Computer networks -- 2007-2008 -- info.uvt.ro/Course 7
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IP packet
edit- packet structure:
- version -- 4 bits;
- header length -- 4 bits:
- expressed in number of 32 bit words;
- priority and type of service -- 8 bits:
- priority -- 3 bits;
- total length -- 16 bits:
- expressed in bytes;
- including header and data;
- identifier -- 16 bits:
- unique IP packet identifier;
- flags -- 3 bits:
- tells if fragmentation has occurred;
- fragment offset -- 13 bits:
- used for packet reassembly;
- time to live -- 8 bits;
- protocol -- 8 bits:
- 1 for ICMP;
- 6 for TCP;
- 17 for UDP;
- header checksum -- 16 bits:
- CRC for the header only;
- source IP address -- 32 bits;
- destination IP address -- 32 bits;
- options -- 0 or 32 bits;
- data;
- links:
IP control protocols
editARP
edit- ARP -- Address Resolution Protocol;
- used to find the hardware address of a host's known network address;
- usable only inside the local network;
- it involves also an ARP cache;
- it works by sending a broadcast asking the node that has that specific IP address to reply;
- ARP is for the network layer as DNS is for the application layer;
- links:
RARP
edit- RARP -- Reverse Address Resolution Protocol;
- it works just like ARP but it inverses the query parameters;
- it could be used by a disk-less machine to find out its IP address based on its MAC;
- links:
ICMP
edit- it is a management protocol;
- it allows the router to inform hosts or other routers of various events:
- destination unreachable;
- hops -- used to indicate that the TTL of a message has reached 0, and the packet hasn't found the destination;
- ping;
- links:
IP routing
edit- routing: the process of moving packets from one network to another;
- router:
- networking device that implements routing;
- it sits at the border of two layer 3 networks;
- it can be an ordinary computer or a dedicated hardware router;
- routing vs routed protocol:
- routing protocol:
- used by routers to dynamically determine the routes;
- they are invisible to the normal user;
- routed protocol: they are responsible for transferring user data once the path has been determined;
- routing protocol:
- routing:
- (see first line);
- it is based on logical -- layer 3 -- addresses, in our case IP addresses;
- routing doesn't take into account the exact destination -- host -- address, but only the network part;
- local vs remote network;
- to be able to route a packet we need to know:
- destination address;
- possible routes to all the remote networks;
- (and it can deduce) the best route to each remote network;
- (maybe) neighboring routers;
- routing table;
- routing types:
- static routing;
- dynamic routing;
- default route;
- asimetric routes;
Simple routing algorithm
edit- case study:
- two hosts:
- 172.16.10.2 -- host A;
- 172.16.20.2 -- host B;
- 172.16.10.1 and 17.16.10.2 -- a router connecting the two networks;
- two hosts:
- host A generates a packet (for example an ICMP echo -- ping -- packet) that it wants to send to the host B; the packet also contains the source and destination address;
- host A determines if the destination is on a local or a remote network; in our case it is on a remote address;
- host A will send the packet to the default router;
- for this the ARP protocol (or the ARP cache) is used to determine the MAC of the router;
- the IP packet is sent to the data link layer together with the MAC address;
- the router receives the packet from the data link layer and hands it to the network layer;
- the router checks the routing table to see where the packet should be sent next;
- in our case the router sends the packet to the host B -- again the whole ARP, data link scenario occures;
- host B receives the packet;
- host B hands the packet to the ICMP protocol module;
- host B generates a reply and the whole scenario happens again;
Static routing
edit- the routing table is managed by an administrator that fills all the possible routes and their metrics;
- advantages:
- low CPU and memory overhead;
- no bandwidth usage;
- greater security;
- disadvantages:
- the network topology must be thoroughly known by the administrator;
- the update process is painful;
- not usable in larger networks;
- it doesn't scale with the number of networks;
- class-ful vs class-less routing:
- sending or not subnetting or VLSM information;
