intro 2 computer networks

Internal Copyright © 2005 Persistent Systems Pvt. Ltd.

Network EssentialsSession 1

R. Venkateswaran

2Internal Copyright © 2005 Persistent Systems Pvt. Ltd.2

Outline

Session 1 7-Layer OSI Model Network Layer protocols (Internet Protocol) Transport Layer protocols (TCP and UDP)

Session 2 Socket Programming – with focus on BSD Sockets

Sample codes in C that work on UNIX/Linux systems


Acknowledgments

This presentation has been adapted from presentations available at:

1. Prof. Shivkumar Kalyanaraman (http://www.ecse.rpi.edu/Homepages/shivkuma/)

2. Prof. Sneha Kumar Kasera (http://www.cs.utah.edu/classes/cs5480/)

3. Prof. David Hollinger (http://www.cs.rpi.edu/~hollingd/netprog)

4. South Asian Network Operators Group (http://ws.edu.isoc.org/workshops/2004/SANOG-IV/ip-services/presentations/ip-intro/ipbasics.ppt)


Network Models

Formal models allow us to deal with various aspects of networks abstractly One such model is the OSI reference model

The OSI reference model is a layered model Divide a task into pieces and then solve each piece

independently Establishing a well defined interface between layers

Major Advantages: Each layer can be implemented independently Adaptability Code Reuse Extensibility


OSI 7-Layer Model

IP

TCP/UDP Virtual End-to-end connectivity

Path selection, Internetworking

Error-free communication links

Transmission of raw signal

Mail, Web, etc.

1

3

2

4

5

6

7 Application

Presentation

Session

Transport

Network

Data Link

Physical

Ethernet

Data encryption,compression

Managing sessions

FuntionalityExamplesLayers


OSI 7-Layer

Application

Presentation

Session

Transport

Network

Link

Physical

Network

Link Link

Network

Link Link

PhysicalPhysical

Host Router Router Host

Application

Presentation

Session

Transport

Network

Link

End to

end

Hop by

hop


TCP/IP Model

Host Router Router

End to

end

No session or presentation layers in TCP/IP model

Host

Hop by

hop

Application

Transport

Network

Link

Physical

Network

Link Link

Network

Link Link

PhysicalPhysical

Application

Transport

Network

Link


TrailerHeader

Header

Header

Packet structure

Application

Transport

Network

Data Link

Data

Transport Layer Data

Network Layer Data

Link Layer Data


Network Layer


Internet Architecture

Packet-switched, connectionless datagram network

IP is the network layer protocol Acts as a glue

Hourglass concept all hosts and routers run IP

Stateless architecture no per flow state inside the

network Hop-by-hop packet forwarding

Header contains all the information


IP - Minimalist Approach

Dumb network Connectivity is the key Network provides minimal functionalities to support

connectivity Addressing, forwarding, routing

Smart end systems Transport layer or application performs more

sophisticated functionalities Flow control, error control, congestion control

Advantages High scalability Works across heterogeneous technologies (Ethernet,

modem, satellite, wireless) Supports diverse applications (telnet, ftp, Web, media

streaming) Decentralized network administration


IPv4 Header

IHL Type of Service Total LengthVersion

Fragment OffsetIdentification Flags

Time to Live Protocol Header Checksum

Source Address

Destination Address

Transport Layer Data…

PaddingOptions

0 4 8 16 32


IP Address

IP address: Unique identification of the end-system from a network-layer perspective

IP address is 32-bits long (version 4) Contains a network ID and host ID

Use subnet mask to detect the network ID

Example of IP address: 133.27.162.125

133 27 162 125

10000101 00011011 10100010 01111101

85 1B A2 7D

Decimal

Binary

HEX

NetID Host ID

Boundary


Network Mask

Define which bits are used to describe the network ID

Different Representations: Decimal dot notation: 255.255.224.0 Number of network bits: /19

Bitwise-AND of 32-bit IP address with 32-bit netmask yields network ID part of the address (truncated appropriately)


Network Mask Examples

137.158.128.0/17 (netmask 255.255.128.0)

1000 1001 1001 1110 1 000 0000 0000 0000

1111 1111 1111 1111 1 000 0000 0000 0000

1100 0110 1000 0110 0000 0000 0000 0000

1111 1111 1111 1111 0000 0000 0000 0000

1100 1101 0010 0101 1100 0001 10 00 0000

1111 1111 1111 1111 1111 1111 11 00 0000

198.134.0.0/16 (netmask 255.255.0.0)

