Network Layer 4-1 Chapter 2 Delivering the data Adapted from slides provided for: All material...

Network Layer 4-1

Chapter 2Delivering the data

Adapted from slides provided for:

All material copyright 1996-2010J.F Kurose and K.W. Ross, All Rights Reserved

Computer Networking: A Top Down Approach 5th edition. Jim Kurose, Keith RossAddison-Wesley

Network Layer 4-2

Chapter goals: understand how data moves between

layers and systems on the network IP address, subnet Routing

Routing table Address resolution Protocols, ports and sockets

Network Layer 4-3

1

23

0111

value in arrivingpacket’s header

routing algorithm

local forwarding tableheader value output link

0100010101111001

3221

Interplay between routing, forwarding

Network Layer 4-4

IP datagram format

ver length

32 bits

data (variable length,typically a TCP

or UDP segment)

16-bit identifier

header checksum

time tolive

32 bit source IP address

IP protocol versionnumber

header length (bytes)

max numberremaining hops

(decremented at each router)

forfragmentation/reassembly

total datagramlength (bytes)

upper layer protocolto deliver payload to

head.len

type ofservice

“type” of data flgsfragment

offsetupper layer

32 bit destination IP address

Options (if any) E.g. timestamp,record routetaken, specifylist of routers to visit.

Network Layer 4-5

IP Addressing: introduction IP address: 32-bit

identifier for network interface

interface: connection between host/router and physical link router’s typically have

multiple interfaces host typically has one

interface• Host with multiple

interfaces can acts as a router

Command:ifconfig

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

223.1.1.1 = 11011111 00000001 00000001 00000001

223 1 11

IP Address vs MAC Address

MAC address Globally unique Statically

configured by manufacturer

flat

IP address Not necessarily

unique Dynamically

assigned Hierarchical:

made up of network part and host part, corresponding to hierarchy in the Internet

Network Layer 4-6

Discussion: difference betweenIP address vs domain name?

[zhang@storm ~]$ ifconfig

em1 Link encap:Ethernet HWaddr B4:99:BA:01:3B:F6

inet addr:150.108.68.26 Bcast:150.108.68.255 Mask:255.255.255.0

inet6 addr: fe80::b699:baff:fe01:3bf6/64 Scope:Link

….

em2 Link encap:Ethernet HWaddr B4:99:BA:01:3B:F8

UP BROADCAST MULTICAST MTU:1500 Metric:1

…

em3 Link encap:Ethernet HWaddr B4:99:BA:01:3B:FA


….

em4 Link encap:Ethernet HWaddr B4:99:BA:01:3B:FC


….

lo Link encap:Local Loopback

inet addr:127.0.0.1 Mask:255.0.0.0

inet6 addr: ::1/128 Scope:Host

…

virbr0 Link encap:Ethernet HWaddr 52:54:00:F2:86:A6

inet addr:192.168.122.1 Bcast:192.168.122.255 Mask:255.255.255.0

…

Network Layer 4-7

Private IP address used inCIS dept. network

IP address within Fordham network

Network Layer 4-8

IP Addresses

0network host

10 network host

110 network host

1110 multicast address

A

B

C

D

class1.0.0.0 to127.255.255.255

128.0.0.0 to191.255.255.255

192.0.0.0 to223.255.255.255

224.0.0.0 to239.255.255.255

32 bits

“class-full” addressing:

Network Layer 4-9

Getting a datagram from source to dest.

IP datagram:

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A

BE

miscfields

sourceIP addr

destIP addr data

datagram remains unchanged, as it travels source to destination

addr fields of interest here

Dest. Net. next router Nhops

223.1.1 1223.1.2 223.1.1.4 2223.1.3 223.1.1.4 2

forwarding table in A

Network Layer 4-10


Starting at A, send IP datagram addressed to B:

look up net. address of B in forwarding table

find B is on same net. as A link layer will send datagram

directly to B inside link-layer frame B and A are directly

connected


223.1.1 1223.1.2 223.1.1.4 2223.1.3 223.1.1.4 2

miscfields223.1.1.1223.1.1.3data

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A

BE


Network Layer 4-11



223.1.1 1223.1.2 223.1.1.4 2223.1.3 223.1.1.4 2

Starting at A, dest. E: look up network address of

E in forwarding table E on different network

A, E not directly attached

routing table: next hop router to E is 223.1.1.4

link layer sends datagram to router 223.1.1.4 inside link-layer frame

datagram arrives at 223.1.1.4

continued…..

