COMS/CSEE 4140 Networking Laboratory Lecture 02 Salman Abdul Baset Spring 2008.
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COMS/CSEE 4140 Networking Laboratory Lecture 02 Salman Abdul Baset Spring 2008
2 Previous lecture… Introduction to the lab equipment A simple TCP/IP example Overview of important networking concepts
3 Previous lecture… Web request Web page A user on host argon.netlab.edu (“Argon”) makes web access to URL http://neon.netlab.edu/index.html. What actually happens in the network? Web clientWeb server
4 Agenda Administrivia MICE access, lab groups. Data Link Protocols Address Resolution Protocol (ARP) Internet Protocol (IP)
5 Terminology Frame Data link layer terminology for a data unit Includes error correction Packet Network layer and above PDU Protocol specific
6 TCP/IP Suite and OSI Reference Model The TCP/IP protocol stack does not define the lower layers of a complete protocol stack How does the TCP/IP protocol stack interface with the data link layer?
7 Data Link Layer The main tasks of the data link layer are: Transfer data from the network layer of one machine to the network layer of another machine Convert the raw bit stream of the physical layer into groups of bits (“frames”)
8 Two types of networks at the data link layer Broadcast Networks: All stations share a single communication channel Point-to-Point Networks: Pairs of hosts (or routers) are directly connected Typically, local area networks (LANs) are broadcast and wide area networks (WANs) are point-to-point
9 Local Area Networks Local area networks (LANs) connect computers within a building or a enterprise network Almost all LANs are broadcast networks Typical topologies of LANs are bus or ring or star We will work with Ethernet LANs. Ethernet has a bus or star topology. Comparing topologies: workstation vs. cable failure? Star LAN
10 MAC and LLC In any broadcast network, the stations must ensure that only one station transmits at a time on the shared communication channel The protocol that determines who can transmit on a broadcast channel are called Medium Access Control (MAC) protocol The MAC protocol are implemented in the MAC sublayer which is the lower sublayer of the data link layer The higher portion of the data link layer is often called Logical Link Control (LLC)
11 IEEE 802 Standards IEEE 802 is a family of standards for LANs, which defines an LLC and several MAC sublayers
12 Ethernet and IEEE 802.3: Any Difference? There are two types of Ethernet frames in use, with subtle differences: “Ethernet” (Ethernet II, DIX) An industry standards from 1982 that is based on the first implementation of CSMA/CD by Xerox. Predominant version of CSMA/CD in the US. 802.3: IEEE’s version of CSMA/CD from 1985. Interoperates with 802.2 (LLC) as higher layer. Difference for our purposes: Ethernet and 802.3 use different methods to encapsulate an IP datagram.
15 Ethernet Speed: 10 Mbps -10 Gbps Standard: 802.3, Ethernet II (DIX) Most popular physical layers for Ethernet: 10Base5 Thick Ethernet: 10 Mbps coax cable 10Base2 Thin Ethernet: 10 Mbps coax cable 10Base-T 10 Mbps Twisted Pair 100Base-TX 100 Mbps over Category 5 twisted pair 100Base-FX 100 Mbps over Fiber Optics 1000Base-FX1Gbps over Fiber Optics 10000Base-FX10Gbps over Fiber Optics (for wide area links)
16 Bus Topology 10Base5 and 10Base2 Ethernets have a bus topology
17 Starting with 10Base-T, stations are connected to a hub in a star configuration Star Topology
18 Ethernet Hubs vs. Ethernet Switches An Ethernet switch is a packet switch for Ethernet frames Buffering of frames prevents collisions. Each port is isolated and builds its own collision domain An Ethernet Hub does not perform buffering: Collisions occur if two frames arrive at the same time. HubSwitch
19 Point-to-Point (serial) links Many data link connections are point-to-point serial links: Dial-in or DSL access connects hosts to access routers Routers are connected by high-speed point-to-point links Here, IP hosts and routers are connected by a serial cable Data link layer protocols for point-to-point links are simple: Main role is encapsulation of IP datagrams No media access control needed
