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Chapter 3Transport, Network and
Informática y Comunicaciones
Transport, Network and Link Layers 3-1
Transport, Network and Link Layers
All material copyright 1996-2012J.F Kurose and K.W. Ross, All Rights Reserved
Chapter 3: outline
3.1 Transport layer
3.2 Network layer
3.3 Link layer
Transport, Network and Link Layers 3-2
Transport services and protocols
� provide logical communicationbetween app processes running on different hosts
� transport protocols run in end systems
� send side: breaks app
application
transport
network
data link
physical
� send side: breaks app messages into segments, passes to network layer
� rcv side: reassembles segments into messages, passes to app layer
� more than one transport protocol available to apps
� Internet: TCP and UDP
application
transport
network
data link
physical
Transport, Network and Link Layers 3-3
Transport vs. network layer
household analogy:� network layer: logical communication between hosts
� transport layer:logical communication
12 kids in Ann’s house sending letters to 12 kids in Bill’s house:
� hosts = housesprocesses = kids
logical communication between processes� relies on, enhances, network layer services
� processes = kids� app messages = letters in
envelopes� transport protocol = Ann
and Bill who demux to in-house siblings
� network-layer protocol = postal service
Transport, Network and Link Layers 3-4
Multiplexing/demultiplexing
use header info to deliverreceived segments to correct socket
demultiplexing at receiver:handle data from multiplesockets, add transport header (later used for demultiplexing)
multiplexing at sender:
application
process
socket
transport
application
physical
link
network
P2P1
transport
application
physical
link
network
P4
transport
application
physical
link
network
P3
Transport, Network and Link Layers 3-5
How demultiplexing works
� host receives IP datagrams� each datagram has source IP address, destination IP address
� each datagram carries one transport-layer segment
source port # dest port #
32 bits
other header fields
transport-layer segment
� each segment has source, destination port number
� host uses IP addresses & port numbers to direct segment to appropriate socket
application
data
(payload)
TCP/UDP segment format
Transport, Network and Link Layers 3-6
UDP: User Datagram Protocol
� “no frills,” “bare bones”Internet transport protocol
� “best effort” service, UDP segments may be:
� lost
� delivered out-of-order
� UDP use:� streaming multimedia apps (loss tolerant, rate sensitive)
� DNS
� SNMP� delivered out-of-order to app
� connectionless:
� no handshaking between UDP sender, receiver
� each UDP segment handled independently of others
� SNMP
� reliable transfer over UDP: � add reliability at application layer
� application-specific error recovery!
Transport, Network and Link Layers 3-7
UDP: segment header
source port # dest port #
32 bits
application
length checksum
length, in bytes of UDP segment,
including header
� no connection establishment (which can
why is there a UDP?
application
data
(payload)
UDP segment format
establishment (which can add delay)
� simple: no connection state at sender, receiver
� small header size
� no congestion control: UDP can blast away as fast as desired
Transport, Network and Link Layers 3-8
UDP checksum
sender:� treat segment contents,
including header fields,
receiver:� compute checksum of
received segment
Goal: detect “errors” (e.g., flipped bits) in transmitted segment
including header fields, as sequence of 16-bit integers
� checksum: addition (one’s complement sum) of segment contents
� sender puts checksum value into UDP checksum field
received segment
� check if computed checksum equals checksum field value:
� NO - error detected
� YES - no error detected. But maybe errors nonetheless?
Transport, Network and Link Layers 3-9
TCP: Overview
� full duplex data:� bi-directional data flow in same connection
� MSS: maximum segment size
connection-oriented:
� point-to-point:� one sender, one receiver
� reliable, in-order byte stream:
� connection-oriented:� handshaking (exchange of control msgs) initssender, receiver state before data exchange
� flow controlled:� sender will not overwhelm receiver
stream:� no “message boundaries”
� pipelined:� TCP congestion and flow control set window size
Transport, Network and Link Layers 3-10
TCP segment structure
source port # dest port #
32 bits
sequence number
acknowledgement numberreceive window
Urg data pointerchecksum
FSRPAUheadlen
notused
URG: urgent data (generally not used)
ACK: ACK #valid
PSH: push data now(generally not used)
# bytes rcvr willing
countingby bytes of data(not segments!)
applicationdata (variable length)
Urg data pointerchecksum
options (variable length)
(generally not used)
RST, SYN, FIN:connection estab(setup, teardown
commands)
rcvr willingto accept
Internetchecksum
(as in UDP)
Transport, Network and Link Layers 3-11
TCP reliable data transfer
� TCP creates rdt service on top of IP’s unreliable service� pipelined segments
� cumulative acks
� single retransmission timer
� retransmissions triggered by:� timeout events
� duplicate acks
Transport, Network and Link Layers 3-12
congestion:� informally: “too many sources sending too much data too fast for network to handle”
� different from flow control!
manifestations:
Principles of congestion control
� manifestations:
� lost packets (buffer overflow at routers)
� long delays (queueing in router buffers)
� a top-10 problem!
