Questions tagged [protocol-theory]

For questions about network protocols published in a Standard or RFC. Do not use this tag merely because the question mentions a networking protocol in it. Questions with this tag *could* be closed if they do not explain why the question is relevant and important to answer; this is necessary to avoid issues with tangential / trivial questions or help vampires.

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141
votes
9answers
77k views

Why do we need a 3-way handshake? Why not just 2-way?

The TCP 3-way handshake works like this: Client ------SYN-----> Server Client <---ACK/SYN---- Server Client ------ACK-----> Server Why not just this? Client ------SYN-----> Server ...
30
votes
5answers
5k views

Why do we still use Ethernet?

There's no doubt the vast majority of Ethernet frames are transporting IP packets. I know there are various other protocols that can be transported over Ethernet as well, but those can be transported ...
38
votes
5answers
35k views

Why was the MTU size for ethernet frames calculated as 1500 bytes?

Is there any specific calculation that that was done to arrive at this number, and what were the factors that were taken into consideration for that calculation.
10
votes
2answers
13k views

How do wifi clients detect SSIDs from APs?

How can my wifi board detect wireless networks that are in range? What protocol is used to communicate with the devices in the area to announce a wifi AP? Does every router transmit on its own ...
6
votes
2answers
2k views

What is the reason for the different order of the Source and Destination in a L2 header?

A TCP/UDP header (Layer 4) lists the Source Port, then the Destination Port: An IP header (Layer 3) lists the Source IP, then the Destination IP: An Ethernet header (L2) and a WiFi Header (L2) lists ...
34
votes
2answers
89k views

What is the actual size of an Ethernet MTU

I think I might be getting confused with terminology surrounding MTU. This definition from Wendell Odom's CCNA book on MTU: The IEEE 802.3 specification limits the data portion of the 802.3 frame ...
12
votes
1answer
24k views

What is the difference between Ethernet II and 802.3 Ethernet?

Can anyone explain to me what is the difference between Ethernet, Ethernet-II and 802.3 Ethernet. I referred to a lot of books, they are in high level language. Can anyone explain to me in a simple ...
18
votes
4answers
23k views

Why does WLAN use Collision Avoidance and not Collison detection?

I am searching for this answer for quite some time now. Why there is CSMA/CD in LAN but CSMA/CA in WLAN? The best explanation I could find is "due to extreme ratio in transmitting and receiving power,...
7
votes
1answer
2k views

Why is the NDP Neighbor Solicitation message sent to the solicited-node address?

As I understand it, when Host Alice knows Host Bob's IPv6 address but not its MAC, Alice sends an NDP Neighbor Solicitation (NS) message to the solicited-node multicast address for Bob. Alice knows ...
11
votes
1answer
10k views

How is BGP “AS_SET” used?

RFC 1771 defines a path attribute type of AS_PATH as follows: AS_PATH (Type Code 2): AS_PATH is a well-known mandatory attribute that is composed of a sequence of AS path segments. Each AS path ...
1
vote
2answers
104 views

Which port determines the protocol?

If we have a common protocol port on both ends.. say source point is 443 and destination port is 80 (hypothetical situation, I know this combination of ports probably makes no sense).. how do we ...
6
votes
4answers
2k views

Why is the ICMP protocol considered to run at layer 3 on the Wikipedia page?

Why is the ICMP protocol considered to run at layer 3 on the Wikipedia page (same layer as IP) when the ICMP protocol data is a payload of the IP packet? This is the article on Wikipedia that states ...
5
votes
5answers
337 views

What blocks a computer from another network in the same switch from answering an ARP request?

Let's suppose I have 3 computers A, B and C connected to the same switch with only the native VLAN active. A and B belong to the same network, let's say that their IP addresses are 192.168.1.1/24 for ...
4
votes
2answers
1k views

Ethernet's frame format, length or EtherType?

Ethernet defines a 2 byte header for the payload's length in its frame format. This header has an ambiguous meaning: if smaller than or equal to 1500 it represents the payload length. if ...
20
votes
4answers
43k views

NACK vs. ACK? When to use one over the other one?

