diff --git a/generate_update_tarball.py b/generate_update_tarball.py
new file mode 100755
index 0000000..4a38b8b
--- /dev/null
+++ b/generate_update_tarball.py
@@ -0,0 +1,158 @@
+#!/usr/bin/python3
+
+import argparse
+import glob
+import os
+import re
+import shutil
+import subprocess
+import sys
+import tempfile
+
+from apt import cache
+from debian import changelog
+from debian import deb822
+from git import Repo
+
+VERSION_MAP = {
+    'any': 'armhf arm64',
+    'linux-any': 'armhf arm64',
+}
+
+ARCHES = [
+    'arm64',
+    'armhf',
+]
+
+def GetDebianDirectories(rootdir):
+    packages_dir = os.path.join(rootdir, 'packages')
+    if not os.path.exists(packages_dir):
+        print('No packages directory found!')
+        sys.exit(-1)
+    debian_dirs = []
+    for package in os.scandir(packages_dir):
+        debian_dir = os.path.join(package.path, 'debian')
+        if os.path.exists(debian_dir):
+            debian_dirs.append(debian_dir)
+    return debian_dirs
+
+def GeneratePackageList(directory):
+    package_name = os.path.split(os.path.split(directory)[0])[1]
+    package_tuples = []
+    with open(os.path.join(directory, 'changelog'), 'rb') as changelog_file, \
+         open(os.path.join(directory, 'control')) as control_file:
+        cl = changelog.Changelog(file=changelog_file)
+        version = str(cl.get_version())
+
+        packages = deb822.Packages.iter_paragraphs(control_file)
+        for p in packages:
+            if 'Package' in p and 'Architecture' in p:
+                arches = p['Architecture']
+                if arches in VERSION_MAP:
+                    arches = VERSION_MAP[arches]
+                arches = arches.split(' ')
+                for arch in arches:
+                    package_tuples.append((package_name, '%s:%s' % (p['Package'], arch), version))
+    return package_tuples
+
+def UpdateNeeded(package_name, package_version, apt_cache):
+    package_name = package_name
+    package = apt_cache.get(package_name)
+    if package:
+        return package_version > package.versions[0].version
+    else:
+        return True
+
+def GetSourceDirectory(package):
+    proc = subprocess.run(['make', package + '-source-directory'], stdout=subprocess.PIPE, universal_newlines=True)
+    proc.check_returncode()
+    for line in proc.stdout.split(os.linesep):
+        if line.startswith('Source directory: '):
+            return line.split(': ')[1]
+
+def CheckVersionTags(rootdir, package, version):
+    source_dir = os.path.join(rootdir, GetSourceDirectory(package))
+    debian_dir = os.path.join(rootdir, 'packages', package)
+    source_repo = Repo(source_dir)
+    debian_repo = Repo(debian_dir)
+    source_tags = source_repo.git.tag('-l')
+    debian_tags = debian_repo.git.tag('-l')
+    if version in source_tags and version in debian_tags:
+        print('Found tags for %s of %s. Checking out tags.' % (version, package))
+        source_repo.git.checkout('tags/' + version)
+        debian_repo.git.checkout('tags/' + version)
+        return True
+    else:
+        print('Did not find tags for %s of %s.' % (version, package))
+        return False
+
+
+def main():
+    parser = argparse.ArgumentParser(description='Find which packages are newer in the local repository than Apt')
+    parser.add_argument('-rootdir', type=str, required=True)
+    parser.add_argument('-sources_list', type=str, required=True)
+    parser.add_argument('-package_dir', type=str, required=True)
+    parser.add_argument('-output_tarball', type=str, required=True)
+    args = parser.parse_args()
+
+    # Find directories of Debian package data.
+    debian_directories = GetDebianDirectories(args.rootdir)
+    packages = []
+    for directory in debian_directories:
+        packages += GeneratePackageList(directory)
+
+    # Check the versions of packages in the local repository,
+    # and compare against the versions of packages in the apt cache.
+    # If a source version that is newer than upstream is present in
+    # the local source repository, check whether git tags exist for that version.
+    packages_to_update = dict()
+    debs_to_update = set()
+    with tempfile.TemporaryDirectory() as tempdir:
+        apt_dir = os.path.join(tempdir, 'etc', 'apt')
+        os.makedirs(apt_dir)
+        shutil.copyfile(args.sources_list, os.path.join(apt_dir, 'sources.list'))
+
+        apt_cache = cache.Cache(rootdir=tempdir, memonly=True)
+        apt_cache.update()
+        apt_cache.open()
+
+        for (package_group, package_name, version) in packages:
+            if UpdateNeeded(package_name, version, apt_cache):
+                if CheckVersionTags(args.rootdir, package_group, version):
+                    packages_to_update[package_group] = version
+                    debs_to_update.add(package_name)
+
+        apt_cache.close()
+
+    # Compile appropriately tagged packages for all arches.
+    for package in packages_to_update:
+        print('make ' + package + '...')
+        for arch in ARCHES:
+            proc = subprocess.run(["make", "USERSPACE_ARCH="+arch, package], stdout=sys.stdout, stderr=sys.stderr)
+            proc.check_returncode()
+
+    # Find the set of output files corresponding to the packages we are going to upload.
+    output_files = set()
+    for deb in debs_to_update:
+        for filename in glob.glob('%s/**/*%s*' % (args.package_dir, deb.split(':')[0])):
+            output_files.add(filename)
+
+    # Generate a tarball appropriate for uploading containing the new packages.
+    with tempfile.TemporaryDirectory() as tempdir:
+        bsp_dir = os.path.join(tempdir, 'packages', 'bsp')
+        core_dir = os.path.join(tempdir, 'packages', 'core')
+        os.makedirs(bsp_dir)
+        os.makedirs(core_dir)
+        for filename in output_files:
+            (path, deb_name) = os.path.split(filename)
+            (_, repository) = os.path.split(path)
+            if repository == 'bsp':
+                shutil.copy(filename, bsp_dir)
+            if repository == 'core':
+                shutil.copy(filename, core_dir)
+        tar_command = "tar -C %s --overwrite -czf %s packages" % (tempdir, args.output_tarball)
+        proc = subprocess.run(tar_command.split(' '), stdout=subprocess.PIPE, stderr=subprocess.PIPE)
+        proc.check_returncode()
+
+if __name__ == '__main__':
+    main()
diff --git a/kokoro/packages.sh b/kokoro/packages.sh
index 7f95673..719e2fc 100644
--- a/kokoro/packages.sh
+++ b/kokoro/packages.sh
@@ -21,11 +21,6 @@
 sudo apt-get install -y haveged
 sudo /etc/init.d/haveged start
 
