Two words commonly used in networking world - Packets and frames.
Can anyone please give the detail difference between these two words?
Hope it might sounds silly but does it mean as below
A packet is the PDU(Protocol Data Unit) at layer 3 (network layer - ip packet) of the networking OSI model.
A frame is the PDU of layer 2 (data link) of the OSI model.
Packets and Frames are the names given to Protocol data units (PDUs) at different network layers
Segments/Datagrams are units of data in the Transport Layer.
In the case of the internet, the term Segment typically refers to TCP, while Datagram typically refers to UDP. However Datagram can also be used in a more general sense and refer to other layers (link):
Datagram
A self-contained, independent entity of data carrying sufficient information to be routed from the source to the destination computer without reliance on earlier exchanges between this source and destination computer andthe transporting network.
Packets are units of data in the Network Layer (IP in case of the Internet)
Frames are units of data in the Link Layer (e.g. Wifi,
Bluetooth, Ethernet, etc).
A packet is a general term for a formatted unit of data carried by a network. It is not necessarily connected to a specific OSI model layer.
For example, in the Ethernet protocol on the physical layer (layer 1), the unit of data is called an "Ethernet packet", which has an Ethernet frame (layer 2) as its payload. But the unit of data of the Network layer (layer 3) is also called a "packet".
A frame is also a unit of data transmission. In computer networking the term is only used in the context of the Data link layer (layer 2).
Another semantical difference between packet and frame is that a frame envelops your payload with a header and a trailer, just like a painting in a frame, while a packet usually only has a header.
But in the end they mean roughly the same thing and the distinction is used to avoid confusion and repetition when talking about the different layers.
Actually, there are five words commonly used when we talk about layers of reference models (or protocol stacks): data, segment, packet, frame and bit. And the term PDU (Protocol Data Unit) is a generic term used to refer to the packets in different layers of the OSI model. Thus PDU gives an abstract idea of the data packets. The PDU has a different meaning in different layers still we can use it as a common term.
When we come to your question, we can call all of them by using the general term PDU, but if you want to call them specifically at a given layer:
Data: PDU of Application, Presentation and Session Layers
Segment: PDU of Transport Layer
Packet: PDU of network Layer
Frame: PDU of data-link Layer
Bit: PDU of physical Layer
Here is a diagram, since a picture is worth a thousand words:
Consider TCP over ATM. ATM uses 48 byte frames, but clearly TCP packets can be bigger than that. A frame is the chunk of data sent as a unit over the data link (Ethernet, ATM). A packet is the chunk of data sent as a unit over the layer above it (IP). If the data link is made specifically for IP, as Ethernet and WiFi are, these will be the same size and packets will correspond to frames.
Packet
A packet is the unit of data that is routed between an origin and a destination on the Internet or any other packet-switched network. When any file (e-mail message, HTML file, Graphics Interchange Format file, Uniform Resource Locator request, and so forth) is sent from one place to another on the Internet, the Transmission Control Protocol (TCP) layer of TCP/IP divides the file into "chunks" of an efficient size for routing. Each of these packets is separately numbered and includes the Internet address of the destination. The individual packets for a given file may travel different routes through the Internet. When they have all arrived, they are reassembled into the original file (by the TCP layer at the receiving end).
Frame
1) In telecommunications, a frame is data that is transmitted between network points as a unit complete with addressing and necessary protocol control information. A frame is usually transmitted serial bit by bit and contains a header field and a trailer field that "frame" the data. (Some control frames contain no data.)
2) In time-division multiplexing (TDM), a frame is a complete cycle of events within the time division period.
3) In film and video recording and playback, a frame is a single image in a sequence of images that are recorded and played back.
4) In computer video display technology, a frame is the image that is sent to the display image rendering devices. It is continuously updated or refreshed from a frame buffer, a highly accessible part of video RAM.
5) In artificial intelligence (AI) applications, a frame is a set of data with information about a particular object, process, or image. An example is the iris-print visual recognition system used to identify users of certain bank automated teller machines. This system compares the frame of data for a potential user with the frames in its database of authorized users.
Related
MTU (Maximum transmission unit) is the maximum frame size that can be transported.
When we talk about MTU, it's generally a cap at the hardware level and is for the lower level layers - DataLink and Physical layer.
Now, considering the OSI layer, it does not matter how efficient are the upper layers or what kind of magic-sauce they are applying, data-link layer will always construct frames of size < 1500 bytes (or whatever is the MTU) and anything in the "internet" will always be transmitted at that frame size.
