Advanced Lab in Computer Communications Meeting 4 Network Layer. Instructor: Tom Mahler

Transcription

1 Advanced Lab in Computer Communications Meeting 4 Network Layer Instructor: Tom Mahler

2 Routing How do packets get from A to B in the Internet? A Internet B

3 Network Layer: Routing Layers model Application Transport Network Data Link Physical Each router makes a LOCAL decision to forward the packet towards destination using Internet routing protocols.

4 Network layer functions Host addressing: numerical label of each device participating in a computer network that uses the Internet Protocol. Path determination: route taken by packets from source to dest. Routing algorithms: Global: link state algorithms (OSPF, IS-IS) Decentralized: distance vector algorithms (RIP, IGRP, EIGRP, BGP) Switching: move packets from router s input to appropriate router output Lookup and Forwarding : Forwarding Table Longest prefix matches

5 IP IP (Internet Protocol) is a Network Layer Protocol. TCP UDP Transport Layer ICMP IP IGMP Network Layer ARP Network Access Link Layer Media IP s current version is Version 4 (IPv4). It is specified in RFC 891. IPv6 is the latest revision of the IP. September 2013 IPv6 surpassed 2% for the first time. IPv6 uses a 128-bit address

6 IP IP is the highest layer protocol which is implemented at both routers and hosts Application Application protocol Application TCP TCP protocol TCP IP IP protocol IP IP protocol IP IP protocol IP Data Link Data Link Data Link Data Link Data Link Data Link Data Link Data Link Network Access Host Router Router Host

7 IP Service IP provide provides an unreliable connectionless best effort service. Unreliable: IP does not make an attempt to recover lost packets Connectionless: Each packet ( datagram ) is handled independently. IP is not aware that packets between hosts may be sent in a logical sequence Best effort: IP does not make guarantees on the service (no throughput guarantee, no delay guarantee, ) Consequences: Higher layer protocols have to deal with losses or with duplicate packets Packets may be delivered out-of-sequence

8 IP Addresses 32 bits version (4 bits) header length TTL Time-to-Live (8 bits) Identification (16 bits) Type of Service/TOS (8 bits) Protocol (8 bits) flags (3 bits) Total Length (in bytes) (16 bits) Fragment Offset (13 bits) Header Checksum (16 bits) Source IP address (32 bits) Destination IP address (32 bits) Ethernet Header IP Header TCP Header Application data Ethernet Trailer Ethernet frame

9 IPv4 Header Identification Original packet ID in case of fragmentation Flags Don't Fragment (DF)/More Fragments (MF) Fragment Offset - specifies the offset of a particular fragment relative to the beginning of the original unfragmented IP datagram. Protocol upper layer protocol e.g. TCP(6), UDP(17), OSPF(89), ICMP(1)

10 IPv4 addresses class-full addressing: Routing between networks Routing within a network

11 IP addressing: CIDR classfull addressing: inefficient use of address space, address space exhaustion e.g., class B net allocated enough addresses for 65K hosts, even if only 2K hosts in that network CIDR: Classless Inter Domain Routing network portion of address of arbitrary length address format: a.b.c.d/x, where x is # bits in network portion of address network part host part /23

12 IP addresses: how to get one? Hosts (host portion): hard-coded by system admin in a file DHCP: Dynamic Host Configuration Protocol: dynamically get address: plug-and-play host broadcasts DHCP discover msg DHCP server responds with DHCP offer msg host requests IP address: DHCP request msg DHCP server sends address: DHCP ack msg

13 IP Addressing: introduction IP address: 32-bit identifier for host, router interface interface: connection between host, router and physical link router s typically have multiple interfaces host may have multiple interfaces IP addresses associated with interface, not host, or router =

14 IP Addressing IP address: network part high order bits host part low order bits What s a network? (from IP address perspective) device interfaces with same network part of IP address can physically reach each other without intervening router LAN network consisting of 3 IP networks (for IP addresses starting with 223, first 24 bits are network address)

15 IP Addressing How to find the networks? Detach each interface from router, host create islands of isolated networks Interconnected system consisting of six networks

16 Routing Example Starting at A, given IP datagram addressed to B: look up network address of B, find B on same network as A link layer sends datagram to B inside link-layer frame frame source, dest. address A B datagram source, dest. address E B s MAC addr A s MAC addr A s IP addr B s IP addr IP payload frame datagram

17 ARP: Address Resolution Protocol Question: how to determine MAC address of B given B s IP address? Each IP node (Host, Router) on LAN has ARP module and table ARP Table: IP/MAC address mappings for some LAN nodes < IP address; MAC address; TTL> <.. > TTL (Time To Live): time of day after which address mapping will be forgotten (typically 20 minutes)

18 ARP protocol A knows B's IP address, wants to learn physical address of B A broadcasts ARP query pkt, containing B's IP address all machines on LAN receive ARP query B receives ARP packet, replies to A with its (B's) physical layer address A caches (saves) IP-to-physical address pairs until information becomes old (times out)

19 IP addresses: how to get one? Network (network portion): get allocated portion of ISP s address space: ISP's block /20 Organization /23 Organization /23 Organization / Organization /23

20 Datagram Forwarding Algorithm for router/hosts Every node (host/router) first checks to see if it is connected to the same physical network as the destination. Routers are connected to several physical networks, while hosts are only connected on a single network. If the destination address is in the same network, send the packet directly to the destination. If the destination address is in a different network, the node forwards the packet to a router that knows how to reach the destination.

