IS-IS - Two different headers?

[Mitrixsen]

Hello, everyone

I've recently began studying IS-IS and I captured one of its messages in Wireshark

From what I understand, it uses the original Ethernet format (not Ethernet 2) together with LLC. In the LLC, the SAP is roughly similar to the EtherType in Ethernet 2, right? The SAP in this case says "ISO Network Layer" so whatever is expected next belongs to an OSI protocol.

My question is, why does IS-IS have two headers? Why does ISO 10589 come first and then the IS-IS hello? Why isn't all this information included in the hello, why is it split between 2 different headers?

Thank you
David

[Cristian Matei · Accepted solution]

Hi,

   By design, IS-IS has both a common standard header which is the same regardless of the IS-IS packet type (what you see above as the ISO 10589 ISIS InTRA Domain Routeing Information Exchange Protocol), follows by the PDU type specific header and associated / specific TLV's ( HELLO PDU which can be L1 LAN, L2 LAN and P2P; LSP PDU which can be L1 LSP and L2 LSP; PSNP PDU which can be L1 PSNP and L2 PSNP; CSNP PDU which can be L1 CSNP and L2 CSNP).

    Why this design has been chosen? Per what I remember understanding by reading the ISO documentation and RFC's, IS-IS uses a common PDU header for essential, universal information needed by all PDUs, while specific headers and TLV's  provide unique details for different packet types, ensuring consistent identification and flexible, extensible parameter encoding without bloating every single packet. Think of it like when sending an e-mail, which has a common header that you as the user don't see as it's built in the protocol and e-mail application you're using, as well as a specific header that you fill in (from / to / CC / BCC) as well as the data / body (aka the TLV's of IS-IS), which keeps e-mail being universally consistent and well structured.  

    The value 0x83 value identifies the common header and it's known as the Intra-domain Routeing Protocol Discriminator (this is how it's being named in the RFC's as well), and serves to identify the specific type of protocol within the OSI (Open Systems Interconnection) network layer, allowing devices to correctly process the incoming data as an IS-IS packet. 

   Fun facts, do you see that in the common header, the term Routeing is used instead of the term Routing? The term Routeing is also used in the ISO documentation, as well as RFC's, due to its origins in the ISO and the OSI protocols, ISO having one of its  official languages be English (using Oxford spelling), thus Routeing is being used as the term "routeing" is the standard British English spelling of the present participle of the verb "to route".

Thanks,

Cristian.

[M02@rt37 · Accepted solution]

Hello David,

The ISO 10589 header come first to identify the protocol, the version, and PDU type and to provide fields shared by every PDU...while the Hello header only contains information relevant to Hellos.

This split avoid duplicating common fields in every PDU; so it implifies parsing, and allows extensibility—new pdu types or tlv can be added without changing the common header...

[Joseph W. Doherty]

@Mitrixsen , BTW, the TLV capability, as explicitly mentioned by both @Cristian Matei and M02@rt37, is one of the most interesting and useful features of IS-IS, especially for service providers.