Internetworking and the Internet Protocol

Network Layer [1]

• Provides the upper layers with independence from data transmission and physical networking technologies
• Responsible for sending data from source to destination (this includes the nodes in between and is therefore not end-to-end)
• Responsible for requesting network facilities such as priority, bit-rate, etc…
• Responsible for routing

Routing [1]

• Autonomous System (AS) – set of networks and routers operated by a single organization
• Interior Router Protocol (IRP) – passing routing information within an autonomous system
• Exterior Router Protocol (ERP) – passing routing information between different autonomous systems

Routing Approaches [1]

• Distance Vector Routing – each node exchanges information with its neighboring nodes (ex: Routing Information Protocol, RIP)
• Link-state Routing – sends link costs of each of its network interfaces to all routers (not just neighboring). Typically used with a Dijkstra-based algorithm (ex: Open Shortest Path First, OSPF)
• Path-vector Routing – router provides information about which networks can be reached by a given router and the autonomous systems that must be crossed (ex: Border Gateway Protocol, BGP)

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Internet Architecture

In a TCP/IP internet, IP Routers provide interconnection among physical networks.[1]

Internet Protocol (IP)

• The most-widely used internetworking protocol

Internetworking Requirements [1] – accommodate the differences among the networks which form the intenetwork.

1. different addressing schemes
2. different maximum packet size
3. different network access mechanism
4. different timeouts
5. different transmission modes (connection oriented, connectionless)
6. error control
7. flow control

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1. Different Addressing Scheme [1]

• Introduce IP Address as a global address
• All hosts on the internet must have a unique IP address
  • NOTE: Techniques such as IP-forwarding allow private IP addresses that might be duplicated somewhere else.

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Subnet and Subnet Masks [1]

• Allow arbitrary complexity of internetworked LANs within an organization
• Insulate overall internet from growth of network numbers and routing complexity
• To rest of internet, site looks like single network
• Each LAN is assigned a subnet number
• Host portion of address is partitioned further into subnet number and host number
• Local routers route within subnetted network
• Subnet mask indicates which bits are subnet number and which are host number by doing a bitwise AND

2. Different Maximum Size [1]

Different networks have different Maximum Transmission Unit (MTU) sizes. What if a packet reaches a network and it exceeds the networks MTU? This can be solved with the following solution:

• Use fragmentation to split large packets into smaller ones
• Use reassembly at the destination only to put the fragments together and build the original packet

Fragmentation and Reassembly [1]

When to re-assemble

• At destination – results in packets getting smaller as data traverses internet
• Intermediate re-assembly
  • Need large buffers at routers
  • Buffers may fill with fragments
  • All fragments must go through same router
    • Inhibits dynamic routing

IP Fragmentation [1]

• IP re-assembles at destination only
• Uses fields in header
  • Data Unit Identifier (ID)
    • Identifies end system originated diagram
      • Source and destination address
      • Protocol layer generating data (ex: TCP)
      • Identification supplied by that layer
  • Data Length
    • Length of user data in octets
  • Offset
    • Position of fragment of user data in original datagram
    • In multiples of 64 bits (8 octets)
  • More flag
    • Indicates that this is not the last fragment

Dealing with Failure [1]

• Re-assembly may fail if some fragments get lost
• Re-assembly time out
  • Assigned to first fragment to arrive
  • If timeout expires before all fragments arrive, discard partial data

3. Different Network Access [1]

• Solved by abstracting networking functions in the networking layer, and relying on Data Link Layer for networking access
• Routers handle the difference in network access mechanism.

Address Mapping [1]

• Sometimes, in order to reach a destination, there is no need to go through an IP router.
• In such a case, the physical address can be used directly

ARP – Address Resolution Protocol

Reverse ARP

4. Different Timeout [1]

• Problem: different networks use different timeout mechanisms.
• Solution: Introduce the concept of Datagram Lifetime.
• Datagrams could loop indefinitely
  • Consumes resources
  • Transport protocol may need upper bound on datagram life
• Datagram marked with lifetime
  • Time to Live (TTL) field in IP
  • Once lifetime expires, datagram discarded (not forwarded)
  • Hop count
    • Decrement time to live on passing through each router

5. Different Transmission Modes [1]

• Use a connectionless architecture
• Advantage:
  • Flexibility
  • Robust
  • No unnecessary overhead
• Disadvantages
  • Not guaranteed delivery
  • Not guaranteed order of delivery
  • Reliability is responsibility of upper layers (ex: TCP)

6. Error Control [1]

• Minimal error control, done only for the header
• Router should attempt to inform source if packet discarded (using ICMP)
• Not guaranteed delivery
• May inform higher layer protocol

7. Flow Control [1]

•  Allows routers and/or stations to limit rate of incoming data
• Limited in connectionless systems
• Send flow control packets
  • Requesting reduced flow
• ex: ICMP

—[1] Prof. Shervin Shirmohammadi, University of Ottawa CEG4183 Course Notes, Lecture 2