15-netwoppppppppppppppppppppppppppprk.ppt
- 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}
- 5. Links have bandwidths and latencies 1 3 2 5 4 6 9 8 7
- 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!
- 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
- 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
- 9. OSI Levels Presentation Transport Network Data Link Physical Application Presentation Transport Network Data Link Physical Application Node A Node B Network
- 10. Layers Application Presentation Session Transport Network Data-Link Physical HTTP TCP IP Ethernet Twisted Pair OSI Reference Reality Packet Format Ethernet Payload IP Payload App data
- 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
- 12. IP Addresses • 32 bits long, split into 4 octets: – For example, 128.95.2.24 • Hierarchical: – First bits describe which network – Last bits describe which host on the network • UW subnets include: – 128.95, 140.142… • UW CSE subnets include: – 128.95.2, 128.95.4, 128.95.219…
- 13. Packet Forwarding • Buffer incoming packets • Decide which output link • Buffer outgoing packets • Send packet
- 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 128.95.2.x in 3 hops”
- 15. Shortest path tree 1 3 2 5 4 6 9 8 7 (1) (2) (1) (1) (2) (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 128.95.x.x – instead of 128.95.2.x, 128.95.3,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
- 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)
- 20. TCP’s Sliding Window • Simple reliability: – Send one packet, wait for acknowledgment, then send the next… • Better performance: – Keep several unacknowledged packets in the network (a window)
- 21. Sliding Window Example Send 1 Send 2 Send 3 Send 4 Send 5 Send 6 Send 7 Send 8 Send 9 Ack 1 Ack 2 Ack 3 Ack 4 Ack 5 Ack 6 Ack 7 Ack 8 • Window size = 3 • Can send up to three packets into the network at a time. • Each packet has a sequence number for ordering
- 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 – 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
- 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.
Editor's Notes
- #3: Circles are nodes, lines are links. Circles represent hosts and routers. Links represent wires, fibers, etc.
- #4: 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.
- #6: 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: That is, the link can be shared between different applications communicating by this sort of time-slicing approach.
- #8: The structure of network design.
- #10: 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.
- #11: 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.
- #13: 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.
- #15: 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!
- #17: 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.
- #19: Previously a typo on source, dest port.
- #20: Previously a typo on the last bullet.
- #23: 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.