Introduction to Computer Networks Yan Yunyi, Ph.D Associate Prof. Xidian University yyyan@xidian.edu.cn
Chapter 5 The Network Layer
Network Layer Design Isues Store-and-Forward Packet Switching Services Provided to the Transport Layer Implementation of Connectionless Service Implementation of Connection-Oriented Service Comparison of Virtual-Circuit and Datagram Subnets
Store-and-Forward Packet Switching fig 5-1 The environment of the network layer protocols.
Services Provided to the Transport Layer Goals: 1) The services should be independent of the router technology. 2) The transport layer should be shielded from the number, type, and topology of the routers present. 3) The network addresses made available to the transport layer should use a uniform numbering plan, even across LANs and WANs. Service Kinds 1) Connectionless Service--Internet community the hosts should accept the fact that the network is unreliable ----Internet 2) Connection-Oriented Service--telephone companies subnet should provide a reliable, connection-oriented service----ATM
Implementation of Connectionless Service Routing within a diagram subnet.
Implementation of Connection-Oriented Service ① ② ② ② ① ① ① ① To: C Label:1 VC 1:H1H2 From: H1 Label:1 VC 2:H3H2 Label Switching Routing within a virtual-circuit subnet.
Comparison of Virtual-Circuit and Datagram Subnets 5-4
Routing Algorithms Properties Desirable for Routing Algorithms The Optimality Principle Shortest Path Routing Distance Vector Routing
Properties Desirable for Routing Algorithms correctness, simplicity, robustness, stability, fairness, and optimality. Robustness Once a network starts, it is expected to work continuously without system-wide failure. All kinds of hardware and software failures cannot affect the network. Stability A stable algorithm reaches equilibrium and stays there. Fairness and optimality
Properties Desirable for Routing Algorithms Conflict between fairness and optimality.
Routing Algorithms Two major classes: nonadaptive and adaptive. Nonadaptive algorithms: do not base their routing decisions on measurements or estimates of the current traffic and topology. Instead, the choice of the route to use is computed in advance, off-line, and downloaded to the routers when the network is booted. This procedure is sometimes called static routing. Adaptive algorithms: change their routing decisions to reflect changes in the topology, and usually the traffic as well.
The Optimality Principle Optimality Principle: if router J is on the optimal path from router I to router K, then the optimal path from J to K also falls along the same route. without regard to network topology or traffic (a) A subnet. (b) A sink tree for router B.
The geographic distance in kilometers Shortest Path Routing How to measure path length? The geographic distance in kilometers The number of hops mean queue length transmission delay
Shortest Path Routing The first 5 steps used in computing the shortest path from A to D. The arrows indicate the working node.
Distance Vector Routing (a) A subnet. (b) Input from A, I, H, K, and the new routing table for J.
Distance Vector Routing (2) NOYES A: YESNO The count-to-infinity problem.
Congestion Control Algorithms General Principles of Congestion Control Congestion Prevention Policies Congestion Control in Virtual-Circuit Subnets Congestion Control in Datagram Subnets Load Shedding Jitter Control
Congestion When too much traffic is offered, congestion sets in and performance degrades sharply.
General Principles of Congestion Control Congestion Control: Open loop and close loop. Close loop: Monitor the system . detect when and where congestion occurs. Pass information to where action can be taken. Adjust system operation to correct the problem.
General Principles of Congestion Control Monitor what: the percentage of all packets discarded, the average queue lengths, the number of packets retransmitted, the average packet delay, the standard deviation of packet delay In all cases, rising numbers indicate growing congestion.
General Principles of Congestion Control How to transfer the information about the congestion: routers in congestion send a packet to the traffic source or sources, announcing the problem. b) routers fill in a bit or field reserved in every packet whenever congestion. c) hosts or routers periodically send probe packets out to explicitly ask about congestion.
Congestion Prevention Policies 5-26 Policies that affect congestion.
Congestion Control in Virtual-Circuit Subnets (a) A congested subnet. (b) A redrawn subnet, eliminates congestion and a virtual circuit from A to B.
Hop-by-Hop Choke Packets Congestion Control in Datagram Subnets Hop-by-Hop Choke Packets (a) A choke packet that affects only the source. (b) A choke packet that affects each hop it passes through.
Load Shedding wine milk old is better than new new is better than old Drop the packets: wine old is better than new FTP milk new is better than old Multimedia
Jitter Control (a) High jitter. (b) Low jitter. For applications such as audio and video streaming, it does not matter much how long delayed as long as the transit time is constant. But higher jitter is unacceptable. (a) High jitter. (b) Low jitter.
Quality of Service Requirements Techniques for Achieving Good Quality of Service Integrated Services Differentiated Services Label Switching and MPLS
How stringent the quality-of-service requirements are. 5-30 How stringent the quality-of-service requirements are.
Flows in Four categories ATM networks classify flows in four broad categories with respect to their QoS demands as follows: 1) Constant bit rate (e.g., telephony). 2) Real-time variable bit rate (e.g., compressed videoconferencing). 3) Non-real-time variable bit rate (e.g., watching a movie over the Internet). 4) Available bit rate (e.g., file transfer).
Techniques for Achieving Good Quality of Service Overprovisioning Buffering Traffic Shaping
Smoothing the output stream by buffering packets.
The Leaky Bucket Algorithm (a) A leaky bucket with water. (b) a leaky bucket with packets.
The Token Bucket Algorithm 5-34 (a) Before. (b) After.