Does the internet's transmission rate really capped at 1500 bytes. Now-a-days, we see speeds in 10-100 Mbps and even Gbps. I wonder for such speeds, does the frames still get transmitted at 1500 bytes, which would mean lots and lots and lots of fragmentation and re-assembly at the receiver. At this scale, how does the upper layer achieve efficiency ?!
[EDIT]
Based on below comments, I re-frame my question:
If data-layer transmits at 1500 byte frames, I want to know how is upper layer at the receiver able to handle such huge incoming data-frames.
For ex: If internet speed in 100 Mbps, upper layers will have to process 104857600 bytes/second or 104857600/1500 = 69905 frames/second. Network layer also need to re-assemble these frames. How network layer is able to handle at such scale.
If data-layer transmits at 1500 byte frames, I want to know how is
upper layer at the receiver able to handle such huge incoming
data-frames.
1500 octets is a reasonable MTU (Maximum Transmission Unit), which is the size of the data-link protocol payload. Remember that not all frames are that size, it is just the maximum size of the frame payload. There are many, many things with much smaller payloads. For example, VoIP has very small payloads, often smaller than the overhead of the various protocols.
Frames and packets get lost or dropped all the time, often on purpose (see RED, Random Early Detection). The larger the data unit, the more data that is lost when a frame or packet is lost, and with reliable protocols, such as TCP, the more data must be resent.
Also, having a reasonable limit on frame or packet size keeps one host from monopolizing the network. Hosts must take turns.
For ex: If internet speed in 100 Mbps, upper layers will have to
process 104857600 bytes/second or 104857600/1500 = 69905
frames/second. Network layer also need to re-assemble these frames.
How network layer is able to handle at such scale.
Your statement has several problems.
First, 100 Mbps is 12,500,000 bytes per second. To calculate the number of frames per second, you must take into account the data-link overhead. For ethernet, you have 7 octet Preabmle, a 1 octet SoF, a 14 octet frame header, the payload (46 to 1500 octets), a four octet CRC, then a 12 octet Inter-Packet Gap. The ethernet overhead is 38 octets, not counting the payload. To now how many frames per second, you would need to know the payload size of each frame, but you seem to wrongly assume every frame payload is the maximum 1500 octets, and that is not true. You get just over 8,000 frames per second for the maximum frame size.
Next, the network layer does not reassemble frame payloads. The payload of the frame is one network-layer packet. The payload of the network packet is the transport-layer data unit (TCP segment, UDP datagram, etc.). The payload of the transport protocol is application data (remember that the OSI model is just a model, and OSes do not implement separate session and presentation layers; only the application layer). The payload of the transport protocol is presented to the application process, and it may be application data or an application-layer protocol, e.g. HTTP.
The bandwidth, 100 Mbps in your example, is how fast a host can serialize the bits onto the wire. That is a function of the NIC hardware and the physical/data-link protocol it uses.
which would mean lots and lots and lots of fragmentation and
re-assembly at the receiver.
Packet fragmentation is basically obsolete. It is still part of IPv4, but fragmentation in the path has been eliminated in IPv6, and smart businesses, do not allow IPv4 packet fragments due to fragmentation attacks. IPv4 packets may be fragmented if the DF bit is not set in the packet header, and the MTU in the path shrinks smaller than the original MTU. For example, a tunnel will have a smaller MTU because of the tunnel overhead. If the DF bit is set, then a packet too large for the MTU on the next link, the packet is dropped. Packet fragmentation is very resource intensive on a router, and there is a set of steps that must be performed to fragment a packet.
You may be confusing IPv4 packet fragmentation and reassembly with TCP segmentation, which is something completely different.
When a computer X sends data through a network to computer Y the data goes down through the OSI layer. This is ok. I understand. But once the data is put on the media as eletric signals then how does the computer Y know what to reassmble, given the headers and trailers of the data model generated in OSI, once it is put on the electric media at layer 1 does not exist any more?
The physical layer is just 1's and 0's as you say - the trick is that there is a pattern that tells the receiver that this is the start of a packet. This is usual referred to as 'Framing'.
Once the receiver knows that, it simply reads in as many bits as its needs for the Layer 2 header and it then has that and so on.
The headers are clear in a typical OSI or networking diagrams, e.g. (https://www.ciscopress.com/articles/article.asp?p=2738463):
So the way the first two layers work on the receiver is:
layer 1 just recognises whether the signal is a one or a zero and creates the stream of ones and zeros.
layer 2 reads this stream and when it recognises the start pattern it then know the following bits are the header and so on and hence it can identify the frames.
