通信网基本概念与主体结构答案

通信网基本概念与主体结构答案每一个抄通信网的学生都是头疼的天使----Laofuzi Communication Networks 2nd Edition Chapter 10 Solutions Solutions to Chapter 10 10.1. Can all traffic in an IntServ router be of guaranteed service type Explain. Solution: All the traffic can be of guaranteed service only if all the flows are shaped and if they strictly comply with the traffic specifications. However in practice the total amount of traffic that can be admitted will have some limit. The reason is that to bound the delay the maximum queue length in the router must be below a certain threshold. If all of the flows are well behaved and without bursts the queue size will always be reasonably small. However if the flows are bursty the queue may grow to levels that can cause large unacceptable delays. To guarantee the delay bounds extra bandwidth should be assigned based to accommodate the burstiness of the flows. This overbooking can sometimes lead to the queue becoming empty and hence result in underutilization of the output link. Thus it is reasonable to fill the output bandwidth during such periods with best effort traffic. So in summary to efficiently utilize the bandwidth and achieve higher throughputs it is necessary to admit best effort traffic alongside guaranteed service traffic. 10.2. Develop a packet-scheduling approach to provide guaranteed service controlled load service and best effort service in a router. Can any mix of these three traffic types be supported Explain. Solution: Separate queues must be provided for the packets from each type of service in order for their packets to be serviced appropriately. The set of queues belonging to guaranteed service should always receive service with a rate R which should be larger than the total guaranteed service traffic. The aggregate rate R is guaranteed and controlled by giving priority to the guaranteed service queues and controlling the service they receive by a leaky bucket shaper with a sustained rate of R. Individual queues within the guaranteed service group are serviced in a round-robin fashion while for each queue its service is controlled by a dedicated leaky bucket shaper with the parameters set based on the requirements of the flow assigned to that queue. The set of controlled load service queues are serviced during the periods that guaranteed service queues are not serviced. The capacity in excess of R is available to controlled load traffic. Flows are assigned to different queues and weighted fair queueing can be used within the group to service the queues depending on their requirements. Best effort traffic is given the lowest priority. The queue is serviced only if the other two sets of queues are empty. 10.3. RSVP signaling is very different from ATM signaling. Discuss the differences and list the advantages and disadvantages of each protocol. Solution: RSVP uses Receiver-Initiated Reservation rather than ATM’s per-hop path request to reserve network resources. RSVP is intended for use with multicasting and is receiver oriented. ATM is designed for unicast traffic and is sender oriented. RSVP receiver-initiated reservation Advantage: interoperates with non-RSVP routers soft state provides robustness with respect to faults requests can be merged reducing the reservation load on the routers. Disadvantage: soft state – requires periodic updates receiver needs to know the path for connection. Leon-Garcia/Widjaja 1每一个抄通信网的学生都是头疼的天使----Laofuzi Communication Networks 2nd Edition Chapter 10 Solutions ATM per-hop path request Advantage: hard state –reserve only once. Disadvantage: Individual VCC is required for each host in multicast application. Source requests the connections so it needs to know the requirements of the receiver. 10.4. Consider the merging of reservations in RSVP. Suppose that there is a single sender and 2n receivers that are reached through a path that consists of a depth n binary tree that has a router located at each node. What is the total number of reservation request messages generated by the receivers Solution: The total number of reservation request messages generated by thereceivers and routers are equivalent to finding the branches of a binary tree with a depth of n. Let Nn be the number of branches in a binary tree with depth n. At level n there 2n new branches and so we have the recursion Nn Nn–1 2n. Draw a picture of a tree to verify this. It is easy to then verify that Nn total number of request messages is 2n1 – 1. 10.5. Suppose RSVP is used to set up a virtual private network to interconnect n users across an Internet. Discuss the appropriateness of the three RSVP reservation styles for this application. RSVP has three reservation styles Wildcard Filter Fixed Filter and Shared Explicit. Wild card filter WF creates a single reservation shared by all senders. It is designed for applications that are unlikely to have multiple senders transmitting simultaneously. It is not appropriate for VPN applications because WF cannot provide the receiver requests if more than one sender is transmitting at the same time. Fixed filter FF creates a distinct reservation for each sender. It is an appropriate solution for VPN requirements. Each sender VPN has its own resource reservation and this resource is shared by number of receivers hosts located in different location belong to the same VPN. Shared explicit SE creates a single reservation shared by a set of explicit senders. It may also be appropriate in VPN applications under certain conditions. The merging of the reservation between senders can result in a more efficient use of network resources. 10.6. Select and justify a choice an RSVP reservation style that is appropriate for the following situations: Solutions follow questions: a A multiuser application where a video stream from each participant must be delivered to every other participant. This situation is a multipoint to multipoint application so the Fixed Filter FF is the appropriate choice. FF creates a distinct reservation for each participant sender and it can merge downstream requests and forward the sum of flow spec upstream. It is an appropriate reservation style because each participant sender will only receive a single reservation request that has been merged by a multiple of other participants. This simplifies the processing overhead for each sender and allows the processing power to focus on accepting multiple video streams for other participants. b An audioconferencing application that is self-limiting in that more than two participants are unlikely to speak simultaneously. This audio conferencing application is best suited with the Wild card Filter WF style. The WF-style is suitable for