Internet Protocol Datagram Format (IPv4 Datagram Format)
- The Internet's network-layer packet is referred to as a datagram.
- IPv4 datagrams have many key fields.
- Time to live sets the max number remaining hops and is decremented at each router.
- Upper layer has the upper layer protocol to deliver payload to.
- 32 bit source IP address and 32 bit destination IP address is also stored.
- Data contains the transport-layer segment to be delivered to the destination.
- Type of Service is the type of data and is used in priority scheduling.
- Overhead is at least 40 bytes; 20 bytes of TCP and 20 bytes of IP.
- Fragmentation is dividing data into multiple chunks.
- Networks have Max Transfer size, so large IP datagram should be divided within the net.
- The fragments are reassembled only at the final destination.
- Starting for IPv6, large datagram is dropped instead of fragmentation because it will cause burden to router.
- The sending host increments the identification number for each datagram it sends.
- The last fragment has a flag bit set to 0, whereas all the other fragments have this falg bit set to 1.
IPv4 Addressing
- The boundary between the host and the physical link is called an interface.
- The boundary between the router and any one of its links is also called an interface.
- A router thus has multiple interfaces, one for each of its link.
- IP requires each host and router interface to have its own IP address.
- Thus, an IP address is technically associated with an interface rather than with the host or router containing that interface.
- However Ethernet switches and WiFi base station does not have IP address because they are layer 2 devices.
- IP address can be divided into 2 parts, subnet part and host part.
- Subnet part is the high order bits while Host part is low order bits.
- Each isolated network is called a subnet.
- Internet's address assignment strategy is known as Classless Interdomain Routing(CIDR).
- CIDR has address format of a.b.c.d/x where x is the number of bits in subnet protion of address.
- Before CIDR was adopted, the network portions of an IP address were constrained to be 8, 16, 24 bits in length, an addressing scheme known as classful addressing.
- In order to obtain a block of IP addresses for use within an organization's subnet, a network administrator might first contact its ISP, which would provide addresses from a larger block of addresses that has already been allocated to the ISP.
- Clearly, there must also be a way for the ISP itself to get a block of addresses.
- IP addresses are managed under the authority of the Internet Corporation for Assigned Names and Numbers.
- Once an organization has obtained a block of addresses, it can assign individual IP addresses to the host and router interfaces in its organization.
- Host addresses can also be configured manually, but typically this is done using Dynamic Host Configuration Protocol(DHCP).
- The goal of DHCP is to allow host to dynamically obtain its IP address from network server when it joins network.
- Because DHCP's ability to automate the network-related aspects of connection a host into a network, it is often referred to as a plug-and-play or zeroconf(zero-configuration) protocol.
- DHCP has a address lease time - the amount of time for which the IP address will be valid.
- The network gets allocated portion of its provider ISP's address space to get subnet part of IP address.
Network Address Translation (NAT)
- All datagrams leaving local network should have the same single source NAT IP address with different source port numbers.
- Datagrams with source or destination in this network have same subnet part of the IP address.
- NAT is a trick to run all the applications which are executed in multiple computers to look like they run in one computer in a global network point of view.
- In other words, NAT change addresses of devices in local network without notifying outside world.
- An NAT router must implement, 1) replace outgoing datagrams, 2) remember NAT translation table, and 3) replace incoming datagrams.
- (Source IP address, port #) should be changed to (NAT IP address, new port #).
IPv6 Datagram Format
- In IPv6, the length of header is fixed to 40 byte, and no fragmentation is allowed.
- Checksum is removed to reduce processing time.
- Options are allowed, but outside of the header indicated by Next Header field.
- Usually IPv4 is used still because the two are not compatible.
- IPv4 routers can not process IPv6 datagrams.
- Such technique to let IPv4 routers to process IPv6 datagrams is called tunneling.
- Tunneling is IPv6 datagram carried as payload in IPv4 datagram among IPv4 routers.
- Invtervening set of IPv4 routers between two IPv6 routers are called tunnel.
Generalized Forward and SDN
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