The Leaky Bucket Algorithm Token Bucket (a) Input to a leaky bucket. (b) Output from a leaky bucket. Output from a token bucket with capacities of (c) 250 KB, (d) 500 KB, (e) 750 KB, (f) Output from a 500KB token bucket feeding a 10-MB/sec leaky bucket. Token Bucket Token Bucket Token + Leaky Bucket
Internetworking How Networks Differ How Networks Can Be Connected Concatenated Virtual Circuits Connectionless Internetworking Tunneling Internetwork Routing Fragmentation
Connecting Networks A collection of interconnected networks. SNA (IBM:System Network Architecture) FDDI (Fiber Distributed Data Interface) A collection of interconnected networks.
Some of the many ways networks can differ. How Networks Differ 5-43 Some of the many ways networks can differ.
How Networks Can Be Connected Data Link Layer, MAC Address, Not need Network protocol Network Layer, IP Address, Network protocol (a) Two Ethernets connected by a switch. (b) Two Ethernets connected by routers.
Concatenated Virtual Circuits Internetworking using concatenated virtual circuits.
Connectionless Internetworking A connectionless internet.
Tunneling a packet from Paris to London.
Tunneling a car from France to England.
(a) Transparent fragmentation. (b) Nontransparent fragmentation.
Fragmentation (2) Fragmentation when the elementary data size is 1 byte. (a) Original packet, containing 10 data bytes. (b) Fragments after passing through a network with maximum packet size of 8 payload bytes plus header. (c) Fragments after passing through a size 5 gateway.
The Network Layer in the Internet The IP Protocol IP Addresses Internet Control Protocols OSPF – The Interior Gateway Routing Protocol BGP – The Exterior Gateway Routing Protocol Internet Multicasting Mobile IP IPv6
Design Principles for Internet Make sure it works. Keep it simple. Make clear choices. Exploit modularity. Expect heterogeneity. Avoid static options and parameters. Look for a good design; it need not be perfect. Be strict when sending and tolerant when receiving. Think about scalability. Consider performance and cost. Simple design to handle various hardware and facilities.
Collection of Subnetworks The Internet is an interconnected collection of many networks.
The IP Protocol The IPv4 (Internet Protocol) header. Header length in 32 bits Do not fragment More Fragments Header +Data 4 Reliability or speed Fragment belongs to which datagram Fragment lies in where TCP or UDP Recomputed at each hop. why??? to limit lifetime (255s max) The IPv4 (Internet Protocol) header.
The IP Protocol (2) 5-54 Some of the IP options.
IP Addresses IP address formats.
IP Addresses (2) Special IP addresses.
A campus network consisting of LANs for various departments. Subnets A campus network consisting of LANs for various departments.
A class B network subnetted into 64 subnets. Subnet mask: 11111111 11111111 11111100 00000000 (255.255.252.0) For example: First in Subnet0: 10000010 00110010 00000000 00000001 (130.50.0.1) First in Subnet1: 10000010 00110010 00000100 00000001 (130.50.4.1) First in Subnet2: 10000010 00110010 00001000 00000001 (130.50.8.1) First in Subnet3: 10000010 00110010 00001100 00000001 (130.50.12.1) …… A class B network subnetted into 64 subnets.
NAT – Network Address Translation to assign each company a single IP address for Internet traffic. Within the company, every computer gets a unique IP address for routing intramural traffic. When a packet exits the company, a network address translation (NAT) takes place. Three private IP arranges:- Can not be used in Internet 10.0.0.0 – 10.255.255.255/8 (16,777,216 hosts) 172.16.0.0 – 172.31.255.255/12 (1,048,576 hosts) 192.168.0.0 – 192.168.255.255/16 (65,536 hosts)
NAT – Network Address Translation True IP address Private IP Placement and operation of a NAT box.
NAT (2) IP:port 192.168.0.1:80 2001 192.168.0.1:21 2002 192.168.0.2:21 2003 192.168.0.3:8008 2004 …… Index process IP:port For example: Remote host 214.3.2.15:5236 NAT http send：192.168.0.1:80 210.27.2.15:2001 NAT http receive： Remote host 210.27.2.15:2001 192.168.0.1:80
Internet Control Message Protocol 5-61 The principal ICMP message types.
ARP– The Address Resolution Protocol DLL hardware does not understand IP address, IP must be mapped onto certain DLL address (MAC address)---ARP Three interconnected /24 networks: two Ethernets and an FDDI ring.
Dynamic Host Configuration Protocol Operation of DHCP.
OSPF – The Interior Gateway Routing Protocol OSPF: Open Shortest Path First (a) An autonomous system. (b) A graph representation of (a).
OSPF (2) The relation between ASes, backbones, and areas in OSPF. on the backbone wholly within one area connect two or more areas talk to routers in other ASes The relation between ASes, backbones, and areas in OSPF.
OSPF (3) OSPF distinguishes four classes of routers: 1)Internal routers are wholly within one area. 2)Area border routers connect two or more areas. 3)Backbone routers are on the backbone. 4)AS boundary routers talk to routers in other ASes. These classes are allowed to overlap
The five types of OSPF messeges. 5-66 The five types of OSPF messeges.
BGP – The Exterior Gateway Routing Protocol BGP: Border Gateway Protocol (a) A set of BGP routers. (b) Information sent to F.
The IPv6 fixed header (required). The Main IPv6 Header The IPv6 fixed header (required).
The IPv6 fixed header (required). The Main IPv6 Header The IPv6 fixed header (required).
IP Address (v6) 8000:0000:0000:0000:0123:4567:89AB:CDEF ::192.168.0.1
Extension Headers 5-69 IPv6 extension headers.
The hop-by-hop extension header for large datagrams (jumbograms). Extension Headers (2) The hop-by-hop extension header for large datagrams (jumbograms).
Summary Datagram subnet and Virtual Circuit subnet Shortest path routing and Distance Vector routing Congestion Control, buffer, leaky bucket, token bucket Quality of Service (QoS), reliability, delay, jitter, bandwidth IPv4, IP address, IP subnet NAT ARP DHCP