DNS – Domain Name System
- humans prefer host names instead of IP addresses
o www.yahoo.com à 204.71.200.68
- DNS does this mapping
- Pre DNS there was a file, /etc/hosts, with the host-to-IP mapping
- This file was maintained manually. It was ok to do it this way when internet was small, but impractical with 1000’s of computers.
- DNS uses a hirearchy of servers that handle the domains of the name. (Each domain of the name can correspond to a different name server.)
o Hostnames are divided in to domains
o EX: ector.cs.purdue.edu
§ edu – domain 0
§ purdue – domain 1
§ cs -- domain 2
§ ector -- host name
- One registers a desired name with a central authority and it is placed in a top level domain.
- Within the “organization” (pets.com) you can:
o Subdivide the domain
§ dog.pets.com
§ cat.pets.com
o arbitrary levels are allowed
o these arbitrary levels are not standardized
o these arbitrary levels are controlled by the local organization
- Name resolution is hirearchical: (DNS Server)
(edu)
/ | \
* * (purdue) * == some other universities for example
/ | \
(cc) (ecn) (cs)
“/”
“(xinu)”
- Domain servers correspond to naming hirearchy
- Individual groups witihn an organization can choose their own hirearchy
DNS Name Server Resolution (lookup)
- DNS uses client/server paradigm
- Client known as “resolver”
- Multiple DNS servers maybe used during a resolution
- Each DNS server corresponds to a contiguous part of the naming hirearchy
- All domain servers ore linked together and they know the IP address of their neighbors.
- DNS lookup can be done using either TCP or UDP
- An application that needs to get a IP ddress from a host name will become a client (resolver).
- There are 2 kinds of resolutions;
1. RECURSIVE
§ if the client chooses recursive resolution, the host asks the immediate DNS server to resolve the hostname.
§ If the immediate DNS server does not have an answer (can’t find the host on its list), this DNS server asks its parent
§ This goes ‘up’ the hirearchy until a DNS server has an answer
o 2. ITERATIVE
§ Green – Do You know who “UCB” is
§ Red – No, but I know a DNS server that might
§ Blue – Yes I know “UCB”
§ if the host asks the immediate DNS server and it cannot answer it will return the address of a DNS server that may know the answer, and so on.
- To reduce DNS traffic, the name to IP address mappings are cached
- The time they are cached depends on the entry and this info is stored in the DNS servers
Nslookup
- A DNS client useful for debugging
- A DNS query may return more than one IP address if the host has multiple interfaces
- Also you specify the type of host in the DNS query:
o Type:
§ any
§ mx, mail exchange
§ etc.
- Example of nslookup use
o > /usr/bin/nslookup
o > set querytype=A (any host)
o > yahoo.com
o Name: yahoo.com
o Address: 216.115.108.245 216.115.108.243
o > set querytype=MX (mail exchange)
o > yahoo.com
o Name: yahoo.com
o mail exchanger=mx2_mail.yahoo.com (might not be ‘2’)
o > set querytype=CNAME (the DNS server)
o > yahoo.com
o ns0.corp.yahoo.com
- DNS cacheing is important otherwise the internet would be flooded with DNS requests
- announcement, professor is to be 15+ min. late
The Evolution of Programming Network Applications
- Sockets is the ‘hard way’ to do network programming
- Sockets:
o Equivalent to programming in assembly language
o You have to take care of every single byte that goes through the network
o With sockets your program uses the network channel as if it where writing to a file or pipe
- RPC – remote procedure calls
o A server exports procedures to the clients
o The client server interaction if modeled as a procedure call
o The programmer writes in a special language called IDL (interface definition language) the header definitions of the procedures that the server will export
o This interface description is preprocessed by a special IDL pre-processor and generates code for the client and server that does the entire communication
o The programmer doen’t need to write any networking code. To program the server it will add code to code already generated, to implement the procedures
o The client will use the prcedures in the server as if they where local
Interface in IDL
void addStudent(string name,
string major,
int age);
int getStudent(string name,
string &major,
int &age);
[Interface in IDL] à preprocessor à server stubs (in C, for example)
client stubs (in C, for example)
Client Server
[client stubs]----------------[server stubs]