205.37.193.128/26 (netmask 255.255.255.192)


Subnets

All device interfaces having the same network ID are part of the same subnet

Devices within a subnet can communicate with each other without an intervening router

192.1.1.2

192.1.1.1

192.1.1.3

192.1.1.4 192.1.2.9

192.1.2.2

192.1.2.1

192.1.3.2192.1.3.1

192.1.3.27

Network consisting of 3 subnets


IP router

A device with more than one link-layer interface Each interface identified by a different IP address (from

different subnets)

Packets arriving at one interface are forwarded out on another interface to get them closer to the destination

Creates and maintains forwarding tables Tables help in making forwarding decisions Tables created and updated based on routing

information exchanged between routers Each router maintains its own forwarding table


Hop by Hop Forwarding


IP Forwarding Rules - I

Destination is in the same subnet (direct connectivity) Recognize that destination IP address is on same subnet Find the destination’s datalink-layer address IP packet encapsulated and sent directly to the

destination’s datalink-layer address

Destination is in a different subnet (indirect connectivity) Recognize that destination IP address is on different subnet Look up destination IP address in a (L3 forwarding) table

to find a match, called the next hop router IP address Find the next hop router’s datalink-layer address IP packet encapsulated and sent directly to the next hop

router’s datalink-layer address


IP Forwarding Rules - II

Problem 1: Recognize if destination is on the same subnet Use netmask to compute network ID of the destination and

match it with device’s network ID

Problem 2: Find a device’s datalink-layer address Static mapping Dynamic mapping using Address Resolution Protocol (ARP)

Sender host broadcasts a request: “What is the Ethernet address of 192.1.1.4?”

The device whose IP address is 192.1.1.4 replies back: “The Ethernet address for 192.1.1.4 is 00-0C-F1-4E-2A-E2”

ARP responses are cached at the senderUse arp command to view/modify the cache


Look up Forwarding Table

Destination =12.5.9.16------------------------------- payload

Prefix Interface Next Hop

12.0.0.0/8 10.14.22.19 eth1

12.4.0.0/15

12.5.9.0/24 attached

eth2

Serial 1/0/7

10.1.3.77

IP Forwarding Table

0.0.0.0/0 10.14.11.33 eth0

even better

OK

better

best!

Longest Prefix Match (Classless) Forwarding


IP Forwarding – Example 1

Forwarding Table on host 192.1.1.1

Note: 127.0.0.1 is the special address of the local interface

192.1.1.2

192.1.1.1

192.1.1.3

192.1.1.4

192.1.3.1

192.1.3.27

Network Destination Netmask Next Hop Metric 0.0.0.0 0.0.0.0 192.1.1.4 20

192.1.1.0 255.255.255.0 192.1.1.1 20 192.1.1.1 255.255.255.255 127.0.0.1 20 192.1.1.255 255.255.255.255 192.1.1.1 20

127.0.0.0 255.0.0.0 127.0.0.1 1

192.1.3.2





192.1.1.2

192.1.1.1

192.1.1.3

192.1.1.4

192.1.3.1

192.1.3.27


192.1.1.0 255.255.255.0 192.1.1.1 20 192.1.1.1 255.255.255.255 127.0.0.1 20 192.1.1.255 255.255.255.255 192.1.1.1 20

127.0.0.0 255.0.0.0 127.0.0.1 1

192.1.3.2





192.1.1.2

192.1.1.1

192.1.1.3

192.1.1.4

192.1.3.1

192.1.3.27


192.1.1.0 255.255.255.0 192.1.1.4 20 192.1.3.0 255.255.255.0 192.1.3.27 20 192.1.1.255 255.255.255.255 192.1.1.4 1

192.1.3.255 255.255.255.255 192.1.3.27 1

192.1.3.2


Routing Protocols

Distance Vector Routing protocol Each router sends a vector of distances to its neighbors Vector contains distances to all the nodes Each router computes next hop towards different nodes Iterative, Asynchronous, Distributed computation

Link State Routing Protocol Each router sends a vector of distances to all the nodes Vector contains distances to only the neighbors Each router has complete topology information Can compute shortest paths to different nodes


Distance vector algorithm - I

Basic idea: Each node periodically sends its own distance vector

estimate to neighbors

When a node x receives new DV estimate from neighbor, it updates its own DV using Bellman-Ford equation:

Dx(z) ← minv{c(x,v) + Dv(z)} for each node z ∊ N

Nexthopx (z) = v

Under natural conditions, the estimate Dx(z) converges to the actual least cost dx(z)


