feat: BGP (Border Gateway Protocol) Header Support

[jecsand838]

This PR introduces initial support for BGP (Border Gateway Protocol version 4) by adding the src/bgp.rs file. This lays the groundwork for structuring BGP messages within the network-types library.

Key Features Added:

• BgpHdr Struct:

  • Represents the common BGP message header (19 octets: marker, length, type) as defined in RFC 4271, Section 4.1.
  • Includes an additional fixed-size byte array (specific_payload_bytes) to hold the initial fixed portion of message-specific payloads (i.e., for OPEN, UPDATE, NOTIFICATION, ROUTE-REFRESH messages). This design avoids heap allocations and unsafe unions while allowing access to these fields.
  • Provides methods to get/set common header fields (marker, length, msg_type).

• Message-Specific Layout Structs:

  • OpenMsgLayout: Defines offsets and total length for fixed fields in BGP OPEN messages (RFC 4271, Section 4.2).
  • UpdateInitialMsgLayout: Defines offsets and total length for the initial fixed field (Withdrawn Routes Length) in BGP UPDATE messages (RFC 4271, Section 4.3).
  • NotificationMsgLayout: Defines offsets and total length for fixed fields in BGP NOTIFICATION messages (RFC 4271, Section 4.5).
  • RouteRefreshMsgLayout: Defines offsets and total length for fixed fields in BGP ROUTE-REFRESH messages (RFC 2918, Section 3).

• Accessor Methods for Message-Specific Fields:

  • BgpHdr now includes methods to safely get and set fields for OPEN, UPDATE (initial part), NOTIFICATION, and ROUTE-REFRESH messages.
  • These methods internally use the msg_type to correctly interpret the specific_payload_bytes array based on the corresponding MsgLayout struct.
  • Getters return Option<T> to handle cases where the msg_type doesn't match or if the payload is unexpectedly short.
  • Setters return bool indicating success or failure ((i.e., if msg_type is incorrect or payload is too short).
  • The update_total_path_attr_len and set_update_total_path_attr_len methods are provided to access the "Total Path Attribute Length" field in UPDATE messages, which requires the full message byte slice due to the preceding variable-length "Withdrawn Routes" field.

• Constants:

  • Defines message type constants (e.g., BGP_OPEN_MSG_TYPE).

• Unit Tests

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This change is 

[jecsand838]

CC: @svencowart

[veronica-m-ef]

@jecsand838 based on the changes, I suggest this text to be the new PR description.

This PR introduces initial support for BGP (Border Gateway Protocol version 4) by adding the src/bgp.rs file, laying the groundwork for structuring BGP messages within the network-types library. The design prioritizes type safety, explicit structure, and efficient parsing.

Key Features Added:

• BgpHdr Enum:

  • Introduces an enum BgpHdr to represent the different types of BGP messages (Open, Update, Notification, KeepAlive, RouteRefresh). This provides a type-safe and ergonomic way to handle various BGP message headers.

• BgpFixedHdr Struct:

  • Defines the BgpFixedHdr struct, which encapsulates the common 19-octet BGP message header (marker, length, type) as specified in RFC 4271, Section 4.1.
  • Includes methods to get and set common header fields (marker, length, msg_type).

• Message-Specific Fixed Header Structs:

  • Introduces dedicated structs, each marked with #[repr(C, packed)], for the fixed-size portions of specific BGP message types:
    • BgpOpenFixedHdr: For BGP OPEN messages (RFC 4271, Section 4.2).
    • BgpUpdateFixedHdr: For the initial fixed fields (Withdrawn Routes Length, Total Path Attribute Length) in BGP UPDATE messages (RFC 4271, Section 4.3).
    • BgpNotificationFixedHdr: For BGP NOTIFICATION messages (RFC 4271, Section 4.5).
    • BgpRouteRefreshFixedHdr: For BGP ROUTE-REFRESH messages (RFC 2918, Section 3).
  • These structs contain the fields specific to their message type and provide direct accessor methods.

• Composite BGP Header Structs:

  • New structs such as BgpOpenHdr, BgpUpdateHdr, BgpNotificationHdr, BgpKeepAliveHdr, and BgpRouteRefreshHdr are introduced.
  • Each of these composite structs combines the BgpFixedHdr with its respective message-specific fixed header (e.g., BgpOpenHdr contains fixed_hdr: BgpFixedHdr and bgp_open_fixed_hdr: BgpOpenFixedHdr).
  • They provide accessor methods for all fields within their scope, offering type-safe and direct access without the need for byte array interpretation.

• parse_bgp_hdr! Macro:

  • A new macro parse_bgp_hdr! is added to simplify the parsing of a byte slice into the appropriate BgpHdr enum variant. This macro efficiently handles the common fixed header and then dispatches to the correct message-specific fixed header parsing based on the message type.

• BgpMsgType Enum:

  • Defines a type-safe BgpMsgType enum (Open, Update, Notification, KeepAlive, RouteRefresh) for clearer representation and usage of BGP message types.

• BgpError Enum:

  • Introduces a BgpError enum for structured error reporting related to BGP header parsing and field access (e.g., IncorrectMessageType, BufferTooShort).

• Unit Tests:

  • Includes comprehensive unit tests to ensure the correctness of header parsing, field accessors, and the overall structure.