Layered Communication

Layers work together

Encapsulation and Deencapsulation

Indirect Communication• Application programs on different

machines cannot communicate directly– They are on different machines!

BrowserBrowser

TransTrans

IntInt

DLDL

PhyPhy

User PC

Web AppWeb App

TransTrans

IntInt

DLDL

PhyPhy

Webserver

HTTP RequestHTTP Request

Layer Cooperation on the Source Host

• Application layer process passes HTTP-request to transport layer process

ApplicationApplication

TransportTransport

InternetInternet

Data LinkData Link

HTTP RequestHTTP Request

PhysicalUser PC

Layer Cooperation on the Source Host

• Transport layer makes TCP segments– HTTP message is the data field– Adds TCP header fields shown earlier– Transport process “encapsulates” HTTP

request within a TCP segment

HTTP RequestHTTP Request TCP-HTCP-H

TCP Segment

DataField

TCPHeader

Encapsulation

• Encapsulation is delivering a message in the data field of another message– TCP encapsulates HTTP request messages

– Can also encapsulate other types of messages

HTTP RequestHTTP Request TCP-HTCP-H

TCP Segment

DataField

TCPHeader

Layer Cooperation on the Source Host

• Transport layer process passes the TCP segment down to the internet layer process

ApplicationApplication

TransportTransport

InternetInternet

Data LinkData Link

TCP segmentTCP segment

PhysicalUser PC

Layer Cooperation on the Source Host

• Internet Layer Process Encapsulates TCP Segment within an IP packet– An IP packet to deliver a TCP segment has a

TCP segment in its data field

TCP segmentTCP segment IP-HIP-H

Data IP Packet

DataField

IPHeader

Layer Cooperation on the Source Host

• The internet layer process passes the IP packet to the data link layer process– Internet layer messages are called packets

ApplicationApplication

TransportTransport

InternetInternet

Data LinkData Link

IP packetIP packet

PhysicalUser PC

Layer Cooperation on the Source Host

• Data Link Layer Encapsulates IP Packet Within a PPP Frame– Data link layer messages are called frames– Data PPP frame has IP packet in data field

PPP Frame Encapsulating an IP Packet

PPP-TPPP-T IP packetIP packet PPP-HPPP-H

Layer Cooperation on the Source Host• The data link layer process passes the PPP frame

to the physical layer process, which delivers it to the physical layer process on the first router, one bit at a time (no message at the physical layer)

ApplicationApplication

TransportTransport

InternetInternet

Data LinkData Link

Physical (10110 …)User PC

PPP framePPP frameTo firstrouter

PPP-TPPP-T

Layer Cooperation on the Source Host

• Recap: Adding Headers and Trailers:

ApplicationApplication

TransportTransport

InternetInternet

Data LinkData Link

HTTP msgHTTP msg

PhysicalUser PC

HTTP msgHTTP msg TCP-HTCP-H

HTTP msgHTTP msg TCP-HTCP-H IP-HIP-H

HTTP msgHTTP msg TCP-HTCP-H IP-HIP-H PPP-HPPP-H

Layer Cooperation on the Source Host

• Encapsulation in Layering– Whenever a process at Layer N (the application,

transport, internet, or data link layer) creates a message,

– That Layer N process passes the message down to the next-lower-layer process, the process at layer N-1

– The N-1 process encapsulates the Layer N message by placing it in the data field of a Layer N-1 message and adding headers and perhaps trailers to create the full Layer N-1 Message

Layer Cooperation on the Source Host

• Small but important detail on naming

• Layer 3 (internet) messages are called packets– IP message is a packet

• Layer 2 (data link) messages are called frames– PPP message is called a frame

Layer Cooperation: Destination Host• Destination host reverses processes on the

sending host– Delivers HTTP message to the webserver

application programApplicationApplication

TransportTransport

InternetInternet

Data LinkData Link

PhysicalUser PC Webserver

Layer Cooperation: Destination Host• Successively pass up layer messages

ApplicationApplication

TransportTransport

InternetInternet

Data LinkData Link

IP-PacketIP-Packet

DL-Frame (protocol unknown)containing IP packet in data field

DL-Frame (protocol unknown)containing IP packet in data field

PhysicalFinal Router Webserver

Data link layer program processes the data link frame’s header and trailer, deencapsulates the IP packet, and passes the IP packet to the next higher layer, the internet layer

Layer Cooperation: Destination Host• Successively pass up layer messages

– Other layers pass successive data fields (containing next-layer messages) up to the next higher layer

ApplicationApplication

TransportTransport

InternetInternet

Data LinkData Link

HTTP msgHTTP msg

TCP segmentTCP segment

IP-PacketIP-Packet

DL-Frame (protocol unknown)DL-Frame (protocol unknown)

PhysicalFinal Router Webserver

Layer Cooperation: Destination Host• Successively pass up layer messages

– Other layers process headers & trailers, pass up message in data field

ApplicationApplication

TransportTransport

InternetInternet

Data LinkData Link

PhysicalFinal Router Webserver

PPP-TPPP-T

HTTP msgHTTP msg

HTTP segHTTP seg TCP-HTCP-H

HTTP msgHTTP msg TCP-HTCP-H IP-HIP-H

HTTP msgHTTP msg TCP-HTCP-H IP-HIP-H PPP-HPPP-H

IP Packet

TCPsegment

HTTP msg

Indirect Communication

• Two processes on different machines cannot communicate directly with one another because they are physically separated

• So they communicate indirectly, using cooperation with lower layers

The First Router• First router receives an IP packet

(encapsulated in a frame) in one port (interface)

• Must make a router forwarding decision: select the port to use to send it back out

B?

D?

C?

Router A

B

C

DPacket

Layer Cooperation on the First Router• So far, we have only looked at hosts

– But deencapsulation and encapsulation also occur on EACH router

• Frame arrives at a port on the first router– Port’s data link layer process receives the

PPP frame containing an IP packet

Data LinkData Link Data LinkData Link

InternetInternet

PPP FramePPP Frame

First Router

Layer Cooperation on the First Router• Incoming Data Link Process on the Router

– Deencapsulates the IP packet from the PPP frame

– Passes the IP packet to the router’ internet layer process

Data LinkData Link Data LinkData Link

InternetInternetIP PacketIP Packet

First Router

Incoming Port on First Router

Layer Cooperation on the First Router• Routers only have physical, data link,

and internet layer processes– So internet layer process is the highest-layer

process on a router for router forwarding– Internet layer process decides where to send

the packet next: another router or the destination host

Data LinkData Link Data LinkData Link

InternetInternet

First Router

Layer Cooperation on the First Router• Internet layer process passes IP packet to

data link layer process on the selected output port that will carry the IP packet to the next router or the destination host

Data LinkData Link Data LinkData Link

InternetInternet

First Router

IP Packet

Selected Output Port on First Router

Layer Cooperation on the First Router

• The data link and physical layer process on the selected port sends the frame encapsulating the IP packet onto the next router (or destination host)

InternetInternet

Data LinkData Link

InternetInternet

Data LinkData LinkFrame

Selected Output PortOn First Router

Input PortOn Next Router

(Or Destination Host)

PhysicalLayer

Layer Cooperation on the First Router

• Notes• For router forwarding, routers only use

physical, data link, and internet processes• Routers First Receive Frames

– Receiving interface deencapsulates the IP packet, passes the packet to the internet layer process

• Routers Then Send Frames Out– On a different output interface (port)

– This requires encapsulating of the IP packet in a data link layer frame