P561: Network Systems “A good network is one that I never have to think about” – Greg Minshall Tom Anderson Ratul Mahajan TA: Colin Dixon True some of the time… 2 Course Goals Project: Fishnet Technology Survey Build an ad hoc wireless network in stages: − How things work − Step 1: basic communication − How they are likely to work in the future − Step 2: routing − Step 3: transport and congestion control − Step 4: applications Design and implementation of network protocols Three modes: − Simulation (all nodes in one process) Research state of the art − Emulation (each node in its own process; interoperability) − Physical (on a PDA or cell phone) Details on the web site; due dates week 3, 5, 7, 10 3 4 Blogs Reading < Class By 5pm before each class, add a unique new comment on one of the questions posted to the Example: Internet has a TTL (time to live) field in web site each packet − Decremented on each hop Example Q: Instead of PPR, why not use smaller − When it gets to zero, router drops packet and sends an error packet back to the source packets? − Essential to correct operation of the Internet, and to its diagnosis Example blog: ? Before class, read the other comments 5 6 1
Pop Quiz #1 Pop Quiz #2 How could you use this to determine link latency? How could you use this to determine link bandwidth? 7 8 Pop Quiz #3 A Systems Approach to Networks How else could you determine link bandwidth? Most interesting applications of computers require: − Fault tolerance − Coordination of concurrent activities − Geographically separated but linked data − Vast quantities of stored information − Protection from mistakes and intentional attacks − Interactions with many people − Evolution over time Networks are no different! 9 10 Network Systems: Design Patterns An Anecdote Scale by connecting smaller pieces together BGP: protocol to exchange routes between ISPs − With no central state − Two primary vendors: Cisco and Juniper Reliability out of unreliability − Monoculture within a given ISP − Stateful: only send updates; 100K routes exchanged − In any system with a billion components, many will be broken at any point in time − And some will fail in bizarre ways When you get a receive an invalid route, what do Interoperability you do? − And what do you think happened in practice? − No single vendor + quasi-formal specs => often unpredictable behavior − Layering to manage complexity − Once standardized, hard to impossible to fix 11 12 2
Another Anecdote Internet Design Patterns Be liberal in what you accept, conservative in what In 1997 and 2001, a small mis-configuration at one ISP you send disrupted Internet connectivity on a global scale − Nothing prevented one ISP from announcing that it can Spread bad news quickly, good news slowly deliver packets for any Internet prefix Internet is still vulnerable to this same problem Use only soft state inside the network − Over half of all new Internet route announcements are misconfigurations! Avoid putting functionality into the network unless − Until recently, Cisco’s Internet prefix was hijacked on a absolutely necessary regular basis 14 Internet Design Patterns in Practice A Brief Tour of the Internet Be liberal in what you accept, conservative in what What happens when you “click” on a web link? you send − Security suggests the opposite Spread bad news quickly, good news slowly request − Inconsistent state is a barrier to improving availability Internet Use only soft state inside the network response − NATs, firewalls, etc. You at home www.msn.com (client) Avoid putting functionality into the network unless (server) absolutely necessary Ubiquitous middleboxes This is the view from 10,000 ft … • 15 16 9,000 ft: Scalability 8,000 ft: Naming (DNS) Caching improves scalability Map domain names to IP network addresses Nameserver Cache www.msn.com “Have it?” “What’s the IP address for www.msn.com?” “No” “Changed?” “It’s 207.68.173.231” “Here it is.” 128.95.2.106 128.95.2.1 We cut down on transfers: All messages are sent using IP addresses Check cache (local or proxy) for a copy − So we have to translate names to addresses first − Check with server for a new version − But we cache translations to avoid next time − 17 18 3
