diff --git a/README.md b/README.md
index 4736ae3..2a80389 100644
--- a/README.md
+++ b/README.md
@@ -46,13 +46,14 @@ The simplest starting point is to set the global tracer. As early as possible, d
 To start a new `Span`, you can use the `StartSpanFromContext` method.
 
 ```php
-    use Psr\Http\Message\RequestInterface;
+    use OpenTracing\Formats;
+    use OpenTracing\GlobalTracer;
 
     ...
 
     $spanContext = GlobalTracer::get()->extract(
-        Propagator::HTTP_HEADERS,
-        HttpHeaders::withHeaders($request->getHeaders())
+        Formats\HTTP_HEADERS,
+        $request->getHeaders()
     );
     
     function doSomething(SpanContext $spanContext, ...) {
@@ -93,7 +94,7 @@ reference.
         $span = GlobalTracer::get()->startSpan(
         	'my_span',
         	[
-        		'child_of' => $spanContext,
+        		'child_of' => $parentSpan,
         	]
         );
         
@@ -105,17 +106,16 @@ reference.
 ### Serializing to the wire
 
 ```php
-    use OpenTracing\Carriers\HttpHeaders as HttpHeadersCarrier;
-    use OpenTracing\Context;
     use OpenTracing\GlobalTracer;
+    use OpenTracing\Formats;
     
     ...
     
     $tracer = GlobalTracer::get(); 
     
     $spanContext = $tracer->extract(
-        Propagator::HTTP_HEADERS,
-        HttpHeaders::withHeaders($request->getHeaders())
+        Formats\HTTP_HEADERS,
+        $request->getHeaders()
     );
     
     try {
@@ -123,14 +123,15 @@ reference.
 
         $client = new GuzzleHttp\Client;
         
-        $request = new \GuzzleHttp\Psr7\Request('GET', 'http://myservice');
+        $headers = [];
         
         $tracer->inject(
-            $span->context(),
-            Propagator::HTTP_HEADERS,
-            HttpHeadersCarrier::withHeaders($request->getHeaders())
-        )
-
+            $span->getContext(),
+            Formats\HTTP_HEADERS,
+            $headers
+        );
+        
+        $request = new \GuzzleHttp\Psr7\Request('GET', 'http://myservice', $headers);
         $client->send($request);
         ...
     } catch (\Exception $e) {
@@ -144,12 +145,12 @@ reference.
 When using http header for context propagation you can use either the `Request` or the `$_SERVER` variable:
 
 ```php
-    use OpenTracing\Carriers\HttpHeaders;
     use OpenTracing\GlobalTracer;
+    use OpenTracing\Formats;
     
     $request = Request::createFromGlobals();
     $tracer = GlobalTracer::get();
-    $spanContext = $tracer->extract(Propagator::HTTP_HEADERS, HttpHeaders::fromRequest($request));
+    $spanContext = $tracer->extract(Formats\HTTP_HEADERS, $request->getHeaders());
     $tracer->startSpan('my_span', [
         'child_of' => $spanContext,
     ]); 
@@ -164,8 +165,6 @@ cause problems for Tracer implementations. This is why the PHP API contains a
 `flush` method that allows to trigger a span sending out of process.
 
 ```php
-<?php
-
 use OpenTracing\GlobalTracer;
 