Transport, Network and Link Layers 3-13
Causes/costs of congestion: scenario 1
� two senders, two receivers
� one router, infinite buffers
� output link capacity: R
� no retransmission
unlimited shared output link buffers
Host A
original data: λin throughput:λout
� maximum per-connection throughput: R/2
Host B
R/2
R/2
λ out
λin R/2
dela
y
λin
� large delays as arrival rate, λin, approaches capacity
Transport, Network and Link Layers 3-14
� one router, finite buffers
� sender retransmission of timed-out packet� application-layer input = application-layer output: λin =
λout
� transport-layer input includes retransmissions : λin λin‘
Causes/costs of congestion: scenario 2
finite shared output link
buffers
Host A
λin : original data
Host B
λout
λ'in: original data, plus retransmitted
data
Transport, Network and Link Layers 3-15
idealization: perfect knowledge
� sender sends only when router buffers available
R/2
R/2
λ out
λin
Causes/costs of congestion: scenario 2
finite shared output link
buffers
λin : original data λout
λ'in: original data, plus retransmitted
data
copy
free buffer space!
Host B
A
Transport, Network and Link Layers 3-16
Idealization: known losspackets can be lost, dropped at router due to full buffers
� sender only resends if packet known to be lost
Causes/costs of congestion: scenario 2
λin : original data λout
λ'in: original data, plus retransmitted
data
copy
no buffer space!A
Host B Transport, Network and Link Layers 3-17
Causes/costs of congestion: scenario 2
Idealization: known losspackets can be lost, dropped at router due to full buffers
� sender only resends if packet known to be lost
R/2
R/2λin
λ out
when sending at R/2, some packets are retransmissions but asymptotic goodput is still R/2
λin : original data λout
λ'in: original data, plus retransmitted
datafree buffer space!A
Host B Transport, Network and Link Layers 3-18
R/2
R/2λin
λ out
when sending at R/2, some packets are retransmissions including duplicated that are delivered!
Realistic: duplicates� packets can be lost, dropped
at router due to full buffers
� sender times out prematurely, sending two copies, both of which are delivered
Causes/costs of congestion: scenario 2
A
λin λoutλ'incopy
free buffer space!
timeout
Host B Transport, Network and Link Layers 3-19
R/2
λ out
when sending at R/2, some packets are retransmissions including duplicated that are delivered!
R/2λin
Causes/costs of congestion: scenario 2
Realistic: duplicates� packets can be lost, dropped
at router due to full buffers
� sender times out prematurely, sending two copies, both of which are delivered
“costs” of congestion:� more work (retrans) for given “goodput”� unneeded retransmissions: link carries multiple copies of pkt
� decreasing goodput
Transport, Network and Link Layers 3-20
� four senders
� multihop paths
� timeout/retransmit
Q: what happens as λin and λin’
increase ?
Host A λout
Causes/costs of congestion: scenario 3
Host B
λin : original data
λ' : original data, plus
A: as red λin’ increases, all arriving
blue pkts at upper queue are dropped, blue throughput � 0
finite shared output link
buffers
Host CHost D
λ'in: original data, plusretransmitted data
Transport, Network and Link Layers 3-21
Causes/costs of congestion: scenario 3
C/2λ o
ut
λ
another “cost” of congestion:� when packet dropped, any “upstream transmission capacity used for that packet was wasted!
C/2λin’
Transport, Network and Link Layers 3-22
Approaches towards congestion control
two broad approaches towards congestion control:
end-end congestion control:
� no explicit feedback
network-assisted congestion control:
� routers provide � no explicit feedback from network
� congestion inferred from end-system observed loss, delay
� approach taken by TCP
� routers provide feedback to end systems
� single bit indicating congestion (SNA, DECbit, TCP/IP ECN, ATM)
�explicit rate for sender to send at
Transport, Network and Link Layers 3-23
fairness goal: if K TCP sessions share same bottleneck link of bandwidth R, each should have average rate of R/K
TCP connection 1
TCP Fairness
bottleneckroutercapacity RTCP connection 2
Transport, Network and Link Layers 3-24
Chapter 3: outline
3.1 Transport layer
3.2 Network layer
3.3 Link layer
Transport, Network and Link Layers 3-25
Network layer
� transport segment from sending to receiving host
� on sending side encapsulates segments into datagrams
� on receiving side, delivers
application
transport
network
data link
physical
network
data link
physical network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
networknetwork� on receiving side, delivers segments to transport layer
� network layer protocols in every host, router
� router examines header fields in all IP datagrams passing through it
application
transport
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physicalnetwork
data link
physical
Transport, Network and Link Layers 3-26
Two key network-layer functions
� forwarding: move packets from router’s input to appropriate router output
routing: determine route
analogy:
� routing: process of planning trip from source to dest
� routing: determine route taken by packets from source to dest.