applications that are unlikely to have multiple senders transmitting simultaneously. In this application the self-limiting feature prevents two participants from speaking simultaneously. Leon-Garcia/Widjaja 2每一个抄通信网的学生都是头疼的天使----Laofuzi Communication Networks 2nd Edition Chapter 10 Solutions c A multiuser application in which each participant must receive explicit permission before transmitting information. The Shared Explicit SE reservation style is best suited for this multiuser application. The SE style creates a single reservation shared by a set of explicit senders. In this application only the participant that receives an explicit permission is allowed to join a single reservation. Any participant that does not receive permission is not allowed to use this single reserved resource.10.7. What information needs to be carried by Resv messages in RSVP to set up guaranteed IP service with a specific bandwidth and a given delay-bound requirement How does a router process this information Solution: The information that must be carried by Resv message in RSVP to set up guaranteed IP service with a specific bandwidth and a given delay bound is: the common header SESSION RSVP_HOP TIME_V ALUES STYLE and Fixed Filter flow descriptor along with flow spec and filter spec. When a router receives this Resv message it reserves the necessary resources based on the flow specification which in turn governs the bandwidth and delay-bound requirements. Next the router exams the filter spec to determine the data packets that correspond to the defined flow spec. 10.8. Discuss the interplay between a QoS routing algorithm RSVP andintegrated services IP. Solution: Integrated services IP provides QoS packet delivery between the source and destination pair. The QoS parameters are dependent on the source requirements for example bandwidth requirement and delay-bound requirement. To provide integrated services it is necessary for the routing protocol network layer to provide QoS routing. QoS routing provides for the selection of paths with specific attributes that can support specified QoS requirements. RSVP provides the means for reserving resources along paths identified by QoS routing. 10.9. Explain how the soft-state feature of RSVP allows it to adapt to failures in the network. Solution: The RSVP soft-state approach requires each node to refresh its reservation periodically by using Path and Resv messages. When a state is not refreshed within a certain time-out period the state is deleted. Upon a link failure within the network a PathErr message is sent to the ingress node source node. The ingress node simply defines a new route by refreshing the Path message to a different next hop. The old failure path will eventually tear down after the timer is expired. This approach provides error recovery upon failure however it is necessary for the ingress node to keep updating the status of the link-state topology and determine the next possible route in case of the primary route fails. 10.10. Discuss whether RSVP provides a scalable means to set up connections to a session that consists of the broadcasting of a given TV program. Solution: Broadcasting a TV program is a point to multipoint multicast application. RSVP does provide a scalable means to set up connections to a session in multicast application. RSVP performs the resource reservation for multicast applications adapting dynamically to changing group membership and changing route. Also RSVP requires the receiver to initiate and maintain the resource reservation rather than obtaining the bandwidth requirement from each receiver. This process avoids an implosion problem and minimizes the processing overhead to the sender. Finally the RSVP routers provide different reservation styles and the requests can merge in several ways according to Leon-Garcia/Widjaja 3每一个抄通信网的学生都是头疼的天使----Laofuzi Communication Networks 2nd Edition Chapter 10 Solutions the reservation style. This prevents duplicate resource reservations if multiple downstream receivers belong to the same multicast group. 10.11. Explain what happens if RSVP is used to set up small-bandwidth flows on a large-bandwidth link How well does RSVP scale in this situation Solution: If RSVP is used to set up small-bandwidth flows on a large-bandwidth link the router will have to update the available resource bandwidth every time there are small-bandwidth flow request for resources. Moreover the router needs to keep track of the available resources and reject resource reservation requests when the available bandwidth cannot support a particular requirement. The router has to keep track of every single flow resource usage during the setup and tear down procedure. All of these activities consume a lot of processing power when the link has large bandwidth being shared by large numbers of small-bandwidth flows. 10.12. Obtain a packet capture file for RSVP either in the lab or downloaded from a web site. Examine the details of Path Resv and refresh messages. Solution: The following text shows the details of the first PATH message for MPLS path setup shown in Figure 2.27. The file is obtained by printing the capture data from Ethereal. We highlight in color the main fields and interesting information. In particular note the explicit route identified in the message and the LABEL REQUEST. The TIME V ALUES field is set to 30000 ms that is 30 second refresh period. Frame 1 306 bytes on wire 306 bytes captured Arrival Time: Feb 10 2000 08:49:03.806994000 Time delta from previous packet: 0.000000000 seconds Time relative to first packet: 0.000000000 seconds Frame Number: 1 Packet Length: 306 bytes Capture Length: 306 bytes Ethernet II Src: 00:90:92:9d:94:01 Dst: 00:d0:63:c3:b8:47 Internet Protocol Src Addr:17.3.3.3 17.3.3.3 Dst Addr: 16.2.2.2 16.2.2.2 Resource ReserVation Protocol RSVP: PATH Message. SESSION: IPv4-LSP Destination 16.2.2.2 Tunnel ID 1 Ext ID 11030303. SENDER TEMPLATE: IPv4-LSP Tunnel Source: 17.3.3.3 LSP ID: 1. RSVP Header. PA TH Message. RSVP Version: 1 Flags: 00 Message Type: PATH Message. 1 Message Checksum: 0xdb58 correct Sending TTL: 254 Message length: 264 SESSION: IPv4-LSP Destination 16.2.2.2 Tunnel ID 1 Ext ID 11030303. Length: 16 Class number: 1 - SESSION object C-type: 7 - IPv4 LSP Destination address: 16.2.2.2 16.2.2.2 Tunnel ID: 1 Extended Tunnel ID: 285410051 17.3.3.3 HOP: IPv4 210.0.0.1 Length: 12 Class number: 3 - HOP object C-type: 1 - IPv4 Neighbor address: 210.0.0.1 Leon-Garcia/Widjaja 4每一个抄通信网的学生都是头疼的天使----Laofuzi Communication Networks 2nd Edition Chapter 10 Solutions Logical interface: 0 TIME V ALUES: 30000 ms Length: 8 Class number: 5 - TIME V ALUES object .。