Chapter-8Network Layer in the Internet

IP Protocol: IP V4

At network layer, internet can be viewed as a collection of sub-networks or Autonomous Systems

Protocol that holds these ASes together is Internet Protocol

IP is the host-to-host network layer delivery protocol for internet

It is unreliable and connectionless datagram protocol

It provides rather best effort service

IP Protocol: IP V4

Best Effort means that IP provide no error-control or flow control

IP uses only error detection mechanism and discards all the corrupted data

IP does its best to deliver a packet to its destination, but doesn’t guarantee

IP depends upon upper layer protocols for the reliable transmission

If reliability is important, IP must be paired with TCP in transport layer

Analogy example of post-office

IP Protocol: IP V4

Uses datagram approach in the packet-switching network

Packets in IP layer are called datagrams A datagram is a variable length packet

consisting of two parts: Header Data

Header is 20-60 bytes long and contains information essential to routing and delivery

Data contains the data sent by the upper layer

IP Protocol: IP V4

The IPv4 (Internet Protocol) header.

IP Protocol: IP V4

VER is the field that contains the IP protocol version. The current version is 4. 5 is an experimental version. 6 is the version for IPv6.

HLEN is the length of the IP header in multiples of 32 bits, without the data field. The minimum value for a correct header is 5 (i.e., 20 bytes), the maximum value is 15 (i.e., 60 bytes).

Service Type: The service type is an indication of the quality of service requested for this IP datagram. It contains the following information.

IP Protocol: IP V4

Precedence specifies the nature/priority: 000: Routine 001: Priority 010: Immediate 011: Flash 100: Flash override 101: Critical 110: Internetwork control 111: Network control

IP Protocol: IP V4

TOS specifies the type of service value: 1000: Minimize delay 0100: Maximize throughput 0010: Maximize reliability 0001: Minimize monetary cost 0000: Normal service The last bit is reserved for future use.

Total Length specifies the total length of the datagram, header and data, in octets.

Identification is a unique number assigned by the sender used with fragmentation.

IP Protocol: IP V4

Flags contains control flags: The first bit is reserved and must be zero The second bit is DF (Do not Fragment) 0 means allow

fragmentation; The third is MF (More Fragments), 0 means that this is

the last fragment Fragment Offset is used to reassemble the full

datagram. The value in this field contains the number of 64-bit

segments (header bytes are not counted) contained in earlier fragments.

If this is the first (or only) fragment, this field contains a value of zero.

IP Protocol: IP V4

TTL (Time to Live) specifies the time (in seconds) the datagram is allowed to travel. In practice, this is used as a hop counter to detect routing loops.

Protocol Number indicates the higher level protocol to which IP should deliver the data in this datagram. E.g., ICMP = 1; TCP = 6; UDP = 17.

IP Protocol: IP V4 Header Checksum is a checksum for the information

contained in the header. If the header checksum does not match the contents, the datagram is discarded.

Source/Destination IP Addresses are the 32-bit source/destination IP addresses.

IP Options is a variable-length field (there may be zero or more options) used for control or debugging and measurement

Padding is used to ensure that the IP header ends on a 32 bit boundary. The padding is zero.

IP Protocol: IP V4 (IP Fragmentation)

IP provides fragmentation/reassembly of datagrams. The maximum length of an IP datagram is 65,535 octets.

When an IP datagram travels from one host to another, it may pass through different physical networks.

Each physical network has a maximum frame size, called maximum transmission unit (MTU), which limits the datagram length.

A fragment is treated as a normal IP datagram while being transported to their destination.

Thus, fragments of a datagram each have a header. If one of the fragments gets lost, the complete datagram is considered

lost. It is possible that fragments of the same IP datagram reach the

destination host via multiple routes. Finally, Since they may pass through networks with a smaller MTU

than the sender’s one, they are subject to further fragmentation.

IP Protocol: IP V4 (IP Fragmentation)

When a datagram is fragmented, each fragment will have their own header with most fields same but some changed

Fragmentation can be done by source host or any router in the path

Reassembly is done only at the destination host Fields in IP header related to fragmentation and

reassembly: Identification Flags Fragmentation Offset

IP Protocol: IP V4 (IP Fragmentation)

When fragmentation is done Identification field is copied to all fragments so that all the fragments can be identified while reassembling at destination

When fragmentation is done DF field of Flag field will be 0 saying, there are fragments which lets the destination know that the datagram is not complete one but rather a fragmented one

Fragmentation field shows the relative position of fragment with respect to whole datagram

Offset of data in original datagram measured in units of 8 bytes

IP Protocol: IP V4 (IP Fragmentation) Suppose a datagram has data

of size 4000 bytes and is fragmented into 3 fragments

The first fragment carries data from 0-1399

The offset would be 0/8=0 Similarly 2nd fragment carries

data from 1400-2799 Offset of 2nd fragment would

be 1400/8=175 Finally 3rd fragment will carry

data from 2800-3999 Offset of 3rd fragment would be

2800/8=350 Offset is carried out by dividing

the position of first byte in fragment by 8

These offsets are used in destination to reassemble in order

IP Protocol: IP V4 (Drawbacks)

IPv4 has a two-level address structure (network part and host part) categorized into 5 classes. The use of address space is inefficient.

