The Network Layer. Internet solutions. Nixu Oy PL 21. (Mäkelänkatu 91) Helsinki, Finland. tel fax.

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1 The Network Layer Nixu Oy PL 21 (Mäkelänkatu 91) Helsinki, Finland tel fax info@nixu.fi

2 OVERVIEW The Internet Protocol IP addresses, address resolution IP in LAN environment Static routing Dynamic routing Copyright 2002 Nixu Oy 2/48 The Network Layer

3 Network Layer Network layer packets are transmitted from the sending network entity all the way to the reciever, spanning several LANs and data link layer technologies There are several network layer protocols Internet Protocol (IP) is currently the most common one X.25 is almost obsolete > Provides realiable, connection oriented packet networking Copyright 2002 Nixu Oy 3/48 The Network Layer

4 IP IP = The Internet Protocol Defined in RFC 791 IP sends simple datagrams over network. It provides unreliable and connectionless delivery service. unreliable = no guarantees, ICMP error messages connectionless = each packet is routed separately Large IP packets may be fragmented and reassembled in transmission Copyright 2002 Nixu Oy 4/48 The Network Layer

5 IP Packet Format version hdr len type of service total length identification flags fragmentation offset time to live protocol header checksum source IP address destination IP address options... padding data Normal size for IP header is 20 bytes, plus options & padding. Copyright 2002 Nixu Oy 5/48 The Network Layer

6 IP Packet Format Version is 4 until IPv6 comes. Type of Service contains quality parameters, like maximize throughput or reliability. Often not implemented. Identification is set by sending host to unique value for each sent IP-packet, usually this is an incremental counter. Flags tell if this packet is fragmented or if this packet should not be fragmented Fragment offset tells how far from the head of original datagram this fragment is Time to Live is decremented by one by every router passed. When 0 is reached, the packet is discarded and an ICMP-message sent back. Protocol may be TCP, UDP, ICMP or one of several others Options are rarely used and not widely supported. They are loose and strict source routing, route recording and timestamping and military security options (RFC 1108). Copyright 2002 Nixu Oy 6/48 The Network Layer

7 IP Addresses IP address identifies a network interface. A host can have several interfaces. Current length 32 bits (IPv4). Future length (IPv6) 128 bits. General syntax: 4 components separated by dots ( dotted quad ) decimal numbers (0-255) for example: Addresses have two components, the network id and the host id. Copyright 2002 Nixu Oy 7/48 The Network Layer

8 Address Classes A B C D E 1 7 network host network host 21 8 network host 28 multicast address 27 reserved for future use first byte: Copyright 2002 Nixu Oy 8/48 The Network Layer

9 Address Classes The network part of the address is used to route a packet to the right LAN The host part tells which host on a LAN should recieve the packet If a host is sending a packet to an address, which network part is not same as the sender s the packet is sent to a gateway (router), if the network part is same, the packet is sent to the LAN There isonly a small number of class A networks but they can have many hosts Class B networks are almost all taken There are quite a few class C networks but they can only have 254 hosts each Class based routing is now mostly obsolete and replaced by clasless routing (CIDR) Copyright 2002 Nixu Oy 9/48 The Network Layer

10 CIDR (Classless InterDomain Routing) Arbitrary length host and network fields instead of A, B and C classes Commonly used to make superblocks of C classes for routing (aka. supernetting) In the future may be used to split unused A classes Network mask marks the boundary For example /22 netmask is The number after the slash (/) tells how many bits in the mask are 1, the rest are 0 RFC 1518, 1519 Copyright 2002 Nixu Oy 10/48 The Network Layer

11 Special Addresses is used for "any" or "no" IP address is local broadcast address 127 followed by hostid is the loopback address (e.g ) netid followed by all zeros is the network address (e.g /24) netid followed by all ones is network broadcast address (e.g /24) Copyright 2002 Nixu Oy 11/48 The Network Layer

12 Subnetting Large networks are often divided into smaller units Subnetting hides the details of internal network organization for example, /16 ( hosts) could be subnetted to /24 (2 8 subnets with hosts in each) netid subnetid hostid default netmask subnet mask host IP address AND network mask = network IP address Copyright 2002 Nixu Oy 12/48 The Network Layer

13 ICMP ICMP = Internet Control Message Protocol Defined in RFC 792 ICMP packet syntax TYPE CODE CHECKSUM DATA Type identifies the message: echo request, echo reply, destination unreachable, etc. Code defines the reason: host unreachable, port unreachable, etc. Data contains part of the IP packet that caused the error. Copyright 2002 Nixu Oy 13/48 The Network Layer