miscfields223.1.1.1223.1.2.3 data

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A

BE


Network Layer 4-12


Arriving at 223.1.4, destined for 223.1.2.2

look up network address of E in router’s forwarding table

E on same network as router’s interface 223.1.2.9 router, E directly

attached link layer sends datagram

to 223.1.2.2 inside link-layer frame via interface 223.1.2.9

datagram arrives at 223.1.2.2!!! (hooray!)

miscfields223.1.1.1223.1.2.3 data Dest. Net router Nhops interface

223.1.1 - 1 223.1.1.4 223.1.2 - 1 223.1.2.9

223.1.3 - 1 223.1.3.27

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A

BE

forwarding table in router

Subnetting

Problem 1: Any network with need for more than 255 hosts, needed class B addresses, or get many class C addresses

Problem 2: Each new network implies additional entry in forwarding table large table

Solution: Share one network number between several

networks.

…Subnetting

Made most sense for large corporations or campuses

Corporation networks share 1 network number Number of other networks within the

corporation, using subnet masks E.g. a class B address, is shared among 8 networks,

by using a 19-bit “subnet mask” (255.255.224.0 = 11111111 11111111 11100000 00000000)

I.e. subnet addresses are defined by 1st 19 bits of the IP address. Host part now has a “subnet” part in it.

Class B network address continues to be advertised to the rest of the Internet, subnetting only used “within campus”

Subnet mask Introduce another level of hierarchy into

IP address

Network Layer 4-15

8 bits are borrowed from the host address field to createsubnet address field.Subnet mask: 255.255.255.0, i.e., all 1’s in upper 24 bits and 0’s in lower 8 bits * 24 bits are network number * 8 bits are host number

Forwarding Ex. with Subnet Masks• Routing Table:

SubnetNumber SubnetMask NextHop

128.96.170.0 255.255.254.0 Intface 0

128.96.168.0 255.255.254.0 Intface 1

128.96.166.0 255.255.254.0 R2

128.96.164.0 255.255.252.0 R3

Default R4D = Dest IP Address For each table entry (subnetNumber, SubnetMask, NextHop)If (D & SubnetMask == SubnetNumber) if NextHop is an interface forward datagram to the interface else deliver datagram to NextHop (a router)

Forwardingpseudocode

Network Layer 4-17

IP addressing: CIDR Classful addressing:

inefficient use of address space, address space exhaustion

e.g., class B net allocated enough addresses for 65K hosts, even if only 2K hosts in that network

CIDR: Classless InterDomain Routing network portion of address of arbitrary length address format: a.b.c.d/x, where x is # bits in network

portion of address

11001000 00010111 00010000 00000000

networkpart

hostpart

200.23.16.0/23

Special IP address within subnet NETWORK ADDRESS

A network address is an address where all host bits in the IP address are set to zero (0).

first and lowest numbered address BROADCAST ADDRESS

all host bits in the IP address are set to one (1). last address in the range of addresses All hosts are to accept and respond to the broadcast

address. This makes special services possible.

Network Layer 4-18

Hosts LOOPBACK ADDRESS

127.0.0.0 class 'A' subnet is used for only a single address, the loopback address 127.0.0.1. used to test the local network interface

device's functionality. All network interface devices should

respond to this address. ping 127.0.0.1 to test network hardware

and software

Network Layer 4-19

Special Use IP addresses

PRIVATE IP ADDRESSES RFC 1918 defines a number of IP blocks set

aside by American Registry of Internet Numbers (ARIN) for use as private addresses on private networks that are not directly connected to t Internet.

Class Start End A 10.0.0.0 10.255.255.255B 172.16.0.0 172.31.255.255C 192.168.0.0 192.168.255.255

Network Layer 4-20

Special Use IP addresses

Multicast IP Addresses set aside for special purposes, such as the

IP's used in OSPF, Multicast, and experimental purposes that cannot be used on the Internet.

Class Start EndD 224.0.0.0 239.255.255.255

Network Layer 4-21

Network Layer 4-22

IP addresses: how to get one?

Q: How does a network get the network part of IP addr?

A: gets allocated portion of its provider ISP’s address space

ISP's block 11001000 00010111 00010000 00000000 200.23.16.0/20

Organization 0 11001000 00010111 00010000 00000000 200.23.16.0/23 Organization 1 11001000 00010111 00010010 00000000 200.23.18.0/23 Organization 2 11001000 00010111 00010100 00000000 200.23.20.0/23 ... ….. …. ….

Organization 7 11001000 00010111 00011110 00000000 200.23.30.0/23

Network Layer 4-23

Hierarchical addressing: route aggregation

“Send me anythingwith addresses beginning 200.23.16.0/20”

200.23.16.0/23

200.23.18.0/23

200.23.30.0/23

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us“Send me anythingwith addresses beginning 199.31.0.0/16”

200.23.20.0/23Organization 2

...

