CS335 Networking & Network Administration Tuesday, May 11, 2010

ARP ARP – Address resolution protocol Translates IP address into a hardware address Physical network hardware does not know how to locate a computer from its protocol address Known as address resolution

ARP Can only resolve hardware addresses for machines on the local physical network

Address Resolution Three techniques of address resolution Table lookup – stored in a table in memory

Table lookup For less than a dozen hosts sequential search suffices In larger networks this requires excessive CPU cycles Hashing – general purpose data structure Hashing Direct indexing

Table lookup Direct indexing – uses the host address as an index into the array

Address resolution Closed-form computation Used when the network interface can be assigned specific hardware addresses Computed by a single Boolean and operation Hardware_address = ip_address & 0xff When a computer connects to a network that uses this, resolution is trivial

Address resolution Message exchange Computers exchange messages across network to resolve an address 3 types of address resolution Table lookup Closed form computation Dynamic message exchange

Address resolution

ARP ARP standard defines 2 basic message types Request – contains an IP address and requests the hardware address Response – has both the IP address and the hardware address

ARP message delivery

ARP message format Although the ARP message format is sufficiently general to allow arbitrary protocol and hardware addresses, ARP is always used to bind a 32 bit IP address to a 48 bit Ethernet address ARP is encapsulated directly in a hardware frame

Identifying ARP frames The type field in the frame header specifies that the frame contains an ARP message

ARP caching ARP software extracts and saves the information Uses small table of bindings in memory Checks cache first before broadcasting an ARP request Improves the efficiency of network traffic

Higher levels use protocol addressing

IP TCP/IP includes both connectionless and connection-oriented services Routers can connect heterogeneous networks so they cannot transmit a copy of a frame that arrives on one network across another IP is a hardware independent packet format

IP datagram Size of a datagram is determined by the application that sends the data Similar to format of a frame Uses IP addresses in header Can contain as little as a single octet of data or at most 64K octets

Forwarding IP datagrams Next hop – either the destination or the next router

IP addresses and routing tables Routing

Destination and Next-Hop addresses The destination address in a datagram header always refers to the ultimate destination When a router forwards the datagram to another router the address of the next hop does not appear in the datagram header.

Best effort delivery IP uses best-effort to describe the service Doesn’t guarantee that it will handle: Datagram duplication Delayed or out-of-order delivery Corruption of data Datagram loss Additional layers of protocol software handle these errors

IP Datagram Header Each field has a fixed size

Encapsulation Network hardware doesn’t understand datagram format or IP addressing Network understands its own frame format and heterogeneous networks may have different formats IP datagram is encapsulated in a frame

Encapsulation Frame type field uses the value reserved for IP Receiver knows the data area contains IP datagram Uses a frame address for next hop obtained by ARP

Transmission across an internet When a datagram arrives in a network frame the receiver extracts the datagram from the frame data and discards the frame header. Frame headers don’t accumulate on the trip.

MTU MTU – Maximum transmission unit Each hardware technology has a limit to the amount of data in a frame Datagram must be smaller than the MTU or it can’t be encapsulated for transmission

Fragmentation In a internet with heterogeneous networks, MTU restrictions can be a problem Routers fragment or divide a datagram into smaller pieces to meet the MTU

Fragmentation Each fragment uses the IP datagram format but carries only part of the data Flags field of the header indicates whether it is fragment or a complete datagram

Reassembly Process of creating a copy of the original datagram from fragments Fragment with the final data has an additional bit set in header so receiver knows all fragments have arrived Ultimate destination host reassembles fragments so the routers

Identifying a datagram IP doesn’t guarantee delivery Fragments can be lost or arrive out of order Sender places a unique identification number in the identification field of outgoing datagram When a router fragments, the identification number and source IP address determines to which datagram a fragment belongs Fragment offset field tells a receiver how to order fragments

Fragment loss Fragments can be delayed or lost IP specifies a maximum time to hold fragments if they are delayed When a fragment arrives receiver starts a timer, if all arrive before timer runs out, datagram is reassembled, otherwise they are discarded No way for receiver to tell sender what fragments didn’t arrive Sender doesn’t know about fragmentation Resent packets may take a different path with different fragmentation Fragments can be fragmented in case of an even smaller MTU

Future IP Let’s go to the lab and research IP v6!