2 0 1

x y z

xyz

0 2 7

∞ ∞ ∞∞ ∞ ∞

from

cost to

from

from

x y z

xyz

0 2 3

from

cost tox y z

xyz

0 2 3

from

cost to

x y z

xyz

∞ ∞

∞ ∞ ∞

cost tox y z

xyz

0 2 7

from

cost to

x y z

xyz

0 2 3

from

cost to

x y z

xyz

0 2 3fr

om

cost tox y z

xyz

0 2 7

from

cost to

x y z

xyz

∞ ∞ ∞7 1 0

cost to

∞ ∞ ∞

2 0 17 1 0

2 0 17 1 0

2 0 13 1 0

2 0 13 1 0

2 0 1

3 1 0

2 0 1

3 1 0

time

x z12

7

y

node x table

node y table

node z table

Dx(y) = min{c(x,y) + Dy(y), c(x,z) + Dz(y)} = min{2+0 , 7+1} = 2

Dx(z) = min{c(x,y) + Dy(z), c(x,z) + Dz(z)} = min{2+1 , 7+0} = 3

∞


Distance Vector: link cost changes

Link cost changes:node detects local link cost change updates routing info, recalculates distance vectorif DV changes, notify neighbors

“goodnews travelsfast”

x z14

50

y1

At time t0, y detects the link-cost change, updates its DV, and informs its neighbors.

At time t1, z receives the update from y and updates its table. It computes a new least cost to x and sends its neighbors its DV.

At time t2, y receives z’s update and updates its distance table. y’s least costs do not change and hence y does not send any message to z.


Distance Vector: link cost changes

Link cost changes:good news travels fast bad news travels slow - “count to infinity” problem!44 iterations before algorithm stabilizes: see text

Poisoned reverse: If Z routes through Y to get to X :

Z tells Y its (Z’s) distance to X is infinite (so Y won’t route to X via Z)

will this completely solve count to infinity problem?

x z14

50

y60


Comparison of LS and DV algorithms

Message complexityLS: with n nodes, E links, O(nE) msgs sent DV: exchange between neighbors only

convergence time varies

Speed of ConvergenceLS: O(n2) algorithm requires O(nE) msgs

may have oscillationsDV: convergence time varies

may be routing loops count-to-infinity problem

Robustness: what happens if router malfunctions?LS:

node can receive incorrect link cost

each node computes only its own table

DV: DV node can advertise

incorrect path cost each node’s table used

by others error propagate thru

network


Transport Layer


Transport Protocols

Protocol implemented entirely at the ends Completeness/correctness of function implementations

User Datagram Protocol (UDP) provides just integrity and demultiplexing

Transmission Control Protocol (TCP) adds… Connection-orientedness (point-to-point) Reliability In-Order delivery Byte-stream Full duplex Flow and congestion control


UDP: User Datagram Protocol [RFC 768]

“No frills”, “bare bones” Internet transport protocol

“best effort” service, UDP segments may be:

lost delivered out of order

to the application connectionless:

no handshaking between UDP sender, receiver

each UDP datagram handled independently of others

Why is there a UDP? No connection establishment

=> Faster communication simple: no connection state

at sender, receiver small 8-byte header (lower

overheads) no congestion control: UDP

can blast away as fast as desired

Streaming Multimedia apps, DNS, SNMP benefit from UDP

0 16 32Source Port Destination Port

Length Checksum

Application Layer Data…


TCP Header

0Source Port Destination Port

Sequence Number

Acknowledgement Number

Data Offset

WindowReserved ACK

URG

EOL

RST

SYN

FIN

Checksum Urgent Pointer

PaddingOptions

4 8 16 32

Application Layer Data…


Connection: Three-Way Handshake

TCP connection-establishment: 3-way-handshake necessary and sufficient for unambiguous setup/teardown even under conditions of loss, duplication, and delay


TCP Connection Setup: FSM

CLIENT

SERVER


TCP – Streams-based

Host A

Seq=100, 20 bytes data

ACK=100

time

Host B


ACK=120

SendBase = 120

Sendbase = 100


TCP is Network-friendly

Reliable transmission Sliding window concept

Flow control Regulated by the receiver

Congestion Control Regulated by the sender Additive Increase, Multiplicative Decrease Fairness of TCP streams


Stop-and-Wait operation

first packet bit transmitted, t = 0

sender receiver

RTT

last packet bit transmitted, t = L / R

first packet bit arriveslast packet bit arrives, send ACK

ACK arrives, send next packet, t = RTT + L / R


Pipelining: increased utilization

first packet bit transmitted, t = 0

sender receiver

RTT

last bit transmitted, t = L / R

first packet bit arriveslast packet bit arrives, send ACK

ACK arrives, send next packet, t = RTT + L / R

last bit of 2nd packet arrives, send ACKlast bit of 3rd packet arrives, send ACK

Increase utilizationby a factor of 3!


