cnp3-ebook/principles/network — English @ Weblate

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72 ../../principles/network.rst:420

	English Each of the packet fragments is a valid packet that contains a header with the source (host `A`) and destination (host `B`) addresses. When router `R2` receives a packet fragment, it treats this packet as a regular packet and forwards it to its final destination (host `B`). Host `B` reassembles the received fragments. 319/10000 · 319
73 ../../principles/network.rst:423

	English These three solutions have advantages and drawbacks. With the first solution, routers remain simple and do not need to perform any fragmentation operation. This is important when routers are implemented mainly in hardware. However, hosts must be complex since they need to store the packets that they produce if they need to pass through a link that does not support large packets. This increases the buffering required on the hosts. 433/10000 · 433
74 ../../principles/network.rst:425

	English Furthermore, a single large packet may potentially need to be retransmitted several times. Consider for example a network similar to the one shown above but with four routers. Assume that the link `R1->R2` supports 1000 bytes packets, link `R2->R3` 800 bytes packets and link `R3->R4` 600 bytes packets. A host attached to `R1` that sends large packet will have to first try 1000 bytes, then 800 bytes and finally 600 bytes. Fortunately, this scenario does not occur very often in practice and this is the reason why this solution is used in real networks. 556/10000 · 556
75 ../../principles/network.rst:427

	English Fragmenting packets on a per-link basis, as presented for the second solution, can minimize the transmission overhead since a packet is only fragmented on the links where fragmentation is required. Large packets can continue to be used downstream of a link that only accepts small packets. However, this reduction of the overhead comes with two drawbacks. First, fragmenting packets, potentially on all links, increases the processing time and the buffer requirements on the routers. Second, this solution leads to a longer end-to-end delay since the downstream router has to reassemble all the packet fragments before forwarding the packet. 641/10000 · 641
76 ../../principles/network.rst:429

	English The last solution is a compromise between the two others. Routers need to perform fragmentation but they do not need to reassemble packet fragments. Only the hosts need to have buffers to reassemble the received fragments. This solution has a lower end-to-end delay and requires fewer processing time and memory on the routers. 327/10000 · 327
77 ../../principles/network.rst:431

	English The first solution to the fragmentation problem presented above suggests the utilization of control packets to inform the source about the reception of a too long packet. This is only one of the functions that are performed by the control protocol in the network layer. Other functions include : 295/10000 · 295
78 ../../principles/network.rst:433

	English sending a control packet back to the source if a packet is received by a router that does not have a valid entry in its forwarding table 136/10000 · 136
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	English sending a control packet back to the source if a router detects that a packet is looping inside the network 107/10000 · 107
80 ../../principles/network.rst:435

	English verifying that packets can reach a given destination 52/10000 · 52
81 ../../principles/network.rst:437

	English We will discuss these functions in more details when we will describe the protocols that are used in the network layer of the TCP/IP protocol suite. 148/10000 · 148
82 ../../principles/network.rst:441

	English Virtual circuit organization 28/10000 · 28
83 ../../principles/network.rst:444

	English The second organization of the network layer, called `virtual circuits`, has been inspired by the organization of telephone networks. Telephone networks have been designed to carry phone calls that usually last a few minutes. Each phone is identified by a telephone number and is attached to a telephone switch. To initiate a phone call, a telephone first needs to send the destination's phone number to its local switch. The switch cooperates with the other switches in the network to create a bi-directional channel between the two telephones through the network. This channel will be used by the two telephones during the lifetime of the call and will be released at the end of the call. Until the 1960s, most of these channels were created manually, by telephone operators, upon request of the caller. Today's telephone networks use automated switches and allow several channels to be carried over the same physical link, but the principles roughly remain the same. 969/10000 · 969
84 ../../principles/network.rst:448

	English In a network using virtual circuits, all hosts are also identified with a network layer address. However, packet forwarding is not performed by looking at the destination address of each packet. With the `virtual circuit` organization, each data packet contains one label [#flabels]_. A label is an integer which is part of the packet header. Routers implement `label switching` to forward `labelled data packet`. Upon reception of a packet, a router consults its `label forwarding table` to find the outgoing interface for this packet. In contrast with the datagram mode, this lookup is very simple. The `label forwarding table` is an array stored in memory and the label of the incoming packet is the index to access this array. This implies that the lookup operation has an `O(1)` complexity in contrast with other packet forwarding techniques. To ensure that on each node the packet label is an index in the `label forwarding table`, each router that forwards a packet replaces the label of the forwarded packet with the label found in the `label forwarding table`. Each entry of the `label forwarding table` contains two pieces of information : 1150/10000 · 1150
85 ../../principles/network.rst:450

	English the outgoing interface for the packet 37/10000 · 37
86 ../../principles/network.rst:451

	English the label for the outgoing packet 33/10000 · 33
87 ../../principles/network.rst:453

	English For example, consider the `label forwarding table` of a network node below. 75/10000 · 75
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	English index 5/10000 · 5
89 ../../principles/network.rst:457 • ../../principles/network.rst:499 • ../../principles/network.rst:511 • ../../principles/network.rst:523 • ../../principles/network.rst:535

	English outgoing interface 18/10000 · 18
90 ../../principles/network.rst:457 • ../../principles/network.rst:499 • ../../principles/network.rst:511 • ../../principles/network.rst:523 • ../../principles/network.rst:535

	English label 5/10000 · 5
91 ../../principles/network.rst:459 • ../../principles/network.rst:501 • ../../principles/network.rst:503 • ../../principles/network.rst:513 • ../../principles/network.rst:525 • ../../principles/network.rst:525 • ../../principles/network.rst:537 • ../../principles/network.rst:537

	English 0 1/10000 · 1

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