14 ICMP ICMP messages are transmitted in IP datagrams. IP header ICMP header ICMP data Communicates error messages and other conditions that require attention. Can be utilized to track network infrastructure (ping, traceroute). Copyright 2002 Nixu Oy 14/48 The Network Layer

15 IP on LAN Usually one physical segment = one IP network Each IP network has a network address and a broadcast address Problem: IP addresses only make sense to the TCP/IP protocol suite, not to the hardware interface Solution: ARP maps IP addresses to hardware addresses If a booting host doesn t know its IP address, RARP, BOOTP or DHCP can be used Copyright 2002 Nixu Oy 15/48 The Network Layer

16 IP on LAN Host interfaces must be activated Loopback interface: ifconfig lo Ethernet interface: ifconfig eth broadcast \ netmask Other interfaces Default route route add default Copyright 2002 Nixu Oy 16/48 The Network Layer

17 IP on LAN gato tsilven 6$ ifconfig -a lo Link encap:local Loopback inet addr: Bcast: Mask: UP BROADCAST LOOPBACK RUNNING MTU:3584 Metric:1 RX packets:77 errors:0 dropped:0 overruns:0 frame:0 TX packets:77 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 eth0 Link encap:ethernet HWaddr 00:60:08:06:2A:36 inet addr: Bcast: Mask: UP BROADCAST RUNNING PROMISC MULTICAST MTU:1500 Metric:1 RX packets: errors:0 dropped:0 overruns:0 frame:0 TX packets:43770 errors:0 dropped:0 overruns:0 carrier:0 collisions:20 Interrupt:5 Base address:0x6c00 gato tsilven 7$ netstat -rn Kernel IP routing table Destination Gateway Genmask Flags MSS Iface U 1500 eth U 3584 lo UG 1500 eth0 Copyright 2002 Nixu Oy 17/48 The Network Layer

18 ARP (Address Resolution Protocol) A host finds other hosts by broadcasting an ARP query for the IP address The host with correct IP address replies with its hardware address The address pair is added to receivers dynamic ARP cache Features: proxy ARP, gratuitous ARP RFC 826 Copyright 2002 Nixu Oy 18/48 The Network Layer

19 ARP Packet Format hard type prot type hard size prot size OP sender MAC address sender IP address target MAC address target IP address Encapsulated into link layer frame Data always 28 bytes hard type = hardware address type (0x0001 = Ethernet) prot type = protocol address type (0x0800 = IP) OP = operation (ARP/RARP request/reply) Copyright 2002 Nixu Oy 19/48 The Network Layer

20 ARP, an Example gato tsilven 15$ arp -a jalopeno.nixu.fi ( ) at 08:00:20:74:F1:2C [ether] on eth0 fajitas.nixu.fi ( ) at 08:00:20:18:06:14 [ether] on eth0 tapas.nixu.fi ( ) at 08:00:09:6D:B6:44 [ether] on eth0 gato tsilven 16$ ping PING ( ): 56 data bytes 64 bytes from : icmp_seq=0 ttl=64 time=3.0 ms 64 bytes from : icmp_seq=1 ttl=64 time=0.7 ms ping statistics packets transmitted, 2 packets received, 0% packet loss round-trip min/avg/max = 0.7/1.8/3.0 ms gato tsilven 17$ arp -a jalopeno.nixu.fi ( ) at 08:00:20:74:F1:2C [ether] on eth0 sueno.nixu.fi ( ) at 00:60:08:54:2D:D9 [ether] on eth0 fajitas.nixu.fi ( ) at 08:00:20:18:06:14 [ether] on eth0 tapas.nixu.fi ( ) at 08:00:09:6D:B6:44 [ether] on eth0 Copyright 2002 Nixu Oy 20/48 The Network Layer

21 ARP, an Example bash-2.02# tcpdump -i eth0 -n -t -q\ host tcpdump: listening on eth0 arp who-has tell arp reply is-at 0:60:8:54:2d:d > : icmp: echo request > : icmp: echo reply > : icmp: echo request > : icmp: echo reply 6 packets received by filter 0 packets dropped by kernel Copyright 2002 Nixu Oy 21/48 The Network Layer

22 Bootstrapping an IP host in the LAN RARP (Reverse ARP), a host broadcasts its hardware address and receives an IP address to use as its own BOOTP (Bootstrap Protocol) is better: IP address and other information can be given Both now replaced by DHCP (Dynamic Host Configuration Protocol) Copyright 2002 Nixu Oy 22/48 The Network Layer

