diff --git a/objectivec/GPBUnknownFieldSet.m b/objectivec/GPBUnknownFieldSet.m
new file mode 100644
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+++ b/objectivec/GPBUnknownFieldSet.m
@@ -0,0 +1,423 @@
+// Protocol Buffers - Google's data interchange format
+// Copyright 2008 Google Inc.  All rights reserved.
+// https://developers.google.com/protocol-buffers/
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#import "GPBUnknownFieldSet_PackagePrivate.h"
+
+#import "GPBCodedInputStream_PackagePrivate.h"
+#import "GPBCodedOutputStream.h"
+#import "GPBUnknownField_PackagePrivate.h"
+#import "GPBUtilities.h"
+#import "GPBWireFormat.h"
+
+#pragma mark CFDictionaryKeyCallBacks
+
+// We use a custom dictionary here because our keys are numbers and
+// conversion back and forth from NSNumber was costing us performance.
+// If/when we move to C++ this could be done using a std::map and some
+// careful retain/release calls.
+
+static const void *GPBUnknownFieldSetKeyRetain(CFAllocatorRef allocator,
+                                               const void *value) {
+#pragma unused(allocator)
+  return value;
+}
+
+static void GPBUnknownFieldSetKeyRelease(CFAllocatorRef allocator,
+                                         const void *value) {
+#pragma unused(allocator)
+#pragma unused(value)
+}
+
+static CFStringRef GPBUnknownFieldSetCopyKeyDescription(const void *value) {
+  return CFStringCreateWithFormat(kCFAllocatorDefault, NULL, CFSTR("%d"),
+                                  (int)value);
+}
+
+static Boolean GPBUnknownFieldSetKeyEqual(const void *value1,
+                                          const void *value2) {
+  return value1 == value2;
+}
+
+static CFHashCode GPBUnknownFieldSetKeyHash(const void *value) {
+  return (CFHashCode)value;
+}
+
+#pragma mark Helpers
+
+static void checkNumber(int32_t number) {
+  if (number == 0) {
+    [NSException raise:NSInvalidArgumentException
+                format:@"Zero is not a valid field number."];
+  }
+}
+
+@implementation GPBUnknownFieldSet {
+ @package
+  CFMutableDictionaryRef fields_;
+}
+
+static void CopyWorker(const void *key, const void *value, void *context) {
+#pragma unused(key)
+  GPBUnknownField *field = value;
+  GPBUnknownFieldSet *result = context;
+
+  GPBUnknownField *copied = [field copy];
+  [result addField:copied];
+  [copied release];
+}
+
+- (id)copyWithZone:(NSZone *)zone {
+  GPBUnknownFieldSet *result = [[GPBUnknownFieldSet allocWithZone:zone] init];
+  if (fields_) {
+    CFDictionaryApplyFunction(fields_, CopyWorker, result);
+  }
+  return result;
+}
+
+- (void)dealloc {
+  if (fields_) {
+    CFRelease(fields_);
+  }
+  [super dealloc];
+}
+
+- (BOOL)isEqual:(id)object {
+  BOOL equal = NO;
+  if ([object isKindOfClass:[GPBUnknownFieldSet class]]) {
+    GPBUnknownFieldSet *set = (GPBUnknownFieldSet *)object;
+    if ((fields_ == NULL) && (set->fields_ == NULL)) {
+      equal = YES;
+    } else if ((fields_ != NULL) && (set->fields_ != NULL)) {
+      equal = CFEqual(fields_, set->fields_);
+    }
+  }
+  return equal;
+}
+
+- (NSUInteger)hash {
+  // Return the hash of the fields dictionary (or just some value).
