Hop - Network jump

Enhanced Definition

In the context of z/OS and mainframe networking, a "hop" refers to a single segment of a data packet's journey across a network, from one network device (such as a router or gateway) to the next. It signifies the passage of a packet through an intermediate device on its path between a source and its destination, which could be the z/OS system itself or a client/server interacting with it.

Key Characteristics

- TCP/IP Stack Relevance: Hops are fundamental to how the z/OS TCP/IP stack routes packets to and from the mainframe, determining the path taken through the interconnected network infrastructure.
- Traceroute Utility: The TRACERTE (or TRACEROUTE) utility, available on z/OS (e.g., via TSO OMVS or NETSTAT commands), is used to identify the sequence of hops and the network devices a packet traverses.
- Performance Impact: Each hop introduces a small amount of latency and processing overhead. A higher number of hops can indicate a less direct or more complex network path, potentially impacting the response times of mainframe-hosted applications.
- Network Topology Reflection: The number and identity of hops directly reflect the underlying network topology and routing decisions between the z/OS LPAR and any remote endpoint.
- Time-to-Live (TTL): Each IP packet contains a Time-to-Live (TTL) field, which is decremented by one at each hop. If TTL reaches zero, the packet is discarded, preventing infinite loops.

Use Cases

- Troubleshooting Connectivity Issues: When a remote client cannot connect to a CICS region, DB2 subsystem, or other mainframe service, TRACERTE can pinpoint the exact hop where the connection is failing (e.g., a router blocking traffic or a misconfigured firewall).
- Performance Analysis: Analyzing the number of hops to critical application servers or client workstations helps identify potential network bottlenecks or suboptimal routing paths that might be affecting mainframe application performance.
- Network Path Verification: Confirming the intended network path for data flowing between distributed application servers and a z/OS data server (e.g., DB2 Connect) to ensure compliance with network policies.
- Firewall Rule Validation: Verifying that firewalls at intermediate hops are correctly configured to allow necessary inbound and outbound traffic to and from the z/OS system.

Related Concepts

Hops are a core concept in TCP/IP networking, which is extensively utilized by z/OS for all external communications. They are directly influenced by routing tables on network devices and the IP addresses assigned to interfaces. The z/OS TCP/IP stack manages the sending and receiving of packets, and the number of hops directly impacts network latency and throughput. Understanding hops is crucial for diagnosing issues related to network segmentation and firewall rules.

Best Practices:

Minimize Hops for Critical Paths: For high-performance or low-latency applications (e.g., CICS transactions, MQ messaging), design the network topology to minimize the number of hops between the client and the z/OS system.
Regular Path Monitoring: Implement network monitoring tools to periodically check the number of hops and path stability for key mainframe services to proactively identify routing changes or performance degradation.
Document Network Paths: Maintain clear and up-to-date documentation of network paths, including expected hops and intermediate devices, for all critical connections to and from z/OS to aid in rapid troubleshooting.
Secure Each Hop: Ensure that all intermediate network devices (routers, switches, firewalls) are properly secured, patched, and configured to protect data in transit to and from the mainframe.
Optimize Routing: Work with network administrators to ensure that routing protocols are optimized to provide the most direct and efficient paths to z/OS resources, avoiding unnecessary hops.