diff --git a/lib/client_connection.ml b/lib/client_connection.ml
index 443aad5e..6d2acd10 100644
--- a/lib/client_connection.ml
+++ b/lib/client_connection.ml
@@ -31,12 +31,15 @@
     POSSIBILITY OF SUCH DAMAGE.
   ----------------------------------------------------------------------------*)
 
+open Sexplib.Std
+
 module Reader = Parse.Reader
 module Writer = Serialize.Writer
 
 module Oneshot = struct
   type error =
     [ `Malformed_response of string | `Invalid_response_body_length of Response.t | `Exn of exn ]
+  [@@deriving sexp_of]
 
   type response_handler = Response.t -> Body.Reader.t  -> unit
   type error_handler = error -> unit
diff --git a/lib/dune b/lib/dune
index b513e7cb..36c2fcaf 100644
--- a/lib/dune
+++ b/lib/dune
@@ -1,6 +1,6 @@
 (library
  (name        httpaf)
  (public_name httpaf)
- (libraries
-   angstrom faraday bigstringaf)
+ (libraries angstrom faraday bigstringaf sexplib)
+ (preprocess (pps ppx_sexp_conv))
  (flags (:standard -safe-string)))
diff --git a/lib/headers.ml b/lib/headers.ml
index e28d0e7c..8a108bcb 100644
--- a/lib/headers.ml
+++ b/lib/headers.ml
@@ -31,10 +31,14 @@
     POSSIBILITY OF SUCH DAMAGE.
   ----------------------------------------------------------------------------*)
 
+open Sexplib.Std
 
-type name = string
-type value = string
-type t = (name * value) list
+
+type name = string [@@deriving sexp]
+type value = string [@@deriving sexp]
+type t = (name * value) list [@@deriving sexp]
+
+let sexp_of_t t = sexp_of_t (List.rev t)
 
 let empty : t = []
 
@@ -70,6 +74,7 @@ module CI = struct
       done;
       !equal_so_far
     )
+  ;;
 end
 
 let ci_equal = CI.equal
diff --git a/lib/headers.mli b/lib/headers.mli
index 9e5ce8a4..d7988ded 100644
--- a/lib/headers.mli
+++ b/lib/headers.mli
@@ -1,4 +1,4 @@
-type t
+type t [@@deriving sexp]
 
 type name = string
 type value = string
diff --git a/lib/httpaf.mli b/lib/httpaf.mli
index 25fd3499..89c1ee4f 100644
--- a/lib/httpaf.mli
+++ b/lib/httpaf.mli
@@ -41,6 +41,8 @@
     1.1 specification, and the basic principles of memory management and
     vectorized IO. *)
 
+
+
 (** {2 Basic HTTP Types} *)
 
 
@@ -58,6 +60,7 @@ module Version : sig
     { major : int (** The major protocol number. *)
     ; minor : int (** The minor protocol number. *)
     }
+  [@@deriving sexp]
 
   val compare : t -> t -> int
 
@@ -95,12 +98,14 @@ module Method : sig
     | `TRACE
     (** {{:https://tools.ietf.org/html/rfc7231#section-4.3.8} RFC7231§4.3.8}. Safe.*)
     ]
+  [@@deriving sexp]
 
   type t = [
     | standard
     | `Other of string
     (** Methods defined outside of RFC7231, or custom methods. *)
     ]
+  [@@deriving sexp]
 
   val is_safe : standard -> bool
   (** Request methods are considered "safe" if their defined semantics are
@@ -147,6 +152,7 @@ module Status : sig
     | `Continue
     | `Switching_protocols
     ]
+  [@@deriving sexp]
   (** The 1xx (Informational) class of status code indicates an interim
       response for communicating connection status or request progress
       prior to completing the requested action and sending a final
@@ -164,6 +170,7 @@ module Status : sig
     | `Reset_content
     | `Partial_content
     ]
+  [@@deriving sexp]
   (** The 2xx (Successful) class of status code indicates that the client's
       request was successfully received, understood, and accepted.
 
