --git a/irc/resume.go b/irc/resume.go
new file mode 100644
index 00000000..7c02181a
--- /dev/null
+++ b/irc/resume.go
@@ -0,0 +1,104 @@
+// Copyright (c) 2019 Shivaram Lingamneni <slingamn@cs.stanford.edu>
+// released under the MIT license
+
+package irc
+
+import (
+	"sync"
+
+	"github.com/ergochat/ergo/irc/utils"
+)
+
+// implements draft/resume, in particular the issuing, management, and verification
+// of resume tokens with two components: a unique ID and a secret key
+
+type resumeTokenPair struct {
+	client *Client
+	secret string
+}
+
+type ResumeManager struct {
+	sync.Mutex // level 2
+
+	resumeIDtoCreds map[string]resumeTokenPair
+	server          *Server
+}
+
+func (rm *ResumeManager) Initialize(server *Server) {
+	rm.resumeIDtoCreds = make(map[string]resumeTokenPair)
+	rm.server = server
+}
+
+// GenerateToken generates a resume token for a client. If the client has
+// already been assigned one, it returns "".
+func (rm *ResumeManager) GenerateToken(client *Client) (token string, id string) {
+	id = utils.GenerateSecretToken()
+	secret := utils.GenerateSecretToken()
+
+	rm.Lock()
+	defer rm.Unlock()
+
+	if client.ResumeID() != "" {
+		return
+	}
+
+	client.SetResumeID(id)
+	rm.resumeIDtoCreds[id] = resumeTokenPair{
+		client: client,
+		secret: secret,
+	}
+
+	return id + secret, id
+}
+
+// VerifyToken looks up the client corresponding to a resume token, returning
+// nil if there is no such client or the token is invalid. If successful,
+// the token is consumed and cannot be used to resume again.
+func (rm *ResumeManager) VerifyToken(newClient *Client, token string) (oldClient *Client, id string) {
+	if len(token) != 2*utils.SecretTokenLength {
+		return
+	}
+
+	rm.Lock()
+	defer rm.Unlock()
+
+	id = token[:utils.SecretTokenLength]
+	pair, ok := rm.resumeIDtoCreds[id]
+	if !ok {
+		return
+	}
+	// disallow resume of an unregistered client; this prevents the use of
+	// resume as an auth bypass
+	if !pair.client.Registered() {
+		return
+	}
+
+	if utils.SecretTokensMatch(pair.secret, token[utils.SecretTokenLength:]) {
+		oldClient = pair.client // success!
+		// consume the token, ensuring that at most one resume can succeed
+		delete(rm.resumeIDtoCreds, id)
+		// old client is henceforth resumeable under new client's creds (possibly empty)
+		newResumeID := newClient.ResumeID()
+		oldClient.SetResumeID(newResumeID)
+		if newResumeID != "" {
+			if newResumeCreds, ok := rm.resumeIDtoCreds[newResumeID]; ok {
+				newResumeCreds.client = oldClient
+				rm.resumeIDtoCreds[newResumeID] = newResumeCreds
+			}
+		}
+		// new client no longer "owns" newResumeID, remove the association
+		newClient.SetResumeID("")
+	}
+	return
+}
+
+// Delete stops tracking a client's resume token.
+func (rm *ResumeManager) Delete(client *Client) {
+	rm.Lock()
+	defer rm.Unlock()
+
+	currentID := client.ResumeID()
+	if currentID != "" {
+		delete(rm.resumeIDtoCreds, currentID)
+	}
+}
diff --git a/irc/server.go b/irc/server.go
index 697f5028..5302a50b 100644
--- a/irc/server.go
+++ b/irc/server.go
@@ -80,6 +80,7 @@ type Server struct {
 	rehashMutex       sync.Mutex // tier 4
 	rehashSignal      chan os.Signal
 	pprofServer       *http.Server
+	resumeManager     ResumeManager
 	signals           chan os.Signal
 	snomasks          SnoManager
 	store             *buntdb.DB
@@ -105,6 +106,7 @@ func NewServer(config *Config, logger *logger.Manager) (*Server, error) {
 
 	server.clients.Initialize()
 	server.semaphores.Initialize()
+	server.resumeManager.Initialize(server)
 	server.whoWas.Initialize(config.Limits.WhowasEntries)
 	server.monitorManager.Initialize()
 	server.snomasks.Initialize()
@@ -271,6 +273,12 @@ func (server *Server) handleAlwaysOnExpirations() {
 //
 
 func (server *Server) tryRegister(c *Client, session *Session) (exiting bool) {
+	// if the session just sent us a RESUME line, try to resume
+	if session.resumeDetails != nil {
+		session.tryResume()
+		return // whether we succeeded or failed, either way `c` is not getting registered
+	}
+
 	// XXX PROXY or WEBIRC MUST be sent as the first line of the session;
 	// if we are here at all that means we have the final value of the IP
 	if session.rawHostname == "" {
diff --git a/irc/version.go b/irc/version.go
index 38c00412..7c352c18 100644
--- a/irc/version.go
+++ b/irc/version.go
@@ -7,7 +7,7 @@ import "fmt"
 
 const (
 	// SemVer is the semantic version of Ergo.
-	SemVer = "2.7.0-rc1"
+	SemVer = "2.7.0-resume1"
 )
 
 var (