From f8f636805992524224aea362539326bc628dc8c3 Mon Sep 17 00:00:00 2001
From: Jet Hughes <jethughes0@gmail.com>
Date: Mon, 17 Oct 2022 12:28:03 +1300
Subject: [PATCH] vault backup: 2022-10-17 12:28:03

---
 content/notes/17-app-layer.md       | 41 ++++++++++++++++++++++++++++-
 content/notes/18-transport-layer.md |  9 +++++++
 content/notes/cosc-203.md           |  2 +-
 3 files changed, 50 insertions(+), 2 deletions(-)
 create mode 100644 content/notes/18-transport-layer.md

diff --git a/content/notes/17-app-layer.md b/content/notes/17-app-layer.md
index 8c75e7f67..23ddaf61e 100644
--- a/content/notes/17-app-layer.md
+++ b/content/notes/17-app-layer.md
@@ -4,6 +4,9 @@ aliases:
 tags: 
 - cosc203
 - lecture
+sr-due: 2022-10-20
+sr-interval: 3
+sr-ease: 250
 ---
 
 # Client-server vs peer to peer
@@ -132,7 +135,43 @@ three components
 ## DNS
 - ip adresses are hard to remember
 - need to map betwen host name and ip address
-- 
+- hierarchical, domain-based naming scheme implemented using a distributed databse system
+- core internet function, but implemented as an application layer protocol 
+
+- tree structure ![](https://i.imgur.com/TtiMt9U.png)
+- domain name space
+	- each node has a label : the root label is a null string
+	- each node has a domain name: a sequence of labels separated by dots, reading from the node up to the root.
+	- ![](https://i.imgur.com/TvHwuPK.png)
+- name servers
+	- name space is divided into non-ovelapping zones
+	- each zone had name servers to hold information
+	- top level dns servers are responsible for domains such as .com .org .net and country domains
+	- authoritative DNS servers: maintained by organization or service providers
+	- ![](https://i.imgur.com/AU5F2Mv.png)
+
+DNS caching
+- once (any) name server learns mapping, it caches mapping, and immediately returns a cached mapping in response to a query 
+	- caching improves response time 
+	- cache entries timeout (disappear) after some time (TTL) 
+	- top-level domain names typically cached in local name servers
+- cached entries may be out-of-date 
+	- if named host changes IP address, may not be known Internet-wide until all TTLs expire! 
+	- best-effort name-to-address translation!
+
+DNS name resolution: recursive query
+- e.g., host at cs.otago.ac.nz wants IP address for ai.cs.waikato.ac.nz
+-  ![](https://i.imgur.com/1jW21AI.png)
 
 
 # Socket programming
+- socket
+	- software component that has
+		- local/remote socket address: IP + port no.
+		- transport layer protocol: TCP or UDP
+- socket api
+	- application programming interface usually provided by the operating system
+	- ![](https://i.imgur.com/ORLYEVH.png)
+
+![TCP socket](https://i.imgur.com/JIOQOGM.png)
+![UDP socket](https://i.imgur.com/SATuwYL.png)
diff --git a/content/notes/18-transport-layer.md b/content/notes/18-transport-layer.md
new file mode 100644
index 000000000..f7933ac4d
--- /dev/null
+++ b/content/notes/18-transport-layer.md
@@ -0,0 +1,9 @@
+---
+title: "18-transport-layer"
+aliases: 
+tags: 
+- cosc203
+- lecture
+---
+
+# 
\ No newline at end of file
diff --git a/content/notes/cosc-203.md b/content/notes/cosc-203.md
index 0537fb664..7a521acb9 100644
--- a/content/notes/cosc-203.md
+++ b/content/notes/cosc-203.md
@@ -46,7 +46,7 @@ tags:
 - [15-basic sql]
 - [16-networks](notes/16-networks)
 - [17-application layer](notes/17-app-layer)
-- [18-transport layer]
+- [18-transport layer](notes/18-transport-layer)
 - [19-network layer data plane]
 - [20-network layer control plane]
 - [21-data link layer and LAN]