+  if (fields_) {
+    return CFHash(fields_);
+  }
+  return (NSUInteger)[GPBUnknownFieldSet class];
+}
+
+#pragma mark - Public Methods
+
+- (BOOL)hasField:(int32_t)number {
+  ssize_t key = number;
+  return fields_ ? (CFDictionaryGetValue(fields_, (void *)key) != nil) : NO;
+}
+
+- (GPBUnknownField *)getField:(int32_t)number {
+  ssize_t key = number;
+  GPBUnknownField *result =
+      fields_ ? CFDictionaryGetValue(fields_, (void *)key) : nil;
+  return result;
+}
+
+- (NSUInteger)countOfFields {
+  return fields_ ? CFDictionaryGetCount(fields_) : 0;
+}
+
+- (NSArray *)sortedFields {
+  if (!fields_) return nil;
+  size_t count = CFDictionaryGetCount(fields_);
+  ssize_t keys[count];
+  GPBUnknownField *values[count];
+  CFDictionaryGetKeysAndValues(fields_, (const void **)keys,
+                               (const void **)values);
+  struct GPBFieldPair {
+    ssize_t key;
+    GPBUnknownField *value;
+  } pairs[count];
+  for (size_t i = 0; i < count; ++i) {
+    pairs[i].key = keys[i];
+    pairs[i].value = values[i];
+  };
+  qsort_b(pairs, count, sizeof(struct GPBFieldPair),
+          ^(const void *first, const void *second) {
+            const struct GPBFieldPair *a = first;
+            const struct GPBFieldPair *b = second;
+            return (a->key > b->key) ? 1 : ((a->key == b->key) ? 0 : -1);
+          });
+  for (size_t i = 0; i < count; ++i) {
+    values[i] = pairs[i].value;
+  };
+  return [NSArray arrayWithObjects:values count:count];
+}
+
+#pragma mark - Internal Methods
+
+- (void)writeToCodedOutputStream:(GPBCodedOutputStream *)output {
+  if (!fields_) return;
+  size_t count = CFDictionaryGetCount(fields_);
+  ssize_t keys[count];
+  GPBUnknownField *values[count];
+  CFDictionaryGetKeysAndValues(fields_, (const void **)keys,
+                               (const void **)values);
+  if (count > 1) {
+    struct GPBFieldPair {
+      ssize_t key;
+      GPBUnknownField *value;
+    } pairs[count];
+
+    for (size_t i = 0; i < count; ++i) {
+      pairs[i].key = keys[i];
+      pairs[i].value = values[i];
+    };
+    qsort_b(pairs, count, sizeof(struct GPBFieldPair),
+            ^(const void *first, const void *second) {
+              const struct GPBFieldPair *a = first;
+              const struct GPBFieldPair *b = second;
+              return (a->key > b->key) ? 1 : ((a->key == b->key) ? 0 : -1);
+            });
+    for (size_t i = 0; i < count; ++i) {
+      GPBUnknownField *value = pairs[i].value;
+      [value writeToOutput:output];
+    }
+  } else {
+    [values[0] writeToOutput:output];
+  }
+}
+
+- (NSString *)description {
+  NSMutableString *description = [NSMutableString
+      stringWithFormat:@"<%@ %p>: TextFormat: {\n", [self class], self];
+  NSString *textFormat = GPBTextFormatForUnknownFieldSet(self, @"  ");
+  [description appendString:textFormat];
+  [description appendString:@"}"];
+  return description;
+}
+
+static void GPBUnknownFieldSetSerializedSize(const void *key, const void *value,
+                                             void *context) {
+#pragma unused(key)
+  GPBUnknownField *field = value;
+  size_t *result = context;
+  *result += [field serializedSize];
+}
+
+- (size_t)serializedSize {
+  size_t result = 0;
+  if (fields_) {
+    CFDictionaryApplyFunction(fields_, GPBUnknownFieldSetSerializedSize,
+                              &result);
+  }
+  return result;
+}
+
+static void GPBUnknownFieldSetWriteAsMessageSetTo(const void *key,
+                                                  const void *value,
+                                                  void *context) {
+#pragma unused(key)
+  GPBUnknownField *field = value;
+  GPBCodedOutputStream *output = context;
+  [field writeAsMessageSetExtensionToOutput:output];
+}
+
+- (void)writeAsMessageSetTo:(GPBCodedOutputStream *)output {
+  if (fields_) {
+    CFDictionaryApplyFunction(fields_, GPBUnknownFieldSetWriteAsMessageSetTo,
+                              output);
+  }
+}
+
+static void GPBUnknownFieldSetSerializedSizeAsMessageSet(const void *key,
+                                                         const void *value,
+                                                         void *context) {
+#pragma unused(key)
+  GPBUnknownField *field = value;
+  size_t *result = context;
+  *result += [field serializedSizeAsMessageSetExtension];
+}
+
+- (size_t)serializedSizeAsMessageSet {
+  size_t result = 0;
+  if (fields_) {
+    CFDictionaryApplyFunction(
+        fields_, GPBUnknownFieldSetSerializedSizeAsMessageSet, &result);
+  }
+  return result;
+}
+
+- (NSData *)data {
+  NSMutableData *data = [NSMutableData dataWithLength:self.serializedSize];
+  GPBCodedOutputStream *output =
+      [[GPBCodedOutputStream alloc] initWithData:data];
+  [self writeToCodedOutputStream:output];
+  [output release];
+  return data;
+}
+
++ (BOOL)isFieldTag:(int32_t)tag {
+  return GPBWireFormatGetTagWireType(tag) != GPBWireFormatEndGroup;
+}
+
+- (void)addField:(GPBUnknownField *)field {
+  int32_t number = [field number];
+  checkNumber(number);
+  if (!fields_) {
+    CFDictionaryKeyCallBacks keyCallBacks = {
+        // See description above for reason for using custom dictionary.
+        0, GPBUnknownFieldSetKeyRetain, GPBUnknownFieldSetKeyRelease,
+        GPBUnknownFieldSetCopyKeyDescription, GPBUnknownFieldSetKeyEqual,
+        GPBUnknownFieldSetKeyHash,
+    };
+    fields_ = CFDictionaryCreateMutable(kCFAllocatorDefault, 0, &keyCallBacks,
+                                        &kCFTypeDictionaryValueCallBacks);
+  }
+  ssize_t key = number;
+  CFDictionarySetValue(fields_, (const void *)key, field);
+}
+
+- (GPBUnknownField *)mutableFieldForNumber:(int32_t)number create:(BOOL)create {
+  ssize_t key = number;
+  GPBUnknownField *existing =
+      fields_ ? CFDictionaryGetValue(fields_, (const void *)key) : nil;
+  if (!existing && create) {
+    existing = [[GPBUnknownField alloc] initWithNumber:number];
+    // This retains existing.
