2. cnp3-ebook
3. principles/network
4. English
5. Zen

53 ../../principles/network.rst:335
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — For example, consider again the network topology above. `A` sends a control packet towards `B`. The initial `record route` is empty. When `R1` receives the packet, it adds its own address to the `record route` and forwards a copy to `R2` and another to `R3`. `R2` receives the packet, adds itself to the `record route` and forwards it to `R3`. `R3` receives two copies of the packet. The first contains the `[R1,R2]` `record route` and the second `[R1]`. In the end, `B` will receive two control packets containing `[R1,R2,R3,R4]` and `[R1,R3,R4]` as `record routes`. `B` can keep these two paths or select the best one and discard the second. A popular heuristic is to select the `record route` of the first received packet as being the best one since this likely corresponds to the shortest delay path. 805/10000 · 805
54 ../../principles/network.rst:337
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — With the received `record route`, `B` can send a `data packet` to `A`. For this, it simply reverses the chosen `record route`. However, we still need to communicate the chosen path to `A`. This can be done by putting the `record route` inside a control packet which is sent back to `A` over the reverse path. An alternative is to simply send a `data packet` back to `A`. This packet will travel back to `A`. To allow `A` to inspect the entire path followed by the `data packet`, its `source route` must contain all intermediate routers when it is received by `A`. This can be achieved by encoding the `source route` using a data structure that contains an index and the ordered list of node addresses. The index always points to the next address in the `source route`. It is initialized at `0` when a packet is created and incremented by each intermediate node. 861/10000 · 861
55 ../../principles/network.rst:339
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — The third technique to compute forwarding tables is to rely on a control plane using a distributed algorithm. Routers exchange control messages to discover the network topology and build their forwarding table based on them. We dedicate a more detailed description of such distributed algorithms later in this section. 318/10000 · 318
56 ../../principles/network.rst:343
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — Flat or hierarchical addresses 30/10000 · 30
57 ../../principles/network.rst:345
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — The last, but important, point to discuss about the `data plane` of the networks that rely on the datagram mode is their addressing scheme. In the examples above, we have used letters to represent the addresses of the hosts and network nodes. In practice, all addresses are encoded as a bit string. Most network technologies use a fixed size bit string to represent source and destination address. These addresses can be organized in two different ways. 453/10000 · 453
58 ../../principles/network.rst:347
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — The first organization, which is the one that we have implicitly assumed until now, is the `flat addressing` scheme. Under this scheme, each host and network node has a unique address. The unicity of the addresses is important for the operation of the network. If two hosts have the same address, it can become difficult for the network to forward packets towards this destination. `Flat addresses` are typically used in situations where network nodes and hosts need to be able to communicate immediately with unique addresses. These `flat addresses` are often embedded inside the network interface cards. The network card manufacturer creates one unique address for each interface and this address is stored in the read-only memory of the interface. An advantage of this addressing scheme is that it easily supports unstructured and mobile networks. When a host moves, it can attach to another network and remain confident that its address is unique and enables it to communicate inside the new network. 1004/10000 · 1004
59 ../../principles/network.rst:349
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — With `flat addressing` the lookup operation in the forwarding table can be implemented as an exact match. The `forwarding table` contains the (sorted) list of all known destination addresses. When a packet arrives, a network node only needs to check whether this address is included in the forwarding table or not. In software, this is an `O(log(n))` operation if the list is sorted. In hardware, Content Addressable Memories can efficiently perform this lookup operation, but their size is usually limited. 507/10000 · 507
60 ../../principles/network.rst:353
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — A drawback of the `flat addressing scheme` is that the forwarding tables linearly grow with the number of hosts and routers in the network. With this addressing scheme, each forwarding table must contain an entry that points to every address reachable inside the network. Since large networks can contain tens of millions of hosts or more, this is a major problem on routers that need to be able to quickly forward packets. As an illustration, it is interesting to consider the case of an interface running at 10 Gbps. Such interfaces are found on high-end servers and in various routers today. Assuming a packet size of 1000 bits, a pretty large and conservative number, such interface must forward ten million packets every second. This implies that a router that receives packets over such a link must forward one 1000 bits packet every 100 nanoseconds. This is the same order of magnitude as the memory access times of old DRAMs. 933/10000 · 933
61 ../../principles/network.rst:355
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — A widely used alternative to the `flat addressing scheme` is the `hierarchical addressing scheme`. This addressing scheme builds upon the fact that networks usually contain much more hosts than routers. In this case, a first solution to reduce the size of the forwarding tables is to create a hierarchy of addresses. This is the solution chosen by the post office since postal addresses contain a country, sometimes a state or province, a city, a street and finally a street number. When an envelope is forwarded by a post office in a remote country, it only looks at the destination country, while a post office in the same province will look at the city information. Only the post office responsible for a given city will look at the street name and only the postman will use the street number. `Hierarchical addresses` provide a similar solution for network addresses. For example, the address of an Internet host attached to a campus network could contain in the high-order bits an identification of the Internet Service Provider (ISP) that serves the campus network. Then, a subsequent block of bits identifies the campus network which is one of the customers of the ISP. Finally, the low order bits of the address identify the host in the campus network. 1260/10000 · 1260