� routing algorithms
to dest
� forwarding: process of getting through single interchange
Transport, Network and Link Layers 3-27
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
routing algorithm determinesend-end-path through networkforwarding table determineslocal forwarding at this router
1
23
0111
value in arrivingpacket’s header
1001 1
Transport, Network and Link Layers 3-28
Connection setup
� 3rd important function in some network architectures:� ATM, frame relay, X.25
� before datagrams flow, two end hosts andintervening routers establish virtual connectionintervening routers establish virtual connection� routers get involved
� network vs transport layer connection service:� network: between two hosts (may also involve intervening routers in case of VCs)
� transport: between two processes
Transport, Network and Link Layers 3-29
Network service model
Q:What service model for “channel” transporting datagrams from sender to receiver?
example services for individual datagrams:guaranteed delivery
example services for a flow of datagrams:in-order datagram � guaranteed delivery
� guaranteed delivery with less than 40 msec delay
� in-order datagram delivery
� guaranteed minimum bandwidth to flow
� restrictions on changes in inter-packet spacing
Transport, Network and Link Layers 3-30
Network layer service models:
NetworkArchitecture
Internet
ATM
ServiceModel
best effort
CBR
Bandwidth
none
constant
Loss
no
yes
Order
no
yes
Timing
no
yes
Congestionfeedback
no (inferredvia loss)no
Guarantees ?
ATM
ATM
ATM
ATM
CBR
VBR
ABR
UBR
constantrateguaranteedrateguaranteed minimumnone
yes
yes
no
no
yes
yes
yes
yes
yes
yes
no
no
nocongestionnocongestionyes
no
Transport, Network and Link Layers 3-31
Connection, connection-less service
� datagram network provides network-layer connectionless service
� virtual-circuit network provides network-layer connection service
� analogous to TCP/UDP connecton-oriented / � analogous to TCP/UDP connecton-oriented / connectionless transport-layer services, but:
� service: host-to-host
� no choice: network provides one or the other
� implementation: in network core
Transport, Network and Link Layers 3-32
Virtual circuits
“source-to-dest path behaves much like telephone circuit”� performance-wise
� network actions along source-to-dest path
� call setup, teardown for each call before data can flow
� each packet carries VC identifier (not destination host address)
� every router on source-dest path maintains “state” for each passing connection
� link, router resources (bandwidth, buffers) may be allocated to VC (dedicated resources = predictable service)
Transport, Network and Link Layers 3-33
VC implementation
a VC consists of:1. path from source to destination
2. VC numbers, one number for each link along path
3. entries in forwarding tables in routers along path
� packet belonging to VC carries VC number � packet belonging to VC carries VC number (rather than dest address)
� VC number can be changed on each link.� new VC number comes from forwarding table
Transport, Network and Link Layers 3-34
VC forwarding table
12 22 32
1 23
VC numberinterfacenumber
Incoming interface Incoming VC # Outgoing interface Outgoing VC #
forwarding table in
northwest router:
Incoming interface Incoming VC # Outgoing interface Outgoing VC #
1 12 3 222 63 1 18 3 7 2 171 97 3 87… … … …
VC routers maintain connection state information!
Transport, Network and Link Layers 3-35
Datagram networks
� no call setup at network layer
� routers: no state about end-to-end connections� no network-level concept of “connection”
� packets forwarded using destination host address
1. send datagrams
applicationtransportnetworkdata linkphysical
applicationtransportnetworkdata linkphysical
2. receive datagrams
Transport, Network and Link Layers 3-36
Datagram forwarding table
routing algorithm
local forwarding tabledest address output link
address-range 1address-range 2address-range 3address-range 4
3221
4 billion IP addresses, so rather than list individual destination address
list range of addresses
(aggregate table entries)
1
23
IP destination address in arriving packet’s header
address-range 4 1
Transport, Network and Link Layers 3-37
Datagram or VC network: why?
Internet (datagram)� data exchange among
computers� “elastic” service, no strict
timing req.
