MPLS Transport Profile (MPLS-TP)
Multi-Protocol Label Switching Transport Profile (MPLS-TP) is a new technology developed jointly by the ITU-T and the IETF. The key motivation is to add OAM functionality to MPLS in order to monitor each packet and thus enable MPLS-TP to operate as a transport network protocol.
Motivations for MPLS Transport Profile
Evolution of SONET/SDH transport networks to packet switching driven by growth in packet-based services (L2/L3 VPN, IPTV, VoIP, etc) and desire for bandwidth and also QoS flexibility in transport network was one of the main drivers of MPLS Transport Profile.
MPLS-TP is a new packet transport mechanism which has the same operational model with TDM based transport networks.
It is useful to understand some of the key differences between MPLS-TP and classic MPLS.
MPLS-TP vs. IP/MPLS
- MPLS-TP uses network management instead of signalling protocols for determinism (know where your packets are). GMPLS is defined as MPLS TP signalling protocol and provides control plane functionality to MPLS TP networks. MPLS TP doesn’t force GMPLS to be used though.
- No routing protocols or IP control plane in MPLS Transport Profile. NMS is used in MPLS TP.
- Also MPLS Transport Profile doesn’t have :
- Penultimate Hop Popping (PHP) – last hop IP meaning no MPLS OAM end to end. Thus there is no PHP in MPLS Transport Profile.
- Equal Cost Multi-Path (ECMP) – makes OAM difficult. That’s why there is no ECMP in MPLS Transport Profile
- Label Switched Path (LSP) merging – makes OAM difficult. Applications
MPLS-TP is a packet transport protocol with capabilities that traditionally belong to transport networks such as SONET/SDH and OTN. The intention with MPLS-TP is to be able to replace such legacy networks as SONET/SDH, though not OTN, while still keeping the advantages of packet transport.
Due to its extensive feature catalogue, MPLS-TP is very flexible and can be used for many different applications. One main advantage is that it uses Pseudowire (PW) as a transport entity. PWs are able to encapsulate any type of traffic, such as Asynchronous Transfer Mode (ATM), Frame Relay, Point-to-Point Protocol (PPP), Ethernet, etc.
MPLS Transport Profile is applicable to situations where reliability, QoS and OAM are the main requirements.
MPLS Transport Profile can be operated/controlled via network management or a control plane, the latter being a main advantage when dynamic provisioning is required. Moreover, MPLS-TP is fully compatible with IP/MPLS networks, which presents many possibilities for network solutions that demand MPLS/MPLS-TP interworking.
MPLS Transport Profile can run over Ethernet, SONET/SDH (G.783) and OTN (G.709, G.872) using Generic Framing Procedure (GFP). In these studies OTN was chosen as the underlying layer and thus the delay from the GFP should be included for the MPLS-TP or the OTN layer.
When MPLS network is controlled by IP/LDP or RSVP, LSP (Label Switched Path) is always unidirectional. Which mean, return traffic requires an additional LSP. Thus return traffic can pass completely different set of routers/nodes than forward traffic.
MPLS TP LSP is a bidirectional LSP which provides deterministic path since the nodes in forward direction is exactly same as reverse direction. Same delay and jitter is provided since the forward and reverse traffic goes through same set of routers/nodes.
MPLS Transport Profile doesn’t change the IP/MPLS data plane. Still 32 bits MPLS Header is used. 20 bits of MPLS Label space, 3 bits for EXP, 1 bit for bottom of stack indicator and 8 bits for TTL.
I mentioned in the beginning of this post, MPLS TP can use GMPLS for control plane function.
Generalized MPLS (GMPLS) provides deterministic and connection oriented behaviour using LSPs (Label Switched Paths).
MPLS-Transport Profile also uses Targeted LDP (T-LDP) to set up pseudowires (PWs) over GMPLS LSPs, to provide VPWS (Virtual Private Wire Service) and VPLS (Virtual Private LAN Service).
MPLS Transport Profile mandates running protocols such as BFD (Bidirectional Forwarding Detection) over GMPLS LSPs and PWs, to provide OAM functionality.
Last but not least MPLS Transport Profile LSPs are connection oriented. Connection oriented is a communication mode in telecommunications and computer networking, where a communication session or a semi-permanent connection is established before any useful data can be transferred, and where a stream of data is delivered in the same order as it was sent.
I recommend below as extra study resources for those who are interested in MPLS Transport Profile