netgo

Window size value : This is the value to receive from other side. (Window size is advertising the RCV buffer space available for the device, how much memory is allocated for the session)

The maximum field for TCP window size = 65535 or 0xFFFF because it is a 2 byte field.  There is a method to make it bigger by negotiating the window using the scaling option. 

I can see in the SYN packet option – Window Scale = 7.  This tells the server it wants to use this option and multiply the window size by 128. And Server agree in the SYN+ACK with the same option = 7.  This tells the client it also will be using the scaling option.

Window scale number 7 means that 2^7 = 128. This number is never changed until fininshing converstation.

After this negotiation, I can look at packet 3 in the TCP section at the Window you will see it shows (211 x 128 = 27008) 

From Server, I can see the window scale option is enabled like below

And middle of the packets, I can see the widnow size more than 65535. 76288 is the sample to use scale option. 

This is normal case to show how the scale option is used.

[Without Window scaling option Case]

This is the sample case to show what the window scale dose not work. In the first syn packet, there is the option. However the syn-ack paket does not have this option like below.

This below is syn-ack packet. There is no window scale option.

So, the last ack packet decide to send packet without the window scale option.

In wireshark, -2 mean that there is no window scale option definition. Please note that -2 always means "no window scale option". If there is no packet syn, syn-ack and ack due to capturing middle of packets, the wireshark determind -2 even if the window size is more than 65535.

Sometimes, I need to see the packet with wireshark tool. I have to calculate sequence numbers. I have learned the IP packet size is 1540 (MSS is 1460). However, I can see the more size in the packet. This post is good for me.

This is whole diagram to understand my situation. This is the reason why I need to capture the packet with switch port.

Please look at the first Syn packet. There is MSS is 8961. In blow, I wrote the MSS size is 1460. However, the wireshark show more than 8961. 

Thus, this is not perfect number. 

[ Reference ]

[ 1 ] https://packetbomb.com/how-can-the-packet-size-be-greater-than-the-mtu/

[ 2 ] https://createnetech.tistory.com/25?category=672583

1. Simple OSPF Parameters to Use

Before configuring the OSPF, I will handle some concept necessary to configure. For interface configuration, there is some term of network type. In this blog, those are compared simply.

In the above, DR/BDR terms are shown. If you did not study about the OSPF, it is not easy. DR has the role to share the OSPF information between the routers and switches. In this blog, it show simple diagram to explain about these.

DR make simple to share routing table information. Because of this, it is useful in Broadcast network. Howerver, I confuse about the Point to Multipoint concept. It looks like Broadcast. I refers this blog. It is different I need to annount the neighbor IP address with "Neighbor" or "Frame-relay" options.

2. Unnumbered Interfaces | net.ipv4.conf.<interface name>.rp_filter value=0

In fact, I did not realize this concept. I met the error message "Please remove network command first" during configure OSPF with FRR. In this post, there are some explain about this.

Unnumberd Interface improve the performance. I need to edit "net.ipv4.conf.<interface name>.rp_filter value=0" in the "/etc/sysctl.conf"

Because of this "unnumbered interface existance", FRRouting give some limitation. After network definition in router ospf part, I can not activate the interface for OSPF.

Reference

[ 1 ] https://docs.cumulusnetworks.com/cumulus-linux-37/Layer-3/Open-Shortest-Path-First-OSPF/

[ 2 ] https://www.freeccnaworkbook.com/workbooks/ccna/configuring-ospf-network-types

[ 3 ] https://www.fir3net.com/Networking/Protocols/ospf-the-dr-and-bdr-roles.html

[ 4 ] https://packetlife.net/blog/2008/jun/19/ospf-network-types/

[ 5 ] http://www.getnetworking.net/tutorials/ospf-point-to-multipoint-over-frame-relay

[ 11 ] https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_ospf/configuration/xe-16/iro-xe-16-book/iro-cfg.html