20 Data Link Protocols for Point-to- Point links SLIP (Serial Line IP) (RFC 1055) First protocol for sending IP datagrams over dial-up links (from 1988) Encapsulation, not much else PPP (Point-to-Point Protocol) (RFC 1661) Successor to SLIP (1992), with added functionality Used for dial-in and for high-speed routers HDLC (High-Level Data Link) (ISO) Widely used and influential standard (1979) Default protocol for serial links on Cisco routers Actually, PPP is based on a variant of HDLC
21 PPP - IP encapsulation The frame format of PPP is similar to HDLC and the 802.2 LLC frame format: PPP assumes a duplex circuit Note: PPP does not use addresses Usual maximum frame size is 1500
22 Additional PPP functionality In addition to encapsulation, PPP supports: multiple network layer protocols (protocol multiplexing) Link configuration Link quality testing Error detection Option negotiation Address notification Authentication The above functions are supported by helper protocols: LCP PAP, CHAP NCP
23 PPP Support protocols Link management: The link control protocol (LCP) is responsible for establishing, configuring, and negotiating a data-link connection. LCP also monitors the link quality and is used to terminate the link. Authentication: Authentication is optional. PPP supports two authentication protocols: Password Authentication Protocol (PAP) and Challenge Handshake Authentication Protocol (CHAP). Network protocol configuration: PPP has network control protocols (NCPs) for numerous network layer protocols. The IP control protocol (IPCP) negotiates IP address assignments and other parameters when IP is used as network layer.
24 Agenda Administrivia Data Link Protocols Address Resolution Protocol (ARP) Internet Protocol (IP)
26 ARP (RFC 826) and RARP (RFC 903) Note: The Internet is based on IP addresses Data link protocols (Ethernet, FDDI, ATM) may have different (MAC) addresses The ARP and RARP protocols perform the translation between IP addresses and MAC layer addresses We will discuss ARP for broadcast LANs, particularly Ethernet LANs
27 Processing of IP packets by network device drivers
28 Topology Web request Web page A user on host argon.netlab.edu (“Argon”) makes web access to URL http://neon.netlab.edu/index.html. What actually happens in the network? Web clientWeb server
29 Address Translation with ARP ARP Request: Argon broadcasts an ARP request to all stations on the network: “What is the hardware address of Router137?”
30 Address Translation with ARP ARP Reply: Router 137 responds with an ARP Reply which contains the hardware address
33 ARP Cache Since sending an ARP request/reply for each IP datagram is inefficient, hosts maintain a cache (ARP Cache) of current entries. The entries expire after 20 minutes. Contents of the ARP Cache: (184.108.40.206) at 00:10:4B:C5:D1:15 [ether] on eth0 (220.127.116.11) at 00:B0:D0:E1:17:D5 [ether] on eth0 (18.104.22.168) at 00:B0:D0:DE:70:E6 [ether] on eth0 (22.214.171.124) at 00:05:3C:06:27:35 [ether] on eth1 (126.96.36.199) at 00:B0:D0:E1:17:DB [ether] on eth0 (188.8.131.52) at 00:B0:D0:E1:17:DF [ether] on eth0
34 Proxy ARP Proxy ARP: Host or router responds to ARP Request that arrives from one of its connected networks for a host that is on another of its connected networks.
35 Things to know about ARP What happens if an ARP Request is made for a non- existing host? Several ARP requests are made with increasing time intervals between requests. Eventually, ARP gives up. On some systems (including Linux) a host periodically sends ARP Requests for all addresses listed in the ARP cache. This refreshes the ARP cache content, but also introduces traffic. Gratuitous ARP Requests: A host sends an ARP request for its own IP address: Useful for detecting if an IP address has already been assigned.
