IP Function and IP Function and ImplementationImplementation

Chuck Davin

UPenn CSE 350

12 April 2001

OSI Reference ModelOSI Reference Model

Application Layer

Presentation Layer

Session Layer

Transport Layer

Network Layer

Data Link Layer

Physical Layer

Unix Network ImplementationUnix Network Implementation

Application Application

Sockets

Protocols

Devices

System Call Interface

Abstract Network Interface

Model vs. ImplementationModel vs. Implementation

Application Application

Sockets

Protocols

Devices

System Call Interface

Abstract Network Interface

Application Layer

Presentation Layer

Session Layer

Transport Layer

Network Layer

Data Link Layer

Physical Layer

OSI Reference ModelOSI Reference Model

Possible Points of MultiplexingPossible Points of MultiplexingApplication Layer

Presentation Layer

Session Layer

Transport Layer

Network Layer

Data Link Layer

Physical Layer

Unix Network ImplementationUnix Network Implementation

Possible Points of MultiplexingPossible Points of Multiplexing

Application Application

Sockets

Protocols

Devices

System Call Interface

Abstract Network Interface

Protocol Layer FunctionsProtocol Layer Functions

Protocol-Specific Functions– UDP– TCP

– IP

Protocol Layer Service Protocol Layer Service InterfaceInterface

XXX_usrreq (so, cmd, …)XXX_input (data, …)XXX_ctlinput (so, cmd, …)XXX_ctloutput (so, cmd, …)XXX_init ()

Protocol User RequestsProtocol User Requests

PRU_ATTACH PRU_DETACH PRU_SEND PRU_RECV PRU_BIND PRU_CONNECT PRU_SHUTDOWN And so forth

Generic Protocol Data Generic Protocol Data StructuresStructures

Application Application

Sockets

Devices

System Call Interface

Abstract Network Interface

ProtocolControlBlock(PCB)

To Protocol-Specific State(if any)

Protocol-Specific Links

IP ObservationsIP Observations

Raw IP sockets are available– But less often used

The most frequent “users” of IP services are UDP and TCP

The most interesting data structures for IP are unrelated to sockets

IP Data FlowsIP Data FlowsApplication Application

TCP

Sockets

Devices

System Call Interface

IP

UDP

Ip_output() XXX_input()

Ifp->if_output() Ip_input()

IP Packets

IP Packets

IP Input ProcessingIP Input Processing

Remove packet from delivery queueValidate IP header checksum, versionCheck packet length for consistencyIs it for me?

– See if_withaddr()Reassemble if packet is a fragmentDeliver entire packets to ULP based on

protocol number

IP Output ProcessingIP Output Processing

Complete IP header, compute IP header checksum

Determine outgoing interface and next hop for this packet

Fragment if packet exceeds MTU of chosen interface

Present packet(s) plus next hop IP address to device via abstract network interface

IP Routing TableIP Routing Table

Destination IP AddressDestination NetmaskRoute Type (e.g., local or remote)Route MetricNext Hop IPCached MTU, Network Device, …

– See if_withaddr()

IP Forwarding ProcedureIP Forwarding Procedure

Find routing entries for which the masked destination address matches the masked destination address of the packet

Of these entries, pick the one with the longest netmask– Contiguous subnet masks

Forwarding table vs. routing table

Network Device FunctionsNetwork Device Functions

Device congestion managementProtocol adaptationBuffer management

Network Device InterfaceNetwork Device Interface

Generic interface presented to ULPs (IP)IF_ENQUEUE()IF_QFULL()Ifp->If_output()Ifp->If_start()…

Network Device Data Network Device Data StructuresStructures

Application Application

Sockets

Protocols

System Call Interface

Abstract Network Interface

IfnetChain

…

IfnetStructure

IfnetStructure

PrivateDeviceData

PrivateDeviceData

Network Device Output Network Device Output ProcedureProcedure

ULP has already enqueued packets and next hop addresses on the output queue

If the medium is multipoint, then resolve the passed ULP address into a local medium address (e.g., ARP)

If the device is currently idle, take a packet from the output queue, frame it and start I/O

Network Device Input Network Device Input ProcedureProcedure

Received frame is checked for integrity and length consistency

Frame is examined to identify ULPFrame is stripped of data link headerFrame Payload plus notation of receiving

device are enqueued on ULP input queue– E.g., ipintrq

Questions to PonderQuestions to Ponder

Private device data is stored contiguously with generic ifnet data rather than according to the linked PCB scheme in upper layers.– What are the tradeoffs?

To what degree should/need a network device do its own routing?

What must a network device do to properly receive striped IP traffic?