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Network Layer 4-
4.1 introduction
4.2 virtual circuit and
datagram networks
4.3 what’s inside a router
4.4 IP: Internet Protocol
datagram format IPv4 addressing ICMP IPv
4.5 routing algorithms
link state distance vector hierarchical routing
4.6 routing in the Internet
RIP
OSPF
BGP
4.7 broadcast and multicast
routing
Chapter 4: outline
Network Layer 4-
Router
two key router functions:
run routing algorithms/protocol
(RIP, OSPF, BGP)
forwarding datagrams from
incoming to outgoing link
4-
Router architecture overview
Main components:
Input ports/Interfaces
Switching fabric
Output ports/Interfaces
Routing processor: (1)executing routing protocol,
(2)maintaining routing information, forwarding tables, etc.
Network Layer Network Layer 4- line termination link layer protocol (receive) lookup, forwarding queueing
Input port functions
Network layer – decentralized switching :
Packet forwarding = decide which output
line to forward each packet based on
packet header.
queuing: if datagrams arrive faster than
forwarding rate into switch fabric
physical layer: bit-level reception data link layer: decapsulation, error checking, etc switch fabric Line card Network Layer 4-
Switching fabrics
Switching fabric function – transfer packets
between input and output line cards
Types of switching fabric
Via memory: datagram is received through input port,
stored in memory, then send to output port – slow.
Via a bus: datagram is sent directly from input to
output via a shared bus – does not scale well
Via a crossbar: interconnection network consisting of
2N busses that interconnect N input and N output
Memory slow memory Bus
10 Gbps Crossbar 320 Gbps (^) Network Layer 4-
Output port
buffering required when datagrams arrive from
fabric faster than the transmission rate
Buffer management decide when and which
packets to drop if there is not enough memory to
store all income packets
scheduling discipline decide which packet, of those
queued to send out next
line termination link layer protocol (send) switch fabric datagram buffer queueing physical layer: bit-level forwarding data link layer: encapsulation, address mapping, etc
Network Layer 4-
4.1 introduction
4.2 virtual circuit and
datagram networks
4.3 what’s inside a router
4.4 IP: Internet Protocol
datagram format IPv4 addressing ICMP IPv
4.5 routing algorithms
link state distance vector hierarchical routing
4.6 routing in the Internet
RIP
OSPF
BGP
4.7 broadcast and multicast
routing
Chapter 4: outline
Network Layer 4-
Host-to-host network-layer delivery protocol for
the Internet with following properties
Connectionless service – each packet is handled
independently
Best-effort delivery service
- Does its best to deliver packet to its destination, but with no guarantees
- Limited error control – only error detection, corrupted packets are discarded
- No flow control
Must be paired with a reliable transport – (TCP) and/or
application-layer protocol to ensure reliability
Internet Protocol (IP)
Network Layer 4-
IP Versions
IPv4, IPv6, Mobile IP
IPv4 – version currently in wide use (formalized in
IPv6 – new version created to correct some of
significant problems of IPv4 such as exhaustion of
address space (formalized in 1996)
Mobile IP – enhanced version of IPv4 which
supports IP in mobile environments (formalized in
Network Layer 4-
IP datagram format
Datagram – IP packet = variable length packet
consisting of header and data
Header – 20 to 60 bytes in length, contains
information essential to routing and delivery
Data – length determined by Maximum
Transmission Unit (MTU) of link layer protocol
(theoretically between 20 to 65536 bytes)
Network Layer 4-
IP datagram format
Data! Network Layer 4-
IP Datagram Fields
Version number – 4-bit field, specifies IP protocol
version of the datagram (IPv4 or IPv6)
Different versions of IP use different datagram
formats
By looking at version number router can
determine how to interpret remainder of
datagram
Header length – 4-bit field, defines total length of
datagram header in 4-byte words
When there are no options header length is 20
HLEN = 5
Service type – 8-bit field, allows different types of
datagram to be distinguished from each other based
on their associated/requested QoS.