� many link types
ATM (VC)� evolved from telephony� human conversation:
� strict timing, reliability requirements
� need for guaranteed service
� “dumb” end systemsmany link types � different characteristics
� uniform service difficult
� “smart” end systems (computers)� can adapt, perform control,
error recovery
� simple inside network, complexity at “ edge”
� “dumb” end systems� telephones� complexity inside network
Transport, Network and Link Layers 3-38
Router architecture overviewtwo key router functions:� run routing algorithms/protocol (RIP, OSPF, BGP)
� forwarding datagrams from incoming to outgoing link
routing processor
routing, managementcontrol plane (software)
forwarding tables computed,pushed to input ports
high-seed switching
fabric
router input ports router output ports
forwarding data plane (hardware)
Transport, Network and Link Layers 3-39
linetermination
link layer
protocol(receive)
lookup,forwarding
queueing
Input port functions
physical layer:
switchfabric
decentralized switching:
� given datagram dest., lookup output port using forwarding table in input port memory (“match plus action”)
� goal: complete input port processing at ‘line speed’
� queuing: if datagrams arrive faster than forwarding rate into switch fabric
bit-level receptiondata link layer:e.g., Ethernet
Transport, Network and Link Layers 3-40
Switching fabrics
� transfer packet from input buffer to appropriate output buffer
� switching rate: rate at which packets can be transfer from inputs to outputs� often measured as multiple of input/output line rate
� N inputs: switching rate N times line rate desirable
� three types of switching fabrics
memory
memory
bus crossbar
Transport, Network and Link Layers 3-41
Switching via memory
first generation routers:� traditional computers with switching under direct control of CPU
� packet copied to system’s memory
� speed limited by memory bandwidth (2 bus crossings per datagram)
inputport(e.g.,
Ethernet)
memoryoutputport(e.g.,
Ethernet)
system bus
Transport, Network and Link Layers 3-42
Switching via a bus
� datagram from input port memory
to output port memory via a shared bus
� bus contention: switching speed limited by bus bandwidthlimited by bus bandwidth
� 32 Gbps bus, Cisco 5600: sufficient speed for access and enterprise routers
bus
Transport, Network and Link Layers 3-43
Switching via interconnection network
� overcome bus bandwidth limitations
� banyan networks, crossbar, other interconnection nets initially developed to connect processors in multiprocessor
� advanced design: fragmenting � advanced design: fragmenting datagram into fixed length cells, switch cells through the fabric.
� Cisco 12000: switches 60 Gbps through the interconnection network
crossbar
Transport, Network and Link Layers 3-44
Output ports
linetermination
link layer
protocol(send)
switchfabric
datagrambuffer
queueing
� buffering required when datagrams arrive from fabric faster than the transmission rate
� scheduling discipline chooses among queued datagrams for transmission
queueing
Transport, Network and Link Layers 3-45
Output port queueing
switchfabric
switchfabric
� buffering when arrival rate via switch exceeds output line speed
� queueing (delay) and loss due to output port buffer overflow!
at t, packets morefrom input to output
one packet time later
Transport, Network and Link Layers 3-46
The Internet network layerhost, router network layer functions:
routing protocols• path selection• RIP, OSPF, BGP
IP protocol• addressing conventions• datagram format
transport layer: TCP, UDP
network
forwardingtable
• RIP, OSPF, BGP • datagram format• packet handling conventions
ICMP protocol• error reporting• router “signaling”
link layer
physical layer
networklayer
Transport, Network and Link Layers 3-47
ver length
32 bits
16-bit identifierheader
checksum
time tolive
32 bit source IP address
head.len
type ofservice
flgs fragmentoffsetupper
layer
IP datagram formatIP protocol version
numberheader length
(bytes)
total datagramlength (bytes)
“type” of data forfragmentation/reassemblymax number
remaining hops(decremented at
data (variable length,typically a TCP
or UDP segment)
32 bit source IP address
32 bit destination IP address
options (if any)upper layer protocolto deliver payload to
(decremented at each router)
e.g. timestamp,record routetaken, specifylist of routers to visit.
how much overhead?� 20 bytes of TCP� 20 bytes of IP� = 40 bytes + app
layer overhead
Transport, Network and Link Layers 3-48
IP addressing: introduction
� IP address: 32-bit identifier for host, router interface
� interface: connection between host/router and physical link
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
223.1.3.27physical link� router’s typically have
multiple interfaces
� host typically has one or two interfaces (e.g., wired Ethernet, wireless 802.11)
� IP addresses associated with each interface
223.1.1.3223.1.2.2
223.1.3.2223.1.3.1
223.1.1.1 = 11011111 00000001 00000001 00000001
223 1 11
Transport, Network and Link Layers 3-49
IP addressing: introduction223.1.1.1
223.1.1.2
223.1.1.3
223.1.1.4 223.1.2.9
223.1.2.2
223.1.2.1
223.1.3.27223.1.1.3
223.1.2.2
223.1.3.2223.1.3.1
wired Ethernet interfaces connected by Ethernet switches
wireless WiFi interfaces connected by WiFi base station
Transport, Network and Link Layers 3-50
Subnets
� IP address:�subnet part - high order bits
�host part - low order bits
�what’s a subnet ?
