Forum of Layer 2 Layer 3 technologies

Virtual circuit Connection Verification (VCCV) : ================================================== VCCV provides connection verification services such as ping ,mpls ping regardless of underlying protocol such as MPLS ,IP tunnel. A network operator may use this to test the liveliness of the network. Ping and other IP messages are encapsulated using the PWE3 encapsulation .These messages are referred to as VCCV messages.VCCV messages are exchanged after negotiation between PEs. MPLS as PSN : ------------- VCCV creates control channel between PWE3 PES to exchange the IP monitoring tools. For more details of this , please refer PWE3 architecture.Packets sent across this channel are IP Packets ,allowing maximum flexibility. When control word is present on VC , it is possible to indicate the control channel by setting the control channel header . this is referred as inband MPLS VCCV as the control channel would be in band. When the control header is not in use , use of MPLS router alert la

L2TP is a protocol that is used to tunnel PPP over public network using IP.L2TP allows the encapsulation of any layer 3 protocol in its packets .The reason is tunnelling is done at layer 2 irrespective of layer 3 protocol. How L2TP provides security Like GRE , L2TP depends IPSec or any application layer mechanism to provide the type of security. Devices in L2TP session : ========================= PC , L2TP access concentrator (LAC),L2TP network server (LNS) The PC establishes a connection to a server known as LAC using dial up , POTS and DSL .The LAC initiates L2TP session to LNS .Typically authentication,authorization and accounting of the end user are done on the LNS itself using AAA server or local database. In running L2TP over IP backbone , UDP is used as carrier of all L2TP traffic which includes the control traffic of session between LAC and LNS. The initiator of tunnel (LAC) uses UDP port 1701. Types of L2TP tunnels : ======================== 1 . Compulsory tunnell

Fragmentation and GRE tunnels : =================================== What is Tunnel : A tunnel is logical interface that provides a way to encapsulate passenger packet inside a transport protocol. Tunneling has three memory components . 1 . Passenger protocol (Apple talk , CLNS,IP or IPX,DECNET) 2 . Carrier protocol – one of the encapsulation protocols GRE ,IP in IP tunnels 3. Transport protocol – The protocol used to carry the encapsulated protocol Original Packet : IP-TCP-Telnet Tunnel Encapsulated packet : IP-GRE-Original packet Where IP is transport protocol , GRE is the encapsulation protocol , IP is passenger protocol Example : Where IP and DECNET are passenger protocols and GRE is carrier protocol. Why tunnelling is required here ? Two non IP discontiguous networks are separated by IP network. The administrator may not want to connect them together by configuring DECNET in the IP ne

Fragmentation and TCP MSS , MPTUD : TCP Maximum segment size and TCP Path MTU Discovery play important role in fragmentation. Let us see first about TCP MSS TCP maximum segment size defines maximum amount of data receiver is willing to accept in single TCP/IP stream . How MSS is calculated for TCP/IP stream : MSS calculation is done based on buffer size in both server and client side During TCP/IP connection setup , The SYN segment contains MSS option. If machine does not want to mention MSS , a default of 536 bytes is assumed. The default value of 536 is derived from 576 minus 40 bytes of header which includes 20 bytes of each ip and tcp header. Please note that MTU of dial up connection is 576 Diagram : 1 . Host A sends MSS of 16 k 2 S 2 . Server receives SYN and sets send MSS of host A to 16k 3 S 3 . Server sends MSS of 8k 4 H4. Host A receives and sets send MSS of server to 8k. Here MSS is based on minimum buffe