-ARCHES="armhf arm64"
+m docker-upstream-delta
 
-for arch in ${ARCHES}
-do
-  USERSPACE_ARCH=${arch} m docker-packages-tarball
-done
-
-cp ${ROOTDIR}/cache/packages.tgz ${KOKORO_ARTIFACTS_DIR}
+cp ${ROOTDIR}/cache/update.tgz ${KOKORO_ARTIFACTS_DIR}/packages.tgz
diff --git a/mendel.list b/mendel.list
new file mode 100644
index 0000000..beb8cef
--- /dev/null
+++ b/mendel.list
@@ -0,0 +1,4 @@
+deb [arch=armhf,arm64 trusted=yes] https://packages.cloud.google.com/apt mendel-animal main
+deb-src [trusted=yes] https://packages.cloud.google.com/apt mendel-animal main
+deb [arch=armhf,arm64 trusted=yes] https://packages.cloud.google.com/apt mendel-bsp-enterprise-animal main
+deb-src [trusted=yes] https://packages.cloud.google.com/apt mendel-bsp-enterprise-animal main
diff --git a/packages.mk b/packages.mk
index 47f9b09..13ea082 100644
--- a/packages.mk
+++ b/packages.mk
@@ -102,7 +102,11 @@
 $(PRODUCT_OUT)/.$1-pbuilder-$(USERSPACE_ARCH): | out-dirs
 endif
 	touch $(PRODUCT_OUT)/.$1-pbuilder-$(USERSPACE_ARCH)
-.PHONY:: $1
+
+$1-source-directory:
+	echo "Source directory: $2"
+
+.PHONY:: $1 $1-source-directory
 endef
 
 # Convenience macro to target a package to the bsp repo
@@ -124,6 +128,10 @@
 	$(ROOTDIR)/build/update_packages.sh
 	tar -C $(PRODUCT_OUT) --overwrite -czf $@ packages
 
+upstream-delta: $(ROOTDIR)/cache/update.tgz
+$(ROOTDIR)/cache/update.tgz:
+	$(ROOTDIR)/build/generate_update_tarball.py -rootdir=$(ROOTDIR) -sources_list=$(ROOTDIR)/build/mendel.list -package_dir=$(PRODUCT_OUT)/packages -output_tarball=$(ROOTDIR)/cache/update.tgz
+
 packages:: $(ALL_PACKAGE_TARGETS)
 
-.PHONY:: packages pbuilder-base
+.PHONY:: packages pbuilder-base upstream-delta upstream-tarball
diff --git a/prereqs.mk b/prereqs.mk
index 4549000..f429017 100644
--- a/prereqs.mk
+++ b/prereqs.mk
@@ -47,6 +47,9 @@
 	python-minimal \
 	python2.7 \
 	python3 \
+	python3-apt \
+	python3-debian \
+	python3-git \
 	python3-setuptools \
 	qemu-user-static \
 	quilt \