Dynamic routing
edit- the router uses a protocol to exchange information with direct neighboring routers, in order to build the routing table;
- advantages:
- no human intervention is needed;
- the topology is not needed to be known in advance;
- the network is optimized based on the actual situation;
- it scales well with the number of networks;
- disadvantages:
- higher CPU and memory usage;
- bandwidth consumption;
- big convergence times;
- AS -- Autonomous System:
- a collection of networks under the same administrative domain;
- types:
- IGP -- interior gateway protocols:
- used inside an AS;
- all the routers share the same routing tables;
- RIP -- Routing Information Protocol;
- IGRP -- Interion Gateway Routing Protocol;
- EIGRP -- Enhanced Interior Gateway Routing Protocol;
- OSPF -- Open Shortest Path First;
- EGP -- exterior gateway protocols:
- used outside an AS, so between different AS;
- BGP -- Border Gateway Protocol;
- IGP -- interior gateway protocols:
- administrative distances:
- a number between 0 and 255;
- 255 it means it will never be used;
- it should represent the thrustworthiness of that link;
- usually a route with the lowest distance is the one used;
- examples:
- connected -- 0;
- static route -- 1;
- EIGRP -- 90;
- IGRP -- 100;
- OSPF -- 110;
- RIP -- 120;
- unknown -- 255;
- metric:
- a value between 0 and 255;
- it should denote the efficiency of the link (bandwidth, delay, etc.);
- routing protocol types:
- distance vector protocols: RIP, IGRP;
- link state protocols: OSPF;
- hybrid: EIGRP;
Distance vector routing protocols
edit- these protocols find the best route based on the estimated distance;
- a good example would be the number of hops;
- general working principle: each router sends to its neighbors its entire routing table;
- also called routing by rumor;
- if all the routes to a network have the same metric, then load balancing is done on these routes;
- because of slow network convergence time routing loops could occure; as a solution:
- any network with a number of hops greater than -- 15 in the case of RIP -- will be deemed unreachable;
- or split horizon could be used -- each router will keep track of where a route was learned from and will not advertise that way;
- route poisoning -- when a link goes down it's hop count is set to 16 and advertised, thus it is marked as unreachable;
- other problems that could occure are when a link keeps changing its state rapidly;
RIP -- Routing Information Protocol
edit- it is a distance vector protocol;
- it is an open standard;
- it sends its table about every 30 seconds;
- it uses hop count as distance;
- it has a maximum hop count of 15;
- any network with over 16 (including) hops is deemed unreachable;
- it is suitable for small networks, but not for larger ones;
- versions:
- v1 -- class-ful routing;
- v2 -- class-less routing;
- timers:
- route update timer -- when it should send its routing table -- about 30 seconds;
- route invalid timer -- when how much it will keep a route without receiving an update about it, and when it elapses it tells to the neighboring routers -- about 180 seconds;
- route flush timer -- how much an invalid route should be still kept -- about 240 seconds;
- as a disadvantage: too expensive because of the bandwidth used and the convergence time;
IGRP -- Interior Gateway Routing Protocol
edit- a Cisco proprietary protocol;
- maximum hop count of 255, but with a default of 100;
- it uses a composite metric: bandwidth, delay, (maybe MTU);
- in general it works like RIP;
- it works in larger networks;
Link state routing protocols
edit- also called shortest path protocols:
- each router keeps three tables:
- directly connected routers;
- entire network topology;
- the actual routing table;
- general working principle: each router sends to all the other routers the state of its links;
- the advantage of these protocols is that they can determine the actual network topology;
OSPF -- Open Shortest Path First
edit- it is an open standard;
- it uses Dijkstra's algorithm for minimum spanning tree;
- it sports VLSM;
- it uses bandwidth as a metric;
- it has fast convergence times;
- it allows hierarchical networks:
- by splitting the network is areas;
- a router keeps a tree for each area it is in;
- the tree contains the best path to that given network;
- the best path is elected based on the metric (bandwidth);