II Sören Schwertfeger 师泽仁
Outline Review Network Layer Routing Transport Layer Applications HTTP Demos
Internet: Huge network of networks Billions of hosts (computers)
Internet Structure Network Edge: End systems with hosts & access networks Access Network: Connect end systems to edge routers Network Core: Interconnected routers Network of networks
The Big Question Networks are complex, with many pieces : hosts routers links of various media applications protocols hardware software Question: How to organize the structure of the network?
Computer Models OSI 7-layer model Simplified 4/5-layer model 7 6 5 4 3 2 1 Application Presentation Session Transport Network Data Physical Application Transport Network Data Physical Application: supporting network applications FTP, SMTP, HTTP Transport: process-process data transfer TCP, UDP Network: routing of datagrams from source to destination IP, routing protocols Data: data transfer between neighboring network elements Ethernet, 802.11(WiFi) Physical: bits on the wire OSI (Open Systems Interconnection) mnemonic: All People Seem To Need Data Processing. 6
Encapsulation
Spanning Tree Protocol STP (Data Layer) Ensure loop-free topology for Ethernet networks Allow spare (redundant) links Algorithm: Select root bridge (connection based on id) Determine the least cost paths to the root bridge Disable all other paths
Network Layer (Layer 3) Main point: packet forwarding: routing! Packages can be passed from one local network to another. Through a router which is connected to more than one network. Packages are encapsulated inside layer 2 frames. Network Layer Protocol: IP (Internet Protocol) 9
Network Layer: IP IPv4: 32bit addresses (4 byte) Broadcast: 255.255.255.255 Loopback: 127.0.0.1 Private networks: 10.0.0.0 192.168.0.0 Link-local: 169.254.0.0 About 4 billion addresses IPv6: 128bit addresses (16 byte) 3.4 10 38 addresses! Other improved features (security, multi-casting, ) Divided into network address and host identifier: E.g.: 192.168.100.0/22 has 1024 IP4 adresses (32-22 = 10 => 2^10 = 1024) between 192.168.100.0 and 192.168.103.255 10
IP Packet Version IHL TOS Length Identification Flag Fragment Offset TTL Protocol Checksum Source Address Destination Address Options and Padding TCP Header & Data Payload Version: This field contains the IP version number, IPv4 or IPv6 IP header length (IHL): The IP header has a minimum size of 20 bytes, but the size can vary depending on the size of the Options field. Type of service (TOS): This field indicates the precedence or priority given to the packet contents. It enables routing protocols to determine the type of path along which to send a packet. Length: The entire IP packet size, which can be up to 65,535 bytes, is provided in this field
Packet encapsulation Net transport Header (ie Ethernet) IP Header TCP or UDP Header Application data TCP Segment Datagram
IP Packet Version IHL TOS Length Identification Flag Fragment Offset TTL Protocol Checksum Source Address Destination Address Options and Padding Identification : IP can convert packets from one size to another for dissimilar networks E.g.: Ethernet packet size 64 to 1518 bytes; Fiber Distributed Data Interface (FDDI) packet up to 4472 bytes => Split message into fragments use identification number to put them together again Flags: Flags are used with fragmentation (1) to convey information and (2) to show when the last fragment in a sequence has been sent (when a packet is fragmented). TCP Header & Data Payload
IP Packet Version IHL TOS Length Identification Flag Fragment Offset TTL Protocol Checksum Source Address Destination Address Options and Padding TCP Header & Data Payload Fragment offset: offset to reconnect fragments Time to live (TTL): This field contains information that prevents a packet from continuously circulating around a network. Each time an IP packet goes through a router, that router reduces the TTL value by a default amount determined by the router or set by a network administrator It is checked by each router through which it passes, so that the packet is discarded when TTL equals 0. Protocol: This field is used to show which protocol- TCP or UDP-is encapsulated in IP Checksum: The checksum is a 16-bit cyclic redundancy check that is the sum of all values contained in every field in the IP header.
IP Packet Version IHL TOS Length Identification Flag Fragment Offset TTL Protocol Checksum Source Address Destination Address Options and Padding Source address: This is the network address and the address of the device that sent the packet Destination address: This field contains the network address and the address of the receiving device Options: There are several options that can be used with IP For example, the time when the packet is created can be entered Padding: Padding fills the options area when there is not enough data to complete the allocated area, because the total size (in bits) of the IP header must be divisible by 32. TCP Header & Data Payload
Admin Academic honesty cases Quiz: Students that confessed academic misbehavior will get a 25% reduction for the signal part. One student is protecting his friend the student gets a 75% reduction for the signal part. Any second offense by any of those students leads to an immediate fail of this course. Everybody also be careful also don t cheat on the homework!