You can see examples of start and stop patterns online e.g. (http://sinauonline.50webs.com/Cisco/Cisco%20Exploration%20Sem1Chap7.html):
I am trying to check the packets coming from a twitch stream. So, I am using ffprobe to probe on it using the m3u8 playlist link for the video.
the source url for the twitch stream
I notice that the video packets I recieve are bigger in size than MTU(quite bigger for I frames or P frames).
packets sizes
Also, one frame is mapped with one packet. I tested it on multiple APs across pubs and starbucks. I always get one packet per frame.
How come ?
I think that the packets are coalesced and hence I see the bigger packets. But I am confused, since I get big packets even on an ethernet connection.
'''
ffprobe -v error -select_streams v -show_packets -show_frames
'''
The term “packet” in ffprobe is not at all related to the term “packet” in TCP. Whether one “packet” is one frame depends on the container and protocol used.
I am working on a project in which i have to send data in bits.
Is it possible to send 1 bit from one computer to another through internet.Most of the people
said to me that minimum internet packet size is 64bytes.If i send 1bit from one computer to another then packet bandwidth is 64bytes.
A TCP or UDP packet consists of a header and data. Maybe you could have one bit of data in the data section, but you would need the header as well. Without the header it would be impossible to send the packet. The header contains all the information required for sending the packet where it is supposed to go and making sure it arrives safely.
I will take "internet packet" to mean Ethernet frame based on the value you give.
An Ethernet frame has a minimum total size of 64 bytes including both header and payload, this is to ensure that the time to transmit a single frame is greater than the round trip time between nodes.
This requirement is a feature of any network that uses CSMA/CD (specifically the CD (collision detection) part), it allows a sensing node to detect a collision whilst still transmitting a frame.
Whilst Ethernet can be used to send a frame smaller than 64 bytes "padding" will be added to ensure the frame is at least 64 bytes.
There are a few things I don't understand about 64/66bit encoding, and failed to find the answers to on the web. Any help/links would be greatly appreciated:
i) how is the start of a frame recognised? I don't think it can be by the initial 10/01 bits called the preamble on wikipedia because you cannot tell them apart (if an idle link is 0, then 0000 10 and 000 01 0 look rather similar). I expect the end of a frame is indicated by a control word, with the rest of the bits perhaps used for the CRC?
ii) how do the scramblers synchronise, and how do they avoid scrambling the same packet the same way? Or to put this another way, why is not possible for a malicious user to induce substantial packet loss by carefully choosing a bad message?
iii) this might have been answered in ii), but if a packet is sent to a switch, and then onto another host, is it scrambled the same way both times?
Once again, many thanks in advance
Layers
First of all the OSI model needs to be clear.
The ethernet frame is a data link layer, while the 64b/66b encoding is part of the physical layer (More precisely the PCS of the physical layer)
The physical layer doesn't know anything about the start of a frame. It sees only data. (The start of an ethernet frame are data bytes which contain the preamble.)
64b/66b encoding
Now let's assume that the link is up and running.
In this case the idle link is not full of '0'-s. (In that case the link wouldn't be self-synchronous) Idle messages (idle characters and/or synchronization blocks ie control information) are sent over the idle link. (The control information encoded with 0b10 preamble) (This is why the emitted spectrum and power dissipation don't depend on if the link is in idle state or not)
So a start of a new frame acts like following:
The link sends idle information. (with 0b10 preamble)
Upper layer (data link layer) sends the frame (in 64bit chunks of data) to physical layer.
The physical layer sends the data (with 0b01 preamble) over the link.
(Note that physical layer frequently inserts control (sync) symbols into the raw frame even during a data burst)
Synchronization
Before data transmission 64b/66b encoded lane must be initialized. This initialization includes the lane initialization which the block synchronization. Xilinx's Aurora's specification (P34) is an example of link initialization.
Briefly receiver tries to match the sync character in different bit-position, and when it match multiple times it reports link-up.
Note, that the 64b/66b encoding uses self-synchronous scrambler. This is why the scrambler (itself) doesn't need to know anything about where we are in the data stream. If you run a self-synchronous (de-)scrambler long enough, it produces the decoded bit stream.
Maliciousness
Note, that 64b/66b encoding is not an encryption. This scrambling won't protect you from eavesdropping/tamper. (Encryption should placed at higher level of the OSI model)
Same packet multiple times
Because the scrambler is in different state/seed when you sending the same packet second time, the two encoded packet will differ. (Theoretically we can creates packets, which sets back the shift register of the scramble, but we need to consider the control symbols, so practically this is impossible.)