Internet must accommodate real-time audio and video transmission requiring minimum delay and reservation of resources which are not provided in IPv4 design

Internet must accommodate encryption and authentication of data for some application. Originally, no security mechanism was provided in IPv4.

IP Protocol: (Rise of IPv6)

Presentation required on this topicWhy IPv6 and what are its significance?Presentation Date: 12th August,09

Other Network Protocols:

ARP (Address Resolution Protocol) & RARP (Reverse ARP)

ICMP (Internet Control Message Protocol)DHCP( Dynamic Host Configuration

Protocol)Presentation on these 3 ProtocolsPresentation Date: 12th August,09

Routing Protocols

Internet is made up of large number of autonomous systems (AS)

Autonomous System is a group of networks and routers under the authority of single administration

Routing inside an autonomous system is called interior routing

Routing between AS is called exterior routing Each AS chooses an interior routing protocol to handle

routing inside the AS (Eg: RIP, OSPF) Only one exterior routing protocol is usually chosen to

handle the routing between Autonomous Systems (Eg: BGP)

Interior Gateway Routing Protocol (OSPF)

Stands for Open Shortest Path First Replaced RIP as Interior Gateway Routing

Protocol Standard in1990 Many router vendors support OSPF Why OSPF over others?

Open Standard Support variety of metrics Dynamic Support Routing based on Type of Service Support Load balancing Support hierarchical system Security

OSPF (Open Shortest Path First)

Special Routers called Autonomous System Boundary Routers are responsible for dissipating information about other autonomous systems into current system

For efficient routing, OSPF divides an AS to areas An area is a collection of networks, hosts and

routers all contained within an AS AS may be divided into many areas Routers inside area flood the area with routing

information At border of each area, Area Border Routers are

used to summarize about the area and send it to other areas

OSPF (Open Shortest Path First)

AS has a special area called backbone area, with which all other areas should be connected

Routers inside backbone area are called backbone routers

Each area in AS has an identification and area identification of backbone area is 0.

OSPF (Open Shortest Path First)

Metrics used in OSPF can be based on type of service Minimum delay Maximum throughput Bandwidth

OSPF (Open Shortest Path First)

Packet Types in OSPF: Hello DBD (Database Description) LSR (Link State Request) LSU (Link State Update) LSAck (Link State Acknowledgements)

OSPF (Open Shortest Path First) Link Types:

Point-to-Point Link Transient Link Stub Link Virtual Link

OSPF (Open Shortest Path First)

Link State Advertisements Router Link Network Link Summary Link to Network Summary Link to AS Boundary Network External Link

Administrative Distance: Administrative distance (AD) is the trustworthiness (or preference) of the route source.

OSPF: 110

OSPF (Open Shortest Path First)

BGP (Border Gateway Protocol)

Protocol used between two Autonomous Systems

Different from interior routing protocol because the goals of both are different

Interior Routing Protocols are just concerned with delivering the information from source to destination efficiently

Interior Routing Protocols don’t bother about the politics or policies implemented

Exterior routing protocols will have to consider politics and polices while delivering the packets not just the costs

BGP (Border Gateway Protocol)

Sometimes secured path is preferred over a shortest path

Some corporate AS might want to send their packet using foreign AS or competitors AS

Some AS will forward packets from only those AS which have paid to make that AS their transit point

These are the policies and politics which only Exterior Routing Protocol mainly BGP is concerned with

All these policies are configured manually on a BGP Routers in the network

BGP (Border Gateway Protocol)

BGP is based on a routing method called Path Vector Routing

Path Vector Routing constitutes of table in which each entry contains the destination network , the next router and the path to reach the destination

Network Next Router Path

N01 R01 AS14, AS23, AS67

N02 R05 AS22, AS67, AS05, AS89

N03 R06 AS67, AS89, AS09, AS34

N04 R12 AS62, AS02, AS09

BGP (Border Gateway Protocol)

One AS boundary router advertises the reachability of the networks within their AS to the neighboring AS boundary routers

Each router that receives a path vector message verifies the advertised path is in agreement with its policy

If it is, it will update its table and modifies the message before sending it to next neighbor

Modification involves adding of AS number to the path and replacing the next router entry with its own identification

BGP (Border Gateway Protocol)

BGP (Border Gateway Protocol)

Loops are prevented in this exterior routing by checking the AS number in the path

If own AS is there in path, then it identifies the loop and ignores the packet

Types of Packets in BGP: Open Message (to create neighborhood relationship) Update Message (to withdraw old advertisements and assign

new routes) Keep-Alive Message (exchanged between BGP routers to tell

each other that they are alive) Notification message (sent by router whenever an error

condition is detected or router wants to close the destination)