Notes - The Network Layer in the Internet

The following are notes from Tanenbaum's Computer Networks 5th Edition.

• Principles of Network Layer Design
• Make sure it works
• do not finalize until approved
• Keep it simple
• Make clear choices
• choose options rather than allowing for several ways to do the same thing
• Exploit modularity
• independent layers
• Expect heterogeneity
• Avoid static options and parameters
• Look for a good design over a perfect one
• Be strict when sending and tolerant when receiving
• Thin about scalability
• Consider performance and cost
• ASes(Autonomous systems)
• connection of internetworks
• Tier 1 networks are ISPs
• following shows connections

• IP(Internet Protocol)
• glues together internet with similarities

The IP Version 4 Protocol

• Version field keeps track of which version of the protocol datagram belongs too
• IHL is provided to tell how long the header is
• Differentiated services field changed its meaning over the years
• originally type of service field
• Total length includes the entire datagram max length 65,535 bytes
• Identification field shows the host where the packet should go, where ti belongs to
• MF is more fragments all fragments except the last one have this bit sets, lets us know when all the fragments of a datagram has arrived
• Fragment offset tells where in the current packet the fragment belongs
• TtL is used to limit packet lifetimes
• counts number of hops
• protocol tells the transport process
• header checksum crc assumed to be 0 on arrival
• source/destination address is IP address
• options field is to allow subsequent version of the protocol to include information
• security tells how secret info is, military use
• strict source routing option gives complete path
• losse source routing gives packet ability to traverse the list of routers specified
• network measurement
• record route tells each router to append its ip address to options field
• timestamp option records the 32 bit timestamp

IP Addresses

• Prefix ip addresses are hierarchal, unlike ethernet
• dotted decimal notation
• subnet mask, prefixes described by their length

• Subnets
• ICANN(Internet Corporation for Assigned Names and Numbers)
• network numbers manager
• splitting prefix example

• CIDR - Classless InterDomain Routing
• routers must be able to determine path from each source to each destination
• tables growing larger and larger
• however, can group into subnets, route aggregation
• CIDR (Classless Inter- Domain Routing)
• router routes between same prefix routers

• when packet comes in routing table is scanned to see if destination lies within prefix
• possible multiple entries will match longest prefix used
• commercial routers use custom VLSI chips with algorithms embedded into hardware
• Classful and Special Addressing

• A allows for up to 128 networks with 16 million hosts
• B allows for up to 16,384 networks with 2 million hosts
• C allows for up to 2 million networks with 256 million hosts
• 3 bears problem, since B is the only network that makes sense for scalability for a company or organization
• Class D networks are being for multicast
• NAT - Network Address Translation
• IP adress are scarce
• dynamically assign IP address to a computer when it is on
• beginning to apply to home users subscribing to ADSL or internet over cable
• problem of running out of IP addresses, solution is to move to IPv6 but this is slowly occurring
• in the meantime we use NAT(Network Address Translation)
• assign each home or business with a single ip address
• within the customer network each computer gets a unique ip address
• these addresses are unavailable

• Operation of NAT
• before packets leave a customer it passes through a NAT box that converts the internal IP to the customer's true IP address

• how does provider send back to customer?
• observed that most IP packets carry TCP or UDP payloads
• contain source destination port
• ports indicate where the TCP connection begins and ends, so we can establish port to port processes rather than ip to ip processes
• ports 0-1023 are reserved, but 1024-65,535 are available for use, so each IP gets this many ports utilized to solve the mapping issue
• issues about using NAT
• breaks end to end connectivity as an outgoing packet is necessary before incoming packets can be allowed
• home user with NAT can make TCP/IP connections to remote web server, but remote user cannot make connections to game server on home network without NAT traversal techniques
• changes from connectionless to connection oriented in an odd way
• violates protocol layering, assumes TCP operational, if TCP is upgraded to a new style, NAT will fail
• processes are not required to use TCP or UDP, if new transport protocol used, NAT cannot handle
• FTP inserts IP addresses into the body of packet, but it cannot rewrite the IP addresses here

IP Version 6

• Goals
• support billions of hosts
• reduce size of routing table
• simplify protocol
• provide better security
• pay more attention to type of service
• aid multicasting by allowing scopes to be specified
• make it possible for hosts to roam
• allow protocol evolution
• permit old and new protocol to coexist until old can be phased out
• SIPP(Simple Internet Protocol Plus) became IPv6
• The Main IPv6 header

• differentiated services distinguishes class of packets
• flow label provides way for source and destination to mark groups of packets
• payload length tells how many bytes follow header
• next header can be optional extension headers, tells transport packets such as the handler where it should pass to next
• Hop limit is Time to live, same idea
• source address destination address with 16 byte addresses
• IPv4 are designated by double collons
• address space so large it won't be used efficiently but its unlikely to run out as there are 1000 IP addresses per square meter of the earth
• Extension Headers

• IPv6 allows for jumbo payload lengths that are used to transmit gigabyte sized packets across internet
• Controversies
• address length compromise
• hop limit field limited to 255 current technology
• remove the IPv4 checksum
• reasoning is that application would have transport layer checksum anyway so no point in doing this over again
• Deployment is the difficulty with IPv6 slow conversion in small islands of IPv6

