TTL Processing in MPLS

TTL Processing in MPLS

Today we are going to discuss on the TTL processing in MPLS network. MPLS stands for Multiprotocol Label Switching. Lets start with the term MPLS and followed by TTL Processing in MPLS

Multiprotocol Label Switching (MPLS)
In order to increase forwarding speed, Multiprotocol Label Switching (MPLS), which has its roots in IPv4, was first suggested. Several network protocols, including IPv6, Internet Packet Exchange (IPX), and Connectionless Network Protocol, can be added to its fundamental technology (CLNP). The phrase "multiprotocol" refers to this.

Layer 2 fast switching and Layer 3 routing and forwarding are both integrated in MPLS, which satisfies the networking needs of several innovative applications.

Fig 1.1- MPLS Network

MPLS TTL processing has two components and these are TTL propagation and ICMP response path.

TTL propagation
An 8-bit TTL field on an MPLS label has the same purpose as an IP packet and is 8 bits long.

When an LSR labels a packet, it replicates the TTL value of the original IP packet or the higher level label to the TTL field of the newly added label, in accordance with RFC 3031, "Multiprotocol Label Switching Architecture". An LSR decreases the TTL value of the label at the top of the stack by 1 when it transmits a tagged packet.

The TTL value of the label at the top of the stack is copied back into the TTL field of the IP packet or lower level label when an LSR pops a label.

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TTL may be used to implement the tracert function as well as prevent routing loops.

  • When IP TTL propagation is enabled at ingress, a packet's IP TTL is reduced by 1 every time it completes a hop along the LSP. As a consequence, the tracert output will show the route that the packet took.
  • As if the ingress and egress were linked directly, when IP TTL propagation is disabled at ingress, a packet's IP TTL does not decrease when it crosses a hop and the results of tracert do not reveal the hops inside the MPLS backbone.
  • TTL propagation takes place between the multi-level labels in an MPLS domain constantly.
  • Regardless of whether IP TTL propagation is enabled or not, the TTL value of a sent local packet is always replicated. This guarantees that the local administrator may use tracert to test the network.
  • The MPLS backbone's architecture might need to be concealed in an MPLS VPN application for network security reasons. TTL propagation for private network packets at ingress is not permitted in this situation.

ICMP response
The second thing here is ICMP response. P routers are unable to transport VPN packets over an MPLS VPN. An ICMP response is produced when the TTL of an MPLS packet expires and is sent down the LSP until it reaches the destination router of the LSP, when IP routing is used to forward it. Such procedure lengthens the packet forwarding latency and increases network traffic.

When the TTL expires for an MPLS packet with a single level of label, the ICMP response message is sent over the IP route.