7,000 ft: Sessions (HTTP) 6,000 ft: Reliability (TCP) A single web page can be multiple “objects” Messages can get lost www.msn.com GET index.html (lost) retransmission GET ad.gif acknowledgment GET logo.gif Fetch each “object” We acknowledge successful receipt and detect and retransmit lost messages (e.g., timeouts); − either sequentially or in parallel checksums to detect corruption 19 20 4,000 ft: Packets (TCP/IP) 5,000 ft: Congestion (TCP) Need to allocate bandwidth between users Long messages are broken into packets − Maximum Ethernet packet is 1.5 Kbytes − Typical web object is 10s of Kbytes How fast can I send? 4. ml 3. x.ht 2. inde 1. GET GET index.html Senders balance available and required Number the segments for reassembly bandwidths by probing network path and observing the response 21 22 3,000 ft: Routing (IP) 2,000 ft: Multi-access (e.g., Cable) May need to share links with other senders Packets are directed through many routers R H H R H H R Headend R R Poll headend to receive a timeslot to send R R H: Hosts upstream H R R: Routers − Headend controls all downstream transmissions H − A lower level of addressing is used … 23 24 4
Different kinds of addresses 1,000 ft: Framing/Modulation Domain name (e.g. www.msn.com) − Global, human readable Protect, delimit and modulate payload as a signal IP Address (e.g. 207.200.73.8) − Global, works across all networks Sync / Unique Header Payload w/ error correcting code Ethernet (e.g. 08-00-2b-18-bc-65) For cable, take payload, add error protection (Reed- − Local, works on a particular network Solomon), header and framing, then turn into a Packet often has all three! signal − Modulate data to assigned channel and time (upstream) a Ethernet Hdr IP Hdr TCP Hdr HTTP Hdr HTTP Payload End of packet Start of packet 26 Protocols and Layering Layering and Protocol Stacks We need abstractions to handle complexity and interfaces to Layering is how we combine protocols enable interoperability. Protocols are the modularity of networks. − Higher level protocols build on services provided by lower levels A protocol is an agreement dictating the form and function − Peer layers communicate with each other of data exchanged between parties to effect Layer N+1 communication e.g., HTTP − Examples: ADSL, ISDN, DS-3, SONET, Frame Relay, PPP, BISYNC, HDLC, Layer N SLIP, Ethernet, 10Base-T, 100Base-T, CRC, 802.5, FDDI, 802.11a/b/g/n, ATM, e.g., TCP AAL5, X.25, IPv4, IPv6, TTL, DHCP, ICMP, OSPF, RIP, IS-IS, BGP, S-BGP, CIDR, TCP, SACK, UDP, RDP, DNS, RED, DECbit, SunRPC, DCE, XDR, JPEG, MPEG, Home PC www.msn.com MP3, BOOTP, ARP, RARP, IGMP, CBT, MOSPF, DVMRP, PIM, RTP, RTCP, RSVP, COPS, DiffServ, IntServ, DES, PGP, Kerberos, MD5, IPsec, SSL, SSH, telnet, HTTP, HTTPS, HTML, FTP, TFTP, UUCP, X.400, SMTP, POP, MIME, NFS, AFS, SNMP, … 27 28 Example – Layering at work Layering Mechanics Encapsulation and decapsulation host host router TCP TCP IP IP IP IP + Layer N+1 PDU Messages Hdr Data Ethernet Ethernet CATV CATV passed becomes between + Layer N ADU Hdr Data layers We can connect different systems: interoperability 29 djw // CSEP561, Spring 2005 30 5
A Packet on the Wire More Layering Mechanics Starts looking like an onion! Multiplexing and demultiplexing in a protocol graph SMTP HTTP Ethernet Hdr IP Hdr TCP Hdr HTTP Hdr Payload (Web object) TCP port number Start of packet End of packet TCP UDP IP protocol field IP ARP This isn’t entirely accurate 802.2 identifier − ignores segmentation and reassembly, Ethernet Ethernet trailers, etc. But you can see that layering adds overhead 31 32 Internet Protocol Framework OSI “Seven Layer” Reference Model Seven Layers: Their functions: Your call Application Many Application Encode/decode messages (HTTP,SMTP) Presentation Manage connections Session Transport TCP / UDP Reliability, congestion control Transport Routing Network IP Network Framing, multiple access Link Many Link Symbol coding, modulation (Ethernet, …) Physical Model Protocols 33 34 Fiber Wireless Different materials absorb, reflect, defract each frequency Long, thin, pure strand of glass differently − Enormous bandwidth available (terabits) 802.11: 20MHz range at 2.4GHz; worst possible RF properties AM FM Twisted Microwave Coax Light source Light detector Fiber Pair TV Satellite (LED, laser) (photodiode) Freq (Hz) 10 4 10 6 10 8 10 10 10 12 10 14 − Vary the glass defraction index to guide waves down Radio Microwave IR Light UV middle of fiber 35 36 6
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