Routing
IP routing
Exterior Gateway Protocol (EGP): Border Gateway Protocol (BGP) A mechanism that allows routers to learn routes from other routers so that they can choose optimal backbone routes A mechanism for routers to inform other routers about hidden networks (internal routes) Autonomous System (AS) has the responsibility of advertising reachability info to other ASs. Each AS has one or more (typically very big) network (IP address range) One+ routers may be designated per AS. Important that reachability info propagates to core routers More than 50,000 AS in the Internet
Purpose of EGP you can reach net A via me AS2 AS1 traffic to A R1 EGP R2 A R3 Share connectivity information across AS table at R1: dest next hop A R2 R border router internal router
EGP Operation Neighbor Reachability: Hellos: j out of m hellos OK => Neighbor UP k out of n hellos NOT OK => Neighbor DOWN Updates/Queries: EGP is an incremental protocol. New info => send updates Each router can query neighbors as well Reachability advertised Requires a tree topology of AS to avoid loops
Why EGP Requires a Tree Structure..
Today s Big Picture Large ISP Large ISP Stub Dial-Up ISP Small ISP Access Network Stub Stub Large number of diverse networks
Nontransit vs. Transit ASes ISP 1 ISP 2 Internet Service providers (ISPs) have transit networks Traffic NEVER flows from ISP 1 through NET A to ISP 2 NET A Nontransit AS might be a corporate or campus network. Could be a content provider
Selective Transit NET B NET C NET A DOES NOT provide transit Between NET D and NET B NET A NET D NET A provides transit between NET B and NET C and between NET D and NET C Most transit AS allow only selective transit key impact of commercialization
Customers and Providers provider provider customer IP traffic customer Customer pays provider for access to the Internet
Customer-Provider Hierarchy provider customer IP traffic
The Peering Relationship Peers provide transit between their respective customers peer provider peer customer traffic allowed traffic NOT allowed Peers do not provide transit between peers Peers (often) do not exchange $$$
Internet providers About 15 Tier 1 Networks, e.g.: Level 3 AT&T Deutsche Telekom NTT No Chinese Company
Internet Exchange Point (IXP): DE-CIX Largest exchange point worldwide Peak traffic throughput more than 5 terabits per seconds More than 700 networks connected in 20 data centers in Frankfurt, Germany 80% for the AS are available (through the peers)
Traffic at DE-CIX
IP: Domain Name Resolution (DNS) Translate between IP address and name DNS name: domains separated by. Distributed directory service DNS record not only for IP addresses. Also: mail server, DNS server, 34
TCP/UDP Layer 4 (transport layer) protocols, run over IP TCP and UDP packets are encapsulated into IP packets Use their own control information, stored in packet headers Port numbers (indicate consuming program in the destination host) TCP is connection-oriented, and provides for reliable, order-preserving transmission of data UDP is not connection-oriented, does not guarantee data arrival, or proper ordering of arriving data TCP for: Web browsing Email Chatting UDP for: Video streaming Audio & Video conference (e.g. Facetime)
Protocols: Laws of Networks
TCP header flags: URG (urgent), ACK (acknowledg-ment), PSH (push function), RST (close the connection), SYN (synchronize sequence numbers), FIN (end of data from sender)
UDP
Ports 16 bit ports determine type of service e.g.: 22: ssh 25: smtp (sending email) 80 (HTTP world wide web) 110: POP3 (getting email) 143 IMAP (getting email) 443 HTTPS secure HTTP High port numbers for outgoing & other connections
Applications HTTP Demos
HTTP HyperText Transport Protocol Language of the Web protocol used for communication between web browsers and web servers TCP port 80 (443 secure) RFC 2616 (ver 1.1)
URI, URN, URL Uniform Resource Identifier Information about a resource Uniform Resource Name The name of the resource with in a namespace Uniform Resource Locator How to find the resource, a URI that says how to find the resource
HTTP - URLs URL Uniform Resource Locator protocol (http, ftp, news) host name (name.domain name) port (usually 80 but many on 8080) directory path to the resource resource name http://xxx.myplace.com/www/index.html http://xxx.myplace.com:80/cgi-bin/t.exe
HTTP - methods Methods GET retrieve a URL from the server simple page request run a CGI program run a CGI with arguments attached to the URL POST preferred method for forms processing run a CGI program parameterized data in sysin more secure and private
HTTP Request Packets Sent from client to server Consists of HTTP header header is hidden in browser environment contains: content type / mime type content length user agent - browser issuing request content types user agent can handle and a URL
HTTP Response Sent by server to client browser in response to a Request Packet Status Header Entities Content-Encoding: Content-Length: Content-Type: Expires: Last-Modified: extension-header Body content (usually html)
Status Header HTTP/1.0 sp code Codes: 1xx - reserved for future use 2xx - successful, understood and accepted 3xx - further action needed to complete 4xx - bad syntax in client request 5xx - server can t fulfill good request
Status Codes 200 OK 201 created 202 accepted 204 no content 301 moved perm. 302 moved temp 304 not modified 400 bad request 401 unauthorized 403 forbidden 404 not found 500 int. server error 501 not impl. 502 bad gateway 503 svc not avail
Demos