@@ -179,6 +186,7 @@ module Status : sig
     | `Use_proxy
     | `Temporary_redirect
     ]
+  [@@deriving sexp]
   (** The 3xx (Redirection) class of status code indicates that further
       action needs to be taken by the user agent in order to fulfill the
       request.
@@ -209,6 +217,7 @@ module Status : sig
     | `I_m_a_teapot
     | `Enhance_your_calm
     ]
+  [@@deriving sexp]
   (** The 4xx (Client Error) class of status code indicates that the client
       seems to have erred.
 
@@ -223,6 +232,7 @@ module Status : sig
     | `Gateway_timeout
     | `Http_version_not_supported
     ]
+  [@@deriving sexp]
   (** The 5xx (Server Error) class of status code indicates that the server is
       aware that it has erred or is incapable of performing the requested
       method.
@@ -237,11 +247,13 @@ module Status : sig
     | client_error
     | server_error
     ]
+  [@@deriving sexp]
   (** The status codes defined in the HTTP 1.1 RFCs *)
 
   type t = [
     | standard
     | `Code of int ]
+  [@@deriving sexp]
   (** The standard codes along with support for custom codes. *)
 
   val default_reason_phrase : standard -> string
@@ -320,7 +332,7 @@ end
     See {{:https://tools.ietf.org/html/rfc7230#section-3.2} RFC7230§3.2} for
     more details. *)
 module Headers : sig
-  type t
+  type t [@@deriving sexp]
 
   type name = string
   (** The type of a case-insensitive header name. *)
@@ -447,6 +459,9 @@ module Body : sig
   module Reader : sig
     type t
 
+    val create : Bigstringaf.t -> t
+    (** [create bs] creates a [t] using [bs] as the internal buffer. *)
+
     val schedule_read
       :  t
       -> on_eof  : (unit -> unit)
@@ -468,6 +483,9 @@ module Body : sig
     val is_closed : t -> bool
     (** [is_closed t] is [true] if {!close} has been called on [t] and [false]
         otherwise. A closed [t] may still have bytes available for reading. *)
+
+    val unsafe_faraday : t -> Faraday.t
+    (** [unsafe_faraday t] retrieves the raw Faraday object from [t]. Unsafe. *)
   end
 
   module Writer : sig
@@ -527,6 +545,7 @@ module Request : sig
     ; target  : string
     ; version : Version.t
     ; headers : Headers.t }
+  [@@deriving sexp]
 
   val create
     :  ?version:Version.t (** default is HTTP 1.1 *)
@@ -573,6 +592,7 @@ module Response : sig
     ; status  : Status.t
     ; reason  : string
     ; headers : Headers.t }
+  [@@deriving sexp]
 
   val create
     :  ?reason:string     (** default is determined by {!Status.default_reason_phrase} *)
@@ -656,6 +676,8 @@ module Reqd : sig
   val respond_with_bigstring : t -> Response.t -> Bigstringaf.t -> unit
   val respond_with_streaming : ?flush_headers_immediately:bool -> t -> Response.t -> Body.Writer.t
 
+  val respond_with_upgrade : ?reason:string -> t -> Headers.t -> unit
+
   (** {3 Exception Handling} *)
 
   val report_exn : t -> exn -> unit
@@ -697,7 +719,7 @@ module Server_connection : sig
   (** [create ?config ?error_handler ~request_handler] creates a connection
       handler that will service individual requests with [request_handler]. *)
 
-  val next_read_operation : t -> [ `Read | `Yield | `Close ]
+  val next_read_operation : t -> [ `Read | `Yield | `Close | `Upgrade ]
   (** [next_read_operation t] returns a value describing the next operation
       that the caller should conduct on behalf of the connection. *)
 
