Internet Routing Internet Routing COS 598A COS 598A Jennifer Rexford Jennifer Rexford http://www.cs.princeton.edu/~jrex/ http://www.cs.princeton.edu/~jrex/ teaching/spring2005 teaching/spring2005 Tuesdays/Thursdays 11:00am-12:20pm Tuesdays/Thursdays 11:00am-12:20pm
31
Embed
Internet Routing COS 598A Jennifer Rexford jrex/teaching/spring2005 Tuesdays/Thursdays 11:00am-12:20pm.
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Internet Routing Internet Routing COS 598ACOS 598A
• Who am I?– Joined the CS faculty in Feb 2005 (i.e.,
today)– Worked for 8.5 years at AT&T Labs—
Research– Research on routing protocols, network
measurement, and network operations
• Who are you, and what do you do?– Introductions…
What is Internet Routing?
• The glue that holds the Internet together
• How routers know where to forward packets
• How operators control the load on their links
• How networks achieve business relationships
1
2
34
5
67
Client Web server
What Does This Course Cover?
• Internet architecture– Best-effort packet-delivery service– Intradomain and interdomain routing
• Network topology– Inside a network, and between networks
• Traffic engineering– Getting the traffic to go where you want
• Convergence– Delay to respond to change– Whether the protocol ever converges
What Does the Course Cover? (Continued)
• Routers– Router hardware and software– Router configuration– Scaling to many destinations, routers, &
networks
• Measurement– Monitoring the routing protocols– Characterizing the routing system– Troubleshooting routing problems
• Routing protocol security• New architectural directions
Emphasis of the Course
• Not so much on the protocols– …though we will cover BGP, OSPF, IS-IS,
MPLS, and various other acronyms of the day
• Or on the routers– …though we will talk about how routers work
• But more on how people manage routing– Selecting which protocols to use– Deciding how to set the parameters– Troubleshooting problems as they arise– Preventing attacks– …
Structure of the Course
• Classroom time– Mixture of lecture and discussion of papers
• Readings– Selected research papers and surveys– Videocasts of presentations (e.g., from NANOG)– Optional short “food for thought” reading each week
• Course project– Literature survey, measurement or simulation study,
protocol design, theoretical analysis, etc.
• Grading– Final course project (written report and oral
presentation)– Class participation (written reviews, class discussion,
etc.)
Today, and Thursday
• Goal– Explain IP best-effort delivery model
• Today– What is the service model?– How can you do anything useful with this?
• Thursday– How do the routers support the service
model?– How do the routing protocols work?
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 on every packet
04/18/23
IP Packet Structure
4-bitVersion
4-bitHeaderLength
8-bitType of Service
(TOS)16-bit Total Length (Bytes)
16-bit Identification3-bitFlags 13-bit Fragment Offset
8-bit Time to Live (TTL) 8-bit Protocol 16-bit Header Checksum
32-bit Source IP Address
32-bit Destination IP Address
Options (if any)
Payload
20-byte20-byteHeaderHeader
usually IPv4 usually 20 bytes
fragments
more later
errorcheckheader
IP Service Model: Why Best-Effort?
• It’s easier not to make promises– Don’t need to reserve bandwidth and memory– Don’t need to do error detection & correction– Don’t need to 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– Higher-level protocol can provide error checking
• Successive packets may not follow the same path– Not a problem as long as packets reach the
destination
• Packets can be delivered out-of-order– Receiver can put packets back in order (if
necessary)
• Packets may be lost or arbitrarily delayed– Sender can send the packets again (if desired)
• No network congestion control (beyond “drop”)– Sender can slow down in response to loss or delay
Layering in the IP Protocols
Internet Protocol
Transmission ControlProtocol (TCP)
User Datagram Protocol (UDP)
TelnetHTTP
SONET ATMEthernet
RTPDNSFTP
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
source network destination
TCP connection
Source and Destination Port Numbers
• Motivation for port numbers– Unique identifier of the TCP connection on each end– Necessary to (de)multiplex packets at the end-
points
• Assigning port numbers– Port numbers below 1024 are assigned– Well-known port numbers for common applications
• Web client contacting a web server– Browser click results in creation of a TCP socket– Client machine assigns an available port (>=1024)– Client machine requests a connection with the
server– Open TCP connection to port 80 at the server
Opening and Closing a TCP Connection
• 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
SYN
SYN
AC
K
AC
KD
ata
FIN
AC
K
AC
K
timeA
B
FIN
AC
K
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 tries to avoid overloading the network
(increase with successful delivery, decrease with loss)– TCP connection starts with small initial congestion window
timecon
gesti
on
win
dow
slow start
congestion avoidance
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
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.foo.com”
• 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
com edu org ac uk zw arpa
unnamed root
bar
west east
foo my
ac
cam
usr
in-addr
12
34
56
generic domains country domains
my.east.bar.edu usr.cam.ac.uk
12.34.56.0/24
DNS Resolver and Local DNS Server
Application
DNS resolver
Local DNSserver
1 10
DNS cache
DNS query
2
DNS response 9
Root server
3
4
Top-leveldomain server
5
6
Second-leveldomain server
7
8
Caching based on a time-to-live (TTL) assigned by the DNS server responsible for the host name to reduce latency in DNS translation.
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, …
Client Server
GET /index.html HTTP/1.1
HTTP/1.1 200 OK
Example: Many Steps in Web Download
Browser cache
DNSresolution
TCPopen
1st byteresponse
Last byteresponse
Sources of variability of delay• Browser cache hit/miss, need for cache
revalidation• DNS cache hit/miss, multiple DNS servers,
errors• Packet loss, high RTT, 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
HTTP
TCP
IP
Ethernetinterface
HTTP
TCP
IP
Ethernetinterface
IP IP
Ethernetinterface
Ethernetinterface
SONETinterface
SONETinterface
host host
router router
HTTP message
TCP segment
IP packet IP packetIP packet
This course focuses on the routers…
Happy Routers Make Happy Packets
• Routers forward packets– Forward incoming packet to outgoing link– Store packets in queues– Drop packets when necessary