223.1.1.1
223.1.1.3
223.1.1.4 223.1.2.9223.1.1.2
223.1.3.27223.1.2.2
223.1.2.1
�what’s a subnet ?�device interfaces with same subnet part of IP address
�can physically reach each other without intervening router network consisting of 3 subnets
223.1.3.2223.1.3.1
subnet
Transport, Network and Link Layers 3-51
recipe
� to determine the subnets, detach each interface from its host or router, creating
Subnets223.1.1.0/24
223.1.2.0/24223.1.1.1
223.1.1.3
223.1.1.4 223.1.2.9223.1.1.2
223.1.3.27223.1.2.2
223.1.2.1
or router, creating islands of isolated networks
� each isolated network is called a subnet
subnet mask: /24
223.1.3.0/24
223.1.3.2223.1.3.1
subnet
Transport, Network and Link Layers 3-52
how many? 223.1.1.1
223.1.1.3
223.1.1.4
223.1.1.2
223.1.7.0223.1.9.2
Subnets
223.1.2.2223.1.2.1
223.1.2.6
223.1.3.2223.1.3.1
223.1.3.27
223.1.7.1223.1.8.0223.1.8.1
223.1.9.1
Transport, Network and Link Layers 3-53
IP addressing: CIDR
CIDR: Classless InterDomain Routing� subnet portion of address of arbitrary length
� address format: a.b.c.d/x, where x is # bits in subnet portion of address
11001000 00010111 00010000 00000000
subnetpart
hostpart
200.23.16.0/23
Transport, Network and Link Layers 3-54
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>tcp/ip->properties
� UNIX: /etc/rc.config
� DHCP: Dynamic Host Configuration Protocol: dynamically get address from as server
� “plug-and-play”
Transport, Network and Link Layers 3-55
DHCP: Dynamic Host Configuration Protocol
goal: allow host to dynamically obtain its IP address from network server when it joins network
� can renew its lease on address in use
� allows reuse of addresses (only hold address while connected/“on”)
� support for mobile users who want to join network (more � 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
Transport, Network and Link Layers 3-56
DHCP client-server scenario
223.1.1.0/24
223.1.1.1
223.1.1.4 223.1.2.9223.1.1.2
223.1.2.1
DHCPserver
arriving DHCPclient needs
223.1.2.0/24
223.1.3.0/24
223.1.1.3
223.1.1.4 223.1.2.9
223.1.3.2223.1.3.1
223.1.3.27223.1.2.2
client needs address in thisnetwork
Transport, Network and Link Layers 3-57
DHCP: more than IP addresses
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 � network mask (indicating network versus host portion of address)
Transport, Network and Link Layers 3-58
IP addresses: how to get one?
Q: how does network get subnet 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 Transport, Network and Link Layers 3-59
Hierarchical addressing: route aggregation
“Send me anything
200.23.16.0/23
Organization 0
Organization 1
hierarchical addressing allows efficient advertisement of routing
information:
“Send me anythingwith addresses beginning 200.23.16.0/20”
200.23.18.0/23
200.23.30.0/23
Fly-By-Night-ISP
Organization 7Internet
ISPs-R-Us“Send me anythingwith addresses beginning 199.31.0.0/16”
200.23.20.0/23Organization 2
...
...
Transport, Network and Link Layers 3-60
ISPs-R-Us has a more specific route to Organization 1
“Send me anything
200.23.16.0/23
Organization 0
Hierarchical addressing: more specific routes
“Send me anythingwith addresses beginning 200.23.16.0/20”
200.23.18.0/23
200.23.30.0/23
Fly-By-Night-ISP
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
...
...
Transport, Network and Link Layers 3-61
IP addressing: the last word...
Q: how does an ISP get block of addresses?
A: ICANN: Internet Corporation for Assigned
Names and Numbers http://www.icann.org/
� allocates addresses
� manages DNS� manages DNS
� assigns domain names, resolves disputes
Transport, Network and Link Layers 3-62
NAT: network address translation
10.0.0.1
10.0.0.210.0.0.4
138.76.29.7
local network(e.g., home network)
10.0.0/24
rest ofInternet
10.0.0.3
138.76.29.7
datagrams with source or
destination in this network
have 10.0.0/24 address for
source, destination (as usual)
all datagrams leaving local
network have same single source NAT IP address:
138.76.29.7,different source port numbers
Transport, Network and Link Layers 3-63
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
NAT: network address translation
� 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)
Transport, Network and Link Layers 3-64
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
NAT: network address translation
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
Transport, Network and Link Layers 3-65
10.0.0.1
S: 10.0.0.1, 3345D: 128.119.40.186, 80
1: host 10.0.0.1 sends datagram to 128.119.40.186, 80
NAT translation tableWAN side addr LAN side addr138.76.29.7, 5001 10.0.0.1, 3345…… ……
2: NAT routerchanges datagramsource addr from10.0.0.1, 3345 to138.76.29.7, 5001,updates table
NAT: network address translation
10.0.0.1
10.0.0.2
10.0.0.3
110.0.0.4
138.76.29.7 S: 128.119.40.186, 80 D: 10.0.0.1, 3345
4
S: 138.76.29.7, 5001D: 128.119.40.186, 802
updates table
S: 128.119.40.186, 80 D: 138.76.29.7, 5001 3
3: reply arrivesdest. address:138.76.29.7, 5001
4: NAT routerchanges datagramdest addr from138.76.29.7, 5001 to 10.0.0.1, 3345
Transport, Network and Link Layers 3-66
� 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
NAT: network address translation
� 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
Transport, Network and Link Layers 3-67
IPv6: motivation� initial motivation: 32-bit address space soon to be completely allocated.