@@ -724,6 +746,7 @@ module Server_connection : sig
   val next_write_operation : t -> [
     | `Write of Bigstringaf.t IOVec.t list
     | `Yield
+    | `Upgrade
     | `Close of int ]
   (** [next_write_operation t] returns a value describing the next operation
       that the caller should conduct on behalf of the connection. *)
@@ -774,6 +797,7 @@ module Client_connection : sig
 
   type error =
     [ `Malformed_response of string | `Invalid_response_body_length of Response.t | `Exn of exn ]
+  [@@deriving sexp_of]
 
   type response_handler = Response.t -> Body.Reader.t  -> unit
 
diff --git a/lib/method.ml b/lib/method.ml
index 46ebe44e..71f56948 100644
--- a/lib/method.ml
+++ b/lib/method.ml
@@ -31,6 +31,7 @@
     POSSIBILITY OF SUCH DAMAGE.
   ----------------------------------------------------------------------------*)
 
+open Sexplib.Std
 
 type standard =  [
   | `GET
@@ -42,11 +43,13 @@ type standard =  [
   | `OPTIONS
   | `TRACE
 ]
+[@@deriving sexp]
 
 type t = [
   | standard
   | `Other of string
   ]
+[@@deriving sexp]
 
 let is_safe = function
   | `GET | `HEAD | `OPTIONS | `TRACE -> true
diff --git a/lib/parse.ml b/lib/parse.ml
index 20f5a104..c0009a8b 100644
--- a/lib/parse.ml
+++ b/lib/parse.ml
@@ -139,7 +139,7 @@ let response =
   lift4 (fun version status reason headers ->
     Response.create ~reason ~version ~headers status)
     (version              <* char ' ')
-    (status               <* char ' ')
+    (status               <* option ' ' (char ' '))
     (take_till P.is_cr    <* eol <* commit)
     (headers              <* eol)
 
diff --git a/lib/reqd.ml b/lib/reqd.ml
index b025ed61..4acd453e 100644
--- a/lib/reqd.ml
+++ b/lib/reqd.ml
@@ -37,14 +37,17 @@ type error =
 module Response_state = struct
   type t =
     | Waiting
+    | Upgrade of Response.t
     | Fixed     of Response.t
     | Streaming of Response.t * Body.Writer.t
 end
 
 module Input_state = struct
   type t =
+    | Waiting
     | Ready
     | Complete
+    | Upgraded
 end
 
 module Output_state = struct
@@ -52,6 +55,7 @@ module Output_state = struct
     | Waiting
     | Ready
     | Complete
+    | Upgraded
 end
 
 type error_handler =
@@ -111,12 +115,14 @@ let response { response_state; _ } =
   match response_state with
   | Waiting -> None
   | Streaming (response, _)
+  | Upgrade response
   | Fixed response -> Some response
 
 let response_exn { response_state; _ } =
   match response_state with
   | Waiting -> failwith "httpaf.Reqd.response_exn: response has not started"
   | Streaming (response, _)
+  | Upgrade response
   | Fixed response -> response
 
 let respond_with_string t response str =
@@ -133,6 +139,7 @@ let respond_with_string t response str =
     Writer.wakeup t.writer;
   | Streaming _ ->
     failwith "httpaf.Reqd.respond_with_string: response already started"
+  | Upgrade _
   | Fixed _ ->
     failwith "httpaf.Reqd.respond_with_string: response already complete"
 
@@ -150,6 +157,7 @@ let respond_with_bigstring t response (bstr:Bigstringaf.t) =
     Writer.wakeup t.writer;
   | Streaming _ ->
     failwith "httpaf.Reqd.respond_with_bigstring: response already started"
+  | Upgrade _
   | Fixed _ ->
     failwith "httpaf.Reqd.respond_with_bigstring: response already complete"
 
@@ -175,6 +183,7 @@ let unsafe_respond_with_streaming ~flush_headers_immediately t response =
     response_body
   | Streaming _ ->
     failwith "httpaf.Reqd.respond_with_streaming: response already started"
+  | Upgrade _
   | Fixed _ ->
     failwith "httpaf.Reqd.respond_with_streaming: response already complete"
 
