Network Layer: Data Plane Flashcards
IP Addressing
IPv4
32-bit identifier associated with each host/router interface
IP Addressing
Interface
- Connection between host/router and physical link
- Each interface has its own IP address
- Routers have multiple interfaces (and therefore multiple IP addresses)
- Host has one or two interfaces
IP Addressing
Subnet
Device interfaces that can physically reach each other wo passing through an intervening router
IP Addressing
IP Address Structure
Subnet Part: devices in same subnet have common high order bits
Host Part: remaining low order bits
IP Addressing
Subnet Mask
Divides the subnet part of the IP address from the host part
IP Addressing
CIDR
- Classless InterDomain Routing
- Subnet portion of address has arbitrary length
- Address Format: a.b.d/x, where x is the # of bits in the subnet portion
IP Addressing
How does a host get an IP address?
- hard-coded by sysadmin in config file
- DHCP gets address from server
IP Addressing
DHCP
- Dynamic Host Configuration Protocol
- Dynamically obtain IP address from network server when it joins the network
- can renew lease on address in use
- allows reuse of addresses (only hold address while connected)
- support
DHCP
DHCP Steps
- host broadcasts DHCP discover message (optional)
- DHCP server responds with DHCP offer message (optional)
- host requests IP address with DHCP request message
- DHCP server sends address with DCHP ACK message
DHCP
DHCP Broadcast
- Optional
- Sent by client
- Asks if there are any DHCP servers
DHCP
DHCP Offer
- Optional
- Sent by DHCP server
- Tells client that its a DHCP server and sends an avaliable IP address
DHCP
DHCP Request
- Required
- Sent by client
- Asks DHCP server to use offered IP address
DHCP
DHCP ACK
- Required
- Sent by DHCP server
- Sends confirmation to client that it has the IP address it requested
DHCP
What can DHCP server return?
- Allocated IP address
- Address of first-hop (closest) router for client
- Name and IP address of DNS server
- Network mask/subnet mask (indicating network vs host portion of address)
Network Address Translation
NAT
- Network Address Translation
- All devices in local network share just one IPv4 address as far as the outside world is concerned
- All datagrams leaving local network have same souce NAT IP address
- All datagrams inside local network have IP address containing subnet mask
Network Address Translation
Private IP Address Examples
- 10.
- 172.
- 192.168
Network Address Translation
Private IP Address Advantages
- Only one IP address needed from ISP for all devices
- Can change addresses of host in local network wo notifying outside world
- Can change ISP without changing addresses of devices in local network
- Security: devices inside local network not directly addressable/visible by outside world
Network Address Translation
Private IP Address
IP address that’s assigned to a device by a network router for use within a private network.
Network Address Translation
NAT Implementation
- Outgoing datagrams: replace (src IP, port#) of every outgoing datagram to (NAT IP, new port #)
- Remember (in NAT table) every (src IP, port #) to (NAT IP, new port #) translation pair
- Incoming datagrams: replace (NAT IP, new port#) in dest fields of every incoming datagrams with corresponding (src IP, port #) in NAT table
IPv6
IPv6 Motivation
- 32-bit IPv4 address space has been completely allocated
- Faster processing/fowarding (40 byte fixed length header)
- Enable different network-layer treatment of flows
IPv6
IPv6 Datagram Format
- priority: identify priority amount datagrams in flow
- 2 128-bit addresses, dest and src
- flow label: identify datagrams in same flow
- no checksum, fragmentation/reassembly, or options
IPv6
Tunneling
- Transition from IPv4 to IPv6
- IPv6 datagram carried as payload in IPv4 datagram among IPv4 routers (packet in a packet)
- usually, datagram is payload in link-layer frame but here in IPv4 datagram
- source/dest changes to start/end of IPv4 router path
Generalized Forwarding
Forwarding Table
- a.k.a flow table
- Contained in router
- Used to determine routes to reach destinations
Generalized Forwarding
Match Plus Action
Abstaction matching bits in arriving packet to action
Generalized Forwarding
Destination-Based Forwarding
Forward based on dest IP address
Generalized Forwarding
Generalized Forwarding
- Many different header fields can determine action
- Many actions possible
Generalized Forwarding
Possible Actions
Actions resulting from header fields:
* Drop
* Copy
* Modify
* Log packet
Generalized Forwarding
Flow
- What actions correspond to what header fields?
- Defined by header field values
Generalized Forwarding
Generalized Forwarding Rules
Match: pattern values in packet header fields
Actions: for matched packet, choose action
Counters: # of bytes and # of packets
Generalized Forwarding
OpenFlow
- match+action: abstraction unifies between different kinds of devices
- Routers, Switches, Firewall, and NAT
Generalized Forwarding
OpenFlow: Routers
match: longest dest IP prefix
action: forward out a link
OpenFlow: Switch
match: dest MAC address
action: forward or flood
Generalized Forwarding
OpenFlow: Firewall
match: IP addresses and TCP/UDP port #s
action: permit or deny
Generalized Forwarding
OpenFlow: NAT
match: IP address and port
action: rewrite address and port