+    [self addField:existing];
+    [existing release];
+  }
+  return existing;
+}
+
+static void GPBUnknownFieldSetMergeUnknownFields(const void *key,
+                                                 const void *value,
+                                                 void *context) {
+#pragma unused(key)
+  GPBUnknownField *field = value;
+  GPBUnknownFieldSet *self = context;
+
+  int32_t number = [field number];
+  checkNumber(number);
+  GPBUnknownField *oldField = [self mutableFieldForNumber:number create:NO];
+  if (oldField) {
+    [oldField mergeFromField:field];
+  } else {
+    // Merge only comes from GPBMessage's mergeFrom:, so it means we are on
+    // mutable message and are an mutable instance, so make sure we need
+    // mutable fields.
+    GPBUnknownField *fieldCopy = [field copy];
+    [self addField:fieldCopy];
+    [fieldCopy release];
+  }
+}
+
+- (void)mergeUnknownFields:(GPBUnknownFieldSet *)other {
+  if (other && other->fields_) {
+    CFDictionaryApplyFunction(other->fields_,
+                              GPBUnknownFieldSetMergeUnknownFields, self);
+  }
+}
+
+- (void)mergeFromData:(NSData *)data {
+  GPBCodedInputStream *input = [[GPBCodedInputStream alloc] initWithData:data];
+  [self mergeFromCodedInputStream:input];
+  [input checkLastTagWas:0];
+  [input release];
+}
+
+- (void)mergeVarintField:(int32_t)number value:(int32_t)value {
+  checkNumber(number);
+  [[self mutableFieldForNumber:number create:YES] addVarint:value];
+}
+
+- (BOOL)mergeFieldFrom:(int32_t)tag input:(GPBCodedInputStream *)input {
+  int32_t number = GPBWireFormatGetTagFieldNumber(tag);
+  GPBCodedInputStreamState *state = &input->state_;
+  switch (GPBWireFormatGetTagWireType(tag)) {
+    case GPBWireFormatVarint: {
+      GPBUnknownField *field = [self mutableFieldForNumber:number create:YES];
+      [field addVarint:GPBCodedInputStreamReadInt64(state)];
+      return YES;
+    }
+    case GPBWireFormatFixed64: {
+      GPBUnknownField *field = [self mutableFieldForNumber:number create:YES];
+      [field addFixed64:GPBCodedInputStreamReadFixed64(state)];
+      return YES;
+    }
+    case GPBWireFormatLengthDelimited: {
+      NSData *data = GPBCodedInputStreamReadRetainedBytes(state);
+      GPBUnknownField *field = [self mutableFieldForNumber:number create:YES];
+      [field addLengthDelimited:data];
+      [data release];
+      return YES;
+    }
+    case GPBWireFormatStartGroup: {
+      GPBUnknownFieldSet *unknownFieldSet = [[GPBUnknownFieldSet alloc] init];
+      [input readUnknownGroup:number message:unknownFieldSet];
+      GPBUnknownField *field = [self mutableFieldForNumber:number create:YES];
+      [field addGroup:unknownFieldSet];
+      [unknownFieldSet release];
+      return YES;
+    }
+    case GPBWireFormatEndGroup:
+      return NO;
+    case GPBWireFormatFixed32: {
+      GPBUnknownField *field = [self mutableFieldForNumber:number create:YES];
+      [field addFixed32:GPBCodedInputStreamReadFixed32(state)];
+      return YES;
+    }
+  }
+}
+
+- (void)mergeMessageSetMessage:(int32_t)number data:(NSData *)messageData {
+  [[self mutableFieldForNumber:number create:YES]
+      addLengthDelimited:messageData];
+}
+
+- (void)addUnknownMapEntry:(int32_t)fieldNum value:(NSData *)data {
+  GPBUnknownField *field = [self mutableFieldForNumber:fieldNum create:YES];
+  [field addLengthDelimited:data];
+}
+
+- (void)mergeFromCodedInputStream:(GPBCodedInputStream *)input {
+  while (YES) {
+    int32_t tag = GPBCodedInputStreamReadTag(&input->state_);
+    if (tag == 0 || ![self mergeFieldFrom:tag input:input]) {
+      break;
+    }
+  }
+}
+
+- (void)getTags:(int32_t *)tags {
+  if (!fields_) return;
+  size_t count = CFDictionaryGetCount(fields_);
+  ssize_t keys[count];
+  CFDictionaryGetKeysAndValues(fields_, (const void **)keys, NULL);
+  for (size_t i = 0; i < count; ++i) {
+    tags[i] = (int32_t)keys[i];
+  }
+}
+
+@end