Internet Routing
COS 598A
Jennifer Rexford
http://www.cs.princeton.edu/~jrex/teaching/spring2005
Tuesdays/Thursdays 11:00am-12:20pm
Who am I, and Who are You?
• 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
4
3
5
2
1
Client
7
6
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
IP Packet Structure
usually 20 bytes
usually IPv4
4-bit
8-bit
4-bit
Version Header Type of Service
Length
(TOS)
3-bit
Flags
16-bit Identification
fragments
8-bit Time to
Live (TTL)
16-bit Total Length (Bytes)
8-bit Protocol
13-bit Fragment Offset
16-bit Header Checksum
20-byte
Header
32-bit Source IP Address
more later
32-bit Destination IP Address
Options (if any)
Payload
error
check
header
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
HTTP
Telnet
FTP
DNS
Transmission Control
Protocol (TCP)
User Datagram
Protocol (UDP)
Internet Protocol
SONET
Ethernet
RTP
ATM
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
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
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
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
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.
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, 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
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
This course focuses on the routers…
IP
Ethernet
interface
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
Reading for Thursday
• Two classic papers
– End-to-end arguments in system design (1984)
– Design philosophy of the DARPA Internet protocols
(1988)
• New perspectives on success of the Internet
– Tussle in cyberspace: Defining tomorrow’s Internet
(2002)
Backup Slides
• TTL and traceroute
Time-to-Live Field
• Potential robustness problem
– Routing loops can cause packets to cycle forever
– Confusing if the packet arrives much later
• Time-to-live field in packet header
– TTL field decremented by each router on the path
– Packet is discarded when TTL field reaches 0…
– …and send “timer expired” message to source
Traceroute: Measuring the Forwarding Path
• Time-To-Live field in IP packet header
– Source sends a packet with a TTL of n
– Each router along the path decrements the TTL
– “TTL exceeded” sent when TTL reaches 0
• Traceroute tool exploits this TTL behavior
TTL=1
source
Time
exceeded
destination
TTL=2
Send packets with TTL=1, 2, 3, … and record source of “time exceeded” message