CS 5520/ECE 5590NA: Network Architecture I Spring Lecture 10: IP Routing and Addressing Extensions

Transcription

1 CS 5520/ECE 5590NA: Network Architecture I Spring 2009 Lecture 10: IP Routing and Addressing Extensions This lecture provides discussion of the mechanisms used to route IP datagrams (Chapter 7). It also discusses methods that are being used to conserve IP addresses. I. The Routing Process Chapter 7 - Internet Protocol: Routing IP Datagrams Routing is the process of choosing a path over which to send packets. Ideal routing would take into account several considerations. Network load QoS Delay, delay variation, loss, etc. Datagram length Type of service requested Practical routing is usually much less sophisticated. Chooses routes with fewest hops in many cases. We will look at details of finding best routes in a later chapter. First of all, we must be clear on how packets are sent along routes to their intended destinations. Remember: Ultimately all datagrams at each hop are sent across some physical network using some type of data link protocol. Routing is accomplished by both host computers and routers. 1. Host computers As Senders: Must determine the over which to send the datagram. Lecture 10, Page 1 of 16

2 Two types of destinations Direct delivery - Send to another machine. - Over a single physical network. - Send datagram straight to its destination over that network. - Checks the network portion of the destination IP address. - If same as the source, datagram is sent over the physical network. Indirect delivery - Send to a router. - So the router can send it onward. - The host must identify a router which can forward the datagram on toward its destination. - Easy if only one router is used to connect to the rest of the Internet. More complicated if there is more than one. As Receivers: Hosts should make sure that packets they receive are destined for them. Host first checks for a match on the datagram's destination IP address. If no match, the datagram is discarded. Hosts do not try to forward the packet on toward their destination. - If a host receives a datagram intended for some other machine, something has gone wrong with internet addressing, routing, or delivery. - Such routing could cause unnecessary network traffic (and may steal CPU time from legitimate uses of the host). - So, such packets are usually just dropped. Lecture 10, Page 2 of 16

3 2. Routers II. The Routing Table Rarely are destinations in themselves, except for testing or sending routing update messages. They send packets on toward their destinations. The also process error messages and share routes with other routers. An Internet routing table (or IP routing table) on each machine stores information about possible destinations and how to reach them. Both hosts and routers maintain routing tables. Need to keep table sizes manageable. Information is not stored about the full path to a destination. Only logs the information. The next hop defines the next router to which to send the datagram. Relies on other routers to know the rest of the path. The router does not know the complete path to the destination. Kind of amazing that it all works. Becomes complicated if all routers do not have up-to-the-second information. - If some are delayed in receiving information. A routing table does not store information about how to reach every possible host. Only stores entries for networks, not individual hosts. The destination network will find the destination host. No information about the end host needs to be known outside its own network. Typically a routing table contains (N, R) pairs. N is the IP address of the destination network. R is the IP address of the next router along the path. R is the place to send the packet next. Lecture 10, Page 3 of 16

4 Routing table for R What is the routing table for Q? Lecture 10, Page 4 of 16

5 Special cases Host-Specific Routes Routing tables can have entries for specific end hosts. Default Routes If no route appears in the routing table, send the datagram to a default router. Routing tables for hosts might need to only have one entry for the default router. Algorithm to forward datagrams: Algorithm: RouteDatagram ( Datagram, RoutingTable ) Extract destination IP address, D, from the datagram and compute the network prefix, N; if N matches any directly connected network address, deliver datagram to destination D over that network (This involves resolving D to a physical address with ARP, encapsulating the datagram, and sending the frame.) else if the table contains a host-specific route for D send datagram to the next-hop specified in the table else if the table contains a route for network N send the datagram to the next-hop specified in table else if the table contains a default route send datagram to the default router specified in table else declare a routing error (no default route and no match) otherwise; Additional Comments This approach goes from most specific to least specific. Since more than one entry could apply to an address. For example, the table might have both an entry for a packet s network and for a host-specific route. All traffic for a given network takes the from a specific router. Since routing tables only route on network address. Lecture 10, Page 5 of 16

