GMPLS

Generalized Multi-Protocol Label Switching (GMPLS) is a set of protocols for routing and signaling in circuit switched networks, like Automatically Switched Optical Networks (ASON) and Automatic Switched Transport Network (ASTN).

The routing protocols distribute topology knowledge among the devices. using that information, a path is computed at a node. Then the signaling protocol takes care of actually setting up a path. This text describes how topology knowledge is distributed in the routing protocols of GMPLS.

In GMPLS, a network consists of Label Switched Routers (LSRs) and links in between them. Examples of LSRs are devices like optical cross connects, DWDM devices, TDM devices or Ethernet devices. Links can carry one or more Label Switched Paths (LSPs). The different LSPs can be distinguished by their label. A label can be a wavelength, VLAN tag or even the physical fiber. LSPs can be nested in another LSP. In fact, each link is considered to be an LSP, and the channels are considered lower order LSPs.

In GMPLS, not the LSR have certain capabilities, but rather the interfaces of an LSP. A device publishes one Link State Announcement (LSA) for each interface, rather then for each link. Since a link has two interfaces, there are two LSAs per link.

an LSA includes information on the switching capability of the interface and the encoding of the link. The LSA does not include information on the (available) labels or the protocol identifier (G-PID), since that is negotiated and decided upon during the signaling phase and not during the routing phase.

Design: decide on labels during signaling, not during routing

In GMPLS, the network topology is described using OSPF-TE Link State Announcements (LSAs). The connections through this network are built using Label Switched Paths (LSPs), which reside on a single layer. All hops of a LSP can be described using a Record Route Object (RRO) in RSVP-TE.

While GMPLS can describe networks in a high detail, the relation between network and network connections was difficult to model, and often insufficient to verify that a connection is valid. The reason is that GMPLS does not explicitly specify the adaptation functions. Instead, GMPLS assumes that any problem can be solved locally during the signaling phase, after the path has been chosen. For example, incompatibilities like WAN PHY versus LAN PHY or STS-24c versus STS-3c-7v are only resolved at the signaling phase. These problem can partly be avoided by using loose hops rather then strict hops in the Explicit Route Object (ERO), and using proprietary parameters in the LSA, but the basic premise is that incompatibilities are resolved at the signaling phase. Thus, the available adaptation functions are not announced in the routing phase and thus not present in the network description.

One of the assumptions of GMPLS is that the path finding algorithm needs no knowledge of the label conversion capabilities of devices, even though that capability may differ greatly amongst different technologies. While all MPLS and ATM devices can covert label between hops (the ID has only link-local meaning), this is not true for VLAN tags and wavelengths. Currently, only very few Ethernet or DWDM devices can covert the label (respectively the VLAN tag or the wavelength). By not spreading the label availability information of a link and the label conversion capability during the routing phase, the path finding algorithm in GMPLS scales better because it needs to take less information into account. The drawback is that the algorithm may come up with a path were not available labels can be found.

Difference between G.805 and ITU-T G.805

GMPLS has one interface per layer, G.805 has one interface per label. For example, given a link with two VLANS: 13 and 14.In G.805, this is described with two interfaces (connection points) on the Ethernet layer: one for each VLAN. (It's obviously only one on a lower layer). In contrast, GMPLS would define only one interface: an LSA announcing Ethernet encoding and L2 switching capability.

References

Working group:

http://www.ietf.org/html.charters/ccamp-charter.html∞

Main GMPLS RFCs on routing:

RFC 3945 - E.Mannie (Editor) et al., "Generalized Multi-Protocol Label Switching (GMPLS) Architecture"
RFC 4202 - L. Berger, "Routing Extensions in Support of Generalized Multi-Protocol Label Switching (GMPLS)"
RFC 4203 - Kompella & Rekhter, "OSPF Extensions in MPLS"

Protocol parameters:

http://www.iana.org/assignments/ospf-traffic-eng-tlvs∞
http://www.iana.org/assignments/ospf-opaque-types∞
http://www.iana.org/assignments/gmpls-sig-parameters∞

Categories

CategoryGMPLS