| (network) |
| addStudent(string name,
| string major,
| int age){
| ……
| }// implementation in server
addStudent(“jim”, “CS”, 21);// client programmer just invokes a procedure
o The Client Stubs:
§ provide a function with the name of the Remote Procedure Call
§ they put the arguments into a network packet(s). This is called serialization, or marshalling. The format of the data in the network can be
· XDR – External Data Representaion. It was created by SUN to represent data in the network
· XML – textual representation of data that may also include semantics
§ The client sends the data arguments to the serverand waits for the answer
§ When the answer comes back from the server, it transilates back the results from the network representation to the machine representation. This process is called deserialization or unmarshalling
§ Then the results are returned to the caller
§ This process is transparent to the program
o The Server Stubs:
§ Wait for an RPC invocation
§ It deserializes the arguments
§ It invikes the procedure indicated by the client
§ It serializes the results and sends the results back to the client
o The programmer doesn’t need to lnow about sockets
- Implementations of RPC:
o Sun RPC
§ Used to implement NFS
§ mountd
§ rusers
Evolution of Programming Networked Apps
- Networking Tools
o Sockets
o RPCs (Remote Procedure Calls)
o RMI (Remote Method/Object Invocation)
- Programming Langauge equivilents to Networking tools
o Assembly Language – like Network Sockets
o Procedural Languages (C, Pascal, etc) – like RPC
o Object Oriented Languages (JAVA, C++, etc) – Like RMI
- RMI (Remote Method/Object Invocation)
o The client server interaction is modeled as a object method invocation.
o The stub or proxy is a local representation of the remote object
o The proxy forwards the call to the remote object
o The client program interacts with the remote object as if it where local
o The proxy code and the code in the server that exports the remote object is generated by a special preprocessor from the description on a interface description language
o RMI Implementations
§ Name Language Generates
§
CORBA
IDL C++
(common Object Request Broker)
§ Java RMI JAVA JAVA
§
DCOM IDL C++, C#, VB
(Microsoft Distributed Common
Object)
- Summary 2nd half of semester
o Protocols/layering
§ ISO-7 layer reference model
§ Internet Reference Model
o Internet
§ What is the Internet?
§ Why the internet is necessary
§ Routers and Hosts
§ IP (Internet Protocol)
· IP addresses (4 bytes in length)
o Network/Host portions of the address
· Classes of IP addresses (A, B, C, D, E)
· Dotted decimal notation
· Special addresses (all 0’s, all 1’s, loopback, etc.)
· IP packet format
o All fields *TTL, SRC, DST
· IP Routing
· IP routing table and default router
· IP semantics
o IP is connectionless
o IP packets can be delayed, come out of order, or even be lost
o Best Effort Delivery
o ARP (Address Resolution Protocol)
§ Transilates IP addresses to local hardware addresses
§ ARP look-up algorithm
§ ARP cache machanics
- IP fragmentation and reassembly
o Why fragmentation is necessary
o MTU – maximum transfer unit
o How Fragmentation works
o Fields
used for reassembly (source
o Reassembly timeout
- ICMP (Internet Control Message Protocol)
o ICMP types:
§ Soirce quench, time exceeded, destination unreachable, echo reply (ping)
o Uses (applications discussed):
§ Traceroute, path MTU discovery
- Transport protocols (UDP, TCP)
o Characteristics
o Advantages/disadvantages of both
- TCP
o Characteristics:
§ Reliable
§ Connection oriented (app-2-app)
§ Full duplex
§ How the reliability is achieved
· Retransmission
· ACKnowledge (positive ACK when packet received)
o Features:*
§ Adaptive retransmission
§ Flow control (window size)
· Slow down the sender if buffer space is not available)
§ Cumulative ACK
§ Congestion Control
· Slow Start
§ Reliable connection start/shut-down
· 3-way handshake
§ TCP header fields *
- DNS (domain name service)
o Transilates host names to IP addresses
o Domain Names
o Domain Name Server hirearchy
o DNS lookup (resolution)
§ Recursive
§ Iterative
o DNS caching
o DNS types
- Evelution of Netwrok Programming
o Sockets
o RPC
o RMI
-
Final Exam Fri. Dec. 19
o Cumulative
o 1st half of semester 20%
o 2nd half of semester 80%
o No specific programming projects,
§ Concepts, “Why do you use JitterBuffer?”, “What are notifications for?”
§ Also study some socket programming (maybe?)