6 Normally, routing tables have no randomness to them. All datagrams for a destination network take the same next hop. - Until the routing table might be updated (every 30 seconds or so). No random selection is made between possible routes. So if datagrams arrive out-of-order or from different routes, this is caused by updates in routing tables. Datagrams traveling from host A to B may travel an entirely different route than those going from B to A. So how can this cause a problem if we wish to set up routing for good QoS (like with RSVP)? We must reserve resources in both directions separately. Use record route and use that path for reservations. (skip Chapter 9 for now) How to compute routes and fill routing tables will be covered in a later lecture Chapter 10 - Classless and Subnet Address Extensions (CIDR) III. Minimizing Network Numbers Remember: Routers keep one routing entry per network. Designers failed to envision network growth Size has been doubling every 9 to 15 months. Designers did not foresee tens of thousands of small networks. Internet design is stressed Immense administrative overhead is required simply to manage all of the network addresses. Lecture 10, Page 6 of 16

7 Routing tables are extremely large - Over 100,000 entries in core network routers. - Core network routers do not use default routes. - They are the routers that need to know the routes to everywhere. Address space will eventually be exhausted. - Maybe not until 2019 as predicted by the textbook author. - Maybe much sooner if IP addresses are assigned to small appliances - cell phones, home devices, etc. There are not enough Class B addresses 2 14 = Class B And Class B addresses are in many cases too large. How many hosts are on a Class B network? 64k But Class C addresses are too small. And there are many of them remaining = 2 million Class C Possible solutions Use one IP network prefix for multiple physical networks. Eliminate wasted addresses for point-to-point networks. Abandon rigid class system for addresses. We will examine the two more prominent solutions Subnet Addressing Use a single network address for multiple physical networks. Classless Addressing Addresses can be saved if the Class A/B/C class framework is not used. IV. Subnet Addressing This is a method for using the same netid for multiple physical networks. Popular because it is very general and has been standardized. Lecture 10, Page 7 of 16

8 Example above A site uses a single class B address, Local site chooses to use the third octet to distinguish between two physical networks. R (but only R) examines this third octet. Remember: A site can choose to use the hostid portion of the IP address however they wish. It is better in this case to consider this the local portion of the IP address, rather than just a hostid. Lecture 10, Page 8 of 16

9 A site may choose to use the most significant bits of the local portion for a subnet address. This is the concept of hierarchical addressing and hierarchical routing. Similar to the U.S. telephone system. 3-digit area code 3-digit exchange 4-digit connection Can allocate bits for the subnet address as desired Depending on - Number of networks. - Number of hosts in each network. - Growth potential. 8-bit network, 8-bit host 254 networks each with 254 hosts. - All 1's and all 0's host addresses are reserved. - All 1's or all 0's subnet addresses are not recommended. Variable or fixed length Most sites that implement subnet addressing used fixed assignments. - Same division of bits for all addresses. Can use variable-length subnetting. - Variability does not mean varying with time, but varies with the subnetwork. - Can use different partitions for different physical networks. - Good idea: Can achieve higher utilization of address space. Lecture 10, Page 9 of 16

10 - But why might this be difficult to implement? Must be careful to avoid address ambiguity - addresses could be interpreted inconsistently. An address could appear to match two different subnets. A site that uses subnet addressing will define a subnet mask. Example: Here 22 bits are defined for the netid and subnet address. Bits are not required to be consecutive, but it is recommended. Can extract the network part of the address using a Boolean bitwise AND. Mask representation Can represent the mask above using dotted decimal Subnet routing The standard routing table has entries of (network address, next hop address). With subnetting, the table must also include the subnet mask. (subnet mask, network address, next hop address). Routers use the subnet mask to first extract network addressing (AND the address with the mask). After that, the network address must be matched. The next hop still must be reachable by a directly connected network (as always). Lecture 10, Page 10 of 16

11 Masks must be shared between routers (inside a group of physical networks that share the same netid) As should addresses. A special control message exists to transfer subnet masks. V. Classless Addressing (Supernetting) The above subnetting technique, while useful, still would not prevent an exhaustion of the IP address space. Subnet addressing was invented in the early 1980's. In 1993, work began on a new version of IP, IPv6, with much larger addresses. 128 bits per address Addresses would never be exhausted. Until the work on IPv6 could be completed, a temporary solution was devised. This "temporary" solution has made a transition to IPv6 seem much less urgent. Along with Network Address Translators (NAT's) which will be discussed in the next lecture. Called classless addressing, supernet addressing, or supernetting. Allows addresses for one organization to span multiple classed prefixes. Example: Medium-sized organization Class C cannot accommodate more than 254 hosts. Class B has more addresses. Class B can support subnetting. Supernetting can support the same objectives without allocating a Class B. 256 Class C s have the same number of addresses as one Class B. - Supernetting allows assignment of 256 Class C addresses instead of one Class B. - So a Class B is not used. Lecture 10, Page 11 of 16