Network Layer 4-
IP fragmentation, reassembly (cont.)
ID
=x offset = fragflag = length = ID =x offset = fragflag = length = ID =x offset = fragflag = length = ID =x offset = fragflag = length = one large datagram becomes several smaller datagrams
example:
4000 byte datagram MTU = 1500 bytes 1480 bytes in data field offset = 1480/ offset = 2960/
Fragmentation offset – 13-bit field, shows relative
position of fragment data with respect to whole
datagram
The offset is measured in units of 8 bytes
Network Layer 4-
4.1 introduction
4.2 virtual circuit and
datagram networks
4.3 what’s inside a router
4.4 IP: Internet Protocol
datagram format IPv4 addressing ICMP IPv
4.5 routing algorithms
link state distance vector hierarchical routing
4.6 routing in the Internet
RIP
OSPF
BGP
4.7 broadcast and multicast
routing
Chapter 4: outline
Network Layer 4-
IP addressing
IP address: uniquely and universally identifies each device
connect to the network
IP Address: 3-bit (4-byte) binary address that identifies a host/router interface to the Internet Two devices on the Internet can never have the same address at the same time; But, a single device can have two IP addresses if it is connected to the Internet via two networks Routers typically have multiple interfaces, e.g. multiple IP addresses 223.1.1. 223.1.1. 223.1.1. 223.1.1.4 223.1.2. 223.1.2. 223.1.2. 223.1.3.1 223.1.3. 223.1.3. Network Layer 4-
IP addressing (cont.)
IP address: Binaay Notation 32-bit/4-byte representation with a
space inserted between each octet (byte). There are about 4
billions possible IP addresses.
IP address: Decimal Notation: 4-number decimal representation
with a decimal dot separating the numbers
Each decimal number, [0,255], corresponding to a byte Network Layer 4-
Classful and Classless IP addressing
Originally, IP addressing used the concept of
classes. This architecture is called classful
addressing.
In the mid 1990s, a new architecture – classless
addressing, was introduced.
Classless Addressing known as CIDR “Classless
InterDomain Routing” addressing – removes class
privileges to compensate for address depletion
CIDR is used for Internet address assignment
Network Layer 4-
Classful IP addressing
Supports addressing of different size networks by
dividing address space into 5 classes: A, B, C, D, E
• An IP address in classes A, B, and C is divided
into Netid and Hostid
Network Layer 4-
Classful IP addressing (cont.)
Recognizing classes
Binary Notation – first few bits of an IP address in binary
notation immediately identify the class of the given address
Decimal Notation – each class has a specific range of numbers in
decimal notation – it is enough to look at the first number to
determine the class
Network Layer 4-
Classful IP addressing (cont.)
Disadvantages of classful network addressing
Lack of a class to support medium-sized organizations
- Class C which supports 254 hosts – too small
- Class B which supports 65534 hosts – too large
A premature depletion of class B addresses has already occurred
- In the early days of the Internet, addresses were freely assigned to those who asked for them without concerns about the eventual depletion of the IP address space
Two existing mechanisms for overcoming the limitations
of classful addressing:
Subnetting - if an organization gets assigned a “big” block of IP
addresses how to distribute them among multiple LAN
Supernetting – how an organization can combine several class C
blocks to create a larger range of address
Network Layer 4-
Subnets
Network divided into several smaller subnetworks each
having its own subnetwork address
Internally, each subnetwork is recognized by its subnetwork
address; to the rest of the Internet all subnetoworks still appear
as a single network
Organization of address space in a subnetted network
A number of HostID bits are borrowed for subnet identification
With m borrowed bits, 2 m^ subnets can be created
Number of hosts in each subnet: 2Hostid- m
Network Layer 4-
Classless addressing: CIDR
CIDR: Classless InterDomain Routing
subnet portion of address of arbitrary length
address format: a.b.c.d/x, where x is # bits in
subnet portion of address
subnet part host part
