1. Kazi Fall 2007 CSCI 370/EENG 480 1 CSCI-370/EENG-480 Computer Networks Khurram Kazi

2. Kazi Fall 2007 CSCI 370/EENG 480 2 Major sources of the slides for this lecture  Interworking with TCP/IP, M9000-02, Global knowledge, training manual, (http://am.globalknowledge.com)http://am.globalknowledge.com  TCP/IP Clearly Explained, Pete Loshin.  The Internet and Its Protocol, Adrian Farrel’s book.

3. Kazi Fall 2007 CSCI 370/EENG 480 3 Reference Network: For discussion purposes

4. Kazi Fall 2007 CSCI 370/EENG 480 4 Internet Control Message Protocol (ICMP)  ICMP is carried inside an IP datagram  Is considered to be at the internet layer (network layer)  ICMP is fundamental to the operation of IP networks and is so closely related to IP that it is not possible to operate hosts within an IP network without supporting ICMP  Used by the routers to identify the hosts and other routers: Most commonly ICMP is used for  Sending error message about unreachable destinations  Sending error and status messages about routes and gateways  Sending echo requests and replies to indicate status of reachable hosts  Sending error message about traffic that has “timed-out” (TTL drops too low)

5. Kazi Fall 2007 CSCI 370/EENG 480 5 ICMP Message format

6. Kazi Fall 2007 CSCI 370/EENG 480 6 ICMP Messages Message TypeMessage 0Echo reply. Sent in direct response to an ICMP echo request message 3Destination Unreachable. An error message sent when a node cannot forward any IP datagram towards its destination 4Source Quench. Sent by a destination node to slow down the rate at which a source node sends IP datagrams 5Redirect. Used to tell a source node that there is a better first hop for it to use when trying to send IP datagrams to a given destination. 8Echo. Sent by a node to probe the network for reachability to a particular destination 9Router Advertisement. Used by a router to tell hosts in its network that it exists and is ready for service. 10Router Solicitation. Used by a host to discover which routers are available for use. 11Time Exceeded. An error message generated by a router when it cannot forward an IP datagram because the TTL has expired. 12Parameter Problem. An error sent by any node that discovers a problem with an IP datagram it has received 13Timestamp Request. Used to probe the network for the transmission and processing latency of message to a give destination. 14Timestamp Reply. Used in direct response to a Timestamp Request Message 15Information request. Used by a host to discover the subnet to which its is attached. 16Information Reply. Used in direct response to an Information Request message 17Address Mask request. Used by a host to discover the subnet mask for the network to which it is attached 18Address Mask Reply. Used in direct response to an Address Mask Request message.

7. Kazi Fall 2007 CSCI 370/EENG 480 7 ICMP  Ground rules for ICMP  ICMP message cannot generate ICMP error messages  ICMP cannot generate broadcast or multicast messages  These rules help avoid “broadcast storms” that can easily flood a network  Unreachability Message  Indicate that there has been a failure somewhere in the process of addressing the datagram that triggers the message.  For example, incorrectly addressed datagrams will cause unreachable messages to be sent  More often, if a host is simply turned off or even when the specified protocol is not available (e.g., trying to connect to a network application port that is prohibited or restricted).

8. Kazi Fall 2007 CSCI 370/EENG 480 8 ICMP  ICMP Routing Messages  Routing messages include requests for a list of available routers and replies that include list of other available routers along with a priority number for each router.  Hosts sometimes make these requests when they boot up to initialize their routing tables  Routers advertise gateways when they bootup.  Routers also periodically broadcast this information.  Another type of routing message is generated when a router becomes overloaded, whether from high-volume stream of traffic from a single host or from a generally high load due to many hosts.  ICMP message may carry routing information when a host sends traffic to one router and a different router advertises a better route (route with fewer hops). This is called redirect. Commonly happens on networks with more than one router, where the hosts start out with only a single router in their routing tables

9. Kazi Fall 2007 CSCI 370/EENG 480 9 ICMP  Ping and ICMP Echo Messages  Ping (or PING) is named from Packet InterNet Groper  Ping sends an ICMP echo request out to a specific host, and the host responds to ICMP echo requests by sending out an ICMP echo reply  Represents the simple level of connectivity possible between two hosts on an internetwork  Useful for testing whether a remote host is reachable or whether the network connection for a local host is properly configured and installed  Command has the format  Ping

