Port numbers at the Transport layer
Written on 04:09 by Ahmet Emir
The Transport band takes the abstracts stream, makes segments out of it, and establishes a reliable
session by creating a basic circuit. It again sequences (numbers) anniversary articulation and uses
acknowledgments and breeze control. If you’re appliance TCP, the basic ambit is authentic by the
source anchorage number. Remember, the host aloof makes this up starting at anchorage cardinal 1024
(0 through 1023 are aloof for acclaimed anchorage numbers). The destination anchorage number
defines the upper-layer action (application) that the abstracts beck is handed to aback the data
stream is anxiously rebuilt on the accepting host.
Now that you accept anchorage numbers and how they are acclimated at the Transport layer, let’s
go aback to Figure 1.30. Once the Transport band attack advice is added to the allotment of
data, it becomes a articulation and is handed bottomward to the Network band forth with the destination
IP address. (The destination IP abode was handed bottomward from the high layers to the
Transport band with the abstracts stream, and it was apparent through a name resolution
method at the high layers—probably DNS.)
The Network band adds a header, and adds the analytic acclamation (IP addresses), to the
front of anniversary segment. Once the attack is added to the segment, the PDU is alleged a packet.
The packet has a agreement acreage that describes area the articulation came from (either UDP or
TCP) so it can duke the articulation to the actual agreement at the Transport band aback it reaches
the accepting host.
Source Anchorage Destination
Port . . .
1028 23
Host A Host Z
Defines high layer
process or application
. . .
SP DP
Defines Basic Circuit
The Arrangement band is amenable for award the destination accouterments abode that dictates
where the packet should be beatific on the bounded network. It does this by application the Abode Resolution
Protocol (ARP)—something I’ll allocution about added in Chapter 2. IP at the Arrangement layer
looks at the destination IP abode and compares that abode to its own antecedent IP abode and
subnet mask. If it turns out to be a bounded arrangement request, the accouterments abode of the local
host is requested via an ARP request. If the packet is destined for a alien host, IP will look
for the IP abode of the absence aperture (router) instead.
The packet, forth with the destination accouterments abode of either the bounded host or default
gateway, is again handed bottomward to the Abstracts Link layer. The Abstracts Link band will add a header
to the advanced of the packet and the allotment of abstracts again becomes a frame. (We alarm it a frame
because both a attack and a bivouac are added to the packet, which makes the abstracts resemble
bookends or a frame, if you will.) This is apparent in Figure 1.29. The anatomy uses an Ether-Type
field to call which agreement the packet came from at the Arrangement layer. Now a cyclic
redundancy analysis (CRC) is run on the frame, and the acknowledgment to the CRC is placed in the
Frame Analysis Sequence acreage begin in the bivouac of the frame.
The anatomy is now accessible to be handed down, one bit at a time, to the Physical layer, which will
use bit timing rules to encode the abstracts in a agenda signal. Every accessory on the arrangement segment
will accord with the alarm and abstract the 1s and 0s from the agenda arresting and body a
frame. After the anatomy is rebuilt, a CRC is run to accomplish abiding the anatomy is okay. If aggregate turns
out to be all good, the hosts will analysis the destination abode to see if the anatomy is for them.
If all this is authoritative your eyes cantankerous and your academician freeze, don’t freak. I’ll be activity over
exactly how abstracts is encapsulated and baffled through an internetwork in Chapter 6.
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