Download jen-network - Princeton University

Computer Networks
Guest Lecture in COS 318
Jennifer Rexford
http://www.cs.princeton.edu/~jrex
Goal of the Lecture
• Brief introduction to data networking
– Best-effort service and the hourglass model
– From sending packets to downloading Web pages
– Internet addressing, routing, and topology
• Teaser for COS 461, offered next term
– MW 1:30-2:50pm
Best-Effort Packet-Delivery Service
IP Service Model: Best-Effort Packet Delivery
• Packet switching
– Send data in packets
– Header with source & destination address
• Best-effort delivery
– Packets may be lost
– Packets may be corrupted
– Packets may be delivered out of order
source
destination
IP network
IP Service Model: Why Packets?
• Data traffic is bursty
– Logging in to remote machines
– Exchanging e-mail messages
• Don’t want to waste reserved bandwidth
– No traffic exchanged during idle periods
• Better to allow multiplexing
– Different transfers share access to same links
• Packets can be delivered by most anything
– RFC 2549: IP over Avian Carriers (aka birds)
• … still, packet switching can be inefficient
– Extra header bits (“envelope”) for every packet
IP Service Model: Why Best-Effort?
• It’s easier not to make promises
– Don’t reserve bandwidth and memory
– Don’t do error detection and correction
– Don’t remember from one packet to next
• Easier to survive failures
– Transient disruptions are okay during failover
• … but, applications do want efficient, accurate
transfer of data in order, in a timely fashion
IP Service Model: Best-Effort is Enough
• No error detection or correction
– Receiver can discard corrupted packets
– Sender can send the packets again
• Successive packets may not follow the same path
– Okay as long as packets reach the destination
• Packets can be delivered out-of-order
– Receiver can put packets back in order
• Packets may be lost or arbitrarily delayed
– Sender can send the packets again
• No network congestion control (beyond “drop”)
– Sender can slow down in response to loss or delay
Layering in the IP Protocols: Hourglass
HTTP
Telnet
FTP
DNS
Transmission Control
Protocol (TCP)
User Datagram
Protocol (UDP)
Internet Protocol
SONET
Ethernet
RTP
ATM
Transport Protocols: Between End Hosts
Transmission Control Protocol (TCP)
• Communication service (socket)
– Ordered, reliable byte stream
– Simultaneous transmission in both directions
• Key mechanisms at end hosts
–
–
–
–
Retransmit lost and corrupted packets
Discard duplicate packets and put packets in order
Flow control to avoid overloading the receiver buffer
Congestion control to adapt sending rate to network load
TCP connection
source
network
destination
Opening and Closing a TCP Connection
B
A
time
• Three-way handshake to establish connection
– Host A sends a SYN to the host B
– Host B returns a SYN and acknowledgement
– Host A sends an ACK to acknowledge the SYN ACK
• Four-way handshake to close the connection
– Finish (FIN) to close and receive remaining bytes , or
– Reset (RST) to close and not receive remaining bytes
Lost and Corrupted Packets
• Detecting corrupted and lost packets
– Error detection via checksum on header and data
– Sender sends packet, sets timeout, and waits for ACK
– Receiver sends ACKs for received packets
– Sender infers loss from timeout or duplicate ACKs
• Retransmission by sender
– Sender retransmits lost/corrupted packets
– Receiver reassembles and reorders packets
– Receiver discards corrupted and duplicated packets
TCP Flow and Congestion Control
• Window-based flow control
– Sender limits number of outstanding bytes (window size)
– Receiver window ensures data does not overflow receiver
• Adapting to network congestion
congestion window
– Congestion window tries to avoid overloading the network
(increase with successful delivery, decrease with loss)
– TCP connection starts with small initial congestion window
congestion avoidance
slow start
time
User Datagram Protocol (UDP)
• Some applications do not want or need TCP
– Avoid overhead of opening/closing a connection
– Avoid recovery from lost/corrupted packets
– Avoid sender adaptation to loss/congestion
• Example applications that use UDP
– Multimedia streaming applications
– Domain Name System (DNS) queries/replies
• Dealing with the growth in UDP traffic
– Interference with TCP performance
– Pressure to apply congestion control
– Future routers may enforce “TCP-friendly” behavior
Converting Host Names to Numerical
Addresses
Domain Name System (DNS)
• Properties of DNS
– Hierarchical name space divided into zones
– Translation of names to/from IP addresses
– Distributed over a collection of DNS servers
• Client application
– Extract server name (e.g., from the URL)
– Invoke system call to trigger DNS resolver code
– E.g., gethostbyname() on “www.cs.princeton.edu”
• Server application
– Extract client IP address from socket
– Optionally invoke system call to translate into name
– E.g., gethostbyaddr() on “12.34.158.5”
Domain Name System
unnamed root
com
edu
org
generic domains
bar
uk
ac
zw
arpa
country domains
ac
inaddr
west
east
cam
12
foo
my
usr
34
my.east.bar.edu
usr.cam.ac.uk
56
12.34.56.0/24
DNS Resolver and Local DNS Server
Root server
3
4
Application
DNS cache
5
1
10
DNS resolver
DNS query
2
6
Local DNS
server
Top-level
domain server
7
DNS response 9
8
Second-level
domain server
Caching based on a time-to-live (TTL) assigned by the DNS server
responsible for the host name to reduce latency in DNS translation.