@@ -183,6 +192,23 @@ let respond_with_streaming ?(flush_headers_immediately=false) t response =
     failwith "httpaf.Reqd.respond_with_streaming: invalid state, currently handling error";
   unsafe_respond_with_streaming ~flush_headers_immediately t response
 
+let respond_with_upgrade ?reason t headers =
+  match t.response_state with
+  | Waiting ->
+    if not (Request.is_upgrade t.request) then
+      failwith "httpaf.Reqd.respond_with_upgrade: request was not an upgrade request"
+    else (
+      let response = Response.create ?reason ~headers `Switching_protocols in
+      t.response_state <- Upgrade response;
+      Body.Reader.close t.request_body;
+      Writer.write_response t.writer response;
+      Writer.wakeup t.writer);
+  | Streaming _ ->
+    failwith "httpaf.Reqd.respond_with_upgrade: response already started"
+  | Upgrade _
+  | Fixed _ ->
+    failwith "httpaf.Reqd.respond_with_upgrade: response already complete"
+
 let report_error t error =
   t.persistent <- false;
   Body.Reader.close t.request_body;
@@ -207,7 +233,7 @@ let report_error t error =
   | Streaming (_response, response_body), `Exn _ ->
     Body.Writer.close response_body;
     Writer.close_and_drain t.writer
-  | (Fixed _ | Streaming _ | Waiting) , _ ->
+  | (Fixed _ | Streaming _ | Waiting | Upgrade _) , _ ->
     (* XXX(seliopou): Once additional logging support is added, log the error
      * in case it is not spurious. *)
     ()
@@ -215,7 +241,7 @@ let report_error t error =
 let report_exn t exn =
   report_error t (`Exn exn)
 
-let try_with t f : (unit, exn) result =
+let try_with t f : (unit, exn) Result.t =
   try f (); Ok () with exn -> report_exn t exn; Error exn
 
 (* Private API, not exposed to the user through httpaf.mli *)
@@ -232,21 +258,41 @@ let persistent_connection t =
   t.persistent
 
 let input_state t : Input_state.t =
-  if Body.Reader.is_closed t.request_body
-  then Complete
-  else Ready
+  let upgrade_status =
+    match Request.is_upgrade t.request with
+    | false -> `Not_upgrading
+    | true ->
+      match t.response_state with
+      | Upgrade _ -> `Finished_upgrading
+      | Fixed _ | Streaming _ -> `Upgrade_declined
+      | Waiting -> `Upgrade_in_progress
+  in
+  match upgrade_status with
+  | `Finished_upgrading -> Upgraded
+  | `Not_upgrading | `Upgrade_declined ->
+    if Body.Reader.is_closed t.request_body
+    then Complete
+    else Ready
+  | `Upgrade_in_progress ->
+    Waiting
 ;;
 
 let output_state t : Output_state.t =
   match t.response_state with
+  | Upgrade _ -> Upgraded
   | Fixed _ -> Complete
   | Streaming (_, response_body) ->
-    if Body.Writer.has_pending_output response_body
+    if Writer.is_closed t.writer
+    then Complete
+    else if Body.Writer.has_pending_output response_body
     then Ready
     else if Body.Writer.is_closed response_body
     then Complete
     else Waiting
-  | Waiting -> Waiting
+  | Waiting ->
+    if Writer.is_closed t.writer
+    then Complete
+    else Waiting
 ;;
 
 let flush_request_body t =
diff --git a/lib/request.ml b/lib/request.ml
index 158fd4c2..385d4bb1 100644
--- a/lib/request.ml
+++ b/lib/request.ml
@@ -31,11 +31,14 @@
     POSSIBILITY OF SUCH DAMAGE.
   ----------------------------------------------------------------------------*)
 