21 Getting a datagram from source to dest. Starting at A, given IP datagram addressed to B: r r r misc fields data look up net. address of B find B is on same net. as A link layer will send datagram directly to B inside link-layer frame m B and A are directly connected A B Dest. Net. next router Nhops E

22 Getting a datagram from source to dest. Starting at A, dest. E: r r r r look up network address of E E on different network m A, E not directly attached routing table: next hop router to E is link layer sends datagram to router inside linklayer frame r datagram arrives at r misc fields data continued.. A B Dest. Net. next router Nhops E

23 Getting a datagram from source to dest. Arriving at , destined for r r r Dest. next misc fields data network router Nhops interface look up network address of E E on same network as router s interface m router, E directly attached link layer sends datagram to inside link-layer frame via interface r datagram arrives at A B E

24 Routing Algorithm classification Global or decentralized information? Global: all routers have complete topology, link cost info link state algorithms Decentralized: router knows physically-connected neighbors, link costs to neighbors iterative process of computation, exchange of info with neighbors distance vector algorithms

25 RIP Routing Information Protocol Distributed with the popular Berkeley Software Distribution version of Unix. Richard Bellman Based on BF algorithm.

26 RIP Routing Information Protocol At start every node is familiar with it s immediate neighbors. Every node holds a distance vector containing the costs to all other nodes. Each node distributes it s distance vector to it s immediate neighbors. The nodes keep updating their distance vectors until convergence is reached. Routers keep sending distance vectors/ updates every 30 sec to handle changes in the network graph topology. Cost metric: number of hops (distance) RIP support 1-15 hops routes (with 16 hops representing infinity).

27 RIP - Example B A C D E Initial distances stored at each node Node Distance to reach a node A B C D E F G A B C D F G F G Initial routing table at node A Destination Cost Next Hop B 1 B C 1 C D - E 1 E F 1 F G -

28 RIP - Example B A C D E Final distances stored at each node Node Distance to reach a node A B C D E F G A B C D F G F G Final routing table at node A Destination Cost Next Hop B 1 B C 1 C D 2 C E 1 E F 1 F G 2 F

29 RIP Convergence Problem A 1 B Node Cost C A 1 C 1

30 IGRP (Interior Gateway Routing Protocol) CISCO proprietary; successor of RIP (mid 80s) Distance Vector, like RIP several cost metrics (delay, bandwidth, reliability, load etc) uses TCP to exchange routing updates Uses a combination of metrics Network delay, bandwidth, reliability and load all factored into the routing decision

31 IGRP Continued Provides a wide range for its metrics For example Reliability value between 1 and 255 Bandwidth values reflecting speeds from 1200 bps to 10 Gbps Delay value from 1 to 224 Contrast with RIP(does not weigh metrics) it only uses one Maximum hop count 255 Contrast with RIP 15 hops only

32 IGRP Metrics and routing Updates Bandwidth by default is given the most importance Router running IGRP sends updates every 90 seconds Declares route inaccessible if an update from first router is not sent within 3 update periods (270 seconds) after 5 update periods (450 seconds) router removes the route from the routing table Poison reverse to speed up convergence

33 Features that Enhance Stability of IGRP Split Horizons with Poison-Reverse Occur when router tries to send information about a route back in the direction that it came Helps prevent routing loops Implemented in IGRP

34 EIGRP EIGRP is a distance vector, classless routing protocol that was released in 1992 with IOS As its name suggests, EIGRP is an enhancement of Cisco IGRP (Interior Gateway Routing Protocol). Both are Cisco proprietary protocols and only operate on Cisco routers. The main purpose in Cisco's development of EIGRP was to create a classless version of IGRP. Although EIGRP may act like a link-state routing protocol, it is still a distance vector routing protocol.

35 EIGRP The Algorithm EIGRP uses the Diffusing Update Algorithm (DUAL). EIGRP does not send periodic updates and route entries do not age out. Only changes in the routing information, such as a new link or a link becoming unavailable ("Hello" packet is not received from a neighbor) cause a routing update to occur. EIGRP routing updates are still vectors of distances transmitted to directly connected neighbors.

36 EIGRP EIGRP s 5 Packet Types Hello packets Used to discover & form adjacencies with neighbors Update packets Used to propagate routing information Query packets Used by DUAL for searching for networks Can use Unicast or Multicast Reply packets Reply packet Can use Unicast only Acknowledgement packets Used to acknowledge receipt of update, query & reply packets

37 EIGRP EIGRP Bounded Updates EIGRP only sends update when there is a change in route status Partial update A partial update includes only the route information that has changed the whole routing table is NOT sent Bounded update When a route changes, only those devices that are impacted will be notified of the change EIGRP s use of partial bounded updates minimizes use of bandwidth

38 OSPF- Open Shortest Path First At first, every node knows it s neighbors and the cost to reach them. Edsger Dijkstra Each router generates link state advertisements (LSAs) which are distributed to all routers LSA = (link id, state of the link, cost, neighbors of the link) Each router maintains a database of all received LSAs (topological database or link state database), which describes the network has a graph with weighted edges. Each router uses its link state database to run a shortest path algorithm (Dijikstra s algorithm) to produce the shortest path to each network

39 Features of OSPF Enables load balancing by allowing traffic to be split evenly across routes with equal cost Type-of-Service routing allows to setup different routes dependent on the TOS field Supports multicasting Allows hierarchical routing

40 Neighbor Discovery and Maintenance Hello Protocol Hello packets sent out every 10 seconds helps to detect failed neighbors RouterDeadInterval (default 40 seconds) neighboring routers agree on intervals hello interval set so that a link is not accidentally brought down

41 OSPF Areas An area is a set of routers that are configured to change link state information among themselves. Area Border Routers (ABRs) are connecting between the different areas. The result: improved scalability rather than absolute optimal shortest paths

42 END