Lecture 8 Modeling & Simulation of Communication Networks.

Lecture 8

Modeling & Simulation of Communication Networks

Layered Tasks

Sender, Receiver, and Carrier

Hierarchy

Services

Figure 2.1 Sending a letter

Internet Model

Peer-to-Peer Processes

Functions of Layers

Summary of Layers

Internet layers

Figure 2.4 An exchange using the Internet model

Figure 2.5 Physical layer

The physical layer is responsible for transmitting individual bits from one

node to the next.

Note:

Figure 2.6 Data link layer

The data link layer is responsible for transmitting frames from

one node to the next.

Note:

Figure 2.7 Node-to-node delivery

Example 1

In Figure 2.8 a node with physical address 10 sends a frame to a node with physical address 87. The two nodes are connected by a link. At the data link level this frame contains physical addresses in the header. These are the only addresses needed. The rest of the header contains other information needed at this level. The trailer usually contains extra bits needed for error detection

Figure 2.8 Example 1

Figure 2.9 Network layer

The network layer is responsible for the delivery of packets from the original

source to the final destination.

Note:

Figure 2.10 Source-to-destination delivery

Example 2

In Figure 2.11 we want to send data from a node with network address A and physical address 10, located on one LAN, to a node with a network address P and physical address 95, located on another LAN. Because the two devices are located on different networks, we cannot use physical addresses only; the physical addresses only have local jurisdiction. What we need here are universal addresses that can pass through the LAN boundaries. The network (logical) addresses have this characteristic.

Figure 2.11 Example 2

Figure 2.12 Transport layer

The transport layer is responsible for delivery of a message from one process

to another.

Note:

Figure 2.12 Reliable process-to-process delivery of a message

Example 3

Figure 2.14 shows an example of transport layer communication. Data coming from the upper layers have port addresses j and k (j is the address of the sending process, and k is the address of the receiving process). Since the data size is larger than the network layer can handle, the data are split into two packets, each packet retaining the port addresses (j and k). Then in the network layer, network addresses (A and P) are added to each packet.

Figure 2.14 Example 3

Figure 2.15 Application layer

The application layer is responsible for providing services to the user.

Note:

Figure 2.16 Summary of duties

Link Layer

5-27

Institutional network

to externalnetwork

router

IP subnet

mail server

web server

Link Layer5-28

Synthesis: a day in the life of a web request

journey down protocol stack complete! application, transport, network, link

putting-it-all-together: synthesis! goal: identify, review, understand protocols

(at all layers) involved in seemingly simple scenario: requesting www page

scenario: student attaches laptop to campus network, requests/receives www.google.com

A day in the life: scenario

Comcast network 68.80.0.0/13

Google’s network 64.233.160.0/19 64.233.169.105

web server

DNS server

school network 68.80.2.0/24

web page

browser

router(runs DHCP)

A day in the life… connecting to the Internet

connecting laptop needs to get its own IP address, addr of first-hop router, addr of DNS server: use DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP, encapsulated in IP, encapsulated in 802.3 Ethernet

Ethernet frame broadcast (dest: FFFFFFFFFFFF) on LAN, received at router running DHCP server Ethernet demuxed to IP demuxed, UDP demuxed to DHCP

router(runs DHCP)

DHCP server formulates DHCP ACK containing client’s IP address, IP address of first-hop router for client, name & IP address of DNS server

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

encapsulation at DHCP server, frame forwarded (switch learning) through LAN, demultiplexing at client

Client now has IP address, knows name & addr of DNS server, IP address of its first-hop router

DHCP client receives DHCP ACK reply

A day in the life… connecting to the Internet

router(runs DHCP)

A day in the life… ARP (before DNS, before HTTP)

before sending HTTP request, need IP address of www.google.com: DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS query created, encapsulated in UDP, encapsulated in IP, encapsulated in Eth. To send frame to router, need MAC address of router interface: ARP

ARP query broadcast, received by router, which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router, so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

router(runs DHCP)

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

IP datagram forwarded from campus network into comcast network, routed (tables created by RIP, OSPF, IS-IS and/or BGP routing protocols) to DNS server

demux’ed to DNS server

DNS server replies to client with IP address of www.google.com

Comcast network 68.80.0.0/13

DNS server

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the life… using DNS

router(runs DHCP)

A day in the life…TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

to send HTTP request, client first opens TCP socket to web server

TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

TCP connection established!

64.233.169.105

web server

SYN

SYN

SYN

SYN

TCPIP

EthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK web server responds with TCP SYNACK (step 2 in 3-way handshake)

router(runs DHCP)

Link Layer5-35

A day in the life… HTTP request/reply

HTTPTCPIP

EthPhy

HTTP

HTTP request sent into TCP socket

IP datagram containing HTTP request routed to www.google.com

IP datagram containing HTTP reply routed back to client

64.233.169.105

web server

HTTPTCPIP

EthPhy

web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

web page finally (!!!) displayed

36

Questions