� additional motivation:� header format helps speed processing/forwarding
� header changes to facilitate QoS
IPv6 datagram format: � fixed-length 40 byte header
� no fragmentation allowed
Transport, Network and Link Layers 3-68
IPv6 datagram format
priority: identify priority among datagrams in flow
flow Label: identify datagrams in same “flow.”(concept of“flow” not well defined).
next header: identify upper layer protocol for dataflow labelpriver
data
destination address(128 bits)
source address(128 bits)
payload len next hdr hop limitflow labelpriver
32 bitsTransport, Network and Link Layers 3-69
Other changes from IPv4
� checksum: removed entirely to reduce processing time at each hop
� options: allowed, but outside of header, indicated by “Next Header” field
� ICMPv6: new version of ICMP� ICMPv6: new version of ICMP� additional message types, e.g. “Packet Too Big”� multicast group management functions
Transport, Network and Link Layers 3-70
Transition from IPv4 to IPv6
� not all routers can be upgraded simultaneously� no “flag days”� how will network operate with mixed IPv4 and IPv6 routers?
� tunneling: IPv6 datagram carried as payload in IPv4 datagram among IPv4 routerstunneling: IPv6 datagram carried as payload in IPv4 datagram among IPv4 routers
IPv4 source, dest addr IPv4 header fields
IPv4 datagramIPv6 datagram
IPv4 payload
UDP/TCP payload
IPv6 source dest addrIPv6 header fields
Transport, Network and Link Layers 3-71
Chapter 3: outline
3.1 Transport layer
3.2 Network layer
3.3 Link layer
Transport, Network and Link Layers 3-72
Link layer: introduction
terminology:� hosts and routers: nodes
� communication channels that connect adjacent nodes along communication path: links
� wired links
global ISP
� wired links
� wireless links
� LANs
� layer-2 packet: frame,encapsulates datagram
data-link layer has responsibility of
transferring datagram from one node
to physically adjacent node over a linkTransport, Network and Link Layers 3-73
Link layer: context
� datagram transferred by different link protocols over different links:
� e.g., Ethernet on first link, frame relay on intermediate links, 802.11
transportation analogy:� trip from Princeton to Lausanne
� limo: Princeton to JFK
� plane: JFK to Geneva
� train: Geneva to Lausanne
� tourist = datagramintermediate links, 802.11 on last link
� each link protocol provides different services
� e.g., may or may not provide rdt over link
� tourist = datagram
� transport segment = communication link
� transportation mode = link layer protocol
� travel agent = routing algorithm
Transport, Network and Link Layers 3-74
Link layer services
� framing, link access:� encapsulate datagram into frame, adding header, trailer� channel access if shared medium� “MAC” addresses used in frame headers to identify source, dest
• different from IP address!• different from IP address!� reliable delivery between adjacent nodes
� we learned how to do this already!� seldom used on low bit-error link (fiber, some twisted pair)
� wireless links: high error rates
Transport, Network and Link Layers 3-75
� flow control:� pacing between adjacent sending and receiving nodes
� error detection: � errors caused by signal attenuation, noise.
� receiver detects presence of errors:
• signals sender for retransmission or drops frame
Link layer services (more)
• signals sender for retransmission or drops frame
� error correction:� receiver identifies and corrects bit error(s) without resorting to
retransmission
� half-duplex and full-duplex� with half duplex, nodes at both ends of link can transmit, but not
at same time
Transport, Network and Link Layers 3-76
Where is the link layer implemented?
� in each and every host
� link layer implemented in “adaptor” (aka network interface card NIC) or on a chip
� Ethernet card, 802.11 card; Ethernet chipset
cpu memory
applicationtransportnetwork
card; Ethernet chipset
� implements link, physical layer
� attaches into host’s system buses
� combination of hardware, software, firmware
controller
physicaltransmission
host bus (e.g., PCI)
network adaptercard
link
linkphysical
Transport, Network and Link Layers 3-77
Adaptors communicating
controller controller
sending host receiving host
datagram datagram
� sending side:
� encapsulates datagram in frame
� adds error checking bits, rdt, flow control, etc.