62 ../../principles/network.rst:357
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — This hierarchical allocation of addresses can be applied in any type of network. In practice, the allocation of the addresses must follow the network topology. Usually, this is achieved by dividing the addressing space in consecutive blocks and then allocating these blocks to different parts of the network. In a small network, the simplest solution is to allocate one block of addresses to each network node and assign the host addresses from the attached node. 463/10000 · 463
63 ../../principles/network.rst:383
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — In the above figure, assume that the network uses 16 bits addresses and that the prefix `01001010` has been assigned to the entire network. Since the network contains four routers, the network operator could assign one block of sixty-four addresses to each router. `R1` would use address `0100101000000000` while `A` could use address `0100101000000001`. `R2` could be assigned all addresses from `0100101001000000` to `0100101001111111`. `R4` could then use `0100101011000000` and assign `0100101011000001` to `B`. Other allocation schemes are possible. For example, `R3` could be allocated a larger block of addresses than `R2` and `R4` could use a sub-block from `R3` 's address block. 689/10000 · 689
64 ../../principles/network.rst:385
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — The main advantage of hierarchical addresses is that it is possible to significantly reduce the size of the forwarding tables. In many networks, the number of routers can be several orders of magnitude smaller than the number of hosts. A campus network may contain a dozen routers and thousands of hosts. The largest Internet Services Providers typically contain no more than a few tens of thousands of routers but still serve tens or hundreds of millions of hosts. 465/10000 · 465
65 ../../principles/network.rst:387
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — Despite their popularity, `hierarchical addresses` have some drawbacks. Their first drawback is that a lookup in the forwarding table is more complex than when using `flat addresses`. For example, on the Internet, network nodes have to perform a longest-match to forward each packet. This is partially compensated by the reduction in the size of the forwarding tables, but the additional complexity of the lookup operation has been a difficulty to implement hardware support for packet forwarding. A second drawback of the utilization of hierarchical addresses is that when a host connects for the first time to a network, it must contact one router to determine its own address. This requires some packet exchanges between the host and some routers. Furthermore, if a host moves and is attached to another routers, its network address will change. This can be an issue with some mobile hosts. 893/10000 · 893
66 ../../principles/network.rst:390
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — Dealing with heterogeneous datalink layers 42/10000 · 42
67 ../../principles/network.rst:392
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — Sometimes, the network layer needs to deal with heterogeneous datalink layers. For example, two hosts connected to different datalink layers exchange packets via routers that are using other types of datalink layers. Thanks to the network layer, this exchange of packets is possible provided that each packet can be placed inside a datalink layer frame before being transmitted. If all datalink layers support the same frame size, this is simple. When a node receives a frame, it decapsulates the packet that it contains, checks the header and forwards it, encapsulated inside another frame, to the outgoing interface. Unfortunately, the encapsulation operation is not always possible. Each datalink layer is characterized by the maximum frame size that it supports. Datalink layers typically support frames containing up to a few hundreds or a few thousands of bytes. The maximum frame size that a given datalink layer supports depends on its underlying technology. Unfortunately, most datalink layers support a different maximum frame size. This implies that when a host sends a large packet inside a frame to its nexthop router, there is a risk that this packet will have to traverse a link that is not capable of forwarding the packet inside a single frame. In principle, there are three possibilities to solve this problem. To discuss them, we consider a simple scenario with two hosts connected to a router as shown in the figure below. 1442/10000 · 1442
68 ../../principles/network.rst:411
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — Consider in the network above that host `A` wants to send a 900 bytes packet (870 bytes of payload and 30 bytes of header) to host `B` via router `R1`. Host `A` encapsulates this packet inside a single frame. The frame is received by router `R1` which extracts the packet. Router `R1` has three possible options to process this packet. 335/10000 · 335
69 ../../principles/network.rst:413
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — The packet is too large and router `R1` cannot forward it to router `R2`. It rejects the packet and sends a control packet back to the source (host `A`) to indicate that it cannot forward packets longer than 500 bytes (minus the packet header). The source could react to this control packet by retransmitting the information in smaller packets. 344/10000 · 344
70 ../../principles/network.rst:415
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — The network layer is able to fragment a packet. In our example, the router could fragment the packet in two parts. The first part contains the beginning of the payload and the second the end. There are two possible ways to perform this fragmentation. 250/10000 · 250
71 ../../principles/network.rst:418
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — Router `R1` fragments the packet into two fragments before transmitting them to router `R2`. Router `R2` reassembles the two packet fragments in a larger packet before transmitting them on the link towards host `B`. 215/10000 · 215
72 ../../principles/network.rst:420
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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