...

Hierarchical addressing allows efficient advertisement of routing information:

Network Layer 4-24

Hierarchical addressing: more specific routes

ISPs-R-Us has a more specific route to Organization 1

“Send me anythingwith addresses beginning 200.23.16.0/20”

200.23.16.0/23

200.23.18.0/23

200.23.30.0/23

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us“Send me anythingwith addresses beginning 199.31.0.0/16or 200.23.18.0/23”

200.23.20.0/23Organization 2

...

...

Longest match

Network Layer 4-25

IP addressing: the last word...

Q: How does an ISP get block of addresses?

A: ICANN: Internet Corporation for Assigned

Names and Numbers allocates addresses manages DNS assigns domain names, resolves disputes

Network Layer 4-26

IP addresses: how to get one?

Q: How does a host get IP address?

hard-coded by system admin in a file Windows: control-panel->network->configuration-

>tcp/ip->properties UNIX: /etc/rc.config

DHCP: Dynamic Host Configuration Protocol: dynamically get address from a server “plug-and-play”

Network Layer 4-27

DHCP: Dynamic Host Configuration Protocol

Goal: allow host to dynamically obtain its IP address from network server when it joins networkCan renew its lease on address in useAllows reuse of addresses (only hold address while connected an “on”)Support for mobile users who want to join network (more shortly)

DHCP overview: host broadcasts “DHCP discover” msg [optional] DHCP server responds with “DHCP offer” msg [optional] host requests IP address: “DHCP request” msg DHCP server sends address: “DHCP ack” msg

Network Layer 4-28

DHCP client-server scenario

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A

BE

DHCP server

arriving DHCP client needsaddress in thisnetwork

Network Layer 4-29

DHCP client-server scenarioDHCP server: 223.1.2.5 arriving

client

time

DHCP discover

src : 0.0.0.0, 68 dest.: 255.255.255.255,67yiaddr: 0.0.0.0transaction ID: 654

DHCP offer

src: 223.1.2.5, 67 dest: 255.255.255.255, 68yiaddrr: 223.1.2.4transaction ID: 654Lifetime: 3600 secs

DHCP request

src: 0.0.0.0, 68 dest:: 255.255.255.255, 67yiaddrr: 223.1.2.4transaction ID: 655Lifetime: 3600 secs

DHCP ACK

src: 223.1.2.5, 67 dest: 255.255.255.255, 68yiaddrr: 223.1.2.4transaction ID: 655Lifetime: 3600 secs

Network Layer 4-30

DHCP: more than IP address

DHCP can return more than just allocated IP address on subnet: address of first-hop router for client name and IP address of DNS sever network mask (indicating network versus

host portion of address)

Network Layer 4-31

NAT: Network Address Translation

10.0.0.1

10.0.0.2

10.0.0.3

10.0.0.4

138.76.29.7

local network(e.g., home network)

10.0.0/24

rest ofInternet

Datagrams with source or destination in this networkhave 10.0.0/24 address for

source, destination (as usual)

All datagrams leaving localnetwork have same single source

NAT IP address: 138.76.29.7,different source port numbers

Network Layer 4-32


Motivation: local network uses just one IP address as far as outside world is concerned: range of addresses not needed from ISP: just

one IP address for all devices can change addresses of devices in local network

without notifying outside world can change ISP without changing addresses of

devices in local network devices inside local net not explicitly

addressable, visible by outside world (a security plus).

Network Layer 4-33


Implementation: NAT router must:

outgoing datagrams: replace (source IP address, port #) of every outgoing datagram to (NAT IP address, new port #). . . remote clients/servers will respond using (NAT IP

address, new port #) as destination addr.

remember (in NAT translation table) every (source IP address, port #) to (NAT IP address, new port #) translation pair

incoming datagrams: replace (NAT IP address, new port #) in dest fields of every incoming datagram with corresponding (source IP address, port #) stored in NAT table

Network Layer 4-34


16-bit port-number field: 60,000 simultaneous connections with a

single LAN-side address! NAT is controversial:

routers should only process up to layer 3 violates end-to-end argument

• NAT possibility must be taken into account by app designers, e.g., P2P applications

address shortage should instead be solved by IPv6

Network Layer 4-35

NAT traversal problem client wants to connect to

server with address 10.0.0.1 server address 10.0.0.1

local to LAN (client can’t use it as destination addr)

only one externally visible NATed address: 138.76.29.7

solution 1: statically configure NAT to forward incoming connection requests at given port to server e.g., (123.76.29.7, port

2500) always forwarded to 10.0.0.1 port 25000

10.0.0.1

10.0.0.4

NAT router

138.76.29.7

Client?