Go-Back-N

Sender:k-bit seq # in pkt header“window” of up to N, consecutive unack’ed pkts allowed

ACK(n): ACKs all pkts up to, including seq # n - “cumulative ACK” may receive duplicate ACKs (see receiver)

timer for each in-flight pkttimeout(n): retransmit pkt n and all higher seq # pkts in window


Go-Back-N


TCP Flow Control

receive side of TCP connection has a receive buffer:

speed-matching service: matching the send rate to the receiving app’s drain rate

app process may be slow at reading from buffer

sender won’t overflow

receiver’s buffer bytransmitting too

much, too fast

flow control


TCP Flow control: how it works

(Suppose TCP receiver discards out-of-order segments)spare room in buffer= RcvWindow

= RcvBuffer-[LastByteRcvd - LastByteRead]

Rcvr advertises spare room by including value of RcvWindow in segmentsSender limits unACKed data to RcvWindow

guarantees receive buffer doesn’t overflow


TCP congestion control:

two “phases” slow start congestion avoidance

important variables: Congwin threshold: defines

threshold between two slow start phase, congestion control phase

“probing” for usable bandwidth:

ideally: transmit as fast as possible (Congwin as large as possible) without loss

increase Congwin until loss (congestion)

loss: decrease Congwin, then begin probing (increasing) again


TCP Slowstart

exponential increase (per RTT) in window size (not so slow!)loss event: timeout (Tahoe TCP)

initialize: Congwin = 1for (each segment ACKed) Congwin++until (loss event OR CongWin > threshold)

Slowstart algorithmHost A

one segment

RTT

Host B

time

two segments

four segments


TCP Congestion Avoidance: Tahoe

/* slowstart is over */ /* Congwin > threshold */Until (loss event) { every w segments ACKed: Congwin++ }threshold = Congwin/2Congwin = 1perform slowstart

TCP Tahoe Congestion avoidance


Where to from here?

IP designed for best-effort service only Affecting new applications like media streaming, VoIP Should the network become application-aware? Should the IP routers look beyond the IP header?

We are already running short of IP addresses Solution: IPv6 – yet to become widespread Temporary fix: Network Address Translation (NAT)

TCP or UDP may not be the best suited for reliable media streaming Answer: Stream Control Transmission Protocol (SCTP) SCTP combines the datagram orientation of UDP with

the sequencing and reliability of TCP SCTP uses multi-streaming, message-oriented routing


Outline

Session 1 7-Layer OSI Model Network Layer protocols (Internet Protocol) Transport Layer protocols (TCP and UDP)

Session 2 Socket Programming – with focus on BSD Sockets

Sample codes in C that work on UNIX/Linux systems


Thank You !


BACKUP


TCP retransmission - I

Premature TimeoutHost A


ACK=100

time

Host B


ACK=120


Seq=

92

tim

eout

ACK=120Seq=

92

tim

eout

SendBase= 120

SendBase= 120

Sendbase= 100


TCP retransmission - II

Lost ACK packet Host A


ACK=100

lossti

meout

Host B

X


ACK=100

time

SendBase= 100


TCP Connection Tear-down

Sender ReceiverFIN

FIN-ACK

FIN

FIN-ACK

Data write

Data ack


TCP Connection Tear-down: FSM

CLOSING

CLOSEWAIT

FINWAIT-1

ESTAB

TIME WAIT

snd FIN

CLOSE

send FIN

CLOSE

rcv ACK of FIN

LAST-ACK

CLOSED

FIN WAIT-2

snd ACK

rcv FIN

delete TCB

Timeout=2msl

send FIN

CLOSE

send ACK

rcv FIN

snd ACK

rcv FIN

rcv ACK of FIN

snd ACK

rcv FIN+ACK


TCP retransmission - III

Cumulative ACKsHost A


ACK=100

loss

tim

eout

Host B

X


ACK=120

time

SendBase= 120


Selective Ack


Selective Ack - Example

intro 2 computer networks

Documents

network hop

router network

key network

ip address ip address

linklayer interface

network essentials session

network mask examples

number of network bits