 // Do application work, buffer spans in memory
@@ -192,10 +191,6 @@ SpanOptions wrapper object. The following keys are valid:
 - `tags` is an array with string keys and scalar values that represent OpenTracing tags.
 
 ```php
-<?php
-
-use OpenTracing\Reference;
-
 $span = $tracer->createSpan('my_span', [
     'child_of' => $spanContext,
     'tags' => ['foo' => 'bar'],
@@ -215,7 +210,7 @@ Tracers will throw an exception if the requested format is not handled by them.
 - `Tracer::FORMAT_HTTP_HEADERS` should represent the span context as HTTP header lines
   in an array list. For two context details "Span-Id" and "Trace-Id", the
   result would be `['Span-Id: abc123', 'Trace-Id: def456']`. This definition can be
-  passed directly to curl and file_get_contents.
+  passed directly to `curl` and `file_get_contents`.
 
 - `Tracer::FORMAT_BINARY` makes no assumptions about the data format other than it is
   proprietary and each Tracer can handle it as it wants.
diff --git a/composer.json b/composer.json
index e81752a..08eeb17 100644
--- a/composer.json
+++ b/composer.json
@@ -11,19 +11,17 @@
     ],
     "minimum-stability": "stable",
     "require": {
-        "php": "^5.6||^7.0",
-        "psr/http-message": "^1.0"
+        "php": "^5.6||^7.0"
     },
     "require-dev": {
         "phpunit/phpunit": "~5.7.19",
-        "guzzlehttp/psr7": "^1.4",
         "squizlabs/php_codesniffer": "3.*"
     },
     "autoload": {
         "psr-4": {
             "OpenTracing\\": "./src/OpenTracing/"
         },
-        "files": ["./src/OpenTracing/Ext/Tags.php", "./src/OpenTracing/Propagation/Formats.php"]
+        "files": ["./src/OpenTracing/Ext/Tags.php", "./src/OpenTracing/Formats.php"]
     },
     "autoload-dev": {
         "psr-4": {
diff --git a/src/OpenTracing/Carriers/HttpHeaders.php b/src/OpenTracing/Carriers/HttpHeaders.php
deleted file mode 100644
index c15b73a..0000000
--- a/src/OpenTracing/Carriers/HttpHeaders.php
+++ /dev/null
@@ -1,58 +0,0 @@
-<?php
-
-namespace OpenTracing\Carriers;
-
-use ArrayIterator;
-use OpenTracing\Propagation\Reader;
-use OpenTracing\Propagation\Writer;
-use Psr\Http\Message\RequestInterface;
-
-final class HttpHeaders implements Reader, Writer
-{
-    private $items = [];
-
-    private function __construct(array $headers)
-    {
-        foreach ($headers as $key => $value) {
-            $this->items[(string) $key] = (string) $value;
-        }
-    }
-
-    /**
-     * @param RequestInterface $request
-     * @return HttpHeaders
-     */
-    public static function fromRequest(RequestInterface $request)
-    {
-        return new self(
-            array_map(function ($values) {
-                return $values[0];
-            }, $request->getHeaders())
-        );
-    }
-
-    /**
-     * @param array|string[] $headers
-     * @return HttpHeaders
-     */
-    public static function fromHeaders(array $headers)
-    {
-        return new self($headers);
-    }
-
-    /**
-     * {@inheritdoc}
-     */
-    public function set($key, $value)
-    {
-        $this->items[(string) $key] = (string) $value;
-    }
-
-    /**
-     * {@inheritdoc}
-     */
-    public function getIterator()
-    {
-        return new ArrayIterator($this->items);
-    }
-}
diff --git a/src/OpenTracing/Carriers/TextMap.php b/src/OpenTracing/Carriers/TextMap.php
deleted file mode 100644
index c4f549e..0000000
--- a/src/OpenTracing/Carriers/TextMap.php
+++ /dev/null
@@ -1,44 +0,0 @@
-<?php
-
-namespace OpenTracing\Carriers;
-
-use ArrayIterator;
-use OpenTracing\Propagation\Reader;
-use OpenTracing\Propagation\Writer;
-
-final class TextMap implements Reader, Writer
-{
-    private $items = [];
-
-    private function __construct(array $textMap)
-    {
-        foreach ($textMap as $key => $value) {
-            $this->items[(string) $key] = (string) $value;
-        }
-    }
-
-    /**
-     * @param array|string[] $textMap
-     * @return TextMap
-     */
-    public static function fromArray(array $textMap)
-    {
-        return new self($textMap);
-    }
-
-    /**
-     * {@inheritdoc}
-     */
-    public function set($key, $value)
-    {
-        $this->items[(string) $key] = (string) $value;
-    }
-
-    /**
-     * {@inheritdoc}
-     */
-    public function getIterator()
-    {
-        return new ArrayIterator($this->items);
-    }
-}
diff --git a/src/OpenTracing/Formats.php b/src/OpenTracing/Formats.php
