diff --git a/ChangeLog.md b/ChangeLog.md
--- a/ChangeLog.md
+++ b/ChangeLog.md
@@ -1,5 +1,10 @@
 # ChangeLog
 
+## 0.2.19
+
+* Compare the minimum packet size on packet size determination.
+  [#81](https://github.com/kazu-yamamoto/quic/pull/81)
+
 ## 0.2.18
 
 * Don't print debug information on stdout as a daemon has already
diff --git a/Network/QUIC/Client/Run.hs b/Network/QUIC/Client/Run.hs
--- a/Network/QUIC/Client/Run.hs
+++ b/Network/QUIC/Client/Run.hs
@@ -158,8 +158,14 @@
     let ver = chosenVersion verInfo
     initializeCoder conn InitialLevel $ initialSecrets ver peerCID
     setupCryptoStreams conn -- fixme: cleanup
-    let pktSiz0 = fromMaybe 0 ccPacketSize
-        pktSiz = (defaultPacketSize peersa `max` pktSiz0) `min` maximumPacketSize peersa
+      -- RFC9000 \S14.2
+      -- "In the absence of these mechanisms, QUIC endpoints SHOULD
+      -- NOT send datagrams larger than the smallest allowed maximum
+      -- datagram size."
+      --
+      -- Thus use 1200 bytes for minimum packet size.
+    let pktSiz0 = fromMaybe (defaultPacketSize peersa) ccPacketSize
+        pktSiz = (defaultQUICPacketSize `max` pktSiz0) `min` maximumPacketSize peersa
     setMaxPacketSize conn pktSiz
     setInitialCongestionWindow (connLDCC conn) pktSiz
     setAddressValidated pathInfo
diff --git a/Network/QUIC/Config.hs b/Network/QUIC/Config.hs
--- a/Network/QUIC/Config.hs
+++ b/Network/QUIC/Config.hs
@@ -4,6 +4,7 @@
 module Network.QUIC.Config where
 
 import Data.IP
+import Data.X509.CertificateStore (CertificateStore)
 import Data.X509.Validation (validateDefault)
 import Network.Socket
 import Network.TLS hiding (
@@ -88,6 +89,7 @@
     --
     -- Default: 'True'
     , ccOnServerCertificate :: OnServerCertificate
+    , ccCAStore :: CertificateStore
     , ccResumption :: ResumptionInfo
     -- ^ Use resumption on the 2nd connection if possible.
     , ccPacketSize :: Maybe Int
@@ -134,6 +136,7 @@
         , ccALPN = \_ -> return Nothing
         , ccValidate = True
         , ccOnServerCertificate = validateDefault
+        , ccCAStore = mempty
         , ccResumption = defaultResumptionInfo
         , ccPacketSize = Nothing
         , ccDebugLog = False
diff --git a/Network/QUIC/Server/Run.hs b/Network/QUIC/Server/Run.hs
--- a/Network/QUIC/Server/Run.hs
+++ b/Network/QUIC/Server/Run.hs
@@ -181,8 +181,14 @@
     initializeCoder conn InitialLevel $ initialSecrets ver cid
     setupCryptoStreams conn -- fixme: cleanup
     let peersa = accPeerSockAddr
+        -- RFC9000 \S14.2
+        -- "In the absence of these mechanisms, QUIC endpoints SHOULD
+        -- NOT send datagrams larger than the smallest allowed maximum
+        -- datagram size."
+        --
+        -- Thus use 1200 bytes for minimum packet size.
         pktSiz =
-            (defaultPacketSize peersa `max` accPacketSize)
+            (defaultQUICPacketSize `max` accPacketSize)
                 `min` maximumPacketSize peersa
     setMaxPacketSize conn pktSiz
     setInitialCongestionWindow (connLDCC conn) pktSiz
diff --git a/Network/QUIC/TLS.hs b/Network/QUIC/TLS.hs
--- a/Network/QUIC/TLS.hs
+++ b/Network/QUIC/TLS.hs
@@ -6,6 +6,7 @@
     serverHandshaker,
 ) where
 
+import Data.X509.CertificateStore (isEmptyCertificateStore)
 import Network.TLS hiding (Version, defaultSupported)
 import Network.TLS.QUIC
 import System.X509
@@ -27,7 +28,13 @@
     -> Bool
     -> IO ()
 clientHandshaker callbacks ClientConfig{..} ver myAuthCIDs establish use0RTT = do
-    caStore <- if ccValidate then getSystemCertificateStore else return mempty
+    caStore <-
+        if ccValidate
+            then
+                if isEmptyCertificateStore ccCAStore
+                    then getSystemCertificateStore
+                    else return ccCAStore
+            else return mempty
     tlsQUICClient (cparams caStore) callbacks
   where
     sni = fromMaybe ccServerName ccServerNameOverride
diff --git a/quic.cabal b/quic.cabal
--- a/quic.cabal
+++ b/quic.cabal
@@ -1,6 +1,6 @@
 cabal-version:      >=1.10
 name:               quic
-version:            0.2.18
+version:            0.2.19
 license:            BSD3
 license-file:       LICENSE
 maintainer:         kazu@iij.ad.jp
@@ -137,6 +137,7 @@
         crypto-token >=0.1.2 && <0.2,
         crypton >=0.34,
         crypton-x509 >=1.7.6 && <1.8,
+        crypton-x509-store >=1.6.11 && <1.7,
         crypton-x509-system >=1.6.7 && <1.7,
         crypton-x509-validation >=1.6 && <1.7,
         fast-logger >=3.2.2 && <3.3,