Personally I don't even feel that there is a need for ACK. It's faster if we just send NACK(n) for the lost packets instead of sending an ACK for each received packet. So when/which situations would ...
25
votes
5answers
58k views

Why is 4096 the max value for VLANs

The Tag Protocol Identifier is set to 16 bits. In the CCNA certification it is said that it's not possible to have more than 4096 VLANs. 4096 in binary is 1000000000000000. Since 16 bits are allocated ...
10
votes
3answers
11k views

Is a TCP three-way handshake required for an HTTP POST?

I don't quite understand if i posting http form data from the browser to the server, does the protocol still need to make three-way handshake (syn-ack-data) or it only works for GET http requests?
10
votes
3answers
10k views

Why don't switches rewrite mac-addresses?

Is there any particular reason why Ethernet switches don't change the MAC address of a packet? Is it for end host identification using the MAC address, or anything else?
12
votes
2answers
13k views

Technical difference between a Collision and Broadcast Domain

Many people new to networking wonder what the technical differences between a broadcast domain and a collision domain are. Specifically: What criteria is used to know the boundaries of a collision ...
8
votes
4answers
6k views

Why is a TCP Socket identified by a 4 tuple?

Newbie to networking here. I'm reading the Computer Networking (3rd edition) book, and in section 3.2 they are discussing multiplexing / demultiplexing for both UDP and TCP. In the UDP protocol, a ...
9
votes
2answers
23k views

Window Size and ACK Number

Copy-pasting from my lecturer's slides: • Receiver indicates the window size is 3000 • Transfer goes ahead • Acknowledge every 3000 bytes • Receiver increases window size to 4000 • 4000 bytes ...
3
votes
1answer
2k views

What is the need for transport Layer?

In 5-layer model. Why we need additional transport layer? I read these two points Users who interact with software have no control over the sub-net; Transport Service can provide a more reliable ...
18
votes
4answers
8k views

Can DHCP Server determine client's OS? [closed]

Is it possible for the DHCP Server to determine the client operating system? I'm working on a monitoring tool for gateways on local networks that is web-based and would like to be able to somehow ...
15
votes
6answers
12k views

Reason for half-duplex mode in Ethernet?

Half-duplex connections were popular at the time of 10Mbps and 100Mbps Ethernet, and, according to standards, it is allowed also in case of 1Gbps Ethernet. Am I correct that half-duplex mode support ...
13
votes
4answers
17k views

MTU and Fragmentation

Consider the following: NAS with 10G interface connected to 10G interface on switch. Client computers connected to switch via gigabit Ethernet. Will there be any gain in performance if MTU size is ...
7
votes
2answers
15k views

Poison reverse example

Can you give me an example when poison reverse is actually necessary? Distance vector routing protocols employ split horizon with poison reverse to minimize the convergence time when a route is no ...
6
votes
2answers
365 views

What Cisco will discard TTL=1 packets for own interface?

There have been a number of questions about router behaviour with IPv4 packets with TTL=1 and apparently contradictory answers. This question is not about what should a host or router do, it is a ...
5
votes
5answers
892 views

How we can assume which network protocol is working in which OSI Layer?

For a long time, I have this kind of silly question: How we can say some network protocols are working in X layer of the OSI model? For example, HTTP working in L7 of OSI, OSPF is L3, ICMP is L4, TCP ...
9
votes
2answers
2k views

Differences and functions of type codes 0800 and 0806

What are the primary differences and functions (or applications) of type codes 0800 and 0806? Through my own research, I found the 0800 was related to the 0800 to the IP/Layer 3 of the OSI model. ...
9
votes
5answers
5k views

Can you reduce RTT by increasing the bandwidth on a link?

Does increasing the bandwidth on a link from, let's say, 1mb to 30mb reduce the RTT? I have found one answer saying no. Can someone please explain? Also, what are the best mechanisms to reduce RTT?
7
votes
2answers
7k views

ARP Replies can they be Broadcast

I have been informed by certain networking professionals that ARP replies can and sometimes actually are broadcast packets instead of unicast. If and when would you use a broadcast ARP reply?
4
votes
1answer
554 views

ARP table aging: What timer can I expect?