new file mode 100644
index 0000000..ce8d844
--- /dev/null
+++ b/src/OpenTracing/Formats.php
@@ -0,0 +1,47 @@
+<?php
+
+namespace OpenTracing\Formats;
+
+/**
+ * Used a (single) arbitrary binary blob representing a SpanContext
+ *
+ * For both Tracer::inject() and Tracer::extract() the carrier must be a `string`.
+ */
+const BINARY = 'binary';
+
+/**
+ * Used for an arbitrary string-to-string map with an unrestricted character set for both keys and values
+ *
+ * Unlike `HTTP_HEADERS`, the `TEXT_MAP` format does not restrict the key or
+ * value character sets in any way.
+ *
+ * For both Tracer::inject() and Tracer::extract() the carrier must be a `array|ArrayObject`.
+ */
+const TEXT_MAP = 'text_map';
+
+/**
+ * Used for a string-to-string map with keys and values that are suitable for use in HTTP headers (a la RFC 7230.
+ * In practice, since there is such "diversity" in the way that HTTP headers are treated in the wild, it is strongly
+ * recommended that Tracer implementations use a limited HTTP header key space and escape values conservatively.
+ *
+ * Unlike `TEXT_MAP`, the `HTTP_HEADERS` format requires that the keys and values be valid as HTTP headers as-is
+ * (i.e., character casing may be unstable and special characters are disallowed in keys, values should be
+ * URL-escaped, etc).
+ *
+ * For both Tracer::inject() and Tracer::extract() the carrier must be a `array|ArrayObject`.
+ *
+ * For example, Tracer::inject():
+ *
+ *    $headers = []
+ *    $tracer->inject($span->getContext(), Formats\HTTP_HEADERS, $headers)
+ *    $request = new GuzzleHttp\Psr7\Request($uri, $body, $headers);
+ *
+ * Or Tracer::extract():
+ *
+ *    $headers = $request->getHeaders()
+ *    $clientContext = $tracer->extract(Formats\HTTP_HEADERS, $headers)
+ *
+ * @see http://www.php-fig.org/psr/psr-7/#12-http-headers
+ * @see http://php.net/manual/en/function.getallheaders.php
+ */
+const HTTP_HEADERS = 'http_headers';
diff --git a/src/OpenTracing/NoopTracer.php b/src/OpenTracing/NoopTracer.php
index 8bbdc53..f97a325 100644
--- a/src/OpenTracing/NoopTracer.php
+++ b/src/OpenTracing/NoopTracer.php
@@ -2,9 +2,6 @@
 
 namespace OpenTracing;
 
-use OpenTracing\Propagation\Reader;
-use OpenTracing\Propagation\Writer;
-
 final class NoopTracer implements Tracer
 {
     public static function create()
@@ -23,14 +20,14 @@ public function startSpan($operationName, $options = [])
     /**
      * {@inheritdoc}
      */
-    public function inject(SpanContext $spanContext, $format, Writer $carrier)
+    public function inject(SpanContext $spanContext, $format, &$carrier)
     {
     }
 
     /**
      * {@inheritdoc}
      */
-    public function extract($format, Reader $carrier)
+    public function extract($format, $carrier)
     {
         return NoopSpanContext::create();
     }
diff --git a/src/OpenTracing/Propagation/Formats.php b/src/OpenTracing/Propagation/Formats.php
deleted file mode 100644
index d432344..0000000
--- a/src/OpenTracing/Propagation/Formats.php
+++ /dev/null
@@ -1,18 +0,0 @@
-<?php
-
-namespace OpenTracing\Propagation\Formats;
-
-use OpenTracing\Carriers\HttpHeaders;
-use OpenTracing\Carriers\TextMap;
-
-const BINARY = 'binary';
-
-/**
- * @see HttpHeaders
- */
-const TEXT_MAP = 'text_map';
-
-/**
- * @see TextMap
- */
-const HTTP_HEADERS = 'http_headers';
diff --git a/src/OpenTracing/Propagation/Reader.php b/src/OpenTracing/Propagation/Reader.php
deleted file mode 100644
index 1d88827..0000000
--- a/src/OpenTracing/Propagation/Reader.php
+++ /dev/null
@@ -1,15 +0,0 @@
-<?php
-
-namespace OpenTracing\Propagation;
-
-use IteratorAggregate;
-
-/**
- * Reader is the extract() carrier for the TextMap builtin format. With it,
- * the caller can decode a propagated SpanContext as entries in a map of
- * unicode strings.
- */
-interface Reader extends IteratorAggregate
-{
-
-}
diff --git a/src/OpenTracing/Propagation/Writer.php b/src/OpenTracing/Propagation/Writer.php
deleted file mode 100644
index f1c9274..0000000
--- a/src/OpenTracing/Propagation/Writer.php
+++ /dev/null
@@ -1,24 +0,0 @@
-<?php
-
-namespace OpenTracing\Propagation;
-
-/**
- * Writer is the inject() carrier. With it, the caller can encode a SpanContext
- * for propagation.
- */
-interface Writer
-{
-    /**
-     * Set a key:value pair to the carrier. Multiple calls to set() for the
-     * same key leads to undefined behavior.
-     *
-     * NOTE: The backing store for the Writer may contain data unrelated