+open Sexplib.Std
+
 type t =
   { meth    : Method.t
   ; target  : string
   ; version : Version.t
   ; headers : Headers.t }
+[@@deriving sexp]
 
 let create ?(version=Version.v1_1) ?(headers=Headers.empty) meth target =
   { meth; target; version; headers }
@@ -80,3 +83,8 @@ let persistent_connection ?proxy { version; headers; _ } =
 let pp_hum fmt { meth; target; version; headers } =
   Format.fprintf fmt "((method \"%a\") (target %S) (version \"%a\") (headers %a))"
     Method.pp_hum meth target Version.pp_hum version Headers.pp_hum headers
+
+let is_upgrade t =
+  match Headers.get t.headers "Connection" with
+  | None -> false
+  | Some header_val -> Headers.ci_equal header_val "upgrade"
diff --git a/lib/response.ml b/lib/response.ml
index 88161943..ee3bfdb9 100644
--- a/lib/response.ml
+++ b/lib/response.ml
@@ -31,11 +31,14 @@
     POSSIBILITY OF SUCH DAMAGE.
   ----------------------------------------------------------------------------*)
 
+open Sexplib.Std
+
 type t =
   { version : Version.t
   ; status  : Status.t
   ; reason  : string
   ; headers : Headers.t }
+[@@deriving sexp]
 
 let create ?reason ?(version=Version.v1_1) ?(headers=Headers.empty) status =
   let reason =
diff --git a/lib/serialize.ml b/lib/serialize.ml
index 61c22131..76039b9d 100644
--- a/lib/serialize.ml
+++ b/lib/serialize.ml
@@ -195,4 +195,6 @@ module Writer = struct
     | `Close         -> `Close (drained_bytes t)
     | `Yield         -> `Yield
     | `Writev iovecs -> `Write iovecs
+
+  let has_pending_output t = Faraday.has_pending_output t.encoder
 end
diff --git a/lib/server_connection.ml b/lib/server_connection.ml
index 839a10b3..952cc947 100644
--- a/lib/server_connection.ml
+++ b/lib/server_connection.ml
@@ -139,21 +139,17 @@ let create ?(config=Config.default) ?(error_handler=default_error_handler) reque
   }
 
 let shutdown_reader t =
-  if is_active t
-  then Reqd.close_request_body (current_reqd_exn t);
   Reader.force_close t.reader;
-  wakeup_reader t
+  if is_active t
+  then Reqd.close_request_body (current_reqd_exn t)
+  else wakeup_reader t
 
 let shutdown_writer t =
-  if is_active t then (
-    let reqd = current_reqd_exn t in
-    (* XXX(dpatti): I'm not sure I understand why we close the *request* body
-       here. Maybe we can write a test such that removing this line causes it to
-       fail? *)
-    Reqd.close_request_body reqd;
-    Reqd.flush_response_body reqd);
+  if is_active t then Reqd.flush_response_body (current_reqd_exn t);
   Writer.close t.writer;
-  wakeup_writer t
+  if is_active t
+  then Reqd.close_request_body (current_reqd_exn t)
+  else wakeup_writer t
 
 let error_code t =
   if is_active t
@@ -162,7 +158,9 @@ let error_code t =
 
 let shutdown t =
   shutdown_reader t;
-  shutdown_writer t
+  shutdown_writer t;
+  wakeup_reader t;
+  wakeup_writer t
 
 let set_error_and_handle ?request t error =
   if is_active t then begin
@@ -225,8 +223,10 @@ let rec _next_read_operation t =
   ) else (
     let reqd = current_reqd_exn t in
     match Reqd.input_state reqd with
+    | Waiting  -> `Yield
     | Ready    -> Reader.next t.reader
     | Complete -> _final_read_operation_for t reqd
+    | Upgraded -> `Upgrade
   )
 