Is the hold time standardized? Is there a standard or reliable recommendation for a timeout? I am well aware that a SOHO non-managed switch requires a shorter timer than a data-center switch. That ...
4
votes
2answers
280 views

IS-IS multi-area

It is well known that an IS-IS L1 router chooses the nearest L1/L2 router to route inter-area traffic, if there is no route leaking as defined in RFC 2966. However, providing that there is no route ...
1
vote
2answers
646 views

Link Local Address using in NDP (IPv6)

How is using of link local address in NDP. We can discover neighbor by using global address. So why we need link local address in NDP?
-3
votes
4answers
11k views

How Layer-2 data will be fragmented?

Got a doubt while studying about LLC layer. From Network layer I came to know that IP fragmentation will be done, For L4 TCP will segment the data. Here, the question is Protocols starting from Layer-...
7
votes
1answer
3k views

How do Endpoints in a TCP conversation determine their MSS?

I understand the MSS is the Maximum Segment Size, which is the size of the Payload TCP will send. I also understand that a "receiving" MSS value is suggested by both parties in a TCP conversation ...
7
votes
4answers
8k views

How OSPF decides a network to be a stub network?

In RFC 2328 it mentions that link-state information of all Loopback interfaces and all interfaces with network mask as /32 (host ip addresses) are advertised as stub network type. Now what about the ...
5
votes
1answer
758 views

IPv6 Link-local addresses

RFC 4291 specifies that IPv6 link-local unicast addresses must have a high-order bit sequence of 1111111010, or fe80::/10. However, the same document specifies that the 54 bits following the first 10 ...
4
votes
2answers
4k views

AODV sequence numbers and loop prevention

I am learning about the AODV (ad-hoc on-demand distance vector) protocol. I saw a "sequence number" with a value like "A 46"; however, I don't understand why incrementing this sequence number can ...
3
votes
1answer
3k views

What is the value of EstimatedRTT at first time?

Although this question might be marked as duplicated! I'm asking again because other question's answers like this couldn't help me. So please consider the case and don't mark it as duplicate and ...
3
votes
1answer
503 views

What is meant by Jumbo frame ? what happen when a frame size is greater than the MTU size?

I know that the frames having size greater than the MTU size should be dropped . i.e, The Ethernet frame having size more than 1522 should be dropped . But my doubt here is what is Jumbo Frame ?
3
votes
2answers
3k views

Is the CSMA/CD protocol active in modern ethernet networks that use LAN switches? If it is active, does it ever detects a collision?

I am interested to know if the CSMA/CD protocol is still active in modern Networks and if it is active why and what is his role now?
2
votes
2answers
520 views

IP Fragmentation

When Device A sends an IP datagram toward Device B, the IP Layer of Device A computes if it must or not fragment the IP datagram to fit the link MTU. Sent to the next hop, the process will be repeated....
2
votes
2answers
2k views

What is the internal structure of a mobile phone call packet/datagram?

Coming from a Networking (IP/TCP/etc) background, I understand that a typical network packet looks something like the following: I further understand that VoIP packets typically do use the same ...
1
vote
2answers
302 views

Sequence numbers of segments in three way handshakes of TCP connection establishment or termination

I was trying to understand TCP connection establishment and termination steps from Forouzan's book. In third step of three way handshake of connection establishment, it says following The client ...
1
vote
1answer
783 views

Using the value of RTT to estimate the latency

Generally speaking, we can calculate the packet delivery time using this formula, Wikipedia: Latency = Transmission delay + Propagation delay + queuing delay + processing delay My question is: Is it ...
1
vote
1answer
55 views

Remote Host Neighbor Cache Entry State after Neighbor Solicitation request

When a Host A sends for the first time a NS request to another Host B, host B answers with a NA. In Host A Neighbor Cache, the state Entry for host B gets from INCOMPLETE to REACHABLE, when NA comes ...
0
votes
1answer
191 views

Computer networking dwdm [closed]

Why do we use digital cross connects, which define permanent cross connect between source and destination in core network while the PSTN allocated circuits dynamically as both are types of circuit ...