Building Applications on Top (e.g., Web)
Application-Layer Protocols
• Messages exchanged between applications
– Syntax and semantics of the messages between hosts
– Tailored to the specific application (e.g., Web, e-mail)
– Messages transferred over transport connection (e.g., TCP)
• Popular application-layer protocols
– Telnet, FTP, SMTP, NNTP, HTTP, …
GET /index.html HTTP/1.1
Client
HTTP/1.1 200 OK
Server
Example: Many Steps in Web Download
Browser
cache
DNS
resolution
TCP
open
1st byte
response
Last byte
response
Sources of variability of delay
• Browser cache hit/miss, need for cache revalidation
• DNS cache hit/miss, multiple DNS servers, errors
• Packet loss, round-trip time, server accept queue
• RTT, busy server, CPU overhead (e.g., CGI script)
• Response size, receive buffer size, congestion
• … downloading embedded image(s) on the page
IP Suite: End Hosts vs. Routers
host
host
HTTP message
HTTP
TCP segment
TCP
router
IP
Ethernet
interface
HTTP
IP packet
Ethernet
interface
IP
TCP
router
IP packet
SONET
interface
SONET
interface
IP
IP packet
Ethernet
interface
IP
Ethernet
interface
Routers, Addressing, and Forwarding
What is a Router?
• A computer with…
– Multiple interfaces
– Implementing routing protocols
– Packet forwarding
• Wide range of variations of routers
– Small LinkSys device in a home network
– Linux-based PC running router software
– Million-dollar high-end routers with large chassis
• … and links
– Serial line
– Ethernet
– Packet-over-SONET
Network Components
Links
Interfaces
Fibers
Ethernet card
Switches/routers
Large router
Wireless card
Coaxial Cable
Telephone
switch
Inside a High-End Router
Processor
Line card
Line card
Line card
Line card
Switching
Fabric
Line card
Line card
Happy Routers Make Happy Packets
• Routers forward packets
– Forward incoming packet to outgoing link
– Store packets in queues
– Drop packets when necessary
• Routers compute paths
– Routers run routing protocols
– Routers compute forwarding tables
• A famous quotation from RFC 791
– “A name indicates what we seek.
An address indicates where it is.
A route indicates how we get there.”
-- Jon Postel
IP Addressing
• 32-bit number in dotted-quad notation (12.34.158.5)
• Divided into network & host portions (left and right)
• 12.34.158.0/24 is a 24-bit prefix with 28 addresses
12
34
158
5
00001100 00100010 10011110 00000101
Network (24 bits)
Host (8 bits)
whois –h whois.arin.net 128.112.136.35
OrgName: Princeton University
OrgID: PRNU
Address: Office of Information Technology
Address: 87 Prospect Avenue
City: Princeton
StateProv: NJ
PostalCode: 08544-2007
Country: US
NetRange: 128.112.0.0 - 128.112.255.255
CIDR: 128.112.0.0/16
NetName: PRINCETON
NetHandle: NET-128-112-0-0-1
Parent: NET-128-0-0-0-0
NetType: Direct Allocation
RegDate: 1986-02-24
Packet Forwarding
• Forwarding tables in IP routers
– Maps each IP prefix to next-hop link(s)
• Destination-based forwarding
– Packet has a destination address
– Router identifies longest-matching prefix
– Cute algorithmic problem: very fast lookups
forwarding table
destination
12.34.158.5
4.0.0.0/8
4.83.128.0/17
12.0.0.0/8
12.34.158.0/24
126.255.103.0/24
outgoing link
Serial0/0.1
Internet Topology and Routing
Autonomous Systems (ASes)
Path: 6, 5, 4, 3, 2, 1
4
3
5
2
7
1
6
Web server
Client
Internet Routing Architecture
• Divided into Autonomous Systems
– Distinct regions of administrative control
– Routers/links managed by a single “institution”
– Service provider, company, university, …
• Hierarchy of Autonomous Systems
– Large, tier-1 provider with a nationwide backbone
– Medium-sized regional provider with smaller backbone
– Small network run by a single company or university
• Interaction between Autonomous Systems
– Internal topology is not shared between ASes
– … but, neighboring ASes interact to coordinate routing
Autonomous System Numbers
AS Numbers are 16 bit values.
Currently around 20,000 in use.
•
•
•
•
•
•
•
•
•
Level 3: 1
MIT: 3
Harvard: 11
Yale: 29
Princeton: 88
AT&T: 7018, 6341, 5074, …
UUNET: 701, 702, 284, 12199, …
Sprint: 1239, 1240, 6211, 6242, …
…
Interdomain Routing (Between ASes)
• ASes exchange info about who they can reach
– IP prefix: block of destination IP addresses
– AS path: sequence of ASes along the path
• Policies configured by the network operator
– Path selection: which of the paths to use?
– Path export: which neighbors to tell?
“I can reach 12.34.158.0/24
via AS 1”
“I can reach 12.34.158.0/24”
2
1
data traffic
12.34.158.5
3
data traffic
Inside an AS: Abilene Internet2 Backbone
Intradomain Routing (Within an AS)
• Routers exchange topology information
– Routers compute “next hop” to other routers
– Path chosen based on link weights (shortest path)
• Link weights configured by network operator
– … to control the flow of traffic
2
3
2
1
1
1
3
5
4
3
Funny Things About the Internet
• Nobody really knows how big it is
– No global registry of the topology
• Hard to know what traffic it carries
– New applications try to hide their identity
• Built based on trust in others
– Do congestion control, announce only the
addresses you own, and so on
• Operators do a lot of things manually
– Half of outages are caused by operator error
• Diagnosing performance problems is hard
– So many things can go wrong, in so many places
Learn More
• COS 461, spring 2006
– MW 1:30-2:50pm