Network Layer 4-36

NAT traversal problem solution 2: Universal Plug

and Play (UPnP) Internet Gateway Device (IGD) Protocol. Allows NATed host to:learn public IP address

(138.76.29.7)add/remove port

mappings (with lease times)

i.e., automate static NAT port map configuration

10.0.0.1

10.0.0.4

NAT router

138.76.29.7

IGD

Network Layer 4-37

NAT traversal problem solution 3: relaying (used in Skype)

NATed client establishes connection to relay

External client connects to relay relay bridges packets between to

connections

138.76.29.7

Client

10.0.0.1

NAT router

1. connection torelay initiatedby NATed host

2. connection torelay initiatedby client

3. relaying established

Network Layer 4-38

ICMP: Internet Control Message Protocol

used by hosts & routers to communicate network-level information error reporting:

unreachable host, network, port, protocol

echo request/reply (used by ping)

network-layer “above” IP: ICMP msgs carried in IP

datagrams ICMP message: type, code

plus first 8 bytes of IP datagram causing error

Type Code description0 0 echo reply (ping)3 0 dest. network unreachable3 1 dest host unreachable3 2 dest protocol unreachable3 3 dest port unreachable3 6 dest network unknown3 7 dest host unknown4 0 source quench (congestion control - not used)8 0 echo request (ping)9 0 route advertisement10 0 router discovery11 0 TTL expired12 0 bad IP header

Network Layer 4-39

Traceroute and ICMP

Source sends series of UDP segments to dest first has TTL =1 second has TTL=2, etc. unlikely port number

When nth datagram arrives to nth router: router discards

datagram and sends to source an

ICMP message (type 11, code 0)

ICMP message includes name of router & IP address

when ICMP message arrives, source calculates RTT

traceroute does this 3 times

Stopping criterion UDP segment eventually

arrives at destination host

destination returns ICMP “port unreachable” packet (type 3, code 3)

when source gets this ICMP, stops.

Network Layer 4-40

Hierarchical Routing

scale: with 200 million destinations:

can’t store all dest’s in routing tables!

routing table exchange would swamp links!

administrative autonomy

internet = network of networks

each network admin may want to control routing in its own network

Our routing study thus far - idealization all routers identical network “flat”… not true in practice

Network Layer 4-41

Hierarchical Routing

aggregate routers into regions, “autonomous systems” (AS)

routers in same AS run same routing protocol “intra-AS” routing

protocol routers in different AS

can run different intra-AS routing protocol

gateway router at “edge” of its own

AS has link to router in

another AS

Network Layer 4-42

3b

1d

3a

1c2aAS3

AS1

AS21a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASes

forwarding table configured by both intra- and inter-AS routing algorithm intra-AS sets entries

for internal dests inter-AS & intra-As

sets entries for external dests

Network Layer 4-43

Intra-AS Routing

also known as Interior Gateway Protocols (IGP) most common Intra-AS routing protocols:

RIP: Routing Information Protocol

OSPF: Open Shortest Path First

IGRP: Interior Gateway Routing Protocol (Cisco proprietary)

Network Layer 4-44

RIP ( Routing Information Protocol) included in BSD-UNIX distribution in 1982 distance vector algorithm

distance metric: # hops (max = 15 hops), each link has cost 1 DVs exchanged with neighbors every 30 sec in response message (aka

advertisement) each advertisement: list of up to 25 destination subnets (in IP addressing sense)

DC

BA

u vw

x

yz

subnet hops u 1 v 2 w 2 x 3 y 3 z 2

from router A to destination subnets:

Network Layer 4-45

OSPF (Open Shortest Path First)

“open”: publicly available uses Link State algorithm

LS packet dissemination topology map at each node route computation using Dijkstra’s algorithm

OSPF advertisement carries one entry per neighbor router

advertisements disseminated to entire AS (via flooding) carried in OSPF messages directly over IP (rather than

TCP or UDP

UNIX routing Principle

1. Search for a matching host address 2. Search for a matching network

address 3. Search for a default entry (specified

as a network entry, with network ID of 0

Network Layer 4-46

netstat

Display routing table[zhang@storm ~]$ netstat -rnKernel IP routing tableDestination Gateway Genmask Flags MSS Window irtt Iface0.0.0.0 150.108.68.1 0.0.0.0 UG 0 0 0 em1150.108.68.0 0.0.0.0 255.255.255.0 U 0 0 0 em1192.168.122.0 0.0.0.0 255.255.255.0 U 0 0 0 virbr0