10. Kazi Fall 2007 CSCI 370/EENG 480 10 Traceroute (“tracert” command DOS)  Used to attempt to trace the route between two hosts on an internetwork  Uses TTL field and multiple passes to collect information.  Starts with a TTL of 1, the neighboring router drops that packet and sends back an ICMP message  Sends the next packet with TTL of 2. This allows the packet to go to the second hop. The second hop router sends an ICMP message back to the origination host that indicates that the offending datagram expired on the network  The origination host keeps on increasing the TTL until it has all the relevant information it is looking for about the route between it and the destination host.  Examining the list of nodes that returned errors gives us the path through the network to the destination.  Examining the turnaround time for the error messages gives a measure of which hops in the network are consuming how much time

11. Kazi Fall 2007 CSCI 370/EENG 480 11 Routing  Routing is the method by which the host or gateway decides where to send the datagram.  It may be able to send the datagram directly to the destination, if that destination is on one of the networks that are directly connected to the host or gateway.  However, the interesting case is when the destination is not directly reachable. In this case, the host or gateway attempts to send the datagram to a gateway that is nearer the destination.  The goal of a routing protocol is very simple: It is to supply the information that is needed to do routing.

12. Kazi Fall 2007 CSCI 370/EENG 480 12 Routing  There are two basic routing problems  Interior Routing  Routing traffic within the organizational internetwork  Exterior Routing  Ensuring traffic is routed within the larger internetwork (Internet)  Internet at large must address exterior routing issues  Interior routing is managed by the local administrators of the organization

13. Kazi Fall 2007 CSCI 370/EENG 480 13 Adding Router means must have the capability to advertise reachability Before addition of the new router….what is the route between network A and Network D? How many hops? When the new router was added, how many hops? Routing protocols allow routers to communicate network connectivity across network boundaries to other routers. Reachability data is passed along, as well as changes in reachbility If a router fails, routing protocols allow neighboring routers to report the failure to the neighbors

14. Kazi Fall 2007 CSCI 370/EENG 480 14 Vector/Distance vs. Link State Routing  Routing protocols can use two basic methods to measure connectivity across internetwork  Vector/Distance  Routers share their routing tables and make additions and corrections based on reports from other routers  Routers are advertised as data pairs, with the router reporting each network it can reach along with the number of hops it takes to reach it  Receiving router looks at the information, compares it with its routing table, if there is new information it updates its routing table, e.g.,  Additional network that did not exist before  Network connectivity showing few hops to a certain destination  This approach can be quite cumbersome as the network grows as routers track connections and pass the information to each other. This results into processing of larger and larger list of network routes

15. Kazi Fall 2007 CSCI 370/EENG 480 15 Distance Vectors: e.g., Routing Table at Router E. Initial routing Distribution while the link between Routers A and B Disabled DestinationOutgoing Interface DistanceNext Hop E127.0.0.10- B10.0.6.21B F10.0.7.11F C10.0.6.22B D10.0.7.12F A 3F Loopback IP/local host address

16. Kazi Fall 2007 CSCI 370/EENG 480 16 Routing Table at Router E. After Full Distribution DestinationOutgoing Interface DistanceNext Hop E127.0.0.10- B10.0.6.21B F10.0.7.11F C10.0.6.22B D10.0.7.12F A10.0.6.22B Loopback IP/local host address

17. Kazi Fall 2007 CSCI 370/EENG 480 17 Vector/Distance vs. Link State Routing  Link State  Keeps the volume of information passed along to other routers to a minimum  Each router periodically checks on the status of neighboring routers, reporting which links are alive to all the other participating routers  With the this information each router can then create its own map of the internetwork

18. Kazi Fall 2007 CSCI 370/EENG 480 18 Routing Protocols  Routing Information Protocol (RIP) is simple and ubiquitous.  Open Shortest Path First (OSPF) protocol is very popular and has a close rival, Intermediate System to Intermediate System (IS-IS), that performs a similar function  Border Gateway Protocol (BGP) is important for hooking together the many Service Provider networks into a single Internet