Internet Control Protocols

• ICMP(Internet Control message protocol) message types are listed as follows

• Time exceeded error message used to create traceroute utility that finds the routers along the paths to a destination, send sequence of packets with counters to reach 0 each time along the packet
• ARP - The Address Resolution Protocol
• Data link layer NICs such as ethernet cards don't understand Internet Addresses
• host 1 outputs a broadcast packet on ethernet to see who owns an IP address
• host 2 responds with its ethernet address
• defined in RFC
• gratuitous ARP
• have every machine broadcast its mapping when configured, make update or entry in everyone's ARP, if error occurs resolved by network manager
• default gateway
• router where off network traffic is sent
• possible to send packet from host to host using proxies, so unable to give direct replies, but can set a router to forward to another network
• DHCP - The Dynamic Host Configuration Protocol
• when computer is started built in Ethernet or link layer address in NIC but no IP
• broadcasts request for IP DHCP DISCOVER packet, which much reach DHCP server
• then it tells the server where it is located
• IP address assignment may be only for a fixed period of time, leasing

Label Switching and MPLS

• MPLS (MultiProtocol Label Switching)
• connection oriented network
• adds a label in front of each packet and forwarding is based on label instead of destination address
• tag switching

• Label field holds the index
• QoS indicates class of service
• S refers to stacking multiple layers
• TtL is number of hops packet can live
• MPLS enhanced packet arrives at LSR(Label Switched Router)
• label is used as an index to determine outgoing line to use
• forwarding
• finds best match for a destination address
• switching
• uses a label taken from the packet as an index into a forwarding table
• simpler and faster but not universal definition
• most hosts don't understand MPLS
• LER(Label Edge Router)
• inspects destination IP address and other fields to see what path the MPLS packet should take
• Flows that belong to the same label are called FEC(Forwarding Equivalence Class)
• traditionally not possible to group several distinct forwarding paths
• can operate in multiple levels at once
• label forwarding tables set up by
• combining routing and connection setup protocols
• when router is booted checks to see which routes for final destination
• creates FECs for each label and pass them to their neighbors

OSPF - An Interior Gateway Routing Protocol

• OSPF (Open Shortest Path First)
• became standard in 1990
• draws on IS-IS(Intermediate System to Intermediate System)
• became ISO standard
• goals
• support variety of distance metrics
• dynamic algorithm
• based on type of service
• load balancing capable
• support for hierarchical systems
• security
• supports point to point links and and broadcast networks
• abstracts collection of networks routers and links into a directed graph in which each arc is assigned a weight in terms of distance/delay
• structure routes to hosts but not through them, only through networks such as LANS
• use a link state method to compute shortest path
• ECMP(Equal Cost MultiPath)
• remembers set of shortest path and splits traffic across them equally
• AS divide itself into numbered areas where an area is a network or set of continuous networks
• routers that lie in a router are called internal routers
• each AS has a backbone Area with backbone routers
• all areas are connected to the backbone
• each router is connected to two or more areas is called an area border router and is also part of the background
• if there is only one router out of an area it is called a stub area
• shortest path algorithms are used in an intra area sense and inter area sense
• find shortest path from area to area
• find shortest path from any host/node within an area to another within the area or from border router to any node in area
• exchange info between adjacent routers, but designates one router as the designated router to be adjacent to all other routers to exchange information so information not duplicated with one backup
• Message types listed in the following diagram

• BGP - The Exterior Gateway Routing Protocol
• BGP(Border Gateway Protocol)
• between ASes
• example policies gateway protocols can handle
• Do not carry commercial traffic on educational network
• don't send information from pentagon through iraq
• use TeliaSonera over Verizon
• don't use AT&T in Australia
• Apple traffic doesn't transit through Google
• ISP pay other ISPs to deliver packets, buy transit service
• interconnections called IXPs(Internet Exchange Points)
• ASes can do peering where they send each others traffic for free
• Multihoming
• company networks connected to multiple ISPs as backup
• Path Vector Protocol
• instead of maintaining routes, maintain path to next hop router and sequence of AS that route followed
• BGP communicates using TCP connections
• iBGP, eBGP internal or external BGP protocol
• internal every router at boundary learns all the routes seen by other boundary routers
• strategies
• routes via peered networks chosen in preference
• routes via shortest path chosen
• routes with lowest cost
• early exit or hot potato routing
• tends to make routes asymmetric

Internet Multicasting

• processes that send from one to a large number of viewers
• IP supports this due to range of IP addresses

• IGMP(Internet Group Management Protocol)
• every about once a minute hosts on LAN reports to a multicast router to identify which group they belong to
• PIM(Protocol Indpendent Multicast
• build spanning tree for multicast
• dense mode pruned reverse path forwarding used
• sparse mode similar to core based trees

Mobile IP

• goals
• each mobile host must be able to use home IP anywhere
• software changes to fixed host not permitted
• not permitted to change router software and tables
• packets should not make detours
• should not have overhead when mobile host is at home
• setup a home agent when roaming
• obtains a new IP address when in a foreign site
• packet for mobile arrives to home, it will tunnel to new site so mobile can send and reply using its home address instead of where it is at currently
• options to solve ip address issue
• create route to specific prefix at moving site, but too many sites possible
• change IP address of mobile, handles mobility at a higher level but breaks some applications whenever mobile moves
• mobility can be solved at link layer
• same as 802.11 wireless network, but degree of mobility is very limited
• IPv4 solution given in RFC 3344
• mobiles uses DHCP to get care of IP address at foreign location, finds an agent to get address from using ICMP
• tunneling achieved using proxy ARP to intercept packets
• Ingress filtering
• a security measure to discard addresses that are incorrect, but mobile will have foreign IPs so mobiles have to use care of address to tunnel packets to home location
• flying router on airplanes is different kind of mobility where the entire network is mobile

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