 and _final_read_operation_for t reqd =
@@ -248,6 +248,7 @@ and _final_read_operation_for t reqd =
       if Reader.is_closed t.reader
       then Reader.next t.reader
       else `Yield
+    | Upgraded -> `Upgrade
     | Complete ->
       advance_request_queue t;
       _next_read_operation t;
@@ -258,7 +259,7 @@ let next_read_operation t =
   match _next_read_operation t with
   | `Error (`Parse _)             -> set_error_and_handle          t `Bad_request; `Close
   | `Error (`Bad_request request) -> set_error_and_handle ~request t `Bad_request; `Close
-  | (`Read | `Yield | `Close) as operation -> operation
+  | (`Read | `Yield | `Close | `Upgrade) as operation -> operation
 
 let rec read_with_more t bs ~off ~len more =
   let call_handler = Queue.is_empty t.request_queue in
@@ -296,17 +297,27 @@ let rec _next_write_operation t =
     | Ready ->
       Reqd.flush_response_body reqd;
       Writer.next t.writer
-    | Complete -> _final_write_operation_for t reqd
+    | Complete -> _final_write_operation_for t reqd ~upgrade:false
+    | Upgraded -> _final_write_operation_for t reqd ~upgrade:true
   )
 
-and _final_write_operation_for t reqd =
+and _final_write_operation_for t reqd ~upgrade =
   let next =
-    if not (Reqd.persistent_connection reqd) then (
+    if upgrade then (
+      if Writer.has_pending_output t.writer then
+        (* Even in the Upgrade case, we're still responsible for writing the response
+           header, so we might have work to do. *)
+        Writer.next t.writer
+      else
+        `Upgrade
+    ) else if not (Reqd.persistent_connection reqd) then (
       shutdown_writer t;
       Writer.next t.writer;
     ) else (
       match Reqd.input_state reqd with
+      | Waiting -> `Yield
       | Ready -> Writer.next t.writer;
+      | Upgraded -> `Upgrade
       | Complete ->
         advance_request_queue t;
         _next_write_operation t;
diff --git a/lib/status.ml b/lib/status.ml
index 318e4067..ed5ba39a 100644
--- a/lib/status.ml
+++ b/lib/status.ml
@@ -31,11 +31,13 @@
     POSSIBILITY OF SUCH DAMAGE.
   ----------------------------------------------------------------------------*)
 
+open Sexplib.Std
 
 type informational = [
   | `Continue
   | `Switching_protocols
   ]
+[@@deriving sexp]
 
 type successful = [
   | `OK
@@ -46,6 +48,7 @@ type successful = [
   | `Reset_content
   | `Partial_content
   ]
+[@@deriving sexp]
 
 type redirection = [
   | `Multiple_choices
@@ -56,6 +59,7 @@ type redirection = [
   | `Use_proxy
   | `Temporary_redirect
   ]
+[@@deriving sexp]
 
 type client_error = [
   | `Bad_request
@@ -80,6 +84,7 @@ type client_error = [
   | `Enhance_your_calm
   | `Upgrade_required
   ]
+[@@deriving sexp]
 
 type server_error = [
   | `Internal_server_error
@@ -89,6 +94,7 @@ type server_error = [
   | `Gateway_timeout
   | `Http_version_not_supported
   ]
+[@@deriving sexp]
 
 type standard = [
   | informational
@@ -97,10 +103,12 @@ type standard = [
   | client_error
   | server_error
   ]
+[@@deriving sexp]
 
 type t = [
   | standard
   | `Code of int ]
+[@@deriving sexp]
 
 let default_reason_phrase = function
  (* Informational *)
diff --git a/lib/version.ml b/lib/version.ml
index 314b8899..e2442d4a 100644
--- a/lib/version.ml
+++ b/lib/version.ml
@@ -31,10 +31,12 @@
     POSSIBILITY OF SUCH DAMAGE.
   ----------------------------------------------------------------------------*)
 
+open Sexplib.Std
 
 type t =
   { major : int
   ; minor : int }
+[@@deriving sexp]
 
 let v1_0 = { major = 1; minor = 0 }
 let v1_1 = { major = 1; minor = 1 }