23 DHCP DHCP (Dynamic Host Configuration Protocol) extends BOOTP: automatic assignment of (permanent) IP addresses dynamic assignment for a limited time Extends vendor-specific area from 64 to 312 bytes RFC 1531 Supports distributed configuration Message forwarding or local servers Not a trivial service to configure for large installations Copyright 2002 Nixu Oy 23/48 The Network Layer

24 DHCP Messages are sent using UDP over IP Server in port 67, client in port 68 The DHCP server on the LAN segment is found using a boradcast First packet to from (client does not know its own address) Message types: DISCOVER, OFFER, REQUEST, DECLINE, ACK, NAK, RELEASE The server returns all necessary information IP address, netmask, gateway to the client DNS server s address also Address assingment for limited time or permanently The IP address can be from a pool or static Copyright 2002 Nixu Oy 24/48 The Network Layer

25 DHCP Event Diagram Server Client Server DHCPDISCOVER DHCPOFFER DHCPREQUEST DHCPDISCOVER DHCPOFFER DHCPREQUEST DHCPACK DHCPRELEASE Copyright 2002 Nixu Oy 25/48 The Network Layer

26 Static Routing When host has an IP datagram to send, it checks the routing table for the correct destination When a host receives an IP datagram, it checks datagram s destination address if there is a match, IP layer deliveres the datagram to correct protocol module else the datagram is silently discarded A (Unix) system can be configured to act as a router in addition to acting as a host routers can forward IP datagrams from one of its interfaces to another Copyright 2002 Nixu Oy 26/48 The Network Layer

27 Router Router is a network component, which passes traffic between networks. Two or more network interfaces For each and every given destination address, router must be able to make routing decision. Where (to what interface) I send this packet? Routing decision might also be: No such destination, cannot send. This applies also to workstations and servers even though they usually have only one network interface. Routing decisions are based on routing table. Data structure, which contains information about possible destinations. Copyright 2002 Nixu Oy 27/48 The Network Layer

28 Routing Table, an Example gato tsilven 19$ netstat -rn Kernel IP routing table Destination Gateway Genmask Flags MSS Iface U 1500 eth U 3584 lo UG 1500 eth0 Copyright 2002 Nixu Oy 28/48 The Network Layer

29 Routing table Can be fixed (configured by hand to each device) Static routing Common at the edges of the network, workstations, servers Not feasible on big and redundant networks Usually very robust Copyright 2002 Nixu Oy 29/48 The Network Layer

30 Routing table... Can also be dynamic (configured by hand, distributed automatically) Routers exchange information using routing protocols Routing protocol events (routing updates) affect directly to routing table. > This causes interesting dynamic problems Debugging can be painful Copyright 2002 Nixu Oy 30/48 The Network Layer

31 Routing table contents For each destination, routing table contains Addresses for this destination > Might be some kind of wild card, e.g. all destinations not mentioned elsewhere in routing table (default route) > Usually expressed as network number / mask > E.g / 24 (class C network, 24 network bits) > Modern routing mechanisms are classless, any number of network bits allowed > Old-fashioned implementations usually are more or less class-bound. Next hop (where to send traffic to this destination) Additional information (cost and/or other administrative information) Copyright 2002 Nixu Oy 31/48 The Network Layer

32 Routing table contents... In practice, most transit providers accept only routes with 24 or less network bits E.g. no routes smaller than class C are accepted Now backbone transit providers are moving towards allowing /16 networks > Usually called superblocks (254 C classes) The routing table has usually a cost associated to each link Not a monetary cost, more like a preference (lowest preferred) Copyright 2002 Nixu Oy 32/48 The Network Layer

33 Routing table... Common case: LAN connected to Internet using serial line Internet s0 R e /24 Routing table is very simple, typical case for static routing: Destination Next hop Comment /24 e0 Local LAN (Ethernet) * s0 Serial line to Internet (default route) Copyright 2002 Nixu Oy 33/48 The Network Layer

34 Routing table... When amount of routers and redundant links increase to non-trivial numbers, something more flexible is needed Inet L2, 2 Mbps R1 L1, 2M bps /24 R2 L3, 64 kbps s1 e0 s0 R /24 Static routing can not handle redundant links nor link faults > Except on some environments (and even there unreliable) Copyright 2002 Nixu Oy 34/48 The Network Layer

