1. Networking Fundamentals

2. Basics
Network – collection of nodes and links that cooperate for communication
Nodes – computer systems
Internal (routers, bridges, switches)
Terminal (workstations)
Links – connections for transmitting data
Protocol – standards for formatting and interpreting data and control information

3. Network = {nodes and links}
Circles are nodes, lines are links. Circles represent hosts and routers. Links represent wires, fibers, etc.

4. Nodes have addresses 7 4 1 6 2 8 5 3
Addresses identify destinations. If a node is presented with data destined for an address, it can tell
Is it me?
Where should I send it instead?
Really, it’s the interfaces, each end of a link that has an address, but this is the basic stuff. 9

5. Links have bandwidths and latencies7 4 1 6 2 8 5 3 9

6. Wires aren’t perfect Attenuation (resistance)
degrades quality of signal
Delay (speed of light * 2/3)
speed of light:
8ms RTT coast-to-coast
8 minutes to the sun
Noise (microwaves and such)
Nodes aren’t perfect either
Unreliability is pervasive!
End nodes might not know when bits start and when they stop, might misinterpret bits.
Some forwarding nodes have been found to swap bytes occasionally.

7. Getting Data Across (imperfect wires)
Split up big files into small pieces
the pieces are called packets
Each packet (~1500 bytes) is sent separately
packets can be corrupted
noise, bugs
packets can be dropped
corrupted, overloaded nodes
packets can be reordered
retransmission + different paths
Allows packets from different flows to be multiplexed along the same link
That is, the link can be shared between different applications communicating by this sort of time-slicing approach.

8. Layers
Each layer abstracts the services of various lower layers, providing a uniform interface to higher layers.
Each layer needs to know:
How to interpret a packet’s payload
e.g., protocol numbers
How to use the services of a lower layer
The structure of network design.

9. OSI Levels Node A Node B Application Application Presentation
Transport
Transport
Network
Network
Data Link
Data Link
Physical
Physical
Network

10. Layers OSI Reference Reality Packet Format App data Application
Presentation
Session
Transport
Network
Data-Link
Physical
HTTP
TCP IP
Ethernet
Twisted Pair
IP Payload
Packet format is the realization of the layered model as bits on a wire.
Physical: bits on a wire
Data-link: frames on a wire (collections of bits traversing a single (conceptual) wire
Network: packets sent across various data-links
Transport: flows / bytestreams / sets of packets logically related, possibly reliable
Session and Presentation: no one knows anymore
Application: http, etc.
Ethernet Payload

11. The Internet Protocol (IP)
Connects disparate networks
Single (hierarchical) address space
Single network header
Assumes data link is unreliable
Provides unreliable service
Loss: A B D E
Duplication: A B B C D E
Corruption: A Q C D E
Reordering: A C D B E
By single, I imply global, universal.
By unreliable, I don’t mean that it fails often, I mean that packets, when sent, don’t necessarily get to their destination intact or at all.

12. IP Addresses 32 bits long, split into 4 octets: Hierarchical:
For example,
Hierarchical:
First bits describe which network
Last bits describe which host on the network
UW subnets include:
128.95, …
UW CSE subnets include:
, , …

13. Packet Forwarding Buffer incoming packets Decide which output link
Buffer outgoing packets
Send packet
How fast does the backplane bus have to be? 5x the speed of the links.
Why input buffering? Packet takes time to arrive.
Why output buffering? Might have packets from different inputs to the same output, want to keep utilization high.

14. Routing
How do nodes determine which output link to use to reach a destination?
Distributed algorithm for converging on shortest path tree
Nodes exchange reachability information:
“I can get to x in 3 hops”

15. Shortest path tree 7 4 (2) 1 (1) 6 2 8 (1) (2) (1) 5 3 9
Packets destined for 6 pass along this tree. 4 would forward a packet destined for 6 via 2.
Such a tree exists for each destination.
3 can choose arbitrarily between 2 and 5 to get to 6. Might be best if it sticks with just one, though.
Problem (segue): one forwarding table entry and one line in a message per destination address.
Need route aggregation! 9
(x) Is the cost to get to 6. The metric (cost per link) here is 1.
Simple algorithm: 6 broadcasts “I’m alive” to neighbors.
Neighbors send “I can get to 6 in 1 hop”, etc.

16. Route Aggregation What hierarchical addressing is good for.
UW routers can advertise x.x
instead of x, ,x, …
Other routers don’t need forwarding table entries for each host in the network.

17. Routing Reality
Routing in the Internet connects Autonomous Systems (AS’s)
AT&T, Sprint, UUNet, BBN…
Shortest path, sort of… money talks.
actually a horrible mess
nobody really knows what’s going on
get high $$ job if you are a network engineer that messes with this stuff
The mess of human intervention implies things will fail.
Cooperation of companies in carrying traffic implies it’s very hard to debug this system (not only is it a distributed system, but its one with distributed administration)
Tracing DoS attacks is nearly impossible, unless you’ve got a lot of money.

18. TCP Service Model
Provide reliability, ordering on the unreliable, unordered IP
Bytestream oriented: when you send data using TCP, you think about bytes, not about packets.

19. TCP Ports Connections are identified by the tuple:
IP source address
IP destination address
TCP source port
TCP destination port
Allows multiple connections; multiple application protocols, between the same machines
Well known ports for some applications: (web: 80, telnet: 23, mail:25, dns: 53)
Previously a typo on source, dest port.

20. TCP’s Sliding Window Simple reliability: Better performance:
Send one packet, wait for acknowledgment, then send the next…
Better performance:
Keep several unacknowledged packets in the network (a window)
Previously a typo on the last bullet.

21. Sliding Window Example
Send 1
Send 2
Send 3
Send 4
Send 5
Send 6
Send 7
Send 8
Send 9
Window size = 3
Can send up to three packets into the network at a time.
Each packet has a sequence number for ordering
Ack 1
Ack 2
Ack 3
Ack 4
Ack 5
Ack 6
Ack 7
Ack 8

22. TCP’s Congestion Control
How big should the window be?
Performance is limited by:
(window size) / round trip time
Performance of bottleneck link (modem?)
If window is too small, performance is wasted.
If window is too big, may overflow network buffers, causing packet loss.

23. Steps for a web access Name lookup TCP Connection setup
Client to local DNS server
Local DNS may return a cached binding, or lookup the name for itself
TCP Connection setup
Client to remote IP, port 80
Send HTTP request
“GET /index.html”
Receive HTTP response
“blah blah blah” maybe several packets
TCP Connection teardown
And as the request goes out to the remote ip, nodes along the path lookup the next hop along the path in the forwarding table they built using a routing protocol.
The http response is a collection of several packets, each with a sequence number (actually, sequence numbers apply to bytes). They are retransmitted if lost, and checked for corruption before being delivered to the web client.
The TCP connection has a teardown phase symmetric to the setup; since state information is stored for each connection, both sides agree to free that state when done.

24. HTTP 1.1 Incremental improvements
“Persistent connections” allow multiple requests over the same connection
Web transfers are often small
Avoid connection setup and teardown overhead
TCP is better the longer you use it: it learns how fast to send to get best performance without overflowing buffers.