[Network week4] ARP, IP Headers, Fragmentation, MTU, PMTUD, ICMP

Network Layer Protocol

Protocols at Network Layer

ARP

• Hosts and routers are recognized at the network layer by IP addresses
  : An IP address is universally unique
• Hosts and routers are recognized at the data-link layer by MAC addresses
  : A MAC address is locally unique

	MAC	IP
Full form	Media Access Control	Internet Protocol Address
Purpose	It identifies the physical address of computer on the Internet	It identifies connection of a computer on the internet
Bits	It is 48 bits hexadecimal address.	IPv4 is a 32-bit address, and IPv6 is a 128-bits address
Address	MAC address is assigned by the manufacturer of NIC card	IP address is assigned by the network administrator or Internet Service Provider.

• Internet routers move the packets from the source network to the destination network
  : IP address is used to find a path to destination
• Packets pass through physical networks to reach hosts and routers
  : MAC address is used to move packets between hosts and routers
• A routing table outputs a next-hop IP address based on destination's IP address
  : Should be able to map an IP address to MAC address

Mapping

• Static
  -- Create a table that associate IP address with MAC address
  -- Table is stored in each machine
  -- MAC address may change
  --> Change NIC card
  --> A mobile computer roams from network to network
  -- Static mapping table needs to be update periodically
• Dynamic
  -- A machine knows one of the two addresses
  -- ARP and RARP

ARP Operation

IP(Internet Protocol)

• Host-to host network layer delivery protocol
• Unreliable and connectionless datagram protocol
  -- Best-effort delivery service
  -- No errol control nor flow control
  -- Only error detection (If detected, discard packet)
• Pair with a reliable protocol (TCP) for a reliability

IP Datagram

• Packets in IP layer are called datagram
• Variable-length packet with 20 to 60 header size
  

IP Headers

• Header length (HLEN) defines the header length in 4-byte words
• Total length filed defines the total length of the datagram including header
  -- The length of IP datagram is limited to 65,555(2^16-1)
• Time To Live (TTL) is approximately 2 times the Max. number of routes between two hosts
• Checksum covers only header not data
  -- Rely on higher-level protocols
  -- Header changes en route not data

Maximum Transfer Unit

• Data link layer (DLL) protocol defines its Max. size of frame that can be transmitted
  : Ethernet has 1500 bytes
• IP datagram must be less that Max. size
  : This size defines Maximum Transfer Unit(MTU)
• Originally, IP has 65,536 bytes
  : We must divide datagram to pass through the network = Fragmentation
  : A datagram can be fragmented by SRC and any routers
  : Reassembly is only done because each fragment becomes independent
  

Fragmentation

• Identification (16bits)
  : All fragments have the same identification number
  : Help DST to reassemble
• Flags (3bits)
  : 1st is reserved
  : 2nd is DF(Don't Fragment)
  : 3rd is MF(More Fragment)
• Fragmentation Offset (13bits)
  : Relative position with respect to whold datagram
  : Measured in units of 8 bytes

Fragmentation Example

• Datagram with 400 bytes
• MTU 1420 bytes
  
  

Notes on Fragmentation

• Performance
  : Single fragment lost means entire packet useless
  : Waste of resources all along the way
• Don't Fragment (DF) bit set
  : Datagram discarded if need to fragment
  : ICMP message generated
  -- May spefify MTU
  : Used to determine Path MTU (both in TCP, UDP)
• Transport and application layer headers do not appear in all fragments.
  : Problem if you peep into those headers

Path MTU Discovery (PMTUD)

• The longer the MTU the better for performance, but the worse for reliability
  : Lost packet means more data to be retransmitted
• IP fragmentation has serious disadvantages
  : It's avoided in practice using DF
• What does router do if it can't deliver a packet and can't fragment it either
  : ICMP sent to SRC
  : SRC adjusts the packet size accordingly
  
  

Options

• Headers of IPv4 is made of 2 parts
  : Fixed and variable
  : Variable part comprises options that can be maximum of 40 bytes to preserve boundary of header
• Option is not required
  : Can be used for network testing and debugging
  

Discussion on IP Header

• If fragmentation is going to be avoided all the time, why not have 4- bytes of fragmentation info as an IP option?
• Is 32-bit address going to be enough?
• Why mess with variable length headers?
  : Can the variability in header length be controlled to allow better encoding?
• Are the IP options really that useful? Why variable length option headers?
• Many of these issues addressed in IPv6

ICMP (Internet Control Message Protocol)

• No error-reporting or error-correcting mechanism in IP
  " What if route must discard a datagram
• Lack of mechanism for host and management queries
  : For example, determining if a router is alive

Error control

• Not guarantee successful delivery of every datagram
• May be discarded a number of reasons
  -- TTL expiration
  -- Congestion
  -- FCS error
  : Not possible to return as SRC address may be damaged
• If datagram is discarded by router, router should attempt to return some information to SRC
  : SRC may use this information to modify transmission strategy

Flow control

• Router and/or receiving station limit the rate at which they receive data
  : Not mandatory and so very limited iin use
• Router sends flow control packets to request reducing data flow to other routers and SRC

ICMP Encapsulation

• ICMP is a network layer protocol
  : But not passed directly to the data link layer
  : Encapsulated inside IP datagrams first

Types of Messages

• Error-reporting messages and query messages
• ICMP always reports error messages to the original source
  

Tunneling

Virtual Private Network

IP-in-IP Tunnel

• Encapsulates IP packets in IP to make a tunnel between two routers