35 Routing table... Routing table for router R3 Destination Next hop Cost Comment /24 e0 0 Directly connected /24 s0 1 Fastest route /24 s1 10 Backup via R2 * s0 1 Fastest route via R1 * s1 10 Slower Cost added to routing table, priorisation of redundant routes How we can know which links are up? > Routing protocol again! Copyright 2002 Nixu Oy 35/48 The Network Layer

36 Routing table... What if we can not have default route at all? Internet "backbone" Multihomed network > Internet connections from many (> 1) ISPs In this case routing table will be very big > And it changes practically all the time Practical example on Internet router ( ) > prefixes (routes) > Routes consume 16MB of memory > Realistic minimum memory for router 64MB Copyright 2002 Nixu Oy 36/48 The Network Layer

37 Routing protocols Routers learn information on network with routing protocols Routing protocols can be divided either by algorithm or by area. By algorithm: No routing protocol (static routes) Link state protocols (SPF) Distance vector protocols (Bellman-Ford) By area: Routing protocols used internally by one AS. Routing protocols used between ASes. Copyright 2002 Nixu Oy 37/48 The Network Layer

38 Distance vector protocols Also known as Bellman-Ford Routers send their entire routing table to their neighbouring routers at regular intervals The routing table is updated based on the information (costs) received from neighbour routers. Only one route to given destination is known at any given time Although changes in network topology change routing table Copyright 2002 Nixu Oy 38/48 The Network Layer

39 Link state protocols Also known as SPF (Shortest Path First) Router sends link state messages to other routers in the network. These packets are flooded to all routers When a router receives link state packet it Builds a table of all routers and networks > Each router knows full network topology Calculates the best route to every network Link state information originates from node, which is "responsible" for the link E.g. nodes connected to that link Copyright 2002 Nixu Oy 39/48 The Network Layer

40 Internal routing protocols Internal routing is routing within one Autonomous System (AS) RIP Routing Information Protocol Old, simple and limited distance-vector protocol > 10 years old RIP-2 Addresses some shortcomings of RIP (E)IGRP (Extended) Interior Gateway Routing Protocol Newer and better distance-vector protocol Proprietary protocol of Cisco Nearly as complex as OSPF Copyright 2002 Nixu Oy 40/48 The Network Layer

41 Internal routing protocols... OSPF Open Shortest Path First Complex but good link-state protocol Developed and recommended by IETF for Internet Good selection of metrics Concept of areas Copyright 2002 Nixu Oy 41/48 The Network Layer

42 External routing protocols External routing is routing between Autonomous Systems Autonomous System is a set of routers and networks under the same administration. Without the concept of AS Internet-routing would become impossible: too many networks. AS3 AS1 AS2 Copyright 2002 Nixu Oy 42/48 The Network Layer

43 External routing protocols... Old protocol: EGP Exterior Gateway Protocol organizes the exchange of reachability information several problems in large networks (looping etc.) New protocol: BGP Border Gateway Protocol Path-vector concept prevents looping Route filtering > Usually a must because somebody will always advertise bogus routes. Copyright 2002 Nixu Oy 43/48 The Network Layer

44 BGP Current version BGP-4 Used between ASes Users: ISPs, transit providers, corporations, multihomed networks Every IP address belongs to one AS For each destination, we find shortest AS path Typical implementation contains gazillion tuning options Route filtering, route advertising, route using, neighborhood,... On some occasions, supports even load balancing Copyright 2002 Nixu Oy 44/48 The Network Layer

45 What routing protocols to implement? RIP Everybody speaks RIP (if anything) Suitable for small environments Simple OSPF Good features Widely used in large environments BGP The external routing protocol Copyright 2002 Nixu Oy 45/48 The Network Layer

46 Special Addresses On the Internet there is an agreement that some addresses are not routed to the backbone / / /12 These addresses are called private networks and used for NAT (Network Address Translation) Copyright 2002 Nixu Oy 46/48 The Network Layer

47 Common routing problems Missing default route On statically routed environments Configuration faults On many platforms, syntax of configuration commands is very hard to manage and/or confusing On some environments, route filtering is a must because some parties advertise bogus routes Routing protocols do not usually have security features Rotten software Some vendors distribute alpha-quality software Bad documentation Copyright 2002 Nixu Oy 47/48 The Network Layer

48 Common routing problems... Lack of expertise Very small amount of Internet specialists actually can manage routing configurations, esp. in non-trivial cases Better documentation needed Bad performance Internet grows all the time and router manufacturers barely keep up with the developments On some environments, some router features (e.g. ACLs) can not be used because of performance issues Copyright 2002 Nixu Oy 48/48 The Network Layer