� receiving side
� looks for errors, rdt, flow control, etc
� extracts datagram, passes to upper layer at receiving side
datagram
frame
Transport, Network and Link Layers 3-78
Error detectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking, may include header fields
• Error detection not 100% reliable!• protocol may miss some errors, but rarely• larger EDC field yields better detection and correction
otherwise
Transport, Network and Link Layers 3-79
Parity checking
single bit parity:� detect single bit
errors
two-dimensional bit parity:� detect and correct single bit errors
0 0
Transport, Network and Link Layers 3-80
Internet checksum (review)
sender:� treat segment contents
as sequence of 16-bit
receiver:� compute checksum of
received segment
goal: detect “errors” (e.g., flipped bits) in transmitted packet (note: used at transport layer only)
as sequence of 16-bit integers
� checksum: addition (1’s complement sum) of segment contents
� sender puts checksum value into UDP checksum field
received segment� check if computed
checksum equals checksum field value:� NO - error detected� YES - no error detected. But maybe errors nonetheless?
Transport, Network and Link Layers 3-81
Multiple access links, protocolstwo types of “links”:� point-to-point
� PPP for dial-up access
� point-to-point link between Ethernet switch, host
� broadcast (shared wire or medium)� old-fashioned Ethernet
� upstream HFC� upstream HFC
� 802.11 wireless LAN
shared wire (e.g., cabled Ethernet)
shared RF(e.g., 802.11 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air, acoustical)
Transport, Network and Link Layers 3-82
Multiple access protocols� single shared broadcast channel
� two or more simultaneous transmissions by nodes: interference
� collision if node receives two or more signals at the same time
multiple access protocol� distributed algorithm that determines how nodes share
channel, i.e., determine when node can transmit
� communication about channel sharing must use channel itself! � no out-of-band channel for coordination
Transport, Network and Link Layers 3-83
An ideal multiple access protocol
given: broadcast channel of rate R bps
desiderata:1. when one node wants to transmit, it can send at rate R.
2. when M nodes want to transmit, each can send at average rate R/Mrate R/M
3. fully decentralized:
• no special node to coordinate transmissions
• no synchronization of clocks, slots4. simple
Transport, Network and Link Layers 3-84
MAC protocols: taxonomy
three broad classes:
� channel partitioning� divide channel into smaller “pieces” (time slots, frequency, code)
� allocate piece to node for exclusive use
� random access� channel not divided, allow collisions� channel not divided, allow collisions
� “recover” from collisions
� “taking turns”� nodes take turns, but nodes with more to send can take longer
turns
Transport, Network and Link Layers 3-85
Channel partitioning MAC protocols: TDMA
TDMA: time division multiple access� access to channel in "rounds" � each station gets fixed length slot (length = pkt trans time) in each round
� unused slots go idle example: 6-station LAN, 1,3,4 have pkt, slots � example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
Transport, Network and Link Layers 3-86
FDMA: frequency division multiple access � channel spectrum divided into frequency bands
� each station assigned fixed frequency band
� unused transmission time in frequency bands go idle
� example: 6-station LAN, 1,3,4 have pkt, frequency bands 2,5,6 idle
Channel partitioning MAC protocols: FDMA
idle
freq
uenc
y ba
nds
FDM cable
Transport, Network and Link Layers 3-87
Random access protocols
� when node has packet to send� transmit at full channel data rate R.� no a priori coordination among nodes
� two or more transmitting nodes ➜ “collision”,� random access MAC protocol specifies:
� how to detect collisionsrandom access MAC protocol specifies: � how to detect collisions� how to recover from collisions (e.g., via delayed retransmissions)
� examples of random access MAC protocols:� slotted ALOHA� ALOHA� CSMA, CSMA/CD, CSMA/CA
Transport, Network and Link Layers 3-88
“Taking turns” MAC protocols
channel partitioning MAC protocols:� share channel efficiently and fairly at high load
� inefficient at low load: delay in channel access, 1/N bandwidth allocated even if only 1 active node!
random access MAC protocolsrandom access MAC protocols� efficient at low load: single node can fully utilize channel
� high load: collision overhead
“taking turns” protocolslook for best of both worlds!
Transport, Network and Link Layers 3-89
polling:� master node “invites”
slave nodes to transmit in turn
� typically used with “dumb” slave devices master
polldata
“Taking turns” MAC protocols
“dumb” slave devices� concerns:
� polling overhead
� latency
� single point of failure (master)
master
slaves
data
Transport, Network and Link Layers 3-90
token passing:� control token passed
from one node to next sequentially.