36 Vulnerabilities of ARP 1. Since ARP does not authenticate requests or replies, ARP Requests and Replies can be forged 2. ARP is stateless: ARP Replies can be sent without a corresponding ARP Request 3. According to the ARP protocol specification, a node receiving an ARP packet (Request or Reply) must update its local ARP cache with the information in the source fields, if the receiving node already has an entry for the IP address of the source in its ARP cache. (This applies for ARP Request packets and for ARP Reply packets) Typical exploitation of these vulnerabilities: A forged ARP Request or Reply can be used to update the ARP cache of a remote system with a forged entry (ARP Poisoning) This can be used to redirect IP traffic to other hosts
37 Agenda Administrivia Data Link Protocols Address Resolution Protocol (ARP) Internet Protocol (IP)
38 IP Addresses Structure of an IP address Classful IP addresses Limitations and problems with classful IP addresses Subnetting CIDR IP Version 6 addresses
41 What is an IP Address? An IP address is a unique global address for a network interface Exceptions: Dynamically assigned IP addresses ( DHCP, Lab 7) IP addresses in private networks ( NAT, Lab 7) An IP address: - is a 32 bit long identifier - encodes a network number (network prefix) and a host number
42 The network prefix identifies a network and the host number identifies a specific host (actually, interface on the network). How do we know how long the network prefix is? Before 1993: The network prefix is implicitly defined (class- based addressing) or After 1993: The network prefix is indicated by a netmask. Network prefix and host number network prefixhost number
43 Dotted Decimal Notation IP addresses are written in a so-called dotted decimal notation Each byte is identified by a decimal number in the range [0..255]: Example: 10001111100000001000100110010000 1 st Byte = 128 2 nd Byte = 143 3 rd Byte = 137 4 th Byte = 144 184.108.40.206
44 Example: ellington.cs.virginia.edu Network address is: 220.127.116.11 (or 128.143) Host number is: 137.144 Netmask is: 255.255.0.0 (or ffff0000) Prefix or CIDR notation: 18.104.22.168/16 Network prefix is 16 bits long Example 128.143137.144
45 Special IP Addresses Reserved or (by convention) special addresses: Loopback interfaces all addresses 127.0.0.1-127.255.255.255 are reserved for loopback interfaces Most systems use 127.0.0.1 as loopback address loopback interface is associated with name “localhost” IP address of a network Host number is set to all zeros, e.g., 22.214.171.124 Broadcast address Host number is all ones, e.g., 126.96.36.199 Broadcast goes to all hosts on the network Often ignored due to security concerns Test / Experimental addresses Certain address ranges are reserved for “experimental use”. Packets should get dropped if they contain this destination address (see RFC 1918): 10.0.0.0 - 10.255.255.255 172.16.0.0- 172.31.255.255 192.168.0.0- 192.168.255.255 Convention (but not a reserved address) Default gateway has host number set to ‘1’, e.g., e.g., 188.8.131.52
47 Subnetting Problem: Organizations have multiple networks which are independently managed Solution 1: Allocate a separate network address for each network Difficult to manage From the outside of the organization, each network must be addressable. Solution 2: Add another level of hierarchy to the IP addressing structure University Network Medical School Library Engineering School
48 Each part of the organization is allocated a range of IP addresses (subnets or subnetworks) Addresses in each subnet can be administered locally Address Assignment with Subnetting University Network Medical School Library Engineering School 184.108.40.206/16 220.127.116.11/24 18.104.22.168/24 22.214.171.124/24 126.96.36.199/24
49 Basic Idea of Subnetting Split the host number portion of an IP address into a subnet number and a (smaller) host number. Result is a 3-layer hierarchy Then: Subnets can be freely assigned within the organization Internally, subnets are treated as separate networks Subnet structure is not visible outside the organization network prefixhost number subnet number network prefix host number extended network prefix
50 Routers and hosts use an extended network prefix (subnetmask) to identify the start of the host numbers Subnetmask
51 Advantages of Subnetting With subnetting, IP addresses use a 3-layer hierarchy: Network Subnet Host Reduces router complexity. Since external routers do not know about subnetting, the complexity of routing tables at external routers is reduced. Note: Length of the subnet mask need not be identical at all subnetworks.