-     * to SpanContext. As such, inject() and extract() implementations that
-     * call the TextMapWriter and TextMapReader interfaces must agree on a
-     * prefix or other convention to distinguish their own key:value pairs.
-     *
-     * @param string $key
-     * @param string $value
-     */
-    public function set($key, $value);
-}
diff --git a/src/OpenTracing/Tracer.php b/src/OpenTracing/Tracer.php
index e2b377f..e3f678d 100644
--- a/src/OpenTracing/Tracer.php
+++ b/src/OpenTracing/Tracer.php
@@ -2,14 +2,10 @@
 
 namespace OpenTracing;
 
-use OpenTracing\Carriers\HttpHeaders;
-use OpenTracing\Carriers\TextMap;
 use OpenTracing\Exceptions\InvalidReferencesSet;
 use OpenTracing\Exceptions\InvalidSpanOption;
 use OpenTracing\Exceptions\SpanContextNotFound;
 use OpenTracing\Exceptions\UnsupportedFormat;
-use OpenTracing\Propagation\Reader;
-use OpenTracing\Propagation\Writer;
 
 interface Tracer
 {
@@ -30,24 +26,27 @@ public function startSpan($operationName, $options = []);
     /**
      * @param SpanContext $spanContext
      * @param string $format
-     * @param Writer $carrier
+     * @param $carrier
      *
-     * @see Propagation\Formats
+     * @see Formats
      *
      * @throws UnsupportedFormat when the format is not recognized by the tracer
      * implementation
      */
-    public function inject(SpanContext $spanContext, $format, Writer $carrier);
+    public function inject(SpanContext $spanContext, $format, &$carrier);
 
     /**
      * @param string $format
-     * @param Reader $carrier
+     * @param $carrier
      * @return SpanContext
-     * @throws SpanContextNotFound when a context could not be extracted from Reader
+     *
+     * @see Formats
+     *
+     * @throws SpanContextNotFound when a context could not be extracted from carrier
      * @throws UnsupportedFormat when the format is not recognized by the tracer
      * implementation
      */
-    public function extract($format, Reader $carrier);
+    public function extract($format, $carrier);
 
     /**
      * Allow tracer to send span data to be instrumented.
diff --git a/tests/Unit/Carriers/HttpHeadersTest.php b/tests/Unit/Carriers/HttpHeadersTest.php
deleted file mode 100644
index 83fa9ba..0000000
--- a/tests/Unit/Carriers/HttpHeadersTest.php
+++ /dev/null
@@ -1,37 +0,0 @@
-<?php
-
-namespace OpenTracingTests\Unit\Carriers;
-
-use GuzzleHttp\Psr7\Request;
-use OpenTracing\Carriers\HttpHeaders;
-use PHPUnit_Framework_TestCase;
-
-/**
- * @covers HttpHeaders
- */
-final class HttpHeadersTest extends PHPUnit_Framework_TestCase
-{
-    const TEST_HEADERS = ['foo' => 'bar'];
-
-    public function testCreationWithHeadersHasTheExpectedValues()
-    {
-        $headers = self::TEST_HEADERS;
-        $httpHeaders = HttpHeaders::fromHeaders($headers);
-
-        foreach ($httpHeaders as $key => $value) {
-            $this->assertEquals('foo', $key);
-            $this->assertEquals('bar', $value);
-        }
-    }
-
-    public function testCreationFromRequestHasTheExpectedValues()
-    {
-        $request = new Request('GET', '', self::TEST_HEADERS);
-        $httpHeaders = HttpHeaders::fromRequest($request);
-
-        foreach ($httpHeaders as $key => $value) {
-            $this->assertEquals('foo', $key);
-            $this->assertEquals('bar', $value);
-        }
-    }
-}
diff --git a/tests/Unit/Carriers/TextMapTest.php b/tests/Unit/Carriers/TextMapTest.php
deleted file mode 100644
index d8fe402..0000000
--- a/tests/Unit/Carriers/TextMapTest.php
+++ /dev/null
@@ -1,34 +0,0 @@
-<?php
-
-namespace OpenTracingTests\Unit\Carriers;
-
-use OpenTracing\Carriers\TextMap;
-use PHPUnit_Framework_TestCase;
-
-/**
- * @covers TextMap
- */
-final class TextMapTest extends PHPUnit_Framework_TestCase
-{
-    public function testCreateATextMapWithItemsSuccess()
-    {
-        $items = ['foo' => 'bar'];
-        $textMap = TextMap::fromArray($items);
-
-        foreach ($textMap as $key => $value) {
-            $this->assertEquals('foo', $key);
-            $this->assertEquals('bar', $value);
-        }
-    }
-
-    public function testSettingAKeyValuePairSuccess()
-    {
-        $textMap = TextMap::fromArray([]);
-        $textMap->set('foo', 'bar');
-
-        foreach ($textMap as $key => $value) {
-            $this->assertEquals('foo', $key);
-            $this->assertEquals('bar', $value);
-        }
-    }
-}