� token message
� concerns:
T
(nothingto send)
“Taking turns” MAC protocols
� concerns:
� token overhead
� latency
� single point of failure (token)
data
to send)
T
Transport, Network and Link Layers 3-91
MAC addresses and ARP
� 32-bit IP address: � network-layer address for interface
� used for layer 3 (network layer) forwarding
� MAC (or LAN or physical or Ethernet) address:� function: used ‘locally” to get frame from one interface to � function: used ‘locally” to get frame from one interface to another physically-connected interface (same network, in IP-addressing sense)
� 48 bit MAC address (for most LANs) burned in NIC ROM, also sometimes software settable
� e.g.: 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each “number” represents 4 bits)
Transport, Network and Link Layers 3-92
LAN addresses and ARP
each adapter on LAN has unique LAN address
1A-2F-BB-76-09-AD
adapter
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Transport, Network and Link Layers 3-93
Ethernet
“dominant” wired LAN technology:
� cheap $20 for NIC
� first widely used LAN technology
� simpler, cheaper than token LANs and ATM
� kept up with speed race: 10 Mbps – 10 Gbps
Metcalfe’s Ethernet sketch Transport, Network and Link Layers 3-94
Ethernet: physical topology
� bus: popular through mid 90s� all nodes in same collision domain (can collide with each other)
� star: prevails today� active switch in center
� each “spoke” runs a (separate) Ethernet protocol (nodes do not collide with each other)do not collide with each other)
switch
bus: coaxial cablestar
Transport, Network and Link Layers 3-95
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
dest.address
sourceaddress
data (payload) CRCpreamble
type
preamble:
� 7 bytes with pattern 10101010 followed by one byte with pattern 10101011
� used to synchronize receiver, sender clock rates
Transport, Network and Link Layers 3-96
Ethernet frame structure (more)
� addresses: 6 byte source, destination MAC addresses� if adapter receives frame with matching destination address, or with broadcast address (e.g. ARP packet), it passes data in frame to network layer protocol
� otherwise, adapter discards frame
� type: indicates higher layer protocol (mostly IP but � type: indicates higher layer protocol (mostly IP but others possible, e.g., Novell IPX, AppleTalk)
� CRC: cyclic redundancy check at receiver� error detected: frame is dropped
dest.address
sourceaddress
data (payload) CRCpreamble
type
Transport, Network and Link Layers 3-97
Ethernet: unreliable, connectionless
� connectionless: no handshaking between sending and receiving NICs
� unreliable: receiving NIC doesnt send acks or nacks to sending NIC
� data in dropped frames recovered only if initial � data in dropped frames recovered only if initial sender uses higher layer rdt (e.g., TCP), otherwise dropped data lost
� Ethernet’s MAC protocol: unslotted CSMA/CD wth binary backoff
Transport, Network and Link Layers 3-98
Ethernet switch� link-layer device: takes an active role
� store, forward Ethernet frames
� examine incoming frame’s MAC address, selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment, uses CSMA/CD to access segmentsegment, uses CSMA/CD to access segment
� transparent
� hosts are unaware of presence of switches
� plug-and-play, self-learning
� switches do not need to be configured
Transport, Network and Link Layers 3-99
Switch: multiple simultaneous transmissions
� hosts have dedicated, direct connection to switch
� switches buffer packets
� Ethernet protocol used on eachincoming link, but no collisions; full duplex
A
BC’
1 2
345
6
full duplex
� each link is its own collision domain
� switching: A-to-A’ and B-to-B’can transmit simultaneously, without collisions switch with six interfaces
(1,2,3,4,5,6)
A’
B’ C
345
Transport, Network and Link Layers 3-100
Interconnecting switches
� switches can be connected together
S
S4
A
B
S1
C D
E
FS2
S3
HI
G
Transport, Network and Link Layers 3-101
Institutional network
to externalnetwork
router
mail server
web server
IP subnet
Transport, Network and Link Layers 3-102
Switches vs. routers
both are store-and-forward:
� routers: network-layer devices (examine network-layer headers)
� switches: link-layer devices (examine link-layer headers)
application
transport
network
link
physical link
physical
switch
datagram
frame
frame
headers)
both have forwarding tables:� routers: compute tables
using routing algorithms, IP addresses
� switches: learn forwarding table using flooding, learning, MAC addresses
network
link
physicalapplication
transport
network
link
physical
frame
datagram
Transport, Network and Link Layers 3-103
Chapter 3: summary
� transport layer
� UDP
� TCP
� Congestion
� network layer
our study of layers now complete!
� network layer
� Routing
� IP
� DHCP
� NAT
� link layer
� Error detection
� MAC
� EthernetTransport, Network and Link Layers 3-104
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