52 Example: Subnetmask 188.8.131.52/16 is the IP address of the network 184.108.40.206/24 is the IP address of the subnet 220.127.116.11 is the IP address of the host 255.255.255.0 (or ffffff00) is the subnetmask of the host When subnetting is used, one generally speaks of a “subnetmask” (instead of a netmask) and a “subnet” (instead of a network) Use of subnetting or length of the subnetmask if decided by the network administrator Consistency of subnetmasks is responsibility of administrator
53 No Subnetting All hosts think that the other hosts are on the same network
54 With Subnetting Hosts with same extended network prefix belong to the same network
55 Different subnetmasks lead to different views of the size of the scope of the network With Subnetting 192: 11000000 144: 10010000 128: 10000000
56 Classful IP Adresses (Until 1993) When Internet addresses were standardized (early 1980s), the Internet address space was divided up into classes: Class A: Network prefix is 8 bits long Class B: Network prefix is 16 bits long Class C: Network prefix is 24 bits long Each IP address contained a key which identifies the class: Class A: IP address starts with “0” Class B: IP address starts with “10” Class C: IP address starts with “110”
58 The old way: Internet Address Classes We will learn about multicast addresses later in this course.
59 Problems with Classful IP Addresses By the early 1990s, the original classful address scheme had a number of problems Flat address space. Routing tables on the backbone Internet need to have an entry for each network address. When Class C networks were widely used, this created a problem. By the 1993, the size of the routing tables started to outgrow the capacity of routers. Other problems: Too few network addresses for large networks Class A and Class B addresses were gone Limited flexibility for network addresses: Class A and B addresses are overkill (>64,000 addresses) Class C address is insufficient (requires 40 Class C addresses)
61 CIDR - Classless Interdomain Routing IP backbone routers have one routing table entry for each network address: With subnetting, a backbone router only needs to know one entry for each Class A, B, or C networks This is acceptable for Class A and Class B networks 2 7 = 128 Class A networks 2 14 = 16,384 Class B networks But this is not acceptable for Class C networks 2 21 = 2,097,152 Class C networks In 1993, the size of the routing tables started to outgrow the capacity of routers Consequence: The Class-based assignment of IP addresses had to be abandoned
62 CIDR - Classless Interdomain Routing Goals: New interpretation of the IP address space Restructure IP address assignments to increase efficiency Permits route aggregation to minimize route table entries CIDR (Classless Interdomain routing) abandons the notion of classes Key Concept: The length of the network prefix in the IP addresses is kept arbitrary Consequence: Size of the network prefix must be provided with an IP address
63 CIDR Notation CIDR notation of an IP address: 192.0.2.0/18 "18" is the prefix length. It states that the first 18 bits are the network prefix of the address (and 14 bits are available for specific host addresses) CIDR notation can replace the use of subnetmasks (but is more general) IP address 18.104.22.168 and subnetmask 255.255.255.0 becomes 22.214.171.124/24 CIDR notation allows to drop traling zeros of network addresses: 192.0.2.0/18 can be written as 192.0.2/18
64 CIDR address blocks CIDR notation can nicely express blocks of addresses Blocks are used when allocating IP addresses for a company and for routing tables (route aggregation) CIDR Block Prefix # of Host Addresses /2732 /2664 /25128 /24256 /23512 /221,024 /212,048 /204,096 /198,192 /1816,384 /1732,768 /1665,536 /15131,072 /14262,144 /13524,288
65 CIDR and Address assignments Backbone ISPs obtain large block of IP addresses space and then reallocate portions of their address blocks to their customers. Example: Assume that an ISP owns the address block 126.96.36.199/18, which represents 16,384 (2 14 ) IP addresses Suppose a client requires 800 host addresses With classful addresses: need to assign a class B address (and waste ~64,700 addresses) or four individual Class Cs (and introducing 4 new routes into the global Internet routing tables) With CIDR: Assign a /22 block, e.g., 188.8.131.52/22, and allocated a block of 1,024 (2 10 ) IP addresses.