12 Or fewer Class C addresses can be allocated if that many are not needed. These Class C addresses are all in (unbroken sequence of addresses). - So a mask can be defined that characterizes all of the addresses. Could come from an ISP that has a batch of Class C addresses it can allocate. - So the organizations (ISP s) can allocate addresses to other organizations. Classless Interdomain Routing (CIDR) Effect of Supernetting on Routing If we allocate many Class C addresses, this increases router demands dramatically. - Much larger routing tables from many more networks. - Many more entries to search and maintain. If all Class C addresses are in one continuous block, we can simplify. CIDR requires two items to specify a block of addresses bit value of the lowest address in the block - 32-bit mask The size of a block of addresses must be a power of two. Example: - The range of addresses above share the same first 21 bits. - A mask for that would have 21 1's This would result in a block of usable addresses of size = Lecture 10, Page 12 of 16

13 CIDR addresses are simplified using a slash notation. - For above, the address could be said to be with a mask of Alternatively, it could be designated as / Since it is a mask with 21 1's. CIDR does not require allocation of Class C address blocks, hence the term. Can even be smaller than Class C, even networks of 4 hosts. Only needs to be in a contiguous range of addresses that can be specified by a mask that covers that range. - This causes some restrictions to exist on how addresses can be assigned (see homework problems). - Example: Allocation of 8 addresses in the range to = = CIDR allocation to include those addresses would have to be / 28, because of the need to create a mask to cover the addresses. Only the first 28 bits are in common. But this would allocate the range of addresses from to (16 addresses instead of 8). A better range of addresses could be from to which would be / 29. Another option would be / 29 for to Both options would only use 8 addresses. UMKC addressing Let's look at some UMKC addresses This is based on a Class B address allocated to UMKC. Lecture 10, Page 13 of 16

14 From these addresses, we would not have to have a Class B network, however. What could it be? How many hosts would it have? = = = = bits shared in common / 17 with 2ˆ15-2 = hosts Instead of /16 (Class B) with 2ˆ16-2 = hosts The real story for UMKC UMKC's primary address is the entire Class B range of Most buildings have subnets of 254 addresses like with a class C, for example, to Some of the larger buildings like Flarsheim Hall have subnets made up of 1024 addresses. Example: The following requests for network address allocations are received (in this chronological order). Requests: Network A Hosts Network B Hosts Network C Hosts Use CIDR address allocation for the requests above. The last allocated address before these requests were received is Give the CIDR address that would be stored in routing tables for each of these networks. Use open spaces that may have been leftover from previous allocations. Start: = Net A => 2046 = 2^11-2 => 11 bits, 3 bits in second octet 2046 addresses will share the first 21 bits Need: xxxxx So next available is xxx uses range (16.0) to (23.255) Lecture 10, Page 14 of 16

15 / 21 unused 11.0 to , and above 24.0 Net B => 1022 = 2^10-2 => 10 bits, 2 bits in second octet Need: xxxxxx So next available (from ) is uses range (12.0) to (15.255) / 22 unused 11.0 to , above 24.0 Net C => 126 = 2^7-2 => 7 bits, only 7 of 8 bits in last octet Need: xxxxxxxx.x So next available (from ) is uses range (11.0) to (11.127) / 25 unused to , above 24.0 Lecture 10, Page 15 of 16

16 Routing information CIDR is used throughout the Internet So CIDR addresses must be transferred inside and between multiple organizations. Routing protocols were modified to send both addresses and masks. Blocks reserved for private networks. The IETF has defined a set of prefixes that are reserved for private networks. Are never used for addresses on the global Internet. A CIDR block was also defined ( / 12) in addition to Class A and B blocks. Next lecture: Two additional approaches to deal with the IP addressing problem NAT and IPv6. Lecture 10, Page 16 of 16