19. Kazi Fall 2007 CSCI 370/EENG 480 19 Classless Interdomain Routing (CIDR)  As covered previously: IP addresses are grouped in classes  Different nibbles/bytes split the address into network portion and the host portion of the IP address  Network mask indicates the length of the network part of the address – know as prefix length  Earlier networks routed traffic based on the classes (A, B or C type)  Subnetting allows the networks to be divided into smaller segments  Subnetting process defines range of addresses assigned to a subnet according to prefix length  Routing using subnetwork addresses is not quite simple as routing as using class addresses, because knowledge of the network mask (prefix length) is not encoded in the address itself  Routing table must consist of a list of subnetwork addresses (i.e. addresses and prefix length), each mapping to a route or path along which packet for that subnet should be forwarded  Routing table will explode if all addresses have to be included in the routing table. e.g., in class A potentially there are 2 22 30-bit prefix subnetworks  The solution within the Internet is to route at an appropriate level of granularity through address aggregation

20. Kazi Fall 2007 CSCI 370/EENG 480 20 Route address aggregation SubnetworkSubnetwork MaskAddress Range 172.19.168.16/28255.255.255.240172.19.168.16 – 172.19.168.31 172.19.168.32/28255.255.255.240172.19.168.32 – 172.19.168.47 172.19.168.32/26255.255.255.224172.19.168.1 – 172.19.168.62 The subnet addresses can be combined/aggregated as a single subnetwork 176.19.168.32/26

21. Kazi Fall 2007 CSCI 370/EENG 480 21 RIP: Routing Protocol RFC 1058  RIP is one of a class of algorithms known as "distance vector algorithms".  RIP is intended for use within the IP-based Internet. The Internet is organized into a number of networks connected by gateways. The networks may be either point-to-point links or more complex networks. Hosts and gateways are presented with IP datagrams addressed to some host.  Limitations of the protocol:  This protocol does not solve every possible routing problem. Its is primary intended for use as an IGP, in reasonably homogeneous networks of moderate size.  The protocol is limited to networks whose longest path involves 15 hops.  It is inappropriate to use this for larger networks  The protocol depends upon "counting to infinity" to resolve certain unusual situations  Routing messages received from  This protocol uses fixed "metrics" to compare alternative routes. It is not appropriate for situations where routes need to be chosen based on real-time parameters such a measured delay, reliability, or load. The obvious extensions to allow metrics of this type are likely to introduce instabilities of a sort that the protocol is not designed to handle.

22. Kazi Fall 2007 CSCI 370/EENG 480 22 RIPv2 message format RIPv2 message consists of a 4-byte header followed by from 1 to 125 route entries

23. Kazi Fall 2007 CSCI 370/EENG 480 23 RIP Details: commands 1 – RequestA request for the responding system to send all or part of its routing table 2– ResponseA message containing all or part of the sender’s routing table. This message may be sent in response to a request or poll, or it may be an update message generated by the sender. 3 – TraceonObsolete (should be ignored) 4 – TraceoffObselete 5 -- reserved Rest of the datagram contains a list of destination, with information about each. Each entry in this list contains a destination or host, and the metric for it.

24. Kazi Fall 2007 CSCI 370/EENG 480 24 RIP Details  RIP can carry routing information for several different protocols. For IP the address family identifier is 2.  The IP address is the usual Internet address, stored as 4 octets in network order.  The metric field must contain a value between 1 and 15 inclusive, specifying the current metric for the destination, or the value 16, which indicates that the destination is not reachable.  “Metric" measuring the total distance to the entity. Distance is a somewhat generalized concept, which may cover the time delay in getting messages to the entity, the dollar cost of sending messages to it, etc.

25. Kazi Fall 2007 CSCI 370/EENG 480 25 Assignment 2: Due date November 2  Find out which RFC defines ICMP. Generate at least 3 different types of ICMP messages. Explain what each message contains and what is it used for.  This does not require you to write any programs. Develop/Write the ICMP messages on paper in similar to slide 5.  Elaborate explanations are encouraged  The assignment should be typed, not hand written