1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
|
---
title: "The problem with the current layered model of the Internet"
author: "Dimitri Staessens"
date: 2020-04-06
weight: 1
description: >
---
```
The conventional view serves to protect us from the painful job of
thinking.
-- John Kenneth Galbraith
```
Every engineering class that deals with networks explains the
[7-layer OSI model](https://www.bmc.com/blogs/osi-model-7-layers/)
and the
[5-layer TCP model](https://subscription.packtpub.com/book/cloud_and_networking/9781789349863/1/ch01lvl1sec13/tcp-ip-layer-model).
Both models have common origins in the International Networking
Working Group (INWG), and therefore share many similarities. The
TCP/IP model evolved from the implementation of the early ARPANET in
the '70's and '80's. The Open Systems Interconnect (OSI) model was the
result of a standardization effort in the International Standards
Organization (ISO), which ran well into the nineties. The OSI model
had a number of useful abstractions: services, interfaces and
protocols, where the TCP/IP model was more tightly coupled to the
Internet Protocol (IP).
### A birds-eye view of the OSI model
{{<figure width="40%" src="/docs/concepts/aschenbrenner.png">}}
Open Systems Interconnect (OSI) defines 7 layers, each providing an
abstraction for a certain *function*, or _service_ that a networked
application may need. The figure above shows probably
[the first draft](https://tnc15.wordpress.com/2015/06/17/locked-in-tour-europe/)
of the OSI model.
From top to bottom, the layers provide (roughly) the following
services.
The __application layer__ implements the details of the application
protocol (such as HTTP), which specifies the operations and data that
the application understands (requesting a web page).
The __presentation layer__ provides independence of data representation,
and may also perform encryption.
The __session layer__ sets up and manages sessions (think of a session
as a conversation or dialogue) between the applications.
The __transport layer__ handles individual chunks of data (think of them
as words in the conversation), and can ensure that there is end-to-end
reliability (no words or phrases get lost).
The __network layer__ forwards the packets across the network, it
provides such things as addressing and congestion control.
The __datalink layer__ encodes data into bits and moves them between
hosts. It handles errors in the physical layer. It has two sub-layers:
Media access control layer (MAC), which says when hosts can transmit
on the medium, and logical link control (LLC) that deals with error
handling and control of transmission rates.
Finally, the __physical layer__ is responsible for translating the
bits into a signal (e.g. laser pulses in a fibre) that is carried
between endpoints.
The benefit of the OSI model is that each of these layers has a
_service description_, and an _interface_ to access this service. The
details of the protocols inside the layer were of less importance, as
long as they got the job -- defined by the service description --
done.
This functional layering provides a logical order for the steps that
data passes through between applications. Indeed, existing (packet)
networks go through these steps in roughly this order.
### A birds-eye view of the TCP/IP model
{{<figure width="25%" src="https://static.packt-cdn.com/products/9781789349863/graphics/6c40b664-c424-40e1-9c65-e43ebf17fbb4.png">}}
The TCP/IP model came directly from the implementation of TCP/IP, so
instead of each layer corresponding to a service, each layer directly
corresponded to a (set of) protocol(s). IP was the unifying protocol,
not caring what was below at layer 1. The HOST-HOST protocols offered
a connection-oriented service (TCP) or a connectionless service (UDP)
to the application. The _TCP/IP model_ was retroactively made more
"OSI-like", turning into the 5-layer model, which views the top 3
layers of OSI as an "application layer".
### Some issues with these models
However, when looking at current networking solutions in more depth,
things are not as simple as these 5/7 layers seem to
indicate. Consider, for instance, __Virtual Private Network__ (VPN)
technologies and SSH __tunnels__. We are all familiar enough with this
kind of technologies to take them for granted. But a VPN, such as
openVPN, creates a new network on top of IP, for instance a layer 2
network over TAP interfaces supported by a Layer 4 (using, for
instance Transport Layer Security) connection to the VPN server.
Technologies such as VPNs and various so-called _tunnels_ seriously
jumble around the layers in this layered model.
Which protocol fits in which layer is also not clear-cut.
Multi-Protocol Label switching MPLS, a technology that makes IP
networks more ATM-like, and allows establishing and managing fixed
paths (circuits), typically sits in between Layer 2 and IP and is
categorized as a Layer 2.5 technology.
QUIC is a protocol that performs transport-layer functions such as
retransmission, flow control and congestion control, but works around
the initial performance bottleneck after starting a TCP connection
(3-way handsake, slow start) and some other optimizations dealing with
re-establishing connections for which security keys are known. But
QUIC runs on top of UDP. If UDP is layer 4, then what layer is QUIC?
The Border Gateway Protocol (BGP) performs (inter-domain)
routing. Routing is a function that is usually associated with Layer
3. But BGP runs on top of TCP, which is Layer 4. There is no real
concensus of what layer BGP is in, some say Layer 3, some (probably
most) say Layer 4, because it is using TCP, and some say it's
application layer. But the concensus does seem to be that the BGP
conundrum doesn't matter. BGP works, and the OSI and TCP/IP models are
_just theoretical models_, not _rules_ that are set in stone.
### Are these issues _really_ a problem?
Well, in my opinion: yes! And _big_ ones too! If there is no
universally valid theoretical model, if we have no clear definitions
of the fundamental concepts and no clearly defined set of rules that
unequivocally lay out what the _necessary and sufficient conditions
for networking_ are, then we are all just _engineering in the dark_.
Progress in developing computer networks is condemned to a sisyphean
effort of perpetual incremental fixes, its fate to remain a craft that
builds on tradition, cobbling together an ever-growing bungle of
technologies and protocols that stretches the limits of manageability.
|