Network Layer 4-47

Transport Layer 3-48

Multiplexing/demultiplexing

application

transport

network

link

physical

P1 application

transport

network

link

physical

application

transport

network

link

physical

P2P3 P4P1

host 1 host 2 host 3

= process= socket

delivering received segmentsto correct socket

Demultiplexing at rcv host:gathering data from multiplesockets, enveloping data with header (later used for demultiplexing)

Multiplexing at send host:


How demultiplexing works host receives IP datagrams

each datagram has source IP address, destination IP address

each datagram carries 1 transport-layer segment

each segment has source, destination port number

host uses IP addresses & port numbers to direct segment to appropriate socket

source port # dest port #

32 bits

applicationdata (message)

other header fields

TCP/UDP segment format


Connectionless demultiplexing recall: create sockets with

host-local port numbers:DatagramSocket mySocket1 = new

DatagramSocket(12534);

DatagramSocket mySocket2 = new DatagramSocket(12535);

recall: when creating datagram to send into UDP socket, must specify

(dest IP address, dest port number)

when host receives UDP segment: checks destination port

number in segment directs UDP segment to

socket with that port number

IP datagrams with different source IP addresses and/or source port numbers directed to same socket


Connectionless demux (cont)

DatagramSocket serverSocket = new DatagramSocket(6428);

ClientIP:B

P2

client IP: A

P1P1P3

serverIP: C

SP: 6428DP: 9157

SP: 9157DP: 6428

SP: 6428DP: 5775

SP: 5775DP: 6428

SP provides “return address”


Connection-oriented demux

TCP socket identified by 4-tuple: source IP address source port number dest IP address dest port number

recv host uses all four values to direct segment to appropriate socket

server host may support many simultaneous TCP sockets: each socket identified

by its own 4-tuple web servers have

different sockets for each connecting client non-persistent HTTP will

have different socket for each request


Connection-oriented demux (cont)

ClientIP:B

P1

client IP: A

P1P2P4

serverIP: C

SP: 9157DP: 80

SP: 9157DP: 80

P5 P6 P3

D-IP:CS-IP: AD-IP:C

S-IP: B

SP: 5775DP: 80

D-IP:CS-IP: B


Connection-oriented demux: Threaded Web Server

clientIP:B

P1

client IP: A

P1P2

serverIP: C

SP: 9157DP: 80

SP: 9157DP: 80

P4 P3

D-IP:CS-IP: AD-IP:C

S-IP: B

SP: 5775DP: 80

D-IP:CS-IP: B


TCP Connection Management

Recall: TCP sender, receiver establish “connection” before exchanging data segments

initialize TCP variables: seq. #s buffers, flow control info

(e.g. RcvWindow) client: connection initiator Socket clientSocket = new

Socket("hostname","port

number"); server: contacted by client Socket connectionSocket =

welcomeSocket.accept();

Three way handshake:

Step 1: client host sends TCP SYN segment to server specifies initial seq # no data

Step 2: server host receives SYN, replies with SYNACK segment

server allocates buffers specifies server initial

seq. #Step 3: client receives SYNACK,

replies with ACK segment, which may contain data


TCP Connection Management (cont.)

Closing a connection:

client closes socket: clientSocket.close();

Step 1: client end system sends TCP FIN control segment to server

Step 2: server receives FIN, replies with ACK. Closes connection, sends FIN.

client

FIN

server

ACK

ACK

FIN

close

close

closed

tim

ed w

ait


TCP Connection Management (cont.)

Step 3: client receives FIN, replies with ACK.

Enters “timed wait” - will respond with ACK to received FINs

Step 4: server, receives ACK. Connection closed.

Note: with small modification, can handle simultaneous FINs.

client

FIN

server

ACK

ACK

FIN

closing

closing

closed

tim

ed w

ait

closed


TCP Connection Management (cont)

TCP clientlifecycle

TCP serverlifecycle


TCP segment structure

source port # dest port #

32 bits

applicationdata (variable length)

sequence number

acknowledgement number Receive window

Urg data pnterchecksum

FSRPAUheadlen

notused

Options (variable length)

URG: urgent data (generally not used)

ACK: ACK #valid

PSH: push data now(generally not used)

RST, SYN, FIN:connection estab(setup, teardown

commands)

# bytes rcvr willingto accept

countingby bytes of data(not segments!)

Internetchecksum

(as in UDP)

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Network Layer 4-1 Chapter 2 Delivering the data Adapted from slides provided for: All material...

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