66 CIDR and Routing Aggregation of routing table entries: 184.108.40.206/16 and 220.127.116.11/16 are represented as 18.104.22.168/15 Longest prefix match: Routing table lookup finds the routing entry that matches the longest prefix What is the outgoing interface for 22.214.171.124/24 ? Route aggregation can be exploited when IP address blocks are assigned in an hierarchical fashion PrefixInterface 126.96.36.199/4interface #5 188.8.131.52/9interface #2 184.108.40.206/17interface #1 Routing table
67 CIDR and Routing Information 220.127.116.11/18 18.104.22.168/15 22.214.171.124/21 Internet Backbone ISP X owns: Company X : 126.96.36.199/22 ISP y : 188.8.131.52/24 Organization z1 : 184.108.40.206/26 Organization z2 : 220.127.116.11/26
68 CIDR and Routing Information 18.104.22.168/18 22.214.171.124/15 126.96.36.199/21 Internet Backbone ISP X owns: Company X : 188.8.131.52/22 ISP y : 184.108.40.206/24 Organization z1 : 220.127.116.11/26 Organization z2 : 18.104.22.168/26 Backbone sends everything which matches the prefixes 22.214.171.124/18, 126.96.36.199/15, 188.8.131.52/21 to ISP X. ISP X sends everything which matches the prefix: 184.108.40.206/22 to Company X, 220.127.116.11/24 to ISP y Backbone routers do not know anything about Company X, ISP Y, or Organizations z1, z2. ISP X does not know about Organizations z1, z2. ISP y sends everything which matches the prefix: 18.104.22.168/26 to Organizations z1 22.214.171.124/26 to Organizations z2
69 IPv6 - IP Version 6 IP Version 6 Is the successor to the currently used IPv4 Specification completed in 1994 Makes improvements to IPv4 (no revolutionary changes) One (not the only !) feature of IPv6 is a significant increase in of the IP address to 128 bits (16 bytes) IPv6 will solve – for the foreseeable future – the problems with IP addressing 10 24 addresses per square inch on the surface of the Earth.
71 IPv6 vs. IPv4: Address Comparison IPv4 has a maximum of 2 32 4 billion addresses IPv6 has a maximum of 2 128 = (2 32 ) 4 4 billion x 4 billion x 4 billion x 4 billion addresses
72 Notation of IPv6 addresses Convention: The 128-bit IPv6 address is written as eight 16-bit integers (using hexadecimal digits for each integer) CEDF:BP76:3245:4464:FACE:2E50:3025:DF12 Short notation: Abbreviations of leading zeroes: CEDF:BP76:0000:0000:009E:0000:3025:DF12 CEDF:BP76:0:0:9E :0:3025:DF12 “:0000:0000:0000” can be written as “::” CEDF:BP76:0:0:FACE:0:3025:DF12 CEDF:BP76::FACE:0:3025:DF1 2 IPv6 addresses derived from IPv4 addresses have 96 leading zero bits. Convention allows to use IPv4 notation for the last 32 bits. ::80:8F:89:90 ::126.96.36.199
73 IPv6 Provider-Based Addresses The first IPv6 addresses will be allocated to a provider- based plan Type: Set to “010” for provider-based addresses Registry: identifies the agency that registered the address The following fields have a variable length (recommeded length in “()”) Provider: Id of Internet access provider (16 bits) Subscriber: Id of the organization at provider (24 bits) Subnetwork: Id of subnet within organization (32 bits) Interface: identifies an interface at a node (48 bits) Registry ID Provider ID 010 Subscriber ID Interface ID Subnetwork ID