diff --git a/content/notes/18-transport-layer.md b/content/notes/18-transport-layer.md
index f7933ac4d..b367ee892 100644
--- a/content/notes/18-transport-layer.md
+++ b/content/notes/18-transport-layer.md
@@ -5,5 +5,68 @@ tags:
 - cosc203
 - lecture
 ---
+chapter 3
+# service provided by transport layer
+services
+- process to process communication
+	- logical communication between application processes
+- multi and demulti plexing
+	- communication for multiple applications of the same host
+- flow control
+	- ensure production reate <= consuption rate
+- congestion control
+	- too many sender, sending too fast
 
-# 
\ No newline at end of file
+protocols
+- TCP transmission control protocol
+- UDP user datagram protocol
+
+## actions and sender
+- is passed an app layer lessage
+- determines segment header fields values
+- creates segment
+- passes segement to IP
+
+## actions at reciever
+- recieves segment from IP
+- checks header values
+- extract app layer message
+- de multiplexes message up to application via socket
+
+# multiplexing and demultiplexing
+sender: handle data from multiple sockets, add transport header
+reciever: user header info to deliver segments to correct socket
+
+![](https://i.imgur.com/f2NqsKE.png)
+
+## how it works
+port number
+- 16 bit integer (0, 66635)
+- segment has a source and a port number
+- host uses IP and port to direct segment to appropriate socket
+
+IANA (internet assign number authority) has divded port number into three ranges
+- 0-1023 well known
+- 1024-49151 registered
+- 49152-65535 dynamic
+
+## principle of reliable data transfer
+![](https://i.imgur.com/ORzC8jm.png)
+
+- compexity of reliable data transfer protocol depends on character istics of unreliable channel
+- sender and reciever do not know the 'state' of each other. they need to communicate via a message - ACKknowledgement
+
+# TCP
+- point to point communicate
+	- one sender, one reciever
+- reliable, in-order byte stream
+	- no message boundaries
+	- each byte has a position in the stream
+- full duplex data
+	- bi directional data flow in the same connection
+	- MSS: maximum segment size
+- cumulative ACKs
+- connection-oriented
+	